The beginning of volcanic activity within Sredinny metamorphic Massif (Sredinny Range,Kamchatka)

For the first time, the age of the beginning of the volcanic activity within Sredinny metamorphic Massif is determined (7–6 Ma). We suppose that this event was caused by the collision of Kamchatka with the Kronotsk arc that started about 7 Ma from accretion of Shipunsky peninsula. We demonstrate that at least two types of rocks were erupted within Sredinny Range of Kamchatka in late Miocene times: typical islandarc rocks were produced in the central and northern parts of the Range, and hybrid type rocks—in its southernmost part.     

Volcanism of the southern part of the Sredinny Range of Kamchatka in the Neogene-Quaternary

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.     

Geology, mineralogy, and PGE mineralization of the copper-nickel occurrences of the Kvinum ore field, Sredinny Range, Kamchatka

The geology and mineralogy of host metamorphic rocks, the mineralogy of sulfide ores, and the distribution of PGE mineralization were studied in detail for the Kvinum-1 and Kvinum-2 copper-nickel occurrences of the Kvinum ore field, which are the most promising targets for the copper-nickel-PGE mineralization of the Sredinny Range of Kamchatka. It was established that stringer-disseminated and massive copper-nickel ores are localized in amphibole peridotites, cortlandites, and form ore bodies varying from tens of centimeters to 5–20 m thick among the layered cortlandite-gabbroid massifs. The massive sulfide ores were found only at the bottom of cortlandite bodies and upsection grade into stringer-disseminated and disseminated ores. Pyrrhotite, chalcopyrite, and pentlandite are the major ore minerals with a sharply subordinate amount of pyrite, sphalerite, galena, arsenopyrite, and löllingite. Besides pentlandite, the Ni-bearing minerals include sulforasenides (gersdorffite), arsenides (nickeline), and tellurides (melonite) of nickel. It was found that PGE mineralization represented by antimonides (sudburyite) and tellurobismuthides (michenerite) of Pd with sharply subordinate platinum arsenide (sperrylite) is confined to the apical parts of massive sulfide zones and the transition zone to the stringer-disseminated ores. Ore intervals enriched in arsenides and tellurides of Ni, Pd, and Bi contain high-purity gold. In the central parts of the orebodies, the contents of PGE and native gold are insignificant. It is suggested that the contents of major sulfide minerals and the productivity of PGE mineralization in the cortlandites are defined by combined differentiation and sulfurization of ultramafic derivatives under the effect of fluids, which are accumulated at the crystallization front and cause layering of parental magmas with different sulfur contents. The fluid-assisted layering of mafic-ultramafic massifs resulted in the contrasting distribution of PGM in response to uneven distribution of sulfur (as well as As, Te, and Bi) during liquid immiscibility. The productivity of PGE mineralization significantly increases with increasing contents of S, As, Te, and Bi (elements to which Pt and, especially, Pd have high affinity) in fluids.     

Two stages of granite formation in the Sredinny Range, Kamchatka: Tectonic and geodynamic setting of granitic rocks

The newly formed continental crust in southern Kamchatka was created as a result of the Eocene collision of the Cretaceous-Paleocene Achaivayam-Valagin island arc and the northeastern Asian margin. Widespread migmatization and granite formation accompanied this process in the Sredinny Range of Kamchatka. The tectonic setting and composition of granitic rocks in the Malka Uplift of the Sredinny Range are characterized in detail, and the U-Pb (SHRIMP) zircon ages are discussed. Two main stages of granite formation—Campanian (80–78 Ma ago) and Eocene (52 ± 2 Ma ago) have been established. It may be suggested that granite formation in the Campanian was related to the partial melting of the accretionary wedge due to its under-plating by mafic material or to plunging of the oceanic ridge beneath the accretionary wedge. The Eocene granitic rocks were formed owing to the collision of the Achaivayam-Valagin ensimatic island arc with the Kamchatka margin of Eurasia. In southern Kamchatka (Malka Uplift of the Sredinny Range), the arc-continent collision started 55–53 Ma ago. As a result, the island-arc complexes were thrust over terrigenous sequences of the continental margin. The thickness of the allochthon was sufficient to plunge the autochthon to a considerable depth. The autochthon and the lower portion of the allochthon underwent high-grade metamorphism followed by partial melting and emplacement of granitic magma 52 ± 2 Ma ago. The anomalously rapid heating of the crust was probably caused by the ascent of asthenospheric magma initiated by slab breakoff, while the Eurasian Plate plunged beneath the Achaivayam-Valagin arc.     

Geochemistry of metabasites of the Kolpakov Group of the Sredinny crystalline Massif in Kamchatka

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.     

New data on the Baraba Formation age (the Sredinnyi Range of Kamchatka)

In the Sredinnyi Range of Kamchatka, the Baraba Formation of continental conglomerates is assumed to be of the late Campanian age based on found flora remains, but data of isotopic geochronology suggest the Eocene age of these deposits. New data on radiolarians from cherty pebbles are considered in this work along with results of fission-track dating of zircons from pebbles and matrix of the Baraba conglomerates. Fission-track dates obtained for zircons from matrix approve the Eocene age of the Baraba Formation, and new dates characterizing pebbles are not contradicting this conclusion. The Baraba Formation structural position can hardly be lower, therefore, than that of the Irunei Formation.     

