Geodynamic setting of high-Cr chromite mineralization in nascent subduction zones: Li isotopic and REE constraints from the Zambales ophiolite, Philippines

Abstract

The Acoje block of the Zambales ophiolite in the Northwest Philippines is a remnant of forearc lithosphere that formed during an Eocene subduction initiation event in the Western Pacific Ocean. It hosts abundant high-Cr chromitites in the uppermost part of the mantle sequence and the overlying dunitic Moho transition zone. The host harzburgites of the uppermost mantle sequence vary upward from Cpx-rich to Cpx-poor varieties. Clinopyroxene grains in both types of harzburgites and the dunites have prevalent finger-like protrusions and diopsidic compositions, indicative of melt impregnation. From the Cpx-rich harzburgites to Cpx-poor harzburgites and dunites, the clinopyroxene grains display increasingly depleted major element and rare earth element (REE) compositions, e.g., Al2O3 contents from 5.15 wt% to 0.84 wt% and Yb concentrations from ~ 0.2 ppm to 1.5 ppm. These changes are compatible with a transition from MORB to boninitic affinity, Olivine grains in the dunites and sparsely disseminated chromitites vary widely in δ7Li values, primarily from ~ +1 to +10. Such a δ7Li range implies that the parental magmas of the high-Cr chromitites were derived from mantle sources that were modified by slab fluids and had Li isotopic signatures either similar to, or heavier than, the normal mantle range (+4 ± 2). Partial melting in the mantle sources was triggered by heating of previously hydrated Cpx-rich harzburgites during slab rollback and subsequent asthenospheric upwelling. The upwelling brought asthenospheric materials and high heat flux into the lithospheric mantle, facilitating formation of high-Ca boninitic magmas and high-Cr chromitites in the nascent mantle wedge. The different Li isotopic domains in the mantle sources of the high-Cr chromitites indicate that the mantle temperatures were elevated rapidly after the hydration event in order to generate the boninitic magmas.