A Geologic Reconnaissance of the Senyavin Metamorphic Complex, Chukotka Peninsula, Northeast Russia

Phillip B. Gans1, Andrew T. Calvert1,
William S. Dinklage1, and V V Akinin2

1Department of Geological Sciences, UCSB, Santa Barbara, CA 93106
2 NEISRI, Magadan, Russia

The origin of sillimanite-grade "gneiss domes" flanked by lower grade Paleozoic and Mesozoic rocks remains a crucial problem in the Bering Strait region. Important questions posed by these metamorphic complexes include their protolith age(s), the timing of peak metamorphism and exhumation, the tectonic significance of pervasive L-S tectonite fabrics, and the nature of the contacts between high- and low-grade rocks. In 1995, we conducted a geologic reconnaissance of the Senyavin uplift, an upper greenschist to amphibolite facies metamorphic terrane situated ~ 30 km north of Providenya on the southern Chukotka Peninsula. This metamorphic complex, interpreted to be Archean basement by previous investigators, is flanked by supracrustal Paleozoic marine deposits and late Cretaceous volcanic rocks.

Metamorphic rocks include marble, banded calcsilicate, calc-schists, pelitic schist, amphibolite, and rare lenses of ultramafic rocks. The amphibolite and associated graphitic schist overlie a predominantly carbonate section, in places, along a conspicuous low-angle, thrust(?) or normal(?) fault with top-to-the-north displacement. A pervasive high strain synmetamorphic fabric is characterized by a well developed, mainly SE-dipping foliation and mainly NE-trending stretching lineation. Metamorphic grade in pelites increases from NW to SE, from chloritoid-gn-bio to ky-stt-gn to finally sil-gn-bio-kspar(?).The metamorphic rocks are depositionally overlain by a volcanic and volcaniclastic succession that includes, in ascending order, andesitic breccias, an extensive dacite ignimbrite, andesite lavas, breccias, and intercalated waterlain tuffs, and finally rhyolitic lavas and tuffs. Subvolcanic rhyolite and dacite porphyry dikes and stocks are ubiquitous in both the volcanic and metamorphic successions and produced extensive hydrothermal alteration. The volcanic rocks and underlying metamorphic basement are cut by a number of N-S trending, W-dipping normal faults with up to 1 km of displacement.Thus, field relations indicate that the metamorphic terrane was unroofed prior to deposition of the Late(?) Cretaceous volcanic rocks. Geochronologic work in progress will better constrain the age of metamorphism and exhumation as well as the precise ages of the volcanic and plutonic rocks in this region.

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