Mg-Carpholite Metapelite and Its Implications for Ancient Oceanic Subduction in the North Qilian Suture Zone, NW China

Abstract

Abstract: A typical Early Paleozoic oceanic suture zone, which consists of ophiolite, arc volcanic rocks and high-pressure low-temperature metamorphic rocks, occurs in the North Qilian Mountains, Northwestern China. Eclogite facies metapelites with index mineral assemblage of Mg-carpholite + garnet + chloritoid + phengite were studied in details in petrology and mineralogy. Using THERMORCAL (V3.23), the authors calculated the P-T conditions of the stability field for Carpholite (cr) + garnet (g) + chloritoid (ct) + phengite (ph) assemblage at P=2.2-2.5GPa and T=530-580℃. Based on the calculated stability fields of carpholite and chloritoid, the authors confirmed that the North Qilian suture zone was a “cold" oceanic subduction zone in Early Paleozoic time with geothermal gradients of 6.5-7.5℃/km, similar to Mesozoic-Cenozoic subduction zones. This study plays an important role in understanding the thermo-structure of the Early Paleozoic subduction zones and dehydration of HP metamorphic pelitic rocks.

Key words: Mg-carpholite, HP metamorphism, P-T conditions, North Qilian Moutains

摘要： 北祁连山发育有缝合带是一条典型的早古生代大洋型俯冲带。针对该缝合带中首次发现的含镁纤柱石高压泥质片岩进行了详细的岩石学和矿物学研究。根据平衡矿物组合，利用THERMORCAL(V3.23)计算出镁纤柱石+石榴石+硬绿泥石+多硅白云母组合稳定的温压条件为P=2.2～2.5GPa，T=530～580℃。利用纤柱石和硬绿泥石的稳定范围，确定了北祁连山早古生代大洋冷俯冲的地热梯度的范围为6.5～7.5℃/km。该研究结果对于了解早古生代时期大洋俯冲带的热结构及其高压变质岩石的脱水过程有重要意义。

关键词: 镁纤柱石, 高压变质, P-T条件, 北祁连山

CLC Number:

P581

YU Xiaoning,SONG Shuguang,WEI Chunjing,ZHANG Lifei. Mg-Carpholite Metapelite and Its Implications for Ancient Oceanic Subduction in the North Qilian Suture Zone, NW China[J]. Acta Scientiarum Naturalium Universitatis Pekinensis.

于孝宁,宋述光,魏春景,张立飞. 北祁连山含镁纤柱石高压泥质岩及其对古大洋俯冲的意义[J]. 北京大学学报（自然科学版）.

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Mg-Carpholite Metapelite and Its Implications for Ancient Oceanic Subduction in the North Qilian Suture Zone, NW China

北祁连山含镁纤柱石高压泥质岩及其对古大洋俯冲的意义

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