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Transformation of refractory cratonic mantle into more fertile lithologies is the key to the fate of cratonic lithosphere. This process has been extensively studied in the eastern North China Craton (NCC) while that of its western part is still poorly constrained. A comprehensive study of newly-found pyroxenite xenoliths from the Langshan area, in the northwestern part of this craton is integrated with a regional synthesis of pyroxenite and peridotite xenoliths to constrain the petrogenesis of the pyroxenites and provide an overview of the processes involved in the modification of the deep lithosphere. The Langshan pyroxenites are of two types, high-Mg# [Mg2+/(Mg2++Fe2+)*100 = ∼90, atomic ratios] olivine-bearing websterites with high equilibration temperatures (880–970 oC), and low-Mg# (70–80) plagioclase-bearing websterites with low equilibration temperatures (550–835 oC). The high-Mg# pyroxenites show trade-off abundances of olivine and orthopyroxene, highly depleted bulk Sr–Nd (εNd = +11·41, 87Sr/86Sr = ∼0·7034) and low clinopyroxene Sr isotopic ratios (mean 87Sr/86Sr = ∼0·703). They are considered to reflect the reaction of mantle peridotites with silica-rich silicate melts derived from the convective mantle. Their depletion in fusible components (e.g. FeO, TiO2 and Na2O) and progressive exhaustion of incompatible elements suggest melt extraction after their formation. The low-Mg# pyroxenites display layered structures, convex-upward rare earth element patterns, moderately enriched bulk Sr–Nd isotopic ratios (εNd = -14·20– -16·74, 87Sr/86Sr = 0·7070–0·7078) and variable clinopyroxene Sr-isotope ratios (87Sr/86Sr = 0·706–0·711). They are interpreted to be crustal cumulates from hypersthene-normative melts generated by interaction between the asthenosphere and heterogeneous lithospheric mantle. Combined with studies on regional peridotite xenoliths, it is shown that the thinning and refertilization of the lithospheric mantle was accompanied by crustal rejuvenation and that such processes occurred ubiquitously in the northwestern part of the NCC. A geodynamic model is proposed for the evolution of the deep lithosphere, which includes long-term mass transfer through a mantle wedge into the deep crust from the Paleozoic to the Cenozoic, triggered by subduction of the Paleo-Asian Ocean and the Late Mesozoic lithospheric extension of eastern Asia. [ABSTRACT FROM AUTHOR]

Igneous activity in the rear-arc of the Paleogene Urumieh–Dokhtar Magmatic Belt of Iran has to date been poorly studied. An example of such activity, Late Eocene potassic mafic to intermediate magmatic rocks in the Lahrud area of NW Iran, is the focus of this work. These lavas include olivine-bearing clinopyroxene-phyric basalts, analcime-bearing leucite–clinopyroxene-phyric basalts, andesites, and trachytes, and Paleocene–Early Eocene pyroclastic rocks. Monzo-syenite plugs (dated here at ∼37 Ma), clinopyroxene-phyric basaltic dikes, and leucite-bearing clinopyroxene-phyric basaltic dikes intrude older lavas and pyroclastic rocks. Olivine-bearing clinopyroxene-phyric basalts and analcime-bearing leucite–clinopyroxene-phyric basalts are characterized by large phenocrysts of olivine, clinopyroxene, leucite, and analcime. Clinopyroxene-rich enclaves and partially resorbed mantle xenoliths also occur. Olivine phenocrysts are zoned from high-Mg# cores (Mg# = 90) to Fe-rich rims (Mg# = 66). Clinopyroxene phenocrysts from the olivine-bearing clinopyroxene-phyric basalts, analcime-bearing leucite–clinopyroxene-phyric basalts and clinopyroxene crystals in the enclaves show complex oscillatory zoning, sieve textures, and resorption textures, but with systematic core–rim compositional trends. Their 87Sr/86Sr isotopic compositions measured in situ range from 0·7037 to 0·7068 (mean = 0·7052 ± 0·0004), suggesting negligible crustal assimilation during fractional crystallization. The Lahrud lavas are potassic and are enriched in light rare earth elements and large ion lithophile elements such as Th, Rb, K and U. High field strength elements (HFSE), such as Nb, are depleted in the olivine-bearing clinopyroxene-phyric basalts and analcime-bearing leucite–clinopyroxene-phyric