Issue 1: February 1996

Abstract

Derivation of A-type granites from a dehydrated charnockitic lower crust: evidence from the Chaelundi Complex, Eastern Australia

B. Landenberger and WJ. Collins Department of Geology, University of Newcastle, Callaghan, N.S.W. 2308, Australia and To whom correspondence should be addressed
ABSTRACT
Triassic I- and A-type granites of the Chaelundi Complex, New England Fold Belt, eastern Australia, were generated in a subduction-related tectonic setting. Although isotopic ages of the suites are indistinguishable, field relations indicate that the A-type is younger. The most mafic granistoids from each suite have similar silica contents (66-68% SiO2), slightly LREE enriched patterns without Eu anomalies, low Rb/Sr and K/Ba ratios, and high K/Rb ratios, suggesting that both represent parental magmas. The A-type is distinguished mineralogically by abundant orthoclase and sodic plagioclase (total >60%), ferro-hornblende, annite and allanite. In contrast, the I-type has more hornblende and biotite, which are more magnesian in composition, and less feldspar. The parental magmas of both suites have many similar geochemical characteristics, although the A-type has slightly higher alkalis, Zr, Hf, Zn and LREE, and lower CaO, MgO, Sr, V, Cr, Ni and Fe³⁺/[Sigma]Fe. The geochemical properties characteristic of leucocratic A-type granites, such as high Ga/Al, Nb, Y, HREE and F contents, are only manifest in the felsic members of the A-type suite. These features were produced by ~70% fractional crystallizaton of feldspar, hornblende, quartz and biotite.

Both granite suites were generated by water-undersaturated partial melting of a similar source, but the A-type parent magma resulted from lower aH2O conditions during partial melting. Generation and rapid ascent of the earlier I-type magma during disequiblibrium partial melting produced a relatively anhydrous, but not refractory, charnockitic lower crust. Continued thermal input from mantle-derived magmas, during continuing subduction, partially melted the 'charnockitized' lower crust at temperatures in excess of 900^oC, to produce A-type magmas. Charnockitic magmas (C-type) form in a similar way to A-type magmas, although their different composition reflects variations in the anhydrous lower-crustal mineral assemblages that remain after the previous (I-type) granite-forming event.

The New England Fold Belt was a subduction-accretion complex until the late Carboniferous, when the deeper parts underwent partial melting to produce S-type granites. As the I-and A-type granites intruded penecontemporaneously, a tonalitic source model for genesis of the Chaelundi A-type is untenable.

KEY WORDS. A-type; charnockitization; eastern Australia

Pages: 145 - 170

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