Petrology and geochemistry of the central Mindano volanic arc, southern Philippines.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This paper presents the regional geology, geochemistry and petrogenesis of volcanic rocks from Central Mindanao Volcanic Arc (CMVA), Philippines. This NNW-trending arc, from Lat 6°N-Long 125ºE to Lat 90 15'-Long 125º30'E occurs parallel to the Philippine Trench and to regional structures E of Lake Lanao. CMVA centres occur 15-200 km above a W-dipping subduction system. Subduction associated with the Philippine Trench probably produced the CMVA. CMVA can be grouped into 3 volcanically and seismically active arc segments from N to S: i) Misamis, ii) Bukidnon and iii) Davao. Camiguin Island is the most active volcanic field and resulted from SE-NW progression of volcanism from late Pliocene-Pleistocene lavas of Butay, Cabuan, Mt Ginsiliban and Mt Mambajao to Pleistocene-Recent lavas of Mt Catarman and Hibok-Hibok. Stratigraphy of historical deposits from Hibok-Hibok shows a range of volcanic products typical of other centres: domes, lava flows and pyroclastic flows. The rocks of CMVA are crystal-rich (40-50 vol% average) with a main phenocryst assemblage of plagioclase (An84-31) + clinopyroxene (augite, diopside) ± olivine (Fo84-72). Timagnetite usually crystallised late. Two types of andesite occur: pyroxene (augite ± enstatite) and homblende (magnesium-ferri homblende) andesite. Two dacite types are also recognised. Type 1 dacite (Mt Katanglad, Butay) usually has fine grained plagioclase + clinopyroxene whereas Type 2 dacite (Hibok-Hibok, Mt Apo) has medium-coarse grained amphibole ± biotite. Phenocryst chemistry shows that with increasing bulk-rock SiO2. plagioclase. olivine, clinopyroxene and Ti-magnetite follow chemical trends consistent with equilibrium crystallisation. Exceptions occur in basaltic andesite and some SiO2-rich andesite in which olivine is Fe-enriched, plagioclase is more calcic than in basalts (An78-84 vs An50-78) and there is a large incidence of reverse zoning in clinopyroxene. The most widespread lavas are medium-K calcalkaline (CA) basalt, andesite and dacite with occasional low-K and high-K CA lavas. Major and trace element variations are similar with other island arc basalt (IAB) and exhibit the characteristic depletion in Zr, Ti, Y and to a lesser extent, of Ce relative to N-MORB. N-MORB normalised trace element contents are generally >1.0 and have low Zr/Nb ratios (7-14) indicating an E-MORB/OIB type mantle wedge. All CMVA basalt-basaltic andesites have lower Mg#, Cr and Ni relative to primitive MORB, probably a result of olivine-dominated fractionation. Silica activity (-0.33 to -0.36 log units) and crystallisation temperatures (1,090 - 1,161°C) of CMVA basalts suggest a source region geochemically between olivine tholelite and alkaline olivine basalt. Basaltic analyses, corrected for olivine fractionation, suggest primitive CMVA compositions resemble magnesian MORSs. Assuming equilibrium batch melting, CMVA parental magmas segregate from residues at 10-15 kbar after 10-14% partial melting. The resultant melts are LIL-element enriched with high Ba/Nb and Ce/Nb ratios indicating a slab contribution although crustal contamination was probably low, based on low Rb/Sr ratios (0.015-0.026) relative to other lAB (0.03) and continental crust (0.12). Andesite and dacite can be modelled satisfactorily by low pressure closed-system crystal fractionation. The main fractionating assemblage is olivine + clinopyroxene + plagioclase ± Timagnetite in basalt-basaltic andesite melts whereas plagioclase + amphibole ± Ti-magnetite fractionate from basaltic andesites to form SiO2-rich andesite and dacite. Magma mixing and assimilation probably occurred at Mt Apo. In summary, the deeper seismic zone beneath CMVA relative to other island arcs may be explained by lateral transport of fluids from subducted slab (Davies and Stevenson, 1992) using a slab-induced convection model (Tatsumi, 1986; Saunders et ai, 1991). Partial melts segregate at depth (spinel peridotite field) to form olivine or nepheline to diopside-normative tholeiites. As these magmas ascend, they cool, become more dense and pond at the lower crust where they differentiate to high-AI basalt and basaltic andesite. Diapirs then migrate from lower crust to shallow magma chambers (< 5 km) where extensive fractional crystallisation results in andesitic magmas. Disequilibrium features of phenocrysts from basaltic andesites support the concept of mixing between a lower basaltic layer and an upper intermediate composition layer. Predominance of intermediate to SiO2-rich lavas suggest that i) most eruptions tap only the upper portion of compositionally-zoned chambers and/or ii) magma chambers are at a mature stage of chemical evolution.