Early Permian Magmatism Research Articles

Petrogenesis of the Bastielieke W-mineralized granitic pluton in the Altai orogen, NW China

Granitic magmatism of the Altai orogen is the key to understanding the tectonic evolution and metallogeny in the Central Asian Orogenic Belt (COAB). In this paper, we conducted comprehensive petrological, geochronological and geochemical investigations on the Bastielieke granitic pluton in the Altai orogen. Zircon U–Pb dating shows an emplacement age of 278–270 Ma for the pluton. Granites of the Bastielieke pluton have S-type granitic affinity and have Zr + Nb + Ce + Y contents (76.6–272 ppm), A/CNK ratios > 1.1, and K2O > Na2O. The low εNd(t) (−2.8 to + 2.9) and high εHf(t) (+7.6 to + 15.9) values with Nd-Hf isotope decoupling, invariable Pb isotopes, and moderate zircon δ18OV-SMOW values suggest crustal recycling of sedimentary rocks in a syn- to post-collision setting. In addition, the pluton is spatially related to the tungsten (W) deposit and also temporally consistent with the mineralization age of 284 ± 6 Ma, indicating a genetic link between the Early Permian magmatism and mineralization in Bastielieke. Furthermore, the source rocks of the pluton which are sedimentary rocks and abundant in W, provide the material basis for the initial enrichment of W in primary magmas. Given high Rb/Sr (>3) and low K/Rb ratios (<200), we interpret that the pluton has experienced high differentiation. Our studies propose that the termination of the subduction-accretion of the Paleo-Asian Ocean and the initiation of a syn- to post-collision regime for the Altai orogen are probably at the Early Permian.

Petrogenesis of the early Permian Hongliujing granite complex in the Chinese Eastern Tianshan orogen: Evidence for crustal growth in the Central Tianshan microcontinent

The generation and modification of silicic magma systems are essential processes in resolving the differentiation of continental crust. This understanding motivated the geochronological and geochemical study of the early Permian Hongliujing granite complex, consisting of quartz monzonite, granite, and leucogranite in the Central Tianshan microcontinent of the southern Central Asian Orogenic Belt. Laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) zircon U-Pb dating of the Hongliujing complex rock units revealed almost identical ages (279 ± 2 Ma to 270 ± 2 Ma). The high-silica leucogranite and granite are characterized by positive Rb and negative Eu anomalies and Ba, Sr, P, and Ti depletions. The zircon trace elements are characterized by relatively low Ti and Th/U and high Yb/Gd. In contrast, the quartz monzonite and its mafic microgranular enclaves display minor negative Ba, Sr, P, Ti, and Eu anomalies, while the zircon trace elements are characterized by relatively high Ti and Th/U and low Yb/Gd. The complex has similar zircon Hf and whole-rock Nd isotopic compositions, with Hf and Nd model ages younger than 1.4 Ga, suggesting that their magmas were derived from an isotopically depleted mantle, with some contributions from crustal melts. The leucogranites further showed relatively large variations of εHf(t) and lower εNd(t) values, implying that their magma was affected by higher amounts of crustal contamination. We suggest that crystal-melt segregation was the major mechanism responsible for the evolution of the magmatic system, and that the early Permian magmatism represents a crust-forming episode triggered by slab rollback of the subducting South Tianshan oceanic plate beneath the eastern Central Tianshan microcontinent. Thus, our study reveals that microcontinents with Precambrian crustal basement were major sites of juvenile continental growth during the accretionary evolution of the Central Asian Orogenic Belt.

Garnet U-Pb dating and magnetite geochemistry: Constraints on the origin of Fe mineralization in the Huogeqi polymetallic deposit, Northern China

The Huogeqi Fe polymetallic deposit, which contains 48.17 Mt Fe, occurs in tremolite-diopside rocks of the Neoproterozoic Langshan Group in the north China. This study focuses on the magnetite mineralogy, texture and chemical composition, and U-Pb geochronology of garnet associated with magnetite and zircon from the granitoid related to mineralization in order to constrain the origin of Fe mineralization in Huogeqi deposit. LA-ICP-MS U-Pb dating of magnetite-bearing garnets yields a lower intercept age of 277.5 ± 8.7 Ma, which is coeval with the emplacement age of the mineralized-related granitoid (275.3 ± 1.3 Ma). Three types of magnetite were formed in the Huogeqi deposit, including Mt-A (coarse-grained disseminated), Mt-B (stratiform and fine-grained dense disseminated), and Mt-C (massive). The Mt-A can be subdivided into high-Ti Mt-A1 and relatively low-Ti porous and fractured Mt-A2. The Mt-B can be subdivided into porous Mt-B1 with relatively high V and low Ti contents, dark porous Mt-B2 with a relatively low V contents, porous inclusion-rich Mt-B3 with low Fe and V contents, and high Si and Mg contents, and less-porous Mt-B4 with low Si and Mg contents. The Mt-C includes porous Mt-C1 with a low Ti contents and high Si, Al, Mn contents and less-porous Mt-C2 with low Si, Al, and Mn contents. The decreasing Ti and V contents in the subsequent magnetite types indicate progressive cooling of the mineralizing fluid, increasing oxygen fugacity, and more intense fluid-rock interaction. Dissolution and re-precipitation played an important role in the formation of different generations of magnetite of Mt-B and Mt-C. The magnetite from Huogeqi deposit has a high Ni/Cr mass ratio, a high Fe content, and a high (Ca + Al + Mn) value, and a low V content, a low Ni/(Cr + Mn) mass ratio, and a low (Ti + V) value. These geochemical characteristics indicate that the magnetite has a hydrothermal origin in a skarn-type deposit, which is related to Early Permian magmatism in the Langshan area in north China.

