Slab Break-off Model Research Articles

Slab break-off of the Kalamaili oceanic slab revealed by the latest Carboniferous mafic–ultramafic rocks in eastern North Tianshan (NW China)

Identifying the tectonic transition from oceanic subduction to collision is crucial for tracking the final stage evolution of ancient orogenic belts. In this study, we present new geochronological and geochemical data for the Mozbaysay mafic–ultramafic complex in the Balikun area, eastern North Tianshan of the southern Central Asian Orogenic Belt. This complex had intruded into the late Carboniferous volcano-sedimentary rocks and is comprised mainly of hornblende gabbro and lherzolite. Zircon U-Pb ages of the hornblende-gabbros reveal that this complex was emplaced at ca. 305 Ma. Geochemical analyses suggest these mafic–ultramafic rocks are characterized by slight enrichment of light rare earth elements (LREEs) and relatively depleted heavy rare earth elements (HREEs), resembling enriched mid-ocean ridge basalt (E-MORB). They also exhibit restricted (87Sr/86Sr)i ratios (0.702396–0.704295) and εNd(t) values (+7.0 to +9.1), indicative of a depleted mantle source with minimal crustal contamination. Incompatible element ratios (i.e., Nb/Ta, Zr/Hf, Rb/Nb, and Ba/Nb) suggest the involvement of subducted slab-derived aqueous fluids in their mantle source. These collectively indicate that the parental magmas of the Mozbaysay mafic–ultramafic rocks may have been generated by a mixed mantle source consisting of the E-MORB-like asthenospheric mantle, subcontinental lithospheric mantle (SCLM), and hydrous fluids from subducted slab. Furthermore, a slab break-off model is proposed to explain the generation of these latest Carboniferous mafic–ultramafic rocks. Integrating these findings with regional geological data, we propose that the tectonic transition from subduction (slab roll-back) to collision (slab break-off) along the Kalamaili suture zone occurred at ca. 305–300 Ma.

Pervasive Neoarchean melting of subducted sediments generating sanukitoid and associated magmatism in the North China Craton, with implications for the operation of plate tectonics

Abstract The Mesoarchean to Neoarchean period (ca. 3.0–2.5 Ga) is the most important stage during the emergence and evolution of plate tectonics. However, plate subduction at this time may have been less stable and perhaps more susceptible to the lubrication effect of sediments than the modern counterpart. Such predictions have not yet been verified by field-based investigations. In this work, we identified two types of rock units (i.e., sanukitoids and associated adakitic suites, exposed in the Eastern Hebei Complex of the North China Craton) and illustrated their petrogenesis and tectonic context through field, geochronologic, geochemical, and isotopic investigations. Laser ablation–inductively coupled plasma–mass spectrometry zircon U-Pb analyses suggest that the two magmatic suites formed within a relatively short time span of ca. 2596–2544 Ma and ca. 2559–2533 Ma, respectively. The sanukitoids are composed of meta-andesites and diorite porphyrites and characterized by relatively high MgO (3.94–5.62 wt%), Mg# (50–61), Cr (73–343 ppm), and Ni (37–111 ppm) values. The adakitic rocks are composed of granodiorite-granite gneisses and have relatively high Sr (316–1001 ppm) and low Y (7–13 ppm) and Yb (0.83–1.37 ppm) contents, and relatively high Sr/Y (36–89) and La/Yb (16–45) ratios. Rocks from both suites exhibit depletions of Nb, Ta, and Ti and have similar Sr-Nd-Hf-Zn isotopes: variable (87Sr/86Sr)i (0.7002–0.7053), weakly positive εNd(t) (+0.3 to +1.7) and εHf(t) (+1.8 to +6.8), and slightly heavy δ66Zn (0.30‰–0.36‰). These geochemical characteristics indicate that the sanukitoids were derived from the melting of subducted sediments followed by melt-mantle interaction, whereas the adakitic rocks were produced by direct partial melting of subducted plate (including tonalite-trondjhemite-granodiorite melts) under a garnet stability field with minor sediments. Such distinct magmatic rock associations, together with the coeval charnockites and tholeiites with diverse compositions in the adjacent area, can be best explained by a slab breakoff model. Further, events associated with slab breakoff are likely to represent a transition of a quasi-plate tectonic regime, characterized by multiple, continuous, and stagnant attempts to start the modern-style subduction on Earth. In addition, the emergence of sanukitoids and associated magmas symbolized the onset of supracrustal recycling into the mantle. Combined with the Nd-Hf-Zn isotopes of diverse magmatic rocks in the North China Craton that are comparable to other Precambrian magmatic rock suites worldwide, we suggest that supracrustal recycling symbolized the onset of plate tectonics since ca. 3.0 Ga, and by inference played a key role in the development of subduction-driven plate tectonics in addition to Earth’s secular cooling.

