Showing papers on "Granulite published in 1984"

A garnet–hornblende geothermometer: calibration, testing, and application to the Pelona Schist, Southern California

Abstract: A garnet–hornblende Fe–Mg exchange geothermometer has been calibrated against the garnet–clinopyroxene geothermometer of Ellis & Green (1979) using data on coexisting garnet + hornblende + clinopyroxene in amphibolite and granulite facies metamorphic assemblages. Data for the Fe–Mg exchange reaction between garnet and hornblende have been fitted to the equation. In KD=Δ (XCa,g) where KD is the Fe–Mg distribution coefficient, using a robust regression approach, giving a thermometer of the form: with very satisfactory agreement between garnet–hornblende and garnet–clinopyroxene temperatures. The thermometer is applicable below about 850°C to rocks with Mn-poor garnet and common hornblende of widely varying chemistry metamorphosed at low aO2. Application of the garnet–hornblende geothermometer to Dalradian garnet amphibolites gives temperatures in good agreement with those predicted by pelite petrogenetic grids, ranging from 520°C for the lower garnet zone to 565–610°C for the staurolite to kyanite zones. These results suggest that systematic errors introduced by closure temperature problems in the application of the garnet–clinopyroxene geothermometer to the ‘calibration’data set are not serious. Application to ‘eclogitic’garnet amphibolites suggests that garnet and hornblende seldom attain Fe–Mg exchange equilibrium in these rocks. Quartzo-feldspathic and mafic schists of the Pelona Schist on Sierra Pelona, Southern California, were metamorphosed under high pressure greenschist, epidote–amphibolite and (oligoclase) amphibolite facies beneath the Vincent Thrust at pressures deduced to be 10±1 kbar using the phengite geobarometer, and 8–9kbar using the jadeite content of clinopyroxene in equilibrium with oligoclase and quartz. Application of the garnet–hornblende thermometer gives temperatures ranging from about 480°C at the garnet isograd through 570°C at the oligoclase isograd to a maximum of 620–650°C near the thrust. Inverted thermal gradients beneath the Vincent Thrust were in the range 170 to 250°C per km close to the thrust.

Precambrian Tectonics and Crustal Evolution in South India

Abstract: About 3.4 Ga ago voluminous calc-alkaline magmas represented by the granitoid gneisses of southern India were emplaced into a now poorly preserved non-continental crust. Unstable ensialic basins, initiated at about 3.0 Ga, were filled with volcanic and sedimentary rocks up to about 2.6 Ga. This basement-cover sequence was deformed at the close of the Archaean, first by northward accretion and thickening of several crustal slabs formerly separated by prisms of stable shelf sediments. Structures produced by this episode were refolded and dislocated by large N-S strike-slip shear belts, to impart intense, steep planar fabrics to large volumes of the crust. Fluids rich in CO₂, possibly derived from sedimentary material driven beneath the crustal slabs by the thickening mechanism, purged the deep crust of H₂O to form granulites. Upward migration of hot H₂O-rich fluids encouraged partial melting at intermediate crustal levels. These late-Archaean tectonic and thermal events began soon after the close of basin f...

Archean granulite gneisses from eastern Hebei Province, China: rare earth geochemistry and tectonic implications

