Transfer of Metasupracrustal Rocks to Midcrustal Depths in the North Cascades Continental Magmatic Arc, Skagit Gneiss Complex, Washington

Abstract

The metasupracrustal units within the north central Chelan block of the North Cascades Range, Washington, are investigated to determine mechanisms and timescales of supracrustal rock incorporation into the deep crust of continental magmatic arcs. Zircon U‐Pb and Hf‐isotope analyses were used to characterize the protoliths of metasedimentary and metaigneous rocks from the Skagit Gneiss Complex, metasupracrustal rocks from the Cascade River Schist, and metavolcanic rocks from the Napeequa Schist. Skagit Gneiss Complex metasedimentary rocks have (1) a wide range of zircon U‐Pb dates from Proterozoic to latest Cretaceous and (2) a more limited range of dates, from Late Triassic to latest Cretaceous, and a lack of Proterozoic dates. Two samples from the Cascade River Schist are characterized by Late Cretaceous protoliths. Amphibolites from the Napeequa Schist have Late Triassic protoliths. Similarities between the Skagit Gneiss metasediments and accretionary wedge and forearc sediments in northwestern Washington and Southern California indicate that the protolith for these units was likely deposited in a forearc basin and/or accretionary wedge in the Early to Late Cretaceous (circa 134–79 Ma). Sediment was likely underthrust into the active arc by circa 74–65 Ma, as soon as 7 Ma after deposition, and intruded by voluminous magmas. The incorporation of metasupracrustal units aligns with the timing of major arc magmatism in the North Cascades (circa 79–60 Ma) and may indicate a link between the burial of sediments and pluton emplacement.

Paleogeotherms of a Midcrustal to Upper‐Crustal Profile Across the Northern North China Block: Implications for the Thermal Structure of Continental Arcs

“…Geochemical analyses and geobarometric determinations on xenolith samples from depths of approximately 2 to 120 km in the Sierra Nevada batholith constructed a depth versus heat production profile for the Jurassic‐Cretaceous crust and show that the mean heat production values first increase, then decrease, with increasing depth (Brady et al, ). Although some midcrustal exhumed rocks (arc plutons and pre‐arc magmatic and metamorphic rocks) are exposed in the Sierra Nevada, the North Cascades, and the Coast plutonic complex within the western North America continental arcs (e.g., Ague, ; Miller & Paterson, ; Paterson & Miller, ; Sauer et al, ; Stowell et al, ), the geothermal history of these midcrustal rocks (especially prearc magmatic and metamorphic rocks) are not well constrained. Thermochronometric and thermobarometric results on the midcrustal to upper‐crustal arc plutons (ca.…”

Provenance and metamorphism of the Swakane Gneiss: Implications for incorporation of sediment into the deep levels of the North Cascades continental magmatic arc, Washington

“…Overall, the Swakane Gneiss is anomalous in structural setting, protolith age, and tectonic history within the North Cascades. The accreted oceanic supracrustal and oceanic arc-related clastic units that form the framework for the North Cascades arc are characterized by mostly Triassic to Lower Cretaceous protolith ages and Mesozoic detrital-zircon age peaks (Tabor et al, 1989;Miller et al, 1994;Gordon et al, 2017;Sauer et al, 2017b). In contrast, the provenance of the Swakane Gneiss protolith includes abundant Mesozoic and Proterozoic detrital zircons with a continental origin (Matzel et al, 2004;Gatewood and Stowell, 2012).…”

Deep-crustal metasedimentary rocks support Late Cretaceous “Mojave-BC” translation

“…Zircon U-Pb data was collected at the University of California, Santa Barbara Petrochronology Laboratory and Hf-isotope data was collected at the Washington State University Radiogenic Isotope and Geochronology Laboratory. The methods used to analyze the PORS samples are the same as described in available in Sauer et al (2017;2018), and are briefly summarized here.…”

“…Zircon grain mounts were first analyzed for U-Pb isotopes using a Nu Instruments Plasma HR multi-collector ICP-MS coupled to a 193 nm Photon Machines excimer laser at the University of California-Santa Barbara. Analytical specifications and data-reduction methods followed the procedure described in Sauer et al (2017). Unknowns were bracketed every 6-8 analyses with standards, using 91500 (ca.…”

“…Representative zircon grains from significant age populations were subsequently analyzed for Hf isotopes at the Radiogenic Isotope and Geochronology Laboratory at Washington State University using a New Wave UP 213 Nd-YAG (213 nm) laser and a ThermoFinnigan Neptune multi-collector ICP-MS. The Hf-isotope data collection and reduction followed the procedures described in Sauer et al (2017), which were modeled after Fisher et al (2011;2014b). The CHUR Lu-Hf values of Bouvier et al (2008) Ma populations correspond to thick, oscillatory-zoned mantles.…”