The first Sm–Nd isotope–geochemical data on the Paleoproterozoic age of metamorphic rocks from the crystalline basement of the Yurovsk rise (Okhotsk massif)

Rocks with Paleoproterozoic Sm–Nd model ages (T _Nd(DM) = 2096–2350 Ma) are the sources of protoliths with gneiss, amphibolite, marble, and calciphyre of Yurovsk rise. This fact makes possible estimation of the lower age boundary of the formation of the crystalline basement of Yurovsk rise as Paleoproterozoic. According to the results of Sm–Nd, U–Pb (SHRIMP), and Pb–Pb geochronological studies, two isotope provinces are distinguished within the crystalline basement of the Okhotsk massif. The Paleoarchean province occupies the territory of Kukhtui rise, in which crust-forming processes played a key role (3250–3650 Ma). The Paleoproterozoic province includes Yurovsk rise, in which the major stages of crust formation have an age of 1900–2350 Ma.     

Paleoproterozoic Granitoids of the Yurovka Swell Basement (Okhotsk Massif): First U–Pb SIMS Geochronological and Nd–Sr Isotope–Geochemical Data

The first results of U–Pb SIMS geochronological and Nd–Sr isotope–geochemical studies of the Yurovka Complex metavolcanics and granitoids of the Luktur Complex belonging to the Yurovka Swell yielded a Paleoproterozoic age of their formation pointing to a considerable Paleoproterozoic continental crust formation event. These data allow us to reconsider existing ideas about the similarity of the composition and age of the basement of the Yurovka Swell and that of the Paleoarchaean Kukhtui Swell of the Okhotsk Massif.     

Main formation stages of the paleoarchean crust in the Kukhtui Inlier of the Okhotsk Massif

Based on U-Pb dating (SHRIMP-II) of euhedral zircon cores from hypersthene-plagioclase granulites of the Kukhtui Inlier (Okhotsk Massif), their igneous protholith of the basic composition is estimated to be Archean in age (3.7 Ga), which is confirmed by the Sm-Nd measurements. Subsequent tectonothermal events are established to occur 3.3–3.2, 2.8–2.7, and 1.9–1.8 Ga ago.     

The isotope composition of Hf in zircon from Paleoarchean plagiogneisses and plagiogranitoids of the Sharyzhalgai uplift (southern Siberian craton): Implications for the continental-crust growth

This paper presents results of U–Pb dating (SHRIMP-II) and Lu–Hf (LA–ICP MS) isotope study of zircon from Paleoarchean plagiogneisses and plagiogranitoids of the Onot and Bulun blocks of the Sharyzhalgai uplift. Magmatic zircons from the Onot plagiogneiss and Bulun gneissic trondhjemite are dated at 3388±11 and 3311±16 Ma, respectively. Magmatic zircons from plagiogneisses and plagiogranitoids of the studied tonalite–trondhjemite–granodiorite (TTG) complexes are characterized mainly by positive values of εHf indicating that felsic melts were generated mainly from juvenile (mafic) sources, which are derived from a depleted mantle reservoir. The variable Hf isotope composition in magmatic zircons and the lower average εHf values in comparison with the depleted mantle values suggest the contributions of both mafic and more ancient crustal sources to magma formation. Metamorphic zircons from the gneissic plagiogranite and migmatized plagiogneiss either inherited the Hf isotope composition from magmatic zircon or are enriched in radiogenic Hf. The more radiogenic Hf isotope composition of metamorphic zircons from the migmatized plagiogneisses is due to their interaction with melt during partial melting. Variations in the Lu–Hf isotope composition of zircon from the Bulun rocks in the period 3.33–3.20 Ga are due to the successive melting of mafic crust or the growing contribution of crustal material to their genesis. Correlation between the Lu–Hf isotope characteristics of zircon and the Sm–Nd parameters of the Onot plagiogneisses points to the contribution of ancient crustal material to their formation. The bimodal distribution of the model Hf ages of zircons reflects two stages of crustal growth in the Paleoarchean: 3.45–3.60 and ~ 3.35 Ga. The isotope characteristics of zircon and rocks of the TTG complexes, pointing to recycling of crustal material, argue for the formation of plagiogneisses and plagiogranitoids as a result of melting of heterogeneous (mafic and more ancient crustal) sources in the thickened crust.     