Abstract: The granulite gneisses and their retrograded products of the Qianxi Group from eastern Hebei Province, China, have been investigated for their isotope and trace element geochemistry. A consistent age of about 2.5 AE has been obtained by the Rb-Sr and Sm-Nd whole-rock isochron methods, in agreement with the zircon U-Pb data (Pidgeon 1980; D.Y. Liu, unpubl.). Geochemical arguments from initial isotopic ratios (ISr and INd) and elemental distribution patterns have led us to conclude that this age of about 2.5 AE represents the time of granulite facies metamorphism, which must have followed closely the primary emplacement of their protoliths. Previous claims for early Archean ages (>3.5 AE) of these granulites are not substantiated. The mineral isotope systematics register an important thermal event at about 1.7 AE, roughly corresponding to the time of the widespread Luliang Orogeny (Ma and Wu 1981) or Chungtiao Movement (Huang 1978). The granulites of the Qianxi Group have diverse compositions ranging from ultrabasic through basic-intermediate to acid. Discriminant function calculations suggest that most analyzed samples have igneous parentage. Only a few show characteristics of metasedimentary rocks. The igneous protoliths apparently belong to two series — tholeiitic and calc-alkaline, with the latter dominating in abundance. The majority of the acid granulites have compositions corresponding to tonalite-granodiorite. Except for ultrabasic and metasedimentary rocks, all REE patterns are significantly fractionated with LREE enrichment. The degree of fractionation, as measured by the (La/Yb)N ratios, is most important in the acid granulites. These rocks often show positive Eu anomalies and HREE depletions that are typical of Archean TTG rocks (tonalitetrondhjemite-granodiorite). The existence of komatiites has been previously reported in this region. Although a few rocks have a major element chemistry similar to that for peridotitic komatiites, the lack of associated members in a komatiitic series and the scarcity of REE data have not confirmed the true komatiite occurrence in this region. Many Qianxi granulites are highly depleted in Rb relative to K and Sr. This preferential Rb depletion during granulite facies metamorphism has led to very high K/Rb and very low Rb/Sr ratios. The most comparable case is found in Lewisian granulites. Although the fractionated REE patterns of the basic granulites somewhat resemble those of continental flood basalts, the highly different abundances in other incompatible elements (Ti, Zr, and Ba) readily distinguish them from each other. Nevertheless, the LREE enriched patterns of the basic granulites may suggest an origin of their protoliths by partial melting of LREE-enriched mantle sources. On the other hand, the REE patterns of acid granulites suggest that their protoliths could be derived by partial melting of quartz eclogite, amphibolite or basic granulite. The close time relationship for a series of geologic events, namely, from initial melting of mantle peridotites, through fractional crystallisation of basaltic magmas, to granulite facies metamorphism, seems to occur in many granulite terrains. This relationship, together with the juxtaposition of lithologies of different origins and the exceptionally high pressure conditions (>10 Kb) can be best explained by crustal underplating combined with intracrustal thin-skinned thrusting and stacking of crustal slices. The “andesitic or island arc” model for the formation of the lower continental crust is not in good agreement with the present geochemical data.

Nd—Sr isotopic composition of granulites and constraints on the evolution of the lower continental crust

Abstract: In a statistical study of granulite facies rocks, we have analysed 32 samples with ages ranging from 2,900 to 200 Myr for Sm, Nd, Rb and Sr and 143Nd/144Nd and 87Sr/86Sr ratios. The Sm/Nd ratio is found to be constant compared with the highly variable Rb/Sr ratio. Using the model age formalism, we show that, in many cases internal differentiation of continental crust follows external differentiation with a large time interval.

Metamorphism of reduced granulites in low-CO 2 vapour-free environment

Abstract: Pervasive flooding of CO2 has been proposed as the cause of granulite facies metamorphism that is capable of producing many distinctive characteristics of the deep continental crust: reduced water activity, orthopyroxene-bearing assemblages, depletion of large-ion lithophile (LIL) elements, and dehydration1–6. The calculations presented here of C–O–H fluid composition for conditions of granulite facies metamorphism show that oxygen fugacity (f O2) estimates from many terranes are sufficiently low that the addition of CO2-rich fluid causes graphite to precipitate. For values of pressure (P) and temperature (T) common to granulites, and with f O2 slightly below the quartz–fayalite–magnetite buffer (QFM), the addition of CO2 sufficient to grow 10 vol. % orthopyroxene requires the precipitation of 1.5 vol. % graphite. As 0.1 vol. % graphite is readily recognizable, but is not reported in most low f O2 granulites, these rocks have not been flooded by CO2, and low f H2O is probably due to extraction of a magma or recrystallization of an already dry rock7,8. Granulite terranes may thus result from a combination of these three processes and the dominance of any one cannot now be demonstrated on a regional basis.

