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    CHAPTER III

    GEOCHEMISTRY OF SEDIMENTARY ROCKS:

    PROVENANCE, PALAEOWEATHERING AND TECTONIC

    SETTING 3.1 INTRODUCTION

    The mineralogical and chemical composition of clastic sedimentary rocks are

    controlled by various factors, including (1) the composition of their source rocks, (2)

    environmental parameters influencing the weathering of source rocks (e.g., atmospheric

    chemistry, temperature, rainfall and topography), (3) duration of weathering (4)

    transportation mechanisms of clastic material from source region to depocenter, (5)

    depositional environment (e.g., marine versus fresh water), and (6) post-depositional

    processes (e.g., diagenesis, metamorphism) (Hayashi et al., 1997). Numerous

    investigations are substantiating the above aspects pertaining to genesis of both ancient and

    modern siliciclastic sediments (e.g., Dickenson et al., 1983; Nesbitt and Young, 1982,

    1984; Bhatia, 1983; Roser and Korsch, 1988; McCann, 1991; Condie et al., 1992; Condie,

    1993; McLennan et al., 1993; Nesbitt et al., 1996; Cullers, 2000; Hessler and Lowe 2006;

    Nagarajan et al., 2007; Spalletti et al., 2008). Several studies have also been focused on the

    identification of palaeotectonic settings of provenances based on geochemical signatures of

    siliciclastic rocks (e.g., Dickinson and Suczek, 1979; Bhatia, 1983; Bhatia and Crook,

    1986; Roser and Korsch 1986; McLennan and Taylor, 1991). Among the terregenous

    sedimentary rocks, shales are considered to represent the average crustal composition of

    the provenance much better than any other siliclastic rocks (e.g., McCulloch and

    Wasserburg, 1978). Shales retain most of the mineral constituents of the source and their

    bulk chemistry preserves the near-original signature of the provenance and more faithfully

    reveal palaeoweathering conditions (e.g., Pettijohn, 1975; Graver and Scott, 1995).

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    The present note examines the geochemistry of sandstones, shales and limestones

    of Lower Jurassic Shemshak Formation, Kerman province, Central Iran and attempts to

    constrain their source rocks, palaeoweathering and tectonic setting of the provenance.

    Owing to limitations of analytical facilities, the present work is based on chemical analyses

    data of major and select trace elements of the investigated sandstones, shales and

    limestones of the study area.

    3.2 METHODOLOGY

    Thirteen representative samples of sandstones, four samples of shales and four

    samples of limestones were collected from fresh outcrops across the strike direction.

    Whole-rock chemical analyses were carried out at the laboratory of the Institute of geology

    and exploration for mineral deposits, Tehran, Iran. Major elements were analyzed by X-ray

    fluorescence spectrometry (XRF), as per the procedure given by Ahmedali (1989). Trace

    element concentrations were determined by ICP-MS (Inductively Coupled Plasma Mass

    Spectrometry) following the procedure described by Jenner et al., (1990). The analytical

    precession is better than 5% for major and trace elements.

    3.3 GEOCHEMISTRY OF SANDSTONES

    Major and select trace element concentrations of 13 samples of the sandstones are

    provided in table 3.1 along with ratios of select pair of major elements. In the analyzed

    samples, concentrations of major elements vary narrowly. The sandstones consist of 74.31

    to 77.32 wt.% SiO2; 7.87 to 10.08 wt.% Al2O3; 0.28 to 0.35 wt.% TiO2; 3.66 to 5.52 wt.%

    Fe2O3; 0.05 to 0.08 wt.% MnO; 2.43 to 2.89 wt.% CaO; 0.53 to 1.18 wt.% MgO; 0.34 to

    1.26 wt.% Na2O; 1.48 to 1.92 wt.% K2O and 0.10 to 0.16 wt.% P2O5. Total iron is

    expressed as Fe2O3. Among the analyzed trace elements, Co varies from 4.88 to 9.55 ppm;

    Ni, from 10 to 22 ppm; Cu from 12 to 28 ppm; Zn from 18 to 46 ppm; and Cr from 58 to

    74 ppm. Correlation coefficient data of major and trace elements are given in table 3.2.

  • 52

    Table 3.1 : Major (wt.%) and trace element (ppm) composition of sandstones.

    Elements PS(S1) PS(S2) PS(S3) PS(S4) PS(S5) PS(S6) PS(S7) PS(S8) PS(S9) PS(S10) PS(S11) PS(S12) PS(S13) Average

    values

    SiO2 74.61 75.08 74.31 76.33 75.63 75.45 74.64 75.29 74.53 75.82 77.32 75.91 74.55 75.34

    Al2O3 9.97 9.43 10.08 8.98 9.25 9.38 9.93 9.40 9.97 9.13 7.87 9.08 9.97 9.42

    TiO2 0.35 0.32 0.35 0.30 0.32 0.32 0.35 0.32 0.35 0.32 0.28 0.32 0.35 0.33

    Fe2O3T 5.37 4.87 5.52 3.95 4.61 4.62 5.37 4.73 5.42 4.41 3.66 4.39 5.41 4.79

    MnO 0.07 0.07 0.06 0.05 0.07 0.07 0.07 0.07 0.08 0.07 0.05 0.05 0.08 0.07

    CaO 2.83 2.79 2.89 2.60 2.70 2.74 2.83 2.74 2.82 2.68 2.43 2.68 2.84 2.74

    MgO 0.82 0.61 1.18 0.92 0.84 0.53 0.78 1.21 0.71 1.05 1.05 1.12 0.95 0.91

    Na2O 0.81 0.74 0.56 0.61 0.34 0.83 1.26 0.49 1.12 0.95 0.70 0.60 0.40 0.72

    K2O 1.88 1.74 1.92 1.57 1.65 1.67 1.87 1.64 1.89 1.62 1.48 1.60 1.90 1.73

    P2O5 0.15 0.14 0.16 0.11 0.13 0.14 0.15 0.13 0.15 0.13 0.10 0.11 0.15 0.13

    SO3 0.07 0.13 0.08 0.12 0.06 0.09 0.07 0.11 0.12 0.09 0.07 0.06 0.06 0.09

    L.O.I 3.01 3.91 2.73 3.96 3.97 3.85 2.59 3.68 2.71 3.52 4.59 3.79 3.12 3.49

    Total 99.94 99.83 99.84 99.50 99.57 99.69 99.91 99.81 99.87 99.79 99.60 99.71 99.78 99.76

    Co 7.84 9.26 5.47 7.59 4.89 6.26 6.31 8.44 5.21 6.84 4.88 7.35 9.55 6.91

    Ni 12.00 12.00 11.00 15.00 11.00 10.00 13.00 16.00 15.00 17.00 12.00 22.00 14.00 14.00

    Cu 12.00 14.00 26.00 23.00 19.00 23.00 20.00 18.00 25.00 19.00 17.00 22.00 28.00 20.00

    Zn 34.00 18.00 31.00 46.00 45.00 23.00 32.00 28.00 29.00 34.00 46.00 32.00 38.00 37.50

    Cr 66.00 73.00 69.00 74.00 69.00 64.00 65.00 58.00 64.00 69.00 71.00 72.00 73.00 68.00

    Al2O3 / TiO2 28.48 29.47 28.80 29.93 28.90 29.31 28.37 29.37 28.48 28.53 28.11 28.37 28.48 28.82

    SiO2 / Al2O3 7.48 7.96 7.37 8.50 8.18 8.04 7.51 8.00 7.47 8.30 9.82 8.36 7.47 8.04

    SiO2 / TiO2 213.10 234.60 212.30 254.40 236.30 235.80 213.20 235.30 212.90 236.90 276.10 237.20 213.00 231.62

    K2O / Na2O 2.32 2.35 3.43 2.57 4.85 2.01 1.48 3.35 1.69 1.70 2.11 2.66 4.75 2.71

    Al2O3 / K2O 5.30 5.42 5.25 5.72 5.60 5.61 5.94 5.73 5.27 5.63 5.31 5.67 5.24 5.51

    Cr / Ni 5.50 6.08 6.27 4.93 6.27 6.40 5.00 3.62 4.05 4.05 5.91 3.27 5.21 5.12

    Moles

    Al2O3 0.10 0.09 0.10 0.09 0.09 0.09 0.10 0.09 0.10 0.09 0.08 0.09 0.10 0.09

    CaO* 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.01 0.02 0.02 0.01 0.01 0.01 0.01

    Na2O 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.01 0.02 0.02 0.01 0.01 0.01 0.01

    K2O 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02

    CIA 53.93 53.56 54.97 54.71 56.03 53.49 51.79 55.40 52.50 52.71 52.31 54.46 55.85 53.98

    PIA 74.95 75.66 81.34 78.46 87.12 73.75 65.62 82.54 68.33 70.29 73.26 78.97 85.84 76.63

    CIW 78.98 79.95 84.58 81.78 89.34 77.55 70.55 85.35 73.07 74.52 77.48 82.25 88.41 80.29

    CIW 88.25 88.59 91.65 89.97 94.37 87.35 82.73 92.10 84.44 85.40 87.31 90.26 93.85 88.94

  • 53

    Table 3.2: Values of Pearsons coefficient of correlation of major and trace elements of sandstones.

    Element SiO2 Al2O3 TiO2 Fe2O3 MnO CaO MgO Na2O K2O P2O5 Co Ni Cu Zn Cr

    SiO2 1.00

    Al2O3 -0.98 1.00

    TiO2 -0.96 0.96 1.00

    Fe2O3 -0.98 0.95 0.97 1.00

    MnO -0.70 0.67 0.66 0.70 1.00

    CaO -0.99 0.98 0.94 0.96 0.67 1.00

    MgO 0.22 -0.22 -0.15 -0.19 -0.43 -0.22 1.00

    Na2O -0.19 0.19 0.27 0.22 0.22 0.16 -0.43 1.00

    K2O -0.95 0.93 0.95 0.98 0.64 0.91 -0.22 0.26 1.00

    P2O5 -0.95 0.92 0.91 0.95 0.74 0.94 -0.31 0.26 0.94 1.00

    Co -0.24 0.24 0.14 0.16 0.21 0.28 0.00 -0.24 0.14 0.10 1.00

    Ni 0.19 -0.14 -0.08 -0.22 -0.26 -0.17 0.48 0.00 -0.28 -0.42 0.26 1.00

    Cu -0.23 0.28 0.28 0.22 0.09 0.23 0.14 -0.13 0.27 0.20 -0.13 0.14 1.00

    Zn 0.52 -0.45 -0.38 -0.45 -0.41 -0.57 0.38 -0.34 -0.36 -0.50 -0.33 0.00 0.10 1.00

    Cr 0.32 -0.30 -0.28 -0.31 -0.43 -0.29 0.00 -0.30 -0.18 -0.33 0.17 0.32 0.10 0.38 1.00

    Sandstones are classified variously based on their chemical composition (e.g.,

    PettiJohn, 1975; Crook, 1974; Blatt et al., 1980; Herron, 1988). Sandstones of the present

    study were classified according to the scheme proposed by Herron (1988). In the bivariate log

    (Fe2O3/K2O) versus log (SiO2/Al2O3) diagram (after Herron, 1988) the sandstones plot in the

    field of litharenite (Fig. 3.1a). Crook (1974) classified the sandstones into 3 types (quartz-rich,

    quartz-intermediate and quartz-poor) based on their Na2O and K2O contents. In the K2O wt. %

    versus Na2O wt. % bivariate diagram (after Crook, 1974) the sandstones of the present study

    plot essentially in quartz-rich field (Fig. 3.1b). However, the sandstones have lower content of

  • 54

    SiO2 (average SiO2 content: 75.34 wt.%) in comparison with typical quartz-rich sandstones

    (average SiO2 content: 89 wt.%; Crook, 1974).

