The Rio de la Plata craton and the adjoining Pan-African/brasiliano terranes: Their origins and incorporation into south-west Gondwana

The Neoproterozoic to Early Cambrian amalgamation of SW Gondwana through the Brasiliano/Pan-African orogeny is reviewed with emphasis on the role of the Río de la Plata craton of South America in the light of new evidence from a borehole at the eastern end of the Tandilia belt (38°S). U–Pb, Hf and O isotope data on zircon indicate that this un-reworked Palaeoproterozoic craton abuts against a distinct continental terrane to the east (Mar del Plata terrane). The craton is bounded everywhere by transcurrent faults and there is no evidence to relate it to the Neoproterozoic mobile belts now seen on either side. The Punta Mogotes Formation at the bottom of the borehole contains 740–840 Ma detrital zircons that are assigned to a widespread Neoproterozoic rifting event. The data suggest that the Mar del Plata terrane rifted away from the southwestern corner of the Angola block at c. 780 Ma. Negative εHf_t values and δ¹⁸O > 6.5‰ suggest derivation by melting of old crust during a protracted extensional episode. Other continental terranes may have formed in a similar way in Uruguay (Nico Pérez) and southeastern Brazil, where the Schist Belt of the Dom Feliciano orogenic belt is probably a correlative of the Punta Mogotes sequence, implying that the Dom Feliciano belt must extend at least as far as 38°S. A new geodynamic scenario for West Gondwana assembly includes at least two major oblique collisional orogenies: Kaoko–Dom Feliciano (580–680 Ma) and Gariep–Saldania (480–580 Ma), the latter resulting from oblique impingement of the Rio de la Plata craton against the Kalahari craton. Assembly of this part of South-West Gondwana was accomplished before the Ordovician (to Silurian?) siliciclastic platform sediments of the Balcarce Formation in the Tandilia Belt covered the southern sector of Río de la Plata craton.     

Low-latitude and multiple geomagnetic reversals in the Neoproterozoic Puga cap carbonate, Amazon craton

Palaeomagnetic study of the carbonates that ubiquitously cap glacial deposits may constrain the latitudinal extent of Neoproterozoic glaciations and the duration of the greenhouse recovery. We present the first palaeomagnetic data on the Neoproterozoic cap carbonates covering the Amazon craton, which are folded along the Paraguay Belt. Samples collected at deformed beds along the Paraguay Belt present a single‐polarity secondary magnetization acquired by the end of the Brasiliano orogeny (540–520 Ma). In the cratonic area, a dual‐polarity component was isolated in dolostones at the base of the sequence. The presence of a stratabound reversal stratigraphy along with high unblocking temperatures strongly suggest that this magnetization is primary. This result implies a low palaeolatitude (22+6/?5°) for the Amazon block just after deposition of Puga diamictites. In addition, the presence of multiple reversals across the first 20 m of the cap carbonate sequence suggests that their sedimentation must have spanned hundreds of thousands of years at least.     

Basin infilling in the southern-central part of the Sergipano Belt (NE Brazil) and implications for the evolution of Pan-African/Brasiliano cratons and Neoproterozoic sedimentary cover

The Neoproterozoic sedimentary cover deposited across the interface of several Pan-African/Brasiliano fold-thrust belts with their respective cratons is strongly similar and has been widely correlated throughout Gondwana. In particular, the upper part of the cratonic cover of the São Francisco Craton has been interpreted as a ring of foreland basin sediments. However, detailed studies carried out around the southern-central part of the Sergipano Belt (NE Brazil) and its interface with the northern margin of the São Francisco Craton demonstrate that: (1) sedimentation records the evolution of a passive continental margin and is divided into two cycles (I and II), each one comprising a basal siliciclastic megasequence overlain by a carbonate megasequence; (2) the cratonic cover comprises cycle I and part of the basal megasequence of cycle II; (3) all of these rocks spread continuously across the craton margin into the Sergipano Belt, where they occur around basement domes and are overlain by a metadiamictite formation and a metacarbonate formation that complete cycle II; and (4) basement and cover underwent the same Brasiliano (670–600 Ma) compressive deformation under sub-greenschist metamorphic conditions. These data deny the foreland basin model for the cratonic sediments to the south of the Sergipano Belt and, coupled with recent data on the evolution of other margins of the craton, indicate that the Neoproterozoic sedimentary cover derives from highs existing close to the centre of the ancient São Francisco Plate. This sedimentary cover was also influenced by highs of an Andean-type margin that evolved ca 900–640 Ma along the western side of the plate. Such evolution also applies to the Neoproterozoic cover of other cratons of the Pan-African/Brasiliano orogeny.     

