Metamorphic Episode Research Articles

Precambrian evolution of the Lhasa terrane, Tibet: Constraint from the zircon U–Pb geochronology of the gneisses

The Lhasa terrane in south Tibet was traditionally considered as the northern margin of the Gondwana Indian continent. However, the Precambrian tectonic evolution of the Lhasa terrane remains poorly constrained. In this paper, we report zircon geochronology and Ti-in-zircon thermometer for the metamorphic rocks from the eastern segment of the Lhasa terrane. The studied three gneisses underwent the amphibolite-facies and granulite-facies metamorphism. Most zircon grains from two gneisses have a structure of core-mantle-rim. The magmatic cores of the zircon yielded the protolith ages of 1784±25Ma and 1782±31Ma; the overgrowth mantles of the zircon yielded the metamorphic ages of 618±3Ma and 604±69Ma; the metamorphic rims of the zircon from one sample yielded an age of 23.2±0.3Ma. Zircon grains from other gneiss have a structure of core-mantle-rim. The mantles of the zircon yielded a metamorphic age of 1117±29Ma. These data show that the Lhasa terrane witnessed Late Paleoproterozoic magmatism, and three episodes of metamorphism in the Late Mesoproterozoic, Late Neoproterozoic and Cenozoic. This results show that the Lhasa terrane and the Indian continent have similar Precambrian tectonic evolution, which was related to the amalgamation of supercontinents Columbia, Rodinia and Gondwana.

Ampandrandava and similar phlogopite deposits in southern Madagascar: Derivation from a silicocarbonatitic melt of crustal origin

The Ampandrandava phlogopite deposit, located near Beraketa in southern Madagascar, and others like it in the region, in both the Androyen and Anosyen domains, were created by the crystallization of pods and dikes of crustally derived silicocarbonatitic melt. The anatectic reaction seems to have involved regional marble with C and O isotopes shifted toward mantle values and, at Ampandrandava, in particular, input from an evaporitic sequence. This hypothesis accounts for the importance of magmatic anhydrite in the pegmatitic pods and the presence of chlorine in the fluorapatite and phlogopite. The pods and dikes were emplaced in phlogopite-bearing clinopyroxenite; the aggressive carbonate–sulfate melt digested the host locally, and the diopside, phlogopite and fluorapatite crystallized quickly owing to a vapor pressure quench rather than a thermal quench. The high-temperature anatectic event occurred after the final episode of regional metamorphism, at a time of exhumation of deep crust, tectonic quiescence accompanying post-orogenic collapse, and anorogenic magmatism once collision and deformation had ceased.

Timing and heat sources for the Barrovian metamorphism, Scotland

New SHRIMP U/Pb zircon ages of 472.2±5.8Ma and 471.2±5.9Ma are presented for the age of peak metamorphism of Barrovian migmatites. 40Ar/39Ar ages for white mica from the Barrovian metamorphic series are presented, and are recalculated using recently-proposed revisions to the 40K decay constants to allow more precise and accurate comparison with U/Pb ages. The 40Ar/39Ar ages are found to vary systematically with increasing metamorphic grade, between c. 465Ma for the biotite zone and c. 461Ma for the sillimanite zone.There is no evidence for any significant metamorphic heating during the first 15Myr of the Grampian Orogeny (before c. 473Ma) or the final 4Myr (after c. 465Ma). The Barrovian metamorphism occurred over a period of ~8Myr within the ~27-Myr Grampian Orogeny. The Barrovian metamorphism records punctuated heating, was temporally and spatially associated with large-scale bimodal magmatism, and developed within crust that was not overthickened. The temporally distinct nature of the Barrovian metamorphic episode within the Grampian Orogeny, and its heating pattern and tectonic context, are not consistent with significant heat contribution from thermal equilibration of overthickened crust. Rather, the Barrovian metamorphism records a transient phase of crustal thermal disequilibrium during the Grampian Orogeny.Temporal and spatial association with Grampian bimodal magmatism is consistent with production of the Barrovian metamorphic series within the middle crust as the result of advection of heat from the lower crust and/or mantle. The Barrovian metamorphic series – the classic example of ‘orogenic regional metamorphism’ – did not form in response to crustal thickening and thermal relaxation, but appears to record large-scale contact metamorphism.

