New Articles for Geosphere Posted Online in December

Boulder, Colo., USA: GSA’s dynamic online journal, Geosphere, posts articles online regularly. Locations studied this month include the North Cascades, USA; the Wrangell Arc; the South Mountain Batholith, Canada; and the Adirondack Highlands, USA. You can find these articles at .

Multiple sediment incorporation events in a continental magmatic arc: Insight from the metasedimentary rocks of the northern North Cascades, Washington (USA)
Ann E. H. Hanson; Stacia M. Gordon; Kyle T. Ashley; Robert B. Miller; Elizabeth Langdon-Lassagne
Abstract : The rheology and composition of arc crust and the overall evolution of continental magmatic arcs can be affected by sediment incorporation events. The exhumed Cretaceous–Eocene North Cascades arc exposes abundant metasedimentary rocks that were incorporated into the arc during multiple events. This study uses field relationships, detrital zircon geochronology, bulk rock geochemistry, geothermometry, and quartz­in­garnet geobarometry to distinguish approximate contacts and emplacement depths for different metasedimentary units to better understand their protolith incorporation history and impact on the arc. The Skagit Gneiss Complex is one of the main deep crustal units of the North Cascades arc. It includes metasedimentary rocks with distinct detrital zircon signatures: Proterozoic–Cretaceous (Group 1) or Triassic–Cretaceous (Group 2) zircon populations. Both metasedimentary groups achieved near­ peak metamorphic conditions of 640–800 °C and 5.5–7.9 kbar; several Group 2 samples reveal the higher pressures. A third group of metasedimentary rocks, which was previously interpreted as metamorphosed equivalents of backarc sediments (Group 3), exhibited unimodal Triassic or bimodal Late Jurassic–Early Cretaceous detrital zircon signatures and achieved near­peak conditions of 570–700 °C and 8.7–10.5 kbar. The combined field and analytical data indicate that protoliths of Group 1 and Group 2 metasedimentary rocks were successively deposited in a forearc basin and underthrusted into the arc as a relatively coherent body. Group 3 backarc sediments were incorporated into the arc along a transpressional step­over zone. The incorporation of both forearc and backarc sediments was likely facilitated by arc magmatism that weakened arc crust in combination with regional transpression.
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Geochronology of the Wrangell Arc: Spatial-temporal evolution of slab-edge magmatism along a flat-slab, subduction-transform transition, Alaska-Yukon
Jeffrey M. Trop; Jeff A. Benowitz; Carl S. Kirby; Matthew E. Brueseke
Abstract : The Wrangell Arc in Alaska (USA) and adjacent volcanic fields in the Yukon provide a long-term record of interrelations between flat-slab subduction of the Yakutat microplate, strike-slip translation along the Denali–Totschunda–Duke River fault system, and magmatism focused within and proximal to a Cretaceous suture zone. Detrital zircon (DZ) U-Pb (n = 2640) and volcanic lithic (DARL) 40Ar/39Ar dates (n = 2771) from 30 modern river sediment samples document the spatial-temporal evolution of Wrangell Arc magmatism, which includes construction of some of the largest Quaternary volcanoes on Earth. Mismatches in DZ and DARL date distributions highlight the impact of variables such as mineral fertility and downstream mixing/dilution on resulting provenance signatures. Geochronologic data document the initiation of Wrangell Arc magmatism at ca. 30–17 Ma along both sides of the Totschunda fault on the north flank of the Wrangell–St. Elias Mountains in Alaska, followed by southeastward progression of magmatism at ca. 17–10 Ma along the Duke River fault in the Yukon. This spatial-temporal evolution is attributable to dextral translation along intra-arc, strike-slip faults and a change in the geometry of the subducting slab (slab curling/steepening). Magmatism then progressed generally westward outboard of the Totschunda and Duke River faults at ca. 13–6 Ma along the southern flank of the Wrangell–St. Elias Mountains in Alaska and then northwestward from ca. 6 Ma to present in the western Wrangell Mountains. The 13 Ma to present spatial-temporal evolution is consistent with dextral translation along intra-arc, strike-slip faults and previously documented changes in plate boundary conditions, which include an increase in plate convergence rate and angle at ca. 6 Ma. Voluminous magmatism is attributed to shallow subduction-related flux melting and slab edge melting that is driven by asthenospheric upwelling along the lateral edge of the Yakutat flat slab. Magmatism was persistently focused within or adjacent to a remnant suture zone, which indicates that upper plate crustal heterogeneities influenced arc magmatism. Rivers sampled also yield subordinate Paleozoic–Mesozoic DZ and DARL age populations that reflect earlier episodes of magmatism within underlying accreted terranes and match magmatic flare-ups documented along the Cordilleran margin.
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The spatial association of accessory minerals with biotite in granitic rocks from the South Mountain Batholith, Nova Scotia, Canada
D. Barrie Clarke; Axel D. Renno; David C. Hamilton; Sabine Gilbricht; Kai Bachmann
