abrupt change in magma generation processes across the...

Contrib Mineral Petrol (1995) 120:378-390 �9 Springer-Verlag 1995

J.A. Walker - M.J. Carr �9 L.C. Patino �9 C.M. Johnson M.D. Feigenson �9 R.L. Ward

Abrupt change in magma generation processes across the Central American arc in southeastern Guatemala: flux-dominated melting near the base of the wedge to decompression melting near the top of the wedge

Received: 8 February 1994/Accepted: 5 December 1994

Abstract. Lavas erupted behind the volcanic front in southeastern Guatemala have many important distinctions from lavas erupted on the volcanic front. These include: generally higher MgO, Nb, Sr, TiO2, and rare earth element concentrations; higher La/Yb and Nb/Y ratios; and lower Ba/La, La/Nb, Ba/Zr and Zr/Nb ratios. These major and trace element distinctions are caused by reduced fractionation during ascent and storage in the crust, lower degrees of melting in the source, and greatly reduced contributions from the subducted Cocos plate in the source. In addition, because all of these important distinctions are even borne in lavas erupted within 20 km of the front, there is little apparent petrogenetic continuity between front and behind-the-front magmas. What little geochemical continuity exists is in radiogenic isotopes: 143Nd/144Nd falls across the arc, Pb isotopic ratios (except 2~176 rise across the arc, and 87Sr/86Sr rise across the arc after an initial discontinuity within 20 km of the front. These continuous across-arc changes in radiogenic isotopes are caused by increased contamination with older, more isotopically disparate rocks, away from the front. Once the effects of crustal contamination are removed, the remaining isotopic variability behind the front is non-systematic and reflects the inherent isotopic heterogeneity of the source, the mantle wedge. Geochemical disconnection in southeastern

J.A. Walker (N~) R.L Ward Department of Geology, Northern Illinois University, DeKalb, IL 60115, USA

M.J. Carr �9 L.C. Patino �9 M.D. Feigenson Department of Geological Sciences, Rutgers University, New Brun- swick, NJ 08903, USA

C. M. Johnson Department of Geology and Geophysics, University of Wisconsin, Madison, WI 53706, USA

Editorial responsibility: T. L. Grove

Guatemala suggests that behind-the-front magmas are produced by decompression melting near the top of the wedge, not by flux-dominated melting near the base of the wedge.

Introduction

A fundamental problem for subduction zone petrolo- gists is the nature of the subcrustal magmatic source (e.g., Ellam and Hawkesworth 1988). The petrological consensus at present is that the primary subcrustal source of subduction zone magmas is the mantle wedge overlying the subducted plate (e.g., Plank and Lan- gmuir 1988). What principally complicates detailed wedge characterization are contributions from the subducted plate or slab. These contributions largely come via water- and alkali-rich fluids which transfer to the wedge a sometimes knotty geochemical signature of subducted oceanic crust and/or subducted sediment (Tatsumi et al. 1986; Morris et al. 1990; Gill et al. 1993).

As contributions from the subducted slab may ob- viously vary between and within individual subduction zones, a useful approach to mantle wedge and overall source characterization has been evaluation of compositional data along the volcanic fronts of individual subduction zones (e.g., Edwards et al. 1993). Similarly, evaluation of the geochemical changes across individual subduction zones or arcs has proven equally useful for source characterization, particularly concern- ing the spatial and compositional variability in the input from the subducted slab (e.g., Leeman et al. 1990). In addition, the character, magnitude and continuity of both "along-arc" and "across-arc" geochemical changes place important constraints on the dynamics of melt extraction in the source (e.g., Spiegelmen and McKenzie 1987).

Carr et al. (1990) Feigenson and Carr (1993) and Leeman et al. (1994) use predominantly the along-arc

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Fig. 1. The volcanoes of Central America: filled triangles -volcanoes of the volcanic front; open triangles behind-the-front volcanoes�9 Rectangular bars are proposed segment boundaries of Stoiber and Carr (1973). Solid lines through central Guatemala are faults associated with the North American/Caribbean plate boundary (e.g., Mann et al. 1990). Inset shows volcanic setting in southeastern Guatemala and western E1 Salvador. Solid lines are faults with horizontal displacements (if any) shown by arrows. Jocotan fault is the southernmost fault associated with the North American/Cari- bbean plate boundary (e.g., Mann et al. 1990). BVF volcanoes broadly subdivided into ]pala Graben (solid circles) and Cuilapa/Barberena (solid triangles) cinder cone fields. See text for discussion

approach to develop a comprehensive picture of magma generation, source mixing and wedge dynamics beneath the Central American subduct ion zone. Here we build on this foundat ion through evaluat ion of across-arc geochemical changes in southeastern Guatemala, where volcanism is most cont inuous across the Central American subduct ion zone (Fig. 1). Al- though volcanism is nearly cont inuous for about 100 km across the arc, many of the geochemical changes are not, suggesting a decoupling of melt generat ion and extract ion across the arc.

Background Volcanoes in Central America can be broadly subdivided into those on the volcanic front (or VF), the trenchward limit of volcanism (Sugimura 1960), and those behind the volcanic front (Fig. 1; Carr et al. 1982). The volcanic front of Central America stretches continu- ously about 1100 km from the Guatemala/Mexico border to central Costa Rica (Fig. 1) and is often associated with large depressions bounded by right-lateral strike-slip faults (e.g., Carr 1976)�9 Behind- the-front (or BVF) volcanism, in contrast, occurs in discontinuous, scattered fields, often associated with graben bounded by N-S nor- mal faults (e.g., Williams et al. 1964). The volcanic front is a direct manifestation of subduction of the Cocos plate beneath the Cari- bbean plate (Carr et al. 1982). BVF volcanism, in comparison, has a more problematic connection to Cocos subduction, particularly BVF volcanism far behind the volcanic front (e.g., Wadge and Wooden 1982). Stoiber and Carr (1973) and Carr et al. (1982) suggest that much of the BVF volcanism is related to segmentation of both the subducting and the overriding plates. Specifically, BVF volcanism, close behind the front, clusters on, or near, proposed segment boundaries (Stoiber and Carr 1973). This may also be true in the northern Andes (Hall and Wood 1985). Alternatively, the location of BVF volcanism may instead be largely controlled by North Ameri- can-Caribbean plate interactions (PlaNer 1976; Burkart and Self 1985; Mann et al 1990). This alternative is most convincing for northern Central America (i.e., Guatemala and Honduras) where regional extension is maximized in proximity to the North American- Caribbean plate boundary (Fig. 1; Plafker 1976; Mann et al. 1990).

380

Indeed the greatest concentration of BVF volcanism in Central America is in southeastern Guatemala, near the North American- Caribbean plate boundary, where BVF volcanoes spread over approximately 3,000 km 2 (Fig. 1). Many of the ubiquitous cinder cones in this region fall within or on the margins of the well-defined Ipala Graben which appears to be the central focus of monogenetic volcanism in northernmost Central America (Fig. 1). Only the cluster of cinder cones near the towns of Barberena and Cuilapa (Fig. 1) could be said to define a separate field, which is supported by statistical analysis (Connor 1987) and by geochemical data (see Results Section, below).

Relatively small calderas, composite cones and shield volcanoes also exist behind the front in southeastern Guatemala (Fig. 1; e.g., Williams et al. 1964; Carr 1974). With the exception of Ayarza Caldera, which is young ( ~ 23,000-27,000 B.P.; Peterson and Rose 1985), the absolute ages of these central volcanoes are unclear at present, but most are clearly older than spatially associated cinder cones (Williams et al. 1964; Carr 1974).

The petrography and major element chemistry of lavas erupted from the BVF cinder cones of southeastern Guatemala and western El Salvador have been previously discussed by Walker (1980; 1981). The important petrological observations and petrogenetic inferences of Walker (1981) remain valid, although olivine-phyric lavas are more abundant than previously thought. The BVF suite is still

dominated, however, by nearly aphyric basalts containing widely scattered phenocrysts of olivine, plagioclase and, very rarely, clinopyroxene, in contrast to petrographic observations, major element variations require the removal of clinopyroxene as well as olivine and plagioclase. To resolve this contradiction, Walker (1981) proposed cotectic crystallization of clinopyroxene, olivine and plagioclase at moderate pressures (5-10 kbar) in the lower crust, fol- lowed by rapid ascent in an extensional environment. During erup- tion, only olivine and plagioclase crystallize because of the expan- sion of the primary phase volumes of olivine and plagioclase relative to clinopyroxene at lowering pressures. Hasenaka and Carmichael (1987) have subsequently presented an identical crustal differentiation model for basaltic magmas feeding a cinder cone field in central Mexico. Hence, the petrogenetic model of Walker (1981) is considered to have general applicability to orogenic cinder cone fields.

Data

New major element, rare earth element and some trace element concentrations were determined by dc-plasma atomic emission spectroscopy at both Rutgers University and Northern Illinois

Table 1 Analyses of representative BVF lavas. Major elements in wt%, trace elements in ppm. Analytical details in text (FeO r total iron as FeO, nd not determined, behind distance behind volcanic front in km)

GUC25 GUC26 GUC601 GUC202 GUC303 GUC309 GUC403 GUC835 GUC30 SALC7 GUC307 GUC404

SiO z 53.70 51.80 51.80 51.20 49.90 52.80 48.60 50.33 49.60 50.00 49.90 50.90 TiO 2 1.27 1.17 1.03 1.49 1.36 0.99 1.58 1.39 1.37 1.13 1.33 1.23 A120 3 17.80 18.20 17.70 17.10 16.50 17.90 17.30 18.06 16.00 16.80 17.50 17.20 FeO T 8.18 9.27 9.20 8.71 8.96 9.40 9.18 9.74 8.81 9.05 8.87 9.45 MnO 0.14 0.16 0.16 0.14 0.16 0.17 0.16 0.17 0.16 0.15 0.16 0.17 MgO 4.19 4.83 5.22 5.88 7.68 5.09 6.73 4.72 8.00 7.06 6.30 5.52 CaO 8.00 8.20 9.00 9.70 9.80 8.70 10.90 9.37 9.50 10.00 9.80 9.20 Na20 3.91 3.64 3.24 3.45 3.23 3.54 3.30 3.93 3.20 3.23 3.54 3.31 KzO 1.97 1.08 1.08 1.20 1.17 0.80 0.73 1.09 0.94 0.84 0.92 1.21 P205 0.42 0.44 0.13 0.21 0.30 0.38 0.13 0.36 0.43 0.17 0.30 0.34

