Koji Kawashita

A review of the geochronology of the Amazonian Craton: Tectonic implications

Abstract The available geochronological data for the Amazonian Craton permit delineation of its main age provinces and their respective tectonic development. Demarcation of the boundaries of each age province is primarily based on Rb-Sr whole-rock isochrons, supported by K-Ar determinations on minerals. Some U-Pb results on zircons, Pb-Pb whole-rock isochrons, and Sm-Nd model ages complement these data in key areas. At the center of the Amazonian Craton lies the Archean age, Central Amazonian Province bounded in its northern part by the Maroni-Itacaiunas mobile belt, which developed during early Proterozoic times. The western and southern margins of the Archean core are bordered by three tectonic provinces: (1) the Rio Negro-Juruena magmatic arc, (2) the Rondonian mobile belt, and (3) the Sunsas mobile belt. Each belt borders the western margin of the preceding belt, and each records a successively younger middle Proterozoic orogeny. Within the Central Amazonian Province, −2.7 Ga age determinations have been obtained from the Serra dos Carajas area. Although the Maroni-Itacaiunas belt includes several reworked Archean nuclei, rock units formed during the 2.2-1.9 Ga Trans-Amazonian Orogeny predominate. The isotopic evidence suggests that a mantle-derived component constituted a significant portion of the crust accreted at that time. The Rio Negro-Juruena Province is of Mid-Proterozoic age (1.75-1.50 Ga), and includes almost entirely mantle-derived material, juxtaposed with the pre-existent continental mass along a series of successive magmatic arcs. The Rondonian and Sunsas provinces both exhibit older sialic basements (>1.5 Ga). The former is characterized by important tectonomagmatic events in the 1.45-1.25 Ga interval, whereas the latter records tectonic reworking at 1.1-0.9 Ga. Finally, the Amazonian Craton is bordered to the east by the late Proterozoic Paraguai-Araguaia (or Araguaia-Tocantins) fold belt.

Potassium-argon dates of basaltic rocks from Southern Brazil

Abstract Potassium-argon ages are reported for 20 basalts (including a few core samples) and 15 diabases from the Parana basin of Southern Brazil. Histograms of both basalt and diabase ages exhibit strong peaks close to 120 m.y. Thus the principal Brazilian volcanism was Mid-Lower Cretaceous and not Jurassic as earlier supposed. While the few dates younger than 120 m.y. are likely due to argon loss, there is a real “tail” of higher ages which shows that there was precursory volcanic activity 147 m.y. ago, in the Upper Jurassic, and perhaps earlier. The span of at least 28 m.y. for the volcanism is similar to that which has been found for South Africa. The basaltic rocks of Brazil differ, however, both in age and potassium content from the older diabase rocks of Southeast Africa, Antarctica and Tasmania which have been dated thus far. Auxiliary studies included a careful evaluation of the analytical precision in our techniques and control experiments in dating series of spaced samples from narrow diabase dikes which represent examples of “instantaneous” cooling. The latter experiments show that whole-rock dating of diabase from Brazil is occasionally susceptible to discrepancies of up to 8 per cent even when fairly rigid thin-section criteria have been exercised in the selection of the samples for dating. An argon diffusion experiment on diabase material gave results characteristic of feldspar. Acid etching was found not to be effective in “rectifying” discordant feldspar separates. Auxiliary studies by electron microprobe showed that a diabase sample, in which only plagioclase feldspar would be noted in cursory visual examination, actually contains feldspars ranging from plagioclase through potash oligoclase and anorthoclase to sodic sanidine, where inconspicuous weathering of the last might be important in creating discrepancies of the type noted.

Potassium-argon ages of alkaline rocks from southern Brazil*

Abstract We present potassium-argon ages for 66 samples (micas, feldspars, fine-grained whole rocks and amphiboles in descending frequency) from 21 alkaline rock localities in southern Brazil. A general geological description is given for each locality. Analytical precision is about one per cent for potassium and two per cent or better for radiogenic argon determinations. It is possible to classify many samples as concordant (within 4 m.y.) or discordant with a preferred age for their localities. On this basis, 41 samples are found concordant and 8 samples, discordant. Four examples of anomalously “old” alkali feldspar were encountered, exhibiting a hitherto unreported reversal of the orthodox mica-feldspar discrepancy in K Ar dating. Pyroxene ages were erratic and have been discarded. Studies with the electron microprobe indicate that the potassium content of pyroxene is much lower than that often reported, the difference being contamination. With some exceptions, the alkaline occurrences fall into two age groups, one Early Cretaceous (122–133 m.y.) and the other Late Cretaceous to Early Tertiary (51–82 m.y.). The older group coincides, in age, with the basaltic vulcanism of the Parana Basin, but the younger group, to which belongs the major part of Brazilian alkaline occurrences, cannot be associated in age with these basalts. The intrusions with older ages are clustered in southern Sao Paulo State, and the younger group, with one exception, lies to the north and east of the older group. At some localities, like Pocos de Caldas, the alkaline magmatism apparently took place at several different stages. There is no close correlation in time between Brazilian and African alkaline intrusions the latter being generally older, but there seem to exist similar age patterns (including coincidence between the vulcanism and the oldest alkaline rocks) in both continents.

