Melting Of Enriched Lithospheric Mantle Research Articles

Permian-Triassic tectonic evolution of the Ailaoshan orogenic belt in SW China: insights from magmatic records

ABSTRACT The Ailaoshan orogen in SW China records the evolution of the Ailaoshan Paleo-Tethyan ocean and the amalgamation history between the South China and Indochina blocks, an important process for the formation of Southeast Asia. However, its tectonic evolution remains ambiguous with various competing models. We have carried out an integrated geochronological–petrological–geochemical study for the granites, dacites-rhyolites, granodiorites and trachyandesites in this orogen to give new constraints on this issue. The 255–254 Ma granites and the 250–248 Ma dacites-rhyolites show similar geochemical features and are characterized by strongly peraluminous nature, continental crust-like trace element patterns and negative zircon εHf(t) values (−14.4 to −6.6), indicating they derived from partial melting of ancient crust. The 229 Ma hornblende-bearing granodiorites are metaluminous to weakly peraluminous (A/CNK = 0.96–1.01) and belong to high-K calc-alkaline I-type granites. They show depleted whole-rock Sr-Nd and zircon Hf isotope compositions (initial 87Sr/86Sr = 0.7027 to 0.7038, εNd(t) = 4.0 to 4.1, εHf(t) = 9.3 to 11.5), suggestive of derivation from partial melting of juvenile mafic crust. The 226 Ma trachyandesites are characterized by high MgO (4.1–4.6 wt.%), high Mg# (56) and arc-type geochemical affinities with enriched isotope compositions (initial 87Sr/86Sr = 0.7108 to 0.7111, εNd(t) = −7.7 to −7.5, εHf(t) = −6.3 to −3.5). These features indicate that their parental magmas derived from partial melting of enriched lithospheric mantle metasomatized by oceanic sediment-derived melts. These new data, in conjunction with spatial-temporal-temperature variations of igneous rocks, metamorphic records and stratigraphic changes in the region, suggest that the Ailaoshan Paleo-Tethyan ocean closed at the Early Triassic (ca. 250 Ma) which was followed by the Late Triassic postcollisional slab breakoff, and the ocean experienced westward unipolar subduction during its evolution with no assumed eastward subduction.

Crust–mantle interaction beneath the Pamir orogenic belt: Insights from the Miocene Dunkeldik carbonatite–syenite complex in Tajikistan

Only some carbonatite–syenite complexes in the world could form carbonatite-related rare earth element (REE) deposits, which is regarded as the main REE origin. The Miocene Dunkeldik carbonatite–syenite complex is located in the Dunkeldik magmatic field in Tajikistan between the Pamir and Tibetan plateaus. So far, the origin, tectonic setting, and REE mineralization potential of the Dunkeldik carbonatite–syenite complex remain unclear due to the limited isotopic values, which inhibit the further understanding the genesis of such carbonatite complex. This study presents zircon U–Pb age, whole-rock geochemical, and Sr–Nd–Pb isotope, and zircon Hf–O isotope data for the Dunkeldik carbonatite–syenite complex on the Pamir Plateau. Zircon U–Pb dating of syenite yielded two concordant ages of 10.27 ± 0.16 and 10.48 ± 0.21 Ma, indicative of formation in a post-collisional extensional setting related to India–Asia collision. Both the carbonatite and syenite exhibit similar enrichments in large-ion lithophile elements (e.g., Sr and Ba) and light REEs, depletions in high-field-strength elements (e.g., Nb and Ta) and heavy REEs, and small Eu anomalies. Whole-rock Sr–Nd–Pb isotopic compositions are highly enriched, with high (87Sr/86Sr)i (0.70971 to 0.70979), low εNd(t) (–8.9 to –9.2), and radiogenic Pb isotope composition (206Pb/204Pb (18.322–18.358), 207Pb/204Pb (15.689–15.695), and 208Pb/204Pb (39.091–39.149)). Zircons Hf-O isotope analyses yield low εHf(t) values (–9.5 to –19.0) and high δ18O value (7.4 ‰–9.5 ‰), respectively. These geochemical and isotopic features indicate that the complex was formed by partial melting of enriched lithospheric mantle during the Miocene India–Asia collision and was assimilated by upper crustal material during magma ascent. The carbonatite and syenite in the complex formed by melt immiscibility. The involvement of crustal material in the magma and less development of fenitization may responsible for the limited REE mineralization potential as compared with other large REE deposits that formed during coeval India–Asia collision.

Petrogenesis of late Permian to Middle Triassic magmatic rocks on northern Hainan Island, South China: Implications for crust–mantle interaction and the tectonic evolution of the Paleo-Tethys

