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.     

1.9-1.8 Ga old strike-slip megashears in the Baltic Shield, and their plate tectonic implications

The tectonic evolution of the ca. 2.0-1.75 Ga old Svecokarelian fold belt is reviewed, and evidence is presented for large-scale intraplate strike-slip movements along ductile megashears. After the formation of the Kola collision suture and the neighbouring Granulite-Tanaelv thrust belt around 1.9 Ga ago, dextral shearing was initiated along N-S trending megashears. Subsequent anticlockwise rotation of the initially NNE-SSW oriented principal compressive stress caused dextral shearing along a NW-SE trending megashear and reversal in the sense of shearing in the N-S trending ones. Further anticlockwise stress rotation (to a total of about 120°) brought an end to sinistral shearing along the N-S megashears around 1.8 Ga ago and caused reversal to sinistral slip along the NW-SE megashear. Both the older (1.9-1.85 Ga) and younger (1.84-1.8 Ga) parts of this evolution are recorded within the Karelian province and its southwestern margin, where consolidation of the lithosphere took place shortly after 1.9 Ga ago. In the Svecofennian province, where crustal accretion did not start until around 1.9 Ga ago, the older movements may have caused synaccretional crustal folding, but with increasing consolidation, the deformation was concentrated along megashears. Although it is still not possible to interrelate the function of active subduction zones and intraplate megashears. the evolution traced so far provides support for plate tectonic interpretations of the Early Proterozoic geodynamics of the Baltic Shield.     

吉林南部变质岩太古宙—元古宙分界时限的确定及过渡性构造演化

吉林南部古老岩石测年数据偏新和太古宙与元古宙之间分界时限的混乱,本文运用Ｓｍ－Ｎｄ等时线法和锆石Ｕ－Ｐｂ法将吉林南部变质岩系划分为４个年龄段：前阜平旋回（〉２６００Ｍａ）;阜平旋回第一幕（２６００￣２５００Ｍａ）;阜平旋回第二幕（２５００￣２４００Ｍａ）;阜平旋回第三幕（２４００￣２３５０Ｍａ）。阜平旋回第三幕归入太古宙并作为太古宙－元古宙之间分界时限,吉林南部古陆构造环境由活动趋向于稳定,新、老     

The Late Paleoproterozoic Orogeny in the North China Craton

Granulite TTG and khondalite formations are the two typical types of Precambrian metamorphic basement in the North China Craton. The former belongs to the basement of the Archean craton, while the latter is a Paleoproterozoic metasedimentary rock series. The formation of the TTG rock series took place during the period of 2500 to 2450 Ma, near the end of the Archean, and it has undergone multiple times of metamorphism. The cooling rate of the TTG lower crust is quite low (less than 0.3°/Ma), developed during slow crustal cooling in a rather stable tectonic environment. The khondalite rock series underwent amphibolite- to granulite-facies metamorphism that is isotopically constrained to 2000 to 1800 Ma followed by fast crustal cooling from 2.4° to 3.4°/Ma. Based on the thermal and dynamic features of different types of metamorphic rocks, the tectonic state of the crust, and the obvious change of crustal cooling rate from Neo-Archean to Paleoproterozoic, it is clear that continental orogeny existed in the North China craton between 2000 Ma and 1800 Ma.     

Middle to Late Ordovician arc system in the Kyrgyz Middle Tianshan: From arc-continent collision to subsequent evolution of a Palaeozoic continental margin

