Density structure of the cratonic mantle in southern Africa: 1. Implications for dynamic topography

The origin of high topography in southern Africa is enigmatic. By comparing topography in different cratons, we demonstrate that in southern Africa both the Archean and Proterozoic blocks have surface elevation 500–700 m higher than in any other craton worldwide, except for the Tanzanian Craton. An unusually high topography may be caused by a low density (high depletion) of the cratonic lithospheric mantle and/or by the dynamic support of the mantle with origin below the depth of isostatic compensation (assumed here to be at the lithosphere base). We use free-board constraints to examine the relative contributions of the both factors to surface topography in the cratons of southern Africa. Our analysis takes advantage of the SASE seismic experiment which provided high resolution regional models of the crustal thickness.We calculate the model of density structure of the lithospheric mantle in southern Africa and show that it has an overall agreement with xenolith-based data for lithospheric terranes of different ages. Density of lithospheric mantle has significant short-wavelength variations in all tectonic blocks of southern Africa and has typical SPT values of ca. 3.37–3.41 g/cm³ in the Cape Fold and Namaqua–Natal fold belts, ca. 3.34–3.35 g/cm³ in the Proterozoic Okwa block and the Bushveld Intrusion Complex, ca. 3.34–3.37 g/cm³ in the Limpopo Belt, and ca. 3.32–3.33 g/cm³ in the Kaapvaal and southern Zimbabwe cratons.The results indicate that 0.5–1.0 km of surface topography, with the most likely value of ca. 0.5 km, cannot be explained by the lithosphere structure within the petrologically permitted range of mantle densities and requires the dynamic (or static) contribution from the sublithospheric mantle. Given a low amplitude of regional free air gravity anomalies (ca. + 20 mGal on average), we propose that mantle residual (dynamic) topography may be associated with the low-density region below the depth of isostatic compensation. A possible candidate is the low velocity layer between the lithospheric base and the mantle transition zone, where a temperature anomaly of 100–200 °C in a ca. 100–150 km thick layer may explain the observed reduction in Vs velocity and may produce ca. 0.5–1.0 km to the regional topographic uplift.     

Density heterogeneity of the cratonic lithosphere: A case study of the Siberian Craton

Using free-board modeling, we examine a vertically-averaged mantle density beneath the Archean–Proterozoic Siberian Craton in the layer from the Moho down to base of the chemical boundary layer (CBL). Two models are tested: in Model 1 the base of the CBL coincides with the LAB, whereas in Model 2 the base of the CBL is at a 180 km depth. The uncertainty of density model is < 0.02 t/m³ or < 0.6% with respect to primitive mantle. The results, calculated at in situ and at room temperature (SPT) conditions, indicate a heterogeneous density structure of the Siberian lithospheric mantle with a strong correlation between mantle density variations and the tectonic setting. Three types of cratonic mantle are recognized from mantle density anomalies. ‘Pristine’ cratonic regions not sampled by kimberlites have the strongest depletion with density deficit of 1.8–3.0% (and SPT density of 3.29–3.33 t/m³ as compared to 3.39 t/m³ of primitive mantle). Cratonic mantle affected by magmatism (including the kimberlite provinces) has a typical density deficit of 1.0–1.5%, indicative of a metasomatic melt-enrichment. Intracratonic sedimentary basins have a high density mantle (3.38–3.40 t/m³ at SPT) which suggests, at least partial, eclogitization. Moderate density anomalies beneath the Tunguska Basin imply that the source of the Siberian LIP lies outside of the Craton. In situ mantle density is used to test the isopycnic condition of the Siberian Craton. Both CBL thickness models indicate significant lateral variations in the isopycnic state, correlated with mantle depletion and best achieved for the Anabar Shield region and other intracratonic domains with a strongly depleted mantle. A comparison of synthetic Mg# for the bulk lithospheric mantle calculated from density with Mg# from petrological studies of peridotite xenoliths from the Siberian kimberlites suggests that melt migration may produce local patches of metasomatic material in the overall depleted mantle.     

Lithospheric structure of the North China Craton: Integrated gravity,geoid and topography data

The lithospheric structure of ancient cratons provides important constraints on models relating to tectonic evolution and mantle dynamics. Here we present the 3D lithospheric structure of the North China Craton (NCC) from a joint inversion of gravity, geoid and topography data. The NCC records a prolonged history of Archean and Paleoproterozoic accretion of crustal blocks through subduction and collision building the cratonic architecture, which was subsequently differentially destroyed during Mesozoic through extensive magmatism. The thermal structure obtained in our study is considered to define the lithosphere-asthenosphere boundary (LAB) of the NCC, and reflects the density variations within the mantle lithosphere. Employing the Moho depths from deep seismic sounding profiles for the inversion, and based on repeated computations using different parameters, we estimate the Moho depth, LAB depth and average crustal density of the craton. The Moho depth varies from 28 to 50 km and the LAB depth varies from 105 to 205 km. The LAB and Moho show concordant thinning from West to East of the NCC. The average crustal density is 2870 kg m^− 3 in the western part of the NCC, higher than that in the eastern part (2750 kg m^− 3). The results of joint inversion in our study yielded LAB depth and lithospheric thinning features similar to those estimated from thermal and seismic studies, although our results show different depth and variations in the thickness. The lithosphere gently thins from 145 to 105 km in the eastern NCC, where as the thinning is much less pronounced in the western NCC with average depth of about 175 km. The joint inversion results in this study provide another perspective on the lithospheric structure from the density properties and corresponding geophysical responses in an ancient craton.     

