The Late Neoproterozoic granitoid magmatism of the Pelotas Batholith, southern Brazil

Geological mapping, petrography, geochemistry, and isotope studies enable the division of the Pelotas Batholith into six granitic suites: Pinheiro Machado (PMS), Erval (ES), Viamão (VS), Encruzilhada do Sul (ESS), Cordilheira (CS), and Dom Feliciano (DFS). The rocks of the PMS show a large compositional range (granite through granodiorite to tonalite), and the suite is considered pre- to syncollisional. Other suites show restricted compositional variations (granite to granodiorite) and are late to postcollisional. In general, the suites are metaluminous to slightly peraluminous (PMS, ES, and VS) or peraluminous (CS) or have alkaline tendencies (ESS and DFS). The magmatic evolution corresponds to high-K calc-alkaline to alkaline magmatism. The suites are enriched in K, Rb, and REE compared with rocks of typical calc-alkaline series. Initial ⁸⁷Sr/⁸⁶Sr ratios vary from 0.705 to 0.716, except in the CS, where they attain values of 0.732–0.740. Sm–Nd T_DM model ages vary between 0.98 and 2.0 Ga, with initial ε_Nd values ranging from −0.3 to −10. U–Pb zircon dates of samples from PMS, VS, and ESS suggest an age between 0.63 and 0.59 Ga for magmatism. Rb–Sr dates of samples of alkaline granites from DFS present ages between 0.57 and 0.55 Ga. The main tectonic controls of the magmatism of the Pelotas Batholith are high-dip sinistral shear zones.     

Geochemical evolution of the early Paleozoic collisional magmatism from autochthonous migmatites and granitoids to multiphase granite intrusions (Sharanur and Aya complexes,Baikal Region)

Overall petrologic and geochemical data indicate that the early Paleozoic magmatism in the Olkhon area of the Baikal Region exhibits diverse types of granitoids, whose time of formation is estimated at a narrow age interval of 500-465 Ma. This magmatism was responsible for the formation of both autochthonous gneiss-migmatite-granitoid suites (Sharanur complex) and multiphase intrusions (Aya complex) emplaced into the upper horizons of the continental crust. In major-element chemistry, K₂O/Na₂O values, and rare-element composition the migmatite-plagiogranites and calc-alkaline and subalkaline granitoids of the Sharanur complex are similar to the host gneisses and schists, as they were likely derived from melting of the ancient metamorphic substratum of the Olkhon series. In new isotope-geochemical characteristics (ICP MS method) the Sharanur granitoids are close to the first-phase biotite granites of the Aya massif, whose further geochemical evolution was governed mainly by intrachamber magmatic differentiation leading to the production of second-phase leucogranites enriched in HREE and HFSE (in particular, Ta and Nb) and depleted in Sr, Ba, Eu, Li, and LREE. The origin of the autochthonous and intrusive granitoids is related to early Paleozoic collision events within the Olkhon metamorphic terrane, while the formation of syncollisional granitoids is best explained by both melting of the crust protolith (Sharanur complex) and magmatic differentiation (multiphase Aya intrusion). All mineralogical and geochemical characteristics indicate that these granitoids are distinguished from rare-metal pegmatoid granites and Li-F and Rb-Be-Nb pegmatites, whose vein bodies crosscut the granitoids, and are regarded as middle Paleozoic rocks, which mark the transition to within-plate magmatism in the Baikal Region.     

A Jurassic garnet-bearing granitic pluton from NE China showing tetrad REE patterns

NE China is characterized by the massive distribution of Phanerozoic granitoids. Most of them are of I- and A-type granites, whereas S-type granites are rarely documented. The present work deals with the Dongqing pluton, a small granitic body emplaced in the southern Zhangguangcai Range. The pluton comprises a two-mica (±garnet) granite and a garnet-bearing muscovite granite; the latter occurs as veins in the former. The pluton shows a gradational contact with the surrounding host granites. Rb–Sr and Sm–Nd isotope analyses on whole-rocks and minerals reveal that the two types of granites were emplaced synchronously at about 160 Ma. The pluton was emplaced coeval with the surrounding I-type granitic pluton, and had a rapid cooling history. It is characterized by an initial Sr isotopic ratio of 0.706, slightly negative _Nd(T) values (−0.5 to −1.9) and young depleted-mantle model ages (970–1090 Ma). This suggests that the parent magma originated from partial melting of relatively juvenile crust, which is largely compatible with the general scenario for much of the Phanerozoic granitoids emplaced in the Central Asian Orogenic Belt.Geochemically, the granites of the Dongqing pluton are peraluminous, with a Shand Index (molar ratio A/CNK) of 1.0–1.1 for the two-mica granites and 1.2–1.3 for the garnet-bearing granites. All the garnet-bearing granites and some of the two-mica granites show tetrad REE patterns (=tetrad group), whereas most two-mica granites show normal granitic REE patterns (=normal group). The normal group granites exhibit depletion in Nb, Ta, P and Ti in spidergrams, and generally weak positive Eu anomalies in REE patterns. By contrast, the tetrad group granites manifest depletion in Ba, Nb, Ta, Sr, P, and Ti and significant negative Eu anomalies. The trace element data constrain the parental magmas to having undergone extensive magmatic differentiation. During their late stage magmatic evolution, intense interaction of residual melts with aqueous hydrothermal fluids resulted in the non-CHARAC (charge and radius controlled) trace element behavior and the tetrad effect in REE distribution patterns. This, in turn, leads to the invalidation of the commonly used tectonic discrimination criteria derived from trace element abundances of normal granites. In view of this and previous studies, we conclude that there were probably no S-type granites produced in NE China during the Phanerozoic. Consequently, weathered sedimentary material did not play an important role in the genesis of the strongly peraluminous granites in the Zhangguangcai Range.     

