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.     

桐柏造山带古元古代变质基性岩的锆石U-Pb年龄及其地质意义

桐柏造山带位于秦岭和大别造山带之间,其北侧对应于北秦岭中高级变质杂岩,南侧对应于大别山高压/超高压变质杂岩.锆石U-Pb定年在一个榴辉岩中获得其原岩侵位年龄和锆石重结晶或生长年龄分别为(1961±23) Ma和(1949±51) Ma,在一个石榴角闪岩(退变榴辉岩？)中获得其变质年龄为(1936±26) Ma,表明这两种基性岩石在二叠-三叠纪高压变质之前经历了古元古代变质作用的影响.由桐柏—大别—苏鲁造山带可利用的年代学资料确定,扬子陆块北缘发育的古元古代构造热事件可分为两幕,第一幕发生在约1.97～1.93 Ga,可能与Columbia超大陆的聚合有关;第二幕发生在约1.85～1.82 Ga,或者代表Columbia超大陆汇聚的继续,或者是陆块边缘增生过程的结果.     

Archaean Kuru-Vaara eclogites in the northern Belomorian Province,Fennoscandian Shield: crustal architecture,timing, and tectonic implications

This paper addresses the relationships between relic amphibole-eclogite facies (AE) eclogites and their host units, Archaean amphibolites, enveloped by Archaean tonalite–trondhjemite–granodiorite (TTG) gneisses, in the Kuru-Vaara study area in the northern Belomorian Province. According to observational constraints, the crystallization of the relic peak omphacite + Mg-garnet ± kyanite assemblage and the subsequent replacement of omphacite by clinopyroxene–plagioclase symplectite occurred before the earliest deformational, metamorphic, and migmatization events that are recorded in the amphibolites. The amphibolites and their TTG hosts have a shared deformational and metamorphic history that is composed of the Archaean and Palaeoproterozoic periods. This history favours the conclusion that the AE metamorphism recorded in the relic eclogites within the amphibolites occurred during the Mesoarchaean to Neoarchaean periods. The deformation and metamorphism of the amphibolite facies of the second period resulted from the Lapland–Kola collisional orogeny at 1.91–1.93 Ga, which led to eclogite–high-pressure granulite (E–HPG) facies conditions in the lowermost portions of the over-thickened crust in Belomorian Province (the southwestern foreland of the Lapland–Kola collisional orogen). The Palaeoproterozoic E–HPG overprint was reported from the Palaeoproterozoic Gridino mafic dikes. Although the ages of the oldest low Th/U zircons are close to the time of the Lapland–Kola collision, the low Th/U 1.9–1.8 Ga zircons reflect a zircon response to regional fluid infiltration in the eclogites during slow exhumation following the Lapland–Kola orogeny and do not record any metamorphic event. Contrary to the Palaeoproterozoic E–HPG overprint, the areal occurrence of the 2.7–2.8 Ga AE eclogites with mid-ocean ridge basalt-like chemistry and their paragenetic link with the TTG gneisses suggest a tectonic regime that involves subduction. This research favours concepts suggesting that the modern-style plate tectonics has operated in some places, at least since the late Mesoarchaean.     

扬子克拉通崆岭杂岩孔兹岩系同位素年代学研究及其地质意义

崆岭杂岩出露有一套覆盖于古老变质结晶基底之上的以含石墨和富铝矿物为特征的孔兹岩系。本文对该套孔兹岩系中代表性岩石类型榴线英岩开展了同位素稀释法(ID-TIMS)矿物–全岩Sm-Nd等时线测年。研究结果表明,榴线英岩形成于2078±31 Ma。榴线英岩Nd同位素组成相对均一(ε_(Nd)(t)值介于-4.9与-2.3之间),对应Nd同位素两阶段模式年龄介于2.91~2.70 Ga之间,表明大多数样品形成于封闭体系中。结合前人在同一套岩石组合中报道的锆石U-Pb年代学和岩石地球化学数据,推测崆岭杂岩孔兹岩系可能沉积于2.13~2.08 Ga,其原岩沉积物主要来自下伏的古老地壳物质,且其源区存在中太古代的初生地壳增长。扬子陆核经历的古元古代(2.08~1.94 Ga)高压麻粒岩相变质作用可能与全球广泛存在的同时期(2.1~1.8 Ga)碰撞造山事件有关,暗示其很可能是古元古代Columbia超大陆的重要组成部分。     

Age and tectonic position of the Stanovoi metamorphic complex in the eastern part of the Central Asian Foldbelt

