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.

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,而未与俯冲岩石圈脱耦的下地壳在板片断离后仍可继续俯冲。俯冲板片断离后,两大陆块在晚三叠世和早-中侏罗世继续汇聚,导致华南陆块下地壳继续俯冲,及已经脱耦并折返至中上地壳的超高压岩片向北仰冲。这一仰冲可能是导致超高压变质岩第二次快速抬升的重要机制。强面理化榴辉岩二次多硅白云母的 Rb-Sr 年龄(182.7±3.6Ma)可能记录了这一超高压岩片仰冲事件发生的时代。惠兰山基性麻粒岩年代学研究揭示了罗田穹隆在早白垩世的快速抬升,与此同时大别山发生了大规模岩浆事件。山体快速抬升与大规模岩浆事件的耦合关系指示了大别造山带早白垩世的去根作用,或岩石圈拆离事件。伴随这一山体快速抬升,大别山超高压变质岩开始大面积出露地表。

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 radiogenic Pb （^206 pb/^204 pb = 15. 844～17. 204 ） in North Dabie zone were derived from subducted lower continental crust. This observation indicates that a detachment or a decoupling in between the subducted upper and lower continental crusts must be occurred during continental subduction. It has been demonstrated by a physical modeling that the decoupled upper crust can be uplifted by thrust along the detachment surface during the continental subduction process. Similarly, it can be referred that a detachment or a decoupling in between the felsic and mafic subducted lower continental crust could be also occurred, because no subducted mafic lower continental crust has been exhumed in Dabie orogen. Several low-viscosity zones occurred at different depths in continental lithosphere （ Meissner and Mooney, 1998 ） provide the conditions for such detachment or decoupling in subducted continental crust. Therefore, the first rapid uplifting of the subducted upper crust and/or part of felsic lower crust is a result of the both detachment or decoupling in subducted continental crust and slab breakoff. These processes can only cause the uplifting and exhumation of the decoupled upper and felsic lower continental crust, while the undecoupled mafic lower continental crust could be subducted continuously after slab breakoff. After slab breakoff, the continuing convergence of two continental blocks in the later Triassic and earlier-middle Jurassic might induce the lithosphere splitting of the SCB and lithospheric-wedging between the south margin of the NCB and north margin of the SCB. This process caused the overthrust of the mid-upper continental crust with exhumed UHPM rocks and underthrust of the deep lithosphere of the SCB. It could be an important mechanism responsible for the second rapid cooling and uplifting of the UHPM rocks. The Rb-Sr age of 182.7 ± 3.6Ma for the secondary phengite from an intensive foliated eclogite may indicates the lithospheric-wedging time. Geochronology study of the Huilanshan marie granulite from the Luotian Dome reveal a fast uplifting in the early Cretaceous. An intense magmatism was occurred simultaneously in the Dabie orogen. The coupling of the mountain fast uplifting and intense magamtism in the Dabie orogen suggests that a mountain root remove or lithospheric delamination event was occurred in the early Cretaceous. Consequently, the UHPM rocks in the Dabie orogen must be further exhumed in a large area with this fast mountain uplifting.