Sequence of magmatic events in the Late Paleozoic of Transbaikalia,Russia (U-Pb isotope data)

The Late Paleozoic intrusive rocks, mostly granitoids, totally occupy more than 200,000 km² on the territory of Transbaikalia. Isotopic U-Pb zircon dating (about 30 samples from the most typical plutons) shows that the Late Paleozoic magmatic cycle lasted for 55–60 m.y., from ~330 Ma to ~275 Ma. During this time span, five intrusive suites were emplaced throughout the region. The earliest are high-K calc-alkaline granites (330–310 Ma) making up the Angara–Vitim batholith of 150,000 km² in area. At later stages, formation of geochemically distinct intrusive suites occurred with total or partial overlap in time. In the interval of 305–285 Ma two suites were emplaced: calc-alkaline granitoids with decreased SiO₂ content (the Chivyrkui suite of quartz monzonite and granodiorite) and the Zaza suite comprising transitional from calc-alkaline to alkaline granite and quartz syenite. At the next stage, in the interval of 285–278 Ma the shoshonitic Low Selenga suite made up of monzonite, syenite and alkali rich microgabbro was formed; this suite was followed, with significant overlap in time (281–276 Ma), by emplacement of Early Kunalei suite of alkaline (alkali feldspar) and peralkaline syenite and granite. Concurrent emplacement of distinct plutonic suites suggests simultaneous magma generation at different depth and, possibly, from different sources. Despite complex sequence of formation of Late Paleozoic intrusive suites, a general trend from high-K calc-alkaline to alkaline and peralkaline granitoids, is clearly recognized. New data on the isotopic U-Pb zircon age support the Rb-Sr isotope data suggesting that emplacement of large volumes of peralkaline and alkaline (alkali feldspar) syenites and granites occurred in two separate stages: Early Permian (281–278 Ma) and Late Triassic (230–210 Ma). Large volumes and specific compositions of granitoids suggest that the Late Paleozoic magmatism in Transbaikalia occurred successively in the post-collisional (330–310 Ma), transitional (305–285 Ma) and intraplate (285–275 Ma) setting.     

Petrology of the Chilliwack batholith,North Cascades,Washington: generation of calc-alkaline granitoids by melting of mafic lower crust with variable water fugacity

Calc-alkaline granitoid rocks of the Oligocene-Pliocene Chilliwack batholith, North Cascades, range from quartz diorites to granites (57–78% SiO₂), and are coeval with small gabbroic stocks. Modeling of major element, trace element, and isotopic data for granitoid and mafic rocks suggests that: (1) the granitoids were derived from amphibolitic lower crust having REE (rare-earth-element) and Sr-Nd isotopic characteristics of the exposed gabbros; (2) lithologic diversity among the granitoids is primarily the result of variable water fugacity during melting. The main effect of f_{H 2 O} variation is to change the relative proportions of plagioclase and amphibole in the residuum. The REE data for intermediate granitoids (quartz diorite-granodiorite; Eu/Eu^*=0.84–0.50) are modeled by melting with f_{H 2 O}<1 kbar, leaving a plagioclase + pyroxene residuum. In contrast, data for leucocratic granitoids (leuco-granodiorites and granites; Eu/Eu^* =1.0–0.54) require residual amphibole in the source and are modeled by melting with f_{H 2 O}=2–3 kbar. Consistent with this model, isotopic data for the granitoids show no systematic variation with rock type (⁸⁷Sr/⁸⁶Sr_i =0.7033–0.7043; Nd(0)=+3.3 to +5.5) and overlap significantly with data for the gabbroic rocks (⁸⁷Sr/⁸⁶Sr_i =0.7034–0.7040; Nd(0)=+3.3 to +6.9). The f_{H 2 O} variations during melting may reflect additions of H₂O to the lower crust from crystallizing basaltic magmas having a range of H₂O contents; Chillwack gabbros document the existence of such basalts. One-dimensional conductive heat transfer calculations indicate that underplating of basaltic magmas can provide the heat required for large-scale melting of amphibolitic lower crust, provided that ambient wallrock temperatures exceed 800°C. Based on lithologic and geochemical similarities, this model may be applicable to other Cordilleran batholiths.     

