Upwelling Of Asthenospheric Mantle Research Articles

How does a soft collision orogen uplift and collapse? Insight from the eastern Central Asian Orogenic Belt

The mechanism of uplift and collapse is critical for understanding orogenic evolution within the Wilson cycle. The Central Asian Orogenic Belt (CAOB) represents one of the largest Phanerozoic accretionary orogens on Earth, experiencing terminal soft collision following the closure of the Paleo-Asian Ocean. However, the timing and mechanism of crustal thickening and thinning in the eastern CAOB remain unclear. Here, we present geochronological, mineralogical, geochemical, and Sr-Nd-Hf isotopic data of the newly identified Late Triassic bimodal dike associations in the easternmost CAOB. The ca. 236−230 Ma mafic dikes can be divided into two groups based on petrographic and geochemical characteristics. Major element modeling using the MELTS software indicates that they evolved via independent differentiation paths. Trace element and isotope simulations reveal that the ca. 236−230 Ma mafic dikes originated from the 4%−10% partial melting of spinel- to garnet-lherzolite lithospheric mantle sources over a range of depths, with varying inputs of asthenospheric mantle materials. Coeval ca. 233 Ma felsic dikes exhibit adakitic geochemical characteristics and strong imprints of crust-mantle interaction, suggesting derivation from melting of a heated juvenile mafic lower crust as a result of the upwelling of asthenospheric mantle. The formation of bimodal dike associations records the transition from lithospheric mantle thinning to delamination. Integrating a large dataset and employing multiple geochemical proxies, our results reveal that the crust of the easternmost CAOB reached a thickness of 54 ± 3 km at ca. 280−255 Ma, likely resulting from magmatic underplating due to rollback of the subducting Paleo-Asian Oceanic slab. This region underwent a further slight increase in crustal thickness to 61 ± 2 km at ca. 254−237 Ma in response to limited tectonic shortening associated with soft collision orogeny before it thinned to 45 ± 13 km at ca. 236−210 Ma due to lithospheric delamination during post-collisional extension. Our findings reveal that the uplift of the eastern CAOB was primarily driven by magmatic underplating, with minimal contribution from tectonic shortening. Lithospheric delamination emerged as an important factor leading to the eventual collapse of the eastern CAOB. Compared to typical hard collisional orogens (e.g., the Himalaya-Tibet orogen), the CAOB experienced significantly weaker tectonic shortening followed by similar lithospheric delamination during post-collisional extension. This study highlights the importance of integrating geochemical and isotopic data in quantifying the complex evolutionary histories of ancient collisional orogenic belts.

The Neogene–Quaternary volcanism of the Carpathian–Pannonian region: from initial plate tectonic models to quantitative petrogenetic explanations

A wide range of volcanic rocks formed in the Carpathian–Pannonian region during the last 20 Myr, closely associated with the tectonic evolution of the area: (1) high-SiO 2 rhyolites and rhyodacites; (2) calc-alkaline-type volcanic suites (basalt–andesite–dacite–rhyolite and adakitic rocks); (3) potassic–ultrapotassic volcanic rocks (shoshonites, latites, leucitites and lamproites); and (4) alkali sodic volcanic rocks (basalts, trachyandesites and trachytes). During the 18.1–14.4 Ma silicic ignimbrite flare-up volcanism, explosive eruptions produced large volume of tephras, which covered extended areas of Europe. Calc-alkaline-type volcanism took place in syn-extensional and post-collisional settings. There is no classic volcanic arc in the Carpathian–Pannonian Region, and so the term ‘back-arc basin’ should be used for the Pannonian Basin only in a structural geology sense. Alkali basalt volcanism occurred mostly in the peripheral areas of the Pannonian Basin primarily owing to edge-driven convective upwelling of heterogeneous asthenospheric mantle. The youngest eruptions took place 100–30 ka ago, mostly in the geodynamically still active southeast Carpathians. After 50 years of intense research, there are still many open questions to be solved. Interdisciplinary understanding of the results of different disciplines is crucial to better constrain the evolution of this tectonically complex area, to reveal its geoenergy and to assess potential natural hazards.

Oligocene magmatism in the eastern Sakarya Zone, NE Turkey: Geodynamic implications

During the Cenozoic, the eastern Sakarya Zone, northeastern Turkey, has witnessed voluminous Eocene and subsidiary Miocene magmatism. By contrast, except in one previous study, Oligocene magmatism has not been reported. Here, we present new data about an Oligocene dyke swarm from the Artvin district in the easternmost part of the Sakarya Zone. Our study improves our understanding of the Oligocene magmatic and geodynamic evolution of the central Tethyan orogenic belt.The Oligocene dykes consist of two distinct rock types: quartz-bearing trachytic and adakitic trachyandesite/andesite, dated at 31.0 ± 0.2 Ma and 31.1 ± 0.2 Ma to 29.2 ± 0.3 Ma by U-Pb zircon geochronology, respectively. Geochemical analysis reveals enrichment in LREE and MREE for quartz-bearing trachytic dykes, whereas adakitic trachyandesite/andesite dykes show depletion in MREE and HREE. Radiogenic isotopic ratios (87Sr/86Sr(i) and 143Nd/144Nd(i)) and trace element data suggest that the source of Oligocene magmatism was linked to low degree (≈10–15 %) partial melting of a metasomatised lithospheric mantle previously enriched by slab-derived melts and fluids. During the magma ascent in the crust, parental magmas of the trachyandesite/andesite dykes have assimilated crustal rocks and undergone fractional crystallization dominated by amphibole. By contrast, quartz-bearing trachytic dykes have recorded plagioclase fractionation.Local extension, in conjunction with continuous foundering of delaminated thickened lower crust beneath the eastern Sakarya Zone, which likely triggered asthenospheric mantle upwelling and resulted in sporadic Oligocene magmatism.Our study highlights the distinct tectonic and magmatic setting of the rare Oligocene magmatism in the eastern Sakarya Zone compared to Oligocene magmatism in the Tauride-Anatolide and South Armenian blocks.

