TESTING TEMPERATURE-TIME-COMPOSITION (T-t-x) RELATIONSHIPS BETWEEN THE HIGH PRESSURE/LOW TEMPERATURE (HP/LT) METAMORPHIC ROCKS IN THE RIO SAN JUAN AND SAMANA METAMORPHIC COMPLEXES, DOMINICAN REPUBLIC

The Rio San Juan (RSJ) and Samana Metamorphic Complexes (SMC) are paleosubduction complexes exposed in northern Hispaniola. The RSJ has been classified as a warm paleosubduction zone, with metamorphic rocks that record thermal gradients as high as ~18 °C/km; whereas the SMC contains metamorphic rocks that record lower thermal gradients. Although rocks in these complexes have been proposed to form in a common subduction zone, the geologic and tectonic relationships between the HP/LT rocks are debated. To evaluate the T-t-x relationships between the HP/LT rocks in these complexes, bulk-rock major and trace element geochemistry, zircon trace element and U-Pb geochronology, and trace element thermometry studies were conducted on blueschist- and eclogite-facies rocks from the RSJ and SMC.

Bulk-rock major and trace element analyses show that metamafic rocks from the RSJ and SMC have geochemical affinities to both N-MORB and arc-related rocks, indicating a mixture of upper and lower plate material as a result of subduction erosion processes. Zircons from a RSJ epidote eclogite record metamorphic ages ranging from 90 – 125 Ma, indicating that subduction-related metamorphism occurred for 35 million years. Zircon cores in this sample record possible protolith ages of ~130 Ma. Zircons from a retrogressed RSJ metamafic yield ages of 101 and 62 Ma. A SMC garnet blueschist zircon yields a similar age of 101 Ma, indicating that metamafic rocks in both complexes likely experienced a similar subduction-related event at this time. Zircons from two SMC metasediment samples yield ages ranging from 28 – 1600 Ma and 22 – 557 Ma, respectively, with prominent age peaks at 22 – 25, 31 – 34, 44 – 45, and 62 – 66 Ma. These age peaks match existing Rb-Sr and Ar-Ar ages from the SMC that have been variably interpreted as reflecting the timing of peak metamorphism for the metasediments and/or periods of exhumation and concomitant fluid-rock interaction. Trace element thermometry results show that SMC metamafics record a lower and narrower range of metamorphic temperatures than RSJ metamafics. Although temperatures for the SMC rocks are consistently lower than those of the RSJ, similarities in bulk-rock composition and metamorphic ages suggest that HP/LT rocks from both complexes had similar origins and experienced metamorphic events at similar times.