Local Anatexis Research Articles

Episodic water-fluxed anatexis recorded by migmatites from the Paleozoic Wuyi-Yunkai Orogeny in South China

The Wuyi-Yunkai Orogeny (WYO) in South China occurred during the assembly of Gondwana, and has abundant granites and migmatites which have been mostly interpreted as products of dehydration anatexis. However, the early Paleozoic migmatites in the Cathaysia Block, investigated in this study, instead document water-fluxed anatexis during the orogeny. The quartzo-feldspathic migmatites are moderately peraluminous, and have ca. 700 Ma and ca. 1800 Ma inherited zircon sub-populations, indicating that they were derived from Middle Neoproterozoic paragneisses. Titanite is common, while other anhydrous peritectic minerals such as garnet and pyroxene are absent in the migmatites and their leucosomes. Both mineral assemblages and microstructures indicate that the rocks were melted through reactions that include quartz + plagioclase + biotite + H2O = titanite + opaque + melt, K-feldspar + plagioclase + quartz + H2O = melt, and K-feldspar + biotite + quartz + H2O = melt. U-Pb ages of the newly-formed zircon and titanite display at least four age populations, recording episodic melting mainly at ca. 466, 437, 425 and 412 Ma, with evidence of local anatexis extending back to at least 480 Ma. Multiple stages of anatexis with short time intervals may have resulted from water-present melting due to episodic fluid influx. Zircon εHf(t) values are negative (-5 to -15) in the 700 Ma and 1800 Ma inherited zircons to nearly zero (0 to -5) in 470–400 Ma newly-formed zircons, and may have resulted from dissolution of the Lu-rich apatite during water-fluxed anatexis. The newly-formed zircons also have high δ18O values (9–12 ‰). These lines of evidence suggest that fluid flux-influenced melting was induced by metamorphic fluids that were released from polyphase metamorphism during crustal thickening and extension in the WYO, and channelized through regional shear zones. This highlights the extent and significance of water-fluxed anatexis and its role in the differentiation of the continental crust during orogeny.

Geochronological constraints for a two-stage history of the Sveconorwegian rare-element pegmatite province formation

Most pegmatites of southern Norway seem to be derived from anatectic melting of metamorphic rocks during the Sveconorwegian orogeny rather than to be highly evolved residual melts derived from granites. We test this hypothesis by providing new age data for thirteen pegmatites and one granite. Based on these new age data, we distinguish two age groups of Sveconorwegian pegmatites (>1,000 m3 in size); Group 1: 1100–1030 Ma and Group 2: 930–890 Ma. All pegmatites except those from the Østfold area crystallized significantly earlier or later than adjacent granites. The Tørdal granite, yielding an age of 946 ± 4 Ma, is about 40 Ma older than the adjacent pegmatites. Field evidence and the age difference between pegmatites and granites supports an anatectic origin for these pegmatites. Sources of these pegmatite melts are biotite- and biotite-amphibole gneisses and amphibolites. Group 1 pegmatites formed in transpressional regimes after peak metamorphism, whereas Group 2 pegmatites formed in an extensional regime and the required heat for partial melting was provided by mafic magma underplating.Differences in the rheological behavior of amphibolite and granitic gneiss during extensional tectonics are the major reason why Group 2 pegmatites occur preferentially in large amphibolite bodies. Under mid-crustal conditions, amphibolite reacts brittle to semi-brittle forming open structures in an extensional tectonic regime where partial melts drained into. Granitic gneisses react in a ductile manner and do not have the ability to drain partial melt.Pegmatite formation in the Grenville Province, i.e., the Laurentian part of the Grenville–Sveconorwegian orogenic belt, formed between ca. 1090 and 980 Ma peaking at 1010 to 980 Ma. Thus, the Grenville peak postdates the Sveconorwegian Group 1 peak by about 30 Ma. These pegmatites formed in similar orogenic settings, implying that similar tectono-metamorphic developments along the Grenville–Sveconorwegian orogenic belt were diachronous.We conclude that local anatexis is the major pegmatite-melt forming process in the Sveconorwegian as well as Grenville orogen. Local anatexis also may be important in other pegmatite provinces.