Tectonic evolution of the Middle-Late Permian orogenic belt in the eastern part of the CAOB: Implications from the magmatism in the Changchun-Kaiyuan area

AbstractVarious magmatisms during the subduction-collision process are crucial to reveal the long-term tectonic evolution of the eastern Central Asian Orogenic Belt. In this paper, we present major and trace elements of whole-rock, zircon U-Pb dating and Hf isotope of the Shanmen pluton. Results imply that the Shanmen pluton consists of quartz diorite and mylonitic granite, with zircon U-Pb ages of 263.7–259.6 Ma. The studied quartz diorite contains high Sr/Y (51.19–90.87) and (La/Yb)N (7.82–13.62) ratios, and belongs to adakitic rocks. Coupled with the positive εHf(t) values of +5.71 to +12.8 with no obvious Eu anomaly, we propose that quartz diorite is the product of the interaction between different degrees of slab melt and the overlying mantle wedge. In contrast, the mylonitic granite has lower MgO (0.28 wt% – 0.47 wt%) contents and positive εHf(t) values of +7.79 to +10.15, indicating an affinity with I-type granite originated by partial melting of the intermediate-basic lower crust. The geochemical characteristics and lithological assemblages, along with the Permian magmatic rocks in the Changchun-Kaiyuan area displaying arc rocks affinity, propose their formation is related to the southward subduction of the Paleo-Asian Ocean (PAO). Based on this study and previous evidence, we lean towards adopting a middle-late Permian slab break-off model, wherein the PAO did not close until the late Permian.

Coexisting Early Cretaceous arc-type and OIB-type mafic magmatic rocks in the eastern Jiangnan Orogen, South China Block: Implications for paleo-Pacific plate subduction

Mafic magmatic rocks can provide important information about the nature of mantle source, material recycling, and mechanism of crust–mantle interaction. However, owing to the scarcity of such rocks, the nature and evolution of the Late Mesozoic mantle source beneath southeastern China have not been well constrained. Here, we present data on zircon UPb ages, whole-rock major and trace elements, and Sr–Nd–Pb isotopes for two types of simultaneous mafic intrusive rocks from the eastern Jiangnan Orogen (EJNO), with the aim of providing insights into their petrogenesis and the relationship of extensive Late Mesozoic magmatism in southeastern China and subduction of the paleo-Pacific plate. The Qukou diabases (~ 128 Ma) show a subalkaline affinity and have a mean Mg# value of 53.4. They exhibit arc-like trace-element distribution patterns, namely, enrichment in light rare earth elements (LREEs), Ba, Th, and U, and depletion in Nb, Ta, P, and Ti, as well as moderately radiogenic Sr and chondritic Nd isotopic compositions, with 87Sr/86Sr(i) = 0.7057–0.7063 and εNd(t) = −1.01 to 1.13. In contrast, the Yangboping gabbros (~ 125 Ma) belong to alkaline basalt and have a mean Mg# value of 56.8. They present OIB-like trace-element characteristics, show enrichment in LREEs, Sr, and Ba, and no depletion in Nb, Ta, Zr, Hf, and Ti, and possess highly radiogenic Sr and weakly nonradiogenic Nd isotopes of 87Sr/86Sr(i) = 0.7096–0.7114 and εNd(t) = −0.97 to −0.11. These geochemical features suggest that the Qukou diabases were generated by partial melting of a pyroxenitic source region, and this pyroxenitic mantle source was formed via the metasomatism by HFSE-depleted hydrous melts of subducted slab-derived terrigenous sediments, whereas the Yangboping gabbros were generated from a metasomatite that formed by metasomatic reaction between depleted MORB mantle and subducted slab-derived HFSE-enriched hydrous melts, in which the ratios of altered oceanic crust-derived melts to sediment-derived melts were 90:10–60:40. The differences in trace-element and isotopic compositions of these two types of mafic magmatic rocks suggest that their mantle sources were metasomatized by hydrous melts from different depths: the HFSE-depleted hydrous melts were from the shallow depths (<80 km), whereas the HFSE-enriched melts were from the greater depths (>200 km), indicating that the subducted oceanic slab would have founded/broken off to provide enough heat to melt the subducted oceanic crust. Consequently, we propose that the flat-slab subduction and slab break-off model of the paleo-Pacific plate was the most likely geodynamic mechanism for Late Mesozoic magmatism in the South China Block.