    Fig. 3.1: Geochemical classification of sandstones. (a) Log (Fe2O3/K2O) versus log (SiO2/Al2O3) bivariate diagram. (b) K2O wt.% versus Na2O wt.% bivariate diagram.

    Chemical analyses data reveal several geochemical features of the sandstones.

    Composition of non-quartz component of the sandstones can be evaluated from the values of

    Index of Compositional Variation (ICV) of Cox et al., (1995) in which ICV = (Fe2O3 + K2O +

    Na2O + CaO + MgO + MnO + TiO2) / TiO2. ICV values of the sandstones of the present study

    vary from 1.10 to 1.23 (average 1.18). Values of ICV more than 1 indicate the presence of less

    clay minerals and more rock forming minerals such as plagioclase, K-feldspar, amphiboles,

    pyroxenes and lithics (Cox et al., 1995). It is known that the values of K2O/Al2O3 of clays are

  • 55

    less than 0.3 and the values of the same ratio of feldspars range from 0.3 to 0.9. The values of

    K2O/Al2O3 ratio of the sandstones vary narrowly from 0.17 to 0.19 (average = 0.18). These

    values indicate preponderance of clay minerals over k-bearing minerals such as K-feldspars

    and micas (Cox et al., 1995).

    Among the major elements SiO2 shows negative correlation with all major elements,

    barring MgO, which it shows weak positive correlation (r = 0.22). MgO, inturn, is negatively

    correlated with all other major elements. All other major elements (ie., Al2O3, TiO2, Fe2O3,

    MnO, CaO, Na2O and K2O) exhibit positive correlations between themselves. Negative

    correlations of SiO2 with major elements (except MgO) confirm that bulk of the SiO2 is

    present as quartz grains. Values of Al2O3/TiO2 ratio of the sandstones are high (average =

    28.82 wt.%) and indicate derivation of the detrital material from a continental source (Fyffe

    and Pickerill, 1993). SiO2 shows strong negative correlation with K2O (r = - 0.95) indicating

    decrease of clay content with increase of quartz. Al2O3 shows strong positive correlation with

    K2O (r = 0.93). This co-variation indicates that K-bearing minerals have significant influence

    on Al distribution and suggests that the bluk of Al and K is primarily contributed by clay

    minerals (e.g., illite) (McLennan et al., 1983; Jin et al., 2006). However, contribution of Al

    and K from feldspars and mica is also indicated from Al2O3/K2O ratios (average = 5.51). The

    values of K2O/Na2O ratio vary from 1.48 to 4.85. Slightly high values of K2O/Na2O ratio

    indicate the presence of K-bearing minerals such as K-feldspar, muscovite and biotite

    (McLennan et al., 1983; Nath et al., 2000; Osae et al., 2000). Low contents of TiO2 (average =

    0.33 wt.%) indicate the presence of phyllosilicates in minor amounts (Dabard, 1990; Condie et

    al., 1992). Na2O exhibits negative correlations with SiO2 (r = - 0.19) and MgO (r = - 0.43).

    MgO shows negative correlation with CaO (r = - 0.22). These relations rule out the possibility

  • 56

    of the presence of smectite. Presence of MgO in sandstones and its positive correlation with

    SiO2 (r = 0.22) indicate the presence of biotite. (Fe2O3 + MgO) which exhibits strong positive

    correlation with Al2O3 (r = 0.90) and this co-variance suggests the presence of chlorite and

    ferromagnesian minerals (eg., biotite, hornblende). High content of Fe2O3 (average 4.79 wt.%)

    indicates that a part of the Fe2O3 was possibly precipitated as limonite/goethite during

    sedimentation and/or diagenesis. CaO exhibits strong negative correlation with SiO2 (r = -

    0.99) suggesting that the carbonates are primary rather than secondary, because the presence

    of secondary carbonates could result in scattered plots in the SiO2 wt.% versus CaO wt.%

    diagram (Feng and Kerrich, 1990).

    Among the analyzed transitional elements (Co, Cu, Ni, Zn, Cr), Co and Cu exhibit

    positive correlations with Al2O3 (r = 0.24; 0.28), TiO2 (r = 0.14; 0.28), Fe2O3 (r = 0.16; 0.22),

    MnO (r = 0.21; 0.09), CaO (r = 0.28; 0.23) and K2O (r = 0.14; 0.27). Positive correlations of

    Co and Cu with Al2O3, TiO2 and K2O indicate that Co and Zn are the adsorbed constituents of

    clay minerals. Ni and Zn show positive correlations with MgO (r = 0.48; 0.38), the latter, as

    mentioned earlier, exhibits positive correlations with SiO2 (r = 0.22). Cr shows positive

    correlations with SiO2 (r = 0.32), Ni (r = 0.32) and Zn (r = 0.38). Ni shows positive

    correlations with Co (r = 0.26) and Cu (r = 0.14). The sandstones have low contents of Cr

    (average = 68 ppm) and Ni (average = 14 ppm). Cr/Ni ratios vary from 3.27 to 6.4 ppm

    (average = 5.12 ppm). These features indicate that Cr, Ni and Zn constitute components of

    clay minerals and other detrital material (e.g., chlorite, ferromagnesian minerals) (McCann,

    1991).

    3.3.1 Provenance

  • 57

    Several investigations have demonstrated that the mineralogical and chemical

    compositions of siliciclastic sedimentary rocks are related to those of their source and this

    tenet has been used to characterize the source rocks from which the investigated sedimentary

    rocks were derived (e.g., Kalsbeek and Frei, 2010). Detrital framework composition of

    sandstones is a reliable indicator of the tectonic setting of the provenance. Detrital mode-based

    ternary diagrams of Dickinson and Suckzek (1979), Dickinson et al., (1983) and Dickinson

    (1985) are widely used to constrain the tectonic regime of the provenance of sandstones.

    Modal analyses of framework components of 4 sandstone samples of the present study were

    carried out (Table 3.3) and the obtained data have been plotted on the Qt-F-L ternary diagram

    of Dickinson et al., (1983). The plots indicate the provenance of recycled orogen for the

    studied sandstones (Fig. 3.2). Quartzolithic composition of the sandstones testifies their

    derivation from a recycled source region. Source components of recycled orogens, according

    to Dickenson (1985) are predominantly sedimentary strata and subordinate volcanic rocks,

    partly metamorphosed, exposed to erosion by the orogenic uplift of fold belts and thrust

    sheets. According to Roser and Korsch (1988), recycled sources represent quartzose sediments

    of mature continental provenance and the derivation of the sediments could be from a highly

    weathered granite-gneiss terrain and / or from a pre-existing sedimentary terrain.

    Table 3.3: Modal data of detrital framework components of sandstones.

    Sample No Quartz Feldspar Rock fragment

    1 66.80 2.50 30.71

    2 69.15 1.68 29.10

    3 65.93 3.12 30.84

    4 66.32 2.32 31.26

    Average 67.05 2.41 30.48

  • 58

    Fig. 3.2: Qt F L tectonic setting discrimination diagram (after Dickinson et al., 1983). Qt = Total quartz, F = Feldspar, L = Lithic fragments including polycrystalline quartz.

    In the published literature several major-, trace-, rare-earth element-based

    discrimination diagrams are proposed to decipher the source rocks / provenance of the

    siliciclastic sedimentary rocks (e.g., Taylor and McLennan, 1985; Floyd and Leveridge, 1987.,

    Roser and Korsch, 1988; Floyd et al., 1989, 1990; McLennan et al., 1993; Condie, 1993; Gu et

    al., 2002; Cingolani et al., 2003; Asieudu et al., 2004). On the major element-based

    provenance discriminant function diagram of Roser and Korsch (1988). The sandstones of the

    present study plot in the field of quartzose sedimentary provenance (Fig. 3.3).

  • 59

    Fig.3.3: Provenance discrimination diagram for sandstones (after Roser and Korsch, 1988). Discriminant Function 1 = (-1.773TiO2%) + (0.607Al2O3%) + (0.76Fe2O3T%) + (-1.5MgO%) + (0.616CaO%) + (0.509Na2O%) + (-1.22K2O%) + (-9.09). Discriminant Function 2 = (0.445TiO2%) + (0.07Al2O3%) + (-0.25Fe2O3T%) + (-1.142MgO%) + (0.432Na2O%) + (1.426K2O%) + (-6.861).

    Ancient quartzose sedimentary source terrains of siliciclastic sediments may be

    composed of varied sedimentary lithounits (e.g., sandstones, shales, argillites, siltstones and

    their metamorphic equivalents) and the detrital components involved in their formation may

    be derived from different crystalline rocks and/or pre-existing sedimentary rocks. For

    example, in the bivariate provenance discrimination diagram of Roser and Korsch (1988) the

    boundaries of the P4 field (quartzo sedimentary provenance) were drawn based on the

    chemical composition data of rock types of a certain terrain in New Zealand. This terrain is

    composed of sandstones and argillites of Ordovician Greenland Group. These sedimentary

    rocks were derived either from a deeply weathered granitic-gneissic terrain (Laird, 1982) or

    from a pre-existing sedimentary terrain (Nathan, 1976). The rock types of the proximal

    quartzose sedimentary provenance of the sandstones of the present study can not be

  • 60

    deciphered from the geochemical data of the sandstones. However, the bulk chemical

    composition (in terms of SiO2 content) of the ultimate crystalline rock provenance of the

    sandstones can be inferred by using a simple equation provided by Hayashi et al., (1997).

    Hayashi et al., while proposing the provenance discrimination equation, provided in brief the

    following justification.

    The Al2O3 and TiO2 contents of the siliciclastic sedimentary rocks are considered as

    significant indicators of their provenance. During weathering of source rocks, Al and Ti

    remain essentially immobile, owing to low solubility of their oxides and hydroxides in low

    temperature aqueous solutions (e.g., Stumm and Morgan, 1981; Yamamoto et al., 1986;

    Sugitani et al., 1996). Hence, the values of Al/Ti ratios of residual soils can be considered to

    be very close to those of their parent igneous rocks. In the residual detrital products of igneous

    rocks, Al typically resides in clays (e.g., kaolinite, illite, smectite), micas and residual

    feldspars. Ti occurs as a chemical constituent of clays and mafic minerals (e.g., biotite,

    chlorite, ilmenite inclusions in silicate minerals). Several studies have shown that during

    fluvial transportation of these minerals Al and Ti involve insignificant fractionation (e.g.,

    Yamamoto., et al., 1986). Hence, the Al2O3/TiO2 ratios of sedimentary rocks derived from

    fluvially transported detrital siliciclastic material should be practically similar to those of their

    magmatic source rocks.

