Polygenetic titanite records the composition of metamorphic fluids during the exhumation of ultrahigh-pressure metagranite in the Sulu orogen
Polygenetic titanite records the composition of metamorphic fluids during the exhumation of ultrahigh-pressure metagranite in the Sulu orogen
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多成因钛岩记录了苏鲁造山带超高压变花岗岩折返过程中变质流体的成分
DOI：
10.1111/jmg.12194
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发表时间：
2016
影响因子：
3.4
通讯作者：
Yang Y. H.
中科院分区：
文献类型：
--
作者：
Chen Y. -X.;Zhou K.;Zheng Y. -F.;Gao X. -Y.;Yang Y. H.
对中国中东部苏鲁造山带超高压变花岗岩中的钛铁矿和锆石进行了 U-Pb 年龄和微量元素的综合研究。这些结果为深俯冲大陆地壳折返过程中变质流体的成分提供了限制。基于 REE 模式和微量元素变化，钛矿有两种域类型，分别为 Ttn-I 和 Ttn-II。这两个域显示出无法区分的 U-Pb 年龄，为 232 ± 14 至 220 ± 8 Ma，与早期折返期间部分熔融事件的深熔锆石 U-Pb 年龄 223 ± 4 至 219 ± 2 Ma 基本一致。 Ttn-I 域的 REE、Th、Ta 和 Sr 显着高于 Ttn-II 域，并且 Th/U 比值更高，表明这两个域是由不同成分的变质流体形成的。对于 Ttn-I 域，Zr-in-titanite 测温法在 2.5 GPa 下产生 773-851 °C 的高温，岩相观察揭示了熔融假象的存在。因此，它们被解释为是在早期折返阶段从含水熔体中生长出来的。相比之下，Ttn-II 域在结构上与角闪岩相矿物（如黑云母和斜长石）平衡，并含有斜长石和石英的包裹体。 Zr-in-titanite 测温法在 1.0 GPa 下产生 627–685 °C 的较低温度。结合它们的稀土元素模式，它们被解释为是在进一步折返过程中在角闪岩相变质条件下从水溶液中生长出来的。 Ttn-I 和 Ttn-II 域之间 Th 和 Sr 含量的差异很明显，表明含水熔体和水溶液之间的成分差异。因此，超高压变质岩中的多成因钛矿为了解大陆俯冲带过程中变质流体的地球化学性质提供了见解。
An integrated study of U–Pb ages and trace elements was carried out for titanite and zircon from ultrahigh‐pressure (UHP) metagranites in the Sulu orogen, east‐central China. The results provide constraints on the composition of metamorphic fluids during the exhumation of deeply subducted continental crust. Titanite has two domain types based on REE patterns and trace element variations, Ttn‐I and Ttn‐II respectively. These two domains show indistinguishable U–Pb ages of 232 ± 14 to 220 ± 8 Ma, in general agreement with anatectic zircon U–Pb ages of 223 ± 4 to 219 ± 2 Ma for the partial melting event during early exhumation. The Ttn‐I domains have significantly higher REE, Th, Ta and Sr, and higher Th/U ratios than the Ttn‐II domains, indicating that the two domains have grown from metamorphic fluids with different compositions. For the Ttn‐I domains, Zr‐in‐titanite thermometry yields high temperatures of 773–851 °C at 2.5 GPa, and petrographic observations reveal the presence of melt pseudomorphs. Thus, they are interpreted to have grown from hydrous melts in the early exhumation stage. In contrast, the Ttn‐II domains were texturally equilibrated with amphibolite facies minerals such as biotite and plagioclase and contain inclusions of plagioclase and quartz. The Zr‐in‐titanite thermometry yields lower temperatures of 627–685 °C at 1.0 GPa. In combination with their REE patterns, they are interpreted to have grown from aqueous solutions at amphibolite facies metamorphic conditions during further exhumation. The differences in Th and Sr contents are prominent between the Ttn‐I and Ttn‐II domains, signifying the compositional difference between the hydrous melts and aqueous solutions. Therefore, the polygenetic titanite in the UHP metamorphic rocks provides insights into the geochemical property of metamorphic fluids during the continental subduction‐zone processes.