Geochemical and U-Pb age constraints on the occurrence of polygenetic titanites in UHP metagranite in the Dabie orogen
Geochemical and U-Pb age constraints on the occurrence of polygenetic titanites in UHP metagranite in the Dabie orogen
复制标题
地球化学和U-Pb年龄对大别造山带超高压变花岗岩中多成因钛岩赋存的制约
DOI：
10.1016/j.lithos.2011.03.020
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发表时间：
2012-04-01
期刊：
影响因子：
3.5
通讯作者：
Guo, Jingliang
中科院分区：
文献类型：
--
作者：
Gao, Xiao-Ying;Zheng, Yong-Fei;Guo, Jingliang
在大别造山带超高压变质花岗岩中，在一些钛矿颗粒的核心中发现了残余岩浆钛矿，其边缘生长着变质钛矿。 LA-ICPMS U-Pb 测年给出了岩浆钛矿的新元古代年龄和变质钛矿的三叠纪年龄。岩浆和变质钛岩由于岩石学和地球化学成分的差异而明显不同。岩浆钛矿以残余岩心形式出现，显示出明亮的 BSE，存在钠长石和石英包裹体，CaO、Al2O3 和 TiO2 含量低，但 Fe2O3 和 MgO 含量高。在微量元素中，岩浆钛矿表现出较高的 REE 和 HFSE 含量、明显的负 Eu 异常、平坦的 MREE-HREE 模式以及较高的 Th/U 比值。相比之下，变质钛矿以均质深色 BSE 的边缘和颗粒形式出现，其中含有绿帘石、石英内含物。钾长石、金红石、黑云母和多硅白云母，CaO、Al2O3 和 TiO2 含量较高，Fe2O3 和 MgO 含量较低，REE 和 HFSE 含量较低，Eu 略为负异常，HREE 相对于 MREE 较弱，Th/U 比值较低。对于岩浆钛矿，锆钛矿测温结果为 727 至 877 摄氏度，0.5 至 1.0 GPa，对于变质钛矿，为 729 至 870 摄氏度，1.5 至 2.0 GPa。新元古代岩浆钛矿 U-Pb 测年系统在三叠纪大陆俯冲带 HP-UHP 变质作用中幸存下来。这表明钛矿 U-Pb 体系的闭合温度相对较高，>800 摄氏度。变质钛矿主要是高压榴辉岩相向角闪岩相过渡过程中减压折返过程中逆变质作用的产物。因此，钛矿在大陆俯冲带变质岩的年代学和岩石成因研究中具有巨大的潜力。 (C) 2011 Elsevier B.V. 保留所有权利。
Relict magmatic titanite was identified in the core of a few titanite grains with the overgrown rim of metamorphic titanite in UHP metagranite in the Dabie orogen. LA-ICPMS U-Pb dating gave Neoproterozoic ages for the magmatic titanite but Triassic ages for the metamorphic titanite. The magmatic and metamorphic titanites are clearly distinct by differences in petrological and geochemical compositions. The magmatic titanite occurs as residual cores that show bright BSE, the presence of allanite and quartz inclusions, low contents of CaO, Al2O3 and TiO2 but high contents of Fe2O3 and MgO. In trace elements, the magmatic titanite exhibits high REE and HFSE contents, distinctly negative Eu anomalies with flat MREE-HREE patterns, and high Th/U ratios. In contrast, the metamorphic titanite occurs as rims and grains of homogeneously dark BSE that contain inclusions of epidote, quartz. K-feldspar, rutile, biotite and phengite, and have relatively high contents of CaO, Al2O3 and TiO2, but low contents of Fe2O3 and MgO, and relatively low REE and HFSE contents, slightly negative Eu anomalies with HREE depletion relative to MREE, and low Th/U ratios. The Zr-in-titanite thermometry yields 727 to 877 degrees C 0.5 to 1.0 GPa for the magmatic titanite, and 729 to 870 degrees C at 1.5 to 2.0 GPa for the metamorphic titanite. The Neoproterozoic U-Pb chronometric system of magmatic titanite survived the Triassic continental subduction-zone HP-UHP metamorphism. This suggests a relatively high closure temperature of >800 degrees C for the titanite U-Pb system. The metamorphic titanite is principally a product of retrograde metamorphism during decompression exhumation at the transition from HP eclogite-facies to amphibolite-facies. Therefore, titanite holds a great potential to geochronological and petrogenetic studies of continental subduction-zone metamorphic rocks. (C) 2011 Elsevier B.V. All rights reserved.