摘要
中国大陆科学钻探工程和苏鲁高压-超高压变质带为大陆岩石圈的深俯冲与折返动力学的研究提供了以下制约:(1)苏鲁高压/超高压变质地体迭置于南、北苏鲁两个不同时代及属性的基底之上;(2)苏鲁巨量表壳岩石深俯冲至200km以下的上地幔深度,并经历超高压变质作用;(3)根据不同类型超高压变质岩石锆石的SHRIMP-U/Pb原位精确定年,获得超高压变质岩石的深俯冲-折返全过程(240~252Ma→230~237Ma→207~218Ma)时限,并建立了新的深俯冲-折返全过程的P-T-t轨迹;(4)富钛铁的辉长岩在大陆地壳的深俯冲过程中,经历了超高压变质作用并转变成了富含金红石的榴辉岩,形成了超高压变质的钛矿床;(5)通过榴辉岩和石榴石橄榄岩的显微构造分析及石榴石、绿辉石和橄榄石EBSD测量,确定深俯冲过程中绿辉石和橄榄石的组构运动学和流变学特征;(6)在大陆的深俯冲过程中,强烈水化的陆壳岩石经历了进变质脱水过程,巨量的地表水带入到>100~200Km的地幔深处,在超高压变质峰期的极端条件下,通过含水超高压变质矿物的分解形成超临界的含水熔体,导致有效的壳-幔物质交换和岩石圈物质分异;(7)苏鲁超高压变质地体在折返阶段形成挤出纳布构造,与岩石圈深俯冲管道流的折返挤出机制有关;(8)提出新的深俯冲-折返动力学模式:陆-陆碰撞的深俯冲剥蚀模式及大陆地壳多重性、分层型和穿时性的俯冲和折返模式。
Detailed studies of ultrahigh-pressure(UHP)rocks from the Maim Hole of the Chinese Continental Scientific Drilling (CCSD-MH)and the Sulu metamorphic terrane have provided following important constraints for the deep subduction and exhumation dynamics of continent:(1)the Sulu HP-UHP metamorphic terrane consists of the south and north Sulu basements with differente protolithic ages and rock associations;(2)The common occurrence of UHP mineral inclusions(e.g.,coesite)in zircons of eclogites and their country-rocks from the Sulu terrane suggests that voluminous continental materials were subducted to the mantle depth (>200km)and has resulted in the UHP metamorphism;(3)Careful SHRIMP U-Pb dating on the zoned zircons separated from various metamorphic rocks identify three discrete and meaningful age groups:the prograde metamorphism of quartz eclogite-facies occurred at 240~252Ma,the UHP peak metamorphic event was at 230~237Ma,and the later amphibolite-facies retrograde overprint took place at 207~218Ma.These age groups combined with previous P-T estimations suggest that voluminous Proterozoic continental materials experienced a fast subduction and rapid exhumation P-T-t evolution.(4)Ti-Fe-rich gabbros were transformed into the rutile- rich eclogite by the UHP metamorphism during the deep continental subduction,and a large titanium deposit was formed by this process;(5)Petrofabric and rheological characteristics of omphacite and olivine during deep subduction have been determined by microstructure analyses of eclogite and garnet-peridotite and EBSD mesuments of omphacite and olivine;(6)During the deep continental subduction,voluminous surface water was transported into the mantle with a depth greater than 100~200km by the hydrous and nominally anhydrous UHP minerals.A supercritical hydrous melt was formed by the breakdown of hydrous UHP minerals at the extremely UHP metamorphic conditions,and has resulted in the material exchange between the crust and mantle,and the lithosphere fractionation;(7)The Sulu HP-UHP metamorphic terrane formed an extrusion nappe structures during the exhumation stage,which was related to the extrusion chennal flow mechanism in the deeply subducted slab;(8)Two new models for the deep continental subduction-exhumation dynamics,i.e.,the deep subduction erosion mode for the continent-continent collision and the polyphase subduction/exhumation mode for continental crust,have been establsihed.