Permian Magmatism Research Articles

Generation and evolution of the Choiyoi granitic magmatism based on U-Pb zircon studies, Cordón del Portillo, Frontal Cordillera (Argentina)

Based on a relevant geochronological U-Pb zircon dataset (n = 47) from a sample (PBL-109) of the Cerro Punta Blanca pluton (CPB), which is part of a calc-alkaline suite, we corroborate the development of protracted magmatic activity with three major crystallization events for this Permian magmatism: 278 ± 1, 283 ± 2, and 289 ± 2 Ma, which outcrop in the Cordón del Portillo, Cordillera Frontal (CF) of Argentina. These ages can be assigned to the lower section of the magmatic record of the Choiyoi magmatism (ca., 290-265 Ma), while the age of 289 Ma represents the oldest known age for the Choiyoi, indicating the start of this magmatism during the Artinskiense. Considering these geochronological data, we postulate the presence of a deep mush reservoir where protracted magmatic activity permitted the prolonged crystallization of antecrysts (ca. 283–289 Ma). Migration of the parental magma from the mush reservoir zone occurred near the time of emplacement and culminated in the formation of an ephemeral magma chamber at shallow levels, where zircon autocrysts crystallized (ca. 278 Ma). Age spectra reported within individual samples support the idea of massive magma migration when conditions were favorable (e.g., thermally matured crust). In this view, the studied “older” Choiyoi magmatism represents a continuous magmatic event lasting 11 Ma and corresponds to a single magmatic episode rather than different periods of magmatic activity and subsequent emplacements. A later alkali-calcic magmatic event is recorded at 265 ± 4 Ma from a sample of the Cerro Bayo pluton (MH-0113), which could represent the end of the lower section of the Choiyoi magmatism. Whole-rock Sm-Nd, Rb-Sr, and Lu-Hf data in zircon, along with ages reported for the studied igneous and inherited zircon from the CPB; together with isotopic data and ages from the detrital zircon found in the Carboniferous accretionary complex of Chile, indicate that the source of the Permian parental magma in this region was a heterogeneous continental crust mainly formed by Devonian and Carboniferous rocks, related probably to a magmatic arc. However, some contribution from the Carboniferous accretionary complex of Chile to the parental magma should be consider.

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.

Evolving Permian magmatism associated with arc migration in the southern Beishan Orogenic Collage, Central Asian Orogenic Belt, NW China

Abstract The Beishan Orogen contains the youngest suture in the Central Asian Orogenic Belt and is vital to understanding its terminal collision history and closure of the last vestiges of the Palaeo-Asian Ocean. Multidisciplinary evidence is provided for a close temporal and spatial linkage between the Liuyuan Complex back-arc ophiolite and the Ganquan Complex arc rocks. The Ganquan Complex mainly comprises felsic volcanic and volcaniclastic rocks and is divided into a lower and upper sequence, both of which exhibit arc-like geochemical signatures and have magmatic zircons characterized by mantle-like δ 18 O values. Their U–Pb zircon ages range between 295 and 286 Ma and between 283 and 281 Ma, with corresponding ε Hf values varying from −4.2 to +1.9 and from +1.3 to +7.6, respectively, indicating a more juvenile magma source over time. The increase of juvenile isotopic signatures in the Ganquan Complex rocks is attributed to arc-trench migration, which progressively removed the arc from influences of an old arc basement contaminant and led to back-arc spreading and formation of the Liuyuan ophiolite. Cessation of the multi-phase Ganquan arc system at c. 280 Ma coincided with the accretion of the continental Baidunzi Complex in the south.

The Fornovolasco area (Alpi Apuane, Northern Apennines): a review and update on its Palaeozoic succession, middle Permian magmatism, and tectonic setting

The Fornovolasco area (Alpi Apuane, Northern Apennines): a review and update on its Palaeozoic succession, middle Permian magmatism, and tectonic setting

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.