Geochronological, geochemical, and Sr–Nd–Hf isotope data are reported for the late Permian to Middle Triassic Juding gabbros, the Xinxing gabbroic–dioritic rocks, and granites and mafic enclaves from the northern part of Hainan Island, South China. Our data were used to constrain the petrogenesis of these rocks, the crust–mantle interactions involved, and the implications for the tectonic evolution of the area. Zircon U–Pb dating gave crystallization ages of ca. 235 Ma for the Juding gabbros, ca. 252 Ma for the Xinxing gabbroic–dioritic rocks, and 253–238 Ma for the granites and mafic enclaves. The Juding gabbros are characterized by weak negative Nb, Ta, and Ti anomalies, low initial 87Sr/86Sr ratios (0.704603–0.704630), and high εNd(t) values (+3.7 to +3.8). The Juding gabbros are inferred to have been derived by partial melting of depleted asthenospheric mantle, with these melts mixing with melts produced from enriched lithospheric mantle. The Xinxing gabbroic–dioritic rocks have relatively intermediate compositions, with initial 87Sr/86Sr ratios of 0.706600–0.708664, εNd(t) values of −6.3 to −5.1, and εHf(t) values of −3.3 to −0.6. The Xinxing gabbroic-dioritic rocks probably derived by partial melting of enriched lithospheric mantle that was modified by subduction-related melts and/or fluids. The granites from the northern part of Hainan Island have extremely high SiO2, low MgO and Ni contents, with initial 87Sr/86Sr ratios of 0.704794–0.711158, εNd(t) values of −8.5 to −6.6, and εHf(t) values of −15.0 to −1.2. They were probably derived by partial melting of a variety of ancient crustal sources with some addition of mantle material. The mafic enclaves in these granites have relatively low SiO2, high MgO and Ni contents, with initial 87Sr/86Sr ratio of 0.707882, εNd(t) value of −6.4, and εHf(t) values of −9.8 to −3.9. They seem to represent the remnants of a mafic component derived from enriched mantle. The gabbroic–dioritic rocks and gabbros formed in response to the subduction and break off of the Paleo-Tethyan oceanic slab. The late Permian to Middle Triassic magmatic rocks from the northern part of Hainan Island record complex multistage crust–mantle interactions and lithosphere–asthenosphere interaction, probably related to subduction during closure of the Paleo-Tethys Ocean and the consequential collision of the South China and Indochina blocks.

Appinitic and high Ba[sbnd]Sr magmatism in central Brazil: Insights into the late accretion stage of West Gondwana

Large amounts of high BaSr magmatism can be produced as the result of late accretion or decompression in the final stages of continental arc evolution in orogenic systems and leads to continental crustal growth. This type of magmatism often occurs associated with mega-shear zones and displays geochemical signatures that indicate magmas deriving from the melting of enriched lithospheric mantle. Such enrichment may be an indication of slab-derived fluid and/or melt interaction, released from subducted material. The high BaSr granitoids of the Brasiliano Paraguay Belt, Tocantins Province, central Brazil, comprise two suites that crystallized approximately 40 My apart from each other. Both suites are composed of metaluminous to slightly peraluminous rocks (ASI = 0.64–1.09) with magnesian character, and present high Ba (~1430 ppm), Sr (~800 ppm), K2O (~4 wt%), LREE and LILE contents. The negative εNd(T) values (−6.32 and − 6.65) associated with high contents of incompatible elements indicate an evolution from partial melting of mantle-derived magma previously metasomatized by subducted slab fluids. Crystallization ages of 557 ± 5.7 Ma and 554 ± 2.9 Ma have been obtained for the older, high MgK calc-alkaline suite. It evolved from high Cr and Ni mantle-derived appinitic magma to coeval granodiorites by crystal fractionation accompanied by minor crustal contamination. The younger, 515 ± 3.6 Ma-old suite displays an elevated concentration in some HFSEs, show alkali-calcic affinity and is represented by high-K monzogranites. A model proposed for the two hybrid suites is based on a slab break-off and a subsequent lithospheric delamination after 40 My, where the heat flux generated by mantle upwelling led to a partial melting of the enriched subcontinental lithospheric mantle beneath the continental crust. The older calc-alkaline suite represents the end process of magmatic arc accretion in the central Tocantins Province. The younger one is compatible with a transitional shoshonitic to A-type affinity, associated to post-collisional collapse stage of the orogen at Cambrian times. The petrogenesis of the investigated rocks requires an ocean closure phase at ca. 590 Ma, which could be linked to the consumption of the Clymene Ocean toward Amazonian Craton, recording a new collisional event in the central West Gondwana.

Melting of metasomatically enriched lithospheric mantle – Constraints from Pan-African monzonites (Damara Orogen, Namibia)

The 560.8 ± 2.4 Ma old Okamutambo Pluton is the easternmost intrusion of the syn-collisional Otjimbingwe Alkaline Complex (OAC; Damara Belt, Namibia) and consists of mafic to intermediate alkaline rocks that belong to the monzodiorite-syenite series. The studied samples are potassic (K2O/Na2O > 1) and have moderately high magnesium (MgO: 5.7–2.6 wt%), nickel (Ni: 66–26 ppm), and chromium (Cr: 223–62 ppm) concentrations. LILE (Ba: 2425–1243 ppm; Sr: 1359–941 ppm) and HFSE (Zr: 447–202 ppm, Nb: 32.8–16.4 ppm, Hf: 4.7–9.4 ppm, Ta: 1.2–2.0 ppm) contents are also high. Strontium and Nd isotope data reveal the existence of two magmatic suites indicating a multi-source origin. Group 1 monzonites-quartz monzonites have moderately evolved Sr and Nd isotopic compositions (initial 87Sr/86Sr: 0.7066 to 0.7073; initial εNd: −3.5 to −5.0) and radiogenic Pb isotope ratios (206Pb/204Pb: 17.65–18.02; 207Pb/204Pb: 15.62–15.67; 208Pb/204Pb: 38.19–38.32). In contrast, group 2 monzodiorites-syenites display more evolved Sr and Nd isotopic compositions (initial 87Sr/86Sr: 0.7088 to 0.7090; initial εNd: −6.7 to −7.1) but similar Pb isotope ratios (206Pb/204Pb: 17.63–17.82; 207Pb/204Pb: 15.64–15.66; 208Pb/204Pb: 38.21–38.37). Differentiation involved AFC processes in group 1 monzonites-quartz monzonites whereas group 2 monzodiorites-syenites were modified by fractional crystallization. Although second-order processes were operative, high total alkali contents, incompatible trace element concentrations in excess of bulk crustal values, and evolved isotopic compositions of the most primitive samples are source-controlled and provide insight into the origin of the parental magmas. With reference to experimental data from the literature, it is inferred that the Okamutambo alkaline rocks represent evolved melts that were generated through melting of enriched lithospheric mantle (phlogopite-lherzolite). The observed negative Nb-Ta and Ti anomalies and positive Pb anomalies in primitive mantle-normalized trace element patterns are in line with a mantle source that contains a recycled crustal component. Isotopic compositions indicate that mantle enrichment is an ancient feature that might be linked to Proterozoic subduction. At 561 Ma, the geodynamic regime in the Damara Orogen was mainly characterized by compression during convergence and continental collision between the Congo and Kalahari cratons which is difficult to reconcile with the generation of the OAC as alkaline magmatism is commonly associated with extensional tectonic regimes. The OAC is, however, associated with a major suture zone that may have been involved in localized transtensional tectonics during oblique flat subduction and thus enabled the generation and ascent of mantle-derived alkaline melts.

