New data on the composition of island-arc volcanic rocks in the Eastern mountain range, Kamchatka

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Neotectonic uplifts of the Vostochnyi (Eastern) mountain range of Kamchatka are composed of Cretaceous‐Paleogene complexes with fold-nappe and imbricate structures that represent fragments of the southern Ozernov‐Valagin segment of the Achaivayam‐Valagin paleoarc [1]. The composition of island-arc volcanic rocks developed within the uplifts has been studied to a variable extent. Among the best studied are the volcanic rocks of the Ozernoi Peninsula and Tumrok Ridge [2‐4]. We obtained new data on the composition of the least studied volcanic rocks of the Kumroch Ridge, as well as on the Tumrok and Valagin ridges, which allowed us to refine the geodynamic conditions of the volcanic sequences of this segment of the paleoarc. The sampling localities of the studied volcanic rocks are shown in Fig. 1 and Table 1. In the Kumroch Ridge, the Late Campanian‐Danian island-arc rocks compose the tectonic nappes of an autochthon (rocks of the Khapitsk sequence) overlain by a serpentinite melange nappe. The nappe includes blocks of Albian‐Cenomanian and Santonian‐Early Campanian siliceous rocks, basalts, diabases, gabbros, and serpentinized peridotites. Volcanic rocks of the Khapitsk sequence are mainly composed of clinopyroxene‐plagioclase porphyritic basalts and basaltic andesites. In the P 2 O 5 ‐TiO 2 diagram [5], these volcanic rocks are plotted in the island-arc rock field. In terms of REE distribution patterns (Fig. 2), the studied samples are ascribed to the calc-alkaline and tholeiitic series. The location of volcanic rocks in the section suggests the simultaneous formation of these two rock groups. In the REE patterns (Fig. 2), the samples of calc-alkaline series demonstrate LREE enrichment relative to HREE, while LREE and MREE contents in tholeiitic rocks show a similar (almost chondrite-type) pattern, with an insignificantly elevated LREE content. The spidergrams of the studied volcanics have island-arc signatures: deep negative anomalies of HFSE (Nb, Ta, Zr, Ti) and REE enrichment relative to LILE (Ba, U, Rb) (Fig. 3). The calc-alkaline rocks have higher contents of Nb, Ta, Ba, U, and Rb. The two rock groups also differ in bulk composition. Relative to the tholeiitic basalts, the calc-alkaline rocks have a more potassic composition. In general, they are depleted in Ti and enriched in P (Table 1).