Tectonometamorphic evolution of the Åreskutan Nappe-Caledonian history revealed by SIMS U–Pb zircon geochronology

Secondary ionization mass spectrometry (SIMS) U–Pb dating of zircons from the Åreskutan Nappe in the central part of the Seve Nappe Complex of western central Jämtland provides new constraints on the timing of granulite–amphibolite-facies metamorphism and tectonic stacking of the nappe during the Caledonian orogeny. Peak-temperature metamorphism in garnet migmatites is constrained to c. 442 ± 4 Ma, very similar to the ages of leucogranites at 442 ± 3 and 441 ± 4 Ma. Within a migmatitic amphibolite, felsic segregations crystallized at 436 ± 2 Ma. Pegmatites, cross-cutting the dominant Caledonian foliation in the Nappe, yield 428 ± 4 and 430 ± 3 Ma ages. The detrital zircon cores in the migmatites and leucogranites provide evidence of Late Palaeoproterozoic, Mesoproterozoic to Early Neoproterozoic source terranes for the metasedimentary rocks. The formation of the ductile and hot Seve migmatites, with their inverted metamorphism and thinning towards the hinterland, can be explained by an extrusion model in which the allochthon stayed ductile for a period of at least 10 million years during cooling from peak-temperature metamorphism early in the Silurian. In our model, Baltica–Laurentia collision occurred in the Late Ordovician–earliest Silurian, with emplacement of the nappes far on to the Baltoscandian platform during the Silurian and early Devonian, Scandian Orogeny lasting until c. 390 Ma.

An evaluation of orogenic kinematic evolution utilizing crystalline and magnetic anisotropy in granitoids

The Silurian-Devonian Galway Granite Complex (GGC ~425-380Ma) is defined here as a suite of granitoid plutons that comprise the Main Galway Granite Batholith and the Earlier Plutons. The Main Batholith is a composite of the Carna Pluton in the west and the Kilkieran Pluton in the east and extends from Galway City ~130km to the west. The Earlier Plutons are spatially, temporally and structurally distinct, situated northwest of the Main Batholith and include the Roundstone, Omey, Inis and Letterfrack Plutons. The majority of isotopic and structural data currently available pertain to the Kilkieran Pluton, several tectonic models have already been devised for this part of the complex. These relate emplacement of the Kilkieran Pluton to extension across a large east-west Caledonian lineament, i.e. the Skird Rocks Fault, during late Caledonian transtension. No chronological data have been published that directly and accurately date the emplacement of the Carna Pluton or any of the Earlier Plutons. There is also a lack of data pertaining to the internal structure of these intrusions. Accordingly, no previous study has established the mechanisms of emplacement for the Earlier Plutons and only limited work is available for the Carna Pluton. As a consequense of this, constituents of the GGC have not previously been placed in a context relative to each other or to regional scale Silurio-Devonian kinematics. The current work focuses on the Omey, Roundstone and Carna Plutons. Here, results of detailed field and Anisotropy of Magnetic Susceptibiliy (AMS) fabric studies are presented. This work is complemented by geological mapping that focuses on fault dynamics and contact relationships. Interpretation of AMS data is aided by rock magnetic experiment data and petrographic microstructural evaluations of representative samples. A new geological map of the the Omey Pluton demonstrates that this intrusion has a defined roof and base which are gently inclined parallel to the fold hinge of the Connemara Antiform. AMS and petrographic data show the intrusion is cross cut by NNW-SSE shear zones that extend into the country rock. These pre-date and were active during magma emplacement. It is proposed that the Omey pluton was emplaced as a discordant phacolith. Pre-existing subvertical D5 faults in the host rock were reactived during emplacement, due to regional sinistral transpression, and served as centralised ascent conduits. A central portion of the Roundstone Pluton was mapped in detail for the first time. Two facies are identified, G1 forms the majority of the pluton and coeval G2 sheets cross cut G1 at the core of the pluton. NNW-SSE D5 faults mapped in the country rock extend across the pluton. These share a geometrical relationship with the distribution of submagmatic strain in the pluton and parallel the majoity of mapped subvertical G2 dykes. These data indicate that magma ascent was controlled by NNW-SSE conduits that are inherently related to those identifed in the Omey Pluton. It is proposed that the Roundstone Pluton is a punched laccolith, the symmetry and structure of which was controlled by pre-exising host rock structures and regional sinistral transpressive stress which presided during emplacement. Field relationships show the long axis of the Carna Pluton lies parallel to mulitple NNW-SSE shear zones. These are represented on a regional scale by the Clifden-Mace Fault which cross cuts the core of this intrusion. AMS and petrographic data show concentric emplacement fabrics were tectonically overprinted as magma cooled from the magmatic state due to this faulting. It is proposed that the Clifden-Mace Fault system was active during ascent and emplacement of the magma and that pluton inflation only terminated as this controlling structure went into compression due to the onset of regional transtension. U-Pb zircon laser ablation inductively coupled mass spectrometry (LA-ICP-MS) data has been compiled from four sample sites. New geochronological data from the Roundstone Pluton (RD1 = ± 3.2Ma) represent the oldest age determination obtained from any member of the GGC and demonstrates that this pluton predates the Carna Pluton by ~10Ma and probably intruded synchronously with the Omey Pluton (~422.5 ± 1.7Ma). Chronological data from the Carna Pluton (CN2 = 412.9 ± 2.5Ma; CN3 = 409.8 ± 7.2Ma; CN4 = 409.6 ± 3.6Ma) represent the first precise magma crystallisation age for this intrusion. This work shows this pluton is 10Ma older than the Kilkieran Pluton and that the supply of magma into the Carna Pluton had terminated by ~409Ma. Chronological, magnetic and field data have been utilised to evaluate the kinematic evolution of the Caledonides of western Ireland throughout the construction of the GGC. It is proposed that the GGC was constructed during four distinct episodes. The style of emplacement and the conduits used for magma transport to the site of emplacement was dependent on the orientation of local structures relative to the regional ambiant stress field. This philosophy is used to critically evaluate and progress existing hypotheses on the transition from regional transpression to regional transtension at the end of the Caledonian Orogeny.

