Orthogneisses in the Spessart Crystalline Complex,north-west Bavaria: Silurian granitoid magmatism at an active continental margin

The Spessart Crystalline Complex, north-west Bavaria contains two orthogneiss units of granitic to granodioritic composition, known as the Rotgneiss and Haibach gneiss, respectively, which are structurally conformable with associated metasediments. The igneous origin of the Rotgneiss is apparent from field and textural evidence, whereas strong deformation and recrystallization in the Haibach gneiss has obscured most primary textures. New geochemical data as well as zircon morphology prove the Haibach gneiss to be derived from a granitoid precursor, which was chemically similar to the Rotgneiss protolith, thus suggesting a genetic link between those two rock units. Both gneiss types have chemical compositions typical of anatectic two-mica leucogranites. They show characteristics of both I- and S-type granites. Rb-Sr whole rock data on the Haibach gneiss provide an isochron age of 407±14 Ma (IR = 0.7077±0.0007; MSWD 2.2), which is slightly younger than the published date for the Rotgneiss (439±15 Ma; IR=0.7048±0.0026; MSWD 4.9). Single zircon dating of six idiomorphic grains, using the evaporation method, yielded a mean ²⁰⁷Pb/²⁰⁶Pb age of 410±18 Ma for the Haibach gneiss and 418±18 Ma for the Rotgneiss. Both zircon ages are within analytical error of the Rb-Sr isochron dates and are interpreted to reflect the time of protolith emplacement in Silurian times. Three xenocrystic zircon grains from the Rotgneiss yielded ²⁰⁷Pb/²⁰⁶Pb ages of 2278±12, 2490±13 and 2734±10 Ma, respectively, suggesting that late Archaean to early Proterozoic crust was involved in the generation of the granite from which the Rotgneiss is derived. Although it is assumed that the granitic protoliths of the two gneisses were formed through anatexis of older continental crust, the relatively low ⁸⁷Sr/⁸⁶Sr initial ratios of both gneisses may also indicate the admixture of a mantle component. The Rotgneiss and the Haibach gneiss thus document granitic magmatism at an active continental margin during late Silurian times.     

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.     

SHRIMP zircon dating and Nd isotopic systematics of Palaeoproterozoic migmatitic orthogneisses in the Epupa Metamorphic Complex of northwestern Namibia

The Epupa Metamorphic Complex constitutes the southwestern margin of the Congo Craton and is exposed in a hilly to mountainous terrain of northwestern Namibia, bordering the Kunene River and extending into southern Angola. It consists predominantly of granitoid gneisses which are migmatized over large areas. This migmatization locally led to anatexis and produced crustal-melt granites such as the Otjitanda Granite. We have undertaken reconnaissance geochemical studies and single zircon U–Pb SHRIMP and Pb–Pb evaporation dating of rocks of the Epupa Complex. The granitoid gneisses, migmatites and anatectic melts are similar in composition and constitute a suite of metaluminous to peraluminous, calc-alkaline granitoids, predominantly with volcanic arc geochemical signatures. The zircon protolith ages for the orthogneisses range from 1861 ± 3 to 1758 ± 3 Ma. Anatexis in the migmatitic Epupa gneisses was dated from a melt patch at 1762 ± 4 Ma, and the anatectic Otjitanda Granite has a zircon age of 1757 ± 4 Ma. Migmatization and anatexis therefore occurred almost immediately after granitoid emplacement and date a widespread high-temperature Palaeoproterozoic event at ∼1760 Ma which has not been recorded elswhere in northern Namibia. The Nd isotopic systematics of all dated samples are surprisingly similar and suggest formation of the protolith from a source region that probably separated from the depleted mantle about 2.4–2.0 Ga ago. A major Archaean component in the source area is unlikely.     

