大别山超高压变质岩的冷却史及折返机制  

Cooling history and exhumation mechanism of the ultrahigh-pressure metamorphic rocks in the Dabie mountains,central China.

作　　者：李曙光[1,2] 李秋立[1,2] 侯振辉[1,2] 杨蔚[1,2] 王莹[1,2] 

机构地区：[1]中国科学院壳幔物质与环境重点实验室 [2]中国科学技术大学地球与空间科学学院,合肥230026

出　　处：《岩石学报》2005年第4期1117-1124,共8页Acta Petrologica Sinica

基　　金：国家重点基础研究发展规划项目(G1999075503);国家自然科学基金(批准号:40373009;40173014)资助

摘　　要：大别山超高压变质岩及其围岩 T-t 冷却曲线显示了超高压变质岩的冷却史从800℃到300℃经历了三个阶段:两次快速冷却(226±3Ma 到219±7Ma 期间从800℃到500℃的第一次快速冷却,180～170Ma 期间从450℃到300℃的第二次快速冷却)和介于二者之间的等温过程。这一具有两次快速冷却的 T-t 曲线已被近年来获得的高精度金红石 U-Pb 年龄(218±1.2Ma)(Li et al.,2003),高压变质和退变质独居石 Th-Pb 年龄(Ayers et al.,2002),和强面理化榴辉岩二次多硅白云母的Rh-Sr 年龄(182.7±3.6Ma)(Li et al.,2001)所证实。超高压变质岩的二次快速冷却事件反映了二次快速抬升过程。在东秦岭及苏鲁地体东端发育的同碰撞花岗岩 U-Ph 年龄为225～205Ma,与超高压变质岩第一次快速冷却时代吻合。考虑到同碰撞花岗岩与俯冲板片断离的成因联系,这种时代耦合关系表明俯冲板片断离可能是超高压变质岩第一次快速抬升和冷却的重要机制之一。大别山 Pb 同位素填图揭示出南大别带超高压变质岩具有高放射成因 Pb 特征,因而源于俯冲的上地壳;而北大别带超高压变质岩具有低放射成因 Pb 特征,源于俯冲长英质下地壳。这表明在陆壳俯冲过程中上、下地壳之间可发生挤离(detachment)或脱耦(decoupling)。已有实验证明脱耦的上地壳在俯冲过程中可沿挤离面逆冲抬升(Chemenda et al.,1995)。同理,由于俯冲镁铁质下地壳在大别山没有出露,可以推测俯冲长英质下地壳和镁铁质下地壳之间也最终发生了挤离或脱耦。大陆岩石圈在不同深度存在若干低粘度带(Meissner and Mooney,1998)是上述俯冲陆壳分层脱耦现象发生的依据。因此,俯冲上地壳及部分长英质下地壳的第一次快速抬升折返是俯冲过程中大陆地壳内部分层脱耦和俯冲板片断离的综合结果。上述过程只能使已脱耦的上地壳及部分长英质下地壳抬升折返,而未与俯冲岩石圈�A T-t cooling path for UHPM rocks and their country rock from Dabie mountains reveals that the cooling history of the UHPM rocks from 800℃ to 300℃ can be subdivided into three stages： two rapid cooling stages （the initial rapid cooling from 800℃ to 500℃ during 226 ± 3Ma to 219 ± 7Ma and the second rapid cooling from 450℃ to 300℃ during 180Ma to 170Ma） and one isothermal stage in between them （ Li S. et al. , 2000）. This T-t path with two rapid cooling processes has been confirmed by the recent geochronological results including a precise U-Pb age of 218 ±1.2Ma （ with Tc = 460℃ ） for rutile from the coesite-bearing eclogite （Li Q. et al. , 2003）, the Th-Pb monazite core age of 223 ±1 Ma （with Tc = 675 ± 25℃） and monazite rim age of 209 ± 3Ma （corresponding to retrograde amphiblite facies metamorphism TM = 500℃～ 450℃） for the jadeite quartzite （Ayers et al. , 2000） and Rb-Sr age of 182.7± 3.6Ma for the secondary phengite from an intensive foliated eclogite （Li S. et al. , 2001 ）. The two rapid cooling events may reflect two rapid uplift processes of the UHPM rocks during the exhumation history. The U-Pb zircon ages of 225 ～205Ma for the syncollisional granites developed in eastern Qinling and eastern Sulu terrane coincide with the first rapid cooling time of the Dabie UHPM rocks. Because syncollisional granite is suggested to be result of slab break off during continental subduction （Davies and Blanckenburg, 1995）, the time consistency between the syncollisional granite formation and the first rapid cooling of the UHPM rocks indicates that the slab break off could be one of the important mechanisms for the initial uplifting and cooling of the UHPM rocks ,. The Pb isotopic mapping in Dabie mountains reveals that the UHPM rocks characterized by relative higher radiogenic Pb （ ^206Pb/^204Pb = 17. 026 ～ 20. 781 ） in South Dabie zone were derived from subducted upper continental crust, while the UHPM rocks characterized by relative lower radiogen

关 键 词：超高压变质岩 冷却史 折返机制 大别造山带 

分 类 号：P588.348[天文地球—岩石学]