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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Origin of zircon megacrysts in alkaline lavas (French Massif Central): Petrology and in situ U-Pb-Hf isotopes
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Origin of zircon megacrysts in alkaline lavas (French Massif Central): Petrology and in situ U-Pb-Hf isotopes

机译：碱性熔岩（法国Massif Central）中锆石甲锭的起源：岩石学和原位U-PB-HF同位素

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In the south eastern Deves volcanic domain of the French Massif Central, we sampled red zircon megacrysts in, both, alluvium from a small stream and neighbouring mugearite lava. LA-ICP-MS U-Pb dating results yield similar ages for both suites of zircons at about 2.6 Ma. Superchondritic signatures of the Hf isotopes clearly indicate a depleted mantle source for these zircons.The mugearite lava also contains xenoliths of mantle (lherzofites) and lower crustal rocks (granulites). The chemical compositions of the minerals within the mugearite lava are similar to the mineral compositions found in these xenoliths (granulites) and granulite and peridotite xenoliths from other volcanic localities of the Massif Central. The mugearite lava also preserves Variscan zircon crystals dated at 285 Ma, originating from recycled lower continental crust.In spite of their mantle origin, as recorded by in situ Hf isotopes, the geochemistry suggests that these Pliocene zircons do not result from crystallization in the mantle or in mantle-derived mafic magmas. In order to explain this inconsistency, we propose that these zircon grains crystallized from felsic melts produced during the ultimate differentiation of mantle-derived mafic magmas in the mid or upper continental crust. Subsequently, a last basaltic batch forced its way to the surface, scavenging upper mantle and lower crust xenoliths as well as zircon xenocrysts formed in these felsic melts which partly dissolved during their ascent.Large amounts of Cenozoic zircon megacrysts are often associated with sapphires in alluvial deposits in many continental volcanic domains, not only in the French Massif Central but also worldwide. Consequently, a huge volume of felsic differentiates issued from mantle-derived basaltic magmas might be suspected in many alkaline volcanic domains at the global scale. (C) 2020 Elsevier B.V. All rights reserved.
机译：在东南部Deves法国Massif Central的火山域，我们从一只小型溪流和邻近的穆阿里斯图拉斯采样了红锆石甲锭剂。 La-ICP-MS U-PB约会结果为ZIRCONS套房的套房相似，ZIRCONS套房相似。 HF同位素的超级特性签名清楚地表明了这些锆石的耗尽的地幔源。乳石熔岩还含有卵石（Lherzofites）和下地壳岩石（肉芽籽）。乳石熔岩内的矿物质的化学组成类似于这些卵泡型（肉芽籽）和颗粒状物中的矿物质组合物，以及来自肿块中央的其他火山区的粒状和恒星Xenoliths。 Mugearite熔岩还保留了在285 mA的瓦斯卡锆晶体，源自回收的较低的欧洲地壳。由于原位HF同位素记录的那样，他们陈述了它们的披风原因，因此地球化学表明这些全茂锌锆石不会导致地幔中的结晶导致结晶或者在地幔衍生的MAFIC岩浆中。为了解释这种不一致，我们建议这些锆石谷物从中间或上部地壳中的地幔衍生的MAFIC岩浆的最终分化期间产生的雀鞘熔化。随后，最后一种玄武岩批次被迫进入表面，清除上部露地和下地壳Xenolith以及在这些肠道熔体中形成的锆石聚物，其在其上升期间溶解.LARGE的新生代锆石甲锭通常与SAPPHIRE相关在许多大陆火山域中存款，不仅在法国大块中央，而且在全球范围内。因此，在全球范围内的许多碱性火山域中可能怀疑来自披风衍生的玄武岩岩浆发出的大量肠差异。 （c）2020 Elsevier B.v.保留所有权利。

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