Massive Eclogites and Their Tectonic Significance in Dabie-Sulu UHP Metamorphic Belt, East-Central China
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摘要: 大别—苏鲁超高压(> 27× 108Pa) 变质带内的榴辉岩, 在大陆深俯冲、碰撞和折返剥露过程中, 大都遭受了强烈的变形和变质作用的重置与再造.但是, 大型榴辉岩体核部以及包裹于大理岩和石榴橄榄岩体内部的块状榴辉岩, 往往保留其初始简单的矿物组合、中-细粒状变晶结构和块状构造.详细地分析了块状榴辉岩的几何学、岩相学及变质作用特征, 指出它们是超高压榴辉岩递进及多期变质变形分解作用的残留体, 位于尺度不同的弱应变域内, 是大陆深俯冲及碰撞作用的真正记录.Abstract: Detailed study of the structures in the Dabie-Sulu UHP metamorphic belt, east-central China indicates that most of Triassic UHP rocks were reworked and reset under eclogite- and amphibolite-facies conditions, following the peak UHP metamorphism. However, some of fresh coesite- and microdiamond-bearing massive eclogite bodies which are enclosed by marbles, foliated eclogites and garnet-peridotites respectively, remained almost undeformed on the centimeter to hundred of meter scale. The massive eclogites consist of garnet, omphacite, rutile and coesite/quartz with medium to fine-grained massive features in appearance. Most of diamond-bearing eclogites come from the eclogite lenses enclosed by the marbles which are surrounded, in turn, by the amphibolite facies gneisses and orthogneisses. Based on the analyses of petrography, structure and metamorphism of the massive eclogites, it can be inferred that these eclogites may record p-T conditions for the deep continental subduction, collision and peak UHP metamorphism. The massive eclogites have been equilibrated in the range of > 800 ℃ and 32×108-40×108 Pa. Furthermore, 250-220 Ma is considered the best estimate for the age of the peak UHP metamorphism and deep continental subduction. A comparative study of deformation features in the massive eclogite lenses and the country rocks shows that metamorphism and deformation partitioning played an important role in the preservation of the fresh massive eclogites as record-keepers of the deep subduction within the Dabie-Sulu UHP metamorphic belt. The episodic or progressive metamorphism and deformation partitioning has resulted in lens-shaped domains of low strain separated by anastomosing domains of high strain. The massive eclogites which occur within the domains, at different scales, of low strain are likely to survive a retrogressive metamorphism and deformation resetting, and consequently to retain the near peak metamorphic signature. Therefore, only these eclogites preserve the record of early UHP metamorphic and deformational events. A possible tectonic model is also given for the Dabie-Sulu UHP metamorphic belt, showing the general geometry of the Dabie-Sulu lithosphere at the time of peak metamorphism during the Triassic continental subduction and collision.
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图 1 大别—苏鲁超高压和高压变质带地壳构造柱
SC.沉积盖层; EB.绿帘-蓝片岩; HP.高压变质岩; UHP.超高压变质岩; CC.核部杂岩
Fig. 1. Cartoon showing the crustal pile of UHP and HP metamorphic belts in the Dabie-Sulu region
图 2 大别山综合横剖面
SKC.中朝克拉通; NHY.北淮阳构造带; DM.大别地块; YC.扬子克拉通; CC.核杂岩单位; UHP.超高压单位; HP.高压单位; EB.绿帘-蓝片岩单位; SC.沉积盖层单位; LD.下拆离带; MD.中拆离带; UD.上拆离带; TD.顶拆离带; BMXF.八里畈-磨子潭-晓天断裂带; GMF.龟梅断裂带; J-K.侏罗-白垩系; C2h.中石炭系胡油房组; Pzg.古生界龟山组; Pzn.古生界南湾组; Pz1.下古生界; Ptq.秦岭群
Fig. 2. Synthetic section across the entire Dabie Mountains, illustrating the post-collisional extensional tectonic style, main petrotectonic units and detachment zones
图 3 潜山县张庄榴辉岩体剖面图
1.块状榴辉岩; 2.局部退变质的面理化榴辉岩; 3.含榴绿帘二云斜长片麻岩; 4.面理化花岗岩; a内方框放大示于b, 其中S2和S3分别代表面理化榴辉岩内及初期折返形成的主面理
Fig. 3. A schematic cross-section of eclogite body showing geometry between the massive eclogite and the foliated eclogite located in the Zhangzhuang area, Qianshan
图 4 安徽省潜山县新店大理岩内的块状榴辉岩块体
1.块状榴辉岩; 2.大理岩
Fig. 4. Outcrop sketch illustrating massive eclogite blocks within marble at Xindian, Qianshan County, Anhui Province
图 5 手标本尺度石榴二辉橄榄岩中的块状榴辉岩透镜体
Fig. 5. Sketch at a hand specimen scale showing massive eclogite lenses in the garnet websterit peridotite, Chijiadian, Rongcheng County, Shandong
图 6 样品S2含柯石英包裹体的块状榴辉岩(a) 和样品M-122具多硅白云母变斑晶的块状榴辉岩(b)
Grt.石榴石; Omp.绿辉石; Rt.金红石; Coe.柯石英; Phe.多硅白云母; 正交偏光, 视域宽5 mm
Fig. 6. Massive eclogite with inclusion of coesite in omphacite in sample S2 (a) and massive eclogite with phengite porphyroblasts (Phe) in which contain inclusion of omphacite in sample M-122 (b)
图 7 苏北青龙山超高压榴辉岩相条件下变质和变形分解作用形成的应变图像
a, b.平面图; c, d.剖面图
Fig. 7. Outcrop sketches showing eclogite strain pictures formed by metamorphic and deformation partitioning under UHP eclogite facies conditions, Qing-longshan, northern Jiangsu
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