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

Abstract

The Swakane Gneiss, interpreted to represent sedimentary strata metamorphosed at 8-12 kbar, is the deepest exposed crustal levels within the exhumed North Cascades continental magmatic arc, yet the nature and age of its protolith and the mechanism by which it was transported to deep-crustal levels remains unclear. Zircons from 11 paragneiss and schist samples were analyzed for U-Pb age and Hf-isotope composition in order to investigate the tectonic history of the Swakane Gneiss from protolith deposition to metamorphism within the North Cascades arc. Zircons interpreted to have crystallized in situ during metamorphism and/or melt-crystallization within the Swakane Gneiss at depth have ca. 74-66 Ma ages. Detrital-zircon age and Hf-isotope characteristics demonstrate provenance shifts that correlate with maximum depositional ages of ca. 93-81 Ma. Samples deposited between ca. 93 and 88 Ma have dominantly Mesozoic age peaks with initial ε Hf values between depleted mantle and chondritic uniform reservoir (CHUR), whereas ca. 86-81 Ma sample show the addition of distinct Proterozoic populations (ca. 1380 and 1800-1600 Ma) and Late Cretaceous zircons with unradiogenic Hf-isotope compositions. Similar detrital-zircon age and Hf-isotope patterns are observed in several Upper Cretaceous forearc and accretionary wedge units between southern California and Alaska along the North American continental margin. The connection between the Swakane Gneiss and these potential protoliths located outboard of Cordilleran arc systems indicate burial by either underplating of accretionarywedge sediments or underthrusting of forearc sediments. Therefore, the protolith and incorporation history for the Swakane Gneiss is likely similar to those of deep crustal metasedimentary units elsewhere in the North Cascades (i.e., the Skagit Gneiss Complex) and to the south along the continental margin (i.e., the Pelona-Orocopia-Rand schists and Schist of Sierra de Salinas). These observations suggest that burial of sediment outboard of continental magmatic arc systems may be a major mechanism for the transfer of sediment to the deep levels of continental arcs.

Along-strike variations in sediment provenance within the Nanaimo basin reveal mechanisms of forearc basin sediment influx events

“…The toolset detritalPy (Sharman et al, 2018) was used to create probability density functions (PDFs) for individual samples and groups of samples presented here, as well as for additional published data sets (e.g., Brown, 2012;Sauer et al, 2018). Previously published data were filtered and presented in the same manner as new data reported here to enable comparisons.…”

“…1); and 3the Cascade Crystalline Core (CCC), which is the most deeply exhumed portion of the Coast Mountains Batholith (Tabor et al, 1987) (Fig. 1), composed of metamorphosed plutonic rocks, volcanic rocks, and forearc sediments that were incorporated into the arc at mid-to-deep-crustal depths (e.g., Swakane Biotite-Gneiss) (Tabor et al, 1987;Matzel et al, 2004;Ducea and Chapman, 2018;Sauer et al, 2017Sauer et al, , 2018. Magmatic activity in the CCC spans 96-45 Ma, and high flux events occurred between 96-89 Ma and 78-71 Ma (Miller et al, 2009).…”

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

“…The U-Pb and Hf-isotope results and sample location information for the Swakane Gneiss are reported in Sauer et al (2018) and housed in GSA Data repository 2018144. Sauer et al (2018) describe how metamorphic and detrital analyses were separated. The MDAs for the Swakane Gneiss samples were calculated using the same method as for the PORS samples.…”

“…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.…”

Discovery of Vestige Sedimentary Archives of the India‐Asia Collision in the Eastern Yarlung Zangbo Suture Zone

“…So the youngest age clusters can be used for estimating the maximum depositional age of the protoliths. On other hand, overgrowth rims (dark under CL) or metamict rims of zircons (Figure 7) can be used to indicate the chronometrical reset by metamorphism (e.g., Breeding et al, 2004; Sauer et al, 2018; Vavra et al, 1999) after deposition, and their U–Pb ages can be used to constrain the minimum depositional age of the protoliths.…”