Xinhua Zhou

SHRIMP U–Pb zircon geochronology of the Fuping Complex: implications for formation and assembly of the North China Craton

Abstract The Fuping Complex, located within the central zone of the North China Craton, is composed of amphibolite to granulite facies orthogneisses, interleaved with minor supracrustal rocks at similar metamorphic grade. The oldest components recognised are hornblende gneiss enclaves within the predominant biotite orthogneiss which have a SHRIMP U–Pb zircon age of 2708±8 Ma. We consider these enclaves to represent fragments of ∼2.7 Ga continental materials incorporated in the biotite gneiss. The biotite gneiss has a SHRIMP U–Pb zircon age of 2513±12 Ma, interpreted to be time of magmatic crystallisation of the igneous precursor, based on the igneous characteristics of the zircons. This indicates a major magmatic episode at 2.52 Ga in the Fuping Complex, identical to the age of felsic volcanism within the low-grade Wutai Complex which crops out immediately to the west. A gneissic granite that intrudes the biotite gneisses has a poorly-defined 207Pb/206Pb age of 2045±64 Ma. This is within error of the age of 2097±46 Ma obtained from a fine-grained gneiss interlayered with amphibolite of the Wanzi Supracrustal Suite (WSS), interpreted to be volcanic in origin. Zircons from both these samples have strong oscillatory zoning and provide the first indication of a Palaeoproterozoic magmatic event in the area, again similar in age to magmatic events recently recognised in the adjacent Wutai Complex. These data indicate a comparable history for the Fuping and Wutai Complexes and support geochemical evidence that they had a common origin and formed part of a Late Archaean arc, affected by later Palaeoproterozoic re-activation. Low-uranium zircons without oscillatory zoning, separated from a sample of biotite gneiss, yield data clustered at 1817±26 Ma, which is interpreted to reflect a period of new zircon growth during a major metamorphic event. These data support the recently-proposed tectonic model that amalgamation of the North China Craton occurred due to collision of the Eastern and Western Blocks along the central zone at ∼1.8 Ga ago.

Secular evolution of the lithosphere beneath the eastern North China Craton: Evidence from Mesozoic basalts and high-Mg andesites

Abstract Geochemical and isotopic data from Mesozoic lavas from the Jianguo, Niutoushan, Wulahada, and Guancaishan volcanic fields on the northern margin of the North China Craton provide evidence for secular lithospheric evolution of the region. Jianguo lavas are alkaline basalts with LILE- and LREE-enrichment ((La/Yb)N=12.2–13.2) and MORB-like Sr-Nd-Pb isotopic ratios ((87Sr/86Sr)i (Zhang et al., 2002) , manifests a vast secular evolution of the lithospheric mantle beneath the eastern NCC from the Paleozoic refractory continental lithosphere to this Mesozoic modified lithosphere. Compared with the cratonic margin, the lithospheric mantle beneath the center of the craton was less extensively modified, implying the secular evolution was related to the subduction processes surrounding the NCC. Therefore, we suggest that the interaction of the slab-derived silicic melt with the old refractory lithospheric mantle converted the Paleozoic cratonic lithospheric mantle into the late Mesozoic fertile mantle, which was also different from the Cenozoic counterpart. A geodynamic model is proposed to illustrate such a secular lithosphere evolution.

Mesozoic crust-mantle interaction beneath the North China craton : A consequence of the dispersal of Gondwanaland and accretion of Asia

We present evidence from zircons entrained within lower-crustal xenoliths in the Cenozoic Hannuoba Basalt of multiple melting events beneath the North China craton in the late Mesozoic. Peak activity was between 180 and 80 Ma, the upper crustal signature of which was the generation of voluminous granitoids and related volcanic rocks, emplacement of dioritic and lamprophyric dikes, and widespread gold mineralization. The process involved partial loss of mantle lithosphere, accompanied by wholesale rising of asthenospheric mantle beneath eastern China. We correlate these events with lithospheric thinning resulting from the breakup and dispersal of Gondwanaland, accompanied by a major mantle overturn, fueled by the destruction of oceanic lithosphere and triggered by its sinking into the lower mantle during the subsequent accretion of Asia.

