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Oxygen fugacity controls the behavior of multivalent elements and compositions of C-O-H fluids in Earth’s mantle, which further affects the cycling of materials between the deep interior and surfac...

We present mineralogical and geochemical compositions of mantle xenoliths from two Cenozoic basalt localities of the northeastern North China Craton. These xenoliths include lherzolite, harzburgite, and websterite. They are generally fertile in major elements and different from the typical cratonic lithosphere, which is consistent with previous hypotheses regarding craton destruction. The ratios of 87Sr/86Sr and (La/Yb)N of clinopyroxenes (Cpx) in one lherzolite are relatively low in the core but high in the rim. The center of the Cpx grain has a high U concentration. Changes in trace elements and Sr isotopes indicate that later stage high 87Sr/86Sr melt metasomatism superimposed on the early hydrous melt/fluid. The Cpxs in some xenoliths are low in Ti/Eu but high in Ca/Al and light rare earth elements, which indicates carbonate melt metasomatism. 87Sr/86Sr is increased in the core and decreased in the rim of most Cpx grains, which reflects the superposition of two-stage metasomatism. The early agent should be high in 87Sr/86Sr, and the recent agent should be low in 87Sr/86Sr. The Cpxs in olivine websterite are low in 87Sr/86Sr (0.70220–0.70320), which reflects the recent metasomatism of asthenosphere-derived melt. Collectively, these observations reflect a three-stage modification of the lithospheric mantle. First-stage hydrous melt/fluid could come from the dehydration of young subducted plates. Second-stage melt/fluid of high 87Sr/86Sr could derive from the partial melting of the subducted altered oceanic crust, and the recent melt/fluid of low 87Sr/86Sr should be from the asthenosphere.

The Archean mantle lithosphere beneath the North China Craton (NCC) was transformed in the Mesozoic, leading to the craton destruction. Despite the significant breakthroughs in the craton studies, lithospheric transformation mechanisms are yet to be fully understood. Compositional variations of mantle-derived rocks and xenoliths provide insights into the nature of the mantle lithosphere before and after the craton destruction. The Paleozoic lithosphere of the NCC is ∼200 km thick. It has a refractory mantle with an evolved isotopic signature. The Mesozoic mantle lithosphere was relatively fertile and highly heterogeneous. In the Cenozoic, the lithosphere in the eastern NCC is about 60–80 km thick. It has an oceanic-type mantle that is fertile in composition and depleted in the Sr-Nd isotopic signature. The Central Zone lithosphere is >100 km thick and has a double-layer mantle with an old upper layer and a new lower layer. The Western Block has a lithosphere of ∼200 km thick. The lithospheric mantle beneath the southern and northern margins and eastern part of the NCC has been transformed significantly by peridotite-melt reactions due to the multiple subductions of adjacent plates since the Paleozoic. Paleo-Pacific subduction and the associated dynamic processes significantly alter the lithosphere based on the distribution of craton destruction. The involved mechanisms include mechanical intrusion of subduction plates, melt fluid erosion, and local delamination. The lithospheric thinning of ∼120 km is relevant to the continental extension caused by subduction plate rollback and trench retreat.

Carbonatite metasomatism plays an important role in modifying the composition of Earth's mantle, however, its effect on mantle Mg isotopic composition is poorly constrained. Here, we report high-precision mineral Mg isotope data for three suites of mantle peridotite xenoliths that experienced variable degrees of carbonatite metasomatism. The δ 26 Mg values of minerals in these xenoliths are variable and range from −0.32 to −0.11‰ in olivine, from −0.28 to −0.09‰ in orthopyroxene, from −0.27 to −0.05‰ in clinopyroxene, from 0.06 to 0.44‰ in spinel and from −0.61 to −0.37‰ in garnet. Calculated bulk-rock δ 26 Mg values of the peridotites vary from −0.27 to −0.10‰, falling within and slightly higher than the normal mantle range (−0.25 ± 0.07‰). The coexisting minerals are in isotopic equilibrium, with clinopyroxene δ 26 Mg values correlated with the carbonatite metasomatic indices such as MgO and Na2O in orthopyroxene. These results suggest that carbonatite metasomatism does not produce light Mg isotopic signature in mantle peridotites as previously suggested, instead it might slightly elevate their δ 26 Mg values. Therefore, carbonatite-metasomatized peridotites in the mantle cannot be the primary source rocks of low- δ 26 Mg mantle-derived magmas. Instead, fractional crystallization and accumulation of chromite during ascent of the basaltic magmas may explain the isotopically light basalts, as supported by the covariations of δ 26 Mg with chemical indices of chromite crystallization (e.g., Cr, V, Fe and Ti). Consequently, chromite crystallization may significantly influence the physiochemical processes on the genesis of basalts, which would require comprehensive evaluation in future studies.

Cratonic lamproites bear extreme Sr−Nd−Pb isotopic compositions widely known as enriched mantle I (EMI), yet the origin of the EMI reservoir remains controversial. Here, we explore this issue by examining Mg−Sr−Nd−Pb isotopic compositions of lamproites from Leucite Hills, Wyoming, USA. The δ26Mg values vary from the range of the normal mantle to lower values (− 0.43 to − 0.18 ‰), correlating with indices of the degree of carbonate metasomatism, an observation that can be best explained through mantle metasomatism by subducted carbonate-bearing sediments. With increasing extent of carbonate metasomatism, these samples display less extreme EMI Sr−Nd−Pb isotopic signatures, arguing for at least two metasomatic events that occurred in their mantle sources. The early metasomatic event associated with subducted continent-derived siliciclastic sediments led to the formation of the EMI Sr−Nd−Pb isotopic signatures while the recent carbonate metasomatism produced the light Mg isotopic signature but diluted the EMI Sr−Nd−Pb isotopic signatures. Our study indicates that a combination of Mg and Sr−Nd−Pb isotopes could be an effective tool in deciphering multiple-stage metasomatic events in mantle sources and places new constraints on the generation of enriched mantle reservoirs.

To investigate the potential of using Barium (Ba) isotopes to trace recycled crustal materials, we report high-precision Ba isotopic compositions for 22 well-characterized samples of Cenozoic basalts from the Taihang Mountains, North China Craton. The basalts have δ138/134Ba [= (138Ba/134Basample/138Ba/134BaSRM3104a – 1) × 1000] of −0.06‰ to +0.03‰(±0.05‰，2SD), lower than the depleted upper mantle (+0.03‰ to +0.15‰). Because Ba isotope fractionation during basaltic magmatic differentiation is negligible, the light Ba isotopic compositions likely reflect the involvement of isotopically light crustal materials in the mantle source. The Ba isotopic variations, combined with the data of elements and Sr Nd isotopes of the basalts, indicate that the light Ba isotopic compositions mainly resulted from marine sediments rather than altered oceanic crust. The quantitative modeling using Sr-Nd-Ba isotopic compositions suggests that about 0.5% to 3% of marine sediments were involved in generating the continental basalts. Our study here highlights the potential of using Ba isotopes to trace recycled materials in the mantle.

To evaluate the role of subducted oceanic crust in the genesis of potassium-rich magmas, we report high-precision Mg isotopic data for a set of Cenozoic volcanic rocks from Northeast China. These rocks overall are lighter in Mg isotopic composition than the normal mantle and display considerable Mg isotopic variations, with δ26Mg ranging from −0.61 to −0.23. The covariation of δ26Mg with TiO2 in these rocks suggests that their light Mg isotopic compositions were derived from recycled oceanic crust in the form of carbonated eclogite in the source region. The strong correlations between δ26Mg and (Gd/Yb)N ratio as well as Sr-Pb isotopes further indicate a multicomponent and multistage origin of these rocks. Magnesium isotopes may thus be used as a novel tracer of recycled oceanic crust in the source region of mantle-derived magmas.

