Margin Of West Gondwana Research Articles

Reply to “Comment on: Passive continental margin subducted to mantle depths: Coesite-bearing metasedimentary rocks from the Neoproterozoic Brasília Orogen, West Gondwana margin” by Schönig (2024)

Reply to “Comment on: Passive continental margin subducted to mantle depths: Coesite-bearing metasedimentary rocks from the Neoproterozoic Brasília Orogen, West Gondwana margin” by Schönig (2024)

Passive continental margin subducted to mantle depths: Coesite-bearing metasedimentary rocks from the Neoproterozoic Brasília Orogen, West Gondwana margin

In this contribution, we investigate the ultra-high pressure metamorphism (UHPM), and subsequent pressure–temperature-time (P-T-t) exhumation path recorded by granulites (rutile-kyanite-garnet-Kfeldspar and rutile-titanite-hornblende-garnet gneisses) of a coherent thrust package (Três Pontas-Varginha Nappe) of the southernmost edge of the Brasília Orogen. The Brasília Orogen, along with Northern Borborema Province, Dahomey, and Hoggar Belts, comprise a widespread linear system of orogens developed during the West Gondwana assembly, recording the diachronic closure of the Tonian-Ediacaran long-lived Goiás-Pharusian Ocean. Optical petrography, Raman spectroscopy, mineral chemistry, Ti-in-zircon and Zr-in-rutile thermometry, and U-Pb dating (LA-ICP-MS) of zircon, monazite, and rutile were combined to identify UHP minerals and textures and to track the decompression/exhumation path. Garnet porphyroblasts preserve UHPM chemical domains and are often radially fractured around rounded quartz inclusions. Remnants of microcoesite were detected by Raman spectroscopy within these inclusions, identified by the diagnostic bands at approximately 170 cm−1, 270 cm−1, and 520 cm−1. Coesite remnants were identified across the nappe stack along with 60° angle-oriented rutile needle inclusions in garnet, which suggests exsolution from Ti-bearing UHP garnets. Soccer ball zircon crystals of Ky-bearing granulites, with flat HREE pattern and negative Eu anomaly, yielded a U-Pb age of around 620 Ma. This age is interpreted as the record of the granulite facies metamorphism (750–805 °C and 15 kbar) post-dating the UHPM from the subduction channel. Zircon and monazite record exhumation under high temperatures around 595–590 Ma, and rutile crystals record the final stages of the decompression path from 590 to 585 Ma. Monazite crystallization occurred until 570 Ma. The new data combined with previously reported metamorphic paths, record a time interval of about 25 m.y. for the exhumation of the Tres Pontas-Varginha Nappe under subsolidus conditions. Our findings represent one of the oldest reports of Neoproterozoic coesite-forming UHPM recorded in metasedimentary rocks buried to mantle depths in the subduction of the passive continental margin.

Terreneuvian skeletonized microfossils from the Central Iberian Zone (Spain) and correlation of the Fortunian Stage throughout southwestern Europe

During the Ediacaran–Cambrian boundary interval, the Iberian margin of West Gondwana recorded oceanic upwelling and phosphogenesis favouring the preservation of originally phosphatic and diagenetically phosphatized shells. In the Abenójar Dome and the Alcudia Anticline of the Central Iberian Zone, the Fuentepizarra Formation comprises several phosphorite and phosphatic limestone interbeds that yield a microfossil assemblage rich in the helcionellid Anabarella plana, the halkieriid Halkieria sacciformis, the hyolith Conotheca subcurvata and chancelloriid debris. The assemblage is correlatable with the Purella antiqua Zone of the Aldan-Lena region and the Purella cristata Zone of the Anabar Uplift (Siberian Platform), slightly predating the Watsonella Zone of the Siberian Platform and the Watsonella crosbyi–Oelandiella korobkovi Zone of the northern Montagne Noire (France), both close to the (not yet globally defined) base of Cambrian Age 2. Despite the intra-Terreneuvian collapse of the Cadomian Orogen, stratigraphically associated with the onset of diachronous gaps, the base and top of the Fortunian Stage is well constrained throughout southwestern Europe based on radiometric ages and the micro- and ichnofossil record.

