SHRIMP U-PB AGES OF PROVENANCE FROM DETRITAL ZIRCON POPULATIONS OF INTRA-BATHOLITHIC METASEDIMENTARY ROCKS, N. PENINSULAR RANGES BATHOLITH, SOUTHERN CALIFORNIA: IMPLICATIONS FOR THEIR TECTONIC SETTING

SHRIMP U-Pb ages were collected from detrital zircon populations from metamorphic rocks on both sides of a suture zone exposed along the north rim of the Eastside Reservoir, Domenigoni Valley, Ca. To the west, metapelitic and minor metasandstone yielded numerous concordant 206Pb/238U ages between 210-240 Ma, and concordant 207Pb/206Pb ages between 1075-1125 Ma, 1375-1430 Ma, and 1615-1735 Ma, although distinct differences in provenance were noted between units. Older 207Pb/206Pb ages obtained were ~2250 and ~2800 Ma. To the east of the suture, high-grade gneisses yielded numerous concordant 206Pb/238U ages between 103-123 Ma and 200-255 Ma, and concordant 207Pb/206Pb ages between 1060-1150 Ma, 1375-1435 Ma, and 1595-1710 Ma. The signatures from these high-grade gneisses are marked by a distinct Pb-loss discordia with a lower-intercept age of ~215 Ma and an upper-intercept age ~1725 Ma. Younger ages between 100 and 105 Ma were obtained from the rims of some grains, suggesting metamorphic zircon growth, and consistent with grt-bt exchange thermometry that record T-P ~625-725°C and 4-7 kb. Euhedral magmatic grains also yield concordant ages between 103 and 125 Ma, indicating derivation from local magmatic sources. The similarity of provenance ages between units west and east of the suture indicate that this fundamental structure probably dissects the same forearc basin that was fed by both arc sediment to the west and continental sediment to the east, and most likely intermixed by longshore currents. Magmatic zircons from a metamorphosed diorite dike that crosscuts layering yielded a SHRIMP 206Pb/238U age of 102.42 ± 0.63 Ma. Euhedral, glassy monazite from a post-metamorphic, crosscutting pegmatite yielded a slightly discordant 207Pb/235U age of 101.85 ± 0.35 Ma and a Th-Pb age of 97.53 ± 0.18 Ma, suggesting that this pegmatite was injected during or just after suturing ceased. Collectively, these results indicate that this sequence of metasedimentary rocks were derived mainly from a late Permian to Early Triassic igneous provenance that was intruded into Proterozoic crust, and were subsequently metamorphosed during the suturing of K-J island-arc rocks to the then Cretaceous continental margin at ~100 Ma, consistent with conclusions of Morton et al. (2000) and Johnson et al. (2000).