Coeval basin formation,plutonism and metamorphism in the Northern Tasmanides: extensional Cambro-Ordovician tectonism of the Charters Towers Province

Abstract

The Charters Towers Province, of the northern Thomson Orogen, records conversion from a Neoproterozoic passive margin to a Cambrian active margin, as characteristic of the Tasmanides. The passive margin succession includes a thick metasedimentary unit derived from Mesoproterozoic rocks. The Cambrian active margin is represented by upper Cambrian–Lower Ordovician (500–460?Ma) basinal development (Seventy Mile Range Group), plutonism and metamorphism resulting from an enduring episode of arc–backarc crustal extension. Detrital zircon age spectra indicate that parts of the metamorphic basement of the Charters Towers Province (elements of the Argentine Metamorphics and Charters Towers Metamorphics) overlap in protolith age with the basal part of the Seventy Mile Range Group and thus were associated with extensional basin development. Detrital zircon age data from the extensional basin succession indicate it was derived from a far-field (Pacific-Gondwana) primary source. However, a young cluster (<510?Ma) is interpreted as reflecting a local igneous source related to active margin tectonism. Relict zircon in a tonalite phase of the Fat Hen Creek Complex suggests that active margin plutonism may have extended back to ca 530?Ma. Syntectonic plutonism in the western Charters Towers Province is dated at ca 485–480?Ma, close to timing of metamorphism (477–467?Ma) and plutonism more generally (508–455?Ma). The dominant structures in the metamorphic basement formed with gentle to subhorizontal dips and are inferred to have formed by extensional ductile deformation, while normal faulting developed at shallower depths, associated with heat advection by plutonism. Lower Silurian (Benambran) shortening, which affected metamorphic basement and extensional basin units, resulted in the dominant east–west-structural trends of the province. We consider that these trends reflect localised north–south shortening rather than rotation of the province as is consistent with the north–south paleogeographic alignment of extensional basin successions.

1. KEY POINTS

2. Northern Tasmanide transition from passive to active margin tectonic mode had occurred by ca 510?Ma, perhaps as early as ca 530?Ma.

3. Cambro-Ordovician active margin tectonism of the Charters Towers Province (northern Thomson Orogen) was characterised by crustal extension.

4. Crustal extension resulted in the development of coeval (500–460?Ma) basin fill, granitic plutonism and metamorphism with rock assemblages as exposed across the Charters Towers Province developed at a wide range of crustal levels and expressing heterogeneous exhumation.

5. Protoliths of metasedimentary assemblages of the Charters Towers Province include both Proterozoic passive margin successions and those emplaced as Cambrian extensional basin fill.

     

Precambrian continent assembly and dispersal events of South Indian and East Antarctic Shields

An Archaean continent ‘SIWA’, an acronym for South India–Western Australia, comprising the Bastar–Dharwar craton, the Yilgarn craton, the Napier Complex, and the Vestfold Hills has been identified from palaeomagnetic and spatio-temporal data. This assembly was dispersed in three phases with the development of the proto-Indian ocean. The first and second events ~2350 and ~2000 Ma were related to the separation of the Yilgarn craton and the Napier Complex, respectively, to form a proto-Indo-Antarctic ocean and the Cuddapah basin. The proto-ocean was closed ~1650 Ma by the collision of the Lambert Terrane of East Antarctica and the Bastar–Dharwar craton. This collision, associated with ultra-high temperature (UHT) granulite facies metamorphism, is identified in the southern domain of the Eastern Ghats and the Oygardens domain of East Antarctica. The third extensional event between 1500 and 1200 Ma was associated with the separation of the Vestfold Hills block and a second phase of opening of the proto-Indian ocean, and the development of a series of basins on the western side of the Eastern Ghats (the Chhatisgarh, Khariar, Ampani, Indravati, and Sabari basins). The closing of this ocean basin during the Eastern Ghats–Rayner orogeny at ~950 Ma was related to the amalgamation of India and East Antarctica to form the supercontinent Rodinia. During the Neoproterozoic, this part of Rodinia was involved in orogenic collapse/extension and deposition of the Sodruzhesvo Group. The Pan-African Prydz Bay orogeny at ~550 Ma caused the closing of the basin to form East Gondwanaland.     