Fluid heterogeneity during granulite facies metamorphism in the Adirondacks: stable isotope evidence

Abstract: The preservation of premetamorphic, whole-rock oxygen isotope ratios in Adirondack metasediments shows that neither these rocks nor adjacent anorthosites and gneisses have been penetrated by large amounts of externally derived, hot CO2-H2O fluids during granulite facies metamorphism. This conclusion is supported by calculations of the effect of fluid volatilization and exchange and is also independently supported by petrologic and phase equilibria considerations. The data suggest that these rocks were not an open system during metamorphism; that fluid/rock ratios were in many instances between 0.0 and 0.1; that externally derived fluids, as well as fluids derived by metamorphic volatilization, rose along localized channels and were not pervasive; and thus that no single generalization can be applied to metamorphic fluid conditions in the Adirondacks.

Origin of Mesozoic and Tertiary granite in the western United States and implications for Pre-Mesozoic crustal structure: 2. Nd and Sr isotopic studies of unmineralized and Cu- and Mo-mineralized granite in the Precambrian Craton

Abstract: In the Cordilleran region of the western United States, Mesozoic and Tertiary peraluminous granitic rocks display regional variations in initial 143Nd/144Nd (eNd); eNd = −10 to −12 in southern Arizona, − 17 to −19 in the northern Great Basin (NGB), and −30 in the northern Rocky Mountains. Initial 87Sr/86Sr values are between 0.710 and 0.721 and show no regional pattern. Metaluminous granitic rocks have a wider range of eNd values extending from values similar to those of the peraluminous granites to much higher values. The 87Sr/86Sr values are mostly fairly low, between 0.705 and 0.710 except in the NGB where values as high as 0.7157 are observed. No systematic differences between the eNd or 87Sr/86Sr values of Cu- or Mo-mineralized and Unmineralized granite were discerned, except for Cu-mineralized granite in eastern Nevada and Mo-mineralized granite in Colorado, which have eNd values higher (∼0) and lower ( ∼−10.0), respectively, than Unmineralized granite in the same region. Comparison to eNd values of exposed Precambrian rock suggests that the peraluminous granite, and the Mo granite in Colorado, were derived exclusively from felsic Precambrian basement rocks and that the regional variations in the eNd values reflect the regional variation in the average crustal age. The Nd data confirm that the Precambrian basement underlying the NGB and eastern California is isotopically distinct from Precambrian crust in the remainder of the western United States. The similarity between the eNd values of peraluminous granite and Precambrian crust also suggests that the high 147Sm/144Nd (>0.13) and the low total light rare earth element (LREE) abundances characteristic of peraluminous granite in southern Arizona were imposed during the chemical evolution of the magmas. Metaluminous granite are interpreted to have formed via mixing of mantle-derived magma and large proportions of low 87Sr/86Sr (granulite facies) lower crust, except in the eastern NGB where the mantle magmas mixed with a lower crustal source with a significantly higher 87Sr/86Sr ratio. REE abundance patterns for metaluminous granite in the NGB are characterized by extreme LREE enrichment, which supports the proposed origin for these rocks by mantle/crust mixing or by remelting of the lower crust alone. No systematic difference exists between the sources of Cu- or Mo-mineralized and Unmineralized metaluminous granite, but the data suggest that the Cu sources are in the mantle and the Mo sources are in preexisting crust. Overall, the Nd data indicate that continental interior granite in the western United States was primarily derived from preexisting crust, and although changes in the thermal structure of the continental mantle may have triggered magma formation, the resulting granites do not represent significant juvenile additions to the continental crust.

