Proterozoic Orogeny Research Articles

Long-lived (>100 m.y.) postcollisional exhumation and cooling in the Paleoproterozoic Trans−North China orogen: Evidence from phase equilibria modeling and monazite petrochronology of granulite-facies metapelites in the Fuping Complex

Long-lived collisional orogens that formed over tens to hundreds of millions of years are common in the geologic record. The Trans−North China orogen marks the collision between the Eastern and Western blocks of the North China craton, and it preserves metamorphic rocks with ages between 1.98 Ga and 1.80 Ga. These units allow detailed assessment of the time scale and duration of crustal thickening, exhumation, and cooling associated with a major Proterozoic orogeny. In this study, we present integrated petrography, mineral chemistry, phase equilibria modeling, and texturally controlled in situ mass spectrometry of monazite U-Th-Pb and trace-element analyses performed on a suite of orthopyroxene-bearing pelitic granulites and garnet-biotite gneisses from the Fuping Complex within the Trans−North China orogen. These rocks record clockwise pressure-temperature (P-T) paths involving granulite-facies peak conditions of 9.9−11.0 kbar and 850−880 °C for pelitic granulites, and 10.9−11.6 kbar and 860−880 °C for garnet-biotite gneisses, followed by postpeak decompression to ∼8−9 kbar and later cooling, with final solidification of melt at <840 °C. Four monazite populations were identified in these samples. Group I grains are irregular and elongated and occur in contact with or embay garnet. They have high rare earth element (REE) and Y contents and metamorphic ages of 1.90−1.86 Ga, which correspond to the breakdown of garnet during postpeak decompression. Most monazite grains crystallized from melt are represented by groups II + III + IV and are associated with orthopyroxene, biotite, plagioclase, and quartz in the matrix. They have crystallization ages between 1.86 Ga and 1.76 Ga and relatively low REE and Y concentrations. These data imply a long-lived (>100 m.y.) postcollisional exhumation and cooling involving decompression from 10−12 kbar to ∼9 kbar during 1.90−1.86 Ga, followed by retrograde cooling from 1.86 to 1.76 Ga under prolonged residence in the middle to lower crust. Initial collision and peak metamorphism occurred before 1.90 Ga, ultimately leading to the final cratonization of the North China craton and its incorporation into the Columbia supercontinent.

High-pressure metamorphic rocks in the Borborema Province, Northeast Brazil: Reworking of Archean oceanic crust during proterozoic orogenies

We present the first evidence of Archean oceanic crust submitted to Proterozoic high-pressure (HP) metamorphism in the South American Platform. Sm–Nd and Lu–Hf isotopic data combined with U–Pb geochronological data from the Campo Grande area, Rio Grande do Norte domain, in the Northern Borborema Province, reflect a complex Archean (2.9 ​Ga and 2.6 ​Ga) and Paleoproterozoic (2.0 ​Ga) evolution, culminating in the Neoproterozoic Brasiliano/Pan-African orogeny (ca. 600 Ma). The preserved mafic rocks contain massive poikiloblastic garnet and granoblastic amphibole with variable proportions of plagioclase ​+ ​diopside in symplectitic texture, typical of high-pressure rocks. These clinopyroxene-garnet amphibolites and the more common garnet amphibolites from the Campo Grande area are exposed as rare lenses within an Archean migmatite complex. The amphibolite lenses represent 2.65 ​Ga juvenile tholeiitic magmatism derived from depleted mantle sources (positive εHf(t) values of +3.81 to +30.66) later enriched by mantle metasomatism (negative εNd(t) values of –7.97). Chondrite and Primitive Mantle-normalized REE of analyzed samples and discriminant diagrams define two different oceanic affinities, with E-MORB and OIB signature. Negative Eu anomalies (Eu/Eu∗ ​= ​0.75–0.95) indicate depletion of plagioclase in the source. Inherited zircon cores of 3.0–2.9 ​Ga in analyzed samples indicate that the Neoarchean tholeiitic magmatism was emplaced into 2923 ​± ​14 ​Ma old Mesoarchean crust (εNd(t) ​= ​–2.58 and Nd TDM ​= ​3.2 ​Ga) of the Rio Grande do Norte domain. The age of retro-eclogite facies metamorphism is not yet completely understood. We suggest that two high-grade metamorphic events are recognized in the mafic rocks: the first at 2.0 ​Ga, recorded in some samples, and the second, at ca. 600 Ma, stronger and more pervasive and recorded in several of the mafic rock samples. The Neoproterozoic zircon grains are found in symplectite texture as inclusions in the garnet grains and represent the age of HP conditions in the area. These zircon grains show a younger cluster of concordant analyses between 623 ​± ​3 ​Ma and 592 ​± ​5 ​Ma with εHf(t) values of +0.74 to –65.88. Thus, the Campo Grande rock assemblage is composed of Archean units that were amalgamated to West Gondwana during Neoproterozoic Brasiliano orogeny continent-continent collision and crustal reworking.