Late Palaeozoic sequence stratigraphy and brachiopod faunas of the Tarim Basin, Northwest China

This thesis deals with the stratigraphy and brachiopod systematic palaeontology of the latest Devonian (Famennian) to Early Permian (Kungurian) sedimentary sequences of the Tarim Basin, NW China. Brachiopod faunas of latest Devonian and Carboniferous age have been published or currently in press in the course of the Ph.D candidature and are herein appendixed, while the Early Permian brachiopod faunas are systematically described in this thesis. The described Early Permian brachiopod faunas include 127 species, of which 29 are new and 12 indeterminate, and six new genera (subgenera) are proposed; Tarimella, Bmntonella, Marginifera (Arenaria), Marginifera (Nesiotia), Baliqliqia and Ustritskia. A new integrated brachiopod biostratigraphical zonation scheme is proposed, for the first time, for the latest Devonian-Early Permian sequences of the entire Tarim Basin on the basis of this study as well as previously published information (including the Candidate's own published papers). The scheme consists of twenty three brachiopod acm biozones, most of which replace previously proposed assemblage or assemblage zones. The age and distribution of these brachiopod zones within the Tarim Basin and their relationships with other important fossil groups are discussed. In terms of regional correlations and biostratigraphical affinities, the Late Devonian to Early Carboniferous brachiopod faunas of the Tarim Basin are closest to those from South China, while the Late Carboniferous faunas demonstrate strong similarities to coeval faunas from the Urals, central Asia, North China and South China. During the Asselian-Sakmarian, strong faunal links between the Tarim Basin and those of the Urals persisted, while at the same time links with central Asia, North China and South China weakened. On the other hand, during the Artinskian-Kungurian times, affinities of the Tarim faunas with the Urals/Russian Platform rapidly reduced, when those with peri-Gondwana (South Thailand, northern Tibet) and South China increased. Thirty lithofacies (or microfacies) types of four facies associations are recognised for the Late Devonian to early Permian sediments. Based on detailed lithostratigraphy, biostratigraphy and facies analysis, 23 third-order sequences belonging to four supcrsequences are identified for the Late Devonian to Early Permian successions, from which sea-level fluctuation curves are reconstructed. The sequence stratigraphical analysis reveals that four major regional regressions, each marking a distinct supersequence boundary, can be recognised; they correspond to the end-Serpukhovian, end-Moscovian, late Artinskian and end-Kungurian times, respectively. The development of these sequences is considered to have been formed and regulated by the interplay of both eustasy and tectonism. Using the system tract of a sequence as the mapping time unit, a succession of 47 palaeogeographical maps have been reconstructed through the Late Devonian to Early Permian. These maps reveal that the Tarim Basin was first immersed by southwest-directed (Recent geographical orientation) transgression in the late Famennian after the Caledonian Orogeny. Since then, the basin had maintained its geometry as a large, southwest-mouthed embayment until the late Moscovian when most areas were the uplifted above sea-level. The basin was flooded again in late Asselian-Artinskian times when a new transgression came from a large epicontinental sea lying to its northwest. Thereafter, marine deposition was restricted to local areas (southwestern and northwestern margins until the late Kungurian, while deposition of continental deposits prevailed and continued through the Middle and late Permian into the Triassic.