Based on the results of Sm–Nd isotopic geochemical and U–Th–Pb geochronological LA–ICP–MS studies, it has been established that the formation of metamorphic rock protoliths of the Stanovoi Complex in the western Dzhugdzhur–Stanovoi Superterrane of the Central Asian Foldbelt took place over the following time spans: 2750–2860 Ma (Ilikan Group of the Ilikan Zone), 1890–1910 Ma (Bryanta Group of the Bryanta Zone), and ~2.0 Ga (Kupuri and Zeya groups of the Kupuri and Zeya zones, respectively). It has been shown that the western part of the Dzhugdzhur–Stanovoi Superterrane was formed ~1.9 Ga ago, as a result of collision of the Neoarchean Ilikan Terrane, the Paleoproterozoic island arc, and the Paleoproterozoic Kupuri–Zeya Terrane. The data make it possible to consider the Kurul’ta, and Zverevo blocks of the Stanovoi Structural Suture and the Ilikan Terrane of the Dzhugdzhur–Stanovoi Superterrane of the Central Asian Foldbelt as constituents of a common terrane.     

Geodynamics of the eastern margin of Sarmatia in the Paleoproterozoic

The eastern margin of Sarmatia comprises the Paleoproterozoic (2.1–2.05 Ga) rock associations of the eastern Voronezh Crystalline Massif, including the Lipetsk-Losevo volcanic-plutonic belt and the adjacent East Voronezh lithotectonic zone composed of metasedimentary rocks of the Vorontsovka Group. The isotopic and geochemical study of the available drill cores that characterize the main rock associations of the Lipetsk-Losevo belt and its nearest framework allowed us to furnish evidence for the formation of this belt in the regime of an island arc at the active margin of the Archean continent above a low-angle subduction zone. The juvenile isotopic and geochemical signatures of metaturbidites of the Vorontsovka Group indicate that only a fast growing mountain edifice with the Lipetsk-Losevo Belt in its highest part (foreland) could have been a provenance of the flysch basin. It is proposed to name this Paleoproterozoic mountain system the East Sarmatian Orogen. The hinterland of this orogen embraced the megablock of the Kursk Magnetic Anomaly as a part of the Voronezh Massif and the Azov Block of the Ukrainian Shield. It has been shown that the East Sarmatian Orogen was formed in the same way as accretionary orogens of the Cordilleran type.     

Ion microprobe UPb zircon geochronology and isotopic evidence for a trans-crustal suture in the Lapland–Kola Orogen,northern Fennoscandian Shield

The Lapland–Kola Orogen (LKO; former Kola craton) in the northern Fennoscandian Shield comprises a collage of partially reworked late Archaean terranes with intervening belts of Palaeoproterozoic juvenile crust including the classic Lapland Granulite Terrane. Rifting of Archaean crust began at c 2.5–2.4 Ga as attested by layered mafic and anorthositic intrusions developed throughout the northernmost Fennoscandian Shield at this time. Oceanic separation was centred on the Lapland Granulite, Umba Granulite (UGT) and Tersk terranes within the core zone of the orogen. Importantly, SmNd data show that Palaeoproterozoic metasedimentary and metaigneous rocks within these terranes contain an important, generally dominant, juvenile component over a strike length of at least 600 km. Evidently, adjacent Archaean terranes, with negative ε_Nd signatures, contributed relatively little detritus, suggesting a basin of considerable extent. Subduction of the resulting Lapland–Kola ocean led to arc magmatism dated by the NORDSIM ion probe at c 1.96 Ga in the Tersk Terrane in the southern Kola Peninsula. Accretion of the Tersk arc took place before c 1.91 Ga as shown by ion probe UPb zircon dating of post-D1, pre-D2 pegmatites cutting the Tersk arc rocks, juvenile metasediments as well as Archaean gneisses in the footwall of the orogen. Deep burial during collision under high-pressure granulite-facies conditions was followed by exhumation and cooling between 1.90 and 1.87 Ga based on SmNd, UPb and ArAr data. Lateral variations in deep crustal velocity and Vp/Vs ratio, together with reflections traversing the entire crust observed in reprocessed seismic data from the Polar Profile, may be interpreted to image a trans-crustal structure — possibly a fossilised subduction zone — supporting an arc origin for the protoliths of the Lapland Granulite, UGT and Tersk terranes and the location of a major lithospheric suture — the Lapland–Kola suture.     

