American Mineralogist, Volume 85, pages 1349–1367, 2000 Geochemistry of pyroxene inclusions from the Warrumbungle Volcano, New South Wales, Australia MOHAMMAD R. GHORBANI* AND ERIC A.K. MIDDLEMOST Division of Geology and Geophysics (FO5), School of Geosciences, University of Sydney, N.S.W 2006, Australia ABSTRACT Pyroxenes provide an elegant key to understanding the tangled petrogenesis of the diverse midalkaline rocks of the Warrumbungle Volcano. These rocks evolved from two primary magmas, and the pyroxenes record the various stages in their origin and evolution. Trace-element abundance patterns for chromian diopside crystals from a spinel-lherzolite xenolith provide evidence of an early mantle-depletion event. Depleted spinel-lherzolite is an appropriate mantle source for the ITEP (incompatible-trace-element-poor) series of rocks. Subcalcic augite megacrysts are common in the basic rocks of this series. They crystallized in a high-pressure, intratelluric environment where they controlled the initial magmatic differentiation of the ITEP series. Aluminian augite occurs as megacrysts and also in gabbronorite xenoliths found in one of the more differentiated rocks of the ITEP series. This pyroxene crystallized in a lower-crustal magma chamber where its host magma was peraluminous and intermediate in composition. A few clinopyroxene (Cpx) megacrysts are Fe- rich and exceptionally enriched in rare-earth elements (REE). They crystallized from a moderately differentiated magma, locally contaminated by crustal material, or were later subjected to a metaso- matic event. One of the nepheline-normative basic rocks of the ITER (incompatible-trace-element- rich) series contains small inclusions of K-rich omphacite. This phase contains one of the highest K contents (0.6–2.3 wt%) ever reported in a Cpx, and crystallized from a K-rich liquid at a greater depth than any of the other pyroxenes, probably deep within the upper mantle. Orthopyroxene (Opx) megacrysts are rare and belong to at least three geochemically distinct types: in association with chromian diopside, with subcalcic augite, and with aluminian augite megacrysts. Each type of Opx crystallized in a separate chemical and physical environment. INTRODUCTION cryptodomes, dikes, and plugs that intrude into, or protrude During the Cenozoic, eastern Australia experienced wide- through, a variety of lava flows and pyroclastic deposits. The spread volcanic activity. An important part of this activity was felsic rocks usually crop out in the central zone whereas the the construction of chains of compound volcanoes made of less-differentiated rocks are more abundant in the distal zone. basalts and hawaiites, lesser amounts of intermediate rocks, Figure 2 shows that these rocks belong to the midalkaline suite, and a variety of felsic extrusives and subvolcanic intrusives. and range from basalt via intermediate types to various tra- Radiometric dating shows that these volcanoes and their coun- chytes and rhyolites. On an (Na2O + K2O) vs. SiO2 diagram terparts in the adjoining Tasman Sea display a progressive de- (Fig. 2), midalkaline rocks occupy the transitional field between crease in age from north to south. It is probable that the the compositional fields of common subalkaline rocks and rare magmatic event responsible for the construction of these vol- alkaline rocks (Middlemost 1997; p. 216). canoes was triggered by a static, long-lasting, mantle plume Examination of the incompatible-trace-element geochem- that impinged on the base of the Australian Plate as it moved istry of the most-primitive basic rocks having high Mg' [Mg' = 2+ northwards away from Antarctica (Wellman and McDougall Mg/(Mg + Fe ); ferric iron is calculated after Middlemost 1974; Johnson et al. 1989). (1989)] values and few phenocrysts reveals that they comprise The well-preserved Warrumbungle Volcano is located in two discrete magmatic series, referred to as the incompatible- northern New South Wales (Fig. 1). It has a low-angle shield trace-element-rich (ITER) and the incompatible-trace-element- form, surmounted by a complex centralized system of domes, poor (ITEP) series, respectively (Figs. 3 and 4). The trace-element patterns of the ITER series are smooth with Nb- Ta enrichments (ocean island basalt signature), whereas the patterns of the ITEP series are more differentiated with depleted * E-mail: c/o M. Saffari [email protected]. Correspond- K and Rb values, and lower LREE/HREE ratios (Fig. 4). Fur- ing author present address: Department of Geology, Tarbiat thermore, on Mg' vs. incompatible element diagrams (e.g., Zr, Modarres University, P.O.Box; 14155-4838 Tehran, Iran. Fax; Rb, K; Fig. 3), the two series display contrasting variation +98 21 8006544. trends: a positive correlation for the ITER and a negative cor- 0003-004X/00/0010–1349$05.00 1349.