Late Carboniferous Magmatism Research Articles

Lithospheric dripping triggered by slab break-off: A possible mechanism for Late Carboniferous magmatism in the eastern Central Asian Orogenic Belt

In this paper, we present zircon UPb dating, elemental geochemistry, and Sr–Nd–Hf isotopic compositions for Late Carboniferous intrusive rocks in the northern Great Xing'an Range, northeast China. We decipher the mechanisms which triggered lithospheric thinning under microblock-microblock collision and provide a new perspective on the Late Paleozoic evolution of the eastern Central Asian Orogenic Belt. Zircon UPb dating indicates that Jifeng gabbroic diorites and Xinlin granites were formed during a short time interval at 322–321 Ma, whereas the Xinlin gabbros were crystallized at 300 ± 2 Ma. The Jifeng gabbroic diorites exhibit typical arc characteristics, depleted in HFSEs and enriched in Pb and have positive εNd(t) (+1.88 to +2.00) and εHf(t) (+6.6 to +10.7) values, suggesting they originated from the partial melting of depleted lithospheric mantle modified by slab fluids. The Xinlin gabbros belong to alkaline series and are characterized by the enrichment in HFSEs with positive Nb and Ta anomalies and depleted NdHf isotopic compositions (zircon εHf(t) = +12.5 to +18.9, whole rock εNd(t) = +8.0 to +8.1). High Nb concentrations and low La/Nb (0.81–0.83) and La/Ta (12.50–14.61) ratios of the Xinlin gabbros indicate an origin from a depleted asthenospheric mantle. The Xinlin granites have high-K calc-alkaline compositions, belonging to I-type granites. These granites have positive εHf(t) values (+0.2 to +15.1) and juvenile two-stage model ages (1314–506 Ma) with low Sr and high Yb contents, indicating they were formed by the partial melting of a juvenile lower crust under low-pressure conditions. These data, together with the temporal and spatial distribution and the geochemistry characteristics of the Carboniferous igneous rocks in the Great Xing'an Range, suggest that the amalgamation between the Erguna–Xing'an and Songnen blocks occurred before the Late Carboniferous. The occurrence of asthenospheric mantle-derived mafic rocks indicates localized thinning process of lithosphere. Late Carboniferous magmatism on the Erguna–Xing'an Block attributed to dripping or piece removal of the lithosphere due to asthenospheric fluctuation triggered by slab break-off.

Crustal contribution for the formation of the Walali Au deposit and implications on the Early Cretaceous Au mineralization in the northern Great Xing’an Range

The northern Great Xing’an Range (NGXR) gold (Au) metallogenic belt is characterized by the wide occurrence of the Early Cretaceous orogenic and arc-magma related epithermal Au deposits. Information of the newly discovered granite-hosted Walali Au deposits in this area could shed new light on the Early Cretaceous Au metallogenesis in NE China. The ore-hosting granite at Walali shows zircon U-Pb ages of 310.6 ± 2.2 Ma, which are significantly different from the ore-bearing pyrite Rb–Sr age (119.9 ± 1.7 Ma). The distinct ages indicate that the Au mineralization occurred at the Early Cretaceous and therefore preclude the genetic link between the Late Carboniferous magmatism and Au ore formation. The fluid inclusion studies show that the ore precitpitated from the hydrothermal fluids with a temperature range of 150–330 °C and a salinity range of 3.4–11.6 wt% equiv. H–O isotopic compositions indicate the involvement of both magmatic and meteoric water in the hydrothermal system. The pyrite and galena S (δ34SV–CDT values: 7.7–10.0‰) and Pb (206Pb/204Pb ratios of 18.177–18.217; 207Pb/ 204Pb ratios of 15.614–15.633) isotopic compositions suggest a crustal contribution for the ore-forming materials at Walali. By comparison, the Walali Au deposits are more comparable with the strata-hosted orogenic Au deposits in the north NGXR, but different from the epithermal- and porphyry-type Au deposits in this area. Combined with the Late Mesozoic tectonic evolution in NE China, we conclude that the Early Cretaceous extensional setting caused the large scale magmatism, which drove the remobilization of crustal materials and resulted in the formation of the granite- and strata-hosted Au deposits in the NGXR.

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.

