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Geochemistry of ultramafic and mafic rocks from the northern Central Asian Orogenic Belt (Tuva, Central Asia) - constraints on lower and middle arc crust formation linked to late Proterozoic intra-oceanic subduction

Abstract:

The Agardagh Tes-Chem complex (ATCC) in Tuva, Central Asia (50.5°N, 95°E) exposes a rare mafic to ultramafic crust-mantle fragment that developed within a late Neoproterozoic (~570 Ma) intra-oceanic island arc system that was accreted to the Tuva-Mongolian microcontinent during the formation of the Central Asian Orogenic Belt. Residual mantle rocks (harzburgites and dunites) are highly refractory with high Cr# (0.59–0.83) and intermediate Mg# (0.46–0.52) in spinel and experienced high degrees of total melt extraction (up to 25%). In ultramafic cumulate rocks (wehrlites and pyroxenites), Cr# and Mg# in spinel are distinctly lower (0.22–0.45 and 0.34–0.37), and rare earth element (REE) abundances of clinopyroxenes range between 0.1 and 1-times chondritic with chondrite normalized La/Yb of ~0.04. Trace element abundances in wehrlite clinopyroxene are lower than in pyroxenite clinopyroxene, whereas all have positive Sr but pronounced negative Zr anomalies. Calculated equilibrium melts of wehrlites and clinopyroxenites suggest strongly depleted hydrous high-MgO high-SiO<inf>2</inf> primary magmas (picrites to boninites) from which these rocks have evolved. Incompatible trace element concentrations in mafic (i.e. gabbroic to dioritic) rocks vary strongly with REE contents between ~0.7 and 20-times chondritic, and chondrite normalized La/Yb between 0.25 and 0.98. Initial ε<inf>Nd</inf> varies between +4.8 and +7.1. These features, along with the broad textural and modal variations indicate that the mafic rocks developed from a single intrusive body that underwent substantial crustal assimilation. Cr and Ni contents in the mafic rocks are low and indicate their formation from already differentiated magmas. The evolution of the ATCC prior to amalgamation is explained by a two stage magmatic process. During the initial stages of melting low degree magmas evolved from a refractory mantle wedge fluxed by hydrous fluids and sediments or sediment derived melts to form enriched upper crustal shallow level intrusive and extrusive rocks. Later stage melting from a less hydrous and more depleted but SiO<inf>2</inf>-enriched source produced the parental picrites/boninites from which the lower and middle arc crustal rocks formed. These magmas differentiated within at least two distinct magma reservoirs at lowermost crustal levels, where cumulate wehrlites formed mostly independently from cumulate pyroxenites. This is evident from distinctly different trace-element compositions of these two rock types. The wehrlite derivative magmas subsequently intruded to mid-crustal levels where they crystallized the mafic rocks within a single intrusion, but accompanied by assimilation of preexisting enriched arc crust.