Cretaceous arc volcanism of Palmer Land, Antarctic Peninsula: Zircon U-Pb geochronology, geochemistry, distribution and field relationships

The Cretaceous marks an episode of enhanced magmatism, sedimentation and tectonic processes along the entire proto-Pacific convergent margin of West Gondwana. Cretaceous magmatism across the Antarctic Peninsula is dominated by the intrusive Lassiter Coast intrusive suite which developed as episodic ‘flare-up’ events during the mid-Cretaceous, at a time of increased convergence. Volcanic rocks of this age are poorly defined, as a consequence of limited field observations and an absence of accurate geochronology. Recent field mapping, combined with unpublished field observations has identified a region >10,000 km2 of dominantly subaerial rhyolitic pyroclastic and epiclastic successions from northern Palmer Land of the Antarctic Peninsula. Volcanic successions up to 1500 m in thickness consist of dominantly silicic ignimbrites, lavas, heterolithic breccias and lahar deposits, fed by caldera-forming eruptions. The volcanic rocks of this region were widely considered to be Early Cretaceous in age based on field relationships and early geochronology. New U-Pb zircon ages identify three distinct volcanic episodes during the Late Cretaceous/Early Cenozoic at ~108 Ma, ~93 Ma and ~64 Ma. Lu-Hf data indicate significant proportions of ancient crust in the petrogenesis of the volcanic rocks and the slightly negative εHf values are consistent with a mid-position (~120 km) within the magmatic arc, relative to more juvenile compositions close to the trench.

The Late Neoproterozoic-Early Paleozoic basin of the western Argentine Precordillera: Insights from zircon U-Pb geochronology

In central-western Argentina, a belt including marine metasedimentary rocks and mafic-ultramafic bodies occurs throughout the western margin of the Precordillera. The belt is considered as the suture zone between the poorly known Chilenia terrane and the Cuyania terrane, part of the composite West Gondwana margin. It is assigned to the Late Neoproterozoic-Early Devonian based on fossil fauna and radiometric ages. In the southern sector of this belt, in the Peñasco area, two units crop out. The Peñasco Formation comprises metasandstone and metapelite spatially associated with mafic metavolcanic and metahyaloclastic rocks. Metagabbro bodies intrude the succession. The Garganta del León Formation consists of metasandstone and scarce metapelite where tractive and deformational sedimentary structures are preserved. Both units are affected by low-grade metamorphism, but the main foliation S1 and crenulation cleavage S2 are better developed in the Peñasco Formation rocks. U-Pb data on detrital zircon of two metasandstone samples from these units show a dominant detrital input from sources with 1.0–1.3 and 0.65–0.53 Ga ages. Detritus may come from reworked sedimentary units or from igneous/metamorphic complexes from the Cuyania terrane basement that was possibly exhumed in the Ocloyic orogen. A Gondwanan provenance for the Late Neoproterozoic-Cambrian population would also be plausible. A ca. 460 Ma zircon population in the Garganta del León Formation is interpreted to be derived from the Famatinian Arc. This would imply that the deposition of the sediment occurred after the collision of the Cuyania terrane against West Gondwana, and that the Ocloyic orogen acted as a barrier for detritus from the Famatinian Arc and other rocks further east.

Low-temperature thermochronology of the South Atlantic margin along Uruguay and its relation to tectonic events in West Gondwana

The geodynamic forces acting during Jurassic-Cretaceous South Atlantic rifting provoked intense transformations in West Gondwana, such as the reactivation of ancient basement structures, voluminous magmatism and general uplift of the new continental margins. Low-temperature thermochronology records cooling associated with uplift syn- and post-breakup along the Brazilian margin, while further south, in Uruguay, mostly pre-breakup uplift is identified. Thermochronometry data are scarce in Uruguay, but previous studies suggest that basement cooling and exhumation preceded West Gondwana breakup by hundreds of millions of years. To improve our knowledge of Uruguay's thermotectonic evolution, in this study we present 19 apatite fission-track ages, 42 apatite and 40 zircon (UTh)/He single crystal ages for the Uruguayan shield (UYS), from which we modeled 19 inverse thermal histories. Our results suggest that the UYS temperatures were below 200 °C since the early Paleozoic, and that cooling below 110 °C started during the Carboniferous, with continuous exhumation of the basement until Early Cretaceous. The onset of this long-term uplift is correlated with orogenesis and terrane accretions in the SW margin of West Gondwana during the Paleozoic. Lithosphere thinning and uplift preceding breakup contributed to the continuous Late Paleozoic to middle Mesozoic exhumation, until the voluminous volcanism of the Paraná-Etendeka Large Igneous Province (c. 133 Ma). This magmatic event, combined with the thermal influence of the Tristão da Cunha mantle plume and rift spreading, likely raised the basement geotherm during the Late Cretaceous. Models suggest a slight increase in temperatures of the UYS from Late Cretaceous until the Oligocene, when a final cooling to surface temperatures took place. Our findings corroborate a long and complex thermal history for Uruguay, with crustal uplift occurring essentially before West Gondwana breakup.