940 Ma Anatexis in 1726 Ma Orthogneiss in the Northern Margin of the Bhilwara Belt and Significance for the Precambrian Evolution in Northwest India

The northwestern domains of India record Proterozoic orogenies that reflect global cycles of convergence and extension. A garnet-biotite migmatitic orthogneiss hosted within the Agucha-Kekri Shear Zone sandwiched between the Bhilwara Belt and the North Delhi Fold Belt (NDFB) has two zircon populations yielding U-Pb ages of 1726 and 938 Ma. The older age is correlated with the intrusion of the migmatite protolith, consistent with the partial melting event recorded in gneisses in the southern margin of the Bhilwara Belt. The younger age is interpreted as the age of partial melting and migmatization. Petrographical observations and pressure-temperature (PT) pseudosection analyses indicate incongruent melting of biotite and plagioclase in the gneiss-produced garnet, potash feldspar, and melt under water-fluxed conditions. The peak conditions of ~9 kbar and ≥700°C estimated for the partial melting are similar to those of coeval migmatization recorded at the northwestern margin of the Bhilwara Belt, but lower than those in the adjacent NDFB. This is interpreted to indicate formation of a migmatitic front along the northwestern margin of the Bhilwara Belt while this was being underthrust under the NDFB. Migmatization under similar PT conditions and, at the same time, estimated for the Central Indian Tectonic Zone implies the presence of several loci of crustal amalgamation leading to the final architecture of peninsular India during Rodinia formation.

Three-dimensional interpretation of tectono-sedimentary evolution and hydrocarbon prospectivity by the integration of airborne gravity gradiometer, regional gravity, magnetic, and two-dimensional seismic data in the Canning Basin, Western Australia

The Canning Basin is a largely unexposed and underexplored frontier basin, formed mostly in the Paleozoic. Geological knowledge of this basin is based predominantly on sparse regional “vintage” two-dimensional seismic and small three-dimensional (3-D) seismic surveys and less than 230 exploration wells. Following seismic interpretation, an integrated interpretation was completed on airborne gravity gradiometer (AGG), magnetic, seismic, well, and complementary data along the southwestern margin of the Fitzroy trough and Gregory subbasin. Seismic data were reinterpreted using AGG data to produce a better constrained geological model. A basement structure map, two intrasedimentary structure maps, and a formation distribution map were produced. The interpretation of seismic profiles, validated through 2.5-dimensional gravity gradiometer modeling, is essential to this workflow. Repeatedly reactivated west–northwest and northwest structural trends, inherited from Proterozoic orogenies, respectively delineate the Fitzroy trough and the Gregory subbasin with its northwestern structural extension into the Fitzroy trough, the Gregory subbasin trend. Subsidence occurred during two periods of extension. An asymmetric extensional system of the Fitzroy trough controlled Ordovician–Silurian deposition of the Carribuddy Group. Devonian–Carboniferous subsidence defines the Gregory subbasin trend. This Pillara extension reactivated structures in the east of the Fitzroy trough. Simultaneous activity of both extensional fault systems and growth faulting controlled the facies and thickness distribution of carbonates and clastics of the early Carboniferous Fairfield Group. The Meda and Fitzroy transpressional phases inverted faults of the Gregory subbasin trend and Fitzroy trough, producing prospects by structural interference. The improved understanding of tectono-stratigraphic relationships, including the 3-D distribution of carbonate reservoirs, benefited the planning of seismic surveys, prospect evaluation, drilling, and acreage relinquishment.