Sedimentological and provenance response to Cambrian closure of the Clymene ocean: The upper Alto Paraguai Group,Paraguay belt,Brazil

Final Gondwana amalgamation was marked by the closure of the Neoproterozoic Clymene ocean between the Amazonia craton and central Gondwana. The events which occurred in the last stage of this closure were recorded in the upper Alto Paraguai Group in the foreland of the Paraguay orogen. Outcrop-based facies analysis of the siliciclastic rocks of upper Alto Paraguai Group, composed of the Sepotuba and Diamantino Formations, was carried out in the Diamantino region, within the eastern part of the Barra dos Bugres basin, Mato Grosso state, central-western Brazil. The Sepotuba Formation is composed of sandy shales with planar to wave lamination interbedded with fine-grained sandstone with climbing ripple cross-lamination, planar lamination, swaley cross-stratification and tangential to sigmoidal cross-bedding with mud drapes, related to marine offshore deposits. The lower Diamantino Formation is composed of a monotonous, laterally continuous for hundreds of metres, interbedded siltstone and fine-grained sandstone succession with regular parallel lamination, climbing ripple cross-lamination and ripple-bedding interpreted as distal turbidites. The upper part of this formation consists of fine to medium-grained sandstones with sigmoidal cross-bedding, planar lamination, climbing ripple cross-lamination, symmetrical to asymmetrical and linguoid ripple marks arranged in lobate sand bodies. These facies are interbedded with thick siltstone in coarsening upward large-scale cycles related to a delta system. The Sepotuba Formation characterises the last transgressive deposits of the Paraguay basin representing the final stage of a marine incursion of the Clymene ocean. The progression of orogenesis in the hinterland resulted in the confinement of the Sepotuba sea as a foredeep sub-basin against the edge of the Amazon craton. Turbidites were generated during the deepening of the basin. The successive filling of the basin was associated with progradation of deltaic lobes from the southeast, in a wide lake or a restricted sea that formed after 541 ± 7 Ma. Southeastern to east dominant Neoproterozoic source regions were confirmed by zircon grains that yielded ages around 600 to 540 Ma, that are interpreted to be from granites in the Paraguay orogen. This overall regressive succession recorded in the Alto Paraguai Group represents the filling up of a foredeep basin after the final amalgamation of western Gondwana in the earliest Phanerozoic.     

燕山中、新元古代沉积盆地构造演化

燕山中、新元古代沉积盆地是一个经历了十亿年演化的长寿盆地。在漫长的地质历史时期中,受华北地台北缘构造环境变化的制约,该盆地经历了三个阶段的演变。裂陷槽阶段(1800Ma～1600Ma)形成一个北东向半地堑式的楔形槽地,团山子-大红峪期火山活动是裂陷作用顶峰的标志。之后,华北地台北侧古大洋板块发生第一次消减作用,裂陷槽迅速消亡,成为一支废弃的裂谷。裂陷槽消亡后盆地向克拉通盆地过渡的阶段(1600Ma～1200Ma),其构造特点是一方面盆地日趋稳定,另一方面还残留了某些裂陷特征。克拉通盆地阶段(1200Ma～800Ma),沉积了典型的地台盖层沉积,沉积物厚度小,厚度变化小,相变也小,盆地表现出整体同升同降的特点,形成数个区域性的间断面。此时,华北地台北侧的古亚洲洋已经封闭消失,沉积盆地古地理格局发生了根本性的变化,转而向东与大洋沟通。     

