PETROCHEMISTRY AND TECTONIC SIGNIFICANCE OF CRETACEOUS ISLAND-ARC ROCKS, CORDILLERA ORIENTAL, DOMINICAN-REPUBLIC

Cretaceous island-arc rocks of the Caribbean island-arc system have be en exposed by Cenozoic faulting in the Cordillera Oriental in eastern Hispaniola. High-K2O intermediate to felsic volcanic rocks (Loma la Ve ga volcanics) are interbedded with marine epiclastic sedimentary rocks and tuffs (Las Guajabas tuffs) and unconformably overlie pre-Aptian s edimentary rocks, low-K2O volcanic rocks (Guamira volcanics) and a gra nodioritic to tonalitic intrusion (El Valle pluton). The petrology and geochemistry of these units, in conjunction with regional stratigraph ic data, are used to speculate on the tectonics of the newly developin g Caribbean island-arc system during Early and Late Cretaceous time. T he Loma la Vega volcanics are characterized by the presence of large p henocrysts of sanidine, and minor amounts of clinopyroxene, opaque oxi des, and rare leucite in a devitrified matrix of chlorite and clay. Al though the volcanic rocks have undergone some low-temperature alterati on/metamorphism, which redistributed some major elements and large-ion -lithophile trace elements, the high-field-strength elements, rare-ear th elements, and radiogenic isotopes appear to have been minimally aff ected. Based on abundances of the relatively immobile elements, trace- element enrichment patterns and isotopic compositions, the Loma la Veg a volcanics are considered part of the high-K, calc-alkaline (CA) or s hoshonitic island-arc volcanic series. In contrast, pre-Aptian (Early Cretaceous?) volcanic and plutonic rocks of the underlying Los Ranchos Formation have chemical characteristics similar to rocks in the islan d-arc tholeiitic or ''primitive island-arc'' (PIA) series that form co eval and along-strike sections of the Early Cretaceous Caribbean islan d arc in other parts of present-day Hispaniola, Cuba, Puerto Rico and the Virgin Islands. An abrupt and regional change in composition from island-arc tholeiites to high-K, calc-alkaline rocks is coincident wit h a hypothesized reversal in subduction polarity in pre-Aptian time. A s inferred from previously published tectonic models. polarity reversa l may have been triggered by attempted subduction of the Caribbean oce anic plateau beneath this segment of the Caribbean island arc. The obs erved magmatic and tectonic effects of the proposed Cretaceous Caribbe an arc reversal are similar to the better documented Neogene subductio n reversal event in the Solomon Islands arc in the southwest Pacific.