http://www.sciencedirect.com/science/article/pii/S1674987114000243

We combine a geological, geochemical and tectonic dataset from 118 ophiolite complexes of the major global Phanerozoic orogenic belts with similar datasets of ophiolites from 111 Precambrian greenstone belts to construct an overview of oceanic crust generation over 4 billion years. Geochemical discrimi- nation systematics built on immobile trace elements reveal that the basaltic units of the Phanerozoic ophiolites are dominantly subduction-related （75%）, linked to backarc processes and characterized by a strong MORB component, similar to ophiolites in Precambrian greenstone sequences （85%）. The remaining 25% Phanerozoic subduction-unrelated ophiolites are mainly （74%） of Mid-Ocean-Ridge type （MORB type）, in contrast to the equal proportion of RiftlContinental Margin, Plume, and MORB type ophiolites in the Precambrian greenstone belts. Throughout the Phanerozoic there are large geochemical variations in major and trace elements, but for average element values calculated in 5 bins of 100 million year intervals there are no obvious secular trends. By contrast, basaltic units in the ophiolites of the Precambrian greenstones （calculated in 12 bins of 250 million years intervals）, starting in late Paleo- to early Mesoproterozoic （ca. 2.0-1.8 Ga）, exhibit an apparent decrease in the average values of incom- patible elements such as Ti, P, Zr, Y and Nb, and an increase in the compatible elements Ni and Cr with deeper time to the end of the Archean and into the Hadean. These changes can be attributed to decreasing degrees of partial melting of the upper mantle from HadeanJArchean to Present. The onset of geochemical changes coincide with the timing of detectible changes in the structural architecture of the ophiolites such as greater volumes of gabbro and more common sheeted dyke complexes, and lesser occurrences of ocelli （varioles） in the pillow lavas in ophiolites younger than 2 Ga. The global data from the Precambrian ophiolites, representative of nearly 50% of all known worldwide greenston     

辽北－吉南地区太古宙花岗岩－绿岩带地质地球化学

辽北－吉南地区是我国典型太古宙花岗岩－绿岩带出露区之一。根据其地质地球化学特征，本区绿岩带可划分为清原型和夹皮沟型，其形成的古构造环境分别为与现代岛弧的大陆边缘活动带和弧后盆地或大陆边缘裂谷相类似的裂谷型构造环境。与绿岩带有关的花岗质岩石可划分为三类：即片麻状花岗质杂岩体、花岗闪长岩和英云闪长岩底辟岩基以及钾质花岗岩。花岗岩－绿岩带的形成时代为２．５－２．９Ｇａ。     

Geochemistry and origin of the Archean Prince Albert Group volcanics,western Melville Peninsula,Northwest Territories,Canada

Major and trace element data on the Archean metavolcanic rocks of the Prince Albert Group (PAG), Northwest Territories. Canada, are reported. The following major groups were found, based on combined field and geochemical evidence: ultramafic flows; basaltic rocks, predominantly tholeiites; andesites; heavy REE depleted dacites; and rhyolites.The ultramafic and basaltic rocks are relatively normal Archean volcanics except for the downward bowed REE patterns of the tholeiitic basalts. The andesites, dacites and rhyolites, however, are not typical of Archean terrains. Comparisons between the andesites of the PAG and other Archean and more recent ones show that those of the PAG are most similar chemically to modern high-K andesites. REE patterns in these rocks suggest that partial melting of assemblages with significant garnet are an unlikely source but it is not possible to ascribe their origin to any simple process. Partial melting of a garnet-poor mafic granulite is an acceptable source for the heavy REE depleted dacites. The geochemical characteristics of the rhyolites cannot be explained by partial melting of a mafic source or by fractional crystallization from the daeites. It is suggested that these rocks originated by partial melting of pre-existing sialic crust.     

Application of geochemical mapping techniques to a complex Precambrian shield area in Labrador, Canada