Paleoproterozoic sediment recycling during multiphase orogenic evolution in Fennoscandia, the Tampere and Pirkanmaa belts, Finland

Geochronology of detrital zircons and their overgrowths combined with whole-rock geochemical and Sm–Nd isotopic data can be used to distinguish different stages of sediment recycling and metamorphism during multiphase orogenic evolution. This approach is applied to the Paleoproterozoic sedimentary rocks of the Tampere and Pirkanmaa belts (southern Finland) in the center of the composite Svecofennian orogen.The lower part of the Tampere belt succession and bulk of the Pirkanmaa belt are characterized by turbidites whereas the upper part of the Tampere belt succession is dominated by 1.90–1.89 Ga mature arc-type volcanic rocks. Detrital zircon geochronology indicates that the Tampere and Pirkanmaa belts have a coeval 1.92–1.89 Ga depositional and tectonic history. Ages of pre-depositional zircon overgrowths vary from 1.91 Ga to 2.0 Ga with clusters at 1.92 Ga and 1.98 Ga. Within the Pirkanmaa belt, post-depositional zircon overgrowths indicate metamorphic culmination at c. 1885 Ma in the Vammala Ni-zone and at c. 1875 Ma in the northern part.The lower conglomerates and graywackes in the Tampere belt and their equivalents in the Pirkanmaa belt are rich both in Neoarchean and Paleoproterozoic grains, the latter ranging in age from 1.9 to 2.1 Ga. Compared to these, a sample from the Vammala Ni-zone has an exotic provenance with at least c. 1.90 Ga, 2.04–2.15 Ga, 2.38–2.48 Ga and 2.57–2.63 Ga components.A sedimentary recycling and tectonic model for the central Fennoscandia is proposed, in which the Paleoproterozoic Keitele + Bergslagen continent was formed during an unnamed orogeny at 1.98–1.97 Ga. The Archean Norrbotten microcontinent was attached to the continent at 1.97–1.93 Ga. Upper Kaleva turbidites, derived from the Lapland-Kola orogen in the north, were deposited before 1.92 Ga on a passive margin of the Archean Karelia craton. The Karelia craton collided with the Keitele + Bergslagen + Norrbotten continent at c. 1.92 Ga forming the Lapland-Savo orogen. Subsequent evolution led to rifting and break-up of the latter continent into two microcontinents in the hinterland. At 1.92–1.91 Ga the rift was developed into a subsiding passive margin of the Keitele microcontinent with voluminous turbidite deposition, now seen as graywackes in the Tampere, Pirkanmaa and Pohjanmaa (western Finland) belts. The turbidite material was derived from the rising Lapland-Savo orogen and included recycled Upper Kaleva, recycled (sandstones) and first-cycle 2.03–1.97 Ga detritus from Keitele, 1.93–1.92 Ga Savo arc material, as well as detritus from the Archean craton and its cover deposits. The collision between Karelia and Keitele caused a subduction reversal and the onset of Tampere arc volcanism at 1.90 Ga. Arc-derived materials started to deposit and were mixed with older sedimentary rocks, and trench-parallel distal turbidites from exotic source were being deposited in the accretionary wedge.     

The Early Paleoproterozoic Monchegorsk layered mafite-ultramafite massif in the Kola Peninsula: Geology, petrology, and ore potential

The Early Paleoproterozoic Monchegorsk Complex is exposed over an area of 550 km² and comprises two layered mafite-ultramafite intrusions of different age: the Monchegorsk pluton of ultramafic and mafic rocks and the predominantly gabbroid Main Range Massif (also referred to as the Moncha-Chuna-Volch??i Tundras Massif), which are separated by a fault. Both massifs consists of intercalating cumulates (first of all, Ol ± Crt, Ol + Opx ± Crt, Opx, Opx + Pl ± Cpx, and Pl), they were produced by similar melts of siliceous high-Mg series but differ in the stratigraphy of their cumulates: while the Monchegorsk pluton is dominated by ultramafites, the Main Range Massif consists mostly of gabbroids, first of all, of gabbronorites. The complex is accompanied by PGE-Cu-Ni ore mineralization, low-sulfide Pt-Pd mineralization, and chromite mineralization. Judging from geological data and isotopic dates, the Monchegorsk Complex is a long-lived magmatic center, which evolved over a time span of 50 Myr at 2.50?C2.46 Ga. The Main Range Massif is younger and likely truncates the western continuation of the Monchegorsk pluton. The complex is spatially restricted to the zone of the Middle Paleoproterozoic regional Central Kola Fault and is now tectonic collage whose rocks were variably affected by overprinted metamorphism in the course of deformations. These processes most significantly affected rocks along the peripheries of the Monchegorsk pluton in the south. These rocks were completely transformed under greenschist-facies conditions but often preserved their primary textures and structures. The processes overprinted both the marginal portions of the pluton itself and the rocks of its second phase, which are accompanied by economic low-sulfide PGE deposits. The PGE-Cu-Ni ore mineralization of the Monchegorsk Complex is genetically related to two distinct evolutionary episodes with a quiescence period in between:

