Geochemistry of pyroclastic deposits in Metro Manila, Philippines: insight on volcanic source

Most of the deposits in Metro Manila (MM) are primary pyroclastics and epiclastics included in the Diliman Tuff member of the Guadalupe Formation. Coring to a 40 m depth intersects a stratigraphic sequence consisting of, from top to bottom: a coarse grained pyroclastic flow deposit with basaltic to dacitic (51 to 66 wt.%\ SiO₂) clasts, a weathered ash - possibly a soil, a thick sequence of fines-rich surge and fall deposit, a fine grained pyroclastic flow consisting of andesitic (59 to 64 wt.%\ SiO₂) clasts and lastly another coarse grained pyroclastic flow deposit. Short cores (10 m) and outcrops in the area support this stratigraphy. The focus of this study is on the uppermost pyroclastic unit from MM. This pyroclastic flow deposit consists of mafic, felsic and banded pumice clasts. The pumices are glassy, finely vesiculated and with few plagioclase crystals. Geochemical variations in the pumice clasts of this MM deposit are different than those from the deposits erupted from the two nearest known calderas in the region, which are Laguna caldera, 40 km to the southeast, and Taal caldera, 60 km south. Both eruptions produced basaltic to dacitic pyroclastic flows. Chemical variation of the MM pyroclastics is distinctly different from Taal caldera deposits. The MM deposits have higher K₂O, Sr and Rb values compared to Taal. In terms of these chemical variables, the MM deposits are similar to the Laguna caldera deposits. Based on other chemical variables, however, MM pyroclastics also do not correlate with Laguna caldera. The pyroclastics in MM have lower TiO₂ and Zr and higher MgO concentrations than Laguna. Concentrations for these elements in MM pyroclastics are intermediate between Laguna and Taal caldera. The geochemical data from MM pyroclastic deposits are not consistent with MM magma sources being related by simple fractional crystallization from either Laguna or Taal caldera sources.