Precambrian geosynclinal metallogeny of Ukrainian shield

The granites of the Ukrainian shield are secondary; the “shield,” in the first instance, is a segment of a fold belt which extends from the Black Sea, via the Voronezh Arch, to the Kola Peninsula. The metamorphic ungranitized anticlines are narrow folds in which are found iron, copper, zirconium and precious metals. Iron, titanium, copper, graphite and mica deposits are found in the folds of granitized gneisses, migmatites, anatektites and granites. The crystalline metal ores are associated with metamorphosed elastics and evaporites; the titanium, copper, cobalt, gold, vanadium, mercury, chromium and nickel deposits – associated with sedimentary volcanics and evaporites. The high-grade metamorphics Contained the deposits of titanium, iron, calcium, magnesium, copper, vanadiuta, lead and zinc. The chromites and nickel silicates are associated with the intrusives. In short, metamorphic history and mineralogy exercised a direct control over the size and grade of ore deposits.     

Principal stages in geological evolution of Ukrainian shield

The first attempt at coordination of the newest geochronological findings with identifiable petrographic complexes within the Ukrainian Shield, here seen as products of geosynclinal evolution and of granitization (in the modern sense), suggests the following characteristic traits of the Precambrian evolution of the earth's crust: an arrest of the upper Archean geosynclinal cycle (the second one of the three) in its initial or early stages; a general non-inversion of geosyncline; a very high geothermal gradient and hence the cardinal role of granitization and the "universality" - involvement of practically all kinds of rocks - of the regional metamorphism. The Shield has behaved as a platfomal entity ever since the end of its last (lower Proterozoic) geosynclinal cycle. The author's historical analysis of the Shield, as distinct from the still current stratigraphic representations, is based among other things on the recognized independence of metamorphic facies from stratigraphic as well as structural boundaries, identifications of formations corresponding to definite stages and zones of the geosynclinal evolution, correlations of geochronological data with petrographic varieties of rocks and with their spatial distribution, examination of migmatites and granitoids from the viewpoints of ultrametamorphism and granitization, proofs of the function of deep fissuring in the character of sedimentation and intrusive activities, and plane analysis of folded structures. Metallogenic analysis of the Shield is facilitated by such identifications and correlations, inasmuch as migrations and accumulations of ore constituents and others were definitely associated with granitization activities. — V. P. Sokoloff     

Geochemistry of zircons from basic rocks of the Korosten anorthosite-mangerite-charnockite-granite complex,north-western region of the Ukrainian Shield

Mineralogy and Petrology - The concentrations of 26 trace elements have been determined by laser ablation ICP-MS in zircons from four samples of basic rocks of the Korosten...     

The origin of basic rocks of the Korosten AMCG complex, Ukrainian shield: Implication of Nd and Sr isotope data