The layered gabbroic intrusion in the Southern Eastern Desert of Egypt: Implications for Fe-Ti-rich magmatism at the end of the East African Orogeny

The Rahaba layered intrusion in the Southern Eastern Desert of Egypt is mainly composed of pyroxene hornblende-, and hornblende gabbros. It hosts thick FeTi oxide ore layers that are composed of titanomagnetite with subordinate ilmenite and magnetite. To understand the petrogenesis of the FeTi rich gabbroic rocks, the nature of their parent high-Fe primitive mantle melts and condition of FeTi oxide layer formation, we obtained new mineral chemistry and whole-rock major and trace element compositions. The geochemical characteristics of the gabbroic rocks resemble those of picro-basalts, suggesting generation from picritic basaltic-like melts of tholeiitic affinity. Their high-Fe-Ti-Mg nature (TiO2, ∼1.91 wt%; MgO, ∼11.0 wt%; Fe2O3, ∼14 wt%) probably indicates elevated mantle potential temperatures due to upwelling of hot asthenospheric mantle. Rare earth elements (REEs) of clinopyroxene show flat patterns with depletion in LREEs, similar to those of the host gabbros and picritic basaltic lava, suggesting a mantle origin for their parental melts. Moreover, clinopyroxene compositions show differentiation trends similar to low-pressure igneous intrusions, and reveal a rift-related origin. Chemistry of plagioclase (An45–62), clinopyroxene (Wo38.4–45.3, En39.9–43, Fs14–19.3; Mg#: 0.71–0.92) and olivine (Fo56–80) shows wide compositional ranges due to variable degrees of crystal fractionation. The calculated equilibrium temperature for the Rahaba Fe-Ti-rich gabbros is mainly 820–1000 °C, at pressure below 5 kbar, reflecting depth of rock solidification of <15 Km. These rocks show both subduction-like and rift-related geochemical signatures comparable to other post-collisional layered intrusions in the Arabian-Nubian Shield. The low average Ce/Pb (8.3) and high Ba/Nb (391) ratios together with the enriched large-ion lithophile elements (Cs, Ba, K and Sr) and depleted high-field strength elements (Th, Nb, Ta and Zr) reflect the contribution from a preceding subduction event. The lineament map, structural lineament and lineament density map reveal main WNW-ESE and NW-SE structural trends of the Najd System in the Rahaba area. The Rahaba FeTi rich gabbroic intrusion was likely formed by initial impact of a mantle plume head with Fe-rich streaks at the base of metasomatized lithospheric mantle during an extensional phase associated with activity of the Najd-related strike-slip faults, at the end of the East African Orogeny. The resultant mantle-derived melt compositions and Najd-related structures possibly controlled the distribution of FeTi ore deposits during this Orogeny. The Rahaba gabbroic intrusion was emplaced during post-collisional extension after slab break-off and lithospheric delamination following collision of accreted island-arcs with the Saharan Metacraton.

Permian−Triassic magmatism above a slab window in the Eastern Tianshan: Implications for the evolution of the southern Altaids

Permian−Triassic metaluminous−peraluminous granitoids, mafic−ultramafic plutons, and Ni-Cu and Au deposits are prominent features in the Eastern Tianshan of the southern Altaids. However, the genetic relationship between coeval granitoids and mafic−ultramafic intrusions, and the geodynamics of magmatism and related mineralization, remain ambiguous. To address these ambiguities, we present petrological, geochemical, and bulk-rock Sr-Nd-Fe and zircon U-Pb-Hf isotope analyses of granitoids from the Shuangchagou Complex and gabbros from the Huangshandong Complex in the Eastern Tianshan. Zircon U-Pb ages demonstrate that the Huangshandong gabbro was emplaced at ca. 277.8 ± 1.4 Ma. In contrast, U-Pb ages determined from zircons in the granitic rocks of the Shuangchagou Complex suggest that the complex crystallized from three stages of magmatism: (1) strongly peraluminous S-type granitic magma represented by early-stage gneiss and granitic veins (ca. 289 Ma), (2) metaluminous to weakly peraluminous I-type granitic magmas represented by the intermediate-stage granitoids (ca. 283−261 Ma), and (3) late-stage granitoids (ca. 250−241 Ma). The intermediate- and late-stage granitoids (ca. 283−241 Ma) show clear enrichments in the light rare earth elements and large ion lithophile elements (e.g., Rb, Th, and U), and depletions in high field strength elements (e.g., Nb, Ta, and Ti), similar to arc magmas, which indicates that the North Tianshan oceanic plate was still subducting during the Middle Triassic. Considering the diversity of magmatic rocks (e.g., mid-oceanic-ridge−type mafic rocks, and I-, S- and A-type igneous rocks), mineralization styles (e.g., Alaskan-type Ni-Cu sulfide deposits and orogenic gold deposits), and the dextral strike-slip faults (e.g., Kanggur Fault) that occurred concurrently in the Eastern Tianshan during the Early Permian to Middle Triassic, we suggest that splitting of the subducted portion of the North Tianshan oceanic plate created a slab window that allowed the upwelling and partial melting of asthenospheric mantle to form the mafic−ultramafic intrusions and related Ni-Cu sulfide deposits. Sustained migration of magma provided the heat necessary to induce partial melting, devolatilization, and desulfurization of crustal materials, producing the Permian−Triassic, high-K to calc-alkaline I- and S-type granitoids, and associated orogenic gold deposits. By integrating the results of this study with published work regarding the Kanggur Accretionary Complex, we suggest that the subduction of the North Tianshan Ocean may have lasted until the Late Triassic.