To investigate the epidemic situation and its epidemic characteristics of malaria in Qingdao City from 2012 to 2017, so as to provide the evidence for the prevention and control of malaria in this city.The data of cases and epidemiological characteristics of malaria in Qingdao City, 2012-2017 were collected from the China's disease prevention and control system infectious disease report information management system and parasitic disease prevention and control information management system, and analyzed.A total of 103 cases of malaria were reported in Qingdao City from 2012 to 2017, and all of them were imported. Among them, 98 (95.15%) were imported from Africa and 5 (4.85%) from Asia. There were 92 laboratory confirmed cases (89.32%) and 11 (10.68%) clinically diagnosed cases. The highest incidence occurred in 2016 (0.28/105) . According to the classification of cases, there were 87 falciparum malaria cases (84.47%), 13 vivax malaria cases (12.62%), and 3 ovale malaria cases (2.91%) . These patients were mainly young men, and the male to female ratio was 19.6∶1. There were malaria case reports in each month, and the peak onset season was not obvious. Newly diagnosed and confirmed units were mainly municipal medical institutions; the median of onset-definite diagnosis was 4 days. There was one death case, but there were no recurrence, reburning, and imported secondary cases.There were no local malaria cases in Qingdao City from 2012 to 2017. The prevention and control of imported malaria is the focus of the future work.[

Clinopyroxene megacrysts in volcanic rocks can provide substantial information on the evolution of the magmatic system at depth. Although considerable attention has been paid to these crystals, their origin is not yet completely resolved. The clinopyroxene megacrysts worldwide can be divided into two major types in general: the green Cr-diopside type and the black Al-augite type. There is a consensus view that the Cr-diopside megacrysts are mantle xenocrysts, whereas two contrasting opinions exist regarding the origin of the black Al-augite megacrysts. One favours a cognate origin, viewing them as crystallization products of the host magmas under high-pressure; while the other argues that they are xenocrysts crystallized from previous alkali basalts or fragments of mantle peridotites, pyroxenites or pegmatite veins. A review study on the clinopyroxene megacrysts in Meso-Cenozoic volcanic rocks from the North China Craton (NCC) and their comparison with those worldwide provides new constraints on their origin, namely, the Cr-diopside megacrysts, as previously thought, are all xenocrysts, representing disaggregated clinopyroxene crystals from clinopyroxene-rich mantle rocks. Contrary to the formerly proposed cognate origin, the Al-augite megacrysts are also xenocrysts, having no direct genetic link to their host rocks. They crystallized from melts that have formed earlier than the host magmas, and probably accumulated in a magma chamber or occurred as sheets or veins filling a fracture network surrounding a magma chamber in the upper mantle. During the subsequent eruption of the host lavas, these previously formed crystals were incorporated into the magma and were brought up to the surface.

This paper reports petrology, major and trace element abundances in bulk-rocks and minerals, and clinopyroxene Sr-Nd-Hf compositions from a suite of mantle xenoliths entrained in Cenozoic Sanyitang alkali basalts, Jining County in the Inner Mongolia Suture Zone. The aim is to constrain the nature and evolution of the lithospheric mantle beneath the western North China Craton. The Sanyitang mantle xenoliths are mainly anhydrous spinel lherzolites with subordinate websterites. Petrological and geochemical features of the spinel lherzolites suggest that they are refractory mantle residues formed by variable degrees of melt extraction (3–14%), followed by silicate and carbonatite metasomatism. The Sr-Nd-Hf isotopic compositions (87Sr/86Sr = 0.7031–0.7037, eNd = +5.2 − +11.2 and eHf = +8.4 − +29.3) of clinopyroxene separates from the majority of the spinel lherzolites lie on, or close to, the mantle array defined by the composition of ocean-island basalts and mid-ocean ridge basalts. On the other hand, clinopyroxene separates from a minority of spinel lherzolites (SYT09-01, SYT09-09, SYT11-01) have high 87Sr/86Sr (0.7040–0.7049), eNd (+12.4 − +21.9) and eHf (+58.2 − +67.8), above the mantle array, reflecting long-term depletion of the mantle. The websterites are characterized by lower MgO (18–22 wt%) and higher Al2O3 contents (4.5–5.6 wt%), and lower Mg# (86.2–87.3) than the spinel lherzolites (Mg# = 89.3–90.8; Al2O3 = 0.9–3.6 wt%). Their convex upward rare earth element patterns, along with lower Ni contents (334–389 ppm) indicate that they were cumulates fractionated from basaltic magmas circulating through the upper mantle. The trace element patterns of clinopyroxenes from three phlogopite-free websterites show enrichment in large ion lithophile and negative anomalies for the high field strength elements. In addition, the phlogopite-free websterite clinopyroxene Sr-Nd-Hf isotope ratios plot in the enriched lithospheric field (87Sr/86Sr = 0.7058–0.7061, eNd = −9.2 to −8.4 and eHf = −5.2 to −3.7 respectively), which implies the contribution of recycled lower crustal materials to their source. The presence of phlogopite in some Sanyitang websterites, coupled with strong enrichments in both light rare earth elements and large ion lithophile elements, as well as high 87Sr/86Sr (0.7065–0.7068) and negative eNd (−1.5 to −0.3) and eHf (−6.1 to −4.8) values of clinopyroxene, provide strong evidence for the existence of subduction-related hydrous fluids/melts metasomatism. This study demonstrates that the Sanyitang mantle xenoliths and their component minerals record the complex petrological history and local heterogeneity of the subcontinental mantle lithosphere under the western North China Craton.

We present detailed major-, trace-element and Sr-Nd-Mg isotopic geochemistry for a suite of pyroxenite xenoliths entrained in Oligocene basalts in Yangyuan county, North China Craton, to reveal their origins and to further constrain the evolution of the lithospheric mantle beneath this area. The Yangyuan pyroxenites exhibit a lithologic spectrum ranging from orthopyroxenite, through orthopyroxene-rich websterite and clinopyroxene-rich websterite to clinopyroxenite. Their equilibrium temperatures (874–975 °C), which are within the temperature range of the upper mantle and lower crust, suggest that they came from the crust-mantle transition zone. The Yangyuan pyroxenite xenoliths can be subdivided into three types based on the compositions of their constituent clinopyroxene. Clinopyroxenes in the type I pyroxenites are characterized by high Mg# (88.3–92.0; Mg# = Mg/(Mg + Fe2+)*100) and Cr2O3 contents (0.86–1.63 wt.%). Their resemblance to those from the peridotite xenoliths, together with their light rare earth element (LREE) enrichment, indicates that the type I pyroxenites should be produced by lithological heterogeneity in the mantle, which may have including metasomatism. Clinopyroxenes in the type II pyroxenites have intermediate Mg# (81.0–86.7) and Cr2O3 contents (0.53–0.73 wt.%), which are similar to those of strongly metasomatized mantle-derived Fe-lherzolite xenoliths. Their LREE-enriched compositions, as well as the high Ni contents of the orthopyroxenes and clinopyroxenes, suggest that the type II pyroxenites were produced by melt-peridotite interactions. Clinopyroxenes from type III pyroxenites have the lowest Mg# (65.2–76.5) and Cr2O3 contents (0.05–0.38 wt.%), which are comparable to those formed by fractional crystallization of primary basalts, suggesting that they are the products of fractional crystallization. The high Mg#, Ti/Eu ratios, the low (La/Yb)N and Ca/Al ratios, and Rb and Ba contents of the clinopyroxenes, as well as the depleted Sr-Nd isotopes of most of the Yangyuan pyroxenites, suggest that asthenosphere or juvenile lithospheric mantle-derived silicate melts were probably involved in their formation, whereas the enriched Sr-Nd isotopes of sample YY17-5 indicate that its source may have mixed with melts derived from recycled ancient lower crust. The light and variable Mg isotopic compositions (δ26Mg = −0.23 to −0.46‰) of the Yangyuan pyroxenites suggest contributions from a subducted oceanic slab. The diverse origins of these pyroxenite xenoliths provide evidence that the deep lithosphere beneath Yangyuan has been modified by multiple melt-rock interactions and magma underplating.

Ultrapotassic igneous rocks can generally be divided into two sub-groups based on the tectonic settings in which they formed. The orogenic sub-group occurs in subduction-related tectonic settings, while the anorogenic sub-group is confined to stable continental regimes. The Pleistocene Xiaogulihe ultrapotassic volcanic rocks outcrop in the western part of Heilongjiang province, northeast China, and are of intraplate origin with respect to its tectonic settings. Previous elemental and isotopic investigations have suggested that the mantle source of these volcanic rocks had been modified by continental-derived sediments resulting from an ancient subduction (at least older than 1.5 Ga). In this contribution, we performed in-situ oxygen isotope analysis on olivine grains in these ultrapotassic rocks using secondary ionization mass spectrometry (SIMS). The olivine grains generally have higher δ 18 O values and CaO contents than those of mantle peridotite xenoliths in the nearby Keluo potassic rocks and show linear correlations between major and trace elements, and Fo, suggesting that they are cognate phenocrysts resulted from fractional crystallization processes. The restricted and non-correlated variations in δ 18 O with the Fo of these olivine grains imply that the fractional crystallization processes might have negligible influence on their δ 18 O values. The relatively higher δ 18 O values of the olivine phenocrysts than the normal mantle imply the addition of an 18 O-rich crustal component into their mantle source after ruling out the crustal contamination of the host magmas. We propose that the high-δ 18 O feature of the olivine phenocrysts was inherited from the subducted crustal component in their mantle source. Given the rapid oxygen isotopic diffusion under high temperature and the long period between mantle metasomatism event and volcanic eruption, it is postulated that the high-δ 18 O signature could only be preserved in the relatively cold and stable subcontinental lithospheric mantle. Such speculation is consistent with our previous inference that the Xiaogulihe ultrapotassic volcanic rocks were mainly generated from the lower subcontinental lithospheric mantle which had been metasomatized by potassium-rich silicate melts derived from ancient subducted continental-derived sediments.