1. The emplacement of large layered mafite-ultramafite intrusions at 2.5?C2.45 Ga. Economic deposits of sulfide Cu-Ni ores with subordinate PGE mineralization occur within the Monchegorsk pluton, and the moderate-grade low-sulfide PGE ores are related to its second evolutionary phase (in the foothills of Vuruchuaivench and in the Moroshkovoe Lake, and Southern Sopcha areas). The primary magmatic ore mineralization is predominantly Cu-Fe-Ni sulfide with PGE bismuthides-tellurides.
2. The Monchegorsk Complex was involved in the zone of the Central Kola Fault at 2.0?C1.9 Ga and was broken in a collage of tectonic blocks. The rocks were sheared along the boundaries of the blocks and were affected by overprinted metamorphism, which proceeded under greenschist-facies conditions in the structures surrounding the Monchegorsk pluton in the south. Thereby the primary PGE-Cu-Ni ore mineralization underwent metamorphic processes was recrystallized with the formation of Pt-Pd arsenides, stannides, antimonides, selenides, etc. This processes was associated with the partial redistribution of PGE with their local accumulation (up to economic concentrations), and the orebodies themselves acquired diffuse outlines. In other words, the second episode was marked by the transformation of the older primary magmatic ore mineralization.

     

Zircon geochronology and Sm–Nd isotopic study: Further constraints for the Archean and Paleoproterozoic geodynamical evolution of the southeastern Guiana Shield, north of Amazonian Craton, Brazil

The eastern part of the Guiana Shield, northern Amazonian Craton, in South America, represents a large orogenic belt developed during the Transamazonian orogenic cycle (2.26–1.95 Ga), which consists of extensive areas of Paleoproterozoic crust and two major Archean terranes: the Imataca Block, in Venezuela, and the here defined Amapá Block, in the north of Brazil.

Pb-evaporation on zircon and Sm–Nd on whole rock dating were provided on magmatic and metamorphic units from southwestern Amapá Block, in the Jari Domain, defining its long-lived evolution, marked by several stages of crustal accretion and crustal reworking. Magmatic activity occurred mainly at the Meso-Neoarchean transition (2.80–2.79 Ga) and during the Neoarchean (2.66–2.60 Ga). The main period of crust formation occurred during a protracted episode at the end of Paleoarchean and along the whole Mesoarchean (3.26–2.83 Ga). Conversely, crustal reworking processes have dominated in Neoarchean times. During the Transamazonian orogenic cycle, the main geodynamic processes were related to reworking of older Archean crust, with minor juvenile accretion at about 2.3 Ga, during an early orogenic phase. Transamazonian magmatism consisted of syn- to late-orogenic granitic pulses, which were dated at 2.22 Ga, 2.18 Ga and 2.05–2.03 Ga. Most of the ε_Nd values and T_DM model ages (2.52–2.45 Ga) indicate an origin of the Paleoproterozoic granites by mixing of juvenile Paleoproterozoic magmas with Archean components.

The Archean Amapá Block is limited in at southwest by the Carecuru Domain, a granitoid-greenstone terrane that had a geodynamic evolution mainly during the Paleoproterozoic, related to the Transamazonian orogenic cycle. In this latter domain, a widespread calc-alkaline magmatism occurred at 2.19–2.18 Ga and at 2.15–2.14 Ga, and granitic magmatism was dated at 2.10 Ga. Crustal accretion was recognized at about 2.28 Ga, in agreement with the predominantly Rhyacian crust-forming pattern of the eastern Guiana Shield. Nevertheless, T_DM model ages (2.50–2.38 Ga), preferentially interpreted as mixed ages, and ε_Nd < 0, point to some participation of Archean components in the source of the Paleoproterozoic rocks. In addition, the Carecuru Domain contains an oval-shaped Archean granulitic nucleus, named Paru Domain. In this domain, Neoarchean magmatism at about 2.60 Ga was produced by reworking of Mesoarchean crust, as registered in the Amapá Block. Crustal accretion events and calc-alkaline magmatism are recognized at 2.32 Ga and at 2.15 Ga, respectively, as well as charnockitic magmatism at 2.07 Ga.

The lithological association and the available isotopic data registered in the Carecuru Domain suggests a geodynamic evolution model based on the development of a magmatic arc system during the Transamazonian orogenic cycle, which was accreted to the southwestern border of the Archean Amapá Block.     

