Heterogeneous lithospheric mantle metasomatism in the eastern North China Craton: He–Ar isotopes in peridotite xenoliths from Cenozoic basalts

The abundances and isotopic compositions of Helium and Argon have been analyzed in a suite of fresh spinel peridotite xenoliths in Cenozoic basalts from the eastern North China Craton (NCC) by step-wise heating experiments, to investigate the nature of noble gas reservoirs in the subcontinental lithospheric mantle beneath this region. The xenoliths include one harzburgite collected from Hebi in the interior of the NCC, two lherzolites from Hannuoba at the northern margin of the craton, and three lherzolites from Shanwang and Nushan on the eastern margin. ³He/⁴He ratios in most of the xenoliths are similar to those of mid-ocean ridge basalts (MORB) or slightly lower (2–10.5 Ra, where Ra is the ³He/⁴He ratio of the atmosphere), suggesting mixing of MORB-like and radiogenic components. One olivine separate from Nushan has a helium value of 25.3 Ra, probably suggesting cosmogenic ³He addition. The ⁴⁰Ar/³⁶Ar ratios vary from atmospheric value (296) to 1625, significantly lower than the MORB value. Available data of the peridotite xenoliths indicate the He and Ar isotopic systematics of the mantle reservoirs beneath the NCC can be interpreted as mixtures of at least three end-members including MORB-like, radiogenic and atmospheric components. We suggest that the MORB-like noble gases were derived from the underlying asthenosphere during mantle upwelling, whereas the radiogenic and recycled components probably were incorporated into the lithospheric mantle during circum-craton subduction of oceanic crust. Available data suggest that the MORB-like fluids are better preserved in the interior of the NCC, whereas the radiogenic ones are more prevalent at the margins. The Paleo-Asian ocean subduction system probably was responsible for the enriched and recycled noble gas signatures on the northern margin of the craton, while the Pacific subduction system could account for the observed He–Ar isotopic signatures beneath the eastern part. Therefore, integration of helium and argon isotopes reflects heterogeneous metasomatism in the lithospheric mantle and demonstrates the critical importance of lithospheric mantle modification related to both circum-craton subduction of oceanic crust and asthenospheric upwelling beneath the eastern NCC.     

Crust–mantle interaction recorded by Early Cretaceous volcanic rocks within the southern margin of the North China Craton

This study presents new geochronological and geochemical data for Early Cretaceous volcanic rocks in the southern margin of the North China Craton (NCC), to discuss the crust–mantle interaction. The studied rocks include pyroxene andesites from Daying Formation, hornblende andesites and andesites from Jiudian Formation, and rhyolites from a hitherto unnamed Formation. These rocks formed in Early Cretaceous (138–120 Ma), with enrichment in light rare earth elements (REE), depletion in heavy REE and arc-like trace elements characteristics. Pyroxene andesites show low SiO₂ contents and enriched Sr–Nd–Pb–Hf isotopic compositions, with orthopyroxene phenocryst and Paleoproterozoic (2320–1829 Ma) inherited zircons, suggesting that they originated from lithospheric mantle after metasomatism with NCC lower crustal materials. Hornblende andesites have low SiO₂ contents and high Mg# (Mg# = 100 Mg/(Mg + Fe²⁺)) values, indicating a lithospheric-mantle origin. Considering the distinct whole-rock Sr isotopic compositions we divide them into two groups. Among them, the low (⁸⁷Sr/⁸⁶Sr)_i andesites possess amount inherited Neoarchean to Neoproterozoic (2548–845 Ma) zircons, indicating the origin of lithospheric mantle with addition of Yangtze Craton (YC) and NCC materials. In comparison, the high (⁸⁷Sr/⁸⁶Sr)_i andesites, with abundant Neoarchean–Paleozoic inherited zircons (3499–261 Ma), are formed by partial melting of lithospheric mantle with incorporation of NCC supracrustal rocks and YC materials. Rhyolites have extremely high SiO₂ (77.63–82.52 wt.%) and low total Fe₂O₃, Cr, Ni contents and Mg# values, combined with ancient inherited zircon ages (2316 and 2251 Ma), suggesting an origin of NCC lower continental crust. Considering the presence of resorption texture of quartz phenocryst, we propose a petrogenetic model of ‘crystal mushes’ for rhyolites prior to their eruption. These constraints record the intense crust–mantle interaction in the southern margin of the NCC. Given the regional data and spatial distribution of Early Cretaceous rocks within NCC, we believe that the formation of these rocks is related to the contemporaneous far-field effect of the Paleo-Pacific Plate.     

Re–Os isotope evidence from Mesozoic and Cenozoic basalts for secular evolution of the mantle beneath the North China Craton

