深部过程和物质架构对大陆碰撞带Cu-REE成矿系统的控制:以冈底斯和三江碰撞带为例

Deep process and lithospheric architectural control of Cu-REE mineralization in continental collision zone: Insights from a case study of the Gangdese and Sanjiang collisional belts

青藏高原是全球最典型的大陆碰撞带,发育世界级规模的斑岩Cu成矿带和REE成矿带,但目前尚不清楚大陆碰撞如何控制它们的形成。基本问题是:触发碰撞增厚的岩石圈熔融的机制,岩石圈架构与Cu-REE成矿的关系以及Cu-REE和挥发分的来源及成矿机制。利用深反射地震和卫星重力数据的联合反演,结合大地电磁(MT)阵列和地球化学数据,对冈底斯正向碰撞带和三江侧向碰撞带的岩石圈结构进行了成像分析,探讨深部过程和物质架构对于Cu-REE成矿的控制。新生代印度大陆-亚洲大陆碰撞过程中,俯冲的印度大陆岩石圈发生了显著的撕裂,从而为软流圈上升流提供了通道,改造了上覆的岩石圈并引发融熔。这一过程产生超钾质熔体,这些熔体上升并在地壳底部积聚,其高的热流值和挥发分释放诱发了上覆新生下地壳的熔融形成富水岩浆,角闪石分离结晶造成岩浆氧化,这种富水高氧逸度的岩浆有利于Cu的迁移和富集。研究表明,三个关键因素形成了与碰撞相关的斑岩矿床:中等角度板片俯冲,板片撕裂和富硫化物新生下地壳的熔融。在三江侧向碰撞带的扬子克拉通边缘,由印度大陆俯冲或地幔对流驱动的热软流圈的垂直上升流和横向流动导致克拉通大陆岩石圈发生热侵蚀和部分熔融。克拉通边缘的大陆岩石圈先前经历了来自再循环海洋沉积物的富含REE和CO_(2)的流体的交代作用,从而富集了REE,后来又被沿着岩石圈不连续面(例如走滑断层、裂谷)上升的碳酸岩熔体携带,形成大型的碳酸岩型稀土矿床。而缺乏源区交代作用的克拉通大陆岩石圈的熔融可能会产生碳酸岩、超钾质岩和镁铁质岩熔体,但它们形成碳酸岩型稀土矿床的潜力有限。

Situated in an archetypal continental collision zone the Tibetan Plateau developed world-class porphyry Cu and carbonate REE metallogenetic belts,yet there is no scientific consensus about the mechanism of continental collision control of Cu-REE mineralization in the region.The outstanding issues include the exact trigger mechanism for melting of the thickened lithosphere,the relationship between the lithospheric framework and Cu-REE metallogenesis,and the source of Cu,REE and volatiles and their depositional processes.To better understand the deep process and lithospheric architectural control of Cu-REE metallogenesis,this paper summarizes the lithospheric architecture of the Gangdese forward and oblique collision zones had been imaged and analyzed using joint inversion of surface wave and satellite gravity data,combined with magnetotelluric(MT)array and geochemical data.During the India-Asia collision in the Cenozoic the subducting Indian lithosphere experienced significant tearing,allowing asthenospheric upwelling to rework the overlying Asian lithosphere and cause melting.The resulting ultrapotassic melt ascended and accumulated at the bottom of the crust,providing high heat flow and volatiles for the melting of the juvenile lower crust.In the hydrous melt,amphibole fractionation led to melt oxidation,promoting Cu recycling and enrichment.The above results revealed three key factors controlling the formation of collisional porphyry deposits:moderate-angle subduction,slab tearing,and reactivation of the sulfide-enriched juvenile lower crust.At the margin of the Yangtze craton,Sanjiang oblique collision zone,vertical upwelling and horizontal flow of asthenosphere,driven by subduction of the Indian plate or mantle convection,caused thermal erosion and partial melting of the cratonic continental lithosphere.The lithosphere beneath the craton margin was REE enriched due to prior metasomatism by REE/CO_(2)-rich fluid from recycled oceanic sediment,and enriched REEs were carried by ascending carbonate melt along the lithosphere discontinuity(e.g.,strike-slip fault,rift)to form large scale carbonate REE deposits.Without prior lithospheric metasomatism,carbonatic,ultrapotassic and mafic melts produced from the melting of the cratonic lithosphere had limited potential to form carbonate REE deposits.