Paleozoic Granitoids Research Articles

Late Paleozoic granitoids from central Qiangtang, northern Tibetan plateau: A record of Paleo-Tethys Ocean subduction

The Longmu Co-Shuanghu suture zone in central Qiangtang, northern Tibetan plateau, is a segment of the main suture zone of the Paleo-Tethys Ocean. Numerous studies have focused on ophiolite and high-pressure metamorphic rocks from this suture zone, whereas the associated subduction-related magmatic rocks have received little attention. In this study, we present new data for the Late Paleozoic granitoids from the Gangma Co, Guoganjianian and Chasum areas along the Longmu Co-Shuanghu suture zone. Zircon U-Pb dating indicates that these granitic rocks were emplaced between 364Ma and 346Ma (Late Devonian-Early Carboniferous). Three granitic plutons exhibit different geochemical compositions, but show uniform Sr-Nd and zircon Hf isotopic features with positive ƐNd(T) and zircon ƐHf(T) values. Furthermore, these rocks were all formed in a volcanic arc setting associated with an oceanic subduction zone. Together with data from coeval volcanic rocks, we suggest that the Late Paleozoic magmatic rocks were formed as a result of northward subduction of the Paleo-Tethys Ocean. Subduction of the Paleo-Tethys Ocean probably commenced in the Late Devonian, continued throughout the Late Paleozoic, and ended in the Triassic.

Phanerozoic granitoids in the central and eastern parts of Central Asia and their tectonic significance

The widespread occurrence of voluminous granitoids is a typical characteristic of the Central Asian Orogenic Belt (CAOB) in the central and eastern parts of Central Asia. The granitoids occupy a total area more than 8 million km2, with three major area peaks at Cambro-Ordovician, Carboniferous-Permian, and Jurassic. Six zircon age peaks at ca. 500, 430–370, 330–300, 290–260, 210–170 and 145–110Ma.The early Paleozoic granitoids (540–440Ma, with age peak at ca. 500Ma) mainly occur in the northwestern CAOB, and are of calc-alkaline I-type. They probably formed in a subduction setting (500–480Ma) within the Paleo-Asian Ocean (PAO) and subsequent microcontinental-collision (ca. 460Ma). The middle Paleozoic granitoids (430–370Ma) predominately occur in the NW and southern CAOB, and are characterized by a high-K calc-alkaline affinity, suggesting an accretionary and/or a collisional setting. The late Paleozoic (350–260Ma) granitoids are widespread in the CAOB and formed in variable tectonic settings within an intra-continental regime or active continental margins in the PAO and in the evolving Mongol-Okhotsk Ocean (MOO). Carboniferous granitoids (350–300Ma) in the northern and northwestern CAOB exhibit A-type, alkaline features and formed in a post-accretionary settings, whereas granitoids in the southern CAOB show I-type, calc-alkaline features and were generated in an arc setting. Permian granitoids (290–270Ma) are predominantly of I-A, A-type with alkaline signatures, associated with coeval mafic magmatism and suggesting a post-collisional or intraplate setting, contemporaneous with a Large Igneous Province (LIP) in the Tarim Craton. Some 270–250Ma calc-alkaline granitoids also occur in the southernmost CAOB. Combined with 250–245Ma high Sr/Y granitoids, these rocks reflect final closure of the PAO during 270–250Ma.Early Mesozoic (mainly Triassic) granitoids occur mainly in the eastern CAOB or eastern Central Asia. The early-phase granitoids (250–230Ma) are of I- and S-types, whereas the mid- and late-phase granitoids (ca. 230–190Ma) are of A-type and transitional I-A type or highly fractionated I-type. All these granitoids occur in variable settings (intraplate, post-orogenic, and continental arc) related to three tectonics regimes, namely the Paleo-Asian Ocean (PAO), the Mongol-Okhotsk Ocean (MOO), and the Paleo-Pacific Ocean (PPO). Late Mesozoic granitoids mainly occur in eastern Central Asia and were emplaced during 6 major magmatic events at 190–180, 180–165, 165–145, 145–135, 135–100 and 100–60Ma. The 190–165Ma granitoids are of I-type, calc-alkaline/high-K calc-alkaline, and evolved along active margins related to the Mongol-Okhotsk and Paleo-Pacific ocean domains. The 145–120Ma granitoids are highly fractionated I-, transitional I-A or, A-types, with high-K calc-alkaline/shoshonitic signatures. The magmatic evolution from the Late Jurassic to Early Cretaceous coincided with a tectonic transition from contractional, thickened crust to extensional thinning. The 120–65Ma granitoid magmatism displays a southeastward younging trend, suggesting a roll-back of the Paleo-Pacific oceanic plate.The granitoids in the CAOB of eastern Central Asia show variable whole-rock Nd and Hf-in-zircon isotopic compositions, and most have juvenile signatures. In some regions (e.g., Altai-Junggar and Great Xing’an), the volume of juvenile and slightly juvenile crust (<1.0Ga) is much higher than that of other orogens (such as the Qinling in Central China. The proportion of juvenile vs. old crust may be a marker to identify a typical accretionary orogen from a collisional orogen.

