ZIRCON AGES, Sr-Nd-Hf ISOTOPIC COMPOSITIONS, AND GEOCHEMISTRY OF GRANITOIDS ASSOCIATED WITH THE NORTHERN OPHIOLITE MELANGE OF CENTRAL CUBA: TECTONIC IMPLICATION FOR LATE CRETACEOUS MAGMATISM IN THE NORTHWESTERN CARIBBEAN

摘要

The geology of Cuba is representative of the orogenic belt fringing the northwesternmost margin of the Caribbean plate. It is characterized by a tectonic collage made up of a continental margin, volcanic arc, and ophiolitic and subduction complexes formed as a result of collision between the leading edge of the Caribbean plate and the Caribeana terrane in the latest Cretaceous to early Tertiary and by oblique convergence with the continental margin of the North American plate during Tertiary times. The northern ophiolite belt of central Cuba consists of a melange containing blocks of eclogite, garnet-amphibolite, amphibolite, blueschist, greenschist, quartzite, metapelite, antigoritite and various types of intrusive rocks that occur as blocks or intrusive gabbro and diabase of the ophiolitic sequence. We present the first combined SHRIMP zircon U-Pb ages, Sr-Nd-Hf isotopic compositions as well as major and trace element data for nine arc-related granitoid rocks from the northern ophiolite melange of central Cuba. A trondhjemite from the Cerro el Chivo area south of Santa Clara, provided a concordant zircon age of ～86 Ma, whereas a granite and a tonalite from the Las Bocas body, north of Placetas, yielded ages of ～75 Ma. Two peraluminous trondhjemitic samples taken from the Tres Guanos area S of Perea yielded zircon crystallization ages of ～72 and ～74 Ma, rare Cambrian (～536 Ma) and Permian (～268 Ma) inherited zircons were found in these samples. Hf isotope ratios were measured in five of the dated zircon grains from the Tres Guanos samples. The three youngest zircons, with ages of ～72 and ～74 Ma, show derivation from a depleted source region, as can be expected for arc granitoids, as well as the oldest xenocryst (～536 Ma) which is less depleted but still from a juvenile Pan-African source. A Permian xenocryst (～268 Ma) has a negative ε_(Hf(T)) value (-2.35), implying that the zircon is derived from a continental source. The initial Nd isotopic composition ε_(Nd(t)) of four of the dated granitoids is almost identical at 7.3 to 7.7 and implies a depleted source, whereas one of the Tres Guanos trondhjemite samples containing an inherited Cambrian zircon has a lower ε_(Nd(t)) of 4.9, possibly reflecting input from an older crustal component. The initial Sr isotopic compositions of all five analyzed samples vary between 0.7031 and 0.7039 which are typical for I-type granitoids. The chemistry of these rocks is compatible with arc-related granitoids typical of a subduction setting. The zircon ages of this study constrain currently discussed geotectonic models and indicate that emplacement of the ophiolite melange must be younger than 71 Ma. Based on current geodynamic models for the Caribbean, our data suggest that the oceanic crust in the northern ophiolite belt is of supra-subduction origin (probably fore-arc) related to a volcanic arc domain that formed during Late Cretaceous time. Apart from the granitoid sample from the Cerro el Chivo that was probably tectonically incorporated into the melange, the remaining samples can be related to a pre-collision/collision calc-alkaline subduction-related environment.