Archean rock homologs in the Kola superdeep borehole section in the northern part of the White Sea mobile belt,Voche-Lambina test site

The Archean Complex homologs of the Kola superdeep borehole (SG-3) were identified in the northern part of the White Sea mobile belt. Tonalite-trondhjemite-granodiorite gneisses of the Voche-Lambina test site and metavolcanic dacite-rhyodacite rocks of the borehole SG-3 were formed at the stages of 2.97–2.82, ∼2.81, and 2.78–2.79 Ga. The Sm-Nd model ages of the studied rocks do not exceed 3.1 Ga, and their positive ɛNd(t) values vary from +0.5 to +3.34. They are characterized by Mg# = 0.20−0.44, similar concentrations (HFSE) of Zr, Nb, Y, and also Rb, Cr, and Ni, and sharply differentiated spectra of the REE distribution (Ce/Sm = 3.2−5.8; Gd/Yb = 2.6−7.1). Primary melts were formed in balance with garnetamphibole restite under P ≥ 15−16 kbar.     

A deep borehole in the East European platform margin: Petrologic and isotopic-geochronologic data

The petrologic and isotopic-geochronologic study of basement rocks that were penetrated by a deep borehole in the marginal part of the East European Platform revealed that its section overlain by the Vendian-Paleozoic sedimentary cover is Early Proterozoic in age and largely consists of aluminous migmatized biotite, biotite-cordierite, and biotite-cordierite-sillimanite gneisses, which are intruded by granites, plagiogranites, and metatonalites. The lower part of the section is dominated by amphibole schists and amphibolites with subordinate nonmetamorphosed dolerite dikes and pegmatite veins. By metamorphism parameters (T = 630–680°C, P = 2–4 kbar), the metamorphic complex may be considered as the shallowest one, compared with other Early Proterozoic complexes, developed at least in the southwestern part of Fennoscandia. The progressive decrease in the mineral-formation pressure observed in the Lower Proterozoic metamorphic rocks southward, away from the Karelian Craton is likely explained by the tectonic transport (thrusting) of the Svecofennides over the margin of the Karelian Craton and their subsequent deeper erosion near the craton. The magmatic crystallization of metamorphic palgiogranites, penetrated at depths of 925–928 and 1004 m, is estimated by the U-Pb ID-TIMS method on zircons to occur 1860 ± 9 Ma ago. It is shown that by their age, the REE composition, and isotopic-geochemical characteristics, these rocks are close to the plagiogranites formed in the southeastern extremity of the Svecofennnian belt in the present-day northern Ladoga region and the Karelian Isthmus. No rocks, which could be correlated by their lithology with the Archean rocks of the Karelian Craton, are found.     

Calc-Alkaline Magmatism at the Archean Proterozoic Transition: the Caico Complex Basement (NE Brazil)

The Paleoproterozoic metaplutonic rocks of the CaicóComplex Basement (Seridó region, NE Brazil) provide importantand crucial insights into the petrogenetic processes governingcrustal growth and may potentially be a proxy for understandingthe Archean–Proterozoic transition. These rocks consistof high-K calc-alkaline diorite to granite, with Rb–Sr,U–Pb, Pb–Pb and Sm–Nd ages of c. 2·25–2·15Ga. They are metaluminous, with high Yb_N, K₂O/Na₂O and Rb/Sr,low I_Sr ratios, and are large ion lithophile elements (LILE)enriched. Petrographic and geochemical data demonstrate thatthey belong to differentiated series that evolved by low-pressurefractionation, thus resulting in granodioritic liquids. We proposea model in which the petrogenesis of the Caicó Complexorthogneisses begins with partial melting of a metasomaticallyenriched spinel- to garnet-bearing lherzolite (with high-silicaadakite melt as the metasomatic agent), generating a basic magmathat subsequently evolved at depth through fractional crystallizationof olivine, followed by low-pressure intracrustal fractionation.A subduction zone setting is proposed for this magmatism, toaccount for both negative anomalies in high field strength elements(HFSE) and LILE enrichment. Mantle-derived juvenile magmatismwith the same age is also known in the São Franciscoand West Africa cratons, as well as in French Guyana, and thusthe Archean–Proterozoic transition marks a very importantcontinental accretion event. It also represents a transitionfrom slab-dominated (in the Archean) to wedge-dominated post-Archeanmagmatism. KEY WORDS: calc-alkaline; magmatism; NE Brazil; Paleoproterozoic; petrogenesis     