Neoproterozoic tectonic synthesis of Uruguay

Neoproterozoic–lower Palaeozoic successions in the Brasiliano fold belts are described and a brief synthesis of these terranes is presented in order to erect a tectonic framework for this region. Tectonic events that occurred around the Río de La Plata craton were diachronous and reflected successive stages of the Brasiliano orogenic cycle. They took place in mobile belts that constituted part of the Gondwana supercontinent. The most thoroughly investigated Neoproterozoic sections are located in the eastern and southeastern regions of Uruguay. The Dom Feliciano Belt shows a tectonic evolution from back-arc to foreland basin characterized by fold-and-thrust, thick-skinned belts developed during the Brasiliano/Pan-African orogenic cycle. The most conspicuous features were late-tectonic high-K calc-alkaline granitoids, HT-LP metamorphism, significant displacements along shear zones, and post-tectonic granitoids. The final stage was characterized by post-collisional basins (molassic sequences) and extensional magmatism related to a phase of crustal stretching. Several lithotectonic units are present as basement inliers in the Dom Feliciano Belt: these include a low-to-medium metamorphic grade sequence (the Zanja del Tigre Formation), granitoids and gneisses (the Campanero Unit), high-grade basement of the Cerro Olivo Complex (Palaeoproterozoic or Neoproterozoic), and a low-metamorphic grade orogenic belt (the Rocha Formation). This paper provides a simplified tectonic map of eastern Uruguay, which we use to describe tectonic evolution from Precambrian to early Palaeozoic time.     

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.      

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.     

西秦岭地区造山型与卡林型金矿床

西秦岭金矿床分为卡林型和造山型两类。卡林型金矿床麇集于南秦岭和松潘—甘孜造山带的东北部。三叠纪和早侏罗世的同构造花岗闪长岩广泛分布于西秦岭中部和南部、松潘—甘孜盆地以及扬子克拉通边缘。造山型脉状金矿床主要分布于西秦岭造山带中的脆韧性剪切带内。大部分粗粒金主要赋存在网格状石英细脉和角砾状围岩中的黄铁矿、磁黄铁矿、毒砂和少量贱金属硫化物中和以分散状分布在蚀变围岩中。同位素资料表明晚三叠世 -中侏罗世与扬子克拉通俯冲有关的作用控制了造山型金矿床的形成。     

Neoproterozoic and Hercynian metamorphic events in the Central Mauritanides: Implications for the geodynamic evolution of West Africa

Field and petrostructural investigations in the Central Mauritanides provide new precisions on the polyorogenic character of the nappe edifice of this belt. The upper structural unit exposed in the Gaouâ area includes anatectic gneisses and high-grade metasediments affected by medium temperature, high-pressure metamorphism and cut by granitoids. They represent a basement unconformably overlain by a monometamorphic cover, the Gaouâ Group, of assumed lower Paleozoic age. Late Paleozoic regional metamorphism of metapelites from this cover is characterized by pyrophyllite–kyanite–chloritoid assemblages equilibrated at T around 420 °C and P = 1 GPa. In the lower structural unit exposed in the south around Boufkerine-Farkâkâ, metapelites of the Gadel Group display slightly retrogressed garnet–kyanite–staurolite–rutile assemblages that equilibrated at T around 600 °C and P ? 1.2 GPa before the intrusion of 639 Ma old plutons . The Gadel Group locally overlies in unconformity polycyclic gneisses. This continental assemblage represents an outboard terrane inserted between two monocyclic greenschist facies terranes: remnants of oceanic lithosphere in the east and arc-derived metasediments and metavolcanics cut by 670 Ma old calc-alkaline plutons in the west. All units have been involved in late Paleozoic nappes emplaced to the east above the West African craton and its late Neoproterozoic to Cambro-ordovician cover. Though an east-directed vergence is locally recorded in the Neoproterozoic units, most early E-W trending stretching and mineral lineations are synchronous with greenschist facies metamorphism developed during the Late Paleozoic and roughly coeval with the Applachian nappe system.     

中国西北地区早—中侏罗世盆地原型分析

文中进行了中国西北地区早—中侏罗世盆地的原型分析 ,着重阐述 3点 :( 1)根据盆地构造特征 ,将中国西北地区早—中侏罗世盆地划分为 :缓断面伸展断陷盆地、陡断面伸展断陷盆地、克拉通内盆地和克拉通周边盆地 4种类型。 ( 2 )这 4类盆地分布于不同的前侏罗纪构造背景之上。第Ⅰ类和第Ⅱ类伸展断陷盆地位于印支晚期造山带和印支晚期复活的古生代造山带 ,它们系造山带伸展垮塌作用的产物。第Ⅲ类克拉通内盆地分布于克拉通块体内部 ,它们具有冷的刚性岩石圈 ,其古地温梯度达不到使岩石圈弱化和产生伸展断陷盆地的临界值。第Ⅳ类克拉通周边盆地位于克拉通和造山褶皱带之间 ,这类盆地的古地温梯度刚达到形成伸展断陷盆地临界值 ,在应变速率低的情况下造成不对称的宽阔凹陷。 ( 3)由于这 4类盆地的地温梯度、演化和改造程度不同 ,它们的含油气远景差别很大 ,其中以克拉通周边盆地最有希望。     