basalts, but enriched in the trachytes and trachytic ignimbrites. The isotopic compositions vary: 87Sr/86Srt from 0·7045 to 0·7052, εNd(t) from +2·8 to +3·3, and εHf(t) from +7·2 to +7·7. The rocks have radiogenic lead 206Pb/204Pb from 18·66 to 18·76, 207Pb/204Pb from 15·58 to 15·62, and 208Pb/204Pb from 38·73 to 38·81. Modeling of major and trace elements using the MELTS algorithm indicates that the geochemical variations in the basaltic to andesitic rocks are reasonably explained by shallow fractional crystallization with some complications owing to source heterogeneity, crustal assimilation, and magma mixing. The isotopic data imply that partial melting of old sub-continental lithospheric mantle was not responsible for the Lahrud potassic magmas; Hf isotopes and Zr/Nb ratios suggest derivation from an enriched mantle wedge, whereas ratios of incompatible trace elements (e.g. La/Yb, Ba/La, Ce/Pb, Th/Yb) and high 87Sr/86Sr suggest mantle metasomatized by slab-derived fluids or melts dominated by a sediment component. Geochemical modeling using the Arc Basalt Simulator version 5 reveals that the HFSE-depleted, olivine-bearing, clinopyroxene-phyric basalts originated from a high-temperature mantle wedge (2·2 GPa, 1310°C) fluxed intensively (5%) by melts from a deep hot slab (6 GPa, 1000°C). The moderately HFSE-depleted, olivine-bearing, clinopyroxene-phyric basalts reflect melting of a lower-temperature mantle wedge (2·2 GPa, 1300°C) with a lesser amount of slab melt flux (4%) from a lower temperature and shallower slab (3 GPa, 866°C). In contrast, the leucite–clinopyroxene-phyric basalts and andesites are from a similar source to the moderately HFSE-depleted, olivine-bearing, clinopyroxene-phyric basalts (3 GPa, 866°C) but with a contribution from a lower temperature mantle wedge (2·2 GPa, 1270°C). During Late Eocene times, slab retreat and upper-plate extension occurred in the rear-arc region of the Urumieh–Dokhtar Magmatic Belt. The Lahrud potassic magmas were generated from a high-temperature mantle wedge, which resulted in melting of the slab; this slab melt flux further promoted melting of the mantle wedge. [ABSTRACT FROM AUTHOR]

The Cadomian orogen of NW Iran includes a series of metamorphic rocks with zircon U-Pb ages between ca 562 and 505 Ma (Ediacaran to middle Cambrian). The Ediacaran-Cambrian basement is intruded by a series of Late Eocene-Late Oligocene I-type granitic rocks. U-Pb geochronology, integrated with geochemical and isotopic data for the basement rocks in NW Iran, provides further evidence of a Cadomian (562-505 Ma) arc-related magmatic event lasting ∼ 60 Myr. Cadomian magmatism in Iran was a part of a ∼ 100 Myr long episode of subduction-related arc magmatism at the northern margin of Gondwana. Zircon Hf-isotope compositions show that during Cadomian magmatic arc activity, juvenile arc magmas interacted with reworked Archean crust to generate the Ediacaran-Cambrian igneous rocks. Our results document both inheritance of old zircons and the presence of zircons with juvenile signatures in NW Iran, suggesting that the geotectonic setting for the Cadomian rocks was an Ediacaran continental magmatic arc and probably a neighboring back-arc basin. The occurrence of Ediacaran ophiolitic slices in NW Iran may provide evidence of back-arc basin opening at that time. Cenozoic plutonism in NW Iran is part of an Eocene-Oligocene magmatic 'flare-up' along the Urumieh-Dokhtar Magmatic Belt in central Iran, which lasted for ca 30 Myr. The melts responsible for the formation of these rocks had an essentially juvenile signature with minor contamination by Archean to Cadomian middle-lower continental crust. Continuous convergence between Arabia and Iran was accompanied by the transition of SW Eurasia from a compressional to an extensional convergent plate margin in Eocene-Oligocene times, leading to orogenic collapse, core-complex formation, exhumation of Cadomian crust and a major increase in arc magmatism. [ABSTRACT FROM AUTHOR]