U-Pb geochronology of zircons from metasediments of the Ladoga Group (North Ladoga Region,Baltic Shield)

During our study we obtained the first age datings of detrital zircons from metasandstones of the Ladoga Group (North Ladoga Region, Russia) with the U-Pb isotopic method using the SHRIMP-II ion microprobe. The data obtained made it possible to clarify the lower age limit of sedimentation and to obtain additional age data for evaluating the chemical composition and an age of source areas. This work presents the results of isotopic-geochemical (Sm-Nd) and geochemical studies of metasediments. High LREE concentrations, a high La/Sc ratio and a low Cr/h ratio, and the presence of a distinct Eu-minimum (Eu/Eu* = 0.54–0.72) indicate a significant role of acidic terrigenous material in source areas. In addition, the data of the isotope analysis of detrital zircons show that sediments of the Ladoga Group accumulated from the destruction of Proterozoic rocks (1.9–2.0 Ga; a proportion in sediments is 60–70%) and, to a lesser extent, Archean rocks (2.54–2.74 and 2.9–3.01 Ga; the proportion in sediments is 30–40%). One of the Archean source areas could be granite-gneisses of the Pitkyaranta-Koirinoya dome structure with the U-Pb zircon age of 2659 ± 15 Ma. We have established the lower age limit of sedimentation as 1.9 Ga within the measurement error. The Sm-Nd model ages obtained (2.5–2.6 Ga for sediments of the Ladoga Group and over 3.4 Ga for granite-gneisses of the dome structure) suggest a significant contribution of ancient crustal source area into source rocks. Our age data agree well with those for svecofennides of Finland.     

The first results of U/Pb dating and isotope geochemical studies of detrital zircons from the neoproterozoic sandstones of the Southern Timan (Djejim-Parma Hill)

This report presents the first results of U/Pb dating, isotope-geochemical, and geochemical studies of detrital zircons from the Neoproterozoic clastic rocks of the Southern Timan. Sixty-one zircon grains were treated, including 51 from red-colored sandstones and 10 grains from aleurosandstones of the Djejim Formation of the southern Chetlas-Djejim zone (Djejim-Parma Hill). It was found that the U/Pb-ages of zircons from the rocks of the Djejim Formation, varied from ∼2.97 to ∼1.20 Ga. The studies of microelement composition in 47 grains (of 61 U/Pb isotope ages obtained), on the basis of several empirical regularities found formerly, show that the detrital zircons had originated from “granites” (22 grains), “diorites” (12 grains), or their volcanic analogues, or more rarely, from “syenites” and “basites” (5 and 8 grains, respectively). The Lu/Hf isotope system of zircons allows one to estimate the model ages (T _DM^C) of the substrate magmatic rocks being parental to the zircons considered. In particular, Archean zircons are characterized by ∼2.84–3.36 Ga model ages of magmaforming rocks. For some of the grains, their model ages (∼2.84 Ga) are close to those of zircons as such (∼2.7–2.8 Ga), which points to the juvenile character of the substrate from which the parent magma of the zircons treated was fused. For Proterozoic (to Middle Riphean) zircons, the Lu/Hf isotope system allows one to estimate the model age of the substrate of their parental rocks within ∼2.00–3.36 Ga, which shows that these rocks were formed under the recycling of the Archean and Early-Proterozoic crust. The ages obtained for detrital zircons, as well as model ages of the substrate of the corresponding parental magmatic rocks, are quite comparable to the age of crystalline complexes of the ancient framework of the East European Platform (EEP), formed in the course of the Archean, Early-Proterozoic, and Early-Middle Riphean tectonomagmatic events. This permits us to conclude that the Neoproterozoic detrital complexes of the Timan were formed owing to the erosion of earlier Neoproterozoic and Early Precambrian complexes constituting the Neoproterozoic Baltica continent, presenting complexes of the passive margin of this continent. A variety of ages of detrital zircons from sandstones and aleurosandstones from the Djejim Formation of Djejim-Parma Hill, and of the estimates of magmatic rocks parental to these zircons, may be characterized as a Baltic Provenance signal.     

