G. Bellieni

Synchrony of the Central Atlantic magmatic province and the Triassic-Jurassic boundary climatic and biotic crisis

The evolution of life on Earth is marked by catastrophic extinction events, one of which occurred ca. 200 Ma at the transition from the Triassic Period to the Jurassic Period (Tr-J boundary), apparently contemporaneous with the eruption of the worlds largest known continental igneous province, the Central Atlantic magmatic province. The temporal relationship of the Tr-J boundary and the provinces volcanism is clarified by new multidisciplinary (stratigraphic, palynologic, geochronologic, paleomagnetic, geochemical) data that demonstrate that development of the Central Atlantic magmatic province straddled the Tr-J boundary and thus may have had a causal relationship with the climatic crisis and biotic turnover demarcating the boundary.

Paraná Magmatic Province-Tristan da Cunha plume system: fixed versus mobile plume, petrogenetic considerations and alternative heat sources

Abstract Paleomagnetic reconstructions demonstrate that the Tristan da Cunha (TC) plume, which is usually related to the genesis of the high- and low-Ti flood tholeiites of the Parana Magmatic Province (PMP), was located ∼1000 km south of the Parana Province at the time of the magma eruptions. Assuming plume mobility, and considering the low-velocity zone identified in the northern portion of the PMP as the TC ‘fossil’ plume (∼20° from the present TC position), the plume migrated southward from 133–132 (main volcanic phase) to 80 Ma at a rate of about 40 mm/yr. From 80 Ma to Present the plume remained virtually fixed, leaving a track (Walvis Ridge) compatible with the African plate movement. However, geochemical and Sr–Nd–Pb isotopic data do not support that the tholeiites from Walvis Ridge, Rio Grande Rise and Parana can result from mixing dominated by the TC plume and mid-ocean ridge basalt components. The similarity among the high-Ti basalts from Rio Grande Rise, part of Walvis Ridge (525A) and the Parana Province suggests that delaminated subcontinental lithospheric mantle must be considered in their genesis. Regional thermal anomalies in deep mantle mapped by geoid and seismic tomography data offer an alternative non-plume-related heat source for the generation of intracontinental magmatic provinces.

Continental Flood Volcanism From the Paraná Basin (Brazil)

In the past years study of the continental flood volcanism in the Parana basin had been mainly concentrated in the southern regions, where thick lava sequences are well exposed. Such research essentially concerned basic geology, petrography, radiometric dating and, subordinately, petrochemistry, isotope geochemistry and paleomagnetism (cf. Almeida, 1983; Amaral et al., 1966; Amaral and Crosta, 1983; Asmus and Baisch, 1983; Compston et al., 1968; Cordani et al., 1980; Creer et al., 1965; Ernesto, 1985; Ernesto et al., 1979; Fodor et al., 1985a, 1985b; Fulfaro et al., 1982; Halpern et al., 1974; Leinz et aL, 1966; McDougall and Ruegg, 1966; Mantovani et al., 1985a, 1985b; Marimon et al., 1983; Marques, 1983; Melfi, 1967; Minioli et al., 1971; Pacca and Hiodo, 1976; Pacca and Ernesto, 1982; Ruegg, 1976; Ruegg and Amaral, 1976; Sartori et al., 1975, 1982; Sartori and Gomes, 1980; Sartori and Filho, 1983; Souza, 1983).

Lower Cretaceous tholeiitic dyke swarms from the Ponta Grossa Arch (southeast Brazil): Petrology, Sr-Nd isotopes and genetic relationships with the Paraná flood volcanics

