Geochemistry of arc volcanic rocks of the Zagros Crush Zone, Neyriz, Iran
H.A. Babaie
a,*, A.M. Ghazi
a, A. Babaei
b, T.E. La Tour
a, A.A. Hassanipak
c aDepartment of Geology, Georgia State University, Atlanta, GA 30303, USAbDepartment of Biology, Geology, and Environmental Sciences, Cleveland State University, Cleveland, Ohio, USA cDepartment of Mining Engineering, University of Tehran, Tehran, Iran
Abstract
The northeastern margin of the Tethyan Neyriz ophiolite complex in southwestern Iran is tectonically juxtaposed under cataclastically-deformed island arc volcanic±volcaniclastic rocks. We document this arc component of the Zagros Crush Zone in the Neyriz area, and describe its petrographic and geochemical characteristics. The arc unit which we call the Hassanabad Unit, is tectonically intercalated with Cretaceous limestone in the cataclastic shear zone around the Hassanabad pass north of Neyriz.
Analyses of the distributions of the major, rare earth and other trace elements in the volcanic rocks of the Hassanabad Unit reveal a dominantly calc-alkaline island arc composition. Volcanogenic sandstone and sedimentary breccia, with clasts of basalt, andesite and diorite, are cataclastically intercalated with pillowed calc-alkaline island arc volcanic rocks, pelagic limestone and radiolarian chert. Trace element geochemistry corroborates the petrographic evidence that the poorly-sorted and angular volcanogenic sediments were derived locally from the island arc volcanic and intrusive rocks. The emplacement of the volcanic arc rocks adjacent to the thrust sheets of the crustal and mantle sequences of the Neyriz ophiolite was probably a result of subduction-related processes during closure of the Tethys ocean during the Late Cretaceous.q2001 Elsevier Science Ltd. All rights reserved.
1. Introduction
Rare earth and other relatively immobile trace elements are useful to elucidate the probable tectonic setting of erup-tion of basalt and other volcanic rocks in ophiolites (Pearce and Cann, 1973; Shervais, 1982; Meschede, 1986; DudaÁs, 1992; Pearce, 1987, 1996).
The geochemistry and petrology of some lavas in the large and well preserved Tethyan ophiolites of Troodos (Cyprus) and Semail (Oman) are signi®cantly different from the oceanic crust that forms at mid oceanic ridge centers (Serri, 1981; Moores, 1982; Bloomer and Hawkins, 1983; Rautenschlein et al., 1985; Coleman, 1984; Pearce et al., 1984; Leith, 1984). These ophiolitic lavas have trace element geochemical systematics that relate more to subduction zone processes than those along mid ocean ridges (Leith, 1984; Bloomer et al., 1995; Kerrich and Wyman, 1996). Pearce et al. (1984) and Rautenschlein et al. (1985), studying lavas in the Semail ophiolite in Oman and Troodos ophiolite in Cyprus, respectively, have docu-mented a subduction zone trace element geochemical signa-ture for these Tethyan ophiolites that are of the same age and
paleo-geodynamic setting as those of the Neyriz ophiolite in Iran Ð the subject of this paper. The ®eld and geochemical work of Alabaster et al. (1980, 1982), Pearce et al. (1981) and Pearce (1982), have revealed that part of the Oman ophiolite complex consists of backarc oceanic crust intruded by products of volcanic arc magmatism.
The Neyriz ophiolite is one of several large allochthonous Tethyan ophiolites that are exposed in the southwestern edge of the NW±SE-trending Sanandaj±Sirjan belt, north-east of the Zagros fold-and-thrust belt in Iran (Ricou, 1971b; Gansser, 1974; Stocklin, 1968, 1977; Stoneley 1981, 1990). Although a paleo-subduction zone has been hypothesized to exist between the Neyriz ophiolite and the Sanandaj±Sirjan belt (Haynes and McQuillan, 1974; Pilger and RoÈsler,1976; Adib et al., 1977; Alavi, 1980), no arc-related volcanic rocks have been documented previously in this area. Desmons and Beccaluva (1983) report arc lavas in the Sahneh ophiolite (near Kermanshah) Ð which is tectoni-cally and temporally related to the Neyriz ophiolite Ð about 600 km to the northwest of Neyriz.
In this paper, we analyze the distribution of immobile trace elements, including the rare earth elements, in volca-nic and volcavolca-niclastic rocks of the Zagros Crush Zone immediately to the northeast of the Neyriz ophiolite complex. We show that these rocks have a subduction zone geochemical signature and probably represent a volca-nic arc within the Neo-Tethys basin.
Journal of Asian Earth Sciences 19 (2001) 61±76
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* Corresponding author. Tel.:1001-404-463-9559; fax:1 001-404-651-1376.
2. Tectonostratigraphy and geologic history of the Neyriz area
The Neyriz ophiolite complex is part of the ªCroissant Ophiolitiqueº (Ricou, 1971b, 1976) that stretches between Oman and the Mediterranean (Gansser, 1974; Stocklin, 1968, 1977; Coleman, 1981; Moores, 1982; Moores et al., 1984; Stoneley, 1990). The term Neyriz ophiolite complex, as used in this paper, represents an imbricate stack of NW-striking, NE-dipping thrust sheets of ophiolitic and related sedimentary, igneous and locally-metamorphosed rocks, exposed immediately to the southwest of the ªMain Thrustº of the Zagros fold-and-thrust belt. Thrust sheets containing mantle and oceanic crustal sequences occur in the middle of the stack and constitute what we call the ophiolite compo-nent of the Neyriz ophiolite complex which includes peri-dotite, gabbro, diorite, plagiogranite and ma®c and acidic volcanic differentiates (including mid ocean ridge basalt, MORB).
