OVERGROWTH TEXTURES, DISEQUILIBRIUM ZONING, AND COOLING HISTORY OF A GLASSY 4-PYROXENE BONINITE DYKE FROM NEW-CALEDONIA

Pyroxenes in a low-Ca boninite from a dyke show remarkable growth and overgrowth textures and chemical zoning, which are used to infer crystallization mechanisms and kinetics. The rock comprises nearly 50 vol.% fresh glass, phenocrysts and glomerocrysts of clinoenstatite sensu lato (approximately 32 vol.%), microphenocrysts of orthopyroxene and Ca-rich clinopyroxene (approximately 17 vol.%), and microlites of amphibole and oxides (approximately 3 vol.%). Clinoenstatite, which inverted from protoenstatite, is skeletal with glass and chrome-spinel inclusions and thin overgrowth zones of Ca-rich clinopyroxene and amphibole; the crystals are occasionally broken. Orthopyroxene is often skeletal, sometimes slightly resorbed with the same inclusions and overgrowth zones. Ca-rich clinopyroxene and amphibole occur mainly in pyroxene overgrowth zones. Chrome-spinel in contact with glass is overgrown by magnetite-maghemite or amphibole. The rock is andesitic with a high mg-number of 0.78, and is quartz- and strongly hypersthene- and feldspar-normative. The is dacitic with an mg-number of 0.15, strongly quartz- and feldspar-normative, and rich in water (approximately 5.6 wt.%). Strong chemical zoning occurs in all minerals, oscillatory zoning occurring only in clinoenstatite and orthopyroxene. The mg-number in clinoenstatite sensu lato ranges from 0.95 (clinoenstatite sensu stricto) to 0.68 (clinohypersthene), the largest range so far described, and from 0.88 to 0.43 in orthopyroxene. The Wo content of the former (0.21-1.56 with rare higher values) is considerably smaller than and does not overlap that of the latter (1.36-4.84), Wo generally increasing with Fs. The cores of both are chromian (up to 0.018 atoms per formula unit), and Cr falls to or below the detection limit for Fs > 25. The Fs-rich zones of clinohypersthene have approximately 2 wt.% Al2O3 and 0.16 wt.% TiO2, whereas those of orthopyroxene reach nearly 10 and 0.4 wt.%, respectively. Ca-rich clinopyroxene varies from pigeonite through subcalcic augite and augite to ferroaugite, the outer zones reaching 12 wt.% Al2O3 and 1.8 wt.% TiO2. Amphibole is magnesio-hornblende on pyroxene or tschermakitic hornblende on oxide. The relative times of nucleation, growth, partial resorption, and fracturing of the crystals were determined from the textures and the chemistry of the overgrowths. Protoenstatite and chrome-spinel nucleated from the melt and were overgrowth in places by all succeeding minerals. Orthopyroxene nucleated mainly from the melt, whereas the other phases occur only as overgrowths. Orthopyroxene may show weak resorption at intermediate stages. Protoenstatite crystals were occasionally broken, the relative times being shown by the nature of the overgrowths. Minor-element concentrations vary strongly with the growth stage, chromium soon being used up in the melt and aluminium and titanium being strongly concentrated in the crystals during the last stages. Plagioclase and quartz are absent because of suppression of their nucleation in a water-rich melt on rapid cooling. Crystallization probably occurred in two main stages, an early one of moderately fast cooling at low undercooling probably in a shallow magma chamber (corresponding to the growth of the protoenstatite phenocrysts) and a later one over a period of less than a day of faster cooling at higher undercooling after uprise and injection into the dyke (corresponding to the fracturing of protoenstatite and growth of the microphenocrysts and microlites).