Earliest Paleoproterozoic supracrustal rocks in the North China Craton recognized from the Daqingshan area of the Khondalite Belt: Constraints on craton evolution

The Upper Wulashan “Subgroup” at Daqingshan in the Khondalite Belt of the Western Block of the North China Craton contains an Early Paleoproterozoic (2.5–2.45Ga) supracrustal sequence, incorporating banded iron formations, which we refer to as the Daqingshan Supracrustal Rocks. They contain rounded to elliptical zircon grains that commonly show core–rim or core–mantle–rim structures and precise SHRIMP U–Pb dating of these domains allows the discrimination of a series of tectono-thermal events that straddle the Archean/Proterozoic boundary. Detrital zircon cores with oscillatory zoning have formation ages ranging from 2.55 to 2.50Ga, indicating derivation from Late Neoarchean magmatic rocks. As a result of exhumation and erosion, these became incorporated in earliest Paleoproterozoic sediments that were deposited between 2.50 and 2.45Ga and then underwent high-grade metamorphism at 2.45–2.40Ga. This event variously recrystallized the cores into two main domains: a dark inner domain and a gray outer domain, as imaged in cathodoluminescence. The gray domains commonly show sector zoning and have Th/U ratios of 0.1–0.5, a feature commonly noted in zircons recrystallized under high-grade metamorphic conditions. Metamorphic mantles are generally more homogeneous in structure and have lower Th/U ratios (commonly <0.1) than the recrystallized domains, although they show similar age distributions. However, they are difficult to distinguish when only two domains are present in the zircon and they represent a continuum of recrystallization. The rocks then underwent a second episode of high-grade metamorphism in the Late Paleoproterozoic, as revealed by thin overgrowth rims that are homogeneous or show only weak zoning, have low Th/U ratios (commonly <0.1), and record ages of 1.95–1.90Ga. This is the first time that two tectono-thermal events of Early and Late Paleoproterozoic age have been identified in single rock samples from the Western Block of the North China Craton. The conclusion that unequivocal earliest Paleoproterozoic sediments are present in the North China Craton is also supported by evidence that the Daqingshan Supracrustal Rocks are cut by anatectic garnet granite, previously dated at 2.39Ga. Combined with evidence obtained here and elsewhere in the North China Craton, we conclude that the 2.45–2.40Ga metamorphism was not an extension of the well-established, craton-wide Late Neoarchean tectono-thermal event, but a previously unrecognized episode that followed a short quiescent period that allowed sedimentation of the Daqingshan Supracrustal Rocks.

Geology of the Monapo Klippe, NE Mozambique and its significance for assembly of central Gondwana

The Monapo Klippe in north-east Mozambique is an ovoid-shaped outcrop measuring approximately 35×40km and is clearly visible on satellite and geophysical images. Based on recent field mapping, geochemical studies and new geochronological data, we present a revision of the lithostratigraphy of the klippe and offer a model for its origin and emplacement in the framework of regional tectonics. There are three main groups of rocks within the klippe: (1) the Metachéria Metamorphic Complex; (2) the Mazerapane Intrusive Suite; and (3) the Ramiane Intrusive Suite. The Metachéria Metamorphic Complex consists of a mélange of granulite gneiss, including mafic, felsic, pelitic and carbonate rocks, characterised by a strong penetrative shear fabric. The largely undeformed Mazerapane and Ramiane Suites have intruded into the Metachéria Metamorphic Complex. The Mazerapane Suite consists of foid-bearing ultramafic and mafic gneisses and intrudes into the western half of the complex, whereas the Ramiane Suite is dominated by alkaline granitic rocks, contains no foid-bearing units and intrudes into the eastern half of the complex. In addition to these three main units, there are a number of minor but structurally important units, the main ones of which include amphibolite-facies tonalitic gneisses and the Evate calcite carbonatite. Underlying all of these units is a narrow, high strain mylonite zone. Undeformed pegmatite bodies and dykes cross-cut all rock types of the Monapo Klippe including the marginal mylonite. Near identical dates for the intrusion of the Ramiane Suite at 637±5Ma and metamorphism of the Metachéria Complex at 634±8Ma indicates a major episode of granulite-facies metamorphism and crust generation at this time. The ∼635Ma age for the granulite-facies metamorphism is comparable to granulite-facies events identified in other parts of the East African Orogen in Tanzania, Madagascar and other parts of northern Mozambique to the north of the Lúrio Belt. The absence of granulite-facies rocks in the underlying Nampula Block is consistent with structural arguments that the Monapo Klippe is the remnant of an allochthonous thrust sheet. In this context, the Monapo Complex is very similar to other granulite-facies “klippe” in East Africa, Antarctica and Sri Lanka, lending support to the idea of a Pan-African mega-nappe formerly existing across greater East Gondwanaland.