Petrology, geochemistry and U–Pb zircon geochronology of Fe–Ti oxide ore‐bearing mafic sill from Saltora‐Mejia area of Chotanagpur Granite Gneissic Complex, eastern India: Implication for late tectonic emplacement of mafic rock of E‐MORB nature in an orogenic belt

An east–west‐trending medium‐grained mafic sill containing co‐genetic Fe–Ti oxide ore lenses is found disposed within granite gneisses around Saltora‐Mejia area in the eastern part of the Chotanagpur Granite Gneissic Complex (CGGC) of eastern India. CGGC is considered as a Proterozoic mobile belt as it witnessed multiple phases of deformation and high‐ grade metamorphism during 1.8–0.8 Ga. Occurrence of such Fe–Ti oxide ore‐bearing mafic sill is unique in the entire CGGC which is a vast Proterozoic orogenic belt and has witnessed many phases of voluminous mafic and felsic magmatisms. The mafic rock is of gabbronorite composition which contains plagioclase, clinopyroxene, orthopyroxene as major constituent primary minerals and amphibole as late magmatic mineral. The rock shows sub‐ophitic, intergranular, mosaic and poikilitic texture (defined by larger pargasitic grain). The gabbronorite shows iron enriched tholeiitic character, low Mg#, low abundances of Ni and Cr, slight enrichment in LILE, LREE and slight depletion in HFSE like Nb and Ti. The computed melt in equilibrium with the studied gabbronorite shows transitional orogenic to anorogenic, within‐plate and E‐MORB‐like geochemical character. In this study, the U–Pb zircon crystallization age (~960 Ma) of the Saltora‐Mejia gabbronorite is reported for the first time which coincides with the late tectonic stage of the most pervasive orogenic activity in the CGGC around 1.2–0.9 Ga. Transitional orogenic to anorogenic geochemical character, late tectonic evolution and other field and laboratory evidences together suggest evolution of the Saltora‐Mejia gabbronorite sill in a late tectonic extensional environment which might have been facilitated by delamination of a subducted plate and upwelling of asthenospheric mantle during the waning stage of a major orogeny in the CGGC.

Timing of Transition from Proto- to Paleo-Tethys: Evidence from the Early Devonian Bimodal Volcanics in the North Qaidam Tectonic Belt, Northern Tibetan Plateau

The transition from the Proto- to the Paleo-Tethys is still a controversial issue. This study reports a new petrology, zircon U–Pb geochronology, and whole-rock geochemistry of volcanic rocks from the Maoniushan Formation in the Nankeke area, northern Qaidam (NQ) of the Tibetan Plateau, to provide new evidence for the transition from the Proto- to the Paleo-Tethys oceans. The volcanic suite consists mainly of rhyolitic crystal lithic tuff lavas and minor basalts. Zircon U–Pb data indicate that the bimodal volcanic rocks were formed during the Early Devonian (ca. 410–409 Ma). Geochemically, the basalts have low contents of SiO2 (48.92 wt.%–51.19 wt.%) and relatively high contents of MgO (8.94 wt.%–9.99 wt.%), TiO2 (1.05 wt.%–1.29 wt.%), K2O (2.35 wt.%–4.17 wt.%), and K2O/Na2O ratios (1.04–2.56), showing the characteristics of calc-alkaline basalts. Their rare earth element (REE) patterns and trace element spider diagrams are characterized by enrichments in LREEs (LREE/HREE = 18.31–21.34) and large ion lithophile elements (LILEs; Rb, Th, and K) and depletion in high-field-strength elements (HFSEs; Nb, Ta, P, and Ti), with slight negative Eu anomalies (Eu/Eu* = 0.82–0.86), which are similar to Etendeka continental flood basalts (CFB). These features suggest that the basalts were most likely derived from low degree (1%–5%) partial melting of the asthenospheric mantle, contaminated by small volumes of continental crust. In contrast, the felsic volcanics have high SiO2 (68.41 wt.%–77.12 wt.%), variable Al2O3 (9.56 wt.%–12.62 wt.%), low MgO, and A/CNK ratios mostly between 1.08 and 1.15, defining their peraluminous and medium-K calc-alkaline signatures. Their trace element signatures show enrichments of LREEs and LILEs (e.g., Rb, Th, U, K, and Pb), depletion of HFSEs (e.g., Nb, Ti, Ta, and P), and negative Eu anomalies (Eu/Eu* = 0.22–0.66). These features suggest that the felsic volcanics were derived from partial melting of the middle crust, without interaction with mantle melts. Considering all the previous data and geochemical features, the Maoniushan Formation volcanic rocks in NQ formed in a post-collisional extensional setting associated with asthenospheric mantle upwelling and delamination in the Early Devonian. Together with the regional data, this study proposed that the Proto-Tethys Ocean had closed and evolved to the continental subduction/collision orogeny stage during the Middle to Late Ordovician, evolved to the post-collisional extensional stage in the Early Devonian, and finally formed the Zongwulong Ocean (branches of the Paleo-Tethys Ocean) in the Late Carboniferous, forming the tectonic framework of the Paleo-Tethys Archipelagic Ocean in the northern margin of the Tibetan Plateau.

