N. N. Kruk

U-Pb Isotopic Age, Composition, and Sources of the Plagiogranites of the Kalba Range, Eastern Kazakhstan

Of special importance in the evolution of the Paleoasian ocean is the Carboniferous‐Permian stage, which reflects the closure of the oceanic basin and formation of the structure of the Central Asian fold belt (CAFB) in its present-day form [1]. This is also the time (320‐280 Ma) when the largest Angara‐Vitim granitoid batholith and conjugate synplutonic basite dikes in Transbaikalia, granitoid intrusions and bimodal basalt‐rhyolite series of the Gobi‐Tien Shan zone of South Mongolia, and the Kalba‐Narym and ZharmaSaur batholithic bodies and preceding subalkaline gabbroids, picritoids, high-Al plagiogranitoids, and paleovolcanic structures of the central type in Eastern Kazakhstan were formed. Unlike the Permian‐Triassic stage, which was proved to be related to the Siberian superplume [2], the geodynamic nature of the Carboniferous‐Permian magmatism of the CAFB remains controversial. The plume model was proposed for the Angara‐Vitim batholith [3], and break-off of lithospheric plates in the collision zone between the Kazakhstan and Siberian paleocontinents was suggested for the batholithic belts of Eastern Kazakhstan [4]. These models believe that the collision-related mantle was supplied either with autonomous plumes or with asthenospheric fingers produced by mantle extinction at the divergent margins of lithospheric plates. The solution of this problem consists in studying key magmatic complexes (composition, structure, age), with a special emphasis on the sources of magmatic melts. One such key complex is the plagiogranite of the Kalba Range traditionally distinguished in the Kunush Complex. One group of researchers suggested an Early Carboniferous age and island arc affinity [5], while others consider them as collision-related Late Carboniferous rocks [6, 7], or as products of the Tarim plume [8]. The analysis of the tectonic position of the magmatic complexes of Great Altai (according to N.A. Eliseev) and their correlation with sources [8, 9] sheds no light on this controversary. This article is aimed at accurate dating of the plagiogranites of the Kalba‐Narym zone for refining their position in the magmatic scheme, as well as for estimating the formation conditions and sources of plagiogranite magmas based on their composition and geochemical modeling of the protolith‐melt system. We studied the plagiogranites from the Zhilandy and Tochka massifs (Fig. 1). The Zhilandy Massif forms an isometric body, with medium-grained biotite granites in the central part and fine-grained porphyritic varieties and plagiogranite porphyry dikes in the marginal parts. The plagiogranites consist of large biotite, short-prismatic zoned andesine‐oligoclase, and single grains of orthoclase‐microperthite and green hornblende. The Tochka Massif is made up of NW-extended minor bodies and dikes of fine-grained porphyritic biotite plagiogranites and plagiogranite porphyries. The plagiogranites of the Tochka Massif differ macroscopically from the Zhilandy rocks owing to superimposed processes of mylonitization; however, microscopically, they have similar petrographic and mineral composition. The dikes of plagiogranite porphyries of both the massifs contain phenocrysts of short-prismatic zoned plagioclase, quartz, and biotite embedded in the microcrystalline groundmass. In both cases, the plagiogranites intrude black shales of the Takyr Formation ( D 3 –C 1 ) and are cut by the Early Permian granitoids of the Kalba batholithic belt [5‐7, 10].

U-Pb and 39Ar/40Ar dating and Sm-Nd and Pb-Pb isotopic study of the Kalguty molybdenum-tungsten ore-magmatic system, Southern Altai

