The Characteristics And Metallogenetic Potential Of The Main-collisional Intrusive Rocks In The Middle-western Gangdese Region

The Gangdese belt is controlled by accreted orogeny related to Tethyan ocean The Gangdese belt has developed magmatism and mineralization strongly controlled by accreted orogeny and continental collisional orogeny related to Tethyan ocean subduction and the India-Asia collision,respectively.During the main collision period of the India-Asia collision?65⁴0Ma?,there were a series of Fe-Cu-Pb-Zn-Mo metallogenic events in Gangdese belt.However,most ore deposits were restricted to the middle and eastern segment?East of 86°E?.Based on a comprehensive research of eight main collisional rocks in four areas of the middle and eastern segment of the Gangdese including systematic field geological survey,petrology and U-Pb isotope chronology,major and trace elements and Sr-Nd-Hf isotopic geochemistry,this paper studies the magmatic source,diagenesis,tectonic background and the geodynamic process.What's more,a simple evaluation is given about the Fe-Cu-Pb-Zn-Mo metallogenic potential of the intrusive rocks in the middle and eastern segment of Gangdese.The magmatism in the main collision strongly developed in the middle and eastern of Gangdese belt.seven plutons in the Yare,Huoerb and Zhongba area are high potassium calc-alkaline and metaluminous I type or evolved I type granite and show typical characteristics of the arc magmatic rocks with“right deviation”REE distribution patterns,enriched Rb,Th,U and Pb and depleted Ba,Nb,Ta,P,Ti in varying degree,of which granite in the Huoerba area is of high silica.One pluton in the Zhada area is middle potassium calc-alkaline and metaluminous diorite and similar to Linzizong volcanic rocks with flat REE patterns and Primitive mantle-normalized trace element diagrams.Six plutons in the Yare and Huoerba area are aged at 39.99-56.36 Ma showing as the products of the main collision period of the Indian and Asian continent collision.Study about Zircon Hf isotopic and the whole rock Sr-Nd-Pb isotopic geochemistry shows the magmatic source of fine grained diorite in the Zhada area and most granitoids in the Yare area is enriched lithosphere mantle and juvenile crust with one granite in the Yare area is mixing source of crust and mantle.In addition,the granite in the Huoerba area originated from the old lower crust.This paper dissect the tectonic background and dynamic process of the intrusive magmatism.In the early stage of the main collision,the subducted Tethys oceanic crust released fluid,metasomatized lithospheric mantle,made it partially melted and formed a primary magmatic system with low(⁸⁷Sr/⁸⁶Sr)i and positive?Nd?t?and?Hf?t?,experienced weak fractional crystallization of apatite and rutile and became fine-grained diorite in the Zhada area at 62.29 Ma.Subsequently,India Plate wedged into the deep mantle northward and generated large frictional heat or mantle-derived water-bearing magma,which caused the partial melting of the juvenile lower crust to generate magma of positive?Hf?t?and weak negative?Nd?t?,underwent different degree mixture of the medium and upper crust and strongly fractional crystallization and became granite in the Yare area at 54.25Ma and 56.36Ma;at the same time,the partial melting of the ancient crustal could generate mixing source of crust and mantle with positive/negative?Hf?t?and negative?Nd?t?in the Zhongba area at 56.68Ma.At⁵2Ma,subduction of the Tethys oceanic crust and the plate fragments from the Indian continental plate,the soft flow ring material upwelling induces the partial melting of the juvenile lower crustal to generate magma of positive?Hf?t?and weak negative?Nd?t?and at last emplace in the Yare area to form granitoids at47.16⁴6.68Ma;meanwhile,the partial melting of the ancient lower crustal in Huoerba produce negative?Hf?t?and?Nd?t?and emplace to produce high silica granite after strongly fractional crystallization.With the crust-mantle action,the ancient crustal was melt partially.It mixed the mantle material to generate mixing source with positive/negative?Hf?t?and negative?Nd?t?and form granite in the Yare area at 39.99Ma.Comprehensive data based previous study and this paper shows the intrusive rocks in the main collision of Gangdese belt have an obvious difference from east to west.In the South Lhasa Terrane,intrusive magmatism is generally strong,and the western and eastern segmentm mainly have the magma source of mantle-derived components or juvenile lower crust.The amount of ancient crustal material gradually increases when close to the Central Lhasa Terrane and the magmatic source in the western middle segment has more ancient crustal material.In the Central Lhasa Terrane,intrusive magmatism generally distributes in the middle and western segment with more ancient crustal material;from east to west,the contribution of the medium-upper crustal material increases.High-K calc-alkaline I-type granites are generally developed in the middle and western segment of Gangdese,and consistent with the types of the Fe-Cu-Pb-Zn ore-forming rocks in the middle and eastern segment.The Sr-Nd-Hf isotope shows that the granite body in the midlle and western segment of the Gangdese mainly originated from the partial melting of the juvenile lower crust with limited addition of medium-upper crust material.This is very similar to the Fe-Cu ore-forming rock bodies in the mid-eastern Gangdese and a large difference from the Pb-Zn-Mo ore-forming rock bodies,indicating that the Fe-Cu mineralization potential of the magmatic system in the Gangdese mid-western segment is huge,but the Pb-Zn-Mo ore-forming potential is small.However,the existence of porphyry biotite granite?in the Huoerba area?derived from the partial melting of the ancient lower crust in the western segment of Gangdese suggests that the magmatic system of the source region has a certain Pb-Zn-Mo metallogenic potential.