Haplogranites of North Queensland, Australia

Extensive, Carboniferous to Permian, felsic I-type granites (>8000 km²) and associated volcanic rocks, occur in north Queensland, Australia. The rocks range from (pyroxene-) biotite-hornblende quartz diorite to biotite syenogranite, but are dominantly haplogranitic, with over 85% containing more than 70% SiO₂. The rocks cluster tightly around the low-pressure granite minimum of Tuttle and Bowen, and with increasing fractionation become strongly depleted in Ti, Fe, Mg, Ca, P, Ba, Sr, Sc, V, Cr, Ni, Eu, and enriched in Rb, Pb, Th, U. The most strongly fractionated rocks, which form a significant component of the intrusive rocks, are further characterised by elevated HFSE, HREE, Na, Na/K and F and lower K.  In any one region the most strongly fractionated units are the oldest. Extended fractionation within these units appears to have occurred largely close to in situ. They commonly contain dyke swarms and numerous pods and sheets of highly evolved leucogranite enclosed in much more voluminous, but less fractionated haplogranite. Magmatism shows a pronounced polarity in age, decreasing from ~340 Ma in the west to 280 Ma in the east. Most rocks, regardless of composition or age, have very similar evolved isotopic signatures, with initial ⁸⁷Sr/⁸⁶Sr ~ 0.710 and ε_Nd ranging from -7.0 to -8.0. The only significant exceptions are where the granites have Proterozoic host rocks, resulting in even more evolved isotopic compositions (ε_Nd of -8.0 to -11.0). The large volume of silicious magma coupled with the similar geochemical and isotopic signatures, and the extended age range imply the involvement of a very large, isotopically homogeneous protolith. The granites appear to be the result of crustal melting in an extensional, possibly back-arc environment.  Petrogenesis involved moderate degrees of partial melting of a quartzofeldspathic source, followed by, high-level, fractional crystallisation ranging from moderate to extensive.