SHRIMP U/Pb Zircon Dating of Neoproterozoic Granitic Magmatism and Collision in the Pelotas Batholith,Southernmost Brazil

The introduction of robust geochronological methods for age determinations of the southernmost segment of the Neoproterozoic terranes of Brazil, namely the Dom Feliciano Belt, provides important clues for unraveling the complex evolution of the Brasíliano/Pan-African orogeny in this southwestern portion of the Gondwana supercontinent. Except for associated small schist belts and post-orogenic foreland basins, the belt is represented in this region of southeastern South America by the Pelotas Batholith. Precise SHRIMP U/Pb zircon geochronological techniques based on the study of 95 individual spots on 74 zircon crystals (three samples) and on Nd-isotopic determinations (three samples) are used to assess the late Neoproterozoic history of the belt, especially the orthogneisses interleaved with the batholithic plutons. Three petrotectonic associations were selected for detailed isotopic investigations—the Pinheiro Machado syncollisional monzogranites, the widespread Piratini gneiss tonalitic xenoliths, and the Arroio dos Ratos (now Encantadas) gneiss. The results allow the establishment of the timing and ages of the metamorphic peak and early magmatism. We demonstrate that evolution of Neoproterozoic magmatism within the Pelotas Batholith occurred through a long-lived crustal recycling process from the Paleoproterozoic Rio de la Plata craton. Three major events are recognized, two corresponding to crustal granite generation by partial melting of Paleoproterozoic protoliths at ～780 and ～610 Ma, and one related to the high-grade syncollisional metamorphic peak at ～630 Ma. The data also yield precise criteria to distinguish between thrust-related granitoids of the Dom Feliciano belt and older orthogneisses, both previously interpreted as a unique, pre-collisional, Brasíliano Cycle magmatic-arc association.     

Shear zone evolution and timing of deformation in the Neoproterozoic transpressional Dom Feliciano Belt,Uruguay

New structural, microstructural and geochronological (U-Pb LA-ICP-MS, Ar/Ar, K-Ar, Rb-Sr) data were obtained for the Dom Feliciano Belt in Uruguay. The main phase of crustal shortening, metamorphism and associated exhumation is recorded between 630 and 600 Ma. This stage is related to the collision of the Río de la Plata and Congo cratons at ca. 630 Ma, which also involved crustal reworking of minor crustal blocks such as the Nico Pérez Terrane and voluminous post-collisional magmatism. Subsequent orogen-parallel sinistral shearing gave rise to further deformation up to ca. 584 Ma and resulted from the onset of the convergence of the Kalahari Craton and the Río de la Plata-Congo cratons. Sinistral shear zones underwent progressive strain localization and retrograde conditions of deformation during crustal exhumation. Dextral ENE-striking shear zones were subsequently active at ca. 550 Ma, coeval with further sinistral shearing along N- to NNE-striking shear zones. The tectonothermal evolution of the Dom Feliciano Belt thus recorded the collision of the Río de la Plata and Congo cratons, which comprised one of the first amalgamated nuclei of Gondwana, and the subsequent incorporation of the Kalahari Craton into Western Gondwana.     

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.     

Peraluminous leucogranites of the Cordilheira Suite: A record of Neoproterozoic collision and the generation of the Pelotas Batholith,Dom Feliciano Belt,Southern Brazil

