REGIONAL CONTACT METAMORPHISM: THE NANAO METAMORPHIC EVENT IN TAIWAN

Young apparent ages of thermochronometers in the high-grade rocks of the Tananao complex, eastern Taiwan have been taken as evidence that tectonic loading and heating was achieved in a short-lived “regional” metamorphic event the “Taiwan orogeny” during the still active collision of the Luzon arc with the continental margin of SE China (e.g. Simoes et al., 2007). However, the discovery that metamorphic zircons in a contact aureole of one of several regionally extensive intrusive granodioritic sheets have the same age as those intrusions suggests that virtually all of the high-grade metamorphism was caused instead by coalescing contact aureoles at ~85 Ma (Wintsch et al., 2011). We test the geodynamic plausibility of this with a 2D transient finite element thermal model which assumes that the sheets were discoid with a diameter equivalent to their strike length and, as anatexis occurs in the contact aureoles, with initial temperature of 850°C. The model begins at 90 Ma with the present erosion level at 15 km, a depth consistent with biotite grade regional metamorphic conditions, with staurolite+kyanite (no andalusite) aureole assemblages, and with metamorphic pressure estimates of 4-5 kbar. We model emplacement both as sills in horizontal strata and dipping sheets in tilted strata. Sills produce a significant increase in metamorphic gradient with depth (i.e. from west to east) that is not observed, suggesting tilting of the entire section occurred during Early Cretaceous loading and predates emplacement. Sheet emplacement raises initial temperatures from <350°C between the sheets to >450°C and to ~600°C near the contact aureoles. The model also produces uplift and cooling curves from 85 to 6 Ma consistent with Cenozoic thermochronologic data. Horizontal sections through the model space predict concave up PT gradients in the contact aureoles. Temperatures estimated from Raman spectroscopy of kerogen (Beyssac et al. 2007) show similar gradients, consistent with heating in these contact aureoles. Thus the model results show that coalescing contact aureoles are a satisfactory explanation for the high grade metamorphism in the Tananao complex which should not be cited as evidence for heating of continental crust under an overriding volcanic arc.