Archived project

Evolution of volcanism in the back-arc part of the contemporary subduction system: Sredinny Range of Kamchatka in Miocene-Quaternary times

Goal: Предлагаемый проект нацелен на изучение условий магмогенерации в тыловой зоне современной островодужной системы на примере южной части Срединного хребта (СХ) Камчатки в неоген-четвертичное время. Планируется изучение эволюции вещественного состава вулканитов, определение причин магмообразования и источников вещества расплавов, выявление особенностей магматической активности и истории геологического развития в пределах южной части СХ, и сравнение новых данных с результатами, полученными ранее по северной части хребта, Восточному вулканическому фронту, Центральной Камчатской депрессии и другим островодужным системам. Для выполнения поставленных задач будет использован комплекс геолого-геоморфологических, петролого-минералогических, геохимических, термобарогеохимических, математических, изотопно-геохронологических методов. Работы по СХ ведутся нашей рабочей группой с 2000 г. За это время собрана представительная коллекция вулканитов миоцен-четвертичного возраста. Она проанализирована методом XRF и частично ICP. В рамках предлагаемого проекта планируется проведение аналитических исследований (ICP, изотопия Sr и Nd) ранее собранного материала. Кроме того запланированы полевые работы для пополнения и дальнейшего изучения коллекции на вулканах Большая и Малая Кетепана, Большой и Малый Чекчебонай, а также в Быстринском хребте. Для наиболее крупных вулканических массивов будет проводиться изотопное датирование лав, что позволит рассмотреть эволюцию вулканизма как во времени, так и в пространстве. В результате выполнения проекта будет предложена петрологическая модель, объясняющая особенности эволюции магматизма в пределах южной части Срединного хребта в неоген-четвертичное время и прогнозирующая возможные пути дальнейшего развития системы.

Date: 1 January 2017 - 31 December 2019

Updates
0 new
0
Recommendations
0 new
0
Followers
0 new
8
Reads
0 new
56

Project log

V. A. Lebedev
added a project goal
Предлагаемый проект нацелен на изучение условий магмогенерации в тыловой зоне современной островодужной системы на примере южной части Срединного хребта (СХ) Камчатки в неоген-четвертичное время. Планируется изучение эволюции вещественного состава вулканитов, определение причин магмообразования и источников вещества расплавов, выявление особенностей магматической активности и истории геологического развития в пределах южной части СХ, и сравнение новых данных с результатами, полученными ранее по северной части хребта, Восточному вулканическому фронту, Центральной Камчатской депрессии и другим островодужным системам. Для выполнения поставленных задач будет использован комплекс геолого-геоморфологических, петролого-минералогических, геохимических, термобарогеохимических, математических, изотопно-геохронологических методов. Работы по СХ ведутся нашей рабочей группой с 2000 г. За это время собрана представительная коллекция вулканитов миоцен-четвертичного возраста. Она проанализирована методом XRF и частично ICP. В рамках предлагаемого проекта планируется проведение аналитических исследований (ICP, изотопия Sr и Nd) ранее собранного материала. Кроме того запланированы полевые работы для пополнения и дальнейшего изучения коллекции на вулканах Большая и Малая Кетепана, Большой и Малый Чекчебонай, а также в Быстринском хребте. Для наиболее крупных вулканических массивов будет проводиться изотопное датирование лав, что позволит рассмотреть эволюцию вулканизма как во времени, так и в пространстве. В результате выполнения проекта будет предложена петрологическая модель, объясняющая особенности эволюции магматизма в пределах южной части Срединного хребта в неоген-четвертичное время и прогнозирующая возможные пути дальнейшего развития системы.
 
V. A. Lebedev
added a research item
We report the chemical and isotopic compositions of volcanic rocks of the Akhtang and Kostina mountain massifs in the Sredinny Range, Kamchatka. The analyzed rocks are similar in composition to the earlier studied volcanics of the eastern flank of the southern part of the Sredinny Range. Results of K–Ar isotope dating reveal three stages of volcanic activity in the two massifs. These stages are divided by long (1.4 and 2.4 Ma) periods of quiescence. In the Akhtang massif, the eruptive activity was at 4.9–4.0, 1.9–1.7, and 0.3–0.2 Ma, and in the Mt. Kostina massif, at ~8.0, 5.6–4.9, and ~3.5 Ma. Two early stages of both massifs are characterized by the eruption of island arc type rocks, and the late stage, by the eruption of rocks of hybrid geochemical type. The Mio-Pliocene (N1–N2 1) rocks of the Mt. Kostina massif are similar in geochemical features to the early Pliocene (N2-1) rocks of the Akhtang massif, and the late Pliocene (N2-2) lavas of the former massif are similar to the middle Quaternary (Q2) rocks of the superimposed monogenetic volcanism zone of the latter massif. For the Akhtang massif it has been first discovered that the volcanic reactivation after the long quiescence periods was accompanied by a change in the composition of rocks and in the type of eruptive activity (from the eruption of plateau-effusives rocks to the formation of stratovolcanoes and monogenetic volcanism zones). The obtained data on the age and composition of rocks as well as some morphological features of the studied massifs suggest that the plateau-effusive rocks of the Sredinny Range might be related to central-type eruptions.
