The eruptive history of Morne Jacob volcano (Martinique Island, French West Indies):Geochronology, geomorphology and geochemistry of the earliest volcanism in the recent Lesser Antilles arc

摘要

Martinique is the Lesser Antilles Island where the most complete volcanic history of the arc can be found from the Oligocene to the present time. In this study, we focused on the construction of Morne Jacob shield volcano in Martinique, which is the largest volcano of the Lesser Antilles. We have dated twenty representative samples from the Morne Jacob, by K-Ar based on the Cassignol-Gillot technique, used geochemical data and obtained morphological constraints that have helped us to reconstruct better the volcanic history of this shield volcano. Our results and the lack of reliable ages on other Plio-Pleistocene islands show that the Morne Jacob is the oldest volcano of the aerial recent arc that has been dated so far. It has a longer history than previously inferred, with different stages ranging between 5.2 and 1.5 Ma. A large basaltic to andesitic shield volcano was first built between 5.2 and 4.0 Ma (J1) with tholeiitic lavas characterized by an increase of SiO_2 content through time. After 800 kyr of repose, calc-alkaline andesites erupted over the first shield from 3.2 to 2.2 Ma (J2a). The accumulation of lavas over a hyaloclastitic basement provoked spreading and northeast creeping of the northern flank of the volcano. The mass movement induced regressive erosion, particularly at the centre of the edifice. Then, between 2.1 and 1.5 Ma (J2b), calc-alkaline lavas, which show a decrease of the SiO_2 content through time, erupted at the central vent and from peripheral fissures, over the previous shield and down to the Caribbean coast. Eruptive volumes were reconstructed, permitting estimates of minimum output rates for the volcano's history. Minimum volume erupted during the first stage is 112 ± 20 km~3 and 33 ± 25 km~3 during the whole second stage, yielding rates of about 0.107 km~3/kyr and 0.019 km~3/kyr, respectively. Considering the entire history of Morne Jacob shield volcano, between 5.14 ± 0.07 and 1.54 ± 0.03 Ma, we obtain a total volume of 145 ± 32 km~3 above the sea level, and a time-averaged construction rate of 0.040 ± 0.008 km~3/kyr. With comparison between the reconstructed paleotopographies and the DEM of present topography, we have calculated an eroded volume of 18 km3 over the outcropping 210 km~2 that have occurred during the last 1.5 Myr. Finally, our set of twenty K-Ar ages, obtained on groundmass separates, allow us to revise the eruptive chronology of Morne Jacob volcano and to date the earliest subaerial volcanic activity so far identified for the recent Lesser Antilles arc.