Rapid transition from oceanic subduction to postcollisional extension revealed by Carboniferous magmatism in East Junggar (NW China), southwestern Central Asian orogenic belt

AbstractKnowledge of the subduction to postcollision tectonic transition in response to oceanic closure is crucial for tracking the final stage of orogenic evolution. Here, we report new geochronology, geochemistry, and isotopic data for Carboniferous magmatism in East Junggar (NW China), southwestern Central Asian orogenic belt, which may record such processes following the closure of the Kalamaili Ocean (a branch of the Paleo-Asian Ocean). The early Carboniferous calc-alkaline volcanic rocks (dominated by basalt and basaltic andesite) yielded zircon U-Pb ages of ca.340–330 Ma and are characterized by arc-like trace-element patterns showing enrichment of light rare earth elements (LREEs) and large ion lithophile elements (LILEs; e.g., Pb) but depletion of high field strength elements (HFSEs; e.g., Nb, Ta, and Ti). Combined with their variable Ba/Nb (9.80–454) and lowNb/La (0.21–0.54) and Sm/Yb (1.77–3.08) ratios as well as depleted mantle–like Sr-Nd-Pb-Hf (whole-rock 87Sr/86Sri = 0.7037–0.7040; εNd[t] = +3.5 to +5.9; 206Pb/204Pbi = 17.728–17.996; zircon εHf[t] = +11.8 to +18.8) isotopic values, we suggest that they were produced by melting of a lithospheric mantle wedge fluxed by slab-derived fluids under spinel-facies conditions. With whole-rock 40Ar/39Ar dating of ca. 320 Ma, the late Carboniferous mafic dikes have geochemical features and Sr-Nd-Pb (87Sr/86Sri = 0.7039–0.7041; εNd[t] = +6.6 to +6.8; 206Pb/204Pbi = 17.905–17.933) isotopic compositions similar to those of the early Carboniferous volcanics, but they show less pronounced Pb anomalies and negative Nb and Ta anomalies. They are interpreted to have formed by partial melting of a spinel-bearing lithospheric mantle metasomatized by limited influx of subduction-related fluids. The late Carboniferous felsic volcanic rocks (dacite and rhyolite) yielded zircon U-Pb ages of ca. 305 Ma and are geochemically equivalent to those of A2-type granites in East Junggar. They have juvenile isotopic compositions (εNd[t] = +4.5 to +6.8; εHf[t] = +13.3 to +18.7) and relatively young Nd and Hf model ages that roughly coincide with the ages of the ophiolites in the area, suggesting that they could have originated from melting of a juvenile basaltic lower crust. Whole-rock geochemistry, assimilation–fractional crystallization (AFC), and isotopic mixing modeling argue for insignificant crustal contamination for the Carboniferous magmatism. We suggest that the early Carboniferous lavas erupted in an island-arc setting related to the northward subduction of the Kalamaili oceanic crust, whereas the late Carboniferous magmatism formed in a postcollisional extensional regime in response to slab breakoff or lithospheric delamination. Combined with regional geological information, we propose that a rapid tectonic transition from oceanic subduction to postcollisional extension may have occurred in East Junggar during the Carboniferous, marking the final closure of the Kalamaili Ocean, which most likely took place ca. 330–320 Ma. This study provides overall geochronological and petrogeochemical evidence to better constrain the amalgamation of the southwestern Central Asian orogenic belt and may be of great importance for understanding the final stage of orogenic evolution elsewhere.

Geochronological, Geochemical and Sr-Nd-Hf Isotopic Studies of the A-type Granites and Adakitic Granodiorites in Western Junggar: Petrogenesis and Tectonic Implications

Late Carboniferous magmatism in the Western Junggar region of the Central Asian Orogenic Belt (CAOB) provides a critical geological record of regional tectonic and geodynamic history. In this study, we determined the zircon U-Pb isotopic compositions, bulk-rock Sr-Nd-Hf isotopic compositions, and major and trace element geochemistry of two granitic bodies in the Western Junggar, with the aim of constraining their emplacement ages, magmatic origin, and geodynamic significance. Radiometric ages indicate that the plutons were emplaced during the Late Carboniferous (322–307 Ma). Plutons in the North Karamay region are characterized by high Sr content (347–362 ppm) and low Y content (15.3–16.7 ppm), yielding relatively high Sr/Y ratios (20.8–23.7). They show consistent Yb (1.68–1.85 ppm), Cr (16–19 ppm), Co (7.5–8.1 ppm) and Ni (5.9–6.6 ppm) content, similar to that of modern adakites. The Hongshan plutons are characterized by high SiO2 (69.95–74.66 wt%), Na2O (3.26–3.64 wt%), and K2O (4.84–5.16 wt%) content, low Al2O3 (12.02–12.84 wt%;) and MgO (0.13–0 18 wt%) content, and low Mg# values (0.16–0.22). This group shows a clear geochemical affinity with A-type granites. All of the studied granitoids have positive εNd(t) (+4.89 to +7.21) and εHf(t) (+7.70 to +13.00) values, with young TDM(Nd) 806–526 Ma) and TDM(Hf) (656–383 Ma) ages, indicating a substantial addition of juvenile material. The adakitic granodiorites in the North Karamay region were likely generated via partial melting of thickened lower crust, while the A-type granites in the Hongshan area may have been derived from the melting of lower-middle crust in an intra-oceanic arc, which consists mainly of oceanic crust. The emplacement of these granitoids represents a regional magmatic “flare up”, which can be explained by the rollback of a subducting slab.

