Aspects of the geology and geochemistry of the proterozoic rocks of the Valley of a Thousand Hills, KwaZulu-Natal.
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Date
1999
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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.
Description
Thesis (Ph.D.)-University of Natal, Durban, 1999.
Keywords
Geology--KwaZulu-Natal--Valley of a Thousand Hills., Geochemistry--KwaZulu-Natal--Valley of a Thousand Hills., Theses--Geology.