The Cordilheira Suite is comprised of peraluminous granites that constitute, together with the Quitéria granite, the beginning of the formation of the Pelotas Batholith in the central portion of the Dom Feliciano Belt. The batholith is composed of seven granitic suites with minor occurrences of gabbro, diorite and subvolcanic rocks. Its evolution between 650 and 550 million years ago is characterised by a ranging from metaluminous to peraluminous rocks and from calc-alkaline to alkaline and peralkaline series. The Cordilheira Suite is composed of the Cordilheira, Arroio Francisquinho, Butiá and Três Figueiras granites, which contain muscovite and/or biotite, with the following accessory minerals: tourmaline, garnet, sillimanite, apatite, zircon, monazite and ilmenite. The granitic bodies are elongate; their ascent and emplacement were controlled by high-angle shear zones oriented at N45-70°E. They have mylonitic structures with magmatic foliation accompanied by a low-angle stretching lineation, indicating that these bodies were emplaced under syn-kinematic conditions during a transcurrent event. The granites have high-K calc-alkaline affinity and are peraluminous. The LILE and REE contents are low. The K₂O/Na₂O and CaO/Na₂O ratios are approximately 1 and less than 0.3, respectively. Pseudosections calculated using the Perple X program suggests that the granites were formed at partial melting temperatures between 740 and 820 °C and pressure between 8.5 and 9 kbar. Petrographic and chemical data suggest that the magmas were generated by the partial melting of the migmatitic pelitic gneisses of the Várzea do Capivarita Complex and, to a lesser degree, orthogneisses of the Arroio dos Ratos Complex, which left a granulitic residue. UHT conditions of granulite facies metamorphism were recorded in the Várzea do Capivarita paragneisses by mineral paragenesis and indicate that temperatures between 900 and 1000 °C and pressures between 4 and 8 kbar were required for the partial melting. It is likely that these medium-pressure conditions resulted from the thickening of the continental crust when the Rio de La Plata Craton collided with the Kalahari Craton to form southwestern Gondwana at the end of the Neoproterozoic.     

Tectonic Structure and Geodynamic Settings of Neoproterozoic Granitoid Magmatism of the Eastern Arctic

Doklady Earth Sciences - The correlation of Neoproterozoic granitoid magmatism of the New Siberian Islands, Wrangel Island, Chukotka, the Chukchi Borderland, and Northern Alaska indicates integrity...     

Zircon U–Pb SHRIMP dating of gneissic basement of the Dom Feliciano Belt, southernmost Brazil

The age of a basement gneiss of the Dom Feliciano Belt along the coast of Rio Grande do Sul has been determined by zircon U–Pb SHRIMP to be about 2.08 Ga for the K-granitic magmatism and 800–590 Ma for the associated low-angle and sub-vertical shear zone deformations. The gneiss is the G3 granitic phase of the Arroio dos Ratos Complex of previous authors, and it now defines a geotectonic environment of juvenile accretion of island arcs in the Paleoproterozoic. The superposition of deformation events during the Neoproterozoic precludes the precise determination of the age of each event in this investigation, but we suggest that the collisional low-angle shear zones occurred at ca. 800 Ma and the sub-vertical shear zones at ca. 600 Ma. Th/U ratios are typically magmatic (about 0.4) in the homogeneous cores of zircons (about 2000 Ma), but are metamorphic (0.01) in the zoned euhedral rims (about 590 Ma).All the Paleoproterozoic gneisses in the region are part of the Encantadas Complex. Archean units, such as the Santa Maria Chico granulites, were all deformed in this major event of the Transamazonian Cycle. The dated gneiss may be correlative with the Epupa Complex north and south of the Kaoko Belt of SW Africa. Ages of the Neoproterozoic deformation are younger in the Kaoko Belt of Namibia than in its Brazilian counterpart.     

阿尔金断裂带东段古生代花岗岩浆作用及其大陆动力学意义

造山带花岗岩浆作用一直是地学的重要研究方向, 它记录了地球动力学深部过程的信息, 开展深入的研究工作可以更好的了解板块汇聚环境的陆壳生长和再造以及壳幔之间的相互作用。北祁连造山带是一典型的早古生代造山带, 先后经历了洋盆的打开到闭合, 敦煌地块则是主要由前寒武纪TTG片麻岩和变质表壳岩组成。北祁连造山带和敦煌地块分别位于阿尔金断裂带东段的东南侧和西北侧, 且均出露有大面积的古生代花岗岩体。本文以阿尔金主断裂两侧产出的花岗岩类为研究对象, 涉及北祁连造山带中的赵家庄二长花岗岩, 石包城复式岩体(花岗岩、正长花岗岩和花岗闪长岩)和红柳河花岗岩, 敦煌地块中的党河水库花岗闪长岩、沙枣园二长花岗岩、安盆沟复式岩体(正长花岗岩和花岗岩)以及小草湖似斑状花岗岩。通过对上述花岗岩体的岩相学、锆石U-Pb年代学、地球化学和锆石Hf同位素的研究, 取得了新的认识:     