Intra-continental back-arc basin inversion and Late Carboniferous magmatism in Eastern Tianshan, NW China: Constraints from the Shaquanzi magmatic suite

The Yamansu belt, an important tectonic component of Eastern Tianshan Mountains, of the Central Asian Orogenic Belt, NW China hosts many Fe–(Cu) deposit. In this study, we present new zircon U-Pb geochronology and geochemical data of the volcanic rocks of Shaquanzi Formation and diorite intrusions in the Yamansu belt. The Shaquanzi Formation comprises mainly basalt, andesite/andesitic tuff, rhyolite and sub-volcanic diabase with local diorite intrusions. The volcanic rocks and diorites contain ca. 315–305 Ma and ca. 298 Ma zircons respectively. These rocks show calc-alkaline affinity with enrichment in large-ion lithophile elements (LILEs), light rare-earth elements (LREEs), and depletion in high field strength elements (HFSEs) in primitive mantle normalized multi-element diagrams, which resemble typical back-arc basin rocks. They show depleted mantle signature with εNd(t) ranging from +3.1 to +5.6 for basalt; +2.1 to +4.7 for andesite; −0.2 to +1.5 for rhyolite and the εHf(t) ranges from −0.1 to +13.0 for andesites; +5.8 to +10.7 for andesitic tuffs. We suggest that the Shaquanzi Formation basalt might have originated from a depleted, metasomatized lithospheric mantle source mixed with minor (3–5%) subduction-derived materials, whereas the andesite and rhyolite could be fractional crystallization products of the basaltic magma. The Shaquanzi Formation volcanic rocks could have formed in an intra-continental back-arc basin setting, probably via the southward subduction of the Kangguer Ocean beneath the Middle Tianshan Massif. The Yamansu mineralization belt might have undergone a continental arc to back-arc basin transition during the Late Carboniferous and the intra-continental back-arc basin might have closed in the Early Permian, marked by the emplacement of dioritic magma in the Shaquanzi belt.

Geochemistry and petrogenesis of Late Carboniferous igneous rocks from southern Mongolia: Implications for the post-collisional extension in the southeastern Central Asian Orogenic Belt

Late Carboniferous is a critical period in terms of the tectonic evolution of the Central Asian Orogenic Belt (CAOB). In this study, we report the petrology, geochronology and geochemistry of Late Carboniferous mafic to felsic rocks from southern Mongolia, to better characterize the Late Carboniferous magmatism in the southeastern CAOB. Three types of igneous rocks have been identified, including diabase dykes, porphyritic granites and quartz monzonite dykes. LA-ICP MS zircon U-Pb dating of these rocks shows that the diabase dykes and porphyritic granites were simultaneously emplaced at 306Ma. The quartz monzonite dykes were subsequently emplaced at 300Ma. The diabases belong to the high-K calc-alkaline series and show strong enrichment of large ion lithophile elements (LILE; e.g., Rb, Ba and Sr) and depletion in high field strength elements (HFSE; Nb, Ta and Ti), with enriched mantle isotopic signature (εNd(t)=−2.6 to +0.5). They were likely derived from the partial melting of metasomatized sub-continental lithospheric mantle (SCLM) and experienced variable degrees of crustal contamination. The porphyritic granites are characterized by high SiO2 contents, strong negative Eu anomalies with low εNd(t) values of −3.0 to −2.7, suggesting that they were derived from crustal anatexis and experienced strong fractional crystallization. Compared with the porphyritic granites, quartz monzonites display lower SiO2 contents, weaker negative Eu anomalies with higher and more variable εNd(t) values (−0.3 to +4.2). This indicates that the quartz monzonites were probably generated by the mixing of crust- and mantle-derived magmas. The petrographic and geochemical data suggest that all the mafic to felsic rocks in the southern Mongolia formed in a post-collisional extensional setting.

