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

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 granitoids 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%), fern-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+/ΣFe. 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 more felsic members of the A-type suite. These features were produced by ∼70% fractional crystallization 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 disequilibrium 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°C, 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.