To evaluate the role of subducted oceanic crust in the genesis of potassium-rich magmas, we report high-precision Mg isotopic data for a set of Cenozoic volcanic rocks from Northeast China. These rocks overall are lighter in Mg isotopic composition than the normal mantle and display considerable Mg isotopic variations, with δ26Mg ranging from −0.61 to −0.23. The covariation of δ26Mg with TiO2 in these rocks suggests that their light Mg isotopic compositions were derived from recycled oceanic crust in the form of carbonated eclogite in the source region. The strong correlations between δ26Mg and (Gd/Yb)N ratio as well as Sr-Pb isotopes further indicate a multicomponent and multistage origin of these rocks. Magnesium isotopes may thus be used as a novel tracer of recycled oceanic crust in the source region of mantle-derived magmas.

The unique eruptions of ultrapotassic volcanic rocks in eastern China reported so far took place in the Xiaogulihe area of western Heilongjiang Province, NE China. These ultrapotassic rocks are characterized by extremely high K 2 O contents (> 7 wt.%), abnormally unradiogenic Pb isotopic compositions ( 206 Pb/ 204 Pb = 16.44–16.55; 207 Pb/ 204 Pb = 15.39–15.46; 208 Pb/ 204 Pb = 36.35–36.61), and moderately high 87 Sr/ 86 Sr ratios (0.7053–0.7057), which can be basically correlated with those of ultrapotassic igneous rocks distributed widely in northwestern America and Aldan Shield. The positive correlation between 187 Os/ 188 Os and 1/Os argues that these ultrapotassic rocks have probably experienced negligible lower continental crust addition (less than 1%) during magma ascent. The high contents of K 2 O and negative correlation between 87 Sr/ 86 Sr and 206 Pb/ 204 Pb of these ultrapotassic rocks indicate the presence of a potassic phase, mostly phlogopite, in their mantle source. Their strong fractionation of rare earth elements and lack of Nd–Hf isotopic decoupling reveal a low-degree partial melting of garnet-bearing source rocks. In addition, the low CaO and Al 2 O 3 contents of whole-rock compositions and low Fe/Mn ratios of olivine phenocryst chemistries suggest peridotites rather than pyroxenites as dominant source rocks for the Xiaogulihe ultrapotassic rocks. Based on these distinctive geochemical characteristics, we thus propose that the ultimate mantle source of the Xiaogulihe ultrapotassic volcanic rocks is phlogopite-bearing garnet peridotite within the lower part of the sub-continental lithospheric mantle (SCLM) that had been metasomatized by potassium-rich silicate melts. Combined with the unradiogenic Pb compositions, the most likely source of these potassium-rich silicate melts is the ancient subducted continental-derived sediments (> 1.5 Ga). These ancient subducted sediments, possessing relatively low initial Pb isotopic compositions, had experienced large U/Pb fractionation during a subduction process, resulting in low-μ ( 238 U/ 204 Pb), and then accumulated in the mantle transition zone. The relatively low 87 Sr/ 86 Sr ratios of these ultrapotassic rocks also imply that their mantle source had evolved with low Rb/Sr ratios, which possibly resulted from the metasomatized melts derived from the ancient subducted sediments. This interpretation is quite different from previous hypotheses that attribute their unusual geochemical features to a dominantly asthenospheric source with a contribution from delaminated ancient SCLM, or a SCLM source that has been metasomatized by melts derived from deep asthenosphere or delaminated ancient lower continental crust.

A detailed study on petrology and mineral chemistry of 32 mantle xenoliths has been conducted to decipher the physical and chemical characteristics of the lithosphere beneath NE Iran. Spinel lherzolite, the most abundant xenolith type, is made up of olivine, orthopyroxene, clinopyroxene, and spinel. Clinopyroxenes in the spinel lherzolites display a primitive mantle-like composition, typical of non-cratonic peridotites. Pyroxenite, another major xenolith type, shows equilibrated textures and highly variable compositions including olivine websterite, websterite and clinopyroxenite. These pyroxenites, together with an equigranular dunite, delineate a clear metasomatic trend, characterized by systematic Mg#, Cr#, Al2O3, and TiO2 variations in the constituent minerals, coupled with light rare earth element enrichment and high field strength element depletion in clinopyroxene. The pyroxenites are therefore suggested to have formed by the interaction between garnet-bearing peridotites within the lithospheric mantle and melts from a stagnant slab within the asthenosphere. The lithospheric mantle may have undergone multiple stages of partial melting. The earliest stage, evidenced by the equigranular dunite, resulted in significant NiO depletion in olivine, low Al2O3 and TiO2 coupled with high Mg# and Cr# in clinopyroxene, and high Cr# in spinel. The second stage occurred more widely and gave rise to the large ion lithophile element depletion in clinopyroxenes of all rock types. The extent of melting is lower in the spinel lherzolites than that in the pyroxenites, implying that the partial melting was not caused by decompression and thus most likely related to Tethyan subduction. A third and more recent melting stage, responsible for the spongy texture in some clinopyroxenes, is attributed to the extensional tectonic regime that started in the middle Miocene in the region. Temperature estimates show that both the spinel lherzolites and pyroxenites equilibrated at ~ 900–1000 °C. Based on our new data and published data we propose the lithospheric structure beneath NE Iran. The xenolith-defined geotherm appears to be hotter than that typical in cratonic and oceanic settings, but colder than that suggested by seismic tomographic modeling for the entire Iranian Plateau. This difference is probably due to the widespread occurrence of pyroxenites within the lithospheric mantle, which we argue could have played a significant role in causing not only the seismic and gravitational anomalies but also late Cenozoic volcanism on the Iranian Plateau.

The nothern margin of the North China Craton (NCC) preseves the tectonic imprints of early Paleozoic to Triassic tectonic processes. Here we investigate the geochronology, geochemistry, zircon hafnium and whole rock Nd isotope data on a suite of magmatic rocks from the Ningcheng pyroxenite–gabbro–diorite complex and the Jiwangyingzi Formation in the Inner Mongolia region of the northern NCC. The zircon U–Pb dating identifies multiple stages of mafic to intermediate magmatism from Early Permian (298 ± 4 Ma) to Middle Triassic (219 ± 1 Ma) in this area and their hafnium isotopes display diverse mantle sources. An integration of field observations, geochronology and elemental and isotopic geochemistry indicates a complex petrogenetic history for the magmatic suite. The Ningcheng pyroxenite–gabbro–diorite complex is a product of multiple pulses of magmas with crystal fractionation, hybridization and assimilation, with the involvement of melts derived from lithospheric mantle and even the asthenosphere. Our data reveal an increasing involvement of mantle component from Late Permian to Triassic. The features of progressive mantle upwelling correlate with tectonic processes associated with the closure of Paleo-Asian Ocean and subsequent post collisional extension, resulting in active magmatism from Carboniferous to Triassic in this region.

The North China Craton (NCC) provides one of the classic examples of craton destruction, although the mechanisms and processes of its decratonization are yet to be fully understood. Here we integrate petrological, geochemical, geochronological and geophysical information from the NCC and conclude that the destruction of the craton involved multiple events of circum-craton subduction, which provided the driving force that destabilized mantle convection and tectonically eroded the lithospheric mantle beneath the craton. Furthermore, subducted-slab-derived fluids/melts weakened the subcontinental lithospheric mantle and facilitated thermo-mechanical and chemical erosion of the lithosphere. The more intense destruction beneath the eastern part of the NCC reflects the crucial contribution of Pacific plate subduction from the east that overprinted the mantle lithosphere modified during the early subduction processes. Our study further establishes the close relationship between lithospheric modification via peridotite–melt reactions induced by oceanic plate subduction and cratonic destruction.