Highly heterogeneous lithospheric mantle beneath the Central Zone of the North China Craton evolved from Archean mantle through diverse melt refertilization

High-Mg# peridotite xenoliths in the Cenozoic Hebi basalts from the North China Craton have refractory mineral compositions (Fo > 91.5) and highly heterogeneous Sr–Nd isotopic compositions (⁸⁷Sr/⁸⁶Sr = 0.7031–0.7048, ¹⁴³Nd/¹⁴⁴Nd = 0.5130–0.5118) ranging from MORB-like to EM1-type mantle, which are similar to those of peridotites from Archean cratons. Thus, the high-Mg# peridotites may represent relics of the ancient lithospheric mantle. Published Re–Os isotopic data for Cenozoic basalt-borne xenoliths show T_RD ages of 3.0–1.5 Ga for the peridotites from Hebi (the center of the craton), 2.2–0 Ga for those from Hannuoba and Jining (north margin of the craton), and 2.6–0 Ga for those from Fanshi and Yangyuan (midway between the center and north margin of the craton). In situ Re–Os data of sulfides in Hannuoba peridotites suggest that whole-rock Re–Os model ages represent mixtures of multiple generations of sulfides with varying Os isotopic compositions. These observations indicate that initial lithospheric mantle beneath the Central Zone of the North China Craton formed during the Archean and was refertilized by multiple melt additions after its formation. The refertilization became more intensive from the interior to the margin of the craton, leading to the high heterogeneity of the lithospheric mantle: more ancient and refractory peridotites with highly variable Sr–Nd isotopic compositions in the interior, and more young and fertile peridotites with depleted Sr–Nd isotopic composition in the margin. Our data, coupled with published petrological and geochemical data of peridotites from the Central Zone of the North China Craton, suggest that the lithospheric mantle beneath this region is highly heterogeneous, likely produced by refertilization of Archean mantle via multiple additions of melts/fluids, which were closely related to the Paleoproterozoic collision between the Eastern and the Western Blocks and subsequent circum-craton subduction events.     

Source of metals in the Guocheng gold deposit,Jiaodong Peninsula,North China Craton: Link to early Cretaceous mafic magmatism originating from Paleoproterozoic metasomatized lithospheric mantle

Widespread Mesozoic Au and other hydrothermal polymetal (Zn–Pb–Cu–Mo–Ag–W–Fe–REE) deposits or smaller prospects occur in association with ancient mobile belts surrounding and cutting through the North China Carton (NCC). Among these, the gold ores of the Jiaodong Peninsula, Shandong Province, eastern NCC, represent the largest gold district in China. However, the genesis of these important gold mineralizations has remained controversial, notably their relationships to widespread mafic magmatism of alkaline affinity.The ore bodies of the Guocheng gold deposit on the Jiaodong Peninsula are fracture-controlled, sulfide-rich veins and disseminations, formed contemporaneously with abundant dolerite, lamprophyre and monzonite dikes at ca. 120 Ma. Dolerite dikes possess mantle-like major element compositions and alkaline affinity, associated with prominent subduction-type trace element enrichments. The dikes show petrographic and chemical evidence of magma mixing that triggered exsolution of magmatic sulfide and anhydrite crystallization, preserved as primary inclusions in phenocrysts. LA-ICP-MS analysis of magmatic sulfide inclusions demonstrates that metal abundance ratios (Ag, As, Au, Bi, Co, Cu, Mo, Ni, Pb, Sb, Zn) largely correspond to those of both unaltered bulk rock and bulk ore. Together with identical Pb isotope ratios of dolerite and bulk ore, this demonstrates that gold mineralization and dolerite dikes share a common source.Lead isotope signatures of the ore sulfides are much less radiogenic (17.08 < ²⁰⁶Pb/²⁰⁴Pb < 17.25, 15.41 <²⁰⁷Pb/²⁰⁴Pb < 15.45, 37.55 < ²⁰⁸Pb/²⁰⁴Pb < 37.93) relative to the Pb signature of Phanerozoic convecting mantle and plot to the left of the Geochron and above the MORB-source mantle Pb evolution line. Forward Monte Carlo simulations indicate three events for the U–Th–Pb isotope evolution: (1) late Archean formation of juvenile crust is followed by (2) subduction of this aged crust at ca. 1.85 Ga along with the assembly of Jiao–Liao–Ji mobile belt (suture within Columbia supercontinent). This late-Archean subducted crust released fluids with drastically reduced U/Pb that metasomatized the overlying depleted mantle, which formed cratonic lithospheric mantle. This metasomatized lithospheric mantle was (3) tapped in response to early Cretaceous extensional tectonics affecting notably the eastern margin of the NCC to generate mafic magmas and associated gold mineralization at Guocheng. Similarly non-radiogenic uranogenic Pb isotope data characterize the contemporaneous mafic dikes and gold deposits in the entire Jiaodong Peninsula, suggesting that our genetic model applies to the entire Jiaodong gold district.We propose that early Cretaceous melting of subcontinental lithospheric mantle metasomatized by subduction fluids during Paleoproterozoic amalgamation of terranes to the eastern NCC along with Columbia supercontinent assembly generated mafic magmatism and associated gold deposits. Given the conspicuous association of Phanerozoic hydrothermal ore deposits associated with reactivated Paleoproterozoic mobile belts, we envisage that our genetic model, which largely corresponds to that which is proposed for the Bingham porphyry-Cu–Au–Mo deposit, USA, may explain much of the magmatic-hydrothermal activity and associated ore formation all around the NCC.     