    It is well known that in normal igneous rocks Al resides mostly in feldspars and Ti in

    mafic minerals (e.g., olivine, pyroxene, hornblende, biotite, ilmenite). Therefore, the Al/Ti

    ratios of igneous rocks gradually increase with increasing SiO2 contents (Poldervaart, 1955;

    Holland, 1984). Al2O3/TiO2 ratios increase (1) from 3 to 8 in mafic igneous rocks (SiO2

    content from 45 to 52 wt.%), (2) from 8 to 21 in intermediate igneous rocks (SiO2 content

  • 61

    from 53 to 66 wt.%) and (3) from 7 to 21 in felsic igneous rocks (SiO2 content from 66 to 76

    wt.%). According to Hayashi et al., (1997) the SiO2 content of normal igneous rocks can be

    evaluated from their Al2O3/TiO2 ratio using the following equation:

    SiO2 (wt. %) = 39.34 + 1.2578 (Al2O3/TiO2) 0.0109 (Al2O3/TiO2)2

    Since Al and Ti are immobile and behave similarly during weathering and

    transportation, the Al2O3/TiO2 ratios of the siliciclastic sedimentary rocks can be considered as

    practically equal to those of their source. Hence, the silica content of the source rocks can be

    constrained from the Al2O3/SiO2 ratios using of the siliciclastic sedimentary rocks of the

    above equation.

    When Al2O3/TiO2 ratios of the sandstones of the present study are substituted in the

    equation of Hayashi et al., (1997), the SiO2 content of the theoretically inferred magmatic

    source rock of the sandstones are found to range narrowly from 66.32 to 69.15 wt. % (average

    67.05 wt.%). These values of SiO2 content suggest that the parent rocks of the quartzose

    sedimentary provenance of the sandstones could be felsic igneous rocks. SiO2 content (~ 67

    wt.%) indicates that the inferred magmatic source rocks constitute lowest SiO2-bearing end

    member of the felsic group, bordering intermediate group of igneous rocks. McLennan et al.,

    (1980), recognizing the significance of Al and Ti in the provenance studies have accordingly

    proposed Al2O3 wt.% versus TiO2 wt.% bivariate discrimination diagram to constrain

    provenance of siliciclastic rocks. On this diagram the sandstones of the present study plot

    along granodiorite trend line (Fig. 3.4a). The abundances of Cr and Ni in siliclastic sediments

    are also considered as an useful indicator in provenance studies. Floyd et al., (1989) proposed

    a bivarite TiO2 wt.% versus Ni (ppm) diagram to evaluate source rock composition. On this

    diagram the sandstones plot falls in the field of acidic igneous rocks (Fig. 3.4b).

  • 62

    Fig.3.4: Provenance discrimination diagrams. (a) TiO2 wt.% versus Al2O3 wt.% bivariate plot

    (after McLennan et al., 1980). The granite line and 3 granite + 1 basalt line are after Schieber, (1992). (b) TiO2 wt.% versus Ni (ppm) bivariate plot (after Floyd et al., 1989).

    The provenance discriminant function diagram of Roser and Korsh (1988), provenance

    discrimination equation of Hayashi et al., (1997), and provenance discrimination diagrams of

    McLennan et al., (1980), and Floyd et al., (1989), together suggest that the detritus involved in

    the formation of sandstones were derived from quartzose sedimentary provenance, the bulk

    chemical composition of which is granodioritic. Granodioritic terrain may constitute the

    lithological provenance of the quartzose sedimentary rocks. Thus, for the sandstones of the

    present study quartzose sedimentary terrain constitutes Proximal provenance, and

    granodioritic terrain, ultimate provenance. Quartzose sedimentary provenances, according

    to Roser and Korsch (1988), constitute suites at either passive continental margins,

    intracratonic sedimentary basins or recycled orogenic provinces.

    3.3.2 Palaeoweathering

    Intensity of chemical weathering of source rocks is controlled mainly by source rock

    composition, duration of weathering, climatic conditions and rates of tectonic uplift of source

    region (e.g., Wronkiewicz and Codie, 1987). About 75% of the labile material of the upper

  • 63

    crust is composed of feldspars and volcanic glass and chemical weathering of these materials

    ultimately results in the formation of clay minerals (e.g., Nesbitt and Young, 1984, 1989;

    Taylor and McLennan, 1985; Fedo et al., 1995). During chemical weathering Ca, Na and K

    are largely removed from source rocks. The amount of these elements surviving in soil

    profiles and in sediments derived from them is a sensitive index of the intensity of chemical

    weathering (Nesbitt et al., 1997). If the siliciclastic sedimentary rocks are free from alkali-

    related post-depositional modifications, then their alkali contents (K2O + Na2O) and

    K2O/Na2O ratios are considered as reliable indicators of intensity of weathering of source

    material (e.g., Lindsey, 1999).

    The degree of source rock weathering is quantified variously. A few indices of

    weathering have been proposed based on molecular proportions of mobile and immobile

    element oxides (Na2O, CaO, K2O and Al2O3). Among the known indices of

    weathering/alteration the Chemical Index of Alteration (CIA; Nesbitt and Young, 1982) is

    well established as a method of quantifying the degree of source weathering. Source

    weathering and elemental redistribution during diagenesis also can be assessed using

    Plagioclase Index of Alteration (PIA; Fedo et al., 1995) and Chemical Index of Weathering

    (CIW; Harnois, 1988). For quantifying source weathering of carbonate-bearing siliciclastic

    rocks, a modified version of CIW (CIW; Cullers, 2000) is also considered (e.g., Jafarzadeh

    and Hosseini-Barzi, 2008). The equations of the above indices are:

    CIA = {Al2O3 / (Al2O3 + CaO + Na2O + K2O)} x 100

    PIA = {(Al2O3 K2O) / ((Al2O3 - K2O) + CaO + Na2O)} x 100

    CIW = {Al2O3 / (Al2O3 + CaO + Na2O)} x 100

    CIW = {Al2O3 / (Al2O3 + Na2O)} x 100

  • 64

    In the above equations the major oxides are expressed in molar propotions and CaO is

    the content of CaO incorporated in silicate fraction.

    The CaO content of the sandstones of the present study varies from 2.43 to 2.89 wt.%

    (average = 2.74 wt.%). The P2O5 content ranges from 0.10 to 0.16 wt.%. There is no direct

    method to distinguish and quantify the contents of CaO belonging to silicate fraction and non-

    silicate fraction (carbonates and apatite). CIA values were determined, assuming the entire

    CaO content of the sandstones as that of silicate fraction and the obtained values vary from

    51.79 to 56.03% (average 53.98%). These unrealistic values underestimate the intensity of

    chemical weathering of the recycled sediment of the present study and hence can not be

    considered. McLennan (1993) proposed an indirect method for quantifying CaO content of

    silicate fraction assuming reasonable values of Ca/Na ratios of silicate material. Procedure for

    quantification of CaO content (CaO) of silicate fraction involves subtraction of molar

    proportion of P2O5 from the molar proportion of total CaO. After subtraction, if the

    remaining number of moles is found to be less than the molar proportion of Na2O, then the

    remaining number of moles is considered as the molar proportion of CaO of silicate

    fraction. If the remaining number of moles is greater than the molar proportion of Na2O,

    then the molar proportion of Na2O is considered as the molar proportion of CaO of silicate

    fraction (CaO). Since Ca is typically lost more rapidly than Na during weathering, this

    procedure of calculation of CIA values is likely to yield minimum CIA values, by upto about 3

    units (McLennan, 1993).

    The above mentioned methodology of quantification of CaO content of non-silicate

    fraction of siliciclastic rocks can provide reliable results if the ultimate source under

    consideration consists of more or less equal proportions of moles of CaO and Na2O. In the

  • 65

    present study granodiorite was inferred as the ultimate source of the sandstones. A typical

    granodiorite consist of more or less equal amounts of CaO (2.95 wt.%) (ie., 0.0524 moles of

    Ca) and Na2O (3.31 wt.%) (ie., 0.534 moles of Na). Following the procedure provided by

    McLennan (1993) CIA, PIA and CIW values have been determined and the obtained results

    along with CIW values are presented in Table 3.1. According to CIA values, the degree of

    source weathering varies from 61.69 to 76.19% (average = 69.07%). PIA values indicate the

    intensity of weathering varying from 65.62 to 87.12% (average = 76.63%). CIW values

    suggest the degree of source weathering in the range from 70.55 to 89.34% (average =

    80.29%). CIW values vary from 82.73 to 94.37% (average = 88.94%). Average PIA and CIW

    values (76.63%; 80.29%) indicate higher degree of source weathering than the degree of

    weathering inferred from CIA values (69.07%). Average CIA value (70%) indicates moderate

    source weathering, whereas the average PIA and CIW values (76% and 80%) suggest

    moderate to intense weathering of the source material. If average CIW values (89%) are

    considered, then we have to infer intense source weathering. The petrographic and chemical

    analyses data (especially Na2O and K2O contents) do not suggest intense weathering of the

    source area. Hence, relying on PIA and CIW values, it can be concluded that the provenance

    of the sandstones was subjected to moderate to intense chemical weathering.

    The above inferred index of chemical weathering/alteration is the sum total of

    intensities of chemical weathering witnessed by lithocomponents during atleast two cycles of

    sedimentation involving (1) chemical weathering of the source rocks, (ultimate granodioritic

    source and proximal quartzose sedimentary source), (2) chemical weathering during fluvial

    transport of the detritus, (3) chemical weathering of the detritus in depocenters, and (4)

    chemical weathering during diagenesis.

  • 66

    PIA monitors and quantifies progressive weathering of feldspars to clay minerals (e.g.,

    Fedo et al., 1995; Armstrong-Altrin et al., 2004). PIA values of the sandstones suggest

    moderate to intense destruction of feldspars during source weathering, transport,

    sedimentation, and diagenesis. During the initial stages of weathering Ca is leached rapidly

    than Na and K. With increasing weathering total alkali content (K2O + Na2O) decreases with

    increase in (K2O/Na2O) ratio. This is due to destruction of feldspars among which plagioclase

    is more preferentially removed than K-feldspars (Nesbitt and Young, 1984; Nesbitt et al.,

    1996).

    Feldspathic material of the sandstones of the present study was subjected to variable

    intensities of weathering during different stages of evolution of the sandstones. Sandstones

    preserve the imprints of chemical weathering witnessed by feldspathic components. The

    mobility of elements during final stages of chemical weathering of previously and variously

    altered feldspars may be visualized from bivariate plots involving Na2O, K2O, CaO, SiO2 and

    PIA. In the (K2O/Na2O) versus PIA diagram the values of (K2O/Na2O) ratio increase with

    increasing values of PIA (r = 0.95; Fig. 3.5a). In the (K2O + Na2O) wt.% versus PIA diagram,

    the total content of alkalies decreases with increasing values of PIA (r = - o.32; Fig. 3.5b).

