Middle Carboniferous crustal melting in the Variscan Belt: New insights from U–Th–Pbtot. monazite and U–Pb zircon ages of the Montagne Noire Axial Zone (southern French Massif Central)

In France, the Devonian–Carboniferous Variscan orogeny developed at the expense of continental crust belonging to the northern margin of Gondwana. A Visean–Serpukhovian crustal melting has been recently documented in several massifs. However, in the Montagne Noire of the Variscan French Massif Central, which is the largest area involved in this partial melting episode, the age of migmatization was not clearly settled. Eleven U–Th–Pb_tot. ages on monazite and three U–Pb ages on associated zircon are reported from migmatites (La Salvetat, Ourtigas), anatectic granitoids (Laouzas, Montalet) and post-migmatitic granites (Anglès, Vialais, Soulié) from the Montagne Noire Axial Zone are presented here for the first time. Migmatization and emplacement of anatectic granitoids took place around 333–326 Ma (Visean) and late granitoids emplaced around 325–318 Ma (Serpukhovian). Inherited zircons and monazite date the orthogneiss source rock of the Late Visean melts between 560 Ma and 480 Ma. In migmatites and anatectic granites, inherited crystals dominate the zircon populations. The migmatitization is the middle crust expression of a pervasive Visean crustal melting event also represented by the “Tufs anthracifères” volcanism in the northern Massif Central. This crustal melting is widespread in the French Variscan belt, though it is restricted to the upper plate of the collision belt. A mantle input appears as a likely mechanism to release the heat necessary to trigger the melting of the Variscan middle crust at a continental scale.     

Neoproterozoic granitic activities in the Xingdi plutons at the Kuluketage block,NW China: Evidence from zircon U–Pb dating,geochemical and Sr–Nd–Hf isotopic analyses

Neoproterozoic igneous rocks are widely distributed in the Kuluketage block along the northern margin of the Tarim Craton. However, the published literature mainly focuses on the ca. 800 Ma adakitic granitoids in the area, with the granites that intrude the 735–760 Ma mafic–ultramafic rocks poorly studied. Here we report the ages, petrography and geochemistry of two granites in the Xingdi mafic–ultramafic rocks, in order to construct a new view of the non-adakitic younger granites. LA-ICP-MS zircon U–Pb dating provided weighted mean ²⁰⁶Pb/²³⁸U ages of 743.0 ± 2.5 Ma for the No.I granite (G1) and 739.0 ± 3.5 Ma for the No.II granite (G2). A clear core-rim texture of similar age and a high zircon saturation temperature of ca. 849 ± 14 °C were observed for the No.I granite; in contrast, G2 has no apparent core-rim texture but rather inherited older zircons and a lower zircon saturation temperature of ca. 763 ± 17 °C. Geochemical analysis revealed that G1 is an alkaline A-type granite and G2 is a high-K calc-alkaline I-type granite. Both granites share similar geochemical characteristics of arc-related magmatic rocks and enriched Sr–Nd–Hf isotopes, likely due to their enriched sources or mixing with enriched magma. Whereas G1 and its host mafic rocks form typical bimodal intrusions of the same age and similar Sr–Nd–Hf isotope compositions, G2 is younger than its host mafic rocks and its Sr–Nd–Hf isotope composition indicates a lower crust origin. Although they exhibit arc-related geochemical features, the two granites likely formed in a rift setting, as inferred from thier petrology, Sr–Nd–Hf isotopes and regional tectonic evolution.     

First evidence for Cambrian rift-related magmatism in the West African Craton margin: The Derraman Peralkaline Felsic Complex

West of the southern, Archean, part of the Reguibat Rise of the West African Craton the Oulad Dlim Massif consists of metamorphic nappes stacked during the Mauritanides (Variscan) orogeny. In the Derraman region, about 12 km west of the nappes, we have found strongly deformed hypersolvus aegirine-riebeckite A1-type granites with SHRIMP zircon U–Pb ages of ca. 525 ± 3 Ma, ε(Nd)_525Ma (− 5.2 to − 6.8.) and Nd model ages T_CR ≈ 1.85 Ga. These granites define two km-sized bodies and a few smaller satellites. One body is emplaced within a 3.12 Ga leucocratic gneiss. The other body and its satellites are emplaced within an Archean low-grade metasedimentary sequence with detrital zircons that have ages that peak at 2.84 Ga, 2.91 Ga, and 3.15 Ga. These Archean gneisses and metapelite rocks define a tectonic unit, hereafter called the Derraman-Bulautad-Leglat (DBL) unit, which was formed from the Reguibat basement at the very margin of the WAC. The ~ 525 Ma Derraman granites are the oldest post-Archean rocks in this unit and were generated in an intraplate rifting environment from melting of crustal fenites during the ubiquitous Cambrian rifting event that affected this part of northern Gondwana. At the present level of knowledge, however, we cannot decide whether the “old” Nd isotope signature of Derraman granites resulted from melting of an old (Paleoproterozoic) fenite source or reflects the signature of the mantle-derived metasomatising fluids. The just-discovered Derraman granites are strikingly similar to other rift-related Cambrian–Ordovician hypersolvus aegirine–riebeckite granites widespread in North Gondwana. Understanding the potential connections between them would help to understand the Cambrian–Ordovician breakdown of northern Gondwana.     