V. A. Lebedev
added a research item
A first set of K–Ar isotopic ages obtained, which allowed to estimate the age of the largest volcanoes of the Anaunsky Dol (3.2, 2.2 and 1.9 Ma) and eruptive centers of the post-caldera stage of Uksichan massif (1.2 and 0.8 Ma), located in the Sredinny Range of Kamchatka. Significant time intervals separating the individual phases of volcanism suggest either repeated activation of the fault zone to which the dated eruptive centers are confined, or the presence of several fault zones here.
Anna O. Volynets
added a research item
Based on the geochemical characteristics of the Miocene-Quaternary volcanic rocks of the Sredinny Range of Kamchatka, we divide it into northern and southern provinces; the latter comprises the “eastern”, “western”, and “central” flanks. We present new data on the composition of Neogene-Quaternary volcanic rocks in the southern part of the Sredinny Range of Kamchatka: Khangar and Icha volcanic massifs and Mt. Yurtinaya on the “western” flank, Bystrinsky and Kozyrevsky Ridges on the “eastern” flank, and Anaunsky Dol and Uksichan massif located in between. We show systematic differences in the composition of rocks from the “western” and “eastern” flanks. During the Neogene, a typical island-arc volcanism took place within the “eastern” flank. Quaternary volcanic rocks of this area have both island-arc and within-plate geochemical features. We propose to call rocks of this type hybrid rocks. Within the “western” flank, hybrid volcanism has been manifested since the Neogene, while typical island-arc rocks are not found. Magma generation processes on the “western” flank of the Sredinny Ridge are influenced by an enriched mantle source; the effect of fluid is less pronounced here as compared to the rocks of the “eastern” flank, where it is clearly traced.
Anna O. Volynets
added 2 research items
Получены первые результаты изучения химического состава продуктов извержений вулканических массивов Большой Чекчебонай и Большая Кетепана, расположенных в южной части Срединного хребта Камчатки.
V. A. Lebedev
added 2 research items
Стратовулкан Ичинский (55o68'N, 157o73'E, 3607 м) – крупнейшая вершина Срединного хребта Камчатки – подстилается эффузивами гор Лаучан, Палец и др. (далее хребет Лаучан). До сих пор было не известно, когда этот вулканический массив начал формироваться. Мы провели изучение и K-Ar датирование ранних порций лав стратовулкана и финальных лав хребта Лаучан.
Изучен химический состав продуктов извержений вулканических массивов гор Ахтанг и Костина, расположенных на юго-восточном фланге южной части Срединного хребта Камчатки. По результатам K-Ar изотопного датирования для двух массивов впервые выявлены три этапа активизации вулканизма, разделенные продолжительными (1.4 и 2.4 млн лет) периодами покоя. В массиве г. Ахтанг эруптивная активность зафиксирована 4.9 – 4.0, 1.9 – 1.7 и 0.3 – 0.2 млн л.н. В массиве г. Костиной – около 8, 5.6 – 4.9 и около 3.5 млн л.н. Установлен эффект хронологического «запаздывания» активизации в более северном массиве.
Anna O. Volynets
added 3 research items
Срединный хребет – один из наименее изученных вулканических районов Камчатки. Большинство исследователей склоняются к тому, что в позднем миоцене-плиоцене в результате аккреции Кроноцкой дуги субдукция под СХ была заблокирована [4, 6 и др.]. У восточных берегов Камчатки образовалась новая зона погружения Тихоокеанской плиты, с которой связана современная вулканическая активность в Восточном вулканическом поясе и Центральной Камчатской депрессии. Причины проявления плиоцен-четвертичной активности в СХ до сих пор остаются дискуссионными. Вулкан Ахтанг (55о 25.643’ с.ш., 158 о 39.210’ в.д., h 1954.6 м) – крупнейшая вершина восточного фланга южной части СХ – расположен на водоразделе рек Сухарики и Караковая. Зона моногенного вулканизма пересекает склоны и подножие вулкана в северо-восточном направлении. Единичные данные о составе пород этого вулканического центра были опубликованы в работе [8] и в объяснительной записке к Государственной геологической карте [2]. Возраст пород, слагающих постройку вулкана, на геологической карте показан как позднечетвертичный, а зона наложенного моногенного вулканизма отнесена к голоцену [2]. Нами проведено K-Ar изотопное датирование лав разных структурно-геоморфологических объектов (Табл.). Полученные результаты позволили выделить несколько этапов вулканической активизации массива Ахтанг, а также впервые документально подтвердить наличие периодов покоя. Зафиксированное начало активности (этап I) датируется плиоценом (4.9 – 4.0 млн л.н., табл.). На этом этапе были сформированы платообразные эффузивы, широко представленные на южном и, в меньшей степени, на западном подножии массива. На завершающей стадии был сформирован главный конус вулкана Ахтанг (h 1954.6 м). После перерыва длительностью около 2 млн лет начался II этап активизации, раннеплейстоценовый (1.9 – 1.7 млн л.н., табл.). На этом этапе в ЗСЗ секторе массива в крупном обвальном цирке плиоценовой постройки образовался лавовый вулкан (h ~ 1500 м), а на завершающей стадии – относительно небольшой лавовый центр на ЮЗ подножии главной вершины Ахтанга (h 1751.3 м). Третий этап активизации (III), среднеплейстоценовый (0.3 – 0.2 млн л.