Long hiatus in Proterozoic sedimentation in India: Vindhyan,Cuddapah and Pakhal Basins — A plate tectonic model

Most of the Proterozoic basins in India, viz. the Vindhyan, the Cuddapah and the Pakhal Basins have experienced long hiatus between the upper and the lower group of rocks. It is proposed that the older group of rocks of Paleoproterozoic period (∼1.9–1.6 Ga) formed during the rifting phase caused by large scale magmatism in respective basins possibly due to plume tectonics. On the other hand, the younger group of rocks of Neoproterozoic (∼1.0–0.7 Ga) are formed during the final phase of convergence after mountain building that supplied sediments. These geological processes explain large scale disturbances in the older group of rocks during subsequent convergence and collision as they usually occurred along the rifted margins of the cratons. These processes also explain the undisturbed nature, devoid of magmatic rocks of the younger group of rocks and hiatus of about 0.5–0.6 Ga in each case. It is suggested that the plume that was responsible for these rifting of the Indian cratons during Paleo-Mesoproterozoic might have also been responsible for the break up of contemporary Columbian agglomeration in this section. Same model can be used to explain the formation of Proterozoic basins and related hiatus any where else where similar geological environment exist.     

Geochemistry of granitoid rocks and crustal evolution, Zhejiang Province, China— II. Proterozoic granitoid rocks

Proterozoic granitoid rocks in Zhejiang Province were formed in the Shengongian period (1.8–1.9 Ga) and the Late Jinningian period (0.6–0.9 Ga), respectively. Petrogenetic problems are discussed based on chemical (major, trace elements and REEs) and Nd-Sr isotopic compositions. The Shengongian granites resulted from partial melting of the Badu Group and the Late Jinningian granites are of mantle derivation with or without contamination of crustal material. The crust in Zhejiang had undergone three major periods of growth during 2.6–2.7 Ga, 0.8–1.1 Ga and 0.10–0.12 Ga after it was generated in Archean time. Compositional fractionation in the process of crust evolution is not evident. The presence of Late Jinningian granites of mantle and mantle-crust-derivation along the Jiangshan-Shaoxing Fault is indicative of crust subduction at that time. This project was finantially supported by both the National Natural Science Foundation of China (No. 9490011) and the Zhongguancun Test Center.     

Sm-Nd and Rb-Sr ages and P-T history of the Archean Sittampundi and Bhavani layered meta-anorthosite complexes in Cauvery shear zone,South India: evidence for Neoproterozoic reworking of Archean crust

 Sittampundi and Bhavani Archean layered meta-anorthosite complexes occur as tectonic lenses within the Cauvery shear zone (CSZ), a crustal scale shear dividing the Precambrian granulite crust of south India into late Archean (> 2.5 Ga) and Proterozoic (c. 0.55 Ga) blocks. They and their host supracrustal-gneiss rocks record at least two stages of tectonometamorphic history. The first is seen as regional scale refolded isoclinal folds and granulite metamorphism (D₁-M₁) while the second stage is associated with dominantly E–W dextral transcurrent shearing and metamorphic recrystallisation (D₂-M_CSZ). Whole rock Sm-Nd isochrons for several comagmatic rocks of the layered complexes yield concordant ages: Sittampundi – 2935±60 Ma, ɛ_Nd + 1.85±0.16 and Bhavani – 2899±28 Ma, ɛ_Nd + 2.18±0.14 (2σ errors). Our Sm-Nd results suggest that: (1) the magmatic protoliths of the Sittampundi and Bhavani layered complexes were extracted from similar uniform and LREE depleted mantle sources; (2) M₁ metamorphism occurred soon after emplacement at c.3.0 Ga ago. P-T estimates on garnet granulites from the Sittampundi complex characterise the M_CSZ as a high-P event with metamorphic peak conditions of c. 11.8 kbar and 830°C (minimum). The M_CSZ is associated with significant isothermal decompression of the order of 4.5–3.5 kbar followed by static high-temperature rehydration and retrogression around 600°C. The timing of M_CSZ is inferred to be Neoproterozoic at c. 730 Ma based on a whole rock-garnet-plagioclase-hornblende Sm-Nd isochron age for a garnet granulite from the Sittampundi complex and its agreement with the 800–600 Ma published age data on post-kinematic plutonic rocks within the CSZ. These results demonstrate that the Cauvery shear zone is a zone of Neoproterozoic reworking of Archean crust broadly similar to the interface between the Napier and Rayner complexes of the East Antarctic shield in a model Proterozoic Gondwana supercontinent. Received: 5 December 1995 / Accepted: 3 May 1996     

