| 西南天山超高压变质带的两类石榴角闪岩 |
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| 引用本文： | 吕增,王凯.西南天山超高压变质带的两类石榴角闪岩[J].地球科学,2018,43(1):150-163. |
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| 作者姓名： | 吕增 王凯 |
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| 作者单位： | 北京大学地球与空间科学学院, 造山带与地壳演化教育部重点实验室, 北京 100871 |
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| 基金项目： | 国家自然科学基金项目41330210国家重点基础研究发展计划（973计划）2015CB856105国家自然科学基金项目41372004 |
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| 摘 要： | 角闪岩是西南天山超高压变质带变基性岩的常见岩石类型之一.野外关系和矿物反应结构表明，大多数角闪岩是由榴辉岩或蓝片岩受到不同程度的钠长绿帘角闪岩相退变质叠加形成的.但对于一些平衡结构发育良好且孤立产出的角闪岩类型（如石榴角闪岩）仍缺乏系统的岩石学研究.本次从岩相学、矿物成分以及热力学模拟几个方面对哈布腾苏河下游地区超高压带内不含钠长石的石榴角闪岩开展了详细的工作.这些石榴角闪岩的主要矿物为绿色角闪石（钙质-钠钙质闪石）、帘石（黝帘石-绿帘石）和石榴石，三者总体积占80%~90%，明显有别于大多数由榴辉岩退变而成的含有钠长石变斑晶的石榴角闪岩.虽然这些角闪岩化学成分十分相近，都具有富钙贫钠和高的Mg/（Mg+Fe）比值，但在结构、构造和矿物组成等方面存在显著差异，据此将它们划分为两类.第一类角闪岩基质中不含石英，保存在变斑晶中的少量残余矿物组合为石榴石+绿辉石+硬柱石+蓝闪石+金红石，指示峰期硬柱石榴辉岩相变质条件，富钛矿物全部为金红石.第二类角闪岩强烈面理化，面理由绿色角闪石、绿帘石和绿泥石以及条带状石英集合体构成.石榴石粒度呈双峰式分布，粗粒比细粒低钙低锰.基质和包体中均未发现高压变质特征矿物绿辉石和蓝闪石.富钛矿物以榍石为主，金红石和钛铁矿仅存在于个别石榴石中.两类角闪岩的石榴石成分具有较大区分度，前者的钙含量较高而镁含量较低.P-T视剖面计算显示它们的峰期条件为480~520 ℃，30~33 kbar，均达到超高压范围，与哈布腾苏河下游及以西地区的榴辉岩相似，表明西南天山超高压变基性岩构成沿中天山南缘断裂延伸数十千米的独立地质单元，不存在所谓的俯冲隧道混杂现象.
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| 关 键 词： | 西南天山造山带 超高压变质地体 石榴角闪岩 相平衡模拟 变质演化 岩石学 |
| 收稿时间： | 2017-07-10 |
Metamorphic Evolution of Two Types of Garnet Amphibolites from UHP Terrane of Southwestern Tianshan,NW China |
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| Abstract: | Amphibolite is one of the common metabasic rock types exposed in the the UHP metamorphic belt of southwestern Tianshan. It is found in the field relationship and mineral reaction textures that most of such amphibolites are retrograded after eclogites or blueschists to various extents at albite-epidote amphibolite facies conditions. However, petrological studies are not systematic and sufficient for those with equilibrated textures from homogeneous blocks. In this paper, it is presented a detailed study of petrography, mineral chemistry and thermodynamic modelling on some albite-free garnet amphibolites in the UHP unit at the lower part of the Habutengsu valley. Results show that they largely consist of green amphibole (Ca-or Na-Ca-amphibole), epidote (or zoisite) and garnet (80% to 90% total volume), substantially different from the equivalents that retrograded from UHP eclogites contain albite porphyroblast. Though these garnet amphibolites are similar in bulk compositions, with high CaO and low Na2O and high Mg content, based on contrasting differences in terms of texture, structure and mineral assemblage, two types are divided herein. Type Ⅰ garnet amphibolite is quartz-free and relict or inclusion assemblage of Grt+Omp+Lws+Gln+Rt indicating peak lawsonite eclogite facies conditions. Rutile is the only Ti-rich phase as inclusion or in the matrix. Type Ⅱ garnet amphibolite is strongly foliated shown by oriented green amphibole, epidote, chlorite and banded quartz aggregates. Garnet is characterized by bimodal grain size and the larger ones have lower Ca and Mn. No HP indicator such as omphacite or sodic amphibole has been detected. Ti-rich phase is dominated by titanite and rare rutile and ilmenite are only found in some garnets. Generally, garnet compositions are distinguishable between the two types of amphibolites, Type Ⅰ with higher Ca but lower Mg. P-T pseudosection calculations show that peak metamorphic conditions of both types are 480 to 520℃, 30 to 33 kbar, in the UHP field akin to other eclogites in the lower reach of the Habutengsu River and adjacent areas to the west. This indicates a coherent UHP unit extending tens of kilometers along the southern Central Tianshan fault, rather than tectonic mélange from a fossil subduction channel. |
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