Integrated Models of Basalt Petrogenesis: A Study of Quartz Tholeiites to Olivine Melilitites from South Eastern Australia Utilizing Geochemical and Experimental Petrological Data

The Tertiary to Recent basalts of Victoria and Tasmania have mineralogical and major element characteristics of magmas encompassing the range from quartz tholeiites to olivine melilitites. Abundances of trace elements such as incompatible elements, including the rare earth elements (REE), and the compatible elements Ni, Co and Sc, vary systematically through this compositional spectrum. On the basis of included mantle xenoliths, appropriate 100 Mg/Mg + Fe+2 (68–72) and high Ni contents many of these basalts represent primary magmas (i.e., unmodified partial melts of mantle peridotite). For fractionated basalts we have derived model primary magma compositions by estimating the compositional changes caused by fractional crystallization of olivine and pyroxene at low or moderate pressure. A pyrolite model mantle composition has been used to establish and evaluate partial melting models for these primary magmas. By definition and experimental testing the specific pyrolite composition yields parental olivine tholeiite magma similar to that of KilaeauIki, Hawaii (1959–60) and residual harzburgite by 33 per cent melting. It is shown that a source pyrolite composition differing only in having 0.3–0.4 per cent TiO2 rather than 0.7 per cent TiO2, is able to yield the spectrum of primary basalts for the Victorian-Tasmanian province by ∼4 per cent to ∼25 per cent partial melting. The mineralogies of residual peridotites are consistent with known liquidus phase relationships of the primary magmas at high pressures and the chemical compositions of residual peridotite are similar to natural depleted or refractory lherzolites and harzburgites. For low degrees of melting the nature of the liquid and of the residual peridotite are sensitively dependent on the content of H2O, CO2 and the CO2/H2O in the source pyrolite. The melting models have been tested for their ability to account for the minor and trace element, particularly the distinctively fractionated REE, contents of the primary magmas. A single source pyrolite composition can yield the observed minor and trace element abundances (within at most a factor of 2 and commonly much closer) for olivine melilitite (4–6 per cent melt), olivine nephelinite, basanite (5–7 per cent melt), alkali olivine basalt (11–15 per cent melt), olivine basalt and olivine tholeiite (20–25 per cent melt) provided that the source pyrolite was already enriched in strongly incompatible elements (Ba, Sr, Th, U, LREE) at 6–9 x chondritic abundances and less enriched (2.5–3 x chondrites) in moderately incompatible (Ti, Zr, Hf, Y, HREE) prior to the partial melting event. The sources regions for S.E. Australian basalts are similar to those for oceanic island basalts (Hawaii, Comores, Iceland, Azores) or for continental and rift-valley basaltic provinces and very different in trace element abundances from the model source regions for most mid-ocean ridge basalts. We infer that this mantle heterogeneity has resulted from migration within the upper mantle (LVZ or below the LVZ) of a melt or fluid (H2O, CO2-enriched) with incompatible element concentrations similar to those of olivine melilitite, kimberlite or carbonatite. As a result of this migration, some mantle regions are enriched in incompatible elements and other areas are depleted. Although it is possible, within the general framework of a lherzolite source composition, to derive the basanites, olivine nephelinites and olivine melilitites from a source rock with chondritic relative REE abundances at 2–5 x chondritic levels, these models require extremely small degrees of melting (0.4 per cent for olivine melilitite to 1 per cent for basanite). Furthermore, it is not possible to derive the olivine tholeiite magmas from source regions with chondritic relative REE abundances without conflicting with major element and experimental petrology arguments requiring high degrees (≧15 per cent) of melting and the absence of residual garnet. If these arguments are disregarded, and partial melting models are constrained to source regions with chondritic relative REE abundances, then magmas from olivine melilitites to olivine tholeiites can be modelled if degrees of melting are sufficiently small, e.g., 7 per cent melting for olivine tholeiite. However, the source regions must be heterogenous from ∼1 to ∼5 x chondritic in absolute REE abundances and heterogerieous in other trace elements as well. This model is rejected in favor of the model requiring variation in degree of melting from ∼4 per cent to ∼25 per cent and mantle source regions ranging from LREE-enriched to LREE-depleted relative to chondritic REE abundances.