岩石密度和超高压岩石折返速率  

Ultrahigh-pressure rock, density, buoyancy, exhumation mechanism, exhumation rate

作　　者：杨建平[1] 金振民[2] 欧新功[3] 徐海军[1] 

机构地区：[1]中国地质大学(武汉)地球科学学院 [2]中国地质大学过程与矿产资源国家重点实验室,武汉430074 [3]中国科学院广州地球化学研究所,广州510640

出　　处：《岩石学报》2005年第2期427-437,共11页Acta Petrologica Sinica

基　　金：国家重点基础研究发展规划项目(2003CB716506)国家自然科学基金(40172068)中国地质大学地质学理科基地项目联合资助

摘　　要：在常温常压条件下对中国大陆科学钻CCSD主孔100-3000米岩心样品进行了密度测量,建立了密度连续剖面,并界定了不同超高压岩石的密度值。通过对比高温高压物性实验资料,岩石密度随着退变程度增强而降低,榴辉岩密度变化序列为3.52g/cm3→3.49g/cm3→3.07g/cm3→2.93g/cm3。超高压长英质岩石密度变化序列为3.00g/cm→2.80g/cm3→2.65g/cm3。上述实验资料是讨论不同折返阶段岩石所受浮力的基础,为研究折返速率大小提供了基本参数。本文通过折返板块运动平衡时,上浮力与粘滞力平衡这一关系式,定量研究了大陆俯冲板块的折返速率,认为密度差产生上浮力从而引起折返,温度对板块折返速率的影响最为显著;密度差大小、折返角度、折返板块大小对折返速率也有直接的影响。定量模拟分析表明,在温度高于850℃时,板块的折返速率可以超过100mm/a;当温度降至700℃时,折返速率则低于1.5mm/a。作者认为在折返早期,温度较高,板块快速折返至60-70km榴辉岩相深度;随着传导散热,温度降低,板块以较慢的速率折返至中下地壳。折返速率的估算表明,浮力是板块折返第一阶段(从>100km深部折返至<40km的中下地壳)的主要驱动力。The densities of cores from main hole (100 - 3000m) of Chinese Continental Scientific Drilling (SSCD) have been performed in the conditions of room temperature and pressure. The continuous section of densities from 100m to 3000m was constructed and densities of several kind of UHP rocks were constrained. Referring to the results of about 30 samples measured under high temperature (600 degrees C) and high pressure (600MPa), we found that the density of eclogite changes in this sequence 3.52 g/cm(3) -> 3.49 g/cm(3) -> 3.07 g/cm(3) -> 2.93 g/cm(3) while UHPM felsic rock changes as 3.00 g/cm(3) -> 2.80 g/cm(3) -2.65 g/cm(3), all in accordance with UHP rocks' retrogression. These results are the prerequisite parameters for the discussion of exhumation rate of UHP rocks. Based on the truth that the gravity, buoyant force is equal to viscous force while the exhumation block moves in balance, we did quantitative analysis of exhumation rate. According to the result, it is clear that the buoyancy associated with density contrast is the driven force for the UHP rock exhumation. Based on density contrast, temperature is the factor that influences the rate most. Density contrast exhumation angle and the volume of exhumation block are all important parameters for the construction of reasonable exhumation model. As the temperature drop from 850 degrees C to 700 degrees C, exhumation rate changes from 100mm/a to 1.51mm/a. So it is thought that in the early stage with high temperature UHP rocks move rapidly up to 60 similar to 70km, then as the temperature drops down, the blocks move slowly up to the middle-lower crust. Buoyancy is inferred to he the dominant. mechanism for the exhumation from deep mantle(> 100km) to lower crust(< 40km).

关 键 词：超高压岩石 密度 浮力 折返机制 折返速率 

分 类 号：P584[天文地球—岩石学] P542.2[天文地球—地质学]