Lake sediment and water geochemical data from a complex area of the Canadian shield in Labrador, Canada, display spatial variation patterns that can be linked to bedrock geology. Composite variables derived by R-mode factor analysis are effective in discriminating large-scale lithotectonic divisions, but single-element raw or residual data (corrected for effects of lake depth, Fe, Mn and LOI) provide better resolution of smaller-scale features and major tectonic boundaries.Archean high-grade gneiss regions are typified by high pH and Ni, coupled with depletion of U and F. This signature is developed most strongly over mafic igneous rocks, but is present also over Archean granitoid orthogneisses. Archean crust affected by Proterozoic structural and thermal reworking retains a high Ni signature, but is not depleted in U and F. An Early Proterozoic belt of felsic intrusive and extrusive rocks is defined by enrichment in F, U, Mo, Pb and Zn. Single-element variations suggest large-scale zonation of the belt, with the strongest enrichment over blocks interpreted to represent high crustal levels. Prominent geochemical boundaries coincide with major faults within this belt. High-grade metamorphic terranes comprising Early to Middle Proterozoic crust affected by the 1.0-Ga Grenville Orogeny show low geochemical relief, and are characterized by strong depletion in incompatible elements.In addition to reflecting the dominant rock types in each domain, some of these patterns may be related to the age, erosion level and orogenic history of the crust. For example, the Archean signature may reflect fundamental contrasts in the compositions of Archean and Proterozoic crust, suggested also by lithogeochemical and petrogenetic studies. Geochemical zonation over Early Proterozoic igneous rocks may be a function of crustal level, with the most differentiated granites, volcanic rocks and hydrothermal mineralization present in the uppermost levels of the belt. Depletion of incompatible elements over both Archean and Proterozoic high-grade metamorphic rocks may reflect expulsion of these elements by dehydration and anatexis.     

Peculiarities of the structure and evolution of the Riphean Ai volcanic complex,South Urals

The Ai volcanic complex is a part of the Ai Formation, which begins the stratotypical Riphean section in the South Urals and lies on the Archean Taratash metamorphic complex. New geochemical and isotope data were obtained for the volcanic rocks. The dominant porphyritic plagioclase and pyroxene trachibasalts associated with dacites are characterized by higher contents of alkalis and titanium, which is typical of rift volcanism. However, other geochemical data, e.g., decreased Ni contents, are beyond of this scheme. The U-Pb (SHRIMP) age of zircons from dacites is 1415 ± 11 Ma.     

The Lapland charnockitic complex: REE geochemistry and petrogenesis

The Lapland charnockitic complex is mainly composed of basic and intermediate granulites, with more restricted types including ultramafic rocks and one high-Mg enderbite occurrence. All these rocks are interbedded with or intrude the metasedimentary granulites (khondalites).The charnockites can be subdivided on the basis of their Y and REE contents into three groups. The first group is characterized by an increase in Y content with progressive differentiation and by fractionated LREE-enriched patterns quite similar to those of some Archean granulites. The REE distribution patterns of these rocks suggests an origin by partial melting of a LREE-enriched mantle source (metasomatism). The other groups and the high-Mg enderbite are depleted in Y and display strongly fractionated REE patterns either with U-shaped HREE and positive Eu anomalies similar to Archean TTG rocks despite more basic major element composition (group II and the high-Mg enderbite) or with strong HREE depletion (group III). The high-Mg enderbite approximates closely high-Mg andesites with respect to major and some trace elements and its REE pattern is identical to those of early Archean group III komatiites of the Onverwacht Group, South Africa. Parental magmas for group II rocks and the high-Mg enderbite are assumed to derive by partial melting of a garnet-enriched mantle source metasomatized by a LREE-enriched fluid, whereas those of group III could be produced by melting of quartz-eclogites or garnet-amphibolites with fractionated REE patterns similar to those of group I.The overall petrographical and geochemical features of the Lapland charnockitic complex are consistent with an evolution of the Belomorian fold belt according to a collision model.     

Geochemical features of metabasite inclusions in gray gneisses of the Baidarik block (Central Mongolia)

The Baidarik block of the Dzavhan microcontinent (Central Asian Fold Belt) includes the Upper Archean Baidaragin gray-gneiss complex. Among gray plagiogneisses, there are metabasic bodies, which are probably relics of early volcanics. By composition, the metabasites are divided into three petrochemical groups, whose protoliths were tholeiitic basalts, Al-undepleted and Al-depleted basaltic komatiites. Only a few samples are similar in REE composition to these protoliths. We have found metabasites with crustal-contamination features. The unusual geochemical properties of the metabasites (LREE enrichment and Nb, Zr, and Ti depletion) are related to their metasomatism.     

