Igneous Geochemistry of Mineralized Rocks of the Baguio District, Philippines: Implications for Tectonic Evolution and the Genesis of Porphyry-Style Mineralization

The Baguio district of the Philippines is one of the worlds premier mineral provinces, containing >35 millionounces (Moz) of gold and 2.7 million metric tons (Mt) of copper in epithermal, porphyry, and skarn depositsthat formed in the last 3.5 m.y. Pliocene and Pleistocene magmatic rocks of the Baguio district that arespatially and temporally associated with mineralization can be broadly subdivided into an intermediate to felsicsuite of mineralized low to medium K intrusions, some of which have adakitic affinities and a suite of maficto intermediate, medium K to shoshonitic hornblende-phyric dikes. The geochemical and isotopic characteristicsof the dikes are consistent with primitive mantle-derived melts that underwent minimal crustal contaminationas they ascended through the arc crust. In contrast, the intermediate to felsic suite has been contaminatedby young arc crust, suggesting ponding and fractionation within shallow-crustal magma chambers.The Philippine arc has formed in a complex tectonic environment and is currently sandwiched between twoactive subduction zones. Eastward-directed subduction of the Scarborough Ridge along the Manila trench iscurrently associated with flattening of the downgoing slab. The formation of the Mafic dike complex is broadlycoeval with the onset of subduction of the Scarborough Ridge and slab flattening. The extinct ScarboroughRidge would have been younger than the downgoing plate and consequently more susceptible to melting.These melts can account for the isotopic recharge of the Pliocene subarc mantle as well as the generation ofthe primitive melts and adakitic rocks found within the Baguio district. The interaction between primitivemafic melts and the more felsic calc-alkaline rocks has