    Weak negative correlation between (K2O + Na2O) wt.% and PIA may be attributed to the

    presence of K-bearing minerals (e.g., muscovite and illite). The behavior of Na during

    progressing weathering of feldspars is clearly seen in the bivariate Na2O wt.% versus PIA

    diagram (Fig. 3.6a). This diagram vividly shows decrease of Na2O content with increase of

    values of PIA (r = - 0.96). In the CaO wt.% and K2O wt.% versus PIA diagrams (Fig. 3.6b and

    c) decrease of CaO and K2O with increase of PIA is noticed and the weakly displayed negative

    correlations between CaO wt.% and PIA (r = - 0.05) and K2O wt.% and PIA (r = - 0.24) may

  • 67

    be the consequence of the presence of carbonates, minor K-bearing primary minerals (e.g.,

    muscovite, biotite) and retention of bulk of the mobilized K by aluminous material leading to

    the formation of illite.

    60 65 70 75 80 85 900

    1

    2

    3

    4

    5

    6

    (a) r = 0.95

    K2O

    / N

    a2O

    PIA

    60 65 70 75 80 85 908

    9

    10

    11

    12

    13

    14

    (b) r = - 0.32

    K2O

    + N

    a2O

    PIA

    Fig.3.5: Bivariate diagrams depicting mobility of elements during weathering of feldspars. (a)

    (K2O/Na2O) wt.% versus PIA. (b) (K2O + Na2O) wt.% versus PIA.

    The above described and documented imprints of progressing chemical weathering of

    detrital feldspars of the sandstones leaves an impression (may be misleading) that major event

    of chemical weathering of detrital feldspars has taken place essentially in the terminal basin

    (depocenter of the sandstones) prior to the lithification of the detritus and during diagenesis.

    The observed systematic depletion of Na2O content with increasing degree of chemical

    weathering of the feldspars (Fig. 3.6a) lends support to the above speculation. Further, it is

    known that detrital feldspar grains can survive more than one sedimentary cycle. Much of the

    alteration of feldspars is usually attributed to weathering contemporaneous with origin of

    sandstones (Blatt et al., 1980).

  • 68

    60 65 70 75 80 85 901.4

    1.6

    1.8

    2.0

    (c) r = - 0.24

    K2O

    %

    PIA

    60 65 70 75 80 85 900.0

    0.4

    0.8

    1.2

    1.6

    2.0

    (a) r = - 0.97

    Na

    2O

    %

    PIA

    Fig.3.6: Bivariate diagrams depicting mobility of Na, Ca and K during processing weathering

    of feldspars. (a) Na2O wt.% versus PIA. (b) CaO wt.% versus PIA. (c) K2O wt.% versus PIA.

    Mobility of elements during the progress of chemical weathering of source material

    and post-depositional chemical modifications of the sandstones can be evaluated by plotting

    the molar proportions of Al2O3, Na2O and CaO (CaO in silicate fraction) in A-CN-K ternary

    diagram (Nesbitt and Young, 1982, 1984). In the A-CN-K diagram (A = Al2O3; CN = CaO +

    Na2O; K = K2O) the sandstones plot above plagioclase-potash feldspar join (Fig. 3.7). The

    plots define a narrow linear trend and the trend line runs slightly at an angle to A-CN edge.

    This is primarly because removal rates of Na and Ca from plagioclase or volcanic glass are

    generally greater than the removal rates of K from microcline or volcanic glass (Nesbitt and

    60 65 70 75 80 85 902.2

    2.4

    2.6

    2.8

    3.0

    (b) r = - 0.05

    CaO

    %

    PIA

  • 69

    young, 1984) edge. The plots trend towards illite on the A-K edge (Fig. 3.7). The plots do not

    show any inclination towards the K apex, thus indicating that the sandstones are free from

    potash metasomatism during diagenesis. The trend line when extended backward intersects the

    plagioclase-potash feldspar join near the field of granodiorite (potential ultimate source).

    Linear weathering trend suggests steady state of weathering conditions where material

    removal matches with production of weathering material (Nesbitt et al., 1997, Nesbit and

    Young, 2004).

    Fig.3.7: A-CN-K ternary weathering diagram. A = Al2O3; CN = (CaO + Na2O); K = K2O (all

    in molar proporations). CaO is the CaO content of silicate fraction. CIA = Chemical Index of Alteration.

    3.3.3 Sediment Maturity and Climatic Conditions During Sedimentation

    SiO2/Al2O3 ratios of siliciclastic rocks are sensitive to sediment recycling and

    weathering process and can be used as an indicator of sediment maturity. With increasing

    sediment maturity, quartz survives preferentially to feldspars, mafic minerals and lithics

    (Roser and Korsch, 1986; Roser et al., 1996). Average SiO2/Al2O3 ratios in unaltered igneous

    rocks range from ~ 3.0 (basic rocks) to ~ 5.0 (acidic rocks). Values of SiO2/Al2O3 ratio > 5.0

  • 70

    in sandstones are an indication of progressive maturity (Roser et al., 1996) As mentioned

    earlier, the average Qt : F : L ratio of the detrital framework components of the sandstones is

    67.25: 2.41: 30.48 (Table 3.3). The SiO2/Al2O3 ratios of the sandstones vary from 7.3 to 9.82

    (average = 8.04). Values of K2O/Na2O ratio range from 1.48 to 4.85 (average = 2.71).

    Presence of substantial quantities of lithics and feldspar (33 wt.%), low values of SiO2/Al2O3

    ratios and high values of K2O/Na2O together indicate low to moderate sediment maturity. It is

    to be noted that the detrital material of the sandstones, which has witnessed atleast two cycles

    of chemical weathering at source regions, during transportation, and in depocenters still

    remain poorly to moderately matured. The low to moderately matured nature of the sandstones

    suggests active uplift of the source region, rapid erosion of the variously weathered source

    material, short distance fluvial transport and poor sorting.

    Weathering indices of sedimentary rocks can provide useful information of tectonic

    activity and climatic conditions in the source region. Increase of degree of chemical

    weathering may reflect the decrease in tectonic activity and/or change of climate towards

    warm and humid conditions which are more favourable for chemical weathering in source

    region (e.g., Jacobson et al., 2003). The sandstones of the present study consist of minor

    quantities (up to 3 wt. %) of fresh and altered feldspars. Presence of fresh and altered feldspars

    would indicate humid and warm climatic conditions (Folk, 1980). Suttner and Dutta (1986)

    proposed a binary SiO2 wt. % versus (Al2O3 + K2O + Na2O) wt. % diagram to constrain the

    climatic condition during sedimentation of siliciclastic sedimentary rocks. On this diagram the

    sandstones plot essentially in the field of humid climate (Fig. 3.8). Deposition of the

    sandstones of the present study under humid climatic conditions can also be inferred from the

    occurrence of co-bedded coal seams near Pabedana and Hashuni areas.

  • 71

    Fig. 3.8: Chemical maturity of sandstones and their palaeoenvironment of deposition based on

    SiO2 wt.% versus (Al2O3 + K2O + Na2O) wt.% bivariate diagram ( after Suttner and Dutta, 1986).

    3.3.4 Tectonic setting

    Several studies have shown that the chemical compositions of siliciclastic sedimentary

    rocks are significantly controlled by plate tectonic settings of their provenances and

    depositional basins, and as a result, the siliciclastic rocks from different tectonic settings

    posses terrain-specific geochemical signatures (Bhatia, 1983; Bhatia and crook, 1986; Roser

    and Korsch, 1986). To infer tectonic setting of provenance of ancient siliciclastic sedimentary

    rocks, several major-trace-rare earth element-based discrimination diagrams have been

    proposed (Maynard et al., 1982; Bhatia, 1983; Bhatia and Crook, 1986; Roser and Korsch,

    1986; McLennan et al., 1990; McLennan and Taylor, 1991, Floyd et al., 1991, Girty and

    Barber, 1993; Kroonenberg, 1994; Murphy, 2000).

    Among the various tectonic setting discrimination diagrams, the major element-based

    discrimination diagrams of Bhatia (1983) and Roser and Korsch (1986) are widely used. In the

    discrimination diagrams of Bhatia (1983) and Roser and Korsch (1986) the bivariates,

    including discriminant functions, are based on immobile and variably mobile major elements,

    including Na2O and K2O. Spalletti et al., (2008) are of the view that usage of highly mobile K-

  • 72

    and Na-based discrimination diagrams has to be considered cautiously. Van de Kamp and

    Leake (1985) observed discrepancies in tectonic settings inferred from the fields proposed by

    Bhatia (1983). Armstrong- Altrin and Verma (2005) tested the functioning of 6 major element-

    based tectonic setting discrimination diagrams and found to yield inconsistent results.

    According to them, the tectonic discrimination fields shown in several major element-based

    discrimination diagrams may not be fully representative of world-wide siliciclastic rocks.

    According to McLennan et al., (1993), the tectonic setting discrimination diagrams can

    provide reliable results for siliciclastic rocks that have not been strongly affected by post-

    depositional weathering/metasomatism/metamorphism. In view of the uncertainties of the

    major element-based discrimination diagrams, the present authors have considered almost all

    the available major element-based discrimination diagrams to arrive at the best possible

    inference on tectonic setting of the provenance of the sandstones of the present study.

    Chemical analyses data of the sandstones have been plotted on 8 tectonic setting

    discrimination diagrams of Bhatia (1983), Roser and Korsch (1986), Kroonenberg (1994),

    Murphy (2000) and Maynard (1982). In the (Fe2O3 + MgO) wt.% versus TiO2 wt.% and

    (Fe2O3T + MgO) wt.% versus Al2O3/SiO2 diagrams of Bhatia (1983) the sandstones plot in and

    near active continental margin (ACM) field ( Fig. 3.9 a and b). In the (Fe2O3T + MgO) wt.%

    versus K2O/Na2O diagram of Bhatia (1983) the sandstones plot near and towards the ACM

    field (Fig. 3.9c). In the discriminant function diagram of Bhatia (1983) the sandstones clearly

    plot in ACM field (Fig. 3.9d). In the widely used K2O/Na2O versus SiO2 wt.% diagram of

    Roser and Korsch (1986) the sandstones clearly plot in passive margin (PM) field (Fig. 3.9e),

    but in the modified version of the same bivariate diagram (after Murphy, 2000) the sandstones

    plot essentially in ACM field ( Fig. 3.9f ). In the SiO2/20 (K2O + Na2O) (TiO2 + Fe2O3 +

  • 73

    MgO) ternary diagram of Kroonenberg (1994) the sandstones plot in the overlapping fields of

    oceanic island arc, continental island arc and active continental margin (Fig. 3.9g). In the

    SiO2/Al2O3 versus K2O/Na2O diagram of Maynard et al., (1982) the sandstones plot in PM

    field (Fig. 3.9h).