Rb 20.4 11.4 22.4 15.0 5.9 10.3 10.8 18.9 11.2 12.4 13.9 23.0 Ba 617 670 783 384 180 436 327 544 307 388 353 597 Sr 582 612 589 546 480 597 497 653 516 519 563 585 Cr 94 39 80 256 162 49 266 23 267 259 167 154 Ni 33 27 40 80 66 16 67 12 108 101 62 47 Zr 244 124 103 189 133 96 131 134 133 97 143 135 Nb 19.2 11.4 5.7 12.9 5.9 6.7 11.4 8.0 10.2 6.6 8.1 7.8

Y 37.4 30.5 26.1 33.8 24.8 30.5 27.9 28.0 25.4 24.4 26.0 34.8 La 25.6 15.8 13.8 21.0 8.5 11.4 10.8 nd 12.1 10.6 12.6 20.6 Ce 51.0 32.8 29.4 44.8 24.1 26.3 27.2 nd 28.9 24.8 30.3 39.0 Nd 26.2 20.4 17.6 25.9 15.0 17.9 15.9 nd 16.7 16.4 18.3 27.4 Sm 5.44 4.67 3.69 5.40 4.03 4.22 4.01 nd 4.38 3.57 4.05 5.96 Eu 1.64 1.64 1.31 1.83 1.27 1.53 1.37 nd 1.37 1.31 1.41 1.92 Gd 5.49 5.29 4.36 5.91 4.30 4.78 4.64 nd 4.41 4.22 4.57 6.45 Dy 5.39 5.01 4.10 5.69 4.68 4.88 4.81 nd 4.39 4.16 4.46 5.97 Er 3.10 2.70 2.50 3.20 2.46 2.80 2.87 nd 2.40 2.30 2.50 3.40 Yb 2.84 2.56 2.22 2.82 1.80 2.51 2.04 nd 2.06 2.18 2.34 3.10

87Sr/S6Sr 0.703810 0.703840 0.704300 0.703710 0.703210 0.703930 0.703370 0.704150 0.703590 0.703620 0.703680 0.704040 143Nd / 144Nd 0.512870 0.512930 0.512800 0.512910 0.512990 0.512840 0.512940 0.512830 0.512880 0.513000 0.512900 0.512890 2o6pb / 2~ 18.701 1 8 . 6 8 1 18.676 1 8 . 6 8 3 18.610 1 8 . 6 1 8 18.692 18.684 18.684 nd nd 19.068 2oTpb / 2~ 15.601 1 5 . 6 1 3 15.582 15.567 15.556 1 5 . 5 6 3 15.582 15.583 15.585 nd nd 15.683 2o*pb / 2~ 38.396 38.405 38.336 38.284 38.183 38.220 38.295 38.337 38.332 nd nd 38.600 Behind 104 109 102 23 22 46 66 70 61 27 15 73

381

Table 2 Analyses of selected crustal rocks from southeastern Guatemala. Major elements in wt%, trace elements in ppm. Analyti- cal details in text (FeO T total iron as FeO, nd not determined)

M1 M2 G1 G2

SiO 2 61.19 81.19 70.84 59.65 TiO z 1.59 1.04 0.26 0.96 AlzO~ 22.34 11.74 13.37 15.74 FeO a~ 1.43 0.39 1.75 6.84 MnO 0.01 nd 0.01 0.14 MgO 0.75 0.30 0.14 3.33 CaO 0.17 0.16 0.49 6.04 Na20 0.31 0.13 4.66 2.93 K20 6.03 2.59 3.94 1.60 P205 0.05 0.03 0.04 0.19

Ba 2950 1377 863 512 Sr 96 37 102 237 Cr 122 46 6 52 Ni 11 31 4 21 Zr 226 426 275 268 Y 69.6 30.5 32.9 38.8

La 75.0 34.1 23.5 19.3 Ce 151.7 78.2 48.9 49.1 Nd 81.5 30.5 22.8 24.2 Sm 15.10 5.86 5.20 6.14 Eu 3.08 1.12 0.91 1.31 Gd 13.30 4.92 4.61 6.23 Dy 12.14 5.18 4.81 6.10 Er 6.56 3.72 3.78 4.04 Yb 6.22 3.04 3.21 3.27

87Sr/86Sr 0.739081 0.742770 0.705955 0.706598 143Nd/lggNd 0 .512128 0.512098 0.512740 0.512632 2~176 19.192 nd nd 18.810 2~176 15.681 nd nd 15.638 2~176 39.060 nd nd 38.605

