Horstpeter H.G.J. Ulbrich

Late Proterozoic granitoid magmatism in the state of São Paulo, southeastern Brazil

Abstract The Precambrian basement in the state of Sao Paulo is made up of a series of metamorphic units exposed at various erosional levels and in part separated by large faults. A strong overprint of the Brasiliano cycle (700-500 Ma) is evident from isotopic data over most of the region; nevertheless, a large part of the metamorphic belts belong to older (Archaean to Middle Proterozoic) reworked sequences. About 30% of the total basement area consists of intrusive granitoids of Late Proterozoic age. These can be grouped into six major groups, on the basis of petrography, geochemistry, rock association, associated manifestations and (whenever possible) age relationships. Gneissic mangerite-charnockite-hastingsite granite associations appear as folded tabular bodies within high-grade regions. The Sao Jose do Rio Pardo suite is chemically comparable to the “aluminous A-type” granitoid series, and its emplacement possibly signals a period of extensional tectonics prior to the onset of the Brasiliano cycle (∼1000 Ma?). A series of elongated batholiths (∼650 Ma) with predominant porphyritic hornblende-biotite granitoids is present mainly in high-grade areas. The compositions are generally granitic to granodioritic, but intermediate (e.g. monzodioritic) rocks are locally common. These high-K calc-alkaline suites are in some aspects comparable to the Cordilleran I-type granitoids, and their origin might be related to a subductional environment. Migmatitic granitoids constitute a peculiar association, occurring as a series of small bodies within high-grade allochthonous domains. These polyphase, heterogeneous, often nebulitic granitoids are source-related products of crustal anatexis. Syn- to late-orogenic Brasiliano granitoids in the low- to medium-grade domains are dominantly porphyritic calc-alkaline biotite granites and granodiorites, reminiscent of Caledonian I-types. In contrast, a peraluminous association (porphyritic biotite granitoid to equigranular two-mica granite) predominates in high-grade domains, which possibly reflects a different tectonic evolution for the two regions. Late- to post-orogenic Brasiliano granitoids intruded after the major orogenic development of the region (∼610-590 Ma). I-type calc-alkaline and aluminous A-type granitoids are geographically associated in the Itu belt, and appear to show some temporal and geochemical (?) overlapping. Wiborgitic facies and some Sn-W mineralization are locally present in the A-type suites. A post-orogenic extensional period is marked in the south of the area by the emplacement of shallow A-type granitoids in the Serra do Mar belt (

Iron(II) oxide determination in rocks and minerals

Abstract The determination of FeO of geologic materials by modern instrumental methods (such as atomic absorption spectroscopy (AAS), inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray fluorescence (XRF), etc.) cannot distinguish between different oxidation states of elements. In many cases, the oxidation state of Fe has to be known in order to perform several chemical calculations (norms, etc.) and discuss the reactions that occur during weathering, hydrothermal alteration and other processes. A modified Wilson method is proposed, giving reproducible results in a much shorter time than the classical method. Back-titration with potassium dichromate and an Fe(II) and ammonia sulphate solution is used, after dissolution of the sample powder in a heated HF/H 3 PO 4 mixture and an ammonium vanadate solution. This modified method, tested with several international reference materials, gives reliable results, equivalent to the ones cited in the literature for the reference materials.

Adaptation of ICP–OES routine determination techniques for the analysis of rare earth elements by chromatographic separation in geologic materials: tests with reference materials and granitic rocks

Abstract The present contribution shows the results of tests on geological materials for determination of the rare-earth elements La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, as well as Y and Sc. The samples were prepared using lithium borate fusion of powdered rock sample followed by dissolution in hydrochloric acid. A separation was performed in cationic exchange columns with resin by hydrochloric and nitric acid gradient concentration and subsequent analysis using inductively coupled plasma atomic emission spectrometry (ICP–OES) and an ultrasonic nebulizer. All potentially interfering elements, except Ba and Zr, were eliminated using this procedure. Spectral interferences generated by Zr and the REE among themselves (Ce on Gd, Nd on Sm–Ce–Er, Gd on Sm, Pr on La–Ce–Nd–Ho–Tb, Tb on Ho, Sm on Tb) were identified and corrected quantitatively. The whole procedure was tested with international geological standard reference materials AC-E and JG-3 (granitic rocks), JB-1a (basalt), JA-2 (andesite), JR-2 (rhyolite) and JGb-1 (gabbro). A Student’s statistical test showed that the obtained analytical results were mostly satisfactory. The methodology, now performed on a routine basis, was applied for the determination of the REE patterns of assorted granitic rocks occurring in the state of Sao Paulo, southeastern Brazil.

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.