Chronological link between deep-seated processes in magma chambers and eruptions: Permo-Carboniferous magmatism in the core of Pangaea (Southern Pyrenees)

In the Southern Pyrenees there are Upper Carboniferous–Lower Permian sedimentary basins with a significant volume of volcanic material derived from explosive eruptions (rhyolitic ignimbrites and andesitic flows). These basins are spatially associated with granodiorites and dacitic dykes emplaced in Variscan basement rocks. U–Pb SHRIMP dating of zircons extracted from three granodiorites, an andesitic flow, a dacitic dyke and six ignimbrites, revealed that magmatism occurred over an extended period of thirty eight million years, from ca. 304 Ma to ca. 266 Ma (Upper Carboniferous–Middle Permian). A scattering of zircon ages in each sample shows that the history of melt crystallization was complex, with more than one zircon-forming event in each magma chamber. The prolonged crystallization history was transferred to the product of the eruptions. A chronological link between the deep-seated magma chambers and processes in eruptions was identified on the basis of four overlapping intervals at: ca. 309–307 Ma (Upper Carboniferous), ca. 304–296 Ma (Upper Carboniferous–Lower Permian), ca. 294–282 Ma (Lower Permian), and ca. 276 Ma (Lower Permian). The variation of zircon U/Th ratios exposes a tendency for an increase in mafic sources as crystallization advances in the Permian. Zircons probably crystallized from melt phases related to both a felsic-intermediate metaluminous source from ca. 310–293 Ma (mostly 0.1 < Th/U < 0.6) to ca. 289–273 Ma (especially in the range 0.6 < Th/U < 1) and a mafic source (mostly 1.2 < Th/U < 1) at ca. 266–265 Ma. U–Pb zircon ages from volcanic and plutonic rocks of the Southern Pyrenees are consistent with the ages of the post-Variscan magmatism of Iberia associated with orocline generation and subduction of the Paleotethys Ocean (ca. 304–283 Ma), and in addition reveal a later magmatic event at ca. 276–266 Ma (Lower–Middle Permian). The location of the Iberian orocline in the core of Pangaea and near the western end of the subduction zone of the Paleotethys Ocean leads to the hypothesis that this later magmatic activity of the Southern Pyrenees could provide the missing link between the Variscan and Cimmerian cycles that acted sequentially in Permo-Carboniferous times.     

Paleoproterozoic granitoids of the Chuya and Kutima complexes (southern Siberian craton): age,petrogenesis, and geodynamic setting

Detailed geochemical, isotope, and geochronological studies were carried out for the granitoids of the Chuya and Kutima complexes in the Baikal marginal salient of the Siberian craton basement. The obtained results indicate that the granitoids of both complexes are confined to the same tectonic structure (Akitkan fold belt) and are of similar absolute age. U–Pb zircon dating of the Kutima granites yielded an age of 2019±16 Ma, which nearly coincides with the age of 2020±12 Ma obtained earlier for the granitoids of the Chuya complex. Despite the close ages, the granitoids of these complexes differ considerably in geochemical characteristics. The granitoids of the Chuya complex correspond in composition to calcic and calc-alkalic peraluminous trondhjemites, and the granites of the Kutima complex, to calc-alkalic and alkali-calcic peraluminous granites. The granites of the Chuya complex are similar to rocks of the tonalite–trondhjemite–granodiorite (TTG) series and are close in CaO, Sr, and Ba contents to I-type granites. The granites of the Kutima complex are similar in contents of major oxides to oxidized A-type granites. Study of the Nd isotope composition of the Chuya and Kutima granitoids showed their close positive values of εNd(T) (+ 1.9 to + 3.5), which indicates that both rocks formed from sources with a short crustal history. Based on petrogeochemical data, it has been established that the Chuya granitoids might have been formed through the melting of a metabasitic source, whereas the Kutima granites, through the melting of a crustal source of quartz–feldspathic composition. Estimation of the PT-conditions of granitoid melt crystallization shows that the Chuya granitoids formed at 735–776 °C (zircon saturation temperature) and > 10 kbar and the Kutima granites, at 819–920 °C and > 10 kbar. It is assumed that the granitoids of both complexes formed in thickened continental crust within an accretionary orogen.     

Early Carboniferous sub- to mid-alkaline magmatism in the Eastern Sierras Pampeanas,NW Argentina: A record of crustal growth by the incorporation of mantle-derived material in an extensional setting

A recently discovered granitic intrusion at Cerro La Gloria in western Sierra de Famatina (NW Argentina) is representative of sub- to mid-alkaline Carboniferous magmatism in the region. The main rock type consists of microcline, quartz and plagioclase, with amphibole, magnetite, ilmenite, biotite, epidote, zircon, allanite and sphene as accessory minerals. We report a U–Pb zircon SHRIMP age for the pluton of 349 ± 3 Ma (MSWD = 1.1), i.e., Tournaisian. Whole-rock chemical composition and Nd isotope analyses are compatible with an origin by melting of older mafic material in the lower crust (εNdt between − 0.58 and + 0.46 and T_DM values of about 1.1 Ga). The pluton is intruded by penecontemporaneous to late alkaline mafic dykes that are classified as back-arc basalts. Coeval, Early Carboniferous A-type granites occur farther east in the Sierras Pampeanas, probably generated during lithospheric stretching. Overall, the Early Carboniferous granitic rocks show a west-to-east mineralogical and isotopic zonation indicating that magma genesis involved a greater contribution of juvenile material of mantle character to the west. Based on the observed patterns of geochronology, geochemistry and field relationships we suggest that A-type magma genesis in the Eastern Sierras Pampeanas was linked to an Andean-type margin where the lithospheric mantle played a role in its generation.     