The Kalguty ore-magmatic system comprises two intrusive complexes: the Kalguty granite-leucogranite complex and Eastern Kalguty complex of dikes and small intrusions. U-Pb dating of individual zircon grains from granites of the main intrusive phase demonstrated that the crystallization age of small grains of magmatic habits and outer rims of large grains is almost concordant and is 216 ± 3 Ma. Ar-Ar isotope study shows that the K-Ar system of biotites from granites of the main phase within the Kalguty ore field was disturbed (radiogenic Ar was partially lost) and gave an age of 202 ± 1 Ma. The Ar-Ar dating of muscovites from intraore and postore dikes of the Eastern Kalguty complex devoid of signatures of postmagmatic recrystallization and superimposed greisenization gave similar ages of 205–201 Ma. This date is considered as the emplacement age of the Eastern Kalguty dikes and associated complex W-Mo-Bi-Be ore mineralization. Sm-Nd and Pb-Pb isotopic study of granites, ongonites, and elvans of the Kalguty ore-magmatic system and host rocks shows that these systems were closed. For example, recalculation of Nd isotopic ratios for corresponding ages of crystallization of magmatic systems (216 and 205 Ma) shows that ɛNd(T) values decrease from −1.9 to −3.5 ... −5.08 with transition from granite-leucogranite to subvolcanic granite porphyry, ongonite, and elvan dikes with corresponding increase of model ages of protoliths from 1.0 to 1.25 Ga. Lead isotopic ratios for leaching residues of whole-rock samples of all rock varieties (206Pb/204Pb = 18.305–18.831; 207Pb/204Pb = 15.527–15.571) are plotted well below the line of average crustal lead evolution according to the Stacey-Kramers model.

Gabbro-granite intrusive series and their indicator importance for geodynamic reconstructions

One of the problems faced by researchers when paleogeodynamic reconstructions are carried out for deeply eroded orogenic terranes is the limited usage of geological and isotopic geochemical data on volcanic associations. The utilization of information on gabbroids and granitoids considered separately also fails to resolve this problem. The convergence of features of arc, collisional, and within-plate magmatic processes leads the researcher to search for indicator plutonic associations, such as paired gabbrogranite intrusive series. The latter were distinguished using geoinformation databases (including those compiled by the authors of this paper), which were composed for the Early Caledonides in the Altai-Sayan folded area and adjacent territories. This makes it possible to characterize mantle-crustal magmatism in suprasubductional and collisional environments and the conditions under which these geodynamic regimes (plate- and plume-tectonic factors) interact. This paper presents estimates of the composition of the parental basic magmas, distinctive features of their differentiation, and the compositional specifics of the accompanying extensive granite-forming processes. The example of the Altai-Sayan folded area and adjacent territories is employed to correlate the composition of the basic-ultrabasic and granitoid magmas and, on this basis, distinguish (a) differentiated gabbro-tonalite-plagiogranite intrusive series corresponding to accretionary-collisional geodynamic environments, and (b) gabbro-monzonite-granosyenite-potassic granite intrusive series, which were produced when the accretionary-collisional system was affected by a plume.

The nature of the continental crust of Sikhote-Alin as evidenced from the Nb isotopy of Rocks of Southern Primorie

The study of the Nb isotopy of rocks of the conti� nental crustal is an effective tool for evaluating the nature of mechanisms of crustforming processes and ages of their manifestation (1). In the first approxima� tion, the Nd model isotope age characterizes the age of formation of the continental crust from the mantle source or, in other words, transformation from oceanic crust to continental. The Nd isotope systematics of sedimentary rocks of upper levels of the continental crust allows us to estimate the average model age and the possible paleogeographic source areas, as well as to determine the lower age limit of sedimentation if there are no paleontological data. Moreover, the use of two� stage Nd model ages of granitoides along with the geochemical and petrological data obtained allow us to obtain data about the deep levels of the continental crust (levels of generation of the granitic melt). In some cases, the combination of these approaches makes it possible to reconstruct the chemical compo� sition and the nature and mechanisms of formation of the continental crust of separate blocks (terranes) and large fold belts (2).

