Cenozoic volcanism of the North-Eastern Asia

Fedorov P.I.*, Koloskov A.V.**

* Geological Institute of RAS, Moscow, Russia; ** Institute of Volcanology and Seismology of Far Eastern Branch RAS, Petropavlovsk-Kamchatsky, Russia

1. At the northeast edge of Asia (Penzhina-Anadyr-Koryak and Eastern Chukotka regions) several stages of Cenozoic magmatic activity are identifiable, linked to various geodynamic regimes.

The Maastrichtian-Mid-Eocene stage consists of high-Ti, low- and moderate-K tholeiites and alkaline olivine basalts erupted in pull-apart systems and are connected with the most early rifting stage [Fedorov, Filatova, 1999, Fedorov, Koloskov, 1999]. Basalts are laterally heterogeneous. The lateral zoning of rocks is expressed in the replacement of K–Na compositions by potassic subalkaline and alkaline rocks going from southeast to northwest, i.e., toward the continent. The tholeiites have high Ti, Hf and low Ta and Nb concentrations. They are characterized by fractionated REE distribution (La_n/Yb_n=1,3-8,7). The isotope ratios of strontium, neodymium and lead are low (⁸⁷Sr/⁸⁶Sr=0,702930-0,704215; εNd=3,8-5,7; ²⁰⁶Pb/²⁰⁴Pb=18,167-18,340; ²⁰⁷Pb/²⁰⁴Pb=15, 462-15,531; ²⁰⁸Pb/²⁰⁴Pb=37,798-38,073; D8/4Pb=20-28). The alkaline olivine basalts are enriched in titanium (2,0-2,4% TiO₂), and are characterized by a strong LREE enrichment and a fractionated REE spectrum (La_n/Sm_n=1,9-3,9; La_n/Yb_n=6,5-19,4).

The Eocene-Oligocene stage is linked to the formation of the Koryak sector of the West Kamchatka-Koryak volcanic-plutonic belt. The belt is made up of rocks of a continuously differentiated andesite-basalt-rhyolite, moderately potassic calcalkaline series and comagmatic intrusive granitoids. The rocks are characterized by a low titanium content (TiO₂=0,1-1,2%), high aluminum, and a strong Ta-Nb minimum. The ⁸⁷Sr/⁸⁶Sr and εNd ratios have strong lateral variability (from 0,70447-0,70709 and 0,9-2,9 in rocks of the central Koryak highland to 0,70275-0,70375 and 8,9-11,2 in rocks of the northern portion of the Koryak highland and the Anadyr basin).

The Miocene areals of the Koryak highland are composed of lava flows of tholeiitic basalts, extrusives and dacite dikes, typical of radiative continental margin rift formation. The basalts are high in titanium (TiO₂=1,3-1,9%), with a highly fractionated distribution of REE (La_n/Sm_n=1,9-3,9;š La_n/Yb_n=3,8-11,8) and a clear, negative Ta-Nb anomaly. The isotope ratios of strontium, neodymium and lead are moderately depleted (⁸⁷Sr/⁸⁶Sr=0,703289-0,703669; εNd=3,8-6,0; ²⁰⁶Pb/²⁰⁴Pb=18,20-18,42; ²⁰⁷Pb/²⁰⁴Pb=15,495-15, 524; ²⁰⁸Pb/²⁰⁴Pb=37,877-38,059; D8/4Pb=16-30).