The volcanic-subvolcanic rocks of the fernando de noronha archipelago, southern atlantic ocean: Mineral chemistry

Fernando de Noronha archipelago presents an older Remédios Formation with subvolcanic intrusions, belonging to two different alkaline series, the sodic (undersaturated: basanites, tephrites, essexites, tephriphonolites, phonolites), and potassic ones (mildly undersaturated to silicic, with alkali basalts, basaltic trachyandesites, trachyandesites, trachytes), and lamprophyres. The upper Quixaba Formation presents nephelinite flows and basanites. A third minor unit, São José, is constituted by basanites carrying mantle xenoliths. Magnesian olivines occur in the Remédios basanites and alkali basalts, and in nephelinites. Melilites are present as groundmass grains in melilite melanephelinites (MEM). Clinopyroxenes (cpx) are mostly salites to titaniferous salites (Remédios sodic series), grading into aegirines in the differentiated aphyric phonolites. Cpx in the lamprophyres show disequilibrium textures. In the Quixaba flows, cpx are salites, enriched in Mg (especially in MEM). Amphiboles, remarkably, are common in tephriphonolites and phonolites and in basaltic trachyandesites, sometimes with disequilibrum zoning textures, and a conspicuous phase in lamprophyres. Dark micas are present as groundmass plates in MEM, OLM and PYM (olivine and pyroxene melanephelinites), with compositional variety (enriched in Ti, Ba, Sr) depending on the composition of the parent rock; BaO can be as high as 16–19%. Feldspars crystallize as calcic plagioclases, sanidines and anorthoclases, depending on the rock types, as phenocrysts and in groundmass, both in Quixaba and Remédios rocks; they are absent in nephelinites. Nephelines are found in Remédios sodic series types and Quixaba rocks. Haüyne and noseane are rarely observed in Remédios rocks.

The Aerogeophysical Project Serra do Mar Sul in the region of the Cunhaporanga Granitic Complex, Paraná, Southern Brazil: analysis of the gamma-ray spectrometric survey

This paper analyses sprectrometric data of the Aerogeophysical Project Serra do Mar Sul (APSMS) in the Meso to Neoproterozoic basement of eastern Parana state, southern Brazil. Three main lithostratigraphic units are found, elongated N30-40E. The easternmost is the Tres Corregos Granitic Complex (TCGC), constituted by calc-alkaline granites and their mega-enclaves. The central unit is the low-grade metamorphic Itaiacoca Group. The western strip is occupied by the mostly calc-alkaline Cunhaporanga Granitic Complex (CGC), also with large enclaves. Further west, the molassic volcano-sedimentary Eopaleozoic Castro Group, deposited in a graben, overlies unconformably the CGC. The Devonian sandstone Furnas Formation, to the west, is the main unit of the Parana Basin in this region. The Cretaceous Banhadao alkaline massif (8 km2) intrudes the TCGC. The heavily weathered central-northern area of the CGC is in part covered by the alluvial deposits of the Iapo drainage basin. Data were corrected with a cosine directional filter (degree 6), and presented as cps (counts per second). U and K give answers that may be spurious, not always correlated with the geology. Most CGC intrusions present rather high Th-U, lower total (TC) and very low K counts, as opposed to the values found in the TCGC. High TC are found in some of the more evolved plutons within the CGC (e.g., the A-type Serra do Carambei Granite). The TCGC granites can be divided into two radiometric domains, one adjacent to the Itaiacoca belt and another to the E-SE (visible in TC and K counts), corresponding to the Sao Sebastiao Granite. The mega-enclaves show lower counts in all channels than the surrounding granites. The Itaiacoca belt is marked in TC and U-Th maps by low to medium counts. The Castro Group shows a complex pattern controlled by lithology. The Furnas sediments show low to medium responses (TC map). The Iapo river deposits stand out clearly by very low counts (TC, U, Th) against the CGC rocks.

Melanite-bearing nepheline syenite fragments and 40Ar/39Ar age of phlogopite megacrysts in conduit breccia from the Poços de Caldas Alkaline Massif (MG/SP), and implications

The Pocos de Caldas Alkaline Massif, located in southeast Brazil, is composed of tinguaites, as well as phonolites and nepheline syenites (NeS), together with assorted pyroclastic rocks. The latter constitute deposits within the Vale do Quartel, and show fragments of alkaline rocks (NeS, tinguaites), basaltic and ultramafic volcanics/subvolvanics, sandstones, and quartz-feldspathic rocks from the crystalline basement. NeS fragments with melanite, previously unknown, and cm-sized fragments of phlogopite megacrysts from related conduit breccia were chosen for detailed studies. The NeS fragments are medium-to coarse grained and contain K-feldspar (Or 95-83), secondary albite (Ab 97), altered nepheline, clinopyroxene (diopside, minor hedenbergite) and partially altered, zoned, idiomorphic melanite. These NeS fragments are typically metaluminous, and point to the existence of similar intrusions hidden at depth, an important input to the massifs petrology. The phlogopite megacrysts are compositionally homogeneous, with high Ti, Mg and Al contents; Ba and F are minor components. Two phlogopite aliquots were dated and yielded identical Ar-40/Ar-39 plateau ages of 87.1(+/- 0.5) Ma and 86.6(+/- 0.5) Ma, which represent minimum and maximum values for phlogopite crystallization and breccia emplacement, respectively. Previously obtained ages for the Pocos de Caldas rocks are concentrated in the 75-86 Ma range interval.