Monazite ages and pressure–temperature–time paths from anatectites in the southern Ruby Range, Montana, USA: evidence for delamination, ultramafic magmatism, and rapid uplift at ca. 1780 Ma

The Wolf Creek ultramafic bodies of the southern Ruby Range, southwest Montana, intruded as high-temperature magma during the late stages of an Early Proterozoic orogeny affecting the Great Falls Tectonic Zone of the western Wyoming craton. Field and petrologic evidence indicate high-temperature metamorphism and deformation uniquely affected metapelitic anatectites adjacent to the ultramafic bodies. Leucosomes are mesoperthite and quartz with garnet. Restite contains sillimanite, garnet, quartz, microcline, and biotite that was partially resorbed by the melt-producing reaction. Late cordierite and sillimanite replace garnet in the migmatites and other pelitic gneiss in the area. Petrogenic grids for the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (NCKFMASH) system indicate that conditions during anatexis were 750–800 °C, 600–900 MPa. Anatexis occurred during deformation of metapelites and associated rocks adjacent to the larger ultramafic bodies, but crystallization and cooling occurred after deformation, as indicated by the preservation of igneous textures and mesoperthite with undeformed lamellae in the leucosome. Element mapping of monazite using electron-probe micro-analysis (EPMA) identifies four distinct chemical domains of monazite growth during the orogenic event in addition to relict, Archaean to earliest Proterozoic cores. U–Th–Pb dating of the domains establishes two periods of growth of matrix monazite in the paleosome that indicate fabric-forming prograde metamorphism at 1829 ± 17 and 1805 ± 4 Ma (2σ). Coarser, igneous monazites from the leucosome also contain two distinct chemical zones visible in yttrium maps, indicating growth with garnet and then during garnet resorption as decompression led to replacement of garnet by cordierite. The domain ages cannot be statistically distinguished. Together they yield an age of 1782 ± 9 Ma (2σ). The data imply that regional thickening leading to upper amphibolite facies metamorphism occurred prior to 1800 Ma. Thickening was followed after ∼25 Ma by the intrusion of ultramafic magmas, contact anatexis, and then decompression. This sequence is consistent with mantle delamination after orogenic thickening.

Spatio-temporal evolution of the Satpura Mountain Belt of India: A comparison with the Capricorn Orogen of Western Australia and implication for evolution of the supercontinent Columbia

Reconstruction of the Neoproterozoic supercontinent Rodinia shows near neighbour positions of the South Indian Cratons and Western Australian Cratons. These cratonic areas are characterized by extensive Paleoproterozoic tectonism. Detailed analysis of the spatio-temporal data of the Satpura Mountains of India indicates presence of at least three episodes of Proterozoic orogeny at ∼2100–1900Ma, ∼1850Ma and ∼1650Ma, and associated basin development and closing. A subdued imprint of the Grenville orogeny (∼950Ma) is also found in rock records of this Mountain Belt. The Capricorn Orogen of Western Australia also shows three episodes of orogeny: Opthalmian–Glenburgh Orogeny (2100–1950Ma), Capricorn Orogeny (∼1800Ma) and Mangaroon Orogeny (∼1650Ma), and basin opening and closing related to these tectonic movements. These broad similarities suggest their joint evolution possibly in a near neighbour position during Paleoproterozoic Era. In view of juxtaposition of the Western Australia along the east coast of India, at the position of the Eastern Ghats, during Archean, it is suggested that the breaking of this Archean megacraton at ∼2400Ma led to northward movement of the broken components and formation of the Satpura–Capricorn Orogen (at ∼2100 and ∼1800Ma) due to the collision of cratonic blocks with the pre-existing northern cratonic nuclei of India and Western Australia. This is also the time of formation of the supercontinent Columbia. A phase of basin opening followed the ∼1800Ma event, followed by another phase of collisional event at ∼1600Ma at the site of the Satpura–Capricorn Orogen. Subsequent evolutions of the Satpura and the Capricorn Orogens differ slightly, indicating separate evolutional history.

