Early-middle Triassic basic magmatism and metamorphism of ultramafic-mafic complexes of the Ust’-Belaya terrane (central Chukotka, NE Russia): ⁴⁰Ar/³⁹Ar ages, petrological and geochemical data, geodynamic interpretations

ABSTRACT

The Ust’-Belaya terrane (Chukotka, NE Russia), belonging to the West Koryak fold belt, is made of mantle and crustal ultramafic-mafic complexes which originated during several discrete episodes of magmatic activity in the Neoproterozoic, the late Neoproterozoic-Cambrian and the early-middle Triassic and accreted to the Asian continental margin in the early Cretaceous. This paper focuses on the latest magmatic episode expressed in intrusion of microgabbro dikes and nearly coeval metamorphism superimposed on both ancient complexes and the dikes. We present original ⁴⁰Ar/³⁹Ar ages, mineral and bulk-rock chemistry of the microgabbro dikes, metamorphosed dike and vein cutting ultramafic-mafic complexes. Dike microgabbros resemble differentiated arc-tholeiitic magmas originated in a subduction setting. Differentiated magmas intruded into much older spinel peridotites and dunites located at mid-crustal levels. Intrusion and crystallization of these magmas was followed by down-going movement of spinel peridotites and rocks of the mantle-crust transition zone towards a mantle wedge where they were metamorphosed at high-P conditions. This metamorphism resulted in transformation of microgabbro to garnet amphibolite, diorite to albite-zoisite-paragonite-pargasite rock and spinel peridotites to metaperidotites. P-T parameters of this metamorphism reconstructed based on mineral assemblage of garnet amphibolite correspond to those of epidote amphibolite – amphibolite – amphibole eclogite facies transition. The peculiar zoisite (clinozoisite)-paragonite mineral assemblage typical of metamorphosed vein rock indicates high-P metamorphic conditions of epidote-amphibolite facies.

KEYWORDS:

• ⁴⁰Ar/³⁹Ar geochronology
• microgabbro
• garnet amphibolite
• arc tholeiite
• high-P metamorphism
• subduction
• early-middle Triassic
• Chukotka

Acknowledgments

The authors are thankful to O. Kuzmina for preparation of whole rocks to chemical investigations, T.G. Kuz’mina and T.V. Romashova for performing XRF analyses, and Ya. V. Bychkova for performing ICP-MS analyses.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplemental Material

Supplementary material can be accessed here.

Additional information

Funding

This study was supported by the Russian Science Foundation under [Grant number 16-17-10251] and the basic state budget under the theme number 0135-2012-0012 (the Geological Institute, RAS) and under the theme number 17 (the V.I. Vernadsky Institute, RAS).