Triggering of episodic back-arc extensions in the northeast Asian continental margin by deep mantle flow

Back-arc extension has been well documented in subduction plate tectonic regimes. However, the reasons why back-arc extensions are associated with some subduction systems but not others have remained elusive. Here, spatio-temporal variations in the composition of Cenozoic basalts in the northeast Asian continental margin are used to constrain the mechanism for episodic back-arc extensions. Using geochemical data sensitive to tectonic affinity, we show that typical volcanic arc compositions are located in the eastern margin of northeast Asia, whereas coeval intraplate volcanic compositions are located in the western part of northeast Asia, and that the intraplate and arc volcanism exhibit two eastward shifts, from 52 Ma to 33 Ma and from 33 Ma to 21 Ma. Intraplate basalts dated at ca. 11 Ma display a weak, arc-like geochemical signature, which suggests that the upwelling of asthenospheric mantle resulted in the remelting of previously melt-extracted lithospheric mantle modified by slab-derived fluids and the cessation of back-arc extensions. Thus, we propose that the eastward mantle flow resulted in eastward shifts of back-arc extensions that led to the development of extensive Cenozoic arc and intraplate volcanism in the northeast Asian continental margin.

Triggering of episodic back-arc extensions in the northeast Asian continental margin by deep mantle flow

Abstract Back-arc extension has been well documented in subduction plate tectonic regimes. However, the reasons why back-arc extensions are associated with some subduction systems but not others have remained elusive. Here, spatio-temporal variations in the composition of Cenozoic basalts in the northeast Asian continental margin are used to constrain the mechanism for episodic back-arc extensions. Using geochemical data sensitive to tectonic affinity, we show that typical volcanic arc compositions are located in the eastern margin of northeast Asia, whereas coeval intraplate volcanic compositions are located in the western part of northeast Asia, and that the intraplate and arc volcanism exhibit two eastward shifts, from 52 Ma to 33 Ma and from 33 Ma to 21 Ma. Intraplate basalts dated at ca. 11 Ma display a weak, arc-like geochemical signature, which suggests that the upwelling of asthenospheric mantle resulted in the remelting of previously melt-extracted lithospheric mantle modified by slabderived fluids and the cessation of back-arc extensions. Thus, we propose that the eastward mantle flow resulted in eastward shifts of back-arc extensions that led to the development of extensive Cenozoic arc and intraplate volcanism in the northeast Asian continental margin.

Sequential melting of deep crustal source rocks in a rift system: An example from southern Tibet

Granitic rocks derived by sequential partial melting are not well-documented worldwide. This paper provides another new important case in southern Tibet. The Yardoi gneiss dome within the Southern Tibet Rift System (STRS) consists of at least four suites of Miocene high-Sr/Y granites, formed at 20.4 Ma, 18.9 Ma, 17.7 Ma, and 17.1 Ma, respectively. Geochemical data indicate that they are Na-rich and peraluminous granites with high Sr/Y and La/Yb ratios. These granites show relatively radiogenic 87Sr/86Sr(t) (0.7091–0.7149) and unradiogenic εNd(t) (−11.1 to −7.7) isotopic signatures. As crystallization ages become younger, they are characterized by decreases in εNd(t) and Sr/Y but increases in 87Sr/86Sr(t) and Rb/Sr. Such temporal trends could be explained by sequential partial melting of first the mafic lower crustal rocks and then progressively shallower metasedimentary rocks. These findings together with literature-derived data, suggest that from north to south along the STRS, as extension proceeded with a heat source from the upwelling of asthenospheric mantle, sequential partial melting produced different Miocene magmatic rocks. This provides new insight into the mechanisms for the generation of geochemical and isotopic variations in spatially confined but temporally progressed granitic rocks.