The base of the complex includes the Pichakun Series (Ricou, 1968b) which represents the abyssal facies of the Neo-Tethys ocean basin and constitutes a series of thrust sheets of late Triassic limestone, middle Jurassic oolitic and
micro-brecciated limestone and middle Cretaceous
conglomeratic limestone. The Pichakun Series is wedged between two thrust sheets of sheared sedimentary meÂlange (e.g. Haynes and McQuillan, 1974; Pamic and Adib, 1982). The two meÂlange units contain clasts of rocks of the Picha-kun Series, radiolarite, pelagic limestone, ocean ¯oor lava, Megalodon- and fusilinid-bearing limestone and mylonitic metamorphic rocks such as marble, amphibolite, mica schist and serpentinite.
The contact of the lower meÂlange with the underlying autochthonous Sarvak Formation, exposed to the west of Lake Bakhtegan (Fig. 1), is de®ned by mylonitic amphibo-lite and represents the sole of the Neyriz ophioamphibo-lite complex. The upper meÂlange, exposed east of the lake above the Pichakun Series, is structurally overlain by thrust sheets of the mantle and oceanic crustal sequences of the ophiolite component. Thermally metamorphosed and mylonitized carbonates are interspersed on top of the ultrama®c and ma®c rocks of the ophiolite component near Tang-e Hana (Ricou, 1971a; Hall, 1981; Arvin, 1982a).
The oceanic mantle and crustal sequences (the ophiolite component) were thrusted onto the continental slope, trench deposits and abyssal facies, represented by the Pichakun Series and the meÂlange, to form the Neyriz ophiolite complex. The assembled complex was then emplaced onto the Cenomanian±Turanian shallow marine, shelf-platform carbonate facies (Sarvak Formation) at the northeastern margin of the Afro±Arabian passive continental margin (Gray, 1949; Ricou, 1968a, 1968b; Hallam, 1976; Stoneley, 1981). The sequential thrusting was due to the Late Creta-ceous, NE±SW-directed contractional tectonics probably along a NE-dipping subduction zone (Stocklin, 1968; Takin, 1972; Haynes and McQuillan, 1974; Alavi, 1980,
1994; Babaie et al., 1996, 1997). The allochthonous complex occurs along the NW±SE trending ªMain Thrustº zone of the Zagros Range between the Sanandaj±Sirjan belt (Haynes and McQuillan, 1974; Pilger and RoÈsler, 1976; Adib et al., 1977; Alavi, 1980) to the northeast and the Zagros Simply-Folded Belt to the southwest (Alavi, 1994). The Neyriz ophiolite complex is overlain uncon-formably by the uppermost Cretaceous anhydritic limestone of the Tarbur Formation in the east and by the Middle Eocene Jahrum Formation in the northwest (James and Wynd, 1965; Alavi, 1994).
The ophiolite complex is structurally overlain by volcanic and volcaniclastic rocks of the Hassanabad unit (the subject of this paper). The unit occurs in several variably-sized tectonic slivers which are intercalated with the Cretaceous limestone. These cataclastically-deformed rocks occur in the Zagros ªCrush Zoneº immediately to the northeast of the ophiolite complex.
The tectonic processes, polarity, paleogeography, timing of subduction initiation and petrogenetic setting of the subduction zone are unknown. It is generally believed that the northeastern margin of the Arabian continental margin is still subducting under the southwestern margin of the Persian segment of the Eurasian plate (i.e. below the south-western edge of the Sanandaj±Sirjan belt), forming the Tertiary±Quaternary Central Iranian magmatic assemblage to the northeast of the Sanandaj±Sirjan belt (Berberian, 1981; Berberian and King, 1981; Kadinsky-Cade and Barzangi, 1982; Lensch et al., 1984; Snyder and Barzangi, 1986; Hempton, 1987; Alavi, 1980, 1994).
3. Background on geochemistry of Arc Magma
Island arc volcanic rocks are markedly different from MORB and ocean island basalt (OIB) in having low abun-dances of high ®eld strength (HFS) elements (Ti, P, Zr, Hf, Nb, Ta) relative to the light rare earth elements (LREE) and the large ion lithophile (LIL) elements (Cs, Rb, K, Ba, Pb, Sr, Th, U; Jakes and Gill, 1970; Gorton 1977; Saunders et al., 1980; Gamble et al., 1993; Thirlwall et al., 1994; Hawkesworth et al., 1993; Pearce and Peate, 1995).
The composition of most parental volcanic arc magmas is controlled by: (i) the composition of a mantle-derived component (e.g. mantle wedge) from which they are derived under the presence of water, and which depends on the fertility of the mantle; (ii) a slab-derived ¯uid advection component (with high LIL/HFS and LREE/ HFS elemental ratios) which depends on the temperature and amount of ¯uid at the slab±wedge interface; (iii) composition of the subducted plate and its sedimentary cover (e.g. Tatsumi et al., 1986; Pearce and Parkinson, 1993; Pearce and Peate, 1995). Slab-derived components
may experience element fractionation and isotope
exchange as ¯uids migrate through the mantle wedge. The frequently observed depletion of the mantle wedge
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above the slab is attributed to earlier melting events (McCulloch and Gamble, 1991).