Triassic Granitic Magmatism at the Northern Margin of the North China Craton: Implications of Geochronology and Geochemistry for the Tectonic Evolution of the Central Asian Orogenic Belt

The early Mesozoic marked an important transition from collisional orogeny to post-orogenic extension at the northern margin of the North China Craton(NCC). In this study, we undertook zircon U-Pb dating and whole-rock majorand trace-element geochemical analyses of early Mesozoic granitic rocks in the Chifeng area to establish their geochronological framework, petrogenesis, and implications for the tectonic evolution of the eastern Central Asia Orogenic Belt(CAOB). Zircon U-Pb dating results show that these rocks were emplaced in three stages during the Triassic:(1) syenogranites during 250–248 Ma,(2) granodiorites during 244–243 Ma, and(3) monzogranites and granodiorites during 232–230 Ma. These Triassic granitoids belong to the high-K calc-alkaline series and are evolved I-type granites. They have high SiO_2 and low Mg O contents with enrichments in light rare-earth elements, Zr, Hf, Rb, Th, and U, and depletions in Ba, Nb, Ta, Sr, and Eu. These geochemical data indicate that the granitoids were derived from partial melting of a lower-crustal source under relatively low-pressure conditions and subsequently underwent extensive fractional crystallization. Considering both the geochemical data and regional geological information, we propose that the 250–248 Ma syenogranites were emplaced in an extensional environment linked to slab break-off after closure of the Paleo-Asian Ocean(PAO) along the Solonker-Xra Moron-Changchun suture zone. The 244–243 Ma granodiorites were formed in a compressional orogenic setting during collision between the Erguna-Xing'an-Songliao composite block and the NCC. The 232–230 Ma granodiorites and monzogranites were emplaced during the transition from compressional orogeny to post-orogenic extension. Overall, the early Mesozoic tectonic evolution of the Chifeng area can be divided into three main stages:(1) closure of the Paleo-Asian Ocean and extension related to slab break-off during the Early Triassic;(2) continuous collisional compression during the Middle Triassic after closure of the PAO; and(3) post-orogenic extension during the Late Triassic, most probably due to lithospheric delamination after amalgamation of the Erguna-Xing'an-Songliao composite block and the NCC.     

Origin and evolution of the granitic intrusions in the Brusque Group of the Dom Feliciano Belt,south Brazil: Petrostructural analysis and whole-rock/isotope geochemistry

In the southern Brazilian state of Santa Catarina the Dom Feliciano Belt, formed by the tectonic juxtaposition of different crustal blocks during the Brasiliano-Pan African Orogenic cycle, can be divided into three domains. In the central domain, three granitic suites intrude the metavolcanosedimentary sequence of the Brusque Group: São João Batista (SJBS), Valsungana (VS) and Nova Trento (NTS), from the oldest to the youngest. This extensive magmatism, here referred to as granitic intrusions in the Brusqe Group (GIBG), is coeval with the thermal peak in the host metamorphic successions, but postdates its main foliation. A progressive deformation starting from the magmatic stage throughout the cooling history points to the influence of the late stages of deformation recorded in the Brusque Group.The SJBS consists of gray to white leucocratic, equigranular granites, with aluminous minerals such as muscovite, garnet and tourmaline. The porphyritic VS is the largest of the suites and is characterized by its cm-sized K-feldspar megacrysts in a coarse-grained biotite-rich matrix. The granites from the NTS are equigranular, light gray to pink in color and have biotite as the main mafic mineral, but magmatic muscovite, tourmaline and hornblende can occur as well.Geochemically, the GIBG are mildly peraluminous and show a calc-alkaline affinity. Most intrusions have a high REE fractionation, but some SJBS granites show a characteristic pattern with no fractionation and strong negative Eu anomalies (“seagull pattern”). Elevated Sr(i) values, between 0.707 and 0.735, and negative ε_Nd values as low as −24 points to the melting of old evolved crust. The Nd (T_DM) ages are scattered between 1.54 and 2.76 Ga, with a predominance of values around 2.0 Ga.The GIBG have a strong crustal signature that most closely connects, within the regional units, to that of the metasedimentary rocks of the Brusque Group and its crystalline basement, the Camboriú Complex. All three suites seem to have been produced during a same regional melting event, but at different crustal levels and reflecting heterogeneities within the same source rocks. Most evidences imply that sedimentary source rocks were especially important to the SJBS, which probably originated in a shallower environment, whilst the VS and NTS represent the melting of deeper crystalline crust, probably sharing some magmatic interaction.     