University (NIU) after Feigenson and Carr (1985). Some preexisting major element data were obtained by atomic absorption spectro- photometry as described by Walker (1981). Consistency between old and new analyses and between Rutgers, Dartmouth and NIU were excellent except for Na20 and Cr. Since neither element is critical to the discussion that follows, the analytical inconsistencies are not pursued further. Zr, Nb, Rb, Sr and Y were determined on pressed- powder pellets by X-ray fluorescence at NIU following Norrish and Chappell (1977). Nd and Sr isotopic ratios were obtained at the mass spectrometry facility at Rutgers University. Sr isotopic ratios are normalized to 86Sr/88Sr of 0.1194 and are reported as measured. NBS SRM 987 is measured at STSr/86Sr = 0.710248. Nd isotopic ratios are normalized to 146Nd/144Nd =0.7219, and reported as measured. La Jolla STD Nd is measured at 14aNd/*44Nd = 0.511852. Internal precision is better than + 0.000010 for Sr and +_ 0.000020 for Nd (two sigma). External

precision (from replicate analyses of standards) is estimated at + 0.000030 for both isotopes. Pb isotopic ratios were determined at

the University of Wisconsin in Madison using single Re filaments and a mixture of dissolved silica gel and H3PO 4 and four collector static (non peak jumping) multi-collection. Data are corrected for mass fractionation by normalizing to NBS-981 and NBS-982 standards. Johnson and Thompson (1991) give analytical precision and accuracy.

BVF samples were collected in 1974 by MJ Carr and by the first author in 1990 and 1991. The recent sampling maximized the geographic coverage, particularly in and around the Ipala Graben. Representative BVF analyses are given in Table 1. Complete analytical data are available from the first author on request. VF samples

selected for comparison are the most mafic or baseline compositions reported by Carr et al. (1990), Bardintzeff and Deniel (1992) and Feigenson and Carr (1993). These include samples from the following VF volcanoes: Pacaya, Tecuamburro and Moyuta in Guatemala, and Santa Ana, Izalco and Boqueron in E1 Salvador (Fig. 1). The Nd and some of the Pb isotopic ratios given by Bardintzeff and Deniel (1992) for lavas from Pacaya are consistently lower and higher, respectively, than corresponding data from this study. For the sake of consistency, only the Rutgers Nd and Wiscon- sin Pb ratios are utilized below. Paleozoic metamorphic and Me- sozoic plutonic rocks from the northernmost reaches of the Ipala Graben (Williams et al. 1964; Clemons and Long 1971) have also been sampled and analyzed as possible magmatic contaminants. These basement lithologies begin to outcrop north of the Jocotan fault (Fig. 1; Donnelly et al. 1990). Analyses of some of these crustal rocks are given in Table 2.

Results

Across -a rc (or V F versus BVF) geochemica l changes in sou theas te rn G u a t e m a l a are unl ike those t rad i t iona l ly stressed (e.g., Gill 1981). Large- ion- l i thophi le (LIL) elements , such as K 2 0 , are no t consis tent ly h igher across the arc (Fig. 2; Ca r r et al. 1979). This is t rue even cons ider ing tha t B V F vo lcan i sm is general ly m u c h m o r e mafic t han tha t on the f ront (Fig. 2; W a l k e r 1981). N b shows the greatest absolu te en r i chmen t in B V F lavas (Fig. 2). TiO2, P 2 0 5 , St, Y and ra re -ear th e lement (REE) concen t r a t i ons are also typical ly higher in B V F lavas (Fig. 2). B V F lavas are best d is t inguished f rom V F lavas, however , by their t race e lement rat ios: lower Ba /La , Ba/Zr , L a / N b , Z r [ N b and higher L a / Y b and N b / Y (Fig. 3). The K / R b ratio, i m p o r t a n t to the pet- rogenet ic m o d e l of Ta t sumi (1986;1989), is m o r e variable beh ind the front, par t icu la r ly in the Cu i l apa /Bar - be rena region (Fig. 3). M o s t significantly, the impor - t an t ac ross -a rc geochemica l changes in t race elements are general ly abrupt , d i scon t inuous and occur less t han 20 k m beh ind the f ront (Fig. 3). Moreove r , t race ele- m e n t concen t r a t i ons and rat ios do not , in general , consis tent ly va ry within the B V F rea lm (Fig 3). Hence, in terms of t race elements, the pe t ro log ic focus shou ld be on B V F / V F dist inct ions, no t on abso lu te dis tance f rom the front.

Rad iogen ic i so topic results compl ica te mat ters . Firstly, B V F lavas are charac te r i zed by cons iderab le i so topic variabi l i ty (Fig. 4). In addi t ion , N d and P b i so topic rat ios display relatively consis tent and con- t inuous ac ross -a rc changes: b o t h 2~176 and 208 204 P b / Pb general ly increase, while 143Nd/144Nd de- clines a w a y f rom the f ront (Fig. 5). In cont ras t , 87Sr/S6Sr, like m a n y of the t race e lement ratios, shows m o r e of an a b r u p t d i scont inu i ty close beh ind the volcanic f ront and thereaf ter tends to rise with dis tance f rom the f ront (Fig. 5). Therefore, all of the rad iogenic i so topes display some across-arc cont inui ty .

Lavas f rom cinder cones near the towns of Bar- be rena and Cui lapa , close "beh ind" P a c a y a and T e c u a m b u r r o Volcanoes , fo rm a s o m e w h a t m o r e corn-

382

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ity is in accord with their geographic and structural separateness (Fig. 1; Connor 1987). Also the low Ba/La and STSr/S6Sr ratios of the Cuilapa/Barberena lavas suggest an immediate lack of connectivity to the volcanic front. At the same time, their higher Zr/Nb, 143Nd/144Nd and lower 2~176 and 2~176 ratios appear to demonstrate just the opposite: a petrogenetic continuity with the volcanic front.