Petrography and geochemistry of the Ngaoundéré Pan-African granitoids in Central North Cameroon: Implications for their sources and geological setting

The Nagoundéré Pan-African granitoids in Central North Cameroon belong to a regional-scale massif, which is referred to as the Adamawa-Yade batholith. The granites were emplaced into a ca. 2.1 Ga remobilised basement composed of metasedimentary and meta-igneous rocks that later underwent medium- to high-grade Pan-African metamorphism. The granitoids comprise three groups: the hornblende–biotite granitoids (HBGs), the biotite ± muscovite granitoids (BMGs), and the biotite granitoids (BGs). New Th–U–Pb monazite data on the BMGs and BGs confirm their late Neoproterozoic emplacement age (ca. 615 ± 27 Ma for the BMGs and ca. 575 Ma for the BGs) during the time interval of the regional tectono-metamorphic event in North Cameroon. The BMGs also show the presence of ca. 926 Ma inheritances, suggesting an early Neoproterozoic component in their protolith.The HBGs are characterized by high Ba–Sr, and low K₂O/Na₂O ratios. They show fairly fractionated REE patterns (La_N/Yb_N 6–22) with no Eu anomalies. The BMGs are characterized by higher K₂O/Na₂O and Rb/Sr ratios. They are more REE-fractionated (La_N/Yb_N = 17–168) with strong negative Eu anomalies (Eu/Eu^* = 0.2–0.5). The BGs are characterized by high SiO₂ with K₂O/Na₂O > 1. They show moderated fractionated REE patterns (La_N/Yb_N = 11–37) with strong Eu negative anomalies (Eu/Eu^* = 0.2–0.8) and flat HREE features (Gd_N/Yb_N = 1.5–2.2). In Primitive Mantle-normalized multi-element diagrams, the patterns of all rocks show enrichment in LILE relative to HFSE and display negative Nb–Ta and Ti anomalies. All the granitoids belong to high-K calc-alkaline suites and have an I-type signature.Major and trace element data of the HBGs are consistent with differentiation of a mafic magma from an enriched subcontinental lithospheric mantle, with possible crustal assimilation. In contrast, the high Th content, the LREE-enrichment, and the presence of inherited monazite suggest that the BGs and BMGs were derived from melting of the middle continental crust. Structural and petrochemical data indicate that these granitoids were emplaced in both syn- to post-collision tectonic settings.     

Geochemistry and zircon geochronology of the Archean granite suites of the Rio Maria granite-greenstone terrane,Carajás Province,Brazil

The Archean granites exposed in the Mesorchean Rio Maria granite-greenstone terrane (RMGGT), southeastern Amazonian craton can be divided into three groups on the basis of petrographic and geochemical data. (1) Potassic leucogranites (Xinguara and Mata Surrão granites), composed dominantly of biotite monzogranites that have high SiO₂, K₂O, and Rb contents and show fractionated REE patterns with moderate to pronounced negative Eu anomalies. These granites share many features with the low-Ca granite group of the Yilgarn craton and CA2-type of Archean calc-alkaline granites. These granites result from the partial melting of rocks similar to the older TTG of the RMGGT. (2) Leucogranodiorite-granite group (Guarantã suite, Grotão granodiorite, and similar rocks), which is composed of Ba- and Sr-rich rocks which display fractionated REE patterns without significant Eu anomalies and show geochemical affinity with the high-Ca granite group or Transitional TTG of the Yilgarn craton and the CA1-type of Archean calc-alkaline granites. These rocks appear to have been originated from mixing between a Ba- and Sr-enriched granite magma and trondhjemitic liquids or alternatively product of interaction between fluids enriched in K, Sr, and Ba, derived from a metasomatized mantle with older TTG rocks. (3) Amphibole-biotite monzogranites (Rancho de Deus granite) associated with sanukitoid suites. These granites were probably generated by fractional crystallization and differentiation of sanukitoid magmas enriched in Ba and Sr.The emplacement of the granites of the RMGGT occurred during the Mesoarchean (2.87–2.86 Ga). They are approximately coeval with the sanukitoid suites (∼2.87 Ga) and post-dated the main timing of TTG suites formation (2.98–2.92 Ga). The crust of Rio Maria was probably still quite warm at the time when the granite magmas were produced. In these conditions, the underplating in the lower crust of large volumes of sanukitoid magmas may have also contributed with heat inducing the partial melting of crustal protoliths and opening the possibility of complex interactions between different kinds of magmas.     

Granitoids in the Dalat zone,southern Vietnam: age constraints on magmatism and regional geological implications

The Dalat zone in southern Vietnam comprises a Cretaceous Andean-type magmatic arc with voluminous granitoids and contemporary volcanic rocks. On the basis of petrographical and mineralogical studies, the granitoids were subdivided into three suites: Dinhquan, Deoca and Cana. Rocks of the Dinhquan suite are hornblende–biotite diorites, granodiorites and minor granites. The Cana suite encompasses mainly leucocratic biotite-bearing granites with scarce hornblende. The Deoca suite is made up of granodiorites, monzogranites and granites. Geochemically, the granitoids are of subalkaline affinity, belong to the high-K, calc-alkaline series, and most of them display typical features of I-type granites. This paper presents the new Rb–Sr mineral and U–Pb zircon and titanite age data for the granitoids, which establish the ages of the plutonic suites as: the Dinhquan at ~112–100 Ma, Cana at ~96–93 Ma and Deoca at ~92–88 Ma. These ages are significantly different from earlier publications, and indicate that the earliest magmatism in the Dalat zone began at ~112 Ma ago, that is ~30–50 Ma later than previously thought. Our geochronological data are also support the continuation of an Andean-type arc running from SE China via southern Vietnam to SW Borneo.     