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.     

Geochronological and geochemical constraints on the Erguna massif basement,NE China – subduction history of the Mongol–Okhotsk oceanic crust

We present new geochronological and geochemical data for granites and volcanic rocks of the Erguna massif, NE China. These data are integrated with previous findings to better constrain the nature of the massif basement and to provide new insights into the subduction history of Mongol–Okhotsk oceanic crust and its closure. U–Pb dating of zircons from 12 granites previously mapped as Palaeoproterozoic and from three granites reported as Neoproterozoic yield exclusively Phanerozoic ages. These new ages, together with recently reported isotopic dates for the metamorphic and igneous basement rocks, as well as Nd–Hf crustal-residence ages, suggest that it is unlikely that pre-Mesoproterozoic basement exists in the Erguna massif. The geochronological and geochemical results are consistent with a three-stage subduction history of Mongol–Okhotsk oceanic crust beneath the Erguna massif, as follows. (1) The Erguna massif records a transition from Late Devonian A-type magmatism to Carboniferous adakitic magmatism. This indicates that southward subduction of the Mongol–Okhotsk oceanic crust along the northern margin of the Erguna massif began in the Carboniferous. (2) Late Permian–Middle Triassic granitoids in the Erguna massif are distributed along the Mongol–Okhotsk suture zone and coeval magmatic rocks in the Xing’an terrane are scarce, suggesting that they are unlikely to have formed in association with the collision between the North China Craton and the Jiamusi–Mongolia block along the Solonker–Xra Moron–Changchun–Yanji suture zone. Instead, the apparent subduction-related signature of the granites and their proximity to the Mongol–Okhotsk suture zone suggest that they are related to southward subduction of Mongol–Okhotsk oceanic crust. (3) A conspicuous lack of magmatic activity during the Middle Jurassic marks an abrupt shift in magmatic style from Late Triassic–Early Jurassic normal and adakite-like calc-alkaline magmatism (pre-quiescent episode) to Late Jurassic–Early Cretaceous A-type felsic magmatism (post-quiescent episode). Evidently a significant change in geodynamic processes took place during the Middle Jurassic. Late Triassic–Early Jurassic subduction-related signatures and adakitic affinities confirm the existence of subduction during this time. Late Jurassic–Early Cretaceous post-collision magmatism constrains the timing of the final closure of the Mongol–Okhotsk Ocean involving collision between the Jiamusi–Mongolia block and the Siberian Craton to the Middle Jurassic.     

Geochemical and Sr–Nd isotope evidences of the suprasubduction nature of mesozoic magmatism in the Mongol–Okhotsk Sector of the Pacific Fold Belt

In this article we present geochemical and isotope characteristics of rocks of the Unerikan, Selitkan and Aezop–Yamalin volcano–plutonic zones of the eastern termination of the Mongol–Okhotsk Orogenic Belt. The obtained data demonstrate that the Mesozoic igneous rocks of the Mongol–Okhotsk sector of the Pacific Folded Belt were formed due to the melting of the continental crust in a tectonic setting corresponding to a suprasubduction one.     

