科研之友
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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: O-18-depleted rocks (as low as delta O-18 = -7.4 degrees for garnet) indicate interaction with cold climate meteoric waters, whereas O-18-normal rocks (with bulk delta O-18 > +5.6 degrees) 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 delta O-18 eclogites usually have higher hydroxyl-mineral contents than the normal delta O-18 rocks, indicating higher water content during UHP metamorphism. Fluid inclusion data suggest that rocks with depleted O-18 compositions have had different fluid histories compared to those with normal delta O-18 values. Rocks with depleted O-18 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 delta O-18 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.
- 出版日期2006-10
- 单位中国地质科学院