During subduction, the incompatible elements and moderately compatible LREE (e.g. La, Ce) are removed from the subducting slab by ¯uids or magma (Kerrich and Wyman, 1996). The subduction zone magma therefore becomes enriched in the so called non-conservative incompatible elements (i.e. those that do not remain in the slab) such as Cs, Rb, Ba, K, Pb and Sr, and to a lesser extent, the moderately non-conservative elements such as Sr, U, Th and the LREE. Tholeiitic volcanic arc basalt (TH±VAB) have negative anomalies of Nb, Zr, Ti and Y relative to the N-MORB (normal MORB), and calc-alkaline volcanic arc basalt (CA-VAB) have higher concen-trations of Nb and Zr, but lower Ti and Y than the N-MORB (Pearce, 1996).
4. Volcanic±volcaniclastic rocks of the Hassanabad Unit
In this paper, we de®ne a new tectonostratigraphic unit (the Hassanabad Unit) which occurs at the southwestern margin of the ªCrush Zoneº, immediately northeast of the Neyriz ophiolite complex. The Hassanabad Unit is desig-nated as ªvsº on Fig. 1 around and to the northwest of the Hassanabad pass on the road from Neyriz to Sirjan. It comprises pelagic sediments and volcanic and volcaniclas-tic rocks which are exposed as tectonic intercalations with the cataclastically-deformed Cretaceous limestone (unit ªKlº on Fig. 1) within the Crush Zone (Falcon, 1967; Wells, 1969; Haynes and McQuillan, 1974; Alavi, 1994). The Crush Zone includes both ªK1º and ªvsº units on Fig. 1. The volcanic±volcaniclastic rocks of the Hassanabad Unit are part of the Cretaceous sedimentary-volcanic unit
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of Valeh and Alavi Tehrani (1985) and the Crush Zone of Haynes and McQuillan (1974), comprising an assemblage of pillow lava and deep water sediments such as radiolarite, red and green marl, turbiditic breccia and conglomerate. The cataclasis has involved brittle faulting which led to tectonic intercalation and brecciation of all rocks. The Cretaceous limestone extends to the Sanandaj±Sirjan block where it is relatively undeformed and lies on Precam-brian basement and its cover rocks (Haynes and McQuillan, 1974).
The Sanandaj±Sirjan belt which includes metamorphic rocks, Triassic and younger volcanic±volcaniclastic rocks, continental arc intrusions and ophiolite, lies northeast of the Hassanabad Unit and southwest of the NW±SE-trending Tertiary±Quaternary Central Iranian andesitic arc volcanic belt (Stocklin, 1968; Haynes and McQuillan, 1974; Alavi, 1980).
The lavas of the Hassanabad Unit [Fig. 2(a)] are not continuous, but rather occur as tectonic lenses in shear zones, intercalated with radiolarian chert, thin limestone, mudstone and volcanogenic sandstone and sedimentary breccia. The dimensions of the cataclastic lenses vary from less than a meter to tens of meters. Within the lenses are numerous minor shear zones and fractures with veins of calcite and quartz; portions of these lenses are brecciated or foliated. The thinly-bedded sedimentary rocks are folded and disrupted by cataclasite and tectonic breccia. Limestone is commonly deformed into cataclasite, but plastically-deformed, foliated calc-mylonite also occurs along some of the shear zones in the unit.
Immediately northeast of the Hassanabad Unit, near Neyriz, rocks of the Sanandaj±Sirjan block are regionally metamorphosed (e.g. Adib, 1978). Although mylonites in the shear zones around several thrust sheets and in the marbles in the Tang-e Hanna area attest to local, fault-related amphibolite facies metamorphism (Pamic et al., 1979; Pamic and Adib, 1980, 1982), no evidence of regional metamorphism exists in Neyriz ophiolite complex or in the Hassanabad Unit.
In this paper we focus only on the geochemistry of the volcanic and volcaniclastic rocks of the Hassanabad Unit and are not concerned with the tectonics of emplacement of these rocks or the geochemistry of the structurally under-lying ophiolitic sequences.
4.1. Petrography
Thin section examination reveals that brecciation and cataclastic shearing have severely modi®ed the textures of the volcanic and volcaniclastic rocks of the Hassanabad Unit but have modi®ed the mineralogy only to a minor extent. From a textural point of view, the volcanic rocks and volcanic clasts in the volcaniclastic rocks have preserved volcanic textures, although vesicle ®lling, shear-related foliation and/or calcite microveins are evident. The non-pelagic rocks of the Hassanabad Unit are divided into
volcanic and volcaniclastic groups (Tables 1 and 2). The volcaniclastic rock group includes sandstone and sedimen-tary breccia with clasts of a variety of volcanic and intrusive rocks.