Age,geochemistry, and sources of clastic materials and accumulation settings of the Glubokin Formation (Eastern Transbaikalia)

We report the results of geochemical, Sm–Nd isotope–geochemical, and U–Pb detrital zircon geochronological studies of metaterrigenous rocks of the Glubokin Formation assigned provisionally to the Upper Riphean. This formation is developed where the Mongol–Okhotsk belt is almost completely squeezed by the surrounding continental structures and, hence, bears important information on its evolution. The obtained results suggest the following conclusions: (1) the Glubokin Formation is Early Pennsylvanian or post-Early Pennsylvanian in age, not Riphean as previously suggested; (2) the Glubokin Formation belongs to the Mongol–Okhotsk Fold Belt rather than to the Argun continental massif, as suggested in the existing schemes of structural zoning; (3) the volcanogenic-terrigenous deposits of the Glubokin Formation were precipitated in a back-arc basin setting above the subduction zone subsiding beneath the southeastern margin of the North Asian Craton; and (4) the main sources of clastic material for the Glubokin Formation were igneous and metamorphic complexes of different ages from the southeastern margin of the North Asian Craton.     

成都市土壤测量Hg元素的发育特征及其来源

成都市是Hg的高背景和民异常区，深、浅层样各具不同的元素组合和发育特征。深层样Hg异常与构造、天然气、地热、矿产和基底隆起有关；浅层样则与城市居民生活，古代颜料、工业、燃煤、农业、河流的污染及上游的矿产等物质来源有关。     

Paleoarchean tonalite-trondhjemite complex in the northwestern part of the Sharyzhalgai uplift (southwestern Siberian craton): results of U-Pb and Sm-Nd study

In the northwestern part of the Sharyzhalgai uplift of the Siberian craton (Bulun block), the earliest sialic crust (grey-gneiss complex) is composed of plagiogneisses, their migmatized varieties, and subordinate plagiogranitoids. The petrochemical, trace-element, and Sm-Nd isotope compositions of rocks were studied, and U-Pb dating of zircons (SHRIMP II) was performed. Plagiogneisses and plagiogranitoids of trondhjemite and, more seldom, tonalite compositions are predominant; their compositions are typical of rocks of Archean tonalite-trondhjemitegranodiorite (TTG) complexes (Al₂O₃ ≥ 15%, Mg# = 28–38, (La/Yb)_n = 23–66, Sr/Y = 27–135, Eu/Eu^? = 0.7–1.1). Plagiogneisses of meta-andesite-rhyodacite association are subordinate (SiO₂ = 59–69%, (La/Yb)_n = 7–32, Sr/Y = 11–24, Eu/Eu^? = 0.5–0.7). Cathodoluminescent study of zircons revealed “magmatic cores” and metamorphic rims; most of the rims differ from the cores in U and Th contents and low or greatly varying Th/U ratios. In migmatized plagiogneisses of trondhjemite composition, two zircon generations of different morphologies have been recognized. The protoliths of the grey-gneiss complex rocks formed in the Paleoarchean as a result of two discrete magmatic events, at ～3.3 and 3.25 Ga, and their metamorphism and migmatization took place at ～3.2 Ga. The isotopic and geochemical features of rocks evidence that the primary melts were produced mainly through the melting of metabasic sources at different depths of the thickened crust. Plagiogneisses of trondhjemite composition apparently resulted from magma generation involving ancient sialic material.     

Sources of the Jurassic terrigenous rocks of the Upper Amur and Zeya–Dep troughs of the eastern part of the Central Asian fold belt: Results of Sm–Nd isotopic–geochemical and U–Pb (LA-ICP-MS) geochronological studies

The results of U–Pb geochronological studies of detrital zircons and Sm–Nd isotopic–geochemical studies of terrigenous rocks of the Upper Amur and Zeya–Dep troughs indicate that the beginning of orogenic processes, which led to the formation of the Mongol–Okhotsk fold belt, occurred in the Early Jurassic.     

