Lithosphere evolution and metallogeny in the Georgetown Inlier and the adjacent Tasman Fold Belt, North Queensland, Australia

The Precambrian Georgetown Inlier is a part of the northeastern edge of the Australia craton in North Queensland, and is adjacent to the Palaeozoic Hodgkinson Province in the Tasman Fold Belt. The age and origin of granitoids and mineralised porphyry intrusions has been determined by analysing zircon grains for isotopic composition. The timing of mantle depletion and metasomatic events in the SCLM, and of underplating events in the lower crust has been determined by analysing upper-mantle peridotite and lower-crustal granulite xenoliths and their minerals for major-, trace-element and isotopic composition. This integrated study has provided a better understanding of the relationships between lithosphere evolution and porphyry deposit genesis on and off the craton through time.

In situ LAM-ICPMS U-Pb and Hf-isotope analyses of detrital zircon grains showed that major crustal events in the Georgetown Inlier occurred ca 154 5-15 8 5 Ma, 4 20 Ma and ca 340 Ma ago. In contrast, the major crustal events in the Hodgkinson Province occurred at ca 305 Ma and at ca 265-280 Ma ago.

Zircon rims and unzoned grains in ca 420 Ma old granitoids from the Georgetown Inlier have a wide range in 176Hf/177Hf almost identical to that defined by their resorbed Mesoproterozoic cores and grains. These data suggest that ca 420 Ma old granitoids were derived by remelting of a heterogeneous lower crust ca 1550 Ma old. The remelting of heterogeneous Proterozoic crust may also explain the Hf-isotope composition of zircon grains in most analysed ca 265-280 Ma granites in the northeastern Hodgkinson Province. The Hf data indicate that the Proterozoic was characterised by crustal growth, whereas Palaeozoic magmatism did not involve juvenile addition to the crust.

Zircon grains in ca 1560 Ma old granites in the Georgetown Inlier, in ca 305 Ma old granitoids in the southwestern Hodgkinson Province, and in other ca 265-280 Ma granites in the northeastern Hodgkinson Province have a narrow range in £Hf values, and show no evidence for juvenile mantle input. They may have been derived by crustal remelting, followed by homogenisation of the magma before zircon crystallised. The mixing of magmas from different crustal sources can explain the Hf-isotope composition of zircon grains in ca 335 Ma gold-mineralised Kidston Median Dyke, and of some grains in ca 305 Ma old mineralised granitoids.

In situ MC-LAM-ICPMS Re-Os and Hf-isotope analyses provided T RD and TMA ages of sulfide grains in spine! lherzolite xenoliths and Hf model ages ofrutile grains in granulite xenoliths respectively. These ages, and whole-rock Hf and Nd model ages of granulites overlap with granitoid events especially at ca 1550 Ma, 1100 Ma, 420 and 340 Ma, implying that origin of granitoid magmas depends on the timing and extent of underplating/overplating in the lower crust. This is in turn triggered by melt depletion and metasomatic events in the SCLM related to the same tectonic process.

The integration of studies of zircons, lower-crustal granulite and spine! lherzolite xenoliths confirms the inferred presence of Georgetown Inlier crust and SCLM beneath the southwestern Hodgkinson Province. However, the Hf-isotope "Event Signature" showed that its northeastern part is a discrete terrane with its own crust/mantle history until ca 1.2 Ga.

Modelling Hf-isotope "Event Signature" indicates that the Broken Hill and Mt Isa terranes have similar crustal history during Mesoproterozoic time. In contrast, the crustal evolution of the Georgetown Inlier and Mt Isa block is more similar during Archean and Palaeoproterozoic time. The most significant reason for the lack of profitable base metal deposits (ca 1.65-1.67 Ga old) in the circum-cratonic Georgetown Inlier is the absence of juvenile mantle input associated with lithosphere thinning and rifting in the back-arc setting and lack of crustal sources that were previously reworked and that can provide a potentially fertile domain at that time. All three terrains are characterised by crustal reworking after 1.65 Ga. The crustal evolution continues in the Georgetown Inlier to Permian time, whereas in two other terranes it ends by Mid- to Late Mesoproterozoic.