Abstract The Seychelles microcontinent is located in the western Indian Ocean and is exposed as a series of granitic islands. The Main Islands of the Seychelles consist of the Mahe and Praslin granite groups. They are characterized as ferroan, metaluminous to peraluminous, alkali-calcic to calc-alkalic, and resemble post-collisional A-type granite but have distinct Sr-Nd-Pb-O isotopic compositions. Previous studies showed that the granites and syn-plutonic dolerites were emplacement at 760–750 Ma, although rocks from Ile aux Recifs yielded an older age (808.8 ± 1.9 Ma). New in situ zircon U-Pb geochronology of granites from northern Mahe (751.9 ± 4.6 Ma, 755.5 ± 5.8 Ma, 756.2 ± 7.7 Ma) and western Praslin (757 ± 11 Ma, 753.0 ± 7.5 Ma, 751.3 ± 5.8 Ma) yielded weighted-mean 206Pb/238U ages similar to previous studies but a significant amount of inherited zircon was identified with ages ranging from ~880 Ma to ~770 Ma. The weighted-mean 206Pb/238U ages of the principal inherited zircon populations from the Mahe (777.5 ± 5.7 Ma, 807.7 ± 7.7 Ma) granites are within uncertainty of those identified from the Praslin (777.5 ± 6.5 Ma, 814 ± 13 Ma) granites indicating there was an older period of magmatism within the Seychelles microcontinent. The Hf isotopic data of the magmatic and inherited zircons from the Mahe granite are generally more depleted (eHf(t) = +6.7 to +13.2) and have younger model ages (TDM2 = 902 Ma to 1219 Ma) than those from the Praslin granite (eHf(t) = +2.2 to +10.9, TDM2 = 986 to 1524 Ma). The differences between the zircon Hf isotopes from the Mahe and Praslin granites is consistent with other isotopic systems and indicates that the isotopic dichotomy is an intrinsic feature of the Seychelles microcontinent that extends back to at least ~840 Ma and possibly to the Mesoproterozoic (1000 Ma to 1400 Ma). It is likely that the Mahe and Praslin granites were emplaced during a period of post-collisional extension/crustal relaxation and that the older inherited zircons are derived from rocks that record an earlier period of magmatism that was likely subduction-related.

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解析塞舌尔微大陆的起源：锆石年代学和 Hf 同位素的洞察

摘要 塞舌尔微大陆位于印度洋西部，暴露为一系列花岗岩岛屿。塞舌尔的主要岛屿由马埃和普拉兰花岗岩群组成。它们的特征是铁质、金属铝质到过铝质、碱钙到钙碱，类似于碰撞后的 A 型花岗岩，但具有不同的 Sr-Nd-Pb-O 同位素组成。先前的研究表明，花岗岩和同深成岩玄武岩在 760-750 Ma 就位，尽管来自 Ile aux Recifs 的岩石产生了更古老的年龄 (808.8 ± 1.9 Ma)。新的原位锆石 U-Pb 年代学来自马埃北部 (751.9 ± 4.6 Ma, 755.5 ± 5.8 Ma, 756.2 ± 7.7 Ma) 和普拉兰西部 (757 ± 11 Ma, 753.0 ± 7.5 Ma, 751. 8 Ma) 产生的加权平均 206Pb/238U 年龄与之前的研究相似，但发现了大量的遗传锆石，年龄范围从 ~880 Ma 到 ~770 Ma。来自 Mahe (777.5 ± 5.7 Ma, 807.7 ± 7.7 Ma) 花岗岩的主要继承锆石种群的加权平均 206Pb/238U 年龄与从 Praslin (777.5 ± 6.5 Ma, 814 ± 13 Ma) 花岗岩中鉴定的锆石年龄相比，在不确定性范围内表明在塞舌尔微大陆内存在较早的岩浆活动时期。Mahe 花岗岩岩浆和继承锆石的 Hf 同位素数据通常比普拉兰花岗岩更贫乏（eHf(t) = +6.7 至 +13.2），模型年龄更小（TDM2 = 902 Ma 至 1219 Ma） （eHf(t) = +2.2 至 +10.9，TDM2 = 986 至 1524 Ma）。Mahe 和 Praslin 花岗岩的锆石 Hf 同位素之间的差异与其他同位素系统一致，表明同位素二分法是塞舌尔微大陆的一个内在特征，至少可以追溯到~840 Ma 并可能延伸到中元古代（1000 Ma到 1400 毫安）。Mahe 和 Praslin 花岗岩很可能是在碰撞后伸展/地壳松弛时期就位的，而较老的继承锆石来自记录了可能与俯冲相关的较早岩浆活动时期的岩石。