Multistage gold mineralization at the Lapa mine, Abitibi Subprovince: insights into auriferous hydrothermal and metasomatic processes in the Cadillac–Larder Lake Fault Zone

The Lapa gold deposit contains reserves of 2.4 Mt at 6.5 g/t Au and is one of the few deposits located directly within the Cadillac–Larder Lake Fault Zone (CLLFZ), a first-order crustal-scale fault that separates the Archean Abitibi Subprovince from the Pontiac Subprovince to the south. Gold mineralization is predominantly hosted in highly strained and altered, upper greenschist–lower amphibolite facies mafic to ultramafic rocks of the Piche Group. Auriferous ore zones consist of finely disseminated auriferous arsenopyrite–pyrrhotite ± pyrite and native gold disseminated in biotite- and carbonate-altered wall rocks. Native gold, which is also present in quartz ± dolomite–calcite veinlets, is locally associated with Sb-bearing minerals, especially at depth ≤1 km from surface where the deposit is characterized by a Au–Sb–As association. At vertical depth greater than 1 km, gold is associated with arsenopyrite and pyrrhotite (Au–As association). The mineralogy and paragenesis of the Lapa deposit metamorphosed ore and alteration assemblages record the superposition of three metamorphic episodes (M1, M2, and M3) and three gold mineralizing events. Spatial association between biotitized wall rocks and auriferous arsenopyrite indicates that arsenopyrite precipitation is concomitant with potassic alteration. The predominant Au–As association recognized across the deposit is related to gold in solid solution in arsenopyrite as part of a pre-M2 low-grade auriferous hydrothermal event. However, the occurrence of hornblende + oligoclase porphyroblasts overprinting the biotite alteration, and the presence of porous clusters and porphyroblasts of arsenopyrite with native gold and pyrrhotite indicate an auriferous metasomatic event associated with peak M2 prograde metamorphism. Late retrograde metamorphism (M3) overprints the hornblende–oligoclase M2 assemblage within the host rocks proximal to ore by an actinolite–albite assemblage by precipitation of free gold and Sb–sulfosalts at lower P–T. The complex relationships between ore, structural features, and metamorphic assemblages at Lapa are related to the tectonometamorphic evolution of the Cadillac–Larder Lake Fault Zone at different times and crustal levels, and varying heat and fluid flow regimes. The Lapa deposit demonstrates that early, low-grade gold mineralization within the Cadillac–Larder Lake Fault Zone has benefited from late gold enrichment(s) during prograde and retrograde metamorphism, suggesting that multi-stage processes may be important to form gold-rich orogenic deposits in first order crustal-scale structures.

High-pressure metamorphism in the Early Variscan subduction complex of the SW Iberian Massif

Several units exposed in the boundary area of the Ossa Morena Zone (OMZ) and the South Portuguese Zone (SPZ) preserve petrographic and thermobarometric evidence for an early metamorphic episode (M1), developed under a high-P, low to intermediate-T gradient, related to Early Variscan subduction in the SW Iberian Massif. In the OMZ, these are the Cubito–Moura Unit (Pmin~9.2kbar and T=395±45°C), blueschists from its basal mélange (Pmin~12.4kbar and T=310±11°C), and the underlying Fuenteheridos Group (P=10.9±0.4kbar and T=449±31°C). The equivalent units in the SPZ are the La Minilla Formation (P=8.7±0.4kbar and T=388±16°C) and the lawsonite pseudomorphs-bearing Pulo do Lobo Formation. All these units formed part of an approximately NE verging orogenic wedge (present coordinates) developed by the accretion of subducted slabs of the outermost margin of Gondwana and other elements of the Rheic Ocean realm, from at least the Middle Devonian to the lowermost Tournaisian. High-pressure rocks were subsequently emplaced on more internal zones of the OMZ that only experienced a younger high-T, low to intermediate-P metamorphism (M2). This high-T event was coeval with magmatic activity from the uppermost Devonian to the Middle Mississippian, probably as a consequence of transtensional lithospheric thinning and/or delamination of the lower crust and mantle lithosphere of the Gondwana margin. Pre-Late Devonian synorogenic sedimentation in forearc and back-arc basins of the subduction complex evolved to a Late Devonian–Middle Mississippian foreland basin system in early collisional stages. Finally, a new Middle–Late Mississippian fold-and-thrust belt with opposite (SW) vergence and new foreland basins developed during late collisional stages.

Timing of metamorphism of the Lansang gneiss and implications for left-lateral motion along the Mae Ping (Wang Chao) strike-slip fault, Thailand

The Mae Ping fault (MPF), western Thailand, exhibits dominantly left-lateral strike-slip motion and stretches for >600km, reportedly branching off the right-lateral Sagaing fault in Myanmar and extending southeast towards Cambodia. Previous studies have suggested that the fault assisted the large-scale extrusion of Sundaland that occurred during the Late Eocene–Early Oligocene, with a geological offset of ∼120–150km estimated from displaced high-grade gneisses and granites of the Chiang Mai–Lincang belt. Exposures of high-grade orthogneiss in the Lansang National Park, part of this belt, locally contain strong mylonitic textures and are bounded by strike-slip ductile shear zones and brittle faults. Geochronological analysis of monazite from a sample of sheared biotite-K-feldspar orthogneiss suggests two episodes of crystallization, with core regions documenting Th–Pb ages between c. 123 and c. 114Ma and rim regions documenting a significantly younger age range between c. 45–37Ma. These data are interpreted to represent possible magmatic protolith emplacement for the Lansang orthogneiss during the Early Cretaceous, with a later episode of metamorphism occurring during the Eocene. Textural relationships provided by in situ analysis suggest that ductile shearing along the MPF occurred during the latter stages of, or after, this metamorphic event. In addition, monazite analyzed from an undeformed garnet-two-mica granite dyke intruding metamorphic units at Bhumipol Lake outside of the Mae Ping shear zone produced a Th–Pb age of 66.2±1.6Ma. This age is interpreted to date the timing of dyke emplacement, implying that the MPF cuts through earlier formed magmatic and high-grade metamorphic rocks. These new data, when combined with regional mapping and earlier geochronological work, show that neither metamorphism, nor regional cooling, was directly related to strike-slip motion.