Geochemistry, zircon U–Pb geochronology, and Hf isotopes of the metavolcanic rocks in the Tongbai orogen of central China: Implication for Neoproterozoic oceanic subduction to slab break-off

Abundant metavolcanic rocks (leptynite, leptite, and amphibolite) occur in the Tongbai orogen, central China, but their age, origin, and tectonic significance are poorly constrained. Here we report geochemistry, zircon U–Pb ages, and Hf isotopes of these metavolcanic rocks to determine their petrogenesis and geodynamic implications. Our data indicate that all the leptynites, leptites, and amphibolites were initially formed at 760–720 Ma and then underwent later metamorphism. These rocks exhibit arc-related geochemical compositions characterized by enriched large ion lithophile elements and light rare earth elements and depleted heavy rare earth elements. Leptynites and leptites present similar characteristics, including high SiO2 (72.14–77.24 wt%), MgO (0.19–0.44 wt%), and Sr (239–827 ppm) compositions and low Y (6.9–16 ppm) and Yb (0.70–1.17 ppm), along with high Sr/Y and (La/Yb)N values. The leptynites exhibit no or slightly negative Eu anomalies in both the whole-rock and zircon. Based on the high K2O (3.75–4.31 wt%) and slightly lower Al2O3 (<15 wt%) contents, as well as high ε(Hf)t values (−0.53 to 5.80), the leptynites conform to C-type adakites and were possibly derived from partial melting of the thickened juvenile mafic lower crust. The leptites demonstrate a distinct negative Eu anomaly and low εHf(t) values of −13.19 to −6.60, implying an ancient middle–lower crustal source. In contrast, the amphibolites exhibit lower SiO2 (52.62–53.33 wt%) and higher MgO (3.88–4.32 wt%) contents, as well as higher Y (19.5–30.0 ppm) and Yb (1.89–3.26 ppm) concentrations, indicating of island arc volcanic rocks. Furthermore, the higher Mg# (31–35), Cr (14–18 ppm), V (200–272 ppm), and Ni (37–59 ppm) contents of the amphibolites indicate more substantial amounts of mantle components in their magma source. The variable zircon εHf(t) values from −10.1 to 1.95 in the amphibolites suggest an enriched mantle source. Based on these new data, we propose that the Tongbai region underwent a continuous oceanic subduction before 750 Ma, followed by a subducted slab break-off during 750–720 Ma, which is inconsistent with the previously proposed single slab break-off model for the South Qinling region

P‐wave Tomography and Azimuthal Anisotropy of the Manila‐Taiwan‐Southern Ryukyu Region

AbstractTaiwan is considered to be created by interactions between the Manila and Ryukyu subduction zones, which form a normal double‐slab subduction system. Because of the two opposite dipping subductions of the Eurasian Plate (EP) and the Philippine Sea Plate (PSP) around Taiwan, subduction polarity reversal beneath northern Taiwan is presumed, which is accounted for by a slab breakoff model. However, this model has not been convincingly supported by tomographic results so far. Moreover, details of the dynamic process and deformation in this normal double‐slab subduction system are still unclear. In this work, we determine high‐resolution 3‐D tomographic images of isotropic P‐wave velocity and azimuthal anisotropy beneath the Manila‐Taiwan‐southern Ryukyu region, which provide substantial observational constraints on this double‐slab subduction system. Our results clearly show the subducted EP breakoff beneath northern Taiwan. We propose two possible factors causing the slab breakoff: the subducted EP rollback under its own gravity after its subduction cessation and collision with the subducted PSP, which may jointly cause the slab breakoff beneath northern Taiwan after a longer collision history. In addition, the subducted EP rollback may be responsible for magmatism and basalts around Taiwan, as well as recent big earthquakes beneath the Taiwan Strait and the South China Sea (SCS). Our results also reveal anisotropy parallel with the SCS fossil ridge beneath the northernmost Luzon Island. We suggest that the fossil ridge and buoyant plateau subduction and/or the ridge‐parallel mantle flow during the SCS seafloor spreading may result in the ridge‐parallel anisotropy.

Silurian-Devonian granites and associated intermediate-mafic rocks along the eastern Kunlun Orogen, western China: Evidence for a prolonged post-collisional lithospheric extension

The eastern Kunlun Orogen hosts numerous granitoid intrusions and associated intermediate-mafic bodies, that bear significant information on the tectonic evolution of this orogenic belt and the genesis of many ore deposits (e.g., Xiarihamu NiCo sulfide deposit), but the timing and petrogenesis of these rocks remain not well understood. In this contribution, the petrogenesis of representative Silurian-Devonian granites and associated intermediate-mafic rocks at Wulonggou, Xiarihamu and Kayakedengtage has been investigated. Zircon UPb dating results reveal that these intrusions were emplaced over a period of ~50 million years (m.y.) from 427 Ma to 382 Ma. Syenogranites at Wulonggou and Xiarihamu as well as quartz monzonites and porphyritic monzogranites at Kayakedengtage have characters of metaluminous A2-type granites. Whole-rock SrNd isotope data indicate that Syenogranites at Wulonggou (427–414 Ma) and Xiarihamu (396–391 Ma) were probably derived from earlier calc-alkaline I-type diorite-granodiorite, whereas quartz monzonites (ca. 424 Ma) and porphyritic monzogranites (ca. 421 Ma) at Kayakedengtage were most likely produced by partial melting of Early Paleozoic granulites. Monzogranites (394 Ma) and granodiorites (397 Ma) at Kayakedengtage and dolerites (426 Ma) from Wulonggou are characterized by positive εNd(t) values, indicative of a depleted mantle origin. The formation of voluminous A2-type granites with mafic rocks from the asthenosphere since 427 Ma occurred immediately after the subduction, supporting a post-collisional setting and a slab break-off model. The extensive magmatism coupled with asthenospheric upwelling and rapid orogenic uplift suggests two large-scale extension events at 427–414 Ma and 403–380 Ma in the orogen. The 411–406 Ma Xiarihamu NiCo sulfide deposit formed during relatively weak lithospheric extension and asthenospheric upwelling between these two extensional events. These results together with 373–357 Ma metaluminous A2-type syenogranites and I-type granodiorites at Qimantagh suggest a prolonged (~70 m.y.) and episodic post-collisional extension in the orogen, which was controlled by cyclic asthenospheric upwelling.

