The petrology and geochemistry of the Reedy Creek granitoids and migmatites

Abstract

The composite, regionally concordant Reedy Creek Pluton comprises three major rock units: (a) the relatively voluminous Reedy Creek Granodiorite, (b) a mafic, commonly foliated diorite and (c) a pink granite occurring in the upper parts of the pluton. Surrounding this complex are migmatites and gneisses belonging to the Cambrian Kanmantoo Group metamorphics. Also intruding these metamorphics, but probably predating the members of the Reedy Creek Pluton, is the Summerfield Granodiorite. This small, concordant dyke-like intrusion displays petrological and chemical features which differ markedly from those of the granitoids belonging to the Reedy Creek Pluton.

Chemical, mineralogical, petrological and field criteria suggest that the granitoids belonging to the Reedy Creek Pluton may be genetically related. Although the gross appearance of these granitoids is consistent with their derivation from igneous source material, i.e. I-type, a derivation invoking two stages of partial melting of a metasedimentary sequence is proposed. The first stage of partial melting, which may have been initiated by the intrusion of the Cambrian-Ordovician? Black Hill Norite (a large mafic intrusion 40km N.E. of Reedy Creek) produced a magma with a composition approaching a minimum melt (this may be represented by the Palmer Granite). Further non minimum melting of this metasedimentary source produced the majority of granitoids in this region. Compositional variation within these granitoids is ascribed to varying degrees of separation of restite from melt during ascent. Since this separation may be very effective at higher temperatures, the refractory minerals may concentrate and so form dioritic rocks. This close genetic relationship between the granodiorites and diorites of the complex is supported by field evidence which suggests that the two rock units are essentially contemporaneous.

In contrast to the I-type granitoids of the Reedy Creek Pluton, the S-type Summerfield Granodiorite displays chemical and mineralogical features which suggest that it may be derived by partial melting of pelitic metasediments which have undergone only a small amount of surface weathering.

Biotite crystallization temperatures of the migmatites (790-810°C) are similar to those of the Reedy Creek granitoids. In addition, migmatite biotites are compositionally similar to biotites of granitic rocks in comparison to those of gneisses and migmatites of the Palmer region (White, 1966). This implies a greater degree of heating for the Reedy Creek migmatites in comparison to those of Palmer and points to an origin involving partial melting.