U-Pb dating of interspersed gabbroic magmatism and hydrothermal metamorphism during lower crustal accretion, Vema lithospheric section, Mid-Atlantic Ridge
U-Pb dating of interspersed gabbroic magmatism and hydrothermal metamorphism during lower crustal accretion, Vema lithospheric section, Mid-Atlantic Ridge
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下地壳增生期间散布的辉长岩岩浆作用和热液变质作用的 U-Pb 定年，Vema 岩石圈剖面，大西洋中脊
DOI：
10.1002/2014jb011668
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发表时间：
2015
期刊：
影响因子：
3.4
通讯作者：
Rioux M
中科院分区：
文献类型：
--
作者：
Rioux M
新的锆石和xenotime的U/Pb分析限制了中大西洋脊下地壳形成过程中岩浆作用、岩浆同化和热液变质作用的时间。研究样品为Vema岩石圈剖面（11°N）的蚀变辉长岩。原生辉长岩矿物几乎完全被多个热液叠层所取代：高温水化反应形成的辉长岩角闪洞和钠长石，首先被干酪岩覆盖，然后被前辉长岩和绿泥石覆盖。在之前的研究中，样品中无包裹体的锆石测得的206pb /238U年龄为13.528±0.101至13.353±0.057 Ma。本文报告的Ti浓度、分带模式和计算的Th/U与这些锆石在火成岩结晶过程中生长一致。为了确定热液变质作用的时间，我们对第二批锆石进行了年代测定，这些锆石中普遍存在< 1-20µm的绿泥石包裹体，以及变质钠长石形成后的锆石夹杂物。富包裹体锆石的结构和包裹体表明，它们是由亚稳火成岩锆石在热液变质过程中或之后的溶蚀-再沉淀耦合形成的。富包裹体锆石的206pb /238U校正年龄范围为13.598±0.012 ~ 13.503±0.018 Ma，除1颗不含包裹体的锆石外，其余锆石均早于结晶，表明富包裹体锆石来自更古老的热液蚀变围岩。xenotime日期对Th校正很敏感，但即使使用最大校正，206Pb/238U日期也在13.341±0.162 ~ 12.993±0.055 Ma之间，并且富含包裹体的锆石和不含包裹体的火成岩锆石都在之后结晶，反映了第二次热液事件。这些资料为下地壳在脊-脊转换交点的吸积过程中，在脊轴或其附近存在岩浆活动和热液变质交替作用提供了证据，表明热液蚀变的地壳被吸收为较年轻的辉长岩岩浆。研究结果表明，高精度U - Pb定年是研究洋中脊岩浆和热液过程时间的有力方法。
New U/Pb analyses of zircon and xenotime constrain the timing of magmatism, magmatic assimilation, and hydrothermal metamorphism during formation of the lower crust at the Mid‐Atlantic Ridge. The studied sample is an altered gabbro from the Vema lithospheric section (11°N). Primary gabbroic minerals have been almost completely replaced by multiple hydrothermal overprints: cummingtonitic amphibole and albite formed during high‐temperature hydration reactions and are overgrown first by kerolite and then prehnite and chlorite. In a previous study, clear inclusion‐free zircons from the sample yielded Th‐corrected206Pb/238U dates of 13.528 ± 0.101 to 13.353 ± 0.057 Ma. Ti concentrations, reported here, zoning patterns and calculated Th/U of the dated grains are consistent with these zircons having grown during igneous crystallization. To determine the timing of hydrothermal metamorphism, we dated a second population of zircons, with ubiquitous <1–20 µm chlorite inclusions, and xenotimes that postdate formation of metamorphic albite. The textures and inclusions of the inclusion‐rich zircons suggest that they formed by coupled dissolution‐reprecipitation of metastable igneous zircon during or following hydrothermal metamorphism. Th‐corrected206Pb/238U dates for the inclusion‐rich zircons range from 13.598 ± 0.012 to 13.503 ± 0.018 Ma and predate crystallization of all but one of the inclusion‐free zircons, suggesting that the inclusion‐rich zircons were assimilated from older hydrothermally altered wall rocks. The xenotime dates are sensitive to the Th correction applied, but even using a maximum correction,206Pb/238U dates range from 13.341 ± 0.162 to 12.993 ± 0.055 Ma and postdate crystallization of both the inclusion‐rich zircons and inclusion‐free igneous zircons, reflecting a second hydrothermal event. The data provide evidence for alternating magmatism and hydrothermal metamorphism at or near the ridge axis during accretion of the lower crust at a ridge‐transform intersection and suggest that hydrothermally altered crust was assimilated into younger gabbroic magmas. The results of this study show that high‐precision U‐Pb dating is a powerful method for studying the timing of magmatic and hydrothermal processes at mid‐ocean ridges.