The primary mineralogy of the volcanic rocks is domi-nated by euhedral-to-subhedral plagioclase feldspar (some normally compositionally zoned) occurring as both pheno-crysts and in the groundmass. Clinopyroxene occurs in the groundmass and to a much lesser extent as phenocrysts. The lack of evidence for moderate-to-high temperature hydra-tion, even in the most highly altered samples, argues against water having played a major role in the alteration. The mineralogical alteration is dominated overwhelmingly by carbonate occurring as in®lling of vesicles, and to a lesser extent, by microveining and partial replacement of plagio-clase phenocrysts and groundmass. The micro-porphyritic volcanic rocks (Table 1) have clusters of plagioclase and pyroxene phenocrysts set within a microlitic groundmass with ®ne plagioclase needles and pyroxene [Fig. 2(b)]. The non-porphyritic, volcanic rocks have a ®ne groundmass made of prismatic plagioclase, augite with hourglass struc-ture and diopside [Fig. 2(c)]. The groundmass is altered and has euhedral hornblende, actinolite, epidote, zoisite, chlorite and calcite. Chlorite, epidote and, in places, pumpellyite and prehnite, represent the low grade prehnite±pumpellyite metamorphic facies (Arvin, 1982b), producing the charac-teristic light green color of these lavas in the ®eld.
The grains in the ®ne-grained, volcanogenic sandstones are moderately- to well-sorted, angular to subrounded quartz, plagioclase (chloritized in places), pyroxene and lithic fragments, cemented with calcite [Fig. 3(a)]. Clasts in the poorly-sorted, volcanogenic, pebbly sedimentary breccia, are angular to subangular lithic fragments [Fig. 3(b)]. These clasts are mostly porphyritic and non-porphyri-tic volcanic rocks and intrusive rocks such as diorite, and granite.
4.2. Analytical methods
Whole rock major elements and some trace elements (Nb, Zr, Ti, V, Cr, Fe, Co, Ni) of the volcanic and volcaniclastic rocks in the Hassanabad Unit were determined by a wavelength dispersive, Rigaku 3070 X-ray ¯uorescence spectrometer utilizing a side-window rhodium target X-ray tube. All analyses were made against standard calibra-tion curves which were prepared using a set of USGS refer-ence standards.
Analysis of the major elements were performed on fused glass disks. The disks were prepared using nine parts lithium borate ¯ux and one part rock powder. The mixture was fused in a crucible of 95% Pt and 5% Au at approx.
11008C for 10±15 min, to form a homogeneous melt. The
melt was then poured into a preheated platinum mold and chilled to a thick glass disk. Analysis of the trace elements were performed on powder pellets, using an equal weight of rock powder and crystalline cellulose binder, pressed at 15
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Table 1
Major and trace elements of volcanic and volcaniclastic rocks of the Hassanabad Unit, Neyriz, Iran
Volcanic rocksP X N Volcanogenic sandstone TS Volcanogenic sedimentary brecciaJ V BHVO-1
Sample 8A 14B A10 A102 Avg.a A9 5A 7 12A Avg. 10A 12B 13 14A Avg. Measurements Recommendations
SiO2 61.8 55.0 54.5 54.8 56.5 58.4 65.5 65.5 63.5 64.8 42.3 47.5 62.4 49.8 50.5 49.9 49.9 TiO2 0.48 0.51 0.74 0.62 0.59 0.62 0.52 0.50 0.32 0.45 0.66 0.97 0.64 1.24 0.88 2.69 2.71 Al2O3 14.7 17.5 16.8 16.0 16.26 17.25 9.4 10.1 4.0 7.8 9.5 14.0 15.1 14.8 13.4 13.8 13.8 FeOt 5.40 6.15 9.91 9.13 7.65 4.18 4.35 4.18 2.61 3.71 6.45 8.78 5.98 10.1 7.8 10.8 11.1 MnO 0.17 0.15 0.13 0.11 0.14 0.15 0.09 0.06 0.05 0.07 0.40 0.13 0.17 0.15 0.21 0.17 0.17 MgO 2.27 2.88 4.76 3.81 3.43 0.49 2.46 3.06 1.61 2.38 2.62 5.77 3.15 6.53 4.52 7.31 7.23 CaO 5.98 6.41 6.68 8.28 6.84 6.48 8.25 6.99 14.8 10.0 19.7 11.6 3.84 8.63 10.92 11.3 11.4 Na2O 4.73 6.15 5.86 6.00 5.69 5.26 1.59 1.92 0.34 1.28 4.03 3.99 2.93 4.05 3.75 2.29 2.26 K2O 0.07 0.17 0.13 0.14 0.13 5.58 1.24 1.23 0.52 1.00 0.65 0.43 1.80 0.40 0.82 0.54 0.52 P2O5 0.15 0.16 0.23 0.21 0.19 0.19 0.13 0.12 0.09 0.11 0.15 0.16 0.15 0.18 0.16 0.28 0.27 LOI 5.48 6.30 3.23 2.49 4.38 4.57 8.83 8.42 13.0 10.1 15.8 8.13 5.65 5.07 8.67 ±b ± Total 101 101 103 102 102 103 102 102 101 102 102 101 102 101 102 99 99 Rb 2.3 3.8 3.6 4.8 3.6 ± 61.7 66.8 23.2 50.6 10.5 11.2 28.0 9.5 14.8 9.5 9.7 Sr 206 331 282 230 262 124 214 218 194 209 215 207 387 285 274 411 420 Y 13.8 13.8 20.3 20.4 17.1 20 19.6 18.6 10.2 16.8 29.7 30.1 32.0 35.1 31.7 26.5 25.8
