AGE AND GEOCHEMISTRY OF AN ANOROGENIC CRUSTAL MELT AND IMPLICATIONS FOR T-TYPE GRANITE PETROGENESIS

The Capoas intrusion is a metaluminous, high-K calc-alkaline, I-type b iotite granite emplaced within Permian-Jurassic sedimentary rocks of t he North Palawan Continental Terrane (NPCT) in the western Philippines . The NPCT is a fragment of the Mesozoic Andean-type margin of southea st China that was separated from the mainland during the late Oligocen e-early Miocene opening of the South China Sea. Zircons from the grani te have xenocrystic cores, and form a discordant array with a lower in tercept age of 15 (+3/-4) Ma. Monazites have concordant (207)pb/U-235 ages with a mean age of 13.4 (+/-0.4) Ma. The late middle Miocene age and the location of the pluton in the NPCT uniquely constrain the form ation of the Capoas granite in a post-rifting, non-collisional tectoni c setting unrelated to any subduction zone. The major and trace elemen t geochemistry of the granite and the presence of apparently Proterozo ic xenocrystic zircon indicate that the pluton is composed largely, if not entirely, of older continental crust. The only viable heat source for crustal melting and/or assimilation was widespread basaltic magma tism that occurred in the area following cessation of seafloor spreadi ng in the South China Sea in early Miocene time. The geochemical affin ity of the Capoas granite with calc-alkaline magmatic are and collisio nal granites is therefore a function of the source rocks that were mel ted to produce the granite rather than the specific tectonic setting i n which the granite was generated. The calc-alkaline source rocks most likely formed in the Mesozoic Andean-type margin of south China and s ubsequently underwent partial melting in late middle Miocene time in a n 'anorogenic' setting. (C) 1997 Elsevier Science B.V.