Carboniferous porphyry Cu (-Au) mineralization of the West Junggar region, NW China: the Shiwu example

ABSTRACTThe West Junggar region, located in the Central Asian Orogenic Belt (CAOB), is characterized by extensive Carboniferous magmatism and porphyry Cu (-Au) deposits. The Shiwu porphyry Cu-Au deposit, located in the east of the Barluk Mountains, the West Junggar region, is not only a newly discovered deposit but also a representative porphyry Cu-Au deposit in this area. The volcanic rocks (including andesite and tuff) and intrusive rocks (including diorite, quartz diorite, quartz diorite porphyry, and tonalite porphyry) occurred in the Shiwu area and the mineralization associated with the quartz diorite porphyry. The secondary ion mass spectrometry (SIMS) zircon U–Pb ages of quartz diorite porphyry and tonalite porphyry are 310.4 ± 2.3 Ma and 310.1 ± 2.4 Ma, respectively, indicating that the Shiwu deposit is related to the Late Carboniferous magmatism. Intrusive rocks, which were characterized by the enrichment of large ion lithophile elements (LILEs) and pronounced negative high field strength elements (HFSEs), belong to the calc-alkaline or tholeiitic series. Their (87Sr/86Sr)i, (143Nd/144Nd)I, and εNd(t) values range from 0.703569 to 0.704311, 0.512488 to 0.512512, and 4.9 to 5.3, respectively. Volcanic rocks, which belong to the calc-alkaline series, have similar geochemical features as the intrusive rocks, and their (87Sr/86Sr)i, (143Nd/144Nd)i, and εNd(t) values, respectively, are 0.703704–0.704071, 0.512520–0.512542, and 5.49–5.92. These characters indicate that the igneous rocks in the Shiwu area derived dominantly from the mantle and formed in an island arc setting. These characters also further confirmed that the Barluk Mountains is still in an island arc setting in the Late Carboniferous and the accretionary orogenesis can exist until 310 Ma at least.

Late Carboniferous N-MORB-type basalts in central Inner Mongolia, China: Products of hydrous melting in an intraplate setting?

Petrogenesis of the ca. 310Ma Benbatu basalts in central Inner Mongolia is crucial for constraining the evolution of the Xing'an Mongolia Orogenic Belt (XMOB), eastern segment of the Central Asian Orogenic Belt. The Benbatu basalts have low initial 87Sr/86Sr ratios (0.7042–0.7048), positive εNd(t) (+8.99 to +9.24) and εHf(t) values (+15.38 to +15.65), and are characterized by relatively flat rare earth element patterns and enrichment of Rb, U, Pb, Zr and Hf, but depletion of Nb, Ta, Sr and Ti, resembling those of the normal Mid-Ocean-Ridge Basalt (N-MORB). Variations of trace element ratios (e.g., Sm/Yb and La/Sm) suggest that the basalts were derived from spinel peridotites, with a melting depth of <60–85km. The characteristics of enrichment of Zr and Hf and depletion of Sr distinguish the Benbatu basalts from typical arc basalts and back arc basin basalts. The arc-like geochemical signatures (i.e., enrichment of large ion lithophile elements and depletion of Nb and Ta) are attributed to hydrated mantle source that may be caused by fluids released from stagnant oceanic slabs in the mantle transition zone. Integrating geological evidences with geochemical and isotopic features of the Benbatu basalts, we proposed that these basalts were produced under an intraplate extensional setting during the Late Carboniferous. The genesis of the Benbatu basalts therefore argues for the pre-Carboniferous accretion of the XMOB and highlights the importance of the deep-Earth recycling water in the generation of the Late Carboniferous magmatism in this region.

Late Carboniferous adakitic granodiorites in the Qiongkusitai area, western Tianshan, NW China: Implications for partial melting of lower crust in the southern Central Asian Orogenic Belt

Carboniferous granitic rocks are wide spread in the western Tianshan, but their tectonic contexts remain controversial. We present zircon U–Pb age, major element, trace element, and in situ zircon Hf isotopic data for the Qiongkusitai granodiorites from the southern part of the Yili block in the western Tianshan. The granodiorites were emplaced in the Late Carboniferous (ca. 314Ma) and are characterized by high SiO2 (62.1–65.7wt.%), high Sr (376–485ppm), low Y (13.7–17.7ppm) and low Yb (1.34–1.81ppm) with Sr/Y ratios of 23–34. Such characteristics imply adakitic affinities. Their low concentrations of Cr (3.85–6.75ppm), Co (7.63–10.7ppm) and Ni (2.94–6.80ppm) suggest little, if any, interaction with a mantle source. Compared to the thickened lower crustal-derived adakitic rocks, the relatively low Nb/Ta ratios of the Qiongkusitai adakitic granodiorites indicate that they were probably generated by partial melting of amphibolite facies rocks, but not eclogite facies rocks. The Qiongkusitai adakitic granodiorites have positive zircon εHf(t) values of +0.7 to +5.1 and TDM2 of 1.28–1.68Ga, and they are interpreted as being derived from remelting of amphibolite facies Mesoproterozoic basement with an input of juvenile material. Late Carboniferous magmatism in the western Tianshan was possibly triggered by asthenospheric upwelling as a result of the roll-back of the subducted southern Tianshan oceanic lithosphere.

