Valderez P. Ferreira

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

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.

Large-scale silicate liquid immiscibility: a possible example from northeastern Brazil

Abstract Ultrapotassic, peralkalic silica-saturated plutons (580 Ma) are widespread in the Cachoeirinha-Salgueiro foldbelt, northeastern Brazil. They consist of alkali-feldspar syenites with pyroxenite as co-magmatic inclusions and syn-plutonic or late-stage dikes. Pyroxenite and syenite have the same mineral phases (aegirine-augite, microcline, sphene, apatite, blue amphibole, magnetite), but only in different proportions. Rare inclusions of a “mixed” rock (about 60% syenite+40% pyroxenite in an emulsion-like texture) are also present. Pyroxenes in the three units are all only slightly zoned, silica-saturated and extremely low in Al2O3 (0.2–1.4%). Amphiboles are mostly K-rich richterite, characterized by high SiO2, low Al2O3 and TiO2 contents and low Mg#. The three rock types have similar REE chondrite-normalized patterns, with negative slopes and lack of Eu anomaly, with the total REE in the pyroxenite greater than that of the syenite. Trace element patterns for the mixed rock are intermediate between those for the pyroxenite and syenite. Major element partitioning between pyroxenite and syenite has the same sense as that one observed between immiscible liquids in volcanic lavas and trace element partitioning is similar to the experimentally determined partition of immiscible liquid pairs. The rocks have similar high δ18O values (avg. w.r.+ 8‰SMOW, corrected from pyroxene), high initial 87 Sr 86 Sr ratios (about 0.710), and low 143 Nd 144 Nd (avg. 0.51104). Field and geochemical characteristics indicate chemical equilibrium among the three rock types and suggest liquid immiscibility between syenite and pyroxenite, the mixed rock representing the original magma composition.

Chromium-isotope signatures in scleractinian corals from the Rocas Atoll, Tropical South Atlantic.

Chromium-isotope compositions (expressed as δ(53) Cr) of recent and ancient skeletal and non-skeletal carbonates are currently explored as a (paleo-) redox-proxy for shallow seawater. The idea behind this approach is that biogenic and non-biogenic carbonates could potentially be used as archives recording the Cr-isotope composition of seawater in which they formed, and with this contribute to the reconstruction of past paleo-environmental changes in the marine realm, and potentially to climate changes on land. However, investigations addressing the behavior and uptake mechanism of Cr, and the potential isotope fractionations between seawater and biogenic carbonates are scarce. Here, we present a study of Cr-isotope variations in three species of corals and contemporary seawater from the Rocas Atoll, NE, Brazil. Cr-isotope values of the studied coral species (Siderastrea stellata, Porites sp., and Montastrea cavernosa) vary from -0.5 to +0.33‰ and point to significant isotopic disequilibrium with coexisting seawater characterized by a Cr-isotope value of +0.92 ± 0.2‰. This isotopic offset requires reduction of hexavalent Cr(VI) in the sequestration process of all the studied coral species. Cr-isotope values in a profile across an S. stellata colony returned homogeneous, slightly positively fractioned δ(53) Cr values of +0.07 ± 0.08‰ (n = 8, 2σ), which we interpret to reflect a constant reductive uptake during the 20-year growth period recorded in this coral. In contrast, samples across a 12-year growth profile from Porites sp. display rather heterogeneous Cr-isotope values with δ(53) Cr varying from -0.50 to +0.10‰, indicating Cr incorporation under changing redox processes during its growth intervals. We propose a mechanism whereby initial photoreduction of isotopically heavy Cr(VI) to isotopically lighter Cr(III) in the endodermal layer of corals must be followed by efficient and effective re-oxidation of reduced Cr species to favor subsequent chromate (CrO42-) substitution during the calcifying processes ultimately leading to the formation of the coral skeleton.

