Complex origin for the south-western Zamboanga metamorphic basement complex, Western Mindanao, Philippines

Amphibolites unconformably overlain by a metasedimentary sequence of quartz -muscovite-feldspar-kyanite schists, metagraywackes and epidote-bearing amp hibolites occur in the northern portion of the south-western Zamboanga meta morphic basement complex, western Mindanao. These amphibolites (here identi fied as the Mount Dansalan amphibolites) display relict magmatic textures i nherited from cumulate gabbro protoliths. Bulk-rock major and trace-element data are consistent with this hypothesis. Together with the chemistry of r elict igneous clinopyroxenes, they indicate a magmatic arc-related signatur e for the gabbro protoliths. Geochemical data allow us to identify various sources for the associated metasediments: the gabbro themselves for the met agraywackes and a continental basement for the quartz-muscovite-feldspar-ky anite schists. Both sources contributed to the genesis of the epidote-amphi bolite metasediments. The compositions of the metamorphic mineral assemblag es suggest that the rocks have undergone metamorphism at temperatures rangi ng from 550 degreesC to 700 degreesC and pressures probably in the range of 5-9 kbar. K-40-Ar-40 isotopic study of amphibole separates from the Mount Dansalan samples document a metamorphic event dated at 24.6 +/- 1.4, 22.2 /- 1.4 and 21.2 +/- 1.2 Ma. Our results are in agreement with plate tectoni c models which describe the south-western Zamboanga metamorphic basement as a continental terrane. However, its evolution was not as simple as it was usually considered. In particular the basement incorporated slivers of magm atic arc crust, which cannot be unambiguously related to any of the Tertiar y arcs documented in the area.