Petrology of basaltic xenoliths in andesitic to dacitic host lavas from Martinique (lesser antilles): Evidence for magma mixing

Abstract

Some recent calc-alkaline andesites and dacites from southern and central Martinique contain basic xenoliths belonging to two main petrographic types: -The most frequent one has a hyalodoleritic texture (~<H type>>) with hornblende + plagioclase + Fe-Ti oxides, set in an abundant glassy and vacuolar groundmass.-The other one exhibits a typical porphyritic basaltic texture (~( B type >>) and mineralogy (oli~_ne + plagioclase + orthopyroxene + clinopyroxene + Fe-Ti oxides and scarce, or absent hornblende).Gradual textural and mineralogical transitions occur between these two types (<< I type >>) with the progressive development of hornblende at the expense of olivine and pyroxenes. Mineralogical and chemical studies show no primary compositional correlations between the basaltic xenoliths and their host lavas, thus demonstrating that the former are not cognate inclusions; they are remnants of basaltic liquids intruded into andesitic to dacitic magma chambers. This interpretation is strengthened by the typical calc-alkaline basaltic composition of the xenoliths, whatever their petrographic type (<<H>>, <<I>> or <<B>>). The intrusion of partly liquid, hot basaltic magma into colder water-saturated andesitic to dacitic bodies leads to drastic changes in physical conditions, The two components; the basaltic xenoliths are quenched and homogeneized with their host lavas with respect to T°; f02 and pH20 condi-

Physical conditions, structure, and dynamics of a zoned magma chamber: Mount Pelée (Martinique, Lesser Antilles Arc)

“…Under these conditions, amphibole is the most abundant phase, exceeding plagioclase in the mineral assemblage (Table 7; see also Holloway and Burnham [1972]). This reproduces one important aspect of mafic enclaves and gabbroic cumulates from the Lesser Antilles arc, namely, the high modal abundance of amphibole [ Arculus and Wills , 1980; Coulon et al , 1984; Fichaut et al , 1989b; Gourgaud et al , 1989]. It has been suggested that fractionation of a Plag + Amph assemblage is responsible for the chemical trends seen in Lesser Antilles lavas [ Arculus and Wills , 1980].…”

“…Information on the mafic part of the chamber comes from mafic enclaves erupted during stage 3 and from mafic lavas (ash‐and‐scoria flows) and gabbroic cumulates from stage 2. Mafic enclaves have phenocryst compositions differing relatively little from phenocrysts in andesites (Figures 4–6, 12, and 13), probably because they were hybridized [e.g., Coulon et al , 1984; Bacon , 1986; Clynne , 1999]. Mafic lavas from stage 2 also carry a minor andesite “component” [ Bourdier et al , 1985], but these, together with gabbroic cumulates, appear to preserve phenocryst and glass compositions representative of the mafic part of the system (Tables 2–5 and see below).…”

Segregation structures in vapor-differentiated basaltic flows

“…In addition, large acicular skeletal or microlitic crystals are observable within C and C±S stuctures, either coated by glass or included within doleritic minerals. This particular texture, which resemble the ªhyalodoleriticº texture defined by Lacroix (1904; see also Coulon et al 1984), has been identified herein as the GDAC texture (glassy or doleritic texture with large acicular crystals). Self et al (1996Self et al ( , 1997 and Kent et al (1998) have also described cylinder sheets and cylinders, respectively, in thicker ( 6 15 m) inflated pahoehoe lava flows.…”

Two types of gabbroic xenoliths from rhyolite dominated Niijima volcano, northern part of Izu-Bonin arc: petrological and geochemical constraints

“…Most of the mafic xenoliths in volcanic rocks from island arc systems fall into one of three categories: cognate xenoliths that are indicative of magma mixing [e.g. 6,7], plutonic blocks derived from middle to lower crustal sources [e.g. 8,9], or cumulate rocks formed by crystallization and precipitation in magma reservoirs [e.g.…”