40Ar/39Ar geochronology at the Instituto de Geociências, USP: instrumentation, analytical procedures, and calibration

Laser heating Ar-40/Ar-39 geochronology provides high analytical precision and accuracy, mum-scale spatial resolution. and statistically significant data sets for the study of geological and planetary processes, A newly commissioned Ar-40/Ar-39 laboratory at CPGeo/USP, Sao Paulo, Brazil, equips the Brazilian scientific community with a new powerful tool applicable to the study of geological and cosmochemical processes. Detailed information about laboratory layout, environmental conditions, and instrumentation provides the necessary parameters for the evaluation of the CPGeo/USp Ar-40/Ar-39 suitability to a diverse range of applications. Details about analytical procedures, including mineral separation, irradiation at the IPEN/CNEN reactor at USP, and mass spectrometric analysis enable potential researchers to design the necessary sampling and sample preparation program suitable to the objectives of their study. Finally, the results of calibration tests using Ca and K salts and glasses, international mineral standards, and in-house mineral standards show that the accuracy and precision obtained at the Ar-40/Ar-39 laboratory at CPGeo/USP are comparable to results obtained in the most respected laboratories internationally. The extensive calibration and standardization procedures under-taken ensure that the results of analytical studies carried out in our laboratories will gain immediate international credibility, enabling Brazilian students and scientists to conduct forefront research in earth and planetary sciences.

PbPb, RbSr, and KAr systematics of the Lagoa Real uranium province (south-central Bahia, Brazil) and the Espinhaço cycle (ca. 1.5-1.0 Ga)

Abstract Geochronologic investigations using PbPb, RbSr, and KAr systems have been carried out on the granite-orthogneiss complex of Lagoa Real, Bahia — one of Brazils most important uranium provinces. RbSr whole rock data on the basement complex yielded an isochron age of about 2700 Ma. The age corresponds to the peak of the Jequie orogenic cycle, when the regional crustal province was probably consolidated. PbPb data for five undeformed examples of the Sao Timoteo granite and seven gneissic samples defined a well-fitted isochron with an age of 1710 ± 100 Ma. The same undeformed samples furnished a similar RbSr whole rock isochron age of 1710 ± 45 Ma. These ages agree with published UPb (zircon) dates, confirming that the intrusion age was around 1700–1720 Ma. The magmas were probably formed largely by melting of crustal components, as shown by ( 87 Sr/ 86 Sr) i and μ 1 values. In complete contrast to the PbPb system, the RbSr system was seriously affected by later events in the orthogneisses. A RbSr isochron apparent age of about 1500 Ma, obtained in one locality, is repeated in other isolated RbSr ages, and also by a UPb (zircon) age obtained in albitites within the region. Other RbSr apparent ages of ca . 1200 and ca . 1000 Ma were obtained, but their real geologic significance remains uncertain. KAr ages in separated minerals fall within the interval 570-500 Ma and show the influence of the Brasiliano tectonothermal cycle.

Radiochronological age and correlation of proterozoic sediments in Brazil

Abstract A review of available Rb—Sr and K—Ar datings obtained on sedimentary sequences, metamorphosed or not, interbedded volcanics and cross-cutting intrusives of the Precambrian of Brazil yields the following conclusions: 1. (1) The Roraima and Rio Fresco Formations, resting upon the Amazonian craton, have been affected by the Trans-Amazonian orogeny and are of Lower Proterozoic age. 2. (2) The Beneficiente Group, in the same region, seems to be of Middle Proterozoic, Lower Riphean age. 3. (3) Upon the Sao Francisco craton, and upon the Lower Proterozoic Rio dos Remedios complex, the Paraguacu Group might be of middle Riphean age and the glacial sequences, Macaubas Group and Bebedouro Formation, date back to between 950 and 1000 Ma. 4. (4) The age of the Bambui and Una Groups, in the same region, remains undetermined. It might be either Upper Riphean, or Upper Riphean and Vendian. 5. (5) The molassic series associated with the Brazilian orogeny are dated back to between the Proterozoic—Phanerozoic boundary and the Ordovician.