[1] The relationship between lithospheric evolution of eastern Eurasia and subduction of the Pacific plate has long been debated. However, the timing and implications of subduction on the tectonics of eastern China are not well constrained. Here we present new zircon U-Pb ages and Hf isotopes, elemental and Sr-Nd-Pb isotopic data on Cretaceous volcanic rocks from the Ningwu basin, eastern China to further address this issue. Our age data reveal rapid eruption of the volcanic rocks within a short duration from 133 to 130 Ma. The rocks, mostly characterized by shoshonitic and high-K calc-alkaline signatures, display light rare earth element and Pb enrichment, Nb, Ta and Ti depletion, highly radiogenic Sr-Pb isotopic ratios and variable eHf(t) (+1.8 to −10), suggesting derivation from an enriched lithospheric mantle metasomatized by marine sediments. The early lavas (133.3 ± 1.1 Ma) show stronger subduction-related signatures than the late lavas (130.1 ± 1.0 Ma), which we interpret to reflect consumption of a significant volume of fusible subducted components in the early melting phase. The large eHf(t) variation of late lavas suggests greater involvement of asthenospheric melts and lower crust in their petrogenesis. The youngest age (130 Ma) appears to coincide with an inferred change in the direction of Pacific-Eurasia convergence, manifested as a change from extension to transpression in eastern China. The narrow window of eruption may signify a rapid change of the tectonic regime in the Early Cretaceous.

article i nfo Studies of mantle xenoliths have confirmed that Archean subcontinental lithospheric mantle (SCLM) is highly depleted in basaltic components (such as Al, Ca and Na) due to high-degree extraction of mafic and ultramafic melts and thus is refractory and buoyant, which made it chronically stable as tectonically independent units. However, increasing studies show that ancient SCLM can be refertilized by episodic rejuvenation events like infiltration of upwelling fertile material. The North China Craton is one of the most typical cases for relatively complete destruction of its Archean keel since the eruption of Paleozoic kimberlites, as is evidenced by a dra- matic change in the compositions of mantle xenoliths sampled by Paleozoic to Cenozoic magmas, reflecting significant lithospheric thinning and the change in the character of the SCLM. The compositional change has been interpreted as the result of refertilization of Archean SCLM via multiple-stage peridotite-melt reactions, suggested by linear correlations between MgO and indices of fertility, covariations of Al2O3 with CaO, La/Yb, 87 Sr/ 86 Sr, 143 Nd/ 144 Nd, 187 Os/ 188 Os and Re-depletion ages (TRD), high Re abundances, scatter in Re-Os isotopic plot, variablein situTRDages of sulfides, andcorrelationbetween TRDagesand olivine Foof peridotite xenoliths in Paleozoic kimberlites and Cenozoic basalts on the craton. By integrating major and trace element, Sr, Nd and Os isotopic compositions of peridotite xenoliths and oro- genic massif peridotites from the continents of Europe, Asia, America, Africa and Australia, together with pre- vious studies of petrology andgeochemistry of global peridotites, we suggest that(1) refertilizationof cratonic and circum-cratonic lithospheric mantle is widespread; (2) Archean SCLM worldwide has experienced a multi-stage history of melt depletion and refertilization since segregation from the convecting mantle; (3) cratonic SCLM may be more susceptible to compositional change caused by refertilization than is generally assumed; (4) the original character of much Archean cratonic mantle has been partly overprinted, or even erased by varying degrees of refertilization, which may play a key role in the rejuvenation and erosion of the SCLM beneath the Archean cratons. Due to the refertilization of ancient SCLM, (1) many published whole-rock Re-depletion ages cannot repre- sent the formation ages of peridotites, but the mixtures of different generations of sulfides. Thus, the chro- nological significance of the Re-Os isotopic composition in individual peridotite should be cautiously interpreted; (2) many kimberlite- and intraplate basalt-borne lherzolite xenoliths, with major element compositions close to primitive mantle, may be the fragments of the ancient SCLM, strongly refertilized by infiltration of asthenosphere-derived melts, rather than newly-accreted SCLM. Consequently, new accre- tion of SCLM beneath ancient cratons such as the North China Craton may be less than was previously assumed.

A comprehensive synthesis of U–Pb geochronology and Hf isotopes of zircons from granulite/pyroxenite xenoliths entrained in Phanerozoic magmatic rocks and inherited xenocrysts from the associated lower crust rocks from various domains of the North China Craton (NCC) provides new insights into understanding the Phanerozoic evolution of the lower crust in this craton. Episodic widespread magma underplating into the ancient lower crust during Phanerozoic has been identified throughout the NCC from early Paleozoic to Cenozoic, broadly corresponding to the Caledonian, Hercynian, Indosinian, Yanshanian, and Himalayan orogenies on the circum-craton mobile belts. The early Paleozoic (410–490 Ma) ages come from xenoliths in the northern and southern margins as well as the central domain of the Eastern Block of the craton which mark the first phase of Phanerozoic magma underplating since the final cratonization of the NCC in the Paleoproterozoic. The magmatism coincided with the northward subduction of the Paleotethysian Ocean in the south and the southward subduction of the Paleoasian Ocean in the north. The subduction not only triggered magma underplating but also led to the emplacement of the diamondiferous kimberlites on the craton, marking the initiation of decratonization. The late Paleozoic event as represented by the 315 Ma garnet pyroxenite and/or lherzolite xenoliths in Hannuoba was restricted to the northern and southern margins of the craton, correlating with the arc magmatism continuous associated with the subduction of the Paleotethysian and Paleoasian Oceans and resulting in the interaction between the melts from subducted slabs and the lithospheric mantle/lower crust. The early Mesozoic event also dominantly occurred in the northern and southern margins and was related with the final closure of the Paleotethysian and Paleoasian Oceans as well as the collisional orogeny between the NCC and the Yangtze Craton. The late Mesozoic (ca. 120 Ma) was a major and widespread magmatic event which manifested throughout the NCC, associated with the geothermal overturn due to the giant south Pacific mantle plume. The Cenozoic magmatism, identified only in the dark clinopyroxenite xenoliths in the Hannuoba, was probably induced by the Himalayan movement in eastern Asia and might also have been influenced by the subduction of the Pacific Ocean to some extent. These widespread and episodic magma underplating or rejuvenation of the ancient lower crust beneath the NCC revealed by U–Pb and Hf isotope data resulted from the corresponding addition of juvenile materials from mantle to lower crust, with a mixing of the old crust with melts. The process inevitably resulted in the compositional modification of the ancient lower crust, similar to the compositional transformation from the refractory lithospheric mantle to a fertile one through the refractory peridotite — infiltrated melt reaction as revealed in the lithospheric mantle beneath the craton.

article i nfo High-Mg# peridotite xenoliths in the Cenozoic Hebi basalts from the North China Craton have refractory min

a b s t r a c t A comprehensive review of U-Pb geochronology and Hf isotopes of zircons separated from granulite xenoliths entrained in Phanerozoic magmatic rocks and inherited xenocrysts from the associated lower crust rocks from various domains of the North China Craton (NCC) provides new insights into under- standing the evolution of the lower crust in this part of the Precambrian nucleus of Asia. The oldest zircons recorded in granulite xenoliths show ages up to 3.6 Ga, slightly younger than the oldest zircons (3.8 Ga) from samples of the exposed Archean upper crust. However, the Hf isotope features indicate that the oldest lower crust might have been derived from a 4.0 to 4.1 Ga primordial crust as indicated by the data from an inherited zircon in the North Qinling Orogenic Belt at the southern part of the NCC. Our synthesis also suggests that ca. 2.5 Ga mark an important period for the evolution of the lower crust of the NCC and the cratonization of the Archean blocks in the NCC, at least for the Eastern Block, with a pos- sible link to mantle plume activity. The occasional presence of zircons with 2.6-3.1 Ga ages in the lower crustal granulite xenoliths suggests a continuous reworking of the Paleoarchean lower crust during the Archean. The comprehensive data also show that 1.8-2.0 Ga period was the first significant event for the evolution of the lower crust after the cratonization of the Archean blocks of the NCC, which affected the whole lower crust of the Eastern NCC. Episodic growth or rejuvenation of the Precambrian lower crust beneath the NCC is also revealed by U-Pb and Hf isotope data, resulting from the corresponding addition of juvenile materials from mantle to lower crust, with a mixing of the old crust with juvenile magma. This compositional modification of the lower crust resembles the compositional transformation from the refractory lithospheric mantle to a fertile one through the refractory peridotite - infiltrated melt reaction as revealed in the lithospheric mantle beneath the NCC.

Mantle xenoliths from the Western Qinling, central China, are dominated by lherzolites, which can be divided into four subgroups—namely, garnet-facies, coexisting spinel-garnet, spinel-facies, and carbonate-bearing ones. All these rocks display light rare earth element enrichment, positive Sr and Ba anomalies, carbonatite-like trace element patterns, and Sr-Nd-Pb isotopic mixing between depleted mantle and enriched mantle type II end members, consistent with the geochemical features resulting from carbonatite metasomatism. The garnet-facies lherzolites show high trace element concentrations but low LaN/YbN ratios, and they show high Sr and Pb isotopic ratios that are similar to those of carbonatites, suggesting that they were highly metasomatized. The spinel-facies group has the lowest trace element concentrations but higher LaN/YbN ratios than the garnet-facies group; their lowest Sr and Pb isotopic ratios are closer to those of the depleted mantle end member, implying low-degree metasomatism. Ge...

