Aspects of the geology and geochemistry of the proterozoic rocks of the Valley of a Thousand Hills, KwaZulu-Natal.

Abstract

A regional field and geochemical study has allowed the identification of three primary units within the Proterozoic basement of the Valley of a Thousand Hills. The Nagle Dam Formation incorporates several chemically distinct orthogneiss series, characterised by limited intragroup fractionation, and derived from discrete sources. Intrusive into the gneisses are the megacrystic A-type granites of the Mgeni batholith, comprising the biotite granites of the Ximba Suite; the hornblende granites and charnockite of the Mlahlanja Suite; and the medium grained leucogranite of the Nqwadolo Suite. Petrogenetic modelling indicates that these are predominately cumulates. A general model for the A-type granites suggests that they were derived through variable MASH processes on an original within plate type basalt. Enclaves within the Mgeni batholith form a distinct series, the Valley Trust Formation, comprising a nongenetic orthogneiss association of amphibolite and crustal sourced quartzo-feldspathic gneiss and locally derived paragneisses. Interaction between the biotite granite and the pelitic enclaves generated a biotite garnet granite. Geothermobarometry suggests temperatures of metamorphism to a maximum of 770°c for the Nagle Dam Formation and c.850°C at a pressure of 6 kb for the Valley Trust Formation. Potential magmatic temperatures of c.760°C at 5 kb are derived for the Mgeni batholith. High Mn garnets within late veins indicate subsequent intrusion at higher levels. Derivation of a tectonic model for the Valley of a Thousand Hills is assisted by a revaluation of the chemical tectonic discrimination plots as source or initiator discriminators. These indicate an origin for the Nagle Dam Formation in an arc environment, while the bimodal orthogneiss association of the Valley Trust Formation and the A-type character of the Mgeni batholith suggests their evolution during extensional events. Geothermobarometry defines an isothermal decompression path, possibly generated during a collision event, superimposed on which is a potential midcrustal heating event, resultant on the intrusion of the Mgeni batholith. These data can be integrated with revised lithotectonic data from the southern portion of the Natal Province to derive a regional model. This comprises: the collision of a number of arcs with associated splitting to form backarcs, sedimentation, and failed rift systems; syn-collisional S-type magmatism, contemporaneous with isothermal decompression of the region; and a series of pulses of post-orogenic granites.