н., табл.), начался после перерыва продолжительностью около 1.5 млн лет. Этот этап характеризуется формированием многочисленных моногенных центров, секущих массив в СВ направлении. Предположительно, этап может состоять из двух или даже трех последовательных эпизодов, когда образовывались существенно лавовые (щитовой вулкан) или существенно пирокластические образования (шлаковые конусы). К этому этапу относится и формирование третьей ЮЗ вершины массива (h 1655.9 м), которую мы называем Молодым Ахтангом. По результатам тефрохронологических исследований вулканизм голоценового времени в массиве Ахтанг не зафиксирован [5]. Изученные вулканические породы принадлежат к умеренно-калиевой известково-щелочной серии и представлены рядом от базальтов до андезитов. Для них характерны умеренные содержания TiO2, Al2O3, CaO и P2O5, слегка повышенные, в сравнении с другими породами Срединного хребта, значения Mg#. В целом же породы Ахтанга отвечают по составу изученным ранее породам восточной ветви Срединного хребта [1]. В серии представительных образцов были измерены содержания микроэлементов. Все изученные образцы имеют типично-островодужный характер распределения микроэлементов с повышенными содержаниями крупноионных литофильных элементов и в различной степени пониженными – высокозарядных элементов (рис. 1). Для пород I этапа активности (рис. 1А) характерны сильно обедненные содержания Nb, Ta, Hf, Zr и РЗЭ и повышенные отношения LILE/HFSE. По характеру распределения микроэлементов эти породы сходны с платобазальтами, извергавшимися в Срединном хребте в неогеновое время [15]. Таким образом, согласно полученным изотопным датировкам возраста и геохимии пород, постройка вулкана Ахтанг может являться центром, из которого происходили излияния платообразных лавовых потоков в период 4.9 – 4.0 млн лет назад. Следующий этап (II) характеризуется проявлениями наиболее кислых для Ахтанга пород андезитового состава с обогащенными спектрами несовместимых микроэлементов, с концентрациями Nb до 8.5 г/т, Ta – 0.53 г/т, однако в породах базальтового состава II этапа распределение микроэлементов имеет островодужный характер (рис. 1А). Для пород III этапа (рис. 1Б) характерны разнообразные спектры распределения микроэлементов. Эпизоды активизации, происходившие 0.31 и 0.24 млн л.н. и представленные породами с наименее фракционированным составом (базальты с содержаниями SiO2 около 51 % и MgO 7,29-7,86 %) характеризуются типично-островодужными спектрами с низкими концентрациями HFSE и высокими – LILE; их состав наиболее близок к составу пород I этапа. Субодновременно с ними были образованы моногенные центры с несколько более эволюционировавшим составом пород (до 54 % SiO2) и более высокими концентрациями высокозарядных элементов; наиболее «продвинутыми» являются лавы щитового вулкана, содержащие 56.56 % кремнезема и 7.9 г/т Nb. При этом, для пород II и III этапов активизации характерны положительные корреляции Nb, Ta, Hf, Zr с содержанием кремнезема, калия, отрицательные – с Ni (тогда как для лав I этапа таких зависимостей не наблюдается). Это дает нам основания полагать, что повышенные концентрации HFSE в андезитах II этапа (1.9 млн л.н.) и лавах щитового вулкана последней стадии активизации, скорее всего, не унаследованы из первичных магм, а приобретены в процессе эволюции магм в открытой системе [10]. Вместе с тем, характер распределения микроэлементов не позволяет ограничить причины вариаций химического состава пород вулкана только процессами фракционирования, даже в открытой системе, предполагая некоторую гетерогенность мантийных и флюидных источников, вовлеченных в магмогенезис на протяжении почти 5 млн лет. Выводы. 1. Впервые получены геохимические данные, характеризующие эволюцию массива Ахтанг на протяжении плиоцен-четвертичного времени. Показано, что в составе пород массива преобладают базальтовые и андезибазальтовые разности с островодужным типом распределения микроэлементов. 2. По результатам впервые проведенного K-Ar датирования выделено три этапа активизации: 4.9 – 4.0, 1.9 – 1.7 и 0.3 – 0.2 млн лет назад. 3. Для платоэффузивов плиоценового времени впервые установлен центр излияний. Доказаны одновозрастность и принципиальное геохимическое сходство пород, слагающих вершинную часть постройки вулкана Ахтанг и платоэффузивов его подножия. 4. На примере массива Ахтанг впервые документально подтверждено наличие длительных (1.5 – 2 млн лет) перерывов в вулканической активности, после которых менялся характер эруптивной активности (от извержений стратовулкана к моногенному вулканизму). Выделенные нами этапы активизации массива Ахтанг коррелируют с региональными эпизодами усиления вулканической активности СЗ Пацифики [12], а среднеплейстоценовый этап оказался синхронен эпизоду активизации, приведшему к образованию Ключевской группы вулканов [7, 9], а также образованию многочисленных моногенных центров на восточной Камчатке [11]. Работа выполнена в соответствии с Госзаданиями по темам ИВиС ДВО РАН № 0282-2016-0004 (анализ данных) и ГИН РАН № 0135-2018-0037 (геохронологические исследования), а также при поддержке гранта РФФИ № 17-05-00112 (аналитические работы). Авторы благодарят Бориса Тагирова за помощь при проведении полевых работ.