In-situ lithium (Li) concentrations and δ7Li of olivine, orthopyroxene (opx) and clinopyroxene (cpx) in mantle xenoliths from the Quaternary Longgang maar provide insights into the nature and evolution of the lithospheric mantle beneath the easterly part of the North China Craton. SIMS analyses show Li partitioning of olivine > opx ≥ cpx and Li isotopic disequilibrium in each sample. Olivines in harzburgite are generally homogeneous, with Li abundances in the range of normal mantle, but their δ7Li (− 0.9 to − 4.0) are lower than normal mantle olivine δ7Li values of ~+4 ± 2. Olivines in cpx-rich lherzolite and pyroxenite have higher Li contents and normal mantle-like δ7Li values. The olivine δ7Li values in the harzburgite and cpx-poor lherzolites are much lower than those published for worldwide peridotites, showing the peculiarity of these peridotites. The Li abundances and δ7Li in the olivines show a positive correlation, indicating mixing between a low-δ7Li melt and normal mantle. Collectively, Li elemental and isotopic compositions of the xenoliths suggest that low δ7Li in olivines reflect a slab-derived Li isotopic signature in the lithospheric mantle. These observations, coupled with geochemical and geophysical evidence, suggest that the lithospheric mantle has been modified three times by (1) low-δ7Li melt likely derived from the subducted Pacific plate (leading to the low δ7Li of olivines), (2) high-degree partial melting (decreasing Li abundances of harzburgite and cpx-poor lherzolites close to the normal mantle), and (3) refertilization by melts mainly from the asthenosphere (producing the normal mantle-like δ7Li and high Li contents in cpx-rich lherzolite and pyroxenite). The melts/fluids derived from the subducted Pacific plate could play a crucial role in the destruction of the craton.

Mineral electron microprobe, bulk rock major, trace elements and Nd isotopes, and zircon U–Pb and Hf isotopic data are reported for granites and dioritic enclaves from the Guangtoushan granitic complex in the northern margin of the North China Craton. The zircon U–Pb dating identifies two stages of magmatism. The early stage enclave and host granite have identical ages (250 ± 2 Ma) and similar bulk rock Nd isotopes (eNd(t) = − 11.5 to − 12.2) as well as similar zircon Hf isotopes (eHf(t) = − 13.3 to − 17.4). However, although the late stage enclave (206 ± 2 Ma), host granite (208 ± 1 Ma) and leucogranite (207 ± 3 Ma) display comparable ages, but exhibit distinct isotopic features: the granite displays moderate bulk rock Nd isotopic value (eNd(t) = − 8.3) and zircon eHf(t) from − 2.6 to − 10.4; the enclave shows least negative bulk rock eNd(t) of − 3.9 and zircon eHf(t) from 0 to − 5.4; and the leucogranite possesses more pronounced negative bulk rock Nd (eNd(t) = − 17.2) and zircon Hf isotopic compositions (eHf(t) = − 6.2 to − 19.0). An integration of field observations, geochronology, geochemistry and zircon Hf isotopic data points to a complex petrogenetic history, where the early stage enclave–granite pair is a product of the early crystallization of felsic magmas that evolved from a mafic magma, whereas the late stage enclave and granite were derived by magma mixing. The late stage leucogranite is considered as a product of remelting of Phanerozoic igneous rocks within this region. The Guangtoushan granitic complex offers robust evidence for a gradual mantle upwelling during Triassic in the northern margin of the North China Craton.

We report U–Pb ages and Hf isotopes of zircons in granulites from two continental suture zones in the North China Craton (NCC)—from granulite xenoliths entrained in the Cenozoic Hannuoba basalts within the Trans-North China Orogen (TNCO) that amalgamates the Western and Eastern Blocks, and from a mafic granulite associated with ultrahigh-temperature (UHT) granulites within the Inner Mongolia Suture Zone (IMSZ) that welds the Yinshan and Ordos Blocks. The zircons in these rocks, together with the inherited zircons from Mesozoic subalkalic intrusives of these regions, show several distinct U–Pb age populations and provide constraints on the evolution of the lower crust beneath the NCC. Parts of the older zircons from the TNCO yield discordant data that project to ages of about 2.5 to 2.7 Ga. These old zircons show Hf TDM model ages of 2.6–2.9 Ga suggesting the existence of Archean lower crust beneath the TNCO, which is coupled with the Neoarchean metamorphic basement and greenstone belt. A major zircon population grew at 1.75–2.0 Ga, consistent with the timing of the Paleoproterozoic collision between the various crustal blocks within the NCC, suggesting that this event had a significant effect on the Archean lower crust. The older inherited zircons from the Western Block also yield discordant data that project to younger ages of about 1.8 to 1.9 Ga. This age is consistent with the timing of intrusion of the mafic magmatic protoliths of the granulites associated with the UHT rocks in the khondalite belt within the IMSZ. These older zircons yield Hf TDM model ages of 2.0–2.5 Ga suggesting that the lower crust beneath the Western Block in the NCC to be dominantly Paleoproterozoic, and decoupled with the Archean metamorphic basement of the region. Yet another population of zircons from the Hannuoba banded granulite xenoliths, garnet-bearing pyroxenite cumulate and dark pyroxenite xenoliths yield a large spread of Phanerozoic concordant ages ranging from 470 Ma to 40 Ma with peaks at 315 Ma, 220–230 Ma, 120 Ma and 46 Ma. The Late Paleozoic zircons predominantly occur in the garnet-bearing pyroxenite xenoliths and yield variable eHf(t) values of − 24 to + 18. Triassic–Cretaceous zircons mainly occur in the banded granulite xenoliths and yield highly variable eHf(t) values, ranging from − 48 to + 16. The Tertiary zircons occur only in the dark pyroxenite xenoliths and show a restricted eHf(t) with a dominant range from 0 to + 18. These data suggest episodic magmatic underplating in the ancient lower crust of the NCC, lasting continuously throughout Phanerozoic, producing zircons from the underplated magmas or providing the heat source for the recrystallization of zircons from the ancient crust. Our data clearly demonstrate the important events of Phanerozoic magmatic underplating beneath the Neoarchean–Paleoproterozoic crust of the NCC.

Geochemical behavior of lithium (Li) in mantle processes is generally explained by: (1) Li diffuses into minerals from melts, faster in clinopyroxene than in olivine, creating isotopically lighter-Li clinopyroxene and heavier-Li olivine; (2) Partitioning of Li changes with temperature, and as cooling proceeds Li diffuses into clinopyroxene from olivine, also resulting in low-δ7Li clinopyroxene and high-δ7Li olivine, similar to the result of process (1). Our results obtained from highly carbonatite-metasomatized peridotite xenoliths from the Western Qinling reveal that the minerals of these xenoliths contain extremely high Li in a range of 2–16 ppm for olivine, 2–43 ppm for orthopyroxene, and up to 75 ppm for clinopyroxene. Secondly, olivines have much lower δ7Li (down to − 42‰) relative to co-existent pyroxenes. Above observations provide important insights into the behavior of Li during mantle processes. One explanation is that the fractionation of Li strongly depends on the metasomatic medium. Alkaline melts, particularly Na-rich carbonatitic melts, which have high Li contents (~ 200 ppm) but mantle level δ7Li (3–5‰), tend to effectively transfer 6Li into olivine and 7Li into pyroxene, while silicate melts behave on the contrary. Alternative might be the existence of a reservoir with a high Li content but low δ7Li, like eclogite, in the mantle since the 7Li released into the overlying mantle wedge during dehydration. This study reveals the distinctive behavior of Li between the carbonatite and silicate metasomatism in the mantle.

Major and trace element compositions of constituent minerals, partly decomposed rims of orthopyroxenes (DRO), ‘closed’ melt pockets (CMP) and open melt pockets (OMP) in some Western Qinling peridotite xenoliths were obtained by LA-ICP-MS. Systematic core-to-rim compositional variations of garnet, clinopyroxene and orthopyroxene demonstrate that these minerals underwent variable degrees of subsolidus breakdown or partial melting. Both DROs and CMPs consist of similar mineral assemblages and are characterized by high TiO2, CaO + Na2O and low MgO contents; they are enriched in LREE and LILE compositions, have positive anomalies in Pb, Sr and particularly Ti, negative Th and U, and variable Zr and Hf anomalies. These chemical features are distinct and reflect reactions involving the orthopyroxenes. Compared to the CMPs, the OMPs, which are composed of a complex assemblage of minerals, display lower FeO and MgO contents, larger ranges in SiO2 and Na2O, higher TiO2, Al2O3, CaO and trace element concentrations, slightly negative Zr and Hf anomalies, and apparently negative Ti anomalies. Modeling calculations of partial fusion of orthopyroxenes and clinopyroxenes suggest that the CMPs most likely originated from the breakdown of orthopyroxenes with variably minor contribution of external melts from the melting of clinopyroxenes, whereas the OMPs were probably formed from the modification of the CMPs through the interaction with large amount of external melts.

