The Geochemistry of the Igneous Rock Suite of St Martin, Northern Lesser Antilles |
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| Authors: | DAVIDSON JON P; BOGHOSSIAN NEVINE D; WILSON MARJORIE |
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| Institution: | 1Department of Earth and Space Sciences, University of California Los Angeles, California 90024-1567
2Department of Earth Sciences, University of Leeds Leeds LS2 9JT, UK |
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| Abstract: | The island of St Martin lies inthe inactive part of the northernLesser Antilles island arc. The island consists of volcaniclasticsediments overlain and intruded by volcanic and plutonic rocks,which are in turn overlain by Miocene limestones. The extrusiveand intrusive rock suites are closely spaced in time (around27 Ma) although field relations suggest that the volcanic rockswere intruded by the plutons. Pluton emplacement gave rise tothermal metamorphism of the volcanic and volcaniclastic carapace,and to widespread hydrothermal alteration throughout the island. Geochemically, the igneous rocks of St Martin form a mildlytholeiitic to calc-alkaline typical subduction-related suite.The extrusive rocks are basalts to andesites, and the magmasappear to have differentiated largely through fractional crystallizationof plagioclase, clinopyroxene, and olivine. The REE displayflat chondrite-normalized patterns, with no significant Eu anomalydespite convincing evidence for plagioclase fractionation. Theplutonic rocks are more silica-rich diorites to granites, containingplagioclase, amphibole, and, less commonly, K-feldspar, sphene,zircon, and pyroxene. REE patterns are slightly LREE enrichedbut display distinct negative Eu anomalies. The fractionationof amphibole and accessory phases may have been important inthe evolution of the plutonic suite, as REE contents do notincrease overall with differentiation. Sr and Nd isotopic ratios of the St Martin suite form restrictedranges which vary little with differentiation, or between theextrusive and intrusive suiteSi 87Sr/86Sr ratios are slightlyhigher and 143Nd/l44Nd slightly lower than for volcanic rocksuites from the currently active northern Lesser Antilles volcanicarc. Some of the high 87Sr/86Sr ratios are explained in termsof hydrothermal alteration involving a high 87Sr/86Sr fluid,associated with pluton emplacement. Pb isotope ratios are similarto those of the currently active northern Lesser Antilles arc,and correlate with SiO2. Such correlations, together with largeranges of incompatible (and immobile) trace element ratios suggestthat open-system differentiation occurred during the evolutionof the St Martin suite. The composition of magma sources in the northern Lesser Antillesarc apparently has not changed significantly over the last 30Ma, despite a westward shift in the locus of arc magmatism.Addition of a slab-derived fluid to the mantle wedge is responsiblefor the high relative abundances of large ion lithophile elements(LILE) and enrichment in radiogenic Pb and Sr relative to mid-oceanridge basalt (MORB). Subsequent differentiation may involveassimilation of the arc basement in St Martin, which is believedto consist of Cretaceous to early Tertiary arc material, similarto that encountered in the Greater Antilles.
*Present address: Department of Geosciences, University of Arizona, Tucson, Arizona 85721 |
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