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.     

Geochronology of the initiation and displacement of the Altyn Strike-Slip Fault,western China

⁴⁰Ar/³⁹Ar dating studies have been carried out along the Dangjin Pass transect across the Altyn Strike-Slip Fault (ASSF). The samples gave ages of 445.2–454.3 Ma in the Northern Belt, 164.3–178.4 Ma in the Mesozoic Shear Zone and 26.3–36.4 Ma in the Cenozoic Shear Zone. Using the piercing point of the Bashikaogong Fault and the Cangma-Heihe Fault an offset of 350–400 km along the ASSF has been estimated. The ⁴⁰Ar/³⁹Ar dating of the syntectonic-growth or syntectonic-resetting minerals from the samples within the ASSF belt, and offset estimations from different age piercing points suggest that the ASSF should be initiated in the Middle Jurassic (178.4–160 Ma). Combined with previously reported ages, our studies show that the ASSF is characterized by multi-phase re-activation during 85–100, 25-40 and 8–10 Ma following its initiation in the Middle Jurassic in the regional tectonic setting of convergence between the Indian and Eurasian continents.     

The early stages of island-arc plagiogranitoid magmatism in Gornaya Shoriya and West Sayan

The structure, composition, and age of Vendian–Early Cambrian plagiogranitoid associations composing the Kshta and Taraskyr massifs of the Yenisei pluton in the Altai–North Sayan island-arc belt are considered. We have established that these associations formed within 550–520 Ma and differ in petrographic composition and sources. Two stages of island-arc plagiogranitoid magmatism are recognized: early (550–540 Ma, formation of plagiogranitoids of the Kshta (545 ± 8 Ma) and Taraskyr (545 ± 7 Ma) massifs) and late (525–520 Ma, formation of plagiogranitoids of the Maina complex of the Yenisei (524 ± 2 Ma) and Tabat plutons). By petrochemical composition and geochemical characteristics, the rocks of the Kshta massif are high-alumina plagiogranitoids similar to adakites. They might have been produced through the melting of metabasites compositionally similar to N-MORB in equilibrium with garnet-containing restite during the subduction of oceanic slab at ≥ 15 kbar. The rocks of the Taraskyr massif are low-alumina plagiogranites. They formed through the melting of metabasites located in the lower layers and(or) the basement of the island-arc system in equilibrium with plagioclase-containing restite at 3–8 kbar. The low-alumina plagiogranitoids of the Yenisei pluton melted out under the same conditions. Isotope-geochemical studies showed that the Vendian–Early Cambrian plagiogranitoids formed at the early stage are characterized by high positive ∑ Nd(T) values (7.5–4.9), Late Riphean model Nd-age (TNd(DM) = 0.64–0.98 Ga), and Sr isotope ratio varying from 0.7040 to 0.7053. These data point to the juvenile parental melts of the rocks and the varying content of ancient crustal material in the magma generation zone.     

Detrital zircon ages and Hf isotopes of the early Paleozoic flysch sequence in the Chinese Altai,NW China: New constrains on depositional age,provenance and tectonic evolution

Subduction–accretion complexes occur widely in the Central Asian Orogenic Belt (CAOB). Due to the scarcity of fossils, the depositional timing of the Habahe flysch sequence of the subduction–accretion complex in the Chinese Altai is poorly constrained, which gave rise to much controversy in understanding the time of the basement and the tectonic evolution of the Chinese Altai. U–Pb dating of detrital zircons from the Habahe sequence in the northwestern Chinese Altai reveals a young zircon population with a mean ²⁰⁶Pb/²³⁸U age around 438 Ma which, together with a mean ²⁰⁶Pb/²³⁸U age of 411 ± 5 Ma for the overlying rhyolite of the Dongxileke Formation, brackets the time of deposition of the sequence between early Silurian and early Devonian. The age of the Dongxileke rhyolite also indicates that the overlying Baihaba Formation possibly began to be deposited in the early Devonian, though U–Pb dating of detrital zircons from this formation gave a maximum depositional age of ～ 438 Ma. The youngest detrital zircons and metamorphic grains of the Habahe sequence reveal different provenance to the sequence in the east. The youngest and metamorphic zircon grains, with early Paleozoic, Neoproterozoic and pre-Neoproterozoic populations, suggest a multi-source for the Habahe sequence. The predominantly early Paleozoic zircons, characterized by concentric zoning, high Th/U ratios and euhedral shapes, imply that the sediments of the sequence were mostly derived from a proximal magmatic source. Based on the age patterns of the Neoproterozoic and pre-Neoproterozoic populations, the Tuva–Mongol Massif, along with adjacent island arcs and metamorphic belts, may be an alternative source region for the Habahe sequence. In view of new geochemical and chronological data for granitoids and advancement in the study of regional metamorphism in the Chinese Altai, we suggest a tectonic model of subduction beneath a huge subduction–accretion complex for the evolution of the Chinese Altai in the early Paleozoic.     