Mesozoic granitic magmatism in extensional tectonics near the Mongolian border in China and its implications for crustal growth

The Yagan area of the southernmost Sino–Mongolian border is characterized by an extensional structure where a large metamorphic core complex (Yagan–Onch Hayrhan) and voluminous granitoids are exposed. New isotopic age data indicate that the granitoids, which were previously regarded as Paleozoic in age, were emplaced in early and late Mesozoic times. The early Mesozoic granitoids have 228±7 Ma U–Pb zircon age, and consist of linear mylonitic quartz monzonites and biotite monzogranites. Their chemical compositions are similar to those of potassic granites and shoshonitic series, and show an intraplate and post-collisional environment in tectonic discrimination diagrams. Their fabrics reveal that they experienced syn-emplacement extensional deformation. All these characteristics suggest that the adjustment, thinning and extensional deformation at middle to lower crustal levels might have occurred in the early Mesozoic. The late Mesozoic granitoids have a U–Pb zircon age of 135±2 Ma, and are made up of large elliptical granitic plutons. They are high-K calc-alkaline, and were forcefully emplaced in the dome extensional setting. Both the early and late Mesozoic granitoids have Nd (t) values of −2.3 to +5, in strong contrast with the negative Nd (t) values (−11) of the Precambrian host rocks. This suggests that juvenile mantle-derived components were involved in the formation of the granitoids. The similar situation is omnipresent in Central Asia. This study demonstrates that tectonic extension, magmatism and crustal growth are closely related, and that post-collisional and intraplate magmatism was probably a significant process for continental growth in the Phanerozoic.     

Geochemical, Sr–Nd and zircon U–Pb–Hf isotopic studies of Late Carboniferous magmatism in the West Junggar, Xinjiang: Implications for ridge subduction?

Voluminous granitic intrusions are distributed in the West Junggar, NW China, and they can be classified as the dioritic rocks, charnockite and alkali-feldspar granite groups. The dioritic rocks (SiO₂ = 50.4–63.8 wt.%) are calc-alkaline and Mg enriched (average MgO = 4.54 wt.%, Mg^# = 0.39–0.64), with high Sr/Y ratios (average = 21.2), weak negative Eu (average Eu/Eu = 0.80) and pronounced negative Nb–Ta anomalies. Their Sr–Nd and zircon Hf isotopic compositions ((⁸⁷Sr/⁸⁶Sr)_i = 0.7035–0.7042, ε_Nd(t) = 4.5–7.9, ε_Hf(t) = 14.1–14.5) show a depleted mantle-like signature. These features are compatible with adakites derived from partial melting of subducted oceanic crust that interacted with mantle materials. The charnockites (SiO₂ = 60.0–65.3 wt.%) show transitional geochemical characteristics from calc-alkaline to alkaline, with weak negative Eu (average Eu/Eu = 0.75) but pronounced negative Nb–Ta anomalies. Sr–Nd and zircon Hf isotopic compositions ((⁸⁷Sr/⁸⁶Sr)_i = 0.7037–0.7039, ε_Nd(t) = 5.2–8.0, ε_Hf(t) = 13.9–14.7) also indicate a depleted source, suggesting melts from a hot, juvenile lower crust. Alkali-feldspar granites (SiO₂ = 70.0–78.4 wt.%) are alkali and Fe-enriched, and have distinct negative Eu and Nb–Ta anomalies (average Eu/Eu = 0.26), low Sr/Y ratios (average = 2.11), and depleted Sr–Nd and zircon Hf isotopic compositions ((⁸⁷Sr/⁸⁶Sr)_i = 0.7024–0.7045, ε_Nd(t) = 5.1–8.9, ε_Hf(t) = 13.7–14.2). These characteristics are also comparable with those of rocks derived from juvenile lower crust. Despite of the differences in petrology, geochemistry and possibly different origins, zircon ages indicate that these three groups of rocks were coevally emplaced at ~ 305 Ma.A ridge subduction model can account for the geochemical characteristics of these granitoids and coeval mafic rocks. As the “slab window” opened, upwelling asthenosphere provided enhanced heat flux and triggered voluminous magmatisms: partial melting of the subducting slab formed the dioritic rocks; partial melting of the hot juvenile lower crust produced charnockite and alkali-feldspar granite, and partial melting in the mantle wedge generated mafic rocks in the region. These results suggest that subduction was ongoing in the Late Carboniferous and, thus support that the accretion and collision in the Central Asian Orogenic Belt took place in North Xinjiang after 305 Ma, and possibly in the Permian.     