Zircon ages,geochemistry, and Nd isotopic systematics of pre-Variscan orthogneisses from the Erzgebirge,Saxony (Germany), and geodynamic interpretation

High grade granitoid orthogneisses occur in several metamorphic units of the Erzgebirge in the Saxothuringian Zone of the Variscan Belt. The determination of protolith ages and the geochemical characterization of these rocks permit a reconstruction of the Neoproterozoic to early Palaeozoic magmatic and geodynamic history of the Erzgebirge. Single zircon Pb-Pb evaporation and SHRIMP ages combined with major and trace element data and Sm-Nd isotope systematics indicate at least two discrete magmatic events concealed in the so-called red gneisses, one at ~550 Ma in rocks of the medium pressure—medium temperature (MP-MT) unit and the other at ~500–480 Ma in rocks of the high pressure units. The transition zones comprise both Neoproterozoic granitoids and early Palaeozoic metarhyolites. The granitoid gneisses represent Neoproterozoic calc-alkaline granitoids with REE patterns similar to those produced in Andean-type continental margins. The early Palaeozoic muscovite gneisses are geochemically distinct from the older granitoids and may be derived from melts generated in a back-arc setting. Initial _Nd values in all samples overlap and range from –4.1 to –9.2, corresponding to crustal sources with average residence times of 1.5 to 1.9 Ga. Zircon xenocryst ages as old as 2992 Ma provide evidence for Grenvillian, Svecofennian-Birimian-Aazonian and older age components and suggest an association of the Erzgebirge with Avalonia.B. Mingram and A. Kröner have shared senior authorship     

Evolution of the Neoproterozoic Delgo suture zone and crustal growth in Northern Sudan: geochemical and radiogenic isotope constraints

In the Delgo basement area of northern Sudan, low to medium grade metamorphosed volcanic, sedimentary and plutonic rocks are surrounded by high grade gneisses. A NNE-SSW trending suture zone can be defined by the lithological, chemical and structural characteristics of several distinct units. The early Proterozoic gneiss terrain is overlain by metasedimentary units, the metamorphism of which has been dated by the Sm-Nd whole rock-mineral technique (702 ± 27 Ma in the west, 592 ± 16 Ma in the east). In the central part, the Abu Sari volcanic rocks show geochemical signatures of formation at an arc, with a protracted tholeiitic, calc-alkaline and shoshonitic evolution. The overlying El Hamri ophiolite contains chemical features of a back-arc tectonic environments. The ophiolite was dated by the Sm-Nd whole rock method on metagabbros at 752 ± 48 Ma. The further extension of this oceanic basin into the Jebel Rahib in the south-west was dated at 707 ± 54 Ma (Sm-Nd whole rock and minerals).Widespread suite of syn-tectonic granitoid intrusives displays subduction-related characteristics. They where emplaced between 650 to 760 Ma (Pb-zircon evaporation method). Their Nd and Sr isotopic compositions indicate a changing pattern of island arc to active continental margin character along an east-west transect and suggest a west to north-west dipping subduction zone. All units were juxtaposed at the minimum age of 600 Ma and rearranged during an extensional event, which was dated by the Rb-Sr thin slab technique (546 ± 19 Ma) on a migmatite. The Delgo suture provides evidence of a complex terrane pattern in north-east Africa and crustal growth during the Pan-African event by the addition of oceanic material to pre-existing continental crust.     

Protolith and deformation age of the Gneiss-Plate of Kartali in the southern East Uralian Zone

The southern East Uralian Zone consists of granite-gneiss complexes that are embedded in geological units with typical oceanic characteristics. These gneisses have been interpreted as parts of a microcontinent that collided during the Uralian orogeny. The gneiss-plate of Kartali forms the south eastern part of the gneiss mantle surrounding the Dzhabyk pluton. Its post-collisional protolith age of 327±4 Ma is inconsistent with the microcontinent model. The deformation of the gneisses took place in 290±4 Ma at the time of the intrusion of the Dzhabyk magmas. Granites and gneisses cooled and were exhumed together. Therefore, we interpret the gneiss complexes of the East Uralian Zone as marginal parts of the granitic batholiths that were deformed during the ascent and emplacement of the pluton. From Nd and Sr isotope constraints we conclude that the magma source of the gneiss protolith was an island arc. Since no evidence for old continental crust has been discovered in the East Uralian Zone, the Uralian orogeny can no longer be interpreted as a continent-island arc-microcontinent collision. Instead, the geochemical data presented within this paper indicate that the stacking and thrusting of island arc complexes played an important role in the Uralian orogeny.     