Large-volume Permian felsic volcanism in the Tisza Mega-unit (East-Central Europe): Evidence from mineralogy, petrology, geochemistry, and geochronology

Permian felsic volcanic rocks are widespread in the Tisza Mega-unit (Carpathian–Pannonian region), covering a relatively large area from southern Transdanubia (Hungary) to the Apuseni Mountains (Romania). The present occurrence and structural position of these Permian volcanic rocks are the result of a complex tectonic evolution (including Alpine nappe stacking and Neogene extension); meanwhile, multiple hydrothermal effects and deformation modified their microtexture and chemical composition. In this study, the same emphasis was given to Permian felsic volcanism as well as the subsequent fluid- and/or deformation-related processes in the Tisza Mega-unit to extend our knowledge in regional issues (e.g., geochronology, stratigraphy, and lithologic correlations) and to investigate if the variably altered volcanic rocks can be used in petrogenetic studies.The zircon U-Pb dating yields ages of ∼270.4–262.9 Ma for the Apuseni samples and ∼268.4–260.2 Ma for those of the Pannonian Basin. Whole-rock geochemistry, considering immobile trace elements, since most of the major element compositions were severely modified by alteration processes, indicates that the studied rocks are dacite to rhyolite. They belong to a voluminous silicic volcanism during the Middle Permian, most probably related to rifts.Petrography, whole-rock geochemistry, and X-ray powder diffraction mineralogy revealed that the predominant alteration of the studied rocks is hydrothermal sericitization, locally superimposed by mylonitization in greenschist to subgreenschist conditions during Alpine orogeny. The K-Ar dating of the separated illite fractions confirms that the ductile deformation acted during the Turonian nappe stacking, as the main Alpine deformation phase in the study area.On a regional scale, the Permian magmatism in the Tisza Mega-unit looks significantly younger than the similar magmatic activity of the stable Europe (∼300–290 Ma).

Geochronology, geochemistry, and Sr–Nd isotopes of Permian dike swarms in the Beishan orogenic belt, southernmost Central Asian Orogenic Belt: Petrogenesis and implications for geodynamic setting

ABSTRACT Permian mafic dikes crop out widely within Beishan area in the southernmost Central Asian Orogenic Belt (CAOB). These mantle-derived products provide important clues for elucidating the genesis and geodynamic setting of Permian magmatism. The mafic dikes within the Beishan area display zircons U–Pb ages of 281 ± 4 Ma. Characterized by low SiO2 (44.88–47.73 wt.%) and high Mg# values (62.2–68.6), the type 1 dikes are relatively depleted in light rare earth elements (LREEs; (La/Yb)n = 0.72–1.43) but enriched in large ion lithophile elements (LILEs). They show high positive εNd(t) values (6.6–8.5) with a range of (87Sr/86Sr) i (0.703477–0.705789) ratios. In contrast, the type 2 dikes have higher SiO2 content (47.47–51.13 wt.%) and lower Mg# (40.9–63.0). They are enriched in LREEs ((La/Yb)n = 1.79–5.02) and LILEs, and have lower εNd(t) values (3.8–7.4) with a variable range of (87Sr/86Sr) i (0.702873–0.706949) ratios. Thus, the type 1 dikes resulted from lower degrees of fractional crystallization and contamination than the type 2 dikes. Considering the LILEs and Pb enrichments, high field strength elements depletions, and high Ba/Th (86–460) ratios, the mantle source was metasomatized by subduction-related fluids. These dikes should be derived from high degrees (>20%) of partial melting of a Sr–Nd isotope-depleted, phlogopite-bearing spinel lherzolite lithospheric mantle according to their high Rb/Sr (0.04–0.24), low Ba/Rb (0.9–3.3), low La/Sm, and constant Sm/Yb ratios in addition to Sr–Nd depletion. Combined with the coeval evidence of regional geology and novel tectonic discriminate diagrams, we suggest that the mafic dikes in Beishan area formed in arc-related setting rather than intracontinental setting during the Permian. The metasomatized lithospheric mantle by subduction-related fluids were heated by upwelling asthenosphere mantle triggered by the slab roll-back, resulting in the formation of the parental magma of the mafic dikes.

