The chemical and mineral compositions of the Late Cretaceous ultramafic volcanics of Eastern Kamchatka were studied. A distinctive feature of the rocks is a wide range of variation for potassium, from low (the predominant type of volcanic rocks) to ultrahigh (subordinate amount). The chemical compositions of minerals and the geochemistry of the rocks indicate that the ultrabasic volcanics with high and low K2O contents were formed from a single mantle substrate. The enrichment of some ultramafic rocks in potassium occurred as a result of the following processes: 1) fluid extraction of potassium from a magmatic melt, 2) stratification of the melt into a K fraction (fluidized) and a low K fraction with the formation of banded sills, 3) separation of the residual magmatic liquid from the crystallizing melt with high potassium contents.

The nature of low-temperature metamorphic rocks in the continent-ocean transition zone has been studied on the example of metamorphosed Late Cretaceous volcanic-sedimentary deposits of Western Kamchatka (Pensantain Sequence).The Pensantain Sequence consists of prevailing albite-actinolite-epidote-muscovite-chlorite schists. Albit-lavsonite-muscovite-chlorite and кiebeckit-epidote-albite-chlorite schists also participate in Pensantain stratum. The temperature and pressure of the metamorphism of the Pensantain metamorphic rocks were determined by computer modeling using the «Selector C» program. P-T conditions of metamorphism (T = 360± 50°С; P = 3800 ± 500 bar) correspond to facies transition from the greenschist to the blueschist facies. Computer modeling was performed for two samples. This is the first application of the «Selector C» program to determine the P-T conditions of low-temperature metamorphism. The geodynamic conditions and the time of metamorphism were reconstructed. It has been established that rocks of the Pensantain Sequence were formed in the Early Paleogene during collisional metamorphism.

Rhythmic layering of an ultrabasic melt in potassium and low-potassium fractions on the example of picrites, Eastern KamchatkaMineral and chemical composition of the strati ed subvolcanic ultrabasic rocks formed in the process of  uidsilicate (liquid) separation of the ultrabasic magma into potassium and low potassium fractions is characterized by the example of the layered picritic sill from the Late Cretaceous formation of the ultrabasic volcanites from Eastern Kamchatka.It is determined that the main potassium concentrator in picrites from potassium layers is residual volcanic glass, the K 2 O content in which reaches as much as 8-9 wt.%, that is unique for ultrabasic melts.

The mineral and chemical compositions of the layered subvolcanic ultrabasic rocks formed through fluid–silicate (liquid) separation of the ultrabasic magma into high-potassium and low-potassium fractions are characterized by the example of the layered picritic sill from the Late Cretaceous ultrabasic volcanic complex of Eastern Kamchatka. It is determined that the main potassium concentrator in the picrites from the high-potassium layers is a residual volcanic glass containing up to 8–9 wt % K 2 O, which is unique for ultrabasic melts.

Проведен детальный анализ изотопно-геохронологических и изотопно-геохимических данных, полученных в последнее время разными исследователями, в том числе авторами настоящей работы, для метаморфических образований Срединного хребта Камчатки, формировавшихся в пределах мелового окраинно-континентального осадочного бассейна. Сделан вывод об одновозрастности всех метаморфических комплексов, которые представляют собой не стратифицированные во времени образования, а последовательность метаморфических фаций от наиболее высокотемпературных в ядре метаморфической зоны до низкотемпературных на ее флангах, что обусловлено процессами метаморфизма, гранитизации и формирования гранитогнейсовых куполов около 52 млн лет назад. Эти процессы привели к резкому воздыманию центральных частей метаморфической зоны хребта, их интенсивной эрозии, обусловив повсеместные тектонические взаимоотношения между глубоко- и слабометаморфизованными образованиями.

Late Mesozoic metapicrites and metapicrobasalts occur in all apoterrigenous rock sequences in Kamchatka’s Sredinnyi crystalline massif, forming sheets, flows, and sills ranging between a few and 100 m or greater in thickness. Metapicrites possess geochemical parameters that are intermediate between mid-oceanic ridge tholeiites and island-arc tholeiites, which is typical of volcanic rock complexes in marginal basins. The host rocks are terrigenous rock sequences of the Kikhchik series and its metamorphosed analogues, viz., the Kolpakova, Kamchatka, and Malka series, which were formed in a common continental-margin sedimentary basin during the late Mesozoic sedimentation cycle with the same source, which was a province in the eastern margin of the Asian continent. The extension of continental crust in the Cretaceous sedimentary basin and intersections by faults of the same age as that of the Okhotsk-Chukotka volcanogenic belt, combined to produce basite and ultrabasite volcanism. Emplacement of mantle material (the ascent of mantle plumages) accompanied by deep hydrogen-rich fluids into the basement of the crust from volcanogenic terrigenous deposits of the marginal basin produced considerable temperature increases in the crust, as well as magmatic replacement of volcanogenic sedimentary rock sequences with the subsequent generation of magma chambers and ascent of acid volcanic and granitoid rocks into the upper crust.