Polymetamorphic evolution of the Lewisian complex,NW Scotland,as recorded by U-Pb isotopic compositions of zircon,titanite and rutile

U-Pb isotopic relations in zircon and titanite of granulite and amphibolite gneisses in the Lewisian complex and bordering Laxford Front reveal complex discordance patterns indicating multiple Late Archean and Early Proterozoic crystallization, overgrowth and Pb-loss events. The earliest stages in the evolution of the complex remain poorly resolved. Zircon ages of 2710 Ma date high-grade metamorphism and magmatism probably related to tectonic and magmatic accretion in a continental arc setting. A distinct event at 2490–2480 Ma, possibly initiated by metamorphism and deformation at high-grade conditions, caused amphibolitization of the granulites and emplacement of granitic pegmatites. This event can be correlated with development of Inverian shear zones and formation of granitoid layers along the Laxford Front. The emplacement of a younger generation of granitoid sheets during the Laxfordian event fromed hydrothermal titanite at 1754 Ma in gneisses south of the Laxford Bridge and partially reset older titanite at Scourie. Growth of secondary titanite and rutile also occurred during subsequent low-grade metamorphism at 1690–1670 Ma.     

Early Paleoproterozoic magmatism in the Quanji Massif, northeastern margin of the Qinghai-Tibet Plateau and its tectonic significance: LA-ICPMS U-Pb zircon geochronology and geochemistry

The Quanji Massif is located on the north side of the Qaidam Block and is interpreted as an ancient cratonic remnant that was detached from the Tarim Craton. There are regionally exposed granitic gneisses in the basement of the Quanji Massif whose protoliths were granitic intrusive rocks. Previous studies obtained intrusion ages for some of these granitic gneiss protoliths. The intrusion ages span a wide range from ~ 2.2 Ga to ~ 2.47 Ga. This study has determined the U-Pb zircon age of four granitic gneiss samples from the eastern, central and western parts of the Quanji Massif. CL images and trace elements show that the zircons from these four granitic gneisses have typical magmatic origins, and experienced different degrees of Pb loss due to strong metamorphism and deformation. LA-ICPMS zircon dating yields an upper intercept age of 2381 ± 41 (2σ) Ma from monzo-granitic gneiss in the Hudesheng area and 2392 ± 25 (2σ) Ma from granodioritic gneiss in the Mohe area, eastern Quanji Massif, and 2367 ± 12 (2σ) Ma from monzo-granitic gneiss in the Delingha area, central Quanji Massif, and 2372 ± 22 (2σ) Ma from monzo-granitic gneiss in the Quanjishan area, western Quanji Massif. These results reveal that the intrusive age of the protoliths of the widespread granitic gneisses in the Quanji Massif basement was restricted between 2.37 and 2.39 Ga, indicating regional granitic magmatism in the early Paleoproterozoic, perhaps related to the fragmentation stage of the Kenorland supercontinent. Geochemical results from the granodioritic gneiss from the Mohe area indicate that the protolith of this gneiss is characterized by adakitic rocks derived from partial melting of garnet-amphibolite beneath a thickened lower crust in a rifting regime after continent-continent collision and crustal thickening, genetically similar to the TTG gneisses in the North China Craton. This suggests that the Quanji Massif had a tectonic history similar to the Archean Central Orogenic Belt of North China Craton during the early Paleoproterozoic. We tentatively suggest that the Quanji Massif and the parental Tarim Craton and the North China Craton experienced rifting in the early Paleoproterozoic, after amalgamation at the end of the Archean. The Tarim Craton and North China Craton might have had close interaction from the late Neoarchean to the early Paleoproterozoic.     

