摘要
岩浆岩在青藏高原的大陆动力学研究中有着重要的作用,它既是构造演化的记录,又是重要构造-岩浆-成矿带的指示.本文主要基于冈底斯带及邻区的地面重力和航磁数据,首先进行地质-地球物理先验信息约束下的重磁2.5维交互式反演,再将2.5维反演结果作为参考模型加入到三维反演计算中,得到地下三维密度和磁化率结构.结合岩浆岩密度、磁化率统计资料和岩浆岩地球化学成果,推断研究区基性岩、I型花岗岩和S型花岗岩的三维分布图,得到如下结论:S型花岗岩主要分布在冈底斯东带和冈底斯弧背断隆带以北;北冈底斯的西部无明显的岩浆活动,而在其南侧和北侧,发现大量的隐伏基性岩和零散分布的I型花岗岩;中生代I型花岗岩在南冈底斯和冈底斯弧背断隆带广泛分布,且到新生代才出现大量的S型花岗岩.上述结果为中生代班公湖—怒江洋壳和新特提斯洋壳的双向剪刀式俯冲模式的观点提供了重要佐证,并认为班公湖—怒江洋壳在北冈底斯西部约84°E—88°E的范围内先后存在向北和向南俯冲的可能,北向羌塘地体下俯冲,南向冈底斯地体下俯冲.
Magmatic rocks play an important role in research on continental dynamics in the Tibetan Plateau,which are records of tectonic evolution as well as the indications of major tectonic-magmatic-metallogenic belts.In this paper,we used large-scale ground gravity andaeromagnetic data to study the characteristics of magmatic rocks in the Gangdise belt and adjacent areas.To obtain the 3 Ddensity and susceptibility models of the study area,we carried out 2.5 D interactive inversion of gravity and magnetic data based on geological-geophysical prior information.The2.5 Dinversion results were then regarded as the reference model for the three-dimensional inversion.Integrating the inversion results with physical property statistics and geochemical data,we mapped the three-dimensional distribution of basic rocks,I type granites and S type granites in the study area.The resulting maps show that S type granites which are related to the Mesozoic collision are mainly distributed in the eastern Gangdise belt and the north of Gangdise back-arc fault uplift.No remarkable intrusive rocks are found west of northern Gangdise.While a large number of hidden basic rocks are present in the south and north sides of the northern Gangdise,and a few I type granites which are related to subduction are scattered there.In the southern Gangdise and Gangdise back-arc fault uplift, Mesozoic I type granites are widely distributed,but the large number of S type granites did not appear till the Cenozoic era.The above results provide important evidence to support the bi-directional scissors-style subduction model of the Mesozoic Bangong Co-Nujiang oceanic crust southward and the Neo-Tethys oceanic crust northward subduction.We consider that the Bangong Co-Nujiang oceanic crust may have subducted northward and southward subsequently in the west of the northern Gangdise in the range of 84°E—88°E.The oceanic crust subducted northward to the Qiangtang terrane and southward to the Gangdise terrane.