The eclogitic terrane near Fairbanks, Alaska, consists of interlayered basic, calcmagnesian, quartzose, and pelitic schists, providing an opportunity to evaluate mineral parageneses in a diverse suite of high pressure metamorphic rocks. The terrane is interpreted to have equilibrated at 600 ± 25°C and 15 ± 2 kbars based on: 1) the jadeite content of omphacite coexisting with quartz + albite and 2) the pelitic assemblage garnet + chloritoid + staurolite + kyanite + quartz. The Ellis and Green (1979) calibration of K D -Fe/Mg of garnet/pyroxene also gives a 600°C temperature for basic schists where X Ca Ga = .30. However, the application of this geothermometer to calc-magnesian schists, where X Ca Ga ranges up to .47, gives temperatures that are much too high > (700°C). The K D values show some scatter, but do not systematically vary with the jadeite content of pyroxene in the range from 8 to 45%, or with the grossularite content of garnet from 25 to 47%. Temperatures derived from biotite-garnet K DS with the calibration of Ferry and Spear (1978), are also approximately 600°C. At such temperatures, glaucophane in the Fairbanks eclogites exceeds the maximum stability limit defined by the experimental studies of Maresch (1977). Comparison of the phase relations and inferred P-T of formation of the Fairbanks rocks with those from other high pressure terranes leads to the derivation of pressure-dependent equilibria that express the transition from amphibolites of the Sanbagawa belt to the substantially higher pressure kyanite eclogites of the Tauern Window. The Fair-banks eclogitic terrane is intermediate in this progression.

Blueschists of the Nome Group in the Seward Peninsula formed in Jurassic time (prior to ~160 Ma ago) in rocks of early Paleozoic to latest Precambrian age (approximately 360 to 720 Ma old). The Sr whole-rock isotopic signature on a plot of 87 Sr/ 86 Sr vs 87 Rb/ 86 Sr ratio—a fan shaped array of orthogneiss points lying between 720 and 360 Ma isochrons and paragneiss points showing a similar scatter and spread toward lower ages—is much like that of the Yukon Crystalline Complex and Cariboo-Omineca Belt in Canada; partial lithologic and historical similarity support the hypothesis of a common origin and tectonic setting marginal to Paleozoic North America. The areas were overridden during latest Triassic to Jurassic time by oceanic and exotic allochthons, and portions are studded with middle to Late Cretaceous plutons. At the same time all have experienced widespread resetting of K-Ar dates and regional uplift. The southern Brooks Range shares many characteristics with the Seward Peninsula—late Precambrian to mostly(?) Paleozoic protoliths, including extensive pelitic and metavolcanic schists, Jurassic (prior to ~120 Ma) blueschist development, and comparable tectonic setting. A late Precambrian metamorphic mineral isochron date for muscovite schist (686 ± 116 Ma) in the Baird Mountains Quadrangle confirms previous K-Ar dating of the same rock by Turner and others (1979). This may be a tectonic fragment of an older blueschist terrane enclosed in a younger blueschist complex, but this area needs further study.

In 1963–1964 I spent a year at the Geological Institute, University of Tokyo, as a National Science Foundation visiting science faculty fellow, sponsored by the late Professor Hisashi Kuno. During that year, as I traveled throughout the Japanese Islands, I was hosted by many resident Japanese geologists and geophysicists whose hospitality and enthusiasm I shall never forget. During the 1966 Pacific Science Conference in Tokyo, Japanese and American geoscientists had their first opportunity to meet several Soviet colleagues, including petrologists and geophysicists, who were doing research on the Kamchatkan volcanoes, and Siberian researchers from the Institute of Geology and Geophysics in Academegorodok, near Novosibirsk. These meetings provided an opportunity to discuss mutual interests and to plan reciprocal visits and cooperative research programs in our respective countries. In 1968 I was invited to visit the Institute of Geology and Geophysics in Academegorodok, hosted by V. S. Sobolev and N. V. Sobolev. The Novosibirsk conference and the field trip to Lake Baikal which followed were among the most exciting and rewarding experiences that I have ever known; I became increasingly aware of the bond that exists between all circumpolar peoples, including Siberians and Alaskans. When our new Geophysical Institute Building was nearing completion, we felt that we were entering a new era at the University of Alaska and that a special event was in order; we decided to invite our Soviet, Japanese, and Canadian friends to a symposium on the geology and geophysics of the Bering Sea region. Coincidentally, the 2d . . .