REE Geochemistry of Precambrian Metamorphic Rocks in Wutaishan Region

The metasedimentary-volcanic series of the Wutai and Hutuo groups experienced regional metamorphism and thus turned into moderate-to low-grade metamorphic rocks.REE abundances and REE distribution patterns in the Shizui and Taihuai Subgroup metasedimentary-volcanic rocks are typical of the Archean,whereas the Gaofan Subgroup and the Hutuo Group show post-Archean REE geochemical char-acteristics.Five types of REE distribution pattern are distinguished:(1)rightward inclined smooth curves with little REE anomaly(Eu/Eu*=0.73-0.95) and heavy REE depletion (e.g.the Late Archean metasedimentary rocks);(2)rightward inclined V-shaped curves with sharp Eu anoma-ly (Eu/Eu*=0.48-0.76) and slightly higher ∑REE (e.g.the post-Archean metasedimentary rocks);(3) rightward inclined steep curves with negative Eu anomaly(Eu/Eu*=0.73-0.76) and the lowest ∑REE (e.g.the post-Archean dolomites);(4)rightward inclined,nearly smooth curves with both positive Eu anomaly and unremarkable positive Eu anomaly(Eu/Eu*=0.95-1.25)(e.g.the meta-basic volcanic rocks);and (5) rightward inclined curves with Eu anomaly(Eu/Eu*=1.09-1.19)and heavy REE depletion(e.g.the meta-acid volcanic rocks).Strata of the two groups are considered to have been formed in an island-arc belt-an instable continental petrogenetic environment.     

4.4 billion years of crustal maturation: oxygen isotope ratios of magmatic zircon

Analysis of δ¹⁸O in igneous zircons of known age traces the evolution of intracrustal recycling and crust-mantle interaction through time. This record is especially sensitive because oxygen isotope ratios of igneous rocks are strongly affected by incorporation of supracrustal materials into melts, which commonly have δ¹⁸O values higher than in primitive mantle magmas. This study summarizes data for δ¹⁸O in zircons that have been analyzed from 1,200 dated rocks ranging over 96% of the age of Earth. Uniformly primitive to mildly evolved magmatic δ¹⁸O values are found from the first half of Earth history, but much more varied values are seen for younger magmas. The similarity of values throughout the Archean, and comparison to the composition of the “modern” mantle indicate that δ¹⁸O of primitive mantle melts have remained constant (±0.2‰) for the past 4.4 billion years. The range and variability of δ¹⁸O in all Archean zircon samples is subdued (δ¹⁸O(Zrc)=5–7.5‰) ranging from values in high temperature equilibrium with the mantle (5.3± 0.3‰) to slightly higher, more evolved compositions (6.5–7.5‰) including samples from: the Jack Hills (4.4–3.3 Ga), the Beartooth Mountains (4.0–2.9 Ga), Barberton (3.5–2.7 Ga), the Superior and Slave Provinces (3.0 to 2.7 Ga), and the Lewisian (2.7 Ga). No zircons from the Archean have been analyzed with magmatic δ¹⁸O above 7.5‰. The mildly evolved, higher Archean values (6.5–7.5‰) are interpreted to result from exchange of protoliths with surface waters at low temperature followed by melting or contamination to create mildly elevated magmas that host the zircons. During the Proterozoic, the range of δ¹⁸O(Zrc) and the highest values gradually increased in a secular change that documents maturation of the crust. After ∼1.5 Ga, high δ¹⁸O zircons (8 to >10‰) became common in many Proterozoic and Phanerozoic terranes reflecting δ¹⁸O(whole rock) values from 9 to over 12‰. The appearance of high δ¹⁸O magmas on Earth reflects nonuniformitarian changes in the composition of sediments, and rate and style of recycling of surface-derived material into magmas within the crust. Electronic Supplementary Material Supplementary material is available for this article at     