Mantle derivation of Archean amphibole-bearing granitoid and associated mafic rocks: evidence from the southern Superior Province,Canada

Amphibole-bearing, Late Archean (2.73–2.68 Ga) granitoids of the southern Superior Province are examined to constrain processes of crustal development. The investigated plutons, which range from tonalite and diorite to monzodiorite, monzonite, and syenite, share textural, mineralogical and geochemical attributes suggesting a common origin as juvenile magmas. Despite variation in modal mineralogy, the plutons are geochemically characterized by normative quartz, high Al₂O₃ (> 15 wt%), Na-rich fractionation trends (mol Na₂O/K₂O >2), low to moderate Rb (generally<100 ppm), moderate to high Sr (200–1500 ppm), enriched light rare earth elements (LREE) (Ce_N generally 10–150), fractionated REE (Ce_N/Yb_N 8–30), Eu anomaly (Eu/Eu^*) 1, and decreasing REE with increasing SiO₂. The plutons all contain amphibole-rich, mafic-ultramafic rocks which occur as enclaves and igneous layers and as intrusive units which exhibit textures indicative of contemporaneous mafic and felsic magmatism. Mafic mineral assemblages include: hornblende + biotite in tonalites; augite + biotite ± orthopyroxene ± pargasitic hornblende or hornblende+biotite in dioritic to monzodioritic rocks; and aegirine-augite ± silicic edenite ± biotite in syenite to alkali granite. Discrete plagioclase and microcline grains are present in most of the suites, however, some of the syenitic rocks are hypersolvus granitoids and contain only perthite. Mafic-ultramafic rocks have REE and Y contents indicative of their formation as amphibole-rich cumulates from the associated granitoids. Some cumulate rocks have skeletal amphibole with X_Mg(Mg/(Mg+ Fe²⁺)) indicative of crystallization from more primitive liquids than the host granitoids. Geochemical variation in the granitoid suites is compatible with fractionation of amphibole together with subordinate plagioclase and, in some cases, mixing of fractionated and primitive magmas. Mafic to ultramafic units with magnesium-rich cumulus phases and primitive granitoids (mol MgO/ (MgO+0.9 FeO^TOTAL) from 0.60 to 0.70 and C_T >150 ppm) are comagmatic with the evolved granitoids and indicate that the suites are mantle-derived. Isotopic studies of Archean monzodioritic rocks have shown LREE enrichment and initial ¹⁴³Nd/¹⁴⁴Nd ratios indicating derivation from mantle sources enriched in large ion lithophile elements (LILE) shortly before melting. Mineral assemblages record lower P_H2O with increased alkali contents of the suites. This evidence, in conjunction with experimental studies, suggests that increased alkali contents may reflect decreased P_H2O during mantle melting. These features indicate that 2.73 Ga tonalitic rocks are derived from more hydrous mantle sources than 2.68 Ga syenitic rocks, and that the spectrum of late Archean juvenile granitoid rocks is broader than previously recognized. Comparison with Phanerozoic and recent plutonic suites suggests that these Archean suites are subduction related.     

准噶尔东部早志留世两类花岗岩的岩石成因及构造意义

哈尔里克山西段早志留世二长花岗岩和正长花岗岩呈北西西向带状展布,侵入奥陶系塔水组（O_1-2t）,LA-ICP-MS锆石U-Pb年龄为438.8±2.3~435.8±3.1 Ma。岩石高硅（SiO₂含量73.0%~77.8%）、富钾（K₂O含量3.31%~4.26%）、低镁（MgO含量0.03%~0.59%）,铝饱和指数A/CNK值1.02~1.08,属高钾钙碱性弱过铝质岩石。二长花岗岩轻重稀土分馏显著,Eu异常中等,亏损Nb、Ta、Ti、P,富集Rb、Ba、K,表现为分异的Ⅰ型花岗岩特征,源区为基性下地壳;正长花岗岩强烈亏损Eu、P、Ti、Sr,不同程度富集Rb、K、Zr、Hf,表现为A型花岗岩特征,其源区为缺水的浅部长英质地壳。结合区域地层不整合资料,认为东准噶尔地区早志留世为后碰撞环境而非岛弧带,后碰撞软流圈上涌带来的热熔融准噶尔年轻地壳形成了岩性丰富的东准噶尔志留纪后碰撞岩浆岩组合。      

Elemental and Sr–Nd–Pb isotopic geochemistry of Late Paleozoic volcanic rocks beneath the Junggar basin, NW China: Implications for the formation and evolution of the basin basement

The basement beneath the Junggar basin has been interpreted either as a micro-continent of Precambrian age or as a fragment of Paleozoic oceanic crust. Elemental and Sr–Nd–Pb isotopic compositions and zircon Pb–Pb ages of volcanic rocks from drill cores through the paleo-weathered crust show that the basement is composed mainly of late Paleozoic volcanic rock with minor shale and tuff. The volcanic rocks are mostly subalkaline with some minor low-K rocks in the western Kexia area. Some alkaline lavas occur in the central Luliang uplift and northeastern Wulungu depression. The lavas range in composition from basalts to rhyolites and fractional crystallization played an important role in magma evolution. Except for a few samples from Kexia, the basalts have low La/Nb (<1.4), typical for oceanic crust derived from asthenospheric melts. Zircon Pb–Pb ages indicate that the Kexia andesite, with a volcanic arc affinity, formed in the early Carboniferous (345 Ma), whereas the Luliang rhyolite and the Wucaiwan dacite, with syn-collisional to within-plate affinities, formed in the early Devonian (395 and 405 Ma, respectively). Positive ε_Nd(t) values (up to +7.4) and low initial ⁸⁷Sr/⁸⁶Sr isotopic ratios of the intermediate-silicic rocks suggest that the entire Junggar terrain may be underlain by oceanic crust, an interpretation consistent with the juvenile isotopic signatures of many granitoid plutons in other parts of the Central Asia Orogenic Belt. Variation in zircon ages for the silicic rocks, different Ba, P, Ti, Nb or Th anomalies in the mafic rocks, and variable Nb/Y and La/Nb ratios across the basin, suggest that the basement is compositionally heterogeneous. The heterogeneity is believed to reflect amalgamation of different oceanic blocks representing either different evolution stages within a single terrane or possibly derivation from different terranes.     