Petrological and geochemical features of the early Paleozoic granitic gneisses and iron ores in the Tianhu iron deposit, Eastern Tianshan, NW China: Implications for ore genesis

This paper reports whole-rock geochemical, zircon U–Pb and Hf isotopic data for ore-hosted granitic gneisses, mineral compositions of oxides, and sulfur isotopic data for sulfides in iron ores from the Tianhu deposit, central part of the Eastern Tianshan. Our results can provide crucial constraints on the genesis of granitic gneisses and early Paleozoic tectonic setting of the Eastern Tianshan. LA-ICP-MS U–Pb dating on magmatic zircons yielded weighted mean 206Pb/238U ages of 463 to 438Ma, interpreted as the crystallization ages of the granitic protoliths and the formation ages of the Tianhu Group. Zircon U–Pb age of ore-hosted granitic gneiss (ca. 459Ma) can provide reliable constrains on upper limit for iron mineralization age in the Tianhu deposit. Geochemical characteristics suggest that the protoliths of the Tianhu granitic gneisses are metaluminous to weakly peraluminous high-K calc-alkaline granitic rocks, exhibiting typical subduction-related features such as strong enrichment in LREE and LILE and depletion in HFSE. Zircon Hf isotopic compositions show a positive trend from ~463 to ~438Ma, indicating that ~460Ma magmas came from both ancient and juvenile sources, whereas ~438Ma magmas involved more juvenile material. Some early Paleozoic granitoids were recently identified in the Eastern Tianshan with the ages between ca. 475 and ca. 425Ma. The formation of these early Paleozoic granitoids was in response to subduction processes, suggesting that subduction of Junggar Ocean probably began in the Early Ordovician and lasted until Late Silurian.Pyrite and pyrrhotite in iron ores have δ34SCDT values from +4.6 to +15.7‰, which are consistent with the marine source, but inconsistent with the magmatic source or those involved evaporites in skarn iron deposit. Geological, geochemical, and isotopic data suggest that the Tianhu iron ores were formed by volcano-sedimentary processes in a subduction environment during the early Paleozoic time, and Tianhu is a metamorphosed volcano-sedimentary iron deposit, rather than a skarn iron deposit as previously proposed.

A-type granitoid in Hasansalaran complex, northwestern Iran: Evidence for extensional tectonic regime in northern Gondwana in the Late Paleozoic

The Hasansalaran plutonic complex is one of the main intrusive bodies with a wide range of granite, monzonite, diorite and syenite that crop out in northwest Iran. This body includes Paleozoic granitoids that are surrounded and cut by Cretaceous granitoids. Zircon U-Pb age dating shows that the crystallization of this body occurred at 360Ma ago in the Early Carboniferous. Whole rock compositions of the investigated intrusive body, show high contents of Ga (11.1-76.3ppm), Zr (73.5–1280ppm), Zn (43.7–358ppm), Y(17.9–177ppm), enrichment of rare earth elements (REEs) together with high Ga/Al ratios and a strong Eu negative anomaly, fairly consistent with typical A-type signature. The low εNd(t=360Ma) value (<+3) and high variation of 87Sr/86Sr(initial) ratios are evidence of the role of the continental component for the evolution of A-type granitoids in the Hasansalaran area. Because of the high contents of Ta, Yb, Nb and Y, all samples are plotted in the within-plate tectonic regime without interfering oceanic released fluids in the subduction zone. These high Nb content rocks (37.2–342ppm without one sample) are classified as A1-type granitoids. Based on the distribution of A1- and A2-type granitoids in the Late Paleozoic in northwest Iran, the existence of some gabbroic rocks with tholeiitic to alkali composition and a long gap for magmatic activities in the area from 550 to 360Ma (approximately 180my.a.) between the Zagros and Tabriz faults, we suggest a new thematic model for evolution of northwest Iran in the Late Paleozoic. Based on our model, the upwelling of a mantle plume, probably due to the proto-Tethys oceanic rollback activity beneath northern Gondwana, had a crucial role in the uplifting of the continental crust and resulted in the crystallization of A-type granitoids with some gabbroic rocks in northwest Iran.