The detailed analysis of the isotope-geochronological and isotope-geochemical data obtained in recent years by different researchers, including the authors of this study, is performed for the metamorphic formations of the Sredinny Range in Kamchatka formed within the Cretaceous sedimentary basin of the continental margin. It is concluded that all metamorphic complexes have the same age. They do not show stratification by the age of their formation, but the sequence of metamorphic facies from the ultra-high-temperature in the core of the metamorphic zone to the low-temperature on the flanges, which resulted from the metamorphic processes, granitization, and the formation of granite-gneiss domes ∼52 Ma ago. These processes resulted in the sharp uplift of the central parts of the metamorphic zone of the Range and their intense erosion, controlling widespread relationships between the deeply and weakly metamorphosed formations.

Рассматривается проблема геохимической типизации гранитоидного магматизма в зоне взаимодействия океанической и континентальной плит на примере мезозойских гранитоидов крутогоровского и кольского интрузивных комплексов Срединного хребта Камчатки. На основании оригинальных геологических, петрологических и геохимических данных (в том числе по распределению в породах изотопов Sr, Nd и Pb) установлено, что исходным протолитом для гранитоидов служили вулканогенно-терригенные породы, формировавшиеся в пределах мелового окраинного бассейна на востоке Азиатского континента. Кристаллизация гранитоидов (U-Pb SHRIMP возраст около 80 млн лет) происходила в условиях андалузит-силлиманитовой фации глубинности, отвечающей давлению около 2 кбар, обусловив контактовый метаморфизм вмещающих толщ, представленных осадочными отложениями с пластовыми телами основных и ультраосновных вулканитов (кихчикская серия и ее метаморфизованные аналоги – колпаковская, камчатская и малкинская серии). Фиксируемая нижняя возрастная граница осадконакопления вмещающих отложений и время проявления базитового вулканизма совпадает с началом формирования Охотско-Чукотского вулканогенного пояса. Такая корреляция не случайна: она отражает закономерную связь процессов магматической активизации, проявленных в окраинно-континентальном осадочном бассейне с формированием на востоке Азии окраинно-континентального вулканического пояса. Развитие основного вулканизма в структуре осадочного бассейна, сопровождаемое подъемом глубинных флюидов, обусловило вовлечение в магматическую деятельность корового субстрата с образованием коровых магматических очагов, проявившихся излияниями средних и кислых лав и становлением малоглубинных гранитоидных интрузивов, имеющих широкое площадное распространение и обусловивших контактовый метаморфизм вмещающих вулканогенно-осадочных отложений. В дальнейшем коллизионные процессы эоцена (60–50 млн лет), связанные с обдукцией океанического сегмента коры переходной зоны на азиатскую континентальную окраину, привели к тектоническому скучиванию пород Центральной Камчатки и резкому увеличению мощности коры, создав благоприятные условия для ее метаморфического преобразования, достигавшего уровня кианит-силлиманитового типа глубинности амфиболитовой фации под воздействием термального фронта и глубинных флюидов, охвативших нижние горизонты коры. Региональный метаморфизм эоцена обусловил не только метаморфические изменения, мигматизацию и гранитизацию отложений Срединного хребта, испытавших на первом этапе только контактовое ороговикование, но и метаморфизм, мигматизацию и интенсивное разгнейсование магматических пород кольского и крутогоровского комплексов, превратив их в гнейсовидные метаграниты.