Geochemistry of the Bafoussam Pan-African I- and S-type granitoids in western Cameroon

The Bafoussam area in western Cameroon is part of the Central African Orogenic Belt. It is dominated by granitoids which belong to the Pan-African syn- to post-collisional post-650 Ma group. Syenogranites are predominant, but alkali-feldspar granite, monzogranite, quartz-monzonite and quartz-monzodiorite occur as well. Four granitoid suites, biotite granitoids and deformed biotite granitoids with amphibole, megafeldspar granitoids with megacrysts and two-mica granitoids with primary muscovite and igneous garnet are distinguished. The granites can be assigned to high-K calc-alkalic to shoshonitic series. The partly shoshonitic biotite granitoids are metaluminous to weakly peraluminous and can be labelled as a highly fractionated I-type suite. The megafeldspar granitoids are weakly peraluminous with I-type character whereas the two-mica granitoids are weakly to strongly peraluminous and belong to an S-type suite. Emplacement ages at 558–564 Ma for the two-mica granitoids have been dated from monazite by the EMP Th–U–Pb method.The REE in the biotite granitoids are moderately fractionated with (La/Lu)_N = 23–38. Enrichment of Nb and Ta varies by one order of magnitude. The megafeldspar granitoids show homogeneous and strongly fractionated REE patterns with (La/Lu)_N = 27–42. The primitive mantle-normalized element patterns are homogeneous with marked negative Ba, Nb, Ta, Sr, Eu and Ti anomalies. The two-mica granitoids are characterized by low to moderate total REE contents with strongly fractionated REE expressed by (La/Lu)_N ranging from 7 to 59. The negative Nb and Ta anomalies are less significant. Nd and Sr whole-rock isotope data confirm different sources for the granitoid suites. The source of the I-type biotite granitoids was probably a juvenile mantle which has been variably metasomatized. The source of the I-type megafeldspar granitoids is characterized by juvenile mantle and lower crust components. Anatectic melts of the upper continental crust with variable contribution of lower continental crust or mantle melts can explain the heterogeneous isotopic signatures of the S-type two-mica granitoids. It is suggested that the melting of these sources was successively initiated by the rising isotherms during a syn- to post-collisional setting which followed a subduction.     

Origin and evolution of post-collisional magmatism: Coeval Neoproterozoic calc-alkaline and alkaline suites of the Sinai Peninsula

Two Late Neoproterozoic post-collisional igneous suites, calc-alkaline (CA) and alkaline–peralkaline (Alk), widely occur in the northernmost part of the Arabian–Nubian Shield. In Sinai (Egypt) and southern Israel they occupy up to 80% of the exposed basement. Recently published U–Pb zircon geochronology indicates a prolonged and partially overlapping CA and Alk magmatism at 635–590 Ma and 608–580 Ma, respectively. Nevertheless in each particular locality CA granitoids always preceded Alk plutons. CA and Alk igneous rocks have distinct chemical compositions, but felsic and mafic rocks in general and granitoids from the two suites in particular cannot be distinguished by their Nd, Sr and O isotope ratios. Both suites are characterized by positive ε_Nd(T) values, from + 1.5 to + 6.0 (150 samples, 28 of them are new analyses), but predominance of juvenile crust in the region prevents unambiguous petrogenetic interpretation of the isotope data. Comparison of geochemical traits of felsic and mafic rocks in each suite suggests a significant contribution of mantle-derived components to the silicic magmas. Model calculation shows that the alkaline granite magma could have been produced by partial (~ 20%) melting of rocks corresponding to K-rich basalts. Material balance further suggests that granodiorite and quartz monzonite magmas of the CA suite could form by mixing of the granite and gabbro end-members at proportions of 85/15. In the Alk suite, alkali feldspar and peralkaline granites have evolved mainly by fractional crystallization of feldspars and a small amount of mafic minerals from a parental syenogranite melt. Thus the protracted, 20 m.y. long, contemporaneous CA and Alk magmatism in the northern ANS requires concurrent tapping of two distinct mantle sources. Coeval emplacement of CA and Alk intrusive suites was described in a number of regions throughout the world.     

