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1.
A new U?CPb SHRIMP age of 551?±?4?Ma on a mylonitic porphyry that intruded into the Sierra Ballena Shear Zone (Southernmost Dom Feliciano Belt, Uruguay) and a review of relevant published data make possible a more refined correlation and reconstruction of Brasiliano/Pan-African transpressional events. Paleogeographic reconstruction, kinematics and timing of events indicate a connection between the shear systems of the Dom Feliciano and Kaoko Belts at 580?C550?Ma. Sinistral transpression recorded in shear zones accommodates deformation subsequent to collision between the Congo and Río de la Plata Cratons. The correlation is strengthened by the similarity of magmatic and metamorphic ages in the Coastal Terrane of the Kaoko Belt and the Punta del Este Terrane of the Dom Feliciano Belt. This post-collisional sinistral transpression brought these units near to their final position in Gondwana and explains the different evolution at 550?C530?Ma. While in the Kaoko Belt, an extensional episode resulted in exhumation as a consequence of collision in the Damara Belt, in the Dom Feliciano Belt, sinistral transpression occurred associated with the closure of the southern Adamastor Ocean due to Kalahari-Río de la Plata collision.  相似文献   

2.
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.  相似文献   

3.
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.  相似文献   

4.
In this study, we challenge the multiple collision model for the tectonic evolution of the Neoproterozoic Ribeira Belt in Southeastern Brazil. New U–Pb SHRIMP data reveal Palaeoproterozoic (2153 ± 15 Ma) and Cryogenian (783 ± 6 and 768 ± 8 Ma) granitic rocks in the Embu Domain, and detrital zircon data of metasedimentary units from the Embu and Costeiro domains suggest a coherent tectonic evolution for the whole Ribeira Belt. Rather than by multiple collisions, these data are best explained by a simpler tectonic model involving continent (craton)‐volcanic arc collisions in the Dom Feliciano and Brasilia belts that led to intracontinental crustal thickening of the adjacent thinned hinterland (Ribeira Belt) at ~640–610 Ma, followed by widespread post‐collisional magmatism and rift‐related sedimentation at ~600–540 Ma. We suggest that intracontinental orogeny is a relevant process during supercontinent assembly, as illustrated here by the evolution of significant parts of the Brasiliano orogen.  相似文献   

5.
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.  相似文献   

6.
ABSTRACT

In many Precambrian provinces, the understanding about the tectonic history is restrained by the limited exposure of rocks so that aerogeophysical data can provide information below the surface cover of sediments, soil, and water to build a tectonic model of the region. However, the tectonic evolution of cratons and shear zones is very complex due to scarce aerogeophysical and geological data. The integrated observation of geology, aerogammaspectrometry, and aeromagnetometry of the Brazilian Shield, states of Santa Catarina, Paraná, and southern São Paulo, is enhanced by a 2011 high-resolution aerogeophysical survey. The Ponta Grossa and Florianópolis dike swarms show two sets of linear high magnetic signal. Peralkaline–carbonatitic complexes are observed in all images, e.g. Jacupiranga, Tunas, and Anitápolis. Three main geotectonic provinces are apparent in the shield. The Dom Feliciano Belt has two domains in terms of radioelements K, Th, and U. This belt includes granitic rocks of the Florianópolis Batholith, Brusque Group schists, and Itajaí Basin of sedimentary and volcanic rocks. The Luís Alves Craton has mostly the Santa Catarina Granulitic Complex and other gneisses. The craton has patches of high emission rates and extends to the northeastern extreme of the studied area with a high-contrast magnetic signal in most of the unit. The Ribeira Belt also has high in gammaspectrometric images, although less homogeneous than the Dom Feliciano Belt. This belt contains granitic complexes, gneisses, and schists and has a low magnetic signal with patches of high analytical signal amplitude. Shear zones delimit different geotectonic units, but are also significant in the Ribeira Belt. The Luís Alves Craton is the most complex unit, as seen in ternary K + eTh + eU images, requiring the future discrimination of small features. This novel study of the Brazilian Shield based on new geophysical data supports the classification of the proposed main geotectonic units.  相似文献   

