EXPLORING THE GEOCHEMICAL VARIATION OF MAGMATIC ROCKS OF IDAHO USING PAIRED GEOCHRONOLOGY-GEOCHEMISTRY OF DETRITAL ZIRCON IN MODERN RIVERS

Previous research has demonstrated the ability of detrital zircon (DZ) age distributions from modern rivers to reproduce, to a first order, the geochronology of the orogenic systems they drain; recent work has applied these methods to the northern Rocky Mountains of Idaho. We conducted an exploratory investigation using laser ablation ICP-MS U-Pb geochronology paired with trace element and Lu-Hf geochemistry. We first assess the level of bias induced by zircon fertility to evaluate the extent to which modern river DZ age distributions are faithful representations of their upstream sediment sources. Then, we place our results into the context of previous interpretations of Late Cretaceous to Cenozoic crustal thickness variations. Three samples from the trunk streams of the Snake, Salmon, and Clearwater rivers yield DZ age distributions dominated by zones within the Idaho batholith (110–53 Ma) and Challis magmatic suite (55–44 Ma). These results are consistent with the spatial extent of the Idaho batholith which dominates outcrops within the drainage basins. However, the Salmon and Clearwater drainages show an outsized ~1370 Ma Mesoproterozoic age mode compared to the relatively small mapped exposure of these rocks in the upstream reaches of their catchments. We find that Mesoproterozoic zircons are distinctive: they show elevated ΣREE + Y and Hf concentrations compared to the younger Idaho batholith zircons, which is consistent with derivation from highly fractionated upper crustal igneous rocks. This is a chemical signature potentially related to high zircon fertility, although this remains to be tested. Ti-in-zircon crystallization temperatures are on average higher for Mesoproterozoic and Challis DZ (~1000°C) compared to the Idaho batholith (700–800°C). Crustal thickness estimates based on in situ geochemical data show crustal thickening to ~50-55 km between 100 and 90 Ma. Crustal thickness calculations based on Eu anomalies in DZ from the Atlanta (85–64 Ma) and Bitterroot (64–53 Ma) lobes are broadly consistent with estimates from chemistry of in situ rocks. Lu-Hf chemistry shows a steady decrease from radiogenic to nonradiogenic between 120 and 80 Ma, which is consistent with increased contribution of older crust to Idaho batholith melts during crustal thickening within this section of the Cordillera.