Geochemistry of Early Cretaceous calc-alkaline lamprophyres in the Jiaodong Peninsula: Implication for lithospheric evolution of the eastern North China Craton

Mesozoic lamprophyres are widely present in gold province in the Jiaodong Peninsula. In this study, we analyzed major and trace elements and Sr–Nd–Pb isotopic compositions of lamprophyres from the Linglong and Penglai Au-ore districts in the Jiaodong Peninsula, in an attempt to better understand Mesozoic lithospheric evolution beneath the eastern North China Craton. These lamprophyre dikes are calc-alkaline in nature, and are characterized by low concentrations of SiO₂, TiO₂ and total Fe₂O₃, high concentrations of MgO, Mg# and compatible element, enriched in LREE and LILE but variably depleted in HFSE. They display initial ⁸⁷Sr/⁸⁶Sr ratios of 0.709134–0.710314, ε_Nd(t) values of − 13.2 to − 18.3, ²⁰⁶Pb/²⁰⁴Pb of 17.364–17.645, ²⁰⁷Pb/²⁰⁴Pb of 15.513–15.571 and ²⁰⁸Pb/²⁰⁴Pb of 37.995–38.374. Interpretation of elemental and isotopic data suggests that the Linglong and Penglai lamprophyres were derived from partial melting of a phlogopite- and/or amphibole-bearing lherzolite in the spinel–garnet transition zone. The parental magma might have experienced fractionation of olivine and clinopyroxene, and minor crustal materials were incorporated during ascent of these mafic magmas. Before ~ 120 Ma of emplacement of these calc-alkaline lamprophyres, the ancient lithospheric mantle was variably metasomatized by hydrous fluids rather than melts from subducted/foundered continental crust. It is proposed that continuous modification by slab-derived hydrous fluids from the Paleo-Pacific plate converted the old cratonic lithospheric mantle to Mesozoic enriched lithospheric mantle. Geodynamic force for generation of these lamprophyres may be related to large scale lithospheric thinning coupled with upwelling of the asthenosphere beneath the North China Craton. Continental arc-rifting related to the Paleo-Pacific plate subduction is favored as a geodynamic force for the cratonic lithosphere detachment.     

Mineral chemistry and zircon geochronology of xenocrysts and altered mantle and crustal xenoliths from the Aries micaceous kimberlite: Constraints on the composition and age of the central Kimberley Craton,Western Australia

The Neoproterozoic (∼ 820 Ma) Aries micaceous kimberlite intrudes the central Kimberley Basin, northern Western Australia, and has yielded a suite of 27 serpentinised ultramafic xenoliths, including spinel-bearing and rare, metasomatised, phlogopite–biotite and rutile-bearing types, along with minor granite xenoliths. Proton-microprobe trace-element analysis of pyrope and chromian spinel grains derived from heavy mineral concentrates from the kimberlite has been used to define a ∼ 35–40 mW/m² Proterozoic geotherm for the central Kimberley Craton. Lherzolitic chromian pyrope highly depleted in Zr and Y, and Cr-rich magnesiochromite xenocrysts (class 1), probably were derived from depleted garnet peridotite mantle at ∼ 150 km depth. Sampling of shallower levels of the lithospheric mantle by kimberlite magmas in the north and north-extension lobes entrained high-Fe chromite xenocrysts (class 2), and aluminous spinel-bearing xenoliths, where both spinel compositions are anomalously Fe-rich for spinels from mantle xenoliths. This Fe-enrichment may have resulted from Fe–Mg exchange with olivine during slow cooling of the peridotite host rocks. Fine exsolution rods of aluminous spinel in diopside and zircon in rutile grains in spinel- and rutile-bearing serpentinised ultramafic xenoliths, respectively, suggest nearly isobaric cooling of host rocks in the lithospheric mantle, and indicate that at least some aluminous spinel in spinel-facies peridotites formed through exsolution from chromian diopside. Fe–Ti-rich metasomatism in the spinel-facies Kimberley mantle probably produced high-Ti phlogopite–biotite + rutile and Ti, V, Zn, Ni-enriched aluminous spinel ± ilmenite associations in several ultramafic xenoliths. U–Pb SHRIMP ²⁰⁷Pb/²⁰⁶Pb zircon ages for one granite (1851 ± 10 Ma) and two serpentinised ultramafic xenoliths (1845 ± 30 Ma; 1861 ± 31 Ma) indicate that the granitic basement and lower crust beneath the central Kimberley Basin are at least Palaeoproterozoic in age. However, Hf-isotope analyses of the zircons in the ultramafic xenoliths suggest that the underlying lithospheric mantle is at least late Archean in age.     

Upper mantle structure of the Saharan Metacraton

The ∼500,000 km² Saharan Metacraton in northern Africa (metacraton refers to a craton that has been mobilized during an orogenic event but that is still recognisable through its rheological, geochronological and isotopic characteristics) is an Archean–Paleoproterozoic cratonic lithosphere that has been destabilized during the Neoproterozoic. It extends from the Arabian–Nubian Shield in the east to the Trans-Saharan Belt in the west, and from the Oubanguides Orogenic Belt in the south to the Phanerozoic cover of North Africa. Here, we show that there are high S-wave velocity anomalies in the upper 100 km of the mantle beneath the metacraton typical of cratonic lithosphere, but that the S-wave velocity anomalies in the 175–250 km depth are much lower than those typical of other cratons. Cratons have possitive S-wave velocity anomalies throughout the uppermost 250 km reflecting the presence of well-developed cratonic root. The anomalous upper mantle structure of the Saharan Metacraton might be due to partial loss of its cratonic root. Possible causes of such modification include mantle delamination or convective removal of the cratonic root during the Neoproterozoic due to collision-related deformation. Partial loss of the cratonic root resulted in regional destabilization, most notably in the form of emplacement of high-K calc-alkaline granitoids. We hope that this work will stimulate future multi-national research to better understand this part of the African Precambrian. Specifically, we call for efforts to conduct systematic geochronological, geochemical, and isotopic sampling, deploy a reasonably-dense seismic broadband seismic network, and conduct systematic mantle xenoliths studies.     

