E. M. Piccirillo

Age of the Ponta Grossa dike swarm (Brazil), and implications to Parana´flood volcanism

The Ponta Grossa Dike Swarm (PGDS) occurs in a NW-trending, 200 km wide zone exposed just east of the Parana´basin in southeastern Brazil. The predominantly basaltic dikes intrude crystalline basement, Paleozoic-Mesozoic sediments, and (rarely) flows of the Parana´-Etendeka flood volcanic province (PEP). The PGDS resembles the failed arm of a rift-rift-rift triple junction, related to the separation of South America and Africa. Detailed geochemical studies of the dikes (including major/minor/trace element and Sr-Nd isotopic analyses) indicate that they probably represent feeders for the voluminous phase of flood volcanism, represented by relatively uncontaminated, predominantly high-TiO2 lavas of the northern PEP, where lava accumulations reach 1700 m thick.40Ar/39Ar stepwise degassing data, using both laser and radiofrequency induction furnace, on plagioclase separates from eighteen dikes and one sill yield seventeen plateau ages: three are between 120.7 ± 1.3 Ma and 125.8 ± 0.6 Ma, and fourteen are clustered between 129.2 ± 0.4 Ma and 131.4 ± 0.5 Ma. Isochron ages are not significantly different from the plateau ages, and plateau ages are adopted in all but two cases as being the most accurate age estimates. The age-probability distribution for the dominant pulse (131.4 ± 0.4 to 129.2 ± 0.4 Ma) shows a pronounced peak at 130.5 Ma; this distribution probably reflects the magma production history in the region. The new geochronologic data are consistent with conclusions based on paleomagnetic and chemical-stratigraphic data that the PGDS is younger than the volumetrically dominant majority of volcanism in the southern PEP, which occurred at 133-132 Ma. The younger (commonly NE-trending) dikes may reflect the initiation of full drift, which was coincident with major basin development in the adjacent continental borderland at 125-120 Ma. The PGDS may indeed represent the failed arm of a rift-rift-rift triple junction, but the triple junction did not coincide exactly in time or space with the site that would be inferred for plume impact.

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.

Intracrystalline Cation Distribution in Natural Clinopyroxenes of Tholeiitic, Transitional, and Alkaline Basaltic Rocks

Thermodynamic studies of Mueller (1962) and Thompson (1969) and the crystallographic work of Ghose (1965) gave new impetus to a rapidly developing branch of mineralogy. Orthopyroxenes were studied by Virgo and Hafner (1969) and Saxena and Ghose (1971) using Mossbauer technique. Site-occupancy data have been used in many mineralogical and petrological problems, e.g., to estimate the thermodynamic solution properties of pyroxenes and to understand the cooling history of rocks. A parallel development in the mineralogy of clinopyroxene did not take place, particularly because the Mossbauer technique proved unsatisfactory for calcic pyroxenes. Recognizing this, we undertook a detailed X-ray crystallographic study of the clinopyroxenes and succeeded to determine crystal-structural parameters for Fe-Mg site occupancies of the nonequivalent Ml and M2 sites. Until now there are no published site-occupancy data on a series of clinopyroxene of intermediate composition. This paper attempts to fill this important gap in the data on clinopyroxenes. Besides our concern for the crystal-chemical and thermodynamic aspects of such a study, we were also motivated by the possible use of pyroxene crystallographic parameters in the classification and characterization of magmatic rocks. With the advent of automation in X-ray crystallography, it is possible to study a number of crystals from one or more rock samples in a relatively short time and provide petrologically useful information.

Mantle source compositions of the Paraná Magmatic Province (southern Brazil): evidence from trace element and Sr–Nd–Pb isotope geochemistry