Early Jurassic A-Type Granites in Southeast China: Shallow Dehydration Melting of Early Paleozoic Granitoids by Basaltic Magma Intraplating

AbstractA-type granites are widespread in most orogenic belts, and their petrogenesis has played an important role in reconstructing orogenic events. However, their sources, even today, are a subject of considerable controversy. At least four models have been proposed for their formation, namely, melting of a previously melted source, fractionation of basaltic magma, anatexis of crustal igneous sources, and magma mixing. Here we present Early Jurassic A-type granites in southeast (SE) China as an example of a shallow (ca. 15–20-km depth), igneous, crust-derived A-type granite. We present detailed laser ablation ICP-MS zircon U-Pb dating and major- and trace-element and Sr-Nd isotopic data for these granites and assess their origin and relationship to the geodynamics of the late Mesozoic extensive magmatism in SE China. The associated granites include the Zhaibei, Keshubei, Zhulanbu, and Guangze plutons, which were emplaced during the interval 191–188 Ma and occur in a northeast-trending belt, parallel to ...

Miocene orbicular diorite in east-central Himalaya: Anatexis, melt mixing, and fractional crystallization of the Greater Himalayan Sequence

The Greater Himalayan Sequence and leucogranite forming the core of the Himalayan orogen provide an excellent natural laboratory in which to study processes related to crustal melting, granitoid formation, and the tectonic evolution of mountain belts. In contrast to most previous studies, which considered the Himalaya-aged granitoids as leucogranites, here we report a Miocene orbicular diorite from the Greater Himalayan Sequence in the east-central Himalaya. The diorite consists of ellipsoidal orbicules in a diorite matrix. The orbicules have a diorite core with or without garnet-sillimanite-biotite schist enclaves, an inner shell of tangentially oriented biotite laths, and an outer shell of radial or plumose plagioclase crystals. The diorite is aluminous and calcic, shows a fractionated rare earth element pattern with a strongly positive Eu anomaly, and has elevated Sr concentration. The schist enclaves underwent high-temperature metamorphism and partial melting under conditions of 9.2–12 kbar and 765–900 °C, followed by a retrograde pressure-temperature path of decompression and cooling. The inherited magmatic cores, metamorphic mantles, and magmatic rims of zircon from the diorite yield a protolith age of ca. 494 Ma, metamorphic ages ranging from ca. 26 Ma to 22 Ma, and melt crystallization ages of ca. 18–14 Ma. The inherited magmatic cores of zircon show variable but mostly negative e Hf ( t ) values, whereas the metamorphic mantles and the magmatic rims of zircon yield variable and lower e Hf ( t ) values. Our study shows that the orbicular diorite is a plagioclase + biotite + cordierite cumulate rock that formed by the fractional crystallization of peraluminous melt, which was generated by mixing of melts derived from the partial melting of both early Paleozoic granitoids and old pelitic rocks. The orbicular diorite and the trapped metapelitic schist have a prolonged high-temperature metamorphic, anatectic, and crystallization history that was initiated at ca. 26 Ma and lasted until ca. 14 Ma. The formation of orbicular structures was probably related to decompression during the ascent of anatectic melt, and subsequent rapid cooling. Melt mixing was also a key factor that aided in the formation of the orbicular structure of the diorite.

Age and tectonic setting of the early Paleozoic magmatism of the Mamyn Terrane, Central Asian Orogenic Belt, Russia

This paper presents new geochemical, U–Pb geochronological, and Sm–Nd isotopic data for early Paleozoic granitoids and acidic volcanic rocks within the Mamyn Terrane that constrain the early Paleozoic tectonic evolution of the eastern central Asian orogenic belt (CAOB). The Mamyn Terrane is usually considered part of the Argun Massif, although our new geochronological data indicate the presence of two magmatic events within the terrane that occurred at the Ediacaran–Cambrian boundary (∼541Ma) and the late Cambrian–Early Ordovician boundary (507–488Ma). Field observations indicate that all of the late Ediacaran–Early Cambrian (∼541Ma) volcanic rocks are deformed whereas the Late Cambrian–Early Ordovician (507–488Ma) intrusive and volcanic rocks are either deformed or undeformed. The ∼541Ma magmatic event in the study area produced rhyodacite, trachyrhyodacite, and trachyrhyolite units that are either high-K calc-alkaline or shoshonitic. These units have εNd(t) values from −7.4 to −8.7, tNd(DM) ages of 1.9–1.8Ga, and formed from primary magmas generated by the partial melting of Mesoproterozoic continental crustal material in a suprasubduction zone setting. The Late Cambrian–Early Ordovician (507–488Ma) magmatic event in this area formed gabbrodiorite, diorite, granodiorite, granite, trachyrhyodacite, and rhyodacite units that are medium-K and calc-alkaline, and have arc-like trace element compositions that are enriched in the large ion lithophile elements (LILEs) and depleted in the high field strength elements (HFSEs). These units have initial 87Sr/86Sr(i) ratios and εNd(t) values that range from 0.7048 to 0.7067 and from −3.3 to −0.2, respectively, yielding tNd(DM) ages of 1.6–1.1Ga. These features indicate that the magmas that formed these units were generated in a subduction zone setting, most likely by the partial melting of Mesoproterozoic crustal material with the addition of some younger juvenile material. In addition, the Late Cambrian–Early Ordovician trachyrhyodacite and rhyodacite units in this area are high-K and calc-alkaline or are shoshonitic and are enriched in K, Rb, the rare earth elements (REE), Y, Nb, Ta, and Zr, and are depleted in Sr and have A-type granite affinities. However, these units do not contain alkali amphibole or pyroxene and have high FeOtot/MgO ratios (6.7–12.7) that indicate they formed from highly fractionated magmas. Therefore, these volcanic rocks represent extreme melt differentiates that also formed the coeval granites. The early Paleozoic magmatic events within the Mamyn Terrane have also been identified in the Argun, Bureya, and Jiamusi areas of the eastern CAOB, suggesting that all of these areas have a common geodynamic history.