7.
The Neoproterozoic geotectonic triad of the Brasiliano Orogen is reconstructed in southern Brazil from studies focused on the Porongos fold and thrust belt. We integrate field geology with isotopic studies of zircon U–Pb SHRIMP and Lu–Hf–O laser determinations in seven metasedimentary and three metavolcanic rock samples. The results indicate that the Porongos palaeo-basin was derived from mixed sources (3200–550 Ma), with major contributions from Rhyacian (2170 Ma) and Ediacaran (608 Ma) sources. Minor contributions from Archaean to Tonian sources are also registered. The maximum depositional age of the Porongos palaeo-basin is established by the age range of 650–550 Ma with TDM model ages between 2.5 and 1.3 Ga. The reworked signature (εHf values = ?34 to ?4) and the characteristic crustal magma reservoirs (δ18O ≥5.3 ‰) indicate that these sediments are equivalent to Neoproterozoic granites of the Dom Feliciano Belt. The episodic depositional history started in the Cryogenian (650 Ma) and lasted until the Ediacaran (most likely 570 Ma). A magmatic event of Tonian age is recorded in rhyodacite samples interleaved with the metasedimentary rocks and dated at 773, 801, and 809 Ma. The crustal evolution of the Sul-Riograndense Shield included mountain building, folding and thrusting and flexural subsidence in the foreland. An orogenic triad is revealed as the Pelotas Batholith, the Porongos fold and thrust belt and the Camaquã Basin, all part of the Dom Feliciano Belt.  相似文献   

8.
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 εHft 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.  相似文献   

9.
In the southernmost Dom Feliciano Belt of Uruguay, highly fractionated calc-alkaline granites, mildly alkaline granites, shoshonitic volcanics, and peralkaline intrusions and volcanics are spatially and temporal associated with the evolution of shear zones. Four representative magmatic unites of this diverse association were petrographic and geochemically investigated: the Solís de Mataojo Complex, a medium to high K2O calc-alkaline granite with signature typical of mature continental arcs and post-collisional settings; the Maldonado granite, highly fractionated calc-alkaline to alkaline, with characteristics that are transitional between both types of series; the Pan de Azúcar Pluton, with characteristics typical of post-collisional alkaline granites and the Las Flores shoshonitic basalts.

Geochemistry and geotectonic setting point out that slab breakoff was most likely the mechanism associated with the generation of high-K calc-alkaline magmas (Solís de Mataojo and Maldonado) shortly after collision. Extension associated to the formation of molassic basins and emplacement of dolerites and basalt flows with shoshonitic affinity (Las Flores) 15and finally a shift to magmas with alkaline signatures (Pan de Azúcar) simultaneous with a second transpressional phase were probably linked with lithospheric thinning through delamination. This evolution took place between 615 and 575 Ma, according to available data. Contrary to previous proposals, which considered this magmatism to represent the root of a continental magmatic arc, a post-collisional environment, transitional from orogenic to anorogenic, during transcurrent deformation is proposed.  相似文献   


10.
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.  相似文献   

11.
《地学前缘(英文版)》2020,11(6):2287-2296
In this contribution we present new insights on the evolution of the Dom Feliciano Belt, southernmost Mantiqueira Province, integrating new whole-rock Sm–Nd isotopic data for the Arroio Grande Ophiolite (Punta del Este Terrane, Brazil/Uruguay border) with previously published bulk-rock and isotope geochemistry of the South Adamastor paleo-ocean metamafic rocks located in Uruguay (Paso del Dragón Complex, Punta del Este Terrane) and Namibia (Chameis Subterrane, Marmora Terrane, Gariep Belt). For the regional geology, the new data corroborate previous hypothesis and demonstrate the depleted mantle features of the amphibolites and metagabbros of the studied ophiolite. The Arroio Grande Ophiolite rocks are compared with its Uruguayan and Namibian counterparts, demonstrating their isotopic and geochemical similarities and differences, and the back-arc affinity of the South Adamastor paleo-ocean. The MORB-affinity amphibolites from the Arroio Grande Ophiolite-Paso del Dragón Complex are, so far, the most juvenile rocks in the eastern sector of the Dom Feliciano Belt, yielding εNd(640 ​Ma) between +7.3 and ​+​9, and high 147Sm/144Nd (>0.169) and 143Nd/144Nd(640 ​Ma) ratios (0.51219–0.51229). The South Adamastor is interpreted in this paper as an internal back-arc ocean, with limited lateral extension, opened at around 750–650 ​Ma as the result of the closure of the older Charrua-Goianide paleo-ocean during the Brasiliano/Pan-African orogenic cycle and final configuration of the West Gondwana paleocontinent.  相似文献   