Middle Cambrian Bradoriida (Arthropoda) from the Franconian Forest, Germany, with a review of the bradoriids described from West Gondwana and a revision of material from Baltica

Bradoriid arthropods (class Bradoriida) are described for the first time from the lower–middle Cambrian boundary interval (regional Agdzian Stage) of the Franconian Forest in eastern Bavaria, Germany. The specimens originate from the Tannenknock and Triebenreuth formations, which are part of a shallow marine succession deposited at the margin of West Gondwana. Five different forms have been distinguished, Indiana aff. dermatoides (Walcott), Indiana sp., Indota? sp., Pseudobeyrichona monile sp. nov., and an undetermined svealutid, all of which belong to families that have previously been reported from and are typical of West Gondwana. However, at the generic level, all taxa are new for the region. Indiana is typical of shallow marine environments. So far it has been reported from Laurentia, Avalonia, and Baltica, and is considered to characterize the paleogeographic vicinity of Cambrian continents. Pseudobeyrichona has previously only been recorded from South China, and its new occurrence corroborates previous documentation of taxa from South China in northern West Gondwana. The presence of Indiana as a typical “western” taxon and Pseudobeyrichona among other typical “eastern taxa” confirms the unique biogeographical position of West Gondwana. The poorly known Indiana anderssoni (Wiman) and Indiana minima Wiman from the late early Cambrian of Scandinavia have been restudied in order to re-evaluate the two species and to refine the definition of Indiana. Indiana anderssoni represents a distinct species of Indiana, whereas I. minima is a pseudo-fossil.

An Ediacaran back-arc basin preserved in the Transversal Zone of the Borborema Province: Evidence from geochemistry, geochronology and isotope systematics of the Ipueirinha Group, NE Brazil

The Ipueirinha Group is a ca. 75 km-long by 13 km-wide succession of metavolcano-sedimentary rocks situated in the westernmost Transversal Zone, Borborema Province, NE Brazil. It is mostly composed of marine terrigenous sedimentary rocks represented by immature metarhythmites and quartzites, local metaultramafites, rhyolitic metavolcaniclastic rocks and rare lenses of metamarl. The metavolcaniclastic rocks show highly fractionated chondrite-normalized REE patterns (LaN/YbN = 3.68–8.94) with slight to moderately negative Eu anomalies (Eu/Eu* = 0.72–0.94), incompatible trace element signatures similar to those of volcanic arc-related rocks, including a Nb-Ta trough, and negative ƐNd(t) ranging from −3.4 to −12.2. Metarhythmites show LREE enrichment (LaN/YbN = 4.83–15.32), moderate Eu anomalies (Eu/Eu* = 0.68–0.81), and incompatible element signatures consistent with provenance from a continental arc. Nd isotope data indicate important detrital contribution from juvenile sources for these metarhythmites, as reflected by the ƐNd(t) values between −3.6 and 0.3. Detrital zircon LA-ICP-MS U-Pb data reveal that the main sources of detritus for the metarhythmites are Neoproterozoic rocks (maximum age peak at ca. 845 Ma and younger zircons at ca. 636 Ma). Hf isotope data obtained from detrital zircons show evidence for an important period of juvenile magmatism in the Borborema Province from 895 to 834 Ma (ƐHf(t) = 2.0–5.7), and suggest that the Neoproterozoic crustal evolution of this province involved mixing between older sources with negative ƐHf(t) and younger juvenile sources. Metaultramafites associated with the Ipueirinha Group show ƐNd(t) ranging from −1.3 to −8.5 and TDM = 1.33–1.64 Ga. The whole unit was submitted to four deformation phases and to heterogeneous metamorphic conditions from lower to upper greenschist facies. Additional geochronological data obtained for a granitic sill (578 ± 13 Ma; zircon U-Pb) along with monazite U-Pb data from a chloritite sample (595 ± 29 Ma) indicate that strike-slip deformation, metamorphism and syn-orogenic melt generation coexisted in the Ipueirinha Group during the Late Ediacaran. We suggest that the Ipueirinha Group comprises a Cryogenian/Ediacaran back-arc basin to the Tamboril-Santa Quitéria magmatic arc, which is one of the few records of initial convergence tectonics in the West Gondwana margin of the Borborema Province.

