Metamorphic history of the metasedimentary rocks in the northern North Cascades, Washington: insights into multiple sediment incorporation events in continental magmatic arcs
AuthorHanson, Ann Elizabeth Hunt
AdvisorGordon, Stacia M
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The construction of a continental magmatic arc is not steady state but rather is affected by fluctuations in magmatic volumes and potentially multiple sediment incorporation events throughout the evolution of the arc. Incorporated sediment introduces a more felsic and rheologically weaker composition into the deep arc crust. Sediment incorporation events are also commonly associated with higher volumes of magmatism in the arc. Thus, deciphering the history of individual sediment incorporation events is important for parsing out the overall evolution of the arc. Abundant metasedimentary rocks are exposed in the northern exhumed Cretaceous–Eocene North Cascades arc. Previous field and detrital zircon studies have suggested multiple sedimentary bodies were incorporated from both the western (forearc and accretionary wedge sediments) and eastern (backarc sediments) sides of the arc. This study utilizes additional field relationships, detrital zircon geochronology, bulk rock geochemistry, thermometry, and high precision quartz-in-garnet barometry to better distinguish the sediment source, the order in which the sediment was incorporated, the differences in their emplacement depths, and the field boundaries of the different metasedimentary units. Skagit Gneiss metasediments, sourced from the west side of the arc, fall into two groups based on their detrital zircon signature: 1) those that reveal Proterozoic–Eocene zircon (Group 1), and 2) those with only Triassic–Cretaceous zircon (Group 2). Both Skagit Gneiss metasedimentary groups reveal near peak metamorphic conditions of 640–800°C and 5.5–7.9 kbar, with several of the Group 2 metasediments having higher pressures of ~7.0–7.9 kbar. These higher pressures combined with field relationships suggest Group 2 metasediments are structurally below Group 1 metasediments. Similar field appearance, geochemistry, and P–T conditions suggest the protoliths of both groups were successively deposited in a forearc sequence, underthrusted into the arc relatively unmixed, and metamorphosed by ca. 65–77 Ma. A third group of metasediments is exposed within the eastern bounding Ross Lake fault zone. These metasediments have been previously described as metamorphosed equivalents to backarc Methow terrane. Meta-Methow terrane Group 3 metasediments exhibited unimodal Triassic or bimodal Late Jurassic–Early Cretaceous detrital zircon signatures. The meta-Methow terrane and adjacent Napeequa Schist reached peak metamorphic conditions of 570–700°C and 8.7–10.5 kbar. Combined U-Pb zircon and geochemistry data with field relationships suggest the Group 3 metasediments were formed as a result of incorporation of backarc sediments to depths of ~31–40 km in a transpressional step-over zone. Overall, the adjacent metasedimentary rocks in the northern North Cascades metasupracrustal units show significant differences in their sediment source, incorporation mechanism, and depth of burial. Both sediment incorporation events are coeval with a magmatic flux event in the arc. The limited or lack of partial melting exhibited by the Skagit Gneiss and meta-Methow terrane, respectively, suggests the high magmatic flux event was not sourced from partial melts of the metasediments, but rather the incorporation of both forearc and backarc sediments was likely facilitated by magmatism weakening the arc crust combined with regional transpression.