Eclogitic rocks occur in a restricted area, some 13 mi north of Fairbanks, as conformable bands and lenses intercalated with amphibolite, impure marble, and pelitic schist. The structural style of the eclogite-bearing terrane is characterized by northwest-trending, isoclinal recumbent folds that have been deformed by open or overturned folding along northeast-trending axes. Crystalline schist masses south of the eclogite-bearing terrane show only the northeast-trending folds and contain mineral assemblages of the greenschist facies. Mineral assemblages from the crystalline schists, which are intimately associated with the eclogitic rocks, are of the lower amphibolite facies. No basic igneous analogs were found for the eclogitic rocks, and their bulk compositions are very different from those reported for other eclogites. On the ACF plot, the calcite-rich variants appear to have been derived from marls, and the calcite-free varieties are compositionally similar to subgraywackes rather than mafic igneous rocks. The eclogitic rocks and associated amphibolites are characterized by the following essential mineral assemblages: (a) garnet-clinopyroxene (± calcite, quartz, sphene); (b) garnet-clinopyroxene-amphibole (± calcite, quartz, sphene); and (c) garnet-amphibole (± calcite, quartz, plagioclase, epidote, rutile). Mica and plagioclase feldspar also occur in some variants. The garnets in these eclogites are compositionally similar to those from “group C” eclogites as defined by Coleman and others (1965), and many plot within the field defined by the above authors for garnets from eclogites within blueschist terrane. The eclogitic clinopyroxenes are true omphacites, averaging 27 percent of the jadeite component. When shown on White’s (1964) diagram, these clinopyroxenes also plot in a field defined by other clinopyroxenes from group C eclogites. The group C affinity of these eclogitic rocks is also reinforced by the garnet-pyroxene tie-line relations in the MgO-CaO-FeO system, and even more conclusively by K D data on garnet-clinopyroxene pairs. Based on experimental data from various authors, and thermodynamic considerations, these eclogitic mineral assemblages were probably crystallized at temperatures of 540° to 590°C at 5.5 to 7.5 kb. K 40 /Ar 40 mica and hornblende dates indicate that earlier deformation of the eclogite-bearing terrane occurred in Paleozoic time, while the later event was of Cretaceous age.

Abstract The basic geologic framework of the Yukon-Tanana upland, Alaska, a mountainous region of about 30,000 sq mi (77,700 sq km) between the Yukon and Tanana Rivers, was delineated primarily by L. M. Prindle and J. B. Mertie, Jr., in the early part of this century. The subsequent recognition of large-scale offset along the Tintina fault, which bounds the eastern upland on the north, has required a reconsideration of the regional stratigraphic and structural relations. The northwestern part of the upland is predominantly underlain by a sedimentary sequence consisting of rocks which range in age from Cambrian to Mississippian. Cretaceous and Tertiary sedimentary rocks unconformably overlie the older sequence. The Cambrian is apparently underlain by a thick section of grits, quartzites, phyllites, and quartz-mica schists. Pre-Silurian volcanic rocks, mafic and ultramafic rocks of probably Devonian age, and Permo-Triassic diabase and volcanic rocks are also present. These sedimentary and igneous rocks are cut by granitic plutons of Cretaceous and Tertiary age. The central and eastern parts of the upland are underlain by a metamorphic complex of rocks which range from lower-greenschist to amphibolite facies. Fossils date the parent sediments of some greenschist-facies rocks as Paleozoic. Radiometric dates from several localities in the metamorphic complex indicate that Precambrian, Ordovician, and Jurassic-Cretaceous thermal events are recorded in the metamorphic history. Mesozoic granodiorite and quartz monzonite batholiths and smaller granitic plutons of Mesozoic and Tertiary age intrude the crystalline schists. Locally, unmetamorphosed Cretaceous and/or Tertiary sedimentary rocks are in unconformable or fault contact with the older rocks. Tertiary volcanic rocks ranging in composition from rhyolife to basalt overlie the older rocks in small but significant parts of the eastern upland. Ultramafic intrusions, mostly small and serpentinized, also occur. Work has progressed to the point where the sedimentary rocks in the upland can reasonably be correlated with those in other parts of Alaska, but interregional correlation of the metamorphic terranes must await additional clarification of structural and petrologie relations.