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首页> 外文期刊>Contributions to Mineralogy and Petrology >Ultrahigh-pressure metamorphic rocks from the Chinese Continental Scientific Drilling Project: II Oxygen isotope and fluid inclusion distributions through vertical sections
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Ultrahigh-pressure metamorphic rocks from the Chinese Continental Scientific Drilling Project: II Oxygen isotope and fluid inclusion distributions through vertical sections

机译：中国大陆科学钻探项目的超高压变质岩：II垂直剖面中的氧同位素和流体包裹体分布

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摘要

In order to reconstruct the formation and exhumation mechanisms of UHP metamorphic terrains, the Chinese Continental Scientific Drilling Program (CCSD) has been carried out in Donghai of the Dabie-Sulu ultrahigh-pressure (UHP) metamorphic belt, East China. Eclogite, gneiss, amphibolite (retrograded from eclogite), ultramafic rocks, and minor schist and quartzite have been drilled. Aiming to reveal the fluid behaviour in a vertical sequence of an UHP slab, we investigated fluid inclusion and oxygen isotope characteristics of selected drillcores from the main hole and the pilot-holes PP2 and ZK 703 of the CCSD. More than 540 laser-ablation oxygen isotope analyses on garnet, omphacite, quartz, kyanite, amphibole, phengite, rutile, epidote, amphibole, plagioclase, and biotite from various rocks in the depth range of 0–3,000 m (mainly eclogite and gneiss) show that the investigated rocks can be divided into two groups: 18O-depleted rocks (as low as δ18O = −7.4‰ for garnet) indicate interaction with cold climate meteoric waters, whereas 18O-normal rocks (with bulk δ18O > +5.6‰) have preserved the O-isotopic compositions of their protoliths. Meteoric water/rock interaction has reached depths of at least 2,700 m. Oxygen isotope equilibrium has generally been achieved. Isotopic compositions of mineral phases are homogeneous on a mm to cm scale regardless of lithology, but heterogeneous on the scale of a few metres. Oxygen isotope distributions in the vertical sections favour an “in situ” origin of the UHP metamorphic rocks. The very negative δ18O eclogites usually have higher hydroxyl-mineral contents than the normal δ18O rocks, indicating higher water content during UHP metamorphism. Fluid inclusion data suggest that rocks with depleted 18O compositions have had different fluid histories compared to those with normal δ18O values. Rocks with depleted 18O mainly have primary medium-to-high salinity inclusions in omphacite, kyanite and quartz, and abundant secondary low-salinity or pure water inclusions in quartz, indicating a high-salinity-brine-dominated fluid system during peak UHP metamorphism; no carbonic inclusions have been identified in these rocks. By contrast, primary very high-density CO2 inclusions are commonly found in the rocks with normal δ18O values. These observations suggest that fluid and oxygen isotope composition of minerals are related and reflect variable degrees of alterations of the Dabie-Sulu UHP metamorphic rocks.
机译：为了重建超高压变质地形的形成和发掘机制，中国大陆科学钻探计划（CCSD）已在中国东部大别苏鲁超高压变质带的东海进行。已钻出榴辉岩，片麻岩，角闪石（由榴辉岩还原），超镁铁质岩石，次片岩和石英岩。为了揭示UHP板在垂直方向上的流体行为，我们研究了CCSD主孔和先导孔PP2和ZK 703中选定钻芯的流体包裹性和氧同位素特征。在石榴石，绿辉石，石英，蓝晶石，角闪石，锂铁矿，金红石，山竹石，角闪石，斜长石和黑云母中，对深度范围为0–3,000 m（主要为榴辉岩和片麻岩）的黑云母进行了超过540种激光烧蚀氧同位素分析结果表明，被调查的岩石可分为两类：贫18的岩石（石榴石的δ18≤O= −7.4‰）表明与寒冷的气候水相互作用，而18 sup> O型常态岩石（体积δ18 O> + 5.6‰）保留了其原岩的O同位素组成。流水/岩石相互作用的深度至少达到了2700 m。通常已经实现了氧同位素平衡。不论岩性如何，矿物相的同位素组成在毫米到厘米范围内都是均匀的，但在几米范围内却是不均匀的。垂直剖面中的氧同位素分布有利于UHP变质岩的“原位”成因。极负的δ18 O榴辉岩通常比正常的δ18 O岩石具有更高的羟基矿物质含量，表明在UHP变质过程中含水量较高。流体包裹体数据表明，与18δsO值正常的岩石相比，组成18's O耗尽的岩石具有不同的流体历史。耗尽18 O的岩石主要在辉绿岩，蓝晶石和石英中具有主要的中至高盐度夹杂物，并且在石英中具有大量的次要低盐度或纯水夹杂物，这说明以高盐度为主导的流体系统在UHP变质高峰期在这些岩石中没有发现碳包裹体。相比之下，通常在δ18 O值正常的岩石中发现非常高密度的主要CO2 夹杂物。这些观察结果表明，矿物的流体和氧同位素组成是相关的，反映了大别-苏鲁超高压变质岩的变化程度。

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