Grey Gneisses Research Articles

Recycling of the Archaean continental crust: the case study of the Gavião, State of Bahia, NE Brazil

The Gavião block, located to the west of the São Francisco Craton (State of Bahia, NE Brazil), is the oldest crustal block so far recognised in South America—3.42 Ga. In its southern part, the Gavião block has been divided into three domains on the basis of 207Pb/ 206Pb dating on single zircons and monazites combined with Sr and Nd isotopic data and major and trace element geochemical modelling. These are: (1) an Archaean juvenile domain which consists of grey gneisses (Bernada massif) which evidence mantle extraction around 3.3 Ga; (2) an Archaean domain (3.24–3.16 Ga) either recycled, or juvenile with crustal contamination, consisting of trondhjemitic grey gneisses (Aracatu massif) and K-rich calc-alkaline granitic gneisses (Mariana and Serra do Eixo massifs); (3) a Paleoproterozoic recycled domain consisting mainly of the Umburanas granites, which yielded inherited zircons ages ranging from 3.1 to 2.8 Ga, whereas the monazite age is ca 2.0 Ga. The Aracatu and Mariana massifs are cut by granites at ca 2.0 Ga the same age of the Serra da Franga massif. The Gavião block is a type example of Archaean continental crust (3.2 Ga) that has been recycled through partial melting events mainly in Paleoproterozoic times during the Transamazonian orogeny (2.0–2.1 Ga). Brasiliano cooling ages are recorded by the Rb–Sr system of biotite–whole rock pairs ca 500 Ma.

Nd isotope mapping of a cryptic continental suture, Grenville Province of Ontario

Nd isotope data are presented for 70 Archean and Paleoproterozoic grey gneisses from the Grenville Parautochthonous belt in Ontario. These data constrain an age boundary which separates partially reworked Archean basement to the northwest from a Paleoproterozoic arc terrane to the southeast. This boundary is interpreted as a crustal-scale continental suture, but its exposure is cryptic due to metamorphic recrystallisation during the Grenville orogeny. Mesoproterozoic stitching plutons injected across the boundary suggest that it has a minimum age of 1.5 Ga, and is probably a Paleoproterozoic structure. Archean basement northwest of the suture was intensely plutonised by the 1.75–1.7 Ga Killarnean ensialic arc. Juvenile Paleoproterozoic basement southeast of the suture has a maximum crustal formation age of less than 2.0 Ga. It is probably an accreted Penokean terrane which was reworked by the same Killarnean ensialic arc.

Geochronological studies of Proterozoic crustal evolution in the northern Outer Hebrides

New data on the Proterozoic thermal and tectonic crustal history of the Lewisian complex have been determined using Sm–Nd and 40Ar– 39Ar isotopic analyses. Igneous assemblages from two mafic/ultramafic masses indicate crystallization at 2.54 and 2.14 Ga from magmas with initial Nd isotopic composition lower than predicted depleted mantle at those times. Sm–Nd isotope systematics ( T DM=2.45 Ga) in two samples of metasediment from the Leverburgh Belt imply that they contain a significant post-Archaean component and cannot therefore be part of an Archaean stratigraphic sequence. Sm–Nd mineral isotope data on three samples confirm the Proterozoic age of high-pressure granulite facies metamorphism in the South Harris igneous complex; the most precise estimate is 1.827±0.016 Ga on a metagabbro which was intruded after deformation of the host anorthosite under granulite facies conditions. Localized granulite facies assemblages in Younger Basic bodies further north are significantly younger at 1.63 Ga. Whole rock Sm–Nd data on two samples from the 1.7 Ga granite injection complex of South Harris indicate ϵ Nd between −11.5 and −13.7 at the time of intrusion, providing a strong indication that they were produced by remelting of pre-existing crust similar to the exposed grey gneisses which had values close to −12 at that time. 40Ar– 39Ar data from 13 hornblendes generally show complex argon release patterns with indications of excess argon in most samples. Flat sections of the spectra range in age between 1.63 and 1.78 Ga. Previous Rb–Sr data on biotite showed a distinct break in age pattern across the Langavat Belt. There is no indication of such a break in the hornblende data and it is inferred that despite variations in the timing of peak metamorphism the whole of the northern Outer Hebrides cooled through hornblende closure temperatures in the interval 1.7–1.6 Ga.

