Ammonite-radiolarian assemblage, Tobago Volcanic Group, Tobago, West Indies—Implications for the evolution of the Great Arc of the Caribbean
An unusually fossiliferous sedimentary sequence is exposed near Scarborough, Tobago, West Indies. It occurs within the Tobago Volcanic Group above an undifferentiated sequence of volcaniclastic breccia and lava and includes volcanogenic sedimentary rocks at the base of the Bacolet Formation. An ammonite-radiolarian assemblage has been recovered from this sedimentary interval in the Tobago Volcanic Group. Ammonites from the Spring Gardens quarry are moderately evolute and have unbranched ribs; they are probably juveniles of Mojsisoviczia , of middle Albian age. Another ammonite from a stratigraphically higher locality (i.e., volcanogenic sedimentary rocks at the base of the Bacolet Formation) is identified as Manuaniceras decsernae Young, of early late Albian age. Radiolarian assemblages from intercalations of black siliceous argillite of the Tobago Volcanic Group are characterized by abundant A rchaeospongoprunum spp. and the common occurrence of Pseudodictyomitra pentacolaensis , Dictyomitra montisserei , Thanarla brouweri , and Pantanellium ventosum . Also present in lesser abundances are other members of the family Archaeodictyomitridae, Xitus , Napora durhami , and Sciadiocapsa speciosa . This assemblage correlates with the Romanus subzone of [O'Dogherty (1994)][1] and Kozurium zingulai zone of [Pessagno (1977)][2], both of which are dated as early to middle Albian.

These newly reported fossils indicate an Albian age for deposition of the Tobago Volcanic Group. Earlier published 40Ar/39Ar radiometric ages from the group are consistent with the stratigraphic age assignment of the fossils and thereby suggest that volcanism associated with the Tobago Volcanic Group began ca. 105 Ma. Possible correlative, accreted fragments of the Mesozoic oceanic-arc of the southern Caribbean include the Curacao Lava Formation, the lower part of the Washikemba Formation (Bonaire), the Tiara Formation of north-central Venezuela, and possibly the subsurface Mejillones complex of the Carupano basin.

The Mesozoic rocks of Tobago consist of two distinct, oceanic-arc rock sequences. The older North Coast Schist (≥120 Ma) exhibits polyphase deformation and lower greenschist facies metamorphism. The structurally overlying Tobago Volcanic Group is not penetratively deformed or folded; it has undergone prehnite-pumpellyite facies metamorphic conditions and has been tilted to moderate dips by various brittle faults. We interpret the hiatus that separates these oceanic-arc rock sequences as representing a significant event in the evolution of the Great Arc of the Caribbean. Consequently, we propose that the commonly postulated reversal in subduction-zone polarity of the Great Arc occurred prior to deposition of the Tobago Volcanic Group (ca. 105 Ma), and that older, more highly deformed and metamorphosed oceanic-arc sequences (e.g., North Coast Schist) served as a basement for younger sequences or as wall rock for intrusive elements of younger magmatic suites of the Great Arc.

 [1]: #ref-23
 [2]: #ref-26