PRESSURE-TEMPERATURE EVOLUTION OF HIGH-PRESSURE BLOCKS OF THE PUERTO NUEVO MÉLANGE COMPLEX (BAJA CALIFORNIA SUR, MEXICO)

Puerto Nuevo mélange complex, in the western part of Baja California, Mexico, is composed of different metamorphic rocks including eclogite, amphibolite, blueschist, ortho-gneisses, and low-grade schists blocks hosted in a sepentinitic matrix. A set of 3 representative samples of the high-grade blocks (one eclogite, one amphibolite, and one blueschist) were selected to explore the pressure-temperature evolution experienced by these blocks.

Eclogite shows a mineral assemblage of grt + omp + amp + ph + ep + ttn + rt. Garnets exhibit decreasing Ca content and increasing Mg content from core to rim. The zoning of garnet indicates the increment of pressure in metamorphic conditions from core to rim. Amphiboles are Na-Ca rich, ranging from pargasite to winchite in composition, and have XMg values greater than 0.5. Phengite has Si values from 3.3 to 3.45 apfu. The amphibolite displays the mineral assemblages of amp + gt + ph/wm + pl + ep + ttn. In contrast to eclogite, garnets from amphibolite display increasing Ca content and decreasing Mg content from core to rim. Amphiboles are Ca-rich and have XMg values ranging from 0.63 to 0.74. Phengite generally has Si values of 3.3 apfu. Blueschist on the other hand shows a mineral assemblage of amp + pl + chl + qz. Amphiboles are glaucophane-rich variety.

Preliminary eclogite phase-equilibrium diagrams show peak conditions ranging between 650-800 °C at 2.2-3 GPa. These peak conditions are in good agreement with the conventional thermobarometric results. Amphibole and epidote along the fractures and the inclusion of garnets indicate retrogression to amphibolite-facies conditions. Preliminary amphibolite phase-equilibrium diagrams yield peak P-T conditions ranging between 550-750° C at 0.45-1.1 GPa. Blueschist-facies blocks form at lower P-T conditions than eclogite- and amphibolite-facies blocks. This study provides the first state-of-the-art quantitative thermobarometric estimates of high-pressure blocks from the Puerto Nuevo mélange complex, providing a unique window to explore subduction-zone processes.