Geochemistry of the upper Diliman Tuff unit in Manila, southwest Luzon, Philippines: Insights on its origin and comparison with Taal and Laguna caldera pyroclastic flows.

摘要

The upper Diliman Tuff is a basaltic to dacitic pyroclastic flow, which overlies the sequence of tuffaceous deposits found north of the southwest Luzon volcanic field in the vicinity of Manila, Philippines. Pumice fragments from this deposit are high-K basalt to dacite (50--65 wt. % SiO 2). These pumices are glassy with 1--3% phenocrysts content, mainly plagioclase and pyroxene. Mingling textures occur and there is variability in glass compositions for a single pumice. Disequilibrium features are also seen in the phenocrysts. Bulk trace element composition and mineral chemistry indicate mingling of magmas. The chemical variation in the deposit can be explained by mixing of melts produced by different degrees of partial melting. Volcanism in Luzon is produced largely from subduction and in the southwestern portion, extension. During subduction, the overriding crust is modified by emplacement of subduction-related magmas and partially melted by hot basaltic intrusions generated in the mantle wedge beneath southwest Luzon. The location of the actual vent or source volcano of the upper Diliman Tuff deposit is uncertain; comparisons show that it is chemically distinct with respect to deposits from adjacent Taal and Laguna calderas. Differences with these volcanic centers are seen in terms of major and trace element compositions.