The Archean lithospheric root of the North China Craton (NCC) has been considerably eroded and modified by Phanerozoic magmatic processes. Here we investigate the decratonization of the NCC through U–Pb and Hf isotopic analyses of zircons from Cenozoic basalts in the Liaodong Peninsula using ion-probe and MC-ICPMS techniques. The U–Pb zircon geochronology identifies three zircon populations: Precambrian, Paleozoic and Mesozoic. The Precambrian zircons yield 207Pb/206Pb ages of 2275–2567 Ma with a peak at around 2.5 Ga. They define a U–Pb discordia with upper intercept ages of 2447 ± 50 Ma to 2556 ± 50 Ma and a wide range of Hf TDM ages with a mode at 2.7–2.8 Ga. Our results clearly demonstrate the presence of an Archean lower crust in the Liaodong region. The Paleozoic zircons from the Liaodong region lack the clear internal zoning and are subhedral to rounded in shape, and yield a narrow 206Pb/238U concordant ages of 419–487 Ma with a weighted mean age of 462 ± 16 Ma. The Mesozoic zircons predominantly show crystallization in the early Cretaceous and yield a relatively large range in 206Pb/238U ages from 100 to 138 Ma (n = 53) with a peak around 120 Ma. Three samples give indistinguishable weighted mean 206Pb/238U ages of 120 ± 5 Ma, 120 ± 4 Ma and 121 ± 2 Ma. These early Cretaceous zircons have enriched Hf isotope compositions with eHf(t) values from −26 to −16. Our results provide important constraints on episodic magmatism during the Phanerozoic in the Liaodong region, which led to substantial reactivation of the Archean basement of the North China Craton.

Mesozoic volcanic rocks are widespread throughout the Great Xing’an Range, NE China. However, precise data constraining the exact eruption ages are limited, especially for those from the southern Great Xing’an Range, which severely hampers our understanding of the petrogenesis and geodynamics of these rocks. In this paper, we report precise in situ LA-ICPMS zircon U–Pb age measurements for these volcanic rocks. Volcanic rocks in the southern Great Xing’an Range were divided into four units from bottom to top, namely, the Manketouebo, Manitu, Baiyingaolao and Meiletu formations. The previous studies suggested that these volcanic rocks were mainly formed in the Late Jurassic. Our data demonstrate that the Manketouebo formation erupted during Late Jurassic to Early Cretaceous time, whereas the other formations are all of Cretaceous age. The southern Great Xing’an Range age dataset, along with recently obtained precise ages for volcanic rocks from the northern Great Xing’an Range indicate that Mesozoic volcanism throughout the Great Xing’an Range commenced in Late Jurassic, but peaked during the Cretaceous. They formed under an extensional tectonic setting which resulted from closure of the Mongol–Okhotsk Ocean and subsequent orogenic collapse. The globally elevated mantle temperature in Cretaceous may provided thermal contributions to the generation of the volcanisms.

This study carried out a comprehensive review of mantle-derived garnet peridotites on the basis of their garnet-spinel compositional relationships. The P-T estimates of the garnet peridotites of both orogenic and xenolithic derivations confirm previously established two garnet-spinel transition (GST) zones. Results of natural samples and experimental studies of all the garnet peridotites plot be- low the first GST at low P (

Two types of melt pockets, closed melt pocket (CMP) and open melt pocket (OMP), are recognized from the peridotite xenoliths entrained in the Cenozoic kamafugites in western Qinling (秦岭), Central China. The Haoti (好梯) CMPs have a mineral assemblage of olivine+ clinopyroxene+amphibole+K-feldspar, whereas the Baiguan (白关) CMPs are composed of olivine+clinopyroxene+ilmenite+carbonate. The components of the OMPs are more complicated. In the Haoti OMPs, there are olivine, clinopyroxene, glass, low modal abundances of amphibole, K-feldspar (Kfs), ilmenite, sulfide, chlorite, perovskite, chromite and phlogopite. The Baiguan OMPs contain olivine, clinopyroxene, glass, chlorite and chromite. Compositionally, olivines in the CMPs and OMPs are both apparently depleted in Ni, and those in the OMPs are also depleted in Fe and Mg, and enriched in Ca compared to the primary ones. Clinopyroxenes display large and systematical compositional variations between the CMPs and OMPs, particularly in Al, Cr, Na, Ca and Ti. Glasses are generally depleted in Si compared to the worldwide glasses in melt pockets, although they still have large variations. Amphiboles and K-feldspars have relatively restricted compositional variations. The petrographical observations and mineral chemistry suggest that the Haoti and Baiguan CMPs were generated by the in-situ decompression melting of orthopyroxenes, olivines and clinopyroxenes, and by the addition of minor external K-rich and Ca-rich melt/fluids. The OMPs formed during the latest metasomatic event in the lithospheric mantle beneath the western Qinling.

Elemental and Li-Sr-Nd isotopic data of minerals in spinel peridotites hosted by Cenozoic basalts allow us to refine the existing models for Li isotopic frac- tionation in mantle peridotites and constrain the melt/fluid- peridotite interaction in the lithospheric mantle beneath the North China Craton. Highly elevated Li concentrations in cpx (up to 24 ppm) relative to coexisting opx and olivine (\4 ppm) indicate that the peridotites experienced metasomatism by mafic silicate melts and/or fluids. The mineral d 7 Li vary greatly, with olivine (?0.7 to ?5.4%) being isotopically heavier than coexisting opx (-4.4 to -25.9%) and cpx (-3.3 to -21.4%) in most samples. The d 7 Li in pyroxenes are considerably lower than the normal mantle values and show negative correlation with their Li abundances, likely due to recent Li ingress attended by diffusive fractionation of Li isotopes. Two exceptional samples have olivine d 7 Li of -3.0 and -7.9%, indicating the existence of low d 7 Li domains in the mantle, which could be transient and gener- ated by meter-scale diffusion of Li during melt/fluid-peri- dotite interaction. The 143 Nd/ 144 Nd (0.5123-0.5139) and 87 Sr/ 86 Sr (0.7018-0.7062) in the pyroxenes also show a large variation, in which the cpx are apparently lower in 87 Sr/ 86 Sr and slightly higher in 143 Nd/ 144 Nd than coexisting opx, implying an intermineral Sr-Nd isotopic disequilib- rium. This is observed more apparently in peridotites having low 87 Sr/ 86 Sr and high 143 Nd/ 144 Nd ratios than in those with high 87 Sr/ 86 Sr and low 143 Nd/ 144 Nd, suggesting that a rela- tively recent interaction existed between an ancient me- tasomatized lithospheric mantle and asthenospheric melt, which transformed the refractory peridotites with highly radiogenic Sr and unradiogenic Nd isotopic compositions to the fertile lherzolites with unradiogenic Sr and radiogenic Nd isotopic compositions. Therefore, we argue that the litho- spheric mantle represented by the peridotites has been het- erogeneously refertilized by multistage melt/fluid-peridotite interactions.

Spongy textures are observed in mantle peridotite xenoliths hosted in Cenozoic kamafugites from the Western Qinling, central China. These textures are mainly developed in clinopyroxenes and spinels, and occur as spongy rims consisting of low-Na clinopyroxene, ilmenite, and bubbles, enclosing nonspongy cores. The ilmenites and bubbles exhibit shapes and sizes that vary with the width of the spongy rims. The spongy-textured minerals preserve primary shapes and well-defined grain boundaries and do not show apparent interaction with contact minerals or observed melts except the subsequent melts forming melt pockets. The xenocrysts display reactive zoning textures with host magmas rather than spongy textures. Compositionally, the spongy rims are enriched in Ca, Ti, and most trace elements, have high Cr#, and are depleted in Na, Al, Fe, AlVI, and AlIV/AlVI compared with the cores. These observations suggest that melts/host magmas did not play any significant role in the formation of the spongy textures. We therefore propose that spongy-textured clinopyroxenes and spinels in Western Qinling peridotite xenoliths developed from a decompression-induced partial melting event prior to formation of melt pockets and xenolith entrainment in host magmas.