Magmatic mixing processes are very important for the petrogenesis of the acid and intermediate rocks. Disequilibrium paragenesises and textural heterogeneity are usually considered as markers of such processes. Melt inclusions study allows one to get direct evidence of the coexistence of the several agents in the magmatic reservoir system. One of the examples of such evidence is the thermobarogeochemical data on melt inclusions of Ichinsky volcanic massif. Ichinsky volcanic massif is located in Sredinny Range of Kamchatka – the largest volcanic structure of the peninsula, composed by Cretaceous-Paleogene metamorphic massif and N-Q volcanic belt; Quaternary period of its evolution is described as post-subduction geodynamic environment. Ichinsky is the largest volcano in Sredinny Range. It is a complex polygenetic volcano of Somma-Vesuvius type. Holocene deposits of the volcano are represented mainly by tephra, sometimes by andesite-dacitic middle-K lava flows. It is surrounded by a large monogenetic volcanic field, which produced in Holocene at least one volumnious eruption, called Southern Cherpouk; lavas of this center (Ol-Pl basaltic) formed spatial lava field and are dated 6500 years BP (Pevzner, 2004, Volynets, 2006). We studied melt inclusions in Pl, Px, Amph and micas of the several horizons of volcanic tephra (Tolstykh, 2018) erupted from 6500 to 4100 years BP (hereafter we use 14C age). The whole rock tephra compositions of these eruptions are very similar (table 2, 1-3). One of the last eruptions of Ichinsky volcano, dated as 4200 y. (table 1, N 2) has two contrast melts composition in its phenocrysts (table 1, columns 4, 5, Fig. 1). This eruption was first after the large-scale activization of this center, happened 6500 y BP. It was so-called “triplex” eruption (Volynets, Pevzner, 2018), which started from the large basic monogenetic edifice formation (Southern Cherpouk) and finished by the dacite tephra eruption from the main polygenetic edifice – Ichnisky volcano (table 1, column 1). Melts of the first type (fig. 1) are found in Cpx and Opx, and in An 57-61 as well. Minerals of this parageneris are frequently form joints; Px are sometimes surrounded by the reaction rims, composed by the spheroidal Amp crystals. Another interesting feature of Pl of this paragenesis is the abundance of the crystalline inclusions of the more acid spar – An 45-57 Ort 3-4. These melts, acid by composition (SiO2 69-76 wt.%) are also characterized by the low K2O content (around 2.5 wt. %) and increased Fe and Mg content (1.5 – 3 and 0.3 – 0.6 wt. %, respectively). Melts of the second type (fig. 1) are found in Pl (An 47-41 Ort 2 – 3), Amph and hydrobiotite, as well as in the groundmass glasses of the dacitic tephra of 4200 14C eruption and in both preceding and following eruptions (shown as fields at Fig. 1). Composition of these glassesare localized in more acid area, belong to the high-K series (K2O 3-4.5 wt. % in most of the inclusions), have lower Fe, Mg, Ti concentrations. Incompatible elements concentrations are different in these two types of the melts as well (fig. 2). Low-K melts (fig. 2, line 3) are characterized by the maximally high for Ichinsky center melts (Fig. 2, field 2) HREE concentrations, average MREE and lowered LREE, Th and U. Besides, acid high-K melts have higher water content (up to 5 wt. % H2O) compared to the low-K type (up to 3 wt. %). It is interesting that acid high-K melts are rock-forming for the rocks of both preceding and following eruptions with respect to 4200 14C y BP event, and the later eruptions melts show a tendency of K2O accumulation (fig. 1). This might be caused by the Amph-Pl paragenesis fractionation. Nevertheless there is no such a fractionation trend which could combine low- and high-K varieties of melts. It is possible to explain the low-K component in the system if we involve a basic Mg component (Southern Cherpouk center, 6500 y). Melts which form inclusions in the olivines of these rocks (Volynets, 2006) are middle-K basalts. Fig. 2 shows similarity of the REE and fluid-mobile element distribution in the basic cone melts (fig. 2, field 1) and low-K type of the acid melts. Unfortunately petrological modeling is impossible in the systems with water-containing mineral phases, therefore we do not have a qualitative assessments of such relationship. Conclusions Products of the 4200 14C y BP eruptions of Ichinsky volcano are hybdid. Part of the phenocrysts is formed from the high-K water-saturated melt of the main crustal chamber, feeding the volcanic center throughout the Holocene times. According to the relationships of the melts compositions, described in this research, we can reconstruct the Holocene part of Ichinsky volcano history. Intrusion of the deep basic magmas into the volcanic center during 6500 y. BP episode, together with monogenetic center formation might trigger the eruptions of the acid magmas of the main chamber. Reasonably small portions of the evolved parts of the basic magmas might join the acid material of the main magmatic chamber of Ichinsky stratovolcano, and formed hybrid rocks of the 4200 y BP eruption. During this event, all “hybrid” material was expended, and the following 4100 y BP eruption shows only high-K acid rocks, which are typical for the whole Holocene stage of the main Icninsky edifice activity.