Origin and evolution of a peraluminous silicic ignimbrite suite: The Violet Town Volcanics

Abstract: The Violet Town Volcanics are a 373 Ma old, comagmatic, S-type volcanic sequence mainly comprising crystal-rich intracaldera ignimbrites. Rock types vary from rhyolites to rhyodacites, all containing magmatic cordierite and garnet phenocrysts. Variation in the suite is primarily due to fractionation of early-crystallized quartz, plagioclase and biotite (plus minor accessory phases) in a high-level magma chamber prior to eruption. Early magmatic crystallization occurred at around 4 kb and 850° C with melt water contents between 2.8 and 4 wt.%. This high-temperature, markedly water-undersaturated, restite-poor, granitic magma was generated by partial melting reactions involving biotite breakdown in a dominantly quartzofeldspathic source terrain, leaving a granulite facies residue.

Fluid inclusions in rocks from the amphibolite-facies gneiss to charnockite progression in southern Karnataka, India: direct evidence concerning the fluids of granulite metamorphism

Abstract: Fluid inclusion studies of rocks from the late Archaean amphibolite-facies to granulite-facies transition zone of southern India provide support for the hypothesis that CO2,-rich H2O-poor fluids were a major factor in the origin of the high-grade terrain. Charnockites, closely associated leucogranites and quartzo-feldspathic veins contain vast numbers of large CO2-rich inclusions in planar arrays in quartz and feldspar, whereas amphibole-bearing gray gneisses of essentially the same compositions as adjacent charnockites in mixed-facies quarries contain no large fluid inclusions. Inclusions in the northernmost incipient charnockites, as at Kabbal, Karnataka, occasionally contain about 25 mol. % of immiscible H2O lining cavity walls, whereas inclusions from the charnockite massif terrane farther south do not have visibile H2O Microthermometry of CO2 inclusions shows that miscible CH4 and N2 must be small, probably less than 10mol.%combined. Densities of CO2 increase steadily from north to south across the transitional terrane. Entrapment pressures calculated from the CO2 equation of state range from 5 kbar in the north to 7.5 kbar in the south at the mineralogically inferred average metamorphic temperature of 750°C, in quantitative agreement with mineralogic geobarometry. This agreement leads to the inference that the fluid inclusions were trapped at or near peak metamorphic conditions. Calculations on the stability of the charnockite assemblage biotite-orthopyroxene-K-feldspar-quartz show that an associated fluid phase must have less than 0.35 H2O activity at the inferred P and T conditions, which agrees with the petrographic observations. High TiO2 content of biotite stabilizes it to lower H2O activities, and the steady increase of biotite TiO2 southward in the area suggests progressive decrease of aH2O with increasing grade. Oxygen fugacities calculated from orthopyroxene-magnetite-quartz are considerably higher than the graphite CO2-O2 buffer, which explains the absence of graphite in the charnockites. The present study quantifies the nature of the vapours in the southern India granulite metamorphism. It remains to be determined whether CO2-flushing of the crust can, by itself, create large terranes of largeion lithophile-depleted granulites, or whether removal of H2O-bearing anatectic melts is essential.

Dehydration melting and the granulite transition in metapelites from southern Namaqualand, S. Africa

Abstract: In a prograde amphibolite-granulite transition zone in the Namaqualand Metamorphic Complex, metapelites show an interbanding of the amphibolite facies association biotite+sillimanite+quartz with the granulite facies association garnet+cordierite+K-feldspar. Relict graded bedding shows that compositional banding is of sedimentary origin. The garnet-cordierite-K-feldspar gneisses contain quartzofeldspathic segregations surrounding garnets, and have more Fe-rich bulk compositions than the biotite-sillimanite schists.

Petrogenesis and evolution of an early Proterozoic collisional orogenic belt: The granulite belt of Lapland and the Belomorides (Fennoscandia)