Zr 81 101 71 58 78 142 181 167 149 166 71 93 110 97 93 167 181
Nb 3.8 4.5 4.3 3.9 4.2 ± 6.5 11.4 7.0 8.3 2.3 3.3 2.3 4.1 3.0 18.5 19.0
Ba 58 58 77 71 66 ± 171 130 62 121 100 81 91 60 83 135 133
Hf 1.2 1.8 ± 2.2 1.7 3.6 2.0 3.4 0.8 2.1 2.2 1.0 2.3 2.6 2.0 4.5 4.4 Pb 10.2 10.3 2.3 2.5 6.3 4.5 21.2 20.1 5.8 15.7 3.2 2.7 5.8 3.2 3.7 2.2 2.1 Th 2.5 1.9 1.8 0.9 1.8 ± 8.3 8.5 3.5 6.8 1.8 0.8 1.6 0.9 1.3 1.1 1.2
U 1.3 1.3 0.3 0.2 0.8 1.2 3.1 3.3 2.6 3.0 0.5 0.2 ± ± ± 0.5 0.4
V 155 157 295 269 219 63 73 83 47 68 90 193 113 228 156 319 317
Cr 19 27 42 39 32 ± 215 202 36 151 ± 227 ± 230 229 289 291
Co 37 33 24 28 31 9 38 18 27 28 25 19 27 25 24 46 45
Ni 35 31 23 27 29 8 101 104 ± 103 ± 52 ± 68 60 118 121
La 11.2 17.5 8.0 8.5 11.29 34.08 15.8 18.1 19.9 18.0 5.7 5.9 5.8 6.4 6.0 15.5 15.8 Ce 22.7 37.3 18.2 16.4 23.64 59.53 30.1 35.9 39.8 35.2 15.9 15.5 15.5 16.5 15.8 38.0 37.8 Nd 12.6 18.2 11.8 10.9 13.4 23.47 14.3 16.1 16.1 15.5 11.0 11.6 11.6 11.7 11.5 24.5 24.8 Sm 2.67 4.18 3.08 2.66 3.15 5.08 3.02 3.39 3.45 3.29 3.26 3.26 3.26 2.92 3.18 6.26 6.10 Eu 0.82 1.35 0.94 0.87 0.99 1.45 0.51 0.66 0.58 0.58 1.21 1.10 1.10 1.07 1.12 2.18 1.98 Gd 2.54 3.97 3.14 2.87 3.13 4.71 2.02 2.75 2.25 2.34 4.24 3.63 3.93 3.76 3.89 6.78 6.40 Dy 2.41 3.52 3.50 2.98 3.1 4.45 2.02 2.19 2.40 2.21 4.93 4.39 4.65 4.42 4.60 4.78 5.20 Er 1.35 1.85 2.00 1.80 1.75 2.16 1.28 1.37 1.41 1.36 2.98 2.79 2.90 2.88 2.89 2.43 2.40 Yb 1.47 1.69 1.99 1.93 1.77 2.11 1.29 1.46 1.58 1.44 3.07 2.98 2.98 3.11 3.03 2.27 2.02 Lu 0.24 0.28 0.32 0.32 0.29 0.29 0.26 0.28 0.31 0.28 0.49 0.47 0.48 0.47 0.48 0.29 0.29
a The average for the volcanic rocks does not include sample A9.
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Table 2
CIPW norms and minerals identi®ed by petrography and/or XRD in the volcanic rocks of the Hassanabad Unit, Neyriz, Iran
8A 14B A10 A102 A9
Mineral Norm Modal & XRD Norm Modal & XRD Norm Modal Norm Modal Norm Modal
Quartz 17.74 Veins of qtz. 0.00 0.00 0.00 0.00
Orthoclase 0.43 1.06 Plag. phenocr. and in the groundmass
0.77 0.83 33.46
Albite 41.73 Plagioclase as phenocrysts and in the groundmass
54.64 49.58 Clots of plag. laths 48.97 Laths and aggregates of plag. 38.82 Zoned and resorbed laths of plagioclase
Anorthite 19.58 20.65 19.07 16.50 7.02
Nepheline 0.00 0.00 Augite altered to amphibole 0.00 Plagioclase as needles in groundmass
1.16 3.39
Diopside-wo 4.38 4.93 5.33 9.88 6.19
Diopside-en 1.90 Pyroxene 2.26 2.48 4.28 Phenocr. of augite 1.24
Diopside-fs 2.47 2.63 Euhedral hornblende 2.80 5.60 5.41 Calcite replacing plagioclase Hypersthene-en 4.02 Chlorite 4.50 1.86 Specs of pyroxene
in groundmass
0.00 Small pyroxene in groundmass 0.00
Hypersthene-fs 5.23 Opaques 5.24 Opaques 2.10 0.00 0.00
Forsterite 0.00 Epidote 0.57 Secondary calcite 5.31 3.73 0.00 Calcite vug and veinlets
Fayalite 0.00 Secondary calcite 0.73 6.62 5.39 Chlorite 0.00
Magnetite 1.23 1.42 Chlorite 2.18 2.02 Calcite veinlets 0.93
Ilmenite 0.95 XRD shows albite & diopside 1.02 XRD shows albite & augite 1.41 Epidote 1.19 1.19 Pyroxene
Apatite 0.34 0.37 0.50 0.46 0.42
tons/square inch in a Spec cap for 4±6 min. In general, the lower limit of detection (lld; for XRF analyses) for the detected trace elements (Nb, Zr, Ti, V, Cr, Fe, Co, Ni) is 4 ppm. The lld is based on peak counts exceeding
back-ground counts outside an error range of^7 S.D. for
aver-aged background counts, for standards containing 4 greater ppm of the element. However, Y, Nb and Zr were also
analyzed using the higher precision inductively coupled plasma mass spectrometer (ICPMS; see below).