    Plottings in majority of the tectonic setting discrimination diagrams indicate that the

    sandstones of the present study evolved in active continental margin setting. The observed

    plottings of sandstones in two distinctly different tectonic setting fields (ie., ACM and PM)

    may be attributed to either deficient functioning of the diagrams and/or abnormal (?) chemical

    composition of the sandstones of the present study. As mentioned earlier, the sandstones

    contain 2.43 to 2.89 wt.% CaO and are characterized by high K2O/Na2O ratios. Plottings in

    discrimination diagrams having K2O/Na2O ratio as one among the two bivariates indicated the

    passive margin setting (Fig. 3.9 e and h). The obtained inconsistent results may be attributed to

    the high CaO and K2O and low Na2O contents of the sandstones. No comment can be made on

    the validity of the field boundaries of tectonic settings shown in the diagrams of the various

    authors.

    In active continental margin settings, sediments are deposited at subduction arc basins,

    strike-slip margins, and in proximal portions in back-arc basins (Alvarez and Roser, 2007).

    During the present study, the tectonic setting inferred for the provenance and depositional

    basin of lower Jurassic sandstones of the Shemshak Formation of the Kerman basin is in

    agreement with the tectonic evolutionary history of the central Iran.

  • 74

  • 75

    Fig. 3.9: Tectonic setting discrimination diagrams based on major element composition of sandstones. (a) (Fe2O3

    T + MgO) wt.% versus TiO2 wt.% diagram (Bhatia, 1983). PM: Passive Margin., ACM: Active Continental Margin., CIA: Continental Island Arc., OIA=Oceanic Island Arc. (b) (Fe2O3

    T + MgO) wt.% versus Al2O3/SiO2 diagram (Bhatia, 1983). Field setting symbols as in Fig 9a. (c) (Fe2O3

    T + MgO) wt.% versus K2O/Na2O diagram (Bhatia, 1983). Field setting symbols as in Fig 9a. (d) Bivariate discriminant functions diagram (Bhatia, 1983). Discriminant Function 1 = (-0.0447SiO2%) + (-0.972TiO2%) + (0.008Al2O3%) + (-0.267Fe2O3%) + (0.208FeO%) + (-3.082MnO%) + (0.140MgO%) + (0.195CaO%) + (0.719Na2O%) + (-0.032K2O%) + (7.510P2O5%). Discriminant Function 2 = (-0.421SiO2%) + (1.988TiO2%) + (-0.526Al2O3%) + (-0.551 Fe2O3%) + (-1.610FeO%) + (2.720MnO%) + (0.881MgO) + (-0.907CaO%) + (-0.117Na2O%) + (-1.840K2O%) + (7.244P2O5%). (e) SiO2 wt.% versus K2O/Na2O diagram (Roser and Korsch 1986). (f) SiO2 wt.% versus K2O/Na2O diagram (Roser and Korsch, 1986), modified after Murphy (2000) CR = Continental Rift. (g) SiO2 / 20 wt.% - (K2O + Na2O) wt.% - (TiO2 + Fe2O3 + MgO) wt.% ternary diagram (Kroonenberg, 1994). A: Oceanic island arc., B: Continental island., C: Active continental margin., D: Passive margin. (h) SiO2/Al2O3 versus K2O/Na2O diagram (Maynard et al., 1982). A1 = arc setting, basaltic and andesitic detritus., A2 = evolved arc setting, Felsic-plutonic detritus.

  • 76

    Iranian plate is a major segment of Cimmerian Microcontinent Collage and was

    originally a part of north eastern Gondwana land. It was detached during the Late Permian and

    moved northwards and finally collided with the Turan Plate ( part of Eurasia) towards the end

    of Middle Triassic (Sengor, 1990; Stampfli et al., 1991; Saidi et al., 1997). The resulting

    orogenic movements (Early Cimmerian orogenic event) brought about a drastic change in the

    sedimentary regime. Thus, the Middle Triassic platformal carbonate sediments were

    succeeded (after a hiatus) by a thick pile of siliciclastic sediments of Late Triassic to Middle

    Jurassic age. These siliciclastic sediments (Shemshak Group), owing to their development

    during the orogenic event (Early and Middle Cimmerian Orogeny), are considered as a

    molasse-type sediment package and constitute fore-land basin fillings (Stampfli 1978; Alavi,

    1996). Rising and rapidly eroding Cimmerian mountain chain north of Kerman basin provided

    the detritus involved in the formation of siliciclastic sedimentary rocks of the Shemshak

    Group. An alternative interpretation is that the Late Triassic to Middle Jurassic siliciclastic

    sediments of the Shemshak group were deposited in a back-arc basin during its initial

    development stage (Brunet et al., 2003).

    3.4 GEOCHEMISTRY OF SHALES

    Major and select trace element concentrations of four samples of shales of the present

    study are provided in table 3.4 along with the ratios of select pairs of major and trace

    elements. In the analyzed samples, concentrations of major elements vary narrowly. The shale

    consist of 65.52 to 67.45 wt.% SiO2; 16.12 to 19.25 wt.% Al2O3; 0.41 to 0.48 wt.% TiO2; 4.78

    to 7.03 wt.% Fe2O3; 0.62 to 1.38 wt.% MgO; 0.18 to 0.79 wt.% CaO; 0.30 to 0.49 wt.% Na2O;

    1.89 to 2.17 wt.% K2O and 0.15 to 0.22 wt.% P2O5. Total iron is expressed as Fe2O3. Among

  • 77

    the analyzed trace elements, Co varies from 14.31 to 15.74 ppm; Cr from 92 to 108 ppm; Cu

    from 25 to 48 ppm; Ni 18 to 33 and Zn from 71 to 85ppm.

    Correlation coefficient data of major and trace elements are given in table 3.5.

    Chemical analyses data reveal the following geochemical features of the shales.

    On the SiO2/Al2O3 versus K2O/Na2O bivariate diagram (Wronkiewicz and Condie,

    1987) the shales as expected, plot in the field of Phanerozoic-Proterozoic shales (Fig. 3.10).

    Fig. 3.11 shows distribution of major and select trace elements contents of the shales

    normalized to PAAS (Post Archaean Australin Shale; Taylor and McLennan, 1985). In

    comparison with the PAAS, the shales of the present study are slightly depleted in TiO2, K2O,

    MnO, Cr and Cu and moderately depleted in MgO, CaO Na2O, Co and Ni contents. Depletion

    of Na2O and CaO in the shales relative to PAAS suggests either lesser amount of plagioclase

    detritus in the shales and/or comparatively intense chemical weathering at the source and

    during fluvial transportation of the detrital material of the shales. Depletion of TiO2, K2O,

    Fe2O3, Co, Cu, Ni and Cr indicates the presence of relatively lesser quantities of

    phyllosilicates and ferromagnesian minerals in the shales (McCann, 1991; Condie et al.,

    1992). Low content of TiO2 also suggests more evolved (felsic) source rocks.

  • 78

    Table 3.4: Major (wt.%) and trace element (ppm) composition of shales.

    Elements S1 S2 S3 S4 Average

    values PAAS

    SiO2 65.52 66.80 63.78 67.45 65.89 62.40

    Al2O3 17.83 18.70 16.12 19.25 17.98 18.78

    TiO2 0.46 0.46 0.41 0.48 0.45 0.99

    Fe2O3T 7.03 4.74 7.24 5.72 6.18 7.18

    MnO 0.05 0.09 0.13 0.07 0.09 0.11

    CaO 0.18 0.61 0.79 0.76 0.59 1.29

    MgO 0.62 0.92 1.38 0.87 0.95 2.19

    Na2O 0.30 0.41 0.49 0.46 0.42 1.19

    K2O 1.89 1.99 2.17 1.73 1.95 3.68

    P2O5 0.16 0.15 0.18 0.22 0.18 0.16

    SO3 0.05 0.08 0.11 0.06 0.08 -

    L.O.I 5.76 4.90 6.48 2.81 4.99 -

    Total 99.85 99.85 99.28 99.88 99.72 -

    Co 14.31 15.74 13.50 15.16 14.68 23

    Ni 18 33 25 24 25 55

    Cu 25 36 48 41 38 50

    Zn 85 71 75 83 79 85

    Cr 97 92 108 104 100 110

    Al2O3 / TiO2 38.76 40.60 39.31 40.10 39.69 -

    SiO2 / Al2O3 3.67 3.57 3.95 3.50 3.67 -

    K2O/Al2O3 0.11 0.11 0.13 0.09 0.11 -

    Ni / Co 1.25 2.09 1.85 1.58 1.69 -

    Cu / Zn 0.29 0.50 0.64 0.49 0.48 -

    ICV 0.58 0.48 0.77 0.52 0.59

    moles

    Al2O3 0.1748 0.1833 0.1580 0.1887 0.1762 -

    CaO* 0.0020 0.0066 0.0079 0.0074 0.0060 -

    Na2O 0.0048 0.0066 0.0079 0.0074 0.0067 -

    K2O 0.0200 0.0211 0.2300 0.0183 0.0724 -

    CIA % 86.62 84.22 80.27 85.04 84.04 -

    PIA % 95.69 92.47 89.52 91.99 92.42 -

    CIW % 96.19 93.27 90.91 92.71 93.27 -

    CIW % 97.30 96.52 95.23 96.22 96.32 - ICV = Index of Compositional Variation, CIA = Chemical Index of Altration (Nesbitt and Young, 1982), PIA = Plagioclase Index of Altration (Fedo et al., 1995), CIW = Chemical Index of Weathering (Harnois, 1988), CIW = Chemical Index of Weathering (Harnois, 1988), PAAS = Post Archaean Australin Shale

  • 79

    Table 3.5: Values of Pearsons coefficient of correlation of major and select trace elements of shale.

    Fig. 3.10: SiO2/Al2O3 versus K2O/Na2O bivariate age discrimination diagram for shales.

    Element SiO2 Al2O3 TiO2 Fe2O3 MnO CaO MgO Na2O K2O Co Ni Cu Zn Cr

    SiO2 1.00

    Al2O3 0.99 1.00

    TiO2 0.94 0.96 1.00

    Fe2O3 -0.80 -0.77 -0.59 1.00

    MnO -0.61 -0.67 -0.83 0.14 1.00

    CaO 0.00 -0.06 -0.26 -0.24 0.72 1.00

    MgO -0.61 -0.67 -0.82 0.23 0.98 0.77 1.00

    Na2O -0.12 -0.20 -0.38 -0.10 0.78 0.99 0.84 1.00

    K2O -0.83 -0.86 -0.94 0.35 0.82 0.20 0.74 0.28 1.00

    Co 0.91 0.90 0.78 -0.96 -0.42 0.00 -0.49 -0.12 -0.61 1.00

    Ni 0.28 0.22 0.00 -0.78 0.47 0.55 0.37 0.46 0.28 0.58 1.00

    Cu -0.28 -0.36 -0.52 0.00 0.86 0.95 0.91 0.98 0.41 -0.27 0.40 1.00

    Zn 0.16 0.21 0.45 0.45 -0.72 -0.49 -0.60 -0.46 -0.67 -0.20 -0.90 -0.47 1.00

    Cr -0.48 -0.51 -0.49 0.62 0.50 0.56 0.64 0.65 0.19 -0.71 -0.35 0.71 0.21 1.00

  • 80

    Fig. 3.11: Distribution of PAAS normalized abundances of major and trace elements of shales.

