Thermochronologic data from Tobago, West Indies: Constraints on the cooling and accretion history of Mesozoic oceanic‐arc rocks in the southern Caribbean
Fission-track thermochronologic studies on mid-Cretaceous dioritic and tonalitic rocks from Tobago, West Indies, provide important temporal constraints on post magmatic Mesozoic cooling as well as the final phase of accretion of these oceanic-arc plutonic rocks onto the continental margin of northern South America. Zircon fission track data indicate that the plutonic rocks of Tobago cooled through the zircon closure isotherm (250° – 200°C) at about 103 Ma. Apatite ages and track length analysis suggest rapid cooling through the apatite annealing zone (120° – 60°C) at about 45 Ma. Furthermore, the track lengths in apatites from these plutonic rocks are all greater than 14 microns, which implies that these rocks have not been above 50°C since 45 Ma. If a geothermal gradient of 25°C/km is inferred, the analyzed samples have been at depths of less than 2 km since 45 Ma. Tobago has occupied an upper crustal structural position since the mid-Cretaceous, and significant final uplift of these Mesozoic oceanic-arc rocks occurred in the mid-Eocene. The tectonothermal history of Tobago since the mid-Eocene has not involved deep burial or a high-temperature thermal flux. This history apparently has chiefly involved translation of the Tobago terrane along the Caribbean - South America Plate Boundary Zone at upper crustal levels.