Quartz-feldspar Gneiss Research Articles

High-pressure metamorphism of Precambrian continental crust in the southwestern part of the Central Asian Orogenic Belt (South Kazakhstan and North Tien Shan) and tectonic implications for the evolution of the Palaeo-Asian Ocean

AbstractIn this study, we investigated the high-pressure (HP) metamorphism of the Precambrian continental crust exposed in the Zheltau terrane in South Kazakhstan (Koyandy complex) and the Chu-Kendyktas terrane in the North Tien Shan of Kyrgyzstan (Aktyuz, Kemin and Kokdzhon complexes) within the SW part of the Central Asian Orogenic Belt. HP quartz–feldspar lithologies of the Koyandy complex consist of migmatized kyanite-bearing garnet–mica paragneisses, garnet–kyanite paragneisses and their derivatives associated with eclogites. Paragneisses demonstrate prograde evolution involving mica dehydration melting and producing magnesium-rich garnet, kyanite and K-feldspar at the near-peak to retrograde stages at pressures of 15–18.5 kbar and temperatures of 800–870°C. The widespread growth of micas in these rocks reflects lower stages of retrogression at P = 10–12 kbar and T = 720–770°C. The age distributions of the cores of detrital zircon grains from the paragneisses indicate a predominance of Neoproterozoic and minor occurrence of Mesoproterozoic and Palaeoproterozoic sources of their protoliths. The ages of ∼487–485 Ma obtained from the zircon rims of the paragneisses reflect the timing of their HP metamorphic re-equilibration. These age clusters are consistent with the age estimates obtained from the rims of zircons in the eclogite-bearing garnet gneisses of the adjacent Aktyuz complex in the North Tien Shan. The P–T paths and zircon ages obtained from the high-grade quartz–feldspar gneisses of the Zheltau and Chu-Kendyktas terranes are thus interpreted to indicate involvement of the crustal material derived from the Precambrian basement (magmatic zircons aged ca. 844 Ma) and its Ediacaran–Cambrian sedimentary cover (detrital zircons with maxima at 1 Ga and 800–600 Ma) in the latest Cambrian subduction processes induced by the closure of the oceanic basins assigned to the Palaeo-Asian Ocean.

The corridor of mineralization in the California gold district, in the Santander Department of Colombia

In the Santander Massif of the Eastern Cordillera of Colombia, intense Pliocene to Holocene sulfide-rich gold mineralization occurs in a six kilometers long section of the La Baja fault zone, where it crosses partially migmatized gneisses and intrusions of Triassic-Cretaceous quartz monzonites and granites. These intrusives are mineralogically and texturally similar to the neosome of the migmatites that they intrude. The mineralization occurs as medium to large veins in the gneisses and as small veins, veinlets and disseminations in the intrusives. The type, volume and metal grades of the veins are dependent on the local structure, and the chemical composition and degree of reactivity of the host rocks. Thicker and higher-grade veins of the Mascota-type occur in chemically favorable magnetite-hornblende gneisses and migmatites, deposited after phases of silicification, and brecciation along the foliation of the host rocks. The Gigante-type shear veins occur in splays of the fault, and are marked by shearing and mylonitization. Lower-grade, flat-banded veins appear in leucocratic quartz-feldspar gneiss and related migmatites. The mineralizing fluids ascended along fault planes, and invaded the gneisses in fractures oblique to them. In all deposits, gold occurs in free form, as electrum and in tellurides, and accompanied by copper and silver.

The flake graphite prospect of Piippum\xe4ki\u2014an example of a high-quality graphite occurrence in a retrograde metamorphic terrain in Finland

The flake graphite occurrence in Piippumäki, Eastern Finland, as indicated by an airborne electromagnetic anomaly, was located during fieldwork by electromagnetic measurements with Slingram. The anomaly is approximately 0.1 × 1 km in size. The flake graphite is hosted by quartz-feldspar gneiss and amphibolite that have been subjected to retrograde metamorphism. This is observed in thin sections as granulite facies (garnet + cordierite + sillimanite + melt) regressing to greenschist facies (epidote, chlorite, albite, and white mica). The graphite (up to 1 mm large flakes) is found in graphite-bearing layers in the gneiss, and to a minor extent disseminated in the amphibolite. The average total sulfur (TS) is 0.33%, total carbon (TC) is 6.49%, and the average content of graphitic carbon (Cg) is 6.41% for the analyzed graphite-bearing rocks. SEM, XRD, and Raman spectroscopy were used for analyzing the flake graphite, indicating that the graphite is almost defect-free, of high quality, and has not been affected by the retrograde metamorphism. The peak metamorphic temperature of 737 °C was determined by a Raman thermometer, and no temperatures of greenschist facies were observed. A pseudosection was constructed from whole-rock chemical composition and indicated equilibration at ca 5 kbar and 740 °C, which corresponds to the observed mineral assemblages.

