Mafic mineral clots and microgranular enclaves in A-type Hiltaba Suite granites from the Gawler Craton, South Australia: Origins and implications

Abstract

The origins of mafic mineral clots and microgranular enclaves in Mesoproterozoic granites were investigated in a case study of felsic A-type Hiltaba Suite granites from the Gawler Craton, South Australia. Mafic mineral clots display interlocked textures comprising plagioclase, amphibole, biotite, apatite and magnetite and occur interstitial to coarse rapakivi feldspar, compositionally zoned plagioclase, and quartz phenocrysts. The microgranular enclaves, which may be partly disaggregated, are rounded, contain partially resorbed K-feldspar xenocrysts and have a similar mineralogy to the clots. Whole-rock geochemistry, mineral chemistry and textural features (e.g., rapakivi and granophyric textures, quartz-amphibole ocelli, multiple generations of biotite, xenocrysts, evidence of undercooling) indicate that mineral clots and enclaves may have similar origins and were sourced via multistage magma mixing. Magma rejuvenation caused mingling with a partially crystalline and ductile mush resulting in the formation of microgranular mafic enclaves which contain numerous xenocrysts. Prolonged input of mafic magma and elevated F within the residual melt promoted partial disaggregation of enclaves and recrystallization of mineral clots. Geochemically distinct biotite generations show this evolution from initially Fpoor annite to F-rich phlogopite dispersed throughout mineral clots and as ‘free’ grains within the Hiltaba Suite granites. Annite-bearing mineral clots formed at higher pressures than phlogopite clots, corresponding to crystallization at greater depths. The abundance of Cu and S in mineral clots and enclaves within Hiltaba Suite granite studied here indicates that repeated magma mixing processes may have provided a source of metals for the formation of the nearby Olympic Dam Fe-oxide Cu-U-Au-Ag deposit.