The late Miocene-early Pliocene melanephelinites of the eastern Chukotka Peninsula are characterized by high titanium and potassium contents (TiO₂=2,6-5%; 2,3-4% K₂O with Na₂O/K₂O=0,8-2,1), strongly fractionated REE (La_n/Sm_n=6; La_n/Yb_n=35), moderately low isotope ratios of strontium (⁸⁷Sr/⁸⁶Sr=0,7031080-0,7032970) and lead (²⁰⁶Pb/²⁰⁴Pb=18,32-18,43; ²⁰⁷Pb/²⁰⁴Pb=15,474-15,509; ²⁰⁸Pb/²⁰⁴Pb= 38,03-38,19; D8/4Pb=23-29) with high neodymium ratios (εNd=7,3-7,9) [Apt et al., 1998]. šIn the northern Priokhotsk area, mid- to late-Miocene melanephelinites are distinguished by higher concentrations of Ta and Nb and a less fractionated REE distribution (La_n/Sm_n=3,5; La_n/Yb_n=17,8) with more depleted isotope ratios of strontium, neodymium and lead (⁸⁷Sr/⁸⁶Sr=0,703036-0,703079; ¹⁴³Nd/¹⁴⁴Nd=0,513052-0,513054; εNd=8,1; ²⁰⁶Pb/²⁰⁴Pb=17,907-17,925; ²⁰⁷Pb/²⁰⁴Pb=15,42-15,47; ²⁰⁸Pb/²⁰⁴Pb=37,505-37,693; D8/4Pb=34-40) [Apt et al., 1998].

The Quaternary stage of volcanism consists of individual lava flows, small cinder cones, necks and dikes making up the Cape Navarin areal, and also the central-type structures, cinder cones and lava flows of the Aluchin and Anuysky volcanoes. The alkaline olivine basalts and basanites of Cape Navarin have high high field strength elements and large-ion lithophilic elements and a strongly fractionated REE distribution (La_n/Sm_n=2,9-4,0; La_n/Yb_n=16-21). The isotope ratios of strontium ⁸⁷Sr/⁸⁶Sr (0,703420-0,703918) and neodymium (εNd=4,5-6,7) are controlled by the mixing in of PREMA and BSE components. The isotopic composition of Pb of these rocks is slightly enriched over chondritic (²⁰⁶Pb/²⁰⁴Pb= 18,29-18,39; ²⁰⁷Pb/²⁰⁴Pb=15,487-15,543; ²⁰⁸Pb/²⁰⁴Pb=38,099-38,256 É D8/4Pb=30-40) [Fedorov et al., 2005]. The subalkaline basalts of the Anuisky volcano and the Monni River lava flow are distinguished by high concentrations of titanium (TiO₂=1,5-1,6%), low aluminum (Al₂O₃=13,7-15,1%), high Nb and Ta, and a fractionated REE distribution (La_n/Sm_n=1,6-2,2; La_n/Yb_n=6,7-9,6). However, the basalts of the Monni River lava flow have lower ⁸⁷Sr/⁸⁶Sr ratios (0,702926) and higher ¹⁴³Nd/¹⁴⁴Nd (εNd=8,7), compared to the basalts of the cone of the Anuysky volcano (0,703769-0,704485 and 3,7-5,4 respectively).

2. The data presented on the composition of the Cenozoic volcanic complexes of the Penzhin-Anadyr-Koryak and Eastern Chukotka regions show the lateral heterogeneity of the volcanism and the change in the types of the volcanic sources over the course of the Cenozoic. Most of these basalts are notable for their compositions which simultaneously contain depleted, intraplate and subduction-zone characteristics [Fedorov, Filatova, 1999].

3. Study of mantle reservoirs for the late-Cenozoic volcanic rocks of the Bering Sea region using Sr-Nd-Pb isotope systematics (Zou et al., 2000) and the ratios ⁸⁷Sr/⁸⁶Sr–²⁰⁶Pb/²⁰⁴Pb allows identificdation of three relatively separate trends, linked at the baseline by comparable values of ⁸⁷Sr/⁸⁶Sr=0,70275-0,70295. This baseline may be thought of as evidence for a common asthenospheric source. The lead isotope ratios define two sets of points, separated by ²⁰⁶Pb/²⁰⁴Pb @ 18,5 and reflecting the influence of two compositionally heterogeneous lithospheric blocks of different ages: the Asiatic and Alaskan [Koloskov, Fedorov, 2008]. Evidence for a primary role of decompression melting of the Pacific asthenospheric mantle in this volcanism is given in a number of studies [Portnyagin et al., 2005, Flower et al., 1998, Koloskov, 2008].

References:

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Portnyagin M., Hoernle K., Avdeiko G. et al. Transition from arc to oceanic magmatism at the Kamchatka-Aleutian junction // Geology. 2005. Vol. 33. P. 25-28.