The lithospheric mantle beneath the southwestern Tianshan area, northwest China

A suite of spinel peridotite xenoliths in Mesozoic basalts of the Tuoyun basin in the Tianshan area of northwest China has a high proportion of amphibole/mica-bearing lherzolites, with high Cpx/Opx ratios (mean 0.74). Many aspects of mineral chemistry in the Tuoyun peridotites are intermediate between those of refractory Archean cratonic mantle and fertile Phanerozoic mantle. These include Ni/Cr and the contents of transition metals and Y in olivine and orthopyroxene and the abundances of elements such as Na, Al, Ti, Y, Sr and LREE in clinopyroxene. The data suggest that the mantle in Tuoyun is moderately depleted in basaltic components relative to both the refractory Archean mantle and the fertile Phanerozoic mantle. The wide variations in the CaO/Al₂O₃ (0.9–3.5) of whole rocks and LREE/HREE (0.8–14.2) and Ti/Eu (971–5,765) of clinopyroxenes in the Tuoyun peridotites are interpreted as the metasomatism of hydrous carbonatitic and potassic melt or the cumulative effects of mantle metasomatism by different agents (carbonatite and small-volume silicate melts) through time. The Tuoyun mantle shows closer affinity to the type of mantle found beneath the Proterozoic Cathaysia block, and especially to that beneath the East Central Asia Orogenic Belt (ECAOB), than to the mantle beneath the Archean North China Craton. This implies that the Tianshan subcontinental lithospheric mantle may have been generated during the accretion of the ECAOB. The high proportion of fine-grained microstructures, high Cpx/Opx ratio, obvious Ca enrichment and lower overall depletion in the Tuoyun mantle relative to that in other parts of the ECAOB reflect stronger mechanical and chemical modification of the Tuoyun mantle, near the translithospheric Talas-Ferghana strike-slip fault, which played a major role in controlling the strength of the mantle lithosphere and has channeled the upwelling mantle.     

Evolution of subcontinental lithospheric mantle beneath eastern China: Re–Os isotopic evidence from mantle xenoliths in Paleozoic kimberlites and Mesozoic basalts

The ages of subcontinental lithospheric mantle beneath the North China and South China cratons are less well-constrained than the overlying crust. We report Re–Os isotope systematics of mantle xenoliths entrained in Paleozoic kimberlites and Mesozoic basalts from eastern China. Peridotite xenoliths from the Fuxian and Mengyin Paleozoic diamondiferous kimberlites in the North China Craton give Archean Re depletion ages of 2.6–3.2 Ga and melt depletion ages of 2.9–3.4 Ga. No obvious differences in Re and Os abundances, Os isotopic ratios and model ages are observed between spinel-facies and garnet-facies peridotites from both kimberlite localities. The Re–Os isotopic data, together with the PGE concentrations, demonstrate that beneath the Archean continental crust of the eastern North China Craton, Archean lithospheric mantle of spinel- to diamond-facies existed without apparent compositional stratification during the Paleozoic. The Mesozoic and Cenozoic basalt-borne peridotite and pyroxenite xenoliths, on the other hand, show geochemical features indicating metasomatic enrichment, along with a large range of the Re–Os isotopic model ages from Proterozoic to Phanerozoic. These features indicate that lithospheric transformation or refertilization through melt-peridotite interaction could be the primary mechanism for compositional changes during the Phanerozoic, rather than delamination or thermal-mechanical erosion, despite the potential of these latter processes to play an important role for the loss of garnet-facies mantle. A fresh garnet lherzolite xenolith from the Yangtze Block has a Re depletion age of ∼1.04 Ga, much younger than overlying Archean crustal rocks but the same Re depletion ages as spinel lherzolite xenoliths from adjacent Mesozoic basalts, indicating Neoproterozoic resetting of the Re–Os system in the South China Craton.     