Abstract: BERBEY, P.; CONVERT, J. ; MOREAU, B.; CAPDEVILA, R. and HAMEURT, J . , 1984: Petrogenesis and evolution of an early Proterozoic collisional orogenic belt: The granulite belt of Lapland and the Belomorides (Fennoscandia). Bull. Geol. Soc. Finland 56, Part 1—2, 161—188. The tectonic and metamorphic pattern of the Precambrian crust exposed in the eastern Baltic Shield is characterized by two adjacent high-grade belts (Tana and Granulite belts) located between the Archean Inari-Kola craton to the east and the South Lapland-Karelia craton to the west. The Tana belt corresponded to a volcanic sequence (tholeiitic and calcalkaline series) subsequently intruded by various plutonic rocks including the Vaskojoki anorthositic suite. The Granulite belt is interpreted as a flysch-like greywacke-shale sequence (khondalite suite) including impersistent volcanic layers later intruded by several igneous suites of both tholeiitic and calc-alkaline affinity (charnockitic complex). These two former belts constitute the axial part of the Belomorides, an early Proterozoic fold belt. Major parts of this mobile belt surrounding the axial zone are composed of intensely reworked Archean material. The Jatulian volcano-sedimentary formations cover large areas on both sides of the mobile belt. Allowing for their epicontinental nature, their early Proterozoic age (2.0—2.4 Ga) and for their deformation by the 1.9 Ga Karelian event, we suggest that the Jatulian formations belong to the Belomorides. We propose to re-interpret the Belomorian fold belt in terms of a Wilson cycle as a collisional orogenic belt comprising a geosynclinal basin with oceanic crust (Tana and Granulite belts), an Archean reactivated basement (Belomorian gneisses) and a platform (Jatulian). The Belomorides represent the oldest fold belt in which a suture has been recognized and constitute an intermediate case between an intracontinental mobile belt orogeny and an active plate margin.

Pressures, Temperatures and Metamorphic Fluids Across an Unbroken Amphibolite Facies to Granulite Facies Transition in Southern Karnataka, India

Abstract: A sampling traverse has been made across the late Archaean regional amphibolite-facies to granulite-facies transition in southern Karnataka, India. The traverse extends from the Peninsular Gneiss-Closepet Granite terrane in the north, through the incipient charnockite localities near Kabbaldurga and southwards into the charnockite massifs of the Biligirirangan and Andhiyur Hills.

Evolution of the lower continental crust: granulite facies xenoliths from the Eifel, West Germany

Abstract: The lower continental crust is a key region to understanding processes of crust–mantle differentiation and subsequent evolution of the crust. The region is metamorphosed into granulite or amphibolite facies1,2. Whereas felsic to mafic granulites with an average intermediate major element composition are typically observed in old exposed lower crustal terranes, mafic compositions are preponderant among granulite facies xenoliths transported by young alkalic basaltic volcanics and kimberlites1. Radiometrie ages for granulite bulk rocks using the Rb–Sr or U–Pb methods are usually interpreted as metamorphic ages. In contrast, Sm–Nd ages obtained on such metamorphics are interpreted to reflect pre-metamorphic events3–7, although a case of severe disturbance of the Sm–Nd isotope system during granulite facies metamorphism has also been reported8. We present here Sm–Nd and Rb–Sr isotope data for a suite of mafic lower crustal xenoliths from the Hercynian belt. Our data suggest that the igneous precursors of these granulites differentiated from depleted mantle ∼1.5 Gyr ago. In most of these granulites this evidence for a mid-Proterozoic age has been obliterated by a young (≺172 Myr) metasomatic overprint by mantle derived fluids or melts.

A geothermobarometric investigation of garnet peridotites in the Western Gneiss Region of Norway