Geochemistry, geochronology and petrogenesis of Maya Block granitoids and dykes from the Chicxulub Impact Crater, Gulf of México: Implications for the assembly of Pangea

The Late Paleozoic tectono–magmatic history and basement of the Maya block are poorly understood due to the lack of exposures of coeval magmatic rocks in the region. Recently, IODP–ICDP Expedition 364 recovered drill core samples at borehole M0077A from the peak ring of the Chicxulub impact crater, offshore of the Yucatán peninsula in the Gulf of México, have been studied comprehensively. In the lowermost ~600 m of the drill core, impact–deformed granitoids, and minor felsite and dolerite dykes are intercalated with impact melts and breccias. Zircon U-Pb dating of granitoids yielded ages of around 326 ± 5 Ma, representing the first recovery of Late Paleozoic magmatic rocks from the Maya block, which could be genetically related to the convergence of Laurentia and Gondwana. The granitoids show the features of high K2O/Na2O, LaN/YbN and Sr/Y ratios, but very low Yb and Y contents, indicating an adakitic affinity. They are also characterized by slightly positive ԑNd(326Ma) of 0.17–0.68, intermediate initial 87Sr/86Sr(326Ma) of 0.7036–0.7047 and two–stage Nd model age (TDM2) of 1027–1069 Ma, which may indicate a less evolved crustal source. Thus, the adakitic granitoids were probably generated by partial melting of thickened crust, with source components similar to Neoproterozoic metagabbro in the Carolina block (Pan–African Orogeny materials) along Peri–Gondwana. Felsite dykes are shoshonitic with typical continental arc features that are sourced from a metasomatic mantle wedge by slab–fluids. Dolerite dykes display OIB–type features such as positive Nb and Ta anomalies and low ThNpm/NbNpm. In our interpretation, the Chicxulub adakitic granitoids of this study are formed by crustal anatexis due to asthenospheric upwelling resulting from slab breakoff. Through comparing sources and processes of Late Paleozoic magmatism along the Peri–Gondwanan realm, a tearing slab breakoff model may explain the discontinuous magmatism that appears to have occurred during the convergence of Laurentia and Gondwana.

Geochronology and petrogenesis of the Xiongmei Cu-bearing granodiorite porphyry in North Lhasa subterrane, central Tibet: Implication for the evolution of Bangong-Nujiang metallogenic belt

Bangong-Nujiang metallogenic belt is a recently recognized post-subduction porphyry-skarn Cu polymetallic metallogenic belt in Tibet. The Xiongmei Cu-bearing granodiorite porphyry was newly found in the middle segment of this belt. In this study, we combined zircon U-Pb ages, Hf isotopic, REEs data, whole-rock Sr-Nd isotopic data, and whole-rock geochemical data to constrain the geochronology, petrogenesis and metallogenic features of the Xiongmei Cu-bearing granodiorite porphyry. Zircon U-Pb and whole rock Rb-Sr isochronal age show Xiongmei Cu-bearing granodiorite porphyry formed during 110–105 Ma. The zircon Hf isotope together with whole rock Sr-Nd isotopic features and high Mg# value (39–59), relatively high Cr (10.9–35.5 ppm) and Ni (10–224 ppm), indicating a melt deriving from mixing between mantle material and an ancient crustal source. The possibility of crustal addition in the magma is further supported by varied Eu anomalies (0.5–1.5), Sr anomalies (0.6–1.9) and high Al2O3 contents (15.50–17.25 wt%). Geochemical composition including K2O contents (0.52–4.72 wt%) and K2O/Na2O ratios (0.2–5.5), and the linear relations between LOI, Cu, Rb and Sr contents indicated that the magma sourced from melting of an amphibolite source, and further suggesting that the juvenile crust derived from an enriched lithospheric mantle. The ratios between incompatible elements together with the absence of Ce anomaly suggested the contribution of fluids from subducted oceanic crust rather than melt. The negative Eu and Sr anomaly features together with relatively high HREEs and Y contents, low (La/Yb)N and Sr/Y ratios are interpreted as low pressure partial melting. Combination with the regional geologic background, we adopted a slab break-off model to explains the diverse contemporaneous magmatic rocks recognized in the North Lhasa subterrane including that of the Xiongmei Cu-bearing granodiorite porphyry.

