喜马拉雅及南藏的地壳俯冲带——地震学证据

地质学的证据表明 ,在喜马拉雅的冲断层带MCT和MBT处有大规模的地壳缩短 ;在雅鲁藏布缝合带附近也观测到冲断层 .但是 ,迄今还不知道这些冲断层向下俯冲多深 .我们根据地震学的证据 ,认为喜马拉雅及南藏的冲断层向下延伸至 80— 1 0 0km ,然后停止 .在MCT、MBT以及雅鲁藏布缝合带下面的冲断层与喜马拉雅以及南藏的多次地壳俯冲有密切关系 .这个现象为印度—欧亚的碰撞过程设定一个十分重要的框架 .该地区的地壳俯冲有一定深度 ,由于入侵的地壳太轻 ,使俯冲不能更深 ;此时由于印度板块的继续向北推进 ,在原俯冲带后方 ,出现另一个新的地壳俯冲带 .喜马拉雅与南藏的多重地壳俯冲与该地区地质活动的多幂性相吻合 .首先 ,在雅鲁藏布缝合带产生地壳俯冲 ,在到达 80— 1 0 0km处停止 .然后 ,在雅鲁藏布以南的MCT和MBT相继产生新的地壳俯冲 .它们也在 80— 1 0 0km的深处停止 .除了喜马拉雅和雅鲁藏布向北倾斜的地震带外 ,另外还观测到一个自地表从唐古拉山向南缓慢倾斜并到达雅鲁藏布地壳底部的地震带 .它可以解释为在唐古拉山附近的地壳向北仰冲 .喜马拉雅及南藏的多重地壳俯冲现象强有力地说明 ,入侵的印度地壳必与它的上地幔分裂开来 ,后者可以另行俯 (仰 )冲到更深的欧亚上地幔 .

湖南猫儿界雪峰期花岗斑岩的发现及其地质意义

猫儿界花岗斑岩具“S”型花岗岩成因特征,构造环境为板缘火山岛弧,侵位于雪峰晚期造陆运动后的拉张性断裂构造带中。该花岗斑岩属偏酸性—酸性铝过饱和型,K2O!Na2O,A/NCK"1.1,副矿物属钛铁矿+独居石型,稀土元素配分模式为右倾“V”型,Eu亏损较明显,Ce分馏不明显,Rb/Sr值为3.6#18.8,87Sr/86Sr初始值为0.7153,年龄834$856Ma。它的发现,基本可确定湖南雪峰期地壳俯冲消减带位于猫儿界以东,发育成对火山—岩浆岩带;雪峰弧形构造带南段的东部边界也相应东移;桃江—城步深断裂带为雪峰弧形褶断带中的一个构造带,最早形成于雪峰期,属拉张型。

Developing plate tectonics theory from oceanic subduction zones to collisional orogens

Crustal subduction and continental collision is the core of plate tectonics theory. Understanding the formation and evolution of continental collision orogens is a key to develop the theory of plate tectonics. Different types of subduction zones have been categorized based on the nature of subducted crust. Two types of collisional orogens, i.e. arc-continent and continent-continent collisional orogens, have been recognized based on the nature of collisional blocks and the composition of derivative rocks. Arc-continent collisional orogens contain both ancient and juvenile crustal rocks, and reworking of those rocks at the post-collisional stage generates magmatic rocks with different geochemical compositions. If an orogen is built by collision between two relatively old continental blocks, post-collisional magmatic rocks are only derived from reworking of the old crustal rocks. Collisional orogens undergo reactivation and reworking at action of lithosphere extension, with inheritance not only in the tectonic regime but also in the geochemical compositions of reworked products(i.e., magmatic rocks). In order to unravel basic principles for the evolution of continental tectonics at the post-collisional stages, it is necessary to investigate the reworking of orogenic belts in the post-collisional regime, to recognize physicochemical differences in deep continental collision zones, and to understand petrogenetic links between the nature of subducted crust and post-collisional magmatic rocks. Afterwards we are in a position to build the systematics of continental tectonics and thus to develop the plate tectonics theory.

Magmatism during continental collision, subduction, exhumation and mountain collapse in collisional orogenic belts and continental net growth: A perspective

Continental orogens on Earth can be classified into accretionary orogen and collisional orogen.Magmatism in orogens occurs in every periods of an orogenic cycle,from oceanic subduction,continental collision to orogenic collapse.Continental collision requires the existence of prior oceanic subduction zone.It is generally assumed that the prerequisite of continental deep subduction is oceanic subduction and its drag force to the connecting passive-margin continental lithosphere during continental collision.Continental subduction and collision lead to the thickening and uplift of crust,but the formation time of the related magmatism in orogens depends on the heating mechanism of lithosphere.The accretionary orogens,on the other hand,have no strong continental collision,deep subduction,no large scale of crustal thrusting,thickening and uplift,and no UHP eclogite-facies metamorphic rocks related to continental deep subduction.Even though arc crust could be significantly thickened during oceanic subduction,it is still doubtful that syn-or post-collisional magmatism would be generated.In collisional orogens,due to continental deep subduction and significant crustal thickening,the UHP metamorphosed oceanic and continental crusts will experience decompression melting during exhumation,generating syn-collisional magmatism.During the orogen unrooting and collapse,post-collisional magmatism develops in response to lithosphere extension and upwelling of asthenospheric mantle,marking the end of an orogenic cycle.Therefore,magmatism in orogens can occur during the continental deep subduction,exhumation and uplift after detachment of subducted oceanic crust from continental crust,and extensional collapse.The time span from continental collision to collapse and erosion of orogens(the end of orogenic cycle)is 50–85 Myr.Collisional orogens are the key sites for understanding continental deep subduction,exhumation,uplift and orogenic collapse.Magmatism in collisional orogens plays important roles in continental reworking and net growth.