V. A. Glebovitsky

Granulite Complexes of the Dzhugdzhur-Stanovoi Fold Region and the Peristanovoi Belt: Age, Formation Conditions, and Geodynamic Settings of Metamorphism

The available data on the age and formation conditions of the granulite complexes in the western Dzhugdzhur-Stanovoi Fold Region (Dambuki and Larba blocks) and the adjacent territory of the Peristanovoi Belt (Kurul’ta, Zverevsky, and Sutam blocks) are systematized. At least three Early Precambrian episodes of high-grade granulite-facies metamorphism dated at 2.85–2.83, 2.65–2.60, and 1.90–1.88 Ga are established in the geological history of the western Dzhugdzhur-Stanovoi Fold Region. Five granulite-facies metamorphic events are documented in the Peristanovoi Belt. The early granulite-facies metamorphism, migmatization, and emplacement of charnockite are related to the first event (2183 ± 1 Ma) in the Kurul’ta Block. The structural transformation and metamorphism of charnockite under conditions of granulite facies correspond to the second event (2708 ± 7 Ma). The enderbite belonging to the Dzhelui Complex (2627 ± 16) and charnockite of the Altual Complex (2614 ± 7 Ma) were emplaced during the third tectonic event, which was immediately followed by the emplacement of the Kalar anorthosite-charnockite complex (2623 ± 23 Ma). The first episode of Early Proterozoic granulite-facies metamorphism of the Sutam Sequence in the tectonic block of the same name was related to the fourth event, probably caused by collision of the Olekma-Aldan continental microplate and the passive margin of the Uchur continental microplate. Finally, granulite-facies metamorphism superimposed on rocks of the Kalar Complex in the Kurul’ta Block and high-pressure metamorphism in the Zverevsky and Sutam blocks (1935 ± 35 Ma) correspond to the fifth metamorphic event. The Late Archean metamorphic events are most likely related to the amalgamation and subsequent collision of the terranes which currently make up the granulite basement of the Dzhugdzhur-Stanovoi Fold Region with the Olekma-Aldan continental microplate. In the Early Proterozoic, the Aldan Shield and the Dzhugdzhur-Stanovoi Fold Region were separated by an oceanic basin. Its closure, and the collision of the Aldan and Stanovoi continental microplates, were accompanied by granulite-facies metamorphism and led to the formation of the Peristanovoi Belt, or Peristanovoi Suture Zone. This collision suture continued functioning in the Phanerozoic (from the Early Jurassic to the Early Cretaceous) with the formation of thick shear zones and greenschist retrograde metamorphism.

Protoliths of the metamorphic rocks of the Fedorov Complex, Aldan shield: Character, age, and geodynamic environments of origin

Geochemical data indicate that the protoliths of the overwhelming majority of the metamorphic rocks composing the Fedorov Complex in the Aldan granulite megacomplex were volcanic rocks of three groups, which occur in different proportions in the complex: (i) volumetrically predominant (no less than 90%) continuous differentiated island-arc basalt-andesite-dacite-rhyolite series, (ii) within-plate basalts, whose composition was similar to that of low-Ti traps, and (iii) basalts of composition similar to that of continentalrift basalts. The U-Pb zircon crystallization age of the metamorphosed basaltic andesites of the Fedorov Complex was estimated at 2006 ± 3 Ma, which testifies, when considered together with preexisting geochronological data, that the complex was produced during a time span of no longer than 25 m.y. A model is proposed according to which the complex was produced within the geodynamic system of the active continental margin of the Olekma-Aldan continental microplate and the Fedorov island arc.

Sm-Nd isotopic provinces of the Aldan Shield

The formation of the Early Precambrian continental crust is among the most topical problems of modern geology and petrology and has remained a subject of debate for several decades. The Sm‐Nd isotopic mapping, which occupies an important place among modern methods developed for the solution of this issue, makes it possible to estimate the relative abundance of rocks related to the formation and transformation of continental crust of a certain age and to select the domains that served as fields of the crust-forming processes of the respective age. In this communication, we discuss the results of the Sm‐Nd isotopic mapping of the Aldan Shield based on isotopic and geochemical study of more than 700 samples of the metavolcanic and metasedimentary rocks and granitoid intrusions. The principles of geological and petrological interpretation of the results of this study were considered in [1‐5 and others]. Here, we only note that the maps of Sm‐Nd isotopic provinces display abundances of rocks with different Nd model ages, but they do not make it possible to recognize domains characterized by crustforming processes of a certain age. To fill this gap, we can use maps of the manifestation of crust-forming processes that demonstrate the fields of abundance of indicator-rocks (hereafter, indicators) of formation and transformation of continental crust. The first type includes magmatic and volcanic rocks, for which model Nd ages coincide at a first approximation with estimates of the crystallization age and e Nd ( T ) values are positive and often close to DM values of the respective age in many cases. In other words, the formation of such rocks is closely related to the formation of continental crust in terms of the Sm‐Nd isotopic systematics. The second type includes intrusive and volcanic rocks with Nd model ages much older than their crystallization ages and with negative T Nd (DM) values. In the modern tectonic schemes [6], the Aldan