EVOLUTION OF THE TALC-CARBONATE ROCKS IN UMM RILAN OPHIOLITE, SOUTH EASTERN DESERT, EGYPT: IMPLICATION FROM MINERALOGY, PETROGRAPHY, GEOCHEMISTRY AND P-T CONDITIONS.

Umm Rilan area is made up of talc-carbonate and talc serpentinite rocks of ophiolitic melange, metavolcanics and granitoid rocks. The derivatives of ultramafic rocks as a result of metamorphism and the neighboring massive metavolcanic of the Late Proterozoic orogeny are related to a dismembered Pan-African ophiolitic melange. In ultramafic rocks talc occurs as lenticular veins and along shear zone planes. Talc-carbonate rocks are whitish green, massive, slightly deformed, very fine- to fine-grained and composed of talc with carbonates as well as iron and manganese oxides as accessories. The talc carbonate rocks consist essentially of talc, malachite, dolomite, tremolite and magnesite. Geochemically, these rocks exhibit mafic-ultramafic cumulate ophiolite rocks of metamorphic peridotite and dunite in composition. The different variation diagrams indicate that these rocks are originated from cumulative komatiite with tendency to be tholeiitic magma and revealed that these rocks are formed in low Ti-ophiolites of tholeiitic environment. The scanning electron microscopic analysis detected that, the presence of Ta, Zn and Fe minerals in association with Cu, Ca and Mg mineralization. The contents of Cu, Zn and Pb show high tendency to be concentrated in the hydrothermally altered products associated with the sulfide minerals. The studied talc-carbonate rocks suffered low grade metamorphism in greenschist facies and was formed under medium pressure and low temperature of low Al2O3/CaO ratio related to the ultramafic rocks. Umm Rilan talc-carbonates are of hydrothermal origin. Tectonic activity plays a major role in the formation of Umm Rilan talc deposits by allowing fluids to penetrate the ultramafic rocks, creating a micro-permeability that facilitates reactions with the host rock (ultramafic-mafic rocks).

Early Proterozoic orogeny and exhumation of Wernecke Supergroup revealed by vent facies of Wernecke Breccia, Yukon, Canada

In the Yukon, the oldest known supracrustal succession, the Wernecke Supergroup, was deposited in a marine basin before 1.71 Ga. The earliest orogenic event to disturb these strata was the Racklan orogeny, which produced folds and fabrics at peak temperatures of 450–550 °C. These features and those of the correlative Forward orogeny are recognized at the surface and in the subsurface throughout much of northwestern Canada. Zones of Wernecke Breccia (hydrothermal breccias, 1.60 Ga) were emplaced into the Wernecke Supergroup after Racklan deformation and metamorphism. Two main types of breccia are recognized: grey sodic breccias and colourful potassic breccias. In the Slab Mountain area, a belt of grey breccias contains abundant megaclasts of country rock including blocks of a subaerial lava succession, the Slab volcanics. These grey breccias are interpreted as a vent facies of Wernecke Breccia, and their emplacement into the stratigraphically lowest unit of the Wernecke Supergroup infers that at least 9 km of exhumation occurred in the core of a major Racklan anticline prior to brecciation. The Slab volcanics are preserved only as clasts in Wernecke Breccia and are interpreted as fragments of a former valley-filling basalt succession which overlay deformed and deeply incised strata of the Wernecke Supergroup.