High-Mg andesites (HMAs) and adakitic rocks are purported to occur exclusively in subduction zones in the modern Earth. In the North China Craton, early Cretaceous HMAs and adakitic dacites were erupted in a continental setting, apparently unrelated to subduction given their location distal (>1000 km) to the trench at that time. Here we report petrological, mineralogical and geochemical data for these rocks with the aim of constraining their petrogenesis and elucidating the role of water in intraplate magmatism and cratonic destruction. The HMAs can be subdivided into olivine (OI-)HMAs and clinopyroxene (Cpx-)HMAs. The former have high MgO (>9.8 wt %) and Mg# (>71), with rare high-Fo (up to 91) olivine phenocrysts, corresponding to (near-)primary magmas that equilibrated with mantle peridotite. The latter have moderate MgO (7.8-8.8 wt %) and Mg# (mostly <70) and low-Fo (mostly<83) olivine phenocrysts. The Cpx-HMAs are interpreted as magmas differentiated from the OI-HMAs by olivine-dominated fractionation at lower-crust levels. P-T-XH2O estimations show that the primary HMAs are melts of shallow (1.1-1.2 GPa), hot (~1250°C) and wet (H2O>3 wt %) lithospheric mantle. The coexisting adakitic dacites are hydrous (H2O≥5 wt %) magmas with high SiO2 (>63 wt %), Sr/Y ratios (≥39) and YbSN (source-normalized), low (Sm/Yb)SN, and negligible Eu anomalies. They also have unradiogenic whole-rock Nd [εNd(t)=-19 to -9] and zircon Hf [εHf(t)=-23 to -21] isotopic compositions consistent with derivation by melting of ancient lower crust at depths<40 km. Melting may have been induced by heating and addition of H2O from underplated HMAs. Mixing between Cpx-HMAs and low-Mg adakitic dacites in magma chambers produced high-Mg adakitic rocks. The petrogenetic model presented here explains the occurrence of intraplate HMAs and adakitic magmas elsewhere in the North China Craton. The P-T-XH2O conditions inferred for HMA generation imply that the subcontinental lithospheric mantle beneath the craton was hot and hydrous in the early Cretaceous, which may have triggered the destruction of the cratonic root. The occurrence of young HMAs and adakitic rocks in an intraplate extensional environment also casts doubts on the common use of a similar igneous rock association as an indicator of subduction processes in Archean time. [ABSTRACT FROM AUTHOR]

Apatites crystallized from different types of igneous rocks show significant variations in the abundances of some minor and trace elements. In this study, electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometry were used to determine the concentrations of 25 minor and trace elements in apatite separated from three principal rock types of the Transhimalayan igneous plutonic suite: S-type granites, the I-type Gangdese batholith and post-collisional adakites. F, Mn, Sr and rare earth elements (REE) in apatite vary systematically with the composition of the host magma and thus have high potential as petrogenetic tracers. More specifically, the F and Mn contents of apatite can be used as an indicator of magma aluminosity or differentiation index. Combined with Sr and REE data, which show significant variations in apatite from different rock types, these elements are useful for constructing ‘discrimination diagrams’. This study also reveals that apatite has the capacity to retain geochemical information about the host magma through the course of magmatic evolution. Systematic variations of Sr and REE in apatite with bulk-rock aluminosity are the results of partition competition with pre-existing and coexisting major and accessory minerals in silicate melts, and thus are useful for more detailed investigations of petrogenetic processes such as fractional crystallization and magma mixing, which is signaled by inconsistent Eu anomalies, Sr abundances and REE patterns relative to bulk-rock compositions. [ABSTRACT FROM PUBLISHER]

