Tectonic Attributes And Significance Of The Wuwamen Ophiolite Melange In The Southern Tianshan Mountains,Xinjiang

The ophiolite is the residual product of the plate convergence collision of the ancient oceanic crust and upper mantle.The ophiolite can record the material exchange,metasomatism and melting of the lower mantle of the ocean basin,the mutual transformation of the crust and mantle,the formation of the oceanic crust and the opening,expansion and closure of the ocean basin.Therefore,the related research of the ophiolite is of great significance to understand the evolution of the oceanic lithosphere,deduce the ancient continental collision splicing and the Wilson cycle.The Central Asian orogenic belt is the largest Phanerozoic accretionary orogenic belt in the world,and the continental crust grows most significantly.The Tianshan orogenic belt is located in the southwestern margin of the Central Asian orogenic belt and is an important part of the Central Asian orogenic belt.Its formation and evolution are closely related to the tectonic evolution of the southern and northern Tianshan Ocean separated by the Yili-Central Tianshan microcontinent.The South Tianshan Ocean is widely regarded as a vast ocean basin,which separated the Karakum-Tarim block in the south of Central Asia from the Central Tianshan block in the north.However,scholars still know little about the tectonic evolution of the Southern Tianshan Ocean,especially when to start.This paper reports the Neoproterozoic oceanic-continental transitional lithosphere remnants discovered from the Wuwamen ophiolite melange in the southwestern Tianshan orogenic belt,Xinjiang,which may record the rifting process related to the opening of the South Tianshan Ocean.In the Wuwamen mantle peridotite,the Si O2 content of peridotite is38.48-44.45 wt.%,belonging to typical ultrabasic rocks.Mg O content is32.93-46.89 wt.%.Among them,the diopside olivine has very low spinel Cr #,high monoclinic pyroxene Na2 O content,and ancient(~ 1.7 Ga)Os model age,indicating that it originates from the Paleocontinental lithospheric mantle(SCLM).The pure peridotite shows more enrichment of Cr # and higher 187 Os / 188 Os ratio in spinel,reflecting the reaction between the protolith of di gabbro and MORB-like osmotic melt.We also found a set of mafic rocks from the Wuwamen melange.The Si O2 content of the basic rock sample was 50.3 – 52.3 wt.%,the Ti O2 content was 1.08 – 1.45 wt.%,the Mg O content was 6.69 – 8.06 wt.%,the Fe2O3 T content was 8.7 – 10.02 wt.%,and the Al203 content was 14.45 – 17.66 wt.%.There was no obvious Eu anomaly,and the δEu was 0.79 – 1.03.(La /Sm)N ratio is1.2-1.8,(Sm / Yb)N ratio is1.8-2.4.The obtained zircon U-Pb age is 730 Ma,and ε Nd(t)is-3.6~-1.5,indicating that they are derived from enriched mantle source areas(for example,metasomatism SCLM).The rock association,together with recently reported N-MORB-type mafic rocks,quite resembles the sequence observed at ocean-continent transitions of magma-poor rifted margins(e.g.,the West Iberia margin).We suggest the documented Wuwamen mafic-ultramafic complex may derive from the distal domain of a Neoproterozoic rifted margin developed along southern Central Tianshan,probably associated with the breakup of the Rodinia and opening of the STO.