Geochemistry and geochronology of migmatites of the Kurul’ta-Nyukzha segment and the problem of correlation between metamorphic events in the Dzhugdzhur-Stanovoi folded area, Eastern Siberia

Crystalline schists of the El’gakan unit (Nyukzha River) were affected by penetrative (volume) replacement by plagiogneisses and granite-gneisses (Lc1) and were then transformed into a polymigmatite complex with successively developing leucosomes Lc2, Lc3, and Lc4. After a thrust-nappe structure was formed in response to collision processes, a new generation of granite veins was produced (Lc5), and then tonalite gneisses Lc6avt and branching migmatites with leucosomes Lc6all were formed along strike-slip fault zones. Zircons from granite-gneisses Lc1 were classified into four types (populations) based on SHRIMP II data. Type I (rhythmically zonal cores) were dated at 2960 and 3010 Ma, which is correlated with the age of the magmatic (predominantly volcanic) protolith. Types II and III were dated at 2703 Ma, which corresponds to granitization under amphibolite-facies conditions and the origin of the Stanoi granite-gneiss. This event is correlated with granulite metamorphism and ultrametamorphism over the whole territory of the Dzhugdzhur-Stanovoi folded area. The most widely spread type IV of the zircons has an age of 1915 Ma, which corresponds to the metamorphism coeval with overthrusting and, hence, with the collision of the Stanovoi plate and a margin of the Siberian Platform. Concentrations of REE, U, and Th and the Th/U ratio were determined to systematically decrease from type I to IV of the zircons (except their type III, whose Th/U ratio increases to >1). Zircons from Lc5 have a concordant age of 139 Ma, which is comparable with the age of the Late Stanovoi granites. The compositional changes from the older cores to younger rims of zircons from Lc5 are analogous to those mentioned above for zircon from Lc1. The concordant age of zircons from Lc6avt is 127–130 Ma. Their Th/U ratio increases from cores (<1) to rims (>1), which suggests that melt may have appeared when Lc6avt was formed. ICP-MS analyses of 53 rock samples reveal differences in the character of the trend (increase/decrease) and magnitude of the changes in the concentrations of trace elements in the distinguished granitization and migmatization series; correlations were revealed between the concentrations of elements and composition of the rock groups. For example, the development of Lc1 was associated with enrichment in Rb, Sr, Ba, LREE, Th, Zr, and Hf at depletion in Nb, Ta, U, and HREE relative to the original rocks. The leucosomes of the Lc2, Lc3, and Lc4 migmatites are depleted in all of these elements except LILE, which is thought to be explained by infiltration-controlled granitization with volume replacement and partial melting at the development of vein leucosome and the subsequent mobilization of the melts together with residues. The different signs of the changes in the LREE and LILE concentrations is unusual for anatectic processes and can be modeled by equilibrium or disequilibrium melting.

Nd isotope variation across the Archaean-Proterozoic boundary in the North Ladoga Area, Russian Karelia

Abstract In order to investigate the boundary between Archaean crust of the Karelian Craton and Paleoproterozoic crust of the Svecofennian Orogen in the area north and west of the lake Ladoga (the North Ladoga Area) 24 samples of mostly granitoid rocks, collected along the 100 km long profile across the inferred suture, were analysed for their Nd isotopic composition. Ten previously published Nd data were incorporated in the dataset. It was established that gneisses from so-called “mantled domes” north of the suture have very low εNd values and represent reworked Archaean basement. North of the Kirjavalahti Dome, the presence of Archaean basement under Kalevian sedimentary cover is registered by abundant Archaean cores in zircons, revealed by a SHRIMP study, and by low εNd values in the rocks of the 1874±13 Ma-old Alattu dyke complex. The transition from Archaean to Proterozoic crust is registered by a shift from very low to Bulk-Earth-type εNd values and occurs within a 10-20 km-wide zone north of the inferred suture. The rocks of the Svecofennian Orogen south of the suture are characterized by relatively uniform initial Nd isotopic composition (average εNd value at 1880-1860 Ma +0.7, n = 17). In terms of Nd isotopic composition, this domain, the Lahdenpohja Domain, is comparable to the Svecofennian terranes to the west: the Central Finland Granitoid Complex and the Accretionary Arc Complex of Southern Finland. The Primitive Arc Complex of Central Finland has, in contrast, significantly more juvenile Nd isotopic composition.

