大别山超高压变质岩折返机制与华北-华南陆块碰撞过程

Exhumation mechanism of the ultrahigh-pressure metamorphicrocks in the Dabie mountains and continental collision processbetween the North and South China blocks

古地磁研究表明华北和华南陆块的碰撞始于三叠纪初 ,止于晚侏罗世 ;同位素年代学研究及大别山北部中—上侏罗统砾岩层中榴辉岩砾石的发现表明大别山超高压变质岩形成于三叠纪初 ,并在中—晚侏罗世出露于地表。因此 ,超高压变质岩是在陆陆碰撞过程中完成它的折返出露过程。揭示超高压变质岩的折返历史与机制有助于我们认识大陆的碰撞过程。大别山超高压变质岩及其围岩θ t冷却曲线显示超高压变质岩从 80 0℃到 3 0 0℃经历了三个阶段 :( 2 2 6± 3 )～ ( 2 1 9± 7)Ma期间从80 0℃到 5 0 0℃的第一次快速冷却 ,1 80～ 1 70Ma期间从 4 5 0℃到 3 0 0℃的第二次快速冷却 )和介于两者之间的等温过程。这一具有两次快速冷却的θ t曲线已被近年来的若干年代学数据所证实。超高压变质岩的两次快速冷却事件反映了两次快速抬升过程。在东秦岭及苏鲁地体东端发育的同碰撞花岗岩U Pb年龄值为 2 2 5～ 2 0 5Ma,与超高压变质岩第一次快速冷却时代吻合。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。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反射成因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。

The palaeomagnetic data suggest that the collision between the North China Block (NCB) and South China Block (SCB) occurred initially in the Early Triassic and they were finally wedded into a single tectonic unit by the time of the Late Jurassic. The geochronological studies of the ultrahigh pressure metamorphic (UHPM) rocks in Dabie-Sulu belt suggest that the continental crust of the SCB was subducted into the depth of >100 km in 230 Ma, while the discovery of eclogite pebbles in the Mid-Late Jurassic Conglomerate in the basin on the north margin of Dabie mountains suggests that the UHPM rocks had been exhumed to the surface in the Middle Jurassic (～ 170 Ma). Therefore, the formation and exhumation of UHPM rock were completely finished during the continental collision process. The unrareling of the process and mechanism of the UHPM rock exhumation may help us to understand the continental collision process that lasted 70 Ma at least. Cooling history study of UHPM rocks is the most direct means to give conunraveling straints on tectonic process of the exhumation. A θ-t 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±3) Ma to (219±7) Ma and the second rapid cooling from 450℃ to 300℃ during 180 Ma to 170 Ma) and one isothermal stage between them (Li S, et al., 2000). This θ-t path with two rapid cooling stages has been confirmed by the recent geochronological results including a precise U-Pb age of (218±1.2) Ma (with θ_c=460℃) for rutile from the coesite-bearing eclogite (Li Q, et al., 2003), the Th-Pb monazite core age of (223±1) Ma (with θ_c=675 ± 25℃) and monazite rim age of (209±3) Ma (corresponding to retrograde amphiblite facies metamorphism θ_M=500～450℃) for the jadeite quartzite (Ayers et al., 2000) and Rb-Sr age of (182.7±3.6) Ma for the second phengite from the 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 225～205 Ma for the syncollisional granites developed in eastern Qinling and eastern Sulu terrane coincide with the first rapid cooling time of the Dabie UHPM rocks, which indicates that the slab breakoff could have been an important mechanism for the initial uplifting and cooling of the UHPM rocks, because syncollisional granite is suggested to be the result of slab breakoff during continental subduction (Davies and Blanckenburg, 1995). The Pb isotopic mapping in Dabie mountains reveals that the UHPM rocks characterized by relative higher radiogenic Pb (N(^(206)Pb)/N(^(204)Pb)=17.026～20.781) in Southern Dabie zone were derived from subducted upper continental crust; while the UHPM rocks characterized by relative lower radiogenic Pb (N((^(206)Pb)/)N(^(204)Pb)=15.844～17.204) were derived from subducted lower continental crust; which indicates that a detachment and a thrust fault in between the subducted upper and lower continental crusts must have occurred during the continental subduction. Similarly, it can be referred that a detachment or thrust between the felsic and mafic subducted lower continental crust could have also occurred because of the density contrast between them. Therefore, the slab breakoff could only have caused the uplifting and exhumation of the subducted upper and felsic lower continental crust, while the mafic lower continental crust could have subducted continuously after slab breakoff. The Sr-Nd-Pb isotopic compositions of the Early Cretaceous mafic-ultramafic intrusions developed in the North Dabie zone (NDZ) suggest that the deep lithosphere underneath the NDZ belongs to the NCB but not the SCB. Therefore, although the surface suture between the NCB and SCB is located on the north side of the NDZ, the subsurface suture between the NCB and SCB should be located to the south side of the NDZ. A continent-collisional litho