Primitive high-K intraoceanic arc magmas of Eastern Kamchatka: Implications for Paleo-Pacific tectonics and magmatism in the Cretaceous

Tectonic reconstructions of the Cretaceous paleo-Pacific system show several significant tectonic and magmatic events, including the formation of several spreading centers, microplates and intra-oceanic arcs, and the initiation (>76–81 Ma) of magmatic activity which formed the Emperor-Hawaiian seamount chain. The Olyutorsky (syn. Achayvayam-Valaginsky, ~80 Ma) and Kronotsky (~90 Ma) intra-oceanic arcs were formed in the NW part of the paleo-Pacific and migrated northwest until their accretion onto the NW Asia margin in the Late Paleocene – Early Eocene and Late Eocene – Early Miocene, respectively. Tectonic remnants of these arcs are preserved as volcanic-sedimentary terranes in the accretionary basement of the Kamchatka Peninsula (Russia). Fragments of the Olyutorsky terrane (OT) are exposed in Eastern Kamchatka, including (from south to north) in the Valaginsky Range (VR), Tumrok Range (TR), and Koryak Highlands (KH). The OT volcanic rocks are characterized by unusually primitive mafic-ultramafic compositions, with low- and high-K characteristics in the north (KH) and high-K in the south (VR and TR), respectively. The primitive compositions of these olivine-phyric (picrites) and basalts provides an opportunity to characterize the magmatic sources of intra-oceanic arcs during the Cretaceous. High-Mg compositions in TR and VR (up to 40 wt% MgO) contain olivine phenocrysts with Fo content up to 94 mol% and high-Cr# spinel (up to 87 mol%), suggesting formation from an unevolved initial melt. Melt inclusions in spinel and olivine are Mg-rich (up to 15 wt% MgO) and have high K₂O contents (1.8–3.7 wt% for TR, and 2.3–3.9 wt% for VR). Picrite whole-rock compositions show strong HFSE depletion and LILE enrichment typical of modern arc rocks. However, LREE-Th enrichment is observed only in VR picrites, suggesting subduction-related modification is particularly strong in the source of magmas erupted in the southern OT. Isotopic compositions in OT picrites (⁸⁷Sr/⁸⁶Sr₈₀ 0.7026 to 0.7039, εNd₈₀ ± 10.6 to +8.4, εHf₈₀ + 20.6 to +16.7) resemble those in modern MORB (particularly Indian MORB) and arc picrites, suggesting that primitive OT arc magmatism originates in mantle wedge peridotite modified by melts and fluids derived from subducted oceanic crust without obvious contributions from continental and oceanic sediments. Compositional similarities between OT arc rocks and coeval early magmatism in the Emperor-Hawaiian melting anomaly suggests that the depleted component of the latter was an Indian-type MORB mantle in the paleo-Pacific region at the start of the Late Cretaceous. Longitudinal variations in OT magmas compositions (e.g. high-K signature, subtly higher ⁸⁷Sr/⁸⁶Sr-²⁰⁶Pb/²⁰⁴Pb, and lower εNd-εHf in VR and TR) may be related to subduction of thicker and possibly older lithosphere in the southern segment of the Olyutorsky arc. Steeper dip angle and stronger deformation/flexure of the subducting plate below this part of the arc could drive the percolation of low-degree alkaline melts from the asthenosphere through the slab, resulting in generation of the “petit-spot” volcanoes, similar to those in the modern Japan Trench. Further subduction of such alkali-volcanoes may produce the high-K geochemical signature recorded in the southern OT arc terranes.