Metamorphic and structural history of continental crust at a Mesozoic collisional margin, the Ruby terrane, central Alaska

Abstract

The Ruby terrane is an elongate fragment of continental crustal rocks that is structurally overlain by thrust slices of oceanic crust. Our results from the Kokrines Hills, in the southcentral part of the Ruby terrane, demonstrate that the low-angle schistose fabric formed under high-P /low-T conditions, at peak conditions of 10.8-13.2 kbar and 425-550" C, consistent with the rare Occurrence of glaucophane. White mica 40Ar/39Ar cooling ages from these blueschists indicate that the metamorphism occurred prior to 144 f 1 Ma. The blueschist facies assemblages are partially replaced by greenschist facies assemblages in the eastern Kokrines Hills. In contrast, in the central and western Kokrines Hills, upper amphibolite to lower granulite facies metamorphism associated with extensive late Early Cretaceous plutonism has completely overprinted any evidence of an earlier high-PIT metamorphic history. Deformation accompanying the plutonism produced recumbent isoclinal folds in the plutonic rocks and pelitic gneisses of the wallrock; decompression reactions in the pelitic gneisses suggest that the deformation Occurred during exhumation. Thermochronological data bracket the time of intrusion and cooling below 500" C between 118 f 3 and 109 f 1 Ma.Our data from the schists of the Ruby terrane support the general assumption of many authors that the Ruby terrane was subducted beneath an oceanic island arc. This tectonic history is similar t o that described for other large continental crustal blocks in northern and central Alaska, in the Brooks Range, Seward Peninsula and Yukon-Tanana Upland. The current orientation of the Ruby terrane at an oblique angle t o these other crustal blocks and to the Cordilleran trend is due to post-collisional tectonic processes that have greatly modified the original continental margin.

Provenance, U-Pb detrital zircon geochronology, Hf isotopic analyses, and Cr-spinel geochemistry of the northeast Yukon-Koyukuk Basin: Implications for interior basin development and sedimentation in Alaska

“…The major difference between the two regions is the 784 presence of late Early Cretaceous (118-109 Ma; Roeske et al, 1995;Roeske et al, 1998; 785 this study) granites and granodiorites that intrude older rocks of the Ruby terrane. The 786 composition of Type 2 sediments combined with their detrital zircon U-Pb ages suggest 787 they were likely derived from both the southern Brooks Range and Ruby terrane.…”

“…to Paleozoic 295 metasedimentary rocks and minor metabasite and quartzofeldspathic gneiss (Dover, 296 1994;Moore and Box, 2016). Thermobarometry by Roeske et al (1995) identified an 297 14 overprinted by greenschist facies assemblages. In the western Kokrines Hills portion of 299 the Ruby terrane, metamorphic assemblages document a later upper amphibolite to lower 300 granulite facies event that likely overprinted the earlier HP metamorphic fabric in the late 301…”

“…Early Cetaceous (~110 Ma; Roeske et al, 1995). These HP and HT metamorphic suites 302 are structurally overlain in fault contact by Devonian slates and phyllites of the slate and 303 phyllite belt and in turn structurally overlain by fault-bound slivers of the Devonian to 304…”

“…Cretaceous time (Moore et al, 1994(Moore et al, , 2015Roeske et al, 1995). This led to the thrust 616 emplacement of oceanic-and possible arc-affinity thrust sheets that make up the highest 617 structural allochthons of the Brookian orogen.…”

Blueschists and Eclogites of the World and Their Exhumation

“…However, the Taylor Highway Map area in Alaska near the Canadian border shows much older 40 Ar/ 39 Ar dates-187 to 213 Ma for hornblende, 109 to 204 Ma for biotite, and 109 to 191 Ma for muscovite (Cushing, 1984;DuselBacon et al, 1989). Dusel-Bacon et al (1995) speculated that underthrusting of the North American continent beneath the Nistling microcontinent formed the A-type blueschist belt, and regarded 166 to 204 Ma 40 Ar/ 39 Ar ages to be cooling ages. The estimated P/T conditions range from 10 kbar, 450° C to 17 kbar, 700° C for high-temperature blueschist and eclogite (Hansen et al, 1991;Hansen, 1992), which clearly belong to the jadeite-glaucophane-type facies series, as documented by the low-temperature assemblages (Paterson and Harakal, 1974).…”