Emplacement mechanism and thermal aureole provoked by the Ediacaran Catingueira pluton, Borborema Province, NE Brazil

Abstract

The Catingueira pluton, with an approximate area of 12 km2, is an Ediacaran peralkaline granite located between the Piancó-Alto Brígida Terrane and the Patos Shear Zone, Borborema Province (NE Brazil). The granite’s intrusion formed a metamorphic aureole over the host metapelitic rocks of the Santana dos Garrotes Formation. The present paper deals with the characterization of this thermal effect by using field relationships, petrography, mineral chemistry and petrophysical properties (thermal conductivity, specific heat, thermal diffusivity and density). An average value of 760 ± 18ºC was calculated for the peralkaline magma’s initial temperature using the zirconium geothermometer. The pluton emplacement provides two regional effects: verticalization of previous low angle structures of the host metapelites, which acquired an E-W direction parallelingthe mylonites of the Patos shear zone; high temperature (450–610ºC) and low pressure (3 kbar) metamorphism up to 2.5 km away from the pluton contact, and development of an assemblage including biotite ± garnet ± staurolite ± cordierite ± sillimanite (+ chlorite, + muscovite). Numerical modelling of the thermal regime was calculated considering two geometric shapes: a vertical cylinder and a horizontal parallelepiped, and on geothermal gradients varying from 30 to 50ºC/km. To achieve thermal equilibrium, cooling times of 781, 1,032 and 1,127 thousand years were calculated. The best fit was obtained for a geothermal gradient of 50ºC/km and a cooling time of 1,127 thousand years. The results obtained here in terms of dimension, shape, depth, and metamorphic assemblages are comparable to other examples of plutonic bodies described in the Borborema Province.