AGES AND DYNAMICS OF DEVELOPMENT OF THE ACTIVE VOLCANOES OF THE KURILE-KAMCHATKA REGION

The deformations of the West Congolian belt (about 600 Ma) and its foreland are described in terms of geometry and dynamics. Their transcurrent regimes, characterized by dextral movements within the belt and transverse sinistral movements in its foreland, are emphasized. On the basis of a Lower Cretaceous fit of the South American and the African continents and of the structural framework of the Brasiliano (Pan-African) domains, links between the West Congo belt and eastern Brazil Brasiliano belts (Sergipano, Rio Pardo) are proposed. In late Proterozoic reconstruction of these domains a geodynamic model of indentation tectonics by the Sao Francisco craton is proposed.     

中国西北地区早—中侏罗世盆地原型分布

文中进行了中国西北地区早—中侏罗世盆地的原型分析,着重阐述3点：(1)根据盆地构造特征,将中国西北地区早—中侏罗世盆地划分为：缓断面伸展断陷盆地、陡断面伸展断陷盆地、克拉通内盆地和克拉通周边盆地4种类型。(2)这4类盆地分布于不同的前侏罗纪构造背景之上。第Ⅰ类和第Ⅱ类伸展断陷盆地位于印支晚期造山带和印支晚期复活的古生代造山带,它们系造山带伸展垮塌作用的产物。第Ⅲ类克拉通内盆地分布于克拉通块体内部,它们具有冷的刚性岩石圈,其古地温梯度达不到使岩石圈弱化和产生伸展断陷盆地的临界值。第Ⅳ类克拉通周边盆地位于克拉通和造山褶皱带之间,这类盆地的古地温梯度刚达到形成伸展断陷盆地临界值,在应变速率低的情况下造成不对称的宽阔凹陷。(3)由于这4类盆地的地温梯度、演化和改造程度不同,它们的含油气远景差别很大,其中以克拉通周边盆地最有希望。     

Tectonic Setting of the Late Proterozoic Lavalleja Group (Dom Feliciano Belt), Uruguay

The Lavalleja Group exposed along the Dom Feliciano orogenic belt is located in the southeast of Uruguay. This group consists of volcano-sedimentary rocks, developed during the Neoproterozoic Brasiliano cycle. The geochemical signature of the igneous rocks of the Lavalleja Group, mainly metagabbros and basic and acidic metavolcanic rocks, indicates a back-arc basin tectonic setting. The metamorphic grade increases to the southeast, from very low grade, lower green-schist facies, in the Minas Formation, to a medium grade, amphibolite facies, in Fuente del Puma and Zanja del Tigre Formations. The metamorphic mineral assemblages correspond to a low-pressure regional metamorphism associated with a high thermal gradient. A compressive deformational event that probably corresponds to the closure of the Lavalleja basin during a continental collision, was recognized. The petrology, geochemistry, metamorphic grade, and tectonic setting are consistent with a back-arc basin setting for the Lavalleja Group.     

Precambrian history of the Zona Transversal of the Borborema Province,NE Brazil: Insights from Sm–Nd and U–Pb geochronology