Diversity of Archean crust in the eastern Tula Mountains,Napier Complex,East Antarctica

The Napier Complex of Enderby and Kemp Lands forms the north-western part of the East Antarctic Shield and consists predominantly of gneisses and granulites metamorphosed during a ca. 2.8 Ga high-grade and a ca. 2.5 Ga ultra-high temperature event. The western segment of the Napier Complex includes coastal outcrops, islands and nunataks around Amundsen and Casey Bays, and the Tula Mountains. This region records some of the highest metamorphic temperatures measured on Earth, affecting a variety of gneisses as old as ca. 3.8 Ga. Five samples of orthogneiss from the less-studied eastern Tula Mountains, including three granitic, one trondhjemitic and one dioritic gneiss, were dated by zircon U-Pb Secondary Ion Mass Spectrometry (SIMS). The three orthogneisses yield protolith ages of 3750 ± 35 Ma (granitic), 3733 ± 21 (trondhjemitic) Ma and 3560 ± 42 Ma (dioritic), whereas the two other granitic orthogneisses record ages of 2903 ± 14 Ma and 2788 ± 24 Ma. Zircon growth during metamorphism occurred at 2826 ± 10 Ma, and also between 2530 Ma and 2480 Ma. Samples from the Tula Mountains can be geochemically subdivided into Y-HREE-Nb-Ta depleted and undepleted groups. Eoarchean granitoids are included in both geochemical groups, as are Meso- and Neoarchean granitoids. The Y-HREE-Nb-Ta depleted granitoids can be generated by medium- to high-pressure melting of mafic crust, whereas undepleted granitoids can be generated by low-pressure melting. However, relatively high potassium contents in most samples, and the presence of xenocrystic/inherited zircon in some, reflect the likely involvement of felsic crustal sources. This diversity in granitoid composition occurs across the Napier Complex. The lack of a simple correlation between protolith age and geochemical type is an indication that magmatism during the Eoarchean (and later) involved diverse sources and processes, including re-melting and recycling of various crustal components, rather than just the formation of juvenile crust.     

SHRIMP U–Pb zircon geochronology of the Strzelin gneiss,SW Poland: evidence for a Neoproterozoic thermal event in the Fore-Sudetic Block,Central European Variscides

Zircon ages recorded in gneissic rocks have recently been used as criteria to define and correlate various tectonic units and crustal blocks in the central European Variscides. A SHRIMP U–Pb zircon geochronological study of the Strzelin gneiss in the Fore-Sudetic Block (SW Poland) indicates the presence of: (1) inherited zircon cores of Palaeo- to Mesoproterozoic ²⁰⁶Pb-²³⁸U ages (between ca. 2,000 and 1,240 Ma), and (2) zoned rims of Neoproterozoic age with two distinct means of 600±7 and 568±7 Ma. The Proterozoic age range of the cores suggests that different Precambrian crustal elements were the source for the protolith of the gneiss. A likely scenario is the erosion of various Proterozoic granites and gneisses, sedimentation (after 1,240 Ma), and partial resistance of the original components to subsequent metamorphic dissolution and/or anatectic resorption (in Neoproterozoic times). The zoned zircon rims of both of the younger Neoproterozoic ages are indistinguishable in the cathodoluminescence images. The data are interpreted in terms of two different thermal events inducing zoned zircon overgrowth at ca. 600 and 568 Ma. In general, the new results confirm earlier assumptions of the Proterozoic age of the gneiss protoliths, and indicate their similarity to orthogneisses in the East Sudetes tectonic domain (e.g. the Velké Vrbno and Desná gneisses). The Neoproterozoic dates are different from the age of 504±3 reported earlier for the Gocicice gneiss from a neighbouring locality in the Strzelin Massif. The new data strongly indicate a Moravo-Silesian (Bruno-Vistulian) affinity for the Strzelin gneiss and support the hypothesis that the Strzelin Massif lies within the tectonic boundary zone between the West- and East Sudetes domains, which represents the northern continuation of the Moldanubian Thrust.     