Antarctica and supercontinent evolution: historical perspectives, recent advances and unresolved issues

Abstract The Antarctic rock record spans some 3.5 billion years of history, and has made important contributions to our understanding of how Earth's continents assemble and disperse through time. Correlations between Antarctica and other southern continents were critical to the concept of Gondwana, the Palaeozoic supercontinent used to support early arguments for continental drift, while evidence for Proterozoic connections between Antarctica and North America led to the ‘SWEAT’ configuration (linking SW USA to East Antarctica) for an early Neoproterozoic supercontinent known as Rodinia. Antarctica also contains relicts of an older Palaeo- to Mesoproterozoic supercontinent known as Nuna, along with several Archaean fragments that belonged to one or more ‘supercratons’ in Neoarchaean times. It thus seems likely that Antarctica contains remnants of most, if not all, of Earth's supercontinents, and Antarctic research continues to provide insights into their palaeogeography and geological evolution. One area of research is the latest Neoproterozoic–Mesozoic active margin of Gondwana, preserved in Antarctica as the Ross Orogen and a number of outboard terranes that now form West Antarctica. Major episodes of magmatism, deformation and metamorphism along this palaeo-Pacific margin at 590–500 and 300–230 Ma can be linked to reduced convergence along the internal collisional orogens that formed Gondwana and Pangaea, respectively; indicating that accretionary systems are sensitive to changes in the global plate tectonic budget. Other research has focused on Grenville-age ( c. 1.0 Ga) and Pan-African ( c. 0.5 Ga) metamorphism in the East Antarctic Craton. These global-scale events record the amalgamation of Rodinia and Gondwana, respectively. Three coastal segments of Grenville-age metamorphism in the Indian Ocean sector of Antarctica are each linked to the c. 1.0 Ga collision between older cratons but are separated by two regions of pervasive Pan-African metamorphism ascribed to Neoproterozoic ocean closure. The tectonic setting of these events is poorly constrained given the sparse exposure, deep erosion level and likelihood that younger metamorphic events have reactivated older structures. The projection of these orogens under the ice is also controversial, but it is likely that at least one of the Pan-African orogens links up with the Shackleton Range on the palaeo-Pacific margin of the craton. Sedimentary detritus and glacial erratics at the edge of the ice sheet provide evidence for the c. 1.0 and 0.5 Ga orogenesis in the continental interior, while geophysical data reveal prominent geological boundaries under the ice, but there are insufficient data to trace these features to exposed structures of known age. Until we can resolve the subglacial geometry and tectonic setting of the c. 0.5 and 1.0 Ga metamorphism, there will be no consensus on the configuration of Rodinia, or the size and shape of the continents that existed immediately before and after this supercontinent. Given this uncertainty, it is premature to speculate on the role of Antarctica in earlier supercontinents, but it is likely that Antarctica will continue to provide important constraints when our attention shifts to these earlier events.

The c. 1000–900 Ma and c. 550–500 Ma tectonothermal events in the Prince Charles Mountains–Prydz Bay region, East Antarctica, and their relations to supercontinent evolution

