Huayun Tang

Anin situ zircon Hf isotopic, U-Pb age and trace element study of banded granulite xenolith from Hannuoba basalt: Tracking the early evolution of the lower crust in the North China craton

Backscattered electron images,in situ Hf isotopes, U-Pb ages and trace elements of zircons in a banded granulite xenolith from Hannuoba basalt have been studied. The results show that the banded granulite is a sample derived from the early lower crust of the North China craton. It is difficult to explain the petrogenesis of the xenolith with a single process. Abundant information on several processes, however, is contained in the granulite. These processes include the addition of mantle material, crustal remelting, metamorphic differentiation and the delamination of early lower crust. About 80% of zircons studied yield ages of 1842 ±40 Ma, except few ages of 3097–2824 Ma and 2489–2447 Ma. The zircons with ages older than 2447 Ma have high εHf (up to +18.3) and high Hf model age (2.5–2.6 Ga), indicating that the primitive materials of the granulite were derived mainly from a depleted mantle source in late Archean. Most εHf of the zircons with early Proterozoic U-Pb age vary around zero, but two have high εHfup to +9.2–+10.2, indicating mantle contribution during the collision and assembly between the Eastern and Western blocks in the early Proterozoic that resulted in the amalgamation of the North China craton.

Age and composition of the Rushan intrusive complex in the northern Sulu orogen, eastern China: petrogenesis and lithospheric mantle evolution

Zircon U–Pb age, whole-rock elemental and Sr–Nd isotopic data are presented for the Rushan intrusive complex from the northern Sulu orogen, China. The intrusion, emplaced at c . 111 Ma, consists mainly of biotite-bearing gabbro and pyroxene-bearing diorite. The rocks are high-K calc-alkaline in major elements, and enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), depleted in high field strength elements (HFSE), and possess narrow ranges of initial 87 Sr/ 86 Sr (0.70746–0.70827) and ɛ Nd ( t ) (−12.37 to −8.26). The complex is interpreted to originate from decompression melting of spinel-facies cratonic mantle that was metasomatized by the subducted Yangtze crustal materials and the melts experienced fractionation of olivine, clinopyroxene, apatite and Fe–Ti oxides. The Rushan complex is similar in composition to other Early Cretaceous mafic-intermediate complexes from the southern margin of the North China Craton, such as the Fangcheng basalts and Yinan gabbros, implying the mantle source of the Rushan intrusion is tectonically affiliated to the southern wedge of the Craton. We infer that the Rushan complex formed in an extensional regime corresponding to the asthenosphere upwelling during gradual erosion and replacement of the cratonic mantle by the fertile lithosphere beneath the eastern North China Craton.

Age and Composition of Peridotites:Implications for the Lithospheric Thinning Accompanying with Mantle Replacement Beneath the Eastern North China Craton

Abstract Olivine-Mg # values and diopside-REE patterns of mantle peridotites from the North China were systematically compared. In situ LAM-MC-ICPMS Re-Os isotopes of sulfides in the Hannuoba peridotitic xenoliths are also reported. Based on these data, the lithospheric thinning mechanism beneath the eastern North China Craton during Mesozoic-Cenozoic times was discussed. The results show that in situ analysis, relative to the whole-rock Re-Os results, is advanced to open out the lithospheric deep processes in detail, and that the existence of thermal events in Mesoproterozoic (1.4 Ga) and in Neoproterozoic (0.7–0.8 Ga) within the subcontinental lithospheric mantle (SCLM) beneath the North China Craton. The coexistence of the lithospheric mantle including refractory, transitional and fertile affinity cannot be well interpreted by the mechanism of lithospheric delamination. On the other hand, simple melt-peridotite interaction is also difficult to interpret abundant diopside with LREE-depletion patterns in the Cenozoic lithospheric mantle, that is, it is irreversible of the transitions of the cratonic mantle with complex evolution history to the “oceanic” one with simpler history. The complex processes including the lateral spreading (extension), melt-rock interaction, asthenospheric erosion and mantle replacement, therefore, must be involved during the lithospheric thinning beneath the North China Craton. These processes may include: 1) the northward subduction and subsequent collision of the Yangtze Craton in Early Mesozoic would result in the North China lithospheric mantle experienced metasomatism or modification (melt-rock interaction) by fluids/melts derived from the subducted Yangtze continent, lithospheric mantle extension and asthenospheric erosion; 2) asthenospheric upwelling related to the subduction of the Kula and the Pacific Plates during Late Mesozoic-Paleogene, erodes the remaining lithospheric mantle (including those modified and newly accreted ones) and results in the huge thinning of North China lithosphere; and 3) asthenospheric cooling in Neogene would slightly lowers the boundary between the lithosphere and asthenosphere, create newly accreted lithospheric mantle and achieve a thinning lithosphere as a whole. The fertile peridotite xenoliths in the Fuxian basalts erupted at 100 Ma indicate that the early mantle replacement beneath the eastern North China Craton took partly place before the time.

