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The Jim Sage volcanic suite (JSVS) exposed in the Jim Sage and Cotterel Mountains of southern Idaho (USA) consists of two volcanic members composed of ~240 km3 of Miocene rhyolite lavas separated by an interval of lacustrine sediments. It is capped by rheomorphic ignimbrite and as much as 100 m of basaltic lava flows probably derived from the central Snake River Plain (SRP) province to the north. The occurrence of volcanic vents in the JSVS links the lava flows to their local eruptive centers, while the adjacent Albion-Raft River-Grouse Creek metamorphic core complex exposes ~3000 km2 of once deep-seated rocks that offer constraints on the composition of the potential crustal sources of these rhyolites. U-Pb zircon ages from the rhyolite lavas of the JSVS range from 9.5 to 8.2 Ma. The Miocene basalt of the Cotterel Mountains has an 87Sr/86Sri composition of 0.7066-0.7075 and εNd(i) = -3.7, and the rhyolite lavas of the JSVS have 87Sr/86Sri = 0.7114-0.7135 and εNd(i) values that range from -6.7 to -7.1. Zircon from the rhyolites of the JSVS range in δ18Ozr (Vienna standard mean ocean water, VSMOW) from -0.5‰ to 5.7‰ and have εHf(i) values ranging from -0.8 to -6.8. Based on geochronology, whole-rock major elements, trace elements, isotopes (Sr and Nd), and in situ zircon O and Hf isotopic compositions, we infer that the JSVS is genetically related to the central SRP province. The eruption of the low-δ18O rhyolites of the JSVS, outside of the main topographic extent of the SRP province (without the large calderas inferred for the SRP rhyolites) implies that there might be an alternative mechanism for the formation of the low-δ18O signature other than the proposed assimilation of hydrothermally altered caldera blocks. One suggestion is that the north to south propagation of SRP-type low- δ18O rhyolitic melt along the Albion fault led to off-axis magmatism. Another possibility is that there was prior and widespread (across a region wider than the SRP) hydrothermal alteration of the crust related to its earlier magmatic and faulting history. The eruption of SRP-type lavas in the hanging wall of an evolving metamorphic core complex helps us outline the role of the SRP magmatic province in the extensional evolution of the northeastern Basin and Range. The lavas of the JSVS imply the addition of basalt, related to the SRP hotspot, to the crust beneath the Raft River Basin that provided a heat source for crustal melting and weakening of the deep crust; this led to a vertical component of crustal flow and doming during extension, after the eruption of the 9.5-8.2 Ma JSVS rhyolites. This younger than 8.2 Ma component of vertical motion during faulting of the Miocene stratified sequence of the Raft River Basin and the rotation of the Albion fault to shallower angles collectively resulted in the subhorizontal detachment structure imaged seismically beneath the Raft River Basin. [ABSTRACT FROM AUTHOR]

The Archean nucleus of Laurentia, the Superior province, formed between 2.7 and 2.6 Ga. There is little record of sediment deposition on the craton for 100 m.yr. until deposition of the Huronian clastic sedimentary rocks (4-12 km thick) began along the south-central margin of the Superior province between 2.5 and 2.2 Ga. Huronian detrital zircon ages ( zircon SHRIMP 207Pbx/206Pbx np266 ages from six samples) are dominantly 2.6-2.7 Ga, with a small population aged 3.0-3.5 Ga in the middle of the Huron section. The mainly younger, and western, Animikie basin (6-12 km thick; 2.21-0.8 Ga) contains a wide range of nearshore and deep-water clastic rocks, including banded iron formations and the 1.85 Ga Sudbury "impactite" deposits with overlying turbidites. Animikie detrital zircon ages (np724 laser ablation ICP-MS 207Pbx/206Pbx zircon ages from 10 samples) contain a complex spectrum, with ages ranging from 1.8 to 3.8 Ga: the basal