LAMPROPHYRES FROM OSPA OPHIOLITE OF THE EAST SAYAN (RUSSIA)

Semen Kovalev1,2, Sergey Zhmodik1,2, Dmitry Belyanin1,2, Eugenia Airiyants1, Olga Kiseleva1, Yury Kulikov1, Alexey Travin1,2

1 N.V. SOBOLEV INSTITUTE OF GEOLOGY AND MINERALOGY SB RAS, GEOLOGY, NOVOSIBIRSK, RUSSIA, ZHMODIK@IGM.NSC.RU

2 NOVOSIBIRSK STATE UNIVERSITY, NOVOSIBIRSK, RUSSIAN FEDERATION

EGU2020: Sharing Geoscience Online, 4-8 May 2020Session GD4.3 Distributed under the Creative Commons

Attribution 4.0 International License

The geographical position

Eastern Sayanmountains are located to the West of Lake Baikal.Eastern Sayanophiolites are a part of the Central-Asian Fold Belt.

Region of work

The geographical position of the Eastern Sayan mountains

The geological position

In the Eastern Sayanmountains (E. Siberia), ophiolite complexes form three extended branches:

1 - Ilchir (MOR ophiolites)2 - Ospa-Khara-Nur(suprasubduction zone (SSZ) and volcanic arc (VA) ophiolites)3 - Shishkhid-Yehe-Shignin(back-arc ophiolites).

Relief map of the south-eastern part of the Eastern Sayan with schematic ophiolite branches position

The history of discovery of the unique rocksLamprophyre (L) dykes or mica peridotites (Shestopalov,

1938) were found in brecciation zone of ophiolites (dunites,harzburgites, serpentinites) of the Ospa-Khara-Nur peridotitecomplex (a part of Ospa-Khara-Nur ophiolite branch) . They formbodies to 1 m thick, and vein-like fragments in intensivelydeformed and altered (serpentinized, tremolitized) ultramaficrocks.

Harzburgite

LamprophyreContact between lamprophyre and harzburgite Lamprophyre sample from Ospa-Khara-Nur ophiolites

Lamprophyre sample from East Sayan. Big amount of mica in gaps between Pl and Amf crystals.

Pl

Phl

Bt

Bt

Amf1 cm

Lamprophyre sample from East Sayan. In that plane there are a lot of medium-grained Pl and Amf, with low amount of mica.

Phl

Phl

Pl

PlAmf

Bt

Amf

1 cm

Amf

Pl

Phl

Lamprophyre sample from East Sayan. Mica in this sample looks like it has filled the voids during magmatic process.

1 cm

Lamprophyre sample from East Sayan. Fine-grained and coarse-grained Pl

Amf

Phl

Pl

Pl

Phl

Phl

Amf

Phl

Phl

1 cm

Phl

Phl

Phl

Phl

Phl

Pl

Pl

Amf

Amf

Xenolith of the host rocks (harzburgite)

Lamprophyre sample from East Sayan. Huge xenoliths of serpentinizedharzburgite with concentric zoning

1 cm

Lamprophyre sample from East Sayan. Huge xenoliths of ultramafic rocks with concentric zoning

Phl

Phl

Phl

Pl

Pl

Pl

Amf

Amf

Amf

Amf

Xenolith of the host rocks (harzburgite)

1 cm

ClassificationDark gray massive porphyric L correspond to the range between ultramafic (UML), alkaline (AL),

Ca alkaline lamprophyre (CAL), and lamproite lamprophyres (LL) according to Rock’s classification(1991) and shows compositional range in MgO-CaO, - Al2O3, - Na2O, - P2O5 diagrams.

CaO/MgO wt.% diagram according to Rock, 1991 (K2O/Al2O3)/(Fe2O3/SiO2) wt.% diagram according to Rock, 1991

Mineral compositionLamprophyre rocks consist of feldspar, phlogopite, orto- and clinopyroxene,

amphibole, with relics of olivine (Fo=45-50, rarely 22-30) and large (up to 1 cm) porphyricphlogopite. In more acid L of CAL type with prevailing hypersthene and fieldspars areassociated by amphiboles metasomatic type (ferro-eckermannite, actinolite, tremolite) andrarely metamorphic glaucophane.