Further Indo-West Pacific palaemonoid shrimps (Crustacea : Decapoda : Palaemonoidea), principally from the New Caledonian region

Based on the material deposited in the Museum national d'Histoire naturelle, Paris, collected from the Indo-West Pacific, principally from the New Caledonian region, the present paper reports 117 palaemonoid shrimp species, which belong, respectively, to Anchistioididae ( one genus, one species), Gnathophyllidae ( one genus, one species), Palaemonidae Palaemoninae ( seven genera, nine species), and Palaemonidae Pontoniinae ( 30 genera, 106 species), including eight new species. The new species are all Pontoniinae: Mesopontonia brevicarpalis sp. nov., Palaemonella komaii sp. nov., Periclimenes crosnieri sp. nov., Periclimenes forgesi sp. nov., Periclimenes loyautensis sp. nov., Periclimenes paralcocki sp. nov., Periclimenes paraleator sp. nov., and Periclimenes pseudalcocki sp. nov. The last six new species are members of the deep-water "Periclimenes alcocki species complex'', which has more than two ( usually four) pairs of dorsolateral telson spines anterior to the posterior telson margin, the cornea is usually reduced, the dactyl of the major second chela is generally flanged and the chela is sometimes covered with small tubercles. The complex is usually found at more than 200m depth in the West Pacific. The species can be distinguished from each other by the armature of ambulatory propod and dactyl, diameter of cornea, rostrum shape and the number of pairs of dorsolateral telson spines. Mesopontonia brevicarpalis sp. nov., from the southeast coast of Africa, is the seventh species of the genus. Palaemonella komaii sp. nov. is very similar to Palaemonella dolichodactylus Bruce, 1991 and Palaemonella hachijo Okuno, 1999. These three species share the features of very long and slender ambulatory pereiopods with the dactyl more than eight times longer than its basal depth and with several long setae on the dorsal dactylar margin.

Post-collisional lamprophyric event in Sierra Norte, Cordoba, Argentina: Mineralogical, geochemical and isotopic characteristics

Over 20 lamprophyre dykes, varying in width between a few centimeters and several meters, have been identified in central Sierra Norte - Eastern Pampean Ranges, Cordoba, Argentina. Their mineralogy and chemistry indicate that they are part of the calc-alkaline lamprophyres clan (CAL). They contain phenocrysts of magnesiohomblende +/- augite set in a groundmass of magnesiohornblende, calcic-plagioclase, alkali feldspar, and opaque minerals, which designate them as spessartite-type lamprophyres. Alteration products include chlorite, calcite and iron oxides after malfic phenocrysts, though some are partially replaced by actinolite. Feldspars are replaced by carbonate and clay minerals. The dykes are relatively primitive, and show restricted major element variation (SiO(2) 51.1-55.3 wt.%, Al(2)O(3) 12-16.6 wt.%, total alkalies 1.5-4.7 wt.%), high Mg# (55-77), high Cr contents (27-988 ppm) and moderate to high Ni contents (60-190 ppm). Lamprophyre LILE (e.g. Rb averages 110 ppm, Sr 211-387 ppm, Ba 203-452 ppm) are high relative to HFSE (e.g., Ta 0.2-1.6 ppm, Nb 4-11 ppm, Y 17-21 ppm), and are enriched in LREE (30-70 times chondrite). They are characterized by relatively high (208)Pb/(204)Pb (38.8-39.9), (207)Pb/(204)Pb(similar to 15.7), and (206)Pb/(204)Pb (18.7-20.1), combined with low (epsilon)epsilon(Nd) (-4.69 to -1.52) and a relative moderately high ((87)Sr/(86)Sr)(i) of 0.7055-0.7074. The Rb-Sr whole rock isochron indicates an Early Ordovician age of 485 +/- 25 Ma. The calculated T(DM) (1.7 Ga) suggests that these rocks appear to have originated from a reservoir that was created during a mantle metasomatism event related to the Pampean orogeny. The Sierra Norte lamprophyres show affinities with a subduction-related magma in an active continental margin. Their geochemical and isotopic features suggest a multicomponent source, composed of enriched mantle material variably contaminated by crustal components. The lamprophyric suite emplacement occurred at the dawning stage of the Pampean orogeny, in a regional post-collisional extensional setting developed in the Sierra Norte-Ambargasta batholith (SNAB) in Early Ordovician times. (C) 2008 Published by Elsevier Ltd.