Newly discovered youngest UHT metamorphism on Earth, Western Sulawesi, Indonesia

Ultrahigh-temperature (UHT) metamorphism represents an extreme crustal thermal event with peak conditions exceeding 900 °C at 7–13 kbar. In the modern-style plate tectonic system, records of the UHT metamorphism are relatively rare due to the secular cooling of Earth. In the Palu region of Western Sulawesi, we newly discovered a series of HT-UHT metamorphic rocks including amphibolite, granulite, eclogites and gneiss. Of them, two granulite samples (18CS14-2, 18CS14-4) with high garnet content (>50 mol%) are chosen for petrographic observation, phase equilibrium modelling, and zircon U-Pb dating. These rocks are characterized by a relic M1 assemblage of Grt + Ky + Bt + Rt and a M2 assemblage of Grt + Sil + Pl + Spl + Crd ± Qtz + Ilm + melt. Phase equilibrium modelling based on effective bulk compositions yields UHT conditions of 7.2–8.5 kbar/940–1080 °C (18CS14-2) and 7.0–7.3 kbar/1000–1040 °C (18CS14-4). U-Pb analysis reveals two generations of metamorphic zircon with evolving REE content that is intimately related to garnet growth and decomposition. Zircon age of 36–5.3 Ma is ascribed to syn- to post-M1 metamorphism, whereas the young zircon age of 5.1–3.8 Ma is linked to syn- and post-M2 stage. The UHT metamorphism was probably the consequence of the upwelling of asthenospheric mantle triggered by post-collisional delamination of lithosphere in the Miocene-Pliocene (ca. 5 Ma). It could represent the youngest known UHT metamorphism on Earth.

Permian ridge subduction-related magmatism in the Eastern Tianshan: Implications for the evolution of the southern Altaids

Permian large-scale transcurrent tectonics and syn -kinematic ultramafic–felsic magmatism are prominent features in the Eastern Tianshan of the southern Altaids. However, the genetic relationship between coeval Ni Cu mineralized mafic-ultramafic intrusions and felsic intrusions, and the geodynamics of the region remains equivocal. To assess these topics and improve our understanding of this geologically complex and economically significant region, zircon Hf isotope and bulk-rock geochemical data from Permian granitoids along the Kanggur–Huangshan Shear Zone (KHSZ) in the Eastern Tianshan are determined and interpreted in the context of previous geochronological–geochemical data. The Kungurian Longdong (ca. 278 Ma) and Hongshi (ca. 273 Ma) granitoids are high-K, calc-alkaline, metaluminous to weakly peraluminous I-type granitoids, whereas the Capitanian Shuangchagoubei (ca. 262 Ma) granitoids are calc-alkaline, peraluminous, adakitic I-type granitoids. All of the granitoid samples are enriched in light rare earth elements and large ion lithophile elements (e.g., Rb, U, and K), and depleted in high field strength elements (e.g., Nb, Ta, and Ti), similar to arc magmas, indicating that subduction-related magmatism was still active in the late Permian. Taking into account the regional geology, and the presence of Permian high-temperature metamorphism, adakites, I-type granites, Alaskan-type Ni Cu sulfide and orogenic Au mineralization, and EW-trending N-MORB type gabbro and mélange along the KHSZ, we suggest that subduction of a spreading center during the Permian generated a slab window, allowing the upwelling of asthenospheric mantle. This permitted the generation and migration of mantle-derived magmas to shallower crustal levels along translithospheric faults (e.g., Kanggur and Yamansu faults), forming the mafic–ultramafic intrusions and associated Ni Cu sulfide deposits. The continuous migration of magma provided the necessary heat to induce partial melting of juvenile crust, generating the Permian high-K to calc-alkaline I-type granitoids. Ridge subduction-induced juvenile crustal reworking likely played a central role in the formation of the granitoids during the late Paleozoic accretionary orogenesis of the southern Altaids. • Subduction-related magmatism was active in the Eastern Tianshan in the late Permian. • Crustal growth and reworking processes must have occurred simultaneously in the Eastern Tianshan during the Permian. • Ridge subduction-induced juvenile crustal reworking likely played an important role in the formation of the granitoids in the southern Altaids.

Early Permian Syn-Subduction Extension in the South Tianshan (NW China): Insights From A-Type Granitoids in the Southern Altaids

A-type granite is an important geodynamic indicator because it requires a high melting temperature that is commonly driven by extensional events. Here we report geochronology, whole-rock geochemistry, and zircon Lu-Hf isotopes of newly identified A-type granitic rocks from the South Tianshan in the southern Altaids. Zircon LA-ICP-MS ages indicate that the granitoids were emplaced at ca. 298–272 Ma. Geochemically, they are metaluminous to slightly peraluminous (A/CNK = 0.95–1.10), and belong to the high-K calc-alkaline to shoshonitic series. They are characterized by relatively high zircon saturation temperatures (824–875°C), K2O + Na2O contents (7.31–9.36%), high field strength elements (HFSE; Zr + Nb + Ce + Y = 365–802 ppm), and Ga/Al ratios (2.8–4.2), which all point to an A-type affinity. In addition, they have slightly enriched Hf isotope compositions (εHf(t) = −10.9 to + 0.6), and corresponding Mesoproterozoic (1,272–1759 Ma) crustal model ages, suggesting they were probably generated by partial melting of mature crust that contained minor mantle-derived magmatic material. The granitoids have distinctive subduction-related trace element signatures, with deep Nb and Ta troughs, elevated large ion lithosphere elements (LILEs), and flat HFSEs patterns, very similar to arc-derived granites in the Lachlan accretionary orogen. Integration of these new sedimentological, structural and geochronological results with relevant published information provides a new data-archive, which indicates that neither the Tarim mantle plume nor post-collisional extension can explain the genesis of these A-type granitoids. Instead, we propose a new more pertinent and robust model according to which they formed due to high temperature gradient in a subduction-related extensional setting probably triggered by southward rollback of the South Tianshan oceanic lithosphere, which caused upwelling of asthenospheric mantle combined with an increased temperature that led to large-scale crustal melting. This process gave rise to a broad magmatic arc in the southern active margin of the Yili-Central Tianshan. Our new data shed light on the retreating accretionary orogenesis of the southern Altaids in the Permian.

