喜马拉雅造山带东构造结拉布拉多期和格林威尔期造山作用的记录

Labradonian and Greenvillian orogenic events in the Namche Barwa Massif of the Himalayan orogenic belt

位于喜马拉雅东构造结西北部的南迦巴瓦复合体,是构造应力最强、隆升和剥蚀最快、新生代变质和深熔作用最强的地区。为厘定该地区早期的变质岩浆作用,本文对南迦巴瓦复合体北部的花岗片麻岩和混合岩进行了岩石学和年代学研究。花岗片麻岩原岩为富钾的偏铝质花岗岩,具有岩浆弧花岗岩的成分特征。花岗片麻岩中的锆石具有岩浆锆石的环带结构,记录了487.9±1.6Ma的一期构造岩浆事件;混合岩的锆石具有明显的核-边结构,核部和边部的不协和线交点年龄分别为1559±13Ma、1154±12Ma。对比印度大陆东部的西隆高原、东高止造山带,发现三者都经历了拉布拉多期、格林威尔期以及泛非期的造山作用。因此,我们认为喜马拉雅东构造结与这两个地区密切相关,可能是他们向北的延伸,这三者可能组成统一的印度大陆东部造山带,一起经历了哥伦比亚超大陆、Rodinia和冈瓦纳超大陆的聚合与裂解过程。

The Namche Barwa Massif, core of the eastern syntaxis of the Himalayan orogenic belt, is notably characterized by intensive and epsisodic Cenozoic deformation, metamorphism, and magmatism which strongly overprint earlier tectonic history in the Indian crystalline basement. To recover the early tectonic events still preserved in the high-grade metamorphic rocks in the Namche Barwa Massif, we have conducted petrological and geochronological investigations on granitic gneiss and migmatite. Bulk-rock element analyses show that the protolith of the granitic gneiss is high-K metaluminous granite with chemical characterization of granitic rocks formed in magmatic arc environment. Zircon grains from the granitic gneiss display textures consistent with that they were crystallied from granitic melt and yield a U-Pb age of 487. 9 ± 1.6Ma. In contrast, zircon grains from the host migmatite are characterized by core- rim texture which yields U-Pb ages of 1559 ± 13Ma and 1154 ± 12Ma, respectively. Pre-collisional tectonic events experienced by the Namche Barwa Massif are similar to those in the Shillong Plateau as well as the Eastern Ghats belts in Indian continent. All of them have undergone Labradirian orogeny, Grenvillian orogeny, and pan-African orogeny, which further implies that the Namche Barwa syntaxis might be northern extension of the Shillong Plateau and Eastern Ghats belts. These three tectonic complexes might constitute an eastern orogenic belt of Indian continent （Eastern Indian Orogenic Belt） that experienced tectonic processes associated with the assembling and breakup of Columbia, Rodinia, and Gonwana supercontinent.