Provenances of metaterrigenous sequences in the Sredinny and Ganalsky uplifts, Kamchatka in the light of New Sm-Nd isotopic data

Despite the long history of studying metamorphic rocks in the Sredinny and Ganalsky uplifts of Kamchatka, their tectonic setting and origin, as well as the time of sedimentation, magmatism, and metamorphism, remain a matter of debate and wide discussion. Our isotopic study shows that composite sections of metaterrigenous rocks of the Sredinny and Ganalsky ranges (Kolpakova, Kamchatka, Malka, Kikhchik, and Ganal groups) reveal no significant difference in the Nd isotopic composition, which is evidence for the geochemical similarity of their provenances in contrast to previous conjectures that these groups vary in age from Archean to Upper Cretaceous and were formed in regions distant from one another and distinct in geodynamic setting. New Sm-Nd isotopic data and recent U-Pb (SHRIMP II) timing of zircons allow us to state that the metaterrigenous rocks of the Sredinny and Ganalsky uplifts actually make up a single terrigenous sequence of a great thickness. This sequence accumulated in the Cretaceous shallow-water epicontinental basin, underwent contact metamorphism affected by intrusions varying in age and composition, was involved in zonal regional metamorphism in the early Eocene, and only in the Pliocene was it dismembered into the Sredinny and Ganalsky uplifts as a result of rifting.     

Age and Thermal History of Eclogites from Tulepsai Complex,Eastern Mugodzhar Range (Western Kazakhstan)

Doklady Earth Sciences - Lenses of eclogites (the Tulepsai complex) that formed at the peak of metamorphism at P = 15 kbar and T = 700–750°C and experienced decompression at 12 kbar...     

Early Eocene magmatism in the Sredinnyi Range,Kamchatka: Composition and geodynamic aspects

Migmatization and granite-forming processes were widespread in the southern Sredinnyi Range of the Kamchatka Peninsula in the Early Eocene (at approximately 52 ± 2 Ma). The paper presents data on the composition and genesis of the Early Eocene granitoids. The Malka Rise contains both equigranular peraluminous garnet-bearing granites, on the one hand, and migmatites and tonalites and trondhjemites (TTG), on the other. The petrography and petrochemistry of most granites in the Malka Rise in the Sredinnyi Range (high SiO₂ concentrations, the presence of muscovite and garnet, the proportions of their Al saturation index ASI and SiO₂, FeO_t + MgO + TiO₂, and SiO₂, Al₂O₃/TiO₂, and CaO/Na₂O), and the composition of biotite in these rocks highlight their similarities with S-granites. The character of the REE patterns and the Sr and Y concentrations suggest that the granites and TTG were formed via the melting of sources of two types: metasediments and metabasites. The metasedimentary nature of the protolith of most of the granitoids also follows from similarities between the REE patterns of the granitoids and host metaterrigenous rocks of the Kolpakova and Kamchatka groups. The variations in the Rb/Ba and Rb/Sr ratios of the granites imply that their protoliths could be sedimentary rocks both depleted and enriched in pelite components. The facts that, along with S-granites, some of the granites are TTG, which likely had mafic protoliths, make the Early Eocene granites generally similar to S-granites of the Cordilleran type. The collision of the Achaivayam-Valaginskii ensimatic island arc with the Kamchatka margin of Eurasia started at 55–53 Ma and predated Early Eocene magmatism. In the course of this collision, arc complexes were obducted over continental marginal rocks, and this resulted in their rapid subsidence, crustal heating, magma generation, and the derivation of the granites, tonalites, and trondhjemites at 52 ± 2 Ma at temperatures of 645–815°C. This rapid heating (duirng no more than 3–5 Ma) required an additional heat source, which was likely the mantle. The latter heated the bottom of the crust at the detachment of the slab. The influx of mantle material resulted in intrusions of the norite-cortlandite association, which was coeval with the granites and was accompanied by Cu-Ni sulfide mineralization. The composition of the granitoids and data on the intrusions of the norite-cortlandite association suggest that mantle material was involved in Early Eocene syncollisional magma generation in Kamchatka. Newly obtained U-Pb zircon SHRIMP dates of the granitoids and recently published data on the age of the norite-cortlandite intrusions indicate that they are coeval and make it possible to recognize an Early Eocene phase of magmatic activity in Kamchatka.     

Meimechite-picrite associations in Siberia,Primorye, and Kamchatka (comparative analysis and petrogenesis)

Analysis of petrochemical and geochemical information on rocks and primary melt inclusions from olivines of meimechite–picrite associations of different ages in Siberia (Maimecha-Kotui province), Primorye (Sikhote-Alin), and Kamchatka was made. It showed that the rocks, despite their similar appearance and identical structural patterns, differ considerably in the contents and distribution of incompatible and rare-earth elements and in the composition and evolution trends of parental high-temperature highly magnesian melts.     

