Titanian aegirine in a teschenite sill

T w o series of pyroxenes have been found in a layered teschenite sill, 22 m thick, emplaced in Late Permian coal measures in the Sydney Basin, New South Wales, Australia. The sill consists of 12 m of homogeneous teschenite, overlain by 6 m of picrite, then by 4 m of a layered sequence. The layering unit of this sequence is teschenite, a pyroxene-rich layer and a narrow leucocratic band. A whole-rock analysis of the lower chilled margin (Table I, anal. 1) gives (Na + K)/A1 of 0.68, and one of the layers containing aegirine (Table I, anal. 2) a ratio of 0.72, indicating that the magma is not peralkaline. The presence in the sill of spinel lherzolite microxenoliths and olivine megacrysts (Fogo), with a wholerock Mg number [100Mg/(Mg+Fe2+)] of 71, evidences the primitive nature of the magma (Frey et al., 1978). Occurrence of the pyroxenes. The most common pyroxene throughout the sill is a pink, sector-zoned titansahlite (Table I, anal. 3). In the pyroxene-rich layers this is accompanied by calcic labradorite, titanomagnetite, titanian biotite, rare olivine, apatite and interstitial sanidine (now largely analcitised). Ti-rich amphibole, kaersutite, may rim titansahlite grains, or occur as stubby euhedra. Some titansahlite grains, commonly near the leucocratic bands, are rimmed by pale green sodian ferrosahlite (Table I, anal. 4). Long prismatic grains of kaersutite are the principal mafic phase, with minor biotite and titanomagnetite, in the labradorite-rich leucocratic bands. Small grains of deep green aegirine-augite (Table I, anal. 5) are nucleated on the amphibole, and also occur in small clusters among the sanidine grains of the pyroxene-rich layers. The widest pyroxene-rich layer (1 m thick) contains numerous cavities, and has the most Tienriched biotite, the lowest Nb/Zr ratio of this layertype, and the highest Rb/Sr ratio of the whole sill. (~) Copyright the Mineralogical Society These characters suggest it is the most evolved layer of the sequence. The cavities, in clots of titansahlite, are filled with Na zeolites and analcite. They are lined with terminated pyroxene prisms, zoned from basal titansahlite, through neutral to deep blue-green tips of titanian aegirine (Table I, anal. 7). These phases are optically continuous. Pyroxene geochemistry. All compositions were determined on a wavelength-dispersive ETEC Autoprobe, with an acceleration voltage of 15 kV, and a 50 na specimen current. Minimum beam diameter used was 10 nm, and grain boundaries were checked with a scanning electron microscope. Following the suggestion of Ferguson (1978), the pyroxenes of single samples have been plotted separately on a N a M g F e 2+ diagram (fig. la). The coexisting kaersutite has been normalized to the same Fe 3 +/Fe 2+ ratio as a kaersutite from the Iki Islands, Japan (Aoki, 1959). The pyroxenes from 19 m above the base of the intrusion have an mg number [100 Mg/(Mg + total Fe)] of 81 to 87 in the titansahlites, and of 50 to 9.6 in the titanian aegirines. Maximum Na and Fe enrichment occur together (Table I, anal. 6), with a ratio of Fe3+/Fe 2+ of 12.7 (calculated stoichiometrically). A marked compositional gap occurs between the Ca-rich and the Na-rich pyroxenes. Kaersutite coexists in the layer, although not in the cavities, and plots in this region of pyroxene instability. A series of pyroxenes from the next lower pyroxene-rich layer (18.6 m above base) is shown for comparison. The m9 number varies from 71 to 67 in titansahlite, and from 66 to 31 in sodian Fe-rich compositions. No region of pyroxene instability is present, and variation from titansahlite to sodian ferrosahlite to aegirine-augite is continuous. However, the coexisting amphibole occupies a position similar to the amphibole of the more sodic series. A Na-Tito ta l -Fe diagram (fig. lb) confirms that the N a F e pyroxenes of the two series do not overlap, and show continuous solid solution between titansahlite and aegirine-augite. On a Na-Ti Fe ~ + plot (fig. ic) the titanian aegirine analyses form a tight group. 530 D. J. M A R T I N TABLE I. Analyses of some rocks and minerals from Scone, N.S.W.