Meso-Cenozoic Granitoid Evolution In Tengchong Block,China And Its Implications For Sn Mineralization

The Tengchong block is located in the southwest Sanjiang Tethys tectonic domain,the granites distributed in it can be divided into three N-S granite belts from east to west according to their ages:the Early Cretaceous Donghe granite belt in the east,the Late Cretaceous Guyong granite belt in the middle and the Paleogene Binlangjiang granite belt in the west.In this thesis,zircon U-Pb age and Hf-O isotopic composition,whole-rock major and trace elements and Sr-Nd isotopic analysis were carried out on the granites in Guyong granite belt and Binlangjiang granite belt,which have a temporal and spatial relationship with tin deposits.The petrogenesis of these granites and their connection with tin deposits and their tectonic-magmatic evolution are discussed.This study indicates that the Guyong and Xiaolonghe granitic intrusions were emplaced in the Late Cretaceous(76 Ma).From Guyong porphyritic monzogranite(GPM)to Xiaolonghe medium-grained syenogranite(XMS)and Xiaolonghe fine-grained syenogranite(XFS),the degree of evolution gradually increased.All the three kinds of granite are depleted in Ba,Sr,P,and Ti,and enriched in large ion lithophile element(LILE).The XFS exhibits obvious rare earth element tetrad effect(TE).The Lailishan granitic intrusion is emplaced in Paleogene(54 Ma-52 Ma).Compared with Lailishan monzogranite(LM),the Lailishan syenogranite(LS)has a higher degree of evolution.They display obvious Eu negative anomalies and strong depletion of Ba,Sr,P,Ti and enrichment of Th,U,Rb.The Guyong and Lailishan granites were both originated from the ancient lower crust,but were formed in different tectonic settings.The subduction of the Neo-Tethys beneath the Tengchong Block changed from flat to steep,which resulted in the upwelling of the asthenosphere that provided heat for the melting of crustal rocks and produced the parental magma(GPM)of Sn-bearing granite,and then the process of crystallization differentiation led to the enrichment of Sn and the formation of Sn-bearing granite(XFS).The LM was produced by the partial melting of ancient lower crustal materials under the background of the India-Asian continental collision in Eocene,and the process of crystallization differentiation produced the LS.Compared with XMS and GPM,the XFS has more obvious TE(TE1-3>1.1),higher degree of evolution(Nb/Ta<5)and Sn content,which provides ore-forming materials(Sn)directly.However,the LM and LS no TE(TE1-3?1.1),and their degree of evolution(Nb/Ta>5)and Sn content are low,which are similar to barren granites.The Lailishan granites magma may provide ore-forming materials,but they are not the metallogenic granite which provides ore-forming materials(Sn)directly.Compared with the regional Late Cretaceous-Early Eocene granite,we found that the Late Cretaceous granite has a high degree of evolution and enriched isotope,which is formed under the background of subduction of the Neo-Tethys,while the Early Eocene granite has various rock types and a large isotopic composition span,the tectonic setting at this time changed from subduction to collision.Therefore,the granitic magmatism and Sn mineralization during this period recorded the entire transition process from subduction to collision of the Neo-Tethys Ocean.