Alpine-type tectonics in the Paleoproterozoic Lapland-Kola Orogen

The Kola region in the northeastern Baltic Shield is characterized by diverse Paleoproterozoic collision processes. The Keivy Terrane is one of the major tectonic units in the northeastern foreland of the Paleoproterozoic Lapland-Kola Collisional Orogen, which markedly differs in a number of parameters from other tectonic units of the Kola region. The study of the Keivy Terrane allowed us to unravel one more basic difference: the large Paleoproterozoic sheath synform of the Serpovidny (Crescentic) Range localized in this terrane. Its core is occupied by volcanic and sedimentary rocks, which correlate with the fill of the Imandra-Varzuga Rift; the limbs are composed of metamorphosed mature sedimentary rocks known as Keivy paraschists of Neoarchean or Paleoproterozoic age. The lower limb of the Serpovidny Synform is strongly squeezed, whereas the upper limb consists of almost undeformed rocks. The deformed rocks underwent ductile flow under conditions of simple or general shear. In the degree of its asymmetry and main parameters, the Serpovidny Synform is similar to the plunging and recumbent anticlines in the Helvetic nappes of the Alps. It is concluded that the Paleoproterozoic core of the Serpovidny Sheath Synform, or plunging anticline, is a fragment of the almost completely eroded deep Serpovidny Nappe of the Helvetic type. During the collision related to the Lapland-Kola Orogeny (1.9–2.0 Ga), this nappe was pushed out northward from the Paleoproterozoic Imandra-Varzuga Rift, which is situated 50 km south of the Serpovidny structure, and thrust over the Keivy paraschists. The latter, together with underlying the Lebyazhka Gneiss, were folded in the process of thrusting and were involved in the structure of the Serpovidny Synform. The Keivy paraschists make up a para-autochthon or a separate nappe of the Pennine type. The Archean Lebyazhka metafelsic volcanics underlie the Keivy paraschists and overlie granitoids of the Archean basement that remained undeformed during thrusting. Most likely, they also belong to the para-autochthon; however, it cannot be ruled out that, like the Keivy paraschists, they occur as a Pennine-type nappe. The large sheath folds known in the Paleoproterozoic and Phanerozoic orogens are genetically related to deep-seated nappes or channel-flow tectonics. Paleoproterozoic and Phanerozoic orogens are similar in this respect.     

Provenance of metasedimentary rocks of the Western Pernambuco-Alagoas Domain: Contribution to understand the crustal evolution of southern Borborema Province

The Borborema Province is a complex neoproterozoic orogen in northeastern Brazil, made of a mosaic of fault-bounded terrains and several metassedimentary sequences. In the present work, new zircon U–Pb provenance data for metasedimentary rocks in the Western Pernambuco-Alagoas Domain, southern part of the Province, are reported. Detrital zircon ages range from Archean to Neoproterozoic. Three samples of the Cabrobó Complex were investigated: (i) sillimanite-kyanite-garnet-biotite schist, which presented mostly Ediacaran and Cryogenian detrital zircon ages (youngest zircon at ca. 554 Ma) indicating erosion of neoproterozoic sources, (ii) garnet-biotite schist, which has a dominant Tonian/Stenian population, a less abundant Cryogenian (youngest zircon age at ca. 643 Ma) as well as Paleoproterozoic and Archean zircon grains, and (iii) tourmaline-muscovite quartzite, which contains detrital zircon varying in age between ca. 2.08 Ga and 1.57 Ga, and an abundant population close to the Meso/Paleoproterozoic boundary, possibly associated with the erosion of rocks formed during the Statherian taphrogenesis, known in the central part of the São Francisco Craton as well as in other areas of the Borborema Province. Two samples of the Riacho Seco Metasedimentary Complex were also investigated: (i) a biotite schist with a dominant population presenting ages mostly between 2.3 and 2.7 Ga (youngest zircon age at ca. 2023 Ma) and (ii) a magnetite-biotite-muscovite quartzite, having detrital zircon grains with ages ranging between ca. 1.9 and 2.7 Ga. The sedimentary rocks of the Riacho Seco Complex may have their origin related to the erosion of sources within the São Francisco Craton. The data for the Riacho Seco metasedimentary rocks, however, are not conclusive with respect to the depositional age of the original sedimentary rocks. The sequence might represent exposure of an old (Paleoproterozoic) sedimentary pile or, alternatively, it comprises a neoproterozoic passive margin sequence, with the original sediments derived from the erosion of the cratonic areas to the south.     

Slab break-off model for the Triassic syn-collisional granites in the Qinling orogenic belt,Central China: Zircon U-Pb age and Hf isotope constraints

Numerous Triassic granitoids in the Qinling orogenic belt related to the Late Triassic collision between the North China Craton (NCC) and the Yangtze Block (YB) are important for determining the crustal composition at depth and the geodynamic processes by which the orogen formed. Most of the Triassic plutons in the Qinling orogen were emplaced between 205 and 225 Ma. The granitoid rocks from the southern margin of the NCC, North Qinling, South Qinling, and the northern margin of the YB that were emplaced during this interval have two-stage Hf model ages of 0.60–2.52 Ga (average 2.19 Ga), 0.90–2.66 Ga (average 1.29 Ga), 0.41–3.04 Ga (average 1.48 Ga), and 1.00–1.84 Ga (average 1.34 Ga), respectively, and mean ε_Hf(t) values of ?14.5, ?0.32, ?1.36, and ?3.98, respectively. The Hf isotope compositions of the granitoids in different tectonic units differ significantly, mirroring the diverse history of crustal growth of the four units.

