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High Mg# andesites and dacites (Mg# = molar Mg/Mg + Fe) from western Aleutian seafloor volcanoes carry high concentrations of Sr (>1000 ppm) that is unradiogenic (87Sr/86Sr < 0.7029) compared to lavas from emergent volcanoes throughout the arc (200–800 ppm Sr, 87Sr/86Sr >0.7030). Data patterns in plots of 87Sr/86Sr vs Y/Sr and Nd/Sr imply the existence of an eclogite-melt source component – formed by partial melting of MORB eclogite in the subducting Pacific Plate – which is most clearly expressed in the compositions of western Aleutian andesites and dacites (Nd/Sr and Y/Sr < 0.02) and which dominates the source budget for Sr in volcanic rocks throughout the arc. When viewed in combination with inversely correlated εNd and 87Sr/86Sr, these patterns rule out aqueous fluids as an important source of Sr because mixtures of fluids from altered oceanic crust with depleted mantle and sediment produce compositions with 87Sr/86Sr higher than in common Aleutian rocks. The unradiogenic nature of Sr in the western Aleutian andesite–dacite end-member may be understood if H2O required to drive melting of the subducting oceanic crust is transported in fluids containing little Sr. Mass balance demonstrates that such fluids may be produced by dewatering of serpentinite in the mantle section of the subducting plate. If the eclogite-melt source component is present throughout the Aleutian arc, melting of the subducting plate must extend into minimally altered parts of the sheeted dike section or upper gabbros, at depths >2 km below the paleo-seafloor. Oxygen isotopes in western Aleutian seafloor lavas, which fall within a narrow range of MORB-like values (δ18O=5.1–5.7), are also consistent with this model. These results indicate that the subducting Pacific lithosphere beneath the Aleutian arc is significantly hotter than indicated my most thermal models.

Archean cratons are dominated by tonalite–trondhjemite–granodiorite (TTG) suites, the products of crustal differentiation that formed early continental crust. These rocks may have been primarily generated by partial melting of hydrated basaltic crust in a variety of settings including subduction zones or the deep lithosphere. Sources are mainly inferred from examination of TTGs alone, as potential source rocks are rarely exposed. In the southern Superior Province, Canada, the Kapuskasing Uplift exposes an important crustal cross-section with upper- to middle-crustal TTGs and lower-crustal metabasites, which show evidence of having produced trondhjemitic anatectic melt. Here, we test the hypothesis that these metabasites were the source of the Mesoarchean to Neoarchean TTGs in the Kapuskasing Uplift by combining phase equilibrium and melt trace element modelling with whole-rock and zircon Lu–Hf isotope analysis and geochronology (garnet Lu–Hf and zircon U–Pb) of metabasic samples. By comparison of the results with existing data from TTGs in the Kapuskasing Uplift, we determined that the metabasites are plausible source rocks for the TTGs. The Lu–Hf systematics of the metabasites and TTGs are the most robust evidence of a genetic connection. Modelling results support an increase in TTG source depth over time. New geochronological data constrain partial melting of metabasite and crystallization of anatectic melt between ca. 2685 Ma and ca. 2600 Ma, coeval with crystallization of only the youngest TTGs. Overall, these results indicate a complex history of intracrustal differentiation in the Kapuskasing Uplift, with partial melting of two isotopically distinct lower-crustal metabasic sources at different times and depths. [ABSTRACT FROM AUTHOR]

New U-Pb zircon ages are reported for granulite facies crustal xenoliths brought to the surface by mafic lavas in the Snake River Plain. All samples yield Meso-to-Neoarchean ages (2.4–3.6 Ga) that significantly expand the known extent of the Archean Wyoming Craton at least as far west as the west-central Snake River Plain. Most zircon populations indicate multiple growth episodes with complexity increasing eastward, but they bear no record of major Phanerozoic magmatic episodes in the region. To extrapolate this work further west to the inferred craton boundary, zircons from southwestern Idaho batholith granodiorites were also analyzed. Although most batholith zircons record Cretaceous formation ages, all samples have zircons with inherited cores—with some recording Proterozoic ages (approaching 2 Ga). These data enhance our perspectives regarding lithosphere architecture beneath southern Idaho and adjacent areas and its possible influence on Cenozoic magmatism associated with the Snake River Plain–Yellowstone "melting anomaly". [ABSTRACT FROM AUTHOR]