Sredinny Range (SR) is the largest volcano-tectonic structure of the Kamchatka peninsula. It consists of the old (Cretaceous – Paleogene) metamorphic massif and volcanic belt, formed in Neogene-Quaternary (N-Q) times. Today, SR is about 400 km away from the contemporary trench. Benioff zone is located at 350-400 km depth in the southern part of SR, up to Khangar volcano latitude (Gorbatov et al., 1997), and is not traced further to the north (Gorbatov et al., 2000). Most of researches agree that Neogene volcanism in SR was caused by the subduction of the Pacific plate, when the active trench was located 200 km to the west from its’ today position. In the Late Miocene-Pliocene time the subduction under SR was blocked due to the accretion of Kronotsk arc (Avdeiko et al., 2006; Legler, 1977; Shapiro, Lander, 2003 and others). A new subduction zone was formed near the eastern shore of Kamchatka; it causes a contemporary volcanic activity in the Eastern volcanic front (EVF) and Central Kamchatka Depression (CKD). Pliocene-Quaternary volcanism of SR is referred to as back-arc (Fedotov, Masurenkov, 1991). From geomorphological point of view, SR can be divided in two parts: northern (SRN) and southern (SRS) (Fig. 1). Northern part of SR is a narrow volcanic range stretching to NE. Southern part is more complicated. There are at least two elements in its structure: (1) “eastern” flank stretching to NE, which is a structural continuation of the SRN; and (2) “western” flank, stretching to NNE. It goes from Sredinny metamorphic massif to N-NE and is marked by large volcanic centers – Khangar, Ichinsky, Kekuknaisky, Ketepana. Between “eastern” and “western” flanks there are Anaunsky Dol, Uksichan and Bolshoy Chekchebonay volcanic massifs, which, probably, mark the intermediate, “central” flank of SRS. This subdivision was confirmed by the geochemical data (major and trace elements). Within the northern part of SR typical island-arc rocks erupted in Neogene and hybrid type rocks in Quaternary time. By hybrid type we understand rocks with both elevated HFS and LIL element concentrations. This can be interpreted as a result of three-component (enriched mantle + depleted mantle + fluid) source interaction (Churikova et al., 2001; Volynets et al., 2010). Degree of source enrichment relative to N-MORB is estimated from HFSE concentrations. Hybrid rocks of the SRN are characterized by the high degree of enrichment (up to 55 % of the enriched mantle in the source (Volynets et al., 2010)). In the SRS, within its “eastern” flank there are as well Neogene island-arc-type rocks; during Pliocene-Quaternary times both island-arc and hybrid type rocks with the low degree of enrichment were erupted (up to 5-14 % OIB-type mantle in the source (Volynets et al., 2010)). “Western” flank of SRS is most likely characterized by the enriched type of the mantle source during the whole time of this structure development starting from Late Miocene (from 10-14 % OIB-type mantle in Miocene to almost 50 % in Quaternary). Though, in Quaternary there are as well less enriched rocks (for ex. within Ichinsky massif - 5-15 % OIB-type mantle, (Churikova et al., 2001)). Pliocene Uksichan volcanic massif, Anaunsky Dol, and Bolshoy Checkchebonay massif are structurally positioned between the “eastern” and “western” flanks, and are subdivided as the ‘central” flank of SRS, but geochemically they are similar to the “eastern” flank rocks. We analyzed Sr-Nd isotopic composition in the representative set of 58 samples of volcanic rocks from virtually all parts of Sredinny Range. Together with the previously published data for 19 samples from the northern part of SR that gives us a unique in scope collection for a regional investigation of the sources of the back-arc volcanism in Kamchatka. The results are presented at fig. 2. Studied rocks of different age form two fields which intersect with the majority of the previously studied rocks from EVF and CKD only partially (Fig. 2). We observe two trends in composition within the different age groups. Neogene rocks are characterized by slightly elevated (compared to primitive) 87Sr/86Sr at more or less constant 143Nd/144Nd. Neogene rocks from the “central” and “eastern” flanks are similar to the Quaternary CKD rocks in isotopic composition with a slight shift towards Pacific MORB compositions. Neogene rocks from the “western” flank have lower 143Nd/144Nd at the same 87Sr/86Sr and thus form a separate trend towards the enriched source. Most SR Quaternary rocks at 143Nd/144Nd vs. 87Sr/86Sr diagram form clear trend from the compositions typical for Pacific MORB (Class, Lehnert, 2012) and Kamchatka mantle wedge (Portnyagin et al., 2015) towards the enriched source with elevated 87Sr/86Sr and unradiogenic 143Nd/144Nd. The enriched character of this source is detected as well by the negative correlations of 143Nd/144Nd with Nb/Y, Ta/Yb and Ce/Pb ratios in the