大陆深俯冲过程中的流体-岩石相互作用——中国大陆科学钻探主孔氧同位素剖面

Fluid-rock interaction during the continental deep subduction: Oxygen isotopic profile of the main hole of the CCSD project.

中国大陆科学钻探工程主孔深5158m,位于苏鲁超高压变质带南部。该钻孔钻进了一个超高压变质岩片,其主要榴辉岩、片麻岩、超基性岩和少量石英和片岩组成。基于连续样品的详细氧同位素分析可得出下列认识:(1)各种变质矿物的氧同位素成分具有很大的变化,其中石榴石为-7．4-+6．2‰,绿辉石为-5．2-+6．5‰,白云母为-4．4-+7．1‰,斜长石为-3．1-+7．6‰,钾长石为4．8-8．3‰,石英为-2．8-+9．6‰。(2)整个钻孔剖面的氧同位素成分是连续和逐渐的变化的,而且与岩石类型无关,氧同位素亏损岩石产出的最大深度为3320m。这个深度之下的岩石均具有正常变质岩的氧同位素值。(3)氧同位素明显亏损的变质岩大多出现在正、副片麻岩及其与榴辉岩的互层带。(4)榴辉岩矿物大多具有平衡的氧同位素分异,大多数高压和低压石英脉体具有与它们的围岩类似的氧同位素成分。(5)利用石英-石榴石氧同位素温度计,所获得的榴辉岩变质温度为598-909℃,片麻岩的变质温度为550-786℃。基于这些事实,并结合氧同位素亏损变质岩在地表和浅钻孔中的广泛分布,可以得出以下重要结论:(1)在苏鲁造山带,有大量的表壳岩石与巨量的寒冷气候下的大气降水发生过交换作用,这为新元古代全球冰期的存在提供了重要证据。(2)许多呈厚层产出的花岗质片麻岩具有正常变质岩的氧同位素成分,并缺少超高压变质作用证据。(3)在大陆深俯冲和折返过程中,只存在通道式的水-岩相作用和非常有限的流体活动,与高压退变质有关的流体是原地形成的。(4)新元古代Rodinia超大陆裂解环境下形成的双峰式岩浆活动为大气降水与表壳岩之间的热液蚀变提供了热源。

The Chinese Continental Scientific Drilling （CCSD） Main Hole with a depth of 5158m located in the Donghai County, the southern Sulu ultrahigh-pressure （UHP） metamorphic belt, has been penetrated into a UHP metamorphic tectonic slab which consists of eclogite, gneiss, garnet peridotite, and minor quartzite and schist. Minerals from various rocks show large variations of oxygen isotopic composition, with ranges from -7.4 to ＋6.2 for garnet, -5.2 to ＋6.5 for omphacite, -4.4 to ＋ 7, 1 for muscovite, - 3.1 to ＋ 7.6 for plagioclase, 4.8 to 8.3 for K-feldspar, and - 2.8 to ＋ 9.6 for quartz. Distinct O-isotopic depletions are observed in the frequently alternated layers of eclogite, paragneiss and orthogneiss, in which the variations of δ^18O values are gradual, regardless of lithologies. Minimum depth of the O-isotopic anomaly has reached down to 3320 m. Eclogite-facies minerals have equilibrium O-isotopic compositions; while minerals from most gneisses have disequilibrium δ^18O values. High-pressure and low-pressure quartz veins have similar or different O-isotopic compositions to their eclogite or gneiss country rocks. The metamorphic temperatures of 598 to 909 C for eclogites and 550 to 786 C for gneisses are estimated using the quartz-garnet thermometer of oxygen isotope. These facts, considering the distribution characters of O-isotopic depleted rocks in the outcrops and shallow drill holes of the Sulu area, suggest that ： （1） a large amount of supracrustal rocks have interacted with the meteoric water during the Neoproterozoic global glaciation; （2）some orthogneisses from this drill hole show normal O-isotopic value, and have no any evidence of UHP metamorphism; （3） the channelized water/rock interactions and very limited fluid mobility was present during subduction and exhumation of the continental crust, The fluids related to the retrograde metamorphism of UHP rocks and the formation of some HP and LP veins are of in-situ origin; （4） the bimodal magmatic activities which were formed during the breakup of Rodinia supercontinent have provided the heating source for the rock - cold meteoric water interactions.