古老造山带洋板块地层*

重点分析和总结了由前寒武纪增生复合体和造山带混杂岩重建的古老造山带洋板块地层,包括由英国威尔士安格尔西岛新元古代莫纳超群混杂岩重建的太平洋洋板块地层、由澳大利亚西北部皮尔巴拉早太古代克里夫维尔绿岩带重建的古印度洋洋板块地层。澳大利亚东皮尔巴拉地块大理石坝地区早太古代玄武岩-硅质岩-碎屑岩序列与日本二叠纪-三叠纪洋板块地层在岩石组成和地球化学特征方面具有高度的相似性,这一认识将为早太古代洋板块地层的沉积环境从高热流洋脊扩张区经过热点向低热流海沟陆源碎屑沉积区转变这一过程提供有力支持。从增生造山带洋板块地层保存的岩石记录看,不同年代洋板块地层的主要物质组成和岩石类型相似,因此在地球38亿年的演化进程中,洋壳扩张、海洋沉积、俯冲及增生的过程并没有显著变化;但随着时间推移,年轻造山带洋板块性质和洋板块地层组成与古老造山带相比,可能会发生一些变化。就古老造山带洋板块地层而言,前寒武纪的地幔温度略高,太古代局部熔融显著,熔融量大大超过洋壳扩张速率,因而没有形成席状岩墙群。     

Sm-Nd ages of two meta-anorthosite complexes around Holenarsipur: Constraints on the antiquity of Archean supracrustal rocks of the Dharwar craton

Whole-rock Sm-Nd isochron ages are reported for two stratiform meta-anorthosite complexes emplaced into the Archean supracrustal-gneiss association in the amphibolite facies terrain around Holenarsipur, in the Dharwar craton, South India. While these metaperidotite-pyroxenite-gabbro-anorthosite complexes are petrologically and geochemically similar, they differ in the intensity of tectonic fabric developed during the late Archean (c. 2.5 Ga) deformation. They also differ in their whole-rock Sm-Nd isochron ages and initial Nd isotopic compositions: 3.285 ± 0.17 Ga,^ɛNd0.82 ± 0.78 for the Honnavalli metaanorthosite complex from a supracrustal enclave in the low-strain zone, and 2.495 ± 0.033 Ga, ɛNd = -2.2 ± 0.3 for the Dodkadnur meta-anorthosites from the high-strain southern arm of the Holenarsipur Supracrustal Belt (HSB). We interpret these results as indicating that the magmatic protoliths of both meta-anorthosite complexes were derived from a marginally depleted mantle at c. 3.29 Ga but only the Dodkadnur rocks were isotopically reequilibrated on a cm-scale about 800 Ma later presumably due to the development of strong penetrative fabrics in them during Late Archean thermotectonic event around 2.5 Ga. Our results set a younger age limit at c. 3.29 Ga for the supracrustal rocks of the HSB in the Dharwar craton.     

Evolution of the Archean crust in Finland

An Archean age for Finnish rocks in the range 2500–3000 Ma has been determined north of the NW-striking Ladoga—Raahe shear belt. The Archean may be divided into two main units: the granitoid association and the greenstone-belt association. The complex is characterized by stockwork tectonics. The granitoid association forms the basement infrastructure and the greenston-belt association forms the suprastructure which is present in synforms between granitoid diapirs. The infrastructure has been subjected to ultrametamorphism, and the second and third generation palingenetic magmas so formed have intruded the suprastructure. The granitoid association contains widespread migmatized relicts of the greenstone-belt association, indicating that the latter originally covered much larger areas, but the granitoids are also thought to be partly transformed primitive ensialic crust on which rocks of the greenstone-belt association were deposited. The Archean rocks have been deformed in at least four subsequent phases, of which part developed in Proterozoic time. The youngest deformation is the overthrust of the granulite belt of Lapland towards SSW. NWSE striking transcurrent faults played a major role in Proterozoic time and affected cratonized Archean crust. On the whole the greenstone belts in eastern and northern Finland form a NNW-trending zone 750 km long. On a geochemical basis the volcanic rocks of the greenstone belts can be divided into two groups: tholeiites with a low potassium content and extremely low aluminium content and a calc-alkalic group with some alkalic affinities.     

Regional geochemical anomalies related to plate tectonic models for the Northwestern Canadian shield