New constraints on the timing of Early Proterozoic tectonism in the Black Hills (South Dakota), with implications for docking of the Wyoming province with Laurentia

A question regarding the 1900–1600 Ma assembly of Laurentia is whether the Wyoming province (west-central United States) was part of the Hearne province (west-central Canada) prior to the Early Proterozoic Trans-Hudson orogeny, or a separate entity welded later to a Hearne-Superior continent (central Canada). New 40 Ar/ 39 Ar mica dates help to address this question by extending a Middle Proterozoic geochronologic front, long established along the southern Wyoming province, into the Black Hills of South Dakota. This suggests that previously unexplained, north-directed fold nappes (F 1 ) in the Black Hills resulted from island-arc accretion to the south ca. 1780 Ma. North-northwest–trending upright F 2 folds, which formed during east-west collision of the Wyoming and Superior provinces, must therefore be younger. New 40 Ar/ 39 Ar hornblende dates, recently published age data, and crustal heat-flow considerations further suggest that this collision began at or before ca. 1770 Ma and culminated with posttectonic magmatism beginning ca. 1715 Ma (the Harney Peak granite). This tectonic-magmatic interval is ∼50–60 m.y. younger than that reported for the Hearne-Superior collision (Trans-Hudson orogeny in Canada). Comparably young metamorphic dates (1810–1710 Ma) also typify the eastern and northern Wyoming province periphery (western Dakotas and southwestern Montana). Collectively, these data suggest that the Hearne and Wyoming provinces were once separate continents that were ultimately welded to the Superior province (and to each other) during distinct Early Proterozoic orogenies. Regional relationships further suggest that final docking of the eastern Wyoming province with Laurentia began during the ca. 1780–1740 Ma interval of island-arc accretion along the southern margin of the growing craton.

Tectonothermal history of the western part of the Limpopo Belt: tectonic models and new perspectives

The westernmost part of the Central Zone of the Limpopo Belt in Botswana is commonly interpreted as a frontal ramp of a westward extruding block or nappe sheet, related to a late Archæan Limpopo Orogeny. In this paper the structural observations from this area are reviewed and new geochronological results presented, in order to test the existing tectonic models. Two tectonometamorphic events can be distinguished. Neoarchæan granulite metamorphism (M 1) is associated with the generation of voluminous granitic bodies at ca 2.6 Ga. The nature of the Archæan tectonometamorphism is difficult to interpret because of a major high-grade metamorphic overprint at 2.0 Ga, which is characterised by a complicated succession of ductile deformational phases. The structural patterns indicate that during the 2.0 Ga evolution of the Limpopo Belt convergence directions changed from north to northwest and west. During the same period metamorphic conditions gradually decreased from upper amphibolite-facies to greenschist-facies. The structural features in the western part of the Central Zone are not compatible with a frontal ramp geometry. Models proposing a single Neoarchæan Limpopo Orogeny do not account for the polyphase tectonometamorphic evolution in the Central Zone and are also rejected. This study suggests that a Proterozoic orogeny involving transpression best explains the geometries encountered in the western Limpopo Belt.

Depth of emplacement and tilting of the Middle Proterozoic (1470 Ma) Wolf River batholith, Wisconsin: 40Ar/39Ar thermochronologic constraints

The 1470 Ma Wolf River batholith of northeastern Wisconsin is one of the oldest and largest intrusive sequences of the Middle Proterozoic magmatic suite that trends across the continent from Labrador to southern California. The presence of miarolitic cavities in portions of the batholith and in associated plutons (Wausau syenite complex) and equilibrium considerations suggest a shallow emplacement depth (i.e., <4 km). However, these data contrast with published Rb/Sr biotite ages, which suggest much of the batholith was uplifted through mid-crustal temperatures well after intrusion. To address this problem better, we have obtained four biotite 40Ar/39Ar dates from a transect across the southwestern portion of the batholith, one date from the ~1520 Ma Wausau syenite complex, and another from rocks metamorphosed during the Early Proterozoic Penokean orogeny. The two oldest dates are concordant (within error) with the crystallization age of the batholith, suggesting rapid cooling after intrusion and hence shallow emplacement depth. Cooling dates young to the south-southeast, however, and the youngest dates within the batholith (~1400 Ma) are concordant with previous Rb/Sr biotite dates and with the Ar/Ar biotite date from the adjacent country rock. The data suggest slow cooling and southeast-side-up tilting of this portion of the exposed batholith from mid-crustal temperatures (~300°C). Postemplacement uplift and tilting of the batholith appear related to differential isostatic uplift driven by substantial original thickness variations of the intrusion.

