GEOLOGICAL SOCIETY OF THE PHILIPPINES
VARIATION IN SULFUR CONTENTS OF MICROPHENOCRYSTIC APATITE IN CENOZOIC IGNEOUS ROCKS OF WESTERN PACIFIC RIM: IMPLICATION TO PORPHYRY COPPER METALLOGENESIS
University of Tokyo
A chain of porphyry Cu deposits extending from lepanto – Far Southeast through Santo tomas II, Dizon, to taysan occurs along the western Luzon arc, Philippines, associated with hydrous intermediate to salicic magmatism. On the other hand, it has been a question why no porphyry cu mineralization has been known in Japanese Island arcs. In order to clarify the cause of such porphyry Cu metallogenesis in island arc setting, more than 11,000 analyses of microphenocrystic apatie have been carried out for Cenozoic intrusive rocks of the western Pacific rim arcs.
Microphenocrystic apatites in shallow instructions associated with porphyry Cu deposits in the western Luzon arc contain S of > 0.1 wt % as SO3. Sulfur is supposed to have been accommodated as oxidized species in hydrous magmas that generated porphyry Cu
Deposits. Such high SO3 contents in microphenocrystic apatite are common characteristics of the hydrous intermediate to silicic magmatism of the western Luzon arc, from plutonic rocks of the central Luzon Cordillera of 15Ma to an active volcano at Mt. Pinatubo.
The SO3 contents of microphenocrystic apatite in hydrous silicic magmas along the volcanic front, in andesite related to native S deposit, and in Miocene to younger shallow granitic intrusions, in the northeastern japan arc, are generally < 0.1 wt %. On the other hand, the SO3 contents of apatite of the kuril arc (Eastern Hokkaido), the Izu arc and the Ryukyu arc (Southern Kyushu) are similar to those from the western Luzon arc.
Apatite commonly occurs as iclusions in other phenocrystic phases. Thus the variation of SO3 contents of the apatite is a primary feature, while modifications are encountered in the discrete apatite in groundmass due to degassing of SO2. The systematic variation in SO3 contents of apatite is found both spatially and temporally. The SO3 contents of microphenocrystic apatite are considered to imply a redox state of magma source or that of the fluid encounterd to magma generation, which is supposed to have been derived from the subducted slab.
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