Late Carboniferous to Early Permian adakitic rocks and fractionated I‐type granites in the southern West Junggar terrane, NW China: Implications for the final closure of the Junggar–Balkhash Ocean

The final closure of the Junggar–Balkhash Ocean resulted in the amalgamation of the circum‐Balkash–Junggar segments of the south‐western Central Asian Orogenic Belt, but the exact timing of its closure remains uncertain. Here, we use zircon U–Pb and whole‐rock chemical data for the adakitic rocks and fractionated I‐type granitoids in the Alashankou area of the southern West Junggar terrane (WJT) to investigate their petrogenesis and tectonic implications for the closure of the Junggar–Balkhash Ocean. The adakitic rocks were emplaced at 313–295 Ma and exhibit high Sr/Y ratios of 121–229. Their low MgO contents (0.91–2.09 wt%), Co, Cr, and Ni concentrations, initial 87Sr/86Sr ratios, and high εNd(t) values, indicate an origin from partial melting of juvenile lower crust under garnet‐amphibolite‐facies conditions triggered by basaltic underplating. By comparison, the fractionated I‐type granitic plutons (283–273 Ma) are weakly peraluminous and distinctive in terms of their low MgO contents (0.31–0.39 wt%) and strongly negative Nb, Ta, P, Eu, and Ti anomalies. These features, along with their higher initial 87Sr/86Sr ratios and lower εNd(t) values, suggest derivation by partial melting of a depleted mantle source and/or of newly underplated lower crust with crustal contamination. Given the Late Carboniferous depositional hiatus and post‐orogenic strike‐slip faulting and absence of post‐Devonian ophiolites and post‐Palaeozoic magmatic events in the WJT, the widespread occurrence of Late Carboniferous to Permian compositionally diverse granitoids (including the adakitic rocks, high‐Mg dioritic rocks, and I‐ and A‐type granitoids) and coeval mafic dykes can be best explained by a post‐collisional slab breakoff model. Together with other lines of geological evidence in the circum‐Balkash–Junggar area, we propose that the Junggar–Balkhash Ocean should have been closed at the beginning of the Late Carboniferous (~320 Ma), consistent with the closure time (~325–300 Ma) for regional major ocean basins.

The Dynamic Sulfide Saturation Process and a Possible Slab Break-off Model for the Giant Xiarihamu Magmatic Nickel Ore Deposit in the East Kunlun Orogenic Belt, Northern Qinghai-Tibet Plateau, China

The Dynamic Sulfide Saturation Process and a Possible Slab Break-off Model for the Giant Xiarihamu Magmatic Nickel Ore Deposit in the East Kunlun Orogenic Belt, Northern Qinghai-Tibet Plateau, China

Petrogenesis of Late Paleozoic diorites and A-type granites in the central Eastern Tianshan, NW China: Response to post-collisional extension triggered by slab breakoff

Geological evolution of the Eastern Tianshan in the Late Paleozoic is significant to understanding the multiple accretions and final formation of the southern Central Asian Orogenic Belt. Here, we report new whole-rock geochemical and Sr–Nd isotopic data, and in situ zircon U–Pb ages for three typical A-type granitic intrusions (syenogranite, K-feldspar granite and monzonitic granite) and one dioritic pluton in the central Eastern Tianshan, NW China, in order to constrain their petrogenesis, tectonic setting and geodynamic evolution. Zircon LA-ICP-MS U–Pb ages indicate that the syenogranites and K-feldspar granites were both formed during a short time interval in the Early Permian (284–286 Ma), whereas the diorites were crystallized at 293 ± 6 Ma. The monzonitic granites were generated at 307 ± 3 Ma, earlier than the other two granitic intrusions. The three kinds of granites (syenogranite, K-feldspar granite and monzonitic granite) have high HFSE, HREE and Y/Nb ratios (1.68–2.84), and remarkably low Sr and Ba contents, with the absence of aluminium-rich minerals and show metaluminous to weakly peraluminous natures, indicating geochemical characteristics of aluminous A2-type granites. These granites show negative εNd(t) (−3.32 to −4.40) values and Mesoproterozoic two stage Nd model ages (TDM2 = 1.36 to 1.43 Ga), which suggest a relatively old crustal source for these rocks. The Sr–Nd isotopic compositions imply that the Mesoproterozoic metamorphic basement of the Central Tianshan Block can be considered as a potential magmatic source for these aluminous A2-type granites. In addition, they have geochemical characteristics similar to the experimental melt that was derived from partial melting of crustal metaigneous rocks at the depth of middle to lower crust levels. Therefore, the granites were most likely produced by partial melting of metaigneous rocks in a post-collisional extensional setting. The diorites have low SiO2 and high MgO, with uniform (87Sr/86Sr)i values (~0.7055), negative εNd(t) values (−4.11 to −4.59) and Mesoproterozoic two stage Nd model ages (TDM2 = 1.41 to 1.45 Ga). They have higher Ti/Zr (42.8–46.7) and Ti/Y (280–359) ratios than continental crust and show negative Nb-Ta-Ti anomalies, which can be explained by partial melting of a relatively fertile mantle modified by old crustal materials of the Central Tianshan Block. The formation of the dioritic pluton was induced by an upwelling asthenosphere in an extensional geodynamic setting. Based on the zircon saturation temperatures of these A-type granites and the distribution of Late Carboniferous to Early Permian mantle-derived intrusions, a slab breakoff model is suggested to interpret the formation of the studied A-type granites and diorites. We infer that the slab breakoff was initiated in the Late Carboniferous and ended in the Early Permian.

