Marcia Ernesto

The age of parana flood volcanism, rifting of gondwanaland, and the jurassic-cretaceous boundary.

The Paran�-Etendeka flood volcanic event produced ∼1.5 x 106 cubic kilometers of volcanic rocks, ranging from basalts to rhyolites, before the separation of South America and Africa during the Cretaceous period. New 40Ar/39Ar data combined with earlier paleomagnetic results indicate that Paran� flood volcanism in southern Brazil began at 133 � 1 million years ago and lasted less than 1 million years. The implied mean eruption rate on the order of 1.5 cubic kilometers per year is consistent with a mantle plume origin for the event and is comparable to eruption rates determined for other well-documented continental flood volcanic events. Paran� flood volcanism occurred before the initiation of sea floor spreading in the South Atlantic and was probably precipitated by uplift and weakening of the lithosphere by the Tristan da Cunha plume. The Parana event postdates most current estimates for the age of the faunal mass extinction associated with the Jurassic-Cretaceous boundary.

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.

Paleomagnetism, geochemistry and dating of the North-eastern Paraná Magmatic Province: tectonic implications

Abstract Seventy sites of sills, flows and dikes from Northeastern Parana Magmatic Province (PMP), were submitted to paleomagnetic, chemical and radiometric analyses. The rocks are high in TiO 2 content, and similar in composition to the rocks from the northern region of PMP. The sills intrude mainly Paleozoic sediments, and can be subdivided into two domains; the northern being characterized by sills showing reversed polarities, and the southern essentially by sills of normal polarities. 40 Ar / 39 Ar dating of three distinct sills gave plateau ages (129.9 ± 0.1, 130.3 ± 0.1 and 131.9 ± 0.4 Ma) that are similar to surface-outcropping flows of the Northern Parana Basin, and the Ponta Grossa dikes. The new paleomagnetic data combined with existing data from the northern PMP allowed the calculation of a paleomagnetic pole at 71.4° E and 83.0° S ( N = 92; α 95 =2.4°; k = 39). This pole is in good agreement with poles for central and southern PMP, which are slightly older than the northern PMP, as well as for the contemporaneous Central Alkaline Province (Paraguay) on the western side of PMP. In contrast, the coeval pole for the Ponta Grossa dikes (eastern border of PMP), however, is slightly displaced from that group of poles, suggesting that dikes in that area may have undergone some tectonic tilting.

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).

Anisotropy of magnetic susceptibility in the Ponta Grossa dyke swarm (Brazil) and its relationship with magma flow direction

Abstract Measurements of anisotropy of magnetic susceptibility (AMS) in 95 mafic dykes (mainly tholeiites 10–200 m in width) from the Mesozoic Ponta Grossa swarm, Southern Brazil, revealed two main types of magnetic fabric. Type I fabric (plane K 1 - K 2 parallel to the dyke plane) represents magma flow within the dykes, whereas Type II (plane K 1 - K 3 parallel to the dyke plane) is compatible with a fabric pattern reflecting vertical compaction of the magma column. Fabric Type I dominates (51% of the dykes) within the swarm, whereas Type II (38% of the dykes) concentrates mainly in the western region where the dykes intrude sediments. Considering the dykes with Type I fabrics, it is concluded that 58% of the dykes were fed by horizontal or sub-horizontal ( K 1 inclinations less than 30°) magmatic flow, and 42% were fed by inclined to vertical ( K 1 inclinations more than 30°) magma flow. The latter are more frequent in the southeastern part of the swarm, suggesting a magma source close to this area, although there may have been other sources in other regions where dykes with inclined flow and distinct chemical characteristics are also found.

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.

Palaeomagnetism of the Mesozoic Serra Geral Formation, southern Brazil

Abstract Palaeomagnetic results from 20 volcanic sequences and 11 intrusive bodies (sills and dykes) of the Serra Geral Formation (Parana Basin) are reported in this paper. The sequences are widespread all over the basin, while sills and dykes (Ponta Grossa arch) come from the northeastern portion. Three mean palaeomagnetic poles were computed for the Serra Geral Formation, which account for a time interval of ∼ 15 Ma. Pole SG1 is located at 85°S,108°E ( α 95 = 1.1°, N = 18) and represents the main phase of the magmatic activity in the basin, with a mean age of ∼ 135 Ma. Pole SG2 is located at 82°S,38°E ( α 95 = 7.8°, N = 2) and represents a younger magmatic phase, with an associated age of ∼ 130 Ma. Pole SG3 (72° S ,37° E ; α 95 = 6.8°, N = 10) is the youngest pole. It is computed from the intrusive rocks and its age is assumed to be not younger than 118 Ma, the lower limit of the ‘Cretaceous normal magnetic interval’. These three poles describe a shifting path, which suggest that the South American platform moved ∼ 5° southwards and rotated ∼ 10° clockwise during the Lower Cretaceous, preceding the South Atlantic opening.

