MULTIPLE PUNCTUATED PULSES OF VOLUMINOUS SILICIC MAGMATISM IN IDAHO: IN SITU GEOCHRONOLOGY AND ISOTOPE GEOCHEMISTRY OF THE IDAHO BATHOLITH

The Idaho batholith is arguably the least understood of the great batholiths of the Cordillera. Age relationships in the batholith have remained particularly elusive due to the pervasive inheritance in its zircons, leading to strong discordance in U-Pb dates (Bickford et al., 1981; Toth and Stacey, 1992). In situ LA-ICPMS dating has enabled us to circumvent this problem and determine the ages of both inherited cores and magmatic overgrowths. Preliminary data suggest that the two lobes of the batholith were constructed during distinctly different intervals of time and consequently different phases in the tectonic evolution of the region. Samples from the larger Atlanta lobe yield Late Cretaceous ages from 86 to 73 Ma. In contrast, samples from the main phases of the northern Bitterroot lobe fall between 62 and 55 Ma. These data suggest that the two lobes represent separate, discrete pulses of magmatism. In addition, samples from several representative epizonal plutons that intrude the batholith and are associated with the Challis volcanics yield ages from 48 to 43 Ma. All zircon samples from the Idaho batholith proper are characterized by inheritance. The majority of inherited cores are between the ages of 1850 and 1600 Ma, which supports the existence of the distinct Paleoproterozoic terrane in central and northern Idaho postulated by Mueller et al. (1995) and Foster et al. (2006). Many zircon samples also contain age components which are slightly older (10 to 20 million years) than the majority of rim ages. These components occur as cores with slightly younger rims, rims surrounding Precambrian cores, and simple whole-grains and suggest that later pulses of magma cannibalized plutons emplaced earlier in the batholith's construction. In contrast to the great complexity shown by zircons from the main phases of the batholith, the zircons from the Challis-related epizonal plutons that we have studied are generally simple and display no inheritance. Preliminary in situ Hf isotope analyses of Paleoproterozoic zircon cores have yielded highly unradiogenic epsilon values of ~ -40, whereas magmatic rims sampled thus far range from -10 to -20. These initial results suggest that the batholith was constructed with some, albeit small, input from the mantle, as suggested by King and Valley (2001) on the basis of oxygen isotopes.