Geochemical Features and Sources of Metasedimentary Rocks of the Western Part of the Tukuringra Terrane of the Mongol–Okhotsk Fold Belt

This work presents the results of geological, geochemical, Sm–Nd isotope-geochemical studies of metasedimentary rocks of the Teploklyuchevskaya, Garmakan, and Algaja formations of the Tukuringra Terrane of the eastern part of the Mongol–Okhotsk fold belt, as well as U–Th–Pb geochronological (LA-ICP-MS) studies of detrital zircons from these rocks. It is established that the lower age boundary of formation of the protolith of metasedimentary rocks of the Teploklyuchevskaya Formation is about 243 Ma (Middle Triassic); those of the Garmakan and Algaja formations are ~175 Ma (Lower–Middle Jurassic boundary) and ~192 Ma (Lower Jurassic), respectively. This makes it possible to correlate the Teploklyuchevskaya, Garmakan, and Algaja formations with the youngest sedimentary complexes of the eastern part of the Mongol–Okhotsk fold belt. In terms of geochemistry, the protoliths of metasedimentary rocks of the above-mentioned formations are the most similar to sedimentary rocks of island arcs and active continental margins. The source terrigenous material was transported from the southern frame of the Mongol–Okhotsk fold belt. It is not improbable that Lower Mesozoic deposits of the western part of the Tukuringra Terrane, in particular, and the eastern part of the Mongol–Okhotsk fold belt, as a whole, are relics of residual basins, preserved in “gaps” in the collision zone between the southern margin of plates of the North Asian Craton and the Amur Superterrane.     

The Early Paleozoic Active Margin of the Khangai Segment of the Mongol–Okhotsk Ocean

Within the northern fringe of the western (Khangai) flank of the Mongol–Okhotsk fold belt, magmatic complexes of intermediate to moderately acidic rocks occur. They comprise widely distributed gabbro–diorites, diorites, tonalites, and granodiorites. Geochronological studies have demonstrated that these rocks were formed in the time span of 437 to 375 Ma. The geochemical affinities of the rocks suggest their formation in subduction tectonic settings; hence, their paleotectonic position corresponds to the continental margin of the Mongol–Okhotsk paleoocean. It has been concluded that this Middle Paleozoic igneous activity occurred in the active continental margin settings, formed by subduction of the paleooceanic plate under the Siberian continent.     

Timing of closure of the eastern Mongol–Okhotsk Ocean: Constraints from U–Pb and Hf isotopic data of detrital zircons from metasediments along the Dzhagdy Transect

The Mongol–Okhotsk Belt, a major structural element of East Asia, is probably the youngest orogenic segment within the Central Asian Orogenic Belt. However, the timing of final closure of the Mongol–Okhotsk Ocean remains unresolved. Here, we present detrital zircon U–Pb–Hf isotopic data and whole-rock geochemical data (major and trace elements and Sm-Nd isotopes) for the metasedimentary rocks from the Un'ya–Bom Terrane, Dzhagdy Terrane, and the eastern part of the Tukuringra Terrane. Our new zircon U-Pb ages suggest that all sedimentary formations along the Dzhagdy Transect are early Mesozoic in age, rather than Paleozoic as previously thought. The detrital zircons from the metasedimentary rocks in the Un'ya–Bom Terrane, the Dzhagdy Terrane, and the eastern part of the Tukuringra Terrane yielded the youngest concordant ages of 194 ± 4, 193 ± 2, and 171 ± 2 Ma, respectively. Moreover, we note that the so-called sedimentary formations of these terranes are not single sedimentary sequences as previously suggested, but a set of an olistostrome or tectonic mélanges composed of rocks of different ages and origins. These sedimentary formations are probably relics of the Mongol–Okhotsk remnant basin that formed in the “gaps” between the southern margin of the North Asian Craton and the Amur Block during their collision. The absence of detrital zircons younger than 171 Ma in the sedimentary rocks of the Mongol–Okhotsk basin implies that the final closure of this basin could have taken place at the boundary of the Early and Middle Jurassic as a result of the collision or the development of the Mongol–Okhotsk orogenic belt in this region. After that, the Mongol–Okhotsk Belt underwent intense deformation related to within-plate strike-slip faulting, which could be attributed to the late Mesozoic rotation of the North Asian Craton relative to the continental massifs of East Asia.     