Late Eocene post-collisional magmatic rocks from the southern Qiangtang terrane record the melting of pre-collisional enriched lithospheric mantle

Abstract Diverse rock types and contrasting geochemical compositions of post-collisional mafic rocks across the Tibetan Plateau indicate that the underlying enriched lithospheric mantle is heterogeneous; however, how these enriched mantle sources were formed is still debated. The accreted terranes within the Tibetan Plateau experienced multiple stages of evolution. To track the geochemical characteristics of their associated lithospheric mantle through time, we can use mantle-derived magmas to constrain the mechanism of mantle enrichment. We report zircon U-Pb ages, major and trace element contents, and Sr-Nd isotopic compositions for Early Cretaceous and late Eocene mafic rocks in the southern Qiangtang terrane. The Early Cretaceous Baishagang basalts (107.3 Ma) are characterized by low K2O/Na2O (<1.0) ratios, arc-like trace element patterns, and uniform Sr-Nd isotopic compositions [(87Sr/86Sr)i = 0.7067–0.7073, εNd(t) = −0.4 to −0.2]. We suggest that the Baishagang basalts were derived from partial melting of enriched lithospheric mantle that was metasomatized by subducted Bangong–Nujiang oceanic material. We establish the geochemistry of the pre-collisional enriched lithospheric mantle under the southern Qiangtang terrane by combining our data with those from other Early Cretaceous mafic rocks in the region. The late Eocene (ca. 35 Ma) post-collisional rocks in the southern Qiangtang terrane have low K2O/Na2O (<1.0) ratios, and their major element, trace element, and Sr-Nd isotopic compositions [(87Sr/86Sr)i = 0.7042–0.7072, εNd(t) = −4.5 to +1.5] are similar to those of the Early Cretaceous mafic rocks. Based on the distribution, melting depths, and whole-rock geochemical compositions of the Early Cretaceous and late Eocene mafic rocks, we argue that the primitive late Eocene post-collisional rocks were derived from pre-collisional enriched lithospheric mantle, and the evolved samples were produced by assimilation and fractional crystallization of primary basaltic magma. Asthenosphere upwelling in response to the removal of lithospheric mantle induced the partial melting of enriched lithospheric mantle at ca. 35 Ma.

Petrology, Geochemistry, and Sr-Nd-S Isotopic Compositions of the Ore-Hosting Biotite Monzodiorite in the Luanjiahe Gold Deposit, Jiaodong Peninsula, China

The Jiaodong Peninsula is one of the most important Au ore provinces in China. There is an ongoing debate on the correlation between ore formation and magmatism in this province, because few intrusive rocks exhibit a clear association with ore deposits. A mineralized biotite monzodiorite (BM) stock, with disseminated ore, pervasive phyllic alteration, and no deformation, was found in a borehole in the footwall of the Zhaoping fault within the Luanjiahe Au deposit, which may shed light on this debate. The biotite monzodiorite contains explosion breccias, miarolitic cavities, skeletal and dendritic quartz, and late-stage evolved aplite dikes, and the in-situ δ34S values of the disseminated pyrite which is associated with Au mineralization are -1.7‰ to 7.3‰ (mean=3.5‰), indicative of a magmatic-hydrothermal system. These findings, combined with the reported age of 123 Ma, show that the intrusion has close spatial, temporal, and geochemical relationships with Au mineralization in the area. The biotite monzodiorite is metaluminous, high-K calc-alkaline and shoshonitic, with enrichment in light rare earth elements (REEs) and large-ion lithophile elements (LILEs), depletion in high-field-strength elements (HFSEs), and enriched Sr-Nd isotopic compositions. The intrusion may be the product of partial melting of enriched lithospheric mantle with a small lower crustal component. The hydrous, Au-bearing, enriched mantle source, and the strongly oxidized magma that was generated, created favorable conditions for Au mineralization.

Mantle Response to Slab Breakoff in the North Qaidam Tectonic Belt: Geochemical Constraints from Syn‐subduction Mafic Igneous Rocks