    Major element compositions of the analyzed samples are controlled mainly by clay

    minerals relative to non-clay silicate phases. This trend can be illustrated from the values of

    the Index of Compositional Variation (ICV; Cox et al., 1995) in which ICV =

    (Fe2O3+K2O+Na2O+CaO+MgO+MnO)/Al2O3. ICV values of the shales of the present study

    are low and vary from 0.48 to 0.77 (average = 0.59). These < 1 values are typical of minerals

    such as kaolinite, illite and muscovite and lower than higher values (>1) expected in rock

    forming minerals such as plagioclase, K-feldspar, amphiboles and pyroxenes. Hence, the

    shales of the present study are enriched in clay in comparison with rock forming minerals.

    Values of K2O/Al2O3 ratio of clays are less than 0.3 and the values of the same ratio of

    feldspars range from 0.3 to 0.9 (Cox et al., 1995). The values of K2O/Al2O3 ratio of the shales

    of the present study vary narrowly from 0.08 to 0.13 (average = 0.10). These values also

    indicate preponderance of clay minerals over K-bearing minerals such as K-feldspars and

    micas (Cox et al., 1995).

    Ratios of abundance of major oxides and correlation coefficient data of major and trace

    elements (Table 3.4 and 3.5) reveal certain other features of the shales. Values of Al2O3/TiO2

    ratio are high (average = 39.95) and indicate derivation of the detrital material from a

    continental source (Fyffe and Pickeril, 1993). SiO2 exhibits strong negative correlation with

  • 81

    K2O (r = 0.83) indicating decrease of K-bearing minerals (in the present case, K-feldspar,

    muscovite and biotite) with increase of quartz. SiO2 shows positive correlation with Al2O3 (r =

    0.99) and TiO2 (r = 0.94). Al2O3 exhibits co-variance with TiO2 (r = 0.96). These linear

    relationships suggest detrital sorting trends. SiO2 shows negative correlation with several

    major elements (Table 3.5) indicating quartz dilution (Kampunzu et al., 2005; Deru et al.,

    2007). Positive correlation of TiO2 with Al2O3 and negative correlations of TiO2 with several

    major elements (Fe2O3, MnO, CaO, MgO, Na2O and K2O) suggest that TiO2 occurs as a

    chemical constituent essentially of clays (kaolinite) rather than of mafic minerals. Low

    contents of TiO2 (average = 0.45 wt.%) indicate presence of phyllosilicates in minor amounts

    (Dabard, 1990; Condie et al., 1992). Al2O3 exhibits strong negative correlation with K2O (r = -

    0.86) and strong positive correlation with SiO2 (r = 0.99). These linear trends suggest the

    presence of kaolinite. The values of K2O/Na2O ratio vary from 3.76 to 6.30 (average = 4.83)

    indicating the presence of k-bearing minerals such as k-feldspar, muscovite and biotite. Na2O,

    even though exhibits positive correlations with CaO (r = 0.99) and MgO (r = 0.84), it shows

    negative correlations with Fe2O3 (r = 0.10), Al2O3 (r = 0.20) and SiO2 (r = 0.12). These

    relationships suggest that smectite may not be present in the shales. CaO shows no correlation

    with SiO2 (r = 0.00) indicating that carbonate minerals, if present, could be of secondary

    origin. Scattered plots of the shales on the CaO wt.% versus SiO2 wt.% diagram suggest the

    presence of secondary carbonates in the shales (Feng and Kerrich, 1990).

    Among the analyzed trace elements (Co, Ni, Zn, Cu and Cr), Co, Ni and Zn show

    positive correlations with SiO2, Al2O3 and TiO2 indicating their presence in clays adsorbed

    components. Cu and Cr positively correlate with MnO, CaO, MgO, Na2O and K2O suggesting

    their occurrence in mafic mineral components of the shales (Table 3.5).

  • 82

    3.4.1 Provenance

    Numerous investigators have demonstrated that chemical composition of siliciclastic

    sedimentary rocks is related to that of their source regions and this tenet has been used to

    characterize the source rocks from which the investigated sedimentary strata were derived

    (e.g., Fralick and Kronberg, 1997 and references there in; Cullers, 2000; Alvarez and Roser,

    2007; Manikyamba et al., 2008; Spalletti et al., 2008; Akarish and El-Gohary, 2008; Paikaray

    et al., 2008; Dey et al., 2009; Kalsbeek and Frei, 2010; Mishra and Sen, 2010). In the

    published literature, several major-, trace-, and rare-earth element-based discrimination

    diagrams have been proposed to infer the source/provenance of siliclastic rocks (Roser and

    Korsch, 1988; Floyd et al., 1989, 1990; McLennan et al., 1993). In the provenance

    discrimination diagram of Roser and Korsch (1988), the formulated discriminant functions

    (i.e., bivariates) are based on concentrations of both immobile and variably mobile major

    elements. On this diagram the shales of the present study plot in the field of mafic igneous

    provenance (Fig. 3.12). On the TiO2 wt.% versus Ni (ppm) diagram (Floyd et al., 1989) the

    shales, on the contrary, plot in the acidic igneous field (Fig. 3.13a). These two contradicting

    inferences (ie., mafic versus acidic) can be resolved with the available chemical analysis data

    of major and select elements utilizing other options. Hayashi et al., (1997) suggested an

    immobile element-based provenance discrimination equation and provided in brief the

    following geological data to justify the proposed equation.

  • 83

    Fig. 3.12: Provenance discrimination diagram for shales (after Roser and Korsch, 1988).

    During weathering of source rocks, Al and Ti remain essentially immobile, owing to

    low solubility of their oxides and hydroxides in low temperature aqueous solutions (e.g.,

    Stumm and Morgan, 1981; Yamamoto et al., 1986; Sugitani etal., 1996). In residual soils Al

    typically resides in clays (e.g., kaolinite, illite, smectite), micas, feldspars, cholorites and to a

    lesser extent in mafic minerals (e.g., augite, anthophyllite, hornblende). Ti occurs as a

    chemical constituent of clays and mafic minerals (e.g., biotite, chlorite, ilmenite inclusions in

    silicate minerals). Several studies have shown that during fluvial transportation of Al and Ti-

    bearing detrital minerals, Al and Ti involve insignificant fractionation (e.g., Yamamoto. et al.,

    1986). Hence, the Al2O3/TiO2 ratios of sedimentary rocks derived from fluvially transported

    detrital siliciclastic material should be practically similar to those of their magmatic source

    rocks.

    In normal igneous rocks Al resides mostly in feldspars, and Ti in mafic minerals (e.g.,

    olivine, pyroxene, hornblende, biotite and ilmenite). Therefore, the values of Al/Ti ratio of

    igneous rocks gradually increase with increasing SiO2 contents (Poldervart, 1955; Holland,

  • 84

    1984). The values of Al2O3/TiO2 ratio increase (a) from 3 to 8 in mafic igneous rocks (SiO2

    content, from 45 to 52 wt.%), (b) from 8 to 21 in intermediate igneous rocks (SiO2 content,

    from 53 to 66 wt.%) and (c) from 7 to 21 in felsic igneous rocks (SiO2 content, from 66 to 76

    wt.%). According to Hayashi et al., (1997) the SiO2 content of normal igneous rocks can be

    evaluated from their Al2O3/TiO2 ratio using the following equation:

    SiO2 (wt.%) = 39.34+1.2578 (Al2O3/TiO2) 0.0109 (Al2O3/TiO2)2

    Since Al and Ti are immobile and behave similarly during residual weathering and fluvial

    transport, the silica content of the source rocks can be inferred from the Al2O3/SiO2 ratios of

    siliciclastic rocks using the above equation. When Al2O3/TiO2 ratios of the shales of the

    present study are substituted in the equation of Hayashi et al., (1997), the SiO2 contents of the

    theoretically inferred magmatic source of the shales are found to range narrowly from 71.72 to

    72.45 wt.% (average 72.09 wt.%). These values of SiO2 content suggest that the source rocks

    of the shales of the present study are felsic igneous rocks. Average SiO2 content (~72 wt.%)

    indicates that the inferred source rocks constitute are high in SiO2 bearing end member of

    felsic group igneous rocks.

  • 85

    Fig. 3.13: Provenance indicating diagrams. (a) TiO2 wt.% versus Ni (ppm) bivariate diagram (Floyd et al., 1989). (b) TiO2 wt.% versus Al2O3 wt.% bivariate diagram (McLennan et al., 1980). (a) The granite line and 3 granite + 1 basalt line are after Schieber, (1992). (c) Al2O3 (CaO+Na2O+K2O) (FeO

    T+MgO) ternary diagram (Hayashi et al., 1997). (d) Fe2O3 K2O Al2O3 ternary diagram (Condie, 1993).

    McLennan et al., (1980), recognizing the significance of Al and Ti in provenance

    studies, proposed Al2O3 wt.% versus TiO2 wt.% bivariate discrimination diagram to constrain

    provenance of siliciclastic rocks. On this diagram the shales of the present study plot along

    granite trend line (Fig. 3.13b). On the Al2O3 (CaO+Na2O+K2O) (FeOT+MgO) ternary

    diagram (Hayashi et al., 1997) the shales plot away from (FeOT+MgO) apex and trend toward

  • 86

    Al2O3 apex (Fig. 3.13c). The plots of the shales on this diagram is another illustration to

    confirm (albeit indirectly) the felsic nature of the source rocks. This inference is further

    confirmed by the plots of the shales on Fe2O3 K2O Al2O3 ternary diagram (Condie, 1993).

    On this diagram the shales plot near the field of granite (Fig. 3.13d). Thus the provenance

    discrimination equation (Hayashi et al., 1997) and plots of the shales in (a) TiO2 wt.% versus

    Ni (ppm) diagram, (b) Al2O3 wt.% versus TiO2 wt.% diagram, (c) Al2O3 (CaO+Na2O+K2O)

    (FeOT+MgO) ternary diagram and (d) Fe2O3 K2O Al2O3 ternary diagram together

    indicate felsic (granitic) nature of the source rocks of the studied shales. Hence, the mafic

    igneous provenance for the shales of the study area, as inferred from the discriminant

    functions diagram of Roser and Korsch (1988) (Fig. 3.12) will not be considered.

