RbSr and KAr geochronologic studies of mantled gneiss domes, Albion Range, Southern Idaho, USA

Mantled gneiss domes of the Albion Range composed of a Precambrian crystalline complex overlain by Paleozoic sediments metamorphosed during the Mesozoic. A post-kinematic adamellite stock intrudes two of the domes. coinciding in a general way with the region of highest metamorphiograde. An isochron for the Precambrian rocks gives a date of 2.4b b.y., an initial ratio (⁸⁷Sr/⁸⁶Sr) of 0.703. Lower Paleozoic schists are badly scattered about a ∼600 m.y. isochron, in reasonable agreement with their stratigraphic age. A good isochron for the post-kinematic adamellite gives a date of 30 m.y., an initial ratio of 0.710. The mass balance involving Sr isotopes permits only a negligible portion of this adamellite to be derived from assimilated or transformed Precambrian rocks. KAr dates decrease from 77 m.y. to ∼30 m y. with increasing stratigraphic depth in Paleozoic sediments, perhaps due to progressive cooling as rocks were brought to the earth's surface by uplift and erosion. KAr dates on Precambrian rocks (45-134 m.y.) are greater than dates on overlying Paleozoic schist. This Reversal of trend illustrates the extent to which factors other than temperature control Ar loss form micas. KAr dates on the 30 m.y. old adamellite average 25.4 m.y. for pegmatitic muscovite, 24.7 m.y. for granitic muscovite, and 22.0 m.y. fergranitic biotite. This progression is presumably a result of gradual cooling of the stock emplaced at depth in a metamorphic terrane.