Abstract: In the Western Gneiss Region of Norway are found numerous peridotite lenses which have been extensively recrystallized under amphibolite fades conditions during the Caledonian Orogeny. However, evidence for an earlier Caledonian high-pressure metamorphism has been recorded by abundant eclogite and granulite relicts within gneiss and by the presence of at least ten garnet perioditite bodies preserved within chlorite peridotites. Two garnet-bearing ultramafic assemblages have been recognized: olivine-orthopyroxene-clinopyroxene-garnet and olivine-ortho-pyroxene-pargasitic-amphibole-garnet. Except for olivine, minerals in the garnet peridotites are compositionally zoned, with relatively uniform cores and compositional gradients generally confined to the outer 200 micrometers, or less, of grains. The most common zoning patterns at grain margins are an increase in Fe/Mg in garnet, an increase in Al2O3 in orthopyroxene, and a decrease in Na2O and Al2O3 in clinopyroxene, although there are exceptions to these patterns at two localities. These zoning patterns have developed mainly in response to cooling and decompression of the garnet peridotites. Application of geothermometers and barometers to the garnet peridotites has yielded temperatures of 770–860° C and pressures of 30–43 kb for cores of grains and consistently lower temperatures and pressures for rims, except for peridotites on Oteroy, where there is an apparent temperature increase from cores to rims. The petrologic and geothermobarometric evidence for most of the investigated garnet peridotites is compatible with their tectonic emplacement from the upper mantle into thickened continental crust during Caledonian collision of the Baltic and Greenland plates.

Uranium-lead and lead-lead investigations of minerals from the Broken Hill lodes and mine sequence rocks

Abstract: A regression analysis of combined rock and lode apatite data for the 207 Pb/ 206 Pb vs. 204 Pb/ 206 Pb plot gives an age of 1,565 + or - 20 m.y. (2sigma ) which is similar, within the experimental error limits, to the minimum U-Pb ages for monazite and Pb-Pb age for sphene from the mine sequence rocks of about 1,595 m.y. The possible differences in apparent age of the monazite-sphene and apatite may reflect the different blocking temperatures of the U-Pb systems in the two minerals. Cooling ages indicate that the Broken Hill Block remained at temperatures in excess of 600 degrees C for over 60 to 70 m.y. and would have allowed extensive concentration and/or recrystallization of the sulfide orebodies. On a concordia plot, the U-Pb data for zircons from two samples of upper granite gneiss and two of Potosi gneiss lie on an array with an apparent age of 1,663 (super +9) (sub -8) m.y. (2sigma ) and with a mean square of weighted deviates of 24.6. This age is interpreted as the time of granulite facies metamorphism. The least magnetic zircon fractions of samples close to mineralization contain from 2.2 to 16 ppm of common Pb. The abundant common lead in the zircons indicates that the sulfides were already in existence by 1,660 m.y., even if only in a disseminated form. An alternative explanation of the isotopic data is that there have been two major metamorphic events at Broken Hill during the middle Proterozoic: the granulite facies at approximately 1,660 m.y. and another at approximately 1,595 m.y. Thirty-five analyses of 21 sphalerites and pyrrhotites show uniform lead isotope ratios throughout the orebody because of the presence of galena inclusions.--Modified journal abstract.

Age and origin of anorthosites, charnockites, and granulites in the Central Virginia Blue Ridge: Nd and Sr isotopic evidence

Abstract: Rb-Sr isotopic data for anorthosites, charnockites, ferrodioritic to quartz monzonitic plutons, and high-grade gneisses of the Blue Ridge of central Virginia show evidence of post-emplacement metamorphism, but in some cases retain Grenville ages. The Pedlar River Charnockite Suite yields an isochron age of 1021 +/-36 Ma, (initial 87Sr/86Sr ratio of 0.7047 +/-6), which agrees with published U-Pb zircon ages. Five samples of that unit which contain Paleozoic mylonitic fabrics define a regression line of 683 Ma, interpreted as a mixing line with no age significance. Samples of the Roseland Anorthosite Complex show excessive scatter on a Rb-Sr evolution diagram probably due to Paleozoic (475 m.y.) metamorphism. Data from the ferrodioritic to quartz monzonitic plutons of the area yield an age of 1009 +/-26 Ma (inital ratio=0.7058 +/-4), which is in the range of the U-Pb zircon ages of 1000–1100 Ma. The Stage Road Layered Gneiss yields an age of 1147 +/-34 Ma (initial ratio of 0.7047 +/- 5). Sm-Nd data for the Pedlar River Charnockite Suite reflect a pre-Grenville age of 1489 +/-118 Ma (ɛ Nd=+6.7 +/-1.2). Data for the Roseland Anorthosite Complex and the ferrodioritic to quartz monzonitic plutons yield Grenville isochron ages of 1045 +/44 Ma (ɛ Nd=+1.0 +/-0.3) and 1027 +/-101 Ma (ɛ Nd=+1.4 +/-1.0), respectively. Two Roseland Anorthosite samples plot far above the isochron, demonstrating the effects of post-emplacement disturbance of Sm-Nd systematics, while mylonitized Pedlar River Charnockite Suite samples show no evidence of Sm-Nd redistribution. The disparity of the Sm-Nd age and other isotopic ages for the Pedlar River Charnockite Suite probably reflects a Sm-Nd “source” age, suggesting the presence of an older crust within this portion of the ca. 1 Ga old basement.

