Abstract
The western margin of Laurentia hosts a series of small-volume Cambrian to Ordovician alkalic intrusions. At least nine of these intrusions occur in Idaho, where they are emplaced in Precambrian metasedimentary rocks of the Belt Supergroup. Cambro-Ordovician intrusions in Idaho are chemically distinct from other intrusive rocks throughout the state due to their significant alkali, rare earth, Zr, and Nb content. Prospects associated with the Cambro-Ordovician intrusions have gained attention for economic sources of rare earth elements and Nb. Lithologies from this suite in Idaho are largely alkali syenite and granite, with a few instances of alkaline mafic rocks. Nb/Ta ratios of the mafic samples and syenites are generally consistent with those of N-MORB compositions and have a narrow range of values, while the granites have lower Nb/Ta ratios. Many of the syenitic and granitic rocks have a negative Eu anomaly, and a flat heavy rare earth element pattern. Alkaline gabbros at the Deep Creek complex are cut by syenite dikes, and signal a progression from a mafic composition enriched in incompatible elements, to an alkali syenite composition. The highest REE concentrations are in the oldest (early Cambrian) and least extensive syenites. U-Pb zircon dating of intrusions of the Arnett Creek pluton reveals three distinct units, forming an intrusive complex spanning from approximately 488 to 477 Ma. The 477 Ma leucogranite at Arnett Creek is the youngest intrusion of the Cambro-Ordovician alkaline suite and post-dates the latest Rodinia breakup extension by approximately 70 million years.