The mechanism and process of lithospheric thinning beneath the North China Craton (NCC) are still debated. A key criterion in distinguishing among the proposed mechanisms is whether associated continental basalts were derived from the thinning lithospheric mantle or upwelling asthenosphere. Herein, we investigate the possible mechanisms of lithospheric thinning based on a systematic Re–Os isotopic study of Mesozoic to Cenozoic basalts from the NCC. Our whole-rock Re–Os isotopic results indicate that the Mesozoic basalts generally have high Re and Os concentrations that vary widely from 97.2 to 839.4 ppt and 74.4 to 519.6 ppt, respectively. They have high initial ¹⁸⁷Os/¹⁸⁸Os ratios ranging from 0.1513 to 0.3805, with corresponding variable γ_Os(t) values (+20 to +202). In contrast, the Re–Os concentrations and radiogenic Os isotope compositions of the Cenozoic basalts are typically lower than those of the Mesozoic basalts. The lowest initial ¹⁸⁷Os/¹⁸⁸Os ratios of the Cenozoic basalts are 0.1465 and 0.1479, with corresponding γ_Os(t) values of +15 and +16, which are within the range of ocean island basalts. These new Re–Os isotopic results, combined with the findings of previous studies, indicate that the Mesozoic basalts were a hybrid product of the melting of pyroxenite and peridotite in ancient lithospheric mantle beneath the NCC. The Cenozoic basalts were derived mainly from upwelling asthenosphere mixed with small amounts of lithospheric materials. The marked differences in geochemistry between the Mesozoic and Cenozoic basalts suggest a greatly reduced involvement of lithospheric mantle as the magma source from the Mesozoic to the Cenozoic. The subsequent lithospheric thinning of the NCC and replacement by upwelling asthenospheric mantle resulted in a change to asthenosphere-derived Cenozoic basalts.     

Evolution of subcontinental lithospheric mantle beneath eastern China: Re–Os isotopic evidence from mantle xenoliths in Paleozoic kimberlites and Mesozoic basalts

The ages of subcontinental lithospheric mantle beneath the North China and South China cratons are less well-constrained than the overlying crust. We report Re–Os isotope systematics of mantle xenoliths entrained in Paleozoic kimberlites and Mesozoic basalts from eastern China. Peridotite xenoliths from the Fuxian and Mengyin Paleozoic diamondiferous kimberlites in the North China Craton give Archean Re depletion ages of 2.6–3.2 Ga and melt depletion ages of 2.9–3.4 Ga. No obvious differences in Re and Os abundances, Os isotopic ratios and model ages are observed between spinel-facies and garnet-facies peridotites from both kimberlite localities. The Re–Os isotopic data, together with the PGE concentrations, demonstrate that beneath the Archean continental crust of the eastern North China Craton, Archean lithospheric mantle of spinel- to diamond-facies existed without apparent compositional stratification during the Paleozoic. The Mesozoic and Cenozoic basalt-borne peridotite and pyroxenite xenoliths, on the other hand, show geochemical features indicating metasomatic enrichment, along with a large range of the Re–Os isotopic model ages from Proterozoic to Phanerozoic. These features indicate that lithospheric transformation or refertilization through melt-peridotite interaction could be the primary mechanism for compositional changes during the Phanerozoic, rather than delamination or thermal-mechanical erosion, despite the potential of these latter processes to play an important role for the loss of garnet-facies mantle. A fresh garnet lherzolite xenolith from the Yangtze Block has a Re depletion age of ∼1.04 Ga, much younger than overlying Archean crustal rocks but the same Re depletion ages as spinel lherzolite xenoliths from adjacent Mesozoic basalts, indicating Neoproterozoic resetting of the Re–Os system in the South China Craton.     

Meso–Cenozoic thermal regime in the Bohai Bay Basin,eastern North China Craton

This article discusses the Meso–Cenozoic thermal history, thermal lithospheric thinning, and thermal structure of the lithosphere of the Bohai Bay Basin, North China. The present-day thermal regime of the basin features an average heat flow of 64.5 ± 8.1 mW m^–2, a lithospheric thickness of 76–102 km, and a ‘hot mantle but cold crust’-type lithospheric thermal structure. The Meso–Cenozoic thermal history experienced two heat flow peaks in the late Early Cretaceous and in the middle to late Palaeogene, with heat flow values of 82–86 mW m^?2 and 81–88 mW m^?2, respectively. Corresponding to these peaks, the thermal lithosphere experienced two thinning stages during the Cretaceous and Palaeogene, reaching a minimum thickness of 43–61 km. The lithospheric thermal structure transformed from the ‘hot crust but cold mantle’ type in the Triassic–Jurassic to the ‘cold crust but hot mantle’ type in the Cretaceous–Cenozoic, according to the ratio of mantle to surface heat flow (q_m/q_s). The research on the thermal history and lithospheric thermal structure of sedimentary basins can effectively reveal the thermal regime at depth in the sedimentary basins and provide significance for the study of the basin dynamics during the Meso–Cenozoic.     

The Mesoproterozoic Zig-Zag Dal basalts and associated intrusions of eastern North Greenland: mantle plume–lithosphere interaction

The lavas of the Zig-Zag Dal Formation of eastern North Greenland constitute a Mesoproterozoic tholeiitic flood basalt succession up to 1,350 m thick, extending >10,000 km², and underlain by a sill complex. U–Pb dating on baddeleyite from one of the sills thought to be contemporaneous with the lava extrusion, gives an age of 1,382±2 Ma. The lavas, subdivided from oldest to youngest into Basal, Aphyric and Porphyritic units, are dominantly basaltic (>6 wt.% MgO), with more evolved lavas occurring within the Aphyric unit. The most magnesian lavas occur in the Basal unit and the basaltic lavas exhibit a generalised upward decrease in Mg number (MgO/(MgO + Fe₂O₃^T)) through the succession. All of the lavas are regarded as products of variable degrees of olivine, augite and plagioclase fractionation and to be residual after generation of cumulates in the deep crust. The basaltic lavas display an up-section fall in the ratio of light to heavy rare-earth elements (LREE/HREE) but an up-section rise in Zr/Nb, Sc, Y and HREE. The older lavas (Basal and Aphyric units) are characterised by low _Nd and _Hf in contrast to higher values in the younger (Porphyritic unit) lavas. The Porphyritic Unit basalts are characterised by a notable enrichment in Fe and Ti. The Zig-Zag Dal succession is inferred to reflect an increase in melt fraction in the sub-lithospheric mantle, with melting commencing in garnet–lherzolite facies peridotites and subsequently involving spinel-facies mantle at increasingly shallow depths. Melting is deduced to have occurred beneath an attenuating continental lithosphere in conjunction with ascent of a mantle plume. Lithospheric contamination of primitive melts is inferred to have diminished with time with the Porphyritic unit basalts being products of essentially uncontaminated plume-source magmas. The high iron signature may reflect a relatively iron-rich plume source.     