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

Single zircons from two orthogneiss complexes, the Grey Gneiss and Red Gneiss, the lowermost tectonic units in the Erzgebirge, were dated. The grey Freiberg Gneiss is of igneous origin and has a ²⁰⁷Pb/²⁰⁶Pb emplacement age of 550±7 Ma. A quartz monzonite from Lauenstein contains idiomorphic zircons with a mean ²⁰⁷Pb/²⁰⁶Pb age of 555±7 Ma as well as xenocrysts ranging in age between 850 and 1910 Ma. Red gneisses from the central Erzgebirge contain complex zircon populations, including numerous xenocrysts up to 2464 Ma in age. The youngest, idiomorphic, zircons in all samples yielded uniform ²⁰⁷Pb/²⁰⁶Pb ages between 550±9 and 554±10 Ma. Nd isotopic data support the interpretation of crustal anatexis for the origin of both units. _Nd(t) values for the grey gneisses are –7.5 and –6.0 respectively, (mean crustal residence ages of 1.7–1.8 Ga). The red gneisses have a wider range in _Nd(t) values from –7.7 to –2.8 (T _DM ages of 1.4–1.8 Ga). The zircon ages document a distinct late Proterozoic phase of granitoid magmatism, similar in age to granitoids in the Lusatian block farther north-east. However, Palaeozoic deformation as well as medium pressure metamorphism ( 8 kbar/600–650° C) are identical in both gneiss units and distinguish these rocks from the Lusatian granitoids. The grey and red gneisses were overthrust by units with abundant high-pressure relicts and a contrasting P-T evolution. Zircon xenocryst and Nd model ages in the range 1000–1700 Ma are similar to those in granitoid rocks of Lusatia and the West-Sudetes, and document a pre-Cadomian basement in parts of east-central Europe that, chronologically, has similarities with the Sveconorwegian domain in the Baltic Shield.     

大别山北部中酸性片麻岩的岩石地球化学特征及其古大地构造意义

大别山北部变质镁铁－ 超镁铁质岩带中的中酸性片麻岩主要可分为三类：①英云闪长质片麻岩、②花岗闪长质片麻岩和③二长花岗质片麻岩,其原岩属于一套钙碱性的中酸性花岗岩系列。岩石地球化学特征表明,它们分别形成于板块碰撞前消减地区（ 消减的活动板块边缘） 、板块碰撞后隆起花岗石区和同碰撞花岗岩区。其中,形成于８００ ～１０００ Ｍａ 左右的英云闪长质片麻岩经受过麻粒岩相变质作用并与该带中属于蛇绿岩成员的变基性熔岩有亲缘关系, 可能属于蛇绿岩中的浅色组分。     

Geology,geochemistry and petrogenesis of late Precambrian granitoids in the Central Hijaz Region of the Arabian Shield

Late Precambrian granitoid rocks occurring within a 44,000 km² area of the western Arabian Shield are subdivided on the basis of geology and petrology into older (820 to 715 Ma) and younger (686 to 517 Ma) assemblages. The older assemblage contains major complexes which can be assigned to either one of a granodioritic or trondhjemitic petrologic association. The earliest granitoid rocks are trondhjemitic tonalites (trondhjemite association), depleted in Ba, Ce, F, La, Li, Nb, Rb, Y and Zr compared to granitoids of the slightly younger granodiorite association, which are related to a calcic, calc-alkaline suite of rocks ranging in composition from gabbro through monzogranite. The plutonic rocks of the older assemblage were probably emplaced in the cores of contemporary island arcs.The younger plutonic assemblage is dominated by three, geochemically distinct, coeval granitic associations: the alkali granite, alkali-feldspar granite and monzogranite associations. The alkali granite association is composed of perthite granites (alkali granites and genetically related alkali-feldspar granites). Rocks of this association are marginally peralkaline or metaluminous and are characterized by low contents of Ba, Co, Li, Rb, Sc and Sr, and high contents of Be, Cu, F, REE, Nb, Sn, Y, Zn and Zr. The alkali-feldspar granite association is mainly composed of alkali-feldspar granites and syenogranites. Rocks of this association are marginally peraluminous or metaluminous and contain low Ba, Sr, and high F, Rb, Sn, Th and U. The monzogranite association consists mainly of monzogranites and granodiorites. Rocks of this association are peraluminous or marginally metaluminous and have the highest contents of Ba, Cu, Co, Li, Sc, Sr, Ta, and V, and the lowest contents of REE, Nb, Rb, Sn, Th, U, Y, Zn and Zr of the three granitic associations.These voluminous granitic magmas, together with the felsic component of a coeval sequence of bimodal volcanic rocks, are partial melts of the earlier island arc terrain produce during a prolonged fusion event. Subsolvus, highca granites of the monzogranite association have I-type features and represent partial melts of previously unfused crust, while low-Ca perthite granites of the alkali granite and alkali-feldspar granite associations have A-type features and represent partial melts of previously fused crust.This type of petrogenetic model can account for much of the petrologic diversity of the Pan-African granitic terrain of the Arabian Shield.     