Conflicting structural and geochronological data from the Ibituruna quartz-syenite (SE Brazil): Effect of protracted ""hot"" orogeny and slow cooling rate?

The Ibituruna quartz-syenite was emplaced as a sill in the Ribeira-Aracuai Neoproterozoic belt (Southeastern Brazil) during the last stages of the Gondwana supercontinent amalgamation. We have measured the Anisotropy of Magnetic Susceptibility (AMS) in samples from the Ibituruna sill to unravel its magnetic fabric that is regarded as a proxy for its magmatic fabric. A large magnetic anisotropy, dominantly due to magnetite, and a consistent magnetic fabric have been determined over the entire Ibituruna massif. The magmatic foliation and lineation are strikingly parallel to the solid-state mylonitic foliation and lineation measured in the country-rock. Altogether, these observations suggest that the Ibituruna sill was emplaced during the high temperature (similar to 750 degrees C) regional deformation and was deformed before full solidification coherently with its country-rock. Unexpectedly, geochronological data suggest a rather different conclusion. LA-ICP-MS and SHRIMP ages of zircons from the Ibituruna quartz-syenite are in the range 530-535 Ma and LA-ICP-MS ages of zircons and monazites from synkinematic leucocratic veins in the country-rocks suggest a crystallization at similar to 570-580 Ma, i.e., an HT deformation >35My older than the emplacement of the Ibituruna quartz-syenite. Conclusions from the structural and the geochronological studies are therefore conflicting. A possible explanation arises from (40)Ar-(39)Ar thermochronology. We have dated amphiboles from the quartz-syenite, and amphiboles and biotites from the country-rock. Together with the ages of monazites and zircons in the country-rock, (40)Ar-(39)Ar mineral ages suggest a very low cooling rate: <3 degrees C/My between 570 and similar to 500 Ma and similar to 5 degrees C/My between 500 and 460 Ma. Assuming a protracted regional deformation consistent over tens of My, under such stable thermal conditions the fabric and microstructure of deformed rocks may remain almost unchanged even if they underwent and recorded strain pulses separated by long periods of time. This may be a characteristic of slow cooling ""hot orogens"" that rocks deformed at significantly different periods during the orogeny, but under roughly unchanged temperature conditions, may display almost indiscernible microstructure and fabric. (C) 2009 Elsevier B.V. All rights reserved.

From Flysch to Molasse - sedimentary and tectonic evolution of late Caledonian - early Hercynian Foreland Basin in North Qilian Mountains

The Late Caledonian to Early Hercynian North Qilian orogenic belt in northwestern China is an elongate tectonic unit situated between the North China plate in the north and the Qaidam plate in the south. North Qilian started in the latest Proterozoic to Cambrian as a rift basin on the southern margin of North China, and evolved later to an archipelagic ocean and active continental margin during the Ordovician and a foreland basin from Silurian to the Early and Middle Devonian. The Early Silurian flysch and submarine alluvial fan, the Middle to Late Silurian shallow marine to tidal flat deposits and the Early and Middle Devonian terrestrial molasse are developed along the corridor Nanshan. The shallowing-upward succession from subabyssal flysch, shallow marine, tidal flat to terrestrial molasse and its gradually narrowed regional distribution demonstrate that the foreland basin experienced the transition from flysch stage to molasse stage during the Silurian and Devonian time.

