The Petrology of Plutonic Blocks and Inclusions from the Lesser Antilles Island Arc

Abstract

Three main groups of plutonic nodules are present in the Lesser Antilles arc and are interpreted as (a) phenocryst clusters, (b) metamorphosed wallrock xenoliths and (c) cumulate textured xenoliths. Large cumulate blocks display felsic-mafic layering, slump structures and auto-intrusive features. The majority of specimens are ad- and heteradcumulates with fewer ortho- and crescumulates. Interstitial scoria and glass are present in a variety of samples. Plagioclase, amphibole, cino- and orthopyroxene, olivine, magnetite, biotite, ilmenite, quartz and apatite are present in various proportions in individual blocks. Plagioclase and amphibole are modally predominant. Significant variation along the arc is displayed in the rarity of orthopyroxene and abundance of amphibole in the southern islands compared with the common presence of two pyroxenes in the northern islands. Plagioclase varies from An100–36 with very low orthoclase component, and usually precedes amphibole in a given crystallization sequence. Only on Grenada are plagioclase-free blocks present. Olivine is restricted to assemblages where coexisting plagioclase is more calcic than An89 and its composition range is Fo90–59. Clinopyroxene is predominantly calcic augite and cinopyroxene, olivine and plagioclase all coexist stably with amphibole. A general trend of decreasing Ca content in clinopyroxene from south to north in the arc is present. Orthopyroxene ranges from En73–49 and is most common in assemblages where the coexisting plagioclase is more sodic than An83 Coexisting pyroxenes define temperatures in the range ⋍800–1050 °C. Amphibole compositions include pargasite, magnesiohastingsite, magnesio-hornblende and tschermakitic hornblende. The K content of the amphiboles increases from north to south in the same sense as the general tholeiitic-calcalkalic-alkalic variation of parental magmas. Magnetite is the dominant spinel phase but ferrian chromite and chromian magnetite are present in some Grenada cumulates and pleonaste is found in rare St. Kitts samples. Ilmenite is present in blocks from several islands; coexisting Fe-Ti oxides define a temperature range of 710°-950 °C at -log10fo2 of 15.5 to 10.0 bars. Biotite, quartz and apatite are restricted to evolve cumulate types. Some modification of interstitial scoria/glass compositions from equilibrium melts has occurred in the majority of samples, but general similarity with erupted lava types is important evidence for the cognate relationship of the cumulate assemblages. The role of H2O is crucial in determining the calcic nature of island arc plutonic plagioclase when compared with relatively dry, layered tholeiitic plutons. Some modal and chemical features of cumulate-lava comparisons suggest plagioclase flotation may be significant. A variety of thermodynamic calculations indicate temperatures and pressures of crystallization in the range 850–1050 °C, 4–10 kb. No evidence exists for systematic along-arc variations in these parameters. Standard amphibole crystalline solution models give unsatisfactory results for aH2Ofluid calculations. Some distinctive contrasts between cumulate and phenocryst modes are present. The abundance of amphibole in equilibrium with basaltic melts in the plutonic situation compared with its rarity in lavas is striking. Plagioclase coexisting with a given melt is more anorthitic in the plutonic than the phenocryst mode. Least squares fractionation tests demonstrate the possibility of relating basalt-andesite-dacite suites by fractional crystallization of the cumulus phases. Trace element systematics of cumulate-lava suites for individual islands also generally support this hypothesis. The suggestion of sole amphibole fractionation for the generation of andesite from basalt is discarded.