Inclusion-bearing nepheline hawaiite from near Woolomin, northeastern New South Wales
Titel:
Inclusion-bearing nepheline hawaiite from near Woolomin, northeastern New South Wales
Auteur:
Morris, Paul A.
Verschenen in:
Australian journal of earth sciences
Paginering:
Jaargang 34 (1987) nr. 1 pagina's 111-118
Jaar:
1987-03
Inhoud:
Nepheline hawaiite southeast of Woolomin, NSW, has been examined in terms of its composition and inclusions. The host is a fine grained, occasionally vesicular nepheline hawaiite (100 Mg/Mg + Fe2+ [henceforth Mg'] = 51.5; ne=11.93) with phenocryst olivine, and groundmass clinopyroxene, brown amphibole, titanomagnetite, and plagioclase. The inclusions are dominantly single crystals of kaersutite, anorthoclase, and rare brown mica, the latter falling into two groups based on TiO2 content. Only two poly-crystalline fragments have been recovered: one is a cumulate-textured hornblende lherzolite (ol + cpx + amph + opx), and the second comprises complexly zoned clinopyroxene enclosing spinel and amphibole. In this second fragment, silicate compositions are similar to phenocrysts and groundmass, and this (combined with textural evidence for rapid cooling) suggests the xenolith is cognate and low pressure. Published experimental work supports both mica and amphibole as liquidus phases in chemically evolved (i.e. low Mg') rocks such as nepheline hawaiites and mugearites. However, as crystals are large, insufficient time would have elapsed for crystallization, Furthermore, two distinct compositions for biotite xenocrysts do not support a cognate origin. A ruptured vein system (suggested for other megacryst occurrences) is not favoured, as polycrystalline fragments are noticeably absent. Instead, it is speculated that both mica and amphibole grew as discrete crystals at low pressure in rock cavities and were mechanically stripped by the magma immediately prior to eruption. The absence of higher pressure (e.g. spinel-bearing) inclusions and the low Mg', Cr, and Ni of the host are consistent with slow magma rise and attendant gravitational settling of inclusions, and mafic mineral fractionation.
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