Provenances of the Mesozoic sediments in the Ordos Basin and implications for collision between the North China Craton (NCC) and the South China Craton (SCC)

To constrain the provenance of the Ordos Basin and the evolution history of the Qinling Orogen Belt from the Triassic to the Jurassic, 10 samples from the Dongsheng area and 28 samples from the Yan’an area were analyzed for U–Pb ages and Lu–Hf and Sm–Nd isotopic compositions. The results indicate that Middle Jurassic sediments in the Dongsheng area were derived from the Khondalite Belt, Langshan Mountain and the Yinshan Terrane. Mesozoic sediments in the Yan’an area consist of two parts. One part is derived from the North China Craton (NCC), which has U–Pb age groups of ∼1.8 Ga and ∼2.5 Ga, and Hf model ages of ∼2.8 Ga. The other part is derived from the Qilian–Qinling Orogenic Belt, which has U–Pb age groups of 600–1500 Ma and 100–500 Ma, and Nd and Hf isotopic model ages of less than 2.2 Ga. Combining the U–Pb ages with the Hf and Nd isotopic model ages, Mesozoic detrital zircons with U–Pb age groups of ∼1.8 Ga and ∼2.5 Ga in the Yan’an area are found to also be derived from the Khondalite Belt, Langshan Mountain and the Yinshan Terrane, not from the Trans-China Orogen Belt. From the late–Late Triassic sediments of the Yan’an area, the low average values of the Hf (2.03 Ga) and Nd (2.03 Ga) model ages and the characteristic age population of 600–1500 Ma reveal that the main collision or continental subduction between the NCC and the South China Craton (SCC) occurred in the late–Late Triassic. After the main collision or continental subduction, the proportion of sediments from the Qinling–Qilian Orogenic Belt began to decrease (recorded in the early Jurassic samples), which may be in response to the gradual slowing of the uplift speed of the Qinling Orogenic Belt. In the early-middle Jurassic, the sediments have a main U–Pb age population of 100–500 Ma, low detrital zircon Hf model ages (average value is 1.17 Ga) and low whole rock Nd model ages (average value is 1.13 Ga), which suggests that the Qilian–Qinling Orogenic Belt may have a fast uplift history in the early-middle Jurassic.     

Petrogenesis of ore-forming and pre/post-ore granitoids from the Kounrad,Borly and Sayak porphyry/skarn Cu deposits,Central Kazakhstan

Ore-forming porphyries and barren granitoids from porphyry Cu deposits differ in many ways, particularly with respect to their adakitic affinity and calc-alkaline characteristics. In this study, zircon U–Pb and molybdenite Re–Os dating, whole rock geochemistry, whole rock Sr–Nd–Pb and zircon O–Hf isotopic analyses were carried out on the ore-forming granitoids from the Kounrad, Borly and Sayak deposits, and also on pre-ore and post-ore granitoids in adjacent regions of Central Kazakhstan. Geochronology results indicate that pre-ore magmatism occurred in the Late Devonian to Early Carboniferous (361.3–339.4 Ma), followed by large scale Cu mineralization (325.0–327.3 Ma at Kounrad, 311.4–315.2 Ma at Borly and 309.5–311.4 Ma at Sayak), and finally, emplacement of the Late Carboniferous post-ore barren granitoids (305.0 Ma). The geochemistry of these rocks is consistent with calc-alkaline arc magmatism characterized by strong depletions in Nb, Ta and Ti and enrichments in light rare earth elements and large ion lithophile elements, suggesting a supra-subduction zone setting. However, the ore-forming rocks at Kounrad and Sayak show adakitic characteristics with high Sr (517.5–785.3 ppm), Sr/Y (50.60–79.26), (La/Yb)_N (9.37–19.62) but low Y (6.94–11.54 ppm) and Yb (0.57–1.07 ppm), whereas ore-forming rocks at Borly and barren rocks from northwest of Borly and Sayak have normal arc magma geochemical features. The Sr–Nd–Hf–O isotopic compositions show three different signatures: (1) Sayak granitoids have very young juvenile lower crust-derived compositions ((⁸⁷Sr/⁸⁶Sr)_i = 0.70384 to 0.70451, ɛ_Nd (t) = + 4.9 to + 6.0; T_DM2 (Nd) = 580 to 670 Ma, ɛ_Hf (t) = + 11.3 to + 15.5; T_DM^C (Hf) = 330 to 600 Ma, δ¹⁸O = 6.0 to 8.1‰), and were probably generated from depleted mantle-derived magma with 5–15% sediment melt addition in the magma source; (2) the Kt-1 granite from northwest of Sayak shows extremely enriched Sr–Nd isotopic compositions ((⁸⁷Sr/⁸⁶Sr)_i = 0.71050, ɛ_Nd (t) = − 7.8, T_DM2 (Nd) = 1700 Ma), likely derived from partial melting of ancient continental crust; (3) other granitoids have transitional Sr–Nd compositions between the Sayak and Kt-1 samples, indicating a juvenile lower crust source with the addition of 10–30% of ancient crustal material. The pre-ore magmatism was probably related to partial melting of juvenile lower crust due to northward subduction of the Junggar–Balkhash Ocean, whereas the ore-forming adakitic rocks at Aktogai, Kounrad and Sayak formed by partial melting of thickened lower crust which subsequently delaminated. The ore-forming rocks at Borly, and the later post-ore barren granites, formed by partial melting of juvenile lower crust with normal thickness. This tectonic setting supports the existence of an Andean-type magmatic arc in the Devonian to the Late Carboniferous, resulting from the subduction of the Junggar–Balkhash oceanic plate. The link between whole rock geochemistry and scale of mineralization suggests a higher metallogenic potential for adakitic rocks than for normal arc magmatism.     