The Lower Cretaceous dykes of the Ponta Grossa Arch, the most important dyke swarms in Brazil, are associated with the flood basalts and rare acid flows of the northern Parana basin. The Ponta Grossa (PG) dykes are formed by two-pyroxene tholeiites and rare acid rocks. The basaltic dykes may be distinguished into two main groups: a dominant, high-TiO2 (> 2 wt.%; HTi) group and a subordinate, low-TiO2 (< 2 wt.%; LTi) group, characterized, for similar MgO content, by high and low incompatible-element contents, respectively. Most PG dykes do not show chemical and isotope evidence supporting important crustal contamination. PG dykes with (87Sr86Sr)0 < 0.7060 plot in the mantle array (ϵSr ≈ + 17 and ϵNd ≈ −3) and can be considered virtually uncontaminated. Intra-dyke chemical variations suggest that the normal or reverse differentiation trends may be related to pre-emplacement low-pressure differentiation processes. The important chemical differences between incompatible-element-poor and incompatible-element-rich basaltic dykes cannot be accounted for in terms of fractional crystallization, but are compatible with different melting degrees of a garnet peridotite mantle source, e.g. 9% and 20% melting for the HTi and LTi tholeiites, respectively. Field distribution of the magmatism, chemistry and Sr-Nd isotope compositions support that the PG dykes may be related to the flood volcanics of northern Parana, and not to those of the central and southern Parana. Paleomagnetic data indicate that PG dykes are younger than the Parana volcanics. All the results suggest that PG dykes were probably feeders of the stratoid volcanics erupted in northern Parana towards the continental margin and later eroded. Chemistry and isotopic data reveal that asthenospheric mantle components were not significant factors in the genesis of PG dykes. Dyke emplacement occurred during early phases of rifting and/or flexuring cutting flood volcanic suites.

Regional variations within the Paraná flood basalts (southern Brazil): Evidence for subcontinental mantle heterogeneity and crustal contamination

Abstract Continental flood volcanism of the Parana basin (Lower Cretaceous) is represented by two-pyroxene tholeiitic basalts (90 vol. %). The Northern Parana Province (NPP) is dominated by basalts high in TiO 2 and incompatible elements (HTiB), while the Southern Parana Province (SPP) is dominated by basalts low in TiO 2 and incompatible elements (LTiB). NPP basalts show relatively small variations of initial (120 Ma) 87 Sr 86 Sr ( R 0 ) and 143 Nd 144 Nd (Nd∗) ratios ( R 0 =0.7051−0.7062 and Nd ∗=0.5124−0.5125 , respectively) relative to those occurring in SPP ( R 0 =0.7046−0.7120 and Nd ∗=0.5122−0.5128 , respectively). The latter basalts show significant positive correlations between R 0 vs. SiO 2 , K 2 O, Rb and Ba, and negative correlations between R 0 vs. (Cr + Ni) and mg-value, believed to be due to crustal “granitic” contamination. In general, the effects of contamination are pronounced in SPP and tend to vanish towards NPP. δ 18 O-values range from +6.5 to +10.0‰ in the basalts from NPP and SPP and essentially reflect water-magma interactions. “Uncontaminated” LTiB and HTiB basalts from NPP are isotopically distinct from the “uncontaminated” analogues from SPP (LTiB) ( R 0 =0.7055 vs. 0.7046; Nd ∗=0.5124 vs. 0.5128, respectively). All chemical data consistently indicate distinct sources and a large-scale mantle heterogeneity. The NPP mantle source is expected to have relatively high content of “enriched” components, possibly related to small-volume melts and metasomatic fluids. Tentatively, the last stabilization age of the Parana mantle heterogeneity is 0.5–1.0 Ga old. The existence of high- and low-TiO 2 basalt suites in both Parana and Karoo provinces in Brazil and southern Africa, respectively, indicates a large-scale heterogeneity in the subcontinental mantle, and suggests that basalt generation occurred in the lithospheric mantle.