Major- and trace-element data on the constituent minerals of garnet peridotite xenoliths hosted in early Paleozoic (457–500 Ma) kimberlites and Neogene (16–18 Ma) volcanic rocks within the North China Craton are compared with those from the pre-pilot hole of the Chinese Continental Scientific Drilling Project (CCSD-PP1) in the tectonically exhumed Triassic (∼220 Ma) Sulu ultrahigh-pressure (UHP) terrane along its southern margin. P–T estimates for the Paleozoic and Neogene peridotite xenoliths reflect different model geotherms corresponding to surface heat flows of ∼40 mW/m2 (Paleozoic) and ∼80 mW/m2 (Neogene). Garnet peridotite xenoliths or xenocrysts from the Paleozoic kimberlites are strongly depleted, similar to peridotites from other areas of cratonic mantle, with magnesium olivine (mean Fo92.7), Cr-rich garnet and clinopyroxene with high La/Yb. Garnet (and spinel) peridotite xenoliths hosted in Neogene basalts are derived from fertile mantle; they have high Al2O3 and TiO2 contents, low-Mg-number olivine (mean Fo89.5), low-Cr garnet and diopside with flat rare earth element (REE) patterns. The differences between the Paleozoic and Neogene xenoliths suggest that a buoyant refractory lithospheric keel present beneath the eastern North China Craton in Paleozoic times was at least partly replaced by younger, hotter and more fertile lithospheric mantle during Mesozoic–Cenozoic times. Garnet peridotites from the Sulu UHP terrane have less magnesian olivine (Fo91.5), and lower-Cr garnet than the Paleozoic xenoliths. The diopsides have low heavy REE (HREE) contents and sinusoidal to light REE (LREE)-enriched REE patterns. These features, and their high Mg/Si and low CaO and Al2O3 contents, indicate that the CCSD-PP1 peridotites represent a moderately refractory mantle protolith. Details of mineral chemistry indicate that this protolith experienced complex metasomatism by asthenosphere-derived melts or fluids in Mesoproterozoic, and subsolidus re-equilibration involving fluids/melts derived from the subducted Yangtze continental crust during UHP metamorphism in the early Mesozoic. Tectonic extension of the subcontinental lithospheric mantle of the North China Craton and exhumation of the Sulu UHP rocks in the early Mesozoic induced upwelling of the asthenosphere. Peridotites sampled by the Neogene basalts represent newly formed lithosphere derived by cooling of the upwelling asthenospheric mantle in Jurassic–Cretaceous and Paleogene time. [ABSTRACT FROM PUBLISHER]

Primary sulphides occur both as micro-inclusions in major silicate and oxide phases and as individual grains in spinel lherzolite and pyroxenite xenoliths from Qilin, southeast China. Most of the lherzolite-hosted sulphide inclusions, typically 20-50 µm across, occur as isolated spheres or spheroids; host grains are olivine, clinopyroxene and orthopyroxene, but not spinel. In contrast, sulphide inclusions in pyroxenite are mostly 20-80 µm across and are almost exclusively hosted by clinopyroxene and spinel. These sulphide inclusions are typically multifaceted polygons, with their shapes epitaxially controlled by the host minerals. Isolated sulphide grains occur only in pyroxenite; they are up to 500 µm across, show evidence of deformation and are spatially associated with spinel. Lherzolite-hosted sulphide grains are polyphase assemblages that consist of pentlandite ± chalcopyrite ± Ni-poor monosulphide solid solution (mss1) ± Ni-rich monosulphide solid solution (mss2) ± cubanite ± heazlewoodite ± millerite ± bornite. Pyroxenite-hosted sulphide grains are pyrrhotite with minor chalcopyrite. All assemblages are likely to be the low-T ( <= 300°C) re-equilibrated products of high-T monosulphide solid solutions (MSS). The bulk compositions of these sulphide grains, estimated using proton microprobe analysis, show no consistent differences between the inclusion suites and intergranular sulphide grains, either in spinel lherzolite or pyroxenite samples. Average values of 111 proton microprobe analyses reveal that the lherzolite-hosted sulphides are rich in Ni (21%), Cu (<9%), Se (110 ppm) and platinum group elements (PGE) ( <= 30 ppm) but poor in Fe (37%) compared with the pyroxenite-hosted sulphides (Ni 1·4%, Cu <4%, Se 35 ppm, PGE absent, Fe 61·5%). Other trace elements (Co, Zn, As, Mo, Ag, Sn, Sb, Te and Pb) show no significant difference between the two suites. Lherzolite-hosted sulphides are inferred to be derived from immiscible sulphide melts trapped in residual mantle during partial melting. The sulphide melts had themselves undergone MSS fractionation before their incorporation into the depleted mantle rocks. In contrast, pyroxenite-hosted sulphides were produced by sulphur saturation during the crystallization of mafic magmas intruded into lherzolitic mantle. [ABSTRACT FROM PUBLISHER]