Petrogeochemical Features And Tectonic Significances Of Baiyanghe Dolerite Dykes In The Northern West Junggar

The West Junggar is located in the southern part of the the Central Asian Orogenic Belt?CAOB?and it is the key area to study the Paleozoic tectonic evolution of the CAOB.Since the Cambrian period,it has experienced long-term and complex plate tectonic process,such as the Paleo-Asian Ocean cracked,subduction,closure and collision,with a large number of magmatic activities.In particular,during the Late Carboniferous to the Early Permian,extensive magmatism has occurred in the West Junggar.Previous studies proposed different type models to account for the Late Carboniferous–Early Permian high temperature magmatism.One view holds that Phanerozoicalkaline granitoids of the West Junggar were generated in a post-collisional setting.Other model,however,suggests that granitoids could be related to mantle plume.Recently,many researches strongly support a ridge subduction regime in the West Junggar during the Late Carboniferous to the Early Permian.This paper takes the Baiyanghe dolerite basic dykes as the research object,combining the new results of regional geological survey closely,guided by the theory of plate tectonics,using comprehensive research methods such as petrology,geochemistry,isotopic geochronology and tectonics,etc.The fresh samples of Baiyanghe dolerite are selected for the petrography,geochemistry and isotope geochemistry analysis.Then,The geochemical characteristics,petrogenesis of Baiyanhe dolerite and tectonic evolution of the western Junggar are discussed.Finally,the following main understanding has been achieved:?1?The U-Pb dating of zircons from the dolerites yielded ages of 272.2±4 Ma and 276.7±6.2 Ma,which indicates the emplacement times;the zircons with Ordovician to Silurian ages?410-462 Ma?may be xenocrysts that were captured from the wall rocks through which the dolerites passed.For the older zircons with Proterozoic to Archean ages,there were possibly gravels or detrital zircons from a neighbouring provenance;?2?The dolerites are characterized by minor variations in SiO₂?46.89 to 49.07 wt.%?,Al₂O₃?13.60 to 13.92 wt.%?and total Fe₂O₃?11.14 to 11.70 wt.%?,and low contents of MgO?2.67to 3.64 wt.%?and total alkalis?Na₂O+K₂O,5.1 to 5.97 wt.%,K₂O/Na₂O=0.37-0.94?,which exhibit the affinities of metaluminous Shoshonites;Most of these samples are enriched in LREE?LREE=208.81-232.42 ppm,LREE/HREE=6.78-7.28,?La/Sm?_N=2.25-2.34,?La/Yb?_N=7.42-8.36?,and display right-inclined REE distribution patterns.Baiyanghe dolerite show no significant Eu anomalies?1.12¹.20?,hign contents of Sr?436 ppm to 787 ppm?and low contents of Cr,Ni indicate that there wree no fractional crystallization of feldspar and the fractional crystallization of olivine and clinopyroxene during magmatic processes.The Baiyanghe dolerite exhibit clear enrichments of large ion lithophile elements?LILEs;e.g.,Rb,Ba and K?and depletions in Nb,Ta,Zr and Hf,showing the affinities of IAB.The Baiyanghe dolerite have Nb/Ta ratios of 15.5¹6.5 and Zr/Hf ratios of 40.3⁴2.4,furthermore,all samples exhibit the“fluid involvement”in the diagrams of Ba/Th vs?La/Sm?_N and Ba/Nb vs Ba/La.Baiyanghe dolerite plotted in the OIB field and and showed the enriched trend in the diagram of Nb/Yb–TiO₂/Yb and plotted in the melting evolution curv of garnet lherzolite and showed the enriched trend in the diagram of La/Sm-Sm/Yb.The positive values of?Nd?t??+5.74 to+7.25?,high initial ⁸⁷Sr/⁸⁶Sr ratios?0.70442 to 0.70682?indicate that the source of Baiyanghe dolerite is lithospheric mantle.Thus,the Baiyanghe dolerite are the products of partial melting of the lithospheric mantle that previously metasomatismed by subducted fluid and The melting should be about 10%;?3?Compared with OIB,The Chondrite-normalized rare earth element patterns,?La/Sm?_N ratios?2.25 to 2.34?and(⁸⁷Sr/⁸⁶Sr)_i ratios?0.70442 to 0.70682?all are consistent with OIB.Furthremore,all samples plotted in OIB filed in the discrimination diagrams of V-Ti/1000,50*Zr-Ti/50-Sm and(⁸⁷Sr/⁸⁶Sr)_i vs.?Nd?t?.Thus,the Baiyanghe dolerite are OIB-type rocks;The Baiyanghe dolerite plotted in the“WPB”field in the tectonic discrimination diagrams of,thus,the West Junggar was in the post-collision stage during the Late Carboniferous to the Early Permian period.