Variation in peak P–T conditions across the upper contact of the UHP terrane, Dabieshan, China: gradational or abrupt?
Variation in peak P–T conditions across the upper contact of the UHP terrane, Dabieshan, China: gradational or abrupt?
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DOI：
10.1111/j.1525-1314.2006.00670.x
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发表时间：
2006-12
影响因子：
3.4
通讯作者：
Y. Shi;Q. Wang
中科院分区：
文献类型：
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作者：
Y. Shi;Q. Wang
大别山南部地体（SDT）此前被分为高压（HP）和超高压（UHP）地体，其区域范围和边界的构造性质一直是争论的焦点。本研究详细绘制了太湖附近100平方公里的榴辉岩含岩区，根据岩性特征将其划分为北区、中区和南区。北带由绿帘黑云片麻岩和榴辉岩块组成，中带由花岗片麻岩、黑云片麻岩、榴辉岩和角闪岩组成，南带主要由含石榴石云母片岩组成。榴辉岩主要以透镜体或块体的形式出现在北部和中部地区。使用 Grt-Cpx Fe2+-Mg 温度计和 Grt-Cpx-Phe 气压计估算了该地区 61 个榴辉岩样品的峰值 P-T 条件。结果显示了三个不同的 P-T 区域：北部地区为 2.82–4.09 GPa/759–942 °C，花岗片麻岩为 2.00–3.54 GPa/641–839 °C，中部地区黑云片麻岩为 1.38–2.36 GPa/535–768 °C。结合该地区榴辉岩的空间分布，榴辉岩的P-T值从南向北不断增加，定义了5°C km−1的参考“地温”。然而，沿该地区南北两条剖面可能会出现一些不合理的表观梯度，并在北部和中部地区之间显示出 P-T 差异。根据该地区榴辉岩的平均 P-T 数据，北部和中部地区之间至少存在 0.3 GPa/20 °C 的差距。相比之下，中部地区榴辉岩的 P-T 值表现出一致的模式，具有从南部高压到北部超高压的过渡特征。我们认为，SDT 在俯冲过程中是一个连贯板片，在折返过程中被一个大断层破坏，该断层是在超高压变质条件下形成的。
The Southern Dabieshan Terrane (SDT) has previously been divided into high‐pressure (HP) and ultrahigh‐pressure (UHP) terranes, and its regional extent and the tectonic nature of its boundaries are hotly debated topics. In this study, an eclogite‐bearing area of 100 km2 near Taihu is mapped in detail, and divided into Northern, Middle and Southern Zones on the basis of lithological characteristics. The Northern Zone consists of epidote‐biotite gneiss and eclogite blocks, the Middle Zone includes granitic gneiss, biotite gneiss, eclogites and amphibolite, and the Southern Zone is composed mainly of garnet‐bearing mica schist. The eclogites occur mainly as lens or blocks in the Northern and Middle Zones. The peak P–T conditions for 61 eclogite samples across the area are estimated using the Grt‐Cpx Fe2+‐Mg thermometers and the Grt‐Cpx‐Phe barometers. The results indicate three different P–T regions: 2.82–4.09 GPa/759–942 °C in the Northern Zone, and 2.00–3.54 GPa/641–839 °C in the granitic gneiss and 1.38–2.36 GPa/535–768 °C in the biotite gneiss from the Middle Zone. Combined with the spatial distribution of eclogites across the area, the P–T values for eclogites increase continuously from the south to the north, defining a reference ‘geotherm’ of 5 °C km−1. However, some unreasonable apparent gradients can be established along two south–north profiles across the area, and display a P–T difference between the Northern and Middle zones. On the basis of the average P–T data for eclogites across the area, a gap of at least 0.3 GPa/20 °C exists between the Northern and Middle zones. By contrast, the P–T values of eclogites from the Middle zone show a coherent pattern with transitional characteristics from HP in the south to UHP in the north. We suggest that the SDT was a coherent slab during subduction, and was broken up by a major fault during exhumation, which was formed under UHP metamorphic conditions.