A cold Early Palaeozoic subduction zone in the North Qilian Mountains, NW China: petrological and U‐Pb geochronological constraints
A cold Early Palaeozoic subduction zone in the North Qilian Mountains, NW China: petrological and U‐Pb geochronological constraints
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DOI：
10.1111/j.1525-1314.2006.00689.x
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发表时间：
2007-03
影响因子：
3.4
通讯作者：
J. Zhang;F. Meng;Y. Wan
中科院分区：
文献类型：
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作者：
J. Zhang;F. Meng;Y. Wan
北祁连高压/低温变质带主要由蓝片岩、榴辉岩和绿片岩相岩石组成。它形成于与洋壳俯冲有关的早古生代增生楔内，被认为是中国保存最完好的高压/低温变质带之一。在这里，我们报告了北祁连山新的含硬柱石的榴辉岩和榴辉岩，这些岩石被包裹在绿帘石蓝片岩中。 5 个样品含有与绿辉石和多硅白云石作为包裹体共存的未蚀变的硬钠石，这表明在另外 11 个榴辉岩和榴辉岩中的石榴石中观察到了榴辉岩相石榴石生长和硬钠石假像。在 420-510 °C 的温度下，由绿钠长石-多硅白云母-石榴石组合估计的峰值压力条件为 2.1-2.4 GPa，位于或接近于绿钠长石榴辉岩的稳定场，这意味着在 6-8 °C km−1 的表观地温梯度下形成，这与冷俯冲带的变质作用一致。对两块含硬钠长石的榴辉岩变碱岩中的锆石进行 SHRIMP U-Pb 定年，得出的年龄分别为 489 ± 7 Ma 和 477 ± 16 Ma。化学发光图像和锆石颗粒中的矿物包裹体表明这些年龄反映了榴辉岩相变质作用。一颗含硬长石假象榴辉岩的火成岩核中记录的锆石年龄为502±16Ma，与北祁连山部分蛇绿岩序列早奥陶世的形成年龄一致，可能与洋壳形成时期有关。岩石学和年代学资料表明北祁连山存在一个寒冷的早古生代俯冲带。
The north Qilian high‐pressure (HP)/low‐temperature (LT) metamorphic belt is composed mainly of blueschists, eclogites and greenschist facies rocks. It formed within an Early Palaeozoic accretionary wedge associated with the subduction of the oceanic crust and is considered to be one of the best preserved HP/LT metamorphic belts in China. Here we report new lawsonite‐bearing eclogites and eclogitic rocks enclosed within epidote blueschists in the North Qilian Mountains. Five samples contain unaltered lawsonite coexisting with omphacite and phengite as inclusions in garnet, indicating eclogite facies garnet growth and lawsonite pseudomorphs were observed in garnet from an additional 11 eclogites and eclogitic rocks. Peak pressure conditions estimated from lawsonite omphacite‐phengite‐garnet assemblages were 2.1–2.4 GPa at temperatures of 420–510 °C, in or near the stability field of lawsonite eclogite, and implying formation under an apparent geothermal gradient of 6–8 °C km−1, consistent with metamorphism in a cold subduction zone. SHRIMP U‐Pb dating of zircon from two lawsonite‐bearing eclogitic metabasites yields ages of 489 ± 7 Ma and 477 ± 16 Ma, respectively. CL images and mineral inclusions in zircon grains indicate that these ages reflect an eclogite facies metamorphism. An age of 502 ± 16 Ma is recorded in igneous cores of zircon grains from one lawsonite pseudomorph‐bearing eclogite, which is in agreement with the formation age of Early Ordovician for some ophiolite sequences in the North Qilian Mountains, and may be associated with a period of oceanic crust formation. The petrological and chronological data demonstrate the existence of a cold Early Palaeozoic subduction zone in the North Qilian Mountains.