Radiometric Ages (Rb-Sr, Sm-Nd, U-Pb) and REE Geochemistry of Archaean Granulite Gneisses from Eastern Hebei Province, China

Abstract: The granulite gneisses and their retrograded products of the Qianxi Group from eastern Hebei Province, China, have been investigated for their isotope and trace element geochemistry. A consistent age of about 2.5 Ga has been obtained by the Rb-Sr and Sm-Nd whole-rock isochron methods, in agreement with the zircon U-Pb data (Pidgeon 1980; Liu et al. 1984). Geochemical arguments from initial isotopic ratios (ISr and INd) and elemental distribution patterns have led us to conclude that this age of about 2.5 Ga represents the time of granulite facies metamorphism, which must have followed closely the primary emplacement of the gneiss protoliths. Previous claims for early Archaean ages (⩾3.5 Ga) of these granulites are not substantiated. The mineral isotope systematics register an important thermal event at about 1.7 Ga, roughly corresponding to the time of the widespread Luliang Orogeny (Ma and Wu 1981) or Chungtiao Movement (Huang 1978).

The structural evolution of the Fyfe Hills‐Khmara Bay region, Enderby Land, East Antarctica

Abstract: Five generations of structure (D1‐D5) have been recognized in the Early Archaean supracrustal gneisses in the Fyfe Hills‐Khmara Bay region in Enderby Land, East Antarctica. The D1, D2 and D3 events comprise the Napier structural episode. They resulted in pervasive deformation during Archaean granulite facies metamorphism, and predated the intrusion of the Middle Proterozoic Amundsen dykes. The Napier metamorphic culmination was broadly coeval with, but outlasted both, D1 and D2. D3 occurred during waning granulite facies metamorphism. The D4 and D5 events comprise the Rayner structural episode, the effects of which are largely restricted to amphibolite facies retrograde shear zones in which the Amundsen dykes are deformed. The Napier Structural Episode is characterized by: (i) mesoscopic, isoclinal, recumbent, F1 folds which formed during intense deformation accompanying the prograde burial of the gneissic sequence; (ii) large scale, tieht to isoclinal, reclined F2 folds formed at deep crustal levels; and...

Seismic-reflection study of the Precambrian crust of central Minnesota

Abstract: A COCORP (Consortium for Continental Reflection Profiling) seismic-reflection survey across the boundary zone between the late Archean Superior Province granite-greenstone terrane and older Archean gneiss and granulite terrane of the Minnesota River Valley shows numerous north-to-northwest–dipping reflection events throughout the crust. A particularly prominent and continuous series of such events projects to the surface near the trace of a fault zone that had been previously identified as the boundary between these two terranes, within the Great Lakes tectonic zone. The moderate dips of these events (about 30°), their correlation with presumed faults near the surface, their continuity over tens of kilometres, and the persistence of these and other reflection zones throughout the crust suggest that these events correspond to fault zones, probably thrusts. They may have originated during late Archean collision between the Superior Province crust and older continental crust to the south. Reflections are sparser and more concordant in the gneiss and granulite terrane than in the Superior Province. There are several prominent reflection groups with gentle northerly dips in the middle and upper crust of the granulite and gneiss terrane. Their distribution appears to correlate with the sources of aeromagnetic and gravity anomalies that are part of a belt that extends west to central South Dakota and that apparently reveal the position of the postulated Archean suture. The axes of deposition and deformation of Proterozoic metasedimentary rocks are approximately parallel to and apparently coincide with the boundary between two Archean terranes farther east in Minnesota, Wisconsin, and Michigan, but Proterozoic (Penokean) deformation has not been distinguished either in the study area or in the seismic sections. The Minnesota COCORP survey has demonstrated the capability of this seismic-reflection method to reveal important but previously unsuspected structures in complexly deformed Archean terranes. This has provided structural evidence that the separate subprovinces of the North American Archean crust might have been assembled by subduction-related collision.