U-Pb ages and Hf isotope compositions of zircons in plutonic rocks from the central Famatinian arc, Argentina

The Famatinian arc formed around the South Iapetus rim during the Ordovician, when oceanic lithosphere subducted beneath the West Gondwana margin. We present combined in situ U–Th–Pb and Lu–Hf isotope analyses for zircon to gain insights into the origin and evolution of Famatinian magmatism. Zircon crystals sampled from four intermediate and silicic plutonic rocks confirm previous observations showing that voluminous magmatism took place during a relatively short pulse between the Early and Middle Ordovician (472–465 Ma). The entire zircon population for the four plutonic rocks yields coherent εHf negative values and spreads over several ranges of initial εHf(t) units (−0.3 to −8.0). The range of εHf units in detrital zircons of Famatinian metasedimentary rocks reflects a prolonged history of the cratonic sources during the Proterozoic to the earliest Phanerozoic. Typical tonalites and granodiorites that contain zircons with evolved Hf isotopic compositions formed upon incorporating (meta)sedimentary materials into calc–alkaline metaluminous magmas. The evolved Hf isotope ratios of zircons in the subduction related plutonic rocks strongly reflect the Hf isotopic character of the metasedimentary contaminant, even though the linked differentiation and growth of the Famatinian arc crust was driven by ascending and evolving mantle magmas. Geochronology and Hf isotope systematics in plutonic zircons allow us understanding the petrogenesis of igneous series and the provenance of magma sources. However, these data could be inadequate for computing model ages and supporting models of crustal evolution.

Evidence of post-Gondwana breakup in Southern Brazilian Shield: Insights from apatite and zircon fission track thermochronology

Apatite and zircon fission track thermochronology studies are applied to basement and sedimentary rocks from the Sul-Rio-Grandense Shield to unravel the tectonic history of the onshore southernmost Brazilian margin. The Sul-Rio-Grandense Shield is a major geotectonic feature of southernmost Brazil that includes Paleoproterozoic basement areas and Neoproterozoic fold belts linked to the Brasiliano/Pan-African orogeny. Crustal reworking and juvenile accretion events related to this cycle were dated in the region between 900 and 500Ma and were responsible for the assembly of southwestern Gondwana in southeastern South America. Apatite fission track (AFT) ages range from 340±33 to 77±6Ma and zircon fission track (ZFT) ages range from ca. 386 to 210Ma. Based on thermal history modeling, the most part of the samples record an early cooling event during the Carboniferous, which reflect the main tectonic activity of the final stages of the Gondwanides at the Pacific margin of West Gondwana. Subsequently, the Permo-Triassic cooling event is related to the last stages of the Gondwanides, with convergence along the southern border of Western Gondwana and consequent reactivation of N-S and NE-SW trending basement structures. The onset of initial breakup of southwestern Gondwana with opening of the South Atlantic Ocean is mostly recorded in the eastern terrain and ZFT ages show that the temperature during this period was high enough for total or at least partial resetting of fission tracks in zircon. The last cooling event of the Sul-Rio-Grandense Shield records the final breakup between South America and Africa, which began during the Late Cretaceous. However, the Cenozoic rapid cooling episode probably is a result of plate adjustment after breakup and neotectonic reactivation of faults associated with South Atlantic rift evolution.

A blind trilobite with Baltic affinities from Cambrian Series 3 of the Iberian Chains, Spain, and its stratigraphical and palaeobiogeographical significance

Three cranidia of a conocoryphid trilobite are described from the lower Caesaraugustan Regional Stage (upper part of provisional Cambrian Stage 5 within Cambrian Series 3) in the Iberian Chains, Spain. The material is from the upper part of the Mansilla Formation and the base of the overlying Murero Formation. It closely resembles Bailiella tenuicincta (Linnarsson, 1879), which has previously been described from the Ptychagnostus gibbus (Cambrian Stage 5) and lower P. atavus zones (lower Drumian Stage) of Baltica (Sweden and Bornholm), and from the P. atavus Zone of West Avalonia (Newfoundland). The record of B. cf. tenuicincta from the Mediterranean margin of West Gondwana provides additional evidence for considering the Acado-Baltic region a discrete biogeographical province, at least during the Cambrian 5 and Drumian ages.

Peralkaline and alkaline magmatism of the Ossa-Morena zone (SW Iberia): Age, source, and implications for the Paleozoic evolution of Gondwanan lithosphere