12.
We constrain the multistage tectonic evolution of the Palaeoproterozoic UHT metamorphic(P=0.9–1.0 GPa,T>1000℃,t=2088–2031 Ma)Bakhuis Granulite Belt(BGB)in Surinam on the Guiana Shield,using large-to small-scale structures,Al-in-hornblende thermobarometry and published fluid inclusion and zircon geochronological data.The BGB forms a narrow,NE–SW striking belt between two formerly connected,~E–W oriented granite-greenstone belts,formed between converging Amazonian and West African continental masses prior to collision and Transamazonian orogeny.Inherited detrital zircon in BGB metasediments conforms agewise to Birimian zircon of West Africa and suggests derivation from the subsequently subducted African passive margin.Ultrahigh-temperature metamorphism may have followed slab break-off and asthenospheric heat advection.Peak metamorphic structures result from layer-parallel shearing and folding,reflecting initial transtensional exhumation of the subducted African margin after slab break-off.A second HT event involves intrusion,at ca.0.49 GPa,of charnockites and metagabbros at 1993–1984 Ma and a layered anorthosite at 1980 Ma,after the BGB had already cooled to<400℃.The event is related to northward subduction under the greenstone belts,along a new active margin to their south.A pronounced syntaxial bend in the new margin points northward towards the BGB and is likely the result of indentation by an anticlinorial flexural bulge of the subducting plate.Tearing of the subducting oceanic plate along this bulge explains why the charnockites are restricted to the BGB.The BGB subsequently experienced doming under an extensional detachment exposed in its southwestern border zone.Exhumation was focused in the BGB as a result of the flexural bulge in the subducting plate and localised heating of the overriding plate by charnockite magmatism.The present,straight NE–SW long-side boundaries of the BGB are superimposed mylonite zones,overprinted by pseudotachylites,previously dated at ca.1200 Ma and 950 Ma,respectively.The 1200 Ma mylonites reflect transpressional popping-up of the BGB,caused by EW-directed intraplate principal compressive stresses from Grenvillian collision preserved under the eastern Andes.Further exhumation of the BGB involved the 950 Ma pseudotachylite decorated faulting,and Phanerozoic faulting along reactivated Meso-and Neoproterozoic lineaments.  相似文献   

13.
Combined analyses of Nd isotopes from a wide range of Neoarchaean–Cretaceous igneous rocks provides a proxy to study magmatic processes and the evolution of the lithosphere. The main igneous associations include the Neoproterozoic granitoids from the southern Brazilian shield, which were formed during two tectonothermal events of the Brasiliano cycle: the São Gabriel accretionary orogeny (900–700 Ma) and the Dom Feliciano collisional orogeny (660–550 Ma). Rocks related to the formation of the São Gabriel arc (900–700 Ma) mainly have a depleted juvenile signature. For the Neoproterozoic collisional event, the petrogenetic discussion focuses on two old crustal segments and three types of mantle components. However, no depleted juvenile material was involved in the formation of the Dom Feliciano collisional belt (800–550 Ma), which implies an ensialic environment for the Dom Feliciano orogeny. In the western Neoproterozoic foreland, records of a Neoarchaean lower crust predominate, whereas a Paleoproterozoic crust does in the eastern Dom Feliciano belt. The western foreland includes two amalgamated geotectonic domains, the São Gabriel arc and Taquarembó block. In the collisional belt, the old crust was intensely reworked during the São Gabriel event. In addition to the Neoproterozoic subduction-processed subcontinental lithosphere (São Gariel arc), we recognize two old enriched mantle components, which also are identified in the Paleoproterozoic intraplate tholeiites from Uruguay and the Cretaceous potassic suites from eastern Paraguay. One end member displays the prominent influence of Trans-Amazonian (2.3–2.0 Ga) or older subduction events, whereas the other can be interpreted as a reenrichment of the first during the latest Trans-Amazonian collisional or younger events. This reenriched mantle is documented in late Neoproterozoic suites from the western foreland (605–550 Ma) and younger suites from the eastern collisional belt (600–580 Ma). The other enriched mantle component with an old subduction signature, however, appears only in older rocks of the collisional belt (800–600 Ma). The participation of the subduction-related Brasiliano mantle as an end member of binary mixing occurred in some early Neoproterozoic suites (605–580 Ma) from the western foreland, but the contribution of the Neoarchaean lower crust increased near the late igneous event (575–550 Ma).  相似文献   