Late Mesozoic collisional granitoids of the upper Amur area: New geochemical data

Geochemical and isotopic data were used for a comparative analysis of Late Mesozoic (150–120 Ma) granitoids in various geological structures of the upper Amur area. The granitoids are metaluminous high-potassic I-type rocks of the magnetite series. They have variable alkalinity and consist of the monzonite-granite and granosyenite-granite associations. The monzonite-granite association consists of calc-alkaline granitoids of normal alkalinity belonging to the Umlekan-Ogodzhinskaya volcanic-plutonic zone and the Tynda-Bakaran Complex of the Stanovoy terrane. The rocks are characterized by negative anomalies of U, Ta, Nd, Hf, and Ti (in patterns normalized to the primitive mantle), with Eu anomalies pronounced weakly in the granodiorites and quartz and monzodiorites and more clearly in the granites: Eu/Eu* = 0.37–0.95, and (La/Yb)_n = 7–24, Tb_n/Yb_n = 1.4–3.2. The granosyenite-granite association comprises of moderately alkaline rocks, which are subdivided into three groups according to their geochemistry. The first group consists of phase-I granosyenites of the Uskalinskii Massif of the Umlekan-Ogodzhinskaya zone with the highest concentrations of Sc, V, Cr, Co, Ni, Cu, Cs, Rb, Sr, Y, Zr, Yb, and Th; negative anomalies at Ba, Ta, Sr, and Hf; Eu/Eu* = 0.50–0.58, (La/Yb)_n = 15–16, and Tb_n/Yb_n = 1.8. The second group comprises of moderately alkaline granitoids of the Umlekan-Ogodzhinskaya zone and the Khaiktinskii Complex of the Baikal-Vitim superterrane. Geochemically, the granitoids of this group are generally similar to the monzodiorite-granite association and differ from it in having lower concentrations of REE and Y, Eu/Eu* = 6.2–1.0, (La/Yb)_n = 28–63, and Tb_n/Yb_n = 2.1–4.5. The third group consists of granitoids of the Chubachinskii Complex of the Stanovoi terrane, which typically show negative Cs, Rb, Th, U, Ta, Hf, and Ti anomalies; the lowest concentrations of V, Cr, Co, and Ni; and the highest contents of Sr. The granosyenites of the first phase display clearly pronounced negative Eu anomalies (Eu/Eu* = 0.53–0.68), (La/Yb)_n = 7–24, and Tb_n/Yb_n = 0.8–2.0. The granitoids of the second phase have (La/Yb)_n = 51–84, no Eu anomalies, or very weak Eu anomalies (Eu/Eu* = 0.97–1.23). The silica-oversaturated leucogranites of the third phase are characterized by elevated concentrations of REE, clearly pronounced Eu anomalies (Eu/Eu* = 0.48), and flat REE patterns (Tb_n/Yb_n = 1.3). The diversity of the granitoids is demonstrated to have been caused largely by the composition of the Precambrian source, which was isotopically heterogeneous. The rocks of the monzodiorite-granite association and first-group granosyenites of the granosyenite-granite association of the Tynda-Bakaran Complex were supposedly derived from garnet-bearing biotite amphibolites. In contrast to these rocks, the source of the second-group granites of the granosyenite-granite association was of mixed amphibolite-metagraywacke composition. The third-group of granitoids were melted out of Early Proterozoic crustal feldspar-rich granulites of variable basicity, with minor amounts of Archean crustal material. The granitoids were emplaced in a collisional environment, perhaps, during the collision of the Amur superterrane and Siberian craton. This makes it possible to consider these rocks as components of a single continental volcanic-plutonic belt. Original Russian Text ? V.E. Strikha, 2006, published in Geokhimiya, 2006, No. 8, pp. 855–872.     

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.     

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.     

再论冀北古缝合带的证据

近南北走向的冀北太古宙麻粒岩相古陆核北侧受到近东西走向的古元古代造山带的交切。在陆缘沉积增生带内发现大量残存的古洋壳残片，包括蛇纹石化方辉橄榄岩、且鬣刺结构的苦橄岩、透闪石岩、橄长岩、异剥钙榴岩、细碧岩、退变榴辉岩、基性枕状熔岩和斜长花岗岩等，与陆缘沉积岩一起构成古蛇绿岩混杂带。推断该蛇绿岩混杂带从古元古代开始直到新元古代末有逐步向北后退发育的特点。     

Zircon age and Nd–Hf isotopic composition of the Yunnan Tethyan belt,southwestern China