The Late Archean history of the Wyoming province as recorded by granitic magmatism in the Wind River Range, Wyoming

The Wyoming province, a small, ca. 500 000 km 2 Archean craton, is the most southwestern of the Archean provinces in North America. It is composed primarily of Late Archean potassium-rich granitic rocks. In contrast to many other Archean provinces, rocks of tonalite-trondhjemite affinity are rare over most of the province and are restricted to rocks older than 2.8 Ga. Field, petrologic, geochemical and isotopic study of the Late Archean granites exposed in the Wind River Range have allowed us to identify at least four periods of potassic calc-alkalic magmatism at ∼2.8, 2.67, 2.63 and 2.55 Ga. Granitic rocks of these ages appear to be widespread across the Wyoming province. The oldest calc-alkalic granites of the Wind River Range, emplaced at ca. 2.8 Ga, appear to be derived predominantly from pre-existing crust. However, Nd isotopic data suggest that these granites cannot be the product solely of partial melting of older tonalitic gray gneisses. During at least two other periods of plutonism, at 2.67 and 2.63 Ga, generation of the Wind River Range batholiths involved the incorporation of substantial amounts of isotopically juvenile material, either from depleted mantle or young continental crust. The information presented below, as well as data available from elsewhere in the Wyoming province, is interpreted to suggest that the Wyoming province, unlike other Archean cratons, is not composed of a tectonic amalgamation of smaller, exotic terranes. Although the Wyoming province did experience crustal addition in Archean time, it was not by lateral accretion, but by incorporation of mantle-derived melts into large granitic batholiths.

Interpretation of Ion-Probe U-Pb Dates from Early Archaean Zircons — Implications for the Age of the Earliest Life on Earth

The age of the proposed earliest life on Earth, preserved as C-isotope anomalies in the Akilia supracrustal rocks of SW Greenland, has been constrained to i> 3.87 Ga by ion-microprobe data on zircons from enclosing orthogneisses (Nutman et al., 1997). These zircons have clearly experienced ancient Pb-loss, yielding a broad range of ages from c. 3.6-3.9 Ga, of which the oldest have been assumed to represent the intrusion age. Using new ion-microprobe analyses coupled with detailed imaging of zircon grains, we demonstrate that there are at least two magmatic events represented by zircons in these grey gneisses, the youngest of which, at c. 3.65 Ga provides the true minimum age constraint upon the Akilia sequence, and hence the Earth's earliest life.

Geochronological evidence for reworking of Archean terrains during the Early Proterozoic (2.1 Ga) in the western Coˆte d'Ivoire (Man Rise-West African Craton)

A period of accretion of continental crust is recognized during the Archean at ca 3.2–3.3 Ga on the basis of wholerock Nd model ages and Pb ages on detrital zircons from various gneisses of the Man area in Coˆte d'Ivoire. There is no significant evidence of any older event, such as the 3.5/3.6 Ga accretion episode recognized in the Archean of the Reguibat Rise in Mauritania. The earliest magmatic events recorded in the northern domain of the Man area correspond to tonalitic grey gneisses of a minimum age ca 3.05 Ga (Leonian event). A subsequent major phase of igneous and metamorphic activity around 2.8 Ga (Liberian event) is recorded in the entire Archean terrain without any evidence of new accretion. This second event began at 2830 ± 7 Ma, its climax is indicated by charnockitic intrusions followed by granodioritic magmas at ca 2800 ± 10 (Pb Pb and U Pb ages on monazite and zircon). Cooling after the Liberian event occurred ca 2.75 Ga ago (K feldspar-whole rock-garnet Sm Nd ages). Paleoproterozoic reworking is mainly found in the southern part of the Archean terrain of the Man area: zircon; monazite; sphene; and garnet geochronometers all indicate an important thermal effect during the Birimian event (i.e. 2.1 Ga). The earliest influence of this event is recorded in a range of 2250 +40/−70 Ma, with a major event ca 2.1 Ga. Partial to total resetting of U Pb and Sm Nd geochronometers indicate that high-temperature conditions were reached during the Birimian event. This metamorphic overprint and the diapiric emplacement of coeval Paleoproterozoic juvenile magmas at 2.1 Ga in the southern domain (Toule´pleu massif) suggest that the 2.1 Ga overprint could be related to major underplating processes under the Archean basement. The thermal flow related to the differentiation and the accretion of these juvenile Birimian magmas may account for the reworking of the surrounding Archean domain which had been possibly thinned during underplating. These results suggest close proximity between the Archean continents and an accretion zone associated with juvenile magmatism 2.1 Ga ago.