Coexisting Nb-enriched basalts and arc volcanic rocks in the northern Qiangtang Terrane, China: Implications for the effects of ambient mantle on subduction zone magmatism

The petrogenesis of Nb-enriched basalts (NEBs) is controversial. NEBs are generally considered to form by melting of either a mixed enriched and depleted mantle source or a mantle wedge metasomatized by adakitic melts. Here we present geochronological, petrological, geochemical, and Sr-Nd-Os isotope data for Early Permian volcanic rocks from the Tuotuohe area in the northern Qiangtang Terrane, China. The studied volcanic rocks can be divided into two groups: arc basalts and basaltic andesites in the lower sequence and NEBs in the upper sequence. Zircon grains from one basaltic andesite sample in the lower sequence yielded a concordia age of 302.0 ± 1.1 Ma. The geochemical and isotopic characteristics suggest that the NEBs were derived from a mixed source consisting of enriched mid-oceanic ridge basalts (MORB)- to oceanic-island basalt-like ambient mantle and slab-derived fluids, whereas the arc basalts and basaltic andesites originated from a source made of the MORB-type ambient mantle modified by slab-derived melts. We propose for the first time that the Nb enrichment of the NEBs was most likely inherited from high-Nb ambient mantle and is independent of the addition of slab-derived components. In addition to slab-derived components, the nature of the ambient mantle is also an important factor controlling the diversity of arc magmatism. Our new findings show that the Late Carboniferous−Early Permian magmatism in the northern Qiangtang Terrane was related to northward subduction of the Paleo-Tethys Ocean, suggesting that subduction might have occurred in at least the Early Permian rather than in the previously thought Middle Permian.

Late Palaeozoic tectonic evolution of the southern Central Asian Orogenic Belt: Constraints from the Early Permian magmatism in the northern Alxa area

The northern Alxa area is the southernmost part of the central segment of the Central Asian Orogenic Belt and is located in a crucial junction to unravel the geological evolutionary history of the Palaeo‐Asian Ocean. Here, we present an integrated study of whole‐rock geochemistry, zircon U–Pb geochronology and zircon–Hf isotope on Early Permian igneous rocks from the Zhusileng–Hangwula Belt of the northern Alxa area. Three syenogranites, two monzogranites and one granodiorite are identified as peraluminous, (high‐potassium) calc‐alkaline I‐type granites. The zircon dating results show that they were formed during the Early Permian (291–285 Ma). Geochemically, these rocks show enrichments in light rare‐earth elements (LREEs), large‐ion lithophile elements (LILEs) and negative Nb–Ta anomalies, as well as low A/CNK values. Furthermore, the syenogranites have almost positive zircon εHf(t) values (−0.4 to +4.8) and Early Palaeoproterozoic–Mesoproterozoic Hf model ages (1,768–1,296 Ma), indicating that they were probably derived from juvenile crustal components, mixed with ancient continental crust. The monzogranites have markedly positive zircon εHf(t) values (+11.2 to +13.5) and Neoproterozoic–Cambrian Hf model ages (716–513 Ma), suggesting that they were generated by rapid reworking of newly formed juvenile material. The granodiorite shows positive zircon εHf(t) values (+4.0 to +7.0) and Mesoproterozoic Hf model ages (1,368–1,090 Ma), implying that the granodiorite was probably derived from the partial melting of the juvenile crust. Our geochronological, geochemical and isotopic data, combined with published data, unravel that a crustal thinning event that was simultaneous with more input of mantle materials in the northern Alxa area, implies that a subduction‐related extensional environment occurred in the Zhusileng–Hangwula Belt which is probably due to the slab rollback of the Paleo‐Asian Ocean during the Late Carboniferous–Early Permian.

Late Carboniferous closure of the Junggar-Balkhash Ocean: Insights from the early Permian post-accretionary magmatism in the Barleik Mountains of West Junggar, NW China

Early Permian magmatic rocks in the West Junggar Terrane (WJT, NW China) contain crucial information about the final closure of the Junggar-Balkhash Ocean (JBO) in the southwestern Altaids, whereas their tectonic settings are inconclusive. In this study, we conduct an integrated study incorporating the geochronology, geochemistry and zircon Hf isotopes of a set of volcanic and intrusive rocks from the northwestern Barleik Mountains in the central WJT. Zircon UPb ages of eight volcanic and felsic–intermediate intrusive samples are ∼292–281Ma, corresponding to a regional magmatic flare-up in the central WJT. The volcanic samples include rhyolitic tuff, andesitic tuff, and rhyolitic andesite, characterized by high-K calc-alkaline to shoshonitic I-type granite features. They yield positive εHf(t) values with a narrow range of 9.40–13.52 and two-stage model ages from 421 Ma to 672 Ma. Above isotopic features, in combination with their relatively low Mg# values of 25–35, demonstrate that they were generated by partial melting of the juvenile lower crust. Felsic intrusive rocks include peraluminous I-type granitic porphyry and A2-type K-feldspar granite, and their low Mg# values (21–35) and trace element ratios suggest the partial melting from lower crustal rocks. By contrast, intermediate intrusive rocks include monzonite, diorite and a dioritic dyke, which represent fractionated partial melts sourced from the underplated depleted mantle-derived magma. Based on the occurrence of I-type and A2-type rocks, together with the coeval extensional strike-slip fault systems and a late Carboniferous angular unconformity, we infer that the early Permian magmatism in the central WJT resulted from an extensional post-accretionary setting. Taken together with similar geological evidence from the WJT and East Kazakhstan, the final closure of the JBO is suggested to occur in the late Carboniferous.