New geological, geochronological and isotopic data reveal a previously unknown arc system that evolved south of the Kyrgyz Middle Tianshan (MTS) microcontinent during the Middle and Late Ordovician, 467–444 Ma ago. The two fragments of this magmatic arc are located within the Bozbutau Mountains and the northern Atbashi Range, and a marginal part of the arc, with mixed volcanic and sedimentary rocks, extends north to the Semizsai metamorphic unit of the southern Chatkal Range. A continental basement of the arc, indicated by predominantly felsic volcanic rocks in Bozbutau and Atbashi, is supported by whole-rock Nd- and Hf-in-zircon isotopic data. ε_Nd(t) of + 0.9 to − 2.6 and ε_Hf(t) of + 1.8 to − 6.0 imply melting of Neo- to Mesoproterozoic continental sources with Nd model ages of ca. 0.9 to 1.2 Ga and Hf crustal model ages of ca. 1.2 to 1.7 Ga. In the north, the arc was separated from the MTS microcontinent by an oceanic back-arc basin, represented by the Karaterek ophiolite belt. Our inference of a long-lived Early Palaeozoic arc in the southwestern MTS suggests an oceanic domain between the MTS microcontinent and the Tarim craton in the Middle Ordovician.The time of arc-continent collision is constrained as Late Ordovician at ca. 450 Ma, based on cessation of sedimentation on the MTS microcontinent, the age of an angular unconformity within the Karaterek suture zone, and the age of syncollisional metamorphism and magmatism in the Kassan Metamorphic Complex of the southern Chatkal Range. High-grade amphibolite-facies metamorphism and associated crustal melting in the Kassan Metamorphic Complex restricts the main tectonic activity in the collisional belt to ca. 450 Ma. This interpretation is based on the age of a synkinematic amphibolite-facies granite, intruded into paragneiss during peak metamorphism. A second episode of greenschist- to kyanite–staurolite-facies metamorphism is dated between 450 and 420 Ma, based on the ages of granitoid rocks, subsequently affected or not affected by this metamorphism. The latest episode is recorded by greenschist-facies metamorphism in Silurian sandstones and granodiorites and by retrogression of the older, higher-grade rocks. This may have occurred at the Silurian to Devonian transition and reflects reorganization of a Middle Palaeozoic convergent margin.Late Ordovician collision was followed by initiation of a new continental arc in the southern MTS. This arc was active in the Early Silurian, latest Silurian to Middle Devonian, and Late Carboniferous, whereas during the Givetian through Mississippian (ca. 385–325 Ma) this area was a passive continental margin. These arcs, previously well constrained west of the Talas-Ferghana Fault, continued eastwards into the Naryn and Atbashi areas and probably extended into the Chinese Central Tianshan. The disappearance of a major crustal block with transitional facies on the continental margin and too short a distance between the arc and accretionary complex suggest that plate convergence in the Atbashi sector of the MTS was accompanied by subduction erosion in the Devonian or Early Pennsylvanian. This led to a minimum of 50–70 km of crustal loss and removal of the Ordovician arc as well as the Silurian and Devonian forearcs in the areas east of the Talas-Ferghana Fault.     

The episodic development of intermediate to silicic volcano-plutonic suites in the Archaean West Pilbara, Australia

Four felsic igneous rock suites in the Archaean West Pilbara have been identified based on geochemistry and geochronology. A voluminous TTG suite formed at ca. 3260 Ma, which appears to be from melting of a mafic-subducted oceanic slab and thus represents generation of new continental crust. A tholeiitic to calc-alkaline volcanic assemblage and coeval granitoids formed at ca. 3120 Ma in an extensional environment. Further TTG magmatism occurred at ca. 3000 Ma, generating both large granitoid complexes and small plutons, again adding new continental crust to the West Pilbara. At 2930-Ma crustal reworking, most likely of the 3000-Ma rocks, generated small plutons that are coeval with layered ultramafic-mafic intrusions in the region. The changes from new crustal material to crustal reworking infer changing tectonic regimes, which is important for models of Archaean continental crust generation. The data presented here indicate that crustal generation mechanisms varied and were episodic in the West Pilbara, implying that early crustal evolution was a result of periodic changes in tectonic regime, which is reflected in the geochemistry of the rocks.     

