Alcides N. Sial

Soft-sediment deformation at the base of the Neoproterozoic Puga cap carbonate (southwestern Amazon craton, Brazil): Confirmation of rapid icehouse to greenhouse transition in snowball Earth

Stratigraphic and isotopic data identify the lower 45 m of the Araras Group, on the southwest margin of the Amazon craton, as a Neoproterozoic platform cap carbonate deposited below wave base upon Varanger glacial diamictites of the Puga Formation. The basal beds consist of moderately deep water pinkish dolomudstone with stratiform to wavy fenestral microbialites locally cut by tube-like structures and fenestral nonmicrobial planar laminites with tepee-like features. Above the basal carbonates are deep-water bituminous lime mudstones with alternating thin calcite crusts and lime mudstone laminae commonly disrupted by calcite crystal fans (pseudomorphs after aragonite). The basal contact of the Puga cap exhibits soft-sediment deformational structures (principally load casts) that are here attributed to rebound-induced seismicity acting upon both recently deposited carbonate sediments and underlying unconsolidated diamictite. These features constitute the first clearly recognized sedimentological evidence for the rapid change from icehouse to greenhouse conditions as postulated in the snowball Earth model of Neoproterozoic glaciation.

Integrated correlation of the Vendian to Cambrian Arroyo del Soldado and Corumbá Groups (Uruguay and Brazil): palaeogeographic, palaeoclimatic and palaeobiologic implications

Abstract The Corumba Group of SW Brazil and the Arroyo del Soldado Group (ASG) of Uruguay are correlated on the basis of litho-, bio- and chemostratigraphy. Both units represent marine sedimentation with alternating siliciclastics and carbonates developed on a stable continental shelf. In the Corumba basin, sedimentation began in the Varangerian, represented by the glaciomarine Puga Formation. A series of sea-level fluctuations coupled with climatic changes are recorded up section. While uppermost deposits of the ASG are of lowermost Cambrian age, sedimentation ceased in the latest Vendian in the Corumba basin. An assemblage of six species of organic-walled microfossils dominated by Bavlinella faveolata and Soldadophycus bossii, three species of vendotaenids and two species of skeletal fossils (Cloudina and Titanotheca) is described from the Corumba Group. The vendotaenid Eoholynia corumbensis sp. nov is described from siltstones of the Guaicurus Formation. An important diversity of skeletal fossils in the Corumba, Arroyo del Soldado and Nama groups points to favourable Vendian palaeoclimatic conditions in SW-Gondwana. Preliminary carbon isotopic data show a series of alternating positive and negative excursions, corroborating the upper Vendian age indicated by fossils for both units. Previously reported strontium isotopic data are also consistent with this age. It is postulated that the Corumba and ASGs were deposited onto the same shelf, which opened to the east. The Rio de la Plata Superterrane (Craton) extends farther to the north than previously expected, or it was already amalgamated with the Amazonian Craton by Vendian times. Collision of the platform with the Parana Block caused closure of the basin during the Cambrian-Early Ordovician. Finally, models of Neoproterozoic glaciations based on enhanced bioproductivity driven by high nutrient availability are discussed.

Magma flow within the Tavares pluton, northeastern Brazil: Compositional and thermal convection

Crystallization coupled with gravity removal of depleted interstitial melt has long been recognized as a mechanism of magma differentiation. Similarly, heat released by synplutonic basaltic magma intrusions has long been recognized as capable of driving convection in granite chambers. Direct evidence of these processes has seldom been described in granites. In the Tavares pluton, we mapped a number of melt extraction structures from pores of a crystal-liquid mush (an effectively solid magma where crystals form an interconnected skeleton) and a variety of flow structures such as (1) meter-scale tear- or mushroom-shaped blobs representing within-chamber diapirs; (2) meter-scale ellipsoids representing frozen thermal plumes of granite, driven by heat released from disrupted diorite intrusions; and (3) ladder dikes and snail structures representing cross sections of several superposed cylindrical magma channels (possibly feeders of diapirs and plume heads). A fundamental feature of the structures in the Tavares pluton is that they are well delineated by mafic schlieren developed at active channel margins. We postulate a new model for the origin of marginal schlieren, which combines shear flow sorting and melt escape from the flowing magma into an effectively solid surrounding mush. Extraction structures (representing melt extraction from mush pores into melt pockets) and schlieren (representing regions where melt escaped into surrounding mush pores) are both favored by magmas that form an interconnected solid framework at low crystal fractions (∼50%), because these mushes are ductile and permeable. Favorable magmas are those with a high wetting angle between melt and solid (∼60°) and a propitious crystal size and shape distribution. We propose a model of compositional and thermal convection that accounts for all described structures.