The Montana metasedimentary terrane in the northern Wyoming Province provides valuable insight into crustal formation and reworking processes along the cratonic margin and offers a unique opportunity to decipher the complex Neoarchean-Paleoproterozoic terrane assembly in southwestern Laurentia. We report new zircon U-Pb dates and Hf isotopes from seven metaigneous samples in the northwestern Montana metasedimentary terrane. The internal textures of zircon in this study are complex; some lack inherited cores and metamorphic overgrowths, while others exhibit core-rim relationships. Based on the cathodoluminescence (CL) features, we interpret these grains to be magmatic populations. These data demonstrate discrete igneous pulses at 2.7 Ga, 2.4 Ga, and 1.7 Ga, which indicate significant crustal formation intervals in the Montana metasedimentary terrane. Zircons at 2.7 Ga have positive εHf values (+2.4 to +0.9) that indicate a depleted mantle source. Most 2.4 Ga and 1.7 Ga samples have negative εHf values (-1.6 to -15.5), which indicate significant contributions from preexisting crust. Two 1.7 Ga samples, however, have near-chondritic εHf values (+0.4 to +0.3) that indicate larger juvenile contributions. The time-integrated Hf isotope trend suggests that the Paleoproterozoic zircons were produced from a mixture of older crust and juvenile mantle inputs. Additionally, the isotopic age fingerprint of the Montana metasedimentary terrane suggests that it differs from northern-bounding terranes. Viewed more broadly, the 2.7 Ga and 1.7 Ga age peaks that the Montana metasedimentary terrane shares with the global zircon age spectrum suggest that the drivers of these events in the Montana metasedimentary terrane were common throughout the Earth and may be associated with the assembly of supercontinents Kenorland and Nuna. [ABSTRACT FROM AUTHOR]

Apatite is a common accessory mineral found in almost all igneous, metamorphic, and clastic sedimentary rocks. It contains concentrations of lutetium and hafnium amenable to dating by the Lu–Hf isochron method. However, the application of this method has been hampered by chemical difficulties associated with the extraction of a clean Hf concentrate from apatite. A novel method of efficient chemical separation utilizing Ln and DGA-normal extraction chromatography columns has been developed for precise isotope analysis and is presented in this study. The routine analysis of sub-ng amounts of Hf maintained high yield close to 100%. The purification of Hf with a DGA-normal chromatography column effectively eliminated the interfering elements (Yb, Lu, and W), typically resulting in less than 1 pg of Lu in the analyzed apatite Hf solution. Apatite Lu–Hf isotope compositions of Otter Lake, Bancroft, and Durango apatites were determined using the reported methodology and yielded isochron ages of 1047.6 ± 3.4 Ma, 1092 ± 17 Ma, and 31.1 ± 1.1 Ma, respectively. All apatite Lu–Hf isochrons are characterized by phenomenally high apatite 176Lu/177Hf and 176Hf/177Hf ratios. Such apatite Lu–Hf isochrons are shown to be not significantly dependent on the initial isotopic composition in determining accurate and precise ages, allowing for anchoring the low end of the isochron with an estimated isotopic composition. [ABSTRACT FROM AUTHOR]

Rocks may undergo complex pressure-temperature (P-T) histories during orogenesis in response to alternating episodes of synconvergent burial and exhumation. In this study, chemical zoning in garnets combined with textural and chemical evidence from the schist of Willow Creek in the Albion Mountains of south-central Idaho (USA), reveals a complex P-T path during the early stages of Sevier orogenesis. The distribution of quartz inclusions combined with internal resorption features establishes a hiatus in garnet growth. Chemical zoning was simulated using a G-minimization approach to yield a P-T path consisting of three distinct pressure changes during increasing temperature, defining an 'N' shape. Lu-Hf isochron ages from multiple garnet fractions and whole-rock analyses in two samples are 132.1 ± 2.4 and 138.7 ± 3.5 Ma. The samples were collected from the hanging wall of the Basin-Elba thrust fault and yielded results similar to those previously obtained from the footwall. This leads to several conclusions: (1) Both the hanging wall and footwall experienced the same metamorphic event, (2) the paths document a previously unrecognized crustal thickening and synorogenic extension cycle that fills an important time gap in the shortening history of the Sevier retroarc, suggesting progressive eastward growth of the orogen rather than a two-stage history, and (3) episodes of extensional exhumation during protracted convergent orogenesis are increasingly well recognized and highlight the dynamic behavior of orogenic belts. Electronic supplementary material The online version of this article (doi:10.1007/s00410-015-1171-2) contains supplementary material, which is available to authorized users., Author(s): Eric D. Kelly[sup.1] , Thomas D. Hoisch[sup.2] , Michael L. Wells[sup.3] , Jeffrey D. Vervoort[sup.4] , Mengesha A. Beyene[sup.5] Author Affiliations: (1) Department of Geological Sciences, University of Texas, [...]