Sur l'origine oceanique des metabasites de la chaine du mayombe (Gabon,Congo, Zaïre,Angola)

The Precambrian Mayombian belt is actually considered to be the result of the closure of an oceanic basin located between a western Eburnean craton and an eastern Archaean craton. The cratonic collision, which took place about 1000 Ma ago, induced the tectonic emplacement of ophiolites along the axis of the belt. This collision favoured the formation of the Nyanga's and Niari's epicontinental basins, deformed later during the Panafrican episode.Geochemial studies of the metabasites of the suture confirm their oceanic origin: they are olivine tholeiites similar to abyssal tholeiites. Different from the tholeiites intruded in a lithospheric stretching model, they are almost identical to the tholeiites of the Red Sea.This evolution model for the Mayombian belt: Kibarian narrow ocean floor basins, which closure leads to the Precambrian superior epicontinental basins, is similar to the Burundian of Burundi and to the Kibarian of Zaïre. It seems to be general for the post-Eburnean belts in Central Africa.     

Single zircon ages from high-grade rocks of the Jianping Complex, Liaoning Province, NE China

The high-grade rocks of the Jianping Complex in Liaoning Provi nce, NE China, belong to the late Archaean to earliest Proterozoic granulite belt of the North China craton. Single zircon ages obtained by the Pb–Pb evaporation method and SHRIMP analyses document an evolutionary history that began with deposition of a cratonic supracrustal sequence some 2522–2551 Ma ago, followed by intrusion of granitoid rocks beginning at 2522 Ma and reaching a peak at about 2500 Ma. This was followed by high-grade metamorphism, transforming the existing rocks into granulites, charnockites and enderbites some 2485–2490 Ma ago. The intrusion of post-tectonic granites at 2472 Ma is associated with widespread metamorphic retrogression and ends the tectono–metamorphic evolution of this terrain. A similar evolutionary sequence has also been recorded in the granulite belt of Eastern Hebei Province. We speculate that the Jianping Complex was part of an active continental margin in the late Archaean that became involved in continental collision and crustal thickening shortly after its formation. There is a remarkable similarity between the 2500 Ma North China granulite belt and the equally old granulite belt of Southern India, suggesting that the two crustal domains could have been part of the same active plate margin in latest Archaean times.     

Depositional and tectonic setting of the Paleoproterozoic Lower Aillik Group,Makkovik Province,Canada: evolution of a passive margin-foredeep sequence based on petrochemistry and U–Pb (TIMS and LAM-ICP-MS) geochronology

The Paleoproterozoic Lower Aillik Group is a deformed metasedimentary–metavolcanic succession located in the Makkovik Province of Labrador, eastern Canada. The group is situated near the boundary between reworked Archaean gneiss of the Nain (North Atlantic) craton and juvenile Paleoproterozoic crust that was both tectonically accreted and formed on or adjacent to this craton during the ca. 1.9–1.78 Ma Makkovikian orogeny. The Lower Aillik Group is structurally underlain by Archaean gneiss and structurally overlain by ca. 1860–1807 Ma bimodal, dominantly felsic volcanic and volcaniclastic rocks of the Upper Aillik Group. We present geochemical data from metavolcanic rocks and U–Pb geochronological data from several units of the Lower Aillik Group in order to address the depositional and tectonic history of this group. U–Pb data were obtained using both thermal ionization mass spectrometry (TIMS) and laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS). Two quartzite units near the structural base of the Lower Aillik Group contain detrital zircons only of Archaean age, and are interpreted to have been deposited on the Nain craton during post-2235 Ma rifting and initiation of a passive continental margin. Overlying mafic metavolcanic rocks contain thin horizons of intermediate tuff, one of which is dated at 2178±4 Ma. This relatively old age, and an inferred stratigraphic relationship with underlying sedimentary units, suggest that the volcanic rocks represent transitional oceanic crust, consistent with their geochemical similarity to tholeiitic rifted margin sequences of Mesozoic age in eastern North America. A package of interlayered psammitic and semipelitic metasedimentary rocks that appears to stratigraphically overlie the mafic volcanic unit is dominated by Paleoproterozoic detrital zircons but also contains Archaean grains. This package was deposited after 2013 Ma, the age of the youngest concordant zircon. The U–Pb data imply a minimum 165 m.y. time gap between mafic volcanism and sedimentation, and are consistent with deposition of the psammite–semipelite unit in an evolving foredeep that heralded the approach of a Paleoproterozoic arc terrane. Accretion of this terrane to the Nain cratonic margin at ca. 1.9 Ga initiated the Makkovikian orogeny. Although the Lower Aillik Group is highly deformed and may contain internal tectonic boundaries or be incomplete, the U–Pb and geochemical data allow quantitative assessment of a prolonged rift-drift-basin closure cycle that characterized the Early Paleoproterozoic evolution of the southern Nain cratonic margin.     