The Korosten complex is a Paleoproterozoic gabbro–anorthosite–rapakivi granite intrusion which was emplaced over a protracted time interval — 1800–1737 Ma. The complex occupies an area of about 12 000 km² in the north-western region of the Ukrainian shield. About 18% of this area is occupied by various mafic rocks (gabbro, leucogabbro, anorthosite) that comprise five rock suites: early anorthositic A₁ (1800–1780 Ma), main anorthositic A₂ (1760 Ma), early gabbroic G₃ (between 1760 and 1758 Ma), late gabbroic G₄ (1758 Ma), and a suite of dykes D₅ (before 1737 Ma). In order to examine the relationships between the various intrusions and to assess possible magmatic sources, Nd and Sr isotopic composition in mafic whole-rock samples were measured. New Sr and Nd isotope measurements combined with literature data for the mafic rocks of the Korosten complex are consistent and enable construction of Rb–Sr and Sm–Nd isochronous regressions that yield the following ages: 1870 ± 310 Ma (Rb–Sr) and 1721 ± 90 Ma (Sm–Nd). These ages are in agreement with those obtained by the U–Pb method on zircons and indicate that both Rb–Sr and Sm–Nd systems have remained closed since the time of crystallisation. In detail, however, measurable differences in isotopic composition of the Korosten mafic rock depending on their suite affiliation were revealed. The oldest, A₁ rocks have lower Sr (⁸⁷Sr/⁸⁶Sr₍₁₇₆₀₎ = 0.70233–0.70288) and higher Nd (εNd₍₁₇₆₀₎ = 1.6–0.9) isotopic composition. The most widespread A₂ anorthosite and leucogabbro display higher Sr and lower Nd isotopic composition: ⁸⁷Sr/⁸⁶Sr₍₁₇₆₀₎ = 0.70362, εNd₍₁₇₆₀₎ varies from 0.2 to − 0.7. The G₃ gabbro–norite has slightly lower εNd₍₁₇₆₀₎ varying from − 0.7 to − 0.9. Finally, G₄ gabbroic rocks show relatively high initial ⁸⁷Sr/⁸⁶Sr (0.70334–0.70336) and the lowest Nd isotopic composition (εNd₍₁₇₆₀₎ varies from − 0.8 to − 1.4) of any of the mafic rocks of the Korosten complex studied to date. On the basis of Sr and Nd isotopic composition we conclude that Korosten initial melts may have inherited their Nd and Sr isotopic characteristics from the lower crust created during the 2.05–1.95 Ga Osnitsk orogeny and 2.0 Ga continental flood basalt event. Indeed, εNd₍₁₇₆₀₎ values in Osnitsk rocks vary from 0.0 to − 1.9 and from 0.2 to 3.4 in flood basalts. We suggest that these rocks being drawn into the upper mantle might melt and give rise to the Korosten initial melts. ⁸⁷Sr/⁸⁶Sr₍₁₇₆₀₎ values also support this interpretation. We suggest that the Sr and Nd isotopic data currently available on mafic rocks of the Korosten complex are consistent with an origin of its primary melts by partial melting of lower crustal material due to downthrusting of the lower crust into upper mantle forced by Paleoproterozoic amalgamation of Sarmatia and Fennoscandia.     

Geochemistry of alkali and nepheline syenites of the Ukrainian Shield: ICP-MS data

We present new geochemical data on alkali and nepheline syenites from various complexes of different age within the Ukrainian Shield. The results reveal a correlation between the content of trace elements in the syenites, their assignment to a particular rock complex, the chemistry of primary melts, and the degree of their differentiation. The data also suggest regional geochemical heterogeneity in the ultramafic-alkaline complexes of the Ukrainian Shield. The alkali and nepheline syenites in the ultramafic-alkaline massifs from the eastern and western parts of the region exhibit similar REE contents and Eu/Eu* ratios but are markedly different in Nb, Ta, Zr, and Hf content and are of the miaskitic type. These rocks have lower REE, Nb, and Zr and higher Sr and Ba compared with early foidolites. The rocks of the gabbro-syenite complexes define a distinct Fe-enrichment fractionation trend from early syenitic intrusions to more differentiated varieties; they are also characterized by lower Sr, Ba, and Eu/Eu* and significantly lower contents of some major elements, e.g., Ti, Mg, and P. The agpaitic index and concentrations of Zr, Nb, Y, and REE increase in the same direction. A similar geochemical feature is observed in the alkali syenites genetically associated with anorthositerapakivi-granite plutons, which show incompatible-element enrichment and strong depletion in Sr and Ba. The distinctive evolutionary trends of alkali and nepheline syenites from different rock complexes of the Ukrainian Shield can be explained by different mechanisms of their formation. The main petrogenetic mechanism controlling the distribution of trace elements in the rocks of ultramafic-alkaline complexes was the separation of parent melts of melanephelinite and melilitite types into immiscible phonolite and carbonatite liquids. The gabbro-syenite complexes and alkali syenites from anorthosite-rapakivi granite plutons evolved via crystallization differentiation, which involved extensive feldspar fractionation.     

