HR: 0800h
AN: V21D-0634    [Abstracts]
TI: Ar/Ar and U/Pb Ages and Geochemistry of the Benton Range Dike Swarm, SE California: New Evidence for an Independence Poly-phased Dike Swarm
AU: * Jourdan, F
EM: fjourdan@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709 United States
AU: * Jourdan, F
EM: fjourdan@bgc.org
AF: Department of Earth & Planetary Sciences, University of California, Berkeley, CA 94709 United States
AU: Renne, P R
EM: prenne@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709 United States
AU: Renne, P R
EM: prenne@bgc.org
AF: Department of Earth & Planetary Sciences, University of California, Berkeley, CA 94709 United States
AU: Mundil, R
EM: rmundil@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709 United States
AB: The Independence dike swarm (IDS) is a locally profuse, mostly NNW striking and ~700 km-long dike swarm occurring throughout southeastern California and possibly extending into northern Mexico. Dike compositions range from mafic to silicic (though strongly bimodal) and span the composition range of the coeval Sierran calc-alkaline arc plutons. Recent geochronological and structural investigations had cast some doubt on the accurate definition of the Independence dike swarm as the swarm more likely represents a poly-phase dike assemblage including at least two generation of dikes (i.e. 90 and 150 Ma; Chen and Moore, 1979; Coleman et al., 2000). To date, most of the geochronological and geochemical investigations available are strongly localized and part of the swarm lacks basic data. Here, we present new ⁴⁰Ar³⁹Ar (n=4) and U/Pb (n=1) ages and detailed major, trace and REE geochemical data on mafic (E-W to N-S) and more abundant silicic (NW-SE to N-S) dikes from the Benton Range dike swarm (BRDS), north-easternmost IDS. Two of the silicic dikes yielded concordant biotite ⁴⁰Ar³⁹Ar mini-plateau and weighted mean ages of 153 ± 2 and 152 ± 3 Ma (2 sigma), in agreement with previous K-Ar biotite ages of 150-155 Ma (Renne et al., 1987) and similar to the "accepted" age for the Independence swarm (~150 Ma). These biotite ages, however, may record a cooling age that significantly post-dates dike intrusion. One silicic dike yields a significantly older, preliminary ²⁰⁶Pb²³⁸U zircon age of 164.6 ± 0.8 Ma from single-crystal analyses. Two E-W striking mafic dikes which cross-cut NNW-striking silicic dikes yield hornblende plateau and mini-plateau ages of 171 ± 2 and 166 ± 2 Ma. BRDS chemical compositions are typical of mafic (SiO₂ = 47-57 wt%; La/Yb_n = 3-14) and granitic (SiO₂ =67-77 wt%; La/Yb_n = 5-31) arc magmas (e.g. Nb anomaly) and typify the known end-member compositions of the IDS. These results, together with compilated published and unpublished geochronological and geochemical data and field relationships suggest that:
(1) The poly-phased Independence dike complex was apparently intruded in several major, magmatic episodes (>170-140 Ma; 115-120 Ma and 100-85 Ma). The first and last diking time periods are correlated with the two major pulses of arc-related igneous activity throughout California (e.g. Ducea, M., 2001) while the second period occurred at the onset of the second major pulse. The dikes were intruded during a time span coinciding with major changes in the orientation of plate convergence (e.g. J2 cusp at ~150 Ma; May et al., 1989). Therefore, the orientation of the dikes is unlikely to be entirely controlled by stress induced by the converging plate. Rather, the data suggest that the dikes are preferentially associated with weakened lithospheric pathways (imposed by the orientation of the free margin?) that have controlled dike orientations over >90 Ma (e.g. Coleman et al., 2000).
(2) Geochemical data are consistent with a lithospheric mantle origin for the mafic magma. Isotopic data (in progress) are required to test whether silicic dikes involve a pre-Mesozoic crustal component or are evolved directly from a mantle melt. As yet, no correlation is observed between the age and the composition of the dikes.

Chen & Moore, Geology 7, 1979
Coleman et al., GSA Bulletin 112, 2000
Ducea et al., GSA today, nov. 2001
May et al., Tectonics 8, 1989
Renne et al., Geology, 1987

DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1065 Major and trace element geochemistry
DE: 1115 Radioisotope geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005