Many of the Guatemalan lavas (both VF and BVF) have slightly elevated 2~176 and 2~176 relative to mid-ocean ridge basalts (MORB) and the Northern Hemisphere reference line (NHRL) of Hart (1984) (Fig. 4). Arc lavas are typically enriched in the former, often, like the Guatemalan lavas, forming relatively steep data arrays (e.g., Kay et al. 1978). In addition, many arc BVF suites are enriched in 2~176 relative to MORBS (e.g., Hochstaedter et al. 1990). Four of the BVF basalts define a trend to unusually high 2~176 2~176 and low 2~176 (Fig. 4). This trend is discussed more fully in the next section.

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positionally homogeneous BVF group (Figs. 3, 5), probably because they are restricted to within 40 km of the front (Fig. 1). In detail, they are distinguished from lavas in and around the Ipala Graben by their somewhat lower Ba/La, 87Sr/86Sr, 2~176162 2~176 and their higher Zr/Nb, and 143Nd/*44Nd (Figs. 3, 5). This geochemical individual-

Causes of BVF/VF and across.arc compositional variation

Degree of partial melting and slab interaction

Some of the incompatible element distinctions of the BVF lavas, such as high Nb concentrations and high Nb/Y and La/Yb, can be largely explained by lower degrees of partial melting, as compared with the degrees of melting producing VF magmas. Figure 6 illustrates that degrees of melting are typically two to ten times smaller behind the front, assuming that Nb and Y are completely invariant across the mantle wedge. A similar contrast in degree of melting between VF and BVF can be inferred from the incompatible element modeling of Carr et al. (1990). The degree of melting does not get progressively smaller behind the front, however, as ratios such as Nb/Y do not systematically increase in the BVF region (Fig. 3).

Some of the remaining incompatible element distinctions of the BVF lavas, such as low Ba/La, are the consequence of greatly reduced source contributions from the subducted Cocos slab. Thus, as would be expected, lower degrees of melting go hand in hand with reduced slab contributions via hydrous fluids or melts. Nevertheless, in northern Central America, slab interaction must take precedence, that is, the connection to the subducted plate must attenuate faster than the degree of melting. Otherwise, the Ba/La systematics between VF and BVF lavas (Fig. 3) would be reversed, or at the least obscured, as Ba is more incompatible than La during melting of mantle peridotite (Sun and McDonough 1989). Stolper and Newman (1991) have indicated a similar precedence for slab interaction (versus degree of melting) from the variation in H20 contents across the Mariana arc. Moreover, the discontinuous nature of many of the across-arc geochemical

384

Fig. 5. Radiogenic isotopic concentrat ions of lavas from southeastern Guatemala versus distance behind the volcanic front

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[]

[] A

I I I I I I I I I 20 40 60 80 100


[]

[]

0.6

o.5 [] / x 2

0.4 [] [] " [] []

0.3 Z

I

0.0 I I I I 0 5 10 15 20 25

Nb

Fig. 6. Nb/Y versus Nb for the lavas of southeastern Guatemala. Solid line is an illustrative partial melting model for garnet peridotite from Gill (1987). Initial source concentrations were 0.66 pp , and 6 pp , for Nb and Y, respectively. Tick marks give degree of melting. Clearly, BVF magmas are the product of lower degrees of melting of the as thenospheric wedge

changes in southeastern Guatemala (Figs. 3, 5) suggests that slab contributions fall off immediately behind the front. Hence, magma generation behind the front may be disconnected (Spiegel,an and McKenzie 1987) from that at the front. This has important implications for

the dynamics of the mantle wedge (see Mantle wedge dynamics, below). The BVF basalts generally have Ba/La ratios > 20 (Fig. 3) and one from E1 Salvador (SALC502) has small amounts of 1~ (4.7x 106 atoms/g, unpublished result), suggesting BVF lavas possess at least a small slab component (Morris et al. 1990; Cart et al. 1990). The slab component may be patchy or may, if the 1~ signal in SALC502 is anomalous, have been incorporated into the wedge during an earlier phase of subduction (Lee , an et al. 1994).

Available 1~ contents and B/Be ratios suggest that subducted sediment can only be a small portion of the slab contributions from the subducted Cocos plate to the source of Guatemalan VF magmas (Morris and Tera 1989; Morris et al. 1990; L e e , an et al. 1994). The steep 2~176 array (Fig. 4) might suggest otherwise (e.g., Kay et al. 1978). Nevertheless, if the latter suggestion were true, the higher 2~176 of many BVF lavas would necessitate larger sediment contributions behind the front, contrary to all other evidence. Also, it will be argued shortly that the high 2~176 of most, if not all, of the BVF lavas is caused by crustal contamination. Therefore, once contamination effects are removed for all Guatemalan lavas, the Pb data are deemed consistent with the less ambiguous interpretations on sediment involvement derived from Be and B data (Morris and Tera 1989; Morris et al. 1990; L e e , a n et al. 1994).

385

0.5131

0.5130

-~ 0.5129 7

0.5128

0.5127 I 0.7030

I I I I I I I I I I I I I

20 ~ . 307~

I I I I I I I I I I I I I 0.7035 0.7040 0.7045

87Sr/a6Sr

Fig. 7. 143Nd/tr versus 87Sr/86Sr for BVF lavas of southeastern Guatemala and closest-fit contamination models with samples GUC303 or SALC7 (Table 1) as the parental or mafic end member compositions. All of the models utilize granite G2 or phyllite M2 (Table 2) as contaminants or felsic end members. AFC1 and AFC2 are assimilation/fractional crystallization (AFC) models which differ in r (assimilation rate/crystallization rate; DePaolo 1981; 0.4 and 0.6, respectively) and in parent and assimilant compositions (SALC7/M2 and GUC303/G2, respectively). Bulk distribution coefficients for both models are 0.8 and 0.03 for Sr and Nd, respectively. Mixl and Mix2 represent bulk mixing of GUC303 and G2, and SALC7 and G2, respectively. Tickmarks show degree of crystallization for AFC models and percentage of G2 for the mixing models.

Crustal contamination and source heterogeneity

The regular across-arc changes in Pb, Nd and Sr isotopic values (Fig. 5) cannot be explained by decreased degrees of partial melting nor by decreased contributions to the mantle wedge from the subducted Cocos plate (e.g., Gill 1981). Lower 143Nd/t44Nd, more radiogenic Pb and even rising STSr/86Sr (after the initial discontinuity behind the front) may instead indicate increasing crustal contamination. Figure 7 illustrates some possible contamination models. Both assimilation-fractional crystallization (AFC) or bulk mixing could explain all of the Sr and Nd isotopic variability displayed by the BVF basalts. Crustal rocks from southeastern Guatemala serve as adequate contaminants, however the required amounts of contamination or fractional crystallization are rather high (Fig. 7). The assimilation/fractional crystallization models can be significantly improved if the bulk distribution coefficient for Sr is raised to 1 or more, that is if plagioclase dominates the bulk extract. Plagioclase domination, however, is inconsistent with the increases in both Sr and A1203 shown by BVF basalts with differentiation (Walker 1981).

The trend toward more radiogenic Pb isotopic compositions exhibited by many of the BVF basalts is also qualitatively consistent with crustal contamination as the local crustal rocks fall nicely along an

extension of the trend (Fig. 4). In addition, the unusual trend toward lower 2~176 and more radiogenic 2~176 and 2~176 (Fig. 4) could also be the result of crustal contamination, but of distinct, granulitic crust (e.g., Tilton and Barreiro 1980; James 1982; Mattinson 1990).

The regular isotopic variations across southeastern Guatemala imply either an absolute or an apparent increase in crustal contamination across the arc. An absolute increase in the amount of contamination seems unreasonable as the tectonic extension behind the front in southeastern Guatemala would logically favor less impeded access to the surface and, hence, less contamination. Therefore, we believe that contamination only appears to increase behind the front probably due to assimilation of more isotopically evolved crust, which outcrops with increasing frequency to the north, away from the trench (e.g., Donnelly et al. 1990). Feely (1993) has suggested an analogous model of regular spatial variations in assimilated basement rocks to explain across-arc trends in a portion of the Central Andes.

Crustal contamination, therefore, can explain the systematic across-arc variation in radiogenic isotopes displayed by the BVF basalts and, perhaps, most of the overall Nd and Sr isotopic variability of the BVF basalts (Fig. 7). Nevertheless, we seriously doubt that it explains the latter. Our doubts stem from the considerable isotopic variation shown by the most mafic BVF basalts ( > 6.0 wt% MgO; Fig. 4). This subgroup of BVF lavas is presumably least affected by contamination (Plank and Langmuir 1988), yet their isotopic variation spans about half of the entire isotopic variation of the BVF lavas (Fig. 4), and hence would require unreasonably large amounts of crustal contamination to explain (Fig. 7). As a consequence, we suggest that the isotopic variation shown by the most mafic BVF basalts, as well as some of their trace element variability, reflects inherent mantle heterogeneity (e.g., Stern et al. 1990). In terms of the source components defined by Carr et al. (1990), the most mafic BVF basalts cluster around enriched mantle (EM) and stretch back toward depleted mantle (DM), with obvi- ously little to no subducted slab (MM) component (Fig. 4), except for one sample, SALC7, which is located approximately 25 km behind Santa Ana volcano, thus in close proximity to the front.

In summary, BVF/VF and across-arc compositional variability in southeastern Guatemala and western E1 Salvador are largely a function of: the degree of melting (lower behind the front), the degree of connection to the subducted Cocos plate (low behind the front), and the degree, or the intricacies, of crustal contamination (greater contamination with older, radiogenic crust with increasing distance from the front). In addition, the compositions of the most mafic BVF basalts reflect the inherent heterogeneity in the mantle wedge.