Accretion and evolution of an Archaean high-grade grey gneiss – amphibolite complex: the Fiskefjord area, southern West Greenland

The Fiskefjord area in southern West Greenland, part of the Akia tectono-stratigraphic terrane, comprises a supracrustal association and two groups of grey quartzo-feldspathic orthogneises c. 3200 and 3000 Ma old. The supracrustal association forms layers and enclaves in grey gneiss and may comprise two or more age groups. Homogeneous amphibolite with MORB-like but LIL element enriched tholeiitic composition predominates; part, associated with cumulate noritic and dunitic rocks, represents fragments of layered complexes. Heterogeneous amphibolite of likely submarine volcanic origin, (basaltic) andesitic amphibolite, leucogabbro-anorthosite, and minor pelitic metasediment occur. Disruption by magmatic and tectonic events and geochemical alteration have obscured primary origin: the supracrustal association may represent oceanic crust.
 Grey orthogneiss of the tonalite-trondhjemite-granodiorite (TTG) association was generated during continental accretion at c. 3000 Ma, most likely by partial melting of wet and hot tholeiitic basaltic rocks subducted in a convergent plate setting. Most dioritic gneiss is c. 220 Ma older. A 3040 Ma dioritic to tonalitic phase, enriched in P2O5 , Ba, Sr, K, Pb, Rb and LREE, probably was derived from metasomatised mantle.
 Intense deformation and metamorphism accompanied the 3000 Ma magmatic accretion.Thrusts along amphibolite-orthogneiss contacts were succeeded by large recumbent isoclines, upright to overturned folds, and local domes with granitic cores. Syntectonic granulite facies metamorphism is thought to be due to heat accumulation by repeated injection of tonalitic magma. Strong ductile deformation produced steep linear belts before the thermal maximum ceased, whereby folds were reorientated into upright south-plunging isoclines. Two large TTG complexes were then emplaced, followed by granodiorite and granite.
 Post-kinematic diorite plugs with unusually high MgO, Cr and Ni, and low LIL and immobile incompatible element contents, terminated the 3000 Ma accretion. Hybrid border zones and orbicular textures suggest rapid crystallisation from superheated magma. The diorites most likely formed from ultramafic magma contaminated with continental crust.
 Widespread high-grade retrogression preserved a granulite facies core in the south-west; to the east the retrogressed gneiss grades into amphibolite facies gneiss not affected by granulite facies metamorphism and retrogression. LIL elements were depleted during granulite facies metamorphism and reintroduced during retrogression, probably transported in anatectic silicate melts and in fluids. Rb-Sr isotope data, and relationships between retrogression, high-strain zones and granite emplacement, show that retrogression took place shortly after the granulite facies metamorphism, before terrane assembly at c. 2720 Ma, probably by movement of melts and fluids into the upper, marginal zone of granulite facies rocks from deeper crust still being dehydrated. Retrogression during Late Archaean terrane assembly was in narrow reactivated zones of ductile deformation; in the Proterozoic it occurred with faulting and dyke emplacement.Geochemical data are presented for Early Proterozoic high-Mg and mafic dykes. A rare 2085 Ma microgranite dyke strongly enriched in incompatible trace elements was formed by partial anatexis of Archaean continental crust.

Crustal evolution in the early Archaean of South America: example of the Sete Voltas Massif, Bahia State, Brazil

The Sete Voltas massif (São Francisco craton, Brazil) appears as a composite crustal segment built in at least three successive accretional events dated by RbSr, 207Pb 206 Pb monozircon and U-Pb SHRIMP methods. (a) At ca 3.4 Ga, generation and emplacement of magmatic precursors to the old grey gneisses. These are the oldest rocks so far recognized in South America. Their composition is typical of Archaean TTG (Tonalite, Trondhjemite and Granodiorite) and geochemical modelling indicates that they were produced by partial melting of an Archaean tholeiite, leaving a hornblende garnet residue. (b) Between 3.17 and 3.15 Ga, younger grey gneisses and porphyritic granodiorites intruded the old grey gneisses. Their geochemical composition is clearly different from that of typical TTG and geochemical modelling shows that they were produced by partial melting of an older continental crust. (c) Grey granite dykes emplaced at ca 2.6 Ga, during a late-stage magmatic event in the Sete Voltas massif. All the units belonging to the Sete Voltas massif yield homogeneous T DM ages at 3.66 Ga which are interpreted as reflecting derivation from older protoliths, or rather their contamination by older pre-existing continental crust. It is tentatively proposed that a 3.66-Ga-old continental crust existed prior to the emplacement of the older grey gneisses. Although such crustal material is unknown at the present day in South America, Nutman and Cordani (1993) have nevertheless reported a single zircon core from Sete Voltas giving a 207Pb 206 Pb age of 3.473 ± 0.008 Ga. The present study, together with some other recent data (Mougeot et al., 1995; Santos Pinto et al., 1995a) indicates that the 3.4-Ga-old South American continental crust, until now considered as of very small geographical extent, in fact made up a true continental block corresponding in size to at least the whole present-day Gavião block. Major and trace element petrogenetic modelling for the young grey gneisses and the porphyritic granodiorites indicates that the precursor magmas are derived from the melting of pre-existing crustal rocks similar to the old grey gneisses. This conclusion is supported by the intense migmatization of the old grey gneisses which took place before the intrusion of the younger intrusives. The calculated residual mineral assemblage in equilibrium with the magmatic liquid is quartz + hornblende + plagioclase, a condition that is achieved between P = 10 kbar, T ≈ 800°C, <5% weight H 2O, or P = 15 kbar, T ≈ 700°C, ∼15% weight H 2O (Johnson and Wyllie, 1988). This suggests that the hydrous melting of the old grey gneisses occurred at ca 3.17 Ga at depths of ca 30–45 km, thus giving a minimum estimate of the thickness of this part of the Archaean crust. This estimate, as well as the development of an intense migmatization and the strong foliation in the old grey gneisses, can be interpreted as classical features of collisional thickening. Taken with the existence of horizontal tectonics in Archaean cratons older than 3.0 Ga, such a conclusion indicates that, more or less modern types of plate tectonic mechanisms operated in the early Archaean in the studied terrain.

