VOLCANIC GEOLOGY OF DOMINICA, LESSER ANTILLES: THE SURFACE EXPRESSION OF AN ISLAND ARC BATHOLITH

The geology of the island of Dominica, at the center of the Lesser Antilles island arc, has been subdivided into four Stratigraphic Divisions: Division 1 (upper Miocene) is dominated by mafic volcanism; Division 2 (upper Pliocene-lower Pleistocene) is characterized by the development of two large stratovolcanoes over a 2 million year period; Division 3 (lower to upper Pleistocene) represents a period of island-wide quiescence except at the north and south of the island where two new centers developed on the flanks of the older stratovolcanoes, and Division 4 (upper Pleistocene-Holocene) characterized by the formation of numerous Pelean centers throughout the island. Three of these centers also produced large volume pumiceous deposits that resulted in the formation of two calderas (Morne Trois Pitons, Wotten Waven). During Divisions 1 and 2 time, basaltic magmas produced by partial meting of the mantle with only minor sediment input ponded at the crust-mantle boundary where subsequent rise and fractionation generated distinct basic and intermediate suites. During Division 3 time the mid-crustal magma chambers beneath the Division 2 stratovolcanoes became barriers to the rise of these basic magmas, thus generating the quiescent period. However the continued rise of magma beneath Dominica resulted in the intrusion of numerous sills into the lower crust eventually producing a “hot zone”. Ultimately intermediate magmas produced in this “hot zone” caused the renewed volcanism during Division 4 time that produced numerous centers dominated almost exclusively by andesites and dacites of such similar compositions that the different centers cannot be geochemically distinguished from each other. The amalgamation of individual magma chambers beneath these different centers appears to have produced a mid-crustal batholith, which resulted in the uplift of the island, the formation of a central half-graben, and the current island-wide distribution of volcanic-related phenomena.