AbstractSlab breakoff originally denotes the detachment of dense subducted oceanic slab from the light subducted continental slab, which is driven by opposing buoyancy forces during continental collision (Davies and von Blanckenburg, 1995; von Blanckenburg and Davies, 1995). The breakoff of subducted oceanic slab can induce the upwelling of sub‐slab asthenosphere through the slab window, and then heat the overriding lithospheric mantle resulting in the melting of its fertile layer within the metasomatic mantle wedge. The decompression partial melting of uprising asthenospheric mantle commonly produce mafic magma with depleted MORB‐like geochemical signatures (Davies and von Blanckenburg, 1995; Cole et al., 2006; Wang et al., 2018), whereas the partial melting of enriched lithospheric mantle will produce mafic magma with alkaline, calc‐alkaline or ultrapotassic features (von Blanckenburg and Davies, 1995). These mafic magmas rise into overlying lower crust and trigger crustal melting to generate the granitic magma. The North Qaidam tectonic belt (NQTB) records the evolutionary process of the South Qilian Ocean from subduction to closure. The subduction of oceanic and continental lithosphere to mantle depths is proven by the identification of oceanic‐type and continental‐type eclogites enclosed in crustal metapelite and gneiss from the North Qaidam tectonic belt (Song et al., 2006; Zhang et al., 2008; Zhang et al., 2010; Zhang et al., 2017). However, details of this process are not exactly constrained, in particularly, the closure timing of South Qilian Ocean. The study of characteristic mafic magmatism, combined with the previous studies of ultra‐high pressure metamorphism, give us an excellent opportunity to trace the detailed processes associated with the transition from oceanic subduction to continental subduction, and assess the feasibility of slab breakoff in the North Qaidam tectonic belt.In this contribution, an integrated study of petrology, geochemistry, geochronology and Sr‐Nd‐Hf isotopes is performed on the mafic igneous rocks from Chahanhe area in the North Wulan gneiss complex. These mafic igneous rocks can be divided into two groups, namely, arc‐like type and E‐MORB type based on their trace element patterns. Arc‐like mafic rocks (441–428 Ma) were characterized by enrichment of light rare earth elements (LREEs), large ion lithophile elements (LILEs) and depletion of heavy rare earth elements (HREEs), high field strength elements (HFSEs). Combined with variable zircon ∊Hf(t) values of −6.17 to +1.58, it is suggested that arc‐like mafic rocks are predominantly derived from the partial melting of the enriched lithospheric mantle, and minor juvenile materials have contributed to their sources. The E‐MORB mafic rocks (440 Ma) exhibit relatively flatted REE patterns and positive ∊Nd(t) values of +1.63 to +4.28, but high (87Sr/86Sr)i ratios of 0.706825 to 0.708979, indicting a derivation from partial melting of asthenospheric mantle, with involvement of enriched components probably derived from ambient lithospheric mantle or stagnant subducted oceanic crust. Collectively, it is proposed that the break‐off of the subducted South Qilian oceanic slab triggered the decompression melting of asthenospheric mantle, and the upwelling of asthenosphere provided heat and induced partial melting of the enriched lithospheric mantle and pre‐existing crust, resulting in generation of arc‐like mafic rocks and widespread granites.

Early Cretaceous bimodal volcanic rocks in the Yinshan belt, North China Craton: age, petrogenesis, and geological significance

Determining the age and petrogenesis of the Mesozoic magmatic rocks may provide crucial evidence regarding the tectonic setting and evolution of the North China Craton (NCC). In this paper, we present new zircon U–Pb ages and geochemical data for bimodal volcanic rocks from Zhuozi area in the eastern Yinshan belt, NCC, to discuss their petrogenesis and tectonic implications. The volcanic rock suite consists of basalts, rhyolites, and trachytes. According to the results of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), the rhyolitic and trachytic rocks have zircon U–Pb ages of 136.9 ± 0.8 Ma and 134.0 ± 1.7 Ma, respectively, representing their eruption ages. The basalts are enriched in large-ion lithophile elements (LILE; Ba, Rb, Sr, and Th) and light rare-earth elements (LREE), and are relatively depleted in high field strength elements (HFSE; Nb, Ta, and Ti). In addition, the basalts are characterized by high fractionations of rare-earth elements (REE), with (La/Yb)N = 24.87–42.85. These features suggest that they were derived from low-degree partial melting of enriched lithospheric mantle metasomatized by subduction-related components. The rhyolitic rocks have low mean Al2O3 (12.26 wt%) and TiO2 (0.14 wt%) contents and Mg# (14.92) values, with relatively low total REE abundances. They have high alkali and Zr, Ce, Y, and Nb contents and high Ga/Al ratios, with notably positive Pb and negative Sr, Ba, and Eu anomalies observed in the primitive mantle-normalized trace element diagram. These features indicate that the rocks exhibit characteristics of A1-type granite and likely derived from the partial melting of a crustal source. The trachytic rocks are show moderate trace elements and SiO2, MgO, Fe2O3T, and TiO2 contents. They were possibly generated by fractional crystallization of the underplating basaltic magmas and assimilated minor crustal materials. The geochemical characteristics, in combination with the regional geology, suggest that the bimodal volcanic rocks formed in an intraplate extensional setting likely associated with the retreat of the subducted Paleo-Pacific plate.

Early Cretaceous mantle upwelling and melting of juvenile lower crust in the Middle-Lower Yangtze River Metallogenic Belt: Example from Tongshankou Cu-(Mo[sbnd]W) ore deposit

The linkage between intracontinental extension and the Early Cretaceous Cu-Mo-W polymetallic metallogenesis in the Middle-Lower Yangtze River Belt (MLYRB) has long been a subject of controversy due to the lack of convincing petrogenetic evidence to identify the nature of magmatic sources and their geological histories during extensional mantle upwelling. Here we present new zircon UPb ages, isotopic and geochemical data for granodiorites, quartz diorites and mafic microgranular enclaves (MMEs) in the Tongshankou area. Comparing the MMEs with their host porphyries, the different ratios of incompatible elements and the similar formation ages, coupled with quenched margins and the xenocrysts in the MMEs, indicate that the MMEs was most likely formed by mixing between mafic magma and their host felsic magma. The MMEs share similar geochemical and isotopic characteristics with the Cretaceous mafic rocks from MLYRB, indicating that MMEs were mostly derived from an enriched lithospheric mantle source without adakitic characteristics. Mixing of a crustal melt derived by melting of an amphibolite bearing juvenile lower crust with a mantle melt derived from melting of enriched lithospheric mantle can account for the generation of the Tongshankou prophyries. The melting of juvenile mafic lower crust and enriched lithospheric mantle is suggested to be caused by upwelling of asthenospheric mantle and the reactivity of trans-lithospheric faults in the intracontinental extensional environment. Our results therefore highlight that juvenile mafic lower crust beneath the Yangtze plate is one of the likely source for ore-forming magmatic rocks in the Early Cretaceous.