Chocolay (Denham, East Branch Arkose) samples contain zircons ranging in age from 2.1 to 3.7 Ga, with a dominant population at ~2.6 Ga; the basal Animikie (Menominee) quartzites contain zircons with ages between 2.1 and 3.8 Ga, with peaks variably at 2.1, 2.3, 2.6, and 3.0 Ga; the upper Animikie (Baraga) turbidites contain zircons with ages between 1.8 and 3.6 Ga, with a dominant peak at 1.85 Ga. The Huron and Animikie basins were intensely deformed by Penokean orogenic (1.85 Ga) accretion from the south. Deposition into the Huron basin was from the north and west for 200 m.yr.; then the depocenter shifted to the west at ~2.2 Ga, with the Animikie basin receiving clastic debris from all directions, including from terranes (e.g., Marshfield and the Wisconsin Magmatic Terrane) approaching from the south. Combined Huron and Animikie detrital zircon results record a migrating eastto- west depocenter, perhaps in conjunction with the dynamics of the central Marathon large igneous province mantle plume and dike swarm (~2.07 Ga), over a period of 500 m.yr. [ABSTRACT FROM AUTHOR]

The Ellsworth-Whitmore Mountain terrane of central Antarctica was part of the early Paleozoic amalgamation of Gondwana, including a 13,000 m section of Cambrian- Permian sediments in the Ellsworth Mountains deposited on Grenville-age crust. The Jurassic breakup of Gondwana involved a regional, bimodal magmatic event during which the Ellsworth-Whitmore terrane was intruded by intraplate granites before translation of the terrane to its present location in central Antarctica. Five widely separated granitic plutons in the Ellsworth-Whitmore terrane were analyzed for their whole-rock geochemistry (X-ray fluorescence), Sr, Nd, and Pb isotopic compositions, and U-Pb zircon ages to investigate the origins of the terrane magmas and their relationships to mafic magmatism of the 183 Ma Karoo-Ferrar large igneous province (LIP). We report high-precision (±0.1 m.y.) isotope dilution- thermal ionization mass spectrometry (IDTIMS) U-Pb zircon ages from granitic rocks from the Whitmore Mountains (208.0 Ma), Nash Hills (177.4-177.3 Ma), Linck Nunatak (175.3 Ma), Pagano Nunatak (174.8 Ma), and the Pirrit Hills (174.3-173.9 Ma), and U-Pb sensitive high-resolution ion microprobe (SHRIMP) ages from the Whitmore Mountains (200 ± 5 Ma), Linck Nunatak (180 ± 4 Ma), Pagano Nunatak (174 ± 4 Ma), and the Pirrit Hills (168 ± 4 Ma). We then compared these results with existing K-Ar ages and Nd model ages, and used initial Sr, Nd, and Pb isotope ratios, combined with xenocrystic zircon U-Pb inheritance, to infer characteristics of the source(s) of the parent magmas. We conclude that the Jurassic plutons were not derived exclusively from crustal melts, but rather they are hybridized magmas composed of convecting mantle, subcontinental lithospheric mantle, and lower continental crustal contributions. The mantle contributions to the granites share isotopic similarities to the sources of other Jurassic LIP mafic magmas, including radiogenic 87Sr/86Sr (0.706-0.708), unradiogenic 143Nd/144Nd (εNd < -5), and Pb isotopes consistent with a low-μ source (where μ = 238U/204). Isotopes and zircon xenocrysts point toward a crustal end member of predominantly Proterozoic provenance (0.5-1.0 Ga; Grenville crust), extending the trends illustrated by Ferrar mafic intrusive rocks, but contrasting with the inferred Archean crustal and/or lithospheric mantle contributions to some basalts of the Karoo sector of the LIP. The Ellsworth-Whitmore terrane granites are the result of mafic rocks underplating the hydrous crust, causing crustal melting, hybridization, and fractionation to produce granitic magmas that were eventually emplaced as post-Ferrar, within-plate melts at higher crustal levels as the Ellsworth-Whitmore terrane rifted off Gondwana (47°S) before migrating to its current position (82°S) in central Antarctica. [ABSTRACT FROM AUTHOR]