A slab break-off model for the submarine volcanic-hosted iron mineralization in the Chinese Western Tianshan: Insights from Paleozoic subduction-related to post-collisional magmatism

The distinct submarine volcanic-hosted iron ore deposits in the Awulale Iron Metallogenic Belt are the most important sources for iron in the Chinese Western Tianshan metallogenetic province, although, the exact tectonic setting for their formation remains poorly constrained. A compilation of published and new geochronological and geochemical data suggests a protracted magmatism from ca. 450 to 290 Ma was in close spatial and temporal proximity to the iron deposits. The different magmatic rocks have broadly similar arc-like geochemical signatures such as LREE and LILE enrichments, HREE and HFSE depletions, Nb–Ta–Ti negative anomalies and flat HREE patterns, all of which are attributed to magmatic differentiation and/or partial melting in relatively thin crust due to long-term extension. The contrasting whole-rock Nd isotopic patterns of the igneous rocks and the well-defined peak metamorphism (ca. 320–310 Ma) of the subduction-zone high-pressure rocks indicate that ca. 450–321 Ma magmatic rocks with an increasingly positive εNd(t) shift were formed during the northward subduction of the South Tianshan Ocean, where slab rollback resulted in an extensional setting in the southern Yili-Central Tianshan Block. In contrast, the synchronicity of the ca. 320–310 Ma subduction-zone peak metamorphism and magmatic flare-up with uniformly positive εNd(t) values is interpreted as the consequence of slab break-off subsequent to the oceanic subduction and the continental collision. By inference, the late-stage (ca. 309–290 Ma) magmatism was likely constructed in a post-collisional setting. The submarine volcanic-hosted iron mineralization exclusively occurred coeval with the peak metamorphism and intensive magmatism, probably suggesting slab break-off as the exact tectonic setting for iron mineralization. Consequently, the western extension of the Awulale Iron Metallogenic Belt occurring roughly parallel to the South Tianshan Orogen may have significant potential for prospecting the submarine volcanic-hosted iron ore deposits.

Slab Breakoff of the Neo‐Tethys Ocean in the Lhasa Terrane Inferred From Contemporaneous Melting of the Mantle and Crust

AbstractOceanic slab breakoff significantly affects the thermal regime of the lithosphere during continental collision. This often triggers extension‐related mafic magmatism and crustal melting. It is generally accepted that the Neo‐Tethyan lithosphere subducted beneath the southern Lhasa Subterrane, resulting in the formation of the Gangdese magmatic arc. However, the timing of slab breakoff is still disputed, due to a lack of evidence for extension‐related mafic magmatism. In this study, we provide comprehensive age, element and Sr–Nd–Hf isotopic data of mafic dikes, felsic intrusions, and enclaves from the Daju area, southern Lhasa Subterrane. The timing of mafic dikes and granitoids are contemporaneous at circa 57 Ma. The mafic dikes are characterized by high Th/U, and Zr/Y ratios, their geochemistry indicates an intraplate affinity rather than arc magmas. Furthermore, the mafic dikes show strongly variable igneous zircon ɛHf(t), and lower whole‐rock ɛNd(t) than granitoids. This evidence suggests that the mafic dikes represent asthenosphere‐derived melts contaminated by various degrees of ancient lithosphere. However, the granitoids were directly derived from the juvenile lower crust. Given the abrupt decrease in the convergence rate between India and Asia, and the surface uplift and sedimentation cessation in the southern Lhasa Subterrane in the early Cenozoic, the occurrence of synchronous mafic dikes and granitoids is best explained by a slab breakoff model. The occurrence of intraplate‐type magmas likely corresponds to the magmatic expression of the initial stage of Neo‐Tethyan slab breakoff. The slab breakoff concept also explains the onset of the magmatic “flare‐up” and crustal growth after 57 Ma.