Early Permian magmatism in northern Inner Mongolia, southeastern Central Asian Orogenic Belt: Implications on lithospheric extension in a post-collisional setting

Identifying the petrogenesis and geodynamic mechanism of the Early Permian igneous rocks widely distributed in the Inner Mongolia–Daxing'an Orogenic Belt (IMDOB) is crucial for understanding the tectonic evolution of the southeastern Central Asian Orogenic Belt. In this study, a combined study of petrology, geochronology, and geochemistry of Early Permian volcanic–plutonic rocks from the Uliastai continental margin, northern IMDOB, was conducted aiming to constrain their petrogenesis and tectonic setting. The UPb zircon data for the magmatic zircons indicate that the Baoligaomiao Formation (BG Fm.) volcanic rocks, including felsic and mafic rocks, erupted at approximately 298–285 Ma, and quartz monzonite porphyries that intruded the BG Fm. were emplaced at approximately 292 Ma. The mafic volcanic rocks exhibit intraplate nature with basaltic compositions transitional between the calc-alkaline and shoshonite series. The geochemical behavior and isotopic data of these volcanic rocks reflect a depleted lithospheric mantle source metasomatized by fluids released from the stagnant slab. Felsic volcanics and quartz monzonite porphyries show affinities with A-type granites and are derived from the partial melting of juvenile crustal materials. Moreover, we suggest that the Early Permian magmatism formed in a post-collisional extensional setting, which is the continuation of the Late Carboniferous extension and was induced by the lithospheric collapse after the slab break-off of the Hegenshan Ocean. Notably, the felsic rocks formed after 290 Ma, with affinity changing from A2-type to A1-type granite, and approximately 288 Ma mafic volcanic rocks exhibited intraplate characteristics. These changes, combined with the development of coeval bimodal volcanic rocks and dike swarms, suggest that the tectonic setting at the end of the Early Permian in the northern IMDOB may have gradually transferred to an intraplate environment.

Two episodes of Late Paleozoic mafic magmatism in the western Tianshan Orogen: From Carboniferous subduction to Permian post-collisional extension

The evolution of the western Tianshan Orogen in the southern Central Asian Orogenic Belt involved prolonged accretion and collision processes in response to the closure of the Paleo-Asian Ocean during the Late Paleozoic. However, the timing for the tectonic transition from subduction to post-collision is still poorly constrained. Herein, we address this issue based on geochronological, geochemical and Sr-Nd-Hf isotopic investigations on representative mafic dikes from the Awulale Mountains in the heart of the western Tianshan. Zircon U-Pb dating suggest two episodes of mafic magmatism during the Late Carboniferous (ca. 324–310 Ma) and Early Permian (ca. 299–290 Ma). The Late Carboniferous gabbro dikes exhibit large variations in MgO (2.79–10.97 wt%) and Ni (7.58–244 ppm), and arc-like trace element patterns with high Th/Yb (0.55–2.26) ratios, relatively low εNd(t) (+0.2 to +2.5) and variable εHf(t) (+1.3 to +10.4) values, indicating that they were derived from fractional crystallization of a peridotite-bearing mantle source metasomatized by sediment-derived hydrous melts. In contrast, the Early Permian diabase dikes have slightly variable MgO (5.88–7.78 wt%) and Ni (59.90–104 ppm) with apparently low Th/Yb ratios (0.20–0.34, except for one sample: 0.86) and depleted εNd(t) (+5.0 to +7.8) and εHf(t) (+8.0 to +14.6) values, which indicates derivation from partial melting of a juvenile lithospheric mantle source with contributions from early subduction metasomatic components. In context with relevant published data, our study indicates that the Late Carboniferous magmatism of the Awulale Mountains was likely induced by breakoff of the North Tianshan oceanic slab during ongoing subduction; conversely, the Early Permian magmatism may have originated from delamination of the lower crust and lithospheric mantle in the subsequent post-collisional setting. Furthermore, the tectonic transition from Carboniferous subduction to Permian post-collisional extension of the western Tianshan suggests that terminal closure of the North Tianshan Ocean took place at Carboniferous-Permian boundary at ∼ 300 Ma.