The Ossa-Morena zone in SW Iberia represents a section of the northern margin of West Gondwana that formed part of a Cordilleran-type orogenic system during the Neoproterozoic (Cadomian orogeny). The crustal section in this zone preserves the record of rifting that led to the opening of the Rheic Ocean in the early Paleozoic and the collision of Gondwana and Laurussia in the late Paleozoic (Variscan orogeny). We present U-Pb zircon data from three alkaline to peralkaline syenites that intruded Neoproterozoic and Cambrian strata and give crystallization ages ranging between ca. 490 Ma and 470 Ma. Lu/Hf isotopic data from these zircons give positive initial e Hf values (0 ≤ e Hf(t) ≤ +11.5) that approach the model values for the depleted mantle at the time of crystallization. This suggests that a significant proportion of the magma was derived from the mantle, with limited mixing/assimilation with crustal-derived melts. Alkaline/peralkaline magmatic suites of similar age and chemical composition intruded other sections of the northern margin of West Gondwana and along the boundaries of the continental blocks that today make up Iberia. These blocks are further characterized by the presence of high-pressure metamorphic belts that formed during accretion and subsequent collision of peri-Gondwanan domains against Laurussia during the Devonian and Carboniferous (Variscan orogeny). Our U-Pb and Lu-Hf data set indicates that during the Cambrian–Ordovician transition, lithosphere extension reached a stage of narrow intracontinental rifting, where deeply sourced magmas, probably coming from the lower crust and/or the upper mantle, intruded continental upper crust across various sections of previously stretched crust. We propose that necking of the Gondwana lithosphere into several continental microblocks with fertile mantle beneath them compartmentalized extension (multiblock model), which favored the onset of early Paleozoic peralkaline and alkaline magmas. The boundaries of microblocks represent zones of inherited crustal weakness that were later reactivated during the late Paleozoic as major accretionary faults related to the amalgamation of Pangea during the Variscan orogeny. Our dynamic model provides an explanation for the unusual spatial relationship between peralkaline and alkaline igneous provinces (usually shallow in the crust) and the occurrence of high-pressure rocks. Our observations suggest that Cordilleran-type orogens subjected to extension after long-lived subduction can develop wide continental platforms that feature multiple continental blocks. In addition, the formation of sequenced high-pressure belts in collisional orogens can be explained as the ultimate consequence of multiple necking events within continental lithosphere during previous collapse of a Cordilleran-type orogen.

The Palaeozoic basement of the Andean Frontal Cordillera at 34º S (Cordón del Carrizalito, Mendoza Province, Argentina): Geotectonic implications

The Cordón del Carrizalito is located in the southern sector of the Andean Frontal Cordillera. In this area, the Andean basement is composed of meta-sedimentary rocks (Las Lagunitas Formation) of Ordovician age. In addition, no- or very low grade metamorphism and less deformed rocks also occur in the study area. We call these rocks Selerpe series, whose characteristics are comparable to other series, late Carboniferous in age, described in nearby areas. The Las Lagunitas Formation is affected by west-verging folds, developed under low-grade metamorphic conditions. These structures can be attributed to the Chanic orogeny (Late Devonian – early Carboniferous). The Selerpe series and Las Lagunitas Formation are deformed by east-verging thrusts and folds developed in narrow bands and generated in the absence or under very low metamorphic conditions. These structures always deform the Chanic structures, and are attributed to the Gondwanan deformation (San Rafael orogeny, late Carboniferous – Permian in age). The Chanic structures of the study area can be placed in the western branch and in the hinterland of the Chanic orogen, which was developed as a result of the accretion of the Chilenia terrane at the west Gondwana margin during Late Devonian and early Carboniferous. The eastern branch of this orogen is located in the Andean Precordillera. The Permo-Triassic cover, deformed by the Andean orogenic cycle (Mesozoic – Cenozoic), rests unconformably on the Palaeozoic basement rocks.

Geochronology of the Cambrian: a precise Middle Cambrian U–Pb zircon date from the German margin of West Gondwana

AbstractA volcanic tuff 1.0 m above the base of the Triebenreuth Formation in the Franconian Forest provides the first precise and biostratigraphically bracketed date within the traditional Middle Cambrian. The first illustration of fossils from the Triebenreuth Formation in this report and their discussion allow a more highly refined correlation within the Middle Cambrian. A weighted mean206Pb–238U date of 503.14±0.13/0.25/0.59 Ma on zircons from this subaerial pyroclastic tuff was determined by U–Pb chemical abrasion isotope dilution mass spectrometry (CA-TIMS) techniques. Atc. 6.0–7.0 Ma younger than the base of the traditional Middle Cambrian in Avalonia, the new West Gondwanan date from east-central Germany suggests that estimates of 500 Ma for the base of the traditional Upper Cambrian and 497 Ma on the base of the Furongian Series may prove to be too ‘old’. Biostratigraphically well-bracketed dates through most of the Middle Cambrian/Series 3 and below the upper Upper Cambrian/upper Furongian Series do not exist. An earlier determined 494.4±3.8 Ma date from the Southwell Group of Tasmania may actually prove to be a reasonable estimate for the age of the base of the traditional Upper Cambrian. Until high precision dates are determined on the base of the traditional Upper Cambrian and base of the Furongian Series, the rates of biotic replacements and geological developments and the durations of biotic zones in the Middle/Series 3 and Upper Cambrian/Furongian Series remain as ‘best guesses’.