An integrated study based on incompatible trace elements and Sr–Nd–Pb isotopes is presented in order to assess the mantle sources involved in the genesis of the Parana Magmatic Province (PMP) tholeiites. Particular emphasis is given to 33 new Pb isotope and concentration data obtained in representative samples of low-TiO2 (LTiB) and high-TiO2 (HTiB) flood basalts that occur in the province. Results show important differences with respect to type and location of these rocks. The LTiB and HTiB from northern PMP exhibit very similar initial Pb isotope ratios (average LTiB vs HTiB: 206Pb/204Pbi=17.78±0.03 vs 17.65±0.02; 207Pb/204Pbi=15.53±0.01 vs 15.52±0.01; 208Pb/204Pbi=38.12±0.03 vs 38.05±0.04). The LTiB from southern PMP, with initial 87Sr/86Sri≤0.7060, show small variation in initial Pb isotope compositions (average 206Pb/204Pbi=18.20±0.07; 207Pb/204Pbi=15.61±0.01; 208Pb/204Pbi=38.32±0.10), which are highly enriched in radiogenic Pb in comparison to the northern PMP analogues. The HTiB from southern PMP have initial Pb isotope ratios (average 206Pb/204Pbi=17.45±0.09; 207Pb/204Pbi=15.50±0.01; 208Pb/204Pbi=37.89±0.03) slightly less radiogenic compared with the HTiB from northern PMP. The data cover a large range of isotope compositions, which are accompanied by systematic changes in incompatible trace element ratios and Sr–Nd isotopes, indicating contributions from different mantle sources. The remarkable chemical and isotope differences between PMP basalts, N-MORB and Tristan da Cunha least evolved volcanics indicate that these asthenospheric sources did not play a significant role in the basalt genesis, suggesting generation from the melting of heterogeneous lithospheric mantle sources. The close similarity between the radiogenic isotopes of the Cretaceous carbonatites that surround the PMP and those of the HTiB rock-types and the LTiB from the northern Parana suggests the involvement of the same mantle components in their genesis: a dominant EMI end member and a radiogenic isotope enriched component of EMII-type, as some phlogopite-peridotite mantle xenoliths (Japecanga) from the Alto Paranaiba Igneous Province. The latter component seems also to have an important role in the origin of the LTiB from the southern Parana, where the other end member is highly depleted in radiogenic lead similar to DMM.

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.

Peridotite clinopyroxene chemistry reflects mantle processes rather than continental versus oceanic settings

Abstract Comparison of 360 trace element analyses of clinopyroxenes from peridotites indicates that clinopyroxene composition cannot discriminate between suboceanic and subcontinental mantle. Clinopyroxenes from abyssal and fore-arc peridotites have the lowest incompatible element concentrations and record melting and basalt extraction as the dominant process. Clinopyroxenes from continental peridotite massifs partially overlap the composition of clinopyroxenes from abyssal peridotites and, in general, behave as a less depleted component in the chemical variation trends defined by the latter. In the peridotite massifs, subsolidus re-equilibration involving phase transitions (spinel to plagioclase facies) may cause significant element variations in clinopyroxene (e.g. Sr and Eu/Eu * decrease, REE, Ti increase). Metasomatic processes cause only comparatively minor trace element variations. Clinopyroxenes from mantle xenoliths in OIB and continental alkali basalts have the highest incompatible trace element concentrations and the largest compositional variations. They primarily record metasomatic enrichment processes, which are similar in suboceanic and subcontinental mantle. These processes induced the highest incompatible element enrichment in the clinopyroxenes from the most refractory peridotites, supporting the chromatographic nature of infiltration metasomatism. This enrichment, however, does not affect Ti, which is in the same concentration range in clinopyroxenes from both xenoliths and abyssal peridotites. The apparent Ti immobility may be due to several causes, such as reduced Ti solubility in hydrous fluids, fractionation of Ti-rich phases from percolating silicate melts, reaction with carbonatite melts formerly equilibrated with amphibole-peridotite. In general, clinopyroxene geochemistry does not allow a clear distinction between different metasomatic agents. The similarity between the geochemical characteristics of xenoliths from continental and oceanic environments supports previous results that the compositions of the pre-metasomatic lithosphere and of the asthenosphere, from where metasomatic agents derive, do not differ in the two environments.

Early Proterozoic dike swarms from western Uruguay: geochemistry, SrNd isotopes and petrogenesis

Abstract Early Proterozoic (1.86 ± 0.12 Ga), unmetamorphosed basic dikes intrude the Precambrian Rio de la Plata Craton of western Uruguay. The dikes define two swarms of tholeiitic andesitic basalt and andesite composition, respectively. Major- and trace-element geochemistry indicates that these two magma types are related via gabbro fractionation. The incompatible trace-element patterns favour a derivation of the parent melts from melting of a garnet peridotite source. The high LILE and LREE content is discussed in terms of crustal contamination of the melts or of the source, or of mantle metasomatism. Mixing calculations do not support important crustal contamination of the melts or of the source. Mantle metasomatism is a preferred process, which operated slightly before or during melting. An anorogenic ensialic environment of emplacement is inferred.