The rare earth elements (REE) and other trace elements such as Rb, Sr, Ba, Hf, Ta, Pb, Th, U, Y, Nb and Zr were determined by an ICPMS using a standard Te¯on vial
acid-digestion procedure involving a mixture of HF-HClO4
-HNO3. All samples were spiked to 50 ng/ml using indium
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as an internal standard. REE analyses for this study were calibrated against a set of working multi-element solutions. The working standards were produced by mixing single element SPEX standard solutions (SPEX Industries, USA). All the solutions were introduced via a peristaltic pump and analyses were performed using a SOLA ICPMS at Georgia State University. The average precision for the major and trace elements during this study is given in Table 1 under the BHVO-1 column. At the 0.5±2 ppm abundance level in the rocks, the relative precision for most elements by ICPMS is 4.5 % or better (range is 1.0±6.0%). An indi-cation of accuracy is given by measured and recommended values for BHVO-1 standard (see columns measurements and recommendations in Table 1). The recommended values
for major oxides are from Gladney and Roelandts (1988) while values for trace elements are principally from Govindaraju (1989). Minerals were identi®ed by a petro-graphic microscope and a Phillips Model 12045 X-ray diffractometer.
4.3. Major elements
Analyzed samples were selected in the ®eld to avoid as much as possible those that appear highly deformed or altered. Major-element analyses of the volcanic rocks clearly reveal the extent of secondary geochemical altera-tion that occurred during the cataclastic deformaaltera-tion in some samples. In the volcanic samples that have had only minor carbonate alteration, the major element chemistry is recognizably basaltic/andesitic in character, albeit altered to a minor degree.
Table 1 gives the major and trace element distribution of the representative, less-altered volcanic±volcaniclastic rocks of the Hassanabad Unit and depicts the symbols used in the geochemical diagrams in the trace element analysis. The LOI values re¯ect secondary alteration of the rocks which is indicated by the presence of veining and vug-®lling by calcite. The Hassanabad Unit's volcanic rocks plot in the island arc calc-alkaline ®eld of the TiO2±
MnO±P2O5diagram [Mullen, 1983; Fig. 4(a)]. Except for
the more alkaline sample A9, the volcanic rocks show
subalkaline compositions on the alkalis vs. SiO2plot [Fig.
4(b)].
4.4. Trace element distributions
Plots of Zr against selected HFS elements such as Ti, Ce, Hf (Pearce and Cann, 1973; Pearce, 1975; Gamble et al., 1993) can potentially discriminate between eruptive envir-onments such as low-K tholeiite (LKT), calc-alkaline basalt (CAB) and ocean ¯oor basalt (OFB). In a Zr±Ti diagram, the volcanic rocks of the Hassanabad Unit plot in the CAB ®eld [Fig. 5(a)]. The volcanogenic sandstones have higher Zr values than volcanic rocks and the breccia.
The ternary diagrams Nb±Zr±Y (Meschede, 1986), Ti± Zr±Y and Ti±Zr±Sr are commonly applied to distinguish OFB, LKT and CAB (Pearce and Cann, 1973; Meschede, 1986). The Nb*2±Zr/4±Y plot [Fig. 5(b)] shows that, except for the Zr-rich sandstones, the volcanic±volcaniclas-tic rocks of the Hassanabad Unit can be attributed to the VAB composition (®elds C and D). Although the concen-tration of Nb in sample A9 was below the detection limits (Table 1), its Y and Zr values are comparable to the other volcanic rocks [Fig. 5(c)]. Almost all volcanic±volcaniclas-tic rocks plot in the CAB ®eld (®elds B and C) of the ternary plot of Ti/100±Zr±Y*3 [sandstones have higher Zr values; Fig. 5(c)]. The ternary diagram, Ti/100±Zr±Sr/2, corrobo-rates the CAB compositions for the volcanic±volcaniclastic rocks [Fig. 5(d)]. The Th±Hf±Nb plot (Fig. 6) relates all the volcanic±volcaniclastic rocks in the Hassanabad Unit to a destructive plate margin and its ®eld of differentiates.
H.A. Babaie et al. / Journal of Asian Earth Sciences 19 (2001) 61±76 69
Normalized multi-element plots, with elements arranged according to increasing incompatibility, have been used to analyze the source of the magma and the origin of arc extrusive rocks in supra-subduction zone ophiolites (Pearce, 1982, 1996). Assuming that elements such as Nb, Zr, Ti, Y, Hf, Yb and HREE are dominantly mantle wedge-derived (i.e. not contributed by the slab), the relative contributions of the mantle and subducting slab in subduction zone basalts can be estimated from the N-MORB-normalized distribu-tion of a series of incompatible elements (Pearce, 1982, 1983; Pearce and Parkinson, 1993).