U–Pb ages of detrital zircons within the Inthanon Zone of the Paleo-Tethyan subduction zone,northern Thailand: New constraints on accretionary age and arc activity

U–Pb dating of detrital zircons was performed on mélange-hosted lithic and basaltic sandstones from the Inthanon Zone in northern Thailand to determine the timing of accretion and arc activity associated with Paleo-Tethys subduction. The detrital zircons have peak ages at 3400–3200, 2600–2400, 1000–700, 600–400, and 300–250 Ma, similar to the peaks ages of detrital zircons associated with other circum-Paleo-Tethys subduction zones. We identified two types of sandstone in the study area based on the youngest detrital zircon ages: Type 1 sandstones have Late Carboniferous youngest zircon U–Pb ages of 308 ± 14 and 300 ± 16 Ma, older than associated radiolarian chert blocks within the same outcrop. In contrast, Type 2 sandstones have youngest zircon U–Pb ages of 238 ± 10 and 236 ± 15 Ma, suggesting a Middle Triassic maximum depositional age. The youngest detrital zircons in Type 1 sandstones were derived from a Late Carboniferous–Early Permian ‘missing’ arc, suggesting that the Sukhothai Arc was active during sedimentation. The data presented within this study provide information on the development of the Sukhothai Arc, and further suggest that subduction of the Paleo-Tethyan oceanic plate beneath the Indochina Block had already commenced by the Late Carboniferous. Significant Middle Triassic arc magmatism, following the Late Carboniferous–Early Permian arc activity, is inferred from the presence of conspicuous detrital zircon U–Pb age peaks in Type 2 sandstones and the igneous rock record of the Sukhothai Arc. In contrast, only minimal arc activity occurred during the Middle Permian–earliest Triassic. Type 1 sandstones were deposited between the Late Permian and the earliest Triassic, after the deposition of associated Middle–Late Permian cherts that occur in the same mélanges and during a hiatus in Sukhothai Arc magmatism. In contrast, Type 2 sandstones were deposited during the Middle Triassic, coincident with the timing of maximum magmatism in the Sukhothai Arc, as evidenced by the presence of abundant Middle Triassic detrital zircons. These two types of sandstone were probably derived from discrete accretionary units in an original accretionary prism that was located along the western margin of the Sukhothai Arc.     

Geochemistry and U–Pb zircon geochronology of the Alvand plutonic complex in Sanandaj–Sirjan Zone (Iran): New evidence for Jurassic magmatism

This paper presents geochemical, Sr–Nd isotopic, and U–Pb zircon geochronological data on the Alvand plutonic complex in Sanandaj–Sirjan zone (SSZ), Western Iran. The gabbroic rocks show a trend of a calc-alkaline magma suite and are characterized by low initial ⁸⁷Sr/⁸⁶Sr ratios (0.7023–0.7037) and positive εNd(t) values (2.9–3.3), which suggest derivation from a moderately depleted mantle source. Geochemical features of the granites illustrate a high-K calc-alkaline magma series, whereas the leucocratic granitoids form part of a low-K series. Granites have intermediate ⁸⁷Sr/⁸⁶Sr ratios (0.707–0.719) and negative εNd_(t) values (−1.0 to −3.4), while leucocratic granitoids have higher initial ⁸⁷Sr/⁸⁶Sr ratio (0.713–0.714) and more negative εNd_(t) values (−3.5 to −4.5). Potential basement source lithologies for the granites are Proterozoic granites and orthogneisses, and those for the leucocratic granites are plagioclase-rich sources such as meta-arkoses or tonalites. The U–Pb dating results demonstrate that all granitoids were exclusively emplaced during the Jurassic instead of being Cretaceous or younger in age as suggested previously. The pluton was assembled incrementally over c. 10 Ma. Gabbros formed at 166.5 ± 1.8 Ma, granites between 163.9 ± 0.9 Ma and 161.7 ± 0.6 Ma, and leucocratic granitoids between 154.4 ± 1.3 and 153.3 ± 2.7 Ma. Granites and leucocratic granitoids show some A-type affinity. It is concluded that the Alvand plutonic complex was generated in a continental-arc-related extensional regime during subduction of Neo-Tethyan oceanic crust beneath the SSZ. The U/Pb zircon age data, recently corroborated by similar results in the central and southern SSZ, indicate that Jurassic granitoids are more areally extensive in this belt than previously thought.     

