Lead isotopic composition of the oldest volcanic rocks of the eastern greater antilles island ARC

Abstract

Pb isotopic analyses for rocks from the early Cretaceous Los Ranchos Formation, which is the basal volcanic assemblage in the Dominican Republic, fall on a short linear array of shallow slope in a 207Pb/204Pb vs. 206Pb/204Pb plot. The array is interpreted to have formed by decay of local U since the time of ore and rock formation ~ 110 Ma ago. Previously reported analyses of sulfide samples from the Pueblo Viejo Au---Ag deposit in the upper part of the Los Ranchos Formation fall on the lower half of the same linear array. In a 208Pb/204Pb vs. 206Pb/204Pb plot the Los Ranchos Formation can be divided into the lower member with an apparent Th/U ratio of 1.2 and the upper three members with an apparent Th/U ratio of 2.6. A cluster of analyses falls at the least radiogenic end of these arrays and is interpreted to represent the initial Pb isotopic composition of the Los Ranchos magmas. This cluster falls close to the Stacey-Kramers growth curve. For the Los Ranchos Pb, a homogeneous source with [mu]=9.6 and Th/U near 3.8, since 3.7 Ga ago, is implied.Previously reported analyses for correlative rocks of the Water Island Formation, the basal volcanic complex in the Virgin Islands at the east end of the Greater Antilles, suggest a steep linear array extending downwards from the Los Ranchos cluster, after correction of the ratios for U decay since formation of the rocks. This array has a slope of 0.250 +/- 0.023, which could have resulted from magma derivation from sources that had been isolated for ~ 3.0 Ga. Younger rock Pb, from the island of Martinique, has a similar slope, although with larger 206Pb/204Pb and 207Pb/204Pb ratios. The data points extend upward from the radiogenic end of the Los Ranchos array.These relationships indicate a complex derivation of magmas in the area. In the Greater Antilles, the proportion of continental material decreases eastward, while even farther east in the Lesser Antilles at Martinique, the proportion of crustal material, as indicated by Pb isotopic composition, is again very high.