Constraints on the timing of crustal imbrication in the central Trans-Hudson Orogen from single zircon 207Pb/206Pb ages of granitoid rocks from the Pelican thrust zone, Saskatchewan

The Pelican thrust is a major ductile high-strain zone in the Reindeer Zone, Trans-Hudson Orogen, northern Saskatchewan. It is interpreted as the main sole thrust separating stacked juvenile Paleoproterozoic allochthons and underlying Archean microcontinental crust in this central part of the orogen. Exposed nonmylonitic rocks in the footwall of the thrust consist of the Sahli monzocharnockite and the smaller, more highly retrograded MacMillan Point granite. Protomylonitic to ultramylonitic gneisses in the thrust zone derive from a variety of prethrust protoliths. A footwall "internal suite" mainly comprises quartzofeldspathic orthogneisses ("Q" gneisses) and high-grade migmatitic paragneisses. Hanging-wall "external suite" mylonitic gneisses include feldspar-porphyroclastic hornblendic grey gneisses probably derived from arc plutons, and laminated amphibolites derived from volcanic rocks. The overlying allochthon mainly comprises protoliths equivalent to those of the porphyroclastic orthogneisses and laminated amphibolites, together with interfolded and overlying Paleoproterozoic paragneisses of the Kisseynew domain. The Sahli monzoeharnockite yields 207Pb/206Pb zircon and whole-rock Rb–Sr ages of ca. 2500 Ma, and the "Q" gneisses give 207Pb/206Pb zircon ages of up to ca 2900 Ma, implying that most of the internal suite (footwall) mylonite protoliths are Archean. In contrast, external suite (hanging wall) porphyroclastic orthogneisses yield ca. 1880–1840 Ma 207Pb/206Pb zircon ages. Main, peak-metamorphic displacement on the Pelican thrust is interpreted to have occurred mainly between 1840 and 1820 Ma, as indicated by 207Pb/206Pb zircon ages from small, highly deformed synthrusting granite–pegmatite neosomal bodies in the thrust zone. Undeformed postcollisional granites and pegmatites were emplaced~1789 Ma. Total duration from arc development to completion of arc–continent collision was ~100 Ma. The Pelican thrust zone may be similar in significance and style to younger, major, ocean closure related thrusts such as the Frontal Pennine thrust of the western Alps and the Main Mantle, Main Boundary, and Main Central thrusts of the Himalayas. As for the Pelican thrust, these displace oceanic rocks over older basement.

Resetting of SmNd systematics during metamorphism of > 3.7-Ga rocks: implications for isotopic models of early Earth differentiation

New SmNd isotopic results for sixteen samples from the Garbenschiefer Amphibolite Unit of the > 3700-Ma Isua supracrustal belt of West Greenland are reported. The data illustrate the difficulty in identifying primary ϵ Nd( T) values in poly-metamorphic and poly-deformed rock sequences. The investigated samples are interpreted as derived from a Mg- and Al-rich igneous protolith which can be argued to have been essentially isotopically homogeneous at least on the outcrop scale at the time of its emplacement. However, the data do not give uniform ϵ Nd( T) values even from single outcrops. The wide variation of ϵ Nd( T) which we obtain (−1.0 to + 5.0) demonstrates that the SmNd systematics of the samples were disturbed by one (or more) metamorphic event(s) which took place some considerable time after the original crystallization of the Isua belt. A SmNd isochron age of 2849 ± 116 Ma has been obtained on plagioclase and amphibole separates which represent the oldest metamorphic assemblage now preserved at Isua. This result indicates that the earliest Amphbolite-facies metamorphism of the Isua belt for which pressure and temperature conditions can be obtained ( P ≈ 5 kbar; T ≈ 500–600°C) is late Archaean, and not early Archaean as had been assumed earlier by some authors. 2850 Ma is also thought to represent the main time of disturbance of the SmNd system in the Garbenschiefer Amphibolite Unit. A consideration of available isotopic data suggests that many of the rocks from Isua underwent some Nd isotopic re-equilibration and/or fractionation of their 147 Sm 144 Nd ratio during the 2850-Ma metamorphic episode. In these rocks, the computed ϵ Nd( T) are apparent values only and need not represent either the primary isotopic composition of their protoliths or early Archaean mantle values. Only Nd-rich rocks (> 20 ppm) such as some felsic schists and the grey gneisses adjacent to the belt are arguably sufficiently robust to retain ϵ Nd( T) values which are close to their original composition. These average +2.0, which may represent a close approximation of the ϵ Nd value of the Earth's mantle at ∼ 3800 Ma. The results presented in this paper show that great care should be exercised in the use of SmNd data from very early Archaean rocks as constraints for isotopic models of early Earth's evolution, particularly in terranes which have been affected by several metamorphic episodes.

