Study On The Early Paleozoic Rocks In Lajishan Orogenic Belt Qinghai Province

Lajishan orogenic belt is locted between The middle Qilian orogenic belt and The South Qilian orogenic belt.The research on the Qilian orogenic belt is mainly concentrated in the North Qilian area and less on the South-Middle Qilian orogenic belt.It is important for studying the tectonic evolution of the Qilian orogenic belt to study the tectonic evolution of the Lajishan orogenic belt.This paper focuses on detailed Petrology,Petrogeochemistry and isotopic ehronology studies on the early Palaeozoico phiolite,voleanices and intrusive rocks from the Lajishan orogenic belt to discussed the petrogenesis of ophiolite of Baijiasi,volcanic rocks of Liudaogou formation and intusive rocks and the tectonic evolution of the Lajishan orogenic belt.The volcanic rocks of Liudaogou formation is widely distributed in the Lajishan orogencic belt.The LA-ICP-MS analysis on zircons from andesite of middle formation is 505Ma±1.6Ma.The formation age of Liudaogou formation is late Cambrian.Nb-Ta element of all The lithology of Liudaogou formation is depleted,which indicated arc volcanic characteristic.La/Nb=1.06-4.2,(Th/Nb)_N=1.93-17.47,which indicated that the Liudaogou formation was derived crustal contamination.We conclude that the tectonic setting of Liudaogou formation wascontinental rift within plate environment.The Lajishan orogenic belt is ophiolite melange belt.In this study we survey the geology section of ophiolite near Baijiasi district.The basalt and plagioclase granite are not depleted in Nb-Ta and the tectonic setting was mid-oceanic ridge.The troctolite and gabbro are depleted in Nb-Ta and the tectonic setting was island-arc environment.The REE of plagioclase is similar to other ophiolite rocks.The age of the plagioclase granite is the formation age of ophiolite melange belt.The age of the plagioclase granite is 474.6±1.5Ma,combined the regional geology,The formation age of ophiolite was started in early Ordovician.The Caledonian granite was widely distributed in the Lajishan orogenic belt and it can be divieded into four stage:The late Cambrian granite,The middle Ordovician granite,The late Ordovician granite and the middle Silurian granite.The late Cambrian granite is I type granite with Ga/Al×104=1.70-1.92,A/CNK=0.82-0.89;The age of late Cambrian granite is 517.4±1Ma.(⁸⁷Sr/⁸⁵Sr)_i=0.7056-0.7061,?Nd(t)=0.6-1.3,?Hf(t)=-7.0-+5.6;The magma source was partial melting of mantle wedge metasomatized by fluid in subduction belt and the tectonic setting is subduction environment.The middle Ordovician granite is I type granite with A/CNK=0.93-1.03,Ga/Al×10⁴=1.58-1.82.The age of late Cambrian granite is 467±1Ma.(⁸⁷Sr/⁸⁵Sr)_i=0.7056-0.7062,?Nd(t)=0.7-1.9,?Hf(t)=12.8-16.1.It was formed by melting of juvenile crusts in the background of subduction of The Lajishan ocean.The MMEs is developed in The late Ordovician granite.The LA-ICP-MS analysis on zircons from both the Saizhisi granodiorite and MMEs yielded similar ages at 446±1.3Ma and 446±1Ma,respectively.The MMEs might have been formed by mixing of mafice and felsic member magmas.The late Cambrian granite is I type granite with A/CNK=0.91-1.03,Ga/Al=2.16-2.35.(⁸⁶Sr/⁸⁷Sr)_i=0.7093-0.7095,?Nd(t)=-1.75?-1.03,?Hf(t)=1.7-6.8,TDM2=995-1750Ma.It was formed in a subduction environment and mostly generated by the mantle which was formed by the lower crust was intruded by the upwelling mantle.The middle Silurian granite shows the characteristics of adakites with Sr=416-862.5?g/g,Y=8.78-12.49?g/g and Its magma came from the thicken crust bottom of partial melting with K₂O=4.80-5.28%,K₂O/Na₂O=1.17-1.34 characteristics.The middle Silurian granite is A type granite with high REE,enriched in Si and Alkali,depleted in Al,Ga/Al=2.31-3.10.The age of the middle Silurian granite is 426.5±3.8Ma.The magma source came from the thicken crust bottom of partial melting influenced by upwelling mantle granite in tensional environment.Combined the regional geology,we conclude that the formation process of Lajishan orogenic belt was"open-close",which was interior rift basin to mid-oceanic ridge environment to subduction environment to intra-continent orogeny.In late Cambrian,The middle-south Qilian orogenic was divided and formed interior rift basin of Lajishan in the background of Souhward subduction of The north Qilian ocean.In this period,the Tadong granite and the Liudaogou formation were formed.The interior rift basin was developed into ocean basin,In The middle Ordovician,the Lajishan ocean subducted toward to the Qilian plate and the Huangcaoshan granite was formed.In the late Ordovician,the Saizhisi granite was formed and the Lajishan ocean was closed.In the early Silurian The tectonic attribute became extensional settings of post-orogenic period and the Yindonggou granite was formed.