Late Paleozoic Shoshonitic Magmatism in the Southwestern Middle Tianshan (Tajikistan) of the Southwestern Altaids: Implications for Slab Roll-Back With Extensional Arc-Related Basins After Flat Subduction

Paleozoic magmatic rocks are widespread in the western Middle Tianshan. Their petrogenesis can provide important insights into the geodynamic evolution of the southwestern Altaids. Here, we present an integrated study of U–Pb zircon geochronology and geochemical and Lu–Hf isotopic compositions for the Late Paleozoic shoshonitic Chorukhdairon pluton and genetically and spatially related quartz porphyry in the southern Chatkal–Kurama terrane, western Middle Tianshan. The Chorukhdairon pluton mainly comprises monzodiorite and quartz monzodiorite (first phase), quartz monzonite (second and main phase), monzogranite (third phase), and leucomonzogranite (fourth phase). LA–ICP–MS zircon dating yielded magma crystallization ages of 294–291 Ma and 286 Ma for the Chorukhdairon pluton and quartz porphyry, respectively. All the rocks possess high K2O content (3.29–5.90 wt.%) and show an affinity with shoshonite series rocks. They display similar trace element compositions characterized by the enrichment of large ion lithophile elements (e.g., Rb, Th, U, and K) and depletion of high-field strength elements (e.g., Nb, Ta, P, and Ti), compatible with typical arc magmatism. Combined with zircon Lu–Hf isotopic data, we suggest that the Chorukhdairon pluton was produced by partial melting of the enriched mantle, followed by fractional crystallization of pyroxenes, amphibole, plagioclase, biotite, and accessory Fe–Ti oxides, apatite, and zircon. The quartz porphyries are similar to highly fractionated I-type granitic rocks, and their parental magma could result from the mixing of different batches of mantle-derived magmas or magmas derived from the mantle and juvenile lower crust. Considering the continuousness of the Middle Carboniferous to Early Permian magmatism in the western Middle Tianshan and other regional geological data, we suggest that the Chorukhdairon pluton and related quartz porphyry probably formed in an oceanic subduction setting. Furthermore, the temporal and spatial evolution of the Paleozoic magmatism imply that the flat-slab subduction that was induced by the subduction/accretion of seamounts probably occurred beneath the Middle Tianshan during the Middle Devonian to Early Carboniferous, after which the southeastward slab roll-back occurred during the Middle Carboniferous to Early Permian. The late slab roll-back was responsible for the southeastward arc magmatism migration and magmatic flare-up in the Chatkal–Kurama terrane, western Tianshan, and led to the formation of arc-related extensional basins and significant crustal growth in the southwestern Altaids.

Metasomatism by Boron-Rich Fluids along Permian Low-Angle Normal Faults (Central Southern Alps, N Italy)

Low-Angle Normal Faults (LANFs) represent in the central Southern Alps area (N Italy) the main structures along which the Variscan basement is in contact with the Upper Carboniferous-Permian volcanic-sedimentary succession. Tourmalinites frequently occur along LANFs, usually replacing former cataclasites. The mineralogy and chemical composition of tourmalinites point to a metasomatic origin. LANFs, together with high-angle faults, controlled the opening of the Permian Orobic Basin and likely acted as a preferred pathway for hydrothermal fluids that triggered the Boron-metasomatism. Along the Aga-Vedello LANF, tourmalinites appear to have formed after the cessation of fault activity, as no brittle post-metasomatism deformation overprint has been observed. These relationships suggest that the circulation of B-rich fluids occurred after the opening of the Orobic Basin that is broadly constrained to the Early Permian. At the same time, ca. 285–270 Ma, a strong magmatic activity affected all the Southern Alps, ranging in composition from mafic to acidic rocks and from intrusions at deep crustal levels to effusive volcanic products. The Early Permian magmatism was likely the source of the late-stage hydrothermal fluids that formed the tourmalinites. The same fluids could also have played a significant role in the formation of the Uranium ore deposit of the Novazza-Vedello mining district, as the ore bodies in the Vedello valley are concentrated along the basement-cover contact.

Basic and Associated Granitoid Magmatism and Geodynamic Evolution of the Altai Accretion–Collision System (Eastern Kazakhstan)

Abstract —The paper summarizes the results of study of the geologic position, composition, and age of basic igneous associations in Eastern Kazakhstan during the late Paleozoic (Carboniferous–Permian). At that time, the Altai accretion–collision system was developed here, which resulted from the interaction of the Siberian and Kazakhstan paleocontinents. The performed studies made it possible to establish three major stages of basic magmatism, corresponding to different stages of evolution of the collision system: early Carboniferous, late Carboniferous, and early Permian. The chemical composition of ultrabasic-basic associations changed, with a successive increase in the contents of K2O, P2O5, TiO2, LREE, Rb, Ba, Zr, Hf, Nb, and Ta. The variations in magma compositions were determined by different compositions of mantle sources (harzburgites, spinel lherzolites, and garnet lherzolites) and different degrees of their melting. The early Permian ultrabasic-basic associations are the most enriched in TiO2 and incompatible components (P2O5, Zr, Hf, Nb, and Ta), which indicates the involvement of relatively enriched mantle sources in the partial melting. All manifestations of mantle magmatism were accompanied by subsynchronous crustal magmatism (granitoid intrusions or silicic volcanics). The major crustal magmatism was manifested in the early Permian; the area of its occurrence was dozens of times larger than the area of Carboniferous crustal magmatism. Possible geodynamic scenarios for magmatism are considered for each stage. The early Carboniferous (C1s) magmatism of the early orogeny stage was manifested locally and was the result of the detachment of the subducting lithosphere (slab) beneath the margin of the Kazakhstan continent. The middle Carboniferous (C2m) magmatism of the late orogeny stage was manifested throughout the area; it was caused by the activation of shear–extension motions along large faults and the orogen collapse. The early Permian magmatism was the result of the interaction of the Tarim mantle plume with the lithosphere, which comprised three stages: initial interaction, maximum interaction, and relaxation. This magmatism in the study area was caused by a combination of thermal disturbance in the upper mantle and the lithosphere extension processes.

