Modeling metamorphism in collisional orogens intruded by magmas: I. Thermal evolution

Abstract

This paper presents the results of two-dimensional (2-D) numerical modeling that reconstructs the thermal evolution and the flow of fluid in a collisional overthrust setting with syn-metamorphic magmatism. The goal of the study is to explore the relationships between regional metamorphism and magmatic activity. Our results demonstrate the strong effects that syn-metamorphic magmatism exerts on the thermal structure of the thickened crust. Magmatic intrusions in the form of repetitive sills of mafic or intermediate-felsic compositions introduced at different levels in the thickened crust section may produce Buchan and Barrovian-type metamorphic se- quences, as well as rocks in the granulite facies of metamorphism. The peak tempera- tures in crustal rocks around intrusions are increased by more than 100 °C from their values for the non-magmatic case; the timescales of peak temperature attainment in rocks at short distances from the intrusions are only 1 Myr or less. The peak temperature attainment is synchronous within the different metamorphic zones over the depth range of up to 20 km. These features differ strongly from those in the overthrust model without magmatism, characterized by broad thermal peaks spanning 5 to 10 Myr and non-synchronous attainment of peak conditions for rocks at different depths, with 5 Myr delay for every 10 km of depth (exhumation rate 1m m yr 1 ). The specific peak metamorphic mineral assemblages that develop in model rock of metapelitic composition as a result of magmatism are largely controlled by the depth of magma emplacement: intrusion into the lower plate of the overthrust zone leads to the development of amphibolite facies mineral assemblages in the staurolite, kyanite and garnet-biotite (without chlorite) zones, as well as considerable volumes of granu- lite facies rocks. Emplacement of intrusions at mid-crustal depths of 15 to 35 km produces minerals of low-pressure/high-temperature (LP/HT) metamorphic zones, such as cordierite-biotite-K-feldspar, in addition to staurolite, kyanite, and sillimanite zones. When mid-crustal magmatism is initiated at earlier times in the model thrust exhumation history, the LP/HT metamorphic zones may not be preserved in the crustal section due to surface erosion. All of the models predict extremely steep, approximately isobaric Metamorphic Field Temperature Gradients (MFTGs) around the edges of the intrusions and along the contacts of thick isolated sills. In some cases, the transition from chlorite to sillimanite zones of metamorphism at the side of the magmatic region may take place in less than 1 km. The magmatism-related MFTGs stand in contrast to the regional metamorphic gradients produced by crustal thickening that form at the flanks of the overthrust zone distal to the magmas. Here, the transition from the chlorite zone to high-grade mineral assemblages takes place over 20 to 40 km, and the pressure of peak temperature attainment increases by as much as 0.3 GPa from low to high grade.