Continental crust and lithospheric mantle interaction beneath North China; isotopic evidence from granulite xenoliths in Hannuoba, Sino-Korean Craton

Mafic granulite and pyroxenite xenoliths from Cenozoic alkaline basalts at Hannuoba, Hebei Province, North China have been selected for a systematic geochemical and Sr–Nd–Pb isotopic study, which provides a unique opportunity to explore nature of the lower crust and the interaction between the continental crust and lithospheric mantle beneath an Archean craton. The major, compatible and incompatible elements and radiogenic isotopes of these xenoliths suggest great chemical heterogeneity of the lower crust beneath the Hannuoba region. Petrological and geochemical evidences indicate a clear cumulate origin, and most likely, they are related to basaltic underplating in different geological episodes. However, the Sr–Nd–Pb isotopic compositions of the xenoliths reveal a profound enriched source signature (EM I) with some influence of EM II, which implies that some portion of pre-existing, old metasomatized subcontinental lithospheric mantle could have played an important role in their genesis. It is suggested that the interaction between continental crust and subcontinental mantle as manifested by basaltic underplating would be closely related to regional tectonic episodes and geodynamic processes in the deep part of subcontinental lithospheric mantle.

Rb-Sr, Sm-Nd, and Pb isotope systematics of pyrite: Implications for the age and genesis of lode gold deposits

Lode gold deposits commonly consist of pyrite and lesser amounts of lead and zinc sulfides accompanied by quartz and calcite. Their exact origin remains controversial mainly because of the scarcity of reliable geochronological data. Here we present direct Rb-Sr dating of pyrite from a lode gold deposit and propose the method as a useful geochronological technique for gold mineralization for which age data are poor. A positive correlation between present-day 87 Sr/ 86 Sr and 87 Rb/ 86 Sr ratios of pyrite (FeS 2 ) subsamples from the Linglong gold mine of China corresponds to an age of 122–123 Ma, which dates the age of gold mineralization. The Sr, Nd, and Pb isotopic compositions of pyrites and the associated rocks suggest that the hydrothermal fluids responsible for the pyrite and gold mineralization were probably derived from a mixed source (i.e., degassing of mafic magmas and meteoric water that had leached the country rocks).

Comprehensive refertilization of lithospheric mantle beneath the North China Craton: further Os–Sr–Nd isotopic constraints

Abstract: Mantle peridotitic xenoliths in Cenozoic basalts from Hannuoba, on the northern margin of the Archaean North China Craton, have variable Re (0.01–0.30 ppb) and Os (2.7–6.2 ppb) abundances and 187Os/188Os (0.1138–0.1236) ratios. These xenoliths yield a range of Proterozoic Re depletion ages of 0.8–2.2 Ga that show a general correlation with olivine Fo. Mantle xenoliths in Cenozoic basalts from the centre and southern margin of the North China Craton also overwhelmingly show Proterozoic Re depletion ages that correlate with olivine Fo. These Re–Os age features are completely different from those of Palaeozoic kimberlite-borne peridotitic xenoliths, which have Archaean ages. This age distinction between xenoliths associated with Palaeozoic and Cenozoic volcanism indicates that the present-day lithospheric mantle was dominantly not newly accreted during the Phanerozoic but instead was transformed from the ancient lithospheric mantle by interaction with melts. The Re–Os data in combination with the geochemical and Sr–Nd isotopic features of peridotitic xenoliths from the Cenozoic basalts of the North China Craton demonstrate the presence of multiple stages of mantle metasomatic overprints in the lithospheric mantle. This study thus further indicates that lithospheric transformation through melt–rock interaction could be an important mechanism for compositional refertilization during the Phanerozoic. Supplementary material: Analytical methods and geochemical data are available at http://www.geolsoc.org.uk/SUP18334.

