K. N. Shatagin

Sources of Archean sanukitoids (High-Mg subalkaline granitoids) in the Karelian craton: Sm-Nd and Rb-Sr isotopic-geochemical evidence

The Sm-Nd systematics of sanukitoids with an age of 2715–2740 Ma in the Western, Eastern, and Central domains of the Karelian craton with various crustal evolutionary histories indicates that the mafic and acid rocks of the sanukitoid series were derived from two contrasting sources: enriched lithospheric mantle and lower crust. The basic sanukitoids of the Western domain were derived from the mantle enriched long before its melting [ɛNd(2715) = −0.48 ± 0.22]. The source of the acid magmas was the young juvenile crust of TTG composition [ɛNd(2715) increases to +1.2]. The mantle source of mafic sanukitoids in the Eastern domain was enriched shortly before melting [ɛNd(2740) = +1.58 ± 0.01], whereas the acid melts came from an ancient crustal source [ɛNd(2740) decreases to −3.0]. For sanukitoids in the Central domain, the time span between the enrichment of the mantle source and its melting was the shortest [ɛNd(2725) = +2.05 ± 0.15], and the contribution of the juvenile TTG crust was insignificant [ɛNd(2725) deceases to +1.7]. The variations in the isotope characteristics of the acid members of the sanukitoid series are consistent with the known age heterogeneity of the crust of the domains. The lateral isotopic-geochemical heterogeneity of the lithospheric mantle source of the sanukitoids is thought to have been related to its two-stage reworking (at 3.2 and 2.8–2.9 Ga) under the effect of TTG granitoids, which are regarded as the melting products of the subducted oceanic crust. The sanukitoids provide information on the geochemical structure of the Archean lithosphere, which is reflected in Archean crust-building processes. The Rb-Sr isotope system of the Neoarchean sanukitoids underwent transformations on the mineralogical scale and within small massifs in the course of at least two Paleoproterozoic tectono-thermal events. A trace of the event at ∼2.1 Ga is left in the Rb-Sr system of monomineralic fractions from a weakly deformed syenite of the sanukitoid series in the Central Domain. Later event (∼1.7 Ga) was recorded in the minerals of the Teloveis sanukitoid massif, which hosts a gold mesothermal deposit in the Western domain. Monomineralic fractions of muscovite and biotite from the wall-rock metasomatites and of plagioclase, microcline, and biotite from metasomatites away from the orebodies yield isochron ages of 1719 ± 60 and 1717 ± 27 Ma. This age of the metasomatic alterations of the Neoarchean sanukitoids is able to explain the broad and unsystematic variations in the Rb-Sr isotope-geochemical characteristics of these rocks. Our data on the Paleoproterozoic age of the mesothermal gold ore mineralization at the Teloveis deposit provide additional lines of evidence for the complex tectonic and metallogenic evolution of the Karelian GGT in the Early Precambrian.

The Middle Riphean Volcanogenic Complex of Kokchetav Massif (Northern Kazakhstan): Structural Position and Age Substantiation

The volcanogenic Kuuspek Formation is a well-defined part of the succession of the Pre-Vendian complexes of the Kokchetav massif (Northern Kazakhstan). The formation is built up of mildly metamorphosed acid lavas, tuffs, and tuffaceous sandstones. At the reference site to the west of the Kokchetav Mountains, the rocks of the Kuuspek Formation compose hinges of small anticlinal folds with sericite-quartz schists of the Late Riphean Sharyk Formation forming the limbs. The Kuuspek Formation lavas are high-alumina rhyolites of high-potassium calc-alkaline series. The U-Pb zircon age of the rhyolites is 1136 ± 4 Ma, thus referring to the Middle Riphean. The Kuuspek rhyolites form the basal part of the Precambrian sedimentary cover of the Kokchetav massif. The cover also comprises schists, limestones, and dolomites of the Sharyk Formation, and quartzites and quartzitic schists of the Late Riphean Kokchetav Formation.