Melt–peridotite interaction in the shallow lithospheric mantle of the North China Craton: evidence from melt inclusions in the quartz-bearing orthopyroxene-rich websterite from Hannuoba

To better understand the origin, migration, and evolution of melts in the lithospheric mantle and their roles on the destruction of the North China Craton (NCC), we conducted a petrological and geochemical study on a quartz-bearing orthopyroxene-rich websterite xenolith from Hannuoba, the NCC, and its hosted melt and fluid inclusions. Both clinopyroxene and orthopyroxene in the xenolith contain lots of primary and secondary inclusions. High-temperature microthermometry of melt inclusions combined with Raman spectroscopy analyses of coexisting fluid inclusions shows that the entrapment temperature of the densest inclusions was ~1215°C and the pressure ~11.47 kbar, corresponding to a depth of ~38 km, i.e. within the stability of the spinel lherzolite. Intermediate pressure inclusions probably reflect progressive fluid entrapment over a range of depths during ascent, whereas the low-pressure inclusions (P < 2 kbar) may represent decrepitated primary inclusions. In situ laser-ablation ICP-MS analyses of major and trace elements on individual melt inclusions show that the compositions of these silicate melt inclusions in clinopyroxene and orthopyroxene are rich in SiO₂, Al₂O₃, and alkalis but poor in TiO₂ and strongly enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), with negative anomalies of high-field strength elements (HFSEs). These characteristics suggest that the silica-rich melts could be derived from the partial melting of subducted oceanic slab. Therefore, this kind of quartz-bearing orthopyroxene-rich websterite may be produced by interaction between the slab-derived melts with the mantle peridotite. This study provides direct evidence for the origin, migration, and evolution of melts in the lithospheric mantle, which may play an important role in the destruction of the NCC.     

Non-marine gastropods from the Cretaceous–Paleogene transition in the Pingyi Basin,eastern China

Our results present a taxonomic and palaeoecological study on non-marine gastropods from the latest Cretaceous to Paleocene deposits of the Pingyi Basin, Shandong Province, eastern China. These gastropods are systematically described: three species belonging to three genera including Physa dongtaiensis Gu, 1989, Hydrobia datangensis Yü, 1977, and the newly established species Cyathopoma pingyiensis sp. nov.; two indeterminata genera and species including Truncatelloidea gen. et sp. indet., and Pomatiopsidae gen. et sp. indet. Among them, Truncatelloidea gen. et sp. indet. and Cyathopoma pingyiensis sp. nov. are the dominant species with the longest record. Cyathopoma pingyiensis, sp. nov. is the earliest representative of this widely distributed Asian extant genus. Sedimentological facies analysis of the gastropod-bearing beds suggested that Truncatelloidea gen. et sp. indet. lived in a small pond with a river inlet and a shallow lake, while Ph. dongtaiensis, ?Pomatiopsidae gen. et sp. indet, and ?H. datangensis only thrived in the shallow lake. Cyathopoma pingyiensis sp. nov. inhabited the land area around the shallow lake. Our results showed that no significant species change of the gastropod fauna across the K/Pg (Cretaceous/Paleogene) boundary was observed in the Pingyi Basin.     

Origin of Late Cenozoic Abaga–Dalinuoer basalts,eastern China: Implications for a mixed pyroxenite–peridotite source related with deep subduction of the Pacific slab