Lower Crustal Accretion and Reworking Beneath the North China Craton: Evidences from Granulite Xenoliths

How has the Earth’s deep continental crust evolved? Most of our knowledge is derived from surface exposures, but xenoliths carried in igneous rocks can be an important source of information. The North China Craton (NCC) is one of the oldest cratons in the world and Phanerozoic igneous rocks with abundant xenoliths are widespread, making it an ideal area to study the formation and evolution of continental crust. Abundant data of U–Pb ages and Hf isotopes in zircons were obtained for lower crustal xenoliths from over ten localities to constrain the history beneath the craton. The oldest components of the NCC may be ~4.0 Ga. The craton experienced complex accretion and reworking processes in its deep crust, accompanied by the formation and differentiation of the ancient continental nucleus. The small size of the NCC, compared with many other cratons worldwide, made it more susceptible to the effects of marginal subduction and collision with surrounding blocks. In the lower crust, the ancient components of the craton were reworked in Paleoarchean (3.80–3.65 Ga) time. The craton also experienced two important accretionary episodes, in the Neoarchean (2.8–2.5 Ga) and the Paleoproterozoic (2.3–1.8 Ga). Asthenospheric upwelling in Neoproterozoic time (0.6 Ga) locally modified the lower crust. Subduction and collision of the surrounding blocks, such as the Yangtze Craton, in Paleozoic and in early Mesozoic time also strongly modified the lower crust, especially along the cratonic margins. Accretion and modification of the lower crust during late Mesozoic–Paleogene were obvious due to the addition of depleted-mantle materials (underplating).

Petrogenesis of the Early Cretaceous Laojunshan monzogranite at the southern margin of the North China Craton: Constrains on the transition of the tectonic regime

Voluminous Mesozoic magmatic rocks containing abundant Au-Mo polymetallic mineralization resources are developed in the Xiaoqinling-Xiong’ershan district of the southern margin of the North China Craton (NCC). The widely distributed intrusions are mainly of intermediate-acidic in composition and formed during the Late Jurassic-Early Cretaceous (Ding et al., 2011; Hu et al., 2012; Li N et al., 2012; Mao et al., 2010; Zhao et al., 2012). Previous studies on representative Late Triassic -early Early Cretaceous plutons such as the Laoniushan, Fangshanyu, Wenyu and Niangniangshan granites in the western and northern regions of the XiaoqinlingXiong’ershan district have revealed that they are characterized by high Sr/Y ratios and thus indicate the existence of thickened crust under compressional setting beneath the Xiaoqinling-Xiong’ershan district prior to ca. 130 Ma (Hu et al., 2012; Zhao et al., 2012). However, precise timing of the tectonic transformation of the lithosphere from compression to extension remain unclear. Here, the detailed petrography and new zircon U-Pb ages and Hf isotope as well as whole rock geochemistry and Sr-Nd-Pb isotopes are presented for the Laojunshan pluton, with the aim toinvestigate their petrogenesis, and to further constrain the geodynamic process of the lithosphere evolution in the southern margin of the NCC.