Paleomagnetism and 40Ar/39Ar dating of the early Cretaceous Florianópolis dike swarm (Santa Catarina Island), Southern Brazil

Abstract The Florianopolis (FL) basaltic dikes are well exposed on Santa Catarina Island, close to the present southeastern border of the Parana Magmatic Province in Southern Brazil, and their intrusion was related to the tectonic and magmatic evolution of rifting in the southern Atlantic. The dikes are vertical or subvertical, and cut the crystalline basement rocks, mainly Late Proterozoic granites. Most of the dikes exhibit NE trends, corresponding to structural trends of the crystalline basement. A few NW-trending dikes can also be observed, occasionally crosscutting the NE dikes. Samples from 73 dikes were collected for paleomagnetic work, and subjected to both alternating field and thermal demagnetization. Characteristic magnetizations of normal and reversed polarities interpreted as original thermoremanences are carried by Ti-poor magnetites showing remanent coercive forces in the range 18–45 mT. The corresponding paleomagnetic pole (FL pole) is located at 3.3°E, 89.1°S (N=65; α95=2.6°; k=47). Samples for 40 Ar/ 39 Ar dating were collected at the same paleomagnetic sites. Plagioclase samples from nine dikes were analyzed by stepwise heating; the apparent age spectra are strongly discordant due to excess 40 Ar contamination, but five samples yielded plateau ages ranging from 119.0±0.9 Ma to 128.3±0.5 Ma. One additional sample yielded an isochron age of 121.5±0.5 Ma. The inferred ages apparently define two modes at ∼119–122 Ma and ∼126–128 Ma, although the data set is probably insufficient to determine whether dike intrusion was continuous or episodic. The ages of 128–119 Ma are coeval with the final stages of the rifting at this latitude, suggesting that the emplacement of the FL dikes is related to extension of the continental crust just prior to the formation of oceanic crust. By comparing the FL pole with the existing paleomagnetic poles for the NW Ponta Grossa dike swarm (mainly 130.5 Ma) to the north, the Parana volcanics (mainly 133–132 Ma), and the Central Alkaline Province (127–130 Ma) on the western side of the Parana Province it is concluded that the differences in pole positions reflect plate displacements, and therefore the FL pole is probably dominated by the dikes with ages younger than 127 Ma.

An Early Cretaceous paleomagnetic pole from Ponta Grossa dikes (Brazil): Implications for the South American Mesozoic apparent polar wander path

Paleomagnetic results from the Early Cretaceous Ponta Grossa dike swarm are reported. These dikes crosscut sediments and basement rocks associated with the Ponta Grossa Arch, a tectonic feature in the eastern border of the Paleozoic-Mesozoic Parana Basin. These dikes are vertical or subvertical and trend mainly NW and NE. Samples from 127 dikes were submitted to both alternating field and thermal demagnetizations ; 121 of them yielded characteristic directions interpreted as original thermal remanent magnetizations. Both normal and reversed polarities as well as intermediate directions (6 dikes) are recorded. The paleomagnetic pole is located at 30.3°E, 82.4°S (N = 115 ; α 95 = 2.0° ; k = 43.8). This pole along with the available paleomagnetic poles for the Parana Basin volcanics (Serra Geral Formation), recalculated in this paper in the light of reported 40 Ar/ 39 Ar ages describe an apparent polar wander path segment corresponding to a clockwise rotation (∼6.9°) of the South American plate. This movement is compatible with the initial phase of the South Atlantic opening with rifting progressing from south to north. However, the calculated plate velocity is higher than the predicted velocities reported so far based on oceanic floor magnetic anomalies. The new data reported in this paper allowed the calculation of a mean Early Cretaceous pole (60.3°E, 83.8°S ; N = 7, α 95 = 2.9°) which can be discriminated from the Middle-Late Jurassic (191.3°E, 86.2°S ; N = 3, α 95 = 7.8°) and Late Cretaceous (346.5°E, 84.1°S ; N = 5, α 95 = 4.9°) mean poles for South America, indicating a more complex movement of the continent than the simple east-west drift generally proposed.