L. M. Samorukova

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.

New data on the age of ultrametamorphic granitoids of the Aldan granulite area (Eastern Siberia), consequences of metamorphic processes and possibilities of regional correlations of geological events

This work presents new U-Pb data (SHRIMP-II) for zircons from products of granitization and leucosomes of migmatites from amphibolite- and granulite-facies zones developed on rocks of the tonalite-trondhjemite group of the unstratified basement and supracrustal formations of the western part of the Aldan granulite area. The age data obtained were interpreted using the data available on the U and Th geochemistry. The main geochemical trend of transition from primary zircons, crystallizing from the melt to the later metamorphic zircons is manifested in increasing U and Th concentrations in zircons. In this case, the Th/U ratio decreases, as do the values of the Ce anomaly and LuN/LaN ratio. By studying the sequence of autochthonous and paraautochthonous granite formation in the amphibolite-facies zone the ancient (3222–3226 Ma) metamorphic event in the Aldan Shield (a manifestation of the ultrametamorphic processes (granitization and migmatization), superimposed on rocks of an ancient infracomplex (3.3–3.4 Ga) and gneisses and schists of supracrustal formations) was established. The data obtained indicate the Middle Archean age of both metamorphosed rock complexes. The ancient period of evolutionary development of the Aldan shield was followed by development of diatectic granitoids with an age of 2450 Ma, which is correlated well with Proterozoic granitoids from the conjunction zone between the Aldan granulite area and Olekma granite-greenstone terrain.The study of similar granitoids from the granulite-facies zone allowed us to determine the age of the last granulite metamorphism as 2030–2100 Ma, which corresponds preliminary to the time of formation of the Fedorovka island arc. According to this, the conclusion has been made that high-temperature and high-gradient metamorphism is the suprasubduction process manifested in the continental margin back-arc. At the final stage of the Paleoproterozoic granite formation the complex of diatectic intrusive chambers (1960 Ma) formed. This resulted in granitoid magmatism, manifested in the central part of the Aldan granulite area simultaneously with the island arc-continental margin collision or at the post-collision stage. The diatectic granitoids contain inherited zircons with an age of over 2677 Ma, indicating the manifestation of a high-gradient metamorphic event in this time, as in adjacent territories.

U-Pb age of autochthonous paleoproterozoic charnockite in the Aldan Shield

Autochthonous and parautochthonous charnockites in granulite facies of the Aldan Shield (the Aldan River upper flow) were dated. According to the geological observation data, the autochthonous and parautochthonous granite formation included successive development of nebulite (Lc1), its melting product such as early diatectite (Lc3), later “layer-by-layer” migmatite (Lc4), and diatectite (Lc5). The concordant ages of Lc1 and Lc3 were estimated at 2436 ± 10 and 2453 ± 14 Ma. The age of Lc5 was estimated by the upper concordia crossing at 1960 ± 8 Ma likely corresponding to the diatectic melt crystallization period. The process is accompanied by repeated high-temperature alterations of nebulite, diatectite, and their zircons yielding a concordant age of 1945 ± 13 Ma. This zircon making up the overgrowth rims is characterized by remarkable enrichment in uranium and thorium. The granulite facies metamorphism is confirmed by dating of monazite from migmatite after metapelite (1947.7 ± 8.7 Ma). The two main stages of the autochthonous and parautochthonous charnockite formation initiated the development of the crust magmatic chambers. The first stage (2430–2450 Ma) was synchronous to allochthonous high-K alkali granite in the Olekma granite-greenstone region. The second stage (1900–1960) implied the formation of autochthonous and parautochthonous charnockites under the granulite facies conditions and development of allochthonous charnockite and granite in the central part of the granulite areal.

Isotope-geochronological timing of metamorphic events in the boundary zone between the Aldan Shield and the Dzhugdzhuro-Stanovoi Folded Area

Previous studies demonstrated [1–3] that age esti� mation of leucosomes of anatectic and diatectic mig� matites and products of granitization (granite gneiss) is an effective way to understand the sequence of the formation of polymetamorphic complexes, because their relative sequence is easily established on the base of simple, uniquely inter preted geological observa� tions. Migmatites of the Stanovoi complex in the zone of direct junction between the Dzhugdzhuro–Stano� voi Folded Area (DSFA) and the Aldan Shield (AS), which were investigated in the section along the Malyi BAM in the area of Nagornyi Village and Kholodni� kan Pass, are the object of this study. We already stud� ied the relations between the Aldan and Stanovoi com� plexes in the early 1960s [4]. It was revealed that over� thrust–sheet structure was developed in this zone and the tectonic plate moderately dips to the south. They are deformed in linear open or isoclinal folds oriented in the latitudal direction parallel to the whole junction zone. Biotite granite gneiss (Lc 1 ) replacing biotite– amphibole gneiss and amphibole–pyroxene crystalline schist is the earliest in the whole sequence of ultrameta� morphogenetic granitoids. At least three generations of anatectic leucosomes (Lc2–4 ) are observed in granite gneiss; they form isoclinal folds in plates of tectonic lay�

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.

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

Geochemical features of zircons from migmatites of the Nimnyr block, Aldan shield

This paper reports the results of chemical and structural study (electron microscopy and ion microprobe) of zircons from different-age generations of migmatite leucosomes in the basement rocks and Kurumkan Formation within the Nimnyr block, Aldan shield. The studied zircons show REE distribution pattern with a positive slope from LREE to HREE and positive Ce anomaly, which is typical of magmatic zircons, but have elevated LREE contents, which implies their crystallization from migmatite melt with subsequent fluid reworking. The transformations of the zircons were caused by fluid, which was separated during crystallization of the last LILE-enriched portions of the melt and inherited the geochemical features of the host rock—leucosome.

