Examples of convective fractionation in high-temperature granites from theLachlan Fold Belt

Igneous rocks derived from high-temperature, crystal-poor magmas of interme diate potassic composition are widespread in the central Lachlan Fold Belt, and have been assigned to the Boggy Plain Supersuite. These rocks range in composition from 45 to 78% SiO2, with a marked paucity of examples in the range 65-70% SiO2, the composition dominant in most other granites of the L achlan Fold Belt. Evidence is presented from two units of the Boggy Plain S upersuite, the Boggy Plain zoned pluton and the Nallawa complex, to demonst rate that these high-temperature magmas solidified under a regime of convec tive fractionation. By this process, a magma body solidified from margin to centre as the zone of solidification moved progressively inwards. High-tem perature near-liquidus minerals with a certain proportion of trapped inters titial differentiated melt, separated from the buoyant differentiated melt during solidification. In most cases much of this differentiated melt buoya ntly rose to the top of the magma chamber to form felsic sheets that overly the solidifying main magma chamber beneath. Some of these felsic tops erup ted as volcanic rocks, but they mainly form extensive high-level intrusive bodies, the largest being the granitic part of the Yeoval complex, with an area of over 200 km(2). Back-mixing of fractionated melt into the main magm a chamber progressively changed the composition of the main melt, resulting in highly zoned plutons. In the more felsic part of the Boggy Plain zoned pluton back-mixing was dominant, if not exclusive, forming an intrusive bod y cryptically zoned from 63% SiO2 on the margin to 72% SiO2 in the core. It is suggested that tonalitic bodies do not generally crystallise through co nvective fractionation because the differentiated melt is volumetrically sm all and totally trapped within the interstitial space: back-mixing is exclu ded and homogeneous plutons with essentially the composition of the parenta l melt are formed.