Detrital and xenocrystic zircon ages from Neoproterozoic to Palaeozoic arc terranes of Mongolia: Significance for the origin of crustal fragments in the Central Asian Orogenic Belt

The Central Asian Orogenic Belt contains many Precambrian crustal fragments whose origin is unknown, and previous speculations suggested these to be derived from either Siberia, Tarim or northern Gondwana. We present an age pattern for detrital and xenocrystic zircons from Neoproterozoic to Palaeozoic arc and microcontinental terranes in Mongolia and compare this with patterns for Precambrian rocks in southern Siberia, the North China craton, the Tarim craton and northeastern Gondwana in order to define the most likely source region for the Mongolian zircons. Our data were obtained by SHRIMP II, LA-ICP-MS and single zircon evaporation and predominantly represent arc-related low-grade volcanic rocks and clastic sediments but also accretionary wedges and ophiolitic environments.The Mongolian pattern is dominated by zircons in the age range ca. 350–600 and 700–1020 Ma as well as minor peaks between ca. 1240 and 2570 Ma. The youngest group reflects cannibalistic reworking of the Palaeozoic arc terranes, whereas the Neoproterozoic to late Mesoproterozoic peak reflects both reworking of the arc terranes as well as Neoproterozoic rifting and a Grenville-age crust-formation event.The 700–1020 Ma peak does not exist in the age spectra of the Siberian and North China cratons and thus effectively rules out these basement blocks as potential source areas for the Mongolian zircons. The best agreement is with the Tarim craton where a major Grenville-age orogenic event and early Neoproterozoic rifting have been identified. The age spectra also do not entirely exclude northeastern Gondwana as a source for the Mongolian zircons, but here the Neoproterozoic age peak is related to the Pan-African orogeny, and a minor Grenville-age peak may reflect a controversial orogenic event in NW India.Our Mongolian detrital and xenocrystic age spectrum suggests that the Tarim craton was the main source, and we favour a tectonic scenario similar to the present southwestern Pacific where fragments of Australia are rifted off and become incorporated into the Indonesian arc and microcontinent amalgamation that will evolve into a future orogenic belt.     

Tectonostratigraphic and geochronologic reappraisal constraining the growth and evolution of Singhbhum Archaean craton, eastern India

Reappraisal of field relationships between the different lithological ensembles supported by available geochronological data, and taking due note of the tectono-metamorphic, magmatic and sedimentation history helped to build up a coherent crustal evolutionary history of the Singhbhum Archaean craton, eastern India. The evolution of the earliest sialic crust, as the isotope ages suggest, was around 3700 Ma or even earlier. Deposition of the oldest, dominantly metasedimentary supracrustals, the Older Metamorphic Group (OMG), was initiated at around 3380 Ma, i.e. after a gap of about 320 million years. The closing of OMG basins synchronously with the emplacement of a granitoid phase was at ca.3285 Ma. No other fabric-forming ductile deformation and metamorphism associated with the development of foliation and mineral lineation is known in the rocks of the Singhbhum Archaean craton subsequent to this event. Formation of the succeeding geological ensembles including the deposition of BIF-bearing Iron Ore Group (IOG) and the emplacement of the post-IOG granitoids at ca.3100 Ma can be described as ??lsnon-orogenic?? event taking place during the phase of tectonic quiescence. Supracrustals like the Dhanjori and Simlipal mafic volcanics with intercalated beds of arenite evolved later during the phase of Plume outburst at around 2800 Ma. The end-Archaean intrusion of Newer Dolerite dykes in conjugate sets and the deposition of Kolhan Group in an N-S oriented basin during an E-W stress system mark the culmination of the Archaean crust-building activity in the Singhbhum Archaean craton.     