Tectonothermal transition from continental collision to post‐collision: Insights from eclogites overprinted in the ultrahigh‐temperature granulite facies (Yadong region, central Himalaya)

AbstractEclogites and granulites are fossil records of orogenies and provide crucial evidence of geodynamic processes. Incomplete knowledge on both hampers the establishment of detailed tectonothermal models for the central Himalaya. Utilizing detailed petrography, pseudosection modelling, mineral thermometers, and zircon petrochronology, the granulitized eclogites discovered firstly at Yadong region at the southern tip of the Yadong–Gulu Rift record five metamorphic stages. The epidote–amphibole eclogite facies metamorphism (M1) is preserved in the garnet core including the relics of omphacite. The peak eclogite facies metamorphism (M2) is represented by the garnet rim and melt‐related polymineralic inclusions. From M1 to M2, the eclogites experienced a prograde path from 1.7 GPa/620°C to 2.1 GPa/750–770°C. The clockwise pressure–temperature path, the occurrence as coherent layers in gneiss, and zircon U–Pb dating imply that the Indian crust subducted to a maximum depth of ~60 km in the central Himalaya, close to the Moho of southern Lhasa block at c. 17 Ma. Afterwards, the exhumation started with decompressional heating. In the high‐pressure granulite facies metamorphism (M3), almost all the omphacite broke down to the symplectites of diopside and plagioclase. The subsequent two‐pyroxene granulite facies metamorphism (M4) is characterized by the intergrowth of clinopyroxenes and orthopyroxenes, high‐Ti amphibole, and antiperthite. The exhumed eclogites eventually reached ultrahigh‐temperature conditions of ~0.8 GPa/950–1,000°C in M4 and were near completely transformed into mafic granulites. At the final stage, these granulitized eclogites cooled in the middle crust to amphibolite facies (M5) of 0.6 GPa/700–750°C. Given the spatio‐temporal consistencies between the granulitized eclogites, post‐collisional magmatism, and north‐trending rifts, the heat source of the ultrahigh‐temperature metamorphism was attributed to the upwelling of asthenospheric mantle due to slab tear of subducted Indian lithosphere. The granulitized eclogites from Yadong evolved from the high‐pressure eclogite to the ultrahigh‐temperature granulite facies, which recorded the transitions from collisional convergence to post‐collisional extension during their exhumation.

Controlling factors of prolonged REE mineralization in the Maoniuping REE deposit: Constraints from alkaline granite in the syenite–carbonatite complex

Previous studies have shown that the formation of Maoniuping giant rare-earth elements (REE) deposit is closely related to the protracted magmatic evolution (>2 Myr) of syenite–carbonatite magma. However, the reasons responsible for the protracted magmatic evolution of the Maoniuping deposit has not yet been well constrained. The REE-barren alkaline granite intruding the Cenozoic REE-mineralized syenite–carbonatite complex provides an opportunity to resolve this problem. In this study, zircon U–Pb geochronology, whole-rock geochemistry, and Sr–Nd–Hf isotope analyses of the alkaline granite and syenite were undertaken to elucidate the long timescales of magmatic evolution in the Maoniuping deposit. Zircon U–Pb dating indicates that the alkaline granite formed slightly later (25.5 ± 0.4 Ma) than the syenite (27.2 ± 0.6 Ma). The former is characterized by high SiO2 (72.8–75.8 wt%), Na2O + K2O (10.5–11.6 wt%), and FeOT (2.24–2.87 wt%) contents, low Al2O3 (10.3–11.8 wt%), MgO (0.08–0.20 wt%), and CaO (0.04–0.33 wt%) contents, and negative Eu, Ba, Sr, Ti, and P anomalies. It has characteristics typical of A-type granitoids, such as high 10000 Ga/Al (3.46–5.16) and FeOT/(FeOT + MgO) (0.93–0.97) ratios, and high (Zr + Nb + Ce + Y) contents (650–1065 ppm). The distinct Sr–Nd–Hf isotopic and compositional signatures of the alkaline granite and syenite indicate that the former is likely a product of direct high-temperature partial melting of shallow crust induced by the upwelling of asthenospheric mantle in a transtensional setting. Combining our results with those of previous studies, we suggest that shallow-seated, continuous underplating of mantle-derived magmas and stable regional thermal anomalies were responsible for the prolonged magmatic evolution and REE mineralization of the Maoniuping syenite–carbonatite complex.

