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Relationships between the mantle, lower crust and upper crust within the Agardagh-Tes Chem ophiolite, Central Asia: Evidence from petrologic, trace element, and isotopic data

Abstract:

Ophiolites are generally regarded as fragments of oceanic crust, ideally comprising a mantle section and overlying lower and upper crustal units. A common assumption is the cogenetic relationship between the mantle and crustal units. However, a recent study of the Internal Ligurian ophiolite in Northern Italy (Rampone et al., 1998) has shown that there are genetic differences between the mantle and crustal units, expressed in different initial isotopic compositions. In this paper, we show for the tectonically dismembered late Proterozoic (569 Ma) Agardagh-Tes Chem ophiolite (ATC ophiolite, location see Fig. 1), that neither the mantle section nor different crustal sections underwent a simple cogenetic magmatic evolution. The mantle section comprises moderately to strongly serpentinized ultramafic rocks (dunites, harzburgites, lherzolites and pyroxenites) and is regarded as a fragment of depleted oceanic upper mantle. The depleted character is expressed by low abundances of modal cpx, high Mg/Si, low Al/Si and high ε(Nd)(t) (around +9). The whole mantle section is embedded within a tectonic melange which consists of sheared pelagic sediments (schists, cherts and minor carbonates) and low-grade metabasalt lenses. Mafic plutonics, sheeted dikes and pillow lavas crop our farther to the northeast and are separated from the mantle section by fault zones. The sheeted dikes and basalts of the upper crustal section are Ol- or Qz-normative basalts and minor basaltic andesites with an island arc affinity. They are characterized by high concentrations of incompatible trace elements (LREE up to 100 times chondritic), enriched REE patterns ((La/Yb)(N) between 4.8 and 9.8, Fig. 2) and a wide range in ε(Nd)(t) from +1.9 to +6.2. In contrast, the plutonic rocks of the lower crustal section (primarily massive Ol- and Hbl-gabbros) have generally low concentrations of incompatible trace elements, slightly depleted REE patterns ((La/Yb)(N) between 0.5 and 2.5, Fig. 2) and generally higher ε(Nd)(t) from +5.8 to +7.1. The distinct isotopic compositions and the fact that equilibrium melts calculated for the gabbros using bulk rock-liquid distribution coefficients do not match the observed basalt compositions (Fig. 2), lead us to conclude that the plutonic and volcanic section of the ATC ophiolite were not derived from the same parental melt. This is confirmed by cpx electron microprobe analyses from Ol- and Hbl-gabbros, which were also used to calculate the composition of the corresponding equilibrium melts in terms of Mg-number vs. Ti and Cr concentration, respectively. These melts have lower Mg-numbers and lower Ti concentrations than the volcanic rocks, underlining the missing genetic link between the lower and upper crustal units of the ATC ophiolite. Inferences on the nature of the mantle source of the volcanic rocks are given by Nd- and Pb-isotopic compositions of the basalts. Highly variable initial ε(Nd), variable 207Pb/204Pb ratios, and a Th/U of 3.0 ± 0.4 (from 208Pb/204Pb vs. 206Pb/204Pb) indicate a heterogeneous mantle source and the involvement of an older crustal component during magma genesis. The most likely mechanism to explain the isotopic signatures is a modification of the source by low amounts of subducted sediments (<1%) prior to melting. However, this model requires an efficient melt extraction mechanism which preserves the primary mantle heterogeneities from the melting region up to the surface. Assimilation of crustal rocks by ascending magmas is ruled out due to high Nd concentrations in the basalts, which would require huge amounts of crustal assimilation (up to 40%). To summarize, we propose that the depleted mantle section of the ATC ophiolite represents a fragment of oceanic lithosphere obducted either from the oceanward or from the back-arc side of a subduction zone. In contrast, the sampled volcanic rocks are produced from a subduction modified, refertilized mantle source beneath the island arc itself. A genetic link between the lower and upper crustal unit is also questionable.