Geochemistry of zircons from ultrametapmorphic granitoids in junction zone of Aldan Shield and Dzhugdzhur-Stanovoi Fold Region

The geochemistry of zircons from autochthonous granite gneiss (Lc1) anatectic (Lc3–4) and injection (Lc5) leucosomes has been studied. Neoarchean prismatic zircon grains with cores that reveal oscillatory zoning and are overgrown by a couple of rims have been seen to occur in Lc3–4. The prismatic grains are occasionally modified into isometric grains with block structure by Paleoproterozoic secondary alteration, which is accompanied by the depletion in HREE, Y, Nb, U; enrichment in Ti, Li, LREE; increasing Th: U ratio and Ce anomaly; and decreasing Eu anomaly. The Paleoproterozoic alteration is related to the low-temperature amphibolite-facies metamorphism followed by partial melting. The Neoarchean prismatic zircons were formed under the conditions of high-temperature amphibolite-facies ultrametamorphism at a temperature of ∼700°C. Judging by the higher Ce/Ce* ratio, the metamorphic rounded zircons were formed at a higher oxygen fugacity as compared with ultrametamorphic zircons from Lc1 and Lc3–4. Specific variation trends of trace element concentrations in prismatic L1 and L3–4 zircons, occasionally with opposite directions, emphasize their different origin. The former are products of metasomatic granitization completed by selective melting with appearance of dispersed melt drops, while the latter are products of anatexis in the open system and by lit-par-lit migmatization. Prismatic zircons L5 are characterized by rhythmic zoning in the core surrounded by rims. The concordant U-Pb age of rims is 129 Ma; the 206Pb/238U age of cores varies from 2213 to 147 Ma. The appreciable enrichment (by a factor of 2–13) of zircons in all minor elements from the core to the rims is caused by the effect of residual postmagmatic fluid, which not only altered zircons, but also facilitated the recrystallization of granite into a pegmatoid variety.

U-Pb age of igneous and metamorphic events on the Fisher Massif (East Antarctica) and its significance for geodynamic reconstruction

This report presents the main results of LA-ICPMS studies of zircon from metamorphosed magmatic rocks of the Fisher Massif in East Antarctica. The minimum age of crystallization for still unexplored granitoid intrusion in the southeastern part of the massif amounts to 1399 ± 11 Ma. The presence of inherited zircon of 1786 ± 23 Ma age in the rocks points to their fusion from a crustal source of Paleoproterozoic age. The time of the eruption of vulcanites of basite composition amounts to 1244 ± 11 Ma. The vulcanites contain xenogenic zircon of Late Archean and Middle Proterozoic age; hence, their initial melt interacted with the heterogeneous continental crust. The earliest metamorphism of the amphibolite facies proceeded 1213 ± 16 Ma ago, and was accompanied with intense shift deformations. The time of volcanism complies with the age of a large basite dike swarm in Vestfold Hills, intruded about 1250 Ma ago, which is associated with the destruction of the hypothetical Paleoproterozoic Nuna (Columbia) continent.

Early migmatites in the prograde metamorphism zone of gneisses in the northern domain of the Ladoga Region: U-Pb evidence based on monazite

The North Ladoga region is the southeastern fragment of the Svecofennian Belt. The metamorphic rocks of this region were traditionally regarded as constituents of a zonal metamorphosed complex composed largely of the Kalevian (late Paleoproterozoic) turbidite formation [1]. Later data [2] provided evidence for tectonic juxtaposition of the high-temperature core of the complex with less metamorphosed rocks. These relationships made it possible to distinguish the northern (ND) and southern (SD) domains [3]. The relatively low-grade metamorphic rocks of the ND are related to the Sveco-Karelian pericratonic zone; the high-grade metamorphic rocks of the SD, to the Svecofennides that are devoid of the Archean granitic basement (Fig. 1). The geochronological data obtained over recent years furnished convincing evidence that ultrametamorphism of rocks in the SD occurred within a rather narrow time interval (1880‐1870 Ma ago) [4]. The results of geochronological study of migmatites in rocks of the Sveco-Karelian pericratonic zone (ND) are reported in the present communication for the first time. The ND structure is controlled by mantled gneiss domes with the Archean granite‐gneiss basement in their cores. The ND is a domain of zonal metamorphism with southward increasing grade from greenschist to amphibolite facies and poorly developed intrusive magmatism (Fig. 1). The thermobarometric estimates of metamorphic grade in the ND and metamorphic mineral assemblages confirm a decrease in metamorphic grade from south to north. The parameters of peak metamorphism in the amphibolite-facies zone are T = 650‐730 ° C and P = 3.8‐5.0 kbar [5]. The ND rocks also bear indications of retrograde transformations of the andalusite‐muscovite subfacies, which are most intense along the narrow submeridional tectonic zones. The SD is characterized by the predominance of granulite-facies metamorphic rocks. Hence, the SD is a high-temperature granulitic block with numerous gabbro, enderbite, tonalite, and granite intrusions. This block is considered a high-temperature core of the zonal metamorphic complex. The SD is thrust over the ND along the low-angle Meier fault zone [3].

