On Arcs and Ophiolites - Petrologic Comprison of Zambales Range and the Mariana Forearc - Evidence of Early Stages of Island Arc Development

The Zambales Range ophiolite (ZRO), a relatively intact, coherent, series of ocean crust and upper mantle rocks, comprises depleted harzburgite-dunite tectonites, overlain by Ol-Opx-Cpx layered cumulates, and Ol-Px-Pl layered and massive rocks. All display evidence for near-solidus, dry, penetrative ductile deformation. Higher crustal levels of ZRO have massive gabbro/norite, diabase sills and dikes, pillow basalts. Many of these have primary amphibole. ZRO has three mappable units separated by high angle faults. A boninite-IAT section, probable forearc; MORB-composition pillows and sills, overlying 3.5 km of gabbro, and a thin cumulate troctolite section, represent a back-arc basin. Thickest section (up to 8 km) probably was a nascent island arc. The upper 2 km has IAT pillows and diabase,many with primary Hbl; these overlie layered and massive Hbl gabbro/ norite, that overlie a 700 m Ol-Opx-Cpx cumulate series. There are many small sills, dikes, and plutons of low-K quartz diorite- tonalite- trondhjemite (QDTT). Deepest levels of ZRO are highly depleted (partly serpentinised) chromite-rich dunite and harzburgite; Fo 90-93, CHR Cr# 0.7-0.74. Ultramafics, cumulates, gabbro/norite, with similar petrology and textures, basalts, boninite, and QDTT have been dredged from inner slopes of the Mariana Trench (MT) 11 o - 18 o N. Petrologic data for ZRO and MT rocks rule out an origin at a mid-ocean ridge setting. Critical data include strong depletion of HFSE and REE in all rocks relative to N-MORB; variable enrichment in water mobile low -ionic potential elements; hydrous magmas are indicated by primary Hbl and late stage high-Ca Plag; petrologic layer thicknesses for ZRO are unlike oceanic crust; e.g., 700m of basal cumulates, 8 km gabbro/norite; ZRO lacks sheeted mafic dikes; but has relatively abundant dikes, sills, small plutons of QDTT. Some MT dredge samples are QDTT; they formed plutonic, volcanic, and water-laid ash fall material. Seismic velocity data suggest that QDTT forms mid-crust of the Izu-Bonin arc; comparable rocks form large plutons of the Tanzawa complex. Boninite,boninite series rocks, and QDTT of ZRO, MT, and Izu-Bonin arc exclude a MOR origin. The ZRO has REPLACED older ocean crust,- it is NOT BUILT ON older crust. Evidence for near-solidus tectonites in ZRO and MT rocks suggest that mantle upwelling, in response to trench roll-back, was an important factor in early arc evolution.