The Itsaq Gneiss Complex of southern West Greenland; the world's most extensive record of early crustal evolution (3900-3600 Ma)

The Itsaq Gneiss Complex of southern West Greenland contains the best-preserved occurrences of ⩾ 3600 Ma crust. Its known area is ∼ 3000 km 2 with almost continuous exposure in some places. SHRIMP UPb zircon geochronology shows that the gneiss complex had a complicated early history, having been added to, and modified in, several events starting at ∼ 3900 Ma. Supracrustal, mafic and ultramafic rocks comprise approximately 10% of the complex and range in age from ⩾ 3870 to ∼ 3600 Ma. A large portion of the Isua supracrustal belt and some other bodies contain sequences of LREE-enriched, mafic (locally pillow-structured) to felsic volcanic and volcaniclastic rocks (some deposited from turbidity currents) and abundant diverse chemical sediments. These sequences might have formed in an environment analogous to present-day volcanic arcs. Other (mostly ⩾ 3800 Ma?) units dominated by LREE-depleted, high Ti/Zr mafic rocks (as found in komatiites and komatiitic basalts free of crustal contamination) with layers of banded iron formation might have been derived from volcanic edifices formed as a result of plume activity. Only in the youngest supracrustal sequences (∼ 3600 Ma) are detrital sediments derived from mixed-provenance clastic sources an important component. Layered anorthosite, gabbro and peridotite units (some ⩾ 3800 Ma) derived from large deep crustal intrusions are also widespread. In addition there are massive dunites and harzburgites. In the rare cases where their original mineralogy and texture are preserved, these contain aluminous spinel, indicating equilibration in the lowermost crust or upper mantle. Approximately 90% of the complex consists of grey gneisses, the dominant protoliths having been tonalites and less abundant trondhjemites, quartz-diorites, diorites and granodiorites. The protoliths were intruded in several events starting at ∼ 3870 Ma. Like other suites of Archaean grey gneisses, they were formed by partial melting, probably in an arc environment, of buried (subducted?) amphibolite, leaving residual hornblende ± garnet. Granites form approximately 10% of the complex. The oldest are 3650 Ma leucogranites, which probably formed by deep crustal anatexis of predominantly tonalitic gneisses. There are also ∼ 3625 Ma augen granites and ferrogabbros, whose chemistry resembles that of some A-type, within-plate granite suites. The evolution of the Itsaq gneiss complex is marked by increasing compositional diversity with time. Pre-3650 Ma lithologies are predominantly mafic and ultramafic rocks, tonalites and diorites. The first recorded regional metamorphic event occurred at 3650 Ma, and was marked by localised partial melting and intrusion of leucogranites. This might record crustal thickening, brought about by collision of different blocks of tonalite-dominated crust. A further thermal event occurred just after 3600 Ma, as shown by PbPb titanite and feldspar ages and local intrusion of granites. Deposition of sediments at ∼ 3600 Ma which were derived from mixed-age sources suggests their derivation from an extensive block of “continental” crust.

Refolded syn-ultrahigh-pressure thrust sheets in the south Dabie complex, China: Field evidence and tectonic implications

The exhumation mechanisms of ultrahigh-pressure (UHP) rocks have been a controversial issue ever since the discoveries of coesite and diamond in metamorphic rocks from several orogenic belts. The lack of consensus principally reflects the limited number of combined structural, petrologic, and geochronologic studies that have been carried out in great detail within these terrains. In this study, we derive a complex structural history for the UHP South Dabie terrain based on detailed (1:10000) mapping in the eastern Dabie Shan, and use this to place new constraints on the exhumation process. Structural analysis demonstrates that syn-UHP south-vergent thrusting (D 1 ) was followed by north-vergent folding under amphibolite facies conditions (D 2 ) and a third generation of folding at shallower level (D 3 ). This supports a two-stage exhumation model in which initially rapid exhumation from mantle depth to lower crustal levels is followed by slower exhumation from the lower crust level to the Earth surface. The remarkable structural concordance between lithological units containing UHP mineral assemblages and the enclosing gray gneiss units implies that the apparent difference in peak pressures recorded by different lithologies is not a consequence of tectonic juxtaposition, but a result of differential reequilibration during decompression.