The temporal and spatial distribution and Hf isotope compositions of the granitoids suggest that there was a unified geodynamic process that triggered the magmatism. Formation of the Triassic granitoid plutons at 225–205 Ma was a consequence of slab break-off or E–W-striking slab tearing, related to slab rollback in the west part of the Qinling orogen and oblique continental collision in the east. Upwelling of the asthenospheric mantle led to partial melting of the subcontinental lithospheric mantle and the lower crust, and mixing and/or mingling of the resulting magmas resulted in the formation of granitoids with diverse geological and geochemical characteristics.     

Late Neoarchean thick-skinned thrusting and Paleoproterozoic reworking in the MacQuoid supracrustal belt and Cross Bay plutonic complex,western Churchill Province,Nunavut, Canada

In the western Churchill Province, Canadian Shield, Neoarchean supracrustal and plutonic rocks, intruded by Paleoproterozoic mafic dykes and granitic rocks, comprise the MacQuoid supracrustal belt and the structurally overlying Cross Bay plutonic complex. They form part of the northwestern Hearne subdomain that occupies an intermediate position between the continental Rae domain to the north and west, and the oceanic central Hearne subdomain to the south and east. New geological mapping and supporting geoscience are compatible with the presence of 2550–2500 Ma, southeast-directed, mid-crustal, thick-skinned thrusting that juxtaposed the plutonic complex over the supracrustal belt. The structural contact between the MacQuoid supracrustal belt and the Cross Bay plutonic complex potentially represents a fundamental boundary between isotopically distinct crustal blocks.The ∼2190 Ma MacQuoid mafic dyke swarm cuts across Neoarchean deformation fabrics, but records ∼1.9 Ga, deep-crustal, regional metamorphism that affected both the supracrustal belt and the plutonic complex. Other Paleoproterozoic deformation events that occurred at ∼1850–1810 Ma are of local extent and appear to be relatively minor manifestations of more important events elsewhere, related to the Trans-Hudson orogen.     

Provenance of latest Mesoproterozoic to early Neoproterozoic (meta)-sedimentary rocks and implications for paleographic reconstruction of the Yili Block

The Yili Block is one of the major Precambrian microcontinents of the Central Asian Orogenic Belt (CAOB). Detrital zircon U-Pb ages and Hf isotopic data of the Meso-Neoproterozoic (meta)-sedimentary units within the Yili Block constrain the tectonic affinity and early history of the block. Detrital zircon U-Pb ages, in combination with related magmatic age data, indicate that the Tekesi and Kusitai groups were deposited during the latest Mesoproterozoic-earliest Neoproterozoic (1040–960 Ma) and early Neoproterozoic (<926 Ma), respectively. Zircons from the Kusitai Group yield major age groups at 941–910 Ma and 1887–1122 Ma, whereas the Tekesi Group have a dominant age group at ca. 2.0–1.1 Ga with age peaks at ca. 1.9 Ga, 1.8 Ga, 1.75–1.70 Ga, 1.58 Ga, 1.5 Ga, 1.47–1.43 Ga and 1.27–1.20 Ga. A minor age peak of ca. 2.5 Ga is also recognized in the middle part of the Tekesi Group. Early Neoproterozoic detrital zircons with relatively uniform ε_Hf(t) values (+0.7 to +3.2) were mainly derived from contemporaneous magmatic rocks in the Yili Block. The Central Tianshan Block provides a likely source for detritus with ages of ca. 1.7–1.4 and 2.5 Ga. The predominant late Paleoproterozoic to latest Mesoproterozoic detrital zircons with positive ε_Hf(t) values (+0.5 to +12.0) in the Yili Block were probably derived primarily from regions exhumed during collisional assembly of Rodinia. These populations are consistent with those from the late Mesoproterozoic-early Neoproterozoic (meta)-sedimentary successions in the Central Tianshan, Kokchetav-North Tianshan and Erementau-Niyaz blocks, and Southeast Siberia and northeastern Laurentia cratons. The Yili Block, together with the Precambrian microcontinents in the southwestern Central Asian Orogenic Belt, was likely located at the margin of Rodinia supercontinent, between the southeast Siberia and northeast Laurentia during the early Neoproterozoic.     

