Growth of metamorphic and peritectic garnets in ultrahigh-pressure metagranite during continental subduction and exhumation in the Dabie orogen
Growth of metamorphic and peritectic garnets in ultrahigh-pressure metagranite during continental subduction and exhumation in the Dabie orogen
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DOI：
10.1016/j.lithos.2016.08.043
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发表时间：
2016-12
期刊：
影响因子：
3.5
通讯作者：
Zhaochu Hu
中科院分区：
文献类型：
--
作者：
Yong-Fei Zheng;James Ashton Van Orman;Haijun Xu;Zhaochu Hu
通过对大别造山带超高压变质花岗岩的岩相学、主量元素和微量元素剖面的综合研究，以及变质花岗岩相平衡模拟，确认大别造山带超高压变质花岗岩中存在两代石榴子石。研究结果使变质和橄榄榴石的基础上的BSE图像，主要和微量元素组成的区别。我们的研究为超高压变质岩在地幔折返过程中脱水熔融导致深熔石榴石的生长提供了新的认识。第一代石榴石（Grt-I）出现在中心的一个广泛的领域，这与俯冲俯冲过程中的变质生长。该石榴石在BSE图像中呈深色，富含钙铝榴石，贫铁铝榴石和镁铝榴石。稀土元素配分模式表现为轻稀土亏损型和中重稀土-重稀土平坦型。第二代石榴石（Grt-II）发生作为自形石榴石的边缘，或作为补丁在Grt-I域，再结晶溶解后，在折返过程中存在的深熔熔体中的预先存在的变质石榴石。BSE图像明亮，钙铝榴石含量低，铁铝榴石和镁铝榴石含量丰富。Grt-II畴微量元素分析表明，Sc、Cr、Y和HREE含量较高，Ti和MREE含量较低。稀土元素配分模式表现为轻稀土亏损型和重稀土-重稀土配分模式。根据石榴石和锆石/钛铁矿的稀土配分，变质和深熔石榴石的最后生长时代分别被锆石和钛铁矿的U-Pb年龄限制在~ 240 Ma和~ 220 Ma。根据深凹显微构造和模拟的P-T假剖面，推测折返过程中发生了2.0- 2.5GPa的脱水熔融作用。熔融作用是通过多硅白云母的包埋反应：石榴石（I）+多硅白云母+斜长石+石英→石榴石（II）+黑云母+钾长石+熔体的分解而发生的。
Two generations of garnet are recognized in ultrahigh-pressure (UHP) metagranite from the Dabie orogen by a combined study of petrography, major and trace element profiles in garnet, and phase equilibrium modeling for metagranite. The results enable distinction between metamorphic and peritectic garnet on the basis of BSE images, and major and trace element compositions. Our research provides new insights into the growth of anatectic garnet due to dehydration melting of UHP metamorphic rocks during exhumation from mantle depths. The first generation of garnet (Grt-I) occurs as a broad domain in the center, which is related to metamorphic growth during prograde subduction. This garnet is dark in BSE images, rich in grossular and poor in almandine and pyrope. The chondrite-normalized rare earth element (REE) patterns show LREE depletion and flat MREE-HREE patterns. The second generation of garnet (Grt-II) occurs as a rim of euhedral garnet, or as patches in Grt-I domains, recrystallized after dissolution of preexisting metamorphic garnet in the presence of anatectic melts during exhumation. It is bright in BSE images, poor in grossular, and rich in almandine and pyrope contents. Trace element analyses on Grt-II domains yield high contents of Sc, Cr, Y and HREE and low contents of Ti and MREE. The chondrite-normalized REE patterns exhibit LREE depletion, and steep MREE-HREE patterns. Based on REE partitioning between garnet and zircon/titanite, the last growth times for metamorphic and anatectic garnets are constrained by zircon and titanite U–Pb ages to be ~ 240 Ma and ~ 220 Ma, respectively. Based on anatectic microstructures and a modeled P-T pseudosection, it is suggested that dehydration melting occurred at 2.0–2.5 GPa during exhumation. Melting occurred through the breakdown of phengite via the peritectic reaction: garnet (I) + phengite + plagioclase + quartz → garnet (II) + biotite + K-feldspar + melt.
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