CONTRIBUTION OF DEVIATORIC STRESSES TO METAMORPHIC P-T PATHS - AN EXAMPLE APPROPRIATE TO LOW-P, HIGH-T METAMORPHISM

P-T paths for a simple situation appropriate to many low-P, high-T (LP HT) terranes in which metamorphism and deformation are localized by ad vection of heat in magmas, has been modelled assuming a medium with a power-law rheology with an inverse exponential dependence of stress an d temperature and capable of sustaining deviatoric stress, tau, in the order of 100 MPa at 400-degrees-C and strain rates of up to 10(-13)s- 1. Numerical simulations and analytical approximations for P-T histori es appropriate for simple convergent deformation histories show that t he destruction of the deviatoric stress field around large intrusions may result in significant decompression near the metamorphic temperatu re peak. Moreover, for a specified strain rate and temperature evoluti on, P-T paths may vary from clockwise to anticlockwise merely as a fun ction of vertical distance from the heat source. Inasmuch as mounting independent evidence suggests that the crust can support deviatoric st resses of up to about 100 MPa at temperatures of 400-500-degrees-C, an d that the shear strength of the crust is strongly temperature-depende nt in the range 400-800-degrees=C, these results suggest that caution should be taken in the tectonic interpretation of P-T paths involving decompression of the order of 100 MPa or less in LPHT terranes. The re sults illustrate a plausible mechanism for the close spatial associati on of both clockwise and anticlockwise P-T paths documented in some LP HT terranes.