Thermal evolution of a Variscan syn-orogenic intracontinental basin (Servoz basin, Western Alps): RSCM geothermometry and geochronology

The Variscan basement of the Aiguilles-Rouges massif (Western Alps) exposes the Servoz syncline which consists of a metavolcano-sedimentary sequence composed of (i) a volcanic unit of unknown age and origin, (ii) Early Carboniferous sedimentary series affected by the Variscan orogeny and intruded by the Montées-Pélissier pluton, and (iii) a Late Carboniferous late-orogenic sedimentary sequence. We combined field investigations, Raman Spectroscopy on Carbonaceous Material geothermometry, and LA-ICPMS U-Th-Pb geochronology on zircon in order to reappraise the sedimentary sequence of the Servoz syncline. Our results allow us to identify three distinct sedimentary formations (F1, F2 and F3). The F1 formation is composed of metagreywackes, bimodal volcanic and magmatic rocks formed during basin opening at an early rifting stage (370–350 Ma) within a back-arc geodynamic setting. This extensional regime was responsible for a high thermal event recorded by a ca. 115 °C/km apparent geothermal gradient. Local anatexis of the basement rocks under the basin is dated at 351 ± 5 Ma. Basin inversion occurred between 350 and 330 Ma in response to oblique collision, with the development of large-scale dextral shear zones and syn-kinematic 340–330 Ma granite intrusions. Subsequent dextral transtension was responsible for the opening of a pull-apart basin between ca. 330 and 310 Ma with the deposition of the F2 phyllite formation that was later deformed by the ongoing dextral transcurrent Variscan tectonics at temperatures between 200 and 350 °C. Finally, the F3 terrigenous sedimentary rocks deposited at ca. 310–290 Ma in a late-orogenic extensional basin. The Alpine-related tectonic event overprinted all the temperatures below 350 °C. Although similar basins have been recognized in other External Crystalline Massifs of the Alps, the Servoz syncline is the first example that allows a major part of the polyphase tectonic evolution, since the early stages of the Devonian, to be recognized. Comparison with similar back-arc basins from the French Central massif, the Vosges massif and the Bohemian massif suggests that the External Crystalline Massifs initially belonged to the Moldanubian hinterlands of the Variscan belt.

Protracted (>60 Myrs) thermal evolution of a Neoproterozoic metasedimentary sequence from eastern Borborema Province (NE Brazil): Thermal and rheological implications for orogenic development

The thermal evolution of orogenic belts is quite varied. While some show rapid cooling, others record high temperatures for tens of millions of years. Here, we report the latter situation based on U-Pb zircon and monazite dating of paragneiss and micaschist from the Surubim Complex (eastern Borborema Province, NE Brazil). The Borborema Province and its African counterparts (Cameroon belt and Nigerian Shield) formed during amalgamation of West Gondwana in the Late Neoproterozoic (Brasiliano/Pan-African Orogeny). The location of the Surubim Complex in the central core of this large orogenic system offers the opportunity to study tectonic processes at work in the inner regions of ancient orogens. Basement-involved thick-skinned tectonics, regional low-angle foliation related to NW-directed thrusting, transcurrent shear zones and intrusion of granitic plutons characterize the Brasiliano Orogeny in the study area. Zircon data indicate provenance of the Surubim Complex mainly from proximal sources, predominantly 2.2–2.0 Ga-old basement rocks (including an orthogneiss here dated at 2132 ± 11 Ma), and from Neoproterozoic extension-related granitoids. The ages of the youngest detrital zircon grains range from 690 to 650 Ma. The weighted mean age (647 ± 4 Ma) of low Th/U zircon overgrowths in one sample is tentatively interpreted as dating syn-extensional metamorphism given the small age difference to the youngest detrital zircon population. A time lag of c. 17 Myrs, between 647 Ma and 630 Ma, suggests a temperature drop due to contractional deformation. The ages of the oldest magmatic grains from a leucosome sample and of monazite grains from a paragneiss sample, together with data from the literature, show that temperatures high enough to promote local anatexis during development of the regional low-angle foliation were reached around 630 Ma. Afterwards, as indicated by zircon and monazite growth and/or recrystallization, temperatures remained relatively elevated until the transition to the transcurrent regime, at c. 590 Ma, and persisted so up to c. 563 Ma. The results outline a protracted thermal history spanning more than 60 Myrs. This evolution is comparable to the tectonothermal evolution of long-lived hot orogens, where persistence of high temperature conditions for a long time has been related to the formation of orogenic plateaus.

The Mesoarchean Amikoq Layered Complex of SW Greenland: Part 1. Constraints on theP–Tevolution from igneous, metasomatic and metamorphic amphiboles

AbstractThe metamorphic history of the Mesoarchean Amikoq Layered Complex within the Akia terrane of SW Greenland was characterised by electron microprobe mineral data and detailed petrography on 12 representative samples, integrated with zircon U–Pb geochronology and petrology. The complex intruded into a >3004 Ma supracrustal association now consisting of granoblastic metabasites with subordinate quartz-rich gneiss. Supracrustal host rocks contain a relict high-temperature assemblage of orthopyroxene–clinopyroxene (± pigeonite exsolution lamellae, exsolved at ~975–1010°C), which is interpreted to pre-date the Amikoq intrusion. Cumulate to granoblastic-textured rocks of the main Amikoq Layered Complex range modally from leuconorite to melanorite, orthopyroxenite to harzburgite/dunite and rare hornblende melagabbro. Observed mineralogy of main complex noritic lithologies is essentially relict igneous with orthopyroxene–biotite and hornblende–plagioclase thermometers yielding temperatures of ~800–1070°C. An anatectic zircon megacryst from a patchy quartzo–feldspathic leucosome hosted in an orthopyroxene-dominated Amikoq rock reflects local anatexis at peak metamorphicP–Tconditions and yields an intrusion minimum age of 3004 ± 9 Ma. Field observations indicate local anatexis of orthopyroxene-dominated lithologies, possibly indicating a post-intrusion peak temperature of >900°C. The last preserved stages of retrogression are recorded in paragneiss plagioclase–garnet, biotite–garnet and host rock ilmenite–magnetite pairs (≤3 kbar and ~380–560°C).The Amikoq Complex intruded a MORB-like crustal section and the former remained relatively undisturbed in terms of modal mineralogy. Preservation of igneous textures and mineralogy are related to an anhydrous, high-grade metamorphic history that essentially mimics igneous crystallisation conditions, whereas local high-strain zones acted as fluid pathways resulting in hydrous breakdown of igneous minerals. There is no evidence of equilibration of the intrusion at sub-amphibolite-facies conditions.

Metamorphic evolution of metasedimentary rocks of the Feira Nova region: Tectonic implications for the Brasiliano Orogeny in eastern Borborema Province, Northeast Brazil

Thermobarometric estimations and X-ray maps were combined to evaluate the metamorphic evolution of metasedimentary rocks (Surubim Complex) in the Feira Nova region, eastern Borborema Province (NE Brazil). Sillimanite-bearing biotite schists, garnet-biotite schists and gneisses were studied. X-ray maps show different growing phases and retrograde zoning patterns for biotite and garnet. High Ca and Mg contents in garnet cores along with high Ti in biotite are interpreted as equilibrium compositions at peak metamorphism. Lower contents of Ca and Mg at garnet rims and of Ti in biotite rims suggest cooling and decompression. Garnet-biotite geothermometer and garnet-Al2SiO5-quartz-plagioclase (GASP) geobarometer constrain high temperature (∼650–760 °C) and medium pressure (∼0.6–0.9 GPa; burial to ∼23–34 km) conditions for the metamorphic peak, which was synchronous to development of a flat-lying foliation and local anatexis. Garnet rims indicate exhumation to ∼19-11 km (∼0.4–0.3 GPa) at ∼590-520 °C, probably during a transpressional regime. The data points to low exhumation and cooling rates. The lack of extensional fabrics in the region right after the Brasiliano Orogeny supports erosional unroofing rather than extensional collapse. Based on kyanite occurrences nearby, we infer a clockwise P-T path related to thickening in a predominant intracontinental setting, followed by exhumation with low erosional and cooling rates.

Multiphase solid inclusions in zoisite-bearing eclogite: evidence for partial melting of ultrahigh-pressure metamorphic rocks during continental collision

Abstract Multiphase solid (MS) inclusions in both garnet and omphacite were investigated for zoisite-bearing eclogite from the ultrahigh-pressure (UHP) metamorphic zone in the Sulu orogen. The results provide petrological evidence for local anatexis during exhumation of deeply subducted continental crust. There are three types of MS inclusions: (1) plagioclase + quartz, (2) plagioclase + quartz + K-feldspar, and (3) barite + plagioclase + K-feldspar ± zoisite/epidote. The host minerals mostly exhibit radial fractures surrounding the MS inclusions. The first and second types of MS inclusions were analyzed for their bulk compositions, yielding high SiO 2 and Na 2 O but very low FeO + MgO + TiO 2 with variable K 2 O for the second type. Trace element analyses of representative MS inclusions yield generally very low concentrations except such large ion lithophile elements as Sr, Ba and Rb. These features suggest different origins for the three types of MS inclusions. The first type of MS inclusions would be primarily derived from dehydration melting of paragonite, whereas the second type of MS inclusions would be derived from dehydration melting of both paragonite and phengite. The third type of MS inclusion may result from the interaction between metamorphic fluid and host mineral in view of the occurrence of barite as a filling phase in the fractures of host minerals. Zoisite breakdown is also indicated by its highly cuspate shape in coexistence with quartz, providing components for growth of garnet during the exhumation. Therefore the anatexis of zoisite-bearing UHP eclogite is primarily driven by the breakdown of hydrous minerals such as phengite and paragonite. The major and trace element compositions of MS inclusions in the eclogites mainly depend on the species of hydrous minerals involved in the mineralogical reactions of dehydration melting in subduction channel.

Continental subduction channel processes: Plate interface interaction during continental collision

The study of subduction-zone processes is a key to development of the plate tectonic theory. Plate interface interaction is a basic mechanism for the mass and energy exchange between Earth’s surface and interior. By developing the subduction channel model into continental collision orogens, insights are provided into tectonic processes during continental subduction and its products. The continental crust, composed of felsic to mafic rocks, is detached at different depths from subducting continental lithosphere and then migrates into continental subduction channel. Part of the subcontinental lithospheric mantle wedge, composed of peridotite, is offscrapped from its bottom. The crustal and mantle fragments of different sizes are transported downwards and upwards inside subduction channels by the corner flow, resulting in varying extents of metamorphism, with heterogeneous deformation and local anatexis. All these metamorphic rocks can be viewed as tectonic melanges due to mechanical mixing of crust- and mantle-derived rocks in the subduction channels, resulting in different types of metamorphic rocks now exposed in the same orogens. The crust-mantle interaction in the continental subduction channel is realized by reaction of the overlying ancient subcontinental lithospheric mantle wedge peridotite with aqueous fluid and hydrous melt derived from partial melting of subducted continental basement granite and cover sediment. The nature of premetamorphic protoliths dictates the type of collisional orogens, the size of ultrahigh-pressure metamorphic terranes and the duration of ultrahigh-pressure metamorphism.

Lithospheric thinning and reworking of Late Archean juvenile crust on the southern margin of the North China Craton: evidence from the Longwangzhuang Paleoproterozoic A‐type granites and their surrounding Cretaceous adakite‐like granites

In the Longwangzhuang area along the southern margin of the North China Craton (NCC), a layer of Cretaceous K‐feldspar granites surrounds Paleoproterozoic granites. The Paleoproterozoic granites are enriched in sodic ferrogedrite and show low Al2O3 (12.3–13.0 wt%) and aluminum saturation index (ASI) (0.93–1.09) with high Zr (609–966 ppm), Y (47.2–96.7 ppm) and Nb (58.7–97.7 ppm) concentrations, resembling A‐type granites. The Cretaceous K‐feldspar granites have relatively high Sr (145–419 ppm), Ba (1252–1660 ppm), Sr/Y (19–127), La/Yb (36–56) ratios, but low Y (4.01–8.88 ppm) and Yb (0.46–0.99 ppm), geochemically resembling adakite‐like rocks. However, these Cretaceous adakite‐like K‐feldspar granites have rather low MgO (0.08–0.26 wt%), Al2O3 (13.6–14.5 wt%), Mg# (11–23), Cr (2.30–4.61 ppm) and high initial 86Sr/87Sr (0.7098–0.7118), as well as abundant K‐feldspar minerals, and are different from typical adakite. Zircon LA–ICP–MS geochronology shows that the two types of granites investigated in this study formed at 1616 ± 20 Ma and 140 ± 1 Ma, respectively. However, zircons from both the Paleoproterozoic and Cretaceous granites yield similar two‐stage Hf model ages at ca. 2.5–2.6 Ga, suggesting the reworking of Late Archean juvenile crust. The Paleoproterozoic A‐type granites show enriched Nd–Hf isotopic features but high whole‐rock oxygen isotopes. These rocks may have formed from partial melting of restitic crustal material during lithospheric thinning, along with extension and experienced crystal fractionation of plagioclase, apatite and magnetite. The Cretaceous adakite‐like K‐feldspar granites were formed by local anatexis of TTG rocks from the Taihua Group, with amphibole as the major residual phase. The local anatexis and accompanying migmatisation suggest crustal compression and possible transpression. Therefore, the lithospheric thinning in the NCC should have taken place after 140 Ma on the southern margin of the NCC. Copyright © 2012 John Wiley & Sons, Ltd.

Geochemistry and petrology of garnet‐bearing S‐type Shir‐Kuh Granite, southwest Yazd, Central Iran

AbstractThe Jurassic Shir‐Kuh granitoid batholith in Central Iran intrudes Lower Jurassic sandstones and shales. The batholith consists of three main facies: (i) a granodioritic facies to the north; (ii) a monzogranitic facies spread throughout the batholith; and (iii) a leucogranitic facies along the northwestern margin. The granodiorites are composed mainly of plagioclase, quartz, K‐feldspar, biotite, and some muscovite, garnet, cordierite, ilmenite, zircon, apatite, and monazite. This facies contains variable amounts of restite minerals which are mainly defined by calcic plagioclase cores and small aggregates of biotite. The monzogranites, with mineral assemblages similar to those in the granodiorites, range from relatively mafic (cordierite‐bearing) to felsic (muscovite‐rich) rocks. The leucogranites, exposed as small stock and dykes, consist mainly of quartz, K‐feldspar, and sodic plagioclase. The batholith is peraluminous, calc‐alkaline, and typical of S‐type, as indicated by Na2O content (2.74%), molecular Al2O3/(CaO + Na2O + K2O) (A/CNK) ratio (1.17), K2O/Na2O ratio (1.39), and isotopic data ([87Sr/86Sr]i = 0.715). The rocks are characterized by enrichment in large ion lithophile elements such as Rb, Th and K and depletion in high field strength elements such as Nb and Ti. Chondrite‐normalized rare earth element (REE) patterns are characterized by light rare earth element (LREE) enrichment, with values of (La/Yb)N between 4.5 and 19.53, unfractionated heavy rare earth element (HREE) with values of (Gd/Yb)N between 0.98 and 2.88, and a distinct negative Eu. The parental magma of the Shir‐Kuh Granite was derived from a plagioclase‐rich metasedimentary source (local anatexis of metagreywacke) in the crust, with heat input from mantle melt components. The separation of restite crystals from the primary melt followed by the fractional crystallization appears to have been an effective differentiation process in the batholith.

Why is amazonitic K-feldspar an earmark of NYF-type granitic pegmatites? Clues from hybrid pegmatites in Madagascar

The presence of amazonitic K-rich feldspar is considered an earmark of evolved granitic pegmatites ofNYF type. Elements of an explanation of the pegmatite-amazonite connection emerged in a study of the classic Anjanabonoina hybrid granitic pegmatite in Madagascar. The NYF assemblages there and at the Sakavalana and Anjahamiary pegmatites contain magmatic U- and Th-rich accessory phases and an amazonitic microcline perthite in miarolitic cavities. The early miarolitic minerals are overgrown by a Li- and Cs-enriched assemblage, depleted in HFSE, containing pale-gray microcline perthite. The amazonitic microcline perthite at Anjanabonoina has a δ 18 O value above 15%o, and that at Sakavalana, close to 13%o, indicative of formation of the NYF-pegmatite-forming magma by the melting of crustal rocks. Locally, the source rocks had become slightly alkaline and enriched in HFSE during periods of metasomatism of the hot granulitic crust accompanying episodes of distension over the course of the protracted Pan-African orogeny. The fertilized crust underwent local anatexis after the culmination of Gondwana re-assembly. The proximity of pyrochlore and other U- and Th-bearing accessory phases in the NYF assemblage provided the ionizing radiation required to interact with H 2 O trapped in the vacancy next to the Pb ions in the structure. The presence of an amazonitic K-feldspar ultimately signals net additions of Pb, U, Th, and the alkalis to the source prior to anatexis during a period of distension after a major orogenic disturbance. Plumbian K-feldspar also occurs in granitic pegmatite bodies formed by anatexis in the vicinity of a galena-bearing orebody (e.g., at Broken Hill, Australia); in this case, the resulting granitic pegmatite is neither of NYF type nor enriched in rare elements, however.

Northward extension of carolina slate belt stratigraphy and structure, south-central virginia: Results from geologic mapping

Geologic mapping in south-central Virginia demonstrates that the stratigraphy and structure of the Carolina slate belt extend northward across a steep thermal gradient into upper amphibolite-facies correlative gneiss and schist. The Neoproterozoic greenschist-facies Hyco, Aaron, and Virgilina Formations were traced northward from their type localities near Virgilina, Virginia, along a simple, upright, northeast-trending isoclinal syncline. This syncline is called the Dryburg syncline and is a northern extension of the more complex Virgilina synclinorium. Progressively higher-grade equivalents of the Hyco and Aaron Formations were mapped northward along the axial trace of the refolded and westwardly-overturned Dryburg syncline through the Keysville and Green Bay 7.5-minute quadrangles, and across the northern end of the Carolina slate belt as interpreted on previous geologic maps. Hyco rocks, including felsic metatuff, metawacke, and amphibolite, become gneisses upgrade with areas of local anatexis and the segregation of granitic melt into leucosomes with biotite selvages. Phyllite of the Aaron Formation becomes garnet-bearing mica schist. Aaron Formation rocks disconformably overlie the primarily felsic volcanic and volcaniclastic rocks of the Hyco Formation as evidenced by repeated truncation of internal contacts within the Hyco on both limbs of the Dryburg syncline at the Aaron-Hyco contact. East-northeast-trending isograds, defined successively by the first appearance of garnet, then kyanite ± staurolite in sufficiently aluminous rocks, are superposed on the stratigraphic units and synclinal structure at moderate to high angles to strike. The textural distinction between gneisses and identifiable sedimentary structures occurs near the kyanite ± staurolite-in isograd. Development of the steep thermal gradient and regional penetrative fabric is interpreted to result from emplacement of the Goochland terrane adjacent to the northern end of the slate belt during Alleghanian orogenesis. This mapping study indicates that the Carolina slate belt does not terminate on the north against through-going faults or rest on higher-grade basement as previously suggested.

Role of the Red River Shear zone, Yunnan and Vietnam, in the continental extrusion of SE Asia

The 1000 km long NW–SE-striking, left-lateral Ailao Shan–Red River shear zone runs from the southeastern corner of Tibet to the Gulf of Tonkin and the South China Sea. It has been used as the prime example of a lithospheric-scale strike-slip fault that has accommodated between 500 and 1000 km of southeastwards extrusion of Indo-China away from the Indian plate indentor. Central to the model of continental extrusion is that such faults cut through the entire lithosphere, that shear heating resulted in high-grade metamorphism and local anatexis, and that the ages of sheared granites along the fault also date the timing of strike-slip shearing. However, structural data from the Red River shear zone clearly show that vertical strike-slip faulting post-dated metamorphism and granite emplacement. Most granites along the shear zone are mantle-related granodiorites or within-plate alkali granites formed prior to shearing along the Red River shear zone. Left-lateral kinematic indicators are ubiquitous within the Red River mylonites, but they are always lower-temperature fabrics, formed after peak sillimanite metamorphism and after granite crystallization. It is suggested that left-lateral strike-slip shearing along the Red River shear zone started after 21 Ma, not at 35 Ma as previously thought, and the fault was purely a crustal structure. None of the geological features used to propose the 500–1000 km offsets are robust, and the total finite offset remains unknown.

Petrogenesis of cordierite-bearing S-type granitoids in Sierra de Chepes, Famatinian orogen, Argentina

During the Lower Paleozoic Famatinian orogeny in the Sierra de Chepes, NW Argentina, emplacement of a major metaluminous igneous suite in the middle crust caused high-temperature (>700 °C) and low-pressure (2–3 kbar) metamorphism. Heating was sufficient to induce local anatexis of the surrounding metasediments. Combined petrology, chemistry/mineralogy, and whole-rock geochemistry studies indicate that the cordierite-bearing granitoids of the Tuaní Granite unit (479 Ma) were mostly produced by in situ partial melting of the surrounding metasedimentary rocks and represent a primary melt. Major (CaO, Na 2O) and trace (Rb, Sr, Ba) element contents in the Tuaní Granite unit strongly suggest anatexis of immature continental platform material. The granitic bodies in the unit are typical S-type granitoids, and the petrogenetic model established here contributes to a better understanding of the transition from high-grade metamorphic to igneous rocks and the link between peraluminous melts and source rocks.

Age and origin of coeval TTG, I- and S-type granites in the Famatinian belt of NW Argentina

Three granitoid types are recognised in the Famatinian magmatic belt of NW Argentina, based on lithology and new geochemical data: (a) a minor trondhjemite–tonalite–granodiorite (TTG) group, (b) a metaluminous I-type gabbro-monzogranite suite, and (c) S-type granites. The latter occur as small cordieritic intrusions associated with 1-type granodiorites and as abundant cordierite-bearing facies in large batholithic masses. Twelve new SHRIMP U-Pb zircon ages establish the contemporaneity of all three types in Early Ordovician times (mainly 470-490 Ma ago). Sr- and Nd-isotopic data suggest that, apart from some TTG plutons of asthenospheric origin, the remaining magmas were derived from a Proterozoic crust-lithospheric mantle section. Trace element modelling suggests that the TTG originated by variable melting of a depleted gabbroid source at 10-12kbar, and the I-type tonalite-granodiorite suite by melting of a more enriched lithospheric source atc.5 kbar. The voluminous intermediate and acidic I-types involved hybridisation with lower and middle crustal melts. The highly peraluminous S-type granites have isotopic and inherited zircon patterns similar to those of Cambrian supracrustal metasedimentary rocks deposited in the Pampean cycle, and were derived from them by local anatexis. Other major components of the S-type batholiths involved melting of deep crust and mixing with the I-type magmas, leading to an isotopic and geochemical continuum.

The Famatinian magmatic arc in the central Sierras Pampeanas: an Early to Mid-Ordovician continental arc on the Gondwana margin

Abstract A new multi-disciplinary study of the central Sierras Pampeanas encompasses fieldwork, petrography, metamorphic and micro-structural analysis, geochemistry and geochronology. Remnants of a low-to-medium grade metasedimentary sequence, which also occurs in the Sierras de Córdoba to the east, are considered regionally equivalent to the Puncoviscana Formation; a ?mid-Cambrian Rb-Sr whole-rock isochron of 513 ± 31 Ma probably dates their main metamorphism. The predominant granitoids of the Los Llanos-Ulapes batholith constitute a calc-alkaline suite representative of the Famatinian subduction-related magmatic arc. The main granodiorite phase of the batholith is associated with an S2 fabric and shear zone formation, and was emplaced late during the deformational history of the metasediments. Conventional and SHRIMP U-Pb zircon dating yielded a combined age of 490 ± 5 Ma. Younger monzogranites gave Rb-Sr whole-rock ages of 470–450 Ma, typical of granites in the Sierra de Famatina, but geochemical continuity with the main granodiorite suite raises the possibility that these are partially reset ages. A minor cordierite granite phase is ascribed to local anatexis caused by heat from the granodiorites. All the calc-alkaline rocks of the Los Llanos-Ulapes batholith have high initial 87 Sr/ 86 Sr (0.7075–0.7105) and low ɛNd t (−4.6 to −6.3), inherited from lower crust. Sm-Nd model ages of 1600–1700 Ma indicate that the underlying crust is identical to that beneath the foreland to the east. This part of the Famatinian arc was thus a continental magmatic arc and was established significantly before the arrival of the allochthonous Precordillera terrane in mid-Ordovician times.

Characterization and tectonic evolution of a Mesoproterozoic island arc in the southern Grenville Orogen, Llano uplift, central Texas

Integrated field and U-Pb geochronologic data document the presence and tectonic evolution of a distinct lithotectonic terrane in the southeastern Llano uplift, here termed the Coal Creek Domain (CCD). The CCD consists of tonalitic and amphibolitic gneisses, foliated gabbroic, tonalitic, and granodioritic intrusions, and the structurally emplaced Coal Creek Serpentinite. The oldest rocks in the CCD consist of tonalitic gneisses with protolith crystallization ages of 1326 Ma and 1301 Ma and pre-1292 Ma amphibolitic gneisses (all ages are U-Pb zircon). They were intruded by dominantly mafic and intermediate with subordinate amounts of felsic low-K plutonic rocks between 1286 Ma and 1275 Ma. High-temperature metamorphism and local anatexis that resulted in the formation of gneisses and migmatites occurred between 1301 Ma and 1286 Ma and is suggested to have occurred at ∼1292 Ma. Metamorphism at 1256 Ma resulted in the growth of zircons in mafic rocks. The regional significance of this metamorphic event is unclear but it may be related to dynamothermal metamorphism of the CCD. The contemporaneous emplacement of low-K gabbro, tonalite, and granodiorite and their spatial association with serpentinite, combined with published geochemical data, strongly suggest that the CCD represents an ensimatic arc. The Coal Creek Serpentinite is interpreted as a fragment of oceanic crust that formed either basement to the arc, or in a related back-arc or intra-arc setting. Combined field, U-Pb geochronologic, isotopic, and geochemical data indicate that the magmatic, metamorphic, and structural history of the CCD differs from that of coeval rocks elsewhere in the Llano uplift. The boundary between the CCD and deformed and metamorphosed granitic and supracrustal rocks to the north is a zone of intense mylonitization and polyphase deformation. This zone is interpreted as the structural boundary between the arc terrane and units to the north.

Late Proterozoic to Early Palaeozoic tectonism in Dronning Maud Land, Antarctica: supercontinental fragmentation and amalgamation

The Mid-Proterozoic-Early Palaeozoic tectonic development of western Dronning Maud Land is examined in the light of recent proposals that suggest geological continuity between Laurentia and Gondwana during the Middle Proterozoic (the SWEAT hypothesis). Gondwana amalgamation from the remaining constituents followed fragmentation and rifting of Laurentia from this supercontinent, culminating in a Late Cambrian orogeny at c . 500 Ma. The Mid-Proterozoic-Early Palaeozoic tectonic development of western Dronning Maud Land was significantly different to that observed in the Transantarctic Mountains, and the c. 500 Ma event in particular is characterized by: (1) a regional isotopic resetting event at c. 470 Ma, (2) scarce coeval magmatism, comprising only one major intrusion of alkaline affinity, and volumetrically small ‘S-type’ granitic sheets derived from local anatexis of gneisses, (3) structural deformation considerably less intense than during the preceding Kibaran (1100 Ma) orogeny, comprising relatively open to gentle folding, (4) reactivation of thrusting initiated during the Kibaran orogeny, with associated metasomatism. It is concluded that the effects of the Late Cambrian ( c . 500 Ma) event are consistent with crustal thickening through large-scale underplating, rather than subduction-related processes as seen in the Ross orogeny of the Transantarctic Mountains. It is also concluded that Gondwana amalgamation occurred along suture lines not currently exposed in outcrop in Dronning Maud Land. These data neither contradict nor support the SWEAT hypothesis as currently argued.

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

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

Oxygen-isotope geochemistry of the Scourian complex, northwest Scotland

The Scourian complex of northwest Scotland underwent granulite-facies metamorphism at 2.7 Ga. Granulite-facies orthogneisses have whole-rock δ 18 O values similar to those of igneous rocks of comparable compositions (ultrabasic gneisses 5.5–5.9‰, basic gneisses 6.4–6.8‰, tonalitic gneisses 9.5–9.7‰), and steep 18 O gradients of up to 3‰in 10cm are recorded at two lithological contacts. These data suggest that large volumes of pervasive metamorphic fluids have not infiltrated the rocks, but do not preclude localized fluid-rock interaction. Scourian paragneisses have lower δ 18 O values (8.5–9.9‰) than typical metasediments, and may have undergone fluid-hosted isotopic exchange with the surrounding orthogneisses. Felsic sheets within tonalitic and basic gneisses have similar δ 18 O values to their hosts: these data, combined with petrological and geochemical information, suggest that the sheets were formed by local anatexis during granulite-facies metamorphism. Fluid infiltration during the 2.5 Ga amphibolite-facies Inverian retrogression caused homogenization of δ 18 O values of the Scourian gneisses. Unaltered tholeiitic Scourie dykes have low δ 18 O values of 2.0‰. It is unlikely that heated, surface-derived fluids infiltrated the Scourian complex, and these dykes were probably emplaced as low-δ 18 O magmas.