Abstract The Prince Charles Mountains (PCM)–Prydz Bay region in East Antarctica experienced the late Mesoproterozoic/early Neoproterozoic ( c. 1000–900 Ma) and late Neoproterozoic/Cambrian ( c. 550–500 Ma) tectonothermal events. The late Mesoproterozoic/early Neoproterozoic tectonothermal event dominates the Rayner Complex and spreads over the main part of the Prydz Belt. This event includes two episodes (or stages) of metamorphism accompanying the intrusion of syn- to post-orogenic granitoids at c. 1000–960 Ma and c. 940–900 Ma. The c. 1000–960 Ma metamorphism in the northern PCM and Mawson Coast records medium- to low-pressure granulite facies conditions accompanied by a near-isobaric cooling path, whereas the c. 940–900 Ma metamorphism in Kemp Land reaches relatively higher P – T conditions followed by a near-isothermal decompression or decompressive cooling path. The late Mesoproterozoic/early Neoproterozoic orogeny (i.e. the Rayner orogeny) involved a long-lived ( c. 1380–1020 Ma) magmatic accretion along continental/oceanic arcs and a protracted or two-stage collision of the Indian craton with a portion of East Antarctica, forming the Indian–Antarctic continental block independent of the Rodinia supercontinent. The late Neoproterozoic/Cambrian tectonothermal event pervasively overprinted on both Archaean–Proterozoic basements and cover sequences in the Prydz Belt. Except for high-pressure granulite boulders from the Grove Mountains, the metamorphism of most rocks records medium-pressure granulite facies conditions with a clockwise P–T path. In contrast, this event is lower grade (greenschist–amphibolite facies) and localized in the PCM. Regionally, the late Neoproterozoic/Cambrian tectonothermal event seems to have developed on the southeastern margin of the Indo-Antarctic continental block, suggesting that the major suture should be located southeastwards of the presently exposed Prydz Belt. The precise dating for different rock types reveals that the late Neoproterozoic/Cambrian orogeny (i.e. the Prydz orogeny) commenced at c. 570 Ma and lasted until c. 490 Ma, which is roughly contemporaneous with the late collisional stage of the Brasiliano/Pan-African orogenic systems in Gondwanaland. Therefore, the final assembly of the Gondwana supercontinent may have been completed by the collision of a number of cratonic blocks during the same time period.

Análise estrutural e metamórfica da região de Sucuru (Paraíba): implicações sobre a evolução do Terreno Alto Moxotó, Província Borborema

Inserted on the Alto Moxoto Terrane of the Borborema Province, the Sucuru region (Paraiba state, Brazil) includes two dominantly pre-Ediacaran tectono-stratigraphic domains, bounded by an expressive thrust shear zone (Carmo SZ). The first domain has a metaplutonic nature, being formed by granodioritic to migmatitic orthogneisses (Floresta Complex) cut by several intrusive suites. These suites comprise a unit of mafic-ultramafic nature (Malhada Vermelha), a second one mainly of granitic-granodioritic composition (Pedra d'Agua) and a last suite whose composition ranges from syenitic to syenogranitic (Serra da Barra). On the second domain predominate migmatitic paragneisses from of the Sertânia Complex. All these units were cut by Cambrian-Ediacaran felsic dykes and A-type granites. Three tectonic events were recognized. The first Dn episode represents a thrusting with tectonic transport to NW-NNW, being important the Sucuru and Carmo shear zones. The Dn+1 episode is a transcurrent event of Ediacaran age, being important in regional context the Coxixola and Congo shear zones. The final progressive late Ediacaran-Cambrian Dn+2 episode, culminated with the emplacement of the Sucuru dyke swarm and the Prata and Serra da Engabelada A-type granites. Petrographic evidences show that the Dn episode reached its metamoprhic peak on the granulitic or eclogitic facies, whose paragenesis was after re-equilibrated to the amphibolite during the Dn+1 event. The Dn+1 episode developed mylonitic corridors on the amphibolite facies with associated migmatization, while the Dn+2, event reached a greenschist facies restrict to influence of the late and post-tectonic intrusives. The occurrence of garnet amphibolites with symplectite texture along the metamorphic path Mn-Mn+1 suggests that the thrust event represented a high pressure metamorphic episode, indicating a probable Paleoproterozoic suture.

U–Pb geochronological constraints on the timing of episodic regional metamorphism and rapid high-T exhumation of the Grand Forks complex, British Columbia

The Grand Forks complex (GFC) is a fault-bounded metamorphic core complex in the southern Omineca Belt of British Columbia, Canada. It experienced prograde metamorphism ranging from upper-amphibolite to granulite facies conditions during the Mesozoic to early Tertiary compressional stage of the Cordilleran orogeny. Peak metamorphism was followed by multi-stage exhumation in the Early Eocene. This study provides U–Pb monazite and zircon constraints on the timing of metamorphic episodes in the GFC and subsequent high-T, amphibolite facies decompression in the Early Eocene. Monazite LA-ICP-MS ages from metapelitic gneisses record episodic metamorphism from the Late Jurassic to Paleocene, with peak metamorphism occurring between ~59 and 50Ma. Peak metamorphism was followed by rapid, near-isothermal decompression of the GFC between ~52 and 50Ma, and leucosome crystallization at ~50Ma. Thermodynamic modeling of metapelites in the system MnNCKFMASHPYCe predicts that monazite was not stable at peak metamorphic conditions, consistent with the dominant population of ~59Ma ages representing growth along the prograde path, most likely at subsolidus conditions. Growth of widespread high-Y monazite rims (~50Ma) is predicted along suprasolidus decompression and cooling paths. Zircon SHRIMP ages from igneous bodies in the GFC and hanging wall of the bounding Kettle River fault (KRF) suggest ductile deformation related to high-T decompression of the GFC was ongoing at 51Ma but had ceased by 50Ma, truncated by post-kinematic granitoids. This high-T deformation predates subsequent greenschist facies extension on the overlying KRF. A pre-KRF, hanging wall ductile shear zone is constrained to ~59–51Ma. It deforms 59Ma Ladybird suite leucogranites and may be related to high-T exhumation of the core complex. Rapid, >100°C/Ma cooling rates are required to accommodate high-T (amphibolite facies) exhumation of the GFC at 52–50Ma followed by low-T (greenschist facies) exhumation at ~49Ma.

