Cretaceous Monzonite-Granite-Migmatite Velitkenay Complex: Petrology, Geochemistry of Rocks and Zircons (U-Pb, Hf, O) as Applied to Reconstructing the Evolution of Magmatism and Continental Crust in Artic Alaska-Chukotka Block

The Velitkenay monzonite-granite-migmatite massif is a granite-core gneiss dome on the Arctic coast of Chukotka and a key outcrop for understanding mid-Cretaceous magmatism and metamorphism of the Arctic Alaska-Chukotka terrane. The petrology and geologic history of the complex were investigated using whole rock major and trace element and isotopic analyses (Sr, Nd and Pb), as well as electron microprobe thermobarometry, zircon U-Pb geochronology, trace element geochemistry, and O and Lu-Hf isotopic methods. The massif consists of two distinctive Albian plutonic suites. Deformed 106-103 Ma monzonitoids represent the early phase of plutonism; they were melted from a relatively mature crustal source with the bulk composition epsilon Nd(i) from -5.5 to -7.9; T-Nd (DM-2st) = 1.4-1.6 Ga, and zircon composition epsilon Hf(i) from -11 to -7, delta O-18 from 10 to 8.4; and they evolved via assimilation and fractional crystallization processes. Late phase 102-101 Ma leucogranites (epsilon Nd(i) from -3.8 to -6.7, T-Nd (DM-2st) = 1.2-1.4 Ga) formed from melting of Neoproterozoic orthogneisses with mantlelike zircon (epsilon Hf(i) from +11 to +13, delta O-18 similar to 5.8) and differ from early phase monzonitoids by the systematic presence of inherited Neoproterozoic (660-600 Ma) zircon xenocrysts. Migmatized Neoproterozoic orthogneisses are exposed in the central part of the dome, whereas the country rock on the flanks of the dome are paragenisses and schists with Devonian protolith ages. The intrusion of monzonitoid magmas was syntectonic with early stages of exhumation of the Velitkenay massif, whereas the leucogranite phase of magma intruded after peak metamorphism and does not exhibit ductile deformation. Based on subhorizontal mineral stretching lineations along the flanks of the dome and overall sigmoidal plan view of the massif, structural doming appears related to localized transtension in a more regional dextral strike-slip geodynamic environment. U-Pb isotope-geochronological data make it possible to distinguish seven episodes of granitoid magmatism, of which three major, in terms of volume (Aptian Bilibino, Albian Chaun, and Turonian-Coniacian Okhotsk-Chukotka granitoid magmatism subprovince), reflect the maximum rates of growth and modification of the Chukotka crust. Subordinate, in terms of volume, magmatic events of the Neoproterozoic, Devonian, Permian-Triassic, Late Jurassic, and Valanginian-Hauterivian have also been reconstructed.