高温再加工过程中附属矿物的抑制响应

IF 3.5 2区 地球科学 Q1 GEOLOGY
Samantha March, Martin Hand, Laura Morrissey, David Kelsey
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引用次数: 0

摘要

铀-铅锆石和独居石地质年代学被认为是制约高温变质事件发生时间的最有效、最可靠的方法之一。然而,这些年代测定器的可靠性与其参与反应的能力息息相关。利用澳大利亚中部瓦鲁米省的花岗岩岩相玄武岩和偏闪长岩岩性,对锆石和独居石的反应能力进行了案例研究。在某些情况下,该地区的玄武质花岗岩是多变质岩,早期的 M1 组合包含正长石、堇青石、生物钛铁矿、石英、钛铁矿和磁铁矿,M2 组合则以石榴石、矽线石、正长石、堇青石、生物钛铁矿、蓝晶石、钛铁矿和磁铁矿为代表。M2 变质作用与 8-10 千巴和 850-915°C 的高温峰值条件有关。这些岩石中的碎屑岩和变质岩锆石和独居石主要记录了1670-1610Ma的U-Pb日期,其微量元素组成表明它们是在岩石中的M2石榴石峰值之前生长的。M2石榴石的Lu-Hf地质年代为大约1150 Ma。因此,锆石和独居石在高温变质过程中基本保持惰性。我们将锆石和独居石在高温中新生代变质过程中相对较小的反应归因于 M1 变质过程中约 1630 Ma 时难熔体成分的局部发展。这就形成了富含 Mg-Al- 的难熔块体成分,尽管其后在约 1150 Ma 时经历了约 900°C 的高温,但仍无法发生显著的部分熔融。与此相反,该地区在M1变质过程中没有形成难熔块体成分的偏闪长岩和变质岩却能够部分熔融,并记录了约1150 Ma的附属矿物U-Pb年龄。这些结果为研究多变质 HT 岩体中锆石和独居石 U-Pb 系统的系统学及其对同位素重置的明显抵抗力的少量但日益增多的案例研究做出了贡献。在不平衡明显的地方,石榴石lu-hf地质年代学可以作为一种重要的工具,用来研究附属U-Pb年龄的意义。在澳大利亚中部的Warumpi省,约1640 Ma的锆石U-Pb年龄以前被解释为反映了在碰撞事件中HT含石榴石花岗岩的形成。相反,含石榴石的集合体形成于中新生代约1150 Ma时期,这使人们对澳大利亚中部是否存在晚古新生代碰撞系统提出了质疑。
本文章由计算机程序翻译,如有差异,请以英文原文为准。

The inhibited response of accessory minerals during high-temperature reworking

The inhibited response of accessory minerals during high-temperature reworking

U–Pb zircon and monazite geochronology are considered to be among the most efficient and reliable methods for constraining the timing of high-temperature (HT) metamorphic events. However, the reliability of these chronometers is coupled to their ability to participate in reactions. A case study examining the responsiveness of zircon and monazite has been conducted using granulite facies metapelitic and metamafic lithologies in the Warumpi Province, central Australia. In some instances, metapelitic granulites from this locality are polymetamorphic, with an early M1 assemblage containing orthopyroxene, cordierite, biotite, quartz, ilmenite and magnetite, and an M2 assemblage represented by garnet, sillimanite, orthopyroxene, cordierite, biotite, sapphirine, ilmenite and magnetite. M2 metamorphism is linked to HT peak conditions of 8–10 kbar and 850–915°C. Detrital and metamorphic zircon and monazite from these rocks dominantly record U–Pb dates of 1670–1610 Ma and have trace element compositions suggesting they grew prior to peak M2 garnet in the rock. Lu–Hf geochronology from M2 garnet gives ages of c. 1150 Ma. Zircon and monazite are therefore suggested to have remained largely inert during HT metamorphism. We attribute the relatively minor response of zircon and monazite during high-temperature Mesoproterozoic metamorphism to the localized development of refractory bulk compositions at c. 1630 Ma during M1 metamorphism. This created refractory Mg–Al-rich bulk compositions that were unable to undergo significant partial melting, despite experiencing subsequent temperatures of ~900°C at c. 1150 Ma. In contrast, metapelitic and metamafic rocks in the area that did not develop refractory bulk compositions during M1 metamorphism were able to partially melt and record c. 1150 Ma accessory mineral U–Pb ages. These results contribute to a small, but growing number of case studies investigating the systematics of the U–Pb system in zircon and monazite in polymetamorphic HT terranes and their apparent resistance to isotopic resetting. Where disequilibrium is apparent, garnet Lu–Hf geochronology can form an important tool to interrogate the significance of accessory U–Pb ages. In the Warumpi Province in central Australia, c. 1640 Ma zircon U–Pb ages had previously been interpreted to reflect the formation of HT garnet-bearing granulites during a collisional event. Instead, the garnet-bearing assemblages formed at c. 1150 Ma during the Mesoproterozoic, calling into question the existence of a late Palaeoproterozoic collisional system in central Australia.

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来源期刊
CiteScore
6.60
自引率
11.80%
发文量
57
审稿时长
6-12 weeks
期刊介绍: The journal, which is published nine times a year, encompasses the entire range of metamorphic studies, from the scale of the individual crystal to that of lithospheric plates, including regional studies of metamorphic terranes, modelling of metamorphic processes, microstructural and deformation studies in relation to metamorphism, geochronology and geochemistry in metamorphic systems, the experimental study of metamorphic reactions, properties of metamorphic minerals and rocks and the economic aspects of metamorphic terranes.
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