Continental intraplate basalts (15.42–0.16 Ma) from Abaga–Dalinuoer volcanic field (ADVF) in central Inner Mongolia of eastern China, as a part of Cenozoic volcanic province along eastern margin of the Eurasian continent, provide a good opportunity to explore potential links between deep subduction of the Pacific slab and continental intraplate volcanism. In this study, we report an integrated dataset of whole-rock K–Ar ages, major and trace elements and Sr–Nd–Pb isotopes, and olivine major and minor elements for the Abaga–Dalinuoer basalts (ADBs), and propose that mantle source lithology of the ADB magmas may consist of both pyroxenite and peridotite. The ADBs display low SiO₂ (42.3–50.2 wt.%), high MgO (7.3–11.4 wt.%) and moderate K₂O + Na₂O (3.8–6.4 wt.%), and can be subdivided into basanites, alkali basalts and tholeiitic basalts that are all characterized by ocean island basalt (OIB)-like rare earth elements (REE) and enrichment in both large ion lithosphile elements (LILE) and high field strength elements (HFSE). Olivine phenocrysts have higher Ni and Fe/Mn and lower Mn, Ca and Ca/Fe relative to those from peridotite melts, but exhibit clearly lower Ni contents (< 2500 ppm) compared with expected Ni range (> 3000 ppm) for olivines crystallized from olivine-free pyroxenite melts. Estimated compositions of the ADB primary magmas, together with olivine compositions, suggest an iron-rich mantle source related with silica-deficient pyroxenite that is most likely derived from deeply subducted Pacific oceanic crust. Additionally, peridotite and recent subducted sediments are also required to account for high Ni and MgO in primary magmas together with their trace elements and Sr–Nd–Pb isotope systematics. We suggest that a mixed pyroxenite–peridotite source lithology can better match observed whole-rock and olivine signatures in the ADB, compared with either peridotite only or olivine-free pyroxenite only source lithology. In our model, pyroxenite melts would either react with or mechanically mix with peridotite, and the proportion of pyroxenite melts may range from 30% to 45% for mechanical mixing scenario. A continuous spectrum from tholeiitic to alkali melts revealed by melt-peridotite reaction experiment can explain formation of primary magmas of basanites, alkali basalts and tholeiitic basalts by increasing melting degree of a similar mantle source. Relatively higher ²⁰⁶Pb/²⁰⁴Pb of the ADB may suggest more significant role of recent (< 0.5 Ga) subducted Pacific oceanic materials, in contrast to other Cenozoic basalts in eastern China (e.g., Changbai basalts) that exhibit varying contributions from ancient (> 1.5 Ga) subducted continental sediments. We emphasize that coupled geochemical and geodynamic links (i.e., subduction polarity) between deeply subducted Pacific slab and continental intraplate volcanism in eastern China may exist, which directly support the involvement of deeply subducted Pacific materials in petrogenesis of the ADB. From the perspective of plate motion kinetics, decompression partial melting of upwelling fragmented Pacific slab (silica-deficient pyroxenite + recent subducted sediments) may be triggered by rollback of deeply subducted Pacific slab during Late Cenozoic times. Continental intraplate volcanism in the ADVF generally started with termination of opening of the Japan Sea, suggesting that deep subduction of the Pacific slab may have been an important geodynamic mechanism responsible for tectono-magmatic evolution of northeastern Asia. We suggest that the ADBs have the potential to shed light on genetic links between continental intraplate volcanism and deep subduction of the Pacific slab in geochemical and geodynamic processes.     

Oceanic lithosphere heterogeneity in the eastern Paleo-Tethys revealed by PGE and Re–Os isotopes of mantle peridotites in the Jinshajiang ophiolite

Platinum group elements (PGE) and Re–Os isotopes of mantle peridotites in the Jinshajiang ophiolite (SW China) were investigated in this study, in order to cons...     

Meso—Cenozoic lithospheric thermal structure in the Bohai Bay Basin,eastern North China Craton

The Bohai Bay Basin is a region where part of the North China Craton has been thinned and destroyed. It has experienced two periods of crustal thinning that occurred during the Cretaceous and Paleogene, but investigations of its Mesozoic and Cenozoic lithospheric thermal structure are limited. Therefore, in this study,the distributions of mantle heat flow, crustal heat flow, and Moho temperatures during the Meso-Cenozoic are calculated based on analyses of the thermal history of the Bohai Bay Basin. The results indicate that the ratio of mantle heat flow to surface heat flow peaked during the late stages of the early Cretaceous and during the middle to late Paleogene. The corresponding mantle heat flow was more than 65% of the surface heat flow. Moho temperatures reached three peaks: 900-1100℃ in the late stages of the early Cretaceous;820-900℃ in the middle to late Paleogene; and(in the Linqing Depression, Cangxian Uplift, and Jizhong Depression) 770-810℃ during the early Neogene. These results reveal that the Bohai Bay Basin experienced significant geological change during the Cretaceous, including the transformation of lithospheric thermal structure from "cold mantle and hot crust" before the Cretaceous to "hot mantle and cold crust" after the Cretaceous. The results also indicate that the basin experienced two large-scale rifting events.Therefore, this work may provide the thermal parameters for further investigations of the geodynamic evolution of eastern China.     

Late Oligocene–Miocene mantle upwelling and interaction inferred from mantle signatures in gabbroic to granitic rocks from the Urumieh–Dokhtar arc,south Ardestan,Iran

The south Ardestan plutonic rocks constitute major outcrops in the central part of Iran’s Cenozoic magmatic belt and encompass a wide compositional spectrum from gabbro to granodiorite. U–Pb laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) dating of zircon three granodiorites yielded ages of 24.6 ± 0.1, 24.6 ± 0.1, and 24.5 ± 0.1 Ma. For tonalitic rocks, internal Rb–Sr isochron ages (biotite, feldspars) indicate cooling ages of 20.4 ± 0.1, 20.5 ± 0.1, and 22.3 ± 0.1 Ma, which are slightly younger than the zircons’ ages. The limited variations in their Sr–Nd isotope ratios indicate derivation from an asthenospheric mantle source. A geodynamic model is presented in which late Oligocene–Miocene rollback of the Neotethyan subducting slab triggered asthenospheric upwelling and partial melting in the south Ardestan. These melts were subsequently modified through fractional crystallization and minor crustal contamination en-route to the surface. Plagioclase + orthopyroxene-dominated fractional crystallization accounts for differentiation of gabbro to gabbroic diorite, whereas fractionation of clinopyroxene, titanomagnetite, and orthopyroxene led to differentiation of gabbroic diorite to diorite. Amphibole fractionation at deeper levels led to the development of tonalites.     