The 1.90–1.88 Ga magmatism in the southernmost Guyana Shield, Amazonas, Brazil: Geology, geochemistry, zircon geochronology, and tectonic implications

The southernmost Guyana Shield-Uatumã subdomain, northeastern Amazonas State, Brazil is dominantly formed by granitoid and volcanic rocks from the Água Branca Suite (ABS), undivided Granite Stocks (GS) and São Gabriel volcano–plutonic system (SGS). The ABS is characterized by a granite series that exhibits comparatively low Fe/(Fe + Mg) ratio, low (Nb/Zr)_N, high Sr values and high Rb/Zr ratio. Its rocks display metaluminous to weakly peraluminous (A/CNK 0.94–1.06), high-K calc-alkaline, I normal-type character and have moderately to strongly fractionated rare earth elements (REE) pattern. The SG granites and SGS effusive–ignimbrite–granite association is metaluminous to weakly peraluminous (A/CNK 0.84–1.18), high-K calc-alkaline, has moderately to weakly fractionated REE trend, higher Fe/(Fe + Mg) ratio, lower Sr content and lower Rb/Zr ratio. The ABS geochemical signature is consistent with formation from volcanic arc rocks and small participation of collisional setting rocks, whereas the SG and SGS have post-collisional tectonic rocks-related geochemical signature. This model is in harmony with a post-collisional extensional regime, started with the 1.90–1.89 Ga Água Branca magmatism, and culminated with the 1.89–1.88 Ga São Gabriel system at an early stage of intracratonic reactivation, which included intrusion of mafic dikes. The Uatumã subdomain was related to mantle underplating with continental uplift and its origin involved contributions of 2.3–2.44 Ga Archean-contaminated Trans-Amazonian, 2.13–2.21 Ga Trans-Amazonian, 1.93–1.94/2.0 Ga Tapajós-Parima. Foliation styles point out that part of the Água Branca granitoids recorded later deformational effects, likely related to the Rio Negro Province formation.     

Early Palaeozoic crustal melting in an extensional setting: petrological and Sm–Nd evidence from the Izera granite-gneisses, Polish Sudetes

The Izera Block in the West Sudetes, which is composed of granites, gneisses (and transitional granite-gneisses) and minor mica schists, is one of the largest outcrops of Early Palaeozoic (ca. 500 Ma) metagranitoid rocks in the basement units of the Variscides of Central Europe. The Izera granites show S-type features: magmatic cordierite, relict garnet and sillimanite, lack of mafic enclaves, and absence of coexisting tonalites and diorites. The paucity of pegmatites indicates that the granitic magma was relatively dry. The S-type character of these granites is further supported by their peraluminous character (A/CNK 1.0–1.63), high content of normative corundum (up to 3.5%) and relatively high ⁸⁷Sr /⁸⁶Sr initial ratio. The chemical variation of these rocks was controlled by the fractional crystallization of plagioclase (CaO, Sr, Eu/Eu*), biotite and cordierite (Al₂O₃, MgO, FeO), zircon (Zr, Hf) and monazite (REE). Initial Nd values range from –5.2 to –6.9 (mean: –5.9, SD=0.6). These largely negative Nd values imply that the granitic magmas emplaced ca. 500 Ma were extracted from a source reservoir that was strongly enriched in LREE (i.e., with low Sm/Nd ratio) on a time-integrated basis. The relatively consistent depleted mantle model ages (1,730–2,175 Ma; mean: 1,890 Ma) is in agreement with the earlier reported presence of ca. 2.1 Ga old inherited Pb component in zircon from the closely related Rumburk granite. This points to an old (Early Proterozoic) crustal residence age of the inferred metasedimentary protoliths of the Izera granitoids, with only minor contribution to their protoliths of juvenile components of Late Proterozoic/Early Palaeozoic age. Although the Izera granites show some trace element features reminiscent of syn-collisional or post-collisional granitoids, they more likely belong to the broad anorogenic class. Our data corroborate some previous interpretations that granite generation was connected with the Early Palaeozoic rifting of the passive margin of the Saxothuringian block, well documented in the region by bimodal volcanic suites of similar age (Kaczawa Unit, eastern and southern envelope of the Karkonosze–Izera Block). In this scenario, granite magmatism and bimodal volcanism would represent two broadly concomitant effects of a single major event of lithospheric break-up at the northern edge of Gondwana.     

Origin and geochemistry of Pan-African granitoid rocks in the Gabal Um Shomer area,Southwestern Sinai,Egypt

Geological, petrological and geochemical studies indicated that there are two distinct types of granitoid rocks: older quartz diorites to granodiorite assemblage and younger granitoids, the latter occurring in two phases. The older granitoids have a meta-aluminous chemistry and a calc-alkaline character, with high MgO, Fe2O3, TiO2, CaO, P2O5, Sr and low SiO2, K2O, and Rb. Their major and trace elements data, together with low 87Sr/ 86Sr ratios (0.7029±0.0008) are indicative of I-type affinities. The second-...     