Depleted and enriched mantle sources for Paleo- and Neoproterozoic carbonatites of southern India: Sr, Nd, C-O isotopic and geochemical constraints

Paleoproterozoic (Hogenakal) and Neoproterozoic (Samalpatti, Sevattur, Mulakkadu–Pakkanadu) carbonatites of Tamil Nadu, southern India, have been investigated for whole-rock geochemistry and Nd, Sr and C–O isotopes. These temporally distinct carbonatite complexes are located close to a tectonically active zone that marks the transition between cratonic noncharnockitic (low- to medium-grade) terrain to the north and the charnockitic mobile belt (granulite facies) to the south. The carbonatites are variably enriched in LREE; the Hogenakal carbonatites being extremely enriched, with the highest SREE among the data while the younger carbonatites show variable enrichment levels and broadly comparable REE patterns. The Hogenakal carbonatites have coherent and typically mantle C- and O-isotopic ratios (y 13 CV-PDBf6xand y 18 OVSMOW f8x). The Neoproterozoic carbonatites are relatively enriched in 13 C( y 13 CV-PDB=5.3xto 3.3x) and also show a range of O-isotopic composition (y 18 OV-SMOW=7.3xto 15.4x; two samples f25x). Higher y 18 O values indicate variable degree of post-magmatic low-temperature alteration. The older carbonatites have marginally positive qNd (+0.54 and +1) and significantly low initial 87 Sr/ 86 Sr ratios (0.70161 and 0.70174) while younger carbonatites have rather low qNd (16.5 to 6.23) and higher 87 Sr/ 86 Sr ratios (0.70486–0.70658). The Sr–Nd isotopic data are compatible with a depleted mantle source for the Hogenakal carbonatites and an EM-I-type enriched mantle component for the younger group. This is the first report of the existence of depleted mantle beneath the southern Indian continental crust. The stable isotopic ratios are interpreted as suggesting a depletion event (crustal extraction) in the south Indian subcontinental mantle f2.6 Ga ago. The depleted mantle was subsequently enriched by metasomatic fluids under the influence of the subducting Dharwar plate (sediments and modified oceanic crust). D 2002 Published by Elsevier Science B.V.

Carbon and Oxygen Isotope Compositions of Some Upper Cretaceous–Paleocene Sequences in Argentina and Chile

The Cretaceous-Paleocene (K-T) transition has been recorded in sedimentary carbonate rocks in northwestern Argentina and southern Chile. In the Yacoraite Basin, Argentina, this transition has been preserved in a 2 m thick marly layer, at the base of the Tunal Formation, which overlies lacustrine/marine carbonates of the Yacoraite Formation (Cabra Corral dam). The K-T transition is also preserved at Maimara, where Tertiary sandstones overlie a 50 m thick limestone bed of the Yacoraite Formation. In the Magellan Basin, Chile, glauconitic sandstones with calcitic cement and limestone concretions of the Maastrichtian Punta Rocallosa Formation are overlain by sandstones, claystones, and limestones of the Chorillo Chico Formation. The K-T transition is preserved in the lower portion of the Chorillo Chico Formation. Carbonates of the Yacoraite Formation display bulk-rock δ13C values from +1 to +2‰ PDB, with a negative incursion (−4‰ PDB) at the K-T transition. δ13C values in the Tunal Formation marls vary from −3 to −1‰ PDB. At Rocallosa Point, δ13C values in limestone strata, calcite cement, and limestone concretions vary from −4 to −33 ‰ PDB, and the lowest value in the Chorillo Chico Formation apparently marks the K-T transition. The δ18O fluctuations in the Yacoraite and Magellan carbonate rocks suggest a temperature drop at the K-T transition, followed by a temperature rise. High 87Sr/86Sr ratios (0.7140-0.7156) characterize the studied profiles of the Yacoraite Formation, documenting an important 87Sr-enriched source of Sr to the water from which these carbonates precipitated. At the Magellan basin, 87Sr/86Sr ratios are closer to the expected values for the global Late Cretaceous-Paleocene ocean.

C-isotope Composition of Carbonates from Indravati Basin, India: Implications for Regional Stratigraphic Correlation

Abstract The Indravati Basin represents an unmetamorphosed and unfossiliferous sequence and shows a broad similarity in lithological association with other Purana Basins of Central India. The carbonates belonging to Indravati Basin have on average, moderately positive δ13 C values (up to +4.5). Numerous successions reported from other parts of world, show similar δ13 C values and contain evidence to suggest that the latest Mesoproterozoic to early Neoproterozoic period may represent an isotopically recognizable interval globally. The elevated δ13 C values of Indravati Basin are also correlatable with other Purana Basins in India including Upper Vindhyan Supergroup.