Episodic Mesozoic thickening and reworking of the North China Archean lower crust correlated to the fast-spreading Pacific plate

A central target in Earth sciences is to understand the processes controlling the stabilization and destruction of Archean continents. The North China craton (NCC) has in part lost its dense crustal root after the Mesozoic, and thus it is a key region to test models of crust–mantle differentiation and subsequent evolution of the continental crust. However, the timing and mechanisms responsible for its crustal thickening and reworking have been long debated. Here we report the Early Cretaceous Yinan (eastern NCC) adakitic granites, for which major/trace elemental models demonstrate that they are complementary to the analogy of the documented eclogitic relicts within the NCC. Based on their Late Archean inherited zircons, depleted mantle Nd model ages of ∼2.8 Ga, large negative ε_Nd(t) values (−36.7 to −25.3) and strongly radiogenic initial ⁸⁷Sr/⁸⁶Sr ratios (0.7178–0.7264), we suggest that the Yinan adakitic granites were potentially formed by the dehydration melting of a thickened Archean mica-bearing mafic lower crust during the Early Cretaceous (ca. 124 Ma), corresponding to a major period (117–132 Ma) of the NCC Mesozoic intrusive magmatism. Combined previous results, it is shown that the thickening and reworking of the North China Archean lower crust occurred largely as two short-lived episodes at 155–180 Ma and 117–132 Ma, rather than a gradual, secular event. These correlated temporally with the superfast-spreading Pacific plate during the Mesozoic. The synchroneity of these events suggests rapid plate motion of the Pacific plate driving the episodic NCC crustal thickening and reworking, resulting in dense eclogitic residues that became gravitationally unstable. The onset of lithospheric delamination occurred when upwelling asthenosphere heated the base of lower crust to form coeval felsic magmas with or without involvement of juvenile mantle material. Collectively, the circum-Pacific massive crustal production could be attributed to the unusually rapid motion of Pacific at 155–180 Ma and 117–132 Ma.     

Re–Os evidence for the age and origin of peridotites from the Dabie–Sulu ultrahigh pressure metamorphic belt,China

Serpentinized garnet peridotites from the Xugou peridotite body of the Sulu ultrahigh-pressure (UHP) metamorphic terrane, central eastern China, are refractory (olivines have Fo_91.7–93.1), indicating their origin as residual mantle. Negative correlations between whole-rock MgO and TiO₂, Al₂O₃, total Fe₂O₃ and CaO (r = − 0.90 to − 0.95) and positive correlations between whole-rock Al₂O₃ and CaO and incompatible elements [Li, V, Cu, Ga, Sr, Y, Zr, heavy rare earth elements (HREEs), Hf, Pb and U] (r = 0.69 to 0.98) likely reflect melt depletion trends. Four highly refractory samples were selected for Re–Os isotopic analysis. Although they show evidence of variable enrichment of incompatible elements during serpentinization/metasomatism, no correlations exist between ¹⁸⁷Re/¹⁸⁸Os or ¹⁸⁷Os/¹⁸⁸Os with either La or Re (r = 0.00 to 0.17). These results indicate that any Re addition was fairly recent and did not affect the Os isotopic composition significantly. The correlation between ¹⁸⁷Os/¹⁸⁸Os and ¹⁸⁷Re/¹⁸⁸Os ratios thus, most likely reflects an ancient melt extraction event.The T_RD, T_MA and errorchron ages of the Xugou peridotites are all similar, suggesting that these peridotites formed around 2.0 Ga ago. This age is similar to Os model ages of mantle peridotites from the Dabie terrane, but contrasts markedly with the Archean ages of the continental lithospheric mantle (CLM) beneath the eastern block of the North China craton (NCC). If we assume that the Dabie–Sulu belt formed by the Triassic collision of the Yangtze craton with the eastern block of NCC and that the Archean aged CLM of the latter persisted until the Triassic, the Paleoproterozoic ages suggest derivation of these Dabie–Sulu mantle peridotites from the Yangtze craton. A Yangtze craton origin is consistent with the existing tectonic model of the Dabie–Sulu UHP belt. Our results support the hypothesis that the crust and underlying lithospheric mantle of the Yangtze craton were subducted to depths of > 180–200 km to form the world's largest UHP belt.     

Nd–Hf–Sr–Pb isotopes and trace element geochemistry of Proterozoic lamproites from southern India: Subducted komatiite in the source

