Petrogenesis And Geotectonic Setting Of Baleigong Mafic-ultramafic Rocks In Southwest Tianshan Orogen

South Tianshan is a prominent intracontinental collisional orogenic belt and the key to understand the central Asia tectonic evolution. It lies between the Kyzylkum and the Kazakhstan cratons on the north and the Karakum and the Tarim cratons on the south.The main focus of this paper is mafic-ultrmafic rocks, located in the southwest part of Kokshal segment, western segment of the Chinese South Tianshan. The main purpose of this study is to present petrological, geochemical and zircon U-Pb chronology data for mafic-ultrmafic rocks in order to known the Petrogenesis, tectonic setting ,and to take an interpretion for the history of the South Tianshan.Baleigong serpentinized peridotite is characterized by lower Ti, Al content, Al₂O₃/CaO and high Cr (1604.05-3863.54), Ni (1719.73²375.23ppm), Mg# (0.90-0.92) in equilibrium with the residue of the highly partial melting mantle. The right slopping spider diagram and the U-shape chondrite-normorlized Ree pattern indicate that the parental magmas represents a mixing magmas of the ascending mantle and the highly partial melting mantle, namely, the pristine rock in composition are predominantly harzburgites. They can be modeled as restites after 20-30% partial melting from already depleted mantle in a supra-subduction zone geotectonic setting.The Baleigong basic volcanic rocks are tholeiitic series, have high Ti, P, Fe and lower Mg#(0.39⁰.53), Cr, Co, Ni, and characterized by Ocean Island Basalt (OIB)-type trace element patterns with significant enrichment in LILE, HFSE and L-MREE, and a slight depletion in HREE, relative to N-MORB. The element ratios in basalts indicate a typical plume tail component, appears to represent a mixture of EM land EM2. High field strength element ratios also indicate an important contribution of melting metasomatised subducted oceanic lithosphere to the Early Paleozoic mantle.Coupled with a geological fieldwork, the overlying sequence units of the SSZ ophiolite may have been disappeared as the subduction relative to diapir ascending of the underlying peridotite. Otherwise, the OIB volcanic is well-preserved in the ophiolitic melange as a result of partial obduction during the oceanic basin close, which implies the basin close event may be occurred in the study area.The zircon U-Pb dating of the diabase in the Baleigong mafic-ultrmafic rocks yielded the age of 449.6 ±1.7 Ma, being a birthday of the OIB volcanic. Associated with the fresh chronology of the south Tianshan orogenic belts, the wide oceanic basin had been yielded in the Late Ordovician and early Silurian. Which signifies the ophiolite or ophiolitic melange all generated in early Paleozoic.The geological field indicates the Tarim and Yili blocks separated from their parent continent in the Cambrian, the Baleigong OIB was a product of the hotspot or plume tail when the South Tianshan Ocean evolved to a mature wide one with more Oceanic seamounts from the Mid-Late Ordovician to Silurian. During the late Silurian-early Devonian, The southern Tianshan oceanic crusts subducted northward beneath the southern margin of the Yili block relative the spreading of South margin of South Tianshan Ocean. By the Late Devonian, the North margin of South ocean basin is close accompanied by the collision between Halik Oceanic seamounts and Yili block, whereas the South ocean basin eventually close events occurred in the Mid-Late Permian as the Tarim and Yili block jointed together.