Ordovician and Triassic mafic dykes in the Wudang terrane: Evidence for opening and closure of the South Qinling ocean basin, central China

We report zircon ages and geochemical composition for mafic dykes that intruded Neoproterozoic volcanic–sedimentary sequences in the southern part of Wudang area, South Qinling. The results indicate that the dykes were emplaced during the Early Paleozoic (c. 460Ma) and Early Mesozoic (c. 220Ma). The dykes share similar major element composition, but have distinctive trace element pattern and Sr–Nd–Pb isotope distribution. Early Paleozoic mafic dykes are characterized by enrichment in LREEs, LILEs and HFSEs and EM II-type isotopic features. These geochemical features suggest derivation from an OIB-type mantle source that had undergone metasomatism during earlier subduction events. The Early Mesozoic mafic dykes can be subdivided into two distinct geochemical groups. Dykes of Group 1 are depleted in LREEs, LILEs and HFSEs and show depleted isotope compositions, indicating an origin by partial melting of asthenospheric mantle material. Dykes of Group 2 have high Rb-, Ba-, and K-contents and EM I-type isotopic features, suggesting input of lower crustal material to the magma source during Mesozoic subduction. We propose that the Early Paleozoic dykes are related to the opening of an oceanic basin separating South Qinling from the Yangtze Block, while the Early Mesozoic dykes were derived from partial melting of up-welling asthenosphere during the final amalgamation of these two blocks in the Early Mesozoic. A slab break-off model could explain not only the petrogenesis of the Mesozoic mafic dykes, but also the distinct geological features between the Dabie–Sulu and South Qinling orogens. We propose that slab break-off occurred at great depth in the Dabie–Sulu orogen and hence rare magmatism occurred. Whereas in South Qinling the break-off occurred at a shallow depth, the asthenospheric mantle material could rise further up into the overlying mantle where it experienced decompression and melting. As a consequence, crustal sections were heated up to produce extensive granitic magmatism.

Early Paleozoic intrusive rocks from the eastern Qilian orogen, NE Tibetan Plateau: Petrogenesis and tectonic significance

Voluminous Early Paleozoic intrusive rocks occur in the Qilian orogenic belt, northeastern margin of the Tibetan Plateau. Their petrogenesis can provide insights into the tectonic evolution of Qilian and its adjacent areas. This paper carries out an integrated study of U–Pb zircon dating, geochemical and Sr–Nd–Hf isotopic compositions for the Bamishan and Heishishan plutons in the eastern Qilian orogen. Both the Bamishan and Heishishan plutons consist of mafic, intermediate and felsic intrusive rocks. U–Pb zircon dating yielded the magma crystallization ages of 459–449Ma for the Bamishan pluton and 447–438Ma for the Heishishan pluton. Their geochemical and Sr–Nd–Hf isotopic data show that the parental magmas of the Bamishan monzogranites and quartz diorites were generated by slab (MORB and overlying sediments) melting. The Bamishan hornblende gabbros have extremely high Cr (1159–1163ppm), Ni (220–353ppm) and Mg# (73–74), indicating a cumulate origin. The Heishishan granodiorite–granite has adakitic geochemical signatures and they were produced by partial melting of the mafic lower crust. The Heishishan hornblende gabbro and quartz diorites were derived from the enriched lithospheric mantle metasomatized by subduction-related fluid/melt. In light of the available data, we propose that there was an Early Paleozoic ocean basin between the Central-South Qilian and Qaidam–West Qiling belts. The northward closure of this basin and subsequent continental collision and post-collisional processes resulted in the generation of the Early Paleozoic magmatism in the eastern Central Qilian. The Late Ordovician to Early Silurian (~460–440Ma) intrusive rocks in the eastern Central Qilian formed in a post-collisional setting. A slab break-off model may account for the magma generation.

The Tertiary dike magmatism in the Southern Alps: geochronological data and geodynamic significance

The relationships between tectonics and magmatic activity in the Alps are still debated. Despite an active subduction since the Late Cretaceous, no arc-related magmatism is recorded prior of the Middle Eocene. The emplacement of plutons along the Insubric Fault in a short time span (~34–28 Ma) has been generally interpreted in terms of the slab break-off model. The Tertiary magmatism, however, is also characterized by the occurrence of widespread calcalkaline dikes not necessarily intruded along the Insubric Fault. The geochemical features of dikes vary along the Alps belt and are interpreted in terms of mantle source heterogeneity and degree of crustal contamination. U–Pb zircon dating of studied dikes indicates intrusion ages in the 42- to 34-Ma time interval. These data provide evidence for a pre-Oligocene magmatic activity that was not solely limited to the Adamello batholith. Moreover, it appears that dikes rejuvenate from SE to NW, in an opposite direction with respect to the Alpine subduction polarity. Thus, a more complex geodynamic scenario than the slab break-off model must be envisaged. The absence of arc magmatism prior to the Middle Eocene can be explained by the low-angle subduction of the Tethyan slab that confined the mantle partial melting zone away from the orogenic wedge. The onset of the Apennines subduction at 55–50 Ma caused the Alpine slab to retreat. The partial melting zone progressively migrated beneath the orogenic wedge and finally reached the axial belt in the Late Eocene, when the Alpine collision was completed. Only at this stage, slab break-off occurred and promoted the intrusion of the Periadriatic plutons.