Geology,geochemistry and 40Ar/39Ar dating of Sevan ophiolites (Lesser Caucasus,Armenia): Evidence for Jurassic Back-arc opening and hot spot event between the South Armenian Block and Eurasia

In the Lesser Caucausus the Sevan-Akera ophiolites of N Armenia have lithological features of a slow-spreading oceanic lithosphere: serpentinites are frequently exposed and hydrothermalized at sea-floor level, plutonic rocks and dykes are rare. A complete differentiation trend is observed from mafic norites evolving to diorites and plagiogranites. Normal faults have exposed some of the deep magmatic rocks at sea-floor level. Geochemically, two distinct lava flow series have been distinguished: (1) a contaminated Mid-Oceanic Ridge Basalt (MORB) series evolving from gabbros to plagiogranites and from basalts to basaltic andesites, exhibiting slight calc-alkaline features (enrichments in Large Ion Lithophile Elements (LILE); negative anomalies in Nb–Ta and Ti relative to N-MORB); (2) an alkaline series evolving from basanites to trachy-andesites (on anhydrous basis). ⁴⁰Ar/³⁹Ar age on amphibole-bearing gabbros evidence a Middle Jurassic age (165.3 ± 1.7 Ma, 2σ) for oceanic crust formation. Structural data, including geological cross-sections and logs of the ophiolite along the northern part of Sevan Lake allow discussing the geodynamic evolution of that segment of the Amassia-Sevan-Akera ophiolitic suture zone.     

Monazite CHIME/EPMA dating of Erinpura granitoid deformation: Implications for Neoproterozoic tectono-thermal evolution of NW India

The in-situ “chemical” Th–U–Pb dating of monazite with the electron microprobe is used to unravel the Neoproterozoic tectono-thermal history of the “Erinpura Granite” terrane in the foreland of the Delhi Fold Belt (DFB) in the NW Indian craton. These granitoids are variably deformed and show effects of shearing activity. Monazites from the Erinpura granitoids recorded two main events; (1) protolith crystallization at 863 ± 23 Ma and (2) recrystallization and formation of new Th-poor monazite at 775 ± 26 Ma during shear overprint. Some components of the Erinpura granitoids, such as the Siyawa Granite and granites exposed near Sirohi town, show evidence of migmatization. This migmatization event is documented by anatexis and associated monazite crystallization at 779 ± 16 Ma. The age data indicate an overlap in timing between anatectic event and ductile shear deformation. The end of the tectono-thermal event in the Sirohi area is constrained by a 736 ± 6 Ma Ar–Ar muscovite age data from the ductile shear zone.     