Evolution of the 3.65–2.58 Ga Mairi Gneiss Complex,Brazil: Implications for growth of the continental crust in the São Francisco Craton

The composition and formation of the Earth’s primitive continental crust and mantle differentiation are key issues to understand and reconstruct the geodynamic terrestrial evolution, especially during the Archean. However, the scarcity of exposure to these rocks, the complexity of lithological relationships, and the high degree of superimposed deformation, especially with long-lived magmatism, make it difficult to study ancient rocks. Despite this complexity, exposures of the Archean Mairi Gneiss Complex basement unit in the São Francisco Craton offer important information about the evolution of South America’s primitive crust. Therefore, here we present field relationships, LA-ICP-SFMS zircon U-Pb ages, and LA-ICP-MCMS Lu-Hf isotope data for the recently identified Eoarchean to Neoarchean gneisses of the Mairi Complex. The Complex is composed of massive and banded gneisses with mafic members ranging from dioritic to tonalitic, and felsic members ranging from TTG (Tonalite-Trondhjemite-Granodiorite) to granitic composition. Our new data point to several magmatic episodes in the formation of the Mairi Gneiss Complex: Eoarchean (ca. 3.65–3.60 Ga), early Paleoarchean (ca. 3.55–3.52 Ga), middle-late Paleoarchean (ca. 3.49–3.33 Ga) and Neoarchean (ca. 2.74–2.58 Ga), with no records of Mesoarchean rocks. Lu-Hf data unveiled a progressive evolution of mantle differentiation and crustal recycling over time. In the Eoarchean, rocks are probably formed by the interaction between the pre-existing crust and juvenile contribution from chondritic to weakly depleted mantle sources, whereas mantle depletion played a role in the Paleoarchean, followed by greater differentiation of the crust with thickening and recycling in the middle–late Paleoarchean. A different stage of crustal growth and recycling dominated the Neoarchean, probably owing to the thickening of the continental crust by collision, continental arc growth, and mantle differentiation.     

内蒙古海拉尔北部八大关地区花岗岩的成岩时代、地球化学特征与成因

内蒙古海拉尔北部八大关地区花岗岩以正长花岗岩和二长花岗岩为主,锆石U Pb定年显示其形成于晚三叠世((212±17)~(2267±16) Ma)。岩石为弱过铝质(A/CNK=101~110),分异指数(DI)介于925~958之间。岩石大离子亲石元素K和Rb以及LREE相对富集,高场强元素Nb、Ta、P、Ti 和Ba、Sr相对亏损,并显示中等铕负异常(δEu=051~071)。较低的10 000 Ga/Al值(平均191)以及较低的锆石饱和温度(平均683 ℃)等特征显示该花岗岩属于高分异Ⅰ型。锆石的εHf(t)值较高(450～1045),Hf二阶段模式年龄为059～097 Ga,反映其源区物质为新元古代增生的基性地壳。综合区域其他地质研究成果,认为研究区晚三叠世花岗岩形成于蒙古—鄂霍茨克洋板块俯冲的活动大陆边缘环境。     

Middle Triassic age of metarhyolite of the Bondikha Formation,Argun Continental Massif,Central Asian Fold Belt

Research works carried out for the first time made it possible to determine the age of one of the key geological complexes in the Argun Continental Massif (Central Asian Fold Belt). It has been found that the metarhyolite of the Bondikha Formation is not Middle–Late Riphean, as previously thought, but is of Middle Triassic age (242 ± 6 Ma). Based upon the geochemical characteristics of the volcanics and regional geology of the Bondikha Formation, it can be assumed that they formed during one of the stages of subduction processes in the history of the Mongol–Okhotsk Fold Belt.     