The N-MORB-normalized multi-element distribution patterns of the volcanic±volcaniclastic rocks of the Hassa-nabad Unit (Fig. 7) indicate depletion of most of the HFS elements with respect to N-MORB, and depletion of Nb relative to Th and Ce. However, as is typical of the
CA-VAB, the absolute concentration of Nb is a little higher than that in the N-MORB. The concentrations of the LIL elements in these volcanic rocks are all greater than those in the N-MORB (Fig. 7).
The CA-VAB-normalized multi-element patterns of the mean volcanic and volcaniclastic rocks [Fig. 8(a)] show that most of the trace element concentrations (except for the LIL) reasonably ®t the CA-VAB pattern. The primitive mantle-normalized patterns of these rocks [Fig. 8(b)] show the relative enrichment of the LIL relative to the HFS elements by a gradual decrease of the concentrations to the right, and the more-or-less uniform behavior of the Eu, Gd, Dy, Y, Er, Yb and Lu.
The fertile MORB mantle (FMM)-normalized patterns of elements, with varying degrees of incompatibility, in arc lavas, give a measure of the relative importance of degree
H.A. Babaie et al. / Journal of Asian Earth Sciences 19 (2001) 61±76
70
of partial melting and source depletion/enrichment events in the mantle (Pearce and Parkinson, 1993). Such patterns show that arcs that do not have proximal backarc basins exhibit N-MORB-like enriched sources. The FMM repre-sents the upper mantle reservoir from which the N-MORB is derived (Pearce and Parkinson, 1993). Under low to moder-ate degrees of partial melting, an undepleted MORB mantle
source produces enrichment in the order VHI.HI.MI,
where VHI are very highly incompatible elements, and HI and MI are the highly incompatible and moderately incom-patible elements, respectively. The patterns becomes ¯at at high degrees of melting (Pearce and Parkinson, 1993; Pearce and Peate, 1995). The FMM-normalized patterns of the volcanic and volcaniclastic rocks of the Hassanabad
Unit [Fig. 8(c)] exhibit the typical VHI.HI.MI trend
representative of the low to moderate degrees of partial melting of an undepleted mantle source.
The ratios of some elements in ophiolitic lavas can be used as discriminants to characterize the composition of the parental magma and magma source and account for the effects of fractionation that occur during partial melting or fractional crystallization (Saunders et al., 1980; Pearce, 1996). The ratios of immobile elements (e.g. HFS, REE) in basaltic systems whose relative abundances remain essen-tially constant (e.g. during hydrothermal alteration or meta-morphism of basalt) to the elements with low bulk distribution coef®cients (e.g. Ti, Zr, P, Nb, Y, La, Ba, REE), are particularly useful as discriminants in determing the tectonic setting of magma eruption (Cann, 1970; Pearce and Cann, 1973; Pearce, 1975; Saunders et al., 1980). A series of selected elemental ratios of the volcanic rocks of the Hassanabad Unit show a good correlation with the ratios of the CA-VAB (Fig. 9), further suggesting their
calc-alka-line af®nity. Such a correlation is not obvious when the elemental ratios are normalized to those in the N-MORB.
Based on the REE distributions, arc lavas are commonly divided into two groups: one with low Ce/Yb ratio in which Ce and Yb vary together, and a second group with higher Ce but similar Yb abundances which may re¯ect either the REE distribution in the source rocks and/or the bulk distribution coef®cients and the degree of melting (Hawkesworth et al., 1991, 1993). As shown in Figs. 10(a) and 10(b), N-MORB-and chondrite-normalized REE distributions of the volcanic and volcaniclastic rocks of the Hassanabad Unit show that the LREE are more enriched than the HREE. The HREE in these rocks have similar abundances as in N-MORB or are a little depleted relative to N-MORB [Fig. 10(a)]. These REE patterns correspond to the second type of arc lava with high CE/Yb ratios.
5. Summary and conclusions
Regional metamorphism of rocks, at all facies below partial melting, generally does not alter the major-element chemistry of the original rocks except for SiO2, CO2, H2O,
and carbonates (Beach, 1974; Hyndman, 1985, p. 467). Even in massive shear zones where original igneous rocks have been converted to extremely ®ne-grained mylonites, chemical changes can be generally minor (e.g. La Tour, 1979). To alter appreciably the bulk chemistry of rocks undergoing deformation, ongoing production or introduc-tion of ¯uid must overwhelm the deformaintroduc-tion process itself. This is a so-called ¯uid-dominated regime. Otherwise, the regime is rock-dominated in which the rock buffers the ¯uid composition, keeping the rock from changing its composi-tion appreciably (Fyfe et al., 1978).
H.A. Babaie et al. / Journal of Asian Earth Sciences 19 (2001) 61±76 71
Fig. 6. Th±Nb±Hf plot showing subduction-related compositions for the volcanic and volcaniclastic rocks in the Hassanabad Unit. See Table 1 for symbols. Fields: AN-MORB; Benriched MORB and within plate basalt (WPB) tholeiite; Calkaline within plate basalt (WPB) and differ-entiates; Ddestructive plate-margin basalt and differentiates.
