Rapid burial of the Swakane protolith is coincident with burial of sediments that formed the Pelona, Orocopia and Rand schists of southern California; however, differences in peak metamorphic temperatures indicate that conditions of burial must have varied along the plate margin. The results are compared to other examples of metasedimentary rocks in the deep levels of North American Cordillera arc segments and suggest that the transfer of upper-crustal material from the forearc side of the arc system is a major mechanism to introduce sediment into the deep levels of continental arc systems. U‐Pb dates obtained in this study from detrital zircons in the Swakane Gneiss indicate that its protolith was deposited as late as 72.5 ± 0.6 Ma, much younger than previous age estimates and well after the juxtaposition of the other arc terranes that comprise the North Cascades core. Among foliated rocks slate is the obvious [17] In order to interpret the petrologic significance of the zircon analyses, zircon grains representing the range of morphologies from samples SW3B and SW2 were imaged using cathodoluminescence (CL) techniques (Figures 5 and 6; Appendix A). As sedimentary provenance information is largely erased for sediments that were assimilated into arc magmas, studies of the intact metasedimentary units in the deep levels of arc systems can be used to differentiate among processes that transfer sediment to depth. L'identification d'un protolithe est un objectif important dans l'étude de la géologie d'une région, il en existe une grande variété. Learn more. Nine of the eleven samples were collected in the Wenatchee block as previous studies have focused on characterizing the Swakane Gneiss in the Chelan block. However, the rates and durations of vertical motion are usually not well constrained, and mechanisms proposed to account for tectonic burial are controversial. Gneiss is such a high grade rock that there are other possible protoliths. The most common path begins with shale, which is a sedimentary rock. 96–87 Ma portions of the western mélange belt have similar peaks to the ca. Zircon cores have a variety of CL patterns including oscillatory, flat (unzoned), sector, convolute, and ghost zoning (c.f. They show a spectrum of textures, from oscillatory zoned to unzoned, and include features such as patchy-sector zoned overprints and clouded, ghost-CL textures (Fig. Linking deep and shallow crustal processes during regional transtension in an exhumed continental arc, North Cascades, northwestern Cordillera (USA). On the basis of previous mapping, Hurlow [1992] suggested that the sill intruded the Swakane‐Napeequa contact after the terranes were juxtaposed [Hurlow, 1992]. The model involving overthrusting of a forearc or back arc basin is most consistent with thermobarometric and isotopic data and the regional geologic setting of the Cascades core. 78–66 Ma rims from two samples (SK14-28 and SK15-61A) have U concentrations similar to detrital zircons, but lower Th concentrations, resulting in U/Th values >10 (Figs. The protolith age of the Napeequa Schist is poorly understood, but is likely Paleozoic–Jurassic, based on correlation with the Mississippian to Jurassic Bridge River–Hozameen terrane (Misch, 1966; Miller et al., 1993) and Triassic protolith ages for amphibolites (metabasalts) (Sauer et al., 2017b). [40] This model can easily account for rapid burial rates. The Swakane terrane crops out in both blocks (Figure 2). Peak P‐T conditions determined from the Cascade River unit reached 8–9 kbar and ∼650°C [Brown et al., 1994; Miller et al., 1993]. 155 to 45 Ma (e.g., Gehrels et al., 2009). The majority of these cores have U and Th concentrations that overlap with detrital zircons in the sample, corresponding to low U/Th ratios (Fig. Localized burial on a steeply dipping shear zone would negate the need for lateral translation, but there is no evidence that a steeply dipping structure was active within the North Cascades at that time and burial on such a structure is incompatible with the shallowly dipping foliation and gently plunging lineation preserved in the gneiss and overlying units [Paterson et al., 2004]. The Swakane Gneiss is an isotopically evolved unit with the lowest ɛNd values (as low as −6 ɛNd at 73 Ma) observed in the Cascades core and has mid to Late Proterozoic Nd depleted mantle model ages [Matzel, 2004; Rasbury and Walker, 1992]. Either scenario may bury accretionary wedge and/or forearc material, and so other observations about the metamorphic history in addition to the protolith origin are required to understand the incorporation of the Swakane Gneiss in the North Cascades arc. Solid circles indicate the location of the U-Pb spot and dashed circles mark the larger Hf-isotope spots. Les lits sombres sont micacés avec la présence éventuelle d'amphiboles. The Cascade River Schist consists of plagioclase–mica schist, metaconglomerate, and amphibole-rich schist, with lesser silicic schist, marble, and amphibolite (Tabor et al., 1989; 2002) that is spatially associated with the Triassic Marblemount–Dumbell plutonic belt. 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). 