Overgrowth Textures, Disequilibrium Zoning, and Cooling History of a Glassy Four-Pyroxene Boninite Dyke from New Caledonia

Pyroxenes in a low-Ca boninite from a dyke show remarkable growthand overgrowth textures and chemical zoning, which are usedto infer crystallization mechanisms and kinetics. The rock comprisesnearly 50 vol.% fresh glass, phenocrysts and glomerocrysts ofclinoenstatite sensu lato ({small tilde}32 vol.%), microphenocrystsof orthopyroxene and Ca-rich clinopyroxene ({small tilde}17vol.%), and microlites of amphibole and oxides ({small tilde}3vol.%). Clinoenstatite, which inverted from protoenstatite,is skeletal with glass and chrome-spinel inclusions and thinovergrowth zones of Ca-rich clinopyroxene and amphibole; thecrystals are occasionally broken. Orthopyroxene is often skeletal,sometimes slightly resorbed with the same inclusions and overgrowthzones. Ca-rich clinopyroxene and amphibole occur mainly in pyroxeneovergrowth zones. Chrome-spinel in contact with glass is overgrownby magnetite-maghemite or amphibole. The rock is andesitic witha high mg-number of 0.78, and is quartz- and strongly hypersthene-and feldspar-normative. The glass is dacitic with an mg-numberof 0.15, strongly quartz- and feldspar-normative, and rich inwater ({small tilde}5.6wt.%). Strong chemical zoning occurs in all minerals, oscillatory zoningoccurring only in clinoenstatite and orthopyroxene. The mg-numberin clinoenstatite sensu lato ranges from 0.95 (clinoenstatitesensu stricto) to 0-68 (clinohypersthene), the largest rangeso far described, and from 0.88 to 0.43 in orthopyroxene. TheWo content of the former (0.21–1.56 with rare higher values)is considerably smaller than and does not overlap that of thelatter (1.36–4.84X Wo generally increasing with Fs. Thecores of both are chromian (up to 0–018 atoms per formulaunit), and Cr falls to or below the detection limit for Fs>25.The Fs-rich zones of clinohypersthene have {small tilde}2wt%A1₂O₃ and (M6wt% TiO₂, whereas those of orthopyroxene reachnearly 10 and 04 wt. %, respectively. Ca-rich clinopyroxenevaries from pigeonite through subcalcic augite and augite toferroaugite, the outer zones reaching 12wt.% A1₂O₃ and 1–8wt% TiO₂. Amphibole is magnesio-hornblende on pyroxene or tschermakitichornblende on oxide. The relative times of nucleation, growth, partial resorption,and fracturing of the crystals were determined from the texturesand the chemistry of the overgrowths. Protoenstatite and chrome-spinelnucleated from the melt and were overgrown in places by allsucceeding minerals. Orthopyroxene nucleated mainly from themelt, whereas the other phases occur only as overgrowths. Orthopyroxenemay show weak resorption at intermediate stages. Protoenstatitecrystals were occasionally broken, the relative times beingshown by the nature of the overgrowths. Minor-element concentrationsvary strongly with the growth stage, chromium soon being usedup in the melt and aluminium and titanium being strongly concentratedin the crystals during the last stages. Plagioclase and quartzare absent because of suppression of their nucleation in a water-richmelt on rapid cooling. Crystallization probably occurred intwo main stages, an early one of moderately fast cooling atlow undercooling probably in a shallow magma chamber (correspondingto the growth of the protoenstatite phenocrysts) and a laterone over a period of less than a day of faster cooling at higherundercooling after uprise and injection into the dyke (correspondingto the fracturing of protoenstatite and growth of the microphenocrystsand microlites).     

Origin and tectonic significance of corundum–kyanite–sapphirine amphibolites from the Variscan French Massif Central

The contact zone between two major allochthonous lithotectonic units in the French Massif Central (FMC) is characterized by the presence of corundum‐bearing amphibolites associated with serpentinites, flaser‐gabbros, eclogites and granulites. These unusual amphibolites are best preserved in the Western FMC, where they are found within the lower oceanic crust of the Limousin ophiolite. Mineralogical observations and thermodynamic modelling of the spinel–corundum–sapphirine–kyanite amphibolites in the CMASH system show that they were formed at peak P–T conditions around 800 °C/10 kbar in response to near isothermal burial followed by a retrogressive anticlockwise path. Metamorphic reactions are controlled both by modification of P–T conditions and by local chemical changes linked to fluid infiltration. Pargasite growth has been enhanced by infiltration of Ca‐ and Al‐rich fluids whereas kyanite‐ and sapphirine‐forming reactions are partly controlled by local inputs of MgO–SiO₂ components, most probably during infiltration metasomatism. By analogy with worldwide ophiolites (Oman, Tethyan, Appalachian) and published numerical models, subduction of a still‐hot oceanic ridge is proposed to form these Al‐rich amphibolites from plagioclase‐rich troctolites. The trace‐element composition of high‐Ti, fine‐grained amphibolites (former fine‐grained Fe–Ti gabbros) adjacent to the corundum‐bearing ones, further indicates that the oceanic crust was initially created at a mid‐ocean ridge (rather than within a back‐arc basin), followed by the emplacement of supra‐subduction zone‐type magmas, probably due to intraoceanic subduction close to the ridge.