U‐PB LA‐ICP‐MS DATING OF ZIRCONS FROM SUBVOLCANICS OF THE BIMODAL DYKE SERIES OF THE WESTERN TRANSBAIKALIA: TECHNIQUE, AND EVIDENCE OF THE LATE PALEOZOIC EXTENSION OF THE CRUST

Our study investigates a possibility of using isotope ratios of U‐Th‐Pb system in zircon analysis by the LA‐ICP‐MS technique and monitoring of precision, convergence and accuracy. New U‐Pb isotope‐geochronological data has been obtained for the salic and mafic members of the bimodal Late Paleozoic series of subparallel dykes in the central part of the Western Transbaikalia. It is shown that a series of subparallel dykes (290–280 Ma) gives evi‐ dence of extension of the continental crust at the final stage of the Late Paleozoic granitoid magmatism. Similar ages of zircons in the salic and mafic subvolcanic rocks confirm the geological indicators of the coexistence and interactions between the magmas of contrasting compositions.

TRACKING DEEP ANCIENT CRUSTAL COMPONENTS BY XENOCRYSTIC ZIRCONS OF PALEOZOIC FELSIC IGNEOUS ROCKS FROM THE ALTAI-EAST JUNGGAR TERRANE AND ADJACENT REGIONS AND ITS TECTONIC SIGNIFICANCE

The deep crustal continental components and architecture of the Central Asian Orogenic Belt have long been a matter of debate [Wang et al., 2009; Kroner et al., 2014; Xiao et al., 2015; Yang et al., 2017]. We present an integrated study of geochronological and Hf-inzircon isotopic data for xenocrystic zircons from the Paleozoic granitoid rocks and associated felsic volcanic rocks of the Chinese Altai, East Junggar and nearby regions. The aim is to trace the age spatial distribution of deep old crustal components in these key parts of the western Central Asian Orogenic Belt.

Detrital zircon U-Pb and Hf isotopic data from the Liuling Group in the South Qinling belt: Provenance and tectonic implications

The Liuling Group is exposed in the Northern part of the South Qinling orogenic belt. LA-ICP-MS U-Pb analysis of detrital zircons from the meta-sandstones in this Group yields ages ranging between 400Ma and 3200Ma, with three prominent age clusters at 500–400Ma, 850–700Ma and 1000–900Ma. A few older zircon populations with U–Pb ages of 1750–1450Ma, 2000Ma and 2600–2400Ma are also present. Age data integrated with cathodoluminescence, trace element data and εHf(t) values of zircon grains show that the Liuling sediments have a complex source. Source rocks mainly include Early Neoproterozoic and Early Paleozoic granitoids, together with minor ultra-high pressure/high pressure (HP-UHP) metamorphic rocks, and paragneiss in the North Qinling belt, and Middle-Late Neoproterozoic magmatic rocks in the South Qinling belt. The dominant population of detrital zircon grains with ages between 500Ma and 400Ma show the characteristics of both magmatic and metamorphic zircons. They show three age clusters at 497Ma, 451Ma, and ca. 420Ma and show marked correlation with the three stages of Palaeozoic magmatism, as well as with the peak and retrograde HP-UHP metamorphic stages in the North Qinling belt. This correlation demonstrates that these Early Palaeozoic granitoids and HP-UHP metamorphic rocks in the North Qinling belt were already exhumed to the surface, underwent erosion prior to Middle Devonian time and were then deposited in an extensional basin. Based on the results from detrital zircon U-Pb dating, combined with geochemical data and the regional geology, the deposition of Liuling sediments is inferred to have occurred in a post-orogenic extensional basin, rather than a subduction-related fore-arc basin or a foreland basin formed during or after continental collision.