Geochronological Framework and Geodynamic Implications of Mafic Magmatism in the Liaodong Peninsula and Adjacent Regions, North China Craton

Mafic rocks are widespread on the Liaodong Peninsula and adjacent regions of the North China Craton. The majority of this magmatism was originally thought to have occurred during the Pre-Sinian, although the precise geochronological framework of this magmatism was unclear. Here, we present the results of more than 60 U–Pb analyses of samples performed over the past decade, with the aim of determining the spatial and temporal distribution of mafic magmatism in this area. These data indicate that Paleoproterozoic–Mesoproterozoic mafic rocks are not as widely distributed as previously thought. The combined geochronological data enabled the subdivision of the mafic magmatism into six episodes that occurred during the middle Paleoproterozoic, the late Paleoproterozoic, the Mesoproterozoic, the Late Triassic, the Middle Jurassic, and the Early Cretaceous. The middle Paleoproterozoic (2.1–2.2 Ga) mafic rocks formed in a subduction-related setting and were subsequently metamorphosed during a ca. 1.9 Ga arc–continent collision event. The late Paleoproterozoic (ca. 1.87–1.82 Ga) bimodal igneous rocks mark the end of a Paleoproterozoic tectono-thermal event, whereas Mesoproterozoic mafic dike swarms record global-scale Mesoproterozoic rifting associated with the final breakup of the Columbia supercontinent. The Late Triassic mafic magmatism is part of a Late Triassic magmatic belt that was generated by post-collisional extension. The Middle Jurassic mafic dikes formed in a compressive tectonic setting, and the Early Cretaceous bimodal igneous rocks formed in an extensional setting similar to a back-arc basin. These latter two periods of magmatism were possibly related to subduction of the Paleo-Pacific plate.     

New constraints for the source characteristics,deposition and age of the 2.1–1.9 Ga metasedimentary cover at the western margin of the Karelian Province

The Paleoproterozoic cover sequence at the 100–150 km wide western margin of the Archean Karelian Province is dominated by deep water Lower and Upper Kaleva metasediments. We present here an interpretation of Sm–Nd isotope and geochemical data on 36 samples, TIMS multi-grain U–Pb zircon analyses on nine samples, and ca. 100 SIMS analysis of detrital zircon grains from four Upper Kaleva and one Lower Kaleva samples.The Lower Kaleva is characterized by autochthonous–parautochthonous, lithologically heterogeneous metaturbidites showing common enrichment in quartz. All the analysed detrital zircons are of a local Neoarchean source but t_DM variation up to 2.4 Ga combined with geochemical data indicate abundant mixing of Paleoproterozoic mafic material, presumably from 2.1 Ga plateau lavas and dykes, in most of the Lower Kaleva samples.The Upper Kaleva is dominantly allochthonous with tectonically enclosed fragments of ophiolite bodies, and it is characterized by lithological and geochemical-isotopic homogeneity. Geochemical, isotopic and detrital zircon data favour material derived from an orogenic domain, comprising both Archean and Proterozoic units, followed by effective mixing during the transport. The Archean zircon grains (25%) are mostly Neoarchean. The Paleoproterozoic grains lack zircons at 2.5–2.2 Ga and plot dominantly (92%) between 1.92 and 2.05 Ga. The indicated maximum deposition ages vary from 1.95–1.94 Ga to 1.92 Ga. The main source area proposed is the Himalaya-type Lapland-Kola orogen (now) in the northeast, which experienced mountain building and erosion at 1.95–1.91 Ga.The western margin of the Karelian Province shows evidence of rifting and lithosphere thinning from 2.1 to 1.95 Ga but it is still under debate whether the craton breakup occurred at 2.06 Ga in a volcanic or later at 1.95 Ga in a non-volcanic margin setting. One hypothesis is that the onset of collision in the northeast changed plate motion and lead to a new spreading within the pre-existing passive margin at 1.97–1.95 Ga. Thus, both a volcanic margin at 2.06 Ga and a non-volcanic margin at ca. 1.95 Ga could have been operated at the western margin of the Karelian Province.     

