MANTLE MELTING PROCESSES DISCERNED FROM A GEOCHEMICAL STUDY OF THE TINAQUILLO PERIDOTITE MASSIF, VENEZUELA

Abstract

The Tinaquillo orogenic spinel peridotite massif in north central Venezuela is a sub-horizontal, 3-km thick sheet overlying and in thrust contact with phyllites of the Cordillera de la Costa belt in the north and underlying and in extensional fault contact with the gabbroic to felsic Tinaco complex to the south (Ostos, 1990). While early workers interpreted the complex as a magmatic or crystal-mush intrusion (e.g., MacKenzie, 1960), Seyler and Mattson (1989, 1993) and Seyler et al. (1998) interpreted it as a fragment of the upper mantle possibly having ascended in the form of a diapir, but having been mylonitized during transit through the lower crust. The ultramafic rocks comprise ca. 75% harzburgite locally grading into lherzolite, 20% dunite, and 5% serpentinite. Pyroxenite and hornblendite veins intruding the complex are volumetrically small, but quite conspicuous where present. Minerals in the peridotite have a bimodal grain-size distribution owing to survival of remnant porphyroclasts of olivine, orthopyroxene, and minor clinopyroxene, and the late development of a mylonitic fabric with considerable grain-size reduction probably related to emplacement. The massif is thought to be a fragment of lithospheric mantle emplaced into the Caribbean belt during the late Cretaceous, but not exhumed until the late Eocene to middle Miocene by northwestward-directed thrusting along the Manrique Fault (Pindell et al., 1988; Pindell and Barrett, 1990). Seven peridoitites and three amphibole-rich veins have been investigated for major and trace element concentrations, and Sr-Nd-Pb-Hf isotopic compositions. The peridotites have geochemical characteristics of residues after moderate degrees of partial melting (estimated to be 8 to 16%) assuming a homogeneous primitive mantle source. In addition, they have strong light rare earth element (REE)- depleted to spoon-shaped REE patterns, but flat heavy REE (La/Yb at 0.04-0.55 times primitive mantle). The (La/Yb)N ratios increase with the refractoriness of the peridotites, reflecting secondary metasomatism, which is a common feature of the sub-continental lithospheric mantle xenoliths world-wide. A primitive mantle-normalized element distribution diagram for the peridotites reveals an overall depletion in the highly incompatible elements relative to less incompatible ones, but moderate enrichments in the strongly incompatible elements (Ba, Th, U), without depletion in Nb and Ta relative to La. The peridotites have isotopic compositions of 87Sr/86Sr = 0.70277-0.71044, 143Nd/144Nd = 0.513095-0.514000, 206Pb/204Pb = 19.00-19.15, 208Pb/204Pb = 38.18-38.75, and 176Hf/177Hf = 0.282692-0.284703. The Nd, Pb and Hf isotopic signatures show ranges that overlap with present-day mid-ocean ridge basalts (MORB) but also extending to more depleted compositions. The unusually radiogenic Sr for some samples appears to be the result of pervasive infiltration by seawater at some stage in the massif’s history. The Sr, Nd, Pb and Hf isotopic signatures of the veins (87Sr/86Sr = 0.70231-0.70256, 143Nd/144Nd = 0.513052-0.513289, 206Pb/204Pb = 18.30-18.52, 208Pb/204Pb = 37.66-37.87, 176Hf/177Hf = 0.282988-0.283303) are similar to the Pacific and Atlantic MORB; lack of the highly radiogenic Sr component in the veins indicates its addition to the peridotites prior to vein emplacement. The Lu-Hf data yield an errorchron suggesting a Phanerozoic age for stabilization of this piece of the lithosphere. We attribute the origin of the amphibole-rich veins in the peridotites to melting during subduction of the proto-Caribbean Plate in a back-arc tectonic setting, fairly recently in the massif’s history, possibly during the late Jurassic.