The distribution, geochronology and geochemistry of early Paleozoic granitoid plutons in the North Altun orogenic belt, NW China: Implications for the petrogenesis and tectonic evolution

Abundant early Paleozoic granitoid plutons are widely distributed in the North Altun orogenic belt. These rocks provide clues to the tectonic evolution of the North Altun orogenic belt and adjacent areas. In this paper, we report an integrated study of petrological features, U–Pb zircon dating, in situ zircon Hf isotope and whole-rock geochemical compositions for the Abei, 4337 Highland and Kaladawan Plutons from north to south in the North Altun orogenic belt. The dating yielded magma crystallization ages of 514Ma for the Abei Pluton, 494Ma for the 4337 Highland Pluton and 480–460Ma for the Kaladawan Pluton, suggesting that they are all products of oceanic slab subduction because of the age constraint. The Abei monzogranites derived from the recycle of Paleoproterozoic continental crust under low-pressure and high-temperature conditions are products of subduction initiation. The 4337 Highland granodiorites have some adakitic geochemical signatures and are sourced from partial melting of thickened mafic lower continental crust. The Kaladawan quartz diorites are produced by partial melting of mantle wedge according to the positive εHf(t) values, and the Kaladawan monzogranite-syenogranite are derived from partial melting of Neoproterozoic continental crust mixing the juvenile underplated mafic material from the depleted mantle. These results, together with existing data, provide significant information about the evolution history of oceanic crust subduction during the 520–460Ma. The initiation of subduction occurred during 520–500Ma with formation of Abei Pluton; subsequent transition from steep-angle to flat-slab subduction at ca.500Ma due to the arrival of buoyant oceanic plateaus, which induces the formation of 4337 Highland Pluton. With ongoing subduction, the steep-angle subduction system is reestablished to cause the formation of 480–460Ma Kaladawan Pluton. Meanwhile, it is this model that account for the temporal–spatial distribution of these early Paleozoic magmatic rocks in the North Altun orogenic belt.

Pb isotope composition of granitoids from the Hercynides of the Central Asian orogenic Belt: Evidence for growth of the juvenile crust

Based on study of the Middle Paleozoic (Hercynian) structures of the Central Asian Fold Belt located in Southwest Mongolia, the problem of the sources of juvenile crust growth registered in the Pb isotope composition of postaccretional granitoids is considered. Our study shows that the Pb isotope system of Late Paleozoic granitoids from the Hercynides of the Central Asian Fold Belt provides evidence for the juvenile nature of the continental crust in this region. The evolution of the Pb isotope composition in the Hercynides of Mongolia corresponds to the model parameter μ = 9.25, according to the Stacey–Kramers twostage model. The juvenile source of the Hercynian crust should be characterized by a lower μ value. In addition, according to the Stacey–Kramers two-stage model, the Th/U value in this crust is 3.58.

Partial melting of an ancient sub-continental lithospheric mantle in the early Paleozoic intracontinental regime and its contribution to petrogenesis of the coeval peraluminous granites in South China

The appinite–granite association has been found in various tectonic regimes related to recent subduction, arc-continent or continent–continent collision and post-collision (orogen), and appinites generally originate from recently subduction-modified lithospheric mantle. We conducted a study on a rarely reported appinite–granite association formed in an intracontinental regime, the Zhangjiafang–Qinglongshan complex (ZQC), in which the appinites were derived from an ancient sub-continental lithospheric mantle (SCLM). The ZQC is located in the western Wugongshan domain, and consists of basaltic to intermediate appinites and granitoids. Ten dated samples (including massive and gneissoid granitoids, hornblende gabbro, and diorite) give zircon 206Pb/238U ages ranging from 444±3Ma to 452±4Ma, indicating that these various lithologies were emplaced synchronously. The basaltic appinites show radiogenic 87Sr/86Sri (0.71016–0.71431) and negative εNd(t) (−6.1 to −8.9) and zircon εHf(t) (−4.2 to −7.5) values. Combined with regional geological background, an origin from the Neoproterozoic metasomatised SCLM can be inferred for the appinites in the Wugongshan domain. The granitoids are peraluminous and almost high-K calc–alkaline to shoshonitic. They exhibit a wide range of isotopic compositions (87Sr/86Sri=0.70828–0.71857, εNd(t)=−6.2 to −10.5, zircon εHf(t)=−9.5 to −26.6). Some of the granitoids display the most evolved Sr–Nd isotopic signatures among all the studied lithologies, which are consistent with those of the middle to upper crust, suggesting a pure crustal origin. Other granitoids show relatively mafic composition and less evolved isotopic signature. The intermediate appinites have intermediate chemical compositions between those of the basaltic appinites and granitoids, and similar Sr–Nd isotopic compositions to those of the basaltic appinites that have relatively evolved composition and isotopic signature. The petrographical and the geochemical studies, as well as magma mixing and mingling between mafic magma and felsic magma observed in field work, indicate that the ZQC was formed by several complex magmatic processes including assimilation and fractional crystallization (AFC) of a basaltic magma derived from an enriched mantle, magma mixing and mingling, fractional crystallization of a crust-derived felsic magma. The contribution of the Neopreoterozoic subduction-modified lithospheric mantle to the early Paleozoic granitoids in South China has been poorly constrained due to its evolved isotopic signatures (such as negative εNd(t) and εHf(t) values, high initial ratio of 87Sr/86Sr) and few studies on the early Paleozoic basaltic–felsic rock association. Our work here suggests that the Wugongshan appinite–granite association was formed in an intracontinental orogen, and the ancient enriched mantle may have had a significant chemical contribution to the formation of some of the Paleozoic peraluminous granites in South China.

