The geochemistry of the Maimón Formation (Central Cordillera, Dominican
BOLETÍN GEOLÓGICO Y MINERO
RESUMEN Los materiales volcánicos de la Formación Maimón, metamorfizados en facies de esquistos verdes, son representativos del magmatismo más antiguo registrado en el arco-isla del Cretácico del Caribe. A partir de nuevos datos geoquímicos de roca total y mediante el uso de elementos traza inmóviles (elementos de alto potencial iónico -HFSE, tierras raras -REE), se reconoce el predominio de protolitos basálticos y una menor proporción de rocas de composición intermedia y ácida.
... da. Geoquímicamente, los protolitos basálticos han sido clasificados como boninitas, toleítas de arco de isla pobres en Ti y LREE, toleítas de arco de isla pobres en Ti normales y escasas toleítas de arco de isla normales. Las rocas ácidas presentan claras afinidades boniníticas y toleíticas y pertenecerían al tipo M (derivadas del manto). La geoquímica de las toleítas pobres en Ti y LREE permite su clasificación como FAB (Fore-Arc Basalts, i.e. basaltos de ante-arco), término recientemente acuñado para describir toleítas con rasgos de MORB (basaltos de dorsal centro-oceánica) identificados en el ante-arco de Izu-Bonin-Mariana. La presencia de FAB y su correlación estratigráfica con boninitas son indicativas de formación en un contexto de ante-arco durante los primeros estadios magmáticos en un régimen extensional ligado al inicio de la subducción y el nacimiento del arco-isla. La Fm. Maimón puede representar la sección superior (volcánica) de una ofiolita tipo "inicio de subducción" . Palabras clave: Arco de Islas Volcánicas, Basaltos de Ante-arco-FAB, Caribe, Geoquímica, Ofiolita. The geochemistry of the Maimón Formation (Central Cordillera, Dominican Republic) revisited ABSTRACT Volcanic rocks from the Maimón Formation, metamorphosed under greenschist facies conditions, represent the most ancient arc magmatism recorded in the Cretaceous Caribbean island arc. From new geochemical data, and on the basis of immobile trace elements (high field strength elements -HFSE, rare earth elements -REE), a predominance of basaltic protoliths over intermediate and acid compositions have been determined. Geochemically, the basaltic protolith includes boninites, low-Ti LREE-depleted island arc tholeiites, normal low-Ti island arc tholeiites, and exiguous normal island arc tholeiites. The acid rocks present a clear boninitic and tholeiitic affinity and are of the M-type (mantle-derived). The geochemistry of low-Ti LREE-depleted island arc tholeiites permits their classification as fore-arc basalts (FAB), which is a term introduced recently to describe tholeiites with MORB (mid-ocean ridge basalt)-like features from the Izu-Bonin-Mariana fore-arc. The presence of FAB and their stratigraphic association with boninites indicate formation in a fore-arc environment during the earliest stage of island arc magmatism in an extensional setting that characterizes the onset of subduction and birth of an island arc. The Maimón Formation could represent the volcanic top of an ophiolite of the "subduction initiation" type. Introduction and Methods The rocks from the Maimón Formation (Central Cordillera, Dominican Republic) are claimed to be part of the oldest and most primitive volcanism in the Caribbean island-arc (Horan, 1995; Lewis et al., 2000 Lewis et al., , 2002 . However, there is still controversy as to its formation geotectonic setting within the island-arc system and its relationship with other lower Cretaceous geologic units cropping out in the Dominican Republic. Here we present a review of the PIA (Primitive Island Arc; Fig. 1 ) series in the Caribbean region, the Dominican Republic and the Maimón Formation as well as new lithogeochemical data obtained from a set of samples representative of the volcanic sequence of the Maimón Formation in its central and southern part. The new data is compared with previous results and interpreted and discussed on the basis of the latest knowledge on intraoceanic subduction initiation and island-arc birth. This study developed from a total of 182 drill core and in situ field samples from the Maimón Formation. (Fig. 2) . Accordingly to descriptions by Draper et al. (1996) and Draper and Gutiérrez-Alonso (1997) , rocks in the Ozama shear zone present higher deformation and metamorphic grade, are conspicuously recrystallized and develop planar foliation structures and mylonitic textures. Deformation progressively diminishes in the el Altar zone, in which foliation is less penetrative and protomylonitic textures, magmatic mineral remnants, amygdales and other primary structures are preserved. Whole rock geochemistry was determined for a total of 36 samples; major element and Zr concentrations were determined by means of X Ray fluorescence (XRF) whereas trace elements where measured by ICP Mass Spectrometry (ICP-MS). Results and Discussion Rocks of volcanic protolith from the Maimón Formation are mainly metabasitic schists and gneissic meta-vulcanites, and less abundant meta-hyaloclastites Rocks from the Maimón Formation have undergone extensive sea-floor metamorphism as well as hydrothermal alteration linked to the formation of the VMS deposits. As a result, a number of elements (Si, K, Na, Ca, Mg, Fe, Rb, Ba, Sr; Fig. 3) are likely to have mobilized and hence those elements considered immobile in such post-magmatic conditions (HFSE, REE, transition elements and Th) are used here for igneous rock classification and tectonic discrimination. According to the Zr/Ti vs. Nb/Y classification diagram of Pearce (1996) , rocks from the Maimón Formation range from subalkaline basalts to rhyodacites (Fig. 4) . Studied samples from the Maimón Formation are predominantly basalts and less abundant andesites and basaltic andesites and rhyodacites. Geochemical analyses of volcanic rocks from the Maimón Formation whose protoliths were identified as basalts returned remarkably low Nb contents (<2 ppm; Table 1), that in the chondrite-normalized extended REE diagrams translate as marked negative Nb anomalies when compared to values of NMORB (Fig. 5) . High V/Ti ratios of studied basalts are indicative of formation in island-arc settings in opposition to low ratios that characterize those basalts formed in MOR or mature back-arcs (Shervais, 1982 ). Basaltic rocks from the Maimón Formation have been classified according to their TiO2 and Cr (used as immobile proxy for MgO) contents into three types: type I or boninites, type II or low-Ti IAT (LOTI) and type II or normal IAT. Type-I basalts or boninites (TiO2<0.5 wt. % and Cr>275 ppm) show general depletions in REE compared to NMORB, with chondrite-normalized REE patterns varying from slightly convex (depletion in MREE relative to LREE and HREE) to mildly LREE-depleted (positive slopes). Studied boninites from the Maimón Formation show similar normalized patterns to those described by Escuder-Viruete et al. (2006) in the Los Ranchos Formation, although LREE values are occasionally higher, similar to boninites from the Izu-Bonin-Mariana (IBM) fore-arc (Pearce et al., 1992; Reagan et al., 2010). However, chondrite-normalized patterns of LREEdepleted boninites show indisputable similarity to basalts described as transitional (from Fore Arc Basalts, FAB, to boninites) in the IBM fore-arc basin by Reagan et al. (2010). Type-II or LOTI basalts (0.4<TiO2<0.8 wt. % and Cr<275 ppm) show conspicuous depletion in REE compared to NMORB. According to their chondrite-normalized REE patterns, LOTI basalts can be grouped into two sub-types: IIa or LREE-depleted LOTI and IIb or normal LOTI. Type-IIa or LREE-depleted LOTI basalts are in the majority among the studied basalts from the Maimón Torró, L., et al., 2017. La geoquímica de la Formación Maimón (Cordillera Central,... Boletín Geológico y Minero, 128 (3): 517-539