Paleoproterozoic gabbronoritic and granitic magmatism in the northern margin of the North China craton: Evidence of crust–mantle interaction

Paleoproterozoic Xuwujia gabbronorites in the northern margin of the North China craton occur as dykes, sills and small plutons intruded into khondalite (aluminous paragneisses, sedimentary protoliths deposited at ca. 2.0–1.95 Ga), and as numerous entrained bodies and fragments of variable scales in the Liangcheng granitoids (ca. 1.93–1.89 Ga). These gabbronoritic dykes are present at all locations where ca. 1.93–1.92 Ga ultra-high-temperature metamorphism is recorded in the khondalite. A gabbronorite sample from the Hongmiaozi dyke gives zircon ²⁰⁷Pb/²⁰⁶Pb mean ages of 1954 ± 6 Ma (core domains) and 1925 ± 8 Ma (rim domains). These ages, as well as previously reported ages, constrain the age of mafic magmatism to be at ca. 1.96–1.92 Ga (∼1.93 Ga). One sample from the Xigou gabbro intruded by the Liangcheng granitoids gives a zircon ²⁰⁷Pb/²⁰⁶Pb mean age of 1857 ± 4 Ma, which is interpreted as the age of a metamorphic overprint. The Xuwujia gabbronorites comprise mainly gabbronorite compositions, as well as some norite, olivine gabbronorite, monzonorite, quartz gabbronorite, and quartz monzonorite. Chemically, they are tholeiitic and can be divided into two groups: a high-Mg group (6.2–22.9 wt.% MgO) and a relatively low-Mg group (2.2–5.7 wt.% MgO). The high-Mg group shows negative Eu-anomalies (Eu/Eu* = 0.53–0.72), slight light rare earth element enrichment (La/Yb_N = 0.56–1.53), and small negative anomalies in high field-strength elements. The ?Nd (t = 1.93 Ga) values vary from +0.3 to +2.4. The low-Mg group shows varied Eu-anomalies (Eu/Eu* = 0.48–1.05), and is enriched in light rare earth elements (La/Yb_N = 1.51–11.98). The majority shows negative anomalies in high field-strength elements (e.g., Th, Nb, Zr, and Ti). Initial ?Nd (at 1.93 Ga) values for low-Mg gabbronorites vary from −5.0 to 0. The Xuwujia gabbronorites possibly experienced assimilation of crust, and fractional crystallization of initially olivine and hypersthene (the high-Mg group), and then olivine, clinopyroxene, and plagioclase (the low-Mg group). The slightly younger Liangcheng granitoids consist of garnet-bearing granite, granodiorite and quartz-rich granitic compositions. They are intermediate to felsic calc-alkaline rocks, thought to be derived from surrounding metasedimentary crust. Xigou gabbro could represent early cumulates. The granitoids have relatively high-Mg numbers (up to 54), and show some chemical affinities with the gabbronorites, which could have resulted from incorporation of gabbronoritic melts. The occurrence and chemical variations of the Xuwujia gabbronorites and Liangcheng granitoids can be interpreted to have resulted from crust–mantle interaction, with mingling and partial mixing of mantle (gabbronoritic) and crustal (granitic) melts. The Xuwujia gabbronorites originated from a mantle region with high potential temperatures (∼1550 °C), possibly associated with a plume or more likely a ridge-subduction-related mantle upwelling event. They could have had extremely high primary intrusion temperatures (up to 1400 °C). Emplacement of these magmas was likely responsible for the extensive crustal anatexis (Liangcheng granitoids) and the local ultra-high-temperature metamorphism. These sequences may have followed ca. 1.95 Ga continent–continent (arc?) juxtaposition and were themselves followed by significant regional uplift and exhumation in the northern margin of the North China craton.     

Geochemistry and Os–Nd–Sr isotopes of the Gaositai Alaskan-type ultramafic complex from the northern North China craton: implications for mantle–crust interaction

We report petrological, chemical and Os–Nd–Sr isotopic data for the Gaositai ultramafic complex from northern North China craton (NCC) to reveal its petrogenesis. The complex shows features of Alaskan-type intrusions, including (1) the concentric zoning from dunite core, to clinopyroxenite and hornblendite in the rim, and the common cumulative textures; (2) the abundance of olivine, clinopyroxene and hornblende, and the scarcity of orthopyroxene and plagioclase, and (3) the systematic decrease in Mg# of ferromagnesian phases from core to rim, accompanied by the Fe-enrichment trend of accessory spinel. The different rock types show highly varied, radiogenic Os isotopic ratios (0.129–5.2), and unradiogenic Nd isotopic composition (ε_Nd(t) = −8 to −15), but are homogeneous in I_Sr ratios (0.7054–0.7066). The (¹⁸⁷Os/¹⁸⁸Os)_i ratios are found to be anti-correlated with ε_Nd(t) values and whole-rock Mg# as well. These data suggest significant crustal contamination during magma evolution. The crustal contaminants are dominantly Archean mafic rocks in the lower crust, and subordinate TTG gneisses at shallower crustal levels. The parental magma was hydrous picritic in composition, derived from an enriched lithospheric mantle source above a subduction zone. The zoned pattern of the complex formed probably through “flow differentiation” of a rapidly rising crystal mush along a fracture zone that was developed as a result of lithospheric extension in a back-arc setting in the northern margin of the NCC at ca. 280 Ma.     