Magmatism and Stages of the Tectonic Evolution in the Donets Folded Structure,Eastern Donets Basin

The structure of the Donets Folded Structure underwent a complicated evolution, which included stages of extension, the origin of rift troughs, compression, collision, inversion uplifts, orogenesis, and stages of stabilization and neotectonic reactivation. The paper presents data on the regional tectono-stratigraphic Late Proterozoic–Cenozoic complexes of the Donets Folded Structure, its magmatic complexes, and their setting in this structure, which is identified using geodynamic and paleotectonic analysis. The paper also reports data on the minerageny of the magmatic complexes, which are indicators of tectonic processes and provide guidelines in exploring for and assessing mineral deposits.     

西拉沐伦成矿带中生代花岗岩浆活动与钼成矿作用

位于华北克拉通北缘的西拉沐伦成矿带内花岗岩浆活动及钼矿化发育,带内主要成矿侵入体岩石类型包括二长花岗岩、斑状花岗岩、花岗斑岩及流纹斑岩,这些岩石属于高钾钙碱性和钾玄岩系列,岩浆源区为古老下地壳和新生地壳。这些侵入体形成于早—中三叠世、晚侏罗世及早白垩世。带内钼矿床包括斑岩型、石英脉型、云英岩型和火山-次火山热液型4种类型,以斑岩型矿床最为发育。带内钼矿床形成时代与相关侵入岩时代一致,也形成于早—中三叠世、晚侏罗世及早白垩世,以早白垩世钼矿床最为发育。3期成岩成矿作用分别形成于华北板块与西伯利亚板块同碰撞至后碰撞构造环境、古太平洋板块向欧亚大陆俯冲的构造环境和中国东部岩石圈减薄构造环境。     

西准噶尔成矿带晚古生代花岗岩类岩浆活动及其构造意义

中亚造山带是晚古生代地壳显著生长与大规模成矿的重要地区。本文采集了中亚造山带西部的西准噶尔成矿带哈图-别鲁阿嘎希及其附近地区11个岩体共33件花岗岩类样品,对其开展了岩石地球化学与同位素示踪等研究,厘定了该地区晚古生代岩浆活动的特点与大地构造环境,并与哈萨克斯坦境内的巴尔喀什成矿带晚古生代岩浆活动进行了对比。研究表明,哈图地区晚石炭世花岗岩类主要为后碰撞伸展构造环境的A型花岗岩类,别鲁阿嘎希等地区存在洋内俯冲与岛弧环境的埃达克岩,显示了西准噶尔晚古生代构造环境时空变化的复杂性。该地区花岗岩类εNd(t)值较高(+4.62~+7.53)、εSr(t)值为(-57.61~+18.21),具有中亚造山带花岗岩类的共同特征,为古生代增生的新生陆壳,其源区与亏损地幔组分具有亲缘关系,这与巴尔喀什成矿带东段的花岗岩类具有一致性。花岗岩的~(206)Pb/~(204)Pb、~(207)Pb/~(204)Pb和~(208)Pb/~(204)Pb比值范围分别为18.2776~19.1677、15.5260~15.5796和38.2080~39.0821,为造山带花岗岩类。     

Deformation Characteristics and Tectonic Evolution of the Eastern Qinling Orogenic Belt

The Qinling Mountains separating the northern from the southern China plate is a key region for the study of structural evolution of eastern Asia. It is composed of the Palaeozoic fold belt in its northern part and the Variscan and Indosinian fold belts in its southern part. The evolution of the former is marked by the closure of a northward subducting oceanic basin in the early stage, followed by southward obduction of ophiolites and intracontinental thrusting during the Variscan; whereas that of the latter is represented by intracontinental, shallow crustal deformation on the basis of a large-scale detachment structure(with a horizontal slip of at least of 100 km). Since the late Palaeozoic, however, both of the belts have been cut by a series of east-west sinistral strike-slip faults.     

