Abstract:

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.

Key words:  metamorphic pTt paths, conventional geothermobarometry, differential thermodynamics, metamorphic phase diagram