Geochemistry, zircon U–Pb geochronology and Hf isotope of the early Permian gabbro and high-Mg diorites from the Zhusileng–Hangwula Belt in the northern Alxa area: Petrogenesis and tectonic implications

AbstractAs the southernmost part of the central segment of the Central Asian Orogenic Belt, the northern Alxa area is characterized by abundant Permian magmatism and records key information on the geological evolution of the Palaeo-Asian Ocean. This study reports new zircon U–Pb and Lu–Hf isotopic and whole-rock geochemical data of the early Permian (285–286 Ma) Huisentala gabbro and Huodonghaer diorites from the Zhusileng–Hangwula Belt in the northern Alxa area. The gabbro is characterized by high Al, Ca, Mg# and light rare-earth elements, and low K, P and high field strength elements (e.g., Ti, Nb and Ta). Furthermore, the gabbro shows heterogeneous zircon ϵHf(t) value (−2.5 to +2.6). The Huodonghaer diorites show high MgO (3.46–6.32 wt%), Mg# (49–58), Sr (408–617 ppm) and Ba (223–419 ppm), and low FeOT/MgO (1.27–1.83) and TiO2 (0.48–0.90 wt%), with geochemical features similar to the high-Mg andesite/diorite. They show radiogenic zircon ϵHf(t) values of +1.2 to +4.9 and high Th/Nb ratios. These features suggest that the Huisentala gabbro and the Huodonghaer diorites were derived from the partial melting of mantle peridotite that was metasomatized by subduction-related fluids and by subducted sediment-derived melts, respectively.

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.

Late Permian plume and Neoproterozoic subduction-modified mantle interaction: Insights from geochronology and Sr-Nd-O isotopes of mafic dikes of the western Emeishan large igneous province

Geochronological investigations of mafic dikes along the southwestern margin of the Emeishan large igneous province (ELIP) in the South China block display a restricted range of U-Pb zircon, baddeleyite, and apatite isotopic ages ranging from 263 to 257 Ma, which overlaps with that of previously studied ELIP basalts and mafic intrusions. The dikes are divided into high-Ti and low-Ti groups, whereby the latter is further divided into two subgroups (low-Ti group-1 and -2). The high-Ti group rocks (Ti/Y &gt; 500) are characterized by ocean island basalt-like trace element patterns with mantle-like zircon δ¹⁸O of 5.0 ± 0.10‰ and slightly enriched ε_Nd(t) values of −1.0 to +1.0. The low-Ti group-1 rocks (Ti/Y &lt; 500) have trace element patterns similar to those of the high-Ti group, yet generally with weak negative Nb-Ta anomalies, lower (Sm/Yb)_N ratios, elevated zircon δ¹⁸O (6.6 ± 0.33 ‰), and highly variable ε_Nd(t) values (−3.9 to +3.2). The low-Ti group-2 rocks (Ti/Y &lt; 500) are characterized by pronounced negative Nb-Ta anomalies, more negative ε_Nd(t) (−8.4 to −6.6) values, and higher initial ⁸⁷Sr/⁸⁶Sr ratios than those of the other two groups. The compositional variations of the high-Ti group and the low-Ti group-1 rocks, in conjunction with the negative correlation between the ε_Nd(t) values and the (Th/Nb)_N ratios, suggest that the two groups were generated from an isotopically heterogeneous mantle plume at different depths, and experienced varying degrees of crustal contamination (but &lt; 20 wt.%). The high-Ti group rocks are considered to have originated from a deeper garnet-stable source, and the low-Ti group-1 rocks from a shallower source. Mixing calculations indicate that the highly enriched Sr-Nd isotopes of the low-Ti group-2 rocks cannot be explained by crustal contamination. A subduction-modified mantle source is required to account for the arc-like geochemical characteristics of this group. This is consistent with the spatial overlap of the low-Ti group-2 rocks and previously studied geochemically similar samples with rocks from the Neoproterozoic subduction zone along the western margin of the South China block. Furthermore, a fertilized mantle is also consistent with the variable δ¹⁸O values of various mafic-ultramafic rocks of the western and central ELIP due to the involvement of recycled oceanic crustal materials. Our results are in accordance with the model that the western ELIP late Permian magmatism was generated by the interaction of two distinct sources, that is, an isotopically heterogenous mantle plume and a Neoproterozoic subduction-modified, Nd isotope-enriched lithospheric mantle with distinct heterogenous oxygen isotope characteristics.

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.