Structural evolution and late Carboniferous magmatism of the Zhongguai arc in the western Junggar Basin, Northwest China: implications for tectonic evolution of the Junggar Ocean

ABSTRACTThe structural evolution and late Carboniferous magmatism of the Zhongguai arc in the western Junggar Basin offer some new constraints on the tectonic evolution of the Junggar Ocean. In this study, we conducted structural analysis of seismic profiles and report new geochronological and geochemical data for the late Carboniferous borehole volcanics (basalts, andesites, dacites, and rhyolites) in the Zhongguai arc. Seismic data suggest that the upper Carboniferous lies between the lower Carboniferous and middle Permian compressional tectonostratigraphic strata. Zircon secondary ion mass spectrometry U–Pb and whole-rock Ar–Ar dating results indicate that they erupted at 309–305 Ma. The andesites are low-Fe calc-alkaline, and characterized by relatively high Mg# (3.24–6.00%), Cr (41.2–70.4 ppm), and Ni (23.3–35.6 ppm) contents, and low TiO2 contents (0.69–0.83%) and TFe2O3/MgO ratios (1.48–2.08), similar to those typical of magnesian andesites. They were likely produced by reaction between slab-derived melts and mantle wedge. The basalts show tholeiitic to calc-alkaline affinity, high εNd(t) values (7.7–8.0) and Nb and Zr contents, variable enrichment in Th, U, and light rare earth elements (LREEs), and significant depletion in Nb and Ta. The felsic rocks display arc-like geochemical features with alkaline grantite, and positive εNd(t) values (5.6–7.1). The basalts are interpreted as resulting from partial melting of a metasomatized mantle by slab-derived melt and fluids with involvement of asthenospheric mantle in an extensional setting, whereas the felsic rocks were derived from juvenile lower crust. The formation of these late Carboniferous rocks is probably related to ridge subduction. Given the widespread presence of such rock association in the area from southern West Junggar terrane to Zhongguai arc and initial southeastward-vergent thrust structures in the Zhongguai arc, we suggest that the southern West Junggar terrane and Zhongguai arc might have been amalgamated as a unified overriding plate above a northwestward ridge subduction zone of the Junggar Ocean at 309–302 Ma. The occurrences of middle Permian terrestrial sediments and thrusting reactivation in the Zhongguai arc are possibly indicative of the final welding of the Zhongguai arc and other units of the Junggar basement.

Detrital zircon geochronology in blueschist-facies meta-conglomerates from the Western Alps: implications for the late Carboniferous to early Permian palaeogeography

In the Western Alps, the Money Complex of the Gran Paradiso Massif, metamorphosed under blueschist facies during the Alpine cycle, is considered to be Permo-Carboniferous in age, but no palaeontological or radiometric data constrain this interpretation. A revision of the lithostratigraphy of the Money Complex allows recognizing a polygenic (graphite-rich) and a monogenic (graphite-poor) meta-sedimentary formation. Detrital zircon U–Pb geochronology in both meta-sedimentary formations shows that (i) the main population is Cambrian and Ordovician in age, (ii) the youngest grains are Silurian and Lower Devonian, and (iii) Carboniferous zircon grains are lacking. A careful study of the age distributions in the Alps suggests that potential source for the detrital material in the Money Complex is the Brianconnais basement. Late Carboniferous magmatism is widespread in the Helvetic Zone of the Alps. Permian magmatism is dominant in the Brianconnais, the Austroalpine and the Southalpine basements. The lack of Carboniferous zircons in the Money Complex suggests that the detritus was not shed from the Helvetic zone, which was separated from the Money basin by the Zone Houillere basin, where the main drainage pattern was developed from south to north and where the depocenters migrated northwards from the Upper Missisippian to Upper Pennsylvanian. We suggest that the Money Complex may had been located to the east of the main river drainage inside the Zone Houillere basin or alternatively may represent a small basin, located on the east of the Zone Houillere.