A suite of peridotite xenoliths from Cenozoic Beiyan basalts within the Tancheng-Lujiang (Tan-Lu) wrench fault zone, eastern North China Craton (NCC), has been studied to provide constraints on the nature and evolution of the lithospheric mantle beneath this region. These xenoliths commonly have porphyroclastic, granuloblastic to resorption textures with the absence of coarse-grained texture. They can be subdivided into three types: lherzolite, clinopyroxene (cpx)-rich lherzolite and wehrlite. Lherzolites are characterized by low forsterite contents (Fo) (88–91) in olivines. Whole rock and cpx separates from lherzolites have convex-upward rare earth element (REE) patterns except for one sample which has the highest Fo in olivine and shows a spoon-shaped REE pattern. The Sr–Nd isotopic compositions of cpx separates are depleted, similar to those of mid-ocean ridge basalts (MORB). These geochemical characteristics indicate that the lherzolites represent fragments of newly accreted lithospheric mantle that makes up much of the Late Mesozoic–Cenozoic lithosphere beneath the Tan-Lu fault zone. Cpx-rich lherzolite and wehrlite reflect the interaction of the lithosphere with melt, as evidenced by relatively lower Fo (

Garnet-bearing mantle peridotites, occurring as either xenoliths in volcanic rocks or lenses/massifs in high-pressure and ultrahigh-pressure terrenes within orogens, preserve a record of deep lithospheric mantle processes. The garnet peridotite xenoliths record chemical equilibrium conditions of garnet-bearing mineral assemblage at temperatures (T) ranging from ~700 to 1,400°C and pressures (P) > 1.6–8.9 GPa, corresponding to depths of ~52–270 km. A characteristic mineral paragenesis includes Cr-bearing pyropic garnet (64–86 mol% pyrope; 0–10 wt% Cr2O3), Cr-rich diopside (0.5–3.5 wt% Cr2O3), Al-poor orthopyroxene (0–5 wt% Al2O3), high-Cr spinel (Cr/(Cr + Al) × 100 atomic ratio = 2–86) and olivine (88–94 mol% forsterite). In some cases, partial melting, re-equilibration involving garnet-breakdown, deformation, and mantle metasomatism by kimberlitic and/or carbonatitic melt percolations are documented. Isotope model ages of Archean and Proterozoic are ubiquitous, but Phanerozoic model ages are less common. In contrast, the orogenic peridotites were subjected to ultrahigh-pressure (UHP) metamorphism at temperature ranging from ~700 to 950°C and pressure >3.5–5.0 GPa, corresponding to depths of >110–150 km. The petrologic comparisons between 231 garnet peridotite xenoliths and 198 orogenic garnet peridotites revealed that (1) bulk-rock REE (rare earth element) concentrations in xenoliths are relatively high, (2) clinopyroxene and garnet in orogenic garnet peridotites show a highly fractionated REE pattern and Ce-negative anomaly, respectively, (3) Fo contents of olivines for off-cratonic xenolith are in turn lower than those of orogenic garnet and cratonic xenolith but mg-number of garnet for orogenic is less than that of off-cratonic and on-cratonic xenolith, (4) Al2O3, Cr2O3, CaO and Cr# of pyroxenes and chemical compositions of whole rocks are very different between these garnet peridotites, (5) orogenic garnet peridotites are characterized by low T and high P, off-cratonic by high T and low P, and cratonic by medium T and high P and (6) garnet peridotite xenoliths are of Archean or Proterozoic origin, whereas most of orogenic garnet peridotites are of Phanerozoic origin. Taking account of tectonic settings, a new orogenic garnet peridotite exhumation model, crust-mantle material mixing process, is proposed. The composition of lithospheric mantle is additionally constrained by comparisons and compiling of the off-cratonic, on-cratonic and orogenic garnet peridotite.

Li contents and its isotopes of minerals in mantle peridotite xenoliths from late Cretaceous mafic dikes, analyzed in situ by Cameca IMS-1280, reveal the existence of melt/rock interaction in remains of refertilized Archean lithospheric mantle in Qingdao, Jiaodong Peninsula, North China Craton. Two groups of peridotites exist, i.e., low-Mg# lherzolite and high-Mg# harzburgites. The low-Mg# lherzolite has a relatively homogeneous Li concentration (ol: 2.01–2.11 ppm; opx: 1.77–1.88 ppm; cpx: 1.75–1.93 ppm) and Li isotopic composition (δ7Li in ol: 4.2–7.6‰; in opx: 6.0–8.3‰; in cpx: 5.3–8.4‰). The similarity in δ7Li value to the fresh MORB provides further evidence for the argument that the low-Mg# lherzolite could be the fragment of the newly accreted lithospheric mantle. The high-Mg# harzburgites have heterogeneous Li abundances (ol: 0.83–2.09 ppm; opx: 0.92–1.94 ppm; cpx: 1.12–4.89 ppm) and Li isotopic compositions (δ7Li in ol: −0.5 to +11.5‰; in opx: −6.2 to +11.1‰; in cpx: −34.3 to +10.1‰), showing strong disequilibrium in Li partitioning and Li isotope fractionation between samples. The cores of most minerals in these high-Mg# harzburgites have relatively homogeneous δ7Li values, which are higher than those of fresh MORB, but similar to those previously reported for arc lavas. These harzburgites have enriched trace elemental and Sr–Nd isotopic compositions. These observations indicate that in the early Mesozoic the lithospheric mantle beneath the southeastern North China Craton was similar to that in arc settings, which is metasomatized by subducted crustal materials. Extremely low δ7Li preserved in cpxs requires diffusive fractionation of Li isotopes from later-stage melt into the minerals. Thus, the Li data provide further evidence that the Archean refractory lithospheric mantle represented by the high-Mg# harzburgites was refertilized through melt/rock interaction and transformed to the Mesozoic less refractory and incompatible element and Sr–Nd isotopes enriched lithospheric mantle.

Lithium isotope geochemistry is increasingly being used to trace deep-earth processes, reflecting the observed large variation of Li isotope ratios in mantle-derived rocks, including peridotite xenoliths associated with ancient continents. We briefly review the Li isotopic compositions of major geochemical reservoirs, the assumed mechanisms of Li isotopic fractionation, and, in particular, the origins of isotopically light Li in mantle-derived rocks based on the latest developments in Li isotope geochemistry. Comparison of Li isotope data with existing Sr-Nd isotope ratios reflects the subduction-recycling of ancient oceanic crust and the reappearance of Li in volcanic rocks. This circulation may play an important role in generating the isotopically light-Li component in the mantle – perhaps the enriched mantle end member defined by the Sr-Nd isotopic compositions of oceanic basalts.

Zoned olivine grains are abundant in the late Mesozoic Shatuo gabbro (southern Taihang Mountains, central North China Craton). Olivine cores are rich in MgO and NiO, rims are rich in FeO and MnO, and both cores and rims have very low CaO contents. The cores invariably have a high Mg no. (92–94), similar to olivine xenocrysts from Palaeozoic kimberlites in eastern China. The compositional features of these olivines imply that they are xenocrysts rather than phenocrysts, namely, disaggregates of mantle peridotites at the time of intrusion. The compositional similarity of olivine cores to xenocrysts from Palaeozoic kimberlites suggests that the lithospheric mantle beneath the central North China Craton is ancient and refractory in nature, and quite different from eastern China, where the mantle is mainly composed of newly accreted materials resulting from large-scale lithospheric removal and replacement. The contrasting features of the lithospheric mantle beneath the eastern and central North China Craton imply that the large-scale lithospheric removal in Phanerozoic times was mainly confined to the eastern North China Craton.

A rare composite xenolith and abundant cumulative pyroxenites obtained from the Mesozoic Fangcheng basalts on the eastern North China Craton record a complex history of melt percolation and circulation in the subcontinental lithospheric mantle. The composite xenolith has a dunite core and an olivine clinopyroxenite rim. The dunite is of cumulative origin and has a granular recrystallized texture and extremely low Mg# [100 Mg/(Mg + Fe) = 81–82] contents in olivines. The olivine clinopyroxenite contains larger clinopyroxene and/or orthopyroxene with a few fine-grained olivine and tiny phlogopite, feldspar, and/or carbonate minerals interstitial to clinopyroxene. The clinopyroxene has low Mg# (83–85). Compositional similarity between dunitic olivine and pyroxenitic one indicates a sequential crystallization of dunite and pyroxenite from a precursor melt. Pyroxenite xenoliths include olivine websterites and clinopyroxenites, both are of cumulative origin. Estimation of the melt from major oxides in olivines and REE concentrations in clinopyroxenes in these composite and pyroxenite xenoliths suggests a derivation from subducted crustal materials, consistent with the highly enriched EMII-like Sr and Nd isotopic ratios observed in the pyroxenites. Occurrence of phlogopite, feldspar and carbonate minerals in some xenoliths requires the melt rich in alkalis (K, Na), silica and volatiles (water and CO2) at the latest stage as well, similar to highly silicic and potassic melts. Thus, the occurrence of these composite and pyroxenite xenoliths provides an evidence for voluminous injection of recycled crustal melts into the lithosphere beneath the southeastern North China Craton at the Late Mesozoic, a reason for the rapid lithospheric enrichment in both elemental and isotopic compositions.