Re–Os dating of chalcopyrite from selected mineral deposits in the Kalatag district in the eastern Tianshan Orogen,China

The Kalatag Cu–Zn–Au district contains a number of economically important Cu deposits in eastern Tianshan in Xinjiang, NW China. Due to the lack of precise mineralization ages, the metallogenesis of this area has long been a matter of debate. In this study, chalcopyrite Re–Os isotope methods are used to date the South Meiling Cu–Zn and Hongshi Cu deposits in the eastern part of Kalatag area.The South Meiling Cu–Zn deposit is hosted in volcanic-sedimentary rocks of the Late Ordovician to Early Silurian Daliugou Formation. The deposit consists of two parts: a concordant massive sulfide ores and discordant vein-type ores located in the footwall strata. The principal ore minerals are pyrite, chalcopyrite, sphalerite, minor tetrahedrite, galena and pyrrhotite. Gangue minerals include quartz, sericite and barite, and minor chlorite, plagioclase and carbonate minerals. The Hongshi Cu deposit represents a hydrothermal vein system hosted in the mafic volcanic rocks of Daliugou Formation. The orebodies are associated with quartz veins and controlled by subsidiary faults of the Kalatag fault. The ore-forming process can be divided into the early, middle and late stages and is characterized by quartz–pyrite, quartz–chalcopyrite–pyrite and quartz–carbonate–gypsum veins, respectively.Re–Os analyses of chalcopyrite from the South Meiling Cu–Zn deposit yield an isochron age of 434.2 ± 3.9 Ma and initial ¹⁸⁷Os/¹⁸⁸Os ratio of 0.647 ± 0.098 (MSWD = 0.59). Re–Os analyses of chalcopyrite from the Hongshi Cu deposit yield an isochron age of 431.8 ± 2.7 Ma and initial ¹⁸⁷Os/¹⁸⁸Os ratio of − 0.165 ± 0.075 (MSWD = 0.77). Since chalcopyrite is the primary copper mineral, we interpret these isochron ages as the timing of Cu mineralization, based on field geology and petrographic evidence. These results suggest that the Re–Os ages presented here provide, for the first time, a direct constraint on an early Paleozoic Cu mineralization event of the eastern Tianshan Orogen. The high initial ¹⁸⁷Os/¹⁸⁸Os ratios (0.647 ± 0.098) ratio of ~ 434 Ma chalcopyrite from the South Meiling deposit suggest that the metal was sourced from a two end-member mixing of crust and mantle materials. Moreover, we propose that the VMS mineral system and hydrothermal vein system of the Kalatag district were related to the south-dipping subduction of the Kalamaili oceanic plate during the Late Ordovician–Silurian.     

Oxygen isotope zoning in garnets from Franciscan eclogite blocks: evidence for rock–buffered fluid interaction in the mantle wedge

The oxygen isotope compositions of eclogite and amphibolite garnets from Franciscan Complex high-grade blocks and actinolite rinds encasing the blocks were determined to place constraints on their fluid histories. SIMS oxygen isotope analysis of single garnets from five eclogite blocks from three localities (Ring Mountain, Mount Hamilton, and Jenner Beach) shows an abrupt decrease in the δ¹⁸O value by ~1–3 ‰ from core to rim at a distance of ~120 ± 50 μm from the rim in nine out of the 12 garnets analyzed. In contrast, amphibolite garnets from one block (Ring Mountain) analyzed show a gradual increase in δ¹⁸O value from core to rim, implying a different history from that of the eclogite blocks. Values of δ¹⁸O in eclogite garnet cores range from 5.7 to 11.6 ‰, preserving the composition of the eclogite protolith. The abrupt decrease in the δ¹⁸O values of the garnet rims to values ranging from 3.2 to 11.2 ‰ suggests interaction with a lower δ¹⁸O fluid during the final stages of growth during eclogite facies metamorphism (450–600 °C). We hypothesize that this fluid is sourced from the serpentinized mantle wedge. High Mg, Ni, and Cr contents of actinolite rinds encasing the blocks also support interaction with ultramafic rock. Oxygen isotope thermometry using chlorite and phengite versus actinolite of rinds suggests temperatures of 185–240 °C at Ring Mountain and Mount Hamilton. Rind formation temperatures together with the lower δ¹⁸O garnet rims suggest that the blocks were in contact with ultramafic rock from the end of garnet growth through low-temperature retrogression. We suggest a tectonic model in which oceanic crust is subducted at the initiation of subduction and becomes embedded in the overlying mantle wedge. As subduction continues, metasomatic exchange between high-grade blocks and surrounding ultramafic rock is recorded in low δ¹⁸O garnet rims, and later as temperatures decrease, with rind formation.     