Variations of Seismicity in the Avachinsky Gulf (Kamchatka, Russia)

Variations of seismic mode in the region of the Avachinsky Gulf (Kamchatka, Russia) are considered. Observed anomalies (seismic quiescence, the ring seismicity, reduction of the slope of the earthquake recurrence diagram) provide a basis to consider this region as a place of strong earthquake preparation. The Kamchatka regional catalogues of earthquakes between 1962–1995 were used in the analysis. A reduced seismicity rate is observed during 10 years in an area of 150 km × 60 km in size. During the last five years, in the vicinity of the area considered, earthquakes with M > 5 occurred three times more often than the average over thirty years. It is interpreted as ring seismicity. The block of 220 km × 220~km in size, including the quiescence zone, is characterized by a continuous decrease of the recurrence diagram slope, which has reached a minimum value for the last 33 years in this region.     

Yurchik Massifs in Kamchatka: Age and affiliation to magmatic associations

Petrographic and isotopic-geochemical data obtained on basic and ultrabasic rocks from the Yurchik Massif in the Ganal block of crystalline rocks in Kamchatka indicate that the distribution of major and trace elements in these rocks are analogous to those in the fractionation products of high-Al tholeiites occurring in island arcs in the eastern continental margin of Eurasia. Allivalites and dunites found as nodules in gabbronorites and gabbro of the massif are thought to be early cumulates of arc basalts. Petrographic and geochemical characteristics of the Yurchik Massif make it different from Ni-bearing Paleocene-Eocene (approximately 50 Ma) norite-cortlandite intrusions in the Sredinnyi Range of Kamchatka. U-Pb zircon and ⁴⁰Ar/³⁹Ar dates for rocks from the massif definitely testify to its younger, Early Miocene (approximately 22 Ma) age.     

Hydrothermal zircon geochronology: Age constraint on Nanling Range tungsten mineralization (Southeast China)

The Nanling Range (Southeast China) is well known for its wolframite-bearing-quartz-vein (WQV) tungsten deposit. This study focuses on the geochemistry and geochronology of zircons from the WQV and challenges the current view of the tungsten mineralization in the Nanling Range. The features of the WQV zircons include: (1) pale brown, murky brown, or orange-red color and translucence under microscope; (2) {110} + {101} type crystal form; (3) weak cathodoluminescence; (4) enrichment of Hf (ranging from 1.97 to 7.83 wt.% HfO₂), U (ranging from 0.02 to 3.97 wt.% UO₂), Th (ranging from 0 to 0.65 wt.% ThO₂), and P (ranging from 0 to 1.82 wt.% P₂O₅); and (5) presence of solid (hydrothermal and ore minerals) and fluid inclusions. These features indicate that the WQV zircons crystallized from hydrothermal fluids during tungsten mineralization. The in-situ LA-ICPMS U–Pb results of the WQV zircons from five different tungsten deposits in the Nanling Range yield similar ages, ranging from 134.4 ± 1.9 Ma to 132.9 ± 1.5 Ma, approximately 20 million years younger than proposed tungsten ore ages (155 ± 5 Ma). Several mineralization characteristics and field observations also cast doubt on the current model — Nanling Range tungsten ore is the result of orthomagmatic processes. The zircon characterization method provided in this study could be applied to tungsten metallogenic research in other parts of the world.     

张广才岭福兴屯组的形成时代、物源及构造背景

前人对张广才岭福兴屯组研究程度较低,限制了对区域古生代构造演化的认识.福兴屯组凝灰岩锆石30个测点加权平均年龄为392±3 Ma,砂岩碎屑锆石最小一组年龄为393 Ma,指示福兴屯组形成于中泥盆世.Al₂O₃/TiO₂平均值为19.58,稀土元素球粒陨石标准化曲线具有轻稀土富集、重稀土稳定和负Eu异常特征,结合碎屑锆石年龄峰值,确定福兴屯组物源主要为晚古生代中酸性火成岩和早古生代花岗质岩.地球化学和区域火山岩特征共同揭示福兴屯组形成于伸展构造环境.松嫩-张广才岭地块和佳木斯地块晚古生代早期地层均广泛分布有489~551 Ma的碎屑锆石,暗示两地块在福兴屯组沉积之前已完成拼合,为两地块于早古生代晚期拼合提供了新的证据.      

Garnet-bearing basites of the Kuvalorog massif (Kamchatka Peninsula )

The petrographic, petrochemical, and mineralogical compositions of the Kuvalorog Ni-bearing cortlandite-norite intrusion and endocontact leucodiorites hosting pyrope-almandine garnet are considered. Lamprophyre dikes with scarce pyrope-almandine garnet, first discovered in the massif, and plagioclase-pyroxene symplectites in garnet porphyroblast areas are studied. Judging from the petrography of rocks and the composition of inclusions rich in incompatible elements in the garnet, the mineral was produced by the reaction of orthopyroxene with the anorthite component of plagioclase at the subsolidus stage of formation of endocontact diorites. This reaction was probably favored by the fluid phase abundant in the parental magma of the Kuvalorog intrusion and, especially, in the zones near its contact with relics of terrigenous rocks, where it was produced as a result of the rock dehydration under the thermal effect of the intrusion.