14.
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.  相似文献   

15.
The Punta del Este Terrane (eastern Uruguay) lies in a complex Neoproterozoic (Brasiliano/Pan-African) orogenic zone considered to contain a suture between South American terranes to the west of Major Gercino?CSierra Ballena Suture Zone and eastern African affinities terranes. Zircon cores from Punta del Este Terrane basement orthogneisses have U?CPb ages of ca. 1,000?Ma, which indicate an lineage with the Namaqua Belt in Southwestern Africa. U?CPb zircon ages also provide the following information on the Punta del Este terrane: the orthogneisses containing the ca. 1,000?Ma inheritance formed at ca. 750?Ma; in contrast to the related terranes now in Africa, reworking of the Punta del Este Terrane during Brasiliano/Pan-African orogenesis was very intense, reaching granulite facies at ca. 640?Ma. The termination of the Brasiliano/Pan-African orogeny is marked by formation of acid volcanic and volcanoclastic rocks at ca. 570?Ma (Sierra de Aguirre Formation), formation of late sedimentary basins (San Carlos Formation) and then intrusion at ca. 535?Ma of post-tectonic granitoids (Santa Teresa and José Ignacio batholiths). The Punta del Este Terrane and unrelated western terranes represented by the Dom Feliciano Belt and the Río de La Plata Craton were in their present positions by ca. 535?Ma.  相似文献   

16.
In the northeastern Dom Feliciano Belt, Santa Catarina/Brazil, Paleoproterozoic rocks (mainly the Camboriú Complex) and Neoproterozoic granitoids – with the older Itapema Granite and the younger Corre-mar, Rio Pequeno and Serra dos Macacos granites – experienced a deformation history from magmatic to greenschist facies temperatures, under different rheological conditions. The concordance of flat amphibolite facies structures of the Camboriú Complex and magmatic and subsolidus structures in the Itapema Granite indicate the late-tectonic character of the latter. Based on tectonic features, the Corre-mar Granite is interpreted as older than the Rio Pequeno Granite and as related to transcurrent tectonics of the Southern Brazilian Shear Belt.In all granites, microstructures point to widespread magmatic alignment, followed by weak subsolidus and, locally, amphibolite to greenschist facies deformation. Magmatic foliations are progressively weaker in the younger granites. Synmagmatic shear zones in the Rio Pequeno Granite are possibly concentrated at the intrusive contact. The weak solid-state deformation at late-magmatic conditions argues for magmatism within a low-strain zone, which is compatible with the location of the area relative to the Major Gercino and Itajaí shear zones. The amphibolite to greenschist facies deformation structures are attributed to continuous deformation within the same low-strain zone during decreasing temperatures.  相似文献   

17.
It has been suggested that large areas of the Earth's lithospheric mantle undergo pervasive dehydration melting during the impact of mantle plumes and the Early-Cretaceous Paraná-Etendeka continental flood-basalt (CFB) province has repeatedly been cited as evidence of this phenomenon. During the Cretaceous, however, southern Brazil experienced two phases of mafic magmatism. These igneous events occurred ～50?Ma apart and therefore represent distinct episodes of melt genesis in the underlying mantle. The first phase of magmatism, in the Early Cretaceous, included the emplacement of lava flows associated with the Paraná-Etendeka CFB province and also the intrusion of small-volume mafic alkaline magmas (e.g. Anitápolis, Jacupiranga and Juquiá) in the Dom Feliciano and Ribeira mobile belts. During the Late Cretaceous, both sodic and potassic mafic magmas were emplaced on the margin of the adjacent Luis-Alves craton and intrude the flood-basalts at Lages. On the basis of variations in incompatible trace-element concentrations (e.g. Ba?=?1000 to 2000?ppm), initial 87Sr/86Sr ratios (0.7048–0.7064) and ?Nd values (?3 to ?12), we suggest that all of the Late-Cretaceous mafic potassic magmas were derived from the subcontinental lithospheric mantle (SCLM) which was metasomatically enriched during the Proterozoic. We propose that these relatively low temperature, volatile-rich, mafic melts provide direct evidence that the underlying SCLM did not melt wholesale during the previous Early-Cretaceous Paraná-Etendeka CFB event. Late-Cretaceous melting of the SCLM beneath southern Brazil may have been caused by heat conduction from either: (1) ponded ～132?Ma Tristan plume-head material; or (2) ～85?Ma Trindade plume-head material channelled southwards between the thick cratonic keels of the Amazonas and São Francisco cratons. The Late-Cretaceous magmatism appears to have been contemporaneous with uplift across southern Brazil and Paraguay; we suggest that both of these phenomena represent the widespread effects of the impact of the Trindade mantle plume on the base of the SCLM. Plate margin stresses and lithospheric extension associated with the opening of the South Atlantic may also have changed the geothermal gradient beneath southern Brazil and contributed to mantle melting.  相似文献   