The problem of the geochemical classification of granitoid magmatism in the zone of interaction of oceanic and continental plates is considered in this paper by the example of Mesozoic granitoids of the Krutogorova and Kol’ intrusive complexes of the Sredinny Range, Kamchatka. Based on new geological, petrological, and geochemical data (including the Sr, Nd, and Pb isotope systematics of rocks), it was shown that the protoliths of the granitoids were volcanic-terrigenous sequences accumulated within a Cretaceous marginal basin in the eastern Asian continent. The granitoids crystallized at ∼80 Ma (SHRIMP U-Pb age) under the conditions of the andalusite-sillimanite depth facies corresponding to a pressure of approximately 2 kbar and induced contact metamorphism in the host sequences, which are made up of sediments with sheetlike bodies of mafic and ultramafic volcanics (Kikhchik Group and its metamorphic analogues of the Kolpakova, Kamchatka, and Malki groups). The lower age boundary of sedimentation of the host sequences and the time of basic volcanism coincide with the beginning of the formation of the Okhotsk-Chukotka volcanic belt. Such a correlation is not accidental and reflects a genetic connection between the processes of magmatic activation in the continental-margin sedimentary basin and the formation of the continental margin volcanic belt in eastern Asia. The development of basic volcanism in the sedimentary basin accompanied by the ascent of deep fluids resulted in the entrainment of crustal materials into magmatic processes and the formation of crustal magma chambers, the activity of which was manifested by the eruption of intermediate and silicic lavas and emplacement of shallow granitoid intrusions of considerable areal extent. These intrusions induced contact metamorphism in the enclosing volcanosedimentary complexes. The subsequent Eocene (60-50 Ma) collision processes related to the obduction of the oceanic segment of the crust of the transitional zone onto the Asian continental margin resulted in the tectonic piling of the rocks of Central Kamchatka and strong crustal thickening, which was favorable for its metamorphic alteration reaching the kyanite-sillimanite depth level of the amphibolite facies under the influence of a thermal front and deep fluids affecting lower crustal zones. The Eocene regional metamorphism caused not only metamorphic transformations, migmatization, and granitization in the sequences of the Sredinny Range, which underwent only contact hornfels formation during the first stage, but also metamorphism, migmatization, and extensive foliation in the igneous rocks of the Kol’ and Krutogorova complexes, which were transformed into gneissic metagranites.

Metabasites (amphibolites, garnet amphibolites, and basic crystalline schists) compose numerous sheeted bodies (often highly boudined) from a few to 100 meters thick in the plagiogneisses and migmatites of the Kolpakov Group. Chemically, they are reconstructed as basalts and picrites that were metamorphosed, as host terrigenous rocks, under the kyanite-sillimanite subfacies of the amphibolite facies ( t = 620–650°C, P s = 5.9–6.9 kbar). Metabasites are dominated by amphibolites and basic crystalline schists distributed throughout the entire section of the Kolpakov Group, whereas garnet amphibolites are more typical of the upper parts of the group, where they are intercalated with amphibolites, basic crystalline schists, plagiogneisses, and quartzites. Metaultrabasites (plagioclase-free amphibolites) occur much more rarely as small boudins up to few meters in size. According to U-Pb SHRIMP zircon dating, the plagiogneiss protolith age corresponds to the end of the Early-Late Cretaceous (90–100 Ma), which is similar in age to the weakly metamorphosed terrigenous deposits of the Kikhchik Group of the Sredinny Range. This allows us to consider the terrigenous rocks of these groups as isofacial sedimentary rocks. The same age (Early-Late Cretaceous boundary) was taken for protoliths of metabasites forming interbeds among metaterrigenous deposits of the Kolpakov Group. The interval of 100−90 Ma coincides with the beginning of the formation of the Okhotsk-Chukotka volcanogenic marginal-continental belt in East Asia. It is shown that the Kolpakov Group possesses the geochemical features of tholeiitic basalts of different geodynamic settings and comprises both typically island arc (low-Ti) and oceanic (moderate to high-Ti) tholeiites associated with ultrabasic volcanic rocks—picrites. Such a chemical peculiarity of basic rocks is typical of the marginal-continental extension zones (pull-apart basin) that were initiated on the sialic crust. It is obvious that similar geodynamic setting of the basite magmatism existed for the Sredinny Range of Kamchatka. The ascent of the mantle matter beneath the extension zone of the continental crust of the sedimentary basin and its intersection by faults that formed simultaneously with the Okhotsk-Chukotka volcanogenic belt served as the beginning of the basite volcanism in the sedimentary basin. They provided an intense fluid effect and a temperature increase in the crust with subsequent granitization and metamorphism of volcanogenic-terrigenous deposits and, finally, the development of the modern structure of the Sredinny Kamchatka Massif. The intense Late Cretaceous basite volcanism and associated granitoid magmatism in Kamchatka were presumably caused by the ascent of mantle plumes bearing hydrogen fluids.