鲁东地区侵入岩形成时代和期次划分——锆石SHRIMPU—Pb年龄的证据

鲁东地区前寒武纪侵入岩其侵入活动集中于新太古代和新元古代,均遭受不同程度的区域变质和韧性剪切带的改造,形成一套花岗质片麻岩类。新太古代早期TTG质花岗岩分布于胶北隆起区莱州一栖霞一带,经受了区域变质作用,形成一套灰色片麻岩,锆石内核SHRIMPU—Pb年龄2726—2707Ma基本代表岩浆结晶年龄,锆石变质边为2500Ma;古元古代侵入岩——莱州基性一超基性岩组合形成于1900～1850Ma左右,锆石SHRIMPU—Pb年龄（1852±9）Ma,（1868±11）Ma;新元古代侵入岩主要为花岗闪长质-二长花岗质片麻岩,出露于胶南-威海造山带,锆石U～Pb定年结果集中为892～605Ma,其中荣成超单元锆石SHRIMPU—Pb年龄720～780Ma。中生代侵入岩以花岗岩类为主,集中出露于半岛北部和东南沿海一带。燕山早期侵入岩文登超单元、垛崮山超单元、玲珑超单元锆石SHRIMPU—Pb同位素年龄值集中在167～150Ma;燕山晚期侵入岩大店超单元锆石SHRIMPU～Pb年龄120±4Ma,崂山超单元锆石SHRIMPU～Pb同位素年龄值为（120±2）～（114±2）Ma。     

U-Pb (zircon) ages for the gneissic terrane west of the Nile,southern Egypt

The first U-Pb zircon ages are reported for the gneissic bedrock inliers previously interpreted as part of the Nile Craton. The inliers crop out in the Egyptian Western Desert, east of the Uweinat area and west of the Eastern Desert. Multi- and single-grain zircon analyses of granitoid gneiss and migmatite from Gebel Um Shagir, Aswan, and another locality approximately 160 km south-west of Aswan, yield simple discordia with near modern day Pb loss trajectories, and the following Neoproterozoic crystallization ages: 626+4/–3, 634 ± 4 and 741 ± 3 Ma. In contrast, multi- and single-grain U-Pb analyses (zircon and sphene) from an anorthositic gabbro at Gebel Kamil (22°46N 26°21E) and an anorthosite at Gebel El Asr (22°46N 31°10E) yield Archean and Paleoproterozoic emplacement ages. The former yield a crystallization age of > 2.67 Ga and a metamorphic age of 2.0 Ga; the latter a metamorphic age of 0.69 Ga and an inheritance age of 1.9–2.1 Ga. Because high grade gneiss and migmatite of Neoproterozoic, Paleoproterozoic and Archean age crop out west of the Nile, pre-Neoproterozoic crust should no longer be identified by its metamorphic grade. By contrast, mapping the anorthosite and related rocks might provide first-order estimates for the extension of pre-Neoproterozoic crust in north-east Africa. It is suggested that Archean and Paleoproterozoic crust of the Uweinat and Congo Craton are contiguous because these U-Pb (zircon) data show no evidence for a Neoproterozoic thermal overprint in the Gebel Kamil area and there is no pronounced Neoproterozoic magmatic activity south of the Uweinat inlier and north of the Congo Craton.     