First 40Ar-39Ar Ages from Low-T Mafic Blueschist Blocks in a Franciscan Mélange near San Simeon: Implications for Initiation of Subduction

The Franciscan Complex of California is a type example of an accretionary prism with widespread high-P/T subduction-zone metamorphism. Low-T, fine-grained, lawsonite-bearing mafic blueschists encased in a shale-matrix mélange near San Simeon have been dated for the first time. 40Ar-39Ar ages were obtained for phengite separated from four blocks and from an actinolitic rind on the margin of one of the blocks. The three blocks with lawsonite + epidote yield ages of between 154 and Ma, while the actinolitic rind yields an age of Ma. These ages are from the part of the Franciscan Complex west of the San Andreas Fault that moved northward at least 300 km with respect to the extensive Franciscan exposures east of the fault. The ages obtained in this study show that some fine-grained, low-T (lawsonite + epidote) Franciscan blueschist blocks formed at the same time as coarse-grained, high-T (garnet ± epidote) Franciscan blueschist blocks. These dated rocks indicate that both high- and low-T mafic blueschists are coeval and probably formed along 1000+ km of the North American Plate margin at ∼160–155 Ma. The episode of dynamic blueschist metamorphism was soon followed by high-Mg rind formation and a long period of nearly static high-P/T conditions.

General relations in the trace-element composition of zircons from eclogites with implications for the age of eclogites in the belomorian mobile belt

Analysis of currently available data (877 individual high-precision zircon analyses) on the composition of zircons from eclogite complexes worldwide reveals general relations in the zircon composition: an anomalous decrease in the Th concentration (no higher than 3 ppm on average) and the Th/U ratio (0.33 on average), a significant decrease in the concentrations of all REE (to 22 ppm) and particularly LREE (<2 ppm), and relatively low concentrations of Y (34 ppm), U (100 ppm), and P (41 ppm) at an elevated Hf concentration (11 400 ppm on average). The REE patterns of eclogitic zircons are noted for pronounced flat HREE patterns, poorly pronounced (if any) negative Eu anomalies, strongly reduced positive Ce anomalies (Ce/Ce* = 11 on average), and U-shaped configurations of LREE patterns up to the development of negative Nd anomalies. The relations detected in the distribution of trace elements and REE in eclogitic zircons are of universal nature and occur irrespective of the rock type (metabasites, metaultrabasites, or gneisses) and the metamorphic pressure (eclogites of high and ultrahigh pressure). The application of the aforementioned criteria makes it possible to reliable distinguish eclogitic zircons from those of magmatic or metamorphic genesis (not related to high-pressure metamorphism). Eclogites in the Belomorian Mobile Belt (in the Salma and Gridino areas) were determined to contain zircons in metagabbro eclogites; the cores of these zircons have an age of 2.8–2.9 Ga and are of magmatic genesis, whereas their outer metamorphic zones have an age of 1.9 Ga and a trace-element composition typical of eclogitic zircons. Hence, the Belomorian Mobile Belt was affected only by single (Svecofennian, at ∼1.9 Ga) episode of eclogite metamorphism of Archean rocks.

Late-Neoarchean magmatism and metamorphism at the southeastern margin of the North China Craton and their tectonic implications

In the present study, two gneiss xenoliths from the Mesozoic Jiagou intrusion at the southeastern margin of the North China Craton (NCC) are examined for in situ zircon SHRIMP U–Pb dating in combination with zircon trace element and Hf isotope analyses. Cathodoluminescence (CL) images and trace element data reveal that most zircons from the dated samples display distinct core–rim structures, i.e., magmatic cores and metamorphic rims. The cores show typical igneous characteristics with oscillatory growth zoning and high rare earth element (REE) contents, whereas the metamorphic rims are characterized by spherical to oval shape, and high and homogeneous luminescent intensity. Zircon SHRIMP U–Pb dating results suggest that the xenoliths formed at 2.55–2.64Ga and experienced high-grade metamorphism at 2.48–2.49Ga. The lower HREE contents, negative Eu anomalies, high Th/U ratios (generally >0.5) and high Ti-in-zircon temperatures (>800°C) of metamorphic zircon rims suggest that the 2.48–2.49Ga event represents an episode of granulite facies metamorphism. These results demonstrate that the lower-crustal rocks in the studied area experienced late-Neoarchean magmatism and subsequent metamorphism. Positive ɛHf(t) values of +2 to +5, with model ages younger than 3.0Ga and formation ages of 2.55–2.64Ga for igneous zircons from the xenoliths indicate that they were derived from juvenile crustal sources and strongly suggest the existence of significant late-Neoarchean crustal growth in the region.

