Geochronological constraints on the evolution of high-pressure felsic granulites from an integrated electron microprobe and ID-TIMS geochemical study
Geochronological constraints on the evolution of high-pressure felsic granulites from an integrated electron microprobe and ID-TIMS geochemical study
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综合电子探针和 ID-TIMS 地球化学研究对高压长英质麻粒岩演化的年代学限制
DOI：
10.1016/j.lithos.2005.08.009
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发表时间：
2006
期刊：
影响因子：
3.5
通讯作者：
K. Mahan
中科院分区：
文献类型：
--
作者：
J. Baldwin;S. Bowring;M. Williams;K. Mahan
通过应用集成电子微探针和同位素稀释热电离质谱 (ID-TIMS) 技术，对加拿大萨斯喀彻温省北部雪鸟构造带的长英质高压麻粒岩进行了地质年代学、地球化学和 Nd 同位素综合研究。所研究的地体是一个 400 平方公里的石榴石-蓝晶石-钾长石含石英长石片麻岩区域。这些麻粒岩中的独居石保留了 2.6 至 1.9 Ga 的复杂生长历史，石榴石中包含良好的铠装、高 Y 和 Th 颗粒，产生最古老的 U-Pb 年代为 2.62 至 2.59 Ga。相比之下，铠装不良的石榴石边缘中的基质颗粒和包裹体的 Y 和 Th 耗尽，并显示出更复杂的 U-Pb 系统学，具有多个年龄范围2.5至2.0 Ga。1.9 Ga独居石仅作为基体晶粒存在。锆石通常比最古老的独居石更年轻（2.58 至 2.55 Ga）。对单个独居石颗粒和整个岩石样品的 Sm-Nd 同位素分析表明，石榴石中太古代独居石的包裹体在同位素组成上与整个岩石特征相似，初始 ɛNd 的初始值略为负值，范围有限。相比之下，含有古元古代成分的颗粒显示出更正的初始εNd，最简单地解释为反映了来自涉及石榴石消耗和重稀土元素普遍消耗的来源的衍生。我们首选的解释是，最古老的独居石年代记录了原岩的火成岩结晶。约。锆石和独居石中的 2.55 Ga 测年记录了一次广泛的熔化事件，在此期间形成了石榴石和三元长石。古元古代 1.9 Ga 的超高压 (>1.5 GPa) 变质作用通过石榴石分解产生了蓝晶石，并导致新独居石和锆石的生长有限。就独居石而言，这可能是由于早期形成的独居石的铠装和隔离，使其无法参与高压事件期间的变质反应，以及由于早期熔化事件后的熔损而导致稀土元素的消耗。
A combined geochronological, geochemical, and Nd isotopic study of felsic high-pressure granulites from the Snowbird Tectonic Zone, northern Saskatchewan, Canada, has been carried out through the application of integrated electron microprobe and isotope dilution thermal ionization mass spectrometry (ID-TIMS) techniques. The terrane investigated is a 400 km2domain of garnet–kyanite–K–feldspar-bearing quartzofeldspathic gneisses. Monazite in these granulites preserves a complex growth history from 2.6 to 1.9 Ga, with well-armored, high Y and Th grains included in garnet yielding the oldest U–Pb dates at 2.62 to 2.59 Ga. In contrast, matrix grains and inclusions in garnet rims that are not well-armored are depleted in Y and Th, and display more complicated U–Pb systematics with multiple age domains ranging from 2.5 to 2.0 Ga. 1.9 Ga monazite occurs exclusively as matrix grains. Zircon is typically younger (2.58 to 2.55 Ga) than the oldest monazite. Sm–Nd isotope analysis of single monazite grains and whole rock samples indicate that inclusions of Archean monazite in garnet are similar in isotopic composition to the whole rock signature with a limited range of slightly negative initial ɛNd. In contrast, grains that contain a Paleoproterozoic component show more positive initial ɛNd, most simply interpreted as reflecting derivation from a source involving consumption of garnet and general depletion of HREE's. Our preferred interpretation is that the oldest monazite dates record igneous crystallization of the protolith. The ca. 2.55 Ga dates in zircon and monazite record an extensive melting event during which garnet and ternary feldspar formed. Very high-pressure (>1.5 GPa) metamorphism during the Paleoproterozoic at 1.9 Ga produced kyanite from garnet breakdown, and resulted in limited growth of new monazite and zircon. In the case of monazite, this is likely due to the armoring and sequestration of early-formed monazite such that it could not participate in metamorphic reactions during the high-pressure event, as well as the depletion of the REE's due to melt loss following the early melting event.