Petrogenesis of granitoids in the eastern section of the Central Qilian Block: Evidence from geochemistry and zircon U-Pb geochronology

The Caledonian-age Qilian Orogenic Belt at the northern margin of the Greater Tibetan Plateau comprises abundant granitoids that record the histories of the orogenesis. We report here our study of these granitoids from two localities. The Qingchengshan (QCS) pluton, which is situated in the eastern section of the Central Qilian Block, is dated at ~430–420 Ma. It has high-K calc-alkaline composition with high SiO2 (> 70 wt%), enrichment in large ion lithophile elements (LILEs), depletion in high field strength elements (HFSEs), and varying degrees of negative Sr and Eu anomalies. The granitoids in the Tongwei (TW) area, 150 km east of the QCS, are complex, the majority of which are dated at ~440 Ma, but there also exist younger, ~230 Ma intrusions genetically associated with the Qinling Orogeny. The Paleozoic TW intrusions also have high SiO2, fractionated REE (rare earth element) patterns, but a negligible Eu anomaly. The whole rock Sr-Nd-Hf isotopic compositions suggest that all these Paleozoic granitoids are consistent with melting-induced mixing of a two-component source, which is best interpreted as the combination of last fragments of subducted/subducting ocean crust with terrigenous sediments. The mantle isotopic signature of these granitoids (87Sr/86Sri: 0.7038 to 0.7100, eNd(t): −4.8 to −1.3, eHf(t): −0.7 to +4.0) reflects significant (~70 %) contribution of the ocean crust derived in no distant past from the mantle at ocean ridges with an inherited mantle isotopic signature. Partial melting of such ocean crust plus terrigenous sediments in response to the ocean closing and continental collision (between the Qilian and Alashan Blocks) under amphibolite facies conditions is responsible for the magmatism. Varying extents of fractional crystallization (±plagioclase, ±amphibole, ±garnet, ±zircon) of the parental magmas produced the observed QCS and TW granitoids. We note that sample HTC12–01 in the TW area shows an A-type or highly fractionated granite signature characterized by elevated abundances and a flat pattern of REEs, weak Nb-Ta anomaly, conspicuous negative Sr and Eu anomalies (Sr/Sr* = 0.09, Eu/Eu* = 0.22), and thus the high 87Sr/86Sr ratio (0.7851), and moderate eNd(t) (−4.9) and eHf(t) (−2.0), pointing to the significant mantle contribution. Compared with the Paleozoic granitoids, the ~230 Ma granitoids in the TW area represented by sample JPC12–02 have higher initial 87Sr/86Sr (0.7073) and lower eNd(t) (−6.2) and eHf(t) (−4.5) values, offering an ideal opportunity for future studies on tectonic effects of juxtaposition of younger orogenesis on an older orogen.