New Lu–Hf and Sm–Nd geochronology constrains the subduction of oceanic crust during the Carboniferous–Permian in the Dabie orogen

Eclogites from the Huwan shear zone in the western Dabie were investigated in terms of their P–T evolution, geochemistry, and combined Lu–Hf and Sm–Nd geochronology. Trace element and isotope data suggest a normal mid-ocean ridge rather than an intraplate or ocean island setting for the protoliths of the eclogites. Electron microprobe analyses of representative garnets show typical prograde zoning profiles. Estimated peak metamorphic temperatures of 540–590 °C most likely did not exceed the closure temperature of the Lu–Hf and Sm–Nd systems. The consistent Lu–Hf and Sm–Nd ages, therefore, most likely reflect garnet growth and are interpreted to reflect high-pressure eclogite-facies metamorphism due to the occurrence of omphacite inclusions from core to rim in garnets and the spherical geometry effect despite the well-preserved prograde zoning in the garnets. The high-pressure mineral assemblage of the eclogite yielded a statistically robust Lu–Hf age of 260.0 ± 1.0 Ma (2σ, 10 points, MSWD = 1.0) and a Sm–Nd age of 260.4 ± 2.0 Ma (2σ, 9 points, MSWD = 1.4), which are younger than the Carboniferous zircon U–Pb ages of ca. 310 Ma. The new Lu–Hf and Sm–Nd data, in combination with published geochronological data, define two distinct Carboniferous and Permian population ages for the oceanic-type eclogites from the Huwan shear zone, which may require that these rocks experienced two episodes of high-pressure metamorphism within less than 50 Myr.     

A geotraverse across two paleo-subduction zones in Tien Shan,Tajikistan

We present first LA-ICP-MS U–Pb zircon ages as well as geochemical and Sr–Nd–Pb isotope data for 14 magmatic rocks collected along ca. 400 km profile across the Chatkal-Kurama terrane in the Mogol-Tau and Kurama ranges and the Gissar Segment of the Tien Shan orogen in Tajikistan. These new data from supra-subduction and post-collisional magmatic rocks of two Late Paleozoic active margins constrain a tectonic model for terrane motions across two paleo-subduction zones: (1) The 425 Ma old Muzbulak granite of the Mogol-Tau range formed in a supra-subduction setting at the northern margin of the Turkestan Ocean. The north-dipping plate was subducted from the Early Silurian to the earliest Middle Devonian. Thereafter the northern side of the Turkestan Ocean remained a passive margin until the Early Carboniferous. (2) In the Early Carboniferous, subduction under the northern margin of the Turkestan Ocean resumed and the 315 to 305 Ma old Kara-Kiya, Muzbek, and Karamazar intrusions formed in a supra-subduction setting in the Mogol-Tau and Kurama ranges. (3) At the same time, in the Early Carboniferous, rifting of the southern passive margin of the Turkestan Ocean formed the short-lived Gissar Basin, separated from the Turkestan Ocean by the Gissar micro-continent. North-dipping subduction in the Gissar Basin is documented by the 315 Ma Kharangon plagiogranite and the voluminous ca. 321–312 Ma Andean-type supra-subduction Gissar batholith. The Kharangon and Khanaka gabbro-plagiogranite intrusions of the southern Gissar range have geochemical and Sr–Nd isotopic compositions (⁸⁷Sr/⁸⁶Sr(t) 0.7047–0.7056, εNd of + 1.5 to + 2.3) compatible with mantle-derived origin typical for plagiogranites associated with ophiolites. The supra-subduction rocks from the Gissar batholith and from the Mogol-Tau Kurama ranges have variably mixed Sr–Nd–Pb isotopic signatures (⁸⁷Sr/⁸⁶Sr(t) 0.7057–0.7064, εNd of − 2.1 to − 5.0) typical for continental arcs where mantle-derived magmas interact with continental crust. (4) In the latest Carboniferous, the Turkestan Ocean and the Gissar Basin were closed. The Early Permian Chinorsay (288 Ma) and Dara-i-pioz (267 Ma) post-collisional intrusions, emplaced in the northern part of the Gissar micro-continent after a long period of amagmatic evolution, have intraplate geochemical affinities and isotopic Sr–Nd–Pb isotopic compositions (⁸⁷Sr/⁸⁶Sr(t) 0.7074–0.7086, εNd of − 5.5 to − 7.4) indicating derivation from Precambrian continental crust which is supported by old Nd model ages (1.5 and 1.7 Ga), and by the presence of inherited zircon grains with ages 850–500 Ma in the Chinorsay granodiorite. The post-collisional intrusions in the southern Gissar and in the Mogol-Tau and Kurama ranges (297–286 Ma), emplaced directly after supra-subduction magmatic series, have geochemical and isotopic signatures of arc-related magmas. The distinct shoshonitic affinities of post-collisional intrusions in the Mogol-Tau and Kurama ranges are explained by the interaction of hot asthenospheric material with subduction-enriched wedge of lithospheric mantle due to slab break-off at post-collisional stage. Despite origination from different tectonic environments, all magmatic rocks have relatively old Nd model ages (1.7–1.0 Ga) indicating a significant proportion of Paleoproterozoic or older crustal material in their sources and their model ages are similar to those of post-collisional intrusions from the Alai and Kokshaal Segments of the South Tien Shan.     

Variscan Sn–W–Mo metallogeny in the gravity picture of the Krušné hory/Erzgebirge granite batholith (Central Europe)