Apt

Bt

Pl

Amf

PlBt

HypAct

Pl

Pl

Bt

Bt

Pl

Act

Results of researches with scanning electron microscope (SEM)

50μm

50μm

Mineral compositionMicas grains from phlogopite to biotite (0.2-1.7% and 2.1-2.8% TiO2) are surrounded by

sericite. Feldspar vary albite to anorthite, and rare grains of orthoclase and Ba-feldspar.Fluorine-apatite (Cl to 0.3%), ilmenite, rutile are common in L but zircon, monazite and Ce-La-epidote are rare. Mineral thermometry range from 1300°C to 950°C for LL then 850°C -560°C and low metamorphic stage.

0,5 mm

Sample scan picture fragment with large amount of mica Mica under the magnification of the microscope

Harker diagrams – TiO2/SiO2

0,0

0,5

1,0

1,5

2,0

2,5

38 40 42 44 46 48 50 52 54 56 58

TIO2

SIO2Harker diagram based on lamprophyres samples – TiO2/SiO2 ratios. The red line indicates a trend

Variation Harker diagrams were built for lamprophyre samples, in order tounderstand the evolution of melt composition.

Harker diagrams – Fe2O3/SiO2

0

2

4

6

8

10

12

14

38 40 42 44 46 48 50 52 54 56 58

Fe2O

3

SiO2Harker diagram based on lamprophyres samples – Fe2O3/SiO2 ratios. The red line indicates a trend

Harker diagrams – MgO/SiO2

0

5

10

15

20

25

30

35

40

45

38 40 42 44 46 48 50 52 54 56 58

MgO

SiO2Harker diagram based on lamprophyres samples – MgO/SiO2 ratios. The red line indicates a trend

Harker diagrams – Al2O3 /SiO2

0

5

10

15

20

25

30

35

38 43 48 53 58

AL 2

O3

SiO2

Harker diagram based on lamprophyres samples – Al2O3 /SiO2 ratios. The red line indicates a trend

Harker diagrams – MnO/SiO2

0,00

0,05

0,10

0,15

0,20

0,25

38 43 48 53 58

MnO

SiO2

Harker diagram based on lamprophyres samples – MnO/SiO2 ratios. The red line indicates a trend

Harker diagrams – CaO/SiO2

0

5

10

15

20

25

38 43 48 53 58

CaO

SiO2

Harker diagram based on lamprophyres samples – CaO/SiO2 ratios. The red line indicates a trend

Harker diagrams – K2O/SiO2

0

1

2

3

4

5

6

38 43 48 53 58

K 2O

SiO2

Harker diagram based on lamprophyres samples – K2O/SiO2 ratios. The red line indicates a trend

Harker diagrams – P2O5/ SiO2

-0,2

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

38 43 48 53 58

P 2O

5

SiO2Harker diagram based on lamprophyres samples – P2O5/ SiO2 ratios. The red line indicates a trend

Harker diagrams – TiO2/ SiO2

0,0

0,5

1,0

1,5

2,0

2,5

38 43 48 53 58

TiO2

SiO2

Harker diagram based on lamprophyres samples – TiO2/ SiO2 ratios. The red line indicates a trend

REE, HFSE and LILE amountsTRE from L shows inclined REE with flat La-Sm, HFSE troughs but high LILE. The acid

CL reveal Eu peak (Eu*=3,2; (La/Yb)n=9). Spider and REE diagram reveal elevated HFSE,Sr, Pb the same high LILE closer to anorthosites and pegmatiod charnokites. Thissuggests that extremely high temperature ML reacted with acid rocks and producedCa-alkaline L type.

1,00

10,00

100,00

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu«Spidergram» of average compositions for lamprophyre. REE normalized to chondrites, using order and

normalizing values of Nakamura (1974)

REE, HFSE and LILE amounts

«Spidergram» of average compositions for lamprophyre normalized to chondrites, using order and normalizing values of Thompson (1982)

0,10

1,00

10,00

100,00

1000,00

Ba Rb Th K Nb Ta La Ce Sr Pr Nd P Sm Zr Hf Ti Tb Y Tm Yb

Comparison with Rock’s (1991) classification

Two of the founded lamprophyre samples were compared withreference samples from classification of Rock N.M.S (1991).