Vendian-Early Paleozoic granitoid magmatism in Eastern Tuva

We summarize results of geological, geochronological, petrogeochemical, and isotope-geochemical (Sr-Nd) studies of Late Vendian-Early Paleozoic granitoid batholiths in Eastern Tuva (Kaa-Khem, East Tannu-Ola, Khamsara, etc.). Analysis of geochronological (U-Pb, Ar-Ar) data has shown that the Late Vendian-Early Paleozoic granitoids in Eastern Tuva formed in several stages in the time interval 562–450 Ma and at different geodynamic stages of the regional evolution: island-arc (562–518 Ma) and accretion-collision (500–450 Ma), with the latter stage characterized by more intense granitoid magmatism. Diorite-tonalite-plagiogranite associations with different petrogeochemical parameters are the most widespread in the region. Petrogeochemical studies of the Late Vendian-Early Paleozoic plagiogranitoid associations have revealed high- and low-alumina varieties reflecting different conditions of formation of parental melts. At the island-arc stage of the regional evolution, only low-alumina plagiogranites of tholeiitic (M-type) and calc-alkalic (I-type) series formed. Their parental melts were generated at 3–8 kbar through the partial melting of N-MORB-type metabasalts in equilibrium with amphibole restite. Isotope-geochemical studies have shown positive £_Nd values (6.9-6.3) and low Sr isotope ratios ((⁸⁷Sr/⁸⁶Sr)₀ = 0.7034-0.7046). The lower (as compared with the depleted mantle) e_Nd values and specific petrogeochemical composition (negative Nb-Ta and Ti anomalies) of the plagiogranites reflect the subduction nature of metabasic substratum and the subordinate role of ancient crustal material. At the accretion-collision stage of the regional evolution, high- and low-alumina plagiogranitoids of calc-alkalic series (I-type) formed. The high-alumina plagiogranitoids are products of melting of N-MORB-type metabasalts in equilibrium with garnet restite at > 15 kbar in the lower part of the collisional structures, and the low-alumina ones formed through the melting of metabasites in equilibrium with amphibole restite at < 8 kbar in the upper part of the same structures. The Sr-Nd isotope data for the high- and low-alumina plagiogranitoids generated at the accretion-collision stage show that the rejuvenation of rocks is accompanied by the decrease in e_Nd (from 6.2 to 3.4) and the increase in their model Nd age !_Nd(DM) (from 0.73 87 86 to 0.92 Ga) and ( Sr/ Sr)0 (0.7036-0.7048). This points to the essentially metabasic composition of the parental substratum, as in the case of the island-arc plagiogranitoids, and the progressive supply of ancient crustal material to the magma generation zone.     

Petrogenesis of post-collisional A-type granitoids from the Urumieh–Dokhtar magmatic assemblage,Southwestern Kerman,Iran: Constraints on the Arabian–Eurasian continental collision

Three plutons (Deh-Siahan, Bande-Bagh and Baghe-Khoshk Sharghi, collectively referred to as the DBB hereafter) in southwestern Kerman, in the southeastern part of the Urumieh–Dokhtar magmatic assemblage (UDMA) of the Zagros orogenic belt differ from the typical calc-alkaline metaluminous, I-type intrusions of the region. The DBB intrusions have a distinct lithological assemblage varying from diorite through monzogranite and monzonite to alkali feldspar syenite and alkali granite. The DBB granitoids are metaluminous to slightly peraluminous, alkaline to shoshonitic in composition and have high total alkali contents with K₂O > Na₂O, high FeO_T/MgO values, and low CaO and MgO contents. They are enriched in some LILEs (such as Rb and Th) and HFSEs (such as Zr, Y and REEs except Eu) and depleted in Sr and Ba relative to primordial mantle, and have low concentrations of transitional metals. These features along with various geochemical discriminant diagrams suggest that the DBB granitoids are post-collisional A-type granitoids, which had not been recognized previously in the UDMA. The chondrite-normalized REE patterns of the DBB granitoids show slightly enriched light REEs [(La/Sm)_N = 2.26–4.13], negative Eu anomalies [(Eu/Eu*)_N = 0.19–0.74] and flat heavy REE patterns [(Gd/Yb)_N = 0.80–1.87]. The negative Eu anomaly indicates an important role for plagioclase and/or K-feldspar during fractional crystallization. Whole-rock Rb–Sr isotope analysis yields an isochron age of 33 ± 1 Ma with an initial ⁸⁷Sr/⁸⁶Sr value of 0.7049 ± 0.0001. Whole-rock Sm–Nd isotope analysis gives ε_Nd^t values from + 2.56 to + 3.62 at 33 Ma. The positive ε_Nd^t and low I_Sr values of the DBB granitoids together with their T_DM of 0.6–0.7 Ga suggest their formation from partial melting of a lithospheric mantle source, modified by fluids or melts from earlier subduction processes. Melting of lithospheric mantle occurred via a dehydration melting process at pressures below the garnet stability field, as a consequence of lithospheric mantle delamination or break-off of a subducted slab and melting of the lithospheric mantle by upwelling of hot asthenosphere. On the basis of Rb/Sr age dating and the post-collisional geochemical signatures of the DBB granitoids, along with extensive pre-collisional volcanic eruptions in Middle Eocene, we suggest Late Eocene for the time of collision between the Arabian and Central Iranian plates. This also implies that the calc-alkaline I-type intrusions in the southwestern Kerman and in other parts of the UDMA may have formed in a post-collisional context.     

