GEMOC - Department of EPS - Faculty of Science

LITHOSPHERE MAPPING IN THE SOUTHERN NEW ENGLAND REGION: PRELIMINARY RESULTS

William J. Powell, GEMOC, Macquarie

Lithosphere mapping is a technique that integrates geochemical, petrological and geophysical information to improve our understanding of the lower crust and upper mantle (O'Reilly & Griffin 1996). The geochemical and petrological information is provided by mantle xenoliths - actual samples of the rocks found at depth which are brought to the surface by basalts. These are also used to provide petrophysical constraints for interpretation of existing and new geophysical data, which can then be used to extend xenolith information laterally. Xenolith geochemistry provides a record of processes in the upper mantle and lower crust such as melting and metasomatism important parts of the geological history of the area. Geochemical data are also used for thermobarometric calculations, allowing samples to be placed in a depth sequence.

Geophysical anomalies in gravity, magnetic and seismic tomography data have been reported for the southern New England region. The tectonic setting has been the subject of much debate over time, and several models for the evolution of the orogen have been proposed. Lithosphere mapping is used to provide new information about the fundamental structure of the area. Basalts are widespread across the various terranes mapped at the surface of the southern New England region, and many xenolith-bearing localities are known, making it an ideal setting for a study of this kind.

Mantle xenolith suites from Allyn River, Lawler's Creek and Wallabadah Rocks have been investigated. Spinel lherzolite is the most common xenolith type at each of the localities, with granulite and pyroxenite xenoliths subordinate. Megacrysts are also common at Lawler's Creek. Modal clinopyroxene in the lherzolites ranges from 1.1-19.6% at Wallabadah, 1.6-14.4% at Allyn River and 2.0-12.0% at Lawler's Creek. The samples lie on the Phanerozoic trend on a Boyd diagram (olivine Mg# against modal olivine %), consistent with their location to the east of the Tasman Line. Preliminary whole-rock data from Wallabadah and Allyn River show simple straight-lie relationships between major elements (eg CaO, Al2O3 and Na2O against MgO), suggesting the lherzolites are residues after extraction of partial melt. Similar relationships between major elements are shown by calculated whole-rock compositions, based on point-counted modes and mineral compositions determined by EMP.

Compositions of mantle minerals vary significantly between the localities, indicating chemical heterogeneity in the mantle beneath New England. Clinopyroxene chromium number (100xCr/Cr+Al) ranges from 4.6-19.9 at Wallabadah, 6.7-35.1 at Allyn River, and 5.8-17.7 at Lawler's Creek. Cr2O3 in spinel ranges from 7.0-32.6 wt% at Wallabadah, 11.9-52.9 wt% at Allyn River, and 8.0-30.5 wt% at Lawler's Creek. Clinopyroxene Cr# and spinel Cr2O3 content are commonly accepted indices of depletion of lherzolites, suggesting higher levels of depletion at Allyn River relative to the other localities.

Equilibration temperatures have been calculated for the xenoliths, but no pressures are calculated as garnet is absent from these suites. Garnet-bearing xenoliths from other localities in eastern New South Wales lie on the Southeastern Australian Geotherm of O'Reilly & Griffin (1985), so this is taken to be representative of the ambient geotherm under New England at the time of eruption. Equilibration pressures for the New England samples are then inferred from the geotherm, allowing their depth of origin to be estimated. At Wallabadah Rocks pyroxenite and granulites are dominant to a maximum depth of ~30 km, below which spinel lherzolites are dominant. On this basis the crust-mantle boundary is placed at 30-35 km beneath Wallabadah Rocks. At Allyn River spinel lherzolite equilibration temperatures are slightly higher, suggesting a thicker crust at this locality, if the samples represent the full sub-MOHO mantle section in the spinel lherzolite zone.

Trace element abundances in clinopyroxene, determined in-situ using laser ablation ICP-MS, are used to fingerprint processes in the upper mantle and lower crust. Rare earth element patterns for Wallabadah Rocks clinopyroxenes show flat HREE patterns, and LREE patterns ranging from enriched to depleted (La/SmN-chon <0.8 to 2.8). Allyn River samples have strong LREE enrichments (La/SmN-chon 0.8 to 16.8), with flat to moderately enriched middle and heavy REEs. Lawler's Creek clinopyroxenes have relatively flat patterns, varying from LREE depleted to enriched (La/SmN-chon 0.5 to 1.7), and HREE depletion in some samples. Differences in REE patterns indicate the mantle has undergone different styles and/or degrees of chemical modification at each of the localities.

References

O'Reilly S. Y. & Griffin W. L. 1985. A xenolith-derived geotherm for southeastern Australia and its geophysical implications. Tectonophysics 111, 41-63.

O'Reilly S. Y. & Griffin W. L. 1996. 4D Lithospheric Mapping: methodology and examples. Tectonophysics 262, 3-18.