The Khao Que-Tam Tao gabbro-granite massif, Northern Vietnam: A petrological indicator of the Emeishan plume

New data obtained on the Khao Que-Tam Tao gabbro-granite pluton (Northern Vietnam) are discussed. It was established that this pluton was formed at the Permian-Triassic boundary (250.5 ± 3.2 Ma, 40Ar/39Ar isotopic age). Morphologically, it represents a hypabyssal fracture intrusion. The first stage was marked by the intrusion of the picrobasaltic melt, the differentiation of which resulted in the formation of the layered peridotite-gabbro series and the quartz-bearing monzodiorites and granophyres in its endocontact at the final stage. At the second stage, the Khao Que peridotite-gabbro massif was broken in its central part by a fault, along which the Tam Tao granodiorite-granite massif was localized. Numerical simulation using the COMAGMAT program for the basic system and geochemical estimates for the granite system allow the statement that the mafic and granitic melts evolved independently, and their final products were quartz-bearing monzodiorite and granophyre, on the one hand, and aplites and pegmatites, on the other hand. The compositional correlation of the Permian-Triassic magmatic associations in Northern Vietnam (the Nui Chua gabbro pluton and the Khao Que-Tam Tao gabbro-granite and Pia Bioc granite plutons) and in Southeast China (flood basalts) allows these complexes to be regarded as a part of a single large igneous province produced by the Emeishan plume activity.

Nd Isotope Systematics in Metamorphic Rocks of the Southern Russian Far East

Sm–Nd isotopic–geochemical investigations are widely used in the study of metamorphic sequences for estimating the age of continental crust formation and the lower age limit of protoliths of metamorphic com� plexes [1]. When the protolith is of primary magmatic origin, the Nd model age reflects in the first approxi� mation the onset of the continental crust formation from the mantle source or, in other words, the time of transformation of the oceanic crust into the continen� tal crust. The Nd isotope systematics of metamorphic terrigenous sedimentary rocks offers the opportunity to estimate the average model age and likely paleogeo� graphic provenances, as well as the lower age limit of sediment accumulation. In this communication, we discuss the data of the Nd isotopic composition in metamorphosed rocks from the three largest blocks located in the southern Far East areas: Matveevka–Nakhimovka, Sergeevka, and Anyui. It should be noted that the studied com� plexes were traditionally considered as representing inliers of the Early Precambrian crystalline basement and only recently have they been regarded as Neogean complexes that were formed by accretion, collision, or postcollision processes [2, 3]. The available geochro� nological data provide no grounds for answering the following question: what is the lag between metamor� phic transformation of rocks and their geological age? The Nd isotope measurements performed for meta� morphic complexes of the southern Far East bring us nearer to assessment of the lower age limits of their formation and to determination of the isotope crust composition for the whole region. The Khanka superterrane is located in southwestern Primor’e region (Fig. 1). It is formed by several ter� ranes of various origins. The southern and northern parts of the superterrane represent fragments of the passive margin of a craton and Early Paleozoic oro� genic belt, respectively. The last of them comprises the Matveevka and Nakhimovka metamorphic terranes that differ from each other in the constituting rocks and degree of their metamorphic transformation [6]. The most intensely metamorphosed rocks of the Matveevka terrane are united into the Iman Group, which is traditionally subdivided into the Ruzhino (diopside–calcite and forsterite–calcite marbles, biotite and biotite–cordierite gneisses with intercala� tions of quartzites and marbles), Matveevka (biotite– sillimanite and biotite–garnet–cordierite gneisses with quartzite and marble intercalations), and Tur� genevka (biotite–amphibole gneisses, crystalline schists, and amphibolites) formations. In the Nakhi� movka terrane, metamorphic rocks are united into the Ussuri Group, which consists of the Nakhimovka (biotite and biotite–amphibolite gneisses with lenses of marbles and amphibolites) and Tat’yanovka (biotite, diopside, and muscovite–graphite crystalline schists) formations. The relationships between the Iman and Ussuri groups remain unknown.

Geochemistry of rocks in the Anuy metamorphic dome, Sikhote-Alin: Composition of the protoliths and the possible nature of metamorphism