Proterozoic orogeny questioned: a view from Scottish Highland Field Workshops, 1995–1996.

Proterozoic orogeny questioned: a view from Scottish Highland Field Workshops, 1995–1996.

Chapter 15 Silurian history

Treatment in this chapter of the Silurian episode in the history of Svalbard must be different from that of the preceding and succeeding chapters because the stratal record is quite minimal - a fact that corresponds to the widespread Caledonian tectogenesis (Fig. 15.1). Indeed, Svalbard was subjected to mid-Paleozoic orogeny which has often obscured the earlier history. This chapter, therefore, concentrates on the tectonic story even where it overlaps earlier and later history and in turn the preceding and succeeding chapters overlap to some extent with the sedimentary record. Historically the metamorphic rocks were first regarded as Archean. Holtedahl (1914) demonstrated their Caledonian nature first in northwestern Spitsbergen and later regionally (Bailey & Holtedahl 1938). It was, however, then a paradigm of tectonic thinking, perhaps by analogy with Wales, that the Caledonian Orogeny was conceived as deforming an Early Paleozoic geosyn-cline (Cambrian through Silurian). This was evident in early interpretations of Scandinavian, Greenland and North American mapping as well as in Svalbard. It was commonly thought that those geosynclines would begin with an initial Cambrian unconformity. Gradually, however, it became clear that the 'Caledonian' geosynclines were more complex and in each of the areas referred to above the major part of the sedimentary pile was found to be Precambrian, often with no great sedimentational break to herald the Phanerozoic Era. Latterly interest has focused on distinguishing, within the Caledonized Precambrian successions, relicts of earlier pre-Cale-donian mainly Proterozoic orogenies referred to here as proto-basement. In contrast to the 80 million years

The building of south China: collision of Yangzi and Cathaysia blocks, problems and tentative answers

This paper aims to give answers to the questions of timing, tectonic style and geodynamic interpretation of south China geologic development. In the middle Jiangnan segment, Yangzi plate and Cathaysia got clearly welded during a Late Proterozoic orogeny of collisional type, marked by HP/LT metamorphism, ophiolite melange obduction, thrusting of greenschist nappes, emplacement of collisional S-type granites. This collision, which built the initial Jiangnan belt, began around 950 ± 40 Ma and was completed at about 770–800 Ma ago. Kinematic study indicates that the Cathaysia plate was underthrust beneath the Yangzi plate. A likely earlier collisional event occurred around 1500 Ma, but is poorly preserved in the studied area. The proposed geodynamic model implies two successive suturings of oceanic domains during the Middle-Late Proterozoic: one about 1500 Ma and one about 950 Ma. A strong remobilization occurred during the Early Paleozoic ‘Caledonian’ orogeny, which induced transpressive ductile deformation. Thin-skinned folding and thrusting took place during the Mesozoic; this intracontinental shortening could be due to collision between the China-Indochina and west Philippines blocks. South China is a composite block, comprising the relics of at least three, maybe four sutures from Jiangnan to the coast.

Event Stratigraphy and Physico-Chemical Characters of Banded Gneissic Complex and Associated Supracrustals in the South Mewar Plains of Rajasthan

Abstract The oldest cratonic nucleus pf the Western Indian shield, familiarly known as the Banded Gneissic Complex (BGC), occupies a large tract in the Mcwar plains of South Rajasthan. It has essentially an old gneissic component (3.3 Ga) and a host of mafic bodies associated with detrital and chemogenic sediments which have been subjected to multiple phases of anatexis and granite intrusions (2.9 Ga and later events). The emplacement of Berach granite and succeeding dolerite dyke swarms mark the end-Archaean cratonization process. This also initiated intracratonic and cratonmarginal basin formation. Later magmatism, especially along the basement-cover interface. is due to local remobilisation of the basement at the time of Proterozoic orogeny.