Peculiarity of ultrametamorphism in the juncture zone of the Aldan shield and Dzhugdzhur-Stanovoi folded area

The processes of ultrametamorphism in the juncture zone between the Aldan shield and Stanovoi folded area are manifested in granitization (volume-for-volume replacement of gneisses by trondhjemite gneisses Lc1) and subsequent migmatization with formation of several leucosome generations Lc2, Lc3, Lc4, and Lc5, which is confirmed by U-Pb zircon dating. It was established that the granitization stage is marked by the input of Si, Na, and Ba and removal of practically all major (including K) and minor elements. Formation of migmatite leucosomes is accompanied by further depletion in transition (Ti, Mg, Fe, V, Cr, Ni) and light rare-earth (La, Ce, Nd, and Eu) elements, and accumulation of HFSE (Pb, U, Th, Nb, Ta, Y) as well as medium and heavy rare-earth elements (Sm, Gd, Yb, Lu). Leucosomes Lc4, in addition, are enriched in K, Rb, and especially HREE due to the appearance of garnet, while Lc5 leucosomes become higher in K, Sr, and Pb. The study of relations of trondhjemite gneisses and migmatite leucosomes with protolith, geochemical features, and opposite trends in variations of Zr/Hf, Zr/Nb, Nb/La, and Eu/Eu*, and LREE/HREE ratios in the series of granitization and migmatization indicate that the trondhjemite gneisses were formed during deep-fluid-assisted infiltration granitization under the amphibolite facies conditions, while migmatite leucosomes were generated during evolving anatexis under conditions of subsequent diatexis and continuing fluid reworking. With time, the composition of the fluid changed changed, the role of K increased, and leucosomes acquired granitic composition. Unlike common K and K-Na types of ultrametamorphism, the considered juncture zone is characterized by specific type of ultrametamorphism-Na type, with formation of granitic leucosomes in subordinate amounts at the final stages.

Zircon geochemistry of anatectic and diatectic stages of migmatite formation in the northwestern Ladoga region

The geochemistry of zircon, which is constantly used for isotopic dating, has been studied insufficiently, especially in leucosomes of migmatites. Investigation of zircons related to progressive ultrametamorphism is attracting interest. How does the zircon composition change during the partial (anatectic) melting of metasediments and formation of migmatites? What happens with zircon during advanced melting (diatexis) and the accompanying rheomorphism? These are questions we attempted to answer based on a study of zircons from outcrop no. 24 in the high-temperature metamorphism zone (Landenpokh zone located southwest of the Tervus two-feldspar granitic pluton in the northwestern Ladoga region) [1‐3]. Repeatedly deformed polymigmatites of cordierite‐ biotite‐garnet gneisses (gn) with a series of concordant and discordant leucosomes of variable thickness are exposed for 600 m. The following generations of leucosomes are recognized: (1) Lc 1 , bedding-plane veins (0.2‐0.4 cm thick), which occupy no more than 5‐10%; (2) Lc 2 , bedding-plane veins 0.3‐0.5 to 1‐5 cm thick (up to 25%) that occasionally crosscut Lc 1 but commonly lie conformably with Lc 1 due to multiple folding; (3) Lc 3 , a series of cross-cutting veins (as thick as 5‐10 cm) developed along shear zones as network migmatites; (4) Lc 4 , veins thicker than Lc 2 (in some cases, 30‐40 cm thick) occupying up to 30‐40% in particular units (they crosscut the earlier roughly banded migmatites and are deformed into isoclinal folds together with early leucosomes); and (5) Lc 5 , sporadic crosscutting veins (up to 20‐40 cm thick) with numerous fragments of country rocks. At the contact with Lc 2 , the country gneiss is somewhat enriched in dark-colored minerals as is typical of melanosome (M). The established sequence of leucosomes in migmatites and their relationships with folding and faulting are in agreement with the sequence established and repeatedly described for the Ladoga migmatites [1‐3]. The overall duration of polymigmatite formation covers an interval of 20 Ma. The early migmatites were formed almost synchronously with peak metamorphism 1880 Ma ago [4, 5]. Biotite‐garnet‐cordierite gneiss (gn) with second