Late Variscan wolframite (± molybdenite) and cassiterite–wolframite greisen, skarn and vein deposits occur in a close spatial association with the granites of the Krušné hory/Erzgebirge batholith (KHEB) in Central Europe. We examined the distribution of the deposits in relation to the gravity field affected by Late Variscan granites using the data from previous gravity and metallogenic studies. Late Variscan granites are differentiated into earlier biotite monzogranites (low-F granites) and later biotite or lithium mica syenogranites (high-F granites) in accordance with the previous classifications. All the outcrops of granites in the KHEB region and their hidden continuation are confined to the Bouguer anomaly contour of − 20 mGal. The Sn–W–Mo (rare metal) deposits and occurrences are within the gravity contour of − 30 mGal with the exception of the Grossschirma stratiform tin deposit in the Freiberg polymetallic ore district. We constructed a geological model based on the gravity data along two profiles across the KHEB showing the position of some rare metal deposits and of outcropping and hidden granite bodies. The models show that the overlapping of earlier and later granites is in the areas of the most intense regional gravity minima. These coincide with the Eastern Volcano-Plutonic Complex (Altenberg minimum), which encloses large volumes of felsic extrusives, microgranite dikes and granites, and the Western Plutonic Complex (Eibenstock minimum), with small volumes of felsic dikes and predominance of earlier and later granites, with no extrusives preserved. There is no distinct relationship between the masses of Late Variscan granites and the distribution and the sizes of associated W ± Mo and Sn–W deposits. We prefer the idea that rare metal mineralization was formed by hydrothermal fluids derived from outside of presently outcropping granites. It originated in two cycles: one connected with the formation of earlier granites producing W ± Mo associations and the other one associated with later granites connected with Sn–W mineralization. Mineralizing fluids were probably generated by mantle–crustal interaction in the crust near the mantle–crust boundary as also indicated by lamprophyric intrusions coeval with the Late Variscan granitic magmatism.     

Geochronology and geochemistry of late Carboniferous volcanic rocks from northern Inner Mongolia,North China: Petrogenesis and tectonic implications

Zircon U–Pb ages, geochemical and Sr–Nd isotopic data are presented for the late Carboniferous Baoligaomiao Formation (BG Fm.) and Delewula Formation (DW Fm.) volcanic rocks, widely distributed in northern Inner Mongolia, in the northern part of the Xing'an–Mongolia Orogenic Belt (XMOB). The BG Fm. rocks mainly consist of basaltic andesites and andesites while the DW Fm. rocks include dacites, trachytes, rhyolites, pyroclastic rocks and minor andesites. New LA-ICPMS zircon U–Pb analyses constrain their eruption to late Carboniferous (317–322 Ma and 300–310 Ma, respectively). The BG Fm. volcanic rocks are characterized by enriched large ion lithophile elements (LILE) and depleted high field strength elements (HFSE), with initial ⁸⁷Sr/⁸⁶Sr ratios of 0.70854–0.70869 and negative εNd(t) (− 2.1 to − 2.4) values. They have low La/Ba (0.03–0.05), high La/Nb (2.05–3.70) ratios and variable Ba/Th (59.5–211) ratios. Such features suggest that they are derived from melting of heterogeneous sources including a metasomatized mantle wedge and Precambrian crustal material. The DW Fm. volcanic rocks are more depleted in HFSE with significant Nb, Ta, P, Ti anomalies. They have high initial ⁸⁷Sr/⁸⁶Sr ratios (0.72037–0.72234) and strong negative εNd(t) (− 11 to − 11.6) values which indicate those igneous rocks were mainly derived from reworking of the Paleoproterozoic crust. The late Carboniferous volcanic rocks have geochemical characteristics similar to those of the continental arc rocks which indicate the northward subduction of the Paleo Asian Ocean may have continued to the late Carboniferous. The volcanic association of this study together with the early Permian post-collisional magmatic rocks suggests that a tectonic transition from subduction-related continental margin arc volcanism to post-collisional magmatism occurred in the northern XMOB between the late Carboniferous and the early Permian.     

U–Pb dating of granites in the Neoproterozoic Sergipano Belt,NE-Brazil: Implications for the timing and duration of continental collision and extrusion tectonics in the Borborema Province

The Sergipano Belt is the outcome of collision between the Pernambuco–Alagoas Massif and the São Francisco Craton during Neoproterozoic assembly of West Gondwana. Field relationships and U–Pb geochronology of granites intruded in garnet micaschists of the Macururé Domain are used to constrain the main collisional event (D₂) in the belt. The granites are divided into two groups, the pre-collisional granites (pre- to early-D₂) and the syn-collisional granites (syn- to tardi-D₂), the latter were emplaced as sheets along the S₂ axial plane foliation or they were collected at the hinge zones of F₂ folds. A U–Pb SHRIMP zircon age of 628 ± 12 Ma was obtained for the pre-collisional Camará tonalite. Two U–Pb TIMS titanite ages were obtained for the syn-collisional granites, 584 ± 10 Ma for the Angico granite and 571 ± 9 Ma for the Pedra Furada granite, and these ages are close to the garnet-whole rock Sm–Nd isochron of 570 Ma found for the peak of metamorphism in the Sergipano Belt. The ages of the Camará tonalite (628 Ma) and the Pedra Furada granite (571 Ma) mark respectively the maximum age for beginning of the D₂ event and minimum age for the end in the Macururé Domain. Using these ages, the main Neoproterozoic D₂ collisional event has been in operation in the Sergipano Belt for at least 57 million years. Correlation with coeval granitoids farther north in the Borborema Province indicate that while in the Sergipano Belt the syn-D₂ granites (ca. 590–570 Ma) were emplaced under compression, in the Borborema Province they emplaced under extensional conditions related to regional strike-slip shear zones. These contrasting emplacement settings for contemporaneous Neoproterozoic granitoids are explained by a combination of continent–continent collision and extrusion tectonics.     

