Fission track evidence for the source of accreted sandstones, Barbados

Abstract

A part of the accretionary wedge associated with the Lesser Antilles arc system is exposed in an area of approximately 50 km² in northeastern Barbados. Zircon separates from sandstones collected from the Scotland District, Barbados, W. I., were analyzed using the fission track technique in order to assess the spectrum of source areas from which these terrigenous sediments were derived and to better constrain the timing of deposition. Results yielded a mixture of ages with strong groupings from 25–80 Ma, 200–350 Ma, and greater than 500 Ma. The youngest population indicates that some of the Scotland beds, previously dated by paleontologic methods as Eocene, may actually be as young as late Oligocene. Possible source areas include the Lesser Antilles arc, the Netherlands‐Venezuelan Antilles arc, and the Caribbean Mountains of Venezuela. The 200–350 Ma population may reflect partially annealed cratonic material, an Andean component, and/or material associated with a Triassic rifting event. The oldest zircons (>500 Ma) and metamict zircons were likely derived from the South American craton. Detrital feldspars separated from sandstone at the base of Chalky Mount were analyzed by the 40Ar/39Ar age spectrum technique. Results yield what is interpreted to be a slow cooling gradient from 1350 Ma to 925 Ma and provides additional evidence of a cratonic source for the accreted sediments. Based on results from this study, and paleogeographical constraints, it is proposed that source areas for the Scotland sandstones of Barbados may have included the Guayana shield, Central Cordillera, uplifted areas of coastal South America, and the Lesser Antilles volcanic arc. We speculate that in Late Oligocene time an immense “proto‐Orinoco” river system flowed in a northeasterly direction from the shield through the northern Venezuelan mountain system (Cordillera de la Costa and Araya‐Paria) finally emerging into the Caribbean Sea in the area now occupied by Unare depression. Sediments comprising this deltaic complex and deep sea fan were either deposited directly into the trench or onto the sea floor where they were soon after caught up in the accretionary wedge of the Lesser Antilles arc system. This article contains supplementary material.