Terrane transfer during the Grenville orogeny: tracing the Amazonian ancestry of southern Appalachian basement through Pb and Nd isotopes

Whole rock Pb isotope data can be used to determine the provenance of different blocks within the Rodinia supercontinent, providing a test for paleogeographic reconstructions. Calculated isotopic values for the source region of the Grenville-deformed SW Amazon craton (Rondonia, Brazil), anchored by published U-Pb zircon ages, are compared to those from the Grenville belt of North America and Grenvillian basement inliers in the southern Appalachians. Both the SW Amazon craton and the allochthonous Blue Ridge/Mars Hill terrane are defined by a similar Pb isotopic signature, indicating derivation from an ancient source region with an elevated U/Pb ratio. In contrast, the Grenville Province of Laurentia (extending from Labrador to the Llano Uplift of Texas) is characterized by a source region with a distinctly lower, time-integrated U/Pb ratio. Published U-Pb zircon ages (ca. 1.8 Ga) and Nd model ages (1.4-2.2 Ga) for the Blue Ridge/Mars Hill terrane also suggest an ancient provenance very different from the rest of the adjacent Grenville belt, which is dominated by juvenile 1.3-1.5 Ga rocks. The presence of mature continental material in rocks older than 1.15 Ga in the Blue Ridge/ Mars Hill terrane is consistent with characteristics of basement rocks from the SW Amazon craton. High-grade metamorphism of the Blue Ridge/Mars Hill basement resulted in purging of U, consistent with observations of the rest of the North American Grenville province. In contrast, the Grenvillian metamorphic history of the Amazon appears to have been much more heterogeneous, with both U enrichment and U depletion recorded locally. We propose that the Blue Ridge/ Mars Hill portion of the Appalachian basement is of Amazonian provenance and was transferred to Laurentia during Grenvillian orogenesis after similar to1.15 Ga. The presence of these Amazonian rocks in southeastern Laurentia records the northward passage of the Amazon craton along the Laurentian margin, following the original collision with southernmost Laurentia at ca. 1.2 Ga. (C) 2004 Elsevier B.V. All rights reserved.

Vertical displacement during late-collisional escape tectonics (Brasiliano Orogeny) in the Ribeira Belt, São Paulo State, Brazil

During the Brasiliano-Pan-African Orogeny, West Gondwana formed by collisional processes around the Sao Francisco-Congo Craton. The Ribeira belt, in southeastern Brazil, resulted from northwestward collision (650-600 Ma), followed by large-scale northeast-southwest dextral strike-slip shear movements related to late-collisional escape tectonics (ca 600 Ma).In São Paulo State, three groups, also interpreted as terranes, are recognised in the Ribeira Belt, the Embu, Itapira and Sao Rogue Groups. The Embu and Itapira Groups are formed of sillimanite-gneisses, schists and migmatites intruded by Neoproterozoic calc-alkaline granitoids, all thrusted northwestward. The Sao Rogue Group is composed of metasediments and metavolcanics in greenschist-facies. Its deformation indicates a transpressional regime associated with tectonic escape. Sub-alkaline granites were emplaced in shallow levels during this regime. Microstructural studies along the Itu, Moreiras and Taxaquara Shear Zones demonstrate the coexistence of horizontal and Vertical displacement components during the transpressional regime. The vertical component is regarded as responsible for the lateral juxtaposition of different crustal levels. (C) 1999 Elsevier B.V. Limited. All rights reserved.

Northwestern overthrusting and related lateral escape during the Brasiliano orogeny north of the Patos lineament, Borborema province, northeast Brazil