A GEOMORFOLOGIA DA SERRA DO ESPINHAÇO EM MINAS GERAIS E DE SUAS MARGENS

The geomorphological evolution of the Serra do Espinhaço is discussed based on: a) a critical analysisof previous papers about the geomorphology of the range, b) new interpretation based on own field data,c) regional interpretation, considering informations about the evolution of adjacent areas west and east ofthe range (São Francisco River, Doce River and Jeqitinhonha-Araçuaí Rivers Basins). The global contextis presented as a synthetical characterization of the main geological conditions, regional physiographyand drainage system organization and anomalies.The regional geomorphological organization is rigid controlled by the geology: a) the shape of therange reproduce the original collisional arc developed during the proterozoic orogeny, b) the existence oftwo strongly diferenciated plateaux (Planalto Meridional and Planalto Setentrional) reflect the litho-structuralcontrast between the southern and northern parts of the range, c) the morphology expresses the associationbetween lithological resistance to erosion and the tectonic organization in all scales.The morphology of the higher portions of the plateaux is characterized by relicts of two tertiaryplanation surfaces. They are probably peneplains resulting from erosion under tropical humid climaticconditions. During the Pleistocene, the valleys recorded the formation of several stepped terraces. thespacial variation on its dislevelments reflects the intensity of crustal uplifts and the position of the streamsin the drainage system hierarchy. The hillsides were, during the Upper Pleistocene, covered by two finecolluvial layers. In both cases, the cenozoic sediments register evidences of humid and hot paleoclimaticconditions.The Serra do Espinhaço acquired its present main physiography after the Miocene, which wascharacterized by tectonic stability. During the Pliocene, a strong compressional tectonic event reactivatedthe main precambrian thrust faults and detachment planes. As a consequence, the high scarp related to thewest thrust front was enhanced together with a eastward tilting of the plateaux. At the same time, in thePlanalto Setentrional, several valley heads of the São Francisco drainage basin were deviated and capturedby the Jequitinhonha Basin drainage system. In the Planalto Meridional, the geological conditions (porouslithology and anticline structure) favoured the capture of streams by the São Francisco Basin drainagesystem.As a consequence of the compressional forces that uplifted the range, an extensional stress fielddeveloped in the eastern border, generating grabens structures where the main rivers installed its valleys.The neotectonic activity is also recorded in several parts of the range and adjacent areas by normal, reverseand strike-slip faults in pleistocenic alluvial and colluvial deposits. Fault planes data analysis indicates ageneral NW-SE compressional stress field that remains up to the present time, as confirmed by results ofseismological data.(*)

Palaeoproterozoic age of the East Baltic granulitic crust

Abstract The crust beneath the East European Platform is one of the largest domains of Precambrian crust in Europe. Granulites of poorly defined ages make up a substantial part of this crust. Representative samples from the granulites in southern Estonia are LREE-enriched and provide Nd T DM -model ages between 2.08 and 2.18 Ga. The equivalent ϵ Nd for T =1.9 Ga range from +0.2 to +2.0. These data are similar to those of the Svecofennian crust in Finland and Sweden, and suggest that the crustal material originated in the Palaeoproterozoic rather than the Archaean. Thus the known volume of juvenile Palaeoproterozoic crust in Europe is expanded considerably. A new aspect is thereby introduced into the geodynamic interpretation of the NE-European Proterozoic orogenies.

Timing of Proterozoic deformation and magmatism in a tectonically reworked orogen, Rayner Complex, Colbeck Archipelago, east Antarctica

The high-grade metamorphic rocks of the Colbeck Archipelago comprise granulite facies metasedimentary gneisses and charnockitic gneisses of three ages. A suite of rocks previously mapped as the Colbeck Gneiss is shown to be composite in age and origin, consisting of metasedimentary and orthogneiss that are intruded by granitic gneiss and charnockitic gneiss. An (?) Archaean granulite facies S 1 gneissosity is preserved in the metasedimentary gneisses and early orthogneiss; the development of S 1 was followed by isobaric cooling from peak metamorphic conditions of T⩾750°C and P=5.1±0.8 kbar. Younger intrusions cut rocks containing S 1 structures, but were deformed by a ≈ 1200 Ma granulite facies D 2 event that resulted in reclined, isoclinal F 2 folds oriented parallel to a pervasive east-trending L 2 mineral and stretching lineation. Rocks bearing S 2 structures are cut by the areally extensive ∼ 960 Ma Mawson Charnockite, which was affected by two upright folding events D 3 and D 4 at ∼ 920 Ma. Events D 2–4 comprise the Proterozoic Rayner Structural Episode, which is characterized regionally by the retrogression of Archaean assemblages. The two pulses of extensive intermediate to felsic magmatism accompanied or immediately preceded the Proterozoic orogenies evident as D 2–4, and it is tempting to infer causal links. However, field relationships are consistent with intrusion having been contemporary with anomalously high conductive heat fluxes, which could be due to advective heating from larger intrusions not exposed at the present structural level. The formation of extensional, granulite facies D 5 ultramylonite±pseudotachylite zones is the last deformation event in the area.