The Central Atlantic Magmatic Province (CAMP) in Brazil: Petrology, Geochemistry, 40Ar/39Ar Ages, Paleomagnetism and Geodynamic Implications

The CAMP tholeiitic magmatism in Brazil (mean 40 Ar/ 39 Ar age of 199.0±2.4 Ma) occurs on the continental margin to ca. 2,000 km into the South American platform, near the boundary between the ancient terrains of the Amazonia craton and Proterozoic/Brazilian-cycle related mobile belts. Geological evidence indicates that this magmatism was preceded, in Permo-Triassic times, by continental sedimentation, indicating a possible regional uplift. The Brazilian CAMP tholeiites are generally evolved and characterized by a low TiO 2 concentration (less than 2 wt%). The Cassipore dykes, which are usually high in TiO 2 (more than 2wt%) are an exception. The Cassipore low- and high-TiO 2 basalts are characterized by a positive Nb anomaly and Sr-Nd isotopes that are parallel to typical mantle array. Except for one sample, all the other Brazilian CAMP tholeiites that are low in TiO 2 , show Sr-Nd isotopes trending towards crustal components. The latter isotopic characteristics could be related to crustal recycling ancient (Middle-Late Proterozoic) subductions, and/or low-pressure crustal interaction. All the Brazilian CAMP tholeiites show a decoupling between their Sr-Nd isotopic composition and Rb/Sr and Sm/Nd values, suggesting mantle metasomatism, and/or subduction-related crustal interaction before mantle melting. Notably, the chemical data show that tholeiites from specific Brazilian regions are related to mantle sources that reflect compositional mantle heterogeneity, including the lower mantle of the lithospheric thermal boundary layer. In general, paleomagnetic poles for CAMP rocks from South America, Africa and North America match an age of ca. 200 Ma, but also show a distribution pattern trending to younger ages (e.g. 190 Ma), especially for the South American poles relative to the CAMP magmatism of the continental edge. The Brazilian CAMP magmatism cannot be easily explained through plume head (active) models, being instead consistent with mantle geodynamic processes where the unstable buoyancy of the Pangea supercontinent played an essential role to approach isostatic stabilization. Therefore, it is proposed that the Brazilian CAMP magmatism was related to hot upper mantle incubation under thick continental lithosphere, and to edge-driven convection between lithospheric domains with different thickness.

The Vedrette di Ries (rieserferner) plutonic complex: Petrological and geochemical data bearing on its genesis

Data on Co, Sc, Rb, Sr, Th, Zr, Ta, Hf and REE contents are reported for tonalites, granodiorites, granites and one diorite from the Alpine plutonic complex of Vedrette di Ries in the Eastern Alps. Co and Sc range respectively between 19.6-1.2 ppm and 32-3.3 ppm showing a good negative correlation with the Differentiation Index (D.I.). Rb values range between 61–197 ppm displaying a positive correlation with D.I. All the samples have variably fractionated light and heavy REE with LaN/SmN=1.7–5.55 and TbN/YbN=0.58–1.65 and no important europium anomalies. On the basis of the HREE fractionation two groups of rocks can be distinguished: one showing higher TbN/YbN values which range between 1.27–1.65 and are positively correlated with the D.I. and a second group with TbN/YbN values less than unity which decrease with the differentiation. The rocks with high TbN/YbN ratio also display a positive Sr versus D.I. variation whereas the samples with flat or upward concave HREE patterns (i.e. with TbN/YbN<1) define negative Sr versus D.I. trends. Th, Ta, Hf, Zr and LREE show an overall tendecy to increase and then to decrease with the differentiation, with a large scattering of values.The data obtained fit the hypothesis that the entire rock series under study is the product of a two-stage crystal/liquid fractionation process starting from one parent magma of tonalitic or dioritic composition. During the first stage, which occurred at high pressure, the separation of hornblende+garnet produced the liquids displaying the positve TbN/YbN and Sr versus D.I. correlation. These liquids during their rise through the crust would have undergone a second stage fractionation with separation of hornblende and plagioclase evolving toward low-TbN/YbN and Sr-poor composition.

Oligocene transitional tholeiitic magmatism in Northern turkana (Kenya): Comparison with the Coeval Ethiopian volcanism

Near Lokitaung in the northernmost part of the Turkana area, transitional tholeiitic basalts about 30 m.y. old overlie the Turkana grits. Coeval, essentially alkaline «basalts» outcrop in the neighbouring zone of Kalin. These rocks represent the southernmost occurrences of the great and extended volcanism which took place in Ethiopia and in part of the Arabian peninsula in Oligocene times.The ascription of the Oligocene Turkana basalts to the Ethiopian volcanic activity is supported by structural, volcanological, geochemical and mineralogical analogies with coeval lavas from N and S Ethiopia. In this respect, clinopyroxene composition resulted to be especially meaningful. In the course of the Oligocene volcanic activity, basic magmas changed their composition at a rate which varied from zone to zone. This rate is considered to be depended on whether the considered zones occupy a more or less peripheral position with respect to the Ethiopian dome.

