Unpairing metamorphic belts: P–T paths and a tectonic model for the Ryoke Belt, southwest Japan
Unpairing metamorphic belts: P–T paths and a tectonic model for the Ryoke Belt, southwest Japan
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DOI：
10.1111/j.1525-1314.1998.00061.x
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发表时间：
1998-01
影响因子：
3.4
通讯作者：
Michael Brown
中科院分区：
文献类型：
--
作者：
Michael Brown
日本西南部被中央构造线（MTL）分为外部和内部区域，这是一个主要的横向断层。外带由Sambagawa（高压中或高P/T型变质作用）、秩父带和四万十带组成。在内带，Ryoke带（红柱石-硅线岩或低P/T型变质作用）主要发育在侏罗纪增生杂岩中。这种高P/T型和低P/T型变质带之间的空间关系使宫代提出了变质带是作为“成对”系统发育的观点。Ryoke带泥质岩中的结构关系和岩石成因上重要的矿物组合意味着最高级变质带的混合岩石榴石-堇青石岩石中的峰值P-T条件为105 kbar和高达850 °C。这是可能的热异常负责Ryoke带的变质作用有关的一段Farallon-Izanagi海岭，因为它俯冲到亚洲大陆的东缘在白垩纪。泥质岩中矿物组合的序列暗示了一个具有浅dP/dT斜率的变质场梯度，推测是由一组嵌套的发夹状“顺时针”P-T路径产生的。这些P-T路径的特征是有限的地幔增厚、峰温时的轻微减压和近等压冷却，这些特征可能是由脊俯冲引起的低P/T型变质带中的典型P-T路径。Ryoke带的山脊俯冲模型意味着Sambagawa带的高P/T变质岩与它并置的结果是混合的结果。在Ryoke和Sambagawa带岩石中记录为同变质、主要为收缩应变的平行带韧性拉伸，以及MTL上的大量左旋位移与左侧斜向会聚一致。穿时性在快速冷却的Ryoke带是隐含的现存的热年代学数据，并推断与渐进SW到NE对接的Sambagawa带。因此，在日本的“成对”变质带的另一种解释是，它们代表横向同生岩，而不是外侧和内侧组件的沟/弧“成对”系统。在前弧沿着大陆边缘的大平移过程中，横向同生岩的合并可以解释地质记录中“成对”变质带的其他例子。
Southwest Japan is divided into Outer and Inner Zones by the Median Tectonic Line (MTL), a major transcurrent fault. The Outer Zone is composed of the Sambagawa (high‐pressure intermediate or high P/T type metamorphism), Chichibu and Shimanto Belts. In the Inner Zone, the Ryoke Belt (andalusite– sillimanite or low P/T type metamorphism) was developed mainly within a Jurassic accretionary complex. This spatial relationship between high P/T type and low P/T type metamorphic belts led Miyashiro to the idea that metamorphic belts were developed as ‘paired’ systems. Textural relationships and petrogenetically significant mineral assemblages in pelites from the Ryoke Belt imply peak P–T conditions of ≈5 kbar and up to 850 °C in migmatitic garnet–cordierite rocks from the highest‐grade metamorphic zone. It is likely that the thermal anomaly responsible for metamorphism of the Ryoke Belt was related to a segment of the Farallon–Izanagi Ridge as it subducted under the eastern margin of the Asian continent during the Cretaceous. The sequence of mineral assemblages developed in pelites implies a metamorphic field gradient with shallow dP/dT slope, inferred to have been generated by a nested set of hairpin‐like ‘clockwise’P–T paths. These P–T paths are characterized by limited prograde thickening, minor decompression at peak‐T , and near‐isobaric cooling, features that may be typical of P–T paths in low P/T type metamorphic belts caused by ridge subduction. A ridge subduction model for the Ryoke Belt implies that juxtaposition of the high‐P/T metamorphic rocks of the Sambagawa Belt against it was a result of terrane amalgamation. Belt‐parallel ductile stretching, recorded as syn‐metamorphic, predominantly constrictional strain in both Ryoke and Sambagawa Belt rocks, and substantial sinistral displacement on the MTL are consistent with left‐lateral oblique convergence. Diachroneity in fast cooling of the Ryoke Belt is implied by extant thermochronological data, and is inferred to relate to progressive SW to NE docking of the Sambagawa Belt. Thus, an alternative interpretation of ‘paired’ metamorphic belts in Japan is that they represent laterally contemporaneous terranes, rather than outboard and inboard components of a trench/arc ‘paired’ system. Amalgamation of laterally contemporaneous terranes during large translations of forearcs along continental margins may explain other examples of ‘paired’ metamorphic belts in the geological record.