Multiple accretion at the eastern margin of the Rio de la Plata craton: the prolonged Brasiliano orogeny in southernmost Brazil

The Neoproterozoic-Eoplalaeozoic Brasiliano orogeny at the eastern margin of the Rio de la Plata craton in southernmost Brazil and Uruguay comprises a complex tectonic history over 300?million years. The southern Brazilian Shield consists of a number of tectono-stratigraphic units and terranes. The S?o Gabriel block in the west is characterized by c.760?C690?Ma supracrustal rocks and calc-alkaline orthogneisses including relics of older, c. 880?Ma old igneous rocks. Both igneous and metasedimentary rocks have positive ??Nd(t) values and Neoproterozoic TDM model ages; they formed in magmatic arc settings with only minor input of older crustal sources. A trondhjemite from the S?o Gabriel block intruding dioritc and tonalitic gneisses during the late stages of deformation (D₃) yield an U?CPb zircon age (LA-ICP-MS) of 701?±?10?Ma giving the approximate minimum age of the S?o Gabriel accretionary event. The Encantadas block further east, containing the supracrustal Porongos belt and the Pelotas batholith, is in contrast characterized by reworking of Neoarchean to Palaeoproterozoic crust. The 789?±?7?Ma zircon age of a metarhyolite intercalated with the metasedimentary succession of the Porongos belt provides a time marker for the basin formation. Zircons of a sample from tonalitic gneisses, constituting the Palaeoproterozoic basement of the Porongos belt, form a cluster at 2,234?±?28?Ma, interpreted as the tonalite crystallization age. Zircon rims show ages of 2,100?C2,000?Ma interpreted as related to a Palaeoproterozoic metamorphic event. The Porongos basin formed on thinned continental crust in an extensional or transtensional regime between c. 800?C700?Ma. The absence of input from Neoproterozoic juvenile sources into the Porongos basin strongly indicates that the Encantadas and S?o Gabriel blocks were separated terranes that became juxtaposed next to each other during the Brasiliano accretional events. The tectonic evolution comprises two episodes of magmatic arc accretion to the eastern margin of the Rio de la Plata craton, (i) accretion of an intra-oceanic arc at c. 880?Ma (Passinho event) and (ii) accretion of the 760?C700?Ma Cambaí/Vila Nova magmatic arc (S?o Gabriel event). The latter event also includes the collision of the Encantadas block with the Rio de la Plata craton to the west. Collision and crustal thickening was followed by sinistral shear along SW?CNE-trending orogen-parallel crustal-scale shear zones that can be traced from southern Brazil to Uruguay and have been active between 660 and 590?Ma. Voluminous granitic magmatism in the Pelotas batholith spatially related to shear zones is interpreted as late- to post-orogenic magmatism, possibly assisted by lithospheric delamination. It marks the transition to the post-orogenic molasse stage. Localized deformation by reactivation of preexisting shear zones continued until c. 530?Ma and can be assigned to final stages of the amalgamation of West Gondwana.     