Regional geochemical maps of a 97,000-km² area of the northwestern Canadian Shield were based on analysis of lake sediment samples collected at 5-km grid points. The maps revealed three global-scale geochemical features: (1) a major anomaly of refractory and chalcophile elements associated with a Proterozoic volcanic-plutonic belt of previously proposed Andean are affinities; (2) a dramatic increase in Ba concentrations on crossing a proposed Precambrian suture zone; and (3) another major anomaly of refractory and chalcophile elements associated with the Archean greenstone belt closest to the supposed suture zone.The regional geochemical anomaly in the Proterozoic volcanic-plutonic belt is considered to have originated during an island-arc-type evolution of the belt by intrusion of magma formed by stages of partial melting of subducted oceanic basalts contaminated by subducted ocean floor sediments. The increase in Ba concentration found on crossing the proposed Precambrian suture is attributed to exposure of deep crust by the post-collision erosion of the uplifted edge of one of the proto-continents. The multielement, geochemical signature of the Archean greenstone belt is tentatively attributed to Archean subduction of oceanic crust and ocean sediments.The regional, multielement anomalies delineated are associated with known or suspected base metal and uranium mineralization. If the geotectonic relationships proposed above are valid, routine geochemical mapping of the Precambrian Canadian Shield will not only be of use in prospecting and mineral potential evaluation but may indirectly assist in metallogenic interpretations. Further, there is a possibility that routine geochemical mapping of the Canadian Shield may define other subduction and suture zones.     

Isotopic-geochemical features and age of zircons in dunites of the platinum-bearing type Uralian massifs: Petrogenetic implications

This paper presents results of isotopic (Cameca IMS1270 NORDSIM and SHRIMP-II ion microprobes) and geochemical (LA-ICP MS) study of zircons in three dunite samples of the Uralian-Alaskan-type massifs of the Urals: Kosva, Sakharin, and Eastern Khabarny. The zircons in the dunites share common features. Each sample contains the following genetic and age groups of zircons: (1) xenogenic zircons of the Archean and Proterozoic age; (2) zircons of magmatic appearance, which in age and geochemistry are close to the zircons from associated gabbroids; (3) postmagmatic zircons that presumably crystallized from hydrothermal solutions. The xenogenic zircons of the Archean age in each of three samples comprise transparent fragments, which are depleted in U and other trace elements and presumably have mantle origin. Xenogenic zircons of the Proterozoic age (1500–2000 Ma) occur as oval grains with surface abrasion, the traces of their redeposition. The geochemical features of the xenogenic zircons unequivocally demonstrate their affiliation to the continental crust—the basement of the Uralian orogen. The zircons of magmatic habit in all the dunite samples are close in age to the associated gabbroids: 435–432 Ma in the Kosva Massif, 378–374 in the Sakharin Massif, and 407–402 Ma in the Eastern Khabarny Massif, and mark the age of dunite formation. In addition, the magmatic zircons from dunites and associated gabbroids share similar geochemical features. These data could serve as additional argument in support of cumulate origin of dunites in the Uralian-Alaskan-type complexes. The postmagmatic zircons are most enriched in trace elements and were presumably formed from a fluid phase, which was responsible for the recrystallization of dunites and redistribution of Cr-spinel and PGE mineralization.     

Modern-style Subduction Processes in the Archean: Evidence from the Shangyi Complex in North China Craton

Abstract: Three fragments of the Archean oceanic crust have been found between the Archean granulite belt and the Paleo-Proterozoic Hongqiyingzi group in North China craton, which spread along the Shangyi-Chicheng ancient fault. This paper presents integrated field, petrology, geochemistry and geochronology evidence of the ancient oceanic fragments. The magma crystallizing age of the tonalite in the Shangyi complex is 2512±19?Ma and the geochemical characteristics suggest that the Nb-enriched basalts may be related to crustal contamination and formed in the intra-oceanic arc of the supra subduction zone setting.     

Magmatic sources of dikes and veins in the Moncha Tundra Massif,Baltic Shield: Isotopic-geochronologic and geochemical evidence

The dike-vein complex of the Moncha Tundra Massif comprises dolerites, gabbro-pegmatites, and aplites. The dolerite dikes are classified into three groups: high-Ti ferrodolerites, ferrodolerites, low-Ti and low-Fe gabbro-dolerites. The U-Pb age of the ferrodolerites is 2505 ± 8 Ma, and the amphibole-plagioclase metagabbroids hosting a ferrodolerite dike are dated at 2516 ± 12 Ma. Data on the U-Pb isotopic system of zircon from the gabbro-pegmatites and titanite from the aplites indicate that the late magmatic evolution of the Moncha Tundra Massif proceeded at 2445 ± 1.7 Ma, and the youngest magmatic events in the massif related to the Svecofennian orogeny occurred at 1900 ± 9 Ma. The data obtained on the Sm-Nd and Rb-Sr isotopic systems and the distribution of trace elements and REE in rocks of the dike-vein complex of the massifs provide insight into the composition of the sources from which the parental magmas were derived. The high-Ti ferrodolerites were melted out of a deep-sitting plume source that contained an asthenospheric component. The ferrodolerites were derived from a mantle MORB-type source that contained a crustal component. The parental melts of the gabbro-dolerites were melted out of the lithospheric mantle depleted in incompatible elements after Archean crust-forming processes above an ascending mantle plume, with the participation of a crustal component. The gabbro-dolerites and the rocks of the layered complex of the Moncha Tundra Massif exhibit similar geochemical characteristics, which suggest that their parental melts could be derived from similar sources but with more clearly pronounced crustal contamination of the parental melts of the rocks of the massif itself. The geochemical traits of the gabbro-pegmatites are thought to be explained not only by the enrichment of the residual magmas in trace elements and a contribution of a crustal component but also by the uneven effect of sublithospheric mantle sources. The aplites were derived from a sialic crustal source.     