386

BVF, southeast Guatemala/western El Salvador (MgO, < 5.1 wt.%)

VF, Pacaya volcano, Guatemala

0.7035 0.7037 0.7039 0.7041 0.7043

Nejapa/Granada cinder cones, Nicaragua

Masaya caldera, Nicaragua

M m , , , , I r l l t l , ,

0,7035 0.7037 0.7039 0.7041 0.7043

87Sr/86Sr

Fig. 8a,b. Histograms of 87Sr/86Sr ratios for: a. evolved BVF lavas of southeastern Guatemala and lavas from a single Guatemalan VF volcano, Pacaya. All analyzed lavas from Pacaya have less than 5.1 wt.% MgO; b. basaltic lavas from Masaya Caldera in Nicaragua and from neighboring non-parasitic cinder cones along the Nicaraguan volcanic front. Note the greater isotopic variability of lavas from "monogenetic" volcanoes.

ogenization during ascent and storage above the as- thenosphere (e.g., Hickey-Vargas et al. 1989; Perfit et al. 1993). For Central America, these two possibilities can be independently assessed using isotopic data for lavas from a small section of the volcanic front in central Nicaragua. There, small cinder cones, not parasitic to larger volcanoes, exist along the volcanic front (Walker 1984). Lavas from these cinder cones display consider- ably more isotopic variability than lavas from adjoin- ing central volcanoes (Fig. 8b). Since melting processes are unlikely to change much along this small portion of the volcanic front (Carr et al. 1990), the isotopic heterogeneity of the cinder cone lavas is more likely caused by the relative lack of mixing and homogenization after melt segregation. The same is reckoned to be true for the volumetrically minor and spatially diffuse BVF lavas of northern Central America (and for monogenetic BVF fields elsewhere). This is consistent with their overall more mafic compositions, particularly since MgO decreases, not increases, with decreasing degrees of mantle melting (e.g., Kinzler and Grove 1992). The obvious corollary is that VF magmas are, as a rule, homogenized at supra-asthenospheric depths, probably in deep crustal, MASH-like magma chambers (e.g., Hickey-Vargas et al. 1989; Davidson and de Silva 1992). Therefore, if one were seeking the worldwide isotopic spectrum of mantle wedges, BVF volcanism, if disconnected from the front and uncontaminated by crustal rocks, would be the least ambiguous focus.

Implications

Mantle wedge heterogeneity

It was shown above that the overall isotopic diversity of the BVF lavas is somewhat larger than that displayed by the contiguous section of the volcanic front (Figs. 4, 5). This appears to be the norm for continental arcs (Notsu et al. 1989; Stern et al. 1990; Nakamura et al. 1990; Davidson and de Silva 1992). In northern Central America the larger isotopic diversity behind the front is particularly evident when the comparison is with individual VF volcanoes and where contamination effects are removed (Fig. 8a). Greater isotopic diversity behind the volcanic front is directly analogous to the considerable isotopic diversity displayed by lavas from off-axis seamounts at divergent plate margins where the comparison is made with lavas from adjacent ridge crests (e.g., Batiza and Vanko 1984). The extent of isotopic heterogeneity in either tectonic setting has two possible explanations. Either (i) smaller degrees of melting, behind the volcanic front and offthe ridge axis, result in less mixing and homogenization during melt production and segregation in the asthenospheric source (Batiza and Vanko 1984), or (ii) lower magmatic fluxes out of the source lead to less mixing and hom-

Mantle wedge dynamics

Feigenson and Carr (1993) present the most recent model for mantle wedge dynamics for the Central American subduction zone. An important premise of the model is that the dip of the subducting Cocos plate, which varies along the arc, controls the fluid flux enter- ing the mantle wedge per unit volume, which, in turn, ultimately controls the degree of source melting. In addition, the dip of the subducting plate also regulates melt streamlines through the wedge (e.g., Spiegelman and McKenzie 1987). The dip of the subducting plate beneath southeastern Guatemala and western E1 Salva- dor is not well constrained, but appears to be about 45 50 ~ (Carr 1976; Burbach et al. 1984; Carr et al. 1990).

If melt streamlines are critically controlled by the angle of subduction, the theoretical approach of Spiegelman and McKenzie (1987) would indicate that BVF volcanism in southeastern Guatemala and western E1 Salvador could be either connected or disconnected to the subducted plate depending on the values chosen for a number of key variables. For example, disconnectedness would be predicted for dimensionless percolation values of about 0.08, which assumes low porosities in the wedge, and subduction angles of 45 50 degrees. In contrast, connectiveness would be predicted

387

for higher porosities (Spiegelman and McKenzie 1987). Porosity of the wedge could be the key variable producing total or partial disconnectivity in southeastern Guatemala, but a more likely critical factor is the tectonic complexity of the region (Fig. 1). Ribe (1989) has argued that active back-arc spreading at a subduction zone would serve to disconnect melt streamlines across the arc. Geochemical data for lavas from the Sumisu Rift, close behind the Izu-Bonin arc, are consistent with disconnectiveness where active spreading is occurring (Hochstaedter et al. 1990). Northern Central America is not experiencing active back-arc spreading, but it is undergoing extensive regional extension, although transverse, not parallel, to the volcanic front (e.g., Mann et al. 1990). By extrapolation of the results of Ribe (1989), transverse extension might also facilitate detachment of melt streamlines across the arc, if the center of maximum extension is far enough behind the front (compare Figs. 4 and 5 of Ribe 1989); moreover, if partial melting behind the front occurs largely at shal- low depths in the mantle wedge (see next section), then the just mentioned qualification is probably moot.

