SHRIMP Zircon U-Pb Dating of Sapphirine-Bearing Granulite and Biotite-Hornblende Gneiss in the Schirmacher Hills, East Antarctica: Implications for Neoproterozoic Ultrahigh-Temperature Metamorphism Predating the Assembly of Gondwana
SHRIMP Zircon U-Pb Dating of Sapphirine-Bearing Granulite and Biotite-Hornblende Gneiss in the Schirmacher Hills, East Antarctica: Implications for Neoproterozoic Ultrahigh-Temperature Metamorphism Predating the Assembly of Gondwana
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DOI：
10.1086/656384
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发表时间：
2010-09
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通讯作者：
S. Baba;T. Hokada;H. Kaiden;D. Dunkley;M. Owada;K. Shiraishi
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作者：
S. Baba;T. Hokada;H. Kaiden;D. Dunkley;M. Owada;K. Shiraishi
采用SHRIMP锆石U-Pb测年方法，对东南极Schirmacher丘陵地区含蓝宝石的斜方辉石石榴石(SOG)麻粒岩和黑云角闪片麻岩(BT-HBL)进行了年龄测定。在BT-HBL片麻岩中，锆石超生边缘和锆石芯获得了一致的年龄，分别具有振荡和不规则的分带。来自BT-HBL片麻岩的低Th/U比值的锆石超长边(Ma)被解释为具有变质成因。振荡带状和/或不规则带状的锆石核可能在Ma的一次火成岩事件中结晶；800 Ma的火成岩事件以前在Dronning Maud Land(DML)内陆Nunatak的中部没有被识别。对SOG麻粒岩中的锆石进行了扇形分带和简单分带分析，获得了一致的Ma年龄。这些锆石具有相对较高的Th/U比值，范围较窄，与石榴石的分解形成堇青石共生。从这些锆石获得的-Ma年龄被解释为峰期变质后不久高Th/U部分熔体的结晶时间。Schirmacher Hills的800 Ma左右的火成岩年龄和660~640 Ma的变质年龄组合不同于DML中部其他邻区的组合。此外，在DML Nunatak中部地区，在650 Ma左右的早期和随后的等压冷却(IBC)阶段，包括超高温的变质PT路径在以前是未知的。约650 Ma的超高温变质事件可能发生在弧后构造环境中，先于DML中部的主碰撞事件(约550~500 Ma)。
We applied SHRIMP zircon U-Pb age dating to ultrahigh-temperature (UHT) sapphirine-bearing orthopyroxene garnet (SOG) granulite and biotite-hornblende (Bt-Hbl) gneiss in the Schirmacher Hills, East Antarctica. In the Bt-Hbl gneiss, concordant ages of and Ma were obtained from zircon overgrowth rims and zircon cores, with oscillatory and irregular zoning, respectively. The zircon overgrowth rims ( Ma) with low Th/U ratios from the Bt-Hbl gneiss are interpreted as having a metamorphic origin. Oscillatory-zoned and/or irregularly zoned zircon cores may have crystallized during an igneous event at Ma; 800-Ma igneous events have not previously been recognized in central Dronning Maud Land (DML) inland nunatak. Zircons in the SOG granulite yielded a concordant age of Ma, using analyses of sector-zoned and simple-zoned grains. These zircons have relatively high Th/U ratios with a narrow range, and they occur in association with garnet breaking down to form cordierite. The -Ma age obtained from these zircons is interpreted as the timing of crystallization from a high-Th/U partial melt soon after peak metamorphism. The combination of a ca. 800-Ma igneous age and 660–640-Ma metamorphic ages obtained from Schirmacher Hills is different from that of other neighboring parts of central DML. In addition, a metamorphic PT path involving ultrahigh temperatures at early and subsequent isobaric cooling (IBC) stages at around 650 Ma has not previously been known in the central DML nunatak region. The ca. 650-Ma UHT metamorphic event probably occurred in a back-arc tectonic setting and predates the main collisional event of central DML (ca. 550–500 Ma).