Eclogite from the Subducted Lower Crust of the Yangtze Plate within the Luotian Dome and its Geological Implication
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摘要: 在“罗田穹隆”中发现了下地壳俯冲成因榴辉岩.榴辉岩呈透镜状或板状产于含石榴子石条带状片麻岩中.新鲜的榴辉岩主要由石榴子石、绿辉石、金红石等组成, 含少量退变质的角闪石、斜长石、紫苏辉石、透辉石、(钛) 磁铁矿和石英等.研究区榴辉岩以保留早期麻粒岩相变质矿物残留体以及经受晚期麻粒岩相和角闪岩相退变为特征, 指示它们由扬子镁铁质下地壳麻粒岩相岩石俯冲到深部发生变质并形成榴辉岩, 然后折返至下地壳发生麻粒岩相退变, 由于麻粒岩相退变质阶段仅以后成合晶形式出现, 因而它们可能在下地壳停留时间不长, 就又进一步被构造抬升至中上地壳而发生角闪岩相退变.大别山造山带乃至扬子板块北缘现今缺乏厚层镁铁质下地壳, 它们也很少出露地表, 推测这些俯冲的镁铁质下地壳可能已拆离再循环进入地幔.从而为“罗田穹隆”的形成和演化以及大别山高压-超高压岩石的形成与折返机制等研究提供了关键性的岩石学证据Abstract: The eclogite produced from the subducted mafic lower crust of the Yangtze continental plate is a new finding within the Luotian dome in northern Dabie Mountains. The eclogites occur as lens in garnet-bearing banded gneisses. The mineral assemblage of the eclogite is garnet, omphacite, rutile and quartz; secondary minerals are hornblende, plagioclase, hypersthene, diopside and ilmenite. The eclogites from the Luotian dome have typical characteristics with relics of earlier hypersthene and plagioclase minerals of granulite-facies and latter overprinting of granulite-facies. No aqueous mineral formed during peak metamorphism may be related to their protoliths, which come from the lower crust. Petrologic evidence suggests that the eclogites studied were produced from the mafic lower crust of the Yangtze continent and suffered from granulite- and amphibolite-facies retrogressive metamorphism. At present, mafic rocks from the lower crust in the Dabie Mountains or even in the northern Yangtze plate have been scarce, which may be delaminated and recycled into underlying convecting mantle. These provide key petrologic evidence for formation and evolution of the Luotian dome, and formation and exhumation processes and uplift of high-grade metamorphic rocks such as eclogite in the northern Dabie Mountains.
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Key words:
- eclogite /
- granulite-facies /
- lower crust /
- Luotian dome /
- northern Dabie Mountains
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图 1 大别山造山带地质简图(据徐树桐等, 2002修改)
Ⅰ.变质复理石; Ⅱ-1.北部杂岩带; Ⅱ-2.苏家河变质带; Ⅲ.南部超高压变质带; Ⅳ.“冷”榴辉岩带; Ⅴ.大别杂岩; Ⅵ.宿松变质带
Fig. 1. Schematic geological map of the Dabie Mountains orogen
图 2 “罗田穹隆”中榴辉岩的显微照片
a.石榴子石中绿辉石包体及其两期退变后成合晶, 样品号LT10;b.石榴子石中绿辉石、金红石和石英包体, 样品号LT8-1; c.含针状石英出溶体的钠质透辉石有角闪石退变边, 样品号LT8-1;d.含针状石英出溶体的钠质透辉石有紫苏辉石退变边, 样品号LT8-1; e.石榴子石中早期紫苏辉石+斜长石矿物包体组合以及麻粒岩相(Hy2+Pl+Di) 和角闪岩相(Hb+Pl) 退变质矿物组合, 样品号LT3;f.含紫苏辉石+斜长石包体的锆石的背散射图像, 样品号LT10;Gt.石榴子石; Omp.绿辉石; Ru.金红石; Zir.锆石; Pl.斜长石; Qtz.石英; Hy1.早期紫苏辉石; Hy2.晚期紫苏辉石; Sy1.由晚期紫苏辉石+透辉石+斜长石组成的麻粒岩相退变后成合晶; Sy2.由角闪石+斜长石+磁铁矿组成的角闪岩相后成合晶.除e照片的视域宽度为0.2 mm外, 其余照片的视域宽度均为3 mm.
Fig. 2. Micro photographs of the eclogites from the Luotian dome in the Dabie Mountains
表 1 “罗田穹隆”中榴辉岩代表性矿物的电子探针分析
Table 1. Electro-probe analyses of major minerals from the eclogites in the Luotian dome
表 2 榴辉岩锆石中代表性矿物包体及一个具成分环带石榴子石的电子探针分析
Table 2. Electro-probe analyses of major mineral inclusions in zircon and a zoned garnet from the eclogites in the Luotian dome
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