T12 High-Pressure and High-Temperature Metamorphism: P–T–t Paths and Tectonics

Sunday, 31 October 2010
8:00 AM - 12:00 PM
Room 203

THE ROLE OF RADIOGENIC HEAT PRODUCTION IN THE GENERATION OF ULTRA HIGH TEMPERATURE CRUSTAL METAMORPHISM

Paper number: T12-9
Program: Technical Sessions
Time: 10:10 AM – 10:25 AM

Authors

Abstract

How the Earth’s crust can reach temperatures greater than 900°C at depths less than 40 km to produce ultrahigh temperature (UHT) metamorphism is a question exercising the minds of many researchers. Many models of continental geothermal gradients fail to account for this type of metamorphism yet natural examples of these rocks are being identified more frequently in orogenic belts around the world. UHT metamorphism is best preserved in rocks of sedimentary origin. This is in part because sedimentary rocks have chemical compositions that generate distinctive mineral phases under conditions of extreme temperature, but there also is a strong indication that this style of metamorphism is often associated with tectonic inversion of a sedimentary basin. It is widely accepted that such high geothermal gradients require thickening of crustal rocks that are either already anomalously hot, or have the potential to become so through elevated concentrations of U, Th and K. The applicability of these models hinges on two key factors (1) that there is a threshold enrichment of the relevant crustal column in U, Th and K and (2) the crust has enough time to respond conductively to the heat generated through the radioactive decay of these elements.

In this presentation we will examine these two factors in an ideal natural laboratory, the Madurai Block of the Southern Granulite Terrane, India. We will constrain the duration of high-geothermal metamorphism through the application of in-situ Sensitive High Resolution Ion Probe (SHRIMP) geochronology linked to the development of UHT mineral assemblages. We will also present 1D numerical models for the temporal evolution of geothermal gradients in these rocks. Our models couple the temperature dependence of thermal conductivity and heat capacity from recent studies with in-situ radiogenic heat production measurements from lithologies within the Madurai Block and integrate the effect of the consumption of heat due to the initiation of partial melting.