俄罗斯白海活动带中的太古宙榴辉岩

在俄罗斯白海活动区发现的迄今为止最古老的太古宙榴辉岩的出露,对整个地质学领域是一次革命性事件。白海活动带位于芬诺斯干地亚地盾东北部太古宙陆核,处于科拉半岛大陆和卡累利阿克拉通之间的太古宙增生碰撞带中,在新太古代和古元古代期间多次受到中高压变质和构造变形作用。榴辉岩出露包括Gridino和Salma两大地区。Gridino榴辉岩区的榴辉岩产状可分为TTG片麻岩围岩中具有复杂成因的太古宙榴辉岩包裹镶体(2.72 Ga),组成强烈构造变形的混合混杂岩体(mélange),以及众多古元古代侵入岩墙岩脉状基性榴辉岩。Salma榴辉岩区的榴辉岩年龄应该晚于2.87 Ga,其中的Fe Ti 榴辉岩年龄测定为约2.80 Ga。两大榴辉岩区的p T演化轨迹比较类似,Gridino榴辉岩的峰期变质温压值(T=740~865 ℃,p=1.4~1.8 GPa)比Salma榴辉岩(T≈700 ℃,p=1.3~1.4 GPa)要高。Salma榴辉岩原岩可能与大洋环境有关。     

华北中部造山带五台-恒山地区古元古代变质作用与构造演化

华北克拉通古元古代造山带的时空分布与构造属性尚有很大争论,一种观点认为华北克拉通从新太古代至古元古代受大洋俯冲闭合控制,在1.85~1.95 Ga之间先后发生3次陆-陆碰撞形成3条造山带,即孔兹岩带、胶-辽-吉带和中部造山带;另一种观点强调华北克拉通这3条元古代造山带在1.80~1.98 Ga之间经历了相同的漫长演化过程,指示当时特有的热俯冲和碰撞环境.通过总结中部造山带中的五台-恒山地区变质作用研究进展,阐述该区古元古代造山时代与构造属性.五台-恒山地区的主要变质岩石-构造单元包括恒山杂岩、五台杂岩和滹沱群.恒山杂岩和五台杂岩主体为新太古代TTG片麻岩和表壳岩,它们在古元古代晚期经历了两期变质作用改造.第一期变质作用为中压型,是由于陆-陆碰撞导致弧后伸展盆地闭合、地壳加厚造山所致,从南向北形成一个递增序列:包括五台群下部和南恒山杂岩南部的低角闪岩相、南恒山杂岩北部高角闪岩相和北恒山杂岩的高压麻粒岩相,其压力峰期所对应的地热梯度为~15 ℃/km.变质锆石所记录的年龄峰值随着变质程度增高而降低,依次为~1.95 Ga、~1.92 Ga和~1.85 Ga,这是因为在变质过程中锆石生长受流体和熔体行为控制:在亚固相线下,变质锆石可记录峰期变质年龄,而在超固相线条件下记录伴随熔体结晶的退变质年龄.由此确定该区中压相系变质作用压力峰期时间为~1.95 Ga,对应地壳加厚造山的峰期.加厚地壳由于重力均衡导致变质岩从深部地壳折返至中部地壳,在P-T轨迹上表现为压力峰期之后发生等温减压（ITD）至0.5~0.7 GPa,岩相学上表现为峰期石榴石分解形成斜长石"白眼圈"等,指示缺流体条件.南恒山北部高角闪岩相岩石中的变质锆石记录的折返时间为~1.92 Ga,指示第一次造山结束.第二期变质作用为中-低压型,系为板内变形所致,表现为折返至中地壳的岩石伴随挤压型剪切变形和流体注入形成平衡矿物组合.朱家坊韧性剪切带就是这次板内变形的强构造域,其中也记录了顺时针型P-T轨迹,但所反映的地壳加厚程度有限,第二期变质-变形峰期时间为~1.85 Ga.由于朱家坊韧性剪切带左行走滑,导致北恒山麻粒岩地体抬升.五台-恒山地区在1.80~1.96 Ga之间经历两期变质-变形事件,这一认识或对讨论华北克拉通其他地区的古元古代造山带演化有一定启示意义.      

Age and provenance of early Precambrian metasedimentary rocks in the Lapland–Kola Belt, Russia: evidence from Pb and Nd isotopic data

²⁰⁷Pb/²⁰⁶Pb ages are presented for detrital zircons (Laser Ablation Microprobe ICP‐MS) and whole‐rock Nd isotopic determinations (TIMS) from samples of Neoarchean and Palaeoproterozoic metasedimentary rocks from the Umba granulite terrane and the Keivy domain of the Central Kola composite terrane, Kola Peninsula, north‐western Russia. Three are samples of rocks from the Umba granulite terrane that were deposited ≈ 2.20–1.90 Ga; they contain Archaean detritus, much of it older than 3.0 Gyr, as well as abundant 2.20–1.95‐Gyr‐old material. Deposition may have occurred on the margin of an Archaean craton with an exposed Palaeoproterozoic magmatic arc source, possibly during orogenesis. Two samples from the Keivy domain have remarkably similar, dominantly Archaean detrital zircon age spectra. One was deposited pre‐2.4 Ga, whereas the other was probably deposited post‐2.01 Ga. Both had similar sources, compatible with the proximal country rocks, and possible shallow‐water (?) cratonic margin depositional settings.