The Borborema province (BP) of northeastern Brazil, located between the São Luís and São Francisco cratons, represents a branching system of Precambrian orogens of the South American platform. It is composed of segments of Archean and Proterozoic crust that were deformed by the convergence of the West African and São Francisco-Congo cratons during assembly of the Brasiliano collage (650 to 500 Ma), a period of intense orogenic activity considered to be the strongest and most pervasive tectonic event that affected the Precambrian of the South American platform. The tectonic and kinematic history of the Brasiliano/Panafrican orogeny is fundamental for reconstructing South American and African Precambrian geology. The correlation between Neoproterozoic tectonic processes occurring in both continents should use structural elements, of regional or local character, with identical kinematic and metamorphic conditions manifested in both basement and supracrustal units. North of the Patos shear zone, subhorizontal Brasiliano thrusts (0.65 to 0.58 Ga) affected the basement and the supracrustal Seridó belt with such related regional D1/D2 structures as foliation, lineation, isoclinal folds, and related metamorphism. Overprinting the previous structures, regional folding with a vertical S3 foliation and an associated strike-slip shear zone were developed (0.58 to 0.52 Ga). The metamorphism is similar for all deformation phases, ranging from upper-greenschist to amphibolite facies with mineral assemblages including biolite and garnet throughout the Seridó fold belt. We propose, on the basis of deformational and kinematic reconstructions, that the structural evolution of the Seridó fold belt was characterized by transition from a syn-collisional to a strike-slip regime. The transition between regimes occurred, progressively or instantaneously, by the switching of the maximum and intermediary strain axes of the strain ellipsoid. The entire tectonic history can be related to a frontal or oblique collision and lateral escape tectonics, with local, syn-collisional transpression and transtension. The Patos shear zone represents a final vertical shearing, juxtaposing different terranes of the northern and southern Borborema province.

U-Pb geochronology of the southern Brasília belt (SE-Brazil): Sedimentary provenance, Neoproterozoic orogeny and assembly of West Gondwana

The Brasília belt borders the western margin of the São Francisco Craton and records the history of ocean opening and closing related to the formation of West Gondwana. This study reports new U-Pb data from the southern sector of the belt in order to provide temporal limits for the deposition and ages of provenance of sediments accumulated in passive margin successions around the south and southwestern margins of the São Francisco Craton, and date the orogenic events leading to the amalgamation of West Gondwana. Ages of detrital zircons (by ID-TIMS and LA-MC-ICPMS) were obtained from metasedimentary units of the passive margin of the São Francisco Craton from the main tectonic domains of the belt: the internal allochthons (Araxá Group in the Áraxá and Passos Nappes), the external allochthons (Canastra Group, Serra da Boa Esperança Metasedimentary Sequence and Andrelândia Group) and the autochthonous or Cratonic Domain (Andrelândia Group). The patterns of provenance ages for these units are uniform and are characterised as follows: Archean- Paleoproterozoic ages (3.4-3.3, 3.1-2.7, and 2.5-2.4Ga); Paleoproterozoic ages attributed to the Transamazonian event (2.3-1.9Ga, with a peak at ca. 2.15Ga) and to the ca. 1.75Ga Espinhaço rifting of the São Francisco Craton; ages between 1.6 and 1.2Ga, with a peak at 1.3Ga, revealing an unexpected variety of Mesoproterozoic sources, still undetected in the São Francisco Craton; and ages between 0.9 and 1.0Ga related to the rifting event that led to the individualisation of the São Francisco paleo-continent and formation of its passive margins. An amphibolite intercalation in the Araxá Group yields a rutile age of ca. 0.9Ga and documents the occurrence of mafic magmatism coeval with sedimentation in the marginal basin. Detrital zircons from the autochthonous and parautochthonous Andrelândia Group, deposited on the southern margin of the São Francisco Craton, yielded a provenance pattern similar to that of the allochthonous units. This result implies that 1.6-1.2Ga source rocks must be present in the São Francisco Craton. They could be located either in the cratonic area, which is mostly covered by the Neoproterozoic epicontinental deposits of the Bambuí Group, or in the outer paleo-continental margin, buried under the allochthonous units of the Brasília belt. Crustal melting and generation of syntectonic crustal granites and migmatisation at ca. 630Ma mark the orogenic event that started with westward subduction of the São Francisco plate and ended with continental collision against the Paraná block (and Goiás terrane). Continuing collision led to the exhumation and cooling of the Araxá and Passos metamorphic nappes, as indicated by monazite ages of ca. 605Ma and mark the final stages of tectonometamorphic activity in the southern Brasília belt. Whilst continent-continent collision was proceeding on the western margin of the São Francisco Craton along the southern Brasília belt, eastward subduction in the East was generating the 634-599Ma Rio Negro magmatic arc which collided with the eastern São Francisco margin at 595-560Ma, much later than in the Brasília belt. Thus, the tectonic effects of the Ribeira belt reached the southernmost sector of the Brasília belt creating a zone of superposition. The thermal front of this event affected the proximal Andrelândia Group at ca. 588Ma, as indicated by monazite age. The participation of the Amazonian craton in the assembly of western Gondwana occurred at 545-500Ma in the Paraguay belt and ca. 500Ma in the Araguaia belt. This, together with the results presented in this work lead to the conclusion that the collision between the Paraná block and Goiás terrane with the São Francisco Craton along the Brasília belt preceded the accretion of the Amazonian craton by 50-100 million years. © 2003 Elsevier B.V. All rights reserved.

