| dc.description.abstract | The Idaho batholith is a composite calc-alkaline plutonic complex that formed as part of a continuous belt of intrusions that extended along the western margin of Mesozoic North America. The limited available data suggest the batholith is mainly of Cretaceous age with later Eocene additions. Further, there is isotopic evidence which suggests that the batholith magmas originated from or were significantly contaminated with Precambrian crustal material. Through two experiments, this study confirms this hypothesis, places age constraints on the old source, and clarifies some of the intrusive age relationships of the northeastern border zone of the batholith. In the first experiment U-Pb (zircon) isoto9e systematics of a suite of inclusions from the batholith were analyzed. The suite, which includes xenoliths ranging from little altered to strongly digested, defined a mixing line between young and old material with ages of 46 +/- 2 and 1636 +/- 43 million years, respectively. Many zircons from granite of the batholith contain old xenocrystic cores, so a second experiment was designed to analyze the old cores and young rims separately. Whereas the stepwise dissolution procedure employed here did not completely accomplish this, the U-Pb data did define a discordia with iii a lower and upper intercept age of 43 +/- 2 and 1523 +/- 90 million years. Although it is questionable that ages obtained from the stepwise dissolution experiment have geological significance, they agree within experimental error with the results of the first experiment. Age data from Precambrian terranes near the Idaho batholith indicate that it is reasonable to postulate the existence of a 1640 million year old source terrane for the old zircon component. Data from this study also indicate the occurrence of previously unrecognized Tertiary intrusions in the northeastern part of the Idaho batholith. | en_US |