Early Permian magmatism above a slab window in Inner Mongolia, North China: Implications for the Paleo-Asian Ocean subduction processes and accretionary crustal growth

Identifying magmatic rock associations in a subduction zone is substantial for understanding the related geodynamic evolution. The final closure time of the Paleo-Asian Ocean and the exact subduction processes have continuously been controversial, hindering our recognition of the tectonic evolution of the Central Asian Orogenic Belt (CAOB). Here we investigated the Early Permian gabbros-granodiorites from Xi Ujimqin in the northern Inner Mongolian region. The gabbros have slightly depleted light rare-earth elements [(La/Yb)N = 0.8] and flat heavy rare earth elements (HREE) [(Gd/Yb)N = 1.1] that are comparable to those of N-MORB. They show depletion in Nb and Ta, positive whole-rock εNd(t) values of +7.7 to +8.7 and zircon εHf(t) values of +8.7 to +10.6. These mafic rocks are interpreted to be products of partial melting of depleted mantle that had been metasomatized by slab-derived fluids, with subsequent fractional crystallization and minor crustal contamination. These granodiorites chemically resemble typical adakites and have MORB-like whole-rock SrNd isotopic compositions (87Sr/86Sri = 0.7028–0.7029; εNd(t) = +8.2–+8.5) and positive zircon εHf(t) (+13.4–+15.9) values, suggesting that they originated from the subducted slab-melts and reacted with mantle wedge peridotite. UPb zircon dating shows emplacement of N-MORB-type gabbros and adakitic granodiorites at ∼297 Ma and ∼290 Ma, respectively. Our new data indicate the presence coeval slab-derived adakites and slab fluid-metasomatized asthenosphere-derived N-MORB-type rocks, indicating that subduction lasted until at least the Early Permian. Such an association along with the extension-related magmatism in northern Mongolia recorded the northward ridge subduction of the Paleo-Asian Ocean during the Late Carboniferous–Early Permian. This model is consistent with the spatial distribution and the ages of magmatic activity with positive εNd(t)–εHf(t) values from this region. Ridge subduction and induced slab windows probably played a key role in Paleozoic crustal growth of CAOB, and by inference in the other accretionary orogens throughout Earth history.

Carboniferous to Early Permian magmatism in the Uliastai continental margin (Inner Mongolia) and its correlation with the tectonic evolution of the Hegenshan Ocean

This study presents a comprehensive analysis of zircon U–Pb–Hf isotopic and whole-rock geochemical data of Carboniferous–Early Permian felsic igneous rocks from the Uliastai continental margin (UCM), southeastern Central Asian Orogenic Belt, to constrain the amalgamation of the Inner Mongolia–Daxing'an Orogenic Belt (IMDOB) in the late Paleozoic. Zircon laser ablation inductively coupled plasma mass spectrometry UPb ages of monzogranite, granite porphyry, felsic volcanic rock, and alkali feldspar granite reveal three stages of magmatism in the Early Carboniferous (ca. 336–320 Ma), Late Carboniferous (ca. 311–307 Ma), and Early Permian (ca. 298 Ma). The Early Carboniferous igneous rocks, including monzogranites and later granite porphyries, exhibit affinity with the high-K calc-alkaline highly fractionated I-type granite, and are depleted in high-field-strength elements and enriched in large-ion lithophile elements. Their zircon εHf(t) values of the Early Carboniferous rocks range from +4.6 to +9.1, indicating that they were generated by partial melting of the juvenile crustal materials in a north-dipping subduction-related environment along with the opening of the Hegenshan Ocean. The Late Carboniferous felsic volcanic rocks are highly fractionated high-K calc-alkaline to alkali-calcic I- and A-type granitoids, with relatively strong peraluminous affinities. Considering the zircon εHf(t) values (+6.2 to +10.6) and relatively low Rb/Ba (0.71–3.57) and Rb/Sr (4.18–8.12) ratios, they were likely derived from a juvenile crust and are comparable to partial melts of clay-poor but plagioclase-rich metaigneous rocks. Notably, the A-type granitoids are gradually increasing in the UCM after 311 Ma, and these magmatic rocks have markedly increased zircon saturation temperatures and (K2O + Na2O)/CaO ratios. These changes may result from the slab break-off of the Hegenshan Ocean, and the Late Carboniferous felsic volcanic rocks were formed in a post-collisional setting. The Early Permian alkali feldspar granites are characterized by typical A-type granite geochemistry, with high Ga/Al ratios; low MgO, Cr, Co, and Ni contents; and high zircon saturation temperatures. These rocks have positive εHf(t) values of +10.2 to +12.7 and record an intracontinental extension setting. In combination with the regional geology, the Late Carboniferous post-collisional magmatic rocks suggest that a change from a subduction-related continental margin to a post-collisional tectonic regime might have occurred in the UCM before 311 Ma.