Avaliação de imagens multipolarizadas de radar (banda L) do sensor SARR99B para mapeamento geológico no Baixo Vale do Curaçá, Bahia

In the tropical semi-arid environment of Brazil geological applications with radar are still rare. The objective of this research was the evaluation of multipolarized L-band radar images acquired by the sensor SAR-R99B (SIVAM-CENSIPAM) for geological mapping in the lower Curaca valley, an important Cu-rich district of the Bahia state. The test-site is characterized by a flat terrain, with moderate to sparse caatinga vegetation and shallow residual soils closely related to changes in the geological substratum. These characteristics favor the evaluation of SAR images for geological mapping. The test-site is part of the Sao Francisco Craton, and encompasses paleoproterozoic granulitic and gneissic rocks and an intrusive sequence of mafics and ultramafics. Subordinately, metasediments with neoproterozoic ages also occur. In this investigation it was evaluated multipolarized L-band images (L-HH, L-VV, L-HV), amplitude format with 6-meter of spatial resolution. The photogeological procedures were applied using the graphical interface of viewing and digital enhancements available in the FOCUS package (PCI Geomatics software), calibrated with bibliographic and field information. The results of the photogeological interpretation showed that L-VV presented the best performance in the discrimination of lithologies given by a higher tonal/textural contrast of more frequent units (mica- schists, quartz-feldspar gneiss) and a better detailment of more restricted ones (marbles, mafic-ultramafics). Regarding the structural characterization, the three polarizations showed a similar performance, with a slightly disadvantage of L-HH revealing the importance of the vertical component of polarization in the L-VV and L-HV images in the enhancement of structural features. The excellent radiometric quality of calibrated SAR data, high spatial resolution and good response sensitivity to variations of macro, micro-topography and moisture were responsible for the results of the analysis.

The Vehmaa rapakivi granite batholith - An assemblage of successive intrusions indicating a piston-type collapsing centre

The 700 km Vehmaa rapakivi granite batholith is located in south-western Finland. It has a concentric structure and is composed of intrusions of five granite varieties (we use the terminology of the rock names according to the published geological map sheet, Lindberg & Bergman, 1993): pyterlite, coarse-grained porphyritic rapakivi granite, medium-grained porphyritic rapakivi granite, even-grained rapakivi granite, and porphyry aplite (Fig. 1). The granite types yield a concordant monazite U-Pb age of 1573 ± 8 Ma (even-grained granite of satellite intrusion) and a zircon U-Pb age of 1582 ± 4 Ma (pyterlite) (Lindberg & Bergman 1993). The batholith is surrounded by mica schists, hornblende gneisses, and quartz-feldspar gneisses, as well as syn-collisional tonalites and late-collisional K-rich granites and migmatites (Ehlers et al., 1993; Selonen & Ehlers, 1998; Vaisanen & Holtta, 1999). In this short communication we discuss the structure and emplacement of the main Vehmaa rapakivi granite batholith as an assemblage of successive pulses of intrusions. The medium-grained porphyritic granite in the centre of the batholith is described in more detail than the other granite types. By careful mapping and by studying geophysical data, two successive intrusion pulses of almost identical types of the medium-grained granite, in contact with each other, have been defined. The two intrusive phases have not previously been described. We think that the mode of intrusion of these granites is important for the understanding the emplacement of the whole batholith, and suggests a more generally applicable model for other intrusions of rapakivi granite in Finland. Recently, Haapala & Lukkari (2005) discussed the evolution of the small, zoned Kymi stock within the Wiborg rapakivi batholith, where both the scale of the intrusion, and the suite of rocks suggested an alternative mode of formation and intrusion. A closer examination of the geochemistry and petrology of the Vehmaa batholith will follow in the near future and is not included in this short note.