The probable sources of some of the famous Indian diamonds are the 1.2 Ga old Krishna lamproites of Southern India, a rare Proterozoic occurrence of lamproites which are usually Cretaceous or younger in age. In this study we report Nd, Sr, Pb and Hf isotopes and multiple trace element concentrations of the Krishna lamproites. The goals are to evaluate mantle-processes and the petrogenesis of these ultrapotassic rocks of extreme chemical composition in light of these geochemical data, including their major element compositions.The Krishna lamproites show nearly uniform, parallel rare earth element (REE) distribution patterns with high concentrations and extreme light-REE enrichment (La/Yb_(N) = 41–88), high average concentrations of Ba (∼ 1200 ppm), Sr (∼ 1200 ppm), Zr (∼ 930 ppm), La (∼ 230 ppm), high U/Pb and Th/U ratios with notable absence of any Eu-anomaly. These rocks are typically porphyritic without any evidence of crystal accumulation, and have moderately high Mg-numbers (59–73) along with high Ni (average ∼ 301 ppm, highest 819 ppm) and Cr (average ∼ 183 ppm, highest 515 ppm) concentrations that show a positive correlation with MgO (wt.%), implying a role of olivine in the melt source. The low SiO₂ content (lowest 37.8%, average 49%) and high Nb (average 147 ppm), Zr, Sr, as well as Ni and Cr in these rocks indicate lack of upper continental crustal contribution in the genesis of these rocks. The initial Pb-isotopic composition of these lamproites is unusual in that in a ²⁰⁷Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb plot, these rocks plot to the left of the 1.2 Ga geochron (age of emplacement), unlike most mantle-derived rocks. This Pb-isotopic signature and the superchondritic Nb/Ta ratios (average 23.6) of these rocks rule out their derivation from a metasomatized sub-continental lithospheric mantle. The high ²⁰⁷Pb/²⁰⁴Pb at low ²⁰⁶Pb/²⁰⁴Pb indicates an Archean component in the source of these rocks. We argue that this Archean crustal component, which produced the low-SiO₂ lamproites along with the high Ni and Cr must have been ultrabasic, and we propose a model in which these lamproites formed by partial melting of metasomatized, subducted Archean komatiite in a peridotite mantle-source assemblage. In addition, these rocks display initial Hf isotopic compositions similar to Al-depleted komatiites, and high Nb/U, Nb/Th, and TiO₂ as well as low Al₂O₃/TiO₂ ratios (1.1–4.2) and average CaO/Al₂O₃ of ∼ 1.6 that are also similar to Archean komatiites. This is also supported by the initial Pb isotopic composition of the Krishna lamproites, requiring evolution in a variably high U/Pb, Th/Pb reservoir early in earth history, possibly resulting from preferential segregation of Pb relative to U and Th in the sulfides of the komatiite.The Al-depleted subducted komatiitic component was enriched by carbonate metasomatism in the peridotitic mantle. This metasomatism was responsible for the observed Nd–Hf isotope characteristics, specifically variable ε_Nd(T) at relatively constant ε_Hf(T) in the lamproites. This Nd–Hf-isotopic characteristic seems to be common in global lamproites of all ages. Our proposed model for the genesis of the Krishna lamproites involving a subducted komatiitic source may also be applicable for other global lamproites from cratonic settings, as older komatiite-bearing subducted crustal components were possibly ubiquitous in the architecture of ancient cratonic mantle.     

Kimberlitic sources of super-deep diamonds in the Juina area,Mato Grosso State,Brazil

The Juina diamond field, in the 1970–80s, was producing up to 5–6 million carats per year from rich placer deposits, but no economic primary deposits had been found in the area. In 2006–2007, Diagem Inc. discovered a group of diamondiferous kimberlitic pipes within the Chapadão Plateau (Chapadão, or Pandrea cluster), at the head of a drainage system which has produced most of the alluvial diamonds mined in the Juina area. Diamonds from placer deposits and newly discovered kimberlites are identical; they have super-deep origins from the upper-mantle and transition zone. Field observations and petrographic studies have identified crater-facies kimberlitic material at seven separate localities. Kimberlitic material is represented by tuffs, tuffisites and various epiclastic sediments containing chrome spinel, picroilmenite, manganoan ilmenite, zircon and diamond. The diamond grade varies from 0.2–1.8 ct/m³. Chrome spinel has 30–61 wt.% Cr₂O₃. Picroilmenite contains 6–14 wt.% MgO and 0.2–4 wt.% Cr₂O₃. Manganoan ilmenite has less than 3 wt.% MgO and 0.38–1.41 wt.% MnO. The ¹⁷⁶Hf/¹⁷⁷Hf ratio in kimberlitic zircons is 0.028288–0.28295 with ε_Hf = 5.9–8.3, and lies on the average kimberlite trend between depleted mantle and CHUR. The previously known barren and weakly diamondiferous kimberlites in the Juina area have ages of 79–80 Ma. In contrast, zircons from the newly discovered Chapadão kimberlites have a mean ²⁰⁶Pb/²³⁸U age of 93.6 ± 0.4 Ma, corresponding to a time of magmatic activity related to the opening of the southern part of the Atlantic Ocean. The most likely mechanism of the origin of kimberlitic magma is super-deep subduction process that initiated partial melting of zones in lower mantle with subsequent ascent of proto-kimberlitic magma.     

Reprint of “Depositional environment and tectonic implications of the Paleoproterozoic BIF in Changyi area,eastern North China Craton: Evidence from geochronology and geochemistry of the metamorphic wallrocks”

