| 拉脊山蛇绿混杂带结构组成、形成时代与形成过程 |
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| 关键词:Lajishan mélange SSZ ophiolite active continental margin tectonic evolution |
| 基金项目:中国地质调查局项目(编号: 1212011120159; 121201140880001; 12120115069401; 12120115027001; 121201102000150020; DD20160022);国家自然科学基金项目(编号: 41272221; 41372207) |
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| The Composition, Age and Tectonic Setting of Lajishan Ophiolitc Mélange |
| The Qilian orogenic belt formed as a result of closure of the Proto-Tethyan Ocean and amalgamation of the Alashan and Qaidam continental blocks along the NE margin of Tibet. It is a classic accretionary orogen in that it consists of various accreted terranes, such as ophiolites, seamounts and island arcs. A dis-tinctive structural feature is the multitude of Protero-zoic microcontinental blocks and slivers, that appear to separate the numerous ophiolite/arc complexes. Ophiolitic mélanges are widely distributed within both the North and South Qilian blocks. In the South Qilian block, Cambrian basalt, andesite, gabbro, ul-tramafic cumulate rocks, chert, limestone, sandstone and rare mantle peridotites are well-exposed and dis-play characteristic features of an ophiolitic mélange. The Lajishan ophiolitic mélange is the largest single zone within the belt. Therefore, its origin and tectonic setting are of a great importance for understanding the tectonic evolution of the orogen. Due to structural complexity, lack of systematic field and petrologic studies, exact lithological assemblages, isotopic ages and tectonic settings remain unclear making regional correlations difficult. For instance, vol-cano-sedimentary units in the Lajishan are assigned to the Cambrian based on trilobite faunas from lime-stones. However, isotopic ages for other lithologies and provenance data are lacking, and whether the limestones are autochthonous or older allochthonous blocks slumped into younger sequences remains to be determined. Geochemical data indicate that the Cam-brian-aged basalts are mainly transitional between MORB and WPB or OIB with some exhibiting is-land-arc affinity. Therefore, several models such as a continental rift, back-arc basin, multi-staged uplifting structure window and a subduction-accretionary com-plex have been proposed. These discrepancies sig-nificantly affect the Cambrian tectonic evolution of the Qilian orogenic belt and the Proto-Tethys Ocean. The fundamental reason lies in lack of combing the ophiolite and island arc as parts of trench-arc-basin systems. |
| FU Chang-lei,YAN Zhen.2017.The Composition, Age and Tectonic Setting of Lajishan Ophiolitc Mélange[J].Acta Geoscientica Sinica,38(s1):29-32. |
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