Constraining cooling histories: rutile and titanite chronology and diffusion modelling in NW Bhutan
Constraining cooling histories: rutile and titanite chronology and diffusion modelling in NW Bhutan
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DOI：
10.1111/j.1525-1314.2011.00958.x
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发表时间：
2012-02
影响因子：
3.4
通讯作者：
C. Warren;D. Grujic;J. Cottle;N. Rogers
中科院分区：
文献类型：
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作者：
C. Warren;D. Grujic;J. Cottle;N. Rogers
对金红石和钛矿的U-Pb分析通常得出的年龄限制了冷却的时间，而不是它们的结晶时间。在不丹西北部大喜马拉雅层序（GHS）并置岩性/构造地层单元中的基性麻粒岩、中间麻粒岩和基性角闪岩中生长于或接近峰值温度条件下的金红石，其LA-MC-ICP-MS U-Pb下截取冷却年龄分别为10.1±0.4、10.8±0.1和10.0±0.3 Ma。受先前发表的温度-时间和Pb扩散数据约束的数值有限差分扩散模型表明，这些年龄最好的解释是，从含麻粒岩单元的14 Ma ~ 800°C和含角闪岩单元的12 Ma ~ 650°C的峰值温度条件快速冷却。模型与分析年龄吻合较好，证实了实验数据表明金红石中Pb的保留率较高。在挖掘相关角闪岩相叠加在邻近Jomolhari地块榴辉岩相矿物组合上生长的钛矿，其U-Pb下截距冷却年龄为14.6±1.2 Ma。扩散模型表明，这个年龄太老，与金红石样品推断的温度-时间路径不一致。相反，钛矿年龄表明，在更早的17-15 Ma时间，从~ 650°C开始冷却，这意味着Jomolhari地块的高品位岩石经历了与不丹西北部GHS其他地区不同的冷却历史。这些数据表明，来自不丹西北部GHS三个明显不同结构水平的高品位岩石在不同时间经历了bb0 - 40°C Ma - 1的快速冷却。最高品位的麻粒岩相岩是在尼泊尔东部以西未暴露、未保存或尚未被识别的较深构造层中发掘出来的。这些热年代学数据暗示了整个造山带的构造制度、变质历史和/或挖掘机制的沿走向递进变化。
U–Pb analyses of rutile and titanite commonly yield ages that constrain the timing of cooling rather than the timing of their crystallization. Rutile which grew at or close to peak temperature conditions in a mafic granulite, intermediate granulite and mafic amphibolite within juxtaposed litho/tectonostratigraphic units in the Greater Himalayan Sequence (GHS) of NW Bhutan yield LA–MC–ICP–MS U–Pb lower intercept cooling ages of 10.1 ± 0.4, 10.8 ± 0.1 and 10.0 ± 0.3 Ma, respectively. Numerical finite‐difference diffusion models constrained by previously published temperature–time and Pb diffusion data suggest that these ages are best explained by rapid cooling from peak temperature conditions of ∼800 °C at 14 Ma in the granulite‐bearing unit and ∼650 °C at 12 Ma in the amphibolite‐bearing unit. The good fit between the model and analysed ages confirms the relatively high retention of Pb in rutile suggested by the experimental data. Titanite that grew during an exhumation‐related amphibolite facies overprint on an eclogite facies mineral assemblage from the neighbouring Jomolhari Massif yields a U–Pb lower intercept cooling age of 14.6 ± 1.2 Ma. Diffusion modelling suggests that this age is too old to be consistent with the temperature–time paths inferred for the rutile‐bearing samples. Instead, the titanite age suggests cooling from ∼650 °C at an earlier time of 17–15 Ma, implying that the high‐grade rocks in the Jomolhari Massif experienced a different cooling history from the rest of the GHS in NW Bhutan. Together these data show that high‐grade rocks from three apparently different structural levels of the GHS in NW Bhutan experienced rapid cooling at >40 °C Ma−1 at varying times. The highest grade granulite facies rocks were exhumed from deeper structural levels that are not exposed, not preserved, or not yet recognized west of eastern Nepal. A progressive along‐strike change in tectonic regime, metamorphic history and/or exhumation mechanism across the orogen is implied by these thermochronologic data.