Magma generation in the Lesser Antilles: geological constraints

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Volcanic activity in the Lesser Antilles has been fairly discontinuous in space and time during the past 22 Ma: 1. (1) for the past 100000 years volcano spacing has been highly irregular (between 130 and 10 km); 2. (2) several sections of the arc, about 100 km long, have proved to be volcanically inactive for 1–4 Ma; 3. (3) the present volcanic production rate appears to be ten times higher in the central portion of the arc than in the northern and southern regions; 4. (4) construction of the arc-ridge proceeds from the juxtaposition of volcanic products emitted in areas generally a few tens of km in length (a small island, or a part of a large island e.g. Martinique, the geology of which is presented in some detail in this paper) during several Ma (up to 10 Ma). Such areas are referred to in this paper as arc-compartments. To reconcile the erratic activity of the arc with the coeval continuous process of subduction of Atlantic crust beneath the Caribbean plate, it is tentatively proposed that partial melting of subducted sediments is a sine qua non condition for generation of orogenic magmas in the Lesser Antilles, instead of dehydration of the descending slab. In this speculative scheme, the length along strike and duration of volcanic activity in an arc-compartment are thought to reflect the width and length of a subducted sedimentary basin passing beneath the arc. From the beginning to the end of activity of this arc-compartment, the volcanics typically evolve from tholeiitic to calc-alkaline series, progressively enriched in incompatible elements and radiogenic isotopes. At the same time, the volcanic front migrates away from the trench (westward in the Lesser Antilles), the volume of lavas decreases, and the acidity of most common lavas and occurrences of magma mixing increase. Mg-rich lavas constitute a group of basanitoids, basalts and basaltic andesites long known to crop out in the southern Lesser Antilles. In the Grenadines and in Martinique, this particular and non-voluminous volcanic activity is controlled by faults transverse to the arc-ridge, and unrelated in time and space to common orogenic series (tholeiites and calc-alkaline lavas). It is assumed that Mg-rich magmas are generated in the upper mantle wedge, and that their eruption is related to the tectonic stress generated on the edge of the Caribbean plate during an oceanic-ridge subduction event. The distribution of volcanic activity and the different types of volcanic series in the Lesser Antilles are best explained if related to the morphological features of the subducted ocean floor i.e. particularly the distribution of oceanic basins where sedimentation is more efficient, and seamounts and aseismic ridges where sediments generally do not pond.