A review of Devonian–Carboniferous magmatism in the central region of Argentina, pre-Andean margin of SW Gondwana

We review a widespread project database for Devonian–Carboniferous magmatism in the Sierras Pampeanas and Frontal Cordillera between 27° and 35°S, including petrological, geochemical, geochronological, and isotope data, with compiled data from the literature and some new results. We distinguish four main magmatic domains: 1) Devonian Arc, 2) Devonian Foreland, 3) Carboniferous Arc, and 4) Carboniferous Retro-Arc. Devonian segmented subduction led to two oceanic slab configurations: 1) flat-slab subduction in the outboard region and resubduction >800 km inland from the trench, including lithosphere delamination of the upper plate and break-off of the subducted oceanic slab, along with asthenospheric mantle upwelling (31°–33° 30′S), and 2) normal subduction (34° - 35°S). In the first configuration the arc magmatism was absent, but voluminous foreland magmatism was developed, including small-scale high silica adakite. The second geodynamic setting took place during the latest Devonian and Carboniferous when the oceanic slab roll-back occurred. Arc and retro-arc magmatism were developed coetaneously between 27° and 31°S, with northwest migration that could be explained by movement relative to hot asthenospheric mantle. The arc is represented by calc-alkaline granitoids, whereas retro-arc magmatism consisted of (a) metaluminous to weakly peraluminous A-type granites, and (b) strongly peraluminous A-type granites, these latter with sometimes incomplete isotopic homogenization of the parental magma. Devonian–Carboniferous magmatic evolution here is explained by segmented tectonic subduction and a switch-off and switch-on geodynamic model. Magmatic activity was mainly continuous from ca. 390 to 320 Ma, although compositional variations occurred through time and space (foreland, arc, and retro-arc). Major involvement of mantle sources in the genesis of the Carboniferous arc granites at 28°–30°S contrast with a dominant continental signature in the granites of the Devonian arc at 34°–35°S. These differences are explained by two different configurations in the subduction system related to advance (28°–30°S) or retreat (34°–35°S) of the subducted ocean slab. The main conclusion of this work is that the complex interaction of oceanic and continental plates can produce different types of magmatism (or its absence): subduction processes do not consist only of an oceanic plate sinking under a continental plate.

Geochemical evolution of the Paradise Mountain Caldera complex, Davis Mountains: Implications for the tectonic and magmatic evolution of Trans-Pecos Texas and adjacent Mexico

The Paradise Mountain Caldera (~20 × 14 km), located west of Fort Davis, Texas, produced two major ignimbrite/lava complexes at ~36.2 and 35.9 Ma. The older unit (Fort Davis Tuff) is composed of rheomorphic tuff and overlying chemically similar lava. The younger unit (Wild Cherry Tuff) forms thick intracaldera deposits and an outflow sheet that spread over an estimated 2000 km2. Near the caldera, the two ignimbrites are separated by trachytic lava of the Mount Locke Formation. The 5-km diameter Point of Rocks intrusion, which crops out within the eastern margin of the caldera, may have fed the Mount Locke flows. Wild Cherry Tuff is overlain by silicic lava (Casket Mountain Formation, ~35.8 Ma), with which is shares chemical similarities. Intracaldera Wild Cherry Tuff was intruded by silicic intrusions and locally intensively silicified and kaolinized. In a revision of stratigraphic nomenclature, we assign all of the above volcanic units to a Barrel Springs Group.Collectively, the above units form a series ranging from trachyte to alkali rhyolite. Glomeroporphyritic trachyte of Mount Locke lavas represents silicic melt incompletely segregated from more mafic material within a source magma chamber. Variation between Mount Locke Formation trachyte and Fort Davis Tuff rhyolite can be modelled by fractionation of a mineral assemblage of plagioclase, clinopyroxene, FeTi oxides, and apatite; variation within the two major tuff units can be modelled almost exclusively by alkali feldspar. Central eruption within the caldera produced intracaldera tuff more evolved than most of its outflow. Alkali rhyolite of the Wild Cherry Tuff and Casket Mountain lavas, although similar in major element chemistry and mineralogy, show wide variation in incompatible trace elements, suggesting that evolution of silicic magmas in terms of major element chemistry had reached an impasse due to fractionation of a mineral assemblage whose bulk composition was similar to that of parental rhyolite.Paradise Mountain units represent a transition from alkalic magmatism in northeastern Trans-Pecos Texas to metaluminous to peraluminous magmatism in southwestern Trans-Pecos Texas and adjacent Mexico. Over a span of 10 Ma, silicic magmatism terminated southwestward over this region at a rate of about 24 km/Ma. This termination may reflect continued foundering of a sinking Farallon slab and associated asthenospheric mantle upwelling. Mafic magma derived by melting of lithospheric mantle formed plutons that, through filter pressing of silicic magma from crystalline mush, generated silicic melts. The transition across the Trans Pecos region from alkalic to metaluminous magmatism may have resulted through preferential modification of lithospheric mantle along the easternmost extent of Laramide subduction.

