Journal of Petrology, Volume 40, Issue 3: March 1999.

Application of the Restite Model to the Deddick Granodiorite and its Enclaves - a Reinterpretation of the Observations and Data of Maas et al. (1997)

ALLAN J. R. WHITE¹, BRUCE W. CHAPPELL² AND DOONE WYBORN²

¹VICTORIAN INSTITUTE OF EARTH AND PLANETARY SCIENCES, SCHOOL OF EARTH SCIENCES, UNIVERSITY OF MELBOURNE, PARKVILLE, VIC. 3052, AUSTRALIA
²ARC NATIONAL KEY CENTRE FOR THE GEOCHEMICAL EVOLUTION AND METALLOGENY OF CONTINENTS (GEMOC), DEPARTMENT OF GEOLOGY, AUSTRALIAN NATIONAL UNIVERSITY, CANBERRA, A.C.T. 0200, AUSTRALIA

Mafic S-type granites of the Lachlan Fold Belt always contain a distinctive suite of enclaves, dominated by schists and gneisses of high metamorphic grade, generally with lesser numbers of microgranular enclaves. Maas et al. (1997, Journal of Petrology 38, 815-841) recently described enclaves from the S-type Deddick Granodiorite. They concluded that the metasedimentary enclaves could be either source lithologies or accidental xenoliths, and that the most common microgranular enclaves formed as globules of hybrid magma that mingled with the host magma. All features of the metasedimentary enclaves are consistent with an origin as lithic restite fragments from the source, and their ubiquity in the mafic S-type granites and their rarity in both felsic varieties and in I-type granites conform with such an origin. The argument that these enclaves are not in chemical equilibrium with the host granite because they do not complement its composition is not valid, as they represent less fertile parts of the source that did not melt to the point of disaggregation. Unaltered cores of microgranular enclaves in the S-type granites have compositions that do not match those of any known igneous rock and an origin by metamorphism of a calcareous sediment is probable. Partial melting during that metamorphism accounts for the pseudo-igneous textures seen in these enclaves.

Keywords: enclave;Lachlan Fold Belt;restite; S-type granite

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