Isotope (U–Pb,Sm–Nd,Rb–Sr) geochronology of alkaline basic plutons of the Kuznetsk Alatau

On the northeastern slope of the Kuznetsk Alatau, small differentiated alkaline basic intrusive massifs form an isometric area ~ 100 km across. They are composed of subalkalic and alkali gabbroids, basic and ultrabasic foidolites, nepheline and alkali syenites, and carbonatites. Results of complex (U–Pb, Sm–Nd, and Rb–Sr) isotope dating suggest that alkaline basic magmatism developed at two stages, in the Middle Cambrian–Early Ordovician (~ 510–480 Ma) and in the Early–Middle Devonian (~ 410–385 Ma). Finding of accessory zircons (age 1.3–2.0 Ga) in alkaline rocks suggests that the ascent of mantle plume was accompanied by the melting of fragments of Proterozoic mature continental crust composing the basement of the Caledonian orogen of the Kuznetsk Alatau. Probably, parental Cambrian–Ordovician alkaline mafic melts initiated metasomatism and lithosphere erosion. During the next melting of lithosphere substrate in ~ 100 Myr, this caused the generation of magmas of similar composition with inherited isotope parameters (ε_Nd(T) ≈ + 4.8 to + 5.7, T_Nd(DM) ≈ 0.8–0.9 Ga) pointing to the similar nature of their matter sources in the moderately depleted mantle.     

U–Pb–Hf zircon study of two mylonitic granite complexes in the Talas-Fergana fault zone,Kyrgyzstan, and Ar–Ar age of deformations along the fault

A 2000 km long dextral Talas-Fergana strike–slip fault separates eastern terranes in the Kyrgyz Tien Shan from western terranes. The aim of this study was to constrain an age of dextral shearing in the central part of the fault utilizing Ar–Ar dating of micas. We also carried out a U–Pb–Hf zircon study of two different deformed granitoid complexes in the fault zone from which the micas for Ar dating were separated. Two samples of the oldest deformed Neoproterozoic granitoids in the area of study yielded U–Pb zircon SHRIMP ages 728 ± 11 Ma and 778 ± 11 Ma, characteristic for the Cryogenian Bolshoi Naryn Formation, and zircon grains analyzed for their Lu–Hf isotopic compositions yielded εHf(t) values from −11.43 to −16.73, and their calculated tHfc ages varied from 2.42 to 2.71 Ga. Thus varying Cryogenian ages and noticeable heterogeneity of Meso- to Paleoproterozoic crustal sources was established for mylonitic granites of the Bolshoi Naryn Formation. Two samples of mylonitized pegmatoidal granites of the Kyrgysh Complex yielded identical ²⁰⁶Pb/²³⁸U ages of 279 ± 5 Ma corresponding to the main peak of Late-Paleozoic post-collisional magmatism in the Tien Shan (Seltmann et al., 2011), and zircon grains analyzed for their Lu–Hf isotopic compositions yielded εHf(t) values from −11.43 to −16.73, and calculated tHfc ages from 2.42 to 2.71 Ga indicating derivation from a Paleoproterozoic crustal source. Microstructural studies showed that ductile/brittle deformation of pegmatoidal granites of the Kyrgysh Complex occurred at temperatures of 300–400 °C and caused resetting of the K–Ar isotope system of primary muscovite. Deformation of mylonitized granites of the Bolshoi Naryn Formation occurred under high temperature conditions and resulted in protracted growth and recrystallization of micas. The oldest Ar–Ar muscovite age of 241 Ma with a well defined plateau from a pegmatoidal granite of the Kyrgysh Complex is considered as a “minimum” age of dextral motions along this section of the fault in the Triassic while younger ages varying from 227 Ma to 199 Ma with typical staircase patterns indicate protracted growth and recrystallization of micas during ductile deformations which continued until the end of the Triassic.     