Mesoproterozoic graphite deposits, New Jersey Highlands: geologic and stable isotopic evidence for possible algal origins

Graphite deposits of Mesoproterozoic age are locally abundant in the eastern New Jersey Highlands, where they are hosted by sulphidic biotite–quartz–feldspar gneiss, metaquartzite, and anatectic pegmatite. Gneiss and metaquartzite represent a shallow marine shelf sequence of locally organic-rich sand and mud. Graphite from massive deposits within metaquartzite yielded δ13C values of –26 ± 2‰ (1σ), and graphite from massive deposits within biotite-quartz-feldspar gneiss yielded δ13C values of –23 ± 4‰. Disseminated graphite from biotite–quartz–feldspar gneiss country rock was –22 ± 3‰, indistinguishable from the massive deposits hosted by the same lithology. Anatectic pegmatite is graphitic only where generated from graphite-bearing host rocks; one sample gave a δ13C value of –15‰. The δ34S values of trace pyrrhotite are uniform within individual deposits, but vary from 0 to 9‰ from one deposit to another. Apart from pegmatitic occurrences, evidence is lacking for long-range mobilization of carbon during Grenvillian orogenesis or post-Grenvillian tectonism. The field, petrographic, and isotope data suggest that massive graphite was formed by granulite-facies metamorphism of Proterozoic accumulations of sedimentary organic matter, possibly algal mats. Preservation of these accumulations in the sedimentary environment requires anoxic basin waters or rapid burial. Anoxia would also favour the accumulation of dissolved ferrous iron in basin waters, which may explain some of the metasediment-hosted massive magnetite deposits in the New Jersey Highlands.

The origin of khondalites: geochemical evidence from the Archean to Early Proterozoic granulite belt in the North China craton

Abstract The dominant rock types in the khondalite suite in the Archean to Early Proterozoic granulite belt of north-central China are sillimanite-garnet gneiss (SGG), quartz-garnet gneiss (QGG), and quartz-feldspar gneiss (QFG). Field and geochemical results from the China khondalites and khondalites from other locations (including those from the type locality in India) are not consistent with a paleosol protolith. Major and trace element distributions in the SGG and QGG suggest shale and feldspathic sandstone protoliths, respectively. REE distributions including negative Eu anomalies in both rock groups are remarkably similar to those in Phanerozoic shales. Element distributions in QFG, including highly fractionated REE patterns, are similar to Archean TTG and these rocks are interpreted to have felsic volcanic protoliths. Sediment protoliths of the SGG and QGG may have been derived from sources in which granite and a smaller amount of basalt dominated, but they cannot have had significant contribution from Archean TTG sources. The China khondalite protoliths appear to have been deposited in a cratonic basin along the northwest margin of the North China craton. Although the khondalites from China and elsewhere chiefly have sedimentary protoliths, different compositions and lithologic proportions between khondalite suites indicate variable sediment provenance, intensity of paleoweathering, and tectonic settings.

The structure of Archean–Ketilidian crust along the continental shelf of southwestern Greenland from a seismic refraction profile

The velocity structure of the continental crust on the outer shelf of southwestern Greenland is determined from dense wide-angle reflection–refraction data obtained with large air-gun sources and ocean bottom seismometers along a 230 km seismic line. This line crosses the geological boundary between the Archean block and the Ketilidian mobile belt. Although the data have high noise levels, P- and S-wave arrivals from within the upper, intermediate, and lower crust, and at the Moho boundary, can be consistently identified and correlated with one-dimensional WKBJ synthetic seismograms. In the Archean, P- and S-wave velocities in the upper crust are 6.0 and 3.4 km/s, while in the intermediate crust they are 6.4 and 3.6 km/s. These velocities match for the upper crust a quartz–feldspar gneiss composition and for the intermediate crust an amphibolitized pyroxene granulite. In the Ketilidian mobile belt, P- and S-wave velocities are 5.6 and 3.3 km/s for the upper crust and 6.3 and 3.6 km/s for the intermediate crust. These velocities may represent quartz granite in the upper crust and granite and granitic gneiss in the intermediate crust. The upper crust is ~5 km thick in the Archean block and the Ketilidian mobile belt, and thickens to ~9 km in the southern part of the Archean. This velocity structure supports a Precambrian collisional mechanism between the Archean block and Ketilidian mobile belt. The lower crust has a small vertical velocity gradient from 6.6 km/s at 15 km depth to 6.9 km/s at 30 km depth (Moho) along the refraction line, with a nearly constant S-wave velocity around 3.8 km/s. These velocities likely represent a gabbroic and hornblende granulite composition for the lower crust. This typical (but somewhat thin) Precambrian crustal velocity structure in southwestern Greenland shows no evidence for a high-velocity, lower crustal, underplated layer caused by the Mesozoic opening of the Labrador Sea.