Isotope analysis of strontium by multicollector inductively-coupled plasma mass spectrometry: High-precision combined measurement of 88Sr/86Sr and 87Sr/86Sr isotope ratios

The paper describes a new high-precision method for the simultaneous precise determination of 88Sr/86Sr and 87Sr/86Sr ratios in a single portion of a geological sample by multicollector inductively coupled mass spectrometry (MC-ICP-MS). The isotope analysis is carried out with mass bias effect correction by a combination of internal normalization to the standard Zr-isotope ratio and bracketing standard method (external normalization). Our results for geochemical IAPSO and BCR-1 standard samples are in a good agreement with the published data. The reproducibility of the 88Sr/86Sr ratio varies from ±0.015 to ±0.05‰ (depending on the sample features) and, together with the analysis accuracy, is superior to the previously reported methods of MC-ICP-MS analysis. Still ahead is only double spike thermal ionization mass spectrometry with its ±0.02‰ reproducibility. However, the new method allows the simultaneous determination of 88Sr/86Sr and 87Sr/86Sr ratios and its productivity is higher by 5 to 6 times. On the other hand, in sample preparation, it is necessary to strive for at least 95% Sr yield from the chromatographic column; otherwise the sorption-desorption process may lead to a 0.6‰ 88Sr/86Sr ratio bias relative to the true value.

Neoproterozic anorogenic rhyolite-granite volcanoplutonic association of the Aktau-Mointy sialic massif (Central Kazakhstan): Age, source, and paleotectonic position

Rhyolite-granite volcanoplutonic association was identified among the Precambrian basement complexes of the Aktau-Mointy Massif, Central Kazakhstan. This association comprises rhyodacites, rhyolites, subalkaline rhyolites, tuffs, and felsic volcanogenic-sedimentary rocks of the Altyn Syngan and Urkendeu formations, as well as granitoids of the Uzunzhal Complex. U-Pb (ID-TIMS) dating of accessory zircons from the volcanic rocks and granites showed that the association was formed in the Neoproterozoic (Tonian, 925–917 Ma). The Neoproterozoic volcanic rocks and granites are the youngest Precambrian magmatic complexes and mark the final stage in the formation of the Precambrian crust of the Aktau-Mointy Massif. In terms of major and trace element composition, the volcanic rocks and granites resemble A-type granites, thus indicating the within-plate settings of their formation. It was established that their primary magma could be derived by melting of metatonalitic or metagraywacke protolith at T ≥ 940°C and P ∼ 8–10 kbar in response to mantle magma underplating. Sm-Nd isotope data on the volcanic rocks and granites (TNd(DM) = 1.9−1.7 Ma, ɛNd(T) from −1.9 to −3.5) testify the Paleoproterozoic age of their crustal protolith. Available data have revealed strong similarity between the Neoproterozoic tectonomagmatic evolution of the Aktau-Mointy Massif and the Congo-São Francisco paleocontinent, which, with other cratons, composed the southern Rodinia supercontinent. This suggests that the formation of the Tonian anorogenic volcanoplutonic association of the Aktau-Mointy sialic massif was related to the global-scale divergent processes in the southern Rodinia supercontinent (Congo-São Francisco paleocontinent).

Cambrian to Lower Ordovician complexes of the Kokchetav Massif and its fringing (Northern Kazakhstan): Structure, age, and tectonic settings

A comprehensive study of the Lower Palaeozoic complexes of the Kokchetav Massif and its fringing has been carried out. It has allowed for the first time to discover and investigate in detail the stratified and intrusive complexes of the Cambrian–Early Ordovician. Fossil findings and isotope geochronology permitted the determination of their ages. The tectonic position and internal structures of those complexes have also been defined and their chemical features have been analyzed as well. The obtained data allowed us to put forward a model of the geodynamic evolution of Northern Kazakhstan in the Late Ediacaran–Earliest Ordovician. The accumulation of the oldest Ediacaran to Earliest Cambrian siliciclastics and carbonates confined to the Kokchetav Massif and its fringing occurred in a shallow shelf environment prior to its collision with the Neoproterozoic Daut island arc: complexes of the latter have been found in the northeast of the studied area. The Early Cambrian subduction of the Kokchetav Massif under the Daut island arc, their following collision and exhumation of HP complexes led to the formation of rugged ground topography, promoting deposition of siliceous–clastic and coarse clastic units during the Middle to early Late Cambrian. Those sediments were mainly sourced from eroded metamorphic complexes of the Kokchetav Massif basement. At the end of the Late Cambrian to the Early Ordovician within the boundaries of the massif with the Precambrian crust, volcanogenic and volcano-sedimentary units along with gabbros and granites with intraplate affinities were formed. Simultaneously in the surrounding zones, which represent relics of basins with oceanic crust, N-MORB- and E-MORB-type ophiolites were developed. These complexes originated under extensional settings occurred in the majority of the Caledonides of Kazakhstan and Northern Tian Shan. In the Early Floian Stage (Early Ordovician) older heterogeneous complexes were overlain by relatively monotonous siliceous–clastic units, that were being deposited until the Middle Darrivilian Stage (Middle Ordovician).

