摘要
喜马拉雅碰撞造山带是世界上最年轻且仍在活跃的陆—陆碰撞型造山带,是研究造山带深部物质如何响应造山作用的野外基地。自印度—欧亚大陆碰撞以来,中—下地壳岩石在构造演化的不同阶段经历了广泛的部分熔融作用,形成多种类型花岗岩。这些花岗岩蕴藏了有关地壳变质和深熔作用、淡色花岗岩侵位等过程及其效应的信息。马拉山—吉隆裂谷带是藏南裂谷系中较典型的一支,位于雅鲁藏布江缝合带(YTS)和主中央逆冲断层(MCT)之间,藏南拆离系(STDS)横贯其中,记录了较完备的有关从缩短增厚向南北向伸展/东西向伸展的构造转换中深部地质作用的信息。该裂谷带中主要花岗岩包括马拉山二云母花岗岩、佩枯错复合花岗岩、藏南拆离系同构造花岗岩和高喜马拉雅淡色花岗岩。马拉山二云母花岗岩的岩浆结晶年龄稍有不同,从18.3Ma到16.0Ma,岩浆寿命约为2.3Ma。年龄谱图具有“阶步式”特征,年龄分布没有空间规律性。展布~10km的二云母花岗岩具有均匀的矿物组成、主量元素、微量元素和Sr-Nd-Hf同位素。同时,裂谷系内其它淡色花岗岩也具有相似的年龄特征。对花岗岩围岩的岩相学观测和年代学测试得出:北喜马拉雅穹窿(NHGD)内变泥质岩未发生部分熔融作用,STDS中眼球状花岗质片麻岩的变质年龄与STDS的活动相关,高喜马拉雅结晶岩系(HHCS)内围岩的变质作用年龄老于淡色花岗岩的结晶年龄,这些事实表明花岗岩的侵位并未引起围岩发生深熔作用和变质作用。综合以上特征表明:花岗岩的形成模型为“量子行为”的岩脉汇聚,非大型岩浆房的底辟。佩枯错淡色花岗岩为一复合岩体,由含电气石淡色花岗岩、二云母淡色花岗岩和含石榴石淡色花岗岩组成,是由于源岩或熔融条件的不同所造成的,并非岩浆结晶分异的结果。马拉山—吉隆裂谷带中淡色花岗岩的岩浆结晶年龄主要集中于37~32Ma、28Ma、21Ma、20~16Ma,部分样品记录了相伴生的多期次变质作用,主要集中于37~32Ma、28~25Ma、23~20Ma、18~16Ma。其中<28Ma的变质年龄位于锆石边部,表明锆石冷凝结晶后受到后期补给岩脉的“热烘烤”发生变质作用,这一现象也支持“岩脉汇聚”模型。同时,花岗岩核部的碎屑锆石(包括变质成因和岩浆成因)和花岗质片麻岩中变质锆石的年龄为448~401Ma,表明喜马拉雅造山曾经历了加里东期构造作用。裂谷带内淡色花岗岩的形成机制包括两类部分熔融作用,白云母脱水熔融作用和水致白云母部分熔融作用。这两类部分熔融产生的花岗岩具有不同的K、Na、Ca、Rb、Sr、Ba、 Eu、Th、LREE、Zr、Hf、Nb、Ta浓度,和Sr、Hf同位素比值,受不同的矿物控制。这些淡色花岗岩中的石榴石为岩浆型,在部分熔融过程中,母岩石榴石会发生差异性溶解。综合喜马拉雅造山带内新生代深熔作用和构造特征得出:在~35Ma以前,印度与欧亚大陆快速汇聚,加厚地壳条件下,以角闪岩部分熔融作用为主,形成具有高Na/K和Sr/Y比值的花岗岩,这些熔体有效地改变了深部岩石的构造物理性质,促使造由带从缩短增厚向伸展减薄转换,引发了STDS的启动。35Ma之后,STDS向北伸展拆离,强烈的伸展减压引起变泥质岩发生大规模白云母脱水部分熔融作用,形成了大量的<30Ma高Rb/Sr比值的花岗岩。其中,在20~16Ma期间,由于藏南裂谷系的东西向开张,地幔热或物质的加入,变泥质岩发生水致自云母部分熔融作用,形成Rb/Sr比值较低但Ba含量较高的花岗岩。
The Himalayan orogenic belt is one of the youngest active collisional belts worldwide, and provides an excellent field laboratory to study the behavior of deep crustal materials as responses to the tectonic evolution of orogenic belts. Since the continental collision between India and Eurasia Plate, the middle to lower crustal materials along this orogenic belt have experienced intensive and episodic partial melting, leading to the formation of various types of granite. These granites have preserved records on the nature of metamorphism and crustal anatexis as well as magmatic processes to generate these granites. The Malashan-Gyirong Rift Zone, one of the branches of N-S trending Southern Tibet Rift Zone (STRZ), across the Yalung-Tsangpo suture (YTS), Southern Tibet Detachment System (STDS), and extending well into the Main Center Thrust (MCT), provides important records on deep geological processes during tectonic transition from compression to extension. Along this rift valley, there is a series of leucogranites including the Malashan two-mica granites, the Paiku composite leucogranite, syn-tectonic leucogranites within the STDS and those within the High Himalayan Crystalline Sequence (HHCS).The Malashan two-mica granite started to crystallize at~18.3Ma and lasted to~16.0Ma, which suggests that its life-time is about~2.3Ma. Zircon crystallization age spectrum displays stepwise/pulse-like behavior but does not show any spatial correlationship. The Malashan two-mica granites show similar mineral assemblages and are characterized by relatively homogeneous element and isotope (Sr-Nd-Hf) geochemistry. The other leucogranites within this rift zone also show similar age patterns. Field relations, petrographic observations, geochronological data from the wall-rocks of these granites all indicate that intermediate-grade metapelites immediately next to the granitc pluton in the Northern Himalaya Gneiss Domes (NHGD) did not experience partial