Geological Map of the Monte Grighini Variscan Complex (Sardinia, Italy)

The study area, belongs to the Nappe zone of the Sardinian Variscan chain at the NW termination of the Flumendosa antiform. The Monte Grighini Complex is a NW-SE trending metamorphic complex made up of three tectonic units and synkinematic magmatic intrusions that show a section of the deepest portion of the Nappe zone. The tectonic units stacked and folded under lower greenschist and upper amphibolite facies conditions, were finally juxtaposed by a kilometer-wide NW-SE trending dextral transtensive shear zone during the Late Variscan tectonics. The 1:25,000 scale geological map, cross sections and shear zone deformation map illustrate the tectonic and metamorphic setting of the area, resulting from the polyphasic Variscan collisional evolution including early nappe stacking and later strike slip and extensional tectonics coeval with a late Carboniferous magmatism. Deformation structures and metamorphic assemblages recorded by the tectonic units as well as synkinematic intrusions, provide constraints of fundamental importance for the reconstruction of southern Variscan chain tectono-metamorphic history.

Petrogenesis of a Late Carboniferous mafic dike–granitoid association in the western Tianshan: Response to the geodynamics of oceanic subduction

Abstract Mafic dike–granitoid associations are common in extensional tectonic settings and provide important opportunities for understanding mantle and crust melting during the tectonic evolution of host orogenic belts. We report results of petrologic, whole rock geochemical, Sr–Nd isotopic data and in situ zircon U–Pb and Hf isotopes for a mafic dike–granitoid association from the Zhongyangchang pluton in the western Tianshan, in order to constrain their petrogenesis and tectonic significance. The intrusive rocks are mainly composed of granodiorite, monzogranite, and minor granitic dikes, with mafic dikes intruded into the pluton. Zircon LA-ICP-MS U–Pb ages indicate that the Zhongyangchang intrusive rocks were all emplaced during a short interval in the Late Carboniferous (317–310 Ma), establishing that the mafic and felsic magmas were coeval. The mafic rocks have low SiO 2 and high MgO concentrations, with low 87 Sr/ 86 Sr ratios from 0.7048 to 0.7053 and positive e Nd (t) and zircon e Hf (t) values from + 2.9 to + 3.8 and + 12.2 to + 13.6, respectively. They are enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs), which can be explained by an origin from melting of a depleted lithospheric mantle source and source fluxing by fluids derived from the down-going slab. Granitoids from the pluton have high SiO 2 contents and low MgO concentrations, suggesting that they were mainly derived from crustal sources. They also have positive whole rock e Nd (t) and zircon e Hf (t) values ranging from + 0.2 to + 2.8 and + 6.6 to + 15.3, respectively, similar to those of the mafic dikes. They were generated by partial melting of juvenile basaltic lower crust as a result of magma underplating. The Late Carboniferous mafic dike–granitoid association was not related to a post-collisional setting, but rather formed in an arc environment related to oceanic subduction. The most likely tectonic model accounting for the genesis of these rocks involves an extensional environment in the western Tianshan during the Late Carboniferous as a tectonic response to the roll-back of subducted Junggar oceanic lithosphere, which could also account for the “flare-up” of Late Carboniferous magmatism.

Geochronological and geochemical constraints on the origin of the 304 ± 5 Ma Karamay A-type granites from West Junggar, Northwest China: implications for understanding the Central Asian Orogenic Belt

U–Pb zircon geochronological, geochemical, and whole-rock Sr–Nd isotopic analyses are reported for a suite of Karamay A-type granites from the Central Asian Orogenic Belt (CAOB) in the western Junggar region of northern Xinjiang, Northwest China, with the aim of investigating the sources and petrogenesis of A-type granites. The Karamay pluton includes monzogranite and syenogranite. Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating yielded a concordant weighted mean 206Pb/238U age of 304 ± 5 Ma (n = 11), defining a late Carboniferous magmatic event. Geochemically, the rock suite is characterized by high SiO2, FeOt/MgO, total alkalies (K2O + Na2O), Zr, Nb, Y, Ta, Ga/Al, and rare earth elements (REEs) (except for Eu), and low contents of MgO, CaO, and P2O5, with negative Ba, Sr, P, Eu, and Ti anomalies. These features indicate an A-type affinity for the Karamay granitic intrusions. Isotopically, they display consistently depleted Sr–Nd isotopic compositions (initial 87Sr/86Sr = 0.7014–0.7022, ϵNd(t) = +5.6–+7.0). Geochronological, geochemical, and isotopic data suggest that the Karamay A-type granites were derived from remelting juvenile lower crust, followed by fractional crystallization. The Karamay A-type granites as well as widespread late Carboniferous magmatism in the western Junggar region of the southwestern CAOB may have been related to ridge subduction and a resultant slab window. This further demonstrates the importance of the late Palaeozoic granitic magmatism in terms of vertical crustal growth in northern Xinjiang.