Multiple Phases of Mafic Magmatism in Gyangze-Kangma Area: Implications for the Tectonic Evolution of Eastern Tethyan Himalaya

正A number of E-W trending subparallel mafic dikes of diabase composition occurred in Gyangze-Kangma area,eastern Tethyan Himalaya,southern Tibet.They intruded into the Tethyan Himalaya sedimentary sequence.Whether they belong to the~132 Ma Comei LIP(Zhu et al.,2009)or     

The Sierra Ballena Shear Zone in the southernmost Dom Feliciano Belt (Uruguay): evolution, kinematics, and deformation conditions

The Sierra Ballena Shear Zone (SBSZ) is part of a high-strain transcurrent system that divides the Neoproterozoic Dom Feliciano Belt of South America into two different domains. The basement on both sides of the SBSZ shows a deformation stage preceding that of the transcurrent deformation recognized as a high temperature mylonitic foliation associated with migmatization. Grain boundary migration and fluid-assisted grain boundary diffusion enhanced by partial melting were the main deformation mechanisms associated with this foliation. Age estimate of this episode is >658 Ma. The second stage corresponds to the start of transpressional deformation and the nucleation and development of the SBSZ. During this stage, pure shear dominates the deformation, and is characterized by the development of conjugate dextral and sinistral shear zones and the emplacement of syntectonic granites. This event dates to 658–600 Ma based on the age of these intrusions. The third stage was a second transpressional event at about 586 to <560 Ma that was associated with the emplacement of porphyry dikes and granites that show evidence of flattening. Deformation in the SBSZ took place, during the late stages, under regional low-grade conditions, as indicated by the metamorphic paragenesis in the supracrustals of the country rocks. Granitic mylonites show plastic deformation of quartz and brittle behavior of feldspar. A transition from magmatic to solid-state microstructures is also frequently observed in syntectonic granites. Mylonitic porphyries and quartz mylonites resulted from the deformation of alkaline porphyries and quartz veins emplaced in the shear zone. Quartz veins reflect the release of silica associated with the breakdown of feldspar to white mica during the evolution of the granitic mylonites to phyllonites, which resulted in shear zone weakening. Quartz microstructures characteristic of the transition between regime 2 and regime 3, grain boundary migration and incipient recrystallization in feldspar indicate deformation under lower amphibolite to upper greenschist conditions (550–400°C). On the other hand, the mylonitic porphyries display evidence of feldspar recrystallization suggesting magmatic or high-T solid-state deformation during cooling of the dikes.     

中亚造山带东缘迪彦庙俯冲增生杂岩带早二叠世洋内弧岩浆作用及构造背景

洋陆转换岩石学证据（洋内弧）的发现使识别、重建、研究洋盆转化为大陆成为可能.对中亚造山带东缘迪彦庙俯冲增生杂岩带内蛇绿岩开展岩石地球化学、Sr-Nd同位素以及锆石U-Pb年代学研究,识别出一套洋内弧火成岩组合.MORB-Like玄武岩锆石U-Pb谐和年龄为286.1±6.1 Ma,代表洋内初始俯冲时代;HMA锆石U-Pb谐和年龄为283.7±4.7 Ma,代表首次岩浆作用后、俯冲程度加深的岩浆作用时代;岛弧拉斑玄武岩（IAT）锆石U-Pb谐和年龄为241±5 Ma,指示古亚洲洋早三叠世逐渐向着正常岛弧岩浆作用转换的大陆化方向发展.从MORB-Like玄武岩到HMA再到IAT的岩石组合序列代表了洋内俯冲作用由浅到深的递进演变以及洋盆向大陆边缘岛弧逐步演化的洋陆转换过程.