There are 5 branches of lamprophyric rocks, according to RockN.M.S:

1. Calc-Alkaline (shoshonitic) lamprophyres – (CAL)2. Alkaline lamprophyres – (AL)3. Ultramafic lamprophyres – (UML)4. Kimberlites – (KL)5. Lamproites – (LL)

Comparison of samples with CAL lamprophyric rocks. All values normalized to chondrites, using order and normalizing values of Rock (1991)

Comparison with Rock’s (1991) classification

0,010

0,100

1,000

10,000

100,000

1000,000

Sc V Cr

Co Ni

Cu Zn Ga Rb Sr Y Zr Nb Cs

Ba La Ce Pr Nd

Sm Eu Gd Tb Dy

Ho Er Tm Yb Lu Hf

Ta Pb Th U

VS-57 VS-49 CAL

Comparison of samples with AL lamprophyric rocks. All values normalized to chondrites, using order and normalizing values of Rock (1991)

Comparison with Rock’s (1991) classification

0,001

0,010

0,100

1,000

10,000

100,000

1000,000

Sc V Cr

Co Ni

Cu Zn Ga Rb Sr Y Zr Nb Cs

Ba La Ce Pr Nd

Sm Eu Gd Tb Dy

Ho Er Tm Yb Lu Hf Ta Pb Th U

VS-57 VS-49 AL

Comparison with Rock’s (1991) classification

Comparison of samples with UML lamprophyric rocks. All values normalized to chondrites, using order and normalizing values of Rock (1991)

0,010

0,100

1,000

10,000

100,000

1000,000

Sc V Cr

Co Ni

Cu Zn Ga Rb Sr Y Zr Nb Cs

Ba La Ce Pr Nd

Sm Eu Gd Tb Dy

Ho Er Tm Yb Lu Hf Ta Pb Th U

VS-57 VS-49 UML

Comparison of samples with LL lamprophyric rocks. All values normalized to chondrites, using order and normalizing values of Rock (1991)

Comparison with Rock’s (1991) classification

0,010

0,100

1,000

10,000

100,000

1000,000

10000,000Sc V C

rC

o Ni

Cu Zn Ga Rb Sr Y Zr Nb Cs

Ba La Ce Pr Nd

Sm Eu Gd Tb Dy

Ho Er Tm Yb Lu Hf Ta Pb Th U

SAMPLES / LL

VS-57 VS-49 LL

Comparison of samples with KIL lamprophyric rocks. All values normalized to chondrites, using order and normalizing values of Rock (1991)

Comparison with Rock’s (1991) classification

0,010

0,100

1,000

10,000

100,000

1000,000

Sc V Cr

Co Ni

Cu Zn Ga Rb Sr Y Zr Nb Cs

Ba La Ce Pr Nd

Sm Eu Gd Tb Dy

Ho Er Tm Yb Lu Hf Ta Pb Th U

VS-57 VS-49 KIL

Age spectraAge spectra for phlogopites were obtained from lamprophyres by 40Ar/39Ar step heating

method. In sample VS-66-2, spectrum reveal intermediate plateaus of 3 stages (32%, 35%, 33%) ofcumulative 39Ar with ages 950 ± 6 and 976 ± 6 Ma, respectively.

In the spectrum VS-57 a good plateau 902 ± 9 Ma is distinguished (79% of cumulative 39Ar). Mostdiscordant spectrum VS-52 reveals 4 stages of creation. Most likely the age of L formation is - 976 ± 6 Maand corresponds to ocean stage. Most likely, 902 ± 9 corresponds to the age of the intensivedeformation later event in subduction zone. Further deformation suggests the complex tectonic-thermal history.

Age diagram

ConclusionWe suggest that late Proterozoic ophiolites, which refer to oceanic

stage of 1100 Ma, were later incorporated to arc complex with the acidbase. At 980 Ma they were subjected to plume event with the creationof ultramafic lamprophyres due to reaction with the crust and then theywere hybridized with acid rocks to produce calc-alkaline lamprophyres.Late alteration produced series of secondary minerals.

Thus, the ultramafic, alkaline, and alc-alkaline lamprophyres provide us with more information about

the history of ophiolites of the Eastern Sayan.

Thank you for your attention!