U-Pb zircon in situ dating with LA-MC-ICP-MS using a mixed detector configuration

The LA-MC-ICP-MS method applied to U-Pb in situ dating is still rapidly evolving due to improvements in both lasers and ICP-MS. To test the validity and reproducibility of the method, 5 different zircon samples, including the standard Temora-2, ranging in age between 2.2 Ga and 246 Ma, were dated using both LA-MC-ICP-MS and SHRIMP. The selected zircons were dated by SHRIMP and, after gentle polishing, the laser spot was driven to the same site or on the same zircon phase with a 213 nm laser microprobe coupled to a multi-collector mixed system. The data were collected with a routine spot size of 25 μm and, in some cases, of 15 and 40 μm. A careful cross-calibration using a diluted U-Th-Pb solution to calculate the Faraday reading to counting rate conversion factors and the highly suitable GJ-1 standard zircon for external calibrations were of paramount importance for obtaining reliable results. All age results were concordant within the experimental errors. The assigned age errors using the LA-MC-ICP-MS technique were, in most cases, higher than those obtained by SHRIMP, but if we are not faced with a high resolution stratigraphy, the laser technique has certain advantages.

Proterozoic crustal evolution at the southern margin of the amazonian craton in the state of mato grosso, Brazil: evidence from RbSr and K:Ar data

Abstract Geologic mapping on a 1:100,000 scale in the Sao Jose dos Quatro Marcos (SJQM) area, southwest of the state of Mato Grosso, central-western Brazil, showed a remarkable variety of Precambrian lithologic units. Predominant are grey tonalitic gneisses with a RbSr isochron age of 1971±770 Ma, an initial 87 Sr 86 Sr ratio of 0.7071, and a KAr biotite cooling age of 1500 Ma. The gneisses gneisses show interlayered “first-generation” amphibolite lenses and bands (KAr amphibole age of 1500 Ma), and some rare relicts of grey gneissic granulites, together with associated pink augen gneisses. Rare outcrops of deformed banded calc-silicate rocks are recrystallized by contact metamorphism. These units are intruded by a granitoid suite constituted by early, usually somewhat deformed tonalites and granodiorites, and late massive pink granodiorites and monzo- and syegogranites (RbSr isochron age=1472 ± 19 Ma, i.r.=0.7037). They show enclaves of grey gneisses and “second-generation” amphibolites. A large amphibolite body to the west is also considered part of this second-generation basic sequence. Scattered rhyolite outcrops to the east represent the southernmost remnants of the volcanic Rio Branco Group. The grey gneisses, here formally identified as the ”Sao Jose dos Quatro Marcos Gneiss“ and probably derived from Lower Proterozoic tonalites, were complexly deformed and recrystallized, at ∼ 2.0 Ga, into predominantly amphibolite-facies rocks. The first- and second-generation amphibolites document two distinct commingling episodes, separated by at least 0.5 Ga, each directly associated with the corresponding host-rock granitoid magmatism. The porphyritic granites were converted into pink augen gneisses before 1500 Ma and are thus not contemporaneous to the 1380 Ma syn-kinematic San Ignacio granitoids from eastern Bolivia. Calc-silicate rocks are interpreted as remnants of supracrustal sequences, perhaps related to the Quatro Meninas Group. The 1500 Ma granitoids, formally named the “Jaboti Intrusive Suite”, invaded under a syn- to late-kinematic regime, determining a unique thermal-magmatic event in this region.

KAr and RbSr geochronology og igneous rocks from the Sierra de Paiman, northwestern Argentina

Abstract Epizonal igneous and metamorphic rocks in northwestern Argentina are exposed in the Sierra de Paiman. The metamorphic rocks are quartzites, phyllites, and slates with soft-body impressions and fossil traces that suggest a late Precambrian-Cambrian age. The igneous rocks were intruded during two major magmatic events according to KAr and RbSr data. The older event is represented by different kinds of granitoids and gabbroids, intruded 437–459 Ma. These rocks were emplaced syntectonically in a shear zone that remained active after emplacement, causing extensive mylonitization on the east side of the range. The granitoids show Sr isotopic disturbances possibly related to magma mixing events. Petrologic, geochemical, and isotopic data for these rocks suggest a volcanic-arc setting, probably related to the back arc of the eastward-dipping continental arc of the Famatina Belt. During the younger event (ca. 379 Ma) stocks and dikes of leucogranites were emplaced post-tectonically with respect to the last episode of mylonitization. The leucogranites have syn-collisional signatures and may thus represent the culmination of the volcanic arc of the Famatina Belt.

Early Holocene ritual complexity in South America: the archaeological record of Lapa do Santo (east-central Brazil)

Abstract Early Archaic human skeletal remains found in a burial context in Lapa do Santo in east-central Brazil provide a rare glimpse into the lives of hunter-gatherer communities in South America, including their rituals for dealing with the dead. These included the reduction of the body by means of mutilation, defleshing, tooth removal, exposure to fire and possibly cannibalism, followed by the secondary burial of the remains according to strict rules. In a later period, pits were filled with disarticulated bones of a single individual without signs of body manipulation, demonstrating that the region was inhabited by dynamic groups in constant transformation over a period of centuries.