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Metabasic and sedimentary rocks of the Duarte Complex of apparent Cretaceous age in the La Vega-Jarabacoa-Janico area have been mapped at a scale of 1:50,000. The Duarte Complex is divided into a lower unit and an upper unit. The lower unit consists of mainly metabasic rocks, greenstones, and amphibolites, partly intruded by granitoid plutons, mainly ionalite. Minor hornblende gabbros and hornblendites are considered to be genetically related to the tonalites.

The lavas forming the lower unit include thick, differentiated high-magnesian flows composed of tholeiitic metaankaramites and picrites. Included in the lower unit is the El Yujo-Duarte subcomplex, which consists of a lower sequence of metabasic lavas, breccias, and tuffs grading through a transitional sedimentary facies into an upper sequence of quartzo-felspathic and quartz-sericite schists. The transitional rocks in drill cores consist of cherts at the base and 70 m of carbonaceous shale with minor chert intervals. In surface exposures the carbonaceous shales are absent and the rocks of the transitional sequence are carbonaceous limestones, breccias, and cherts. The structure of the El Yujo section is interpreted to be an asymmetrical synform. The preferred hypothesis is that the El Yujo section is a conformable transitional sequence, reflecting a change from basic volcanism (possibly a seamount) up into a series of arc rocks of acidic composition.

The lower part of the upper unit, exposed in the Guaigui-Rio Camu area, shows a sequence of rock types, which include, from south to north, a dike complex, vitric basalts (including pillow lavas and breccias), interbedded lavas and hyaloclastites, intrusive diabases, and metatuffs, apparently grading into a thick sedimentary section of mainly shales and cherts with intercalated basalt flows. To the northeast, the Rio Yami–Loma La Monja area (Rio Yami subcomplex) consists of multiple blocks of serpentinized peridotite intercalated with sedimentary and basaltic rocks. The sedimentary rocks are indurated clastic rocks, mainly shales, less commonly cherts intercalated with diabases and basalts. Most of the lavas in the upper unit are aphyric or only weakly porphyritic. Both the lavas and hypabyssal rocks are tholeiites, low in MgO (<11 percent in aphyric lavas), but have variable TiO2 (0.78 to 4.24 percent).

Basaltic rocks throughout the Duarte Complex in the area show remarkably similar trace element patterns indicating that the basalts evolved from a single magma system over a limited time interval. The rocks are characterized by high concentrations of the large ion lithophile (LIL) elements relative to the high field strength (HFS) elements. The association of mafic lavas, pillow lavas, hyaloclastites, and hemipelagic sedimentary rocks indicates that these rocks formed in a marine environment and represent the upper part of an ophiolite sequence.

The Duarte sequence is interpreted to represent either a seamount or the early formed lavas of the arc structure juxtaposed against an uplifted back-arc basin (ocean floor) now represented by the Loma Caribe serpentinized peridotite and by vitric basalt in the southeast part of the Median Belt.

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