Paleoproterozoic anorogenic granitoids of the Zheltav sialic massif (Southern Kazakhstan): Structural position and geochronology

The basement of the Zheltav sialic massif (Southern Kazakhstan) is composed of different metamorphic rocks united into the Anrakhai Complex. In the southeastern part of the massif, these rocks form a large antiform with the core represented by amphibole and clinopyroxene gneissic granite varieties. By their chemical composition, dominant amphibole (hastingsite) gneissic granites correspond to subalkaline granites, while their petroand geochemical properties make them close to A-type granites. The U–Pb geochronological study of accessory zircons yielded an age of 1841 ± 6 Ma, which corresponds to the crystallization age of melts parental for protoliths of amphibole gneissic granites of the Zheltav Massif. Thus, the structural–geological and geochronological data make it possible to define the Paleoproterozoic (Staterian) stage of anorogenic magmatism in the Precambrian history of the Zheltav Massif. The combined Sm–Nd isotopic—geochronological data and age estimates obtained for detrital zircons indicate the significant role of the Paleoproterozoic tectono-magmatic stage in the formation of the Precambrian continental crust of sialic massifs in Kazakhstan and northern Tien Shan.

Isotopic geochronological evidence for the Paleoproterozoic age of gold mineralization in Archean greenstone belts of Karelia, the Baltic Shield

The Rb-Sr age of metasomatic rocks from four gold deposits and occurrences localized in Archean granite-greenstone belts of the western, central, and southern Karelian Craton of the Baltic Shield has been determined. At the Pedrolampi deposit in central Karelia, the dated Au-bearing beresite and quartz-carbonate veins are located in the shear zone and replace Mesoarchean (∼2.9 Ga) mafic and felsic metavolcanic rocks of the Koikar-Kobozero greenstone belt. At the Taloveis ore occurrence in the Kostomuksha greenstone belt of western Karelia, the dated beresite replaces Neoarchean (∼2.7 Ga) granitoids and is conjugated with quartz veins in the shear zone. At the Faddeinkelja occurrence of southern Karelia, Aubearing beresite in the large tectonic zone, which transects Archean granite and Paleoproterozoic mafic dikes, has been studied. At the Hatunoja occurrence in the Jalonvaara greenstone belt of southwestern Karelia, the studied quartz veins and related gold mineralization are localized in Archean granitoids. The Rb-Sr isochrons based on whole-rock samples and minerals from ore-bearing and metasomatic wall rocks and veins yielded ∼1.7 Ga for all studied objects. This age is interpreted as the time of development of ore-bearing tectonic zones and ore-forming hydrothermal metasomatic alteration. New isotopic data in combination with the results obtained by our precursors allow us to recognize the Paleoproterozoic stage of gold mineralization in the Karelian Craton. This stage was unrelated to the Archean crust formation in the Karelian Block and is a repercussion of the Paleoproterozoic (2.0–1.7 Ga) crust-forming tectonic cycle, which gave rise to the formation of the Svecofennian and Lapland-Kola foldbelts in the framework of the Karelain Craton. The oreforming capability of Paleoproterozoic tectonics in the Archean complexes of the Karelian Craton was probably not great, and its main role consisted in reworking of the Archean gold mineralization of various genetic types, including the inferred orogenic mesothermal gold concentrations.

