COMPOSITIONAL CHANGES DURING FRACTIONATION OF FELSIC GRANITES FROM WESTERN TASMANIA, AUSTRALIA

Dr Doone Wyborn and Professor Bruce W. Chappell, GEMOC ANU

Geochemical and Sr isotopic signatures of Late Devonian Granites of western Tasmania indicate they were derived from four main source-rocks, two metaluminous (I-type Housetop and Meredith Suites) and two peraluminous (S-type Pieman and Interview Suites). All granites are felsic with SiO2 greater than 70%, and show fractionation trends indicating fractionation near the ternary minimum. The behaviour of trace elements during the fractionation is dependent on the aluminium saturation index and oxygen fugacity of the magmas. In the oxidised metaluminous Housetop Suite Rb, Th and Y, and possibly Nb and U are incompatible. The weakly metaluminous Meredith Batholith has only Rb, Y and U incompatible. Rb, Nb, Sn and U are incompatible in the weakly peraluminous Pieman Suite, and in the strongly peraluminous Interview Suite P2O5, Rb, Nb, Sn, and U are incompatible.

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