THE ND-ISOTOPIC AND SR-ISOTOPIC COMPOSITION OF I-TYPE MICROGRANITOID ENCLAVES AND THEIR HOST ROCKS FROM THE SWIFTS CREEK PLUTON, SOUTHEAST AUSTRALIA

Initial Nd- and Sr-isotopic compositions have been determined for microgranitoid enclaves and their host rocks from the Siluro-Devonian Swifts Creek Pluton within the Lachlan Fold Belt of southeastern Australia. Enclaves are of quartz dioritic to tonalitic composition and their textures support an igneous origin. Host rocks range from tonalite to granite. The isotopic data form an array in ε{lunate}Nd 87Sr 86Sr space with enclaves systematically displaced towards higher ε{lunate}Nd- and lower 87Sr 86Sr values. Enclaves range in ε{lunate}Nd (t=405 Ma) from -7.3 to -3.1 with a narrow range in 87Sr 86Sr from 0.70613 to 0.70718 whereas host rocks range from -9.0 to -6.4 and 0.70761 to 0.70946 in ε{lunate}Nd and 87Sr 86Sr, respectively. The isotopic relations rule out simple crystal-liquid fractionation or restite unmixing alone as the major genetic link between enclaves and host rocks. The data are also incompatible with an origin of the enclaves by crystal accumulation or liquid immiscibility. Instead, mixing between a mafic, mantle-derived and a crustal-derived end-member to generate the host rock-enclave spectrum is compatible with the data. Because the initial isotopic ratios of enclaves and host rocks are well within the range of crustal rocks, we favour the alternative view that both enclave and host-rock magmas are of crustal origin. Consequently, we avoid extrapolations to hypothetical end-members. We suggest that the enclaves from the Swifts Creek Pluton are derived by partial melting of a mafic lower crust and mixed at or near the site of magma generation with a chemically less primitive and isotopically more evolved component to generate the enclave and host-rock spectrum. The isotopic heterogeneities within the enclave and host rocks survived the superimposed effects of crystal-liquid fractionation. Model ages (tDM) for enclaves and host rocks are almost identical averaging 1.7 and 1.6 Ga, respectively. These model ages indicate that both the enclave and host-rock protoliths were derived from a mantle reservoir during the Proterozoic. © 1990.