Reassembly of the Dharwar and Bastar cratons at ca. 1 Ga: Evidence from multiple tectonothermal events along the Karimnagar granulite belt and Khammam schist belt,southern India

The northern part of the Nellore–Khammam schist belt and the Karimnagar granulite belt, which are juxtaposed at high angle to each other have unique U–Pb zircon age records suggesting distinctive tectonothermal histories. Plate accretion and rifting in the eastern part of the Dharwar craton and between the Dharwar and Bastar craton indicate multiple and complex events from 2600 to 500 Ma. The Khammam schist belt, the Dharwar and the Bastar craton were joined together by the end of the Archaean. The Khammam schist belt had experienced additional tectonic events at \(\sim \)1900 and \(\sim \)1600 Ma. The Dharwar and Bastar cratons separated during development of the Pranhita–Godavari (P–G) valley basin at \(\sim \)1600 Ma, potentially linked to the breakup of the Columbia supercontinent and were reassembled during the Mesoproterozoic at about 1000 Ma. This amalgamation process in southern India could be associated with the formation of the Rodinia supercontinent. The Khammam schist belt and the Eastern Ghats mobile belt also show evidence for accretionary processes at around 500 Ma, which is interpreted as a record of Pan-African collisions during the Gondwana assembly. From then on, southern India, as is known today, formed an integral part of the Indian continent.     

北秦岭高压-超高压岩石的时空分布、P-T-t演化及其形成机制

北秦岭构造带早古生代的构造格局和演化过程一直是地学界比较关注也是存在较大争议的问题之一.在已有研究基础上,系统总结了本课题组近年来在北秦岭早古生代高压-超高压变质作用研究方面的进展,从变质作用角度对北秦岭早古生代的构造演化提供重要限定.丹凤斜长角闪岩中柯石英的发现为区内超高压变质作用的存在提供了最直接的矿物学证据;东秦岭秦岭杂岩中的斜长角闪岩普遍经历了高压-超高压榴辉岩相变质,具面状分布的特征,是陆壳俯冲/深俯冲作用的产物;高压-超高压榴辉岩和围岩片麻岩都记录了顺时针的P-T-t轨迹,峰期变质时代为500~490 Ma,之后主体又经历约470~450 Ma和约420~400 Ma两期抬升退变质叠加和部分熔融作用;高压-超高压岩石两期退变质和部分熔融发生的时代与北秦岭460~440Ma和~420Ma的两期岩浆事件的时代一致,说明北秦岭早古生代岩浆作用是深俯冲陆壳板片断离和碰撞造山结束后地壳伸展作用的岩浆响应;高压-超高压榴辉岩原岩形成时代约800 Ma,具有与南秦岭新元古代中晚期岩浆岩一致的地球化学特征,北秦岭超高压岩石的形成可能是商丹洋关闭后洋壳拖曳着南秦岭陆壳物质向北发生大陆深俯冲的结果,商丹洋在500 Ma主体应该已经关闭;秦岭岩群是部分而不是整体经历了大陆的深俯冲,现今的秦岭岩群是一个俯冲碰撞杂岩带而不是一个岩石地层单元或微陆块;北秦岭早古生代造山作用在中泥盆世已经结束,整体处于构造隆升后的剥蚀阶段,是南秦岭刘岭群碎屑岩的主要蚀源区,刘岭群沉积盆地形成于碰撞造山后的伸展构造背景而非弧前环境.      

A 2.44 Ga syn-tectonic mafic dyke swarm in the Kolvitsa Belt,Kola Peninsula,Russia: implications for early Palaeoproterozoic tectonics in the north-eastern Fennoscandian Shield