Zircon U–Pb ages and Sr–Nd isotope ratios for the Sirstan granitoid body,NE Iraq: Evidence of magmatic activity in the Middle Cretaceous Period

The Sirstan granitoid (SG), comprising diorite and granodiorite, is located in the Shalair Valley area, in the northeastern part of Iraq within the Sanandaj–Sirjan Zone (SSZ) of the Zagros Orogenic Belt. The U–Pb zircon dating of the SG rocks has revealed a concordia age of 110 Ma, which is interpreted as the age of crystallization of this granitoid body during the Middle Cretaceous. The whole-rock Rb–Sr isochron data shows an age of 52.4 ± 9.4 Ma (MSWD = 1.7), which implies the reactivation of the granitoid body in the Early Eocene due to the collision between the Arabian and Iranian plates. These rocks show metaluminous affinity with low values of Nb, Ta and Ti compared to chondrite, suggesting the generation of these rocks over the subduction zone in an active continental margin regime. The SG rocks are hornblende-bearing I-type granitoids with microgranular mafic enclaves. The positive values of ?_Nd (t = 110 Ma) (+0.1 to +2.7) and the low (⁸⁷Sr/⁸⁶Sr)_i ratios (0.7044 to 0.7057) indicate that the magma source of the SG granitoids is a depleted subcontinental mantle. The chemical and isotope compositions show that the SG body originated from the metasomatic mantle without a major role for continental contamination. Our findings show that the granitoid bodies distributed in the SSZ were derived from the continuous Neo-Tethys subduction beneath the SSZ in Mesozoic times and that the SSZ was an active margin in the Middle Cretaceous.     

Cretaceous to Cenozoic evolution of the northern Lhasa Terrane and the Early Paleogene development of peneplains at Nam Co,Tibetan Plateau

Highly elevated and well-preserved peneplains are characteristic geomorphic features of the Tibetan plateau in the northern Lhasa Terrane, north–northwest of Nam Co. The peneplains were carved in granitoids and in their metasedimentary host formations. We use multi-method geochronology (zircon U–Pb and [U–Th]/He dating and apatite fission track and [U–Th]/He dating) to constrain the post-emplacement thermal history of the granitoids and the timing and rate of final exhumation of the peneplain areas. LA-ICP-MS U–Pb geochronology of zircons yields two narrow age groups for the intrusions at around 118 Ma and 85 Ma, and a third group records Paleocene volcanic activity (63–58 Ma) in the Nam Co area. The low-temperature thermochronometers indicate common age groups for the entire Nam Co area: zircon (U–Th)/He ages cluster around 75 Ma, apatite fission track ages around 60 Ma and apatite (U–Th)/He ages around 50 Ma. Modelling of the thermochronological data indicates that exhumation of the basement blocks took place in latest Cretaceous to earliest Paleogene time. By Middle Eocene time the relief was already flat, documented by a thin alluvial sediment sequence covering a part of the planated area. The present-day horst and graben structure of the peneplains is a Late Cenozoic feature triggered by E–W extension of the Tibetan Plateau. The new thermochronological data precisely bracket the age of the planation to Early Eocene, i.e. between ca. 55 and 45 Ma. The erosional base level can be deduced from the presence of Early Cretaceous zircon grains in Eocene strata of Bengal Basin. The sediment generated during exhumation of the Nam Co area was transported by an Early Cenozoic river system into the ocean, suggesting that planation occurred at low elevation.     