A slab detachment and delamination model for the generation of Carboniferous high-potassium I-type magmatism in the Eastern Pontides, NE Turkey: The Köse composite pluton

The Hercynian Köse composite pluton (KCP) is located in the Eastern Pontides, Turkey, and consists of two units of high-K calc-alkaline, primarily peraluminous granites: (i) the internal body, and (ii) the external body. The internal body, which was emplaced at 322–318 Ma ( 40Ar/ 39Ar ages on biotite and hornblende, respectively), displays a wide compositional range (49–71 wt.% SiO 2) and contains several lithologies: hybrid equigranular rocks, microgranular magmatic enclaves, mafic dikes, porphyry dikes and mylonites. The external body, which was emplaced at 306.7 Ma ( 40Ar/ 39Ar age on K-feldspar), consists exclusively of monzogranite (> 71 wt.% SiO 2). Field relationships, mineralogy, major- and trace element geochemistry, and initial Sr–Nd isotope values ( I Sr = 0.70821 to 0.71002, e Nd(t) = −6.6 to −8.0) show that the internal body was differentiated and evolved by crystal fractionation and magma mixing processes. The end-members of the mixing process were a mafic rock and a felsic rock. Mafic magma was derived from a relatively deep-seated (25–30 km) crustal storage reservoir, not directly from the mantle, and underwent significant differentiation by fractional crystallization and crustal contamination before mixing. In addition, these magma storages probably supplied the additional heat necessary to initiate crustal melting. Some of the additional heat may have also been released by the radiogenic decay of heat producing elements. Eventually, the existing felsic magma from the melting of K-bearing meta-greywackes was raised to its emplacement level at a depth of ~ 10–16 km. After partial crystallization, it was sporadically intruded by modified mafic magma from the deeper crustal reservoir to generate hybrid rocks. The hybrid rocks were then elevated to a shallower depth by normal faults during the collapse of the orogen and erosion. Mylonites that were later overprinted by pseudotachylites are typically constrained to the internal body and are regarded as markers of this event. The external body is characterized by a significantly less radiogenic and limited range of Sr–Nd isotope values ( I Sr = 0.70639 to 0. 70792, e Nd(t) = −4.4 to −6.5) than those of the internal body and a lack of rocks documenting the open system differentiation processes. Fractional crystallization is the exclusive process responsible for the elemental range within the body. The rocks also contain less biotite relative to those of the internal body. All these involve less K-bearing mid-crustal rocks (orthogneisses) in their source, which was probably located at depths near the lower crust. The absence of purely lower crustal-derived melts can be explained by the removal of this type of material during the formation of the parental melt. This melt later ascended to its emplacement level at a depth of around ~ 5–10 km and cut the hybrid rocks of the internal body and regional metamorphic rocks that had been raised previously due to ongoing erosion. The melt that injected into the cracks of the internal body crystallized into porphyries because there was not enough time for the entire crystallization of magma. The data presented here indicate that late Early Carboniferous and Late Carboniferous magmatism occurred in a collisional setting. Slab detachment and subsequent delamination seem to be the most plausible mechanisms for the generation of the Hercynian high-K calc-alkaline magmatism in the Eastern Pontides, Turkey.

Geochemical, Sr–Nd and zircon U–Pb–Hf isotopic studies of Late Carboniferous magmatism in the West Junggar, Xinjiang: Implications for ridge subduction?