Chemostratigraphy of the Lower Arroyo del Soldado Group (Vendian, Uruguay) and Palaeoclimatic Implications

Abstract New C- and O-isotopic determinations from the Vendian lower Arroyo del Soldado Group are reported and combined with sedimentologic and biostratigraphic data. On the basis of different geochemical and petrographic criteria, the primary nature of the C- and Sr-isotopic signature is shown. Positive d 13 C-values characterize the mainly siliciclastic upper Yerbal Formation, which contains oxide-facies BIF and a diverse assemblage of skeletal fossils, including Cloudina riemkeae. A series of positive and negative d 13 C-excursions occur up-section in the overlying Polanco Formation, mainly composed of limestones and limestone-dolostone rhythmites. The transition to the overlying Barriga Negra and Cerro Espuelitas Formation, which consists of conglomerates and shales/cherts/BIF respectively, is marked by a further negative excursion. On the basis of sedimentary structures, a correlation of d 13 C with palaeobathymetry is established. Positive d 13 C -peaks are associated with high sea-level stand, high organic-carbon burial and relatively higher microfossil diversity, while negative d 13 C -excursions occur in carbonates with less organic matter, less microfossil diversity and are always associated to regressions. These observations can be readily explained by palaeoclimatic models which postulate that enhanced bioproductivity due to higher availability of nutrients (P, N, Fe) was the key factor controlling Neoproterozoic glaciations. The mentioned models are discussed in view of the new data. The occurrence of at least four cold periods in the upper Vendian is envisaged. These cold periods led to sea-level fall and possibly glaciation at higher latitudes. The absence of glacigenic rocks associated to negative d 13 C-excursions in the Arroyo del Soldado Group is probably due to the tropical setting of the basin. Finally, the upper Vendian age of the lower Arroyo del Soldado Group is confirmed, on the basis of C- and Sr-isotopes. The onset of carbonate deposition at the base of the Polanco Formation is estimated at 580 Ma by comparison with existing global isotopic curves.

Geochemical and isotopic signatures of Proterozoic granitoids in terranes of the Borborema structural province, northeastern Brazil

Two large E–W trending megastructures, the Patos and Pernambuco shear zones, subdivide the Borborema Province (BP), northeastern Brazil, into three main domains: Northern (ND), Transverse Zone (TZD) and Southern (SD). These domains evolved through reworking and amalgamations, during the Brasiliano cycle (0.70–0.50 Ga), of several tectonostratigraphic terranes and major crustal blocks previously deformed during Meso and Paleoproterozoic orogenic cycles. Petrological and geochemical characteristics of granitoids and syenitoids in these domains allow their classification into nine groups. These groups are enriched in K and Ba and have low Nb (usually 1.0×10−3SI, eNd(0.6 Ga) from −15 to −20, and tDM model ages>2.0 Ga. Within the TZD, in contrast, calc-alkalic and high-K calc-alkalic mEp-bearing granitoids, respectively in the Cachoeirinha–Salgueiro and Alto Pajeuterranes, display lower MS (0.4×10−3SI), high δ18O (+10 to +13‰SMOW), lower eNd (−1 to −4), and younger tNd (1.1–1.4 Ga). In high-K mEp-free granitoids, eNd values cluster around −12 in the Granjeiro terrrane and from −12 to −15 in the Capibaribe terrane. The Alto Pajeuterrane in the TZD is characterized by intrusions of peralkalic ultrapotassic syenitoids that show negative eNd (−15 to −19), tDM from 2.1 to 2.4 Ga, and MS from 0.7 to 1.0×10−3 SI, and are regarded as derived from a metasomatized lithospheric mantle source. Granitoids in the SD are isotopically more complex. Three tDM Nd model-age intervals (1.0–1.5, 1.8–2.2 and 2.4–2.5 Ga) are found in the Pernambuco–Alagoas terrane, the largest one within this domain. MS values are rather low, approaching zero in leucocratic granitoids, and up to 15×10−3, in high-K metaluminous syenitoids. Calc-alkalic mEp-bearing plutons in the Macurureterrane are similar to those in the TZD in all respects and distinguish the Macurureterrane from the neighbouring terranes. Altogether, the geochemical and isotopic characteristics of the different granitoid types suggest that: (1) the ND is rather homogeneous, at least vertically, in spite of lateral differences in terms of composition of source rocks and magmatic processes that produced the granitoids; (2) the TZD has lateral and vertical heterogeneities, except for the Cachoeirinha–Salgueiro terrane, which is very homogenous; (3) the SD is even more heterogenous than the TZD. The SD had a long and complex accretionary history, as suggested by the presence of a large number of in situ, leucocratic melts associated with large areas of migmatization, both northern and southern boundaries of the Pernambuco–Alagoas terrane, coupled with a large volume of high-K calc-alkalic granitoids emplaced in several pulses.

