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首页> 外文期刊>Contributions to Mineralogy and Petrology >Combined U/Pb and (U–Th)/He geochronometry of basalt maars in Western Carpathians: implications for age of intraplate volcanism and origin of zircon metasomatism
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Combined U/Pb and (U–Th)/He geochronometry of basalt maars in Western Carpathians: implications for age of intraplate volcanism and origin of zircon metasomatism

机译：喀尔巴阡山脉玄武岩的U / Pb和（U-Th）/ He联合年代学：对板内火山时代和锆石交代作用的影响

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摘要

The age of intraplate volcanism in northern Pannonian Basin of Carpathians is revisited using a combination of zircon U/Pb, zircon (U–Th)/He and apatite (U–Th)/He dating techniques, complemented by electron microprobe (EMP) characterisation of dated minerals. A total of six maar structures and diatremes in the South-Slovakian Volcanic Field (SSVF) were dated and the obtained new ages yielded the following key findings: Two isolated maars in SE part indirectly dated by geomorphologic constraints to Late Pleistocene are actually of Pliocene (2.8 ± 0.2 Ma) and Late Miocene (5.5 ± 0.6 Ma) ages. In contrast, two maars in NW part of the study area are of Late Pliocene age (4.1 ± 0.4 and 5.2–5.4 Ma), younger than the Late Miocene age (~6.5 Ma) inferred previously from K/Ar data on the proximal basaltic lava flows. These maars therefore belong to the second volcanic phase that was previously identified only in SE part of the SSVF. In the light of the new geochronologic data, it seems likely that the Pliocene phreatomagmatic eruptions may have occurred along extension-related, NW- and NE-trending orthogonal faults. EMP analyses and imaging revealed an extensive syn- and post-growth metasomatic replacement by dissolution-reprecipitation in the majority of zircons. Abundant silicate melt inclusions in porous metasomatised parts of the zircons are diagnostic of magmatic rather than hydrothermal metasomatism. Consistent ages of the metasomatised and non-metasomatised zones do not indicate disturbance of the U–Pb system during the metasomatism. Enrichment in U and Th loss in the metasomatised zircons are diagnostic of an increasing oxygen fugacity triggered by degassing of the volatile residual melt during the final stages of alkali basalt fractionation. Rare zircon-to-baddeleyite transformation was probably connected with lowered silica activity in carbonated basaltic magmas in south-eastern part of the study area.
机译：使用锆石U / Pb，锆石（U–Th）/ He和磷灰石（U–Th）/ He测年技术，结合电子微探针（EMP）表征，重新探讨了喀尔巴阡山脉北部Pannonian盆地板内火山时代。过时的矿物。在南斯洛伐克火山场（SSVF）中总共确定了六个maar结构和异常点，并且获得的新年龄产生了以下关键发现：东南部的两个孤立的maars是由晚更新世的地貌限制间接测年的（实际上是上新世（ 2.8±0.2 Ma）和中新世晚期（5.5±0.6 Ma）年龄。相比之下，研究区域西北部的两个马氏体是上新世晚期（4.1±0.4和5.2-5.4Ma），比之前根据近端玄武岩的K / Ar数据推断的中新世晚期（〜6.5Ma）要年轻。熔岩流。因此，这些岩浆属于第二个火山相，以前仅在SSVF的SE部分中被确定。根据新的年代学数据，似乎上新世的浅岩浆喷发可能发生在与伸展有关的，西北向和东北向的正交断层上。 EMP分析和成像显示，在大多数锆石中，通过溶解-再沉淀作用，广泛地进行了生长后和生长后的交代置换。锆石的多孔交代部分中大量的硅酸盐熔体包裹体是岩浆而不是热液交代作用的诊断。交代区域和非交代区域的一致年龄并不表示在交代过程中U–Pb系统受到干扰。交代锆石中U和Th损失的富集可以诊断为在碱性玄武岩分馏的最后阶段对挥发性残留熔体进行脱气而引起的氧气逸度增加。在研究区的东南部，碳酸盐玄武岩岩浆中稀有锆石到baddeleyite的转变可能与二氧化硅活性的降低有关。

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