The coherent ultrahigh-pressure terrane of the Tianshan meta - ophiolite belt, NW China
The coherent ultrahigh-pressure terrane of the Tianshan meta - ophiolite belt, NW China
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DOI：
10.1016/j.lithos.2018.06.004
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发表时间：
2018-08
期刊：
影响因子：
3.5
通讯作者：
Z. Lü;K. Bucher
中科院分区：
文献类型：
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作者：
Z. Lü;K. Bucher
在高压和超高压(HP-UHP)下变质的基性岩块通常嵌在变质沉积岩或花岗岩中，这些岩石没有显示出HP变质的证据。这些孤立的岩石碎片(通常是榴辉岩)与周围岩石达到的最大深度形成鲜明对比，这通常被视为一种构造混杂作用的证据。在中国西北部的天山变质蛇绿岩带中，一些与柯石英相邻的榴辉岩和泥质长英质片岩显然缺乏超高压条件的证据。它们推算出的最大压力为1.52.5 Gpa。这些高压块体被认为是被肢解的蛇绿岩碎片，每一块蛇绿岩块都有自己的变质历史，是由俯冲通道中的强迫返回流产生的。在这里，我们重新评估了以前由各种P-T工具得出的这些已报道的高压岩石的P-T估计，包括泥质片岩、镁铁质榴辉岩和火山碎屑成因的榴辉岩。用Na2O-CaO-K2O-FeO-MgO-Al_2O_3-SiO_2-H_2O系(热解3.33版)等值线体系中的P-T假截面来推算峰值P-T条件。研究发现，所有样品都经历了超高压变质作用(2.8~3.5 GPa5 2 0~5 80 °C)。用于这些P-T估计的热力学数据集也被证明对世界上典型的UHP-LT岩石是有效的。我们认为，由石榴石等温压计得出的P-T估计与柯石英等超高压矿物学证据一致，但对石榴石溶液模型和有效块体成分敏感。涉及钠基单斜辉石和多硅白云母的不一致的P-T结果和相关工具在许多情况下(如果不是全部的话)应予以排除。重新评估的P-T数据的结果是，榴辉岩(包括层间和块状榴辉岩)和相关的寄主岩石组成了一个连贯的天山超高压地体，而不是一个高度破碎的混杂岩。
Blocks of basic rocks metamorphosed at high- and ultrahigh-pressures (HP-UHP) are often embedded in metasedimentary or granitic rocks that do not show evidence for HP metamorphism. The contrasting maximum depth reached by these isolated rock fragments, normally eclogites, and the enveloping rocks is commonly seen as evidence for a form of tectonic mélange. In the Tianshan meta-ophiolite belt of NW China, some reported eclogites and pelitic-felsic schists adjacent to coesite-bearing rocks apparently lack evidence of UHP conditions. Their deduced maximum pressures are 1.5–2.5 GPa. These HP blocks have been regarded as dismembered ophiolite fragments each with its own metamorphic history generated by a forced return flow in a subduction channel. Here we reappraise the P-T estimates, formerly derived by various P-T tools, of these reported HP rocks, including pelitic schists, mafic eclogites and eclogitic rocks of volcaniclastic origin, from the Akeyazi area. P-T pseudosections in the system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O contoured for garnet core or mantle composition (thermocalc version 3.33) have been used for deducing peak P-T conditions. It was found that all investigated samples were subject to UHP metamorphism (2.8–3.5 GPa at 520–580 °C). The used thermodynamic dataset for these P-T estimates has also been testified valid for typical UHP-LT rocks worldwide. We suggest that garnet isopleth thermobarometry-derived P-T estimates are potentially robust in accordance with UHP mineralogical evidence such as coesite, but are sensitive to garnet solution models and effective bulk compositions. Inconsistent P-T results and related tools involving sodic clinopyroxene and phengite should be dismissed in many cases, if not all. The consequences of the reevaluated P-T data are that eclogites, both interlayered and blocky, and the associated host rocks comprise a coherent Tianshan UHP terrane, not a highly disrupted mélange.