Carbon and oxygen isotope profiles across Meso-Neoproterozoic limestones from central Brazil: Bambuı́ and Paranoá groups

We present carbon and oxygen isotopic data on carbonates along three profiles of the Neoproterozoic Bambui group in central Brazil. This unit covers an area of more than 300 000 km2 and comprises carbonate–silicliclastic sediments at the base that grade into siliciclastic sediments towards the top. The Bambui group overlies by unconformity the Paranoa group, which consists mostly of siltstone, quartzite and minor limestone. The data presented here improve the stratigraphic correlation within the Bambui basin and show that it evolved in an environment significantly different from that of the Paranoa basin. Our data show large fluctuations of δ13CPDB in limestones from the Bambui Group (from +0.8 to +13.5‰) in all the three studied areas. Some of these fluctuations represent stratigraphic markers that can be used as a chronostratigraphic tool within a basin scale. This observation is relevant considering the lack of fossil record and other stratigraphic markers in Neoproterozoic sequences. We also present the first isotopic profiles along the Paranoa–Bambui transition, which shows that the δ13CPDB values grade from +1.0‰ in the Paranoa group, to +2.6‰ in the lower portion of the Bambui group, increasing up to +12‰ in the upper part of this unit. Based on our carbon isotope data, as well as other geological, mineralogical and Nd isotope studies, we argue that the sediments of the Paranoa group were deposited on an open platform that was fully connected to the ocean. On the other hand, the sediments of the Bambui group were deposited in an epicontinental sea and during a tectonic inversion in a foreland basin at about 790–600 Ma. This unit displays an increased amount of clastic sediments upwards. We argue that the high carbon isotope values observed in limestones and marlstones from the Bambui group are correlated to worldwide high carbon isotope values reported for the Neoproterozoic. However, we also point out that novel marine conditions induced by the tectonic inversion of the basin may also have contributed to increase the carbon isotopic composition of the Bambui carbonates.

Emplacement, petrological and magnetic susceptibility characteristics of diverse magmatic epidote-bearing granitoid rocks in Brazil, Argentina and Chile

Magmatic epidote mEp -bearing granitoids from five Neoproterozoic tectonostratigraphic terranes in Northeastern NE . Brazil, Early Palaeozoic calc-alkalic granitoids in Northwestern NW Argentina and from three batholiths in Chile have been studied. The elongated shape of some of these plutons suggests that magmas filled fractures and that dyking was probably the major mechanism of emplacement. Textures reveal that, in many cases, epidote underwent partial dissolution by host magma and, in these cases, may have survived dissolution by relatively rapid upward transport by the host magma. In plutons where such a mechanism is not evident, unevenly distributed epidote at outcrop scale is armoured by biotite or near-solidus K-feldspar aggregates, which probably grew much faster than epidote dissolution, preventing complete resorption of epidote by the melt. Al-in-hornblende barometry indicates that, in most cases, amphibole crystallized at PG 5 kbar. Kyanite-bearing thermal aureoles surrounding plutons that intruded low-grade metamorphic rocks in NE Brazil support pluton emplacement at intermediate to high pressure. mEp show overall chemical variation from 20 to 30 mol% mole . . percent pistacite Ps and can be grouped into two compositional ranges: Ps and Ps . The highest Ps contents are 20 - 24 27 - 30 in epidotes of plutons in which hornblende solidified under P- 5 kbar. The percentage of corrosion of individual epidote crystals included in plagioclase in high-K calc-alkalic granitoids in NE Brazil, emplaced at 5-7 kbar pressure, yielded estimates of magma transport rate from 70 to 350 m year y1 . Most of these plutons lack Fe-Ti oxide minerals and Fe q3 is . mostly associated with the epidote structure. Consequently, magnetic susceptibility MS in the Neoproterozoic granitoids in NE Brazil, as well as Early Palaeozoic plutons in Argentina and Late Palaeozoic plutons in Chile, is usually low

Carbon Isotopic Profile Across the Bilara Group Rocks of Trans-Aravalli Marwar Supergroup in Western India: Implications for Neoproterozoic — Cambrian Transition

Abstract The rocks of Marwar Supergroup in the trans-Aravalli sector in western India are presumed to span the time interval between Neoproterozoic and early Cambrian. This, predominantly unfossiliferous, marine sedimentary sequence is characterized by a lower arenaceous facies (Jodhpur Group), middle carbonate facies (Bilara Group) and upper argillaceous— arenaceous facies (Nagaur Group) rocks. The sedimentation has been essentially in a shallow basin, described either as the fore-land slope of the rising Aravalli mountains or a sag-basin which developed and evolved due to subsidence of the updomed crust during Neoproterozoic Malani magmatism that failed to open rifts. The carbon isotopic profile for the Bilara Group carbonate rocks in the lower part shows marked oscillations and broadly negative δ13C character with negative anomalies as low as

