西藏冈底斯南缘碰撞后渐新世成矿作用--来自帕南钼矿床的Re-Os同位素年龄证据

Post-collisional Oligocene metallogenic process in southern margin of Gandise belt: Evidence from Re-Os dating of molybdenite from the Pa'nan porphyry Mo deposit

帕南矿床是西藏山南地区冲木达-努日矿集区东缘新发现的一个斑岩型钼矿床.野外勘查发现矿区内与成矿作用有关的二长花岗斑岩呈脉状侵入到古新世花岗闪长岩中,并引起斑岩体内部和围岩发生强烈的辉钼矿矿化和热液蚀变作用.辉钼矿矿化主要产于平直石英细脉内,部分呈浸染状分布于脉体围岩中.本文对帕南斑岩型钼矿床的4件辉钼矿样品进行了Re-Os同位素测试,其模式年龄为31.34～31.83 Ma,平均值为31.66±0.22 Ma,并有较好的等时线年龄（32.4±1.5Ma,MSWD=1.5）.等时线年龄与模式年龄在误差范围内一致,表明该年龄应该代表帕南矿床的矿化年龄.这一年龄值与冲木达-努日成矿带的成矿作用时间大致相当（30～23 Ma）,显示出帕南矿床与成矿带内其它矿床可能具有相似的成矿动力学背景.然而,帕南矿床的成矿时代略早于区内其它矿床,表明冲木达-努日成矿带的成矿作用应始于32 Ma左右,而成矿作用则可能与碰撞后的加厚岩石圈地幔拆沉有关.

The Pa＇ nan deposit is a newly-discovered porphyry Mo deposit at the eastern edge of the ChongmudaNuri ore field in Shannan Prefecture of Tibet.According to field work,the vein-like monzonitic granite-porphyry related to mineralization intruded into the Paleocene granodiorite,resulting in strong molybdenite mineralization and hydrothermal alteration in the porphyry and the contact zone.The molybdenite mineralization was mainly related to the straight quartz vein where most molybdenite occurred with a minor impregnation in quartz veins near the surrounding rock.In this paper,direct Re-Os isotopic dating of molybdenites collected from the quartz-molybdenite veins in the Pa＇nan porphyry Mo deposit was carried out,which yielded model ages ranging from 31.34 to 31.83 Ma,a weighted average age of 31.66 ± 0.22 Ma and an isochron age of 32.4 ± 1.5 Ma （MSWD =1.5）.The model ages and isochron age are consistent within the error range.Therefore the data represent the ore-forming age of the Pa＇ nan deposit,which is basically consistent with the previous studies which suggest that the mineralization of the Chongmuda-Nuri ore field occurred in Oligocene （30～23 Ma）.These data indicate that the Pa＇ nan deposit had a similar deep dynamic progress to the other deposits in the ChongmudaNuri ore belt.Nevertheless,the metallogenic epoch of the Pa＇nan porphyry deposit is slightly earlier than that of the other deposits in the Chongmuda-Nuri ore belt,suggesting that mineralization was probably started at ca.32 Ma and related to the delamination of post-collision thickened mantle lithosphere.