The age and geochemistry of the mid-Cretaceous volcanic rocks in the Jinan Basin: Implications for the mid-Cretaceous tectonic environments of the Korean Peninsula and Northeast Asia

Most of the mid to late Cretaceous igneous rocks in the southeastern margin of the Northeast Asian continent are interpreted to be formed due to upwelling of asthenospheric mantle caused by slab rollback and associated back-arc extension. However, the tectonic environments for the mid-Cretaceous igneous rocks in the Korean Peninsula is uncertain. The Jinan Basin is one of the Cretaceous pull-part basins in the southern Korean Peninsula. The mid-Cretaceous volcanic rocks occur in association with the Jinan Basin. In this study, we investigated whole-rock major and trace element geochemistry and Sr-Nd-Pb isotope compositions of the mid-Cretaceous volcanic rocks in the Jinan Basin to understand their petrogenesis and tectonic environments. Also, U-Pb zircon age of sedimentary and volcanic rocks in the Jinan Basin was determined to constrain the age of sedimentation and igneous activities. The results show that sedimentation started at least from 97.7 Ma and continued until 89.5 Ma. The sedimentary sequences were intermittently extruded by rhyolitic and andesitic magma 90–89 Ma, and later intruded by basaltic trachyandesite 84 Ma. The volcanic rocks in the Jinan Basin show geochemical features of alkaline to high-K calc-alkaline magma series with strong enrichment in large ion lithophile elements and light rare earth elements relative to high field strength elements. The mafic and intermediate volcanic rocks are distinguished from the typical continental arc magmas by enrichment in incompatible elements such as Y and Zr with high Zr/Y, suggesting that they originated from an enriched source. The basaltic trachyandesites and andesite have Sr isotope ratios (87Sr/86Sri) of 0.708769–0.709484 and Nd isotope ratios (143Nd/144Ndi) of 0.511782–0.511886, which indicate that they formed from mantle which was contaminated with crustal component in their petrogenesis. The Pb isotope data (206Pb/204Pb = 17.572–18.158, 207Pb/204Pb = 15.535–15.640 and 208Pb/204Pb = 38.366–38.921) suggest that the crustal component should have experienced a prolonged evolution with a low μ value. The Mg# (46–57) and Cr contents (40–180 ppm) in the mafic to intermediate rocks higher than typical crustal melts at a given range of SiO2, showing that they evolved from a mantle-derived primary magma. We attribute the crust-like geochemical and isotopic signature of the Jinan mafic and intermediate igneous rocks to the partial melting of enriched lithospheric mantle metasomatized by subduction components and subsequent crustal assimilation during magma ascent and pooling at the mid-crustal depth of ~9–10 km. The geochemical and isotopic features of the Jinan volcanic rocks are similar to those of the mid-Cretaceous basaltic volcanic rocks in the Yeongdong and Gyeongsang Basins, South Korea and the southeastern margin of South China Craton (SCC). This may suggest that the partial melting of the enriched lithospheric mantle, induced by slab rollback of the paleo-Pacific plate and upwelling of the asthenospheric mantle, was widespread underneath the southeastern Korean Peninsular as well as the southeastern margin of SCC.

Origin of Devonian mafic magmatism in the East Kunlun orogenic belt, northern Tibetan Plateau: implications for continental exhumation

AbstractThis paper presents a comprehensive study of the zircon geochronology, geochemistry and Sr–Nd isotope geology of Devonian mafic rocks developed in the East Kunlun orogenic belt, northern Tibetan Plateau, and reveals their mantle sources, petrogenesis and geodynamic implications for continental exhumation. The zircon geochronology of typical samples indicates that these mafic rocks crystallized at 406∼408 Ma. They can be classified into two different groups based on petrographic observations and geochemical compositions. Group 1 rocks exhibit low TiO2 and FeOt contents and Nb/Y ratios and have enriched mid-ocean ridge basalt (E-MORB)-like compositions with slight negative Nb and Ta anomalies. However, Group 2 rocks have distinctly high TiO2 and FeOt contents and Nb/Y ratios, comparable to typical Fe–Ti-rich mafic rocks worldwide. All the samples exhibit weak enrichments in light rare earth elements, Nb and Ta relative to the primitive mantle. Based on geochemical and isotopic studies, Group 1 rocks are suggested to be derived from depleted asthenospheric mantle that was metasomatized by c. 3–5 % continental crustal components, while Group 2 rocks originated from partial melting of enriched lithospheric mantle. The high contents of Fe, Ti and Nb for Group 2 rocks could be attributed to a high degree of olivine crystallization under low fO2 conditions with delayed nucleation of Fe–Ti oxides. Combining those results with other geological data, we conclude that slab break-off was the key factor causing exhumation of eclogites and triggering flare-up of the Devonian magmatism, and that continental collision or continental subduction may have initiated at 431∼436 Ma.