The Kolvitsa Belt in the south-western Kola Peninsula formed coeval with the earliest Palaeoproterozoic rift-belts in the Fennoscandian Shield. The Palaeoproterozoic history of this belt comprises the deposition of the 2.47 Ga Kandalaksha amphibolite (metabasalt) sequence onto 2.7 Ga granitoid gneisses, the intrusion of the 2.45–2.46 Ga Kolvitsa Massif of gabbro-anorthosite and the subsequent multiple injection of mafic dykes and magmatic brecciation, followed by the intrusion of 2.44 Ga dioritic dykes, and extensive shearing at 2.43–2.42 Ga. The gabbro-anorthosite and dykes contain high-pressure garnet-bearing assemblages that have previously been considered as evidence for metamorphism in a compressional setting of the Kolvitsa Belt at 2.45–2.42 Ga, i.e. coeval with the formation of the Imandra–Varzuga rift-belt and layered mafic intrusions in an extensional setting. The Kochinny Cape study area on the White Sea coast presents an unique remnant of a 2.44 Ga mafic dyke swarm that endured ca. 1.9 Ga collision but preserved its primary structural pattern well. All these dykes were intruded along numerous NW-trending shear zones within the Kolvitsa Massif and contain angular xenoliths of sheared gabbro-anorthosite. Every new batch of mafic melt underwent shearing during or immediately after solidification, and later dykes intruded into already sheared dykes. Thus, rocks of the Kolvitsa Massif and its dyke complex were successively injected into a large-scale shear zone which was active from ca. 2.46 to 2.42 Ga. Multiple injection of mafic melts, the presence of mutually intruding, composite, sheared mafic dykes, of magmatic breccias with gabbroic groundmass, and of host rocks fragments (showing no evidence of tectonic stacking at the time of brecciation), all indicate an extensional setting. Shearing was also extensional as it occurred simultaneously with the multistage magmatism. The asymmetric morphology of deformed dykes, and asymmetric flexures within weakly deformed lenses show that all these extensional shear zones, apart from a few exceptions, are dextral, were formed in a transtensional setting and are attributed to general W–E to WSW–ENE extension. Structural data available for 2.4–2.5 Ga magmatic rocks elsewhere in the Kola region suggest that the same kinematics operated on a regional scale. The presence of the garnet-bearing assemblages in gabbro-anorthosite and dykes may be explained by crystallisation and shearing of the magmatic rocks at deep crustal levels. Alternatively, corona development might have occurred much later as a result of tectonic loading due to the juxtaposition and overthrusting of the Umba Granulite Terrane onto the Kolvitsa Belt at ca. 1.9 Ga. In view of the field evidence and published ages, an overall extensional setting rather than a combination of compressional and extensional zones is preferable for Palaeoproterozoic tectonics in the north-eastern Fennoscandian Shield at 2.5–2.4 Ga.     

华北克拉通的变质沉积岩及其克拉通的构造划分

早前寒武纪花岗质岩年龄统计结果显示,华北克拉通经历了3.8,3.3,2.9,2.5和1.8~1.9 Ga等多个旋回才从陆核成长为陆台,与之对应沉积岩也由少变多,大约以500 Ma为一周期。由于沉积作用出现在成陆间歇期,所以二者在时间上相间互补,其状如同显生宙超大陆裂解和拼合的周期交替。这一现象不但是地壳演化的普遍规律,而且也可反过来用沉积岩反映陆壳的演化。然而,早前寒武纪尤其是太古宙的沉积岩毕竟太少,无法用来恢复当时古陆块的面貌,但古元古代的特别是陆缘沉积的孔兹岩,尽管已进入下地壳并成为克拉通基底的组成,则以保存甚多、分布延续,使其重塑克拉通的拼合成为可能。已有的华北克拉通的构造划分方案多种多样,但以陆缘沉积的古元古代孔兹岩作为地块的边界,理当最能反映当时古陆块的面貌。因此,以孔兹岩为主要依据,并综合考虑岩石组合、构造环境、变质p-T轨迹、同位素年龄、以及不变质的沉积盖层等地质特征,将华北克拉通主体从西往东划分为：鄂尔多斯地块 / 晋蒙弧形拼合带 / 冀鲁豫地块 /（郯庐断裂）/ 胶辽地块群等构造单元,所得到的不同于以往的构造轮廓,显示华北陆台并非一统的太古宙克拉通,而是吕梁运动拼合成的古元古代大陆。     

The Olkhon metamorphic terrane in the Baikal region: An Early Paleozoic collage of Neoproterozoic active margin fragments

