Geochemistry and petrogenesis of late Pleistocene to recent volcanism in Southern Dominica, Lesser Antilles

Dominica is the most productive volcanic island in the Lesser Antilles arc and contains the largest number of potentially active volcanoes, but is not well studied petrologically. Quaternary volcanic activity on Dominica has been dominated by intermediate to felsic magmas, erupted as large-volume ignimbrites (Roseau, Grand Savanne, Grand Bay) and dome complexes. One of the largest concentrations of these deposits, and one of the currently most active areas in terms of shallow seismicity and hotspring activity, is the Plat Pays Volcanic Complex (PPVC) at the southern tip of the island. The PPVC is made up of medium-K caic-alkaline silicic andesites and rarer dacites. The major and trace element compositions define regular variation trends that can be explained by fractionation of observed phenocryst phases (plagioclase, clinopyroxene, orthopyroxene, Fe-Ti oxides). The contemporary Morne Anglais center erupted a wider range of magmas, extending from silicic andesites (identical in composition to the PPVC) to basalt. The new geochemical and Sr-Nd-Pb isotope data presented here cover all main units of the PPVC and Morne Anglais, and give a reconnaisance view of three other important Quaternary volcanic centers on Dominica that are associated with large-volume ignimbrite eruptions: Diablotins, Wotten Waven/Micotrin and Trios Pitons. Geochemical modelling of variation trends from the PPVC and Anglais centers indicates these are consistent with derivation from a basaltic parent magma like that of Morne Anglais. With a few exceptions at the mafic end of the spectrum, Sr, Nd and Pb isotope ratios remain constant with fractionation from 52 to 64 wt-% SiO2. The epsilon Nd values range from +2 and +4, and Sr-87/Sr-86 between 0.7041 and 0.7047. The range of values for Pb-208/Ph-204, Pb-207/Pb-204 and Ph-206/Pb-204 are 39.2-39.4, 15.71-15.75, and 19.4-19.5, respectively. Exceptions to this are basalt scoria clasts from Anglais and an andesite dike cutting PPVC domes. These samples have higher eNd and less radiogenic Pb isotope ratios. This, together with other geochemical anomalies observed in the Anglais scoria samples and field evidence for magma mixing from their sample locations, indicates a more open-system behavior of the Anglais system, perhaps involving magma recharge. Application to the PPVC and Morne Anglais samples of magnetite-ilmenite, hornblende-plagioclase and two-pyroxene geothermometry yielded temperature estimates in the range of 800 to 890 degrees C for silicic andesite and dacite, and 970-1100 degrees C for basaltic andesites from Morne Anglais. Pressure estimates from magnetite-ilmenite-orthopyroxene equilibria are 200-300 MPa, similar to an estimate of 200 MPa derived from dissolved Cl and H2O contents in melt inclusions, and consistent with the 5-6 km depth of volcanic seismicity observed in the PPVC area between 1998 and 2000. The similarity of chemical and isotopic compositions, regular differentiation trends and P-T conditions for the different units of the PPVC suggest that recurrent eruptions in the last 40 ka have tapped a common, rather homogeneous magma source. Variations are due to local differences in the degree of differentiation before eruption. The long eruptive history at the PPVC, including episodic dome growth over the last 40 ka, may indicate rejuvination of the system by mafic magma recharge and this is supported by the proximity to the contemporaneous mafic Anglais center which itself shows some evidence of basaltic magma recharge. Despite this, we find no convincing petrographic or geochemical evidence in its eruptive products for mafic recharge into the PPVC magma system. (c) 2005 Elsevier B.V. All rights reserved.