Weathered crust on ultrabasic rocks at southern margin of Ukrainian crystalline shield

The chiefly residual, nickel-bearing, weathered “crust” (30 to more than 80 m thick) on the hyperbasites of the South Bug Basin at the southern margin of the Ukrainian shield was formed during the Upper Cretaceous and part of the Tertiary. Compositions for “crusts” on each of the following rocks are given: apoharzburgite-serpentinite, amphibole pyroxenites and peridotites, phlogopite rocks, chlorite rocks, carbonatites, and chrome-spinel rocks. Vertical zoning, present in the weathering products, appears to be related to stages in the weathering processes that reflect changes in the external mediuni, especially in its redox potential and pH. Predictions may be made as to the composition of bedrock on the basis of the composition of the “crust.” — R.V. Dietrich.     

Geochemistry of the volcanoplutonic association of the Shaw massif (East Antarctica): Composition,genesis, and geodynamic interpretation

This paper reports the first petrological and geochemical evidence for the Meso-Neoproterozoic metamorphic and metaintrusive rocks of the Shaw Mountain massif (Prince Charles Mountains, East Antarctica). It was shown that the orthogneisses (plagiogneisses) and metabasites of the massif were formed as constituents of a volcanoplutonic complex, which included a variety of igneous rocks of normal and subalkaline groups, from ultrabasic to silicic and was assigned to the volcanic tholeiite basalt-andesite-rhyolite, plutonic peridotite-gabbro, and late (?) calc-alkaline gabbro-diorite-plagiogranite associations. The distribution of major and compatible trace elements indicates the fractionation of the primary melts that produced the volcanoplutonic association of the Shaw massif. With respect to the distribution of REE and trace elements and some trace element ratios, the metabasic rocks of the Shaw massif correspond mainly to enriched and normal basalts of mid-ocean ridges, continental rifts, and ocean islands, which suggests a contribution from a plume mantle source. It was found that the region of the Shaw massif is a high-grade metamorphosed margin of the Fisher volcanoplutonic complex, a Mesoproterozoic structure of single geodynamic nature. This is supported by the spatial proximity of the Shaw and Fisher regions, the similar behaviors of most major elements and distribution patterns of trace elements and REE in comparable magmatic associations, and the similar ages of some plutonic associations. Based on the petrological and geochemical data, an alternative geotectonic model was proposed for this region. According to this model, the Fisher complex was formed in a setting of continental rifting coupled with the processes of mantle diapirism, which was subsequently changed by the compression stage. During rifting, the structure could experience significant opening accompanied by ultrabasic-basic tholeiitic magmatism with a significant contribution of mantle material. A subsequent inversion resulted in that the rift structure underwent considerable horizontal compression accompanied by calc-alkaline magmatism and the formation of narrow intracratonic fold zones. The cyclic character of rifting processes and superposition of young rift systems on older ones was also established in the Phanerozoic geotectonic history of the region of the Prince Charles Mountains.     

Hydrocarbons and other volatile components in alkaline rocks from the Ukrainian shield and Kola Peninsula

Gas chromatography and other analytical techniques (EMR, PMR, and IR spectroscopy) were used to examine volatile components (CH₄, C₂-C₃, CO₂, CO, H₂, H₂O, and others) in alkaline rocks and minerals from the Ukrainian Shield (eight massifs and dikes of grorudites) and from the Khibina and Lovozero massifs in the Baltic Shield. The alkaline rocks from the Ukrainian Shield are mostly of Proterozoic (1.7–2.1 Ga) age. The alkaline rocks from the Kola Peninsula were confirmed to be rich in methane (21 ± 14 μl/g on average) and other hydrocarbons, whereas the analogous rocks from the Ukrainian Shield are poor in methane (2.1 ± 1.6 μl/g on average at a maximum of 14 μl/g). The latter rocks are richer in CO₂, which is one of the major volatile components of alkaline rocks, including agpaitic nepheline syenites from the Kola Peninsula. The rocks from the Ukrainian Shield often have elevated contents of nitrogen (up to 20 μl/g). The reasons for the differences in the composition of volatile components of rocks from the Kola Peninsula and Ukrainian Shield are as follows: the agpaitic crystallization trends of large massifs in the Kola Peninsula and much less clearly pronounced agpaitic trends in the small massifs in the Ukrainian Shield, the affiliation of these rocks with different complexes, the deeper erosion levels of the Ukrainian alkaline massifs, different ages of these rocks, etc.     

