Determining the significance of high-grade shear zones by using temperature-time paths, with examples from the Grenville orogen

Abstract

Ductile shear zones preserve essential information on processes that are active in orogenic roots, but the significance of these zones is often difficult to interpret. Structural, petrologic, and geochronologic data from shear zones yield elements of the history that are not necessarily synchronous. However, by combining these data with temperature-time ( T-t ) paths, insights are obtained into the nature of shear zones, the relation between bounding blocks, and orogenic evolution of the deep crust. This procedure is illustrated with two examples from the mid-Proterozoic Grenville orogen. T-t paths from ∼1160 to ∼900 Ma are based on U-Pb dating of metamorphic minerals—including garnet (closure temperature, T c , >800 °C), monazite ( T c ≅ 725 °C), sphene ( T c ≅ 600 °C), and rutile ( T c ≅ 400 °C)—and 40 Ar/ 39 Ar hornblende ages ( T c ≅ 480°C). Comparison of T-t paths from adjacent blocks allows predictions about the significance, kinematics, and timing of displacement of shear zones. In the Grenville orogen, T-t paths can distinguish between major terrane boundaries (e.g., the Carthage-Colton shear zone) and within-terrane shear zones (e.g., the Bancroft shear zone). Thus, these data can also be used to identify individual tectonic terranes in the deep crust. This integrated approach to analysis of shear zones provides constraints needed to determine the nature and rate of deep orogenic processes in areas that are complicated by high metamorphic grades.