Abstract
Abstract Most metasedimentary rocks in the southern Coast Mountains batholith are of uncertain tectonic affinity because they occur in discontinuous pendants surrounded by large intrusive bodies, and many protolith features are obscured by regional deformation and metamorphism. This study uses U-Th-Pb ages and Lu-Hf isotope signatures of detrital zircons in metasedimentary rocks in Bute, Loughborough, and Knight Inlets in an effort to test possible correlations with the adjacent Wrangellia, Alexander, Taku, Yukon-Tanana, and Stikine terranes. Detrital zircons from metasedimentary samples yield ages that belong to age groups of 590-528 Ma (peak age of 560 Ma), 485-432 Ma (peak age of 452 Ma), 356-286 Ma (peak age of 307 Ma), and 228-185 Ma (peak ages of 215 and 198 Ma). A small number of ~1.1-1.9 Ga grains are also present. εHft values of the 590-185 Ma grains yield a progression from intermediate (0 to +5) values to more juvenile (mostly +4 to +15) values from Neoproterozoic through early Mesozoic time. The Comparison of these results with similar data sets from adjacent terranes demonstrates that primary connections with the Yukon-Tanana and Taku terranes are unlikely but are consistent with primary connections with the Wrangellia, Stikine, and/or Alexander terranes. Unfortunately, the available constraints are not sufficient to eliminate any of these options or the possibility that the pendants are a unique tectonic fragment. Zircons from the metasedimentary samples also yield U-Th-Pb ages of 165-128 Ma (peak age of 152 Ma) and 114-88 Ma (peak age of 102 Ma). εHft values of these zircon domains are mostly juvenile (+7 to +13). Comparison of U concentrations, U/Th values, and CL textures of zircons from the metasedimentary samples, leucocratic sills that intrude the pendants, and surrounding plutonic bodies suggests that most of the young grains, as well as widespread younger rims on older grains, grew during metamorphism associated with emplacement of the adjacent plutonic bodies. Some young grains were derived from thin felsic sills or veins that were unintentionally included in the sampled material.
Highlights
The Coast Mountains of western British Columbia and southeast Alaska are underlain mainly by Jurassic, Cretaceous, and early Tertiary plutons that intrude a variety of metasedimentary and metavolcanic rocks (Figure 1)
The similarity of ages, U/Th values, U concentrations, and CL textures in zircons from the metasedimentary samples and leucocratic sills suggest that many of the grains in our metasedimentary samples are igneous in origin and were inadvertently incorporated into the samples despite careful selection of material on the outcrop and during sample processing
The higher U/Th values, U concentrations, and convolute CL textures seen in some whole grains and the rims of many grains indicate that metamorphic zircon is present
Summary
The Coast Mountains of western British Columbia and southeast Alaska are underlain mainly by Jurassic, Cretaceous, and early Tertiary plutons that intrude a variety of metasedimentary and metavolcanic rocks (Figure 1). This study attempts to evaluate these tectonic affinities and possible connections with the nearby Alexander or Yukon-Tanana/Taku terranes, by presenting U-Th-Pb geochronologic data and Hf isotopic data on detrital zircons that have been extracted from metasedimentary rocks in Bute, Loughborough, and Knight Inlets (Figure 1). The first assemblage, extending across the southwestern part of the study area and in upper Knight Inlet, is found in pendants that consist mainly of interlayered quartzite and marble (Figure 3(b)) with subordinate pelitic schist and local calcsilicate. A third assemblage consists of two belts of metabasalt and subordinate marble that crop out in Bute and Knight Inlets (Figure 2) These layers are present near sequences dominated by quartzite and marble, but continuity with adjacent units could not be established. Marble, or pelitic schist have been found in this assemblage, suggesting that these rocks comprise a distinct tectonostratigraphic unit
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