HR: 09:30h
AN: T11D-07    [Abstracts]
TI: Evolution of intrusive activity in the Greater Antilles: Zircon U-Th-Pb Ages for island arc granitoids in Puerto Rico, USA
AU: * Cavosie, A J
EM: acavosie@uprm.edu
AF: University of Puerto Rico, Dept. of Geology PO Box 9017, Mayaguez, PR 00681, Puerto Rico
AU: Perez, R J
EM: perezrafa@gmail.com
AF: University of Puerto Rico, Dept. of Geology PO Box 9017, Mayaguez, PR 00681, Puerto Rico
AU: Gehrels, G
AF: Univ. of Arizona, Dept. of Geosciences 1040 E. 4th, Tucson, AZ 85721, United States
AB: Understanding the timing of island arc construction in the Greater Antilles is central to unraveling the complex history and evolution of the Caribbean plate. In this study, we present zircon U-Pb ages for oceanic island arc granitoids from Puerto Rico to address both the compositional variation and timing of intrusive activity along the northeast margin of the Caribbean plate. Prior geochronology studies of plutonic rocks in PR have used the K-Ar or Ar-Ar methods on minerals and WR, which are susceptible to thermal resetting (Cox et al., 1977; Smith et al., 1998). In this study 14 plutons were analyzed from the three main igneous provinces of Puerto Rico, the southern (SIP) central (CIP), and northeast (NIP) igneous provinces, and include the following stocks/batholiths: SIP: Las Tunas, Tibes, Tea Road; CIP: Utuado, Ciales, Morovis, Barranquitas, Coamo Arriba, Cuyon, Caguas, San Lorenzo, Vieques; NIP: Rio Blanco. While the igneous bodies vary in size (1 to 500 km²), whole-rock analyses show little variation, as the plutons are primarily intermediate in composition (56 to 66 wt. % SiO₂), metaluminous (avg. A/CNK = 0.91, n=135), and range from diorite to granodiorite. Exposures of gabbros associated with the above plutons are rare. Zircons were mounted in epoxy rounds, and imaged for internal zoning characteristics prior to U-Pb analysis. Cathodoluminescence images show that the zircons are mostly oscillatory and/or sector zoned, and record simple growth histories, with no single grain demonstrating evidence of inheritance. Zoning patterns are similar within a population, but differ from among samples, regardless of WR composition or pluton size. Zircons were analyzed for age by MC-ICPMS at the Univ. of Arizona. Spot analyses were made on 20-30 grains per sample. The zircon U-Pb ages record a nearly continuous interval of Cretaceous to Eocene intrusive activity from 85 to 39 Ma. Three main pulses of magmatism are distinguished: (1) intrusion of small, intermediate to felsic stocks from 85-83 Ma (Santonian) in the CIP (n=3); (2) intrusion of large granodiorite batholiths from 75 to 67 Ma (Campanian/ Maastrichtian) in the CIP (n=5); and (3) intrusion of small intermediate stocks at ca. 48 Ma (Middle Eocene) in the CIP and NIP (n=3). The three small intermediate stocks from the SIP yield U-Pb ages of 79, 60, and 39 Ma, ages not recorded in the CIP and NIP. The difference in pluton crystallization ages suggests the SIP records a disparate history of arc evolution from the CIP and NIP. In both the CIP and NIP, an unexpected correlation was found between the average Th/U ratio for the population of zircons analyzed (20 to 30 grains per sample), and the composition of the arc basement intruded by the pluton. Plutons intruded into ˇ®primitive island arc' rocks (the PIA series of Donnelly and Rogers, 1980) uniformly have avg. Th/U(zircon) ˇÜ0.45 (n=7); plutons intruded into younger calc-alkaline basement have avg. Th/U(zircon) >0.45 (n=6). The observed correlation of zircon Th/U composition with basement lithology occurs over the entire age range of plutons analyzed in the two provinces, irrespective of size and major element composition, and thus does not appear to be related to any temporal evolution or compositional changes in the granitoid source region through time. This unusual correlation suggests that the composition of the arc basement influenced the evolution of the granitoids, and is thus interpreted as a previously unrecognized record of crustal recycling during granitoid genesis in the Puerto Rico section of the Greater Antilles island arc.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 8021 Melanges
DE: 8104 Continental margins: convergent
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting