Granitic Orthogneisses Research Articles

Petrology of the Neoproterozoic granulites from Central Dronning Maud Land, East Antarctica – Implications for southward extension of East African Orogen (EAO)

Suturing of east and west Gondwana occurred during the Neoproterozoic along the East African Orogen (EAO) through continent–continent collision. The southern extension of EAO is projected in central Dronning Maud Land (CDML) in east Antarctica. CDML is considered to be a magmato-metamorphic terrain where high-grade metamorphism occurred during Mesoproterozoic. Granulite grade metamorphic assemblages from metapelites (garnet+sillimanite+K feldspar (perthite)+graphite±plagioclase+quartz±melt), mafic granulite (orthopyroxene+garnet±clinopyroxene+plagioclase+quartz) and granitic orthogneiss (garnet+Fe-clinopyroxene+K feldspar+plagioclase+quartz) in the Humboldt Mountains in CDML have been used to deduce the peak metamorphic conditions as ∼850°C and 6–8kbar pressure. In situ chemical dating of monazite in matrix and inclusions in garnet indicates metamorphism to have occurred between 640 and 580Ma and was partially overprinted at ∼540Ma which is correlatable with extensive charnockite and A-type granite emplacements. This Neoproterozoic metamorphism and its correlation with the coast margin Neoproterozoic granulites at Schirmacher oasis is inferred as the extension of EAO in east Antarctica.

Two-stage breakdown of monazite by post-magmatic and metamorphic fluids: An example from the Veporic orthogneiss, Western Carpathians, Slovakia

The initial to advanced stage of monazite breakdown was identified in a granitic orthogneiss from the pre-Alpine basement in the Veľký Zelený Potok Valley (the Veporic Unit, Western Carpathians, central Slovakia). Monazite-(Ce) formed during Variscan metamorphism of the original Cambrian to Ordovician granitic rock. Two younger, Permian post-magmatic hydrothermal, and Cretaceous metamorphic-hydrothermal events caused a breakdown of the monazite to secondary egg-shaped coronal structures (100 to 500μm in diameter) with concentric newly-formed mineral phases. Two principal breakdown stages and newly formed mineral assemblages are recognizable: (1) partial to complete replacement of primary monazite with an internal apatite+ThSiO4 (huttonite or thorite) zone and an external allanite-(Ce) to clinozoisite zone; (2) hydroxylbastnäsite-(Ce) partly replacing apatite+ThSiO4 and allanite to clinozoisite aggregates. The monazite breakdown was initiated by fluid sources differing in composition. Stage (1) originated due to post-magmatic hydrothermal fluids, whereas stage (2) indicates an input of younger, CO2-bearing metamorphic-hydrothermal fluids.

Microstructural and metamorphic evolution of a high‐pressure granitic orthogneiss during continental subduction (Orlica–Śnieżnik dome, Bohemian Massif)

AbstractA microstructural and metamorphic study of a naturally deformed medium‐ to high‐pressure granitic orthogneiss (Orlica–Śnieżnik dome, Bohemian Massif) provides evidence of behaviour of the felsic crust during progressive burial along a subduction‐type apparent thermal gradient (∼10 °C km−1). The granitic orthogneisses develops three distinct microstructural types, as follows: type I – augen orthogneiss, type II – banded orthogneiss and type III – mylonitic orthogneiss, each representing an evolutionary stage of a progressively deformed granite. Type I orthogneiss is composed of partially recrystallized K‐feldspar porphyroclasts surrounded by wide fronts of myrmekite, fully recrystallized quartz aggregates and interconnected monomineralic layers of recrystallized plagioclase. Compositional layering in the type II orthogneiss is defined by plagioclase‐ and K‐feldspar‐rich layers, both of which show an increasing proportion of interstitial minerals, as well as the deformation of recrystallized myrmekite fronts. Type III orthogneiss shows relicts of quartz and K‐feldspar ribbons preserved in a fine‐grained polymineralic matrix. All three types have the same assemblage (quartz + plagioclase + K‐feldspar + muscovite + biotite + garnet + sphene ± ilmenite), but show systematic variations in the composition of muscovite and garnet from types I to III. This is consistent with the equilibration of the three types at different positions along a progradeP−Tpath ranging from <15 kbar and <700 °C (type I orthogneiss) to 19–20 kbar and >700 °C (types II and III orthogneisses). The deformation types thus do not represent evolutionary stages of a highly partitioned deformation at constantP−Tconditions, but reflect progressive formation during the burial of the continental crust. The microstructures of the type I and type II orthogneisses result from the dislocation creep of quartz and K‐feldspar whereas a grain boundary sliding‐dominated diffusion creep regime is the characteristic of the type III orthogneiss. Strain weakening related to the transition from type I to type II microstructures was enhanced by the recrystallization of wide myrmekite fronts, and plagioclase and quartz, and further weakening and strain localization in type III orthogneiss occurred via grain boundary sliding‐enhanced diffusion creep. The potential role of incipient melting in strain localization is discussed.

Delineating crustal domains in Peninsular India: Age and chemistry of orthopyroxene-bearing felsic gneisses in the Madurai Block

The Madurai Block of southern India is rich in granitic orthogneiss, much of which is orthopyroxene-bearing (charnockite). This study has identified that orthogneiss from the northwest of the Madurai Block (broadly defined as north and west of the Karur–Kambam–Painavu–Trichur lineament) has a very different origin from that in the south-eastern Madurai Block.To the north and north-west of the Madurai Block, charnockites record Neoarchaean (∼2.7–2.5Ga) crystallization ages with metamorphic overprinting at ∼535Ma. Their Archaean Nd depleted mantle model ages (2.51–3.01Ga), high ɛNd(T) values (between −3.28 and +4.55) and low to moderate initial 87Sr/86Sr ratios (0.7017–0.7106) indicate a primarily mantle-derived origin with variable involvement of Archaean crustal material. The south-east Madurai Block samples have much younger crystallization ages of 1007±23Ma and 784±18Ma and a range of Nd depleted mantle model ages between 1.38 and 1.69Ga, suggesting a significantly different isotopic evolution of this terrane. The isotopic signature of these charnockites (ɛNd(T)=+2.58 to −3.28, initial 87Sr/86Sr=0.7054–0.7084) suggests the involvement of a mantle source with variable input from the contemporary crust. All samples show subsequent zircon growth, or isotopic disturbance, due to Ediacaran–Cambrian metamorphism (∼535Ma).The sharply contrasting primary igneous U–Pb ages of orthogneisses identify two distinct terranes; an Archaean basement terrane to the north and west and a Proterozoic terrane dominated by metasedimentary rocks to the south and east of the Madurai Block. The isotopic boundary between the two is here defined as a broad zone south of the Karur–Kambam–Painavu–Trichur (KKPT) lineament and appears as a southerly dipping series of seismic reflectors. A correlation is made with central Madagascar where Proterozoic metsedimentary rocks are thrust over Neoarchaean orthogneisses of the Antananarivo Block.

U–Pb geochronology of basement rocks in central Tibet and paleogeographic implications

The ages and paleogeographic affinities of basement rocks of Tibetan terranes are poorly known. New U–Pb zircon geochronologic data from orthogneisses of the Amdo basement better resolve Neoproterozoic and Cambro-Ordovician magmatism in central Tibet. The Amdo basement is exposed within the Bangong suture zone between the Lhasa and Qiangtang terranes and is composed of granitic orthogneisses with subordinate paragneisses and metasedimentary rocks. The intermediate-felsic orthogneisses show a bimodal distribution of Neoproterozoic (920–820 Ma) and Cambro-Ordovician (540–460 Ma) crystallization ages. These and other sparse basement ages from Tibetan terranes suggest the plateau is underlain by juvenile crust that is Neoproterozoic or younger; its young age and weaker rheology relative to cratonic blocks bounding the plateau margins likely facilitated the propagation of Indo-Asian deformation far into Asia. The Neoproterozoic ages post-date Rodinia assembly and magmatism of similar ages is documented in the Qaidaim–Kunlun terrane, South China block, the Aravalli–Delhi craton in NW India, the Eastern Ghats of India, and the Prince Charles mountains in Antarctica. The Amdo Neoproterozoic plutons cannot be unambiguously related to one of these regions, but we propose that the Yangtze block of the South China block is the most likely association, with the Amdo basement representing a terrane that possibly rifted from the active Yangtze margin in the middle Neoproterozoic. Cambro-Ordovician granitoids are ubiquitous throughout Gondwana as a product of active margin tectonics following Gondwana assembly and indicate that the Lhasa–Qiangtang terranes were involved in these tectono-magmatic events. U–Pb detrital zircon analysis of two quartzites from the Amdo basement suggest that the protoliths were Carboniferous–Permian continental margin strata widely deposited across the Lhasa and Qiangtang terranes. The detrital zircon age spectra of the upper Paleozoic Tibetan sandstones and other rocks deposited in East Gondwana during the late Neoproterozoic and Paleozoic are all quite similar, making it difficult to use the age spectra for paleogeographic determinations. There is a suggestion in the data that the Qiangtang terrane may have been located further west along Gondwana’s northern boundary than the Lhasa terrane, but more refined spatial and temporal data are needed to verify this configuration.

Long-lived Paleoproterozoic granitic magmatism in the Seridó-Jaguaribe domain, Borborema Province–NE Brazil

Abstract The northeastern part of the Borborema Province is dominated by Paleoproterozoic migmatitic tonalitic to granitic orthogneisses (Caico Complex) that are the basement for younger, metavolcanosedimentary rock assemblages. Within this complex gneissified, porphyritic metagranitoid rocks (the G2-type augen gneisses) are fairly common and supposed to define a synorogenic magmatism at c . 2.0 Ga. New U–Pb (SHRIMP) on zircons and Sm–Nd data shows that these augen gneisses do not differ significantly from the metaplutonic rocks of the basement complex regarding their nature and emplacement age of the primary magmas. U–Pb ages cluster in a time interval between 2.17 and 2.25 Ga and are correlated with Nd parameters (older t DM model ages and negative initial eNd values) indicating an origin from recycling of an older, probably Neoarchean crust. The regional host rocks that accommodate the augen gneiss could be c . 2.4–2.3 Ga supracrustal sequences presently preserved as small remnants in the Caico Complex. The Paleoproterozoic magmatic activity extends to the Late Paleoproterozoic with the intrusion of the Serra Negra pluton (now a coarse augen gneiss) which yielded a (semi) concordant U–Pb age of c . 1.75 Ga. This magmatic activity, until now unsuspected in the Caico Complex, was probably related to crustal extension and rifting over large areas that include the NE Brazil and the Nigerian shield in Africa.

Early Paleozoic metamorphic rocks of the Erguna block in the Great Xing'an Range, NE China: Evidence for the timing of magmatic and metamorphic events and their tectonic implications

The Erguna block is a crustal unit located along the eastern margin of the Central Asian Orogenic Belt (CAOB) in the far north of China. It contains a variety of khondalitic rocks that include sillimanite- and garnet-bearing gneiss, biotite–plagioclase gneiss and carbonate, in association with hornblende–plagioclase gneiss and a variety of granitic orthogneisses. These rocks are collectively referred to as the Mohe Complex and make up the basement rocks in this poorly exposed terrane. Four samples were analyzed for this study, including two samples of biotite–plagioclase gneiss, one sample of garnet–sillimanite gneiss, and a sample of hornblende-plagioclase gneiss. All samples provide evidence of metamorphism in the form of discrete zircon grains or metamorphic rims around detrital cores and they record remarkably consistent 206Pb/ 238U ages between 495 ± 2 Ma and 497 ± 6 Ma. Detrital zircon cores with oscillatory zoning from the two biotite–plagioclase gneiss samples yield magmatic ages that range from 608 ± 8 Ma to 1015 ± 12 Ma, whereas those in the garnet–sillimanite gneiss range from 678 ± 8 to 1373 ± 17 Ma. These results establish that a range of Meso- to Neoproterozoic magmatic rocks were present in the source area and contributed to the sedimentary protoliths. The youngest zircon core with an age of 608 ± 8 Ma limits the maximum possible age of deposition, indicating that the rocks of the Mohe Complex cannot be Late Archean to Early Proterozoic as previously considered. The ~ 500 Ma age for the granulite facies metamorphic event in the Mohe Complex is identical to that recorded in the Mashan Complex of the Jiamusi block and the Hutou Complex of the Khanka block, > 1000 km to the south-southeast, implying that an extensive early Paleozoic khondalite belt extended across much of the eastern CAOB. Rocks of similar age and metamorphic grade have also been recorded from the Sayang–Baikal Orogen along the southern margin of the Siberia Craton, > 1000 km to the west. It is unclear at present whether these two belts are directly related but, whatever the relationship, it is evident that extensive tracts of khondalitic rocks were present in the eastern CAOB. They were subsequently affected by high-grade metamorphism in the Early Paleozoic, forming collisional accretionary terranes that were accreted during the Late Pan-African global event.

U-PB SHRIMP ZIRCON DATING OF MESOPROTEROZOIC MAGMATIC ROCKS FROM THE SCOTTBURGH AREA, CENTRAL MZUMBE TERRANE, KWAZULU-NATAL, SOUTH AFRICA

Research Article| September 01, 2010 U-PB SHRIMP ZIRCON DATING OF MESOPROTEROZOIC MAGMATIC ROCKS FROM THE SCOTTBURGH AREA, CENTRAL MZUMBE TERRANE, KWAZULU-NATAL, SOUTH AFRICA B.M. EGLINGTON; B.M. EGLINGTON University of Saskatchewan, Saskatoon, Canada, e-mail: bruce.eglington@usask.ca Search for other works by this author on: GSW Google Scholar R.J. THOMAS; R.J. THOMAS British Geological Survey, Keyworth, Notts, NG12 5GG, United Kingdom, e-mail: bthomas@bgs.ac.uk Search for other works by this author on: GSW Google Scholar R.A. ARMSTRONG R.A. ARMSTRONG Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia, e-mail: Richard.Armstrong@anu.edu.au Search for other works by this author on: GSW Google Scholar South African Journal of Geology (2010) 113 (2): 229–235. https://doi.org/10.2113/gssajg.113.2.229 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation B.M. EGLINGTON, R.J. THOMAS, R.A. ARMSTRONG; U-PB SHRIMP ZIRCON DATING OF MESOPROTEROZOIC MAGMATIC ROCKS FROM THE SCOTTBURGH AREA, CENTRAL MZUMBE TERRANE, KWAZULU-NATAL, SOUTH AFRICA. South African Journal of Geology 2010;; 113 (2): 229–235. doi: https://doi.org/10.2113/gssajg.113.2.229 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract New U-Pb zircon SHRIMP dates are presented for two critical meta-igneous rock units from the Scottburgh area, central Mzumbe Terrane, Natal Sector of the Namaqua-Natal Metamorphic Province (“Natal belt”). A sample of the oldest syntectonic granitic orthogneisses of the Mzumbe Terrane, the Mzimlilo Granite, gave a date of 1147 ± 8 Ma. This is interpreted to represent the crystallisation age and thus gives a reliable minimum estimate of the age of the first, collision-related tectono-metamorphic episode (D1) in the Mzumbe Terrane. The date is younger than the earliest foliation in the Tugela Terrane to the north, but is in accordance with previous Ar-Ar dating of shear zones in that terrane. This implies that the early fabrics in the Tugela Terrane were rejuvenated at this time. Magmatic zircons from melanorite of the Equeefa Suite provided a SHRIMP crystallisation date of 1083 ± 6 Ma. The Equeefa Suite comprises a large mafic-ultramafic body west of Scottburgh, associated with a swarm of deformed mafic (amphibolitic) dykes. The suite forms a valuable time marker, having been emplaced after D1 but during the early stages of the major, long-lived oblique sinistral shearing episode, which led to the final juxtaposition of the Mzumbe and Margate Terranes. This event has been dated between ca. 1.05 and 1.0 Ga from previous studies. The Equeefa date is thus consistent with the previous data and defines a period of possible crustal extension between the major tectono-thermal events. Both dates obtained provide important constraints on tectono-magmatic events in this part of the Natal belt and are consistent with previous data. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Mesoproterozoic geology of the Nampula Block, northern Mozambique: Tracing fragments of Mesoproterozoic crust in the heart of Gondwana

The Nampula Block covers over 100,000 km 2, making it the largest Mesoproterozoic crustal segment in northern Mozambique and an important component of the Neoproterozoic to Cambrian (Pan-African) East African Orogen. It is bounded in the north by the WSW–ENE trending Lúrio Belt. The oldest rocks (Mocuba Suite) are a polydeformed sequence of upper amphibolite-grade layered grey gneisses and migmatites associated with intrusive trondhjemite-tonalite-granodiorite and granitic orthogneisses. A banded gneiss, interpreted as a meta-volcanic rock, yielded a U-Pb SIMS zircon date of 1127 ± 9 Ma. Metamorphic rims, dated at ca. 1090 Ma, probably grew during a later magmatic phase, represented by the tonalitic Rapale Gneiss, two samples of which were dated at 1095 ± 19 and 1091 ± 14 Ma, respectively. The earliest (D 1) deformation that took place at approximately this time, was associated with high grade metamorphism and migmatisation of the Mocuba Suite. The geochemistry of these rocks suggests that they were generated in a juvenile, island-arc setting. The Mocuba Suite is interlayered with extensive belts of meta-pelitic/psammitic, calc-silicate and felsic to mafic meta-volcanic paragneisses termed the Molócuè Group. U-Pb data from detrital zircons from a calc-silicate paragneiss gave a bimodal age distribution at ca. 1100 and 1800 Ma, showing derivation from rocks of the same age as the Mocuba Suite and a Palaeoproterozoic source region. The age of the Molócuè Group has been directly determined by dates of 1092 ± 13 and 1090 ± 22 Ma, obtained from two samples of the leucocratic Mamala Gneiss (meta-felsic volcanics?), one of its major constituent components. The final phase of Mesoproterozoic activity is represented by voluminous plutons and sheet-like bodies of foliated megacrystic granite, augen gneiss and granitic orthogneiss of the Culicui Suite, which have A-type granite geochemical characteristics and are interpreted to have been generated in a late tectonic, extensional setting. Three samples from the suite gave identical ages of ca. 1075 Ma. The Nampula Block was extensively re-worked during the major (D 2: Pan-African) collision orogen in Late Neoproterozoic to Cambrian times, when the major regional fabrics were imposed upon the Mesoproterozoic rocks under amphibolite-facies metamorphic conditions. In the dated samples, this orogenic event is represented by metamorphic zircon rim ages of ca. 550 to 500 Ma. The new data indicate that the Mesoproterozoic rocks of the Nampula Block were originally accreted to a Palaeoproterozic crustal Block and the Nampula Block only reached its current position, separated from the other Mesoproterozoic blocks of NE Mozambique by the Lúrio Belt, during Neoproterozoic collision and plate movements. The geological history of the Nampula Block is comparable with that described from other parts of the Mesoproterozoic orogenic belts of the Kalahari craton and helps to constrain an integrated model of their evolution.

Middle Proterozoic and Lower Paleozoic of the Sierra de Umango, Andean Foreland (29°S), Argentina: U-Pb ages and isotopic characteristics.

Igneous and metamorphic units formed mainly during the Middle Proterozoic and Early Paleozoic times have been distinguished in the Sierra de Umango crystalline basement. These include granitic orthogneisses, a sequence of schists and marbles, amphibolites, and three small granitic plutons. Geochemical analyses of granitoids and amphibolites, as well as age determinations using U-Pb and K-Ar methods were carried out. The U-Pb age in zircon of the Juchi Orthogneiss (1108±13 Ma) confirms the previous Rb-Sr Mesoproterozoic dating (1030±30 Ma). The U-Pb in zircon age of El Penon Granite (473±17 Ma) is slighty older than the Rb-Sr age (469±9 Ma). The K-Ar mouscovite dates from El Penon Granite (409±14 and 381±11 Ma) are related to a Devonian deformation and metamorphism. On the basis of Nd and Sr isotopic characteristics, two different sets of units can be identified, one related to the Grenvillian cycle and the second to the Pampean-Famatinian cycles. According to the data obtained for the Sierra de Umango, we interpret that in this part of the Western Sierras Pampeanas there are relics of a Mesoproterozoic basement (Grenville cycle), together with more important exposures of a schists and marble succession, with amphibolites and small granitic plutons, that were formed during Late Proterozoic to Devonian times (Pampean and Famatinian cycles). The major basement structure, formed during Ordovician to Devonian, shows a clear tectonic vergence toward the west, and consists of thrust sheets (nappes) typical of collisional tectonics. These sheets are transported from the inner parts of the orogen (Sierras Pampeanas Orientales) toward the contiguous foreland of the interacting terrane. The Upper Ordovician to Devonian sedimentary rocks of the Precordillera (foreland basin) could complete the geodynamic environment.

Early Oligocene partial melting in the Main Central Thrust Zone (Arun valley, eastern Nepal Himalaya)

The Main Central Thrust Zone (MCTZ) is a key tectonic feature in the architecture of the Himalayan chain. In the Arun valley of the eastern Nepal Himalaya, the MCTZ is a strongly deformed package of amphibolite- to granulite-facies metapelitic schist and granitic orthogneiss. This package is tectonically interposed between the underlying, low-grade, Lesser Himalaya sequences and the overlying, high-grade and locally anatectic, Higher Himalayan Crystallines (HHC). The MCTZ is characterized by a well documented inverted metamorphism from the Grt–Bt zone, across the Ky-in, St-in and -out, Kfs-in, Ms-out and Sil-in isograds. Partial melting with local occurrence of migmatitic segregations has been rarely reported from the highest structural levels of the MCTZ. While it is widely accepted that thrusting along the MCT occurred during the Miocene, geochronological data constraining the timing of crustal anatexis in the upper portion of the MCTZ are still lacking. In order to understand the link between partial melting in the MCTZ and the Miocene activation of the MCT, we present the P– T–time evolution of a kyanite-bearing anatectic gneiss occurring at the highest structural levels of the MCTZ, along the Arun–Makalu transect (eastern Nepal). Microstructural observations combined with P– T pseudosection analysis show that dehydration partial melting occurred in the kyanite-field. After reaching peak conditions at about 820 °C, 13 kbar, the studied sample experienced decompression accompanied by cooling down to 805 °C, 10 kbar, which caused in situ melt crystallization. SHRIMP monazite and zircon geochronology provides evidence that the anatexis affecting the upper portion of the MCTZ occurred during Early Oligocene (∼ 31 Ma). These results demonstrate that in the upper MCTZ, at least in the eastern Himalaya, crustal anatexis was earlier than, and not a consequence of, decompression linked to exhumation along the MCT.

Structural development of high-temperature mylonites in the Archean Wyoming province, northwestern Madison Range, Montana

The Crooked Creek mylonite, in the northwestern Madison Range, southwestern Montana, is defined by several curved lenses of high non-coaxial strain exposed over a 7-km-wide, northeast-trending strip. The country rocks, part of the Archean Wyoming province, are dominantly trondhjemitic to granitic orthogneiss with subordinate amphibolite, quartzite, aluminous gneiss, and sills of metabasite (mafic granulite). Data presented here support an interpretation that the mylonite formed during a period of rapid, heterogeneous strain at near-peak metamorphic conditions during an early deformational event (D1) caused by northwest–southeast-directed transpression. The mylonite has a well-developed L-S tectonite fabric and a fine-grained, recrystallized (granoblastic) texture. The strong linear fabric, interpreted as the stretching direction, is defined by elongate compositional “fish,” fold axes, aligned elongate minerals, and mullion axes. The margins of the mylonitic zones are concordant with and grade into regions of unmylonitized gneiss. A second deformational event (D2) has folded the mylonite surface to produce meter- to kilometer-scale, tight-to-isoclinal, gently plunging folds in both the mylonite and country rock, and represents a northwest–southeast shortening event. Planar or linear fabrics associated with D2 are remarkably absent. A third regional deformational event (D3) produced open, kilometer-scale folds generally with gently north-plunging fold axes. Thermobarometric measurements presented here indicate that metamorphic conditions during D1 were the same in both the mylonite and the country gneiss, reaching upper amphibolite- to lower granulite-facies conditions: 700 ± 50° C and 8.5 ± 0.5 kb. Previous geochronological studies of mylonitic and cross-cutting rocks in the Jerome Rock Lake area, east of the Crooked Creek mylonite, bracket the timing of this high-grade metamorphism and mylonitization between 2.78 and 2.56 Ga, nearly a billion years before the 1.78-Ga Big Sky orogeny, which overprinted the basement rocks exposed in adjacent ranges of the Wyoming province.

Geology of the western Arunachal Himalaya in parts of Tawang and west Kameng districts, Arunachal Pradesh

The Western Arunachal Himalaya comprises migmatitic gneisses of the Sela Group intruded by Tertiary leucogranite (tourmaline, muscovite and K-feldspar bearing), metasedimentaries of Lumla and Dirang Formations and granite gneiss of the Bomdila Group. The litho units of Sela Group are completely migmatised with distinct leuco, melano and mesosomes (Neosome). The mineral assemblages indicate a sedimentary protolith (para gneiss). Concordant metabasic bodies are observed (rarely discordant) indicating older dykes/sills. The migmatites of Sela Group are of Significantly higher grade than the tectonically underlying Dirang Formation, characterized by kyanite ± biotite± staurolite ± sillimanite assemblages. The tourmaline bearing pegmatitic High Himalayan leucogranite crops out in the higher altitudes of the High Himalayan Sequence. The primary mineral assemblages recorded are tourmaline + muscovite + biotite + garnet apart from quartz and K-feldspar. This intrudes the migmatite of Sela Group parallel as well as oblique to earlier gneissosity as lens shaped bodies. The Sela's are thrust over the metasedimentary rocks of Dirang Formation. Earlier workers marked this thrust as the Main Central Thrust (MCT). During the present studies the whole Dirang Formation was found to be a high strained zone with many shear planes, almost all the lithounits are sheared with prolific graphite/carbon emplacement along the permeable shear planes. The Dirang Formation comprises quartzite, kyanite ± staurolite + garnet ± biotite schist, crystalline limestone and carbon phyllite. Basic and ultrabasic rocks intrude these lithounits. The Bomdila Gneiss, which forms a part of the Lesser Himalayan Sequence, is a large body of granite orthogneiss. The primary mineral assemblage includes quartz, K-feldspar, biotite and garnet. Though the gneisses have an intrusive relationship with the Dirang metasediments the contact is marked by a thrust. Metamorphic grade increases from the base of Dirang to the top of the Sela Group. The base of the Dirang Formation is characterized by biotite + garnet assemblages followed by kyanite + staurolite at the top the Dirang Formation. The base of the Sela Group is marked by kyanite ± garnet ± biotite followed by sillimanite ± biotite at the top of Sela Group. Two phases of deformations are recorded from Sela and Dirang rocks with type III interference pattern folds.

Evidence for prolonged mid-Paleozoic plutonism and ages of crustal sources in east-central Alaska from SHRIMP U–Pb dating of syn-magmatic, inherited, and detrital zircon

Sensitive high-resolution ion microprobe (SHRIMP) U–Pb analyses of igneous zircons from the Lake George assemblage in the eastern Yukon–Tanana Upland (Tanacross quadrangle) indicate both Late Devonian (∼370 Ma) and Early Mississippian (∼350 Ma) magmatic pulses. The zircons occur in four textural variants of granitic orthogneiss from a large area of muscovite–biotite augen gneiss. Granitic orthogneiss from the nearby Fiftymile batholith, which straddles the Alaska–Yukon border, yielded a similar range in zircon U–Pb ages, suggesting that both the Fiftymile batholith and the Tanacross orthogneiss body consist of multiple intrusions. We interpret the overall tectonic setting for the Late Devonian and Early Mississippian magmatism as an extending continental margin (broad back-arc region) inboard of a northeast-dipping (present coordinates) subduction zone. New SHRIMP U–Pb ages of inherited zircon cores in the Tanacross orthogneisses and of detrital zircons from quartzite from the Jarvis belt in the Alaska Range (Mount Hayes quadrangle) include major 2.0–1.7 Ga clusters and lesser 2.7–2.3 Ga clusters, with subordinate 3.2, 1.4, and 1.1 Ga clusters in some orthogneiss samples. For the most part, these inherited and core U–Pb ages match those of basement provinces of the western Canadian Shield and indicate widespread potential sources within western Laurentia for most grain populations; these ages also match the detrital zircon reference for the northern North American miogeocline and support a correlation between the two areas.

Precambrian crystalline basement in southern Mongolia as revealed by SHRIMP zircon dating

Single zircon ages determined by ion microprobe (SHRIMP II) for granitoid gneisses from the southern slope of the Baga Bogd massif (Gobi-Altai, southern Mongolia) reveal several episodes of zircon growth, ranging from late Palaeoproterozoic to late Cambrian. The oldest events are documented by a zircon crystallization age for a gneiss protolith at 1519 ± 11 Ma and by a xenocrystic zircon from a dark grey augen-gneiss yielding an age of c. 1701 Ma. Discrete igneous events are recorded in granite-gneisses with protolith emplacement ages of 983 ± 6, 956 ± 3 and 954 ± 8 Ma. These ages provide the first record of early Neoproterozoic magmatic activity in this region. A much younger and discrete magmatic event is recorded by several dioritic to granitic orthogneisses which are tectonically interlayered with the older gneisses and have protolith emplacement ages between 502 and 498 Ma. These late Cambrian granitoids of calc-alkaline affinity are likely to have been emplaced along an active continental margin and suggest that the Baga Bogd Precambrian crustal fragment was either docked against the southward (present-day coordinates) growing margin of the CAOB or was a large enough crustal entity to develop an arc along its margin. We speculate that the Precambrian gneisses of this massif may be part of a crustal fragment rifted off the Tarim Craton.

U–Pb zircon ages (SHRIMP) for Cadomian and Early Ordovician magmatism in the Eastern Pyrenees: New insights into the pre-Variscan evolution of the northern Gondwana margin

New geochronological data from low- to medium-grade metamorphic areas of the Eastern Pyrenees (Canigó, Roc de Frausa and Cap de Creus massifs) confirm the presence of two significant pre-Variscan igneous events: Ediacaran–Early Cambrian and Early Ordovician. The Ediacaran–Early Cambrian (580–540 Ma) magmatism is characterized by metavolcanic plagioclasic gneisses (metatuffs) coeval with sedimentation and by sheets of granitic orthogneisses emplaced in the lower part of the metasedimentary series. In the Canigó and Roc de Frausa massifs, the metatuffs are spatially associated with metabasites. Both lithologies occur as massive layers of lava flows, discontinuous lense-shaped, subvolcanic, gabbroic bodies or volcanoclastic tuffs interbedded in the lower and middle part of the pre-Upper Ordovician metasedimentary succession. This magmatism is bimodal and has a tholeiitic and calc-alkaline affinity. The granitic orthogneisses represent thick laminar intrusions of subaluminous and aluminous composition. Early Ordovician (475–460 Ma) magmatism is represented by laccoliths of aluminous granitic orthogneisses emplaced in the middle part of the pre-Upper Ordovician succession. These geochronological data reveal the existence of an Ediacaran metasedimentary sequence and Cadomian magmatism in the Pyrenees and allow their correlation along the Eastern Pyrenean massifs. The data also show ages ranging from Neoproterozoic to Early Ordovician of the large bodies of granitic orthogneisses that intruded into the series at different levels. Both events represent the final stages of the Cadomian orogeny and its transition to the Variscan cycle in the Eastern Pyrenees. A Cambrian rifting event linking both cycles has not been identified in the Pyrenees to date. Our findings provide a better fit for the pre-Variscan sequences of the Pyrenees with those of the Iberian Massif and allow their comparison with other pre-Variscan massifs in Europe.

Padrões Isotópicos Sm-Nd no limite entre os terrenos Alto Pajeú e Alto Moxotó, PB

Sm-Nd isotopic studies were carried out to recognize and define two different groups of geologictectonic terranes of the eastern most part of the Transversal Zone (central sub-province) of the Borborema Province, northeast of the South American Platform. The first recognized group (I), formed by granitic orthogneisses and supracrustal rocks is the Alto Pajeu Terrane - APT - that exhibits an isotopic signature with T DM values between 2.28 and 1.37 Ga, high negative values for eNd(0) ( ; 0.12 that indicates a possible isotopic fractioning. The second group (II) is composed mostly by orthogneisses (a minor supracrustal belt -Sertânia complex), the Alto Moxoto Terrane -AMT - exhibits an isotopic signature with higher T DM values, between 3.0 and 2.0 Ga, very negative values for eNd , between -20 and -35, and 147Sm/144Nd lower than those of the group I, i.e. between 0.12 and 0.08. The eNd(t) values calculated for the assumed ages of both groups are respectively: group I eNd (950)= -14 up to +1.28; group II - eNd (2000) = -11.28 up to +1.73. Additionally, these distinct isotopic patterns worked out as a complementary tool for the geological mapping, as well as, they have gradually been reiterated by means of the study of the Brasiliano granitic plutonism (Ediacaran in age) that occurs piercing both, APT and AMT. Data obtained by other geochronological methods (U-PB and Rb-Sr) for this granites and for the pre-Brasiliano terranes themselves were in good agreement with these above discriminated patterns.

Padrões isotópicos Sm-Nd no limite entre os terrenos Alto Pajeú e Alto Moxotó (PB)

Sm-Nd isotopic studies were carried out to recognize and define two different groups of geologictectonic terranes of the eastern most part of the "Transversal Zone" (central sub-province) of the Borborema Province, northeast of the South American Platform. The first recognized group (I), formed by granitic orthogneisses and supracrustal rocks is the Alto Pajeú Terrane - APT - that exhibits an isotopic signature with T DM values between 2.28 and 1.37 Ga, high negative values for εNd(0) ( ; 0.12 that indicates a possible isotopic fractioning. The second group (II) is composed mostly by orthogneisses (a minor supracrustal belt -Sertânia complex), the Alto Moxotó Terrane -AMT - exhibits an isotopic signature with higher T DM values, between 3.0 and 2.0 Ga, very negative values for εNd , between -20 and -35, and 147Sm/144Nd lower than those of the group I, i.e. between 0.12 and 0.08. The εNd(t) values calculated for the assumed ages of both groups are respectively: group I εNd (950)= -14 up to +1.28; group II - εNd (2000) = -11.28 up to +1.73. Additionally, these distinct isotopic patterns worked out as a complementary tool for the geological mapping, as well as, they have gradually been reiterated by means of the study of the Brasiliano granitic plutonism (Ediacaran in age) that occurs piercing both, APT and AMT. Data obtained by other geochronological methods (U-PB and Rb-Sr) for this granites and for the pre-Brasiliano terranes themselves were in good agreement with these above discriminated patterns.

U–Pb SHRIMP and Sm–Nd geochronology of granite–gneiss complexes and implications for the evolution of the Central Brazil Archean Terrain

The paper is a first attempt to unravel the Archean multi-stage metaplutonic assemblage of the Meso/Neoarchean terrane of the State of Goiás, Central Brazil, by means of the U–Pb SHRIMP zircon and Sm–Nd techniques. Two stages of granitic plutonism, spanning ca. 140 m.y., were precisely established for the accretion of the gneiss protoliths. The earliest stage embraces tonalitic to granodioritic and minor granitic orthogneisses with Nd juvenile signature, emplaced from ca. 2845 to ca. 2785 Ma, interpreted as the roots of an early arc. Inherited zircon xenocrysts and Nd isotopic data indicate that the juvenile magmas underwent contamination from a sialic crust as old as 3.3 Ga, from which there are, so far, no recognizable exposures. The second stage comprises granodioritic to granitic gneisses and lasted from ca. 2711 to 2707 Ma. Based on their Nd isotopic signatures and on inherited zircon crystals, their protoliths are interpreted as dominantly crustal-derived. The SHRIMP data from zircon crystals did not depict a Paleoproterozoic overprinting on the Archean gneisses, which is due to geological processes with prevailing temperatures below the isotopic stability of the U/Pb/Th system in the mineral. These processes comprise crustal extension and intrusion of a mafic dike swarm at ca. 2.3 Ga, followed by low grade events mostly related to shear zones between ca. 2.15 and 2.0 Ga. The study also revealed the extent of the Pan- African tectono-thermal overprinting on the Archean orthogneisses. Most of the zircon populations show morphological evidence of metamorphic peripheral recrystallization dated between ca. 750 and 550 Ma. One of the banded gneisses with a crystallization age of ca. 2700 Ma (2 σ) has a more complex zircon population including magmatic new grains, which yielded a precise 206Pb/238U crystallization age of 590 ± 10 Ma (2 σ). These new grains are interpreted to have grown in anatectic veins injected within strongly sheared gneiss.The data characterize a widespread Pan-African-aged metamorphic overprinting, culminating with localized anatexis of the Archean orthogneisses.

The Vila de Cruces Ophiolite: A Remnant of the Early Rheic Ocean in the Variscan Suture of Galicia (Northwest Iberian Massif)

The Vila de Cruces ophiolite is one of the ophiolitic units involved in the Variscan suture of the northwest Iberian Massif. This ophiolite consists of a tectonically repeated succession of greenschist facies volcanic rocks, common alternations of metasediments of pelitic or siliceous character, and scarce orthogneisses, metagabbros, and serpentinites. The protolith age of a granitic orthogneiss that intruded the mafic rocks is dated at $$497\pm 4$$ Ma (U‐Pb in zircons). This age can be considered a reference for the generation of the ophiolite. According to their contents of some of the most immobile trace elements, the greenschist and the metagabbros are derived from basaltic magmas with compositions similar to those of island‐arc tholeiites. The influence of a subduction zone in the generation of the original basaltic magmas can be deduced from the marked negative Nb anomaly observed in all the metabasic rocks of this ophiolite. The granitic orthogneisses can also be genetically related to the basic rocks because they are similar to granitic rocks generated in volcanic arcs. The Vila de Cruces ophiolite is interpreted as a suprasubduction zone ophiolite generated in Late Cambrian times, during the early stages of the opening of the Rheic Ocean. The ophiolite was probably generated in a back‐arc basin developed during the first stages of the pulling apart and later drift of one or more peri‐Gondwanan terranes, one of them represented by the upper allochthon of the northwest Iberian Massif.