The Rb-Sr isochron of ore and pyrite sub-samples from Linglong gold deposit, Jiaodong Peninsula, eastern China and their geological significance

Here we report the first direct Rb-Sr dating of pyrites and ores using sub-sampling from lode gold deposits in Linglong, Jiaodong Peninsula, which is a supra-large lode gold deposit and propose this as a useful geochronological technique for gold mineralization with poor age constraint. The Rb-Sr data of pyrites yield an isochron age of (121.6±8.1) Ma, whereas those of ore and ore-pyrite spread in two ranges from 120.0 to 121.8 Ma and from 110.0 to 111.7 Ma. Studies of characteristic of gold deposit and microscopy of pyrite and quartz indicate that the apparent ages of ore and ore-pyrite are not isochron ages, only mixed by two end members, i.e. the primitive hydrothermal fluids and wall rocks, whereas the isochron age of pyrite sub-samples constrains the age of gold mineralization (121.6±8.1) Ma, i.e. early Cretaceous, which is in good agreement with the published SHRIMP zircon U-Pb ages.

Cratonic peridotites and silica-rich melts: Diopside-enstatite relationships in polymict xenoliths, Kaapvaal, South Africa

Abstract Silica-rich metasomatism is invoked as a mechanism for the conversion of clinopyroxene-bearing peridotites to orthopyroxene-rich, clinopyroxene-poor peridotites. While harzburgites are a major constituent of the garnet-diamond facies “keel” of Archaean (> 2500 Ma) cratons, metasomatic conversion of lherzolite to harzburgite is not widely documented from cratonic rocks. We report on the replacement of diopside by enstatite in polymict peridotites from Kimberley, South Africa and provide elemental and isotopic constraints on the nature of this process. Silica-rich metasomatism appears to have occurred at mid-craton depths and to have involved a silica-rich derivative of incompatible element-rich melts like kimberlite. The preservation of elemental and isotopic disequilibria in these mantle rocks may arise from the nearly synchronous occurrences of the kimberlite entrainment process and fluid-assisted deformation/crack propagation resulting in polymict formation.

Osmium isotope determination on mantle-derived peridotite xenoliths from Panshishan with N-TIMS

The187Os/188Os ratios of spinel lherzolite xenoliths from Panshishan determined with N-TIMS are lower than that of the primitive mantle, which shows depleted mantle characteristics. Their positive correlation with Al2O3/MgO suggests that the geochemical behavior of trace elements Re and Os is similar to that of Al2O3 and MgO respectively during the magmatic evolution and Re-Os isotopic system is largely immune to mantle metasomatism. A model age of 2.8–3.4 Ga obtained by187Os/188Os-Al2O3/ MgO correlation might represent the homogeneous age of the mantle lithosphere beneath the area.

Mantle-Derived Fluids Involved in Large-Scale Gold Mineralization, Jiaodong District, China: Constraints Provided by the He-Ar and H-O Isotopic Systems

The Jiaodong super-large gold metallogenic region is located on the eastern margin of the North China craton and the western margin of the Circum-Pacific tectonic belt. Ore-forming chronology indicates that these deposits formed at 120 ± 10 Ma. Helium and argon isotope compositions of fluid inclusions in pyrite range in 3He/4He ratios from 0.38 to 2.36 R/Ra (R: 3He/4He ratios of samples; Ra: 3He/4He ratios of Beijing air, 1.4 x 10-6) and in 40Ar/36Ar ratios from 310 to 1148. δ18O and δD values of fluid inclusions in quartz are -2.16 to +6.28‰ and -65 to -98‰, respectively. A consistent relationship between H-O and He-Ar isotopic systems show that mantle-source fluids were involved in large-scale metallogeny in the Jiaodong region. The fluids were produced accompanying delamination, lithosphere thinning, and crust-mantle interaction that occurred in the eastern North China craton during the Early Cretaceous (140-120 Ma).