Geology, geochemistry and petrogenesis of Late Precambrian S-, I- and A-type granitoids in the Saldania belt, Western Cape Province, South Africa

Abstract

Late Precambrian granitoids occurring as a series of batholiths and plutons in the southwestern portion of the Saldania belt were subdivided into S-, I- and A-type associations according to geological and petrological criteria. Field relationships, multicationic interpretation, major and trace element geochemistry, rare earth element composition and age were also used to obtain the subdivision. Six major granitic associations in four batholiths were identified, namely two S-type ( Sa and Sb), two I-type ( Ia and Ib) and two A-type associations ( Aa and Ab). The Sa association is further subdivided into the Sal and Sa2 types. P 2O 5 and Th are used as the principal discriminating elements and Na 2O, K 2O, Zr, Nb, Y and FREE for further characterization. The S-type granitoids dominate in the western Tygerberg tectonic terrane, the I-type granitoids in the central Swartland, as well as the eastern Boland terranes, and the A-types are intrusive in both the Swartland and Tygerberg ter apes. Plutonism in the southwestern Saldania belt is related to three major phases. Granitoids of phases I and III were precursed by mafic to intermediate magmas of tholeiitic and calc-alkaline affinity, respectively. Mafic and intermediate rocks of phase I are characterized by low K 2O and granophile elements. The older (∼600 m.y.) Sa granitoids of this phase have features corresponding to typical S-type granites. Phase I ended with the intrusion of Sb granitoids (540 Ma), which are seen as contaminated metaluminous melts with a high degree of fractionation. Phase II granitoids are typically metaluminous, high-K calc-alkaline quartz monzonites ( Ia association) and granites with I-type features. These granitoids are high in radioelements, especially Th. This phase of magmatism ended with the intrusion of the Ib association at ∼517 Ma, typically highly evolved plutons intrusive into slightly older Ia quartz monzonites. Phase III commenced with the intrusion of a K-series of gabbros to quartz syenites at ∼519 Ma. These rocks are high in alkalis as well as Nb, Zr and REE. Phase III culminated with the intrusion of two associations of A-type granites, the Nb, Zr and REE enriched Aa association and the Th, Y and REE enriched Ab association. Plutonism in the southwestern Saldania belt is subduction related. Older mafic and intermediate associations were intrusive in an arc environment, followed by the intrusion of peraluminous late orogenic to anorogenic S-type granitoids. The metaluminous I-type granites are late orogenic and intruded during a long time span (560-517 Ma) in a marginal position with respect to a Proterozoic subduction zone. Granitoids related to the Aa and Ab associations have the typical characteristics of anorogenic alkaline granites.