Detrital zircon U–Pb ages of metasedimentary rocks from Sierra de Valle Fértil: Entrapment of Middle and Late Cambrian marine successions in the deep roots of the Early Ordovician Famatinian arc

We obtained detrital zircon ages from three metasedimentary rocks with siliciclastic protoliths that were buried to paleodepths of 20–27 km during Early Ordovician Famatinian arc magmatism. In all three samples, U–Pb zircon age distributions are polymodal but the dominant peaks make well-defined clusters, matching the ages of orogenic systems that characterize West Gondwana. The most prominent peaks of constituent age populations at 520 Ma, 600 Ma, and 1050 Ma reflect derivation from sources that are characteristic of the West Gondwana margin. Moreover, grains with Meso- and Paleo-Proterozoic ages appear as small fractions (<5%). The siliciclastic protoliths to metasedimentary rocks from the deep Famatinian crust are dominantly composed of detritus derived from two sources: 1) a nearby magmatic arc or a rapidly exhuming orogen, and 2) sedimentary recycling of Neoproterozoic–Lower Cambrian sedimentary successions broadly grouped as Puncoviscana Formation. Our findings reveal that granulite–facies metasedimentary rocks in the deep crust of the Famatinian arc have detrital zircon patterns closely resembling those found in nearby Middle to Upper Cambrian quartz-rich turbidite sequences from central Famatina, the Cordillera Oriental and the Puna. At the regional scale, these sedimentary successions yield maximum depositional ages corresponding to the Middle Cambrian (<520 Ma). This indicates that metasedimentary rocks in the Sierra de Valle Fértil were rapidly buried to deep levels after deposition. Our results are also consistent with the existence of a crustal scale paleo-suture located at the western margin of the Sierras Valle Fértil and La Huerta that explains the differences of detrital zircon age patterns between metasedimentary rocks in the Valle Fértil and age-equivalent sedimentary units in the nearby Cuyania Terrane.

Geological, Petrological and Geochemical Evidence for Progressive Construction of an Arc Crustal Section, Sierra de Valle Fertil, Famatinian Arc, Argentina

The petrogenesis of calc-alkaline magmatism in the Famatinian arc is investigated in the central Sierra Valle Fe¤ rtil, a major, lower to middle crustal section of the Early Ordovician active margin of West Gondwana. Large-scale field relationships show a gradual and continuous compositional variation of the plutonic sequence, ranging from olivine-bearing gabbronorites to hornblendeand biotite-bearing granodiorites. Distinctive lithostratigraphic units are, however, discernible as one compositional type of plutonic rock dominates over mappable areas.These results allow us to identify a continuous plutonic arc stratigraphy that progressively exposes shallower paleo-depths towards the east. At all the exposed levels, calc-alkaline plutonic rocks are volumetrically dominant, interrupted only by granulite-facies migmatites and leucogranites. The migmatites are interpreted to be refractory remnants of supracrustal sedimentary successions, whereas the peraluminous leucogranites have field relationships and chemical and isotopic compositions suggesting that they were produced via anatexis of metasedimentary packages. Massbalance calculations predict that a parental gabbroic magma after progressive closed-system fractionation would crystallize about 80% of the original mass to yield a granodioritic daughter. Because the crystallizing mineral assemblage comprises hornblende and plagioclase, mass balance suggests a volume of residual amphibole-rich gabbroic rocks much larger than that observed, suggesting that differentiation is significantly driven by open-system processes. Indeed, the combination of field and petrographic observations with bulk-rock geochemistry and petrogenetic modeling demonstrates that most dioritic and tonalitic rocks are hybrids formed by either (1) bulk assimilation of metasedimentary materials into gabbroic magmas, or (2) multi-stage and complex interactions between gabbroic rocks and metasedimentary-derived leucogranitic melts. The source region of the granodioritic magmas is located at the transition zone between a tonalite-dominated intermediate unit and a granodiorite-dominated silicic unit. Typical granodiorites have a hornblende-bearing mineralogy, metaluminous chemical signature and isotopic compositions [Sr/Sr(T)1⁄4 0·7075^0·7100 and eNd(T) 5·0] broadly overlapping those of the tonalites of the intermediate rock unit. These major compositional features of the granodiorites can be best explained if three end-member components contribute to their generation. As field observational data suggest, primitive gabbroic rocks, metaluminous intermediate magmas and anatectic leucogranitic melts mixed to produce the calc-alkaline granodiorites; however, the exact petrological process generating the granodioritic magmas is unclear because the mafic end-member may have been incorporated as mafic inclusions in the intermediate magmas or as syn-magmatic dikes, or both.The polygenetic nature of the intermediate to silicic plutonic rocks, along with the preponderance of parental gabbroic rocks at the inferred base of the plutonic column, suggests an upward growth of the intermediate to silicic crust that involved the complete reconstitution of the pre-existing crustal configuration. The main implication of this study is that intermediate and silicic plutonic rocks in the Valle Fe¤ rtil section formed within a crustal column in which the mass transfer and heat

