Samail蛇绿岩俯冲起始的时间演化:变质底的高精度U-Pb锆石年代学

IF 3.5 2区 地球科学 Q1 GEOLOGY Journal of Metamorphic Geology Pub Date : 2023-03-10 DOI:10.1111/jmg.12719
Matthew Rioux, Joshua M. Garber, Michael Searle, James L. Crowley, Sally Stevens, Mark Schmitz, Andrew Kylander-Clark, Kayla Leal, Tyler Ambrose, Andrew J. Smye
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引用次数: 2

摘要

蛇绿岩变质底的高精度定年可以为蛇绿岩的构造演化和俯冲带过程提供深入的认识。为了了解年轻的、保存完好的、具有良好特征的蛇绿岩的俯冲起始,我们对来自Samail蛇绿岩(阿曼-阿拉伯联合酋长国)变质底的25个样品进行了耦合锆石激光烧蚀电感耦合质谱痕量元素分析和高精度同位素稀释-热电离质谱U-Pb测年。角闪岩-麻粒岩相(0.8 ~ 1.3 GPa, ~700 ~ 900℃)的锆石颗粒,含石榴石-斜辉石角闪岩样品(n = 18)的重稀土元素斜率(HREE)呈系统递减趋势;Yb/Dy)随着Yb浓度的降低,反映了在石榴石生长过程中稀土元素的逐渐耗竭。对于一半的石榴石-斜辉石角闪岩样品,Ti - in -锆石温度升高,U-Pb年龄随着HREE斜率的减小而变年轻,这与锆石和石榴石在进变质过程中的耦合生长一致。在其余样品中,Ti - In -锆石温度随HREE斜率的减小没有明显变化,U-Pb和地球化学数据表明,锆石结晶沿变质P-T-t路径的渐近至峰或渐近至初始逆行部分。新资料将变质底最高品位岩石中前进式石榴石和锆石生长的时间定在96.698±0.094 ~ 95.161±0.064 Ma之间,而之前发表的地质年代学表明前进式变质作用在~104 Ma。鞋底较低品位(但仍为较高品位角闪岩相)部分的无石榴石角闪岩和白垩质角闪岩均富集稀土元素(Yb/Dy bbb5),比同一地点的较高品位岩石年轻~0.5 ~ 1.3 Myr,限制了高、低品位单元变质作用和并置的时间偏移。除一块角闪岩外,其余角闪岩的正锆石εHf(+6.5 ~ +14.6)与鞋底的洋玄武岩原岩一致。我们的新数据表明,前进单变质作用(96.7-95.2 Ma)早于上覆蛇绿岩壳(96.1-95.2 Ma)的发育和重叠。蛇绿岩北部单一变质作用开始与蛇绿岩岩浆活动开始之间的~600 ky偏移量比先前提出的~8 Ma短一个数量级,与自发俯冲起始或诱导俯冲起始的短暂初始逆冲期相一致。综上所述,底壳和蛇绿岩壳保存了前~1.5 Myr的俯冲记录。从西北(96.7 Ma)到东南(96.0-95.7 Ma)的高变质作用起始梯度可能记录了新生俯冲带的扩展和/或俯冲速率沿蛇绿岩长度的变化。
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The temporal evolution of subduction initiation in the Samail ophiolite: High-precision U–Pb zircon petrochronology of the metamorphic sole

High-precision dating of the metamorphic sole of ophiolites can provide insight into the tectonic evolution of ophiolites and subduction zone processes. To understand subduction initiation beneath a young, well-preserved and well-characterized ophiolite, we performed coupled zircon laser-ablation inductively coupled mass spectrometry trace element analyses and high-precision isotope dilution-thermal ionization mass spectrometry U–Pb dating on 25 samples from the metamorphic sole of the Samail ophiolite (Oman-United Arab Emirates). Zircon grains from amphibolite- to granulite-facies (0.8–1.3 GPa, ~700–900°C), garnet- and clinopyroxene-bearing amphibolite samples (n = 18) show systematic trends of decreasing heavy rare earth element slope (HREE; Yb/Dy) with decreasing Yb concentration, reflecting progressive depletion of the HREE during prograde garnet growth. For half of the garnet-clinopyroxene amphibolite samples, Ti-in-zircon temperatures increase, and U–Pb dates young with decreasing HREE slope, consistent with coupled zircon and garnet growth during prograde metamorphism. In the remaining samples, there is no apparent variation in Ti-in-zircon temperature with decreasing HREE slope, and the combined U–Pb and geochemical data suggest zircon crystallization along either the prograde to peak or prograde to initial retrograde portions of the metamorphic P–T–t path. The new data bracket the timing of prograde garnet and zircon growth in the highest grade rocks of the metamorphic sole between 96.698 ± 0.094 and 95.161 ± 0.064 Ma, in contrast with previously published geochronology suggesting prograde metamorphism at ~104 Ma. Garnet-free amphibolites and leucocratic pods from lower grade (but still upper amphibolite facies) portions of the sole are uniformly HREE enriched (Yb/Dy > 5) and are ~0.5–1.3 Myr younger than the higher grade rocks from the same localities, constraining the temporal offset between the metamorphism and juxtaposition of the higher and lower grade units. Positive zircon εHf (+6.5 to +14.6) for all but one of the dated amphibolites are consistent with an oceanic basalt protolith for the sole. Our new data indicate that prograde sole metamorphism (96.7–95.2 Ma) immediately predated and overlapped growth of the overlying ophiolite crust (96.1–95.2 Ma). The ~600 ky offset between the onset of sole metamorphism in the northern portion of the ophiolite versus the start of ophiolite magmatism is an order of magnitude shorter than previously proposed (~8 Ma) and is consistent with either spontaneous subduction initiation or an abbreviated period of initial thrusting during induced subduction initiation. Taken together, the sole and ophiolite crust preserve a record of the first ~1.5 Myr of subduction. A gradient in the initiation of high-grade metamorphism from the northwest (96.7 Ma) to southeast (96.0–95.7 Ma) may record propagation of the nascent subduction zone and/or variations in subduction rate along the length of the ophiolite.

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来源期刊
CiteScore
6.60
自引率
11.80%
发文量
57
审稿时长
6-12 weeks
期刊介绍: The journal, which is published nine times a year, encompasses the entire range of metamorphic studies, from the scale of the individual crystal to that of lithospheric plates, including regional studies of metamorphic terranes, modelling of metamorphic processes, microstructural and deformation studies in relation to metamorphism, geochronology and geochemistry in metamorphic systems, the experimental study of metamorphic reactions, properties of metamorphic minerals and rocks and the economic aspects of metamorphic terranes.
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