The rare earth elements (except La), and other trace elements such as Y, Ti, Zr, Hf, Nb, Cr and Th are believed to be the least sensitive to secondary processes that occur within subduction zones (Muecke et al., 1979; Saunders et al., 1980; Campbell et al., 1984; MacLean and Kranidiotis, 1987; Pearce, 1996; Kerrich and Wyman, 1996). Of these, Th, Nb, Zr and Ti are believed to be less mobile than the LREE and behave like the HREE during sea¯oor hydrother-mal alteration under low to moderate water/rock ratios and low grade metamorphism (Pearce, 1983; Humphris, 1984; Kerrich and Wyman, 1996).
Despite the zeolite and greenschist facies metamorphic alterations, the major and trace element geochemistry of the
volcanic±volcaniclastic rocks of the Hassanabad Unit give consistent geochemical evidence for subduction-related magmatism and sedimentation, suggesting that major redis-tributions of trace elements have not occurred. The subduc-tion-related section of the Hassanabad Unit includes calc-alkaline volcanic and volcanogenic sandstone and breccia with clasts of arc volcanic rocks and diorite. The trace element distributions of the volcaniclastic rocks are similar to those of the volcanic rocks indicating: (1) that the igneous clasts in the volcaniclastic rocks have the same chemical composition as the associated volcanic rocks; (2) that sedi-mentation and subsequent alterations during diagenesis and low temperature metamorphism did not redistribute the
H.A. Babaie et al. / Journal of Asian Earth Sciences 19 (2001) 61±76
72
trace elements in the clasts or matrix of the volcanogenic sandstone and breccia.
The presence of arc-related volcanogenic sandstone and sedimentary breccia, intercalated with pelagic sediments and arc volcanic rocks in the Hassanabad Unit, indicates that the arc volcanic rocks were erupting into a deep basin where radiolarian chert was being deposited and into which coarse-grained volcaniclastic debris was probably trans-ported by turbidity currents from nearby arc volcanoes.
Because volcaniclastic sediments are absent in the adja-cent, structurally underlying crustal and mantle sequences (the ophiolite component) of the Neyriz ophiolite complex, we infer the presence of a source of the volcanic debris (i.e. a volcanic arc) to the northeast of the Neo-Tethys trough and its Late Cretaceous subducting crust, implying a north-east-dipping subduction zone. The subduction polarity and the presence of the volcanic arc have important implications for the tectonic interactions in this part of the Neo-Tethys basin during the Late Cretaceous, and for the emplacement of the Neyriz ophiolite complex. Although this is the subject of another paper, we speculate that the arc-related andesitic-basaltic rocks were erupting in the northeastern margin of a narrowing ocean basin where turbidity currents transported the volcaniclastic debris from an adjacent arc which lied to the northeast, and deposited them intercalated with radio-larian chert and pelagic sediment.
The tectonic intercalation of the Hassanabad Unit with the Cretaceous limestone and their thrust contact with the crustal and mantle sequences of the Neyriz ophiolite complex along northeast-dipping thrust faults, suggest that the basin in which the Hassanabad rocks were deposited lied above (i.e. in a forearc basin) a northeast-dipping subduc-tion zone. The underlying ophiolite component probably represents offscraped or underplated slivers of the Neo-Tethyan subducting oceanic lithosphere. The tectonic
juxta-position of the Hassanabad Unit and the ophiolite compo-nent may have occurred at the time of subduction in the Late Cretaceous, or later.
Further narrowing of the Neo-Tethyan oceanic basin emplaced the disrupted slivers of the oceanic crust, possibly with their overlying arc volcanic±volcaniclastic unit, onto abyssal and continental slope, and later, the shelf facies of the passive continental margin of the Afro±Arabian plate. The ®nal contraction of the area during the Miocene led to the thrusting of parts of the arc and emplacement of the Hassanabad Unit and its carbonate platform above the ophioite complex. The Crush Zone, which includes cata-clastically-deformed rocks of both the southwestern edge
H.A. Babaie et al. / Journal of Asian Earth Sciences 19 (2001) 61±76 73
Fig. 10. Mean rare earth element (REE) distributions in the volcanic± volcaniclastic rocks of the Hassanabad Unit: (a) normalized to the N-MORB (1); (b) normalized to the chondrite. Normalizing values are from Sun and McDonough (1989). See Table 1 for the symbols. Fig. 9. Distributions of the elemental ratios (CeYb means the Ce/Yb ratio)
of the Sanandaj±Sirjan block and the northeastern part of the ophiolite complex, formed as a result of the Miocene contractional tectonics. The Miocene contractional defor-mation led to renewed thrusting and slicing in the ophiolite complex and the cataclastic intercalation of the volcaniclas-tic±volcanic rocks with the Cretaceous limestone at the southeastern edge of the Sanandaj±Sirjan block.
Acknowledgements
We are grateful for funds provided to Hassan Babaie by Georgia State University and to Abed Babaei by Cleveland State University and the United Nations' T.O.K.T.E.N program for their ®eld work. Acknowledgement is made to Mohammad Reza Sayyad and A. Afrasiabian, both at the Ministry of Energy, Water Resources Research Organi-zation, Shiraz, Iran, for providing logistical support for Hassan Babaie's ®eld work. Abed Babaie thanks Mohsen Arvin and M. Hashemi-Tangestani for logistical support in the ®eld. The ICPMS facility at GSU was partially funded by the NSF grant EAR 94-05716 awarded to A.M. Ghazi and D.A. Vanko. We thank Drs. Simon Haynes and U. Knittel for their valuable, critical and constructive comments and reviews that led to signi®cant improvement of this manuscript.
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