Adakitic火成岩对大陆地壳增厚过程的指示:以青藏北部火山岩为例

Adakitic火成岩可以通过几种不同的岩浆作用方式产生,其中下地壳镁铁质岩石的直接部分熔融和拆沉下地壳的部分熔融可能是两种重要的adakitic火成岩形成方式。在一个大陆厚地壳背景,adakitic火成岩的产生指示了它们的岩浆源区位于大于40 km的下地壳之中,因此,暗示该大陆地壳的最小厚度超过40 km。青藏高原腹地的羌塘地区分布有40 Ma左右的“低镁”和“高镁”adakitic安山岩-英安岩-流纹岩,它们应分别是青藏高原厚大陆地壳下部镁铁质岩石直接部分熔融和拆沉的下地壳脱水熔融的产物。这套adakitic火山岩的厘定指示出在40 Ma左右时,青藏羌塘地区或更大范围的大陆地壳已经加厚到超过40 km,其地表在当时或稍后可能已经开始了隆升。

TRACING THE THICKENING PROCESS OF CONTINENTAL CRUST THROUGH STUDYING ADAKITIC ROCKS:EVIDENCE FROM VOLCANIC ROCKS IN THE NORTH TIBET

Adakitic rocks can be produced through several different magmatisms, including "lower crust melting" and "melting of delaminated lower crust section" besides the" slab melting". In a continental setting, adakitic rocks suggest that their magmas were derived from low crust at depth over 40 km. Therefore, the adakitic rocks in continental setting indicate that the least thickness of crust is over 40 km. It is in Qiangtang area of the Tibetan Plateau that the adakitic volcanic rocks of the Cenozoic era have been found. Our study revealed that these adakitic volcanic rocks in the Qiangtang area were generated by lower crust melting and dehydration melting of delaminated lower crust section. The adakitic volcanic rocks of 40 Ma infer that the continental crust of the Qiangtang area in the Tibetan Plateau has been thickened to over 40 km, and the plateau likely started uplifting at that time.