134 and 96 Ma, with the exception of one sample with a ca. Overall, zircons from the Swakane Gneiss samples yield dates that are dominantly Mesozoic or Proterozoic (Figs. 1B). (2009). Only SW3B analysis z25 displays sector growth zonation typical of metamorphic zircon [Corfu et al., 2003]. to produce the metamorphic one. Processes in Geophysics, Atmospheric Since there is little evidence of early top‐to‐WSW kinematics in the Swakane Gneiss, the sense of motion during burial is unconstrained and may have been accomplished along a thrust contact that is no longer exposed. These schists are thought to underlie a large region beneath southern California and into Arizona [Cheadle et al., 1986; Ehlig, 1981; Haxel and Dillon, 1978; Malin et al., 1995]. (1896-1977), Chinese Journal of Geophysics (2000-2018), International It is characterized by alternating light and dark bands The youngest zircon grains in the Swakane Gneiss are essentially coeval with plutons in these source terranes and may have been derived from their volcanic cover. The western contact with the overlying Napeequa complex is an unnamed fault. Transfer of Metasupracrustal Rocks to Midcrustal Depths in the North Cascades Continental Magmatic Arc, Skagit Gneiss Complex, Washington. The Geology of North America, vol. 86–80 Ma mantles are interpreted as detrital, in contrast with the ca. [22] Potential sedimentary protoliths of the appropriate age for the Swakane Gneiss are present, but not abundant, in the region. These dates range from 72.5 ± 0.6 Ma to 191.6 ± 0.2 Ma. Stars represent data from sample SW3B. The following considers these previous interpretations and the relationship of the Swakane Gneiss to other components of the North Cascades crystalline core using the new detrital zircon U-Pb and Hf-isotope data presented here. 86–80 Ma mantle zones, (C) ca. Mihai Ducea, David Pearson, and editor Laurent Godin provided helpful reviews that improved this manuscript. 4, 5, and 6). Similarly, quartzite is made only of quartz, and [1] The metamorphic core of the North Cascades largely comprises island arc and oceanic terranes juxtaposed prior to circa 96 Ma magmatism. Underplating occurs at plate-tectonic rates; for example, the PORS schists reached 30–33 km depth in less than 3 m.y. (A) Geologic map modified from Haugerud and Tabor (2009) and Cater and Crowder (1967), showing sample locations throughout the Swakane Gneiss. 1400–1300 and 1800–1600 Ma. (Matzel et al., 2004). Pelitic wall rocks of the 91–88 Ma Eldorado Orthogneiss, which intrudes the Napeequa Complex, Cascade River unit, and Skagit paragneisses contain andalusite replaced by kyanite indicating shallow emplacement of the Eldorado orthogneiss followed by regional burial metamorphism. Two possible mechanisms considered for this rapid burial (∼7 mm/yr) are overthrusting of a forearc or back arc basin by older crystalline rock or underthrusting of trench sediments during low‐angle subduction. Gneiss is such a high grade rock that there are Zircon grains were picked in ethanol under a binocular microscope and sorted by their morphology, color, clarity, and inclusion characteristics. [20] The question of whether the Swakane protolith was a sedimentary rock (arkose or greywacke) or a predominantly silicic volcanic rock has been long debated [Cater, 1982; Mattinson, 1972; Miller et al., 2000; Sawyko, 1994; Tabor et al., 1987a, 1987b; Waters, 1932; Whitney et al., 1999]. In addition, the Skagit protoliths reached depth between ca. (2016). 1380 Ma and a broad population between ca. A thin swath of rocks mapped as the Holden unit of the Cascade River Schist separates the Swakane Gneiss from the Entiat fault (Cater and Crowder, 1967). Physics, Comets and The fact that the Swakane Gneiss lacks Cretaceous or younger arc plutons is difficult to explain by this model. Sediment underthrusting within a continental magmatic arc: Coast Mountains batholith, British Columbia. [37] This model is difficult to apply in detail to the evolution of the North Cascades because a series of small basins has not been recognized as such. On trouve généralement des gneiss dans les parties profondes, érodées, des chaînes de montagnes. The same could be true for 145–120 Ma; Brown and Gehrels, 2007; Paterson, 2014). Compiled from Brown and Gehrels (2007), DeGraaff-Surpless et al. been metamorphosed to a very low grade. As the Swakane Gneiss samples do not yield significant Archean zircons, we interpret that the Proterozoic zircons in the Swakane Gneiss most closely match Mesozoic and Proterozoic sediment sources in southwestern Laurentia, but a northwestern Laurentian provenance cannot be ruled out. Le mot est attesté en français pour la première fois en 1750 sous la forme Kneiss, puis en 1774 et 1779[2], sous sa forme actuelle gneiss. In addition to variable CL textures, zircons record a range of U/Th ratios. 81 Ma zircon population was the result of mixed analyses. The Cascade River Schist also includes the Holden unit and Twentyfive Mile unit of Miller et al. All leucogranite sheets were carefully avoided during sampling. Peak P‐T results from metapelitic outcrops within the Skagit Gneiss Complex are 9–10 kbar and >700°C [Whitney, 1992; Whitney et al., 1999]. 86 Ma) from dominantly a Mesozoic detrital-zircon signature to one that includes significant Proterozoic and unradiogenic Late Cretaceous populations demonstrates the addition of new sediment sources that are not observed in rocks of the North Cascades or Coast Plutonic Complex (Fig.
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