Journal of Metamorphic GeologyVolume 13, Issue 1 p. 1-8 Cordilleran high-pressure metamorphic terranes: progress and problems B. E. PATRICK, B. E. PATRICK Department of Geological Sciences, University of California, Santa Barbara, CA 93106, USASearch for more papers by this authorH. W. DAY, H. W. DAY Department of Geological Sciences, University of California, Davis, CA 95616, USASearch for more papers by this author B. E. PATRICK, B. E. PATRICK Department of Geological Sciences, University of California, Santa Barbara, CA 93106, USASearch for more papers by this authorH. W. DAY, H. W. DAY Department of Geological Sciences, University of California, Davis, CA 95616, USASearch for more papers by this author First published: January 1995 https://doi.org/10.1111/j.1525-1314.1995.tb00201.xCitations: 13AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat REFERENCES Archibald, D. A., Garver, J. G. & Schiarizza, P., 1991. Ar-Ar dating of blueschist from the Bridge River Complex, SW British Columbia and its tectonic implications. Geological Society of America Abstracts with Programs, 23, A136. 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New U-Pb zircon age determinations of 676 +/- 15 Ma anti 681 +/- 3 Ma (upper intercept ages) for two granitic orthogneiss bodies on Seward Peninsula confirm a minimum age of Late Proterozoic for the lower Nome Group, a continental margin sequence in northwestern Alaska. Discordance of U-Pb data is attributed to Pb loss (lower intercepts of 117 +/- 17 Ma and 124 +/- 6 Ma) induced by high-pressure, low-temperature metamorphism during the Jurassic-Cretaceous Brookian orogeny. Age correlation of the Seward Peninsula bodies with Late Proterozoic orthogneiss bodies in the Brooks Range and eastern Siberia (Chukotka) provides an early paleogeographic link between these regions and indicates the existence of a Late Proterozoic magmatic belt in Arctic Alaska-Chukotka. This magmatic belt is temporally distinct from known Late Proterozoic magmatism in the North American Cordillera and Canadian Arctic Islands, but it is strikingly similar in age to pre-Caledonian magmatism in eastern North America and in Svalbard. We suggest that the pre-Cretaceous Arctic Alaska-Chukotka crustal fragment restores to the Barentian continental margin and is thus exotic to western North America. Our reconstruction supports models calling for a Canadian Arctic Islands transform margin (rather than rotational models) during Cretaceous opening of the Canada Basin.

A reaction producing jadeitic pyroxene in metagreywackes of the northern Diablo Range has been identified on the basis of mineral distribution, isograd patterns and composition of coexisting minerals. The appearance of jadeitic pyroxene (∼Jd80) is closely followed by the disappearance of pumpellyite, which indicates that pumpellyite plays a major role in the pyroxene‐producing reaction. A new projection from hematite, lawsonite, chlorite, quartz and H2O on to the NaAlO2‐FeO‐MgO ternary confirms the role of pumpellyite in pyroxene production and suggests a reaction of the form: 1.00 pumpellyite + 0.31 chlorite + 8.71 albite + 0.70 hematite + 2.00 H2O = 8.54 jadeite + 0.57 glaucophane + 3.09 lawsonite + 5.26 quartz. Metagreywackes of the northern Diablo Range were metamorphosed under conditions of PH2O=Ptotal at 200‐300 °C and 7.5‐10.0 kbar. Despite the low temperatures attained during metamorphism, the assumption of equilibrium yields results consistent with field observations and phase relations.

Blucschists and greenschists of the central Seward Peninsula (the Nome Group) crop out over an 8000 km2 area. The protolith package for the mid-Jurassic high P/T metamorphism consisted largely of Cambrian, possibly also Precambrian, to Devonian sedimentary rocks (limestones, marls, pelites, carbonaceous siltites, quartzites) of shallow water miogeoclinal origin, which maintained a coherent lithostratigraphy during metamorphism and concurrent high-strain, noncoaxial deformation. Several experimentally and empirically calibrated geothermobarometers have been integrated with textural and microstructural observations in order to derive a pressure-temperature (P-T) path for the metamorphism of the Nome Group. These data, coupled with previously obtained Rb-Sr phengite-whole-rock isochron ages, yield a P-T-t path showing a steep burial segment followed by a prolonged period (˜55 Ma) of isothermal decompression. Peak pressures, obtained from the jadeite content of clinopyroxene in rare, isofacial eclogites, and from the celadonite content in phengite yield ˜12 kb. Temperatures at this pressure, gained from garnet-clinopyroxene and microcline-plagioclase pairs, were 460±30°C. Decompression temperatures were essentially the same, as evidenced by calcite-dolomite and garnet-biotite thermometry. Uniformity of metamorphic conditions throughout the Seward Peninsula blueschist terrane coupled with the miogeoclinal nature of the protolith package indicate burial beneath a flat-lying (A-type) subduction zone. The derived P-T-t path is difficult to reconcile with one-dimensional thermal models of collisional blueschists, where peak P and T are not generally coincident.

Blueschist-facies rocks of the central Seward Peninsula cropout over 8000 km2. Protoliths were Lower Paleozoic-Precambrian(?) shallow-water miogeoclinal sediments that were metamorphosed during the Middle Jurassic. Thermobarometric estimates yield ‘peak’ metamorphic conditions of 10–12 kbar at 460 ± 30°C. Crystallization of blueschist-facies minerals was synkinematic with development of a transposition foliation. This foliation is parallel to lithologic contacts and is axial planar to recumbent mesoscopic isoclinal folds. These folds are refolded by larger scale recumbent tight to isoclinal folds. Both fold sets have hinges parallel to a well-developed N—S stretching lineation. Sheath folds are also present. The long axes of the sheath folds also parallel the stretching lineation. This deformation was non-coaxial as indicated by microstructures and quartz c-axis fabrics. Folds nucleated, then rotated into parallelism with the stretching direction. Kinematic indicators show unequivocal top-to-the-north shear sense, compatible with blueschist formation during mid-Jurassic collision between the Brooks Range continental margin and a N-facing island arc (Yukon-Koyukuk). Convergence of these two plates is believed to have been nearly N—S (in present co-ordinates).

ABsrRAcr Orthogneisses of granite, granodiorite and tonalite composition, in the lorr-grade schist terrane of the Seward Peninsula, Alaska, underwent structurally coherent, progressive regional blueschist-facies metamorphism in tle mid-to-late Jurassic, followed by partial re