喜马拉雅造山带的核心由高级变质岩系和淡色花岗岩构成,是研究碰撞造山作用和板块构造的天然实验室.本文评述了喜马拉雅造山带变质作用和部分熔融研究取得的新进展和存在的争议,主要内容包括:(1)造山带核部具有“三明治”结构,高级变质和部分熔融的高喜马拉雅系列(GHS)夹持在较低级变质的特提斯喜马拉雅系列(THS)和低喜马拉雅系列(LHS)之中,GHS的变质作用程度具有向上和向下部构造层位降低的特征.高喜马拉雅系列主要由高压麻粒岩相到榴辉岩相的变质岩组成,具有＞ 1.2～ 1.6GPa和＞700 ～ 800℃峰期变质条件,顺时针型变质作用P-T轨迹,其进变质以增温增压为特征,退变质早期为近等温或增温降压过程,晚期为降温降压和近等压降温过程;(2)在造山带西段,紧邻缝合带产出的超高压变质岩具有4.4 ～4.8GPa和560 ～ 760℃的峰期变质条件和顺时针型P-T轨迹,并在退变质中期出现加热过程;(3)尽管造山带的高压和超高压变质岩形成在中、高温条件下,但岩石中的石榴石都保存有明显的主量和微量元素生长成分环带特征;(4)造山带变质核下部发育反转的中、高压型变质序列;(5)在造山带核部,变泥质和长英质麻粒岩的强烈部分熔融主要是增压、增温进变质过程中的白云母和黑云母脱水熔融,和近等温或增温降压过程中的黑云母脱水熔融,可以形成花岗质和英云闪长质熔体.加厚下地壳的高变质温度足以使各种成分岩石(包括基性岩)发生深熔,而不需要外来热源;(6)造山带变质核经历了长期的变质演化过程,其进变质始于～ 47Ma,峰期变质发生在～25 Ma,退变质持续到～15Ma.这些岩石也记录了持续的(超过20Myr)高温变质和部分熔融过程.在造山带西段的超高压变质岩具有～46Ma的峰期变质年龄和～40Ma的退变质年龄,所以经历了一个快速俯冲与折返过程;(7)印度大陆西缘与岛弧的碰撞(造山带西段)和印度大陆东缘与大陆弧的碰撞时间一致,为～50Ma;(8)在造山带西段,印度大陆的深和陡俯冲形成了超高压变质岩;而在造山带中段,印度大陆的平缓俯冲形成了中高压变质岩;(9)构造变质不连续在变质核中广泛存在.多重有序逆冲和无序逆冲导致的岩片叠置控制着造山带的地壳结构;(10)现有的构造模型,包括楔形挤出、隧道流、临界楔和构造楔模型,都不能全面合理地解释造山带变质核部的折返机制.%The core of the Himalayan orogen consists of high-grade metamorphic rocks and lcucogranites,and is a natural laboratory for studying collisional orogeny and plate-tectonics.This paper reviews recent research progress and controversies about metamorphism and partial melting of the orogenic core.The main contents are as follows:(1) the metamorphic core of the orogen can be considered a metamorphic sandwich,in which low-grade Tethyan Himalayan Sequence (THS) and Lesser Himalayan Sequence (LHS) rocks bound intervening high-grade Greater Himalayan Sequence (GHS) rocks.The GHS consists mainly of high-pressure (HP) granulite-facies and eclogite-facies rocks.The rocks have peak-metamorphic conditions of ＞ 1.2 ～ 1.6GPa and ＞ 700 ～ 800℃,and clockwise P-T paths,with a prograde process of both pressure and temperature increase,early retrogression of nearly isothermal or heating decompression,and late retrogression of nearly isobaric cooling;(2) In the western segment of the orogen,ultrahigh-pressure (UHP) metamorphic rocks immediately south of the Indus-Tsangpo suture zone have peak-metamorphic conditions of 4.4 ～4.8GPa and 560 ～760℃,and clockwise P-T paths with heating during the exhumation;(3) Although the HP and UHP rocks from the orogen formed under medium-to high-temperature conditions,garnets from the rocks have marked growth zoning for major and trace elements;(4) The lower structural level of the metamorphic core preserves a complete inverted medium to high pressure metamorphic sequence;(5)Pelitic and felsic rocks in the orogenic core underwent intensively partial melting,characterized by muscovite-and biotite-dehydration melting during the heating and burial prograde metamorphism,and biotite-dehydration melting during the nearly isothermal or slight heating decompression.The generated melts have granitic and tonalitic compositions.Metamorphic temperatures of the thickened lower crust are high enough for regional anatexis of various rocks,including meta-mafic rock;(6) The metamorphic core of the orogen underwent long-lived metamorphic process with a prograde process initiated at ～47Ma,peak-stage at ～25Ma,and retrograde process lasted to ～ 15Ma.The orogenic core also recorded a prolonged high-temperature (HT) metamorphic and anatectic process over ～ 20Myr.The UHP rocks in the western segment of the orogen have a peak-metamorphic age of ～46Ma and retrograde age of ～40Ma,witnessing a rapid subduction and exhumation process;(7) The collision between the western margin of Indian continent and the island arc was synchronous with the collision between the eastern margin of the Indian continent and the continental magmatic arc;(8) Steep and deep subduction of the Indian continent resulted in the formation of UHP rocks in the western Himalaya,whereas shallow subduction of the Indian continent generated the HP rocks in the central Himalaya;(9) Tectonometamorphic discontinuities are present in the metamorphic core of the orogen.Duplexing of LHS and GHS resulted from multiple in-sequence and out-of-sequence thrusting forms crustal architecture of the Himalayan orogen;(10) The existing tectonic models,including the wedge extrusion,channel flow and tectonic wedging,cannot reasonably interpret the exhumation mechanism of the Himalayan metamorphic core.
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