Geochronological, geochemical and geological constraints on models of ophiolite generation and ARC growth: Evidence from the northern Philippines

Concordant zircon U-Pb ages determined on basement in northern Luzon provide constraints on the tectonic development of the Philippine arc complex. The Acoje and Coto blocks of the Zambales ophiolite are 44.2 {dollar}pm{dollar} 0.9 (2{dollar}sigma{dollar}) Ma and 45.1 {dollar}pm{dollar} 0.6 Ma, respectively, the Angat ophiolite is 48.1 {dollar}pm{dollar} 0.5 Ma, and the Itogon ophiolite is 26.8 {dollar}pm{dollar} 0.4 Ma. These results, along with regional geologic constraints, suggest that the Zambales-Angat ophiolite formed during Eocene back-arc spreading within Cretaceous oceanic lithosphere. The Agno batholith/Itogon ophiolite is the product of limited extension within the Eocene lithosphere.; The Zambales ophiolite consists of 'blocks' dominated by arc-like crust or MORB (mid ocean ridge basalt)-like crust. This study shows that the arc-like areas have higher {dollar}sp{lcub}207{rcub}{dollar}Pb/{dollar}sp{lcub}204{rcub}{dollar}Pb and {dollar}sp{lcub}87{rcub}{dollar}Sr/{dollar}sp{lcub}86{rcub}{dollar}Sr, but identical {dollar}sp{lcub}143{rcub}{dollar}Nd/{dollar}sp{lcub}144{rcub}{dollar}Nd relative to the MORB-like areas, suggesting that a hydrous subduction component was involved in generation of the arc-like areas. Arc-like and MORB-like melts may be formed closely in space and time at a back-arc spreading center without invoking direct fluxing by fluids from a subjacent slab. When mantle enriched by subduction zone fluids is drawn up beneath the spreading center, its lower solidus allows melting at a depth greater than 'normal' MORB melting. The enriched melts rise through MORB-depleted peridotite and consequently generate arc-like magmas by remelting of and/or reaction of melts with the depleted peridotite.; Kyanite-garnet amphibolites exposed at the base of the Palawan ophiolite preserve peak metamorphic conditions of 700-760{dollar}spcirc{dollar}C and 9-10 kbar. The {dollar}sp{lcub}40{rcub}{dollar}Ar/{dollar}sp{lcub}39{rcub}{dollar}Ar ages of hornblende and muscovite from amphibolite and kyanite schist, respectively, are concordant at 34.0 {dollar}pm{dollar} 0.6 Ma. Available data indicate rapid uplift and cooling of the metamorphic rocks following their generation at and during the inception of a south-directed subduction zone within pre-South China Sea lithosphere. Activation of this subduction zone accommodated the opening of the South China Sea.; The Capoas granite in Palawan intruded continental rocks that rifted from China during the opening of the South China Sea. Zircon and monazite U-Pb systematics indicate a post-spreading crystallization age of 13.6 {dollar}pm{dollar} 0.5 Ma. The geochemistry of the granite and a 2 Ga component in zircon cores indicate substantial melting of the lower continental crust. Granite generation was coeval with widespread basaltic magmatism in the South China Sea area. This suggests that the granite is the product of crustal melting by basaltic underplating.