Re–Os dating of sulphides from the Yushui Cu-polymetallic deposit in eastern Guangdong Province,South China

The Yushui Cu-polymetallic deposit, which is associated with Ag, Pb, and Zn, is located in the middle part of the Yongan–Meixian Late Paleozoic Hercynian depression. It was discovered in eastern Guangdong Province in the late 1980s and is one of the richest copper deposits in China with high-grade copper averaging 3.25% and locally reaching 50–60%. The main ore body is located along the unconformity between the Upper Carboniferous Hutian Group limestone and the Lower Carboniferous Zhongxin Formation quartz sandstone with a bedded and lenticular morphology. The ores exhibit massive textures dominated by chalcopyrite, bornite, chalcocite, pyrite, sphalerite, galena, and a trace amount of argentite. Although researchers began studying the Yushui deposit in the early 1990s, the ore genesis remains controversial because of the lack of precise mineralisation age constraints. In this study, direct Re–Os dating of Cu sulphides aided in facilitating a better understanding of the timing of formation of the Yushui deposit. This study is the first attempt to use the Re–Os isotopic system for directly dating chalcopyrite and bornite ores for the Yushui deposit. The contents of Re, common Os, ¹⁸⁷Re and ¹⁸⁷Os in nine sulphides are 1.68–219.35 ppb, 0.003–0.427 ppb, 1.05–137.31 ppb, and 0.045–0.734 ppb, respectively. The isotope data yielded an isochron age of 308 ± 15 Ma (mean square weighted deviates = 2.4) using the ⁸⁷Re/¹⁸⁸Os–¹⁸⁷Os/¹⁸⁸Os plot, which is interpreted to represent the age of formation for these sulphides, suggesting that the mineralisation age of the Yushui deposit is close to the age of the host rocks. The ¹⁸⁷Os/¹⁸⁸Os initial value obtained from the Re–Os isochron is 1.81 ± 0.34, which corresponds to the γOs value of + 1349. This value indicates that the ore-forming materials were derived from the crust without mixing with materials from the mantle, and that the Yushui massive sulphide deposit may be of sedimentary exhalative origin.     

A unique Paleoproterozoic HP–UHT metamorphic event recorded by the Bengbu mafic granulites in the southwestern Jiao–Liao–Ji Belt,North China Craton

The Wuhe Complex in the Bengbu area of the Jiao–Liao–Ji Belt, southeast North China Craton, contains garnet-bearing mafic granulites that have undergone high-pressure (HP) and ultrahigh-temperature (UHT) metamorphism. These granulites also experienced partial melting and occur as lenses within marbles. Petrographic observations and quantitative phase equilibria modeling reveal clockwise P–T paths, involving an inferred HP stage followed by decompressional, medium-pressure, granulite-facies metamorphism and subsequent cooling. The HP assemblage of garnet + clinopyroxene + plagioclase + K-feldspar ± amphibole ± quartz ± rutile indicates P–T conditions of 840–980 °C and 12–17 kbar. This was followed by post-peak, near-isothermal decompression with the development of orthopyroxene + clinopyroxene + plagioclase + K-feldspar + garnet + amphibole + ilmenite at 850–960 °C and 7–10 kbar, resulting in the development of orthopyroxene rims on resorbed garnet. Pyroxene and ternary feldspar thermometry yielded high temperatures of ~1150 °C and 1055–1087 °C at 10 kbar, respectively, which constrain the minimum crystallization temperatures of the igneous protoliths. The host and lamellae of the pyroxene and ternary feldspar are relict magmatic minerals/textures that survived metamorphism due to the silica-undersaturated bulk-rock conditions. Zr-in-rutile thermometry yielded temperatures of ~935 °C and 800 °C, with the former being consistent with the predicted peak metamorphic temperatures. Small amounts of melts (up to 5%) were generated during decompression of the Bengbu mafic granulites. The generated partial melts were mainly (quartz) monzonite at 900–920 °C, and the silica contents of the melts were controlled by the quartz stability field in P–T pseudosections. The partial melts were enriched in Na and strongly depleted in Fe–Mg at the peak pressure of ~14 kbar and 920 °C, and later evolved to Fe–Mg-rich and high-K compositions during decompression. The melt compositions in the studied rocks are similar when the pressures reached ~9 kbar. The modal proportion of amphibole increased as the melt H₂O content decreased at lower pressures, indicating that the limited H₂O remaining in the host rocks was consumed to produce amphibole. U–Pb geochronology of zircon containing inclusions of clinopyroxene, plagioclase, and apatite constrains the timing of metamorphism to 1930–1840 Ma, as is the case for HP granulites from Shandong, Liaoning, and southern Jilin in the central and northeastern Jiao–Liao–Ji Belt. The Wuhe HP–UHT mafic granulites were ultimately sourced from upwelling asthenosphere-derived magma at ~2.1 Ga, which intruded and crystallized at shallower depths. The igneous protoliths were then buried to middle–lower crustal levels and experienced HP–UHT granulite-facies metamorphism and partial melting at 1.95–1.90 Ga related to continental subduction and overthickening. The HP–UHT mafic granulites were rapidly exhumed at ~1.85 Ga and generated small volumes of (quartz) monzonite during decompression. The newly discovered Paleoproterozoic HP–UHT mafic granulites associated with partial melting suggest that the continent materials were deeply subducted to the lower crustal levels and that additional heating was not involved. The finding of the HP–UHT granulites, together with the widespread distributions of the granulite-facies metamorphic rocks and the determination of the clockwise P–T–t paths, reveal that the Paleoproterozoic Jiao–Liao–Ji orogenic belt extends at least 1000 km, starting from southern Jilin, passing through the southeastern Liaoning and Jiaobei terranes, and elongating to the Bengbu area in Anhui.     

