Reference 1620

Abstract

The Prairie Creek stock, located in central Idaho along thesoutheastern margin of the Idaho batholith, is a shallow Eocenegranitic intrusion. Directly overlying the granite is a gradationalunit of dacite prophyry and granodiorite. Previous work in the areasuggests that the granite and granodiorite are co-magmatic. However,trace element, radiogenic, and stable isotope data indicate distinctpetrogenetic histories and source materials for the two suites.Granite suite initial super 87 Sr/ super 86 Sr ratios of 0.7095 to0.7097 and epsilon sub Nd near -14 imply that the granites wereproduced by melting within the lower crust. Pb-Pb isochron ages super 206 Pb/ super 204 Pb = 18.19 to 18.24, super 207 Pb/super 204 Pb = 15.59 to 15.61, and super 208 Pb/ super 204 Pb =38.76 to 38.87 and Nd model ages of 1.5 to 1.7 Ga from the granitesuggest that the lower crust in the region is Middle Proterozoic.delta super 18 O quartz values of 8.6 to 9.9 per mil also indicatethe granites were derived from crustal material. The granite magmasmay also have assimilated some middle-upper crust during their ascent.Limited data suggest that the granodiorites followed a similarhistory, however, during their genesis incorporated smallerproportions of upper crustal material than the granite suite. deltasuper 18 O values quartz-feldspar fractionations ranging from 1.6to 7.6 per mil from the granite show that some of the rocks werealtered by meteoric hydrothermal fluids. Crustal thickening and/orEocene lithospheric extension, documented by various workers, may havesteepened the geothermal gradient in the area. This could havefacilitated melting in the lower crust and produced the magmas thatformed the Prairie Creek area igneous rocks.

89 samples from this reference