Records Of Early Paleozoic Ocean Activities In The North Qaidam

The North Qaidam tectonic belt,extending from southest to northwest and bounded by Qaidam Block in the south and Qilian Block in the north,is a newly-established UP-UHP metamorphic belt in western China.A set of early Paleozoic metamorphic volcanic-sedimentary rocks called Tanjian Group is closely associated with the HP-UHP metamorphic rocks spatially and temporally in this belt.Based on detailed field surveys,we discovered a suite of ultramafic to intermeidiate-basic magmatic rocks,which is related to the formation and evolution of the intra-oceanic arc.On the basis of systematic mineralogy,petrology,geochemistry and zircon U-Pb chronology studies on various rocks in this ophiolite,we discussed the petrogenesis,formation ages and tectonic significance of this ophiolite.The results provide important constraints on the formation and evolution history of the Paleozoic ocean in the North Qaidam.The main achievements of this thesis can be sum up in the following aspects:1.Based on detailed field geological survey and petrolography study,we confirm that the ultramafic and intermeidiate-basic magmatic rocks in Kaipinggou are mainly composed of peridotites,gabbros,basalts,boninites,esporphyrites and plagiogranites,which constitute a suite of ophiolite rock assemblage.2.Geochemical data show that both basalts and homogeneous gabbros in Kaipinggou are enriched in LILE and depleted in HFSE.They have flat or even LREE-depleted REE distribution patterns,which are similar to the MORB-like geochemical characteristics of fore-arc basalts.Boninites show characteristics of for-arc boninites with high Si and Mg,but low Ti and Ca contents.The esporphyrites are enriched in LILE(Rb,Ba,U,K,Pb and Sr)but depleted in HFSE(Nb and Ta),similar to island arc magmatic rocks.Plagiogranites have low trace elements contents.The characteristics of slight LREE enrichment,low Sr content and obvious positive Eu anomalies,are similar to the “shear-type” plagiogranites,indicating these plagiogranites were formed by partial melting of amphibolite facies hydrous gabbros in shear zone during the intra-oceanic metamorphism and deformation.Combined with published data of the Kaipinggou peridotites,we propose that these rocks constitute a suite of SSZ-type ophiolite formed in fore-arc environment of intra-oceanic arc.3.LA-ICP-MS zircon U-Pb dating results show that the crystallization ages of the cummulate gabbros(14QH-21 and 14QH-22)and homogeneous gabbro(16QH-48)are 513±3Ma,510±3Ma and 510±9Ma,respectively.A formation age of 492±8Ma is also yielded for the plagiogranite.Therefore,the age of the ophiolitic belt is restricted to about 513 Ma,indicating the initial subduction of the Early Paleozoic ocean the North Qaidam should be early than 513 Ma.