左权-赞皇变质杂岩的地球化学特征及其构造意义

左权变质杂岩位于华北克拉通中部造山带的中南段,赞皇变质杂岩西南。两杂岩区出露的早元古代——晚太古代变质岩石类型主要有:长英质片麻岩、黑云斜长片麻岩、斜长角闪岩、石榴角闪岩、云母片岩和长石石英岩等。通过详细地野外地质调查、岩相学以及地球化学研究发现,左权变质杂岩与赞皇变质杂岩有类似的地球化学性质,其中,长英质片麻岩、黑云斜长片麻岩和角闪岩的原岩均有正、有副,按原岩性质可分为变质沉积岩、变质花岗质岩石和变质基性岩三类。变质沉积岩的原岩为粘土岩或杂砂岩,物源以上地壳的长英质成分为主,REE配分型式与PAAS以及上地壳平均成分类似,原岩在形成过程中经历了中——低等程度的风化作用,沉积背景为有演化岛弧发育的活动大陆边缘;变质花岗质岩石的原岩为中酸性侵入岩,形成于大陆边缘弧环境,与变质沉积岩呈侵入接触关系;变质基性岩的原岩是拉斑——钙碱玄武质岩石,其稀土总量较低、轻稀土轻微富集,地壳混染作用明显,总体形成环境类似于现代大陆边缘的岛弧构造环境。基于以上地球化学特征推测左权——赞皇变质杂岩形成于典型碰撞造山环境,卷入了华北克拉通东部陆块和西部陆块之间的俯冲——碰撞过程。     

Paleomagnetic and tectonophysical evidence of geodynamics of Sarmatia in the Devonian (based on the example of the Ukrainian shield)

As a result of paleomagnetic investigations the positions of Devonian magmatic complexes of the Ukrainian shield (Sarmatia) in the tropical latitudes of the Southern Hemisphere in the Late Devonian were reconstructed. The moderate-temperature magnetization components in complexes of different ages of the Ukrainian Shield and recorded manifestations of the Devonian tectonomagmatic events were determined.     

Geochemistry of the Rio Espinharas hybrid complex, northeastern Brazil

The Rio Espinharas pluton, northeastern Brazil, belongs to the shoshonitic series and consists mainly of syenogranite, quartz–monzonite and porphyritic quartz–monzonite, but diorite, quartz–monzodiorite, quartz–syenite and microsyenogranite also occur containing microgranular enclaves, except for the diorite. Most variation diagrams of rocks, amphiboles, biotites and allanites show linear trends, but K, Zr, Sr and Ba of rocks display curved scattered trends. The rocks ranging from diorite to syenogranite define a pseudo-errorchron and have similar REE patterns. Syenogranite and microsyenogranite are derived from two distinct pulses of granite magma with initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7083±0.0003 and 0.7104±0.0007, respectively. Modelling of major and trace elements shows that the syenogranite evolved by fractional crystallization of plagioclase, microcline, edenite, biotite and titanite, whereas quartz–monzonite, porphyritic quartz–monzonite, quartz–monzodiorite and quartz–syenite resulted from simple mixing between an upper mantle-derived dioritic magma and the upper crust-derived syenogranite magma. Dioritic enclaves are globules of a mafic magma from the upper mantle.     

Geochemistry and petrogenesis of the Laramie anorthosite complex,Wyoming

A geochemical investigation of the Laramie anorthosite complex determined that monsonite associated with the complex are characterized by positive Eu anomalies and display a regular variation in composition with distance from the monzonite/county rock contact. Anorthositic rocks have major and trace element abundance typical of similar complexes. The internal variations in the monzonite were produced by in situ fractionation and contamination. The data indicate that anorthosite and monzonite cannot be comagmatic. It is proposed that the anorthosite and monzonite of the complex evolved from two distinct magmas, and that two stages of anatectic melting contributed to the evolution of the monzonite. An initial stage of partial melting was induced by intrusion of a gabbroic anorthosite magma into the lower crust; a second partial melting event occurred after emplacement where heat from the intrusions melted country rocks resulting in extensive contamination ofthe monzonite.     

