U-PB APATITE AND SPHENE THERMOCHRONOLOGY DOCUMENTING MID-CRUSTAL TEMPERATURE GRADIENTS DURING ARCHEAN LITHOSPHERIC STABILIZATION, KAAPVAAL CRATON, SOUTHERN AFRICA

The time-scales of Archean lithospheric assembly and stabilization remain poorly known. U-Pb thermochronometry of apatite and sphene are used to track the cooling histories of basement rocks on opposite sides of the Barberton Greenstone Belt (BGB), a proposed continental suture, in order to document syn- and post-orogenic crustal exhumation and to explore the relaxation of complex isotherms generated during lithospheric assembly of the southeast Kaapvaal craton in the Mesoarchean.

High-precision ID-TIMS U-Pb zircon, sphene and apatite data from the Kaap Valley pluton, located north of the BGB, are consistent with published thermochronology, indicating that it cooled through the closure of apatite (~450-500 ˚C) at ~3.211 Ga, shortly after intrusion at 3.2272±0.0003 Ga. In contrast, a sample from the Ancient Gneiss Complex (AGC) from south of the BGB which intruded at 3.2177±0.0025 Ga, yields concordant apatite dates between ~3.150 and ~3.105 Ga. A relationship between single-grain apatite dates and their grain-size suggests that their Pb isotope systematics were controlled by thermally activated volume diffusion. Numerical thermal modelling in combination with imaging, subgrain sampling, and isotopic analysis have helped to constrain the importance that thermal resetting and late-stage overgrowth had on the apparent apatite cooling ages.

This work indicates that the AGC immediately south of the BGB resided above the closure temperature for Pb diffusion in apatite for at least 60 Myr longer than basement rocks north of the BGB, revealing that a transient thermal state existed in the middle-crust for at least ~100 Myr following lithospheric assembly ca. 3.2 Ga. Subsequent differential exhumation most likely occurred by some combination of late-stage extensional denudation and gradational unroofing due to isostatically-compensated erosion. Further constraints derived from field mapping and U-Pb thermochronology throughout the southeastern Kaapvaal craton will help decipher the mechanisms of differential exhumation as well as the time-scales over which this portion of crust reached a cratonic thermal character.