The Borborema Province has three major subprovinces. The northern subprovince lies north of the Patos shear zone and is comprised of Paleoproterozoic cratonic basement with Archean nuclei, plus overlying Neoproterozoic supracrustal rocks and Brasiliano plutonic rocks. The central subprovince occurs between the Patos and Pernambuco shear zones and is mainly comprised of the Zona Transversal. The southern subprovince occurs between the Pernamabuco shear zone and the São Francisco craton and is comprised of a tectonic collage of various blocks, terranes, or domains ranging in age from Archean to Neoproterozoic. This report focuses on the Zona Transversal, especially on Brasiliano rocks for which we have the most new information.Paleoproterozoic gneisses with ages of 2.0–2.2 Ga occur discontinuously throughout the Zona Transversal. The Cariris Velhos suite consists of metavolcanic, metasedimentary, and metaplutonic rocks yielding U–Pb zircon ages of 995–960 Ma. This suite is mainly confined to a 100 km wide belt that extends for more than 700 km within the Alto Pajeú terrane. Sm–Nd model ages in metaigneous rocks cluster about 1.3–1.6 Ga, indicating that older crust was involved in genesis of their magmas. Brasiliano supracrustal rocks dominate the Piancó-Alto Brígida terrane, and they probably also constitute significant parts of the Alto Pajeú and Rio Capibaribe terranes. They are only slightly older than early stages of Brasiliano plutonism, with detrital zircon ages at least as young as 620 Ma; most T_DM ages range from 1.2 to 1.6 Ga.Brasiliano plutons range from ca. 640 to 540 Ma, and their T_DM ages range from 1.2 to 2.5 Ga. Previous workers have shown significant correlations among U–Pb ages, Sm–Nd model ages, petrology, and geochemistry, and we are able to reinforce and extend these correlations. Stage I plutons formed 640–610 Ma and have T_DM ages less than 1.5 Ga. Stage II (610–590 Ma) contains few plutons, but coincides with the peak of compressional deformation, metamorphism, and formation of migmatites. Stage III plutons (590 to ca. 575 Ma) have older T_DM ages (ca. 1.8–2.0 Ga), as do Stage IV plutons (575 to ca. 550 Ma; T_DM from 1.9 to 2.4 Ga). Stage III plutons formed during the transition from compressional to transcurrent deformation, while Stage IV plutons are mainly post-tectonic. Stage V plutons (550–530 Ma) are commonly undeformed (except along younger shear zones) and have A-type geochemistry. The five stages have distinct geochemical properties, which suggest that the tectonic settings evolved from early, arc-related magma-genesis (Stage I) to within-plate magma-genesis (Stage V), with perhaps some intermediate phases of extensional environments.     

Pre-variscan,Variscan and Early Alpine thermo-tectonic history of the north-eastern Bohemian Massif: An 40Ar/39Ar study

The Orlica-Snieznik and Jeseník Mountains correspond to three main domes from west to east: the Snieznik, Keprnfk and Desna domes. They are composed of a basement of autochthonous gneisses, a thick series of blastomylonites and a supposed para-autochthonous or allochthonous metamorphic pre-Devonian to Devonian cover. Their broad direction is NNE-SSW. ⁴⁰Ar-³⁹Ar radiometric measurements allow three main groups of ages to be defined. (1) 300–310 Ma, represented in the Keprník and Desná domes. This age is interpretated following the constraints on the age of the metamorphism, which is linked with the extensional process occurring during the Westphalian. (2) 320–340 Ma, represented mainly in the Snieznik Dome, but not in the Keprnfk Massif. The nappe structure of Orlik-Vysoká hole, in the northern area of the Desna Dome, also exhibits this age, which is interpretated as reflecting the period of the major Variscan Barrowian metamorphism, which accompanied the compressional process. It is only represented in the zones where the extensional process was not strong enough to result in a complete overprinting. (3) 340–440 Ma, corresponding to a very strictly defined area in the eastern rim of the Desná Dome occupied by ultramylonites and mylonites. These ages, obtained on muscovites, result from an incomplete resetting of the minerals developed during the cooling of a granitic protolith and mylonitized during the extensional process. A laser probe analysis confirms the extreme inhomogeneity of the ages of the muscovites and their different resetting from one grain to another. The Late Alpine overprinting is more discrete, but can be deciphered through the low extraction temperatures with ages between 80 and 120 Ma. These ages can be compared with Alpine ages in the close Western Carpathians.     

