Travis Hudson

Paleogene metamorphism of an accretionary flysch terrane, eastern Gulf of Alaska

Reconnaissance field studies have identified a regional metamorphic complex, at least 200 km long and 25 to 50 km wide, in the eastern Chugach Mountains, southern Alaska. The metamorphic complex was developed within the Cretaceous Valdez Group, a flysch sequence that was accreted to the continental margin during the Late Cretaceous or early Tertiary. The metamorphic complex shows the progressive development from subgreenschist and green-schist facies slate, phyllite, and semischist that is regionally characteristic of the Valdez Group, to biotite-plagioclase-quartz schist (± muscovite, cordierite, andalusite, staurolite, garnet, and sillimanite) in an intermediate zone, and amphibolite facies muscovite-biotite-plagioclase-quartz schist and gneiss (± sillimanite and garnet) in a higher-grade central zone. K-feldspar is characteristically absent in the amphibolite facies schists, but it is present in migmatitic rocks that are common to the central zone. The mineral assemblages and petrographic relations indicate that metamorphism took place under low-pressure intermediate-facies series conditions; partial melting took place at the highest grade of metamorphism. Melting produced granodioritic magmas in migma-titic rocks of the central zone that were emplaced as discrete plutons in both the intermediate zone and peripheral rocks of the complex. The timing of metamorphism as indicated by field relations and K-Ar data is about 50 to 65 m.y. ago and overlaps or is coeval with the emplacement, throughout the Gulf of Alaska region, of early Tertiary plutons that are thought to be anatectic in origin. The distribution of these plutons suggests that a high-temperature low-pressure metamorphic belt, about 1,800 km long, developed along the leading edge of the continental margin in late Paleocene to early Eocene time.

Paleogene anatexis along the Gulf of Alaska margin

Early Tertiary plutons of biotite tonalite, granodiorite, and granite are found in a curvilinear 2,000-km-long belt along the margin of the Gulf of Alaska. These plutons intrude flyschoid rocks that were accreted to the continental margin during Late Cretaceous and/or early Tertiary time. Field, petrologic, age, and Rb-Sr data on plutonic and metamorphic rocks in eastern Chugach Mountains suggest that the granitic magmas were produced by partial melting of deeper parts of the accretionary prism after it was deformed against the continent. Such magmas may be a common product of heating at the termination of certain major accretionary episodes; they are not subduction-related magmas from subcrustal sources.

Tin granites of Seward Peninsula, Alaska

Seven granite plutons, spatially and genetically related to tin metalization, are exposed in a 170-km-long belt across northwestern Seward Peninsula, Alaska. These plutons are cupolas and epizonal composite stocks that consist of several textural varieties of biotite granite, including medium- to coarse-grained seriate biotite granite, porphyritic biotite granite with an aplitic groundmass, and fine- to medium-grained equigranular biotite granite. The common accessory minerals are fluorite, allanite, apatite, and zircon. Other accessory minerals that are locally present include tourmaline, sphene, opaque oxide minerals, and late-forming (deuteric) muscovite and chlorite. The granites range in major-element contents as follows: SiO 2 , 72.5% to 76.6%; A1 2 O 3 , 12.7% to 14.3%; Na 2 O, 2.9% to 4.0%; K 2 O, 3.9% to 5.6%; and CaO, 0.6% to 1.2%. The sum of FeO + Fe 2 O 3 + MgO ranges from 0.3% to 2.4%; and the K 2 O to Na 2 O ratio from 1.1 to 1.8. The 0.1% to 0.9% F and 0.01% to 0.2% Cl reflect the over-all volatile-rich nature of the granites. The granites contain average or below-average concentrations of Co, Sc, Cr, and Zn, and generally above-average to distinctly high concentrations of Th, U, Hf, and Ta. The large cations emphasize the evolved nature of the granites; the Rb/Sr ratio is as high as 90 in some samples. Initial 87 Sr/ 86 Sr ratios range from 0.708 to as high as 0.720. The three Rb-Sr isochrons defined by the data agree with K-Ar age determinations and show that the stocks were emplaced during the Late Cretaceous, between about 70 and 80 m.y. ago. The field, petrologic, and geochemical data indicate that the plutons had a multistage origin that involved large-scale melting of sialic crust, emplacement of magmas derived from batholithic fractionation at depth, and subsequent evolution of these magmas to generate small volumes of more highly evolved residual magmas. Although evolution of the granite complexes was largely governed by crystal-melt fractionation, some minor-element variations in the highly evolved granites cannot be explained by this process. For example, the distribution of rubidium and the light rare-earths appears to have been influenced by volatile depletion at the final stages of crystallization. The field data, petrologic data, and variation trends, such as distinct shifts toward higher albite contents in the residual granites, suggest that the coexistence of a volatile phase was important in their evolution. These results require that models seeking to explain compositional gradients in high-level granite (rhyolite) systems fully consider the role of a coexisting volatile phase.

Mesozoic plutonic belts of southern Alaska

Five calc-alkalic plutonic belts, developed mostly during Mesozoic time, are definable in southern Alaska. These belts may represent parts of magmatic arcs developed during episodes of subduction, and at least four appear to be temporally and spatially distinct. Age relations indicate that 10- to 30-m.y. intervals are sufficient for the generation and emplacement of the individual plutonic belts. Any overlapping sequence of tectonism, volcanism, and sedimentation that may have accompanied regional plutonism apparently ended within an interval of 20 to 55 m.y. The space-time relations of Mesozoic plutonism in southern Alaska suggest that more flexible evolutionary models and greater temporal resolution are needed to reconstruct the tectonic history of the region.

Tectonic controls on greenhouse gas flux to the Paleogene atmosphere from the Gulf of Alaska accretionary prism

The late Paleocene to early Eocene (ca. 61-56 Ma) was a period of long-term global warming, perhaps the warmest in the Cenozoic. Recent modeling suggests that methane loading of the atmosphere, and related development ofpolar stratospheric clouds, could have been an important forcing mechanism for this period of warm climate. The Gulf of Alaska accretionary prism contained ∼6 x 10 6 km 3 of siliciclastic sediments deposited in trench and slope settings along Alaskas Maastrichtian and Paleogene continental margin. These sediments underwent complex deformation, accretion, and unusual high heat flow soon after deposition. Accretion processes thermally overmatured the sediments during a time that overlaps the 61-56 Ma period of long-term global warming. Assuming a modest average organic carbon content of 0.3 wt% in these sediments, an estimated 8.35 x 10 1 5 kg of methane were generated in the accretionary prism over an ∼5 m.y. period. This methane was not effectively trapped, and migration pathways to the atmosphere were developed through complexly deformed and emergent continental borderlands. The Gulf of Alaska accretionary prism is a possible source of the atmospheric methane needed to force Paleocene and early Eocene global warming and an example of how tectonic processes can significantly recycle carbon from the geosphere.