Dating mafic magmatism by integrating baddeleyite, zircon and apatite U–Pb geochronology: A case study of Proterozoic mafic dykes/sills in the North China Craton
Dating mafic magmatism by integrating baddeleyite, zircon and apatite U–Pb geochronology: A case study of Proterozoic mafic dykes/sills in the North China Craton
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综合斜锆石、锆石和磷灰石U-Pb年代学测定镁铁质岩浆作用——以华北克拉通元古代镁铁质岩脉/基台为例
DOI：
10.1016/j.lithos.2020.105820
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发表时间：
2020-10
期刊：
影响因子：
3.5
通讯作者：
Jian-Xin Zhao
中科院分区：
文献类型：
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作者：
Linlin Li;Yuruo Shi;J. Lawford Anderson;Teresa Ubide;Alex;er A. Nemchin;John Caulfield;Xuan-Ce Wang;Jian-Xin Zhao
基性岩墙/岩床的直接定年对于理解地幔地球动力学演化和壳幔相互作用至关重要，但也具有挑战性，特别是在古老的样品中。本文通过对共生斜锆石、锆石和磷灰石的U-Pb同位素和元素分布图的综合研究，为元古宙基性岩墙/岩床的定年提供了可靠的方法。本文以华北地区古-新元古代基性岩墙（岩槛）为研究对象，通过高分辨离子探针（SHRIMP）207 Pb/206 Pb定年，确定了岩墙（岩槛）的结晶年龄。通过激光烧蚀四极电感耦合等离子体质谱（LA-Q-ICP-MS）元素测绘，确定了斜锆石沿沿着和裂缝的常见铅侵入可能是年龄不确定性的来源。而元古宙镁铁质侵入体中的岩浆锆石则受到变质作用的影响，表现出较高的α剂量（Dα），因而具有复杂的U-Pb同位素特征。强烈变质锆石易受铅损失，由于流体渗透，清楚地说明了相反的富集趋势的Pb和U的锆石地图。在变质锆石不受流体蚀变影响的情况下，它们能够保持原始的207 Pb/206 Pb比值，从而对古岩墙/岩床的侵位提供高精度的年龄限制。U-Pb数据定义Tera-Wasserburg等时线，截距年龄较低，与寄主岩石的结晶相匹配。该方法的年龄不确定性在于仪器分析误差所造成的低浓度的U，铅和低比例的放射成因铅的总铅，这些参数通常表现出一致的富集趋势，从晶体核心到边缘。通过U-Pb定年和元素填图相结合的方法，突出了普遍存在的磷灰石的潜力，为斜锆石和锆石的误差分析和年龄解释提供了新的手段，为基性岩的年代学研究提供了新的途径。
Direct dating of mafic dykes/sills is crucial for understanding mantle geodynamic evolution and crust-mantle interaction, but is also challenging, especially in ancient samples. Here, we provide a reliable approach for dating Proterozoic mafic dykes/sills through an integrated investigation of U–Pb isotopes and elemental distribution maps of the coexisting baddeleyite, zircon and apatite. This study focuses on Paleo- to Neo-Proterozoic mafic dykes/sills in the North China Craton.In the studied samples, baddeleyite is undoubtedly magmatic, with sensitive high resolution ion microprobe (SHRIMP)207Pb/206Pb ages that may record crystallization ages of the dykes/sills. Common lead incursion along rims and fractures of baddeleyite is identified via laser ablation quadrupole inductively coupled plasma mass spectrometry (LA-Q-ICP-MS) elemental mapping as the likely origin of age uncertainties. In contrast, magmatic zircons from the Proterozoic mafic intrusions are somewhat affected by metamictization as roughly indicated by high alpha dose (Dα), and thus show intricate U–Pb isotopic features. Intensely metamict zircons are susceptible to Pb loss due to fluid infiltration, as clearly illustrated by opposing enrichment trends of Pb and U in zircon maps. Where metamict zircons are unaffected by fluid alteration, they are able to preserve the original207Pb/206Pb ratios and thus give high precision age constraints on emplacement of the ancient dykes/sills.As for apatites, abundant euhedral coarse-grained crystals were separated from the mafic rocks, rendering them suitable for LA-Q-ICP-MS measurement. U–Pb data define Tera-Wasserburg isochrons with lower intercept ages that match crystallization of the host rocks. Age uncertainties of this method lie in instrumental analysis errors caused by low concentrations of U, Pb and a low proportion of radiogenic Pb in the total Pb, with these parameters commonly showing consistent enrichment trends from crystal cores to rims. By combining U–Pb dating and elemental mapping, this study provides a novel approach to the geochronology of mafic rocks, by highlighting the potential of ubiquitous apatite, offering additional means for error analysis and age interpretation of baddeleyite and zircon.
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影响因子：
2.7
作者：
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影响因子：
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作者：
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通讯作者：
Han Bao