冈底斯带南部新特提斯洋早期俯冲有关岩浆特征与成矿分析

Magmatic properties and metallogenic analyses of magmatism triggered by early Neo-Tethys subduction in South Gangdese, Tibet

冈底斯南缘经历了新特提斯洋俯冲及其后的陆陆碰撞,发育众多后碰撞环境的斑岩铜矿床,而与洋壳俯冲有关斑岩型矿床却发现不多,目前仅有雄村斑岩型铜金矿床。本工作在冈底斯南缘东西向200多千米范围内发现多个铜矿化岩体及火山岩。岩体锆石U-Pb年龄显示其侵位于早中侏罗世。这些铜矿化岩体及火山岩多发育脉状帘石化蚀变,而其周围新生代岩基多未发生蚀变及矿化,显示矿化及蚀变早于新生代岩基;而磁铁矿-赤铁矿-黄铜矿矿物组合表明铜矿化与岩浆磁铁矿的氧化还原过程有关,矿化时代与岩体基本同期。矿化岩体多富含角闪石,发育磁铁矿-赤铁矿组合,显示岩浆富水氧化特征,而岩浆期榍石、磷灰石及石英中发育原生流体包裹体,显示富水岩浆较早出溶挥发分。综合区内早中生代岩浆岩的时空分布,冈底斯南缘存在东西向的侏罗纪弧岩浆岩带。该套岩浆岩具有典型弧岩浆地球化学特征,锆石?Hf(t)值(+10^+16)显示近亏损地幔端元的源区特征。上述证据表明,冈底斯南缘在早中生代发生了大范围的岩浆活动和同期铜矿化,新生下地壳或亏损地幔源区可提供良好的Cu金属来源,高氧化富水的母岩浆有利于亲铜元素的富集与成矿,为斑岩铜矿床形成的有利条件。据此,我们提出冈底斯南缘在早中侏罗世可能存在一个包括雄村斑岩型铜金矿床在内的高氧化岩浆岩和矿化带,其与新特提斯洋洋壳在侏罗纪俯冲引发的大规模的岩浆活动相关,该带具备良好的斑岩铜矿找矿前景。鉴于冈底斯南缘在白垩纪之后经历了大规模抬升和剥蚀,在该地区寻找新的俯冲型斑岩铜金矿床应主要集中在早中侏罗世火山岩出露区。

South Gangdese has experienced the Neo-Tethys subduction and subsequent collision between the Indian and Eurasian plates. Many post-collision-related porphyry Cu-Mo deposits have been found in the South Gangdese belt. However, only the Xiongcun super-large porphyry Cu-Au deposit was identified as relating to the Neo-Tethys subduction in the South Gangdese belt. This work reports several Cu-mineralized intrusions in the South Gangdese belt, which are distributed widely from the west to the east. Zircon U-Pb ages show that they were emplaced in the Early Mesozoic. Such Cu-mineralized igneous rocks generally develop quartz-epidote vein alterations, while the surrounding Cretaceous-Cenozoic Gangdese granitic batholith did not undergo any corresponding alteration and mineralization. This finding suggests that Cu mineralization and alteration occurred earlier than the batholith＇s formation. The chalcopyrite-magnetite-hematite mineral assemblage shows that Cu mineralization was related to the redox process of the magmatic magnetite. Thus, the mineralization was coeval with the intrusions. These Jurassic rocks have high hornblende content and quartz-magnetite-titanite and hematite-magnetite mineral assemblages, indicating they originated from highly oxidized and water-rich magmas. The primary fluid inclusions found in the magmatic sphere and apatite of mineralized intrusions indicate that their parental water-rich magmas existed in an exsolved volatile phase during the early stage of the magma evolution process. The temporal and spatial distributions of the Early Mesozoic magmatic rocks show the existence of a west-east magmatic rock belt parallel to the Yarlung Zangbo suture zone in the South Gangdese. These Early Mesozoic magmatic rocks show typical arc magma characteristics, and have high positive zircon εHf（t） （＋10 -＋16）, providing evidence of a mantle-derived magma source. The above findings imply that South Gangdese developed large-scale magmatism associated with Cu mineralization in the Early Mesozoic. These Jurassic magmatic rocks originated from a juvenile lower crust, which provided a good Cu metal source. Further, their parental magmas were highly oxidized water-rich magmas, favoring the concentration and mineralization of chalcophile elements during the magma evolution process. Thus, it is proposed that the Early-Middle Jurassic subduction of the Neo-Tethys slab induced a west-east oxidized arc magmatic rock belt and contemporary Cu mineralization in the South Gangdese belt. The results reveal there was good potential for the creation of subduction-related porphyry Cu deposits. Given the strong uplift and denudation in South Gangdese after the Cretaceous, the domains of the Early-Middle Jurassic volcanic rocks or intrusions associated with Cu mineralization in this area are a good prospecting target for subduction-related porphyry Cu deposits.