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[1] New high-precision single grain U-Pb zircon geochronology and whole rock Nd isotopic data provide insight into the magmatic and tectonic development of the Samail ophiolite. The analyzed rocks can be broadly divided into two groups based on their structural position, dates, and isotopic composition: an older group related to on-axis magmatism and a younger group of post-ridge dikes, sills, and stocks. Isotopic data suggest two distinct sources for the post-ridge magmas: five of the gabbros and tonalites from the crust have e Nd (96 Ma) = 6.90 AE 0.12 to 7.88 AE 0.16, and two trondhjemites from the upper mantle and metamorphic sole have e Nd (96 Ma) = À7.77 AE 0.08 and À7.01 AE 0.16. The negative e Nd (t) and U-Pb dates from the mantle dike require that subduction or thrusting was established below the ophiolite ≤0.25-0.5 Ma after formation of the crust. The bimodal isotopic composition of post-ridge magmas may reflect coeval decompression and/or fluid fluxed melting of the mantle and melting, dehydration, or assimilation of sediment in the down going plate at this time. The new data place temporal constraints on mid-ocean ridge and supra-subduction zone models for ophiolite formation.

[1] New high-precision U/Pb zircon geochronology from the Oman-United Arab Emirates (U.A.E.) ophiolite provides insight into the timing and duration of magmatism and the tectonic setting during formation of the lower crust. The new data come from a well-preserved and exposed crustal section in the center of the Wadi Tayin Zircon dates from upper-level gabbros are most consistent with the ophiolite forming at a fast spreading ridge with half-rates of 50-100 km/Ma. Dates from tonalites/trondhjemites and from a gabbroic pegmatite associated with a wehrlite intrusion overlap with dates from adjacent upper-level gabbros, suggesting that any age differences between these three magmatic series are smaller than the analytical uncertainties or intrasample variability in the dates. Three of the dated upper-level gabbros and a single gabbroic pegmatite from the base of the crust have >1 Ma intrasample variability in single grain dates, suggesting assimilation of older crust during the formation or crystallization of the magmas. Whole rock ɛ Nd (t) of seven samples, including the upper-level gabbros with variable zircon dates, have tightly clustered initial values ranging from ɛ Nd (96 Ma) = 7.59 AE 0.23 to 8.28 AE 0.31. The ɛ Nd values are similar to those from other gabbros within the ophiolite, suggesting that any assimilated material had a similar isotopic composition to primitive basaltic magmas. The new dates suggest that the studied section formed at a fast spreading mid-ocean ridge between $96.4-95.5 Ma. The large intrasample variability in zircon dates in some samples is unexpected in this setting, and may be related to propagation of a younger ridge into older oceanic lithosphere.Citation: Rioux, M., S. Bowring, P. Kelemen, S. Gordon, F. Dudás, and R. Miller (2012), Rapid crustal accretion and magma assimilation in the Oman-U.A.E. ophiolite: High precision U-Pb zircon geochronology of the gabbroic crust,

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Rare ultrahigh-temperature-(near)ultrahigh-pressure (UHT-near-UHP) crustal xenoliths erupted at 11 Ma in the Pamir Mountains, southeastern Tajikistan, preserve a compositional and thermal record at mantle depths of crustal material subducted beneath the largest collisional orogen on Earth. A combination of oxygen-isotope thermometry, major-element thermobarometry and pseudosection analysis reveals that, prior to eruption, the xenoliths partially equilibrated at conditions ranging from 815°C at 19 kbar to 1100°C at 27 kbar for eclogites and granulites, and 884°C at 20 kbar to 1012°C at 33 kbar for garnet-phlogopite websterites. To reach these conditions, the eclogites and granulites must have undergone mica-dehydration melting. The extraction depths exceed the present-day Pamir Moho at 65 km depth and suggest an average thermal gradient of 12-13°C km )1 . The relatively cold geotherm implies the introduction of these rocks to mantle depths by subduction or gravitational foundering (transient crustal drip). The xenoliths provide a window into a part of the orogenic history in which crustal material reached UHT-(U)HP conditions, partially melted, and then decompressed, without being overprinted by the later post-thermal relaxation history.

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The U-Pb age and Hf-isotope composition of detrital zircons from Jurassic to Upper Cretaceous sedimentary rocks adjacent to the southern North Cascades-Coast Plutonic Complex continental magmatic arc document shifting provenance, the tectonic evolution of the arc system, and translation along the continental margin. Systematic changes in the detrital-zircon data provide insight that the western margin of North America evolved from: marginal basins adjacent to continent-fringing oceanic arcs (ca. 160-140 Ma); forearc basins adjacent to mid-Cretaceous (ca. 120-90 Ma) Andean-type continental arcs; and addition of a cratonic source to forearc and accretionary wedge units to Cordilleran arc systems in the mid-Late Cretaceous (ca. 85 Ma). Jurassic Methow terrane, Nooksack Formation, and western mélange belt units dominantly contain detrital zircons derived from accreted oceanic terranes, whereas Lower Cretaceous strata from the same units have age peaks that correspond to known pulses of magmatism in Cordilleran continental magmatic arc systems. The age peaks and Hf-isotope signature of the Jurassic and Lower Cretaceous strata are comparable to multiple sources exposed along the margin. In contrast, the Upper Cretaceous western mélange belt has distinct Precambrian zircon populations and unradiogenic Late Cretaceous zircons that are more similar to southwestern than northwestern Laurentian sources. Statistical comparisons confirm provenance similarities between rocks of the North Cascades and those 700-2000 km to the south and, thus, support marginparallel translation from as far as the latitude of southern California.

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