西南天山造山带超高压变质作用研究新进展

New progress in the study of UHP metamorphism of China's southwestern Tianshan orogenic belt

低温超高压变质岩具有极低的地热梯度,其变质演化对于认识板块间相互作用的动力学过程以及弧地壳的生长机制具有重要意义。西南天山造山带发育了世界上少有的经历过深俯冲作用且具有洋壳属性的典型低温超高压变质地质体。近几年来对该造山带中的超高压变质岩开展了大量深入细致的岩石学研究工作,取得了一系列新进展。变基性岩和变沉积岩系岩石中柯石英的普遍发现,直接证明西南天山变质蛇绿混杂岩曾经俯冲到上地幔深度,且具有极低的地热梯度,与热力学模拟结果一致。柯石英的稀少以及大量不同类型柯石英假像的存在,说明在折返过程中发生了强烈的退变质叠加,只有刚性较大且没有经历碎裂-愈合作用的矿物(如石榴石)才有可能保存柯石英。综合岩相学证据和相平衡计算结果,确定西南天山造山带北部的高压地质体(即哈布腾苏-科布尔特单元)整体经历过超高压变质作用,南部的高压地质体峰期压力未达到柯石英稳定域。超高压和高压变质地体的空间分布特点指示了古天山洋由南向北的俯冲极性。这些基础岩石学研究工作的开展对于揭示冷俯冲带的深部物理化学过程以及建立中亚南天山造山带演化的精细模型具有重要意义。

Though geophysical observation and numerical modeling suggest that the oceanic crust have been subducted along convergent continental margins into the deep Earth, the exhumed UHP metamorphic terranes of oceanic affinity are rarely exposed. The UHP metamorphic belt of China＇ s southwestern Tianshan was inherited from the ophiolitic complex as a result of the northward subduction of the paieo-Tianshan ocean beneath the Yili-Central Tianshan plate. With the in-depth investigations of many aspects in recent years, the UHP metamor- phism of China＇s southwestern Tianshan has been better understood. In this paper, the authors focused the discussion on the progress in petrological studies of UHP rocks from this orogenic belt. The ubiquitous discovery of coesite in metabasaltic and metasedimentary rocks indicates that the metaophiolitic belt of China＇s southwestern Tianshan have reached the mantle depths with extremely low goetherms, which is consistent with thermodynamic forward modeling. The rarity of coesite and the development of various eoesite pseudomorphs suggest the intensive retrograde overprint at low temperatures. Only in the unfractured domain of porphyroblastic garnet could coesite be preserved. On the basis of detailed petrographical observation and phase diagram model, the UHP metamorphism in the north of the metamorphic belt of China＇s southwestern Tianshan is confirmed and a coherent UI-IP unit （referred to as Kebuerte-Habutengsu UHP unit） is proposed. To the south of this unit, no UHP evidence has been identified in eelogite or peliticfelsic schist. The imbricate sequence relation between HP and UHP units points to the northward subduction polarity of the paleo-Tianshan ocean. The progress in the study of petrology is fundamental for further unraveling the physiochemical processes of cold oceanic crust subdueted to great depths and for constructing a more reasonable tectonic model of the South Tianshan orogenic belt in Central Asia.