Age and isotopic character of Early Proterozoic basement gneisses in the southern Monashee Complex, southeastern British Columbia

The southern Monashee Complex is a fault-bounded exposure of upper-amphibolite-grade basement gneisses (core gneisses) and unconformably overlying mantling metasedimentary gneisses. The eastern margin is marked by the Early Eocene ductile to brittle Columbia River fault zone; the western and southern margins are marked by the Monashee Decollement and structurally higher Selkirk allochthon.The basement gneisses are exposed in the cores of large, northeast-verging nappes that subsequently have been overprinted towards the east by the Columbia River fault zone. The basement gneisses are a supracrustal sequence intruded by at least two distinct orthogneisses: (i) a biotite granite gneiss ("gray gneiss") dated by U/Pb zircon at 1874 ± 21 Ma; and (ii) a ±hornblende–biotite K-feldspar augen gneiss dated by U/Pb zircon at 1934 ± 6 Ma.The supracrustal gneisses are predominantly heterogeneous biotite–quartz–feldspar gneiss interlayered with less common pelitic schist and calc-silicate gneiss. U/Pb zircon data on detrital zircon populations from this heterogeneous supracrustal sequence give 207Pb*/206Pb* ages of less than 2.2 Ga. Whole-rock Pb isotopic data indicate an age of approximately 2.0 Ga. Whole-rock Sm/Nd model ages on the two intrusive suites indicate separate sources, the 1874 Ma gneiss having been produced from similar-age juvenile Early Proterozoic material (TDM ≈ 2.2 Ga). In contrast, Nd data from the 1934 Ma augen gneiss clearly indicate interaction with a component of older (late Archean) material (TDM ≈ 2.8 Ga). Whole-rock Sm/Nd data from the supracrustal gneisses follow this same pattern, with one group (seven samples) similar to the 1874 Ma gneiss (with TDM ≈ 2.3–2.6 Ga) and a second group (five samples) showing provenance or derivation from an Archean source (TDM = 2.8–3.3 Ga). The age of the intrusive suites, combined with the Nd data, strongly argues for a correlation with the Early Proterozoic Wopmay orogenic belt in northern Canada.

Radioelement distribution in U, Th, Mo, and rare-earth-element pegmatites, skarns, and veins in a portion of the Grenville Province, Ontario and Quebec

Gamma-ray spectrometer measurements were obtained at and in the vicinity of 104 of the 124 U, Th, Mo, and rare-earth-element (REE) occurrences examined in the Central Metasedimentary Belt of the Grenville Province. Spatial, temporal, mineralogical, and geochemical relationships among granitic pegmatites, phlogopite – scapolite – Ca pyroxene skarns, and fluorite – apatite – calcite veins hosting U, Th, Mo, and REE minerals indicate a common magmatic–hydrothermal origin. Quartz–feldspar gneisses in the Central Metasedimentary Belt (n = 54) have low abundances of uranium (1–7 ppm) and thorium (4–27 ppm) suggesting that partial melting, fractional crystallization, and volatile phase separation were responsible for the enrichment of uranium (2–37 ppm) and thorium (5–102 ppm) in uncontaminated granitic pegmatites (n = 163) derived during ultrametamorphism. The U/Th ratio is probably inherited from the source quartz–feldspar gneiss protolith and enhanced during fractionation.Average U and Th concentrations and U/Th ratios at numerous localities show significant positive correlations among pegmatites, skarns, and veins, providing further evidence for a related origin. The interaction of the pegmatite-derived hydrothermal fluids with host rocks produced a spectrum of types and styles of alteration, which include (i) hybridization and (or) endoskarnification along pegmatite margins; (ii) marble- and clinopyroxenite-hosted exoskarn; and (iii) fluorite–apatite–calcite veins. The deposition of U, Th, Mo, and REE from the evolving hydrothermal fluid is responsible for the heterogeneous distribution of U, Th, and REE minerals and molybdenite within pegmatites, skarns, and veins at each locality. Secondary enrichment of uranium in association with hematitized sheared pegmatites and veins may be responsible for the observed large variation in U/Th ratios at some sites.