辽宁鞍山地区陈台沟蛇绿岩地球化学特征

岩石化学、地球化学资料表明,古太古代陈台沟蛇绿岩由绿泥滑石片岩和蛇纹岩（橄榄岩）、角闪岩类岩石（拉斑玄武岩）、石英岩（远洋深海硅质岩）组成,它们具备蛇绿岩套层序的基本特征,可能为古太古代洋壳残片.陈台沟蛇绿岩具有贫硅、贫碱、低钛,而富铁、镁,稀土总量很低的特点.微量元素分析结果表明该蛇绿岩中的玄武岩富集K、Rb、Ba等大离子亲石元素,亏损Nb、Hf等高场强元素,具有岛弧型火山岩的特点,其在稀土元素球粒陨石标准化配分图解中主要显示平坦型曲线.陈台沟蛇绿岩的形成环境可能为岛弧环境.     

Composition,Age, and Origin of Cretaceous Granitic Magmatism on the Eastern Chukchi Peninsula

New geochronological and isotopic geochemical data are given, which make it possible to recognize two types of granitic rocks on the eastern Chukchi Peninsula. Early Cretaceous Tkachen and Dolina granitic plutons with zircon ages (U–Pb SIMS) of 119–122 and 131–136 Ma are related to the first type. They cut through Devonian–Lower Carboniferous basement rocks and are overlain by the Aptian–Albian Etelkuyum Formation. Basal units of the latter contain fragments of granitic rocks. Late Cretaceous Provideniya and Rumilet granitic plutons, which contain zircons with ages of 94 and 85 Ma (U–Pb SIMS), respectively, belong to the second type. They cut through volcanic–sedimentary rocks of the Etelkuyum and Leurvaam formations pertaining to the Okhotsk–Chukotka Volcanic Belt. In petrographic and geochemical features, the Early Cretaceous granitic rocks of the Tkachen Pluton are commensurable with I-type granites, while Late Cretaceous granite of the Rumilet Pluton is comparable to A₂-type granite. The Sr–Nd isotopic data provide evidence that from the Early Cretaceous Tkachen and Dolina plutons to the Late Cretaceous Provideniya and Rumilet plutons, the degree of crustal assimilation of suprasubduction mantle-derived melts increases up to partial melting of heterogeneous continental crust enriched in rubidium. An unconformity and various degrees of secondary alteration of volcanic–sedimentary rocks have been established in the Okhotsk–Chukotka Volcanic Belt, and this was apparently caused by transition of the tectonic setting from suprasubduction to a transform margin with local extension.     

The U–Pb age and Lu–Hf isotope composition of detrital zircon from metasedimentary rocks of the Onot greenstone belt (Sharyzhalgay uplift,southern Siberian craton)

We present data on the composition of metasedimentary rocks from the greenstone belt of the Onot terrane (Sharyzhalgay uplift) and results of U–Pb dating (SHRIMP II) and Lu–Hf isotope study of detrital zircon from garnet–staurolite schists. The metasedimentary rocks of the Onot greenstone belt are dominated by garnet- and staurolite-bearing schists alternating with amphibolites (metabasalts) in the upper part of the section. Compositionally the protoliths of garnet–staurolite schists correspond to sedimentary rocks, ranging from siltstone to pelitic mudstone. The trace-element characteristics of the garnet–staurolite schists indicate that the terrigenous material was derived from three different rock types, such as tonalite–trondhjemite plagiogneisses (elevated Gd/Yb ratios), mafic rocks (elevated Cr/Th ratios and reduced Th/Sc ratios), and felsic igneous rocks formed by crustal melting (the presence of a Eu minimum), which agrees with the set of potential source rocks from the Onot terrane. The age of predominant detrital zircon reflects the erosion of mainly Neoarchean igneous rocks; this fact, combined with the poor rounding of zircon and tectonically active sedimentation conditions accompanied by mafic volcanism, suggests that the probably depositional age is ca. 2.7 Ga. Older source rocks (2.80–3.35 Ga) contributed to the sediment deposition along with the Neoarchean ones. According to the Hf isotope composition of detrital zircon from the garnet–staurolite schists, the source provenances had different crustal prehistories. The source provenances include Paleoarchean and juvenile Neoarchean crust and rocks formed by the mixing of melts from ancient and juvenile crustal sources.