Isotopic constraints on the age and origin of the Brattskarvet intrusive suite, Dronning Maud Land, Antarctica

The alkali granitoids of the Brattskarvet intrusive suite constitute the largest intrusion (∼ 100 km 2) post-dating Middle Proterozoic orogeny in the Sverdrupfjella region of the Maudheim tectonic province, western Dronning Maud Land. RbSr whole-rock data indicate an intrusive age of 518 ± 15 Ma ( R o = 0.7081), and SmNd whole-rock data an age of 522 ± 120 Ma ( R o = 0.5111). RbSr mineralogical data (biotite, alkali feldspar, sphere) yield ages of 482−465 Ma. The intrusive age of the suite is strongly indicated to be ∼ 519 Ma, with subsequent cooling at ∼ 476 Ma. PbPb whole-rock data yield no age information, but indicate a lower-crustal source with low U/Pb ratios. These data support field evidence that intrusion post-dated the early orogeny (Kibaran, 1100-1000 Ma), but predated the less intense deformation synchronous with the Ross or Pan-African event in Cambriant times. The country rocks, paragneisses of the Sverdrupfjella Group, retain a Kibaran whole-rock RbSr overprint of ∼ 1170 Ma ( R o = 0.7036), as do gneissic xenoliths enclosed in the intrusion ∼ 1046 Ma ( R o = 0.7105). SmNd and PbPb whole-rock data from these rocks are equivocal; PbPb whole rock data indicate a source similar to upper-crustal rocks. U-Pb data from two zircon fractions extracted from the parageneiss confirm the dominance of Kibaran ages. Two hornfelsic xenoliths have undergone complete RbSr resetting during the magmatism, but have retained Sm-Nd characteristics of the country rocks. Although biotite and feldspar from the country rocks underwent RbSr resetting durin the Ross event, muscovite from one sample has retained a Kibaran age, and thus suggests a narrow range for the crustal temperatures during the younger event. Combined Sr and Nd data indicate that the intrusion is isotopically discrete from the country rock, precluding an origin through local anatexis or contamination/mixing of an externally derived magma with the paragneisses. Derivation from an antique Sr- and Nd-depleted granulitic terrane, possibly attenuated continental crust predating the accretionary Kibaran orogeny, is favoured. The A-type geochemical character of the suite supports this interpretation.

GEOTHERMS OF CONTINENTAL LITHOSPHERE BASED ON XENOLITH THERMOBAROMETRY

The thermobarometry of garnet lherzolite and pyroxenite xeno-liths from kimberlitic explosion pipes and volcanoes has been used to determine the geotherms, values of the surface heat flow, and the thickness of the lithosphere in regions of Archean orogeny (the Kaapvaal Craton, South Africa and Lesotho, and the Siberian Platform in the vicinity of the Udachnaya and Mir pipes), Proterozoic orogeny (Somerset Island, N.W.T., Canada; Gibeon Province, Namibia; and the Colorado Plateau, USA), and Paleozoic orogeny (southeastern Australia, and the Hangay Highlands, Mongolia). In each case, during the period of volcanism, the geotherm remains, as before, conductive through the full thickness of the lithosphere, but consists of two branches: a stationary low-temperature branch and a disturbed high-temperature branch, although not everywhere represented by xenoliths. It may be demonstrated that the minerals of the peridotite association enclosed in diamonds, were equilibrated at temperatures and pressures which prev...