Middle and Upper Jurassic Ostracods from Western Kachchh, Gujarat, India: Biostratigraphy and paleobiogeography

This paper presents a study of the Middle and Upper Jurassic Ostracods from Western Kachchh, India. The first part discusses the biochronological importance of the Ostracod assemblages, while the second part reports the paleobiogeographical results.Benthic foraminiferal biozones are used to calibrate the biostratigraphical distribution of ostracod species and genera from sections in the Aalenian–Tithonian interval located in Western Kachchh (or Kutch) (Mundham Anticline, Jumara Dome, Jhurio Dome and Habo Dome) and on Pachchham Island (Khavda Nala), Gujarat, India. A total of 71 species are listed, including 32 previously published species and 38 species in open nomenclature, belonging to 23 genera. The ostracod assemblages indicate that faunal communication was first established during the Bathonian and Callovian (Middle Jurassic) and continued until the early Cretaceous. These faunal exchanges occurred between Laurasia (USA, Europe, Russia and Siberia) and the Indo-East African Province, extending along the Western Tethyan carbonate shelves (bordering West and East Gondwana) through to the Pacific Ocean margin of West Gondwana.

The Parautochthonous Gondwanan origin of the Cuyania (greater Precordillera) terrane of Argentina : a re-evaluation of evidence used to support an allochthonous Laurentian origin

A substantial, diverse body of evidence has been interpreted as suggesting that the Cuyania terrane of northwestern Argentina, which includes the Argentine Precordillera, rifted from the Ouachita embayment of Laurentia in the Early Cambrian, drifted across the Iapetus ocean as a microcontinent, and docked with the proto-Andean margin of Gondwana in the Mid to Late Ordovician. This is the so-called Laurentian microcontinent model. However, several lines of evidence (basement age and affinity, stratigraphic, paleomagnetic and paleobiogeographical records) also point to a parautochthonous origin of this terrane. In this parautochthonous model, Cuyania migrated along a transform fault from a position on the southern margin of West Gondwana (present coordinates) in the Mid Ordovician to its modern position outboard of the Famatina magmatic belt in Devonian time. With regard to basement age and characteristics, recently acquired U-Pb geochronology of detrital zircons from Cambrian and Ordovician sandstones and of zircons from igneous clasts in an Ordovician conglomerate are difficult to explain with the Laurentian model and indicate, instead, a Gondwanan origin of the Cuyania terrane. Furthermore, potential basement rocks of Cuyania of Neoproterozoic to Early Cambrian age and Early Mesoproterozoic age are characteristic of Gondwana, rather than Laurentia. Pb isotopic ratios of Grenvillian-age basement rocks are not only similar to those of Grenvillian basement in Laurentia but also to those in other areas of West Gondwana. In terms of the stratigraphic record, the similarity of the Cambrian-Ordovician carbonate platform succession of Cuyania to that of Laurentia reflects similar paleolatitude and eustatic histories but not a direct connection. Moreover, the Middle-Upper Ordovician siliciclastic successions of Cuyania do not represent a peripheral foreland basin, but instead were deposited in strike-slip related basins in a transform fault zone. Middle Ordovician K-bentonites do not indicate that Cuyania was approaching the Famatina Magmatic arc from the west (modern coordinates), but instead that it was located to the southeast. In light of paleomagnetic data, the Cambrian paleolatitude of Cuyania is consistent not only with the location of the Ouachita embayment of Laurentia but also with the southern margin of West Gondwana. Finally, most of the paleobiogeographic criteria used to support the Laurentian model must be reconsidered. Brachiopod and conodont faunas in lower Middle Ordovician strata of the Precordillera have many more genera in common with Laurentia than those in Lower Ordovician strata. Cambrian trilobites faunas of Cuyania are of very limited abundance and diversity in comparison to correlative faunas of southeastern Laurentia; many species are endemic to Cuyania; olenellid trilobites considered to be restricted to Laurentia probably had the ability to disperse between paleoplates with similar environments. Mid Ordovician graptolites of the Precordillera on the one hand and of the Famatinian belt and Cordillera Oriental on the other belong to different oceanic provinces and likely did not live in close proximity.

