An analysis of the characteristics of the rock-structure association of the Dunhuang orogenic belt and their implications for tectonic setting
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摘要:
敦煌地区位于北山造山带和祁连造山带之间，对其大地构造属性长期以来多存争议。对敦煌地区三危山—红柳峡一带的岩石-构造组合进行了研究，包括三危山—红柳峡北部地区和红柳峡南部地区2个构造单元。三危山—红柳峡北部地区，表现为变形强烈的变沉积岩以基质形式包裹不同类型、不同年龄的岩块和不同岩性的岩片相互堆叠，整体显示混杂带“blocksin-matrix”的岩石-构造组合特征。红柳峡南部地区，岩石的变质程度向南逐渐降低，北侧主要为云母石英片岩和长英质片麻岩组合，南端出露晚石炭世含煤岩系与碳酸盐岩沉积组合，可能形成于陆缘滨海环境，且被斜长角闪岩推覆体所覆盖；该区由北向南构造形式依次表现为倾向北的强剪切带→双冲构造→逆冲叠瓦扇，且变形强度呈逐渐减弱趋势，表现出前陆褶皱冲断带的岩石-构造组合特征。据此认为，敦煌地区自北向南存在三危山-红柳峡北部混杂带和红柳峡南部前陆褶皱冲断带两个大地构造单元，碰撞作用应发生于晚石炭世之后。
Abstract:Located between the Beishan orogenic belt in the north and the Qilian orogenic belt in the south, the Dunhuang region possesses a crucial tectonic position. However, the geotectonic attribution of this region has long been controversial. In this paper, the authors investigated the characteristics of the rock-structure association of the Sanweishan-Hongliuxia area in the Dunhuang region, which includes the Sanweishan-Northern Hongliuxia Area (SNHA) and the Southern Hongliuxia Area (SHA). The observation in the SNHA shows that the intensively sheared metasedimentary rocks are well exposed as matrix which enclose the blocks with different lithological types and ages, and the structural slices with different lithological types are tectonically superimposed upon each other. This rock-structure association with the"blocks-in-matrix"fabric integrally shows the typical characteristics of mélange. In the SHA, the metamorphic grade of the exposed rocks decreases southward, and the northern part of the SHA is dominated by the micaquartz schist and felsic gneiss association, while the Late Carboniferous coal-bearing strata and carbonate succession regarded as being formed in a continental margin setting are exposed in the southern part of the SHA, which are overthrusted by the plagioclase amphibolite nappe. In the SHA, the structural forms from north to south are from the north dipping intense shear zone through the duplex to the imbricate thrust faults, accompanied by the southward decreasing deformation intensity. The characteristics of the rock-structure association of the SHA are consistent with those of the foreland fold-and-thrust belt. The authors thus hold that there exist two tectonic facies units, i.e., the Sanweishan-northern Hongliuxia mélange in the north and the Hongliuxia foreland fold-andthrust belt in the south of the Dunhuang region. The collision process within the Dunhuang orogenic belt should occur after the Late Carboniferous period.
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图 图版Ⅰ a.云母石英片岩包裹斜长角闪片麻岩透镜体；b.大理岩与基性麻粒岩混杂堆叠；c.斜长角闪岩包裹片麻状斜长花岗岩岩块；d.绿帘石化辉长岩透镜体产于花岗片麻岩中；e.长英质片麻岩包裹石榴角闪岩透镜体；f.斜长角闪岩、花岗片麻岩、大理岩构造堆叠；g.大理岩透镜体产于强烈变形的长英质片麻岩中；h.花岗片麻岩呈透镜状产于云母石英片岩中；i.变硅质岩包裹斜长角闪岩岩块
图 图版Ⅱ a.斜长角闪片麻岩；b.石墨大理岩；c.片麻状斜长花岗岩；d.绿帘石化辉长岩；e.黑云斜长片麻岩；f.长英质片麻岩；g.云母石英片岩；h.变质杂砂岩。显微照片均为正交偏光。Hbl—角闪石；Pl—斜长石；Ep—绿帘石；Ab—钠长石；Grp—石墨；Cal—方解石；Qtz—石英；Ms—白云母；Bt—黑云母
图 图版Ⅲ a.大理岩碎块形成不对称旋转碎斑系（干洪沟）；b.剪切带内大理岩块形成“布丁”构造（干洪沟）；c.长英质片麻岩和泥质片麻岩的早期片麻理形成褶皱 (蘑菇台)；d.剪切带内大理岩片形成小型背形堆垛双重逆冲构造 (antiformal stack duplex)（红柳峡北部）；e、f.大理岩和云母石英片岩构造接触，形成向SW推覆的小型逆冲断层（红柳峡北部）；g、h.斜长角闪岩、泥质片岩和花岗岩片麻岩形成的逆冲断层（大安朋沟）
图 图版Ⅳ a.长英质片麻岩和云母石英片岩构造接触；b.含煤岩系与灰岩沉积组合；c.长英质片麻岩（糜棱岩化）；d.长英质粉砂岩；e.灰岩；f.浅变质泥岩。Pl—斜长石；Qtz—石英；Cal—方解石；Ms—白云母
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