The Early Paleozoic basite magmatism of Western Transbaikalia: Composition, isotope age (U-Pb, SHRIMP RG), magma sources, and geodynamics

Remnants of the Early Paleozoic gabbro and gabbromonzonite with an age of 514.6 ± 7.2 Ma (U-Pb, Zrn, SHRIMP-RG, Turka Massif) were identified among basites spatially associated with Late Paleozoic granitoids of Western Transbaikalia. Obtained geochronological data are close to those of felsic subvolcanic rocks of the Early Cambrian volcanotectonic structures of the Uda-Vitim paleoisland arc and gabbro of the Dzhida island arc in Central and Southwestern Transbaikalia. As compared to the Late Paleozoic analogues, the Early Paleozoic gabbromonzonite is characterized by the moderately low potassic alkalinity, fractionated REE pattern, and LILE enrichment relative to HFSE. The Early Paleozoic gabbro and gabbromonzonite are depleted in Nb, Ta, Zr, and Hf and enriched in Pb and Sr, which is typical of suprasubduction magmatic rocks. Geochemical data indicate a contribution of crustal (subducted) material in a magma source. A combination of geological, geochemical, and isotope-geochronological data indicates that the Early Paleozoic gabbromonzonite was formed in the Uda-Vitim paleoisland arc system in a suprasubduction setting. The geochemical similarity of the Early Paleozoic rocks and Late Paleozoic basites, which are associated with the Late Paleozoic granitic rocks of the Angara-Vitim batholith and were formed 200 Ma later, is attributed to inheritance of mantle source.

Paleozoic collisional and intraplate granitoids of the Baikal area: comparative geochemistry and petrogenesis

Early Paleozoic granitoids of autochthonous and allochthonous facies in the Baikal area (Ol’khon Island, Khamar-Daban Ridge) are in close spatial association with gneisses, migmatites, and plagiogranites and are usually confined to granite–gneiss domes. They are virtually not subjected to magmatic differentiation. Formation of granitoids of the Solzan massif and Sharanur complex lasted 26–28 Myr, which might be considered an indicator of collisional granitoid magmatism. Collisional granitoids of different provinces have a series of indicative features: They are peraluminous and highly potassic and are enriched in crustal elements (Rb, Pb, and Th) but sometimes have low contents of volatiles. In contrast to collisional magmatism, petrogenesis of intraplate granitoids does not depend on the composition and age of the enclosing rocks. The geochemical evolution of intraplate granitoid magmatism in the Baikal area is expressed as an increase in contents of F, Li, Rb, Cs, Sn, Be, Ta, Zr, and Pb and a decrease in contents of Ba, Sr, Zn, Th, and U during the differentiation of multiphase intrusions. The geochemical diversity of these granitoids formed both from crustal and from mantle sources and as a result of the mantle–crust interaction, might be due to the effect of plume on the geologic evolution of intraplate magmatism. The wide range of compositions and geochemical types of igneous rocks (from alkali and subalkalic to rare-metal granitoids) within the Late Paleozoic Baikal magmatism area suggests its high ore potential.

Formation of adakitic granitoids in the collisional orogens: Evidence from the Early Paleozoic granitoids of the Munku–Sardyk Range, Eastern Sayan

The classical models of adakite formation by melting of basaltic layer of oceanic lithosphere in the subduction zone were verified using geochemical and Sr–Nd isotope data on the Early Paleozoic granitoids of Eastern Sayan. The presence of adakites in fold belts is usually regarded as geochemical proxy for paleogeodynamic reconstruction. The formation of felsic derivatives with adakitic signatures in the collisional orogens is inconsistent with these models and requires their revision. It is shown that the composition of the granitoids and their evolution cannot be described with these models. In order to solve this problem, two hypotheses of granitoid formation by mixing of two geochemically contrasting reservoirs were proposed and verified. According to the first hypothesis, the granitoids represent the mixing products between alkaline olivine basalts and partial melts of the gray gneiss basement of this region. The second model relates the formation of the granitoids with melting of geochemically 2700 Ma-old enriched source in the subcontinental lithospheric mantle. In spite of differences, both these hypotheses are based on the remobilization of sources formed at the previous stages of the geological evolution of the region. In both cases, adakitic geochemical characteristics of forming felsic magmas are determined by the composition of protolith rather than by their geodynamic position. Obtained preliminary results place constraints on genetic models and geochemical reservoirs participating in the formation of the granitoids.

Geotechnical and landslide aspects in weathered granitoid rock masses (Serre Massif, southern Calabria, Italy)

