Formula omitted] Thermochronological constraints on the structural evolution of the Mesoproterozoic Natal Metamorphic Province, SE Africa

Abstract

Ten 40 Ar 39 Ar age spectra are presented for hornblende grains separated from mylonitized and weakly deformed amphibolites from the Mesoproterozoic (∼ 1.2-1.0 Ga) Natal Metamorphic Province (NMP). Since the proposal of the SWEAT hypothesis, the NMP has become a crucial area in which to study Grenville-aged (locally termed ‘Namaquan’) accretion in this part of Rodinia/Gondwana. Unlike many segments of the World's ∼ 1.1 Ga belts, the NMP is particularly suited to such studies since it is devoid of a high-temperature Pan-African (∼ 500 Ma) overprint. In this study we have dated hornblendes from within the earliest (D1) NE-directed thrust-dominated structural domains of the belt. The D1 domains are characterized by NE-directed recumbent fold and thrust nappes and a pervasive, generally S- to SW-dipping, metamorphic foliation with down-dip stretching lineations. The new data constrain the minimum age of this early event (which includes obduction of the northernmost Tugela Terrane onto the Kaapvaal Craton) to 1135 ± 9 Ma. In the D1 domains of the Mzumbe Terrane to the south, cooling to below ∼ 550°C had only been attained by ∼ 1005 Ma. We also dated amphibolites from the later (D2) structural domains which are characterized by sub-vertical sinistral mylonite belts with sub-horizontal to oblique stretching lineations. Our data show that D2 oblique shearing commenced at 1050-1035 Ma in the Mzumbe Terrane and only terminated at ∼ 980 Ma. The youngest movements also included reactivation of major D1 structures such as the Melville Thrust (Mzumbe-Margate Terrane boundary) at ∼ 990 Ma. Finally, indications of minor Pan-African resetting was detected during the first ∼ 5% of the ArAr spectra from mylonites of the Lilani-Matigulu shear zone (Tugela-Mzumbe Terrane boundary), indicating the polyphase nature of this major structure. It has been suggested that the D1 event records the early NE-SW directed arc-continent collision history of the belt whilst the D2 event represents a continuation of essentially the same convergence vectors after extensive crustal thickening. The new data place important time constraints on the major tectonic events in the NMP which appear comparable in many ways to other parts of the global ‘Grenvillian’ orogen.