兴蒙陆内造山带

本文提出了"兴蒙陆内造山带"的新概念(Xing-Meng Intracontinent Orogenic Belt,XMIOB),从大地构造、沉积建造、岩浆作用和变质作用等方面论述了XMIOB从晚古生代到中生代初的陆内伸展及陆内造山过程,为探讨晚古生代构造演化提供了新模式。根据对内蒙古中西部晚古生代构造格局的总体认识,可将XMIOB划分为五个构造单元即:早石炭世二连-贺根山裂谷带、晚石炭世陆表海盆地、早二叠世艾力格庙-二连伸展构造带、早-中二叠世盆岭构造带和晚二叠世索伦山-乌兰沟伸展构造带。晚石炭世末-二叠纪在兴蒙造山带基底上发育三期伸展构造:第一期见于内蒙古北部二连-艾力格庙地区,形成陆内裂谷盆地及其盆缘三角洲沉积,发育时代为302～298Ma;第二期在内蒙古中西部广泛分布,以隆起与凹陷相间分布的盆岭构造为特征,发育时代为290～260Ma;第三期见于内蒙古南部索伦山到温都尔庙乌兰沟一带,形成主动裂谷背景下的红海型小洋盆,发育时代为260～250Ma。晚古生代与伸展过程有关的岩浆活动可分四期:1)早石炭世贺根山期:以蛇绿岩为主,发育于具有前寒武纪古老基底和早古生代造山带年轻基底的陆壳伸展区; 2)晚石炭世达青牧场期:主要沿北造山带分布,以基性和酸性岩浆构成的双峰式侵火成岩为特征; 3)早二叠世大石寨期:形成的岩石种类多样,分布广泛,包括双峰式火山岩、双峰式侵入岩和碱性岩; 4)二叠纪末-三叠纪初索伦山期:形成陆缘型蛇绿岩或基性岩-超基性岩组合,产生于软流圈上涌造成的主动裂谷背景。兴蒙陆内造山带的构造变形可分为两期,第一期为晚古生代地层大范围褶皱变形,造成盆-岭构造带的缩短;第二期为沿盆-岭构造的边界强烈剪切变形,产生向东逃逸的挤出构造,其构造背景是北部蒙古-鄂霍茨克造山带和南部大别-秦岭中央造山带的远距离效应引起的被动闭合作用。兴蒙陆内造山带的变质作用分为两个阶段,早期变质作用主要表现为石炭纪期间与陆内伸展有关的低压高温变质,晚期为二叠纪末到三叠纪初区域大面积的低压绿片岩相变质以及沿构造边界的局部中-低压型低温变质。     

High frequency peritidal cycles of the upper Araras Group: Implications for disappearance of the neoproterozoic carbonate platform in southern Amazon Craton

The Araras Group is an extensive carbonate platform developed at the southeastern margin of the Amazon Craton during the Neoproterozoic. The Nobres Formation corresponds to the upper unit of the Neoproterozoic Araras Group. It is exposed in road cuts and quarries in the Northern Paraguay Belt, and is characterized by meter-scale shallowing upward cycles. Forty-four fourth-to fifth-order parasequence cycles are enclosed into three third order sequences/megacycles, unconformably overlain by siliciclastic deposits of the Alto Paraguay Group. The cycles are generally of peritidal type, limited by exposure surfaces composed of asymmetrical tidal flat/sabkha lithofacies in the basal Nobres Formation. They consist of fine dolostone, intraclastic dolostones with megaripples, stromatolites biostrome, sandy dolostone with enterolithic structures and silicified evaporite molds. Upsection, the cycles progressively become symmetrical, comprising arid tidal flat deposits with abundant stromatolite biostrome, fine-grained sandstone and rare evaporitic molds. The stacking patterns for hundreds of meters indicate continuous and recurrent generation of accommodation space, probably triggered by subsidence concomitant with relative sea-level changes. Palynomorphs found in the upper part of Nobres Formation comprehend spheroidal forms, such as Leiospharidia, rare filamentous and acanthomorphous acritarchs, mostly Tanarium correlated to the Ediacaran Complex Acantomorph Palynoflora of ∼580–570 Ma. Previous data of carbon isotopes and paleogeographic reconstructions, and also the presence of evaporites and storm-influenced deposits in the Araras Group, suggest a wet to tropical setting for Amazonia during the Mid-Ediacaran, which is incompatible with previous claims for Gaskiers-related glacial sedimentation in the region. During the final stages of evolution of the Araras carbonate platform, a progressive input of terrigenous has occurred in the peritidal setting likely due tectonic activity in the vicinity of the basin and the rapid uplift of source areas to the west and to the east of the basin preceding the progradation of the siliciclastic fluvial-coastal environments related to the Alto Paraguay sedimentation. This event resulted in erosion of shallow carbonate basins and, potentially, the demise and disintegration of the Neoproterozoic carbonate platforms in southern Amazon Craton.