melting. Metamorphism to form augen granitic gneiss within the STDS as well as a variety of rock types within the HHCS experienced metamorphism postdated the intrusion of a number of leucogranites, which indicates in many circumstances, intrusion of these leucogranitic magma did not result in the metamorphism and partial melting of wall-rocks. All these line of evidence presented above indicate the relatively large leucogranitic pluton possibly was assembled at relatively shallow levels by diking, not by diapirism of large and mobile bodies of magma. The Paiku leucogranite is a composite pluton, consist of tourmaline-bearing leucogranite, two-mica granite, and garnet-bearing leucogranite. They were derived from different source rocks combined with different melting conditions and did not result from differentiation of magma.Leucogranites along the Malashan-Gyirong Rift Zone formed at mainly37~32Ma、28Ma、21Ma, and20~16Ma. A number of leucogranites also show records suggesting that their source rocks also experinenced simultaneous metamorphism at37~32Ma、28~25Ma、23~20Ma and18~16Ma, respectively. A number of zircon grains from these leucogranites contain rims with textures consistent with metamorphic origin and yielded ages younger than28Ma. This observation implies that though young the zircons from these leucogranites were, they also could undergo metamorphism due to heating or magma influx from later recharging within a very short time of periods. Detrital zircons (metamorphic and magmatic origin) from these granites as well as from granitic gneisses yield ages from448~401Ma, which provides the first evidence that the Himalaya orogenic belt could have experienced a previously unrecognized tectonic process at Caledonian time.Leucogranites in this Rift Zone were formed through two types of partial melting reactions, dehydration melting versus fluxed melting of muscovite. Leucogranites formed prior to~20Ma were derived from dehydration melting, whereas those younger than~20Ma and older than~15Ma from fluxed melting of muscovite. These broadly different types of granites show distinct geochemistry not only in major and trace element but also in radiogenic isotope compositions. Garnets in these leucogranites are magmatic origin and contain composition of metamorphic garnets from source rocks due to melting of garnet in the crustal anatexis.Combined with data on the Cenozoic crustal anatexis and characteristics of tectonic evolution in the Himalayan Belt, we can conclude that (1) prior to35Ma, the partial melting of dominantly amphibolite occurred in the thickened crustal conditions due to continental collision between the India and Eurasia plate, which produced granites with high Na/K ratio and Sr/Y ratio. These melting processes effectively change the physical properties of deep crustal rocks and trigged the tectonic transition from compression to extension and initiated the movement of STDS;(2) after35Ma, with further extension along STDS, rapid exhumation of deep crustal material resulted in larger-scale dehydration melting of muscovite in the metapelites and the formation of granites with high Rb/Sr ratios;(3) during20~16Ma, E-W extension along the STRZ, possibly with additional heat or material from the mantle results in fluxed-melting of muscovite in metapelites and produced granites with lower Rb/Sr ratio but elevated Sr and Ba contents.