Latest precambrian (Cadomian) zircon ages, Nd isotopic systematics and P-T evolution of granitoid orthogneisses of the Erzgebirge, Saxony and Czech Republic

Single zircons from two orthogneiss complexes, the ‘Grey Gneiss’ and ‘Red Gneiss’, the lowermost tectonic units in the Erzgebirge, were dated. The grey Freiberg Gneiss is of igneous origin and has a 207Pb/206Pb emplacement age of 550±7 Ma. A quartz monzonite from Lauenstein contains idiomorphic zircons with a mean 207Pb/206Pb age of 555±7 Ma as well as xenocrysts ranging in age between ∼ 850 and ∼ 1910 Ma. Red gneisses from the central Erzgebirge contain complex zircon populations, including numerous xenocrysts up to 2464 Ma in age. The youngest, idiomorphic, zircons in all samples yielded uniform 207Pb/206Pb ages between 550±9 and 554±10 Ma. Nd isotopic data support the interpretation of crustal anatexis for the origin of both units. ɛNd(t) values for the grey gneisses are −7.5 and −6.0 respectively, (mean crustal residence ages of ∼1.7–1.8 Ga). The red gneisses have a wider range in ɛNd(t) values from −7.7 to −2.8 (TDM ages of 1.4–1.8 Ga). The zircon ages document a distinct late Proterozoic phase of granitoid magmatism, similar in age to granitoids in the Lusatian block farther north-east. However, Palaeozoic deformation as well as medium pressure metamorphism (∼ 8 kbar/600–650° C) are identical in both gneiss units and distinguish these rocks from the Lusatian granitoids. The grey and red gneisses were overthrust by units with abundant high-pressure relicts and a contrasting P-T evolution. Zircon xenocryst and Nd model ages in the range 1000–1700 Ma are similar to those in granitoid rocks of Lusatia and the West-Sudetes, and document a pre-Cadomian basement in parts of east-central Europe that, chronologically, has similarities with the Sveconorwegian domain in the Baltic Shield.

Rb-Sr redistribution during amphibolite-grade mylonitization: An example from the Hope Valley Shear Zone, Massachusetts, U.S.A.

The Sr and Pb isotopic systematics of minerals and whole rock slabs cut from three large samples of the Ponaganset Gneiss near the Hope Valley Shear Zone (HVSZ) in south-central Massachussets indicate that isotopic redistribution along a Late Paleozoic mylonitic zone in the gneiss occurred in two distinct episodes, via two discrete mechanisms. The Rb-Sr data also provide additional geochronologic evidence for high-grade Alleghanian metamorphism in SE New England. At the sample locality, the Ponaganset Gneiss was sheared at the contact between two lithologically distinct gneiss units. A narrow (<10 cm) band of mylonitic mafic schist also occurs intermittently at the contact. A whole rock age from thin slabs of the mylonitized gray gneiss (274 ± 14) agrees well with a mineral age from the mylonitic gray gneiss (268 ± 5), and with other published age data on Alleghanian metamorphism in southeastern New England. The mafic schist Rb-Sr mineral age (306 ± 4) is significantly older, and may represent the age of the initial episode of deformation and recrystallization. The age-corrected Sr and Pb isotopic signatures of the mylonitized gray gneiss are consistently more radiogenic than those of the adjacent units. Therefore, it is possible that the isotopic alteration of the mylonitic zone involved Sr and Pb from an outside source. Closer examination of the mylonitic gneiss-mafic schist contact also revealed a zone of less radiogenic Sr within 2–3 cm of the contact. The shapes of Rb-Sr isotopic profiles across the contact are indicative of diffusive redistribution which occurred during, or after, the foliation-parallel alteration of Sr and Pb isotopic signatures in the mylonitized gray gneiss. Nonlinear least-squares fits of the data to a diffusion equation yield Dt of approx. 8 × 10 5 m 2 ( D = effective diffusivity of Sr in m 2/s, t = time in s). Attempts to use this result to estimate the duration of Rb-Sr diffusion produced results that are not consistent with geochronological data, which indicate that these rocks experienced high temperatures for up to 10 7 yr.

