A mica, pyroxene, ilmenite megacryst-bearing lamprophyre from Mt Woolooma, northeastern New South Wales.

Abstract

A narrow lamprophyre dyke, dated at 85 m.y., intrudes early Carboniferous sediments at Mt Woolooma in E New South Wales. The lamprophyre contains megacrysts of titanbiotite (Mg18-21, 7-8% TiO2) and rare Ti-Al salite (Mg67.72, 6-9% Al2O3), ilmenite (5% MgO), titanphlogopite (Mg61, 9% TiO2) and apatite. The host-rock consists of lamprophyric-textured titanphlogopite (Mg62-70, 7-8% TiO2), diopside-salite, olivine, kaersutite and Ti-magnetite in a K-feldspar-rich base. The rock resembles a minette (mica-lamprophyre), but its bulk composition is richer in SiO2 and Al2O3 (53-54% SiO2 anhydrous, approx 16% Al2O3), and poorer in MgO (3%), Ni and Cr (30 ppm) than many minettes elsewhere. This minette forms part of a diverse assemblage of alkaline igneous rocks from the Scone-Gloucester area. These include alkali olivine basalt, teschenite, biotite alnoite, kimberlite and leucite monchiquite, many of which carry mantle xenoliths/xenocrysts and/or megacrysts. (Authors' abstract)-C.N. Show more