whole rocks. Therefore, our large-scale regional investigation of the Sr-Nd composition of volcanic rocks of Sredinny Range has shown that this structure has a complex geological history, with various sources involved in magma generation in different structural parts of the Range in different periods of time. In Miocene-Pliocene times, when SR represents a frontal part of the active subduction zone, we observe typical island-arc rocks with slightly elevated Sr isotopes, most likely inherited from the fluid addition, and rocks with the enriched mantle signature in the western parts of SR, probably representing back-arc at those time. In Quaternary times, the enriched mantle source signature becomes dominating in isotopic composition of the erupted rocks in the whole Range. This shift reflects a change in geodynamic environment caused by Kronotsky terrains accretion. Special thanks to V. Ladygin and O. Dirksen for providing samples for this research, to V. Rodin and B. Tagirov for the help in the field work. This work is performed in accordance with the research themes of the IVS FAB RAS № 0282-2016-0004 and GIN RAS № 0135-2018-0037 and is supported by RFBR grants № 17-05-00112 and 17-05-01163.
Anna O. Volynets
added 2 research items
One of the actual problems of the nature of monogenetic volcanism is a problem of its genetic relationship to the adjacent large polygenetic volcanoes. A comparison of the melt-forming environments using thermobarogeochemistry may serve as a possible way for the solution of this problem. We studied melt inclusions in minerals of lava and tephra of monogenetic cones and stratovolcanoes of the largest volcanic centers in Kamchatka – Shiveluch and Ichinsky volcanoes. Shiveluch is located at the north-eastern part of Kamchatka at the triple junction between Kurile-Kamchatka and Aleutian island arcs. It is the biggest active andesitic volcanic center in Kamchatka. Its products are represented mainly by Mg Pl-Amf middle-K andesites (SiO2 ≥ 55%). At the same time, in some Holocene soil-pyroclastic covers at the foot of this volcano a basic tephra horizon was identified, deposited 3600 14C years BP (Volynets et al., 1997; Ponomareva et al., 2007). Judging by its deposition area and size distribution of basic ash and lapilli particles it was supposed that this tephra is a result of several sub-synchronous eruptions of the hidden monogenetic center located in the SW part of Shiveluch massif. The composition of this tephra corresponds to high-Mg high-K Ol-Pl-Px basalt (SiO2 51%), with small amounts of phlogopite. Ichinsky volcanic massif is located in Sredinny Range of Kamchatka – the largest volcanic structure of the peninsula, composed by Cretaceous-Paleogene metamorphic massif and N-Q volcanic belt; Quaternary period of its evolution is described as post-subduction geodynamic environment. Ichinsky is the largest volcano in Sredinny Range. It is a complex polygenetic volcano of Somma-Vesuvius type. Holocene deposits of the volcano are represented mainly by tephra, sometimes by andesite-dacitic middle-K lava flows. It is surrounded by a large monogenetic volcanic field, which produced in Holocene at least one volumnious eruption, called Southern Cherpouk; lavas of this center (Ol-Pl basaltic) formed spatial lava field and are dated 6500 14C years BP (Pevzner, 2015, Volynets et al., this meeting). We studied melt inclusions in different minerals of the rocks with contrast composition: 1) In Px and Pl of basic lapilli formed during 4600-3600 14C years BP interval (Shiveluch massif) 2) In different minerals of andesitic tephra younger than 4000 14C years BP (Shiveluch) 3) In Ol of the initial and final stages of the monogenetic eruption at Ichinsky massif (South Cherpouk, 6500 14C years BP) 4) In different minerals of dacitic tephra of the summit crater of Ichinsky volcano (6500 and 4200 14C years BP) Melt inclusions were studied according to (Sobolev, 1996) method. A large dataset on melt inclusions compositions of Shiveluch center is presented in (Tolstykh et al., 2015) Melts of monogenetic and polygenetic edifices have very different composition both in Ichinsky and Shiveluch massifs (both in SiO2 and femiс components, fig. 1, 2). Concentrations of major elements in Ichinsky massif melts form continuous trends which may be interpreted in terms of fractionation, while Shiveluch melts demonstrate clear genetic difference of the sources of melts (fig. 1d). Trace elements distribution diagrams show melts characteristics even more clear (fig. 2). Basic melts in both cases are enriched by MREE and HREE. Monogenetic centers of Ichinsky massif are characterized by relative depletion by incoherent elements, while Shiveluch basalts have rather high LIL and fluid-mobile element concentrations. Such spectra can’t be described in terms of fractional crystallization, which would be expressed by sub-parallel position of spidergrams with