Peculiarities in the Behavior of Trace and Rare Earth Elements during Granitization and Migmatization of the Elgakan Complex (East Siberia, Nyukzha River)

The behavior of major elements in the process of Neoarchean amphibolite-facies metamorphism was considered previously for occasionally pyroxene-bearing biotite‐amphibole‐plagioclase schists and amphibolites, which occur in the middle and lower reaches of the Nyukzha River and are identified as the Elgakan Unit [1]. Ultrametamorphism took place in several stages. At the first stage, plagiogranite gneisses (Lc 1 ) and rocks of an intermediate composition (sch*) were sequentially developed from the volume replacement of schists (sch). At the second stage, the stromatic, network, and coarse-banded migmatite leucosomes (Lc 2 , Lc 3 , and Lc 4 , respectively) developed progressively in schists and plagiogranite gneisses [1, 2]. The two stages were separated by emplacement of intermediate and silicic dikes and minor intrusions. This paper is focused on behavior of trace and rare earth elements in these processes. Without dwelling on the major element geochemistry of crystalline schists and amphibolites (27 analyses), let us note that their compositions correspond to basalt‐trachybasalt and basaltic andesite‐basaltic trachyandesite in the TAS diagram: SiO 2 50.10 ± 3.50 and 55.40 ± 1.85 wt %, respectively; (Na 2 O + K 2 O) 5.22 ± 1.86 and 5.48 ± 1.27 wt %, respectively. Correlation links are typical of igneous rocks of this composition: Si displays negative correlation with Fe 3+ , Fe 2+ , Mn, Mg, and Ca; positive correlation with Na; and the absence of significant correlation with K, Ti, and Al. The absence of significant positive correlation with Ca, Na, and Al testifies to the insignificant effect of plagioclase fractionation. The Lc 1 group (26 samples) includes plagiogranite and granite gneisses with biotite (occasionally, amphibole). In terms of composition, this group varies from tonalite to trondhjemite and granite with predominance of rocks containing 68‐73 wt % SiO 2 . In terms of proportions of normative Ab, An, and Or, the distribution of samples is as follows: trondhjemite 39%, granite 23%, and tonalite and granodiorite 19% each. The sum total of alkali metals shows a positive correlation with K and a negative correlation with Na, resulting in a negative correlation between K and Na (‐0.60). In terms of ASI values, most samples fall within a range of 0.9‐1.2 (maximum 1.0‐1.1). Four samples (out of 26) are characterized by ASI = 1.2‐1.4. The Fe index ( F ) varies from 50 to 70%, and the degree of Fe oxidation ( f 0 ) is 10‐40% (maximum 20‐30%). Si has negative correlation with most of the major elements but no correlation with Na and K. Leucosomes Lc 2 (20 samples) always contain >68 wt % SiO 2 . In comparison with Lc 1 , they are characterized by the prevalence of trondhjemite (50%), a higher content of granite (35%), and a lower content of tonalite (15%). The correlation between K and Na is negative (‐0.58). Samples with ASI = 1.0‐1.1 and 1.2‐ 1.3 are most abundant. In terms of F and f 0 , these rocks are close to Lc 1 but marked by more mafic compositions and higher percentages of rocks with elevated contents of alkali metals. Si is negatively correlated with Al, Fe 2+ , and Mg. Rocks of group Lc 3 (8 samples) are similar to Lc 1 in composition. They differ from Lc 2 in their higher Mg and lower Si contents. Group Lc 4 (14 samples) is close in many parameters to Lc 2 . However, the percentage of granite is higher (64%), while tonalite (7%) and trondhjemite (29%) are subordinate. K and Na are not correlated. As in the preceding group, Si lacks any significant correlation with all major elements except Al. The samples are uniformly distributed in three ASI intervals (1.0‐1.1, 1.1‐1.2, and 1.2‐1.3). The maximums of F (60‐70%) and f 0 (20‐30%) are distinct. The compositional variation of basic rocks during the granitization and evolution of leucosomes may be traced by comparing the average major element contents in the rock series from the basic protolith to the granitized basic rocks, plagiogranite gneisses, and leu

Zircon from charnockite gneiss, charnockite, and leucosome of migmatite in the Nimnyr Block of the Aldan Shield

The microgeochemistry of zircon was studied in three samples: charnockite gneiss (1594), charnockite (1594a), and migmatite leucosome Lc4 (1594c). Prismatic (Zrn I) and oval (Zrn II) zircon morphotypes are distinguished in the first two samples. Most zircon grains consist of two-phase cores and overgrowth rims variable in thickness. The average weighted concordant U–Pb age of Zrn II cores from charnockite gneiss is 2436 ± 10 Ma. The concordant ages of Zrn I and Zrn II cores from charnockite are 2402 ± 16 Ma and 2453 ± 14 Ma, respectively. Some overgrowth rims are 1.9–2.1 Ga in age. In leucosome Lc4, all measured prismatic zircon crystals yielded a discordant age of 1942 ± 11 Ma (the upper intersection of discordia with concordia). These zircons are strongly altered and anomalously enriched in U and Th. Zrn I grains are enriched relative to Zrn II in REE, Li, Ca, Sr, Ba, Hf, Th, and U. Zrn I is considered to be a product of melt crystallization or subsolidus recrystallization in the presence of melt. Zrn II is relict or crystallizing from melt and then partly fused again. Zrn I from charnockite gneiss and especially from charnockite are markedly altered and have a more discordant age than Zrn II. This is probably related to concentration of fluid in the residual melt left after zircon crystallization.