Early Permian Magmatism in Northern Inner Mongolia, Southeastern Central Asian Orogenic Belt: Implications on Lithospheric Extension in a Post-Collisional Setting

Early Permian Magmatism in Northern Inner Mongolia, Southeastern Central Asian Orogenic Belt: Implications on Lithospheric Extension in a Post-Collisional Setting

Late Paleozoic tectonic transition in East Junggar, NW China: Insights from I- and A-type granitic magmatism in the Karamaili region

The closure time of the Karamaili Ocean is the key to understand the accretionary history of the East Junggar at the southern Central Asian Orogenic Belt in the Paleozoic. In this study, we report geochronology, whole-rock elemental and Sr-Nd isotopic data and zircon Lu-Hf isotopic data of four Late Carboniferous to Early Permian I- and A-type granitic plutons in the Karamaili region in East Junggar, NW China. The Carboniferous Hongliuxia gneissic granite and the Hongliugou granodiorite yield zircon 206Pb/238U ages of 324 ± 1 Ma and 306 ± 1 Ma, respectively, and both of them are high-K calc-alkaline and belong to I-type granitoids. These granites show (87Sr/86Sr)i values from 0.7034 to 0.7046, ɛNd(t) values from 4.4 to 6.3, with Nd TDM2 model ages of 559–727 Ma; and ɛHf(t) values from 9.4 to 12.8, with Hf TDM2 model ages of 481–710 Ma. The variable positive Nd-Hf isotopic signature and young TDM2 model ages suggest that the I-type granitoids were derived from partial melting of juvenile crustal source, probably earlier arc intrusion. The Early Permian Daliugou and Mawangmiao plutons have 206Pb/238U ages of 297 ± 1 Ma and 284 ± 2 Ma, respectively, show high-K calc-alkaline to shoshonitic characteristics and belong to A2-type granite. They exhibit (87Sr/86Sr)i values from 0.7040 to 0.7065, ɛNd(t) values from 2.1 to 5.4, with Nd TDM2 model ages of 612–887 Ma; and ɛHf(t) values from 5.2 to 11.5, with Hf TDM2 model ages ranging from 548 Ma to 955 Ma, and the Nd-Hf isotope data also suggest that they were derived from juvenile crustal source. Mafic microgranular enclaves in the Daliugou pluton yield zircon 206Pb/238U age of 299 ± 2 Ma, and show similar geochemistry affinity with the granite host. The presence of enclaves and reverse zoning K-feldspars indicate that the magma mixing may have played an important role in the evolution of the Daliugou pluton. In this study, only the Hongliuxia gneissic granite was deformed, and a Nd-Hf isotopic transition occurred at ~300 Ma. Furthermore, the Late Carboniferous I-type granitoids from the Hongliuxia and the Hongliugou plutons show an arc affinity, and the Early Permian A-type granitoids from the Mawangmiao and the Daliugou plutons show post-collisional affinity. These lines of evidence enable us to argue that the Karamaili Ocean probably closed at a period between 324 Ma and 306 Ma. The post-collisional Early Permian magmatism in East Junggar was independent from the Tarim mantle plume.

Age, origin and tectonic implications of Late Carboniferous-Early Permian felsic magmatic rocks from central Inner Mongolia, south-eastern Central Asian Orogenic Belt

ABSTRACT Permo-Carboniferous felsic magmatic rocks are widely exposed on the northern half of the south-eastern Central Asian Orogenic Belt. However, their origin and spatiotemporal distribution are unknown. Herein, zircon U-Pb-Hf isotopic data, major and trace element compositions of the whole-rock, and Sr-Nd isotopic data derived for felsic volcanic rocks and granites of the Chaganaobao and Baiyinaobao areas in central Inner Mongolia are combined with the data from previous studies. The integration of these results reveals that the Uliastai Continental Margin, Erenhot-Hegenshan Ophiolite Belt, and Northern Orogenic Belt have coeval magmatic lull or trough at c. 307 Ma, which overlap with the closure timing of the Hegenshan Ocean. The Latest Carboniferous−Early Permian (c. 303−295 Ma) felsic volcanic rocks in the Chaganaobao area of the Uliastai Continental Margin comprise dacite, trachydacite, and rhyolite, while demonstrating positive whole-rock εNd(t) (+2.9 to +4.7) and zircon εHf(t) values (+7.65 to +13.78). Early Permian granites of the Baiyinaobao area located within the Northern Orogenic Belt show positive εHf(t) values (+ 2.70 to +11.33) and low TFe2O3 + MgO + TiO2 contents. The geochemical composition reveals that those felsic magmatic rocks mainly originate from the partial melting of juvenile crustal components with subordinate input of mantle-derived melts. Those c. 305−275 Ma felsic magmatic rocks from the north of the south-eastern Central Asian Orogenic Belt exhibit affinities of post-collision type granites in geochemical nature. Therefore, the flare-up of the Early Permian magmatism surrounding the Erenhot-Hegenshan Ophiolite Belt could be related to the extension after the final closure of the Hegenshan Ocean.