The Changyi banded iron formation (BIF) in the eastern North China Craton (NCC) occurs within the Paleoproterozoic Fenzishan Group. Three types of metamorphic wallrocks interbedded with the BIF bands are identified, including plagioclase gneisses and leptynites, garnet-bearing gneisses and amphibolites. Protolith reconstruction suggests that the protoliths of the plagioclase gneisses and leptynites are mainly graywackes with minor contribution of pelitic materials, the garnet-bearing gneisses are Fe-rich pelites contaminated by clastics, and the amphibolites are tholeiitic rocks. Trace elements of La, Th, Sc and Zr of the plagioclase gneisses and leptynites and the garnet-bearing gneisses support that these meta-sedimentary rocks were probably derived from recycling of Archean rocks with felsic and mafic materials differentiated into different rock types. ²⁰⁷Pb/²⁰⁶Pb ages of detrital zircons from the meta-sedimentary rocks concentrate at 2.7–3.0 Ga, confirming their derivation from the Archean rocks. The presence of several Paleoproterozoic detrital zircons (2240 to 2246 Ma), however, also suggests minor involvement of Paleoproterozoic materials. The Archean detrital zircons have ε_Hf(t) values varying from − 0.7 to 7.6, which mainly fall between the 3.0 Ga and 3.3 Ga average crustal evolution lines on the age vs. ε_Hf(t) diagram, further illustrating that the rocks providing materials for the meta-sedimentary rocks mainly originated from partial melting of a Mesoarchean crust. This is strongly supported by their crust-like trace element distribution patterns (such as Nb, Ta, P and Ti depletion) and ancient Nd depleted mantle model ages (T_DM = 2.9–3.4 Ga). In addition, the remarkably high ε_Hf(t) values (7.5 to 9.3) of the Paleoproterozoic detrital zircons constrain the Paleoproterozoic materials to originate from a depleted mantle. The amphibolites show low SiO₂ (46.5 to 52.8 wt.%) and high MgO (5.68 to 10.9 wt.%) contents, crust-like trace element features and low ε_Nd(t) values (− 4.5 to − 0.3), suggesting that these ortho-metamorphic rocks were mainly derived from subcontinental lithospheric mantle with some contamination by Archean crustal materials. Since an intra-continental environment was required for the formation of the above metamorphic rocks, these rocks not only confine the depositional environment of the Changyi BIF to be an intra-continental rift, but also support the rifting processes of the eastern NCC during Paleoproterozoic.     

Coexistence of the moderately refractory and fertile mantle beneath the eastern Central Asian Orogenic Belt

Relative to the North China Craton, the subcontinental lithospheric mantle (SCLM) beneath the Central Asian Orogenic Belt is little known. Mantle-derived peridotite xenoliths from the Cenozoic basalts in the Xilinhot region, Inner Mongolia, provide samples of the lithospheric mantle beneath the eastern part of the belt. The xenoliths are predominantly lherzolites with minor harzburgites, and can be subdivided into three groups, based on the REE patterns of clinopyroxenes. Group 1 peridotites (LREE-enriched), with low modal Cpx (3–7%), high Mg^# in olivine (> 90.6) and Cr^# in spinel (> 43.8), low whole-rock CaO + Al₂O₃ contents (1.62–3.22 wt.%) and estimated temperatures of 1043–1126 °C, represent moderately refractory SCLM that has experienced carbonatite-related metasomatism. Group 2 peridotites (LREE-depleted), with high modal Cpx (9–13%), low Mg^# in olivine (< 90.6) and Cr^# in spinel (< 20.0), high whole-rock CaO + Al₂O₃ contents (4.93–6.37 wt.%) and estimated temperatures of 814–970 °C, show affinity with Phanerozoic fertile SCLM that has undergone silicate-related metasomatism. Group 3 peridotites (convex-upward REE patterns), show wide ranges of olivine-Mg^# (88.4–90.6), spinel-Cr^# (11.5–47.6), and modal Cpx (3–14%) that overlap Groups 1 and 2. Their spinels have high TiO₂ contents (> 0.41 wt.%), implying involvement of reactions between melt and peridotites. The estimated temperatures of Group 3 (1033–1156 °C) are similar to those of Group 1. We suggest that the pre-existing moderately refractory lithospheric mantle (i.e., Group 1) beneath the eastern part of the Central Asian Orogenic Belt was strongly penetrated by upwelling asthenospheric material, and the cooling of this material produced fertile lithospheric mantle (i.e., Group 2). The present lithospheric mantle of this area consists of interspersed volumes of younger fertile and older more refractory lithosphere, with the fertile type dominating the shallower levels of the mantle.     

Cratonic reactivation and orogeny: An example from the northern margin of the North China Craton

Reactivation of cratonic basement involves a number of processes including extension, compression, and/or lithospheric delamination. The northern margin of the North China Craton (NCC), adjacent to the Inner Mongolian Orogenic Belt, was reactivated in the Late Paleozoic to Early Mesozoic. During this period, the northern margin of the NCC underwent magmatism, N–S compression, regional exhumation, and uplift, including the formation of E–W-trending thick-skinned and thin-skinned south-verging folds and south-verging ductile shear zones. zircon U–Pb SHRIMP ages for mylonite protoliths in shear zones which show ages of 310–290 Ma (mid Carboniferous–Early Permian), constraining the earliest possible age of deformation. Muscovite within carbonate and quartz–feldspar–muscovite mylonites from the Kangbao–Weichang and Fengning–Longhua shear zones defines a stretching lineation and gives ⁴⁰Ar/³⁹Ar ages of 270–250 Ma, 250–230 Ma, 230–210 Ma, and 210–190 Ma. Deformation developed progressively from north to south between the Late Paleozoic and Triassic. Exhumation of lower crustal gneisses, high-pressure granulites, and granites occurred at the cratonic margin during post-ductile shearing (~ 220–210 Ma). An undeformed Early Jurassic (190–180 Ma) conglomerate overlies the deformed rocks and provides an upper age limit for reactivation and orogenesis. Deformation was induced by convergence between the southern Mongolia and North China cratonic blocks, and the location of this convergent belt controlled later deformation in the Yanshan Tectonic Province. This province formed as older E–W-trending Archean–Proterozoic sequences were reactivated along the northern margin of the NCC. This reactivation has features typical of cratonic basement reactivation: compression, crustal thickening, remelting of the mid to lower crust, and subsequent orogenesis adjacent to the orogenic belt.     

