Volume 38: January - December 1997

Issue 7: July 1997

Abstract

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

  • 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

    • R. Maas, IA. Nicholls and C. Legg

    Vieps School of Earth Sciences, La Trobe University, Bundoora, Victoria 3083, Australia and Vieps Department of Earth Sciences, Monash University, Clayton, Victoria 3186, AustraliaCorresponding author Email: rmaas@mojave.geol.latrobe.edu.au

    ABSTRACT

    The Deddick Granodiorite, a mafic S-type pluton in the Lachlan Fold Belt, contains abundant enclaves. Most are derived from high-grade metasediments of pelitic-psammitic composition, and migmatites are common. Among these, discrete fragments of melanosomes rich in cordierite and garnet are restites from partial melting. However, compositional and Nd-Sr isotopic data indicate they are not restite in equilibrium with the melt component of the host magma, but may have formed part of a spectrum of diverse magma source lithologies. Alternatively, they may be accidental xenoliths. Regardless of their origin, data for these enclaves suggest their precursors could represent deeply buried age equivalents of the ubiquitous Ordovician turbidites of the Lachlan Fold Belt. Three types of microgranular enclaves with igneous textures are distinguished. The most common type are small and rounded mafic enclaves of tonalitic composition. They carry xenocrysts derived from the host magma, and some contain high-Mg pyroxene derived from a mafic magma. Isotopic data form an array with ENd from -6 to -12, and 87Sr/86Sr from 0.7130 to 0.7167 (host rock -10 and 0.715, respectively). These enclaves formed as globules of hybrid mafic magma commingled with, and contaminated by, the more felsic host magma. Other enclave types were derived from disrupted syn-plutonic dykes having a distinct isotopic composition, and from a cogenetic marginal facies of the host.

    Keywords: enclaves; magma mixing; xenocrysts; Nd-Sr isotopes; S-type granites; Lachlan Fold Belt

    Pages: 815 - 841

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