Neoproterozoic collisional S-type granitoids of the Yenisei Ridge: petrogeochemical composition and U-Pb,Ar-Ar,and Sm-Nd isotope data

Collisional granitoid magmatism caused by the Early Neoproterozoic orogeny in the west of the Siberian craton is considered. New data on the petrogeochemical composition, U-Pb (SHRIMP II), Ar-Ar, and Sm-Nd isotopic ages of the Middle Tyrada granitoid massif in the northwestern Yenisei Ridge are presented. Plagiogranites, granodiorites, and quartz diorites of the massif are of calcareous and calc-alkalic composition. The elevated alumina contents and presence of accessory garnet permit them to be assigned to S-type granitoids. Their spidergrams show Rb, Ba, and Th enrichment, minimum Nb, P, and Ti contents, and no Sr depletion. The granitoids formed through the melting of plagioclase-enriched graywacke source, obviously Paleoproterozoic metaterrigenous rocks of the Garevka Formation and Teya Group (T_Nd(DM) = 2.0-2.5 Ga), judging from the isotope composition of the granitoids (T_Nd(DM-2st) = 2200 Ma and 8_Nd(T) = − 6.0) and the presence of ancient zircon cores (1.80-1.85 Ga). Formation of granitoids took place in the final epoch of the Grenville collision events in the late Early Neoproterozoic (U-Pb zircon age is 857.0 ± 9.5 Ma). In the Late Neoproterozoic, the granitoids underwent tectonothermal reworking caused by Vendian accretion and collision events on the southwestern margin of the Siberian craton, which explain the younger K-Ar biotite age, 615.5 ± 6.3 Ma.     

Age and composition of Antarctic sub-glacial bedrock reflected by detrital zircons,erratics, and recycled microfossils in the Ellsworth Land–Antarctic Peninsula–Weddell Sea–Dronning Maud Land sector (240°E–0°–015°E)

The age and composition of the 14 × 10⁶ km² of Antarctica's surface obscured by ice is unknown except for some dated detrital minerals and erratics. In remedy, we present four new analyses (U–Pb age, T_DM^C, εHf, and rock type) of detrital zircons from Neogene turbidites as proxies of Antarctic bedrock, and review published proxies: detrital hornblendes analysed for Ar–Ar age and bulk Sm–Nd isotopes; Pb isotope compositions of detrital K-feldspars; erratics and dropstones that reflect age and composition; and recycled microfossils that reflect age and facies. This work deals with the 240°E–0°–015°E sector, and complements Veevers and Saeed's (2011) analysis of the 70°E–240°E sector. Each sample is located in its ice-drainage basin for backtracking to the potential provenance. Gaps in age between sample and upslope exposure are specifically attributable to the provenance. The major provenance of detritus west of the Antarctic Peninsula (AP) is West Antarctica, and of detritus east of the AP East Antarctica. We confirm that the Central Antarctic provenance about a core of the Gamburtsev Subglacial Mountains (GSM) and the Vostok Subglacial Highlands (VSH) contains a basement that includes igneous (mafic granitoids) and metamorphic rocks with peak U–Pb ages of 0.65–0.50, 1.20–0.9, 2.1–1.9, 2.8–2.6, and 3.35–3.30 Ga, T_DM^C of 3.6–1.3 Ga, and mainly negative εHf. The potential provenance of zircons of 650–500 Ma age with T_DM^C ages of 1.55 Ga, and of zircons of 1200–900 Ma age with positive εHf lies beneath the ice in East Antarctica south and southeast of Dronning Maud Land within the Antarctic part of the East African–Antarctic Orogen. Zircons with the additional ages of 1.7–1.4 Ga, 2.1–1.9 Ga, and 3.35–3.00 Ga have a potential provenance in the GSM.     