Melting model

Although they are too often sidestepped, important considerations for modelling melting in the mantle wedge are the location and cause (or causes) of partial melting. For the production of VF magmas, the cause of melting is ultimately dehydration of the subducting plate at about 100 120 km which adds fluids to and spurs melting in the adjacent mantle wedge which, in turn, is the main location of melting (Tatsumi 1986; Tatsumi et al. 1986; Plank and Langmuir 1988; Davies and Stevenson 1992). For BVF magmas, the ultimate cause and location may be substantially the same except that dehydration and melting occur at somewhat greater depths (e.g., Tatsumi 1986,1989; Ryan and Lan- gmuir 1993). A tight genetic coherence between BVF and VF magma generation along the slab/wedge interface beneath southeastern Guatemala, however, is not consistent with the notable geochemical discontinuities across the arc (Figs. 3, 5), unless the subducted plate is efficiently stripped of the necessary tracers during VF melt production. In addition, at deeper levels along the slab/wedge interface, phlogopite likely assumes influence in the source and, hence, extracted magmas might reflect this influence with lower K/Rb ratios (Tatsumi et al. 1986,1992). K/Rb ratios in southeastern Guate- mala and western E1 Salvador, however, are not lower behind the front (Fig. 3). Therefore, the source of BVF magmas may well be removed from the subducting and dehydrating Cocos plate.

Our preferred location for BVF magma generation is nearer the upper part of the mantle wedge where asthenospheric mantle is replacing that removed by

slab-induced counterflow (e.g., Davies and Stevenson 1992). The cause of melting in this region could be decompression if counterflow has a substantial upward component (e.g., Reagan and Gill 1989) or temperature increase if counterflow results in migration into a substantially hotter portion of the wedge (e.g., Davies and Stevenson 1992).

A finetuned model for magma generation and wedge dynamics is presented in Fig. 9a. The mantle source is heterogeneous on a small scale. The more enriched portions are likely veins or streaks with slightly higher garnet to clinopyroxene ratios (Feigenson and Carr 1993). The mantle source has a weak, patchy slab component, perhaps inherited from earlier subduction. Magma generation and ascent behind the front, however, is totally or partially decoupled from that at the front.

The model presented in Fig. 9a may not apply to magma generation below the slightly older central volcanoes of southeastern Guatemala. Although the requi- site geochemical data do not exist, at present, to test this statement directly, data from other central volcanoes in similar tectonic settings provide some in- direct guidance. Amak, Bogoslof (the Aleutians), Mt. Arayat (the Philippines), Makalia (New Britain) and Lanin (southern Andes) are all central volcanoes located well behind the fronts of other subduction zones. All display geochemical connections to their subducted plates (Morris and Hart 1983; Hickey-Var- gas et al. 1989; Morris et al. 1990; Gill et al. 1993; Bau and Knittel 1993). Hence, the alternative melting model illustrated in Fig. 9b presents an alternative picture of magma generation beneath these BVF central volcanoes, and perhaps for the central volcanoes of southeastern Guatemala as well. Therefore, there may have been a temporal shift behind the front of southeastern Guatemala from flux-dominated melting near the base of the wedge, analogous to what occurs beneath the volcanic front except deeper (Fig. 9b), to decompression melting near the top of the wedge (Fig. 9a). The development of widespread crustal extension behind the front would have facilitated the proposed shift.

Conclusions

Magma generation behind the volcanic front in southeastern Guatemala and western E1 Salvador is at present totally or partially decoupled from that at the volcanic front. Behind the front (BVF) volcanism is produced by smaller degrees of melting of an asthenospheric source near the top of the wedge where there is a much smaller slab overprint. Crustal extension behind the front in southeastern Guatemala and western E1 Salvador is the major cause of petrogenetic decoupling across the arc. Extension facilitates detachment of melt streamlines across the mantle wedge and could

388

Fig. 9a. Model for BVF magma generation and mantle wedge dynamics for southeastern Guatemala. Parental BVF magmas are generated in the upper part of the wedge via decompression melting. The distribution of BVF volcanoes is purely illustrative, b. Possible contrasting model for magma generation of parental magmas for slightly older central volcanoes of southeastern Guatemala where BVF parental magmas are produced at or near the slab/wedge interface

100

200

BVF monoqenetic VF

BVF central VF

100

200

100

200

~ - - mantle f low lines --*-* magma f low lines ~ - slab fluid f low lines

100

200

also facilitate pressure-release melting of the wedge at shallower depths behind the front. This view contrasts with the conventional view of BVF magma generation where melting occurs at deep levels along or near the slab/wedge interface (e.g., Tatsumi 1989; Ryan and Langmuir 1993).

Although rising in an extensional setting, BVF magmas are quite susceptible to crustal contamination with increasing susceptibility of contamination by older, more radiogenic crust with increasing distance behind the front. In addition, the compositional effects of contamination on BVF magmas are not softened by supra-mantle homogenization in sizable magma chambers. Hence, BVF lavas provide a less ambiguous win- dow into crustal, as well as, mantle differentiation at subduction zones.

Acknowledgements Many thanks to the following: Lisa Paulson for drafting the figures; Otoniel Matias, Mike Conway and Rich Mar-

key for expert help in the field; Nell Dickey and Bob Bailey for analytical assistance; and two anonymous reviewers for helping us connect our thoughts. This work was supported by NSF grants EAR-9205053 (Carr) and EAR-8917588 (Walker) and by Northern Illinois University.

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