The Mesozoic lithospheric mantle beneath the North China craton remains poorly constrained relative to its Palaeozoic and Cenozoic counterparts due to a lack of mantle xenoliths in volcanic rocks. Available data show that the Mesozoic lithospheric mantle was distinctive in terms of its major, trace element, and isotopic compositions. The recent discovery of mantle peridotitic xenoliths in Late Cretaceous mafic rocks in the Jiaodong region provides an opportunity to further quantify the nature and secular evolution of the Mesozoic lithospheric mantle beneath the region. These peridotitic xenoliths are all spinel-facies nodules and two groups, high-Mg# and low-Mg# types, can be distinguished based on textural and mineralogical features. High-Mg# peridotites have inequigranular textures, high Mg# (up to 92.2) in olivines, and high Cr# (up to 55) in spinels. Clinopyroxenes in the high-Mg# peridotites are generally LREE-enriched ((La/Yb)N>1) with variable REE concentrations, and have enriched Sr–Nd isotopic co...

The Cenozoic Haoti kamafugite field (23 Ma) is situated at the western Qinling Orogen, Gansu Province in China, which is a conjunction region of the North China Craton, the Yangtze Craton and the Tibetan Plateau. Fresh peridotitic xenoliths entrained in these volcanic rocks provide an opportunity to study the nature and processes of the lithospheric mantle beneath the western Qinling. These xenoliths can be divided into two groups based on the petrological features and mineral compositions, type 1 and type 2. Type 1 xenoliths with strongly deformed texture have higher Fo (90–92.5) contents in olivines, Mg# (91–94) and Cr# (15–35) of clinopyroxenes, and Cr# (36–67) of spinels than the weakly deformed type 2 xenoliths, which have the corresponding values of 89–90, 89–91.5, 10–15 and 5–15 in minerals, respectively. CaO contents in fine-grained olivines are slightly higher than 0.10 wt% compared with coarse-grained ones (less than 0.10 wt%). Fine-grained clinopyroxenes have low Al2O3 + CaO contents (generally 23 wt%). Fo contents in fine-grained olivines mainly in the melt pocket of the type 1 xenoliths are higher than those in coarse-grained ones, which is somewhat contrary to the type 2 xenoliths without melt pocket. Clinopyroxenes of the type 2 display higher Na2O contents (1.7–1.9 wt%) than those of the type 1 ( 10%), modal metasomatic and deformed characteristics, and may represent the old refractory lithospheric mantle. In contrast, the type 2 peridotites show fertile features with low degree of partial melting (

Precise age dating and systematic geochemical investigation were performed on the widely distributed late Mesozoic volcanic rocks in the North Great Xing’an Range (NGXR). In situ zircon U–Pb age measurements indicate that the volcanic eruption commenced from 163 Ma ago and lasted to 113 Ma ago. These volcanic rocks show a wide range in compositions from basaltic andesite, trachyandesite and trachydacite to rhyolite. The majority of volcanic rocks exhibit high-K calc-alkaline affinity with the subordinate showing shoshonitic features. The volcanic rocks are characterized with low MgO contents, LILE, LREE enrichment and HFSE depletion. Elemental and isotopic variations suggest that fractional crystallization with the predominant removal of olivine and orthopyroxene play an important role in the evolution of magma. Most of the basic and intermediate volcanic rocks are generated from an enriched lithospheric mantle which was metasomatised by fluids released from subducted slabs during the closure of the Paleo-Asian and Mongol-Okhotsk oceans. The generation of such widely distributed volcanic rocks was caused by the decompressional partial melting of enriched lithospheric mantle in an extensional regime, which resulted from the gravitational collapse and upwelling of asthenosphere after the final closure of the Mongol-Okhotsk oceans in late Jurassic and from then the Mongol-Okhotsk orogen turned into the post-orogenic stage.

Late Mesozoic volcanism is widespread throughout northeastern China, mainly clustering in the Great Xinggan Range, where the major period of volcanic eruption was during the Early Cretaceous. The spatial distribution of volcanic rocks shows a NNE-extending pattern. The lithology of Mesozoic basaltic rocks in the Great Xinggan Range ranges from trachytic basalts, basalts to basaltic andesites. On the basis of lithological associations and spatial relationships, we divide the basalts into four groups, i.e. Tahe, Genhe, Zalute and Xiwu. The Early Cretaceous volcanism in the Great Xinggan Range is characterized by significant large ion lithophile elements (LILE) and light rare earth elements (LREE) enrichment and high field strength elements (HFSE) depletion, with a slightly enriched to depleted Nd–Hf and weakly enriched Sr isotopic compositions (initial 87Sr/86Sr = 0.7046–0.7079, eNd(t) = + 3.6 to − 6.9 and eHf(t) = + 8.9 to − 4.6). The Pb isotopic compositions are also variable with 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values of 17.791–18.530, 15.477–15.597 and 37.761–38.497, respectively. The Sr–Nd–Pb–Hf isotopic data indicate that the magmatic sources are heterogeneous, showing an enriched to depleted signature of continental lithospheric mantle. The magmatic sources of the Genhe and Zalute groups are characterized by mixing among PM, EM II and DM, whereas the Tahe and Xiwu groups derive from enriched lithospheric mantle that had experienced metasomatism by fluids or melts from subducted Paleo-Asian and Mongol–Okhotsk ocean slabs and possible influence of Precambrian blocks. The Late Mesozoic volcanic rocks in the Great Xinggan Range resemble those of the Cenozoic calc-alkaline magmatism in the Basin and Range Province of the United States. We therefore suggest that the widespread Mesozoic volcanic rocks in the Great Xinggan Range were generated in an extensional tectonic setting.

The occurrence of the Pishikou mafic dike in the Qingdao region, China provides important constraints on the origin of Late Cretaceous (86–78 Ma) mafic magmatism on the eastern North China craton. The Pishikou mafic dike is distributed in the Cretaceous Laoshan granitoid body, Qingdao region and contains peridotitic and granulitic xenoliths, xenocrysts, and megacrysts. Rocks from the Pishikou mafic dike are basanites and have low SiO2 ( 8 wt%), total alkalis (Na2O + K2O > 4.8 wt%, Na2O/K2O > 1), TiO2 (> 2.5 wt%), CaO (> 9 wt%) and P2O5 (> 1 wt%). In trace element abundances, they are highly enriched in large ion lithophile elements (LILEs) and light rare-earth elements (LREEs) (ΣREE = 339–403 ppm, (La/Yb)N = 39–42) without high field strength element (HFSE) depletion. These rocks have radiogenic Sr and Pb, and less radiogenic Nd isotopic compositions [(87Sr/86Sr)i > 0.7059, eNd ≈ 2.7–3.8 (206Pb/204Pb)i ≈ 18.0 ± 0.1]. The diagnostic elemental ratios, such as Nb/La, Nb/U, and Nb/Th, are compatible with those of mid-oceanic ridge basalts (MORBs) and oceanic island basalts (OIBs). Therefore, the Pishikou mafic dike has a geochemical feature completely different from those of the Early Cretaceous mafic dikes from the Qingdao region, but similar to those of back-arc basalts from the Japan Sea. This geochemical feature suggests that the Pishikou mafic dike was derived from an asthenosphere source, but contaminated by materials from the subducted Pacific slab. The discovery of this mafic dike thus provides a petrological evidence for the contribution of subducted Pacific slab to the Late Cretaceous magmatism in the Qingdao region of the eastern North China craton.

The petrology and geochemistry of peridotite xenoliths in the Cenozoic basalts from Fanshi, the central North China Craton (NCC), provide constraints on the evolution of sub-continental lithospheric mantle. These peridotite xenoliths are mainly spinel-facies lherzolites with minor harzburgites. The lherzolites are characterized by low forsterite contents in olivines (Fo 91), high-Cr# spinels and clinopyroxenes with low abundances of heavy REE (HREE). These features are similar to those from old refractory lithospheric mantle around the world, and thus interpreted to be relics of old lithospheric mantle. The old lithospheric mantle has been chemically modified by the influx of melts, as evidenced by the Sr–Nd isotopic compositions of clinopyroxenes and relatively lower Fo contents than typical Archean lithospheric mantle (Fo > 92.5). The Sr–Nd isotopic compositions of harzburgites are close to EM1-type mantle, and of the lherzolites are similar to bulk silicate earth. The latter could be the result of recent modification of old harzburgites by asthenospheric melt, which is strengthened by fertile compositions of minerals in the lherzolites. Therefore, the isotopic and chemical heterogeneities of the Fanshi peridotite xenoliths reflect the refertilization of ancient refractory lithospheric mantle by massive addition of asthenospheric melts. This may be an important mechanism for the lithospheric evolution beneath the Central NCC.