Abstract

Within the Amguema–Kanchalan volcanic field(AKVF) of the Okhotsk–Chukotka volcanic belt(OCVB), the Senonian–Paleogene contrasting seriescan be recognized to include the Paleogene bimodalsuite of the marginal–continental–rifting type [5, 6, 8].The relevance of this issue is enhanced by theimportance of theoretical aspects of the problem ofgenesis of acid magmatites, either structurally orgenetically related to basites. Acid alkaline rocks ofthis suite were considered earlier [6, 7] to be productsof strong differentiation of trachybasalt magmas orderivatives of lowdegree partial melting of the mantlesource common with trachybasalts. The intermediateand salic rocks of subalkaline series were supposed tohave been derived from mantle and crustal sourcesthrough the mechanism of parataxis. The new isotope–geochemical characteristics suggest the need forrevision of this point of view and for updating theunderstanding of the sources of alkaline–salic melts.The bimodal suite is associated with the belt areasresting on the crust with a thick granitemetamorphiclayer ([4] and others). It is dominated by trachybasalts,trachydacites, and trachyandesidacites. Trachyandesibasalts, trachyandesites, and trachyrhyolites, as well as pantellerites, comendites, and alkalinegranites, are less common. Manifestations of Paleogene sodium alkaline and subalkaline volcanoplutonism within the AKVF were primarily controlled bythe nearEW and NS extensional structures and werealmost synchronous with the opening of the “pullapart basins,” which hosted intensive trachybasalt volcanism. The succession of magmatic events has beenconfirmed by the results of isotope dating [7, 9].Basaltoids of the bimodal suite are mafictointermediate sodium trachybasalts (including highTi) andtrachyandesibasalts. The acid members of the bimodalsuite are represented within the AKVF by subalkaline(trachyandesidacite (trachyte)–trachydacite–trachyrhyolite) and alkaline (pantellerite–comendite–alkaline granite) rock associations. Previous studiesrevealed significant differences in the trace elementminerals and whole rock compositions of these rocks([5–7] and others).Paleogene subalkaline basites form shield and fissure volcanoes in the nearfault troughs and volcanicgrabens. The subalkaline rocks of more acidic composition, from trachyandesibasalts to trachyrhyolites, areassociated with centraltype structures on the flanks ofthe graben, such as stratovolcanoes and ring calderasdeveloped on the relics of shield volcanoes. Basaltsand trachybasalts occur locally in these structures.Comendites and alkaline granites within the AKVFmake up large volcanic and plutonic domes along thesouthern periphery of the Belye Uvali volcanic grabenand on the eastern flank of the Varenay volcano–tectonic trough ([5] and others). Pantellerites, formingsubvolcanic domes and dikes, occur in the nearfaulttroughs and volcanic grabens, within the relics ofbasaltic volcanoes.The geochemical comparison of alkaline acid rocksfrom the Paleogene contrasting series to the rocksfrom typical geodynamic settings showed them to bealmost analogous to acid members of the intraplate

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