The age and tectonic setting of the Puncoviscana Formation in northwestern Argentina: An accretionary complex related to Early Cambrian closure of the Puncoviscana Ocean and accretion of the Arequipa-Antofalla block

Abstract TIMS and SHRIMP U–Pb zircon geochronology of selected parts of the Puncoviscana Formation suggest its deposition took place mainly during the Early Cambrian, coeval with 540–535 Ma calc-alkaline Pampean arc volcanism mainly preserved as tuff beds in the oldest identified parts of this unit. Syn- to post-tectonic plutons constrains the Tilcarian–Pampean orogeny to have occurred between ca. 530 Ma and deposition of the unconformably overlying Middle-Upper Cambrian Meson Group. Deposition of the Puncoviscana Formation continued after the onset of the Tilcarian–Pampean orogeny. We propose that the Puncoviscana Formation rocks older than 530 Ma were deposited in the arc-trench gap of the west-facing Pampean arc and/or the associated trench, whereas the rocks younger than 530 Ma were deposited in a syn-collision foreland basin. The Puncoviscana Formation rocks were progressively assembled into a west-younging accretionary complex, consistent with the style of deformation and low-grade metamorphism. The age of the syn-collision plutons (≤530 Ma) suggest the foredeep deposits record the transition from trench to foreland basin, due to arrival of the Arequipa-Antofalla block at the west-facing trench at ca. 530 Ma. Our geochronological and Pb-isotope investigations suggest that the Arequipa-Antofalla terrane was a coherent, ribbon-shaped crustal block that also included the western part of the Pampia terrane. A compilation of existing U–Pb zircon studies suggests that the Pampean arc extended along the length of the proto-Andean margin of West Gondwana, represented by the previously amalgamated Amazonia and Rio de La Plata cratons, and probably was initiated during the late Ediacaran after 600 Ma. Following earlier workers, we reaffirm that the Arequipa-Antofalla block was originally separating Laurentia and Amazonia in Rodinia. It probably rifted from Laurentia during the Ediacaran between 600 and 570 Ma, following an earlier departure of Amazonia (∼650 Ma?). The separation of Arequipa-Antofalla from Amazonia and Laurentia opened the Puncoviscana and Iapetus oceans respectively.

Detrital Quartz and Zircon Combined: The Production of Mature Sand with Short Transportation Paths Along the Cambrian West Gondwana Margin, Northwestern Argentina

Abstract Quartz, the most common mineral in most sandstones, rarely is used in provenance studies. This study demonstrates the provenance-discriminatory potential of combining cathodoluminescence (CL) color wavelength spectra of quartz with morphology and in situ U-Pb ages of zircon. The Cambrian Meson Group in northwestern Argentina is used for the test of this combined methodology, because it is composed of nonmetamorphosed sandstones with > 90% quartz and a dominance of zircon among the heavy minerals. Correlation of the results from the two methods is evident: (1) both CL spectra typical for bright-luminescent quartz (red to blue; > 90%) and oscillatory zircon growth zoning (80–90%) indicate a dominance of magmatic detritus. (2) Upsection, diminishing input from euhedral 510–600 Ma zircons (70% at the base) correlates with lower amounts of volcanic quartz (40% red and violet grains at the base). Instead, plutonic quartz and abraded zircons of 550–700 Ma age prevail. The data point to the occurrence of a (nearly) synsedimentary volcanic phase, which has not been known previously. Furthermore, initial short transportation paths from nearby magmatic bodies (100–200 km) can be assumed, because of the low degree of grain abrasion and an age correlation with magmatic bodies in the region, such as the Santa Rosa de Tastil batholith, here dated at 513 +4/−5 Ma. The proximity to the source area at an initial stage indicates first-cycle sand. The maturity was probably reached by intense chemical weathering. The upsection-diminishing young, volcanic input indicates erosion of the volcanic source within a time period of millions of years. At that stage, mainly up to 1000-km-long coast-parallel transport from the Sierras Pampeanas in the south fed the depositional basin. The change in—and expansion of—the main source areas record a change to production of multicycle sand. Hence, the maturity was partly caused by sedimentary recycling. More generally, the provenance-discriminative effect of the CL of quartz, which has been doubted by some researches, can be confirmed. Therefore, the results imply that both quartz and zircon should be taken into account in provenance studies of quartz arenites.