U–Pb (zircon) and geochemical constraints on the age,origin, and evolution of Paleozoic arc magmas in the Oyu Tolgoi porphyry Cu–Au district,southern Mongolia

Uranium–Pb (zircon) ages are linked with geochemical data for porphyry intrusions associated with giant porphyry Cu–Au systems at Oyu Tolgoi to place those rocks within the petrochemical framework of Devonian and Carboniferous rocks of southern Mongolia. In this part of the Gurvansayhan terrane within the Central Asian Orogenic Belt, the transition from Devonian tholeiitic marine rocks to unconformably overlying Carboniferous calc-alkaline subaerial to shallow marine volcanic rocks reflects volcanic arc thickening and maturation. Radiogenic Nd and Pb isotopic compositions (ε_Nd(t) range from + 3.1 to + 7.5 and ²⁰⁶Pb/²⁰⁴Pb values for feldspars range from 17.97 to 18.72), as well as low high-field strength element (HFSE) contents of most rocks (mafic rocks typically have < 1.5% TiO₂) are consistent with magma derivation from depleted mantle in an intra-oceanic volcanic arc. The Late Devonian and Carboniferous felsic rocks are dominantly medium- to high-K calc-alkaline and characterized by a decrease in Sr/Y ratios through time, with the Carboniferous rocks being more felsic than those of Devonian age. Porphyry Cu–Au related intrusions were emplaced in the Late Devonian during the transition from tholeiitic to calc-alkaline arc magmatism. Uranium–Pb (zircon) geochronology indicates that the Late Devonian pre- to syn-mineral quartz monzodiorite intrusions associated with the porphyry Cu–Au deposits are ~ 372 Ma, whereas granodiorite intrusions that post-date major shortening and are associated with less well-developed porphyry Cu–Au mineralization are ~ 366 Ma. Trace element geochemistry of zircons in the Late Devonian intrusions associated with the porphyry Cu–Au systems contain distinct Th/U and Yb/Gd ratios, as well as Hf and Y concentrations that reflect mixing of magma of distinct compositions. These characteristics are missing in the unmineralized Carboniferous intrusions. High Sr/Y and evidence for magma mixing in syn- to late-mineral intrusions distinguish the Late Devonian rocks associated with giant Cu–Au deposits from younger magmatic suites in the district.     

Contrasting SHRIMP U–Pb zircon ages of two carbonatite complexes from the peri-cratonic terranes of the Reguibat Shield: Implications for the lateral extension of the West African Craton

The Oulad Dlim Massif of the Western Reguibat Shield contains several carbonatite complexes of previously unknown age. The largest and best studied are Gleibat Lafhouda, composed of magnesiocarbonatites, and Twihinate, composed of calciocarbonatites. Gleibat Lafhouda is hosted by Archean gneisses and schists. It has a SHRIMP U–Th–Pb zircon crystallization age of 1.85 ± 0.03 Ga, a Nd model age of T_CR = 1.89 ± 0.03 Ga, and a Sm–Nd age of 1.85 ± 0.39 Ga. It forms part of the West Reguibat Alkaline province. Twihinate, on the other hand, is much younger. It is hosted by Late Silurian to Early Devonian deformed granites and has a zircon crystallization age of 104 ± 4 Ma, which is within error of the age of the carbonatites of the famous Richat Structure in the southwest Reguibat Shield. Like these, the Twihinate carbonatites are part of the Mid-Cretaceous Peri-Atlantic Alkaline Pulse. The Twihinate carbonatites contain abundant inherited zircons with ages that peak at ca. 420 Ma, 620 Ma, 2050 Ma, 2466 Ma, and 2830 Ma. This indicates that their substratum has West African rather than, as previously suggested, Avalonian affinities. It has, however, a Paleoproterozoic component that is not found in the neighboring western Reguibat Shield. The 421 Ma to 410 Ma gneissic granites hosting Twihinate are epidote + biotite + Ca-rich garnet deformed I-type to A-type granites derived from magmas of deep origin compatible, therefore, with being generated in a subduction environment. These granites form a body of unknown dimensions and petrogenesis, the study of which will be of key importance for understanding the geology and crustal architecture of this region.     

Zircon and titanite age evidence for coeval granitization and migmatization of the early Middle and early Late Proterozoic Saharan Metacraton; example from the central North Sudan basement

Central North Sudan, west of the Keraf suture, is part of the Saharan Metacraton whose crystalline basement encompasses migmatite gneisses and granites. Granites intrude migmatites in form of small plutons, veins, lenses and pods, indicating a complex chronology. This study, based on whole rock element concentrations, isotope geochemistry and single mineral geochronology, is aimed to unravel the petrogenesis of these basement rocks.Whole rock geochemistry indicates an I-type potassic calc-alkaline meta- to peraluminous composition. Granite zircon U–Pb and Pb–Pb evaporation analyses yield an identical age range (597 ± 25–602 ± 3.5 Ma). Similar ages (597 ± 8.6–603.8 ± 2 Ma) are obtained for the migmatite gneisses. Titanite U–Pb ages are also similar in both rock types, but are younger or closely conform with zircon ages. Biotite Rb/Sr ages are younger and identical (566 ± 11–570 ± 17 Ma). These age data suggest coeval granitization and migmatization during the Pan-African period and somewhat later cooling of the central North Sudan basement. Older zircon U–Pb ages, ranging from 613 to 1322 Ma, are thought to be signatures of inheritance, while younger ones (336–594 Ma) suggest radiogenic Pb loss. Sr initial ratios (0.70257–0.72102) and ε_Nd values (−2.3 to −8.8), calculated for the zircon crystallization age of ∼600 Ma indicate a crustal signature. Coupled with Nd model ages of 1460–1990 Ma, isotope data indicate that the central North Sudan basement is recycled Middle to Late Proterozoic continental crust.     

