Petrology and age of volcanic-arc rocks from the continental margin of the Bering Sea: implications for Early Eocene relocation of plate boundaries

Abstract

Eocene volcanic flow and dike rocks from the Beringian margin have arc characteristics, implying a convergent history for this region during the early Tertiary. The extrusive rocks are basalt, basaltic andesite, andesite, and minor dacite and rhyolite. The intrusive sample is from a quartz diorite dike intruding serpentinized peridotite. Major-element oxide contents, particularly FeO*/MgO versus SiO,, identify both tholeiitic and calc-alkalic basalt; more silicic lavas have calc-alkalic affinities. Consistent with volcanic-arc compositions, spidergrams show pronounced Nb-Ta depletion and alkali enrichment relative to light-rare-earth-element (LREE) abundance. Chondrite-normalized REE plots show relatively flat patterns, with only slight LREE enrichment for tholeiitic compositions and greater LREE enrichment and lower heavy-rare-earth-element (HREE) abundance for calc-alkalic compositions. The samples, particularly those with calc-alkalic compositions, are rich in plagioclase that is strongly zoned; the more silicic samples contain orthopyroxene, clinopyroxene, and primary amphibole. The quartz diorite dike contains iron-rich almandine phenocrysts that appear to be magmatic, suggesting emplacement at great depth near the base of the crust or upper mantle.Chemical and mineralogical compositions are similar to those of modem Aleutian-arc lavas. They also resemble volcanicarc compositions from western mainland Alaska, although greater chemical diversity and a stronger continental influence are observed in the Alaskan mainland rocks.Early Eocene ages of 54.4-50.2 Ma for the Beringian samples are well constrained by conventional K-Ar ages of nine plagioclase separates and by concordant 40Ar/39Ar incremental heating and total-fusion experiments. A concordant U -Pb zircon age of 53 Ma for the quartz-diorite dike is in good agreement with the K-Ar data.Plate motion studies of the North Pacific Ocean indicate more northerly directed subduction prior to the Tertiary and a continuous belt of arc-type volcanism extending from Siberia, along the Beringian margin, into mainland Alaska. Around 56 Ma (chron 25 -24), subduction changed to a more westerly direction and subduction-related volcanism ceased for most of mainland Alaska. The increasingly oblique angle of convergence should have ended subduction along the Beringian margin as well. However, consistent ages of 54-50 Ma indicate a final pulse in arc-type magmatism during this period of plate adjustment, which may be explained by three different models: (1) The northern and central part of the Beringian margin maintained a higher angle of convergence, allowing a final pulse of arc-type magmatism. (2) The rocks erupted in an early, or proto, Aleutian arc and were rafted against the continental margin along transform faults. (3) The rocks erupted along a leaky transform fault, analogous to calc-alkalic volcanism in the southern California borderland.Les roches d'bge kockne d'une coulee volcanique et d'un dyke de la marge bkringienne prksentent les caractkristiques d'un arc, ce...

Reconstruction of Subduction and Back‐Arc Spreading in the NW Pacific and Aleutian Basin: Clues to Causes of Cretaceous and Eocene Plate Reorganizations

“…Fore‐arc deposits exposed on the Komandorsky Islands were dated as Late Paleocene to Early Eocene using bio‐magnetostratigraphy (Bazhenov et al, ; Minyuk & Stone, ; Rostovtseva & Shapiro, ), although the interpretation of the biostratigraphy is debated (Scholl, ). The oldest radiometric ages from rocks interpreted to belong to the Aleutian arc of ~46 Ma are only a few million years younger than the youngest arc volcanics found on the Beringian margin (Figure ), where K‐Ar and U‐Pb zircon ages from dredged arc volcanics are 54.4 to 50.2 Ma (Davis et al, ). Therefore, conventional tectonic models of the Bering Sea region suggest that subduction initiation below the Aleutian Trench was related to a jump of the subduction zone from the Beringian margin in the north toward the Aleutian Trench in the south around 50–46 Ma, likely related to the obduction of the Olyutorsky arc on Kamchatka (Scholl, ; Worrall, ).…”

“…The proto‐Komandorsky basin is a triangular basin, suggesting that its opening was associated with a ~40° counterclockwise rotation of the Shirshov Ridge—and consequently also the Aleutian basin lithosphere—relative to the Olyutorsky arc (Figures b and c) around a pole at the position of the Bering Strait. This requires convergence between the Aleutian basin and North America, which was accommodated by northeastward subduction at the Beringian margin until ~50 Ma (Davis et al, ; Miller et al, ; Scholl, ; Worrall, ). We therefore model the opening of the proto‐Komandorsky basin largely between 60 and 50 Ma.…”

Continental shelves as detrital mixers: U–Pb and Lu–Hf detrital zircon provenance of the Pleistocene–Holocene Bering Sea and its margins

“…The Bering Sea occupies a marginal ocean basin that was separated from the north Pacific by the development of the Aleutian Arc during Eocene time (Davis et al, 1989; Scholl, 2007). The Bering Sea can be subdivided into two main physiographic regions: the shallow‐marine Bering Shelf and deep‐marine basins and ridges (Figure 2).…”

Crustal structure of the Bering and Chukchi shelves: Deep seismic reflection profiles across the North American continent between Alaska and Russia

“…Magmatism is believed to be at least partly related to subduction beneath the North Paciµc margin, possibly above a southward-retreating subduction zone (Rubin et al, 1995;Amato and Wright, 1997;Bering Strait Geologic Field Party, 1997;Miller et al, this volume, Chapter 17;Amato et al, this volume). Late Cretaceous (65-77 Ma) and early Tertiary (61 Ma) arc magmatic rocks presumably linked to the Okhotsk-Chukotsk volcanic belt are found on Saint Matthew Island (Patton et al, 1976) and in dredge samples from the continental slope (Davis et al, 1989). On the Chukotka and Seward Peninsulas, Cretaceous magmatism was contemporaneous with ductile thinning and uplift of mid-crustal rocks in the Koolen and Kigluaik gneiss domes (Amato et al, this volume; Akinin and Calvert, this volume).…”

“…The red unconformity emerges at the sea×oor at ∼km -935, so that prered strata crop out on the lower slope below ∼2.8 s (2.1 km water depth). Carapace rocks including Cretaceous limestones and siltstone, and early Eocene arc volcanic rocks, have been dredged in this position (Jones et al, 1981;Davis et al, 1989). No structure is visible within the pre-red unconformity complex, here presumed to include a pre-late Eocene accretionary complex, although rock dredging has not recovered any such samples (Jones et al, 1981).…”