Petrological-tectonic units in the coesite-bearing metamorphic terrain of the Dabie Mountains, central China and their geotectonic implications

Four main faults outline the metamorphic terrain in the Dabie Mountains, central China. The difference of metamorphic grade in the northern, central and southern parts is well-established. This study suggests that these metamorphic rocks belong to three individual geological-tectonic units, which are the northern Andean-type magmatic arc, the central UHPM rock-bearing melange zone and the mobilized sedimentary cover and basement of Yangtze southern continental margin. An old ductile sheared zone through Wuhe and the south of Yuexi-Shuihou divides the Dabie metamorphic complex into northern and central units. The northern unit is predominantly occupied by tonalitic-trondhjemitic grey gneisses and amphibolites facies metasediments. Several approximately E-W-trending ductile-sheared mylonitic belts cut through the grey gneisses, and are especially developed in the northern and southern margins of the unit (high-strain zones). There is a series of lenses of basic granulites, serpentinites and Grt-pyroxenites in the high-strain zones. They do not have the petrological and geochemical characteristics of ophiolitic rocks. However, they may represent the root of an arc combined with grey gneisses. Our study also demonstrates that all rocks from the northern block did not undergo eclogite facies metamorphism. Mesozoic post-collisional granites occur extensively in the Dabie terrain, especially in the northern unit. Coesite and diamond-bearing eclogites in the central unit indicate a regional ultra-high-pressure metamorphism of pressures of > 30 kbar and temperature of 600–800°C. Except for eclogites, various ultra-high-pressure metamorphic (UHPM) rocks include ultramafic rocks, marbles and other metamorphosed sedimentary rocks. They, as large-scale blocks, are mixed with lower P-T encompassing host rock orthogneisses. The central block is a UHPM rock-bearing melange zone. The south block is a metasedimentary sequence of continental margin of the Yangtze plate. The boundary between the central and southern blocks is located in the north from Huangzhen, extending eastwards through the dam of Huangliangting reservoir. In addition, the Foziling Group, north of the Dabieshan terrain was possibly deposited in an extensional basin on an extensional shear zone, comparable to the basin in the Variscan of France and the Caledonides of S. Norway (Dewey, Ryan and Anderson, Geological Society Special Publication No. 76, pp. 325–343, 1993). The authors hope that this information will provide more valid evidence of regional tectonic evolution.

Two Types of Archaean Grey Gneiss in North China and Their Geological Significance1,2

Abstract Grey gneisses are tonalitic intrusive rocks that have Archaean characteristics and tectonic significance. Archaean grey gneisses in North China may be classified into two types: the Zunhua type and the Hengshan type. Grey gneisses of the Zunhua type alternate with basic granulites, forming a bimodal migmatitic suite. The rock is characterized by complex mineral composition, highly varied REE patterns and positive Eu anomaly and originated from autochthonous or partly autochthonous deep ‐ level migmatization with a high degree (30% ±) of fusion. As the sub ‐ stratum in the root of the early ‐ stage high ‐ grade greenstone belt, they prevented basic rocks from sinking. Grey gneisses of the Hengshan type are characterized by a relatively distinct intrusion form, large ‐ amplitude upward emplacement, rather simple mineral composition, only slight changes of REE patterns and absence of Eu anomaly. The rock is a product of subduction and underthrusting of the high ‐ grade greenstone belt and then fusion and emplacement with a low degree (15% ±) of fusion. In this paper an attempt is made to furnish important information about palaeoplate tectonics and early crustal evolution by means of a study of grey gneisses.

The early evolution of the Southwest Swedish Gneiss Province: geochronological and isotopic evidence from southernmost Sweden