gradual enrichment from basic to acid varieties (as, for ex., is shown in (Tolstykh, 2012)). Genetic relationship between these melts is not reproducible using Petrolog or MELTs software. Conclusions: Monogenetic centers in Ichinsky and Shiveluch massifs serve as conduits for primitive mantle melts, whose characteristics reflect the composition of the primary mantle source. Large magma chambers of Ichinsky and Shiveluch volcanoes are not produced by fractionation of the above mentioned primary mantle melts. There is no direct evidence of the participation of these basic primitive melts in magmatic mixing, which produces volcanic rocks of the polygenetic volcanic centers of Ichinsky and Shiveluch. Financial support by RFBR grant # 17-05-0012
Kamchatka subduction system is located at the north-western part of the Pacific at the convergent boundary of the Okhotsk and Pacific plates. The latter is presently subducting under Kamchatka at the rate of 8-9 cm/year (Scholl, 2007, etc.). Quaternary volcanism in Kamchatka occurs in three zones, parallel to the trench: Eastern Volcanic Front, graben-like Central Kamchatka Depression and Sredinny Range (SR) in the back-arc. Today, SR is about 400 km away from the contemporary trench. Benioff zone is located at 350-400 km depth in the southern part of the Range, up to Khangar volcano latitude (Gorbatov et al., 1997), and is not traced further to the north (Davaille, Lees, 2004; Gorbatov et al., 2000). Ichinsky volcano, the largest volcanic edifice in SR, is located in its southern part. It is a complex stratovolcano of Somma-Vesuvius type. Numerous monogenetic centers occur in its vicinity. Until recent time, Ichinsky was considered as the only active volcano in SR due to its fumarolic activity and fresh geomorphologic surfaces of lava flows. A tephrochronological and radiocarbon dating accomplished during the last decades allowed to identify Holocene age of 14 summit eruptions of Ichinsky volcano and of two large eruptions at the monogenetic field to the south from the stratovolcano, which are called Southern Cherpouk and Northern Cherpouk (Pevzner, 2015). A subject of this study, the triple eruption Southern Cherpouk (SCh) – Northern Cherpouk (NCh) – Ichinsky volcano (ICH) happened 6500 14C years BP. Southern Cherpouk is a large cinder cone with a 22 km–long lava flow, covering the area of at least 56 km2. The total volume of this eruption is estimated by Pevzner (2015) as ~2.5 – 2.65 km3. Products are represented by basaltic andesites. Northern Cherpouk is cinder cone built on one of the thick older lava flows of Ichinsky volcano. It has a huge 18 km – long lava flow produced by a large bokka located 0.7 km to the SW from the main cone. It covers the area of 31 km2 and the total volume of the erupted products is estimated by Pevzner (2015) as ~2 km3. Remarkably for such a voluminous eruption in a monogenetic zone, lava and cinder are predominantly andesitic to dacitic in composition. Most voluminous eruption of Ichinsky stratovolcano in Holocene (ICH) happened right after NCh, with a center at the SW slope of somma. It is represented by block-and-ash flow, pyroclastic waves and several small dacitic lava flows. The total volume of this eruption is ~3.5 km3. Ash and cinder of the NCh eruption lay without interruption on the SCh deposits and are covered by the ash of ICH eruption; therefore, Pevzner (2015) proposed a sub-synchronous triplex eruption within Ichinsky massif 6500 14C years BP. At the same time, both SCh and NCh eruptions were usually considered as eruptions of the monogenetic zone which probably triggered the activity of the stratovolcano. Here we discuss the geochemical compositions of rocks of this triplex event. Lava compositions form three distinct groups on Harker diagrams with a gap in SiO2 between SCh and NCh and almost continuous trend from NCh to ICH compositions. At the same time, the youngest portions of NCh lava as well as earliest ejected lapilli are close in composition to SCh. Trace element distribution patterns provide us with further evidence of the genetic relationships between the three eruptions. SCh lava has elevated HFSE content and lower U/Nb, Th/Tb, Th/Ta, Ba/Nb ratios at higher La/Sm than NCh and ICH lavas, which are, in turn, rather close to each other in composition, with higher Cs, Rb, Th, U, Pb and lower REE. Trace element patterns of SCh are similar to those of the basaltic monogenetic zone volcanic rocks with various degree of enrichment by OIB-like mantle source (Churikova et al., 2001). NCh and ICH lavas have very similar trace element distribution patterns, almost identical in LILE part but crossing each other at HREE part of the spidergram. Most basic product of this triple event – SCh lava – have highest REE concentrations and therefore can’t serve as a parent melt for more acid varieties of NCh and ICH eruptions. Relationship between NCh and ICH can hardly