The authors analyzed detrital zircon grains from 10 metasedimentary rock samples of the Priest River complex and three other amphibolite-facies metamorphic sequences in north-central Idaho to test the previous assignment of these rocks to the Mesoproterozoic Belt-Purcell Supergroup. Zircon grains from two samples of the Prichard Formation (lower Belt) and one sample of Cambrian quartzite were also analyzed as controls with known depositional ages. U-Pb zircon analysis by laser ablation--inductively coupled plasma--mass spectrometry reveals that 6 of the 10 samples contain multiple age populations between 1900 and 1400 Ma and a scatter of older ages, similar to results reported from the BeltPurcell Supergroup to the north and east. Results from the Priest River metamorphic complex confirm previous correlations with the Prichard Formation. Samples from the Golden and Elk City sequences have significant numbers of 15001380 Ma grains, which indicates that they do not predate the Belt. Rather, they are probably from a relatively young, southwestern part of the Belt Supergroup (Lemhi subbasin). Non-North American (1610-1490 Ma) grains are rare in these rocks. Three samples of quartzite from the Syringa metamorphic sequence northwest of the Idaho batholith contain zircon grains younger than the Belt Supergroup and support a Neoproterozoic age. A single Cambrian sample has abundant 1780 Ma grains and none younger than ~1750 Ma. These results indicate that the likely protoliths of many high-grade metamorphic rocks in northern Idaho were strata of the Belt-Purcell Supergroup or overlying rocks of the Neoproterozoic Windermere Supergroup and not basement rocks. Les auteurs ont analyse des grains de zircons detritiques de 10 roches metaseedimentaires du complexe de Priest River et de trois autres sequences au facies des amphibolites dans le centre- nord de l'Idaho afin de verifier l'assignation anterieure de ces roches au Supergroupe Belt-Purcell (Mesoproteerozolque). Des grains de zircons de deux echantillons de la Formation de Prichard (Belt infeerieur) et un echantillon de quartzite du Cambrien ont aussi ete analyses en tant que controles dont les ages de deposition etaient connus. Les analyses U-Pb sur zircon par ablation au laser et spectromeetrie de masse avec un plasma a couplage inductif revelent que six des dix echantillons contiennent des populations d'ages multiples entre 1900 et 1400 Ma et une dispersion d'ages plus anciens, semblables aux resultats obtenus du Supergroupe BeltPurcell vers le nord et l'est. Les resultats obtenus du complexe metamorphique Priest River confirment les correlations anterieures avec la Formation de Prichard. Des echantillons des sequences Golden et Elk City ont de nombreux grains datant de 1500-1380 Ma, ce qui indique qu'ils ne sont pas anterieurs au Belt, mais qu'ils proviennent probablement plutot d'une partie sud-ouest, relativement jeune du Supergroupe de Belt (sous bassin de Lemhi). Des grains ne provenant pas de l'Ameerique du Nord (1610-1490 Ma) sont rares dans ces roches. Trois exemples de quartzite de la sequence metamorphique de Syringa, au nord-ouest du batholite d'Idaho, contiennent des grains de zircon plus jeunes que ceux du Supergroupe de Belt et ils renforcent l'ideee d'un age Neoproterozoique. Un seul echantillon provenant du Cambrien a d'abondants grains de 1780 Ma et aucun plus jeune que ~1750 Ma. Ces resultats indiquent que les protolites probables de nombreuses roches a meetamorphisme eleve dans le nord de l'Idaho etaient des strates du Supergroupe de Belt-Purcell ou des roches sus-jacentes du Supergroupe de Windermere (Neeoproteerozolque) et non des roches du socle. [Traduit par la Redaction], Introduction The Mesoproterozoic Belt-Purcell Supergroup is exposed throughout much of northern Idaho, western Montana, northeastern Washington, and southeast British Columbia (Fig. 1). Its age is known to be about 1470-1400 [...]

We present a new method for Lu-Hf geochronology using lawsonite, an index mineral of high-pressure, low-temperature metamorphism. The method uses common cation exchange chromatography to purify Lu and Hf from the sample and analysis by muiticollector inductively coupled plasma--mass spectrometry. Lawsonite [sup.176]Lu/[sup.177]Hf ratios are elevated and are similar to other minerals commonly used in Lu-Hf geochronology such as garnet or apatite. We obtain an age of 145.5 [+ or -] 2.4 Ma for lawsonite-blueschist facies metamorphism at Ring Mountain within the Franciscan Complex, California, the lawsonite type locality. The age agrees well with published geochronology and thermochronology for the Franciscan Complex. The new method provides a powerful tool to investigate processes and time scales of blueschist to eclogite facies metamorphism, crust-mantle recycling via subduction, and the geodynamics of convergent margins. doi: 10.1130/G30292A.1