Search for: All records

Abstract The Sanak-Baranof belt includes a series of near-trench plutons that intrude the outboard Chugach–Prince William terrane over ~2200 km along the southern Alaskan margin. We present new petrological, geochronological, and geochemical data for comagmatic microgranitoid enclaves and granitoid rocks from the Crawfish Inlet (ca. 53–47 Ma) and Krestof Island (ca. 52 Ma) plutons on Baranof and Krestof Islands, as well as the Mount Stamy (ca. 51 Ma) and Mount Draper (ca. 54–53 Ma) plutons and associated mafic rocks that intrude the Boundary block at Nunatak Fiord near Yakutat, Alaska, USA. These data suggest that intrusion of the Sanak-Baranof belt plutons is actually a tale of two distinct belts: a western belt with crystallization ages that young systematically from west to east (63–56 Ma) and an eastern belt with crystallization ages ranging from 55 to 47 Ma, but with no clear age progression along the margin. Hf isotope analyses of magmatic zircon from the western Sanak-Baranof belt become increasingly evolved toward the east with εHft = 9.3 ± 0.7 on Sanak Island versus εHft = 5.1 ± 0.5 for the Hive Island pluton in Resurrection Bay. The Hf isotope ratios of eastern Sanak-Baranof belt rocks also vary systematically with age but in reverse, with more evolved ratios in the oldest plutons (εHft = +4.7 ± 0.7) and more primitive ratios in the youngest plutons (εHft = +13.7 ± 0.7). We propose that these findings indicate distinct modes of origin and emplacement histories for the western and eastern segments of the Sanak-Baranof belt, and that the petrogenesis of eastern Sanak-Baranof belt plutons (emplaced subsequent to 57–55 Ma) was associated with an increasing mantle component supplied to the youngest eastern Sanak-Baranof belt magmas. These plutons reveal important information about offshore plate geometries and a dynamic period of plate reorganization ca. 57–55 Ma, but a clearer picture of the tectonic setting that facilitated these Sanak-Baranof belt intrusions cannot be resolved until the magnitude and significance of lateral translation of the Chugach–Prince William terrane are better understood. 

The Upper Cretaceous to Paleocene Yakutat Group contains a flysch unit and a mélange unit with an unknown source terrane. The provenance of detrital zircons may be the key to understanding the age of clastic units, their source terrane, and correlative rocks along the margin. Two samples were collected from remote and difficult to access areas in Glacier Bay National Park, and these samples can be compared to samples from Harlequin Lake, Russell Fiord, and Yakutat Bay to the north. We dated detrital zircons using standard LA-ICPMS methodology. A sample of Yakutat Group flysch (YGf) from the Grand Plateau Glacier is from quartzofeldspathic turbidites adjacent to the Grand Plateau pluton. It has an MDA of ~66 Ma (Maastrichtian-Paleocene), and the grain-age distribution is dominated by a broad mid-Cretaceous population with ages from ~91 to ~114 Ma, it also has a Jurassic component at ~166. A unique attribute of this sample is that 23% of the zircons are Precambrian with a bimodal population at ~1397 Ma and ~1702 Ma. A sample of sandstone from the Yakutat Group mélange (YGm) from Lituya Bay, was collected from an assemblage of dark lithic sandstones interbedded with basalt, and dark-gray bedded chert. This sample has an MDA of ~108 (Albian), and its grain-age distribution is dominated (88%)by Jurassic dates ranging from ~156 to ~188 Ma. Both samples can be correlated to similar dated units in the area in and around Yakutat Bay. The YGf sample is correlative to the primary zircon facies common to arkosic rocks in both the Yakutat Group flysch and mélange, which we refer to as the Russell zircon facies, with an MDA range from 61-72 Ma, and distinctive Precambrian populations. The YGm sample is more complicated, but it appears to belong to the Shelter Cove zircon facies, dominated by mid-Cretaceous lithic sandstones that occur only in the mélange. The Yakutat terrane has been translated along the margin of the Cordillera, and candidate correlative rocks are to the south. We are intrigued that similar facies with similar grain-age distributions occur in the Western Mélange Belt in the North Cascades foothills in WA. We evaluate the correlation and connection between the Yakutat and the WMB and post Paleocene translation of part of this once contiguous unit.