Tectonophysics, 192 (1991) 359-366 Elsevier Science Publishers B.V., Amsterdam 359 Xenolith geotherms and crustal models in Eastern Australia J.P. Cull a, Suzanne Y. O’Reilly b and W.L. Griffin ’ ’ Department of Earth Sciences, Monash University, Clayton, Vie. 3168, Australia h School of Earth Sciences, Macquarie University, Sydney, NS W, 2109, Australia ’ Division of Exploration Geoscience, C.S.I.R.O., North Ryde, 2113, Aurtralia (Received October 12, 1990; revised version accepted February 14.1991) ABSTRACT Cull, J.P., O’Reilly, S.Y. and Griffin, W.L., 1991. Xenolith geotherms and crustal models in Eastern Australia. Tectonophysics, 192: 359-366. Lower crust and upper mantle rocks occurring as xenoliths in basaltic host rocks from eastern Australia have been used to provide pressure and temperature estimates which allow the construction of an empirical geotherm consistent with variations in the deep-seated stratigraphy. High gradients in the resulting geotherm at shallow depths (24-30 km) are inconsistent with thermal conduction models for heat flow. Consequently new models are proposed incorporating heat input from basaltic intrusions around the crust-mantle boundary. The best fit to the empirical geotherm is obtained if the basaltic magma is assumed to form a layer 4 km thick at 28 km depth with rates of intrusion approaching 900 m/Ma. This model is consistent with tectonic uplift rates for the eastern Highlands and with seismic reflection profiles showing a layered lower crust. Introduction The structural history and the tectonic response of any lithospheric terrane varies with the physi- cal/mechanical state of the crust and uppermost mantle. Variations in mineralogy generate signifi- cant rheological anomalies dominating any re- gional response (Kirby, 1985; Carter and Tsenn, 1987; Ranalli and Murphy, 1987). However, varia- tions in temperature also cause significant anoma- lies in the rheological profile (e.g., Vetter and Meissner, 1979) resulting in complex interpreta- tions of the surface geology. Consequently, precise geothermal models are required for detailed stud- ies of lithosphere dynamics and regional deforma- tion (e.g. Royer and Danis, 1988; Lister and Etheridge, 1989; Lambeck, 1984). Suitable geo- thermal data are now available for the lithospheric terrane of eastern Australia (Fig. 1) including em- pirical geotherms obtained from xenolith suites, observations of surface heat flow, and MAGSAT anomalies. Conventional methods for estimating the geo- thermal gradient are based primarily on extrapola- tion from measurements of the surface heat flow. Few observations are available in Australia and most are subject to significant error (Cull, 1982). Xenoliths of deep-seated rock types provide inde- pendent estimates of temperature and pressures (e.g., O’Reilly and Griffin, 1985; Jones et al., Fig 1. Heat flow data and xenolith samples used to constrain geothermal models in Australia. Heat flow domains: (A) = Archaean/Precambrian, (B) = Central Proterozoic Cratons, (C) = Palaeozoic Folds; B/G = Bullenmerri/Gnotuk Xeno- liths. 0040-1951/91/$03.50 0 1991 - Elsevier Science Publishers B.V.