硅质大火成岩省的形成机制及其与资源环境的关系

Genesis of Silicic Large Igneous Provinces and effects of resources and environment

硅质大火成岩省是以流纹质熔结凝灰岩为主体的(>80vol.%),覆盖面积大于10^5km^2、体积大于2.5×10^5km^3的巨型岩浆岩建造,多呈条带状产出在大陆边缘,可能与大陆裂解和相邻镁铁质大火成岩省密切相关;其形成时间相对镁铁质大火成岩省较长,可达~40Myr。岩石中有黑云母、角闪石等含水斑晶,多为钙碱性系列,具有从Ⅰ型向A型花岗质岩石过渡的地球化学特征。本文在前人工作的基础上,整理了五个主要硅质大火成岩省的样品数据,综合研究认为其形成是玄武质岩浆底侵(或地幔柱上升),使得前期由于俯冲形成的富水下地壳重熔后经历不同程度分离结晶的结果。硅质大火成岩省的形成暗示深部有"隐伏的镁铁质大火成岩省",因为上部硅质岩浆的阻碍和富水地壳不易形成岩浆运移通道所以未能喷出,但这些镁铁质岩浆向上输送了大量的SO2气体。大规模的硅质岩浆活动一方面能够将这些SO2气溶胶送入平流层吸收太阳辐射,另一方面能够使海洋富铁后通过光合作用吸收大气中的CO2,从而导致全球降温。此外,硅质大火成岩省还是一个大型的热液成矿系统,形成大量的低硫化型贵金属浅成低温热液矿床,具有重要的经济意义。

Silicic Large Igneous Provinces(S-LIPs)are magmatic formations with areal extents greater than 10^5km^2,volumes greater than 2.5×10^5km^3,and more than 80vol.%are composed of rhyolitic ignimbrites.Most of the S-LIPs are located along the continental margins,which may be related to the breakup of super-continent and adjacent Mafic Large Igneous Provinces(M-LIPs).Their life span is relatively long(~40Myr)compared with M-LIPs.The igneous rocks from S-LIPs usually belong to calc-alkaline series,and contain hydrous phenocrysts such as biotite and amphibole,and they show transitional geochemical features fromⅠ-type to A-type granitoids.Based on previous studies,we compile whole-rock geochemical data from five S-LIPs,and propose that the remelting of the subduction-related hydrous lower crust triggered by magma underplating(or mantle plume)and followed different degrees of fractional crystallization is the possible main mechanism for the formation of S-LIPs.Because the upper silicic magmas act like density barrier and the magma conduit cannot be easily formed in hydrous crust,these mafic magmas are unable to erupt through the crust.However,they do transfer large amounts of SO2 gas upward.Thus,the S-LIPs can also be considered as the presence of"hidden"M-LIPs.On the one hand,the large-scale silicic magmatism could inject these SO2 aerosols into the stratosphere,on the other hand,it could prompt the photosynthetic conversion of CO2 to organic carbon by iron fertilization of ocean,and both of them could cause global cooling.Furthermore,S-LIPs could also be considered as one large-scale hydrothermal mineralization system,forming extensive low-sulfur epithermal precious metal deposits,which are of great economic significance.