变质作用p-T-t轨迹的研究方法与进展

Approaches and advancement of the study of metamorphic p-T-t paths.

变质作用p-T-t轨迹理论的提出完全改变了人们对变质作用过程的认识。地壳加厚区(造山带)内区域变质作用发生于地壳从热扰动到热松弛的动态演化过程中,岩石的p-T-t轨迹是地壳加厚的方式及机制、热松弛速率和岩石折返速率的综合函数。一维热模拟假设引起热扰动的构造作用在瞬间完成,岩石在折返过程中开始变质作用演化。二维热模拟结果表明在地壳加厚过程中(岩石埋藏阶段)伴随明显热效应,岩石在折返过程中有少量加热,达到温度峰值。反演变质作用的p-T-t轨迹包括3种方法:传统地质温压计方法,吉布斯/微分热力学方法和变质相图方法。无论哪种方法,都必须以详细岩相学研究为基础,在岩石中划分出两期以上矿物组合。传统地质温压计方法被广泛使用,但在确定不同期次矿物组合的平衡和p-T条件上有若干不确定性。吉布斯/微分热力学方法理论上非常完善,依据矿物(石榴石)的生长环带计算岩石的p-T-t轨迹,但是由于难以确定矿物生长阶段的矿物组合变化,以及缺少复杂固溶体的活度模型等,致使该方法实用性较差。目前反演岩石p-T-t轨迹的最好方法是变质相图方法,该方法依据p-T视剖面图上矿物等值线温压计,模拟由矿物的世代关系和生长环带所记录的p-T条件变化,并可以定量模拟变质过程中的矿物组合演化、变质反应和流体行为。对不同中压型变质带和超高压型地体中岩石p-T-t轨迹的反演模拟表明,岩石在构造埋藏阶段应伴随明显热效应,发生一系列递增变质作用,几乎同时达到压力与温度峰值,岩石折返过程以等温降压型(ITD)为主。这与一维热模拟结果很不相同,而与二维热模拟结果大体相似,但一般折返速率更快。

Development of the metamorphic p-T-t path has completely modified the understanding of metamorphic processes.The regional metamorphism in a thickened crustal region（orogenic belt） is considered to occur in a dynamic process from a tectonic perturbation to subsequent thermal relaxation.Thus,a p-T-t path for a rock is a function of models and mechanism of crustal thickening,of the rate of thermal relaxation and the rate of rock exhumation.Results from one-dimensional thermal modeling suggest that the tectonic perturbation would occur almost instantaneously and metamorphism starts to evolve during exhumation of rock.Two-dimensional thermal modeling indicates that there is pronounced heating during the crustal thickening（burial stage） and a rock reaches the maximum temperature during its exhumation as a result of a slight heating.The reverse modeling of metamorphic p-T-t paths involves three approaches：the conventional geothermobarometry,Gibbs/differential thermodynamics and metamorphic phase diagram.Modeling of a p-T-t path for a rock using any of the three approaches should be on the basis of detailed petrographic observations,for instance,recognizing at least two stages of mineral assemblage.The conventional geothermobarometry approach has been widely used,but there are uncertainties in the determination of equilibrium domain and p-T conditions for different stages of mineral assemblage.The Gibbs/differential thermodynamics approach,that is to calculate a p-T-t path based on a growth zoning of mineral（garnet）,has been considered to be perfect theoretically.However,this approach is difficult to be used because：（i） it is hard to determine the evolvement of mineral assemblage during the growth of zoned minerals;and（ii） there is the lack of available activity models for complicated solid solutions.At present,the metamorphic phase diagram approach is the best one to model the p-T-t path.With this approach,a p-T-t path recorded in mineral generations and growth zoning can be well determined by isopleth-thermobarometry of mineral composition in a p-T pseudosection.Moreover,evolvement of mineral assemblage,metamorphic reaction and fluid flowing during a metamorphic process can also be quantitatively modeled.The p-T-t paths reversely modeled from several medium-pressure metamorphic belts and ultrahigh-pressure terranes indicate that during tectonic burial,there is pronounced heating,coupled with a series of metamorphic evolvements,and the maximum pressure in a rock is reached almost simultaneously as the maximum temperature.The exhumation of rock is characterized by isothermal decompression（ITD）.These are distinct from the results by one-dimensional thermal modeling,and approximate to those from two-dimensional thermal modeling except for the rate of exhumations being much faster.