Indosinian Tectonic-magmatism And Regional Tectonic Evolution In The Qinghainanshan Tectonic Belt

The Qinghainanshan tectonic belt,situated in the northern margin of Gonghe basin,was in the conjunction area among the West Qinling Orogen,South Qilian tectonic belt and Zongwulong tectonic belt.A large quantity of intrusive rocks,varying continuously from basic to acid rocks,outcropped in the Qinghainanshan tectonic belt,which recorded the Indosinian tectonic evolution of this area.Guided by the Plate Tectonic theory,we carried out systematic studies on field work,petrology,geochemistry and zircon U-Pb isotope geochrononology for the basic rocks,intermediate-basic rocks and intermediate-acid rocks in the Qinghainanshan tectonic belt.Based on the above studies and previous work by other researchers,some new points have been achieved as follows:1.The mafic-ultramafic rocks in the Dabuzuhu complex,which consist of gabbro and pyroxenite,represent the Late Early Triassic-early Middle Triassic continental margin-arc basic rocks.Both the gabbro and pyroxenite belong to subalkalic series and island-arc tholeiitic series and show characteristics similar to boninite.The trace element geochemistry is evidently characterized by positive anomalies of LILEs and negative anomalies of HFSEs.The mafic magma was derived from partial melting of the mantle wedge which was metasomatized by hydrous fluids from dehydration of subducted slab.The gabbro and pyroxenite were formed by 5%~10% partial melting of the spinel lherzolite,and subsequent low-degree fractional crystallization of olivine and pyroxene.Mineral compositions show characteristics of island arc or magmatic rocks formed at active continental margins.A comprehensive analysis suggests that Lari basic rock mass were formed in the front arc of subduction zone.The LA-ICP-MS zircon U-Pb age of the gabbro and pyroxenite are 247.7±2.8Ma and 247.9±2.5Ma.2.The Early-Middle Triassic Dabuzuhu and Qianbulusi intermediate-basic igneous complexes consist of gabbro diorites,diorites,gabbros and so on.The rocks of both complexes belong to subalkalic series and island-arc tholeiitic/calc-alkaline series,originated from a common source via fractional crystallization.Their primary magma was derived from the 5%~15% partial melting of garnet-spinel lherzolite in the mantle wedge which was metasomatized by hydrous fluids and subducted sediments from the subducted slab.Geochemical characteristics show that the evolution process is controlled by the fractional crystallization and plus a slight and inhomogeneous crustal contamination.The LA-ICP-MS zircon U-Pb age of the Dabuzuhu gabbro diorite is 243.7±2.2Ma.The LA-ICP-MS zircon U-Pb age of the Qianbulusi diorite is 246.0±2.5Ma.All of these island-arc magmatic rocks represent the product of the limited basin subducting southward during the Early to Middle Triassic.3.The granitic plutons in the Qinghainanshan tectonic belt were products of the contemporaneous magmatism.The Dangjiasi granitic pluton consisits of granodiorite and monzogranite.The Jiangxigou pluton dominantly consists of granodiorite,multiple-porphyritic monzogranite,sparsely-porphyritic monzogranite,porphyritic-bearing monzogranite,monzogranite and syenogranite.The Heimahe pluton consisist of granodiorite and quartz diorite.The Gouhou pluton consists of gabbro,gabbro-diorite,quartz diorite and granodiorite.The majority of granitic rocks are metaluminous-weakly peraluminous,and belong to high-Kcalc-alkaline series.A few granitic rocks from the Jiangxigou granitic pluton are strongly peraluminous,which might be caused by highly evolution.The primary magma of the granitic rocks had undergone fractional crystallization of plagioclase,K-feldspars and hornblende.The mantle-derived magmas may play an important role in the formation of the granitoids.Mineralogical researches on the biotite,amphibole and plagioclase selected from the host rocks and MMEs show that the tested minerals are all likely respondent to magma mixing.The granitic rocks from the four plutons all display enrichment of LILEs(Rb,Th,K)and depletion of HFSEs(Nb,Ta,Ti),showing features of magmatic rocks in island arc or active continental margins.The LA-ICP-MS zircon U-Pb dating show that the crystallization ages of the four granitic plutons are 245Ma~240Ma,belonging to the Middle Triassic.4.The Early Indosinian intrusive rocks in the Qinghainanshan tectonic belt are mainly composed of granite,minor quartz diorite,diorite and gabbro,which are similar to the igneous rock association in active continental margins.Moreover,the zircon ages(248Ma~240Ma)from the intrusive rocks are close to the ages of continental marginal arc magnetic rocks in the East Kunlun Orogen and the Zongwulong tectonic belt.Geochemical characteristic of the Early-Middle Triassic sandstone in this area show that their source rocks were formed at active continental margins.Taking into consideration of previous data on strata and magmatic rocks,we suggest that there existed an unified active continental margin in Zongwulong tectonic belt,Qinghainanshan tectonic belt and Northern Margin of Western Qinling.5.Combined with regional geological background,we consider that the Early Indosinian tectono-magmatic event in the Qinghainanshan tectonic belt were controlled by the southward subduction of the Zongwulong oceanic basin.The tectonic evolution of the Late Paleozoic-Early Mesozoic of the Qinghainanshan tectonic belt and its adjacent area can be divided into three stages: 1)initial disintegration of the unified continent and the oceanic crust expansion stage from Late Devonian to Middle Permian;2)the oceanic crust subduction stage from Late Permian to Middle Triassic;3)syn-collision and post-collision stage from the latest Middle Triassic to the end of the Late Triassic.