Zircon U–Pb geochronology, major-trace elements and Sr–Nd isotope geochemistry of Mashhad granodiorites (NE Iran) and their mafic microgranular enclaves: evidence for magma mixing and mingling

ABSTRACT Mashhad granitoids and associated mafic microgranular enclaves (MMEs), in NE Iran record late early Mesozoic magmatism, was related to the Palaeo-Tethys closure and Iran-Eurasia collision. These represent ideal rocks to explore magmatic processes associated with Late Triassic closure of the Palaeo-Tethyan ocean and post-collisional magmatism. In this study, new geochronological data, whole-rock geochemistry, and Sr–Nd isotope data are presented for Mashhad granitoids and MMEs. LA–ICP–MS U–Pb dating of zircon yields crystallization ages of 205.0 ± 1.3 Ma for the MMEs, indicating their formation during the Late Triassic. This age is similar to the host granitoids. Our results including the major and trace elements discrimination diagrams, in combination with field and petrographic observations (such as ellipsoidal MMEs with feldspar megacrysts, disequilibrium textures of plagioclase), as well as mineral chemistry, suggest that MMEs formed by mixing of mafic and felsic magmas. The host granodiorite is a felsic, high K calc-alkaline I-type granitoid, with SiO2 = 67.5–69.4 wt%, high K2O (2.4–4.2 wt%), and low Mg# (42.5–50.5). Normalized abundances of LREEs and LILEs are enriched relative to HREEs and HFSEs (e.g. Nb, Ti). Negative values of whole-rock εNd(t) (−3 to −2.3) from granitoids indicate that the precursor magma was generated by partial melting of enriched lithospheric mantle with some contributions from old lower continental crust. In the MMEs, SiO2 (53.4–58.2 wt%) is lower and Ni (3.9–49.7 ppm), Cr (0.8–93.9 ppm), Mg# (42.81–62.84), and εNd(t) (−2.3 to +1.4) are higher than those in the host granodiorite, suggesting a greater contribution of mantle-derived mafic melts in the genesis of MMEs.

Geochemical and isotopic imprints of early cretaceous mafic and felsic dyke suites track lithosphere-asthenosphere interaction and craton destruction in the North China Craton

The North China Craton (NCC) underwent significant lithospheric destruction during the Mesozoic, although the mechanisms and timings associated with this process remain debated. The extensive magmatism associated with craton destruction includes dyke suites of diverse composition derived from lithospheric and asthenospheric sources. Here we investigate lamprophyre, dolerites and felsic dyke suites from the Laiyuan complex in North Taihang Mountain of the NCC. Zircon UPb dating shows weighted mean ages in the range of 110–115 Ma, 117–126 Ma and 127–130 Ma for the lamprophyres, dolerites and felsic dykes and their εHf (t) values show a range of −17.2 to −3.7, −23.3 to −14.2 and − 22.3 to −17.2, respectively, with an evident increase through time suggesting increasing input of asthenospheric components in the mafic suite. We present major, trace and REE data on these rocks which suggest that the mafic dykes experienced limited crustal contamination and were dominated by olivine and clinopyroxene fractional crystallization. The Laiyuan mafic dykes were derived through partial melting mantle previously enriched by subduction-related fluids, within amphibole- and garnet-stability field with increasing input of asthenospheric material through time. The felsic dykes show adakitic feature with magma generation through the reworking of ancient basement rocks induced by heat input from underplated mafic magma which was derived from the partial melting of enriched lithospheric mantle. Subsequently, the enriched mantle-derived magmas migrated through lithospheric faults and were emplaced as dolerite dykes at ~117 Ma. The asthenosphere upwelling contributed to the thermo-mechanical erosion along weak zones, and the limited lithosphere-asthenosphere interaction generated the lamprophyres during ~115 Ma to 110 Ma. Our data suggest that both delamination and thermo-mechanical erosion contributed to the craton destruction of the NCC during Early Cretaceous.

Postcollisional delamination and partial melting of enriched lithospheric mantle: Evidence from Oligocene (ca. 30 Ma) potassium-rich lavas in the Gemuchaka area of the central Qiangtang Block, Tibet

Postcollisional potassium (K)-rich magmatic rocks have important implications for unveiling the deep geodynamic processes active during the evolution of orogenic belts and the surface uplift of plateaus. However, their petrogenesis remains highly controversial. Here we report on zircon U-Pb and mineral Ar-40-Ar-39 age, zircon Hf-O isotope, mineral composition, whole-rock major and trace element composition, and Nd-Sr isotope data for the postcollisional K-rich lavas in the Gemuchaka area of the central Qiangtang Block (central Tibet). Age dating suggests that these lavas were generated in the late Early Oligocene (ca. 30 Ma). Two series of rocks were recognized: silica-undersaturated and -saturated lavas. Both series are geochemically characterized by continuous variation trends. All rocks are enriched in light rare earth elements (LREEs) relative to heavy REEs (HREEs) and depleted in Ta-Nb-Ti, with negligible Sr and Eu and obviously positive Pb anomalies. The rocks have higher Th/ La (0.2-0.8) and Nb/Ta (18.6-19.8) values than asthenospheric mantle-derived rocks but slightly variable values of epsilon(Nd) (t) (-6.0 to -4.6), (Sr-87/Sr-86)(i) (0.7076-0.7083), and zircon epsilon(HF)(t) (-7.2 to +0.8) and delta O-18 (7.1-8.5 parts per thousand). We suggest that the parental magmas of these K-rich lavas were likely derived by variable partial melting of lithospheric mantle metaso-matized by subducting continental sediment-derived melts with the addition of minor asthenospheric components. Element modeling results suggest that the basic end members were likely generated by fractionation processes dominated by olivine and minor clinopyroxene from parental magmas, and some other evolved rocks were generated by crystal (olivine, clinopyroxene, Fe-Ti oxides) fractionation from basic magmas. Combining geological and Cenozoic magmatic rock data in the Qiangtang Block, we propose that the Gemuchaka magmatism was most likely triggered by the delamination of lithospheric mantle thickened by early (Paleocene- Eocene) intracontinental subduction and subsequent asthenospheric upwelling.