The Pancarli nickel-copper sulfide mineralization, eastern Turkey

The Pancarli Ni-Cu mineralization is located in the metamorphic sequence of the Bitlis massif consisting of biotite gneiss, quartz-feldspar gneiss, amphibolite, and metagranitic rocks. The rocks are probably Precambrian in age and have been affected by regional amphibolite-facies metamorphism and by a later cataclasis. There is also evidence of an earlier eclogite-facies metamorphism. The Ni-Cu mineralization occurs as massive sulfide lenses aligned parallel to the penetrative foliation along various levels. The lenses are up to 3 m in length and 2 m in width. The host rock in the immediate vicinity of the orebodies is generally quartz-feldspar gneiss or more rarely quartz-feldspar gneiss and amphibolite. The mineral paragenesis of the ore consisting of pyrrhotite, pentlandite, and chalcopyrite and the low As and Co contents of the ore indicate an orhomagmatic origin of the mineraliza tion. The textures and chemistry of the spinel minerals in the Pancarli deposit together with Cu/(Cu+Ni) ratio of 0.29 of the ore suggest that the mineralization is genetically related to a basic magma. The only mafic rocks in the area are amphibolites that are characterized by a tholeiitic basaltic chemical composition. However, no imprignation and network ores are present in any of the amphibolite bodies. A model based on the segregation of a sulfide melt fraction from a basaltic magma in a magma chamber has been proposed. The subsequent and independent intrusions of the two fractions into the country rocks account for the absence of the impregnation and network ores in the amphibolites, and explain the similarities in geologic setting of the ore and amphibolites as well as their observed field relationships. Structural and textural features of the ores indicate that they have been subjected to regional metamorphism and the later cataclastic deformation that affected the country rocks. The author believes that the ores probably formed during the Pan-African orogenic development of the Bitlis massif in Late Precambrian time. In their present state, the Pancarli ores exhibit varying degrees of weathering with supergene assemblages.

Regional geochemical and isotopic characteristics of high-grade metamorphics of the Prydz bay area: The extent of proterozoic reworking of Qrchaean continental crust in East Antarctica

Abstract Preliminary isotopic data for Late Proterozoic (∼ 1100 Ma) granulite-facies metamorphics of the Prydz Bay coast indicate only very minor reworking (i.e., remetamorphism) of Archaean continental crustal rocks. Only two orthopyroxene—quartz—feldspar gneisses from the Rauer Group of islands, immediately adjacent to the Archaean Vestfold Block, show evidence for an Early Archaean origin (∼ 3700—3800 Ma), whereas the vast majority of samples have Middle Proterozoic crustal formation ages (∼ 1600–1800 Ma). The Prydz Bay rocks consist largely of garnet-bearing felsic gneisses and interlayered aluminous metasediments, although orthopyroxene-bearing gneisses are common in the Rauer Group; in contrast, Vestfold Block gneisses are predominantly orthopyroxene-bearing orthogneisses. The extensive Prydz Bay metasediments may have been derived by erosion of Middle Proterozoic rocks, such as the predominantly orthogneiss terrain of the Rauer Group, and deposited not long before the Late Proterozoic metamorphism. Data from nearby parts of the East Antarctic shield also suggest only limited Proterozoic reworking of the margins of the Archaean cratons. As in the Prydz Bay area, high-grade metamorphies in nearby parts of the East Antarctic shield show a secular increase in the sedimentary component. Archaean terrains like the Vestfold Block consist mainly of granitic orthogneisses derived by partial melting of igneous protoliths (I-type), whereas Late Proterozoic terrains (such as the Prydz Bay coast) include a much higher proportion of rocks derived either directly or by partial melting (S-type granitic orthogneisses) from sedimentary protoliths. Related chemical trends include increases in K 2 O 2 , Rb, Pb, and Th, and decreases in CaO, Na 2 O 2 and Sr with decreasing age, essentially reflecting changes in the proportions of plagioclase and K-feldspar.

The origin of the early Proterozoic supracrustal rocks in the Garpenberg district, south central Sweden

Abstract The early Proterozoic supracrustal sequence in the Garpenberg district, south central Sweden, consists mainly of quartz-feldspar gneissses (‘leptites’) and subordinate amphibolites, quartz gneisses, mica schists, skarns and marbles. Stratabound iron and sulfide deposits are intercalated within this sequence. Most of the quartz-feldspar gneisses and amphibolites are interpreted as acid and basic metavolcanic rocks. The quartz gneisses and mica schists represent the metamorphic equivalents of originally altered felsic volcanic and sedimentary rocks. The geologic development of the Garpenberg district was dominated by extensive acid volcanism of calc-alkaline affinity, which was interrupted by periods of intense hydrothermal activity, ore deposition and sedimentation. Most of the volcanic activity occurred was submarine in a shallow-water environment. Subsequently, the supra crustal sequence was affected by the Svecokarelian orogeny (1800–1750 Ma) and the rocks were metamorphosed in the amphibolite ...