Early Permian continental arc magmatism in the Zhaojinggou area of the northern North China Craton: Implications for crust-mantle interactions during southward Paleo-Asian plate subduction

Early Permian magmatism in the Zhaojinggou area of Inner Mongolia, northern China produced intrusive gabbroic diorite and granites including biotite granite, K-feldspar granite and fine-grained granite. We present zircon U–Pb geochronology, whole-rock geochemistry, and Sr–Nd–Hf isotopic compositions of these rocks to resolve complex crust–mantle processes beneath the northern margin of the North China Craton (NCC). Zircon U–Pb dating reveals that the rocks were emplaced in the Early Permian, with gabbroic diorite (291.1 ± 1.8 Ma) forming ~10 Myr earlier than the granites (biotite granite, 280.0 ± 1.6 Ma; K-feldspar granite, 279.7 ± 2.1 Ma; fine-grained granite, 282.2 ± 2.9 Ma). Geochemical data indicate that the rocks have calc-alkaline to high-K calc-alkaline and metaluminous to peraluminous affinities, with enrichment in light rare-earth elements (LREE) and large-ion lithophile elements (LILE), and depletion in high-field-strength elements (HFSE), typical of continental arc magmatic rocks. The gabbroic diorite is characterized by low–moderate Mg# values (38–59, mean 50), moderate average Cr (45.0 ppm) and Ni (36.5 ppm) contents, and moderately enriched Sr–Nd–Hf isotopic compositions with (87Sr/86Sr)i values of 0.704719–0.705497; εNd(t) values from −7.8 to −7.0; and εHf(t) values from −11.2 to −6.7. These features, combined with highly variable Rb/Y, Ba/La, Th/Yb, Th/Ce and Th/Sm ratios and Ba contents, indicate that the gabbroic diorite was derived from subduction-related enriched sub-continental lithospheric mantle (SCLM), previously metasomatized by slab fluids and ~5% sediment melts. Weakly negative to strongly positive Eu anomalies among gabbroic diorite samples indicate no residual plagioclase accumulation or fractionation during magma evolution. The granites have similar ages and enriched isotopic features (87Sr/86Sri = 0.703611–0.707427; εNd(t) = −11.4 to −9.2; εHf(t) = −12.8 to −7.2) with old Nd and Hf model ages of 1980–1801 Ma and 1730–2088 Ma, respectively, indicating that they originated through partial melting of ancient mafic lower crust. Both biotite granite and K-feldspar granite exhibit weakly negative to highly positive Eu anomalies and relatively low Zr saturation temperatures of 618–648 °C. In contrast, the fine-grained granite has negative Eu anomalies and relatively high Zr saturation temperatures (692–709 °C). These features imply that the magma source of the fine-grained granite (with plagioclase accumulation) was deeper than that of the biotite granite and K-feldspar granites (without plagioclase accumulation). Different mantle and crustal sources involved in the generation of continental arc magmatism in the Zhaojinggou area have implications for extensive and complex crust–mantle interactions beneath the northern margin of the NCC related to southward subduction of the Paleo-Asian oceanic crust during the Early Permian.

Bimodal magmatism produced by delamination: geochemical evidence from late Palaeozoic volcanic rocks from the Yili Block, Western Tianshan, Northwestern China

AbstractThe Western Tianshan orogenic belt is essential for understanding the evolution of the Central Asian orogenic belt. However, no agreement exists among geologists about its tectonic environment during the Late Palaeozoic. The volcanic rocks of the Yishijilike and Wulang Formation in the Yili Block, Western Tianshan, formed in the Late Carboniferous to Early Permian, mainly consist of a bimodal suite of basalts – basaltic andesites and rhyolites, with only some intermediate rocks. Mafic rocks are slightly enriched in light rare earth elements (LREE) and depleted in Nb, Ta, Zr and Hf, suggesting a subduction-modified depleted mantle source. Some mafic samples in the Early Permian bimodal volcanic rocks have high Ti contents with relatively high concentrations of Nb and high field strength elements (HFSE) and low contents of heavy rare earth elements (HREE). These rocks are similar to the continental flood basalts, which suggests that they formed from an asthenospheric mantle. This paper indicates that mafic members were created by the partial melting of the asthenospheric mantle material and subduction-modified lithospheric mantle mixture. Some rhyolites and dacites in the Wulang formation were enriched in Ga, Nb, Zr, Ce and Y and depleted in Sr and Eu. Additionally, they showed fractionation of rare earth elements (REE) with negative Eu anomalies, which is indicative of an A-type affinity of felsic rocks. The genesis of mafic members and an A-type affinity of felsic members indicate that the Late Carboniferous – Early Permian magmatism in the Western Tianshan area formed as a result of an extensional setting. This study also reveals bimodal magmatism produced by delamination in an extensional tectonic setting.