Geochemistry and petrology of Mesozoic-Cenozoic magmatic complexes of the southern framing of the Aldan shield: Geodynamic problems

This paper summarizes the results of long-term geological, petrological, and geochemical investigations of the Mesozoic-Cenozoic complexes of the Stanovoy Range in order to determine the main reasons for their generation and evolution. The analysis of this material showed that the compositionally variable Late Mesozoic igneous complexes of the Stanovoy Range were formed in various depth facies, from abyssal to surficial. The majority of their salic complexes show minor compositional variations, whereas the mafic complexes are more variable, especially in the southeast of the region. The southeastern Stanovoy Range comprises comparable amounts of both subalkaline and low-alkali igneous rocks, whereas the central part is dominated by subalkaline rocks, and the northwestern part contains rocks only of the shoshonite-latite series. This zoning is fundamentally different from that of typical island arcs, which are characterized by the occurrence of volcanic rocks of similar alkalinity in each zone. Extrusive and intrusive rocks with similar alkali and silica contents (and schlieren-like inclusions in the granitoids of the region) were formed from common magmas of corresponding chemical compositions. In addition, the mafic and most of the salic magmas were formed as independent melting products, whereas the magmas of intermediate composition were formed mainly by mixing of chemically contrasting liquids (i.e., salic and basic). It was shown that the available information on the magmatism of the region is best interpreted in terms of the model of mantle diapirism. In particular, mantle diapirs ascended rather slowly during the Mesozoic and occurred over the whole territory of the Stanovoy Range during the Jurassic-Cretaceous stage (J₃-K₁), when alkaline and subalkaline basalts were formed. During the Early-Late Cretaceous stage, mantle diapirs produced alkali-poor basalts in the central and eastern parts. During the Cenozoic, the diapir ascended rather rapidly but only in a small area in the eastern part of the region forming alkali basalts. In contrast to the Cenozoic, the Earth’s crust was strongly affected by mantle diapirs and related mafic magmas in the Mesozoic. As a result, crustal sequences were reworked by fluids and subsequently yielded tremendous volumes of compositionally corresponding salic magmas, which interacted and mixed with mafic magmas producing the corresponding chemical zoning. The maximum generation of crustal magmas was confined to the axial zones of ascending diapirs, where the highest energy effects took place, whereas the role of autochthonous gneissic granites increased away from the axis at the expense of typical intrusive complexes.     

Geochemistry of an ophiolitic complex from New Caledonia

The ophiolites of New Caledonia are composed of ultramafics overlain by mafic rocks, all of which were affected by low P metamorphism. The mafic rocks studied (gabbroic cumulates, and basaltic flows and dikes) from Montagne des Sources are similar to recent mid-ocean ridge rocks. They are olivine-normative with Mg/Mg+Fe²⁺ ratios ranging from 0.69 in lavas to 0.90 in gabbroic cumulates and show tholeiitic fractionation trends such as a negative correlation of Ti and V with the Mg/Fe ratio. The lavas have a flat REE pattern with a slight depletion of light REE and a La/Yb ratio <2. The dikes have three different types of REE patterns. The first type is nearly parallel to that of lavas, the second one is enriched in LREE (La/Yb4) and the third type with the lowest REE contents and a distinct LREE depletion is similar to that of cumulitic pyroxene gabbro. The variations in chemical compositions of the mafic rocks can be accounted for by the dynamic partial melting process of Langmuir et al. (1977). In agreement with structural and tectonic observations, the geochemical data suggests that the ophiolites were formed during the spreading of a mid-ocean ridge with a spreading half-rate of about 1 cm/ year.     