Voluminous granitic intrusions are distributed in the West Junggar, NW China, and they can be classified as the dioritic rocks, charnockite and alkali-feldspar granite groups. The dioritic rocks (SiO 2 = 50.4–63.8 wt.%) are calc-alkaline and Mg enriched (average MgO = 4.54 wt.%, Mg # = 0.39–0.64), with high Sr/Y ratios (average = 21.2), weak negative Eu (average Eu⁎/Eu = 0.80) and pronounced negative Nb–Ta anomalies. Their Sr–Nd and zircon Hf isotopic compositions (( 87Sr/ 86Sr) i = 0.7035–0.7042, ε Nd( t) = 4.5–7.9, ε Hf( t) = 14.1–14.5) show a depleted mantle-like signature. These features are compatible with adakites derived from partial melting of subducted oceanic crust that interacted with mantle materials. The charnockites (SiO 2 = 60.0–65.3 wt.%) show transitional geochemical characteristics from calc-alkaline to alkaline, with weak negative Eu (average Eu⁎/Eu = 0.75) but pronounced negative Nb–Ta anomalies. Sr–Nd and zircon Hf isotopic compositions (( 87Sr/ 86Sr) i = 0.7037–0.7039, ε Nd( t) = 5.2–8.0, ε Hf( t) = 13.9–14.7) also indicate a depleted source, suggesting melts from a hot, juvenile lower crust. Alkali-feldspar granites (SiO 2 = 70.0–78.4 wt.%) are alkali and Fe-enriched, and have distinct negative Eu and Nb–Ta anomalies (average Eu⁎/Eu = 0.26), low Sr/Y ratios (average = 2.11), and depleted Sr–Nd and zircon Hf isotopic compositions (( 87Sr/ 86Sr) i = 0.7024–0.7045, ε Nd( t) = 5.1–8.9, ε Hf( t) = 13.7–14.2). These characteristics are also comparable with those of rocks derived from juvenile lower crust. Despite of the differences in petrology, geochemistry and possibly different origins, zircon ages indicate that these three groups of rocks were coevally emplaced at ~ 305 Ma. A ridge subduction model can account for the geochemical characteristics of these granitoids and coeval mafic rocks. As the “slab window” opened, upwelling asthenosphere provided enhanced heat flux and triggered voluminous magmatisms: partial melting of the subducting slab formed the dioritic rocks; partial melting of the hot juvenile lower crust produced charnockite and alkali-feldspar granite, and partial melting in the mantle wedge generated mafic rocks in the region. These results suggest that subduction was ongoing in the Late Carboniferous and, thus support that the accretion and collision in the Central Asian Orogenic Belt took place in North Xinjiang after 305 Ma, and possibly in the Permian.

Primary Carboniferous and Permian paleomagnetic results from the Yili Block (NW China) and their implications on the geodynamic evolution of Chinese Tianshan Belt

In order to better understand the tectonic role of the Yili Block on the Paleozoic evolution of the Chinese Tianshan Belt, we performed a primary paleomagnetic study on Carboniferous and Permian rocks from different areas in the Yili Block, NW of China. More than 320 sedimentary and volcanic samples were collected from 39 sites. Except for the Ordovician samples showing a weak and unstable magnetic remanence, the majority of this collection presents characteristic remanent magnetization carried by magnetite and hematite. In the study area, though positive fold test has been observed on the Early Carboniferous rocks, a general re-magnetization of these rocks has been identified and attributed to the Late Carboniferous magmatism. Moreover, all Early and Late Carboniferous samples from the interior of the Yili Block yield stable and coherent magnetic directions with exhaustively reverse magnetic polarity. The Late Carboniferous (C 2) is considered as the magnetic remanence age since these rocks are covered or intruded by synchronous magmatic rocks of the Yili arc, which lasted until to ∼ 310 Ma. The C 2 paleomagnetic pole is therefore calculated at 68.6°N, 290.6°E with A 95 = 6.4° and n = 15. The Late Carboniferous rocks located close to a deformation zone present a consistent magnetic inclination but significant different declination with respect to other areas and are suspected to have probably experienced a local rotation. Although no fold test can be performed due to the monoclinal bedding, stable magnetic components are isolated from Late Permian (P 2) red beds in the interior of the Yili Block with also a solo reverse magnetic polarity, the P 2 paleomagnetic pole of the Yili Block has been, therefore, calculated from the characteristic remanent magnetization: 79.7°N, 172.0°E with A 95 = 11.3° and n = 5. Keeping important uncertainties in mind, comparisons of the C 2 and P 2 paleomagnetic poles of the Yili Block with available coeval poles of Junggar, Tarim and Siberia indicate (1) no significant relative motion between the Yili and Junggar blocks since the Late Carboniferous, (2) no significant or weak latitudinal relative motion occurred since the Late Carboniferous among these blocks, but (3) the 46.2° ± 15.1° and the 31.6° ± 15.1° counterclockwise rotations of the Yili–Junggar blocks with respect to Tarim and Siberia took place during C 2 to P 2. These rotations are accommodated by the Permian dextral strike–slip faults along the northern and southern sides of Tianshan Belt and sinistral strike–slip faulting along the Erqishi Fault of Altay Belt, resulting in about 1000 km and 600 km lateral displacements in the Tianshan and Altay belts, respectively.