Granitoid evolution in Sinai,Egypt, based on precise SHRIMP U–Pb zircon geochronology

Late-stage Pan-African granitoids, including monzogranite, syenogranite and alkali granite, were collected from four separate localities in Sinai. They were selected to represent both the calc-alkaline and alkaline suites that have been viewed as forming separate magmatic episodes in the Eastern Desert of Egypt, with the transition to alkali granite at ~ 610 Ma taken to mark the onset of crustal extension. Although intrusive relations were observed in the field, the emplacement ages of the granitoids cannot be distinguished within analytical uncertainty and they all formed within a restricted time span from 579 to 594 Ma. This indicates that the two suites are coeval and that some calc-alkaline rocks were also likely generated during the late extensional phase. These ages are identical to those recently obtained from similar rocks in the North-Eastern Desert, confirming that Sinai is the northern extension of the Eastern Desert Pan-African terrane of Egypt. Rare inherited zircons with ages of ~ 1790 and ~ 740 Ma are present in syenogranite from northeastern Sinai and indicate that older material is present within the basement. A few zircons record younger ages and, although some may reflect later disturbance of the main zircon population, those with ages of ~ 570 and 535 Ma probably reflect thermal events associated with the extensive emplacement of mafic and felsic dykes in both northeastern and southern Sinai.     

U-Pb and Pb-Pb zircon dating of the older orthogneiss suite in the Silvretta nappe,eastern Alps: Cadomian magmatism in the upper Austro-Alpine realm

Two orthogneiss suites dominate the Silvretta nappe. Primary crystallization of the larger suite (younger orthogneisses) is assumed to be Ordovician in age. The second, adjacent magmatic suite consists of older, alkaline to calc-alkaline, ultrabasic, basic to intermediate and granitic rocks known as older orthogneisses. U-Pb data of multigrain zircon fractions, as well as single zircon stepwise evaporation ²⁰⁷Pb/²⁰⁶Pb results suggest a latest Proterozoic to early Cambrian intrusion age for the protoliths of the older orthogneisses as both dating methods yield early Cambrian crystallization ages of 526±7 and 519±7 Ma for an alkaline granite gneiss; similar results were obtained for two neighbouring calc-alkaline orthogneisses (²⁰⁷Pb/²⁰⁶Pb ages of 533 ± 4 and 568 ± 6 Ma, respectively). The crystal habitus corresponds to P5, S19 and S9 zircons of magmatic origin. Whole-rock initial Sr isotope ratios indicate a primitive source. The igneous protoliths of these older orthogneisses represent a fragment of a Cadomian (Pan-African) crust found in places within the basement of the European Hercynides.     

Constraints of Mantle and Crustal Sources Interaction during Orogenesis of Pre- and Post-collision Granitoids from the Northern Arabian-Nubian Shield: A Case Study from Wadi El-Akhder Granitoids,Southern Sinai,Egypt

