New Ages on Old Arc Rocks, Preliminary Results From the Southern IBM Arc

The first results of our reevaluation of the geochemistry and geochronology of Eocene to Miocene volcanic rocks from the southern Izu-Bonin-Mariana (IBM) arc include preliminary ⁴⁰Ar/³⁹Ar ages on rocks from Guam, and DSDP sites 447, 448, 458, 459B. Our new ⁴⁰Ar/³⁹Ar age of 44 Ma on a boninite series basalt from the Facpi formation, supports earlier work by Meijer et al. (1983, AGU Mono. 27, p349) who obtained a 44 Ma K-Ar age on a similar basalt from the same area in western Guam. Our ⁴⁰Ar/³⁹Ar age of 20 Ma age on a tholeiite dike in the southern portion of the Facpi formation is in the range of K-Ar ages in Cosca et al. (1998 Is. Arc 7 p.579) and Meijer et al. (1983) of 36 Ma to 16 Ma for the Facpi. These ages are younger than the Late Middle Eocene age assigned to the Facpi, and indicates either that a previously unrecognized unit of tholeiitic arc volcanics exists in southern Guam, or that zeolite facies metamorphism reset the ages. If the former is true, then this unit has ages within the gap in arc volcanism previously postulated for the late Oligocene and early Miocene on Guam. Our preliminary results on DSDP cores when combined with ⁴⁰Ar/³⁹Ar and K-Ar dates from Cosca et al. (1998,), and older K-Ar ages from Meijer et al. (1983) suggest that tholeiitic to boninitic volcanism occurred at site 458 and perhaps 459 to east of Guam at about 50 Ma. Similarly, Cosca et al. (1998), showed that volcanism along the Palau-Kyushu ridge dates back to about 50 Ma. Thus, magmatism appears to have occurred around 5 Ma later on Guam than to the east and west, which is consistent with a center of Eocene forearc spreading near Guam. A single preliminary ⁴⁰Ar/³⁹Ar age of 20 Ma for a relatively intact diabase at site 447 and dates reported in Fujioka et al., (1999, Geology 27, p.1135) and Hickey-Vargas (1998 JGR 103, p. 20963) presents the possibility that volcanism continued in the West Philippine Basin into the early Miocene.