Tectonomagmatic Dynamics Process Of Complex In Gangdese,Southen Tibet

Baidui complex is located at the west of Lhasa,where is part of southern Gangdese.A series of sub-parallel,W-E trending dikes of dioritic-granodioritic composition intruded into the Baidui granodiorite complex with its dioritic enclaves,which looks like ?bar code‘ from far away.There is also a series of N-S trending dikes of leucogranite intruded into Baidui complex and mineralization of Cu and Au in granodioritic dikes.These dikes indicate a dramatically extension in the process of the subduction of Neo-Tethys.Field investigations and zircon U/Pb dating indicate that:(1)the mineralized granodioritic dikes formed at ~82 Ma;(2)the granodioritic dikes formed from 86 Ma to 69 Ma;(3)the magma formed dioritic dikes continues from 84 Ma to 18.5 Ma,which can be divided into 3 episodes about 85~60Ma,~30Ma and ~18.5Ma;(4)the leucogranite fromed at ~89Ma and ~63Ma.The mineralized granodioritic dikes,the granodioritic dikes and the dioritic dikes show the similar characteristics of geochemistry.Almost all specimens of The mineralized granodioritic dikes,the granodioritic dikes and the dioritic dikes are High-K to medium-K.The mineralized granodiorite,the granodiorite and the diorite are sub-alkaline.They all shows relatively high Sr,low Y,high Sr/Y,enriched in large ion lithophile elements(LILEs),but highly depleted in high field strength element(HFSEs).The mineralized granodiorite,the granodiorite and the series-(40)leucogranite are adakite-like.They all shows weak negative Eu anomalies and enriched in LREE,depleted in HREE,nearly flat distribution patterns for elements from Ho to Yb.The mineralized granodiorite,the granodiorite and the diorite has remarkable positive zircon ?Hf(t)(>9.5)and signally young one stage model ages(44~618Ma),which suggest the source of the magma is the thickened lower crust with characteristics of depleted.Both the zircon saturation thermometer and monazite saturation thermometer yield the there are tow soars from Late Cretaceous to Early Paleogene.The first soar,from 700? to 800?,may cause by the mantle-derived magma upwelling of the delamination of the lower crust at 85 Ma.The second soar,from 750? to 780?,may explain as the products of the roll-back of the Neo-Tethys plat at 65 Ma.From Late Cretaceous to Early Paleogene,there are there episodes of magamatism occurred in the southern part of Gangdese.Firstly,at 90 Ma,the melting of thickened lower crust formed the series(40)leucogranite.Secondly,from 85 Ma to 70 Ma,in the process of the delamination of the root of the hanging wall,the mineralized granodiorite,the granodiorite and the diorite generated.Finally,at 65 Ma.This series(40)(40)leucogranite might be resulted from roll-back of northward subduction of the Neo-Tethys plat.The ore-forming hydrothermal solution of the Baidui mineralization may be derived from the magam.The initial magma formed from the melting of the thickened lower crust.Then,the Cu and Au enriched in the ore-forming hydrothermal solution with the magma evolution.These illustrate that there is a real potential of the mineralization in the process of the melting of the thicken lower crust associated with northward subduction of the Neo-Tethys plate.