Mesozoic lithospheric mantle of the northeastern Siberian craton (evidence from inclusions in kimberlite)

Several thousand clinopyroxene, garnet, and phlogopite inclusions of mantle rocks from Jurassic and Triassic kimberlites in the northeastern Siberian craton have been analyzed and compared with their counterparts from Paleozoic kimberlites, including those rich in diamond. The new and published mineral chemistry data make a basis for an updated classification of kimberlite-hosted clinopyroxenes according to peridotitic and mafic (eclogite and pyroxenite) parageneses. The obtained results place constraints on the stability field of high-Na lherzolitic clinopyroxenes, which affect the coexisting garnet and decrease its Ca contents. As follows from analyses of the mantle minerals from Mesozoic kimberlites, the cratonic lithosphere contained more pyroxenite and eclogite in the Mesozoic than in the Paleozoic. It virtually lacked ultradepleted harzburgite-dunite lithologies and contained scarce eclogitic diamonds. On the other hand, both inclusions in diamond and individual eclogitic minerals from Mesozoic kimberlites differ from eclogitic inclusions in diamond from Triassic sediments in the northeastern Siberian craton. Xenocrystic phlogopites from the D’yanga pipe have ⁴⁰Ar/³⁹Ar ages of 384.6, 432.4, and 563.4 Ma, which record several stages of metasomatic impact on the lithosphere. These phlogopites are younger than most of Paleozoic phlogopites from the central part of the craton (Udachnaya kimberlite). Therefore, hydrous mantle metasomatism acted much later on the craton periphery than in the center. Monomineral clinopyroxene thermobarometry shows that Jurassic kimberlites from the northeastern craton part trapped lithospheric material from different maximum depths (170 km in the D’yanga pipe and mostly < 130 km in other pipes). The inferred thermal thickness of cratonic lithosphere decreased progressively from ~ 260 km in the Devonian-Carboniferous to ~ 225 km in the Triassic and to ~ 200 km in the Jurassic, while the heat flux (Hasterok-Chapman model) was 34.9, 36.7, and 39.0 mW/m², respectively. Dissimilar PT patterns of samples from closely spaced coeval kimberlites suggest different emplacement scenarios, which influenced both the PT variations across the lithosphere and the diamond potential of kimberlites.     

Inhibited eclogitization and consequences for geophysical rock properties and delamination models: Constraints from cratonic lower crustal xenoliths

Studies on lower crustal and mantle xenoliths as well as geophysical data provide important information on the cratonic lithosphere. While geothermobarometric calculations of a majority of mantle xenoliths are in agreement with the typically low surface heat flow values of a craton (~ 40 mW/m²), P–T estimates for lower crustal xenoliths deviate significantly from the cratonic geotherms. Independent from the individual cratonic history, the temperatures are ~ 200–300 °C higher than what is expected at the base of the lower crust (~ 500–600 °C at ~ 1.3–1.6 GPa). Possible explanations may be a lack of equilibration to the cratonic geotherm or a relatively recent localized heat input. The presence of granulitic rocks under eclogite-facies conditions which are expected to prevail in the lower cratonic crust has consequences for the interpretation of geophysical rock properties. A mafic granulite which has been preserved under eclogite-facies conditions has densities and P-wave velocities similar to a felsic composition equilibrated to eclogite-facies conditions. Furthermore, phase diagrams calculated from xenolith bulk compositions demonstrate that eclogitization at relatively high temperatures as required for delamination of continental crust can only be triggered at significantly higher pressures than lithostatic at the base of the lower crust. As long as P–T conditions and the rock composition entail the assemblage to be granulitic, the addition of fluid at temperatures above 800 °C will not result in eclogitization, but rather in melt generation. This can also lead to an increase in density of up to 3%, however, this is strongly dependent on the amount of water saturation.     

Petrology and geochemistry of peridotite xenoliths from the Lianshan region: Nature and evolution of lithospheric mantle beneath the lower Yangtze block

Lithospheric thinning beneath the North China Craton is widely recognized, but whether the Yangtze block has undergone the same process is a controversial issue. Based on a detailed petrographic study, a suite of xenoliths from the Lianshan Cenozoic basalts have been analyzed for the compositions of minerals and whole rocks, and their Sr–Nd isotopes to probe the nature and evolution of the subcontinental lithospheric mantle beneath the lower Yangtze block. The Lianshan xenoliths can be subdivided into two Types: the main Type 1 xenoliths (9–15% clinopyroxene and olivine-Mg# < 90) and minor Type 2 peridotites (1.8–6.2% clinopyroxene and olivine-Mg# > 90). Type 1 peridotites are characterized by low MgO, high levels of basaltic components (i.e., Al₂O₃, CaO and TiO₂), LREE-depleted patterns in clinopyroxenes and whole rocks, and relatively high ¹⁴³Nd/¹⁴⁴Nd (0.513219–0.513331) and low ⁸⁶Sr/⁸⁷Sr (0.702279–0.702789). These features suggest that Type 1 peridotites represent fragments of the newly accreted fertile lithospheric mantle that have undergone ~ 1% of fractional partial melting and later weak silicate–melt metasomatism, similar to Phanerozoic lithospheric mantle beneath the eastern North China Craton. Type 2 peridotites may be shallow relics of the older lithospheric mantle depleted in basaltic components, with LREE-enriched and HREE-depleted patterns, relatively low ¹⁴³Nd/¹⁴⁴Nd (0.512499–0.512956) and high ⁸⁶Sr/⁸⁷Sr (0.703275–0.703997), which can be produced by 9–14% partial melting and subsequent carbonatite–melt metasomatism. Neither type shows a correlation between equilibration temperatures and Mg# in olivine, indicating that the lithospheric mantle is not compositionally stratified, but both types coexist at similar depths. This coexistence suggests that the residual refractory lithospheric mantle (i.e., Type 2 peridotites) may be irregularly eroded by upwelling asthenosphere materials along weak zones and eventually replaced to create a new and fertile lithosphere mantle (i.e., Type 1 xenoliths) as the asthenosphere cooled. Therefore, the subcontinental lithospheric mantle beneath the lower Yangtze block shared a common evolutional dynamic environment with that beneath the eastern North China Craton during late Mesozoic–Cenozoic time.     