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.     

太古宙TTG 能否与埃达克岩对比？——全球数据给出的结果

文中收集了全球太古宙TTG 和后太古宙埃达克岩的数据,研究对比表明,太古宙TTG 不同于埃达克岩,早先学术界认为太古宙TTG 相当于现代的埃达克岩,是一个错误的见解,是一个伪命题。TTG 术语最早出现时并没有类似于埃达克岩的见解,后来发现了埃达克岩才引出二者类似的认识。为什么会出现这种见解？推测有两种可能：1）早先的研究均来自抽样的研究,即从典型到一般的推理方法。但是,抽样研究可能没有代表性。2）TTG平均值的误导,TTG 的Sr的平均值的确＞400×10^-6,相当于埃达克岩,但是,通过分析TTG 全球数据表明,TTG 中的Sr含量变化很大,与埃达克岩没有关系。TTG 主要出现在太古宙,后太古宙很少;TTG 是太古宙岩石地壳的主体;而埃达克岩主要出现在后太古宙,是火山岩中很少的类型。这表明,TTG 和埃达克岩分别处于不同的构造体制。太古宙异常的热,可能还没有板块构造。太古宙的岩浆岩（包括TTG）即使与后太古宙某些岩浆岩具有类似的地球化学特征,可能也不能照搬现代板块构造的解释。     

Evidence for Archean ocean crust with low high field strength element signature from diamondiferous eclogite xenoliths

Late Archean (2.57 Ga) diamond-bearing eclogite xenoliths from Udachnaya, Siberia, exhibit geochemical characteristics including variation in oxygen isotope values, and correlations of δ¹⁸O with major elements and radiogenic isotopes which can be explained by an origin as subducted oceanic crust. Trace element analyses of constituent garnet and clinopyroxene by Laser-ICPMS are used to reconstruct whole-rock trace element compositions, which indicate that the eclogites have very low high field strength element (HFSE) concentrations and Zr/Hf and Nb/Ta ratios most similar to modern island arcs or ultradepleted mantle. Although hydrothermal alteration on the Archean sea floor had enough geochemical effect to allow the recognition of its effects in the eclogites and thus diagnose them as former oceanic crust, it was not severe enough to erase many other geochemical features of the original igneous rocks, particularly the relatively immobile HFSEs. Correlations of the trace element patterns with oxygen isotopes show that some, generally Mg-richer, eclogites originated as lavas, whereas others have lower δ¹⁸O and higher Sr and Eu contents indicating an origin as plagioclase-bearing intrusive rocks formed in magma chambers within the ocean crust. Major and trace element correlations demonstrate that the eclogites are residues after partial melting during the subduction process, and that their present compositions were enriched in MgO by this process. The original lava compositions were picritic, but not komatiitic, whereas the intrusives had lower, basaltic MgO contents. The HFSE signature of the eclogites may indicate that ocean floor basalts of the time were relatively close to island arcs and recycled material, which would be consistent with a larger number of smaller oceanic plates. Their composition appears to indicate that komatiitic ocean crust compositions were restricted to the early Archean which is not known to be represented among the eclogite xenolith population.     