Petrogenesis of the Nanling Mountains granites from South China: Constraints from systematic apatite geochemistry and whole-rock geochemical and Sr–Nd isotope compositions

The widespread Mesozoic granitoids in South China (∼135,300 km²) were emplaced in three main periods: Triassic (16% of the total surface area of Mesozoic granitoids), Jurassic (47%), and Cretaceous (37%). Though much study has been conducted on the most abundant Jurassic Nanling Mountains (NLM) granites, their rock affinities relative to the Triassic Darongshan (DRS) and Cretaceous Fuzhou–Zhangzhou Complex (FZC) granites which are typical S- and I-type, respectively, and the issue of their petrogenetic evolution is still the subject of much debate. In this study, we discuss the petrogenesis of NLM granites using apatite geochemistry combined with whole-rock geochemical and Sr–Nd isotope compositions. Sixteen apatite samples from six granite batholiths, one gabbro, and three syenite bodies in the NLM area were analyzed for their major and trace element abundances and compared with those collected from DRS (n = 7) and FZC (n = 6) granites. The apatite geochemistry reveals that Na, Si, S, Mn, Sr, U, Th concentrations and REE distribution patterns for apatites from DRS and FZC granites basically are similar to the S and I granite types of the Lachlan Fold Belt (Australia), whereas those from NLM granites have intermediate properties and cannot be correlated directly with these granite types. According to some indications set by the apatite geochemistry (e.g., lower U and higher Eu abundances), NLM apatites appear to have formed under oxidizing conditions. In addition, we further found that their REE distribution patterns are closely related to aluminum saturation index (ASI) and Nd isotope composition, rather than SiO₂ content or degree of differentiation, of the host rock. The majority of apatites from NLM granites (ASI = 0.97–1.08 and εNd(T) = −8.8 to −11.6) display slightly right-inclined apatite REE patterns distinguishable from the typical S- and I-type. However, those from few granites with ASI > 1.1 and εNd(T) < −11.6 have REE distribution patterns (near-flat) similar to DRS apatites whereas those from granites with ASI < 1.0 and εNd(T) > −6.6 and gabbro and syenite are similar to FZC apatites (strongly right-inclined). In light of Sr and Nd isotope compositions, magmas of NLM intrusives, except gabbro and syenite, and few granites with εNd(T) > −8, generally do not involve a mantle component. Instead, they fit with a melt derived largely from in situ melting or anatexis of the pre-Mesozoic (mainly Caledonian) granitic crust with subordinate pre-Yanshanian (mainly Indosinian) granitic crust. We suggest that an application, using combined whole-rock ASI and εNd(T) values, is as useful as the apatite geochemistry for recognizing possible sources for the NLM granites.     

Petrogenesis of gold-mineralized magmatic rocks of the Taerbieke area,northwestern Tianshan (western China): Constraints from geochronology,geochemistry and Sr–Nd–Pb–Hf isotopic compositions

Many Late Paleozoic Cu–Au–Mo deposits occur in the Central Asian Orogenic Belt (CAOB). However, their tectonic settings and associated geodynamic processes have been disputed. This study provides age, petrologic and geochemical data for andesites and granitic porphyries of the Taerbieke gold deposit from the Tulasu Basin, in the northwestern Tianshan Orogenic Belt (western China). LA-ICP-MS zircon U–Pb dating indicates that the granitic porphyries have an Early Carboniferous crystallization age (349 ± 2 Ma) that is broadly contemporaneous with the eruption age (347 ± 2 Ma) of the andesites. The andesites have a restricted range of SiO₂ (58.94–63.85 wt.%) contents, but relatively high Al₂O₃ (15.39–16.65 wt.%) and MgO (2.51–6.59 wt.%) contents, coupled with high Mg^# (57–69) values. Geochemically, they are comparable to Cenozoic sanukites in the Setouchi Volcanic Belt, SW Japan. Compared with the andesites, the granitic porphyries have relatively high SiO₂ (72.68–75.32 wt.%) contents, but lower Al₂O₃ (12.94–13.84 wt.%) and MgO (0.10–0.33 wt.%) contents, coupled with lower Mg^# (9–21) values. The andesites and granitic porphyries are enriched in both large ion lithophile and light rare earth elements, but depleted in high field strength elements, similar to those of typical arc magmatic rocks. They also have similar Nd–Hf–Pb isotope compositions: ε_Nd(t) (+0.48 to +4.06 and −0.27 to +2.97) and zircons ε_Hf(t) (+3.4 to +8.0 and −1.7 to +8.2) values and high (²⁰⁶Pb/²⁰⁴Pb)_i (18.066–18.158 and 17.998–18.055). We suggest that the Taerbieke high-Mg andesitic magmas were generated by the interaction between mantle wedge peridotites and subducted oceanic sediment-derived melts with minor basaltic oceanic crust-derived melts, and that the magmas then fractionated to produce the more felsic members (i.e., the Taerbieke granitic porphyries) during late-stage evolution. Taking into account the Carboniferous magmatic record from the western Tianshan Orogenic Belt, we suggest that the formation of the Early Carboniferous andesites and granitic porphyries in the Taerbieke area were related to the Paleo-Junggar Oceanic plate southward subduction under the Yili–Central Tianshan plate. The close association of the Early Carboniferous magmatic rocks and Au mineralization in the Taerbieke area suggests that the arc magmatic rocks in the Tulasu basin may have a high potential for Au mineralization.