Ion-microprobe U-Pb zircon geochronology and correlation of Archaean gneisses from the Lewisian Complex of Gruinard Bay, northwestern Scotland
Ion-microprobe U-Pb zircon geochronology and correlation of Archaean gneisses from the Lewisian Complex of Gruinard Bay, northwestern Scotland
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DOI：
10.1016/s0016-7037(97)00251-2
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发表时间：
1997-10
影响因子：
5
通讯作者：
M. Whitehouse;S. Claesson;T. Sunde;J. Vestin
中科院分区：
文献类型：
--
作者：
M. Whitehouse;S. Claesson;T. Sunde;J. Vestin
本文对苏格兰西北部格吕纳德湾晚白垩世刘易斯期片麻岩杂岩中的三个样品进行了离子探针U <$Pb锆石年代学研究，试图解决奥长花岗岩和角闪石组分的Sm <$Nd和Pb <$Pb等时线年龄之间的差异。全岩样本暗示约。2.8佐治亚州两种成分的年龄（怀特豪斯等人，1996年），但分离的矿物产生了约。2.4-2.5 Ga为奥长花岗岩，3.3 Ga为角闪石（Burton et.例如，1994年）。从格吕纳德湾的三种代表性岩性（奥长花岗岩、角闪石岩和条带状英云闪长岩）中分离出的锆石产生一致或接近一致的U-Pb数据，207 Pb-206 Pb年龄主要在约100 - 100 μ m之间。2.85 Ga和ca。2.75 Ga和ca处的一些离群值。2.65佐治亚州奥长花岗岩中锆石的背散射电子图像可以区分钙钛矿和钙钛矿。2.85 Ga芯和Ca。2.75 Ga过度生长;后者被解释为火成的基础上的领域关系的奥长花岗岩的其他样品。角闪石锆石的形态更符合在变质条件下的结晶，而不是在约火成条件下。2.8 Ga，尽管怀特豪斯等人（1996）先前关于原岩年龄不会大大超过这一年龄的论点仍然有效。新的锆石数据表明，在格吕纳德湾岩浆活动发生在一个相对较短的时间间隔，在雅阁与以前的意见，从全岩同位素数据。分析的锆石中没有证据表明大约。奥长花岗岩和角闪石的岩浆年龄分别为2.4- 2.5Ga和3.3Ga。奥长花岗岩的矿物等时线年龄可能代表了与区域热演化有关的变质复位作用。2.5 Ga高级别事件记录在中部地区。对于角闪石，矿物Sm <$Nd年龄被证实为假的，从结晶后的开放系统同位素行为的矿物尺度上。刘易斯复合体内年龄的区域比较表明，≤5 ca。2.85 Ga的Gruinard湾TTG片麻岩可能与中部≥ 2.96 Ga的片麻岩在一个约1000年前或期间合并。2.76 Ga事件和随后经历了类似的地质演化。
We present ion microprobe UPb zircon geochronology for three samples from the late Archaean Lewisian gneiss complex at Gruinard Bay, northwestern Scotland, in an attempt to resolve the discrepancy between SmNd and PbPb isochron ages for trondhjemite and hornblendite components. Whole-rock samples imply ca. 2.8 Ga. ages for both components (Whitehouse et al., 1996), but separated minerals have yielded ca. 2.4–2.5 Ga for trondhjemite and 3.3 Ga for hornblendite (Burton et. al., 1994). Zircons separated from three representative lithologies at Gruinard Bay (trondhjemite, hornblendite, and banded tonalite) yield concordant or near concordant UPb data with207Pb206Pb ages mostly between ca. 2.85 Ga and ca. 2.75 Ga, and a few outliers at ca. 2.65 Ga. Backscattered electron images of zircons from the trondhjemite allow differentiation between ca. 2.85 Ga cores and ca. 2.75 Ga overgrowths; the latter are interpreted as igneous on the basis of field relationships of the trondhjemite to the other samples. Morphology of the hornblendite zircons is more consistent with crystallisation under metamorphic rather than igneous conditions at ca. 2.8 Ga, although the previous arguments of Whitehouse et al. (1996) for a protolith age not greatly in excess of this remain valid. The new zircon data suggest that magmatism at Gruinard Bay occurred over a relatively short time interval, in accord with the previous observations from whole-rock isotopic data. There is no evidence from the analysed zircons for ca. 2.4–2.5 Ga or 3.3 Ga magmatic ages in the trondhjemite and hornblendite, respectively. The mineral isochron ages for the trondhjemite probably represent metamorphic resetting related to the regional ca. 2.5 Ga high-grade event recorded in the central region. For the hornblendite, the mineral SmNd ages are confirmed as spurious, resulting from post-crystallisation open-system isotope behaviour on the mineral scale. Regional comparison of ages within the Lewisian Complex suggests that the ≤5 ca. 2.85 Ga Gruinard Bay TTG gneisses might have been amalgamated with ≥ 2.96 Ga gneisses of the central region before or during a ca. 2.76 Ga event and subsequently have experienced a similar geological evolution.