Magma mixing revealed from in situ zircon U–Pb–Hf isotope analysis of the Muhuguan granitoid pluton, eastern Qinling Orogen, China: implications for late Mesozoic tectonic evolution

In this paper, we present zircon U–Pb age and Hf isotope data to document the significance of magma mixing in the formation of Late Jurassic granitoid intrusions in the eastern Qinling Orogen, China. The Muhuguan granitoid pluton from this orogen consists of monzogranite and lesser biotite granite and granodiorite, all containing abundant hornblende-rich cumulates, dioritic xenoliths, and mafic magmatic enclaves (MMEs). The monzogranite and granodiorite are intruded by a number of lamprophyre dykes. Both a cumulate and a dioritic xenolith samples have concordant zircon U–Pb ages of ca. 161 ± 1 Ma, but possess contrasting Hf isotopic compositions. The cumulate has more radiogenic zircon Hf isotopes with negative ε _Hf(t) values (?7.9 to ?2.5) and T _DM1 ages of 0.9–1.1 Ga, indicating its derivation likely from basaltic rocks of the Neoproterozoic to early Paleozoic Kuanping Group in the area. The dioritic xenolith has much lower zircon ε _Hf(t) values of ?19.5 to ?8.8 and T _DM2 ages of 2.4–1.7 Ga, consistent with a juvenile Paleoproterozoic crust source presumably represented by the metabasic rocks of the Qinling Group in the area. Individual samples of the monzogranite, MME, and a lamprophyre dyke have U–Pb ages of 150 ± 1, 152 ± 1, and 152 ± 1 Ma, respectively, demonstrating coeval mafic and felsic magmatism in the Late Jurassic. The lamprophyre dyke has homogeneous, highly negative zircon ε _Hf(t) values (?29.8 to ?24.8) and Archean T _DM2 ages (3.0–2.7 Ga), and its genesis is interpreted as partial melting of an ancient enriched subcontinental mantle source. Zircons from the fine-grained MME show a large range of ε _Hf(t) between ?29.1 and ?9.8, overlapping values of the monzogranite and lamprophyre dyke samples. Zircon U–Pb age and Hf isotopes of the MMEs are consistent with their formation by mixing of crustal- and enriched mantle-derived magmas. The main group of zircons from the monzogranite has ε _Hf(t) values (?17.9 to ?9.3) and T _DM2 ages (2.3–1.8 Ga) that are compatible with the dioritic xenoliths, indicating that the former was produced by partial melting of Paleoproterozoic crustal source with involvement of mantle-derived magmas. Mafic magmatism revealed from the Muhuguan pluton indicates that the eastern Qinling Orogen was dominated by lithospheric extension during the Late Jurassic. Compilation of existing geological and geochronological data suggests that this extensional event started in Late Jurassic (ca. 160 Ma) and persisted into the Early Cretaceous until ca. 110 Ma. The Jura-Cretaceous extension may have resulted from the late Mesozoic westward subduction of the Pacific plate beneath the East Asian continental margin.     

Mesozoic collision-related magmatism and crust–mantle interaction along the Anhui segment of the Yangtze River Valley,east-central China

The middle segment of the Yangtze River Deep Fault Belt, located in the foreland of the Dabie orogen, contains widely exposed volcanic–intrusive complexes that formed during two episodes of magmatism (post-collisional and post-orogenic), reflecting crust–mantle interactions during the Late Jurassic (J₃) to Early Cretaceous (K₁). This article summarizes research on the Mesozoic igneous suites and xenolith suites in the area along the Yangtze River. ‘Post-collisional magmatism’ occurred during lithospheric extension at ～145–130 Ma. Its beginning and end are marked by gabbroic xenoliths and pyroxene cumulates within intrusions at Tongling, and by alkali-rich magmatic rocks. The association includes peraluminous silicic rocks and metaluminous mafic–felsic igneous suites, ranging from medium-K to high-K calc-alkaline to shoshonitic compositions. Taking the Tongling region as an example, quartz monzodiorite yields a sensitive high resolution ion microprobe (SHRIMP) zircon U–Pb age of 139.5 ± 2.9 Ma, and granodiorite yields an age of 135.5 ± 4.4 Ma. These intrusive rocks contain 52.79–66.46 wt.% SiO₂, 13.12–17.73 wt.% Al₂O₃, 1.37–4.62 wt.% MgO, 3.86–6.84 wt.% FeO^T, and 4.71–7.87 wt.% total alkalis (Na₂O?+?K₂O). ACNK values range from 0.62 to 1.20, and ANK values from 1.45 to 3.48. ‘Post-orogenic magmatism’ occurred during lithospheric delamination at ～130–120 Ma. The start of magmatism was marked by the formation of gabbro containing spinel lherzolite xenoliths in the Nanjing–Wuhu Basin (NWB), and its end was marked by the generation of feldspathoid phenocryst-bearing phonolite in the NWB and the Lujiang–Zongyang Basin (LZB), respectively. The association that formed during this episode ranges from alkaline to peralkaline. Taking the Niangniangshan Formation in the NWB as an example, the Nosite phonolite yields a whole-rock monomineral Rb–Sr isochron age of 120 ± 9 Ma, and contains 49.92–60.09 wt.% SiO₂, 17.67–20.65 wt.% Al₂O₃, 0.08–2.45 wt.% MgO, 1.32–6.62 wt.% FeO^T, and 9.24–13.92 wt.% total alkalis (Na₂O?+?K₂O). ACNK values range from 0.72 to 1.24, and ANK values from 1.03 to 1.35.

The two magmatisms correspond to two episodes of crust–mantle interaction. The first involved intensive interaction between middle–lower crust and underplated basaltic magma derived from the upper mantle lithosphere, whereas the second involved minor interaction between the middle–lower crust and basaltic magma derived from the lower lithospheric mantle.