Petrology, isotope characteristics, and K-Ar ages of the Maranhão, northern Brazil, Mesozoic basalt province

Northern Brazil contains remnants of Mesozoic flood basalts and hypabyssal rocks that were apparently emplaced during tectonism related to opening of the Atlantic Ocean. Analyses and new K-Ar ages reveal that this ∼700x250 km Maranhão province (5°–8°S) has low-Ti basalts (∼1.1 wt% TiO2) in the western part that range about 160 to 190 Ma, and high-Ti basalts (3.4–4.4 wt% TiO2) in the eastern part about 115–122 Ma. Low-Ti basalt compositions are less evolved and have a smaller range, Mg# 62-56, than the high-Ti basalts, Mg# 44–33. General characteristics of the least evolved members of low- and high-Ti groups include, respectively, Zr 100 and 250 ppm, Sr 225 and 475 ppm, Ba 200 and 500 ppm, Nb 10 and 26 ppm, Y 29 and 36 ppm, La/Yb(n) 4.2 and 8.8, where La(n) is 30 and 90. Overall compositions resemble the low- and high-Ti basaltic rocks of the Mesozoic Serra Geral (Paraná) province in southern Brazil. The Maranhão low-Ti basalts have more radiogenic Sr and Pb and higher δ18O than the high-Ti basalts. Respectively, low- vs high-Ti: ɛSr26−54 vs 15−18; 206Pb/204Pb=18.25–.78 vs 18.22–.24; and δ18O 8.9–12.6 vs 6.5–8.6. Nd isotopes overlap: ɛNd−1.6 to −3.8 vs −2.1 to −3. Ages, compositions, and isotopes indicate that the low- and high-Ti groups had independent parentages from enriched subcontinental mantle. However, both groups can be modeled from one source composition if low-Ti basalt isotopes reflect crustal contamination, and if the parentages for each group were picritic liquids that represent either higher (for low-Ti) or lower (for high-Ti) percentages of melting of that single source. When comparing Pb isotopes of Maranhão and Serra Geral high-Ti basalts (uncontaminated) to evaluate the DUPAL anomaly, Maranhão has Pb Δ7/4=4.6–11, and Pb Δ8/4=72–87; Serra Geral has Pb Δ7/4=10–13, and Pb Δ8/4=95–125. The small difference is not enough to conform to DUPAL contours, and is inconsistent with large-scale isotopic heterogeneity of mantle beneath Brazil prior to rifting of South America from Africa. Maranhão low-Ti magmas probably relate to the opening of central North Atlantic, and high-Ti magmas to the opening of equatorial Atlantic. The proposed greater percentage of source melting for low-Ti basalts may reflect a Triassic-Jurassic hotspot, while lesser melting for high-Ti magmas may relate to Cretaceous decompressional (rifting) melting.

Common Crustal Source for Contrasting Peraluminous Facies in the Early Paleozoic Capillitas Batholith, NW Argentina

Abstract The Ordovician Capillitas batholith, part of the northern Pampean ranges, NW Argentina, exhibits two peraluminous granitic facies in its eastern portion: (a) coarse- to medium-grained, porphyritic mafic facies with biotite, cordierite and aluminosilicates, carrying sillimanite-, cordierite-, and andalusite-bearing migmatitic enclaves and schlieren and (b) enclave free, mica poor, coarse-grained, porphyritic felsic facies, with andalusite and sillimanite. Banded hornfels aureoles contain cordierite poikiloblasts, biotite, plagioclase and quartz. The low-P mineral assemblage in these granites, enclaves and restites, suggests partial fusion of a supracrustal protolith. The two facies plot as two separate groups in geochemical variation diagrams, suggesting that they evolved from different magma batches derived from the same source, rather than from in situ fractional crystallization. The composition of felsic facies granites corresponds to pelite and metagraywacke-derived melts, whereas cordierite-bearing mafic granites follow a trend indicating mixing of pelite-derived melts and corresponding restites. The mafic-facies granites approach more the continental crust composition than the felsic-facies ones, which display more pronounced Ba and Sr negative anomalies. The average (La/Yb) N ∼11 rules out a high pressure garnet-rich source and the low normalized Sr contents, in both granitic facies, suggest a recycled metasedimentary protolith. Absence of mafic intrusives that could have assimilated pelitic schists, allow us to infer that melting took place at rather low T and under high water activity. Heat to trigger partial fusion could have been radiogenically generated and stored in the upper crust during deformation and thickening of the continental crust, with further release during decompression. The Capillitas batholith, emplaced close to an I, S-type granite boundary line in this region appears to be an Argentinean analogue of the Cooma Series supersuite in the Lachlan Fold Belt, emplaced close to the eastern Australian I, S-type granite boundary line.