We report data from the Khadarta, Khoboi, and Orso metamorphic complexes of the Olkhon terrane in the western Baikal region. High-grade rocks in the three complexes may have been derived from active continental margin rocks (island arc–backarc basin system). The backarc basin history possibly began at 840–800 Ma, according to SHRIMP-II U-Pb zircon ages of the Orso gneiss. Many tectonic units in the Olkhon terrane belonged to the active margin of the Barguzin microcontinent which rifted off the Aldan province of the Siberian craton in the early Neoproterozoic. The accretion of the microcontinent to the craton was accompanied by high-grade metamorphism recorded in the Khadarta and Khoboi granulites. The 507 ± 8 Ma and 498 ± 7 Ma SHRIMP-II U-Pb zircon ages of the latter complexes, respectively, may refer to the earliest evolution stage of the Olkhon metamorphic terrane. New data for the Olkhon terrane agree well with the ages of other high-grade complexes along the southern Siberian craton (Slyudyanka, Kitoikin, Derba) and correspond to the initiation of the Central Asian orogen. With these data, the Olkhon metamorphic terrane has been interpreted as an Early Paleozoic collisional collage of fragments of the microcontinent’s Neoproterozoic active margin.     

http://www.sciencedirect.com/science/article/pii/S1674987110000022

<正>The Early Cretaceous Hohhot metamorphic core complex(mcc) of the Daqing Shan(Mtns.) of central Inner Mongolia is among the best exposed and most spectacular of the spatially isolated mcc's that developed within the northern edge of the North China "craton".All of these mcc's were formed within the basement of a Late Paleozoic Andean-style arc and across older Mesozoic fold-and-thrust belts of variable age and tectonic vergence.The master Hohhot detachment fault roots southwards within the southern margin of the Daqing Shan for an along-strike distance of at least 120 km.Its geometry in the range to the north is complicated by interference patterns between(1) primary,large-scale NW-SE-trend-ing convex and concave fault corrugations and(2) secondary ENE-WSW-trending antiforms and synforms that folded the detachment in its late kinematic history.As in the Whipple Mtns.of California, the Hohhot master detachment is not of the Wernicke(1981) simple rooted type:instead,it was spawned from a mid-crustal shear zone,the top of which is preserved as a mylonitic front within Carboniferous metasedimentary rocks in its exhumed lower plate.~(40)Ar—~(39)Ar dating of siliceous volcanic rocks in basal sections of now isolated supradetachment basins suggest that crustal extension began at ca.127 Ma, although lower-plate mylonitic rocks were not exposed to erosion until after ca.119 Ma.Essentially synchronous cooling of hornblende,biotite.and muscovite in footwall mylonitic gneisses indicates very rapid exhumation and at ca.122—120 Ma.Contrary to several recent reports,the master detachment clearly cuts across and dismembers older,north-directed thrust sheets of the Daqing Shan foreland fold-and-thrust belt.Folded and thrust-faulted basalts within its foredeep strata are as young as 132.6±2.4 Ma,thus defining within 5—6 Ma the regional tectonic transition between crustal contraction and profound crustal extension.     

The early proterozoic greenstone belt of the Northern Guiana Shield,South America

A large part of the northern Guiana Shield is underlain by metavolcanic and metasedimentary sequences steeply folded and metamorphosed in the greenschist facies during the 2000 Ma old Trans-Amazonian orogeny. It is suggested that this extensive early Proterozoic belt represents a back-arc basin environment marginal to a supercontinent; that is to say, this belt was possibly intercratonic with respect to existing Archaean basements, whereas elsewhere in the interior of the continent, as in Brazil, fold belts of identical age were intracratonic. The formation of the volcano-sedimentary pile and subsequent orogeny to which the Guiana belt was subjected are ascribed to plate convergence and subduction. This interpretation is supported by the nature of the volcanic pile and the presence of a possible fossil suture in northern Guiana.     

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.