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 v olcanogenic sedimentary rocks at the base of the Bacolet Formation. An ammo nite-radiolarian assemblage has been recovered from this sedimentary interv al in the Tobago Volcanic Group, Ammonites from the Spring Gardens quarry a re moderately evolute and have unbranched ribs; they are probably juveniles of Mojsisoviczia, of middle Albian age. Another ammonite from a stratigrap hically higher locality (i.e., volcanogenic sedimentary rocks at the base o f 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 abun dant Archaeospongoprunum spp, and the common occurrence of Pseudodictyomitr a pentacolaensis, Dictyomitra montisserei, Thanarla brouweri, and Pantanell ium ventosum. Also present in lesser abundances are other members of the fa mily Archaeodictyomitridae, Xitus, Napora durhami, and Sciadiocapsa specios a, This assemblage correlates with the Romanus subzone of O'Dogherty (1994) and Kozurium zingulai zone of Pessagno (1977), both of which are dated as early to middle Albian. These newly reported fossils indicate an Albian age for deposition of the T obago Volcanic Group. Earlier published Ar-40/Ar-39 radiometric ages from t he group are consistent with the stratigraphic age assignment of the fossil s and thereby suggest that volcanism associated with the Tobago Volcanic Gr oup began ca, 105 Ma. Possible correlative, accreted fragments of the Mesoz oic oceanic-are of the southern Caribbean include the Curacao Lava Formatio n, the lower part of the Washikemba Formation (Bonaire), the Tiara Formatio n of north-central Venezuela, and possibly the subsurface Mejillones comple x of the Carupano basin. The Mesozoic rocks of Tobago consist of two distinct, oceanic-are rock sequ ences, The older North Coast Schist (120 Ma) exhibits polyphase deformation and lower greenschist facies metamorphism. The structurally overlying Toba go 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 separ ates these oceanic-are rock sequences as representing a significant event i n the evolution of the Great Are of the Caribbean, Consequently, we propose that the commonly postulated reversal in subductionzone polarity of the Gr eat Are occurred prior to deposition of the Tobago Volcanic Group (ca, 105 Ma), and that older, more highly deformed and metamorphosed oceanic-are seq uences (e.g., North Coast Schist) served as a basement for younger sequence s or as wall rock for intrusive elements of younger magmatic suites of the Great Are.