Metamorphism and Oceanic Crust ExhumationConstrained by the Jilang Eclogite and Meta-Quartzite from the Sumdo (U)HP Metamorphic Belt
Metamorphism and Oceanic Crust ExhumationConstrained by the Jilang Eclogite and Meta-Quartzite from the Sumdo (U)HP Metamorphic Belt
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森多(U)HP变质带吉郎榴辉岩和变石英岩制约的变质作用和洋壳折返
DOI：
10.1007/s12583-019-0894-9
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发表时间：
2019
影响因子：
3.3
通讯作者：
Yang Jingsui
中科院分区：
文献类型：
--
作者：
Li Yang;Zhang Cong;Liu Xiaoyu;Shen Tingting;Qiu Tian;Yang Jingsui
藏南桑多榴辉岩高压变质带横贯拉萨造山带中段100多km，在拉萨造山带南北两侧形成了二叠纪-三叠纪大洋俯冲带，重新解释了拉萨造山带的构造演化。前人的研究表明，榴辉岩的温度和压力条件在四个区域有显著差异，桑多、新大多、白朗、吉朗地区。研究松岛带榴辉岩峰期变质的P-T条件和路径，对揭示拉萨盆地古特提斯洋俯冲折返机制具有重要意义。本文以松岛带吉浪地区榴辉岩为例，研究了其变质演化过程。榴辉岩的矿物组合为石榴子石、绿辉石、多硅白云母、角闪石、绿帘石、石英和少量黑云母。石榴石有一个“脏”的核心，有丰富的包裹体，如绿帘石，角闪石，斜长石和一个“清晰”的边缘，有几个包体绿辉石和多硅白云母。从核部到边缘，石榴子石中镁铝榴石含量逐渐增加，钙铝榴石含量逐渐减少，呈现出典型的生长分带。石榴子石的边缘被斜长石+闪长岩的冕状包裹，在退变质过程中表现为角闪岩相的叠加。变质作用可分为三个阶段：（1）变质阶段，以石榴石核及其矿物包裹体为代表;（2）变质高峰阶段，以石榴石边、绿辉石、硬硅钙石、多硅白云母和石英为代表;（3）变质退变质阶段，以硬硅钙石分解为绿辉石、绿辉石合晶和冕边石榴石为特征。用石榴石中钙铝榴石和镁铝榴石含量等值线描绘的AP-T假剖面，将近峰P-T条件限定在2.85GPa，575 °C。吉朗榴辉岩在折返过程中表现为近等温减压的顺时针P-T轨迹。结合变质石英岩碎屑锆石的年龄峰值分别为583、911和1134 Ma，推测吉朗榴辉岩存在俯冲通道折返后浅部折返的陆缘物质，并可能进一步指示松岛榴辉岩带的俯冲方向为自北向南。
The Sumdo eclogite-bearing (U)HP metamorphic belt extends over 100 km across the middle part of the Lhasa terrane in southern Tibet, which forms a Permian-Triassic oceanic subduction zone between the south and the north Lhasa sub-terranes, leading to the reinterpretation of the tectonic evolution of the Lhasa terrane in the Tibetan-Himalayan orogeny. Previous studies show that there are significant differences in temperature and pressure conditions of the eclogites in four areas, e.g., Sumdo, Xindaduo, Bailang and Jilang areas. Studying the peak metamorphicP-Tconditions and path of eclogite in the Sumdo belt is of great significance to reveal the subduction and exhumation mechanism of Paleo-Tethys Ocean in the Lhasa terrane. In this contribution, eclogite in the Jilang area of the Sumdo belt is chosen as an example to study its metamorphic evolution. The mineral assemblage of the eclogite is garnet, omphacite, phengite, hornblende, epidote, quartz and minor biotite. Garnet has a “dirty” core with abundant inclusions such as epidote, amphibole, plagioclase and a “clear” rim with few inclusions of omphacite and phengite. From the core to the rim, pyrope content in garnet increases while grossular content decreases, showing typical growth zoning. The rim of garnet is wrapped by the pargasite+plagioclase corona, showing amphibolite facies overprint during retrogression. Three stages of metamorphism are inferred as (1) prograde stage, represented by the core of garnet and mineral inclusions therein; (2) peak stage, represented by the garnet rim, omphacite, lawsonite, phengite, and quartz; (3) retrograde stage characterized by decomposition of lawsonite to zoisite, followed by symplectite of omphacite and corona rimmed garnet. AP-Tpseudosection contoured with isopleths of grossular and pyrope contents in garnet is used to constrain the near peakP-Tcondition at 2.85 GPa, 575 °C. In general, the Jilang eclogite shows a clockwiseP-Tpath with a near isothermal decompression process during exhumation. Combined with the age peaks of 583, 911, and 1 134 Ma from the detrital zircons of the country metaquartzite, a continental margin material involving exhumation process at shallow depth after the subduction channel exhumation is inferred for the Jilang eclogite and may further indicate that the subduction direction of the Sumdo eclogite belt is from north to south.