The Baoshan block of the Tethyan Yunnan, southwestern China, is considered as northern part of the Sibumasu microcontinent. Basement of this block that comprises presumably greenschist-facies Neoproterozoic metamorphic rocks is covered by Paleozoic to Mesozoic low-grade metamorphic sedimentary rocks. This study presents zircon ages and Nd–Hf isotopic composition of granites generated from crustal reworking to reveal geochemical feature of the underlying basement. Dating results obtained using the single zircon U–Pb isotopic dilution method show that granites exposed in the study area formed in early Paleozoic (about 470 Ma; Pingdajie granite) and in late Yanshanian (about 78–61 Ma, Late Cretaceous to Early Tertiary; Huataolin granite). The early Paleozoic granite contains Archean to Mesoproterozoic inherited zircons and the late Yanshanian granite contains late Proterozoic to early Paleozoic zircon cores. Both granites have similar geochemical and Nd–Hf isotopic charateristics, indicating similar magma sources. They have whole-rock T _DM(Nd) values of around 2,000 Ma and zircon T _DM(Hf) values clustering around 1,900–1,800 and 1,600–1,400 Ma. The Nd–Hf isotopic data imply Paleoproterozoic to Mesoproterozoic crustal material as the major components of the underlying basement, being consistent with a derivation from Archean and Paleoproterozoic terrains of India or NW Australia. Both granites formed in two different tectonic events similarly originated from intra-crustal reworking. Temporally, the late Yanshanian magmatism is probably related to the closure of the Neotethys ocean. The early Paleozoic magmatism traced in the Baoshan block indicates a comparable history of the basements during early Paleozoic between the SE Asia and the western Tethyan belt, such as the basement outcrops in the Alpine belt and probably in the European Variscides that are considered as continental blocks drifting from Gondwana prior to or simultaneously with those of the SE Asia.     

太行-五台山区不同时代花岗岩类岩石学和地球化学的特征对比及陆壳演化

通过对太行－五台山区太古宙、元古宙和中生代等不同时代的花岗岩类岩石学和地球化学特征的对比，并将晚太古时期花岗岩类同国内外同时期花岗岩类进行对比，确定了本区晚太古时期花岗岩类岩石组合为英云闪长岩－奥长花岗岩－花岗闪长岩或英云闪长岩－奥长花岗岩－花岗闪长岩－花岗岩，并以奥长花岗岩演化趋势为主，叠加了部分钙碱性演化；而元古宙、中生代花岗岩类岩石组合则主要为花岗闪长岩－花岗岩或花岗岩，以钙碱性演化趋势为特征。可以认为晚太古时期的太行－五台山区正处于陆壳演化的初始阶段和成熟阶段之间的过渡性阶段。     

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.     

Catagenesis and burial metamorphism of terrigenous and volcanic sequences (section recovered by the En-Yakhinskaya parametric borehole SG-7)

Secondary alterations of Cretaceous, Jurassic, and Triassic terrigenous complexes recovered by borehole SG-7 were studied from the depth of 3620 m to 6920 m (roof of basalts). Down to the depth of ∼6770 m, the section shows a gradual intensification of catagenetic alterations of sandy rocks: the formation of pressure dissolution textures and regeneration of clastic quartz. Intensification of the transformation of clay minerals is not observed in this mineral. Variations in the contents of illite, hydromicas, mixed-layer minerals, smectite, chlorite, and kaolinite at different depths of the recovered section are related to changes in the provenance during the accumulation of sedimentary complexes. The Middle Triassic coarse-grained sandy rocks (suprabasalt sequence) are more intensely transformed: they are marked by microstylolitic textures of pressure dissolution, recrystallization blastesis of clastic quartz grains, and newly formed zoisite. The composition of clay minerals is also characterized by variation: micas are represented by sericite with ΔD = 0; Fe-chlorite and kaolinite are noted. These features suggest the absence of linear positive correlation of T and P with the subsidence depth of sedimentary complexes. Intense heating (up to 200–300°C) of the Middle Triassic suprabasalt rocks is likely caused by trap activity (intense effusion of basalt lavas). Investigation of secondary minerals in basalts recovered by borehole SG-7 revealed that the grade of their transformation matches the medium-temperature subfacies of the greenschist facies.