The paper presents geological, geochemical, and isotopic data on metamorphic rocks in the Anuy block (dome) in the Northern Sikhote-Alin and the surrounding sedimentary rocks of the Samarka accretionary prism. The geochemistry and isotopic composition of the amphibolite-facies metamorphic rocks (variably migmatized gneisses and crystalline schists) in the Anuy block and unmetamorphosed Jurassic-Cretaceous sediments surrounding the block are proved to be similar. All of them corresponded to the erosion products of the transitional-type crust (mature island arcs and active continental margins), have similar major- and trace-element compositions, and Nd model ages of 1.25–1.4 Ga. The geochemistry and isotopic parameters of metapelites in the Anuy block are principally different from those of analogous rocks in the Khanka Massif (the latter rocks are erosion products of the mature crust and have a Nd model age of 1.7–1.9 Ga). The metabasites, which are found as beds and lenses in gneisses and crystalline schists in the Anuy block and among sedimentary rocks surrounding the block, have a composition corresponding to oceanic basalts of the N- and E-MORB types. Based on the synthesis of geological, geochemical and isotopic data it was suggested that the Anuy block could be not a fragment of the basement of an ancient continent (as was believed previously) but rather a complex of the Early Cretaceous granite-metamorphic core of the Cordilleran type.

New data on the age and geodynamic interpretation of the Kalba-Narym granitic batholith, eastern Kazakhstan

Geological and new geochronological data are summarized for the Kalba-Narym granitic batholith in eastern Kazakhstan, and their geodynamic interpretation is suggested. In the structure of the batholith, we consider (from late to early) the Kunush plagiogranitic complex, the Kalguta granodiorite-granitic association, and the Kalba granitic, Monastery leucogranitic, and Kainda granitic complexes. The granitic complexes of the Kalba-Narym batholith were formed between the Carboniferous-Permian and the Early-Middle Permian (∼30 Ma). New data indicate that formation of the Kalba-Narym batholith was related to the activity of the Tarim mantle plume. Heating of the lithosphere by the plume coincided with postcollision collapse of the orogenic structure and led to the crust melting and formation of the studied granitic complexes in a relatively short period.

Petrology of volcanic and plutonic rocks from the Uimen-Lebed’ terrain, Gorny Altai

The Uimen-Lebed’ volcanoplutonic terrane is located at the junction of the Gorny Altai, Gornaya Shoriya, and western Sayan structures and is part of the Devonian-Early Carbonaceous Salair-Altai volcanoplutonic belt. The volcanic facies of the terrane composes the contrasting Nyrnin-Sagan Group, which includes basalt-basaltic andesite and basalt-rhyolite associations. The plutonic facies makes up the multiplet Elekmonar Group, which includes two independent complexes: monzogabbro-monzodiorite-granodiorite-granite and granodiorite-granite-leucogranite. The volcanic and plutonic rocks are asymmetrically distributed: volcanic sequences fill inherited depressions in the eastern part of the terrane, whereas plutonic complexes are located in its western part at the fault system branching from the transregional Kuznetsk-Teletsk-Kurai fault zone. The basalts of the Nyrnin-Sagan Group show geochemical signatures of both suprasubduction and rift-related rocks. The evolution of basaltoid magmatism reflects the formation and development of a suprasubduction mantle wedge in the inner part of an active continental margin accompanied by the influence of an intraplate mantle source. The silicic volcanism was generated under lower crustal conditions (P > 10 kbar) at the expense of metabasic materials and was accompanied by the influx of potassium into the anatectic zones. The gabbroids of the Elekmonar Group show suprasubduction geochemical features and no signatures of rift-related structures. The composition of the Elekmonar granitoids indicates significantly shallower (compared with the silicic volcanics) depths of their generation. The Uimen-Lebed’ volcanoplutonic terrane in the northeastern part of Gorny Altai was formed in the inner part of an active continental margin of the Andean type. Its magmatic complexes were formed over a considerable time range, from the early Emsian, when the formation of the active continental margin began, to the end of the Eifelian or, more likely, the beginning of the Givetian stage.

Geological position, geochemistry, and geodynamic formation environments of late givetian-early frasnian basalts in the central gornyi altai region

1151 The active continental margins (ACMs) represent one of the most illustrative examples of convergent boundaries between lithospheric plates. It is tradition ally believed that diverse magmatism of these struc tures is determined by successive dehydration of the subsided oceanic plate in response to the increasing depth of primary magma generation away from the edge of the continent, on the one hand, and the pecu liar structure and composition of the continental lithosphere, which are responsible for the specific composition of silica melts and crust–mantle interac tion, on the other [1 and others].