Omar Bartoli, Leo J Millonig, Bruna B Carvalho, Horst R Marschall, Axel Gerdes
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引用次数: 0
摘要
在辉绿岩和花岗岩中,熔体产生、萃取和结晶的速率,以及熔体转移和与其残余物相互作用的规模都在不断变化,影响着作为时间胶囊的独居石和锆石的行为。因此,附属矿物计时仪在概述高品位变质岩的温度-时间演化方面可能是模糊和不完整的。在这项研究中,我们将石榴石原位 U-Pb 定年的新技术应用于意大利西北部伊夫雷亚-韦尔巴诺区(IVZ)的典型下大陆地壳。在伊夫雷亚-维尔巴诺区(IVZ),由于构造、岩浆、变质和变质过程在一亿多年的时间里相互作用,花岗岩成因变质作用与黑云母底侵之间的时间关系一直存在争议。从黑云母和辉绿岩花岗岩中提取的石榴石的 U-Pb 年龄介于 287.4 ± 4.9 Ma 和 280.1 ± 12.4 Ma 之间,与所提出的黑云母复合体的形成时间(286-282 Ma)在不确定的范围内重叠。这些结果表明,花岗岩的热高潮是由黑云母下伏和随之而来的星体层上涌造成的,而不是从后石炭纪演化中继承而来。这项研究提供了低至 4 ng/g U 的石榴石可靠年代测定,证明了石榴石岩石年代学在解析高品位陆相复杂构造-变质历史方面的优势。它还标志着在将石榴石确定为原位 U-Pb 地质年代的一部分方面又向前迈进了一步。
The Age of Granulite-Facies Metamorphism in the Ivrea-Verbano Zone (NW Italy) Determined through in-Situ U–Pb Dating of Garnet
Rates of melt production, extraction and crystallization, as well as scales of melt transfer and interaction with their residuum change continuously in migmatite and granulite, affecting the behavior of monazite and zircon as time capsules. Therefore, accessory mineral chronometers may be ambiguous and incomplete in providing an overview of the temperature–time evolution of high-grade metamorphic rocks. In this study, we applied the novel technique of in-situ U–Pb dating of garnet to the archetypal lower continental crust of the Ivrea-Verbano Zone (IVZ), NW Italy. In the IVZ, the temporal relationship between granulite-facies metamorphism and mafic underplating has long been debated, because of the interplay between tectonic, magmatic, metamorphic and metasomatic processes over a period of more than a hundred million years. Garnet from mafic and pelitic granulites yielded U–Pb ages between 287.4 ± 4.9 Ma and 280.1 ± 12.4 Ma, overlapping within uncertainty the time proposed for the emplacement of the Mafic Complex (286–282 Ma). These results indicate that the thermal climax in granulitic rocks was caused by mafic underplating and concomitant asthenospheric upwelling, rather than being inherited from the post-Variscan Carboniferous evolution. Providing robust dating of garnet with as low as 4 ng/g U, this study demonstrates the strength of garnet petrochronology in resolving complex tectono-metamorphic histories of high-grade terranes. It also represents a further step forward towards establishing garnet as part of the in-situ U–Pb geochronology repertoire.
期刊介绍:
The Journal of Petrology provides an international forum for the publication of high quality research in the broad field of igneous and metamorphic petrology and petrogenesis. Papers published cover a vast range of topics in areas such as major element, trace element and isotope geochemistry and geochronology applied to petrogenesis; experimental petrology; processes of magma generation, differentiation and emplacement; quantitative studies of rock-forming minerals and their paragenesis; regional studies of igneous and meta morphic rocks which contribute to the solution of fundamental petrological problems; theoretical modelling of petrogenetic processes.