    3.4.2 Palaeoweathering

    The CaO content of the shales of the present study varies from 0.18 to 0.79 wt.%

    (average = 0.58 wt.%). The P2O5 content ranges from 0.15 to 0.22 wt.%. There is no direct

    method to distinguish and quantify the contents of CaO belonging to silicate fraction and non-

    silicate fraction (carbonates and apatite). CIA values were determined, assuming the entire

    CaO content of the shales as that of silicate fraction and the obtained CIA values vary from

    82.60 to 86.15% (average 82.27%).

    Following the procedure of McLennan (1993), the CIA, PIA and CIW values of the

    shales have been determined and the obtained results along with CIW values are provided in

    table 3.4. According to CIA values, the degree of source weathering varies from 80.27 to

    86.62% (average = 84.03%). PIA values indicate the intensity of alteration of source material

    varying from 89.52 to 95.69% (average = 92.41%). CIW values suggest the degree of source

    weathering in the range from 90.91 to 96.19% (average = 93.27%). CIW values vary from

  • 87

    95.23 to 97.30% (average 96.31%). Avarage PIA and CIA values (92% and 93%,

    respectively) indicate higher degree of weathering than the degree of weathering inferred from

    CIA values (84%). Phyllites of the present study consist of reasonably significant amount of

    alkalies (average = 2.30 wt.%). Hence, the CIW values (average = 96%), which indicate very

    high degree of chemical weathering, can not be considered. Relying on CIA, PIA and CIA

    values ranging from 84 to 93%, it can be said that the litho-components of the shales were

    subjected to intense chemical weathering.

    PIA monitors and quantifies progressive weathering of feldspars and volcanic glass to

    clay minerals (Fedo et al., 1995). PIA values of shales suggest intense destruction of feldspars

    during the course of source weathering, fluvial transport, sedimentation and diagenesis.

    During the initial stages of weathering of feldspar-bearing source material Ca is leached

    rapidly than Na and K. With increasing weathering total alkali content (K2O+Na2O) decreases

    with increase in K2O/Na2O ratio. This is due to destruction of feldspars among which

    plagioclase is more preferentially removed than K-feldspars (Nesbitt and Young, 1984).

    Detrital grains of feldspars of the shales preserve imprints of varied degrees of

    alteration witnessed at source region and during transport, sedimentation and diagenesis. The

    mobility of elements during progressing chemical weathering of clastic feldspars in the shales

    may be visualized from bivariate plots involving Na2O, K2O, CaO and PIA. In the K2O/Na2O

    versus PIA diagram the values of K2O/Na2O ratio increase with increasing values of PIA (r =

    0.78; Fig. 3.14a). In the (K2O+Na2O) wt.% versus PIA diagram, the total content of alkalies

    decreases with increasing values of PIA (r = 0.79; Fig. 3.14b). The mobility of Na, Ca and K

    during progressing weathering is clearly seen in the bivariate Na2O wt.% versus PIA, CaO

    wt.% versus PIA and K2O wt.% versus PIA diagrams (Fig. 3.14 c,d,e). These diagrams vividly

  • 88

    show decrease of Na2O, CaO and K2O contents with increase of values of PIA (r = 0.96;

    0.93 and 0.52, respectively). Mobility of MgO during chemical weathering of

    ferromagnesian minerals can be demonstrated in the bivariate MgO wt.% versus CIA diagram.

    This diagram clearly shows decrease of MgO with increase of values of CIA (r = 0.99; Fig.

    3.14f).

    Mobility of elements during the progress of chemical weathering of source material

    and post-depositional chemical modifications of the shales can be evaluated by plotting the

    molar proportions of Al2O3, Na2O and CaO (CaO in silicate fraction) in A-CN-K ternary

    diagram (Nesbitt and Young, 1982, 1984). In the A-CN-K diagram (A = Al2O3; CN = CaO +

    Na2O; K = K2O) the shales plot above plagioclase-potash feldspar join (Fig. 3.15). The plots

    define a narrow linear trend and the trend line runs slightly at an angle to A-CN edge. This is

    primarly because removal rates of Na and Ca from plagioclase are generally greater than the

    removal rates of K from microcline (Nesbitt and young, 1984). The plots trend towards A

    apex (Fig. 3.15). The plots do not show any inclination towards the K apex, thus indicating

    that the shales were not subjected to potash metasomatism during diagenesis. The trend line

    when extended backward intersects the plagioclase-potash feldspar join near the field of

    granite (potential source rock). Linear weathering trend suggests steady state of weathering

    conditions where material removal matches with production of weathering material (Nesbitt et

    al., 1997, Nesbit and Young, 2004).

  • 89

    84 88 92 96 1003

    4

    5

    6

    7

    (a) r = 0.78K

    2O

    / N

    a2O

    PIA

    84 88 92 96 1002.0

    2.2

    2.4

    2.6

    2.8

    3.0

    (b) r = - 0.79

    K2O

    + N

    a2O

    PIA

    84 88 92 96 1000.0

    0.2

    0.4

    0.6

    (c) r = - 0.96

    Na

    2O

    %

    PIA

    84 88 92 96 1000.0

    0.2

    0.4

    0.6

    0.8

    1.0

    (d) r = - 0.93

    CaO

    %

    PIA

    84 88 92 96 1001.0

    1.5

    2.0

    2.5

    3.0

    (e) r = - 0.52

    K2O

    %

    PIA

    78 81 84 87 900.0

    0.5

    1.0

    1.5

    2.0

    (f) r = - 0.99

    Mg

    O %

    CIA

    Fig. 3.14: Bivariate diagrams showing the mobility of Na, Ca, K and Mg during progressing

    weathering of lithocomponents of shales. (a) K2O/Na2O versus PIA bivariate diagram. (b) (K2O + Na2O) wt.% versus PIA bivariate diagram. (c) Na2O wt.% versus PIA bivariate diagram. (d) CaO wt.% versus PIA bivariate diagram. (e) K2O wt.% versus PIA bivariate diagram. (f) MgO wt.% versus CIA bivariate diagram.

  • 90

    Fig. 3.15: A CN K ternary diagram showing weathering trend. A = Al2O3; CN = (CaO +

    Na2O); K = K2O (all in molar proporations). CaO = CaO content of silicate fraction. Stars: G = Granite; A = Andesite; B = Basalt.

    3.4.3 Sediment maturity and depositional environment

    Weathering indices of sedimentary rocks can provide useful information on tectonic

    activity and climatic conditions in the source region. Increase of degree of chemical

    weathering may reflect the decrease in tectonic activity and/or change in climate towards

    warm and humid conditions (e.g., Jacobson et al., 2003). Suttner and Dutta (1986) proposed a

    binary SiO2 wt.% versus (Al2O3+K2O+Na2O) wt.% diagram to constrain the climatic

    condition during sedimentation of siliciclastic rocks. On this diagram the shales plot in the

    field of semiarid climate (Fig. 3.16), indicating that the shales of the present study were

    deposited under semiarid conditions.

  • 91

    Fig. 3.16: SiO2 wt.% versus (Al2O3 + K2O + Na2O) wt.% bivariate palaeoclimate

    discrimination diagram ( after Suttner and Dutta, 1986).

    Palaeo-redox conditions during sedimentation of siliciclastic rocks can be evaluated

    based on their chemical analyses data. According to Jones and Manning (1994) values of

    Ni/Co ratio below 5 indicate oxic environment where as values of the same ratio above 5

    suggest suboxic and anoxic environment. The values of Cu/Zn ratio are also considered as an

    indicator of palaeo-redox conditions. According to Hallberg (1976) high values of Cu/Zn ratio

    indicate reducing conditions, while low values of the same ratio suggest oxidizing conditions.

    The values of Ni/Co ratio of the shales vary from 1.25 to 2.09 (average = 1.70) and the values

    of Cu/Zn ratio vary from 0.29 to 0.64 (average = 0.48). These low values of Ni/Co and Cu/Zn

    ratio suggest deposition of the shales of the present study under oxic conditions.

    3.4.4 Tectonic setting

    Several studies have shown that the chemical compositions of siliclastic rocks are

    significantly controlled by plate tectonic settings of their provenances, as a result, the

    siliclastic rocks from different tectonic settings posses terrain-specific geochemical signatures

    (Bhatia, 1983; Bhatia and Crook, 1986; Roser and Korsch 1986).

  • 92

    Tectonic settings of provenances of ancient siliclastic rocks can be inferred from

    several major-, trace-, and rare earth-based discrimination diagrams proposed by several

    investigators (Maynard et al., 1982; Bhatia, 1983; Bhatia and Crook, 1986; Roser and Korsch,

    1986). These diagrams classify the provenances into three or four categories on the basis of

    the bulk geochemistry contrasts (e.g., oceanic island arc, continental island arc, active

    continental margin, passive margin; Bhatia and Crook, 1986). On the bivariate discriminant

    functions diagram of Bhatia (1983), the shales of the present study plot in the field of active

    continental margin (Fig. 3.17). In active continental margin settings, sediments are deposited

    at subduction arc basins, strike-slip margins, and in proximal portions in back-arc basins

    (Alvarez and Roser, 2007). During the present study, the tectonic setting inferred for the

    provenance of the shales of Shemshak Formation of the Kerman Province is in agreement with

    the tectonic evolutionary events witnessed by the central Iran during Jurassic period.

    Iranian plate is a major segment of Cimmerian Microcontinent Collage and was

    originally a part of north eastern Gondwana land. It was detached during the Late Permian and

    moved northwards and finally collided with the Turan Plate (part of Eurasia) towards the end

    of Middle Triassic (Sengor, 1990; Stampfli et al., 1991; Saidi et al., 1997). The resulting

    orogenic movements (Early Cimmerian orogenic event) brought about a drastic change in the

    sedimentary regime. Thus, the Middle Triassic platformal carbonate sediments were

    succeeded (after a hiatus) by a thick pile of siliciclastic sediments of Late Triassic to Middle

    Jurassic age. These siliciclastic sediments (Shemshak Group), owing to their development

    during the orogenic event (Early and Middle Cimmerian Orogeny), are considered as a

    molasse-type sediment package and constitute fore-land basin fillings (Stamplfi 1978; Alavi,

    1996). Rising and rapidly eroding Cimmerian mountain chain located north of Kerman basin

  • 93

    provided the detritus involved in the formation of siliciclastic sedimentary rocks of the

    Shemshak Group. An alternative interpretation is that the Late Triassic to Middle Jurassic

    siliciclastic sediments of the Shemshak group were deposited in a back-arc basin during its

    initial development stage (Brunet et al., 2003).

    Fig. 3.17: Tectonic setting discrimination diagram (Bhatia, 1983). Discriminant Function 1 =

    (-0.0447SiO2%) + (-0.972TiO2%) + (0.008Al2O3%) + (-0.267Fe2O3%) + (0.208FeO%) + (-3.082MnO%) + (0.140MgO%) + (0.195CaO%) + (0.719Na2O%) + (-0.032K2O%) + (7.510P2O5%). Discriminant Function 2 = (-0.421SiO2%) + (1.988TiO2%) + (-0.526Al2O3%) + (-0.551 Fe2O3%) + (-1.610FeO%) + (2.720MnO%) + (0.881MgO) + (-0.907CaO%) + (-0.117Na2O%) + (-1.840K2O%) + (7.244P2O5%).

