Sm nd dating garnet
Here, magmatic fabric patterns and anisotropy of magnetic susceptibility (AMS) in the Wallowa batholith record three phases of progressive deformation of the host Wallowa terrane during Early Cretaceous.First is terrane-oblique ~NE–SW shortening, interpreted as recording attachment of the amalgamated oceanic and fringing, continental-margin terranes to the continental margin during dextral convergence at ~140 Ma.In a nearby amphibolite, garnets with a two-stage growth history consist of inclusion-rich cores surrounded by discontinuous, inclusion-free overgrowths.
Caledonian orogenesis in Scotland is currently interpreted in terms of a Mid-Ordovician arc–continent collision (Grampian event) followed by the Silurian collision of Laurentia with Baltica (Scandian event). 475–460 Ma obtained from prograde garnets in metasedimentary successions and metabasic intrusions within the Northern Highland and Grampian terranes confirm that the Mid-Ordovician Grampian orogenic event was approximately synchronous in the two terranes. 450 Ma obtained from prograde garnets within the Moine Nappe of the Northern Highland terrane provide evidence for a hitherto unrecognized Late Ordovician regional metamorphic event.
The existing two-stage Grampian–Scandian model for Caledonian orogenesis in northern Scotland is thus an oversimplification, and the new ages imply a more complex structural evolution.
Instead, we propose that the Coon Hollow Formation is part of a belt of suprasubduction-zone extensional back-arc basins that formed in latest Jurassic time due to awell-documented period of trench retreat in the western United States.
Our new data require that the underlying Wallowa terrane was accreted to and received detritus fromwestern North America by ca. This minimum estimate for the age of terrane accretion in western Idaho and eastern Oregon is substantially earlier than previous estimates (∼135–118 Ma).
Our geochronologic results show that metamorphism of arc-derived rocks in western Idaho was episodic and significantly older than in arc rocks along the eastern margin of the Wrangellian Superterrane in British Columbia and Alaska.
The pre-144 Ma event may be an expression of the late Jurassic amalgamation of marginal oceanic arc-related terranes (e.g., Olds Ferry, Baker, Wallowa) during the initial phases of their collision with North American rocks.
Three garnet fractions with apparent ages of 144, 141, and 136 Ma are thought to represent mixtures between late Jurassic (pre-144 Ma) inherited radiogenic components preserved within garnet cores and early Cretaceous (128 Ma) garnet overgrowths.
These observations confirm the resilience of garnet to diffusive exchange of trace elements during polymetamorphism at amphibolite facies conditions.
The Sm–Nd isotopic compositions of plagioclase, apatite, hornblende, and garnet define a precise, 8-point isochron of 1283 Ma (MSWD=1.2) interpreted as mineral growth at the metamorphic peak.