PT conditions and trace element variations of picroilmenites and pyropes from placers and kimberlites in the Arkhangelsk region,NW Russia

Compositions of picroilmenite and pyrope concentrates from Carboniferous sandstones in the Arkhangelsk kimberlite province were analyzed by EPMA and LAM ICP MS in Analytic Center of V.S. Sobolev’s Institute of Geology and Mineralogy, SD RAS, Novosibirsk. The results from single grain thermobarometry (Ashchepkov et al., 2010, Ashchepkov et al., 2011, Ashchepkov et al., 2012) for garnet, spinel, ilmenite and clinopyroxene suggest heating of the base of the lithospheric mantle to 1400 °C (45 mw/m²) at 7.0–7.5 GPa and to 900 °C (35 mw/m²) at 3.5–5.5 GPa in an interval corresponding to a lens enriched in chromite and clinopyroxene. The pipes from the eastern fields reveal smoother mantle geotherms and lower temperature PT paths. Mantle columns beneath the kimberlites from northern (Verkhotinskoe field) and western pipes (Kepinskoe field) show heating from the lithosphere base to 5.0 GPa and stepped PT paths shown by chromites probably due to interaction with magmas which caused local Ti-enrichment near 3.0 and 5.5 GPa. The PT paths in the mantle columns beneath the alnöite pipes reveal higher temperature and relatively shallow PT conditions with two major clusters around 3.0 and 5.0 GPa. Trace element patterns for garnets vary from S-type typical of harzburgites to those with a hump in MREE (middle REE) typical for pyroxenites. Lherzolitic garnets with sinusoidal decrease of LREE show distinctive HFSE enrichment. Trace element ratios (Sm/Er)_n and (La/Yb)_n of garnets correlate positively with pressures estimates by single grain thermobarometry (Ashchepkov et al., 2010, Ashchepkov et al., 2011, Ashchepkov et al., 2012) but only poorly with Cr₂O₃ content. Enrichment in HFSE of all garnets is related to metasomatism that accompanied the picroilmenite-forming event.Ilmenites reveal two compositional trends. One corresponds to fractionation within conduits at the lower mantle (6.0–7.0 GPa) without contamination. A second trend at <6.0 GPa, formed due to assimilation fractional crystallization (AFC), is characterized by Fe and Cr increase with decreasing pressure. Similar trace element patterns of the various in HREE in ilmenites, possibly partly due to garnet assimilation from wall rock peridotites. The PT conditions and geochemistry for the minerals from the Carboniferous sediments are similar to those from the Lomonosovskoe deposit and Arkhangelskaya pipe (Lehtonen et al., 2009).     

Refertilization of lithospheric mantle beneath the Yangtze craton in south-east China: Evidence from noble gases geochemistry

The Yangtze craton (YC), in eastern China, is one of the oldest cratons in the world and is characterized by a complex tectonic and geodynamic evolution. This evolution regards most of the eastern China craton, which since Mesozoic time has undergone significant thinning (> 200 km) of Archean lithosphere. This thinning favored the refertilization of the old refractory subcontinental lithospheric mantle (SCLM) by the upwelling of younger fertile asthenosphere. Whether this feature is localized only beneath certain areas of eastern China or is a more widespread characteristic of the mantle, including the YC, is a matter of debate.In order to constrain the history of the YC SCLM, we have measured the He- and Ar-isotopic compositions of fluid inclusions hosted in mantle xenoliths in the Lianshan area, which is part of the poorly investigated YC in south-east China. We also report new mineral chemistry and trace element compositions of clinopyroxenes from the same suite of samples, for comparison with noble gases. Two distinct types of xenoliths can be identified: Type 1, characterized by mantle-like He-isotopic (³He/⁴He) ratios (up to 9.1 Ra), represents fragments of a fertile lithospheric mantle; Type 2, showing ³He/⁴He values in the SCLM range (³He/⁴He < 7 Ra), represents shallow relicts of a refractory mantle. The patterns of rare-earth elements as well as the Y and Yb concentrations in the clinopyroxenes normalized to primitive mantle (Y_N and Yb_N, respectively) indicate that fractional partial melting might have affected the local mantle by < 3% in Type 1 and up to 20% in Type 2 xenoliths from Lianshan, respectively. The range of ⁴He/⁴⁰Ar* (⁴⁰Ar* is corrected for atmospheric contamination) ranges from 4.9 × 10^− 4 to 3.6 × 10^− 1, which is below the typical production ratio of the mantle (⁴He/⁴⁰Ar* = 1–5); this range is however compatible with this fractional partial melting. The variable ³He/⁴He and ⁴He/⁴⁰Ar* values in Lianshan xenoliths suggest that the local mantle source was also influenced by kinetic fractionation, possibly triggered by metasomatic melts. Metasomatism associated with carbonatitic melts, together with fluxing by CO₂-rich fluids, have permeated the mantle beneath Lianshan, generating the observed decoupling between noble gases and trace elements. The interpretative framework is also applicable for other mantle xenoliths from eastern China, indicating that the refertilization of the SCLM by ascending mantle-like melts is common also to YC, which can be identified using noble gases.