The Egyptian older and younger granitic rocks emplaced during pre- and post-collision stages of Neoproterozoic Pan-African orogeny, respectively, are widely distributed in the southern Sinai Peninsula, constituting 70% of the basement outcrops. The Wadi El-Akhder, southwestern Sinai, is a mountainous terrain exposing two granitoid suites, namely the Wadi El-Akhder Older Granites (AOG) and the Homra Younger Granites (HYG). The AOG (granodiorites with subordinate tonalite compositions) have geochemical characteristics of medium-K calc-alkaline, metaluminous to mildly peraluminous granitoids formed in an island-arc environment, which are conformable with well-known Egyptian older granitoids rocks, whereas the HYG display calc-alkaline to slightly alkaline nature, peraluminous syeno-, monzogranites and alkali feldspar granites matching well those of the Egyptian younger granites. With respect to the AOG granitoids, the HYG granites contain lower Al2O3, FeO*, MgO, MnO, CaO, TiO2, Sr, Ba, and V, but higher Na2O, K2O, Nb, Zr, Th, and Rb. The AOG are generally characterized by enrichment in LILE and LREE and depletion in HFSE relative to N-MORB values (e.g., negative Nb and Ta anomalies). The geochemical features of the AOG follow assimilation-fractional crystallization (AFC) trends indicative of extensive crustal contamination of magma derived from a mantle source. The chemical characteristics of the AOG are remarkably similar to those of subduction-related granitoids from the Arabian-Nubian Shield (ANS). The compositional variations from monzogranites through syenogranites to alkali feldspar granite within HYG could not be explained by fractional crystallization solely. Correlating the whole-rock composition of the HYG to melts generated by experimental dehydration melting of meta-sedimentary and magmatic rocks reveals that they appear to be derived by extended melting of psammitic and pelitic metasediments, which is similar to the most of younger granitic suites in the ANS.     

The augen gneisses of the Peloritani Mountains (NE Sicily): Granitoid magma production during rapid evolution of the northern Gondwana margin at the end of the Precambrian

The medium- to high-grade polymetamorphic basement rocks of the Peloritani Mountains, northern Sicily, include large volumes of augen gneiss of controversial age and origin. By means of a geochemical and SHRIMP zircon study of representative samples, the emplacement age of the original granitoid protoliths of the augen gneisses and the most likely processes and sources involved in that granitoid magmatism have been determined. U–Pb dating of three samples from widely spaced localities in the Peloritani Mountains yielded igneous protolith ages of 565 ± 5, 545 ± 4 and 545 ± 4 Ma, respectively. These late Ediacaran/early Cambrian ages are much older than was previously assumed on geological grounds, and are typical of the peri-Gondwanan terranes involved in the geodynamic evolution of the northern Gondwana margin at the end of the Avalonian–Cadomian orogeny. Major and trace element compositions and Sr–Nd isotopic data, in combination with zircon inheritance age patterns, suggest that the granitoid protoliths of the Sicilian and coeval Calabrian augen gneisses were generated by different degrees of mixing between sediment- and mantle-derived magmas. The magmas forming the ca. 545 Ma inheritance-rich granitoids appear to have had a significant contribution from partial melting of paragneiss that is the dominant rock type in the medium- to high-grade Peloritanian basement. The closeness of the inferred deposition age of the greywacke protoliths of the paragneisses with the intrusion age of the granitoids indicates rapid latest Precambrian crustal recycling involving erosion, burial, metamorphism to partial melting conditions, and extensive granitoid magmatism in less than ca. 10 Ma.     

Geochemistry and Origin of Neoproterozoic Granitoids of Meghalaya, Northeast India: Implications for Linkage with Amalgamation of Gondwana Supercontinent

Many granitic bodies intrude the basement gneisses in Meghalaya Plateau, Northeast India. Rb-Sr whole-rock isotopic ages of the granitoids range from 881 to 479 Ma while the ages of the basement orthogneisses vary from 1714 to 1150 Ma. All the plutons are dominantly metaluminous and show geochemical variation. Oxygen isotopic compositions in the granitoids and gneisses are concordant (d¹⁸O: + 5.78% to + 8.70%). However, the gneisses from high-grade terrain have low d¹⁸O value of +2.52% to +5.31%. Initial ⁸⁷Sr/⁸⁶Sr (I_Sr) ratios of the plutons vary from 0.70459 to 0.71487 and tend to increase with progressive younging in age. The geochemical characters suggest derivation of the granites from lower crustal source. The fractionated rare earth patterns observed in the granitoids can be obtained by partial melting of gneisses or diorites. Some gneiss samples have experienced interaction with hydrothermal fluids resulting in lowering d¹⁸O. The isotopic ages of granite plutonism in Meghalaya are similar to the plutonic and tectonothermal events in other parts of India, southwestern Australia and document final amalgamation events of the Gondwana Supercontinent.