THE RECYCLING OF CHROMITITES IN THE PUERTO NUEVO OPHIOLITE (BAJA CALIFORNIA SUR, MEXICO)

The presence of crustally-derived zircons in mantle-wedge chromitites is considered an evidence of recycling of crustal material within the subduction zones. Crustal contamination of the mantle wedges above supra-subduction zones can be produced by 1) oceanic crust and sediments dragged down by the subducting slab, 2) subduction erosion of the fore-arc regions, or 3) delamination of dense lower crust beneath a continental arc. Here, we present a comprehensive study of zircons from high-Cr chromitites (Cr#= 0.61–0.69) hosted in the mantle peridotites from the Puerto Nuevo sequence (Baja California Sur, Mexico) (250-221 Ma), formed in the back-arc region of a supra-subduction zone.

Zircons recovered from these chromitites yield ages (2263±44 to 278±4 Ma) and Hf-O compositions (ɛHf(t)= −18.7 to +9.1 and δ18O= 5.9 to 12.4 ‰) that broadly match with those zircons reported in the nearby exposed Arteaga Complex (i.e., Potosi fan). Thus, we interpret that the chromitite zircons from the Puerto Nuevo sequence are xenocrysts from continent-derived sediments delivered into the mantle via subduction and transferred to the mantle wedge by cold plumes. In this scenario, chromites containing zircons were formed by the mixing of a Cr-rich boninitic melt formed by dissolution of orthopyroxene from harzburgite, and a more SiO₂-rich melt formed by partial assimilation of the plume contaminated with crustal material. This is in agreement with the minor and trace elements composition of chromites, which are similar to those from high-Cr ophiolitic chromitites crystallized from melts similar to high-Mg island-arc tholeiites (IAT) and boninites in supra-subduction zone. In addition, the presence of clinopyroxene lamellae with preferred crystallographic orientation in our chromites evidence that the Puerto Nuevo chromitites have experienced HT-UHP conditions (<12 GPa and ~1600 °C). Hence, we propose a tectonic scenario that involves: 1) the formation of chromitite in the supra-subduction zone mantle wedge underlying the Vizcaino intra-oceanic arc ca. 250 Ma ago (Early Triassic), 2) deep mantle recycling, and 3) subsequent diapiric exhumation in the intra-oceanic basin (the San Hipólito marginal sea) generated during an extensional stage of the Vizcaino intra-oceanic arc ca. 221 Ma ago (Late Triassic).