Shrimp U–Pb zircon geochronology,geochemistry, and Nd–Sr isotopic study of contrasting granites in the Emeishan large igneous province,SW China

The Cida A-type granitic stock (∼ 4 km²) and Ailanghe I-type granite batholith (∼ 100 km²) in the Pan-Xi (Panzhihua-Xichang) area, SW China, are two important examples of granites formed during an episode of magmatism associated with the Permian Emeishan mantle plume activity. This is a classic setting of plume-related, anorogenic magmatism exhibiting the typical association of mantle-derived mafic and alkaline rocks along with silicic units. SHRIMP zircon U–Pb data reveal that the Cida granitic pluton (261 ± 4 Ma) was emplaced shortly before the Ailanghe granites (251 ± 6 Ma). The Cida granitoids display mineralogical and geochemical characteristics of A-type granites including high FeO^⁎/MgO ratios, elevated high-field-strength elements (HFSE) contents and high Ga/Al ratios, which are much higher than those of the Ailanghe granites. All the granitic rocks show significant negative Eu anomalies and demonstrate the characteristic negative anomalies in Ba, Sr, and Ti in the spidergrams. It can be concluded that the Cida granitic rocks are highly fractionated A-type granitoids whereas the Ailanghe granitic rocks belong to highly evolved I-type granites.The Cida granitoids and enclaves have Nd and Sr isotopic initial ratios (ε_Nd(t) = − 0.25 to + 1.35 and (⁸⁷Sr/⁸⁶Sr)_i = 0.7023 to 0.7053) close to those of the associated mafic intrusions and Emeishan basalts, indicating the involvement of a major mantle plume component. The Ailanghe granites exhibit prominent negative Nb and Ta anomalies and weakly positive Pb anomalies in the spidergram and have nonradiogenic ε_Nd(t) ratios (− 6.34 to − 6.26) and high (⁸⁷Sr/⁸⁶Sr)_i values (0.7102 to 0.7111), which indicate a significant contribution from crustal material. These observations combined with geochemical modeling suggest that the Cida A-type granitoids were produced by extensive fractional crystallization from basaltic parental magmas. In contrast, the Ailanghe I-type granites most probably originated by partial melting of the mid-upper crustal, metasedimentary–metavolcanic rocks from the Paleo-Mesoproterozoic Huili group and newly underplated basaltic rocks.In the present study, it is proposed that petrogenetic distinctions between A-type and I-type granites may not be as clear-cut as previously supposed, and that many compositional and genetically different granites of the A- and I-types can be produced in the plume-related setting. Their ultimate nature depends more importantly on the type and proportion of mantle and crustal material involved and melting conditions. Significant melt production and possible underplating and/or intrusion into the lower crust, may play an important role in generating the juvenile mafic lower crust (average 20 km) in the central part of the Emeishan mantle plume.     

Late Neoproterozoic alkaline magmatism in the Arabian–Nubian Shield: the postcollisional A-type granite of Sahara–Umm Adawi pluton, Sinai, Egypt

The Sahara–Umm Adawi pluton is a Late Neoproterozoic postcollisional A-type granitoid pluton in Sinai segment of the Arabian–Nubian Shield that was emplaced within voluminous calc-alkaline I-type granite host rocks during the waning stages of the Pan-African orogeny and termination of a tectonomagmatic compressive cycle. The western part of the pluton is downthrown by clysmic faults and buried beneath the Suez rift valley sedimentary fill, while the exposed part is dissected by later Tertiary basaltic dykes and crosscut along with its host rocks by a series of NNE-trending faults. This A-type granite pluton is made up wholly of hypersolvus alkali feldspar granite and is composed of perthite, quartz, alkali amphibole, plagioclase, Fe-rich red biotite, accessory zircon, apatite, and allanite. The pluton rocks are highly evolved ferroan, alkaline, and peralkaline to mildly peraluminous A-type granites, displaying the typical geochemical characteristics of A-type granites with high SiO₂, Na₂O + K₂O, FeO*/MgO, Ga/Al, Zr, Nb, Ga, Y, Ce, and rare earth elements (REE) and low CaO, MgO, Ba, and Sr. Their trace and REE characteristics along with the use of various discrimination schemes revealed their correspondence to magmas derived from crustal sources that has gone through a continent–continent collision (postorogenic or postcollisional), with minor contribution from mantle source similar to ocean island basalt. The assumption of crustal source derivation and postcollisional setting is substantiated by highly evolved nature of this pluton and the absence of any syenitic or more primitive coeval mafic rocks in association with it. The slight mantle signature in the source material of these A-type granites is owed to the juvenile Pan-African Arabian–Nubian Shield (ANS) crust (I-type calc-alkaline) which was acted as a source by partial melting of its rocks and which itself of presumably large mantle source. The extremely high Rb/Sr ratios combined with the obvious Sr, Ba, P, Ti, and Eu depletions clearly indicate that these A-type granites were highly evolved and require advanced fractional crystallization in upper crustal conditions. Crystallization temperature values inferred average around 929°C which is in consistency with the presumably high temperatures of A-type magmas, whereas the estimated depth of emplacement ranges between 20 and 30 km (upper-middle crustal levels within the 40 km relatively thick ANS crust). The geochronologically preceding Pan-African calc-alkaline I-type continental arc granitoids (the Egyptian old and younger granites) associated with these rocks are thought to be the crustal source of f this A-type granite pluton and others in the Arabian–Nubian Shield by partial melting caused by crustal thickening due to continental collision at termination of the compressive orogeny in the Arabian–Nubian Shield.