U–Pb ion-microprobe zircon geochronology from the basement inliers of eastern Graham Land, Antarctic Peninsula

New geological mapping combined with U–Pb ion microprobe zircon geochronology on the isolated but locally extensive exposures of crystalline basement inliers of eastern Graham Land has greatly improved our understanding of the region’s early crustal evolution and has allowed a more thorough evaluation of Patagonia–Antarctic Peninsula connections prior to Gondwana break-up. At Eden Glacier, diorite gneisses yield Early Ordovician protolith ages of 487 ± 3 and 485 ± 3 Ma and represent the oldest in situ rocks recorded on the Antarctic Peninsula, and indicate a significant spatial extension of Famatinian-age magmatism of Patagonia. Zircon overgrowths in the Early Ordovician protoliths and granitic leucosomes developed within them record two phases of Permian metamorphism at c . 275 and c . 257 Ma, coincident in part with diorite plutonism of the area at 272 ± 2 Ma. At Adie Inlet, granitic leucosomes from paragneiss have been dated at 276 ± 3 Ma, and these are in turn cut by 257 ± 3 Ma xenolith-rich diorite gneiss. The diorite intruded during a second phase of deformation, which folded the paragneiss leucosomes into tight folds. This whole assembly is cut by intensely brecciated megacrystic granodiorite, which yielded a 259 ± 3 Ma age. South of Cabinet Inlet a very different sequence of events is evident, with Triassic magmatism at c . 236 Ma extensive along the Joerg Peninsula. Migmatitic leucosomes are dated at c . 224 Ma and magmatism and deformation events apparently continued to c . 209 Ma at Cape Casey. Our data indicate that the Devonian and Carboniferous magmatism at Target Hill, considered to represent the ‘classic’ basement complex of the Antarctic Peninsula, is not representative regionally. The Target Hill crustal block contains a major break along Cabinet Inlet; to the north, Ordovician and Permian protoliths were variably migmatized during two episodes of Permian deformation and metamorphism, whereas to the south, Triassic protoliths and Triassic metamorphism are encountered. Supplementary material: U–Pb isotopic data are available at http://www.geolsoc.org.uk/SUP18526 .

U–Th–Pb geochronology of meta-carbonatites and meta-alkaline rocks in the southern Canadian Cordillera: A geodynamic perspective

U–Pb and Th–Pb ages of zircons from seven meta-carbonatite and three meta-alkaline rock samples provide evidence for three distinct episodes of carbonatite and alkaline magmatism in the southern Canadian Cordillera spanning a period of ~460Ma. The earliest, Neoproterozoic event occurred at ~800–700Ma and coincides with the postulated initial break-up of Rodinia. The second, previously undocumented, event of carbonatitic magmatism is constrained to the Late Cambrian at ~500Ma and corresponds to a period of extensional tectonics that affected the western continental margin of North America from the Canadian Cordillera to the southwestern United States. The youngest and most prevalent period of alkaline igneous activity occurred in Late Devonian to Early Carboniferous times at ~360–340Ma and resulted from extensional tectonics, presumably caused by slab rollback. In addition, different episodes of amphibolite-facies metamorphism subsequently affected the igneous rocks between ~155 and 50Ma. This dataset puts new constraints on the timing of carbonatite and alkaline igneous activity and the evolution of (ancestral) North America's western continental margin from Neoproterozoic to Carboniferous times.

Zircons U-Pb and Lu-Hf isotopic and whole-rock geochemical constraints on the Gantaohe Group in the Zanhuang Complex: Implications for the tectonic evolution of the Trans-North China Orogen