Sr-Nd-Pb-Ca Isotopes of Holocene Basalts from Jingpohu, NE China: Implications for the Origin of Their Enriched Mantle Signatures

The geochemistry on Holocene lavas from the Jingpohu volcanic field in NE China are compared with other Cenozoic lavas from across the back-arc rift of NE China, in order to constrain their enriched mantle sources. Holocene lavas within Jingpohu volcanic field comprise two separate “Crater Forest” (CF) and “Frog Pool” (FP) volcanic areas. FP lavas have lower MgO, CaO, and heavy rare earth elements and higher Al2O3, Na2O, K2O, and large-ion lithophile elements than CF lavas. Yet, both CF and FP lavas share similar isotopic signatures, with depleted Sr and Nd isotopes (87Sr/86Sr = 0.703915–0.704556, 143Nd/144Nd = 0.512656–0.512849) and unradiogenic Pb isotopes (208Pb/204Pb = 37.79–38.06, 207Pb/204Pb = 15.45–15.54, 206Pb/204Pb = 17.49–18.15), similar to oceanic island basalts. An important new constraint for the Jingpohu lavas lies in their Ca isotopes of δ44/40Ca from 0.63‰ to 0.77‰, which are lower than that of the bulk silicate earth (0.94 ± 0.05‰). By comparing the isotopic signatures of sodic lavas with that of the potassic lavas across NE China, we propose a three-component mixing model as the source for the sodic lavas. In consistence with geophysical results, we propose that subducting Pacific plate induces asthenospheric mantle upwelling of an upper depleted mantle (DM), including subducted ancient sediments (EM I), which partially melted upon ascent. These primary melts further interacted with the lithospheric mantle (EM II), before differentiating within crustal magma chambers and erupting.

Magnesium isotope constraints on contributions of recycled oceanic crust and lithospheric mantle to generation of intraplate basalts in a big mantle wedge

Despite numerous studies, the origin of intraplate basalts from eastern Asia is still elusive. This includes the relative importance of subducted oceanic crust and metasomatized lithospheric mantle in generation of these lavas. To address this important issue, we have carried out an integrated Mg-Sr-Nd isotope study of Jiaohe garnet pyroxenite xenoliths and Cenozoic basalts from the Changbaishan-baoqing volcanic belt (CVB), NE China. The Jiaohe garnet pyroxenite xenoliths exhibit low Hf/Hf* ((HfN/(SmN × NdN)0.5, 0.7–0.9), depleted SrNd isotopic compositions (87Sr/86Sri = 0.7035–0.7041; εNd = +2.9 − +8.6) and low δ26Mg (−0.61 to −0.39‰). These features reflect both protolith heterogeneity and the effect of isotopic exchange with carbonates during subduction. The CVB basalts display variable Mg isotopic compositions. Low-Si basalts are characterized by low δ26Mg (−0.27 to −0.44‰), variable Hf/Hf* (0.7–1.4) and depleted SrNd isotopic compositions (87Sr/86Sri = 0.7039–0.7050; εNd = +1.0 − +4.7), which were attributed to partial melting of upwelling asthenospheric mantle containing recycled oceanic crustal components. In contrast, high-Si basalts display low Hf/Hf* (0.6–0.9), EM1-like SrNd isotopic compositions (87Sr/86Sri = 0.7047–0.7054; εNd = −2.4 - -0.2) and normal mantle-like δ26Mg values (−0.18 to −0.33‰). Such features can be explained by participation of metasomatized lithospheric mantle in their source. The transition from low-Si melts with low-δ26Mg to high-Si melts with normal mantle-like δ26Mg for the CVB basalts are genetically related to continental rifting, asthenospheric mantle upwelling and mechanical-chemical erosion of basal metasomatized lithosphere induced by subduction of the Pacific oceanic slab. Recycled oceanic crust and basal lithospheric mantle both played a fundamental role in generation of the intraplate basalts in eastern Asia.

Ore‐forming mafic plutons emplaced at syn‐collisional compressive setting in Kalatongke Ni–Cu sulphide district, Southern Altay, CAOB: New evidence from the Late Carboniferous granitic porphyries

The Kalatongke deposit is the largest Ni–Cu sulphide deposit in the Central Asian Orogenic Belt (CAOB) in northern Xinjiang. The tectonic setting of the Kalatongke deposit and the collision period of the Altai and East Junggar terranes remain controversial. This study involves petrological, geochemical, and geochronological evaluations of granitic porphyries in the Kalatongke. The granitic porphyries are divided into three types according to the characteristics of petrology and geochemistry, dated at ~339, ~325, and ~310 Ma. Type‐1 granitic porphyry exhibits evident syn‐collisional characteristics, with plagioclase phenocrysts extending in one direction. This type is metaluminous (A/CNK &lt;1.1), characterized by low SiO2 and K2O contents, and depleted in Nb, Ta, P, and Ti. Type‐2 granitic porphyry is strongly peraluminous (A/CNK &gt;1.1) and characterized by high SiO2 and Al2O3 but low Sr contents. Type‐3 granitic porphyry is also metaluminous (A/CNK &lt;1.1) and has high SiO2 and K2O contents. The estimated crustal thickness of the Kalatongke area increased from 40 to 55 km from 339 to 310 Ma, thereby indicating that its existence within the collisional orogeny period. According to the estimated emplacement depth of the granitic and mafic intrusions, the crust had been thickening since then. During the late collision period, lithospheric detachment caused by terrane collision resulted in asthenospheric mantle upwelling and formed regional mafic intrusions along the Irtysh fault.