The Anari and Tapirapuã Jurassic formations, western Brazil: paleomagnetism, geochemistry and geochronology

The Anari and Tapirapua formations are very similar from the point of view of paleomagnetic, geochronological and geochemical results. They date from around 197 Ma and the flows are mainly tholeiitic basalts with a low TiO2 and incompatible element content. The magnetic carriers in rocks from these two formations were highly oxidized titanomagnetites, maghemites and, probably, titanomaghemites. Paleomagnetic analysis has shown that the magnetizations are all normal in polarity and virtual geomagnetic poles obtained for both formations are indistinguishable at the 95% confidence level. The calculated mean for these poles is 250.3°E, 65.5°S (N = 15; A95 = 3.6°; KSC = 1578). This pole is compatible with Jurassic poles which have been determined for South America or transposed from Africa.

Potassic dyke swarm in the Sapucai Graben, eastern Paraguay: petrographical, mineralogical and geochemical outlines

Abstract The western side of the Parana Basin of Brazil extends to central Paraguay, where repeated and widespread magmatic activity developed from Lower Cretaceous to Oligocene, associated with late Mesozoic crustal extension trending NE-SW. In central Paraguay this trend is characterized by a zone of NW-SE normal faults which formed the Asuncion-Sapucai graben, up to 45 km wide and 200 km long, where alkaline rocks occur as volcanic domes, complexes, lava-flows and dykes. These rocks, 128 Ma aged, are dominantly potassic and ne-normative. A swarm of at least 200, mainley NW-SE trending, dykes occurs in the Sapucai region and seems to be formed by two main lineages: tephrite to phonolite (and peralkaline phonolite) and alkali basalt to trachyphonolite. They are characterized by ubiquitous diopside to ferrosalite, consistently yielding Al enrichment trends; common olivine, Fo 81-69 in tephrites and alkaline basalts, and up to Fo 65 in phonolites; zoned megacrysts of hastingsitic hornblende (core) to kaersutite (rim), associated with accessory groundmass pargasite in tephrites and phonotephrites; K-rich hastingsite and K-rich ferroan pargasite in the phonolites. Accessory groundmass mica falls in the annite-phlogopite range, and consistently yields insufficient (Si + Al) to satisfy the expected T site occupancy of 8.00 a.f.u. FeTi oxides are Ti-magnetite, rarely ilmenite or haematite. Phenocrystal, i.e. xenocrystal plagioclase is An 70-20 , and An 74-42 in the tephrites and phonolites, respectively; coexisting groundmass microlites are An 22-14 , associated with sodasanidine and sanidine. Feldspathoids include analcimized leucite and nepheline; accessories Ti-andradite and sphene. The two main lineages, recognized by distinctive mineralogical variations, are consistent with the petrochemical variations. Complex interaction of discrete and independently evolving magma batches are indicated by intra- and/or interphase chemical variations, suggesting multiple equilibrations of the crystallizing phases under shallow level, volcanic pressure regime. The observed geochemical trends are quite similar to those of “Roman Region type magma” with the same negative anomalies of Ta, Nb, Zr and Ti. The most likely mantle source is a garnet-peridotite characterized by different enrichment in incompatible elements and which suffered low degree of partial melting (3–7%), which has geochemical and isotopic features distinct from those of the adjoining tholeiitic basalts (130 Ma) and nephelinites (61-39 Ma). The similarities of the Sapucai dyke suite with Bartons “Roman Region type magma” supports the view that this magma type may not be formed as a result of orogenic and/or subduction-driven activity in this region. Therefore, a causal relationship of the latter activity with “Roman Region type magma” is not supported and remains questionable.