The iron-rich suite from the Amîtsoq gneisses of southern West Greenland: early Archaean plutonic rocks of mixed crustal and mantle origin

Abstract: A distinctive group of augen gneisses and ferrodiorites (termed the iron-rich suite) is a component of the early Archaean Amitsoq gneisses of southern West Greenland. The iron-rich suite outcrops south of the mouth of Ameralik fjord in an area that underwent granulite facies metamorphism in the early Archaean. The iron-rich suite forms approximately 30% of the Amitsoq gneiss of this area and occurs as sheets and lenses up to 500 m thick. The rest of the Amitsoq gneisses are predominantly tonalitic-granodioritic, banded grey gneisses. Despite intense deformation and polymetamorphism, there is local field evidence that the iron-rich suite was intruded into the grey gneisses after they had been affected by tectonism and metamorphism. The banded grey gneisses are interpreted as 3,700 to 3,800 Ma old; U-Pb zircon ages from the iron-rich suite give concordia intercepts at circa 3,600 Ma.

Sapphirine bearing granulites from the Sipiwesk Lake area of the late Archean Pikwitonei granulite terrain, Manitoba, Canada

Abstract: Sapphirine occurs in the orthopyroxene-cordierite and feldspar-sillimanite granulites in the Sipiwesk Lake area of the Pikwitonei granulite terrain, Manitoba (97°40′W, 55°05′N). The orthopyroxene-cordierite granulites have extremely high Al2O3 (24.5 wt%) and MgO (24.6 wt%) contents and contain sapphirine (up to 69.2 wt% Al2O3), aluminous orthopyroxene (up to 8.93 wt% Al2O3), cordierite, spinel, phlogopite, and corundum. Sapphirine forms coronas mantling spinel and corundum. Corona sapphirine is zoned and its composition varies through the substitution (Mg, Fe, Mn) Si=2 Al as a function of the phases with which it is in contact. Textural and chemical relationships of sapphirine with coexisting phases indicate that spinel + cordierite reacted to form orthopyroxene + sapphirine under conditions of increasing pressure. Moreover, decreasing core to rim variation of Al2O3 in orthopyroxene porphyroblasts suggests decreasing temperature during sapphirine formation. On the basis of experimentally determined P-T stability of the assemblage enstatite + sapphirine + cordierite, and the Al content of hypothetical Fe2+-free orthopyroxene associated with sapphirine and cordierite, metamorphic temperatures and pressures are estimated to be 860–890° C and 3.0–11.2 kbar.

Radioactive element distribution in the Archean granulite terrane of Jequié — Bahia, Brazil

Abstract: The radioactive element distribution in the Archean granulite facies rocks of Jequie, Brazil, has been determined. The K/Rb ratios show a large spread with majority of the values falling within the normal field (K/Rb<300). The Th/U ratios fall into two groups, one group with values in the range of 1.7 to 2.6, whereas the other group show Th/U values greater than 7 (7 to 25). These distributions can be explained on the basis of the presence of accessory minerals apatite and zircon. The relatively high concentrations of the radioactive and other elements in Jequie granulites probably represents the relict primary geochemical character, little modified during metamorphism. The radiogenic heat production data for the area are calculated and using the measured regional heat flow value from nearby area an estimate of the relative thickness of the continental lithosphere is made using the model developed by Oxburgh (1981).