Geochronology, geochemistry and Sr-Nd-Pb-Hf isotopes of the Early Paleogene gabbro and granite from Central Lhasa, southern Tibet: petrogenesis and tectonic implications

ABSTRACTThe Tibetan Plateau is a composite orogenic belt that has experienced prolonged subduction, obduction, and collisional processes, during the opening and closure of successive Tethyan oceans. We present new zircon U-Pb ages and Hf isotopes, and whole-rock geochemical and Sr-Nd-Pb isotopic data from the Early Paleogene Longge’er gabbro and Qingduxiang granite of Central Lhasa, southern Tibet. Together these allow us to refine existing models for widespread magmatic activity associated with the subduction of the Neo-Tethyan Ocean. The Longge’er gabbro (53.5 ± 1.6 Ma) belongs to the low-K tholeiitic to medium-K and metaluminous series, while the Qingduxiang granite (54.5 ± 0.9 Ma) is characterized as high-K, calc-alkaline, metaluminous, and of I-type affinity. Both intrusions are enriched in the LREE and depleted in the HREE with negative Eu, Ba, Nb, Ta, Sr, and Ti anomalies. Trace elements characteristics and enriched whole-rock Sr-Nd-Pb and zircon Hf isotopic compositions demonstrate that the gabbro was derived from partial melting of enriched lithosphere mantle metasomatized by Central-Lhasa ancient crustal materials, while the I-type granite was generated by partial melting of Central-Lhasa ancient lower crust combined with magmas derived from Southern-Lhasa juvenile crust. Geochemical compositions of the gabbro and granite reveal the Early Paleogene magmatism was emplaced in a shallow extensional setting related to slab break off following the closure of the Neo-Tethyan Ocean. Combined with previous studies, we can infer slab rollback occurred from Late Cretaceous (~69 Ma) to Early Eocene (55 Ma), while slab break off was shortly lived at ca. 55–49 Ma. Consequently, the India-Asia collision must not have started later than ca. 55 Ma.

Geochronology and geochemistry of Ordovician plutons in the Erguna Block (NE China): further insights into the tectonic evolution of the Xing’an–Mongolia Orogenic Belt

ABSTRACTThe Ordovician plutons in the Erguna Block, NE China, can be classified into two groups: Early Ordovician diorites with zircon U–Pb ages ranging from 486 to 485 Ma and Middle Ordovician gabbros and granites with zircon U–Pb ages ranging from 466 to 463 Ma. The diorites are calc-alkaline in nature and are characterized by weak to moderate enrichments of large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high field strength elements (HFSE) and heavy rare earth elements (HREE). The gabbros and granites have high total alkali contents, and all samples are enriched in LREE and LILE and depleted in HFSE such as Nb, Ta, and Ti. Isotopically, Early Ordovician diorites display values that are less radiogenic [εHf(t) = + 9.9–+16.8] compared to those of Middle Ordovician gabbros [εHf(t) = − 3.0–+5.0]. Middle Ordovician granites have positive εHf(t) values of +1.4 to +4.3 and two-stage Hf model ages (TDM2) of 1167 to 1356 Ma. These data indicate that the diorites may have been generated by the partial melting of a recently metasomatized mantle source, whereas the gabbros and granites may have been formed by the partial melting of enriched lithospheric mantle and Mesoproterozoic crust, respectively. Our results, combined with other regional results, suggest that Early Ordovician magmatism was likely associated with the northward subduction of the Heihe–Xilinhot oceanic plate beneath the Erguna–Xing’an Block, whereas the Middle Ordovician gabbros and granites were most likely formed in an extensional setting controlled by the rollback of this subducted oceanic plate.

Petrogenesis of Cretaceous volcanic-intrusive complex from the giant Yanbei tin deposit, South China: Implication for multiple magma sources, tin mineralization, and geodynamic setting

The giant Yanbei tin ore deposit is the largest porphyry-type tin deposit in South China. The orebodies are hosted by the granite porphyry in the central part of the Yanbei volcanic basin in southern Jiangxi Province. The Yanbei volcanic-intrusive complex mainly consists of dacitic-rhyolitic volcanic rocks, granite, granite porphyry and diabase dikes. In previous papers, the granite porphyry was considered as subvolcanic rocks, which came from the same single magma chamber with the volcanic rocks. In this study, zircon U-Pb ages and Hf isotope data, as well as whole-rock geochemical and Sr-Nd isotopic compositions of different magmatic units in the Yanbei complex are reported. Geochronologic results show that various magmatic units have different formation ages. The dacite yielded a zircon U-Pb age of 143±1Ma, and the granite porphyry has the emplacement age of 138±1Ma. Diabase dikes which represented the final stage of magmatism, yielded a zircon U-Pb age of 128±1Ma. Distinctive whole rock Sr-Nd and zircon Hf isotopic compositions suggest that these magmatic units were derived from different magma sources. The volcanic rocks were mainly derived from the partial melting of Paleoproterozoic metasedimentary rocks without additions of mantle-derived magma. The granite porphyry has an A-type geochemical affinity, and was derived from remelting of Paleo-Mesoproterozoic crustal source with involvement of a subordinate mantle-derived magma. The granite porphyry is also a typical stanniferous granite with high F (4070–6090ppm) and Sn (7–39ppm) contents. It underwent strongly crystal fractionation of plagioclase, K-feldspar, and accessory minerals (like apatite, Fe-Ti oxides), which may contribute to the tin mineralization. The diabase was derived by partial melting of enriched lithospheric mantle which had been metasomatised by slab-derived fluids. The change of magmatic sources reflected an increasing extensional tectonic environment, perhaps induced by slab rollback of subducted paleo-Pacific plate.

