| 华南陆块北缘大陆裂断带高温低压变质作用 |
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| 引用本文： | 贺强,郑永飞.华南陆块北缘大陆裂断带高温低压变质作用[J].地球科学,2019,44(12):4186-4194. |
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| 作者姓名： | 贺强 郑永飞 |
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| 作者单位： | 中国科学院壳幔物质与环境重点实验室, 中国科学技术大学地球和空间科学学院, 安徽合肥 230026 |
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| 基金项目： | 科技部“973”计划项目2015CB856102 |
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| 摘 要： | 高温低压变质岩的形成要求高的热梯度（>30℃/km），所对应的构造环境一直受到地质学界的关注.本文总结了我们对华南陆块北缘新元古代Rodinia超大陆裂解（breakup）时期形成的变质花岗岩和变质玄武岩所进行的岩石学和地球化学研究成果，强调大陆裂断（rift）带是形成高温低压变质岩最可能的构造环境.高温低压变质作用主要记录在含铝硅酸盐矿物的变质花岗岩中，其中所含的红柱石和夕线石为变质成因，由白云母脱水反应产生.根据含铝硅酸盐矿物的峰期矿物组合和视剖面计算，得到变质温压条件为560~660℃/1.0~3.5 kbar.变质红柱石具有非常负的δ18O值，并且与岩浆锆石处于氧同位素不平衡状态，进一步证明它是岩浆结晶后变质作用的产物.变质榍石U-Pb定年得到高温低压变质作用的年龄为751±11 Ma，与Rodinia超大陆裂解峰期年龄一致.变质玄武岩显示岛弧型微量元素分布特征，指示其源区为受俯冲大洋地壳来源流体交代的地幔楔，因此地幔源区形成于格林威尔期Rodinia超大陆聚合过程中.由此可见，导致超大陆裂解的大陆裂断是在古俯冲带基础上发育的.通过对比形成变质峰期矿物组合所需的热流值和变质花岗岩中产热元素提供的热流值，得知大陆裂断带确实存在来自软流圈地幔的异常高热流，这使得超大陆裂解过程可以发育高温低压变质作用.
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| 关 键 词： | 大陆裂断 超大陆裂解 高温低压变质 高热梯度 铝硅酸盐矿物 岩石学 |
| 收稿时间： | 2019-09-24 |
High-Temperature/Low-Pressure Metamorphism in a Continental Rift in the Northern Margin of the South China Block |
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| Abstract: | The formation of high-temperature (HT)/low-pressure (LP) metamorphic rocks requires high thermal gradients of > 30℃/km. It is intriguing which tectonic setting is responsible for such geological processes. This paper presents a summary of our petrological and geochemical studies on metagranite and metabasalt from the northern margin of the South China block, which were formed during breakup of Rodinia supercontinent in the middle Neoproterozoic. The results demonstrate that continental rifts are the most plausible setting for the production of HT/LP metamorphic rocks. The HT/LP metamorphism is mainly recorded in alumino silicates-bearing metagranites, in which metamorphic andalusite and sillimanite were produced by muscovite dehydration reaction. Metamorphic P-T conditions of 1.0-3.5 kbar and 560-660℃ were obtained from the petrology of aluminosilicates-bearing peak mineral assemblages in combination with pseudosection calculations. The metamorphic andalusite shows very negative δ18O values in O isotope disequilibrium with magmatic zircon, further demonstrating that it is the metamorphic product after magma crystallization. The U-Pb dating of metamorphic titanite yields concordant ages of 751±11 Ma for the HT/LP metamorphism, consistent with the peak age of the Rodinia breakup. The metabasalt shows island arc basalts-like trace element distribution patterns, indicating that its source was generated by metasomatic reaction of the mantle wedge peridotite with fluids derived from the subducting oceanic crust. Therefore, the mantle source was formed during the Grenvillian assembly of Rodinia supercontinent. In this regard, the continental rifting that resulted in the supercontinental breakup was developed in the former subduction zone. By comparing heat flow required to form the metamorphic peak mineral assemblages with that provided by heat producing elements in the metagranites, it appears that anomalously high heat flow was indeed delivered from the asthenospheric mantle to the continental rift, leading to the HT/LP metamorphism during the Rodinia breakup. |
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