Origin(s) and evolution of granitic melts: theoretical considerations and field examples from Mount Isa, Australia

Abstract

Felsic igneous rocks are formed from a variety of processes including: (1) partial melting and melt extraction from source regions in the crust and mantle, (2) host-rock assimilation, (3) magma mixing, and (4) fractional crystallisation. These processes do not generally occur in isolation, but often overlap in space and time. As a result determining the relative contributions of these processes to granite petrogenesis remains challenging. Here, a combination of geochemical proxies, mineral chemistry, and field data were used to assess the key parameters that have contributed to the origin and magmatic evolution felsic melts of the Proterozoic Mount Isa Inlier of northern Australia. An evaluation of the available geochemical data for the main granite suites of the western Mount Isa Inlier provides insight into the crustal evolution of the eastern margin of the proto-North Australian Craton (NAC). The 1860 Ma Kalkadoon Suite are geo- chemically similar to I-type granites of the Lachlan Fold Belt but exhibit anomalously low-Mg#, Na2O, as well as anomalously high-K2O, LREE, and Th/U for an I-type gran- ite. The modified I-type character of the Kalkadoon Suite reflects the high-geothermal gradient nature of the compressional 1890–1840 Ma Barramundi Orogeny. The 1740 Ma Wonga and the 1680–1650 Ma Sybella suites are compositionally similar to Lachlan Fold Belt A-type granites, but exhibit extreme enrichment in U and Th. The Sybella Suite is also extremely enriched in F and LREE. The anomalous geochemistry of these two suites reflects the interaction of mantle derived melts with an incompatible element-enriched ensialic crust. Part of the Sybella Suite, the zoned Queen Elizabeth Granite exhibits complex textural relationships which record incremental emplacement, differentiation, deformation, mix- ing and crystallisation of compositionally distinct magmatic phases during active rifting in the western Mount Isa Inlier. Emplacement of voluminous porphyritic to equigranular syenogranite was followed by emplacement of a leucogranite phase along the northern and eastern margins of the pluton. A high-strain zone developed contemporaneously with intrusion and crystallisation of an enclave-rich monzogranite phase in the paleo-floor zone of the pluton during continued extensional deformation. Intermingled doleritic, hybrid granitoid, and pegmatite lithologies along the western edge of the pluton highlight the bimodal character of the Queen Elizabeth Granite magmatism. Late-stage melt-drainage networks link the development of pegmatitic segregations in the marginal leucogranite phase to the accumulation of voluminous pegmatite bodies within the adjacent Mica Creek Pegmatite Field. Mineral compositional data highlight the mineral-scale variability of the main granite phases of the Queen Elizabeth Granite. Major minerals host most of the Rb, Ba, and Sr. Accessory minerals including allanite, titanite, zircon, and monazite are the major hosts of Y, Th, U, and REE. Hornblende contains a significant proportion of the HREE budget in the main syenogranite phase of the Queen Elizabeth Granite. Combined in situ zircon U-Pb-Hf-O isotopic analysis of pre-1600 Ma felsic igneous rocks from western Mount Isa Inlier provide insights into the magmatic evolution of the eastern margin of the proto-NAC during the Paleoproterozoic. Barramundi-aged granitoids are dominantly sourced from reworking of ca. 2.5 Ga, isotopically evolved continental crust. Zircon isotope data suggest that the Sybella Batholith was emplaced during two magmatic cycles at 1680–1670 Ma and 1660–1650 Ma with each cycle associated with a broad trend towards increasing juvenile mantle input with time. The geochemical, isotopic, and field data are consistent with the presence of a long-lived, mantle driven, transcrustal magmatic plumbing system active within the eastern proto- NAC crust during the late Paleoproterozoic. This mantle driven model contrasts with existing models that rely on derivation of A-type melts from protoliths within the crust and directly links bi-modal magmatism, crustal assimilation, sedimentation, and extensional tectonics within the western Mount Isa Inlier during the Paleoproterozoic.