18.
The Ribeira Belt (Brazil) is a Neoproterozoic collisional-related feature that was located in a south-central position in West Gondwana. We present quantitative data on finite strain, flow vorticity and deformation temperatures for the Curitiba Terrane, a major segment of the southern Ribeira Belt. Six deformation phases (D1-D6) related with crustal thickening and exhumation were recognized. D1 and D2-related microstructures are preserved exclusively within porphyroblasts, in part grown during stages of high-pressure (∼9–12 kbar) isobaric heating after crustal thickening. D3 phase was active from peak metamorphism attained in contrasting crustal levels (810–400 °C), to the early stage of exhumation (500–400 °C), as indicated by petrological, microstructural and quartz c-axis fabric evidence. Kinematic vorticity results indicate that the SL3 mylonitic fabric resulted from a simple shear-dominated deformation related with westward thrusting. North-verging overturned D4 folds with E-W-trending subhorizontal axes derived from a pure shear-dominated deformation. Regional D5 open folds with subvertical axes and NNE-SSW-trending traces were produced by indentation tectonics. D6 phase comprises retrograde orogen-parallel transcurrent shear zones related with scape tectonics. Geochronological data indicate that D3-D6 phases occurred between 584 and 580 Ma, suggesting a fast exhumation rate of ∼8 mm/year for the deepest rocks from the southern Ribeira Belt.  相似文献   

19.
张波  张进江  钟大赉  郭磊 《地质科学》2009,44(3):889-909
滇西澜沧江构造带自北向南沿碧螺雪山和崇山连续延伸; 按照构造几何学特点和运动学特征我们把该构造带分为3段：北段、中段和南段; 本文对各段的构造、组构、运动学及构造年代学进行了翔实研究,得到以下认识：构造带呈双变质岩带,核部为强变形高级变质岩带,两侧为强变形低级变质岩带,部分剖面几何形态似“花状”构造; 宏观和微观组构特征均指示构造带北段和中?南段存在明显的运动学差异,北段为右旋走滑剪切,中、南段为左旋走滑剪切; 同构造浅色花岗质糜棱岩中分选出白云母(北段)和黑云母(中段),进行单颗粒矿物的激光熔化40Ar?39Ar定年,结果显示,糜棱岩化造成了花岗质岩石同位素时钟的重置和部分重置; 表面年龄指示了该构造带中新世的构造变形事件; 其中,北段右旋韧性剪切作用年龄为17.8～13.4 Ma或更早,至少持续到13.4 Ma,构造带中段记录了17.9～13.1 Ma的左行韧性剪切事件; 构造变形时代表现出同时代和同期次特点。综合分析认为,位于印度与欧亚大陆斜向汇聚带东缘的澜沧江构造带,是调节印支块体陆内变形的重要变形区域,为典型的新生代剪压应变区; 与区内哀牢山-红河构造带新生代左旋走滑相对应; 剪压应变和应变分解过程中,构造带东-西向减薄作用通过韧性物质垂向挤出和沿剪切方向的挤出平衡,垂向挤出导致地壳增厚和高应变体的抬升,形成现今的地貌高位,统一的陡立面理和亚水平拉伸线理是韧性物质沿剪切方向挤出的流变学响应; 构造带南段和北段运动学差异是澜沧江构造带新生代左旋剪压应变分解的必然产物和运动学要求。  相似文献   

20.
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.  相似文献   

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