AR-40 AR-39 THERMOCHRONOLOGICAL CONSTRAINTS ON THE STRUCTURAL EVOLUTION OF THE MESOPROTEROZOIC NATAL METAMORPHIC PROVINCE, SE AFRICA/

Ten Ar-40/Ar-39 age spectra are presented for hornblende grains separa ted from mylonitized and weakly deformed amphibolites from the Mesopro terozoic (similar to 1.2-1.0 Ga) Natal Metamorphic Province (NMP). Sin ce the proposal of the SWEAT hypothesis, the NMP has become a crucial area in which to study Grenville-aged (locally termed 'Namaquan') accr etion in this part of Rodinia/Gondwana. Unlike many segments of the Wo rld's similar to 1.1 Ga belts, the NMP is particularly suited to such studies since it is devoid of a high-temperature Pan-African (similar to 500 Ma) overprint. In this study we have dated hornblendes from wit hin 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, m etamorphic foliation with down-dip stretching lineations. The new data constrain the minimum age of this early event (which includes obducti on of the northernmost Tugela Terrane onto the Kaapvaal Craton) to 113 5 +/- 9 Ma. In the D1 domains of the Mzumbe Terrane to the south, cool ing to below similar to 550 degrees C had only been attained by simila r to 1005 Ma. We also dated amphibolites from the later (D2) structura l domains which are characterized by sub-vertical sinistral mylonite b elts with sub-horizontal to oblique stretching lineations. Our data sh ow that D2 oblique shearing commenced at 1050-1035 Ma in the Mzumbe Te rrane and only terminated at similar to 980 Ma. The youngest movements also included reactivation of major D1 structures such as the Melvill e Thrust (Mzumbe-Margate Terrane boundary) at similar to 990 Ma. Final ly, indications of minor Pan-African resetting was detected during the first similar to 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 t he D1 event records the early NE-SW directed are-continent collision h istory of the belt whilst the D2 event represents a continuation of es sentially the same convergence vectors after extensive crustal thicken ing. The new data place important time constraints on the major tecton ic events in the NMP which appear comparable in many ways to other par ts of the global 'Grenvillian' orogen. (C) 1997 Elsevier Science B.V.