Isotope-geochronological (U-Th-Pb, Lu-Hf) study of the zircons from the archean magmatic and metasedimentary rocks of the Podolia domain, Ukrainian shield

Zircons from the oldest magmatic and metasedimentary rocks of the Podolia domain of the Ukrainian shield were studied and dated by U-Pb method on a NORDSIM secondary-ion mass spectrometer. The age of zircon cores in the enderbite gneisses taken in the Kazachii Yar and Odessa quarries on the opposite banks of the Yuzhnyi Bug River reaches 3790 Ma. Cores of the terrigenous zircons in the quartzites from the Odessa quarry as well as in the garnet gneisses from the Zaval’e graphite quarry have an age within 3650–3750 Ma. Zircon rims record two metamorphic events at 2750–2850 Ma and around 1900–2000 Ma. Extremely low U content in the zircons of the second age group indicates conditions of the granulite-facies metamorphism in the Paleoproterozoic within the Podolia domain. Obtained data on the orthorocks (enderbite-gneiss) and metasedimentary rocks unambiguously suggest the existence of ancient Paleoarchean crust in the Podolia (Dniester-Bug) domain of the Ukrainian shield. They contribute in our knowledge of scales of the formation and geochemical features of the primordial crust.     

Geochemistry and petrology of the jequie granulitic complex (Brazil): an archean basement complex

The Jequie granulitic complex is part of the extensive high-grade metamorphic terrain located within the Sao Francisco craton of northeastern Brazil. Some Jequie rocks appear to have been formed in the middle Archean ( 3.1 Ga) from preexisting sialic crust. The dominant mineral composition of these rocks is quartz-microcline-plagioclase-hyperstene and occurs over an extensive area ( 2,000 km²).Scattered enrichment of normal granites with many minor elements (e.g. Rb, Y, Zr, Nb, Ba, REE), and the non-depletion of other elements (e.g. Cs, U), normally considered mobile during granulite facies metamorphism, must lead either to the reconsideration of regional metasomatism subsequent to granulite facies metamorphism, or at least raise some doubts about common wisdom concerning the distribution of heat-producing elements at depth. The region includes large-scale thrust structures, which could play a part in influencing high-level emplacement of the rocks and their regional metasomatism and structures.     

Geochemistry,petrogenesis and tectonic significance of the Newer Dolerites from the Singhbhum Orissa craton,eastern Indian shield

The Singhbhum Orissa craton, eastern India contains rocks as old as 3.6 Ga. The Newer Dolerites occur in two distinct orientations (NE/SW and NW/SE) in the Singhbhum Granitoid Complex (SBGC). These dikes are mostly tholeiites and quartz-normative dolerites associated with subordinate norites. We recognize three geochemical groups of the Newer Dolerites that were emplaced in the SBGC. Group I dikes contain lower SiO₂ ( < 53.29%) and higher Mg #, Ni and Cr than group II dikes. Group III dikes have higher SiO₂ than groups I and II. A few investigated samples show boninitic geochemical features. They have high-MgO (>8%), high-SiO₂ (>52%) and low-TiO₂ ( ≤ 0.5%) bulk-rock compositions. The main feature of the Newer Dolerite spidergrams is enrichment in the large-ion lithophile elements (LILE, e.g. Rb, K and Ba) relative to high field-strength elements (HFSE), resulting in high LILE/HFSE ratios. These geochemical characteristics suggest that the Newer Dolerites are subduction related. High La/Ta ratios (21–66) support a non-plume source. Therefore, we conclude that the Newer Dolerites show geochemical signatures similar to those of back-arc basalts.     

Geochemistry of the Ilesha granite gneiss in the basement complex of southwestern Nigeria

The Ilesha granite gneiss comprises a varied series ranging from porphyroblastic alkali gneiss and granitic gneiss to banded and strongly foliated melanocratic rocks. Deformation is intense and the dominant structural trend is approximately N-S.Chemical data show essentially a systematic variation reflecting the differences in the petrographic character of the outcrops. SiO₂, Na₂O, K₂O and related trace elements, particularly Rb, are higher in the alkali and granitic varieties, whereas the melanocratic types have lower contents of these elements. The basic rocks are likewise significantly enhanced in TiO₂, Fe, MgO, CaO, Cr and Ni concentrations, with some values being comparable to those of basaltic rocks.