Geochronology and geochemistry of late Jurassic adakitic intrusions and associated porphyry Mo–Cu deposit in the Tongcun area,east China: Implications for metallogenesis and tectonic setting

The genesis of adakites and associated Mo–Cu mineralization in non–arc settings in China is poorly constrained. Here, we present geochronology, geochemistry and Sr–Nd–Pb isotopes for the Tongcun intrusive complex, and report Pb isotopes and ⁴⁰Ar–³⁹Ar age for the Tongcun Mo–Cu deposit. The Tongcun intrusive complex is composed mainly by granodiorite and monzogranite (phase 1 and phase 2), with emplacement age of 160 Ma to 148 Ma. The Tongcun complex can be classified as typical high–K calc–alkaline I–type granitoid and also shows adakitic geochemical features. Moreover, the negative Nb, Ta, Ti, and P anomalies and enriched initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7083–0.7092 of the Tongcun intrusive complex are consistent with those of the subduction–related magmatism. The ⁴⁰Ar–³⁹Ar dating of sericite, which is intergrown with chalcopyrite, indicates that the late Cu mineralization event occurred at ~ 155.5 Ma. The early Mo (Cu) and the late Cu mineralization events in this deposit were temporally, spatially and genetically associated with the emplacement of monzogranite (phase 1). There are no obvious linear correlation between SiO₂ and most of the major and trace elements, and all rock samples fall within the fields of unfractional crystallization felsic granites in Zr + Nb + Ce + Y versus FeO^T/MgO and (K₂O + Na₂O)/CaO diagrams, indicating that partial melting rather than fractional crystallization has played an important role for the formation of the Tongcun intrusive complex. Magmatic inherited zircons from the Tongcun granitoids with the age peaked at 780–812 Ma, imply that the Neoproterozoic igneous rocks in the lower crust have been incorporated into the magma source. The uniform ε_Nd(t) (− 6.3 to − 7.3), initial ⁸⁷Sr/⁸⁶Sr, ²⁰⁷Pb/²⁰⁴Pb (15.596–15.621), and ²⁰⁸Pb/²⁰⁴Pb (38.374–38.650), as well as high K₂O contents (3.36–4.10 wt.%) and relatively high Mg# values (35.40 to 40.30) suggest the Tongcun intrusive complex was derived from partial melting of the thickened lower continental crust triggered by basaltic magma underplating plus additional input from the EM II mantle-derived basaltic melts. The Tongcun area was controlled by a compression setting related to the subduction of the Paleo–Pacific Plate in Mesozoic period.     

Zircon U-Pb ages and geochemistry of the Wenquan Mo-bearing granitioids in West Qinling,China: Constraints on the geodynamic setting for the newly discovered Wenquan Mo deposit

East Qinling is the largest porphyry molybdenum province in the world; these Mo deposits have been well documented. In West Qinling, however, few Mo deposits have been discovered although granitic rocks are widespread. Recently, the Wenquan porphyry Mo deposit has been discovered in Gansu province, which provides an insight into Mo mineralization in West Qinling. In this paper we report Pb isotope compositions for K-feldspar and sulfides, S isotope ratios for sulfides, the results obtained from petrochemical study and from in situ LA-ICP-MS zircon U-Pb dating and Hf isotopes. The granitoids are enriched in LILE and LREE, with REE and trace element patterns similar to continental crust, suggesting a crustal origin. The Mg^# (40.05 to 56.34) and Cr and Ni contents are high, indicating a source of refractory mafic lower crust. The ε_Hf(t) values of zircon grains from porphyritic monzogranite range from ? 2.9 to 0.6, and from granitic porphyry vary from ? 3.3 to 1.9. The zircons have T_DM2 of 1014 to 1196 Ma for the porphyritic monzogranite and 954 to 1224 Ma for the granitic porphyry, implying that these granitoids were likely derived from partial melting of a Late Mesoproterozoic juvenile lower crust. The Pb isotope compositions of the granitoids are similar to granites in South China, showing that the magma was sourced from the middle–lower crust in the southern Qinling tectonic unit. The Pb isotopic contrast between the Mo-bearing granitoids and ores shows that the Pb in the ore-forming solution was derived from fractionation of a Triassic magmatic system. δ³⁴S values of sulfides are between 5.02 and 5.66‰, similar to those associated with magmatic-hydrothermal systems. LA-ICP-MS zircon U-Pb dating yields crystallization ages of 216.2 ± 1.7 and 217.2 ± 2.0 Ma for the granitoids, consistent with a previously reported molybdenite Re-Os isochron age of 214.4 ± 7.1 Ma. This suggests that the Mo mineralization is related to the late Triassic magmatism in the West Qinling orogenic belt. In view of these geochemical results and known regional geology, we propose that both granitoid emplacement and Mo mineralization in the Wenquan deposit resulted from the Triassic collision between the South Qinling and the South China Block, along the Mianlue suture. Since Triassic granitoid plutons commonly occur along the Qinling orogenic belt, the Triassic Wenquan Mo-bearing granitoids highlight the importance of the Triassic tectono-magmatic belt for Mo exploration. In order to apply this metallogenic model to the whole Qinling orogen, further study is needed to compare the Wenquan deposit with other deposits.     