Abstract : We use mineral liberation analysis (MLA) to quantify the spatial association of 15,118 grains of accessory apatite, monazite, xenotime, and zircon with essential biotite, and clustered with themselves, in a peraluminous biotite granodiorite from the South Mountain Batholith in Nova Scotia (Canada). A random distribution of accessory minerals demands that the proportion of accessory minerals in contact with biotite is identical to the proportion of biotite in the rock, and the binary touching factor (percentage of accessory mineral touching biotite divided by modal proportion of biotite) would be ~1.00. Instead, the mean binary touching factors for the four accessory minerals in relation to biotite are: apatite (5.06 for 11,168 grains), monazite (4.68 for 857 grains), xenotime (4.36 for 217 grains), and zircon (5.05 for 2876 grains). Shared perimeter factors give similar values. Accessory mineral grains that straddle biotite grain boundaries are larger than completely locked, or completely liberated, accessory grains. Only apatite-monazite clusters are significantly more abundant than expected for random distribution. The high, and statistically significant, binary touching factors and shared perimeter factors suggest a strong physical or chemical control on their spatial association. We evaluate random collisions in magma (synneusis), heterogeneous nucleation processes, induced nucleation in passively enriched boundary layers, and induced nucleation in actively enriched boundary layers to explain the significant touching factors. All processes operate during the crystallization history of the magma, but induced nucleation in passively and actively enriched boundary layers are most likely to explain the strong spatial association of phosphate accessories and zircon with biotite. In addition, at least some of the apatite and zircon may also enter the granitic magma as inclusions in grains of Ostwald-ripened xenocrystic biotite.
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Cooling of the continental plate during flat-slab subduction
Xiaowen Liu; Claire A. Currie; Lara S. Wagner
Abstract : Most flat-slab subduction regions are marked by an absence of arc volcanism, which is consistent with closure of the hot mantle wedge as the subducting plate flattens below the continent. Farther inland, low surface heat flow is observed, which is generally attributed to cooling of the continent by the underlying flat slab. However, modern flat slabs have only been in place for <20 Ma, and it is unclear whether there has been sufficient time for cooling to occur. We use numerical models to assess temporal variations in continental thermal structure during flat-slab subduction. Our models show that the flat slab leads to continental cooling on timescales of tens of millions of years. Cool slab temperatures must diffuse through the continental lithosphere, resulting in a delay between slab emplacement and surface cooling. Therefore, the timescales primarily depend on the flat-slab depth with shallower slabs resulting in shorter timescales. The magnitude of cooling increases for a shallow or long-lived flat slab, old subducting plate, and fast convergence rates. For regions with flat slabs at 45–70 km depth (e.g., Mexico and Peru), shallow continental cooling initiates 5–10 Ma after slab emplacement, and low surface heat flow in these regions is largely explained by the presence of the flat slab. However, for the Pampean region in Chile, with an ~100-km-deep slab, our models predict that conductive cooling has not yet affected the surface heat flow. The low heat flow observed requires additional processes such as advective cooling from the infiltration of fluids released through dehydration of the flat slab.
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Ultrahigh-temperature granulite-facies metamorphism and exhumation of deep crust in a migmatite dome during late- to post- orogenic collapse and extension in the central Adirondack Highlands (New York, USA)
Ellen P. Metzger; Mary L. Leech; Michael W. Davis; Jackson V. Reeder; Brandon A. Swanson ...
Abstract: This study combines field observations, mineral and whole-rock geochemistry, phase equilibrium modeling, and U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon geochronology to investigate sillimanite-bearing felsic migmatites exposed on Ledge Mountain in the central Adirondack Highlands (New York, USA), part of an extensive belt of mid-crustal rocks comprising the hinterland of the Mesoproterozoic Grenville orogen. Phase equilibrium modeling suggests minimum peak metamorphic conditions of 960–1025 °C and 11–12.5 kbar during the Ottawan orogeny—significantly higher pressure-temperature conditions than previously determined—followed by a period of near-isothermal decompression, then isobaric cooling. Petrography reveals abundant melt-related microstructures, and pseudosection models show the presence of at least ~15%–30% melt during buoyancy-driven exhumation and decompression. New zircon data document late Ottawan (re)crystallization at ca. 1047 ± 5 to 1035 ± 2 Ma following ultrahigh-temperature (UHT) metamorphism and anatexis on the retrograde cooling path. Inherited zircon cores give a mean date of 1136 ± 5 Ma, which suggests derivation of these felsic granulites by partial melting of older igneous rocks. The ferroan, anhydrous character of the granulites is similar to that of the ca. 1050 Ma Lyon Mountain Granite and consistent with origin in a late- to post-Ottawan extensional environment. We present a model for development of a late Ottawan migmatitic gneiss dome in the central Adirondacks that exhumed deep crustal rocks including the Snowy Mountain and Oregon anorthosite massifs with UHT Ledge Mountain migmatites. Recognition of deep crustal meta-plutonic rocks recording UHT metamorphism in a migmatite gneiss dome has significant implications for crustal behavior in this formerly thickened orogen.
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GEOSPHERE articles are available at . Representatives of the media may obtain complimentary copies of GEOSPHERE articles by contacting Kea Giles at the address above. Please discuss articles of interest with the authors before publishing stories on their work, and please refer to GEOSPHERE in articles published. Non-media requests for articles may be directed to GSA Sales and Service,

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For Immediate Release
4 January 2022
GSA Release No. 22-02

Kea Giles