Abstract The continental crust in southwestern Sweden has a complicated history of formation, apparently commencing ∼ 1.75 Ga ago and continuing until the end of the Sveconorwegian orogeny ∼ 0.9 Ga ago. Most of the rocks are polymetamorphic gneisses, but massifs of intrusive rocks can commonly be recognized. This study reports new UPb zircon ages from the province of Skane in the southernmost part of the region. Two gneiss samples and four samples of granitoid rocks were dated by the UPb zircon method. In the two gneiss samples, selected single zircon crystals were analyzed in addition to the standard multi-grain size fractions. The multigrain zircon analyses yielded an upper intercept age of 1613 ± 6 Ma for a grey gneiss from the central part of the province of Skane, and 1557−27+32 Ma for a red aplitic gneiss from southern Skane. Individual zircon crystals from these samples gave 207Pb 206Pb -ages between 1335 and 1669 Ma. For the grey gneiss, the individual zircons define a separate discordia with an upper-intercept age of 1640±16 Ma; in the red gneiss sample they scatter around a line with an upper intercept at 1675±25 Ma. Core-bearing zircons from a red gneissic granite in central Skane yielded an age of 1575−61+77 Ma, while similar zircons from a strongly foliated red granite from the Kullaberg horst in northwestern Skane gave an age of 1497−34+47 Ma. Core-free zircons from the Beden granodiorite in southern Skane have an age of 1449−11+23 Ma, and core-bearing zircons from a gneissic charnockite near Orkelljunga in northern Skane yielded an age of 1452−50+350 Ma. Although the latter age is poorly defined, it allows for similar intrusion ages of the Orkelljunga charnockite and the Beden granodiorite. SmNd isotopic analyses yield TDM ages of 1.84 o 1.99 Ga, and TCHUR ages of 1.48 to 1.60 Ga for these rocks, while RbSr analyses give TUR ages between 1.46 and 1.71 Ga. The combined evidence suggests that the main crust-forming episode in this part of southwestern Sweden occurred 1.6 to 1.7 Ga ago, with presumably subduction-related magmatism forming the protoliths of the red and grey gneisses. Around 1.5 to 1.6 Ga ago red anatectic granites were generated. The Beden granodiorite and the Orkelljunga charnockite, as well as the Varberg charnockite farther north, were all formed ∼ 1.45 Ga ago, possibly by mantle-generated anorogenic magmatism. Some 0.9 Ga ago, after the Sveconorwegian crustal thickening and high-pressure metamorphism, uplift within the Southwest Swedish Gneiss Province exposed lower- to mid-crustal granulitic and charnockitic rocks in its southern part.

An Early Proterozoic U-Pb zircon age from an Eskolabreen Formation gneiss in southern Ny Friesland, Spitsbergen

A preliminary U/Pb zircon age determination has been carried out on a grey gneiss of the Eskolabreen Formation, the lowest observable lithostratigraphic unit of Precambrian metamorphic rocks in southern Ny Friesland, NE Spitsbergen. The obtained age, ca. 2, 400 Ma, is considered to be a metamorphic age and suggests an Early Proterozoic tectonothermal event.

Granulite facies middle-lower crustal xenoliths from Nushan alkali basalt in northeastern Anhui Province, China

Metamorphic xenoliths within the Nushan alkali basalt of northeastern Anhui (NEA), China, are from the middle-lower crust. They could be divided into two end-members: basic and acid. Intermediate xenoliths are scarcely found. Basic two-pyroxene granulites (pyriclasites) were formed at 720–810° C and 7–8 kb. Petrological and geochemical studies indicate that the primary magma of the protoliths of basic granulites was derived from the metasomatized upper mantle, while the parental magma of the acid end-member was probably produced by partial melting of the basic rocks. The protoliths of charnockites and grey gneisses represent respectively the early and late crystallization products of the granitic magma. The Nushan granulites are much different in many aspects from the granulites exposed in the northern part of North China, which implies the inhomogeneity regarding to the early evolution of the North China terrane.

High- P—high- T anatexis of Archaean tonalitic grey gneisses: the Eseka migmatites, Cameroon

The Eseka migmatites provide a good example of high- P—high- T anatexis of Archaean quartz-plagioclase-biotite grey gneisses. Partial melting resulted in the development of neosomes comprising a quartzo-feldspathic fraction of granodioritic to trondhjemitic composition and ferromagnesian minerals represented by hornblende mainly (15–45 vol.%), garnet in lesser amounts (4–8 vol.%) and scarce clinopyroxene. Thermobarometry suggests that anatexis occurred close to the amphibolite-granulite facies transition, under conditions estimated at around 9 kbar and 750°C. A late retrogression is documented by the growth of epidote and chlorite and by plagioclase albitization. Textural relations, mineralogical data and recalculation of the primary mineral assemblages suggest that the biotite dehydration melting conditions were reached leading to the generation of granodioritic melts in equilibrium with K-feldspar, hornblende and garnet (±clinopyroxene). In some cases, this was followed, during melt crystallization, by a retrograde reaction involving the granodioritic melt and hornblende to produce biotite-epidote assemblages and trondhjemitic neosomes. Compared to similar migmatites, the Eseka neosomes are characterized by higher Al and Ti contents of hornblende, the presence of garnet and the lack of sphene which can be related to higher P, T conditions. Geochemical data point to the segregation and removal of some melt in the Eseka migmatites resulting in limited concentration of hornblende and garnet at the site of melting. This together with previous field observations and experimental data, could suggest that some hornblende-rich gneisses or amphibolites found in the lower middle crust may represent a residue left after extraction of anatectic melts. Geochronological and petrological data further suggest that large portions of the Archaean TTG rocks from the Congo craton have been reworked under high-grade conditions during the Paleoproterozoic.