Pyroxenite Veins in the Loma Caribe Peridotite Massif of the Caribbean Island Arc: a Record of Boninitic Magma Crystallization, Melt-Harzburgite Reaction and Lithospheric Deformation during Embryonic Intra-Oceanic Subduction

Abstract

In the supra-subduction zone environment, the formation of pyroxenite veins is linked to reactions between boninitic melts and mantle peridotites. Because boninite volcanism is tied to subduction initiation, studying these pyroxenite veins provides crucial insights into both boninite petrogenesis and the onset of subduction, a pivotal yet poorly constrained geodynamic process. Here we combine a field structural study with microstructural, mineralogical, and geochemical analyses to characterize pyroxenite veins in the Loma Caribe peridotite massif, which represents a mantle section of the Early Cretaceous intra-oceanic Caribbean island arc. The studied pyroxenite veins range from orthopyroxenite to websterite and are characterized by highly depleted whole-rock and mineral major- and trace-element compositions. Trace-element modeling indicates that orthopyroxenites crystallized from Mg- and Si-rich, hydrated, ultra-depleted melts produced by the aggregated, non-modal fractional melting of a depleted source analogous to the clinopyroxene-rich host harzburgites. Websterites, in contrast, crystallized from melts with boninitic affinity, representing a mixture of an infiltrating melt (i.e. the melt in equilibrium with orthopyroxenites) and a melt derived from the partial melting of the host harzburgites. Thermobarometric calculations yield conditions for vein formation under spinel peridotite facies conditions at the base of the sub-arc lithosphere. Subsequent subsolidus reequilibration records the cooling and exhumation of the peridotites. The subhorizontal shearing recorded by the pyroxenite veins indicates that boninitic magma emplacement either predated or was synchronous with the development of the porphyroclastic foliation in the host harzburgites, which was driven by lithospheric extension within the Caribbean island arc. Broadly, these findings demonstrate that the boninitic parental melts reacted with partial melts of the depleted host peridotites to acquire their highly depleted compositions, meaning that these pyroxenite veins mark the pathways of melt migration through the lithospheric mantle during the onset of intra-oceanic subduction.