Igneous and Metamorphic Enclaves in the S-type Deddick Granodiorite, Lachlan Fold Belt, SE Australia: Petrographic, Geochemical and Nd-Sr Isotopic Evidence for Crustal Melting and Magma Mixing

Abstract

The Deddick Granodiorite, a mafic S-type pluton in the Lachlan INTRODUCTION Fold Belt, contains abundant enclaves. Most are derived from Enclaves in granites and volcanic rocks can provide high-grade metasediments of pelitic–psammitic composition, and petrogenetic information not readily available from their migmatites are common. Among these, discrete fragments of host rocks (Didier, 1973; Didier & Barbarin, 1991). For melanosomes rich in cordierite and garnet are restites from partial example, metasedimentary enclaves can place constraints melting. However, compositional and Nd–Sr isotopic data indicate on the age, composition and deformational history of they are not restite in equilibrium with the melt component of the magma source regions within continental crust (Fleming, host magma, but may have formed part of a spectrum of diverse 1991; Steele et al., 1991). Enclaves formed by acmagma source lithologies. Alternatively, they may be accidental cumulation of early phenocrysts (‘autoliths’) document xenoliths. Regardless of their origin, data for these enclaves suggest the early evolution of the host magma, whereas others their precursors could represent deeply buried age equivalents of the may represent commingled magmas and illustrate the ubiquitous Ordovician turbidites of the Lachlan Fold Belt. Three possible role of hybridization (Didier, 1987). types of microgranular enclaves with igneous textures are disEnclave suites in the relatively mafic S-type granitoids tinguished. The most common type are small and rounded mafic of SE Australia are often diverse, including abundant enclaves of tonalitic composition. They carry xenocrysts derived from enclaves of metasedimentary origin and microgranular the host magma, and some contain high-Mg pyroxene derived from enclaves (Phillips et al., 1981; Price, 1983; Chen et al., a mafic magma. Isotopic data form an array with eNd from –6 to 1989; Wyborn et al., 1991). The metasedimentary en–12, and Sr/Sr from 0·7130 to 0·7167 (host rock –10 and claves are lithologically diverse, including gneissic– 0·715, respectively). These enclaves formed as globules of hybrid migmatitic types and biotite-rich ‘surmicaceous’ types. mafic magma commingled with, and contaminated by, the more Apart from their potential as indicators of crustal structure felsic host magma. Other enclave types were derived from disrupted and composition, they are of interest as possible fragments syn-plutonic dykes having a distinct isotopic composition, and from of source material and/or refractory residue from which a cogenetic marginal facies of the host. peraluminous melts were generated and extracted. By