Ages and petrogenesis of Jurassic and Cretaceous intrusive rocks in the Matsu Islands: Implications for lower crust modification beneath southeastern China

Major and trace element, whole-rock Sr-, Nd- and Hf-isotope, zircon U-Pb age and Hf-O isotope data are reported for the intrusive rocks from the Matsu Islands in the coastal area of southeastern (SE) China, in order to study the ages, sources and petrogenesis of these rocks and evolution of the lower crust. The rocks include gneissic granite, massive granite, brecciated granite and diabase. Secondary ion mass spectrometer (SIMS) zircon U-Pb dating reveals that the rocks in the Matsu Islands were emplaced at ∼160 Ma, ∼130 Ma and ∼94 Ma. The Jurassic granites (∼160 Ma) have high SiO2 (74.1–74.5 wt%) and K2O+Na2O (8.32–8.33 wt%) contents and high Rb/Sr ratios of 0.6–1.2 and (La/Yb)CN ratios of 12.6–19.4. Their relatively high initial 87Sr/86Sr ratios (0.7074–0.7101), variable and negative εNd(t) values (−9.2 to −5.4), and variable zircon εHf(t) (−17.0 to +5.2) and δ18O (4.7–8.1‰) values indicate they were mainly derived from an ancient lower crustal source, but with involvement of high εHf(t) and low δ18O materials. The Early Cretaceous diabase (∼130 Ma) has SiO2 content of 56.5 wt%, relatively high MgO concentration, low initial 87Sr/86Sr ratio and negative εNd(t) value, similar to geochemical features of other Cretaceous mafic rocks in the coastal area of SE China. Zircons from the diabase have high εHf(t) values (−5.5 to +0.2) and relatively low δ18O values of 4.2–5.0‰. These characteristics indicate that the parental magma of the diabase was generated by partial melting of enriched lithospheric mantle, which have been metasomatised by altered oceanic crust-derived low-δ18O fluids. For the Cretaceous granitoids (∼130 Ma and 94 Ma), they have relatively low SiO2 (68.0–71.3 wt%) and K2O+Na2O (5.30–7.55 wt%) contents and low Rb/Sr ratios and (La/Yb)CN ratios of 5.8–7.1. They have low initial 87Sr/86Sr ratios (0.7071–0.7082), homogeneous εNd(t) (−4.3 to −4.5) and relatively high zircon εHf(t) values (−3.7 to +1.2) and low δ18O values (4.6–5.9‰). Their isotopic compositions are similar to those of the diabases in this study as well as other Cretaceous mafic rocks in the coastal area of SE China, suggesting that the sources of the Cretaceous granitoids might be the newly formed lower crust related to the underplated mafic rocks. Whole-rock geochemical, Sr-Nd and zircon Hf-O isotopic compositions indicate that the Jurassic granitoids are most likely generated by partial melting of relatively ancient basement rocks, whereas the Cretaceous granitoids were generated by partial melting of relatively young lower crustal rocks with addition of mantle-derived magma. This distinction implies that the pre-existing ancient lower crust beneath the coastal area of SE China has been modified by large-scale mafic magma underplating. Therefore, underplating of mantle-derived mafic magma would result in modification of the pre-existing ancient lower crust and formation of the relatively juvenile lower crust.

Early Cretaceous diabases, lamprophyres and andesites-dacites in western Shandong, North China Craton: Implications for local delamination and Paleo-Pacific slab rollback

Early Cretaceous tectono-magmatic activity in the eastern North China Craton has long been linked to cratonic destruction, but no systematic investigations about the geodynamic processes have been performed. Early Cretaceous diabases, lamprophyres, andesites and dacites are distributed in the western Shandong Peninsula, and these rocks have a close relationship as they formed synchronously. Zircon U-Pb dating results show that the diabases and lamprophyres formed dykes or sills at 133–127Ma and that the andesites erupted at ∼125Ma. The diabases could be divided into high- and low-Mg# value groups. The high-Mg# (>60) diabases have lower SiO2 (48.44–54.68wt%) and higher MgO (7.41–19.30wt%) contents than the low-Mg# (<60) ones (SiO2: 52.90–56.41wt%; MgO: 2.70–4.37wt%). The lamprophyres (Mg# values of 58–70) exhibit low SiO2 (45.04–49.99wt%) and moderate MgO (4.77–9.87wt%) contents. The diabases and lamprophyres are enriched in large ion lithophile elements (e.g., Ba, Th and U) and light rare earth elements, are depleted in Nb and Ta, and feature negligible Eu anomalies. They have lower zircon εHf(t) values (−12 to −19) and whole-rock εNd(t) values (2 to −12), with (87Sr/86Sr)i ratios of 0.7066–0.7097. The andesites and dacites have intermediate SiO2 contents (60.10–64.95wt%), high Mg# values (56–65), high Sr (1005–1300ppm), and low Y (11.3–12.9ppm) contents, with high Sr/Y ratios (78–115), showing adakitic affinity. They have lower zircon εHf(t) (−12 to −26) and whole-rock εNd(t) (−9 to −13) values and high (87Sr/86Sr)i ratios (0.7109–0.7111). We suggest that the diabases and lamprophyres were derived from the partial melting of enriched lithospheric mantle, and the parental magmas of some of them had more contribution from the asthenosphere. The andesites and dacites were derived from the partial melting of delaminated thickened lower crust with addition of mantle material. Since the late Mesozoic, local delamination may have occurred beneath the Shandong Peninsula. During the Early Cretaceous, the rollback process of the Paleo-Pacific slab beneath the Shandong Peninsula could have facilitated delamination of the lithospheric mantle, and enhances the activity of trans-lithospheric faults.