Geology, petrology, U-Pb (shrimp) geochronology of the Morrinhos granite -Paraguá terrane, SW Amazonian craton: implications for the magmatic evolution of the San Ignácio orogeny

O Granito Morrinhos é um corpo batolítico levemente alongado segundo a direção NNW, de aproximadamente 1.140 km², localizado no município de Vila Bela da Santíssima Trindade, estado de Mato Grosso. Situa-se no Terreno Paraguá, Província Rondoniana-San Ignácio, na porção SW do Cráton Amazônico. Essa intrusão exibe uma variação composicional entre tonalito a monzogranito, textura inequigranular média a grossa, localmente, porfirítica, tendo biotita como máfico predominante em uma das fácies e hornblenda na outra, ambas metamorfizadas na fácies xisto verde. As rochas estudadas caracterizam uma sequência intermediária a ácida formada por um magmatismo subalcali no, do tipo álcali-cálcico, metaluminoso a levemente peraluminoso evoluído por meio de mecanismos de cristalização fracionada. Dados estruturais exibem registros de duas fases deformacionais, representadas pela foliação penetrativa (S₁) e dobras abertas (D₂) ambas, provavelmente, relacionadas à Orogenia San Ignácio. A investigação geocronológica (U-Pb SHRIMP) e geoquímica isotópica (Sm-Nd) dessas rochas indicaram, respectivamente, idade de cristalização 1.350 ± 12 Ma, T_DM em torno de 1,77 Ga e valor negativo para εNd(_1,35) de -2,57, sugerindo uma geração relacionada com processo de fusão parcial de uma crosta continental paleoproterozoica (estateriana). Os resultados aqui obtidos indicam que o Granito Morrinhos foi gerado em arco magmático continental, em estágio tardi a pós-orogênico, da Orogenia San Ignácio e permite reconhecê-lo como pertencente à Suíte Intrusiva Pensamiento.

Detrital Zircon Evidence for Non-Laurentian Provenance, Mesoproterozoic (ca. 1490-1450 Ma) Deposition and Orogenesis in a Reconstructed Orogenic Belt, Northern New Mexico, USA: Defining the Picuris Orogeny

Detrital zircon and igneous zircon U-Pb ages are reported from Proterozoic metamorphic rocks in northern New Mexico. These data give new insight into the provenance and depositional age of a >3-km-thick metasedimentary succession and help resolve the timing of orogenesis within an area of overlapping accretionary orogens and thermal events related to the Proterozoic tectonic evolution of southwest Laurentia. Three samples from the Paleoproterozoic Vadito Group yield narrow, unimodal detrital zircon age spectra with peak ages near 1710 Ma. Igneous rocks that intrude the Vadito Group include the Cerro Alto metadacite, the Picuris Pueblo granite, and the Penasco quartz monzonite and yield crystallization ages of 1710 +/- 10 Ma, 1699 +/- 3 Ma, and 1450 +/- 10 Ma, respectively. Within the overlying Hondo Group, a metamorphosed tuff layer from the Pilar Formation yields an age of 1488 +/- 6 Ma and represents the first direct depositional age constraint on any part of the Proterozoic metasedimentary succession in northern New Mexico. Detrital zircon from the overlying Piedra Lumbre Formation yield a minimum age peak of 1475 Ma, and similar to 60 grains (similar to 25%) yield ages between 1500 Ma and 1600 Ma, possibly representing non-Laurentian detritus originating from Australia and/or Antarctica. Detrital zircons from the basal metaconglomerate and the middle quartzite member of the Marquenas Formation yield minimum age peaks of 1472 Ma and 1471 Ma, consistent with earlier results. We interpret the onset of ca. 1490-1450 Ma deposition followed by tectonic burial, regional Al2SiO5 triple-point metamorphism, and ductile deformation at depths of 12-18 km to reflect a Mesoproterozoic contractional orogenic event, possibly related to the final suturing of the Mazatzal crustal province to the southern margin of Laurentia. We propose to call this event the Picuris orogeny.