Mineral textural evolution and PT-path of relict eclogite-facies rocks in the Paleoproterozoic Nagssugtoqidian Orogen, South-East Greenland
Mineral textural evolution and PT-path of relict eclogite-facies rocks in the Paleoproterozoic Nagssugtoqidian Orogen, South-East Greenland
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DOI：
10.1016/j.lithos.2017.11.008
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发表时间：
2018
期刊：
影响因子：
3.5
通讯作者：
Sascha Müller;A. Dziggel;J. Kolb;S. Sindern
中科院分区：
文献类型：
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作者：
Sascha Müller;A. Dziggel;J. Kolb;S. Sindern
格林兰东南部的那萨木托起甸造山带是一个深侵蚀的古元古代碰撞造山带。它由各种太古宙和古元古代岩石组成，以TTG片麻岩为主，并由各种上地壳岩石和基性岩脉组成。本研究旨在通过对纳苏木-祁甸造山带库穆特地体中出露的石榴石辉石岩、后退榴辉岩和角闪岩相岩石的变质演化研究，为该造山带的地球动力学过程和俯冲深度提供新的认识。石榴石辉石岩矿物组合以石榴石、正辉石、斜辉石和角闪石为主，石榴石角闪石片岩和石榴石蓝闪石片岩分别由石榴石、角闪石、斜长石和石英以及石榴石、蓝晶石、黑云母和石英组成。在基性变质火山岩和古元古代不和谐基性岩脉中，经常发现榴辉岩相矿物组合的残余物和伪晶。在后退的榴辉岩中，后退的矿物反应在完成之前停止，导致两个结构域的形成。斜辉石结构域由透辉-斜长石复长石组成，它们被解释为是以辉长石为代价生长的。正长石被角闪石和斜长石所包围，部分被角闪石和斜长石所取代。红辉石（XJd25-42）保存在一个富钠样品中，它出现在大斜辉石的核心中，并作为石榴石和角闪石的包裹体。在石榴石域，石榴石被斜长石的内冕和角闪洞+/−正辉石和斜辉石的外冕所替代。这两个区域的退退程度以及退退组合的类型似乎取决于流体活动。大的石榴石颗粒保留了富钙岩心，说明其起源是进阶的，而富镁石榴石边缘形成于榴辉岩相变质和后来的再平衡过程中。拟剖面模拟结合常规地温测量结果显示，pt演化过程为顺时针方向，包括17-19 kbar和740-810°C的榴辉岩相条件，随后是近等温减压到中压麻粒岩相条件（13.8-15.4 kbar, 760-880°C），随后是减压并轻微冷却到高压角闪岩相等级（8.8-10.9 kbar, 660-840°C）。这些数据表明，库姆努特地体的岩石在中下地壳70 ~ 30公里处被挖掘出来。pt路径表明，挖掘最初是快速的，受构造控制。
The Nagssugtoqidian Orogen in South-East Greenland is a deeply eroded, Paleoproterozoic collision orogen. It consists of a variety of Archean and Paleoproterozoic rocks, most notably TTG gneiss, a variety of supracrustal rocks and basic dykes. This study aims at providing new insight into the geodynamic processes and subduction depth of this orogen by investigating the metamorphic evolution of garnet pyroxenite, retrogressed eclogite and amphibolite-facies rocks that are exposed within the Kuummiut Terrane of the Nagssugtoqidian Orogen. The garnet-pyroxenite has a dominant mineral assemblage of garnet, orthopyroxene, clinopyroxene and hornblende, while garnet-amphibolite and garnet-kyanite schist are made up of garnet, hornblende, plagioclase and quartz, and garnet, kyanite, biotite and quartz, respectively. Relicts of, and pseudomorphs after, eclogite-facies mineral assemblages are frequently found within basic metavolcanic rocks and Paleoproterozoic discordant basic dykes. In the retrogressed eclogite, the retrograde mineral reactions ceased prior to completion, resulting in the formation of two domains. A clinopyroxene domain consists of diopside-plagioclase symplectites, which are interpreted to have grown at the expense of omphacite. The symplectites are surrounded and partly replaced by hornblende and plagioclase. Omphacite (XJd25–42) is preserved in a Na-rich sample, where it occurs in the core of large clinopyroxene and as inclusion in garnet and hornblende. In a garnet domain, garnet is variably replaced by an inner corona of plagioclase and an outer corona of amphibole +/− orthopyroxene and clinopyroxene. The degree of retrogression as well as the type of the retrograde assemblage in both domains appears to be dependent on fluid activity. Large garnet grains preserve Ca-rich cores, interpreted as prograde in origin, while Mg-rich garnet rims formed during eclogite-facies metamorphism and later re-equilibration. Pseudosection modelling combined with conventional geothermobarometry reveals a clockwise PT-evolution, involving eclogite-facies conditions of 17–19 kbar and 740–810 °C, followed by near-isothermal decompression to medium-pressure granulite-facies conditions (13.8–15.4 kbar, 760–880 °C) and subsequent decompression with minor cooling to high-pressure amphibolite-facies grades (8.8–10.9 kbar, 660–840 °C). These data show that rocks of the Kuummiut Terrane were exhumed from 70 to about 30 km into the mid- and lower crust. The PT-path implies that exhumation initially was rapid and tectonically-controlled.