The Zanhuang complex is located at the eastern margin of the middle segment of the Trans-North China Orogen, along which the Western and Eastern Blocks collided to form the North China Craton. The complex has been divided into the Archean Zanhuang Group and the Paleoproterozoic Gantaohe and Dongjiao Groups. The Gantaohe Group is composed of meta-volcanic rocks, basal conglomerates, meta-tuffaceous siltstones, meta-sandstones, phyllites, slates and dolomites. Geochemistry of the meta-sedimentary rocks indicates that most of the source rocks are felsic, displaying continental island arc signature, whereas a few of them are mafic and have oceanic island arc feature. U-Pb ages of detrital zircons from the Gantaohe meta-sedimentary rocks yielded three age populations of 2600–2400Ma, 3010–2600Ma and 2400–1900Ma. The primary 2600–2400Ma detrital zircons were most probably sourced from the TTG gneisses in the Zanhuang and adjacent Fuping Complexes. The subordinate 3010–2600Ma detrital zircons were most likely directly or indirectly from the Meso- and Neo-Archean crust of the Eastern Block, and minor amounts of 2400–1900Ma detrital zircons may have derived from the Paleoproterozoic granitic rocks in the Fuping Complex. The crystallization age of 2088±8Ma of a phenocrystic rhyolite and the youngest detrital zircon age peak of 2077±37Ma of a quartzite from the Nansizhang Formation suggest that the sedimentation of the Gantaohe Group began at ~2.09Ga. In combination with the ~1850Ma major metamorphic episode in the Trans-North China Orogen, the depositional age of the group can be constrained between ~2.09 and ~1.85Ga. Taking into account the lithostratigraphic feature, provenance and depositional age, the Gantaohe Group is interpreted as having deposited in a back-arc basin, which is consistent with the model that the unique collision between the Eastern and Western Blocks to form the coherent basement of the North China Craton occurred at ~1.85Ga.

The Early Baikalian crystalline complex in the basement of the Dzabkhan microcontinent of the Early Caledonian orogenic area, Central Asia

Fragments of continental blocks or microcontinents are represented in the Early Caledonian orogenic area of Central Asia (or Early Caledonian superterrane); the largest of these are the Dzabkhan and Tuva-Mongolian microcontinents, with Early and Late Precambrian crystalline basements, respectively. In the linkage zone of these microcontinents, crystalline rocks of the Tarbagatai and Songino blocks that are considered as units of the Early Precambrian ensialic basement of the superterrane are also known. They are composed of strongly metamorphosed rocks formed during the Early Baikalian orogeny about 790 to 820 Ma. U-Pb zircon dating and Nd isotope studies revealed, within the northwestern Dzabkhan microcontinent, the Dzabkhan-Mandal zone of crystalline rocks associated with the Riphean crust-forming process. The age of the gneiss substrate of this zone is estimated as 1.3 to 0.86 Ga. An early episode of metamorphism is dated at about 856 ± 2 Ma. The data available so far indicate a heterogeneous structure of the Dzabkhan microcontinent basement represented by Early Precambrian and Early and Late Baikalian crystalline formations.

Variation of illite/muscovite 40Ar/39Ar age spectra during progressive low-grade metamorphism: an example from the US Cordillera

40Ar/39Ar step-heating data were collected from micron to submicron grain-sizes of correlative illite- and muscovite-rich Cambrian pelitic rocks from the western United States that range in metamorphic grade from the shallow diagenetic zone (zeolite facies) to the epizone (greenschist facies). With increasing metamorphic grade, maximum ages from 40Ar/39Ar release spectra decrease, as do total gas ages and retention ages. Previous studies have explained similar results as arising dominantly or entirely from the dissolution of detrital muscovite and precipitation/recrystallization of neo-formed illite. While recognizing the importance of these processes in evaluating our results, we suggest that the inverse correlation between apparent age and metamorphic grade is controlled, primarily, by thermally activated volume diffusion, analogous to the decrease in apparent ages with depth observed for many thermochronometers in borehole experiments. Our results suggest that complete resetting of the illite/muscovite Ar thermochronometer occurs between the high anchizone and epizone, or at roughly 300 °C. This empirical result is in agreement with previous calculations based on muscovite diffusion parameters, which indicate that muscovite grains with radii of 0.05–2 μm should have closure temperatures between 250 and 350 °C. At high anchizone conditions, we observe a reversal in the age/grain-size relationship (the finest grain-size produces the oldest apparent age), which may mark the stage in prograde subgreenschist facies metamorphism of pelitic rocks at which neo-formed illite/muscovite crystallites typically surpass the size of detrital muscovite grains. It is also approximately the stage at which neo-formed illite/muscovite crystallites develop sufficient Ar retentivity to produce geologically meaningful 40Ar/39Ar ages. Results from our sampling transect of Cambrian strata establish a framework for interpreting illite/muscovite 40Ar/39Ar age spectra at different stages of low-grade metamorphism and also illuminate the transformation of illite to muscovite. At Frenchman Mtn., NV, where the Cambrian Bright Angel Formation is at zeolite facies conditions, illite/muscovite 40Ar/39Ar data suggest a detrital muscovite component with an apparent age ≥967 Ma. The correlative Carrara Fm. is at anchizone conditions in the Panamint and Resting Spring Ranges of eastern California, and in these locations, illite/muscovite 40Ar/39Ar data suggest an early Permian episode of subgreenschist facies metamorphism. The same type of data from equivalent strata at epizone conditions (greenschist facies) in the footwall of the Bullfrog/Fluorspar Canyon detachment in southern Nevada reveals a period of slow-to-moderate Late Cretaceous cooling.