Peridotite-basalt association at MAR between 19°42′ and 19°59′ N: Evaluation of petrogenetic conditions and material balance during hydrothermal transformation of the oceanic crust

Petrogenetic conditions are evaluated for a poorly studied segment of the MAR axial zone. Mantle peridotites from MAR at 19°42′–19°59′ N are determined to show low to moderately low degrees of melting. The geochemistry of the peridotites is close to that of typical abyssal peridotites hosting active hydrothermal fields in the Atlantic (such as the Ashadze-Semenov-Logachev), except that plagioclase lherzolite occurs among the peridotites. Some of our samples collected in the area represent magnesian metabasites which could have been produced by metasomatic transformation (rodingitization) of harzburgite at contact with gabbroids. Cr and Ni concentrations in serpentine were proved to be indicative of the precursor mineral of the serpentine. The distribution of the isotopic composition data points of the MAR abyssal peridotites in the 87Sr/86Sr-143Nd/144Nd diagram suggests that outcrops of mantle peridotites and related plutonic rocks in the crest zone of the ridge expose blocks of deep-sitting rocks that were not simultaneously brought to the surface. The MAR axial segment between 19°42′ and 19°59′ N can be viewed as prospective for searches for hydrothermal ore mineralization. Compositional parameters of basalt in our rock collection correspond to those of the most depleted MORB varieties in the Atlantic.

Pillow lavas of the Sierra Leone test site, Mid-Atlantic Ridge, 5°–7°N: Sr-Nd isotope systematics, geochemistry, and petrology

Based on detailed petrological, geochemical, and isotope-geochemical study, fragments of fresh pillow lavas with chilled glass margins dredged at the Sierra-Leone test site in the axial MAR rift zone between 5° and 7°N correspond to MORB tholeiites, which are not primitive mantle melts but were differentiated in intermediate magmatic (intrusive) chambers. Small-scale geochemical and Sr-Nd isotope heterogeneities were established for the first time in the basalts and their glasses. It was shown that some samples show significant nonsystematic differences in the 87Sr/86Sr ratio between the basalts and their chilled glasses and less significant difference in ɛNd; higher Sr ratios can be observed both in the glasses and basalts of the same lava fragments. No significant correlation is observed between the isotope characteristics of the samples and their geochemistry; it was also shown that seawater did not affect the Sr and Nd isotope composition of the chilled glasses of the studied pillow lavas. It is suggested that such differences in isotope ratios are related to a small-scale heterogeneity of the melts owing to incomplete homogenization during their rapid ascent to the surface. The heterogeneity of the basaltic melts is explained by their partial contamination by the older plutonic rocks (especially gabbroids) of the lower oceanic crust, through which they ascended to the ocean floor surface. The wider scatter of the Sr isotopic ratios relative to Nd is related to the presence of xenocrysts of calcic plagioclase; correspondingly, the absence of a Nd mineral carrier in the rocks results in less distinct Nd isotope variations. It was shown that all of the studied basalts define a single trend along the mantle correlation array in the Sr-Nd isotope diagram. The causes of this phenomenon remain unclear.

The formation processes and isotopic structure of continental crust of the Chingiz Range Caledonides (Eastern Kazakhstan)

According to this paper, the juvenile crust of the Chingiz Range Caledonides (Eastern Kazakhstan) was formed due to suprasubduction magmatism within the Early Paleozoic island arcs developed on the oceanic crust during the Cambrian–Early Ordovician and on the transitional crust during the Middle–Late Ordovician, as well as to the attachment to the arcs of accretionary complexes composed of various oceanic structures. Nd isotopic compositions of the rocks in all island-arc complexes are very similar and primitive (εNd(t) from +4.0 to +7.0) and point to a short crustal prehistory. Further increase in the mass and thickness of the crust of the Chingiz Range Caledonides was mainly due to reworking of island-arc complexes in the basement of the Middle and Late Paleozoic volcanoplutonic belts expressed by the emplacement of abundant granitoids. All Middle and Late Paleozoic granitoids have high positive values of εNd(t) (at least +4), which are slightly different from Nd isotopic compositions of the rocks in the Lower Paleozoic island-arc complexes. Granitoids are characterized by uniform Nd isotopic compositions (<2–3 ε units for granites with a similar age), and thus we can consider the Chingiz Range as the region of the Caledonian isotope province with an isotopically uniform structure of the continental crust.