    3.5 GEOCHEMISTRY OF LIMESTONES

    The chemical composition of carbonate components of ancient limestones may serve

    as a potential tool for evaluating the degree of diagenesis and for detecting the original

    mineralogy of the different stabilized carbonate phases. The Mg and Sr elements are useful in

    carbonate facies studies (Bathurst, 1975; Chilingar et al., 1979). Sr and Mn are diagenetic

    indicators because of their widely divergent partition coefficients, their acceptance in to the

    carbonate lattice, and their large compositional differences in marine and meteoric water

  • 94

    (Bondine et al., 1965; Turekian, 1972).The micritisation by recrystallzation is common in

    shallow water carbonate grains in tropical regions around the world (Pamela and Macintyre

    1998).

    3.5.1 Major Elements

    During the present study four limestone samples were analysed for their major and

    trace element contents and the obtained data are presented in Table 3.6 and 3.7. In the

    limestones SiO2 content varies narrowly from 9.88 to 12.25wt % (av. = 10.96 wt%). Al2O3

    content varies from 1.70 to 2.64 wt % (av. = 2.14 wt%) which suggests the presence of minor

    amount of clay minerals. Fe2O3 content varies from 5.28 to 5.77 wt% (av. = 5.56 wt%)

    indicate ferruginous nature of the limestone. CaO content ranges from 41.22 to 44.86 wt %

    (av. = 42.90 wt%). High CaO content indicates CaO rich nature of the limestone. MgO content

    varies narrowly between 0.51 to 0.71 wt% (av. = 0.61 wt%) which suggesting appreciable

    variation in their MgO content. Na2O content also varies narrowly from 0.02 to 0.03 wt % (av.

    = 0.03 wt%). The presence of sodium is considered are possible index of diagenetic influx.

    K2O content varies from 0.12 to 0.14 wt% (av. = 0.13 wt%). P2O5 content varies from 0.02 to

    0.08 wt% (av. = 0.05 wt%). TiO2 content ranges from 0.09 to 0.18 wt% (av. = 0.14 wt%). The

    loss on ignition (LOI) varies from 35.90 to 36.98 (av. = 36.51 wt%).

  • 95

    Table 3.6 : Major (wt.%) element data of limestone.

    Elements PS(Li1) PS(Li2) PS(Li3) PS(Li4) Average

    values

    SiO2 9.88 11.43 10.26 12.25 10.96

    Al2O3 2.44 1.76 2.64 1.70 2.14

    TiO2 0.09 0.16 0.11 0.18 0.14

    Fe2O3 5.28 5.73 5.46 5.77 5.56

    MgO 0.71 0.54 0.66 0.51 0.61

    CaO 44.86 42.33 43.18 41.22 42.90

    Na2O 0.03 0.02 0.03 0.02 0.03

    K2O 0.12 0.13 0.12 0.14 0.13

    P2O5 0.02 0.06 0.04 0.08 0.05

    MnO 0.12 0.18 0.15 0.22 0.17

    SO3 1.25 1.11 1.18 0.96 1.13

    L.O.I 35.90 36.66 36.48 36.98 36.51

    Total 100.70 100.11 100.31 100.03 100.29

    K2O/Al2O3 0.11 0.11 0.13 0.09 0.11

    SiO2 / Al2O3 3.67 3.57 3.95 3.50 3.67

    Log (SiO2 / Al2O3) 0.56 0.55 0.59 0.54 0.56

    Fe2O3 / K2O 3.72 2.38 3.33 3.31 3.19

    Log (Fe2O3 / K2O) 0.57 0.37 0.52 0.51 0.49

    Al2O3 / TiO2 38.76 40.60 39.31 40.10 39.69

    K2O/Na2O 6.30 4.85 4.43 3.76 4.64

  • 96

    Table 3.7 : Trace Element (ppm) data of limestone.

    3.5.2 Geochemical interpretation

    The average contents of CaO (41.22 to 44.86), SiO2 (9.88 to 12.25) and Al2O3 (1.70 to

    2.64) indicate the influx of sediments from terrigenous source as suggested for similar

    limestones elsewhere. The Fe2O3 content ranges from 5.28 to 5.77 wt % suggesting that the

    iron content might have been incorporated with calcite during primary precipitation in

    reducing condition as a result of input and sedimentation.

    The major element data of limestone of the study area are plotted in various bivariate

    diagrams to know the provanence for the limestones of the study area. The SiO2 versus MnO,

    P2O5 and K2O diagrams (Fig. 3.18a) indicate a positive correlation suggesting the terrigenous

    influx into the basin. The SiO2 versus MgO , Na2O and SO3 diagrams (Fig. 3.18b) suggest that

    a sulfur, magnesium and sodium possibly associated with the detrital components.

    CaO versus Al2O3, MgO and SO3 diagrams (Fig. 3.18c) suggest positive correlations

    and CaO vs. TiO2, P2O5 and K2O (Fig. 3.18d) exhibit a negative correlations suggesting that

    Elements PS(Li1) PS(Li2) PS(Li3) PS(Li4) Average

    values

    Hg 0.33 0.28 0.16 0.24 0.25

    As 12.95 9.68 14.23 12.43 12.32

    Cd 8.36 7.54 11.65 7.31 8.72

    Co 41 28 35 29 33

    Cr 184 161 173 188 176.50

    Cu 24 36 35 29 31

    Mn 962 869 1012 811 913.50

    Ni 34 43 32 38 36.75

    Sr 251 326 284 221 270.50

    Se 2.17 2.67 2.88 2.14 2.47

    Zn 73 89 84 93 84.75

  • 97

    CaCO3 content possibly increased at the sites of fossiliferous sediment deposition. There is an

    increase in the SiO2, TiO2, P2O5, Fe2O3 and K2O contents with a proportionate decrease in

    CaO. This indicates that SiO2 which is largely remained in the insoluble fraction and therefore

    could be related to the detritial influx during sedimentation (Nagendra and Nagarajan, 2003).

    The Al2O3 versus Fe2O3 and SiO2 diagram (Fig. 3.18e) have negative correlation while

    Al2O3 versus MgO, Na2O and SO3 (Fig. 3.18f) exhibits positive trend indicating that the

    limestone of the study area might have been derived from the fossils sources. Various

    physicochemical process influenced variation in the concentration of trace elements in these

    carbonate sediments during weathering and leaching of the rocks and their deposition.

    Adsorption of some of the trace elements was mainly influenced by principal adsorbents like

    the carbonate minerals, Clay, iron oxides and silica minerals.

    3.4.2 Trace elements

    The trace elements like Hg, As, Cd, Co, Cr, Cu, Mn, Ni, Sr, Se and Zn are determined

    in the limestones of the study area and presented in Table 4.5. The trace element data

    presented in Table 4.5 indicate that the Hg varies from 0.16 to 0.33 ppm, (average = 0.25

    ppm). The As varies from 9.68 to 14.23 ppm, (average = 12.32 ppm). The Cd varies from 7.31

    to 11.65 ppm, (average = 8.72 ppm). The Co varies from 28 to 41 ppm, (average = 33 ppm).

    Cr varies from 161 to 188 ppm, (average = 176.5 ppm). The Cu varies from 24 to 36 ppm,

    (average = 31 ppm). Mn varies from 869 to 1012 ppm, (average = 913.50 ppm). The Ni varies

    from 32 to 43 ppm, (average = 36.75 ppm). The Sr varies 221 ppm to 326 ppm, (average =

    270.5 ppm). The Se varies from 2.14 to 2.88 ppm, (average = 2.47 ppm). The Zn varies from

    73 to 93 ppm, (average = 84.75 ppm).

  • 98

    Fig 3.18a Fig 3.18b

    9.5 10.0 10.5 11.0 11.5 12.0 12.5

    0.02

    0.04

    0.06

    0.08

    0.10

    0.12

    0.14

    0.16

    0.18

    0.20

    0.22 SiO2VsMnO

    SiO2VsP

    2O

    5

    SiO2VsK

    2O

    Mn

    O/P

    2O

    5/K

    2O

    SiO2 (wt%)

    9.5 10.0 10.5 11.0 11.5 12.0 12.50.0

    0.2

    0.4

    0.6

    0.8

    1.0

    1.2

    SiO2VsMgO

    SiO2VsNa

    2O

    SiO2VsSO

    3

    MgO

    /Na

    2O

    /SO

    3

    SiO2 (wt%)

    Fig 3.18c Fig 3.18d

    41 42 43 44 450.4

    0.6

    0.8

    1.0

    1.2

    1.4

    1.6

    1.8

    2.0

    2.2

    2.4

    2.6

    2.8

    CaOVsAl2O

    3

    CaOVsMgO

    CaOVsSO3

    Al 2O

    3/M

    gO

    /SO

    3

    CaO (wt%)

    41 42 43 44 45

    0.02

    0.04

    0.06

    0.08

    0.10

    0.12

    0.14

    0.16

    0.18 CaOVs TiO

    2

    CaOVs K2O

    CaOVs P2O

    5

    TiO

    2/K

    2O

    /P2O

    5

    CaO (wt%)

    Fig 3.18e Fig 3.18f

    1.6 1.8 2.0 2.2 2.4 2.6 2.85

    6

    7

    8

    9

    10

    11

    12

    Al2O

    3VsFe

    2O

    3

    Al2O

    3SiO

    2

    Fe

    2O

    3/S

    iO2

    Al2O

    3 (wt%)

    1.6 1.8 2.0 2.2 2.4 2.6 2.80.0

    0.2

    0.4

    0.6

    0.8

    1.0

    1.2 Al

    2O

    3VsMgO

    Al2O

    3VsNa

    2O

    Al2O

    3VsSO

    3

    Mg

    O/N

    a2O

    /SO

    3

    Al2O

    3 (wt%)

    Fig. 3.18a,b,c,d,e,f : Diagrams for major elemental variation in Limestones of the study area.

  • 99

    The trace element data of limestones of the study area are compared with the trace

    element data of PAAS. Limestone are characterized by higher concentration of Hg, Cr, Mn

    and Se and lower concentration of Cd, Cu, Ni, Sr and Zn in ratio to PAAS. The variation in

    chemical composition represents changes in the supply of material and a variation in physico

    chemical environment of deposition. The pattern of geochemical haviour of individual

    elements suggest that most of the trace elements that found their way into the ancient

    sediments appear to have invaded the lattices of carbonates, silicates and clay minerals and

    combined with them structurally.

    Significance trace element in limestones indicates that presence of these elements may

    be due to their inter-element affinities. The Mn can be used as indicator of original carbonate

    mineralogy and associated depositional and early diagenetic environments. In the study area

    Mn content ranges from 811 ppm to 1012 ppm. The variation of Mn values in limestone are

    characterised by the rate of precipitation of diagenetic calcite (Rao, 1990). The high

    concentration of Mn is related to vicinity of coal depositional environment.