LEAD ISOTOPE STUDY OF THE PALEOGENE IGNEOUS ROCKS OF THE SIERRA MAESTRA, SOUTHEASTERN CUBA

The Sierra Maestra of southeastern Cuba occupies a key position between the remnants of the Greater Antilles arc accreted terranes and the Cayman strike-slip belt that constitutes the present day northern Caribbean plate boundary. An isotopic study has been undertaken to constrain the paleotectonic setting and the source of the Sierra Maestra structure. Lead isotope ratios were determined for a set of lithologies representative of each major magmatic complex of the Sierra Maestra. Both ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios are restricted to a narrow, well-defined array of values in a band parallel to the North Atlantic Reference Line NHRL The slight enrichment in ²⁰⁷Pb/²⁰⁴Pb ratios represent the selective mobilization U with respect to Th, (U/Th ratios ranges from 0.8 to 1). Overall Pb isotope ratios for the Sierra Maestra are homogeneous, indicating the existence of a single magma source. This observation correlates with the low Ce/Yb values that characterize primitive arcs with varying degrees of the PREMA (Primitive fertile mantle) component. There is a significant overlap between the isotopic ratios of the Sierra Maestra and the Cretaceous island arc tholeiites of Puerto Rico indicating that the Paleogene arc rocks have a similar mantle source.

Site	Rock Type	206Pb/204Pb		207Pb/204Pb	208Pb/204Pb		SiO2
Daiquiri	Gabbro	18.719	15.591			38.33	48.36
Daiquiri	Qz-diorite	18.919	15.571			38.402	63.29
Daiquiri	Andesite	18.983	15.597			38.516	57.4
Nima-Nima	Qz-diorite	18.991	15.625			38.566	64.01
Nima-Nima	Basalt	18.672	15.592			38.329	51.22
Guama	Qz-diorite	19.107	15.635			38.723	62.52
Guama	Bas-And.	18.977	15.627			38.519	54.98
Turquino	Gr-diorite	19.29	15.634			38.719	74.62
Turquino	Basalt	18.818	15.596			38.385	50.89
Turquino	Diabase	18.879	15.581			38.344	51.52
Turquino	Qz-diorite	18.506	15.597			38.119	51.59