be explained by fractional crystallization only because of the HREE behavior, but the similarity of their major and LIL element concentrations suggests some kind of the genetic link between these two magmas. Moreover, composition of products of the youngest eruptions of Ichinsky stratovolcano in some cases is even closer to NCh than to ICH. These observations lead us to a conclusion that NCh is not a monogenetic edifice, but rather a side eruption of the stratovolcano, produced by the same magma chamber that is feeding the summit activity of Ichinsky. We propose that the eruption of the Southern Cherpouk cinder cone, which is beyond any doubt an example of monogenetic volcanic field activity, has served as a trigger for the further activity of Ichinsky stratovolcano. Geochemical affinities of the volcanic rocks of the triple eruption can be explained by the admixture of basic SCh magma to the andesitic-dacitic long-lived magma chamber under the stratovolcano, which is confirmed by the pyroclastic deposits compositions and existence of banded pumices and cinder in NCh deposits. This conclusion is a vivid example of the coexistence of the large stratovolcano with a complicated and long-lived magma plumbing system (Dobretsov et al., 2016) with the spacious monogenetic volcanic field with the deeper roots and contrast composition of magmas. Ichinsky volcano therefore can be opposed to Tolbachik volcanic massif, where the monogenetic volcanic field captured the extinct stratovolcano edifice and uses its magmatic system as one of the evacuation channels for magma supply to the surface (Volynets, Kugaenko, this meeting; Kugaenko, Volynets, submitted to JVGR). Financial support by RFBR grant # 17-05-0012.
Dmitry Melnikov
added a research item
Monogenetic volcanic fields are frequently located in the faulted area and in clusters which are associated with the particular geometry of the magmatic chambers and structures of the magma plumbing system in the crust. The method of cluster analyses of the spatial distribution and morphometric characteristics of the cinder cones was used in our research of the conditions of origin and evolution of one of the largest monogenetic fields in Kamchat-ka back-arc-the Anaunsky Dol, or Anaun MVF. Kamchat-ka subduction system is located at the northwestern part of the Pacific at the convergent boundary of the Okhotsk and Pacific plates. Today, Sredinny Range represents its back-arc part and is characterized by the wide distribution of the monogenetic volcanic fields: it has more than 1000 cinder cones, which deposits cover the area of about 8500 km2 (Laverov, 2005; Ogorodov et al., 1972) (Fig. 1). Sredinny Range has a complex structure with several volcanic provinces with different geological history and variable composition of products. Anaun monogenetic volcanic field occupies one of the lowest sections of the whole Sredinny Range. The youngest volcanism in this area (according to the geological map, it was formed in Quaternary times, although our geochemical research and isotopic dating shows its earlier age) is confined to the lowered block of basement rocks. Shield volcanoes, volcanic ridges, cinder and lava cones are located on a low-laying volcanic dale. We made an attempt to make a spatial analysis of distribution of the volcanic edifices and to quantitatively estimate the structural control of the magma plumbing channels. Based on a digital relief model (DEM SRTM, spatial resolution 30 m) we distinguished more than 100 morphometrically expressed cinder cones. For them, using semi-automatic mode, we estimated the morphometric characteristics: height, diameter of the basement, height/basement ratio, angle of the slope, volume of the edifice. With time, cinder cones change their shape due to the erosion processes. Therefore, finally the edifice height is decreased while the basement diameter increased. Determination of the morphometric parameters allowed us to compose a relative age scale for the cinder cones located in Anaun monogenetic volcanic field. Spatial analysis has shown that cones tend to form series of clusters, which are associated with the systems of lineaments. Statistically significant patterns in the cinder cones distribution were then compared with the strike of lineaments to estimate possible location of the magma feeding channels.
Anna O. Volynets
added a research item
Впервые определён возраст начала вулканической активности в пределах Срединно-метаморфи- ческого массива Камчатки (7–6 млн л.н.). Предположительно это событие было обусловлено кол- лизией Камчатки с Кроноцкой дугой, начавшейся ~ 7 млн л.н. с причленения Шипунского п-ва. Впервые установлено, что в позднем миоцене в пределах Срединного хребта Камчатки происхо- дило извержение не менее двух разновидностей пород: типично-островодужных в центральной и северной частях хребта, а также пород гибридного типа на самой его южной оконечности.