Geological and geochemical data indicate that the formation of the granulite-like rocks in the contact aureole of the Yurchik gabbronorite intrusion of the Ganal Range, Kamchatka, was caused by the contact metamorphism, metasomatism, and local melting of the initial volcanosedimentary rocks of the Vakhtalka Sequence of the Ganal Group. The temperature in the inner part of the aureole reached 700–800°C and caused the transformation of the basic volcanic rocks of the sequence into two pyroxene-plagioclase, clinopyroxene-amphibole-plagioclase, and amphibole-plagioclase hornfelses, while sedimentary rocks were converted into garnet-biotite ± cordierite hornfelses. The hornfelsed basic volcanic rocks were locally subjected to metasomatic alteration and magmatic replacement with formation of biotite-orthopyroxene-plagioclase metasomatic bodies containing biotite-orthopyroxene-plagioclase ± garnet veinlets and aggregates. During these processes, sedimentary interlayers were converted into garnet enderbites at 700–800°C and 3.2–4.8 kbar. The comparison of the chemical composition of basic volcanic rocks of the Vakhtalka Sequence and their transformation products indicates that the metasomatic alteration and magmatic replacement correspond to siliceous-alkaline metasomatism (granitization) and cause subsequent and uneven influx of SiO2, Al2O3, Na2O, K2O, Rb, Ba, Zr, Nb, and Cl and removal of Fe, Mg, Mn, Ca, Cr, Co, Ti, Y, and S. REE data on basic metavolcanic rocks, hornfelses, and metasomatites suggest that the processes of hornfelsation, metasomatism, and magmatic replacement of the initial volcanic rocks were accompanied by significant increase in LREE and slight decrease in HREE. The Sr and Nd isotope study of the rocks in the aureole showed that the initial basic volcanic rocks of the Vakhtalka Sequence are isotopically close to both mature island arc tholeiites and mid-ocean ridge basalts. The metasomatic alteration and magmatic replacement of volcanic rocks in the aureole lead to the decrease of 143Nd/144Nd and increase of 87Sr/86Sr approximately parallel to mantle array. Pb isotopic ratios in the studied rocks become more radiogenic from initial metavolcanic rocks to metasomatites.

The metavolcanic rocks of the Kvakhona Formation exposed on the western slopes of the Sredinnyi Range metamorphic massif are represented by two sequences. The lower sequence occupying the most part of the exposed formation is dominated by porphyric and aphyric clinopyroxene-plagioclase metabasalts and their tuffs with subordinate metapicritic basalts, metaandesites and their tuffs, and metadacites. The latter form isolated bodies in the northern part of the Kvakhona exposures. The upper sequence is composed of metaandesites, metabasalts, and their tuffs intercalated with terrigenous rocks (siltstones, sandstones, and carbonate graywacke) and metadacite bodies. The rocks were subjected to intense metamorphism under the greenschist facies conditions (t = 250–420°C, P s around 1 kbar) with the replacement of clinopyroxene phenocrysts (or their clasts in the tuff varieties) by actinolite, chlorite, and epidote, while plagioclase phenocrysts are replaced by albite, muscovite, chlorite, and epidote. In the metabasalts enriched in ore minerals, clinopyroxene is replaced by very thin veinlets and the finest grains (about 20–30 μm) of Ca-Na and Na amphiboles (winchite, ferrowinchite, glaucophane, and ferroglaucophane). The groundmass of the rocks (or tuff cement) consists of variable combinations of titanite, magnetite, chlorite, epidote, silpnomelane, and albite. The metabasalts of the formation belong to the high-Fe and often high-Ti rocks, which makes them similar to the tholeiitic basalts of mid-ocean ridges or rifting zones. The metaandesites and metadacites also preserved an elevated Fe content and belong to calc-alkaline series rocks typical of island arcs and active continental margins. It is suggested that the primary rocks of the Kvakhona Formation were formed within volcanic centers on the floor of a vast Cretaceous epicontinental marginal basin, which accumulated thick sequences of terrigenous rocks. The detrital material for these rocks was supplied from the northeastern Asian continent. The geological and geochemical data testify to the similarity of the Kvakhona metavolcanic rocks and the greenstone altered volcanic rocks of the Pensantain Range of Western Kamchatka, which are dated by the U-Pb SHRIMP method at 90–100 Ma. The protolith of the metavolcanic rocks of the Kvakhona Formation was presumably formed within the same age interval.

The metamorphic complexes of eastern Kamchatka exposed on the Khavyven Highland and Karaginsky Island, as well as on the Kamchatka and Ozernoi peninsulas, compose large (up to 1.5 km) elongated blocks spatially associated with ophiolitic peridotite and gabbroic rock bodies (the Khavyven Highland and Karaginsky Island) or make up isolated fragments and blocks among serpentinite melange (the Ozernoi and Kamchatka peninsulas). The degree of metamorphism of the primary rocks varies from the greenschist/amphibolite boundary facies (Karaginsky Island and the Khavyven Highland) to the high-pressure amphibolite facies (the Ozernoi and Kamchatka peninsulas).