Older orogens cooled slower: new constraints on Orosirian tectonics from garnet diffusion modeling of metamorphic timescales, Jiaobei terrain, North China Craton
Older orogens cooled slower: new constraints on Orosirian tectonics from garnet diffusion modeling of metamorphic timescales, Jiaobei terrain, North China Craton
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DOI：
10.1007/s00410-021-01846-w
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发表时间：
2021-10
影响因子：
3.5
通讯作者：
Yinqiao Zou;Qiu-li Li;Xu Chu;M. Zhai;R. Mitchell;Lei Zhao;Ligang Zhou;Yuquan Wang;Bo Liu-Bo-L
中科院分区：
文献类型：
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作者：
Yinqiao Zou;Qiu-li Li;Xu Chu;M. Zhai;R. Mitchell;Lei Zhao;Ligang Zhou;Yuquan Wang;Bo Liu-Bo-L
变质时间尺度为量化岩石圈过程和识别构造机制提供了重要信息。在2.1-1.8 Ga期间，以全球造山活动而闻名的奥陶纪可能标志着全球尺度板块构造的出现。虽然变质压力-温度（P-T）路径和t /比值已被广泛用于表征2.1-1.8 Ga造山带，但其详细的变质时间尺度仍未得到充分利用。本文采用石榴石扩散模拟方法，明确探讨了华北克拉通胶辽基造山带西南段胶北地体优势泥质麻粒岩的变质时间尺度。泥质麻粒岩的高压麻粒相变质作用为13-14 kbar，温度为860-870℃，随后快速减压（4-21 mm yr - 1），并在一定程度上加热至7-8 kbar，温度为865-950℃（1.85 Ga）的中压麻粒相沉积。随后的冷却几乎是等压的，但可能是非线性的，最初更快，后来更慢的冷却（17-32°C Myr - 1,950 /865 - 700°C; 5-7°C Myr - 1,700 - 600°C）。确定的顺时针ep - t路径为收敛的板块边界过程提供了证据，快速发掘可能与伸展环境有关。通过扩散计时法获得的相对较快的高温掘出和冷却速率代表了2.1 ~ 1.8 Ga造山带中迄今报道的最快速率，比在现代造山带中观察到的速率慢几倍到一个数量级。根据最近的数值模拟，我们认为。2.1 - 1.8 Ga JLJB造山岩的这些较慢的变质速率与发生在较高地幔温度下的造山作用特征相一致，例如造山热构造温度较高，造山地壳过厚较少，应变分布模式分布。因此，JLJB的时间尺度信息表明，较高温度下的古元古代板块构造可能导致了与现代不同的俯冲和碰撞造山作用。值得注意的是，通过扩散模拟获得的详细的古代变质时间尺度不能用地质年代学来解决，这突出了应用扩散年代学来量化古代构造过程的必要性。
Metamorphic timescales harbor crucial information for quantifying lithospheric processes and identifying tectonic regimes. The Orosirian Period is known for global orogenesis during 2.1‒1.8 Ga that possibly signifies the emergence of global-scale plate tectonics. While the metamorphic pressure–temperature (P–T) paths andT/Pratios have been extensively used to characterize 2.1–1.8 Ga orogens, their detailed metamorphic timescales remain underutilized. In this study, we use garnet diffusion modeling to explicitly explore the metamorphic timescales of dominant pelitic granulites in the Jiaobei terrane, the southwest part of the Jiao-Liao-Ji orogenic belt (JLJB), North China Craton. The pelitic granulites record high-pressure granulite-facies metamorphism of 13–14 kbar and 860–870 °C, followed by fast decompression (4‒21 mm yr−1) with some degree of heating to medium-pressure granulite-facies retrogression of 7‒8 kbar and 865‒950 °C atc.1.85 Ga. The subsequent cooling is nearly isobaric, but likely nonlinear with initially faster and later slower cooling (17‒32 °C Myr−1, 950/865–700 °C; 5–7 °C Myr−1, 700–600 °C, respectively). The determined clockwiseP–T–tpath provides evidence for convergent plate boundary processes and the fast exhumation is likely related to an extensional setting. The relatively fast exhumation and cooling rates at high temperatures, as obtained via diffusion chronometry, represent the fastest rates yet reported from 2.1 to 1.8 Ga orogens and are several times to one order of magnitude slower than those observed in modern orogens. In the light of recent numerical modeling, we suggest that these slower metamorphic rates of thec.2.1‒1.8 Ga JLJB orogenic rocks are consistent with the features of orogenesis occurring at higher mantle temperatures, such as hotter orogenic thermal structures, less over-thickened orogenic crusts, and distributed strain patterns. Therefore, the timescale information from the JLJB indicates that Paleoproterozoic plate tectonics under higher temperatures might have caused subduction and collisional orogenesis to be distinct from their modern style. Notably, the detailed ancient metamorphic timescales achieved by diffusion modeling cannot be resolved geochronologically, highlighting the necessity of applying diffusion chronometry to quantify ancient tectonic processes.