The Shakhtama porphyry Mo ore-magmatic system (eastern Transbaikalia): age,sources, and genetic features

Two intrusive complexes are recognized at the Shakhtama deposit: Shakhtama and ore-bearing porphyry. The U–Pb zircon dates (SHRIMP II) are 161.7 ± 1.4 and 161.0 ± 1.7 Ma for the monzonites and granites of the Shakhtama complex and 159.3 ± 0.9 and 155.0 ± 1.7 Ma for the monzonite- and granite-porphyry of the ore-bearing complex. The igneous complexes formed in a complex geodynamic setting in the late Middle Jurassic and early Late Jurassic, respectively. The setting combined the collision of continents during the closure of the Mongol-Okhotsk ocean and the influence of mantle plume on the lithosphere of the Central Asian orogenic belt. The intrusion of the Shakhtama granitoids took place at the end of the collision, and the intrusion of porphyry of the ore-bearing complex, during the change of the geodynamic setting by a postcollisional (rifting) one. The complexes are composed of monzonite–granite series with similar geochemical characteristics of rocks. The performed geological, geochemical, and isotope-geochemical studies suggest that the sources of magmas were juvenile crust and Precambrian metaintrusive bodies. The juvenile mafic crust is considered to be the predominant source of fluid components and metals of the Shakhtama ore-magmatic system. The granitoids of both complexes include calc-alkalic high-K rocks with typical geochemical characteristics and with characteristics of K-adakites. These geochemical features indicate that the parental melts of the former rocks were generated at depths shallower than 55 km, and the melts of the latter, at depths of 55–66 km. K-adakite melts resulted from the melting of crust submerged into the mantle during the lithosphere delamination, which was caused by the crust thickening as a result of the repeated inflow of basic magma into the basement of the crust and tectonic deformations in its upper horizons. The high-Mg monzonitic magma produced under these conditions ascended and was mixed with melts generated in the upper horizons, which accounts for the high Mg contents of the Shakhtama granitoids. The similar compositions and petrogeochemical characteristics of the granitoids of the Shakhtama and porphyry complexes point to the same sources, transport paths, and evolution trend of their parental melts. This indicates that the igneous rocks of both complexes are products of the same long-living magmatic system, which produced Mo mineralization at the final stage. The favorable conditions for the ore production in the magmatic system during the formation of the porphyry complex appeared as early as the preceding stage—during the formation of the Shakhtama complex, which we regard as a preparatory stage in the evolution of the ore-magmatic system.     

Petrology and mineralogy of ophiolites in pull-apart structures of the Cayman Trough

New geological, mineralogical, and geochemical data on rocks dredged in the local spreading zone of the Cayman Trough (Caribbean Sea) make it possible to use this ophiolite complex as a reference one for pull-apart structures. The ophiolites form fault-bounded narrow elongated zones. Separate magmatic complexes within these zones can be of different ages, which decrease along the strike from the flanks to the central parts. The base of the ophiolite complex is mostly composed of lherzolites, while the gabbroid complex demonstrates clear magmatic layering formed through crystallization differentiation. At the same time, the crystalline and igneous rocks define a single geochemical series, which is characterized by the accumulation of lithophile and light rare-earth elements in more differentiated varieties. The REE distribution patterns are strongly correlated with the mineral composition of the rocks. The geochemical characteristics of the ophiolites indicate their affiliation to the plume type.     

Early Precambrian granite-gneiss complexes in the Central Aldan Shield

The central portion of the Aldan Shield hosts very widely spread Archean and Early Proterozoic granitoids, much of which are granite-gneisses. Geochemical lines of evidence, data on inclusions in minerals, and Sm-Nd isotopic geochemical data suggest that the protoliths of granite-gneisses in the central part of the Aldan Shield were granitoids that had various composition, age, and were derived from distinct sources and under different parameters and were then emplaced in different geodynamic environments. The granitoids belong to at least two types of different composition that occur within spatially separated areas. The protoliths of granite-gneisses in the western part of the Western Aldan Megablock and the junction zone of the Chara-Olekma and Aldan geoblocks (granite-gneisses of type I) had the same age and affiliated to the same associations as the within-plate granitoids of the Nelyukinskii Complex. Their parental melts were derived at 2.4–2.5 Ga by the melting of Archean tonalite-trondhjemite orthogneisses of the Olekma and Aldan complexes. The protolith of granite-gneisses in the eastern portion of the Western Aldan Megablock (granite-gneisses of type II) can be subdivided into two groups according to their composition: granitoids with geochemical characteristics of subduction- and collision-related rocks. The protoliths of the type-II granite-gneisses with geochemical characteristics of subduction granitoids were produced simultaneously with the development of the Fedorovskaya island arc (at 2003–2013 Ma), whereas the protoliths of the type-II granite-gneisses with geochemical characteristics of collision granitoids were formed in the course of accretion of the Fedorovskaya island arc and the Olekma-Aldan continental microplate at 1962–2003 Ma, via the melting of magmatic rocks of the Fedorovskaya unit and older continental crustal material.