First terrestrial occurrence of roedderite in volcanic ejecta of the Eifel, Germany

Idiomorphic crystals of roedderite occur in melt-coated cavities of xenoliths of contact-altered quartz-sillimanite and quartz-feldspar gneisses which were ejected with the tephritic lava of the Bellerberg volcano. Physical and chemical properties of three different sets of crystals agree generally with those of roedderites from meteorites, in which so far the mineral had been found exclusively. In detail, however, there are characteristic chemical differences amongst the Eifel roedderites with one set of crystals matching closely the ideal formula (Na,K)2Mg5 Si12O30, a second set containing excess alkalies according to the substitution Na+→0.5 Mg2+, and a third set richer in iron having an alkali deficiency following Fe3+→Fe2++Na+. The terrestrial roedderites are considered to be precipitates from highly alkaline, MgSi-rich, but Aldeficient gas phases that evolved through contact heating of the gneisses by the tephrite magma.

Old K–Ar Mineral Ages from the Grenville Province, Ontario

Hornblende-rich concentrates from quartz–feldspar gneisses of the Grenville Province near Brent Ontario, have yielded K–Ar apparent ages of 1570 to 1480 ± 80 m.y., while coexisting biotite- and feldspar-rich separates give 'normal' Grenville K–Ar ages near 900 ± 40 m.y. Comparison with the nearest Rb–Sr isochron dates suggests that the indicated hornblende K–Ar age represents a minimum age for time of crystallization of the gneisses in the Brent area and that the younger ages for minerals with lower blocking temperatures indicate a later thermal event in the metamorphic history of the Grenville Province.

Geology of North-Central Guayana Shield, Venezuela

The geological work for this paper was based on the reconnaissance mapping of more than 8000 square miles of savanna during a period of 7 months by one geologist, who was equipped with a helicopter for a quarter of this time. In the north-central part of the Guayana Shield in Venezuela, the lowest sequence of rocks, referred to as the Imataca Complex, comprises quartz-feldspar gneisses, amphibolite and amphibole-pyroxene gneisses, iron-formation, and migmatite. The grade of metamorphism ranges from almandine-amphibolite to granulite, with granites restricted to areas of lower-grade metamorphism. Overlying the gneisses on the south are strata of the Pastora-Carichapo Assemblage. The Carichapo Formation consists of amphibolite derived from mafic lavas and is interlayered with sedimentary, including manganiferous, strata. Perhaps unconformable above the Carichapo Formation is the Pastora Series, a sequence comprising volcanic wackes, shale, and mafic flows, all in the greenschist facies. On the north side of the Imataca Complex are quartzites, conglomerates, and amphibolites of the Real Corona-El Torno Assemblage. From considerations of metamorphic grade, these rocks undoubtedly overlie the Imataca sequence and from lithologic considerations, may be equivalent to the Pastora-Carichapo Assemblage. The occurrence of aluminous hematitic pebbles in one of the conglomerates suggests that the conglomerates have not been derived from the Imataca terrain. Both elongate, gneissic folds and more equidimensional domes have formed in the Imataca Complex. Some of the folds are strongly overturned but show no consistent regional pattern. The Complex is traversed by the El Pao and the Ciudad Piar-Guri faults, the former associated with as much as 3 km of flaser gneiss and mylonite and the latter with several hundred meters of mylonite. Despite these differences in the widths of cataclastic belts, the El Pao fault has been the locus of less movement, for like a splay from the Ciudad Piar-Guri fault, it disappears in fold structures. From a consideration of structural relationships and isotopic age determinations, it appears that Imataca rocks must have been folded prior to 2.0 b.y. before the present (B.P.), that Imataca and Carichapo rocks were folded together about 2.0 b.y. B.P., and that Pastora strata may have been recrystallized again about 1.3 b.y. B.P. in some places. The Roraima Formation of quartzites and shales delimits a very old, stable, tectonic element to the south. The major lithologic differences and major faults are indicated also by regional gravity and magnetic data.