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.     

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.     

Sources and petrogenesis of Jurassic granitoids in the Yusufeli area,Northeastern Turkey: Implications for pre- and post-collisional lithospheric thinning of the eastern Pontides

Small granitoids emplaced into the early Jurassic volcani-clastic succession in the Yusufeli area, northeastern Turkey, can be temporally and geochemically classified into two groups: early Jurassic low-K and late Jurassic high-K. ⁴⁰Ar–³⁹Ar hornblende analyses yielded 188.0 ± 4.3 Ma for the Dutlup?nar intrusion, dating the subduction related rifting in the region. It comprises metaluminous to weakly peraluminous (ASI = 0.94–1.11) granodiorite and, to a lesser extent, tonalite whose K₂O-poor (< 2.04 wt.%) nature and weak negative Eu anomalies (Eu/Eu? = 0.9–0.7) preclude derivation by fractional crystallization from a K-rich melt. Sr, Nd and Pb isotopic data reveal derivation by partial melting from an already cooled tholeiitic basic rocks which had mantle-like isotope signature. The Sumbated intrusion formed in the late Jurassic (153.0 ± 3.4 Ma) and consists chiefly of metaluminous (ASI = 0.84–0.99) quartz monzodiorite. Medium to high-K₂O, relatively high MgO and Sr contents, flat HREE patterns without prominent Eu anomalies, slightly positive ε_Nd(t) values (+ 1.5 to + 2.5) and low I_Sr ratios (0.7046–0.7056) are consistent with an origin by dehydration melting of a juvenile source, above the garnet stability field, dominated by likely K-amphibole bearing calc-alkaline mafic rocks. Geochemical data show that fractional crystallization from a Sumbated-like quartz monzodioritic magma is the fundamental process responsible for the evolved compositional range of the Keçikaya intrusion. The geochemical and geochronological data presented here indicate that the late Jurassic magmatism occurred in a post-collisional setting. Slab-breakoff, which was followed by shortly after collision, seems to be the most plausible mechanism for the generation of medium to high-K calc-alkaline rocks of the Sumbated and the Keçikaya intrusions, indicating a switch in the geodynamic setting, e.g., from pre-collision to post-collision in the middle Jurassic in the eastern Pontides.     

Continental crust of Gorny Altai: stages of formation and evolution; indicative role of granitoids

The composition and mechanisms of formation of continental crust in Gorny Altai and the role of granitoid magmatism in its evolution are considered. Geochemical and isotope data for major types of rocks of primary crust and for Early–Middle Paleozoic granitoids of the region are presented. The role of granitoids as indicators of the different stages of the continental-crust evolution is discussed. A review of the main models of continental crust formation is maid, and their applicability to the Gorny Altai segment of the Central Asian Fold Belt is shown. Based on the complex of geological, geochemical, isotope, and geochronological data, it has been established that the formation of continental crust in the Early and Late Caledonian terranes of Gorny Altai proceeded nearly synchronously (in the Middle–Late Devonian).In the Early Caledonian terranes, this process was the consequence of the multistage fractionation of primary juvenile crust of basic composition, and in the Late Caledonian ones it was the result of one-cycle intracrustal melting of hybrid andesitic crust rich in recycled material.