Paleoproterozoic S-type granites in the Helanshan Complex, Khondalite Belt, North China Craton: Implications for rapid sediment recycling during slab break-off

S-type granites, typically derived from the rapid recycling of sedimentary rocks, are sometimes accompanied by contemporary mafic magmatism and granulite metamorphism. However, the geodynamic context for such rock suites is often highly disputed, with various model proposed, including back-arc basin opening, lithospheric delamination, mantle plume and continental rifting. The Paleoproterozoic Khondalite Belt in the North China Craton provides an example of synchronous mafic and felsic magmatism that was accompanied by granulite-facies metamorphic events for which the tectonic affinities of these rocks remains unclear. This study integrates in situ zircon Hf–O isotope analyses, whole-rock geochemistry and Nd isotope results for the earliest two-mica granites (ca. 1.95Ga) in order to provide constraints on the above issues. The granites are strongly peraluminous (A/CNK value >1.1), and characterized by high zircon δ18O values of 7.3–10.6‰, corresponding to calculated magmatic δ18O values of 9.1–12.3‰, similar to those of typical S-type granites. They have relatively high and homogeneous ɛNd(t) values of −1.1 to +0.9 and highly variable zircon ɛHf(t) values ranging from −1.0 to +8.3. In situ zircon Hf–O isotopic compositions indicate that the S-type granites may contain some mantle or juvenile crustal components in addition to a sediment component. Based on the new results and published data, a slab break-off model is proposed to explain the rapid recycling of sedimentary precursors and the generation of the ca. 1.95Ga S-type granites.

From crustal anatexis to mantle melting in the Variscan orogen of Corsica (France): SIMS U–Pb zircon age constraints

High-precision SIMS U–Pb zircon age determinations are conducted in this study on migmatites and Mg–K magmatic suites from the Variscan orogen of Corsica (France). Zircons from leucosomes of four migmatites yield consistent crystallization ages of ca. 345Ma. Four Mg–K granitoid rocks and one monzogabbro enclave from the Calvi-IIe Rousse pluton of NW Corsica yield indistinguishable U–Pb zircon ages of ca. 330Ma. These new SIMS zircon U–Pb dating results indicate that the regional crustal anatexis under amphibolite-facies condition occurred synchronously at ca. 345Ma throughout the Corsican Variscan chain. There is a ca. 15m.y. time interval between anatexis of the thickened crust and partial melting of the metasomatized mantle and overlying crust to form the Mg–K rock suites. In combination with the previous dating results for the high-temperature and high-pressure granulite-facies metamorphism, three major discrete tectonothermal events at ca. 360Ma, ca. 345Ma, and ca. 330Ma are acknowledged. We argue for a slab break-off model accounting for thermal and mechanical evolution of the crust within the Variscan orogen of Corsica. A tearing of the subducting oceanic lithosphere initiates the asthenosphere rise to fill the void and causes the granulite-facies metamorphism of the overlying continental crust at ca. 360Ma. Final breaking of the subducting oceanic lithosphere results in exhumation of the subducted crust to the thickened lower to middle crustal-level. Internal radiogenic heating causes crust-scale migmatization at ca. 345Ma. Slab break-off and foundering enhance the rise of hot asthenosphere, resulting in conducted thermal perturbation that leads to melting of the metasomatised enriched mantle lithosphere to form Mg–K mafic–granitoid magmatism at ca. 330Ma.

Early Cretaceous (100–105 Ma) Adakitic magmatism in the Dachagou area, northern Lhasa terrane, Tibet: implications for the Bangong–Nujiang Ocean subduction and slab break-off

We performed U–Pb zircon dating and whole-rock major and trace element geochemical analyses of monzonites and granodiorites from the Dachagou area, northern Lhasa terrane, Tibet. Zircon U–Pb dating of two plutons yielded magmatic crystallization ages of 100 and 105 Ma, indicating that the intrusive rocks were emplaced in the Early Cretaceous. The monzonites and granodiorites are characterized by high Al2O3 and Sr contents, low Yb and Y contents, and high Sr/Y ratios, and secondarily by low K2O contents, high Mg # values, and relatively high Cr and Ni abundances. Major and trace element geochemical analyses indicate that the rocks have adakitic signatures, suggesting generation by partial melting of an oceanic slab. However, there are some differences between the monzonites and granodiorites, which imply that the monzonitic magmas were contaminated by enriched mantle and crustal components and that the granodioritic magmas were contaminated by enriched mantle melts without a significant crustal contribution. Based on these data and the regional tectonic setting, we propose that the oceanic slab break-off model best explains adakitic magma generation in the Dachagou area. Identification of adakites in the area provides important evidence of slab break-off during the southward subduction of the Bangong–Nujiang Ocean plate.