Geochronology and petrogenesis of the composite Zuluhong Granite, North Xinjiang Province of China: Implications for the crust-mantle interaction and continental crustal growth in Western Tianshan Orogen

The mineralogy, whole rock geochemistry, zircon U-Pb-Hf isotopic compositions, and Sr–Nd isotopes were analysed from the composite Zuluhong Granite at the Alataw Mountains in NW China. The aim is to determine the granite's nature and tectonic setting, and gain a better understanding of the continental growth of the Western Tianshan Orogen (WTO), which is commonly regarded as part of the Central Asian Orogen (CAO). The Zuluhong Granite is divided into a monzogranite and porphyritic granodiorite. The monzogranite yields a zircon U–Pb date of 320 ± 2 Ma interpreted as its emplacement age, and the porphyritic granodiorite yields a zircon U–Pb date of 309 ± 1 Ma interpreted as its emplacement age. The monzogranite is characterised by its high SiO2 content (72.7–74.4 wt%), moderate 10,000 × Ga/Al values(2.59–2.81), strongly negative Eu anomaly (0.39–0.43), and relatively low zircon saturation temperatures (Tzr) of 726–807 °C, which are indicative of a highly fractionated monzogranite. The combined arc-like geochemical signatures (e.g. enrichment in LILE and LREE relative to HFSE, negative anomalies in Nb and Ta), and relatively high positive εNd(t) values of +4.4 to +4.8, indicate that the monzogranite originated from the partial melting of arc rocks in the lower crust. In contrast, the granodiorite has higher Al2O3 assays averaging 16 wt% and Sr/Y ratios averaging 40, and lower Yb assays of 1.38–1.49 ppm with a weak Eu anomaly, which are characteristics of adakite-like rocks. The combination of the granodiorite's relatively lower SiO2 content of 63.7–65.3 wt%, higher Mg# values of 53–57, higher εHf(t) values of +10.9 to +13.4, and complex oscillatory zoned plagioclase phenocrysts are indicative of a source derived from material derived from the mantle and lower crust. Our data on the Zuluhong Granite, coupled with the results of previous studies of the Late Carboniferous to Early Permian magmatism in the WTO, indicate that a regional tectonic transition from compression to extension took place in the CAO during the end of the Early Carboniferous (ca. 320 Ma). Furthermore, substantial post-collisional reworking of the Precambrian rocks along the northeastern margin of the Yili Terrane is more pronounced than the West Junggar Terrane to the east. The generation of widespread magmatism and continental growth in the CAO in a post-collisional tectonic setting including the recycling of earlier juvenile-arc complexes, and underplating of mantle-derived magma.

Permian post-collisional basic magmatism from Corsica to the Southeastern Alps

Post-Variscan Early Permian magmatism is widespread from Corsica to the Eastern Alps. After the formation of the Variscan orogenic belt between Laurussia and Gondwana, felsic and mafic bodies were emplaced during an extensional tectonic phase. This study focuses on a mafic dyke swarm that intruded in the region of Ajaccio (Corsica, France) and on a gabbroic intrusive complex outcropping in the Eastern Alps (Bressanone/Brixen, South Tyrol, Italy) both of which were emplaced during this extensional event. New U-Pb data from zircon show that both of these intrusions were emplaced at ca. 282 Ma. Most Ajaccio dykes display a calc-alkaline affinity and are characterized by enrichment of LILE (Large Ion Lithophile Elements) and LREE over HREE (Light and Heavy Rare Earth Elements) and HFSE (Nb, Ta, Hf, Zr; High Field Strength Elements). Two dykes show tholeiitic affinity and are characterized by marked LREE and Th-U depletion and by minor negative Nb-Ta anomalies on multi-element diagrams, which is typical for within-plate basalts. Bressanone/Brixen gabbros display similar geochemical features to the Ajaccio calc-alkaline samples with LREE enrichment over MREE (Medium Rare Earth Elements) and HREE, and HFSE depletion. Such characteristics are also confirmed by trace element data obtained by LA-ICP-MS on clinopyroxene and plagioclase crystals.The calc-alkaline to tholeiitic dykes are characterized by enriched to depleted Sr-Nd-Pb isotopic compositions, respectively. Some Ajaccio dykes and the Bressanone/Brixen gabbros are clearly affected by crustal contamination. However, the combination of trace element and isotopic variations suggests that mantle source heterogeneities were also involved. Isotopic compositions of the Ajaccio dykes reveal the presence of two distinct mantle source signatures: an enriched mantle (EM) component, which dominates the composition of the calc-alkaline-like suite, while a DMM (Depleted MORB Mantle) component dominates the source of the tholeiitic suite. The EM mantle source likely formed during Variscan subduction, however the composition of the tholeiitic suite requires a source composed dominantly of depleted mantle, with a minor contribution from both the subducting slab and supra-subduction mantle wedge. The mantle source of the tholeiitic magmas possibly originated from greater depths (compared to the calc-alkaline-like suite) during the post-orogenic extensional phase potentially due to break-off and/or roll-back of the Variscan slab or mantle flow from the front or margins of the subducted slab.Notably, coeval Permian mafic intrusive bodies from throughout Corsica (Bocca di Tenda, Porto, Pila Canale) and from the Australpine and Southalpine Alps intruded at a similar time and have geochemical features that are very similar to the Ajaccio dyke swarm and Brixen gabbro. This indicates that a widespread Permian magmatic province developed in a post-orogenic extensional tectonic setting at the margin of the former Variscan belt. During the Middle-Late Triassic magmatism with similar enriched geochemical signatures occurred in the Southern Alps, Dinarides, Australpoine domains and at the end of the Triassic over central Pangea (Central Atlantic Magmatic Province). This would highlight the importance that the Variscan orogeny had in the development of a more fertile mantle.