Petrogenesis of Early Permian basalts in the Turpan-Hami basin, NW China: Implications for the spatial limits of the Tarim mantle plume

Controversies remain as to the area of the Tarim Large Igneous Province (TLIP), in particular, the relationship between extensive Permian magmatism in the Central Asian Orogenic Belt (CAOB) and the activity of Tarim plume. Here we report the geochronological, mineralogical and geochemical characteristics of the Permian basalts from the Turpan-Hami (Tu-Ha) basin, located in southwestern CAOB. LA-ICP-MS zircon U-Pb dating yielded an age of 298.4 ± 2.6 Ma, temporally close to that of the Tarim plume. The basalts could be classified into lower porphyritic basalt with phenocrysts of plagioclase, olivine, minor clinopyroxene, and upper aphyric basalts. Geochemically, the former is characterized by relatively higher Na2O + K2O (3.97–5.13 wt%), low (La/Yb)N (∼2) and depleted Sr-Nd values [εNd(t)= +8.2 to +8.7; (87Sr/86Sr)i = 0.7032–0.7034], whereas the latter have lower Na2O + K2O (4.19–4.43 wt%) content, higher (La/Yb)N (∼4–6) and more enriched Sr-Nd values [εNd(t)= +4.6 to +6.2; (87Sr/86Sr)i = 0.7033–0.7048]. The compositional variation of plagioclase phenocrysts and pressure estimation (∼7kbar) indicate that Tu-Ha magmas were stored and evolved in a lower crustal magma chamber which experienced at least one-time magma replenishment. The estimated mantle potential temperature (1270–1480 °C) in Tu-Ha basin is similar to MOR, with no obvious thermal anomaly. Furthermore, higher Na/Ti ratio of quartz-normative Tu-Ha basalts suggest a shallower melting depth for Tu-Ha basin than central area of TLIP, which against a typical plume model. These characteristics alongside regional geologic records collectively suggest that the formation of Permian Tu-Ha basalts is more closely linked post-orogenic extension, implying that the extensive Permian magmatism in the CAOB is unrelated to the Tarim plume.

Middle Permian magmatism in the Tangjia-Sumdo region, Tibet: evidence for intra-oceanic subduction

ABSTRACT The late Paleozoic tectonic evolution of the Lhasa terrane and subduction of Sumdo Paleo-Tethys ocean remains controversial due to limited data and variable Permian magmatism in central and southern Lhasa. We report new zircon U–Pb ages and Hf-isotope ratios as well as whole-rock major element, trace element, and Sr–Nd isotope data from middle Permian (266 ~ 257 Ma) arc-related magmatism, including high-Mg quartz diorite and trondhjemite in the Tangjia-Sumdo area. The quartz diorite has high SiO2 (53.92–57.90 wt.%), MgO (3.69–8.03 wt.%) and low TiO2 (0.60–1.01 wt.%) contents. It is enriched in the large ion lithophile elements (LILEs, e.g. Rb, Ba, Sr) and depleted in the high field strength elements (HFSEs, e.g. Nb, Ta, Ti). We interpreted these chemical features, in combination with uniform whole-rock (87Sr/86Sr)i (0.70375–0.70452), εNd(t) (+4.48 – +5.55), and zircon εHf(t) (+11.2 – +13.7) values, as indicative of dioritic melt derived by the interaction of fluids expelled from the subducting slab with the overlying mantle wedge in an island arc setting. The trondhjemite has high SiO2 (71.16–73.52 wt.%), Al2O3 (14.92–16.24 wt.%) and K2O + Na2O (7.34–8.27 wt.%) contents, is enriched in the light rare-earth elements (LREEs) and is depleted in Th and heavy rare-earth elements (HREEs). These rocks are characterized by positive εNd(t) (+5.91 – +6.68) and εHf(t) (+12.0 – +16.1) values and may have been derived from partial melting of subducted oceanic crust. Based on the elemental and isotopic composition of the quartz diorite and trondhjemite, we suggest that middle Permian magmatism occurred in an intra-oceanic subduction setting in the Sumdo Paleo-Tethys ocean. The Sumdo Paleo-Tethys ocean was probably a small ocean basin within the eastern Lhasa terrane and not a vast ocean basin that completely separated central Lhasa from the southern Lhasa subterrane.

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.