The geochemistry of the Atlanta Lobe of the Idaho Batholith in the western United States, Cordillera

Isotope, major and trace element studies of the Late Cretaceous and Tertiary granitoids of the Atlanta lobe of the Idaho batholith in the western United States cordillera have led to new. insights on the nature of the lower crust and have placed constraints on granitoid magma genesis during a transition from a compressional to an extensional regime. The Cretaceous (-90-75Ma), calc-alkaline, tonalitic to granitic, ' metaluminous to peraluminous granitoids are characterisedb y high LI]LE/HFSE and LREE/HREE ratios, low Rb/Sr and Snx/Ndr atios and F-Nda nd ESro f -4.9 to -11.6 and 35 to 120 respectively. Only the tonalites and silicic granodiorites to granites underwent significant degrees of within suite fractional crystallization, the former dominated by hornblende and plagioclase and the latter by alkali feldspar, quartz, plagioclase and biotite. The Cretaceous magmas have been successfully modelled as 25%-45% partial melts of tonalitic to gabbroic, biotite, garnet and homblende-bearing meta-igneous*s ources. Thermal conditions suggestt hat anatexis was induced by both Sevier tectonic crustal thickening and ponding of subduction-relatodb asalts at the base of the crust. The inferred source regions were relatively trace element enriched, with high Sr and LlLE/HFSE ratios; this volcanic-arc signature was presumably attained during generation of the source region by subduction related processes. Little intra-crustal reprocessing of the source region can have occurred, but unsupported Sr and Pb isotope ratios suggest that minor depletion of Rb, U, and Th by high-grade metamorphism may have occurred. Small differences between the trace element compositions of the Cretaceous intrusive phases are thought to reflect minor heterogeneitiqs within the source regions. Small volume leucogranites with distintive REE and isotope compositions are interpreted as upper crustal, partial melts of Archaean and Proterozoic metasediments. Inferred garnet-bearing granulitic residues after partial melting cause Sm/Nd ratios to be significantly reduced from source to magma, but correcting for this produces TLM in agreement with Pb-Pb ages of -1.5Ga. This mid-Proterozoic age of the lower crust indicates that major age province boundaries with the Wyoming and Snake River Plain Archaean provinces exist to the east and close to the southern margin of the Atlanta lobe. The Tertiary bimodal granitoids consist of a 55Ma-47Ma, metaluminous dioritic suite characterised by high Sr and Ba, low LILE and moderate LREE/HREE ratios, and a 48Ma- 42Ma, peraluminous granitic suite are characterised by lower Ba and Sr abundances and LREE/HREE ratios, higher HFSE and Rb, U and Th abundancesw, ith large LILE variation and minor to large negative Eu/Eu* anomalies. F-Nd and ESr of -4 to - 11.8 and 25 to 65 respectively, in the dioritic suite, systematically vary with major and trace elements and are interpreted to reflect AFC processes, with dioritic initial magmas derived from the subcontinental mantle lithosphere and high proportions of old continental crust assimilants. The Tertiary granitic suite have higher ESr and lower ENd and Pb isotope ratios than the Cretaceousg ranitoids, but NU similar mid-Proterozoic T6M. They are modelled as anatectic melts at the base of a thinned, isotopically distinct region of a trace element enriched lower crust similar to the source of the Cretaceous granitoids to leave a mafic granulitic residue without garnet leading to distinctly higher Y and Yb abundancesin the Tertiary granitoids. The Cretaceous to Tertiary transition from lower crustal granitoids generatedin a tectonically thickened crust with ponding of subduction-related basalts, to generation of granitoids in an attenuated lithosphere, due to rapid uplift, themial and gravitational relaxation and extension in the latest Cretaceous, is reflected in the change to increasing HREE abundances, the incorporation of mantle-derived material and the epizonal levels of emplacement and association with the coeval Challis volcanics.