The weathering processes produce important chemical and physical transformations in rock masses producing a decay of their strength and then stability. In this study different methodologies were employed to define the physical-mechanical properties and the minero-petrographical changes at each grade of weathering, useful to determine a geological model of the weathered granitoids involved in mass movements. The paper focuses on a multidisciplinary research on the weathering profiles of the Paleozoic granitoid rocks surrounding the village of Fabrizia located in the Serre Massif (southern Calabria, Italy). Fabrizia lies on an old erosion surface composed of basement rocks made up of weathered granitoids, belonging to the Serre Massif granitoid complex, and overlain by thin eluvial-colluvial deposits.The tectonic uplift of the area combined with Mediterranean climatic conditions, both active at least since the Quaternary, are the main causes of the deep weathering of the plutonic rocks. The weathered profiles of the granitoid complex have been studied through field observations, borehole explorations with geotechnical tests and seismic surveys coupled to minero-petrographical analysis and laboratory tests. Weathered profiles are mainly composed by rock masses varying from completely weathered and residual soil (classes V–VI) to moderately weathered rocks (class III), characterized by a progressive increase of geotechnical properties (e.g., seismic wave velocity, rock quality designation, internal friction angle, uniaxial compressive strength) with a decrease of weathering. The weathered rocks display a wide variety of mass movement. Landslide types are rockfalls for the fresher rocks (class III); translational slides occur between different weathering degrees (classes IV and V) and rotational slides dominate at the top of the slopes and generally involve the weathering grades from class IV to classes V–VI. The shallow horizons are composed of thin eluvial-colluvial deposits and more weathered granitoids (classes V–VI) that may generate dangerous debris flows or debris avalanches during intense and prolonged rainfall. The landslide surface is usually at a depth lower than 2m in the debris flow type, while deeper landslides (depth>2m) are rotational and translational types with sliding surfaces occurring mainly in the transition between the classes IV and V.The field survey coupled to geotechnical, geophysical and minero-petrographical data allowed the recognition of the different weathered horizons and relative geotechnical properties useful to the reconstruction of the reliable geological model. The model represents an important support for the study of the landslide processes and for a sustainable management of the territory. The used approach provided results to set a general reference framework for establishing more precise correlations between mass movement phenomena and weathering processes.

Late Paleozoic granitoid magmatism of Eastern Kazakhstan and Western Transbaikalia: plume model test

We present results of a comparative study of Late Paleozoic granitoids of Eastern Kazakhstan and Western Transbaikalia composing the large Kalba-Narym and Angara-Vitim batholiths. We have established that despite the different geologic history of these regions, granitoid magmatism there proceeded nearly synchronously at the Carboniferous/Permian boundary (330–280Ma) and was accompanied by mantle magmatism. The regularities of its evolution are considered in terms of the plume model and different stages of interaction of mantle plumes with the lithosphere. The major principles of plume-lithosphere interaction in accretion-collision fold belts have been formulated: (1) Plume-lithosphere interaction results in large-scale melting of sublithospheric mantle, lower lithosphere, and crustal substrates warmed by the preceding orogenic processes; (2) The processes last 30 to 50 Myr and produce large volumes of igneous rocks, mostly granitoids; (3) The sequence of formation of granitoid and basic igneous complexes and the metallogenic specialization can be different and depend on the lithosphere structure and preceding geologic history of the region.

Geochemistry, zircon U–Pb dating and Hf isotopies composition of Paleozoic granitoids in Jinchuan, NW China: Constraints on their petrogenesis, source characteristics and tectonic implication

Granitoids are widely distributed in Jinchuan at the southwestern margin of the North China plate, which is also an important area of mineral deposits. The research subject of this article are two Paleozoic granitoids, a cataclastic syenogranite and a granodiorite porphyry. This study presents whole rock geochemistry and zircon U–Pb–Hf isotope data for the two granitoids to determine their petrogenesis, source characteristics and tectonic significance. The cataclastic syenogranite is characterized by metaluminous composition with high potassium, and LaN/YbN from 39 to 48. The composition with strong negative Eu anomalies and Zircon saturation temperatures (TZr) from 947 to 1072°C classify this intrusion as an A-type granite. The granodiorite porphyry is metaluminous with high sodium, sub-alkaline, LaN/YbN ratios from 27 to 32. These I-type intrusions have no Eu anomalies and TZr ranges from 818 to 845°C. Both the cataclastic syenogranite and granodiorite porphyry show enrichment of LREE and LILE and depletion of HREE and HFSE, except Hf and Zr. Using single zircon LA-ICP-MS U–Pb dating, the emplacement age of the cataclastic syenogranite and granodiorite porphyry are determined at 433.4±3.7Ma and 361.7±4.6Ma, respectively. Zircons from the cataclastic syenogranits have uniform negative εHf(t) values (−11±0.5 to −9±0.5), implying the involvement of an old Palaeoproterozoic crustal source in magma genesis. The zircons from the granodiorite porphyry have εHf(t) values that range from −8±1.0 to +10±0.6, suggesting heterogeneous source materials involving both juvenile and ancient crust reworked crustal components. Based on the geological significance of granites at the southwestern margin of the North China plate, the closure of the North Qilian Ocean occurred at ∼444Ma. Geochemical features suggest that the cataclastic syenogranite and granodiorite porphyry formed in an intraplate extensional and compressional setting, respectively. Hence after a period of extensional post-collisional intraplate evolution, strong compressive forces affected the area.