Abstract

Peralkaline granites exhibit the hallmark features of A-type igneous rocks, but strongly differentiated chemistry and intense hydrothermal alteration camouflage their ultimate origins. We present the first in situ Hf isotope data from zircons of peralkaline granites, aimed at clarifying the protoliths of these plutons and their genetic relationship to associated metaluminous/weakly peraluminous granites. This study used rocks of the Devonian Narraburra Complex in southeastern Australia, and found that correlations between Hf isotopes and trace element ratios reveal fundamentally different origins for the nonperalkaline and peralkaline granites. The latter have a depleted mantle-like ancestry, whereas a weakly peraluminous rock formed from melts of older arc crust that were modified by interaction with juvenile, probably alkaline magmas. Juxtaposition of crust- and mantle-derived magmas reflects the high heat flow and lithosphere-scale faults associated with continental extension, and explains the diversity of A-type granites.

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