The Paleozoic Tectonic Evolution History Of Back-arc Basin In South Chinese Tianshan:Insights From Structural And Chronological Studies Of The Wuwamen Ophiolitic M(?)lange

A series of ophiolitic melange are discontinuously distributed in Chinese South Tianshan from west to east.The remnants of the ophiolite suite and associated siliceous rocks have been subjected to high grade metamorphic and intense structural deformation.It is critical and necessary to study the ophiolitic melange belt,in order to reconstruction evolution history of the ocean and Tianshan orogen.The whole South Tianshan is dominated by deep marine deposits.In the northern part of the deep marine sequences,the beddings were intensively folded showing South-dipping axes and were partially replaced by S-dipping schistosity,and a North-vergent shearing is therefore suggested;obsecued stretching lineation or crenulation can be recognized on the surface of greenschist-facies meta-basalts and/or meta-shales.In addition,an earlier phase of ductile deformation can be recognized in the ophiolitic melange and accretionary zones.And these kinematic criteria suggest a top-to-the-North shearing along the major foliation,possibly related to the south subduction of the south tianshan ocean.Besides,the stratum in accretionary zones showing E-W-striking steep foliation and shallow stretching lineation,with kinematic indicators,including the highly deformed calcareous conglomerate,a-type gravel and quartz clasts,demonstrating a dextral sense of shear.These dextral structures are in agreement with the fabrics produced by the Permian ductile shearing along the Baluntai strike-slip fault.The basic-ultrabasic rocks in Wuwamen ophiolitic melange are typically characterized by obvious depletions in Nb and Ta,but slightly enriched in LILE(Rb,Ba,Th).One N-MORB-type basalt sample(12TS51-2)also has highly positive ?Nd(t)value(+11.6),indicating that the basalt was produced from a depleted mantle source.Such MORB-derived basalts with arc features were documented to form in supra-subduction zone where mantle wedge modified by fluid released from subducting slab and associated ocean floor sediments were involved in the formation of oceanic crust of fore-arc or back-arc basin.So the meta-basalts are constituents of a SSZ-type ophiolitic melange.The andesitic rocks from the Wuwamen ophiolitic melange display steeply rightward decreasing REE patterns with arc-like signatures(depletion inTa,Nb and enrichment of Th,Rb,Ba).And,near-zero(+0.6)and negative(-11.3)?Nd(t)values and Proterozoic single-stage Nd model ages(?940 Ma and?2160 Ma)of the andesitic rocks suggest involvement of continental crust.Geochemical and isotope characteristic imply that magma source(s)of the andesitic rocks were most likely influenced by subduction-related materials.A continental rifting setting above a subduction zone can be suggested for the formation of these andesitic rocks.Their LA-ICP-MS U-Pb zircon U-Pb ages are concentrated in two stages:?324Ma to?363Ma and?401 Ma to?480Ma,corresponds to multiple magmatic events in the provenance.Considering the regional geological background,continental arc magmatism(ca.475-420Ma)and underformed late Paleozoic granitic plutons(?370Ma to?300Ma)were intruded into the Central Tianshan.We deduced that the deposit sediment of the accretionary wedge came from the nearby continental arc in the Chinese Central Tianshan.And the youngest zircon(Th/U=0.53)gave a U-Pb age of 324 ± 2Ma,which can be considered the maximum depositional age,with suitable caution.The ophiolitic gabbros(?332 Ma)in the Guluogou melange was the youngest age date of South Tianshan ophiolitic melange(Jiang et al.,2013).In this study,an OIB-like andesite with arc signature occurring as block included in the sheared basaltic matrix of the Wuwamen ophiolitic melange is dated at 309 ± 4 Ma,indicating that ophiolitic melange was tectonically emplaced later than?310 Ma.In addition,one samples of undeformed granitic dike crosscutting both the blocks and matrix of the Wuwamen ophiotic melange yielded a concordant mean age at 300 ± 4 Ma,which is interpreted as the timing of granitic injection.Thus,the emplacement of the ophiolitic melange should be earlier than?300 Ma.Meanwhile,syn-/post-collision granite widely intrude in Chinese South Tianshan.