| New data ({dollar}>{dollar}300 samples) were integrated with published data from thirty-two Pliocene-Recent volcanic centers in the Philippines in order to provide clues to (1) thermal and compositional histories of the asthenosphere, (2) mass transfer between lithospheric slabs and the convecting asthenosphere, and (3) fundamental causes of melting in arc settings. Volcanoes were grouped into a tripartite scheme according to whether they were associated with (1) micro-continent collisions (Luzon-Taiwan; Palawan-Mindoro), (2) subducting oceanic lithosphere with pelagic sediment cover (Philippine Sea Plate) and (3) subducting 'transitional' marginal basin with continent-derived sediment cover (South China Sea basin). These groups were compared in terms of processes affecting their source regions, utilizing enrichment, depletion and partial melting models on the basis of global studies and relevant experimental and geophysical constraints on mantle hydration and dehydration reactions.; The findings of this study can be summarized: (1) Melting above subduction zones results from solidus reduction due to the addition of H{dollar}sb2{dollar}O and adiabatic decompression triggered by hydrous phase breakdown and lithosphere stretching, (2) ambient arc melt fractions and production rates may be comparable to those at mid-ocean ridges. Large melt fractions ({dollar}>{dollar}0.25) of a MORB-like source cannot explain elevated LILE/HFSE ratios in arc lavas. Most arc lavas are probably generated by melt fractions ranging from 10-20%, (3) Magma sources are confined to the mantle 'wedge' and, as seen in variable FeO* and HFSE depletion, range from fertile to refractory. In the Philippines, the frontal arc volcanoes have more refractory sources and larger melt fractions than the secondary (back-side) volcanoes, (4) High-field strength element depletion in arc magmas is best explained by the variably refractory character of their sources, and not by retention of HFSE by accessory phases (rutile, sphene, etc.), (5) Arc LILE and LREE enrichment comprises slab-derived fluids (probably delivered at shallow depths), and sediment melting at deeper depths. This enrichment is added into a variably refractory MORB residue, (6) Most primitive arc lavas are generated at relatively shallow depths (40-60 km), especially those that are related to rifting and (7) By synthesizing data from reasonable isotherm distributions, earthquake hypocenter data, seismic tomography, dehydration reactions and solidus relationships for peridotite and basalt compositions, melt segregation depths and temperatures, variably refractory source compositions, and fingerprinting of slab contributions, it can be concluded that refractory lithosphere mantle, weakened by hydration and entrained by asthenospheric corner flow, is the probable source for most arc magmas. |
