| The destruction of cratonic lithosphere beneath the North China craton (NCC) is one of the central themes in the study of continental geodynamics. The widespread generation of Mesozoic igneous rocks accompanied the cratonic destruction, which provides some insights into the evolution of the craton during lithospheric thinning. Data on mineral chemistry, major and trace elements and Sr-Nd isotopes of whole rocks, and in-situ U-Pb age and Hf-isotope analyses of zircons are reported for Triassic-Jurassic volcanic rocks in the Western Liaoning, in order to investigate their sources, petrogenesis and implications for the Phanerozoic evolution of northern part of NCC.Late Triassic high-Sr/Y Shuiquangou lavas, mainly trachytes with minor pyroxene andesite and rhyolite, are found at Lingyuan, in the Yanshanian fold-and-thrust belt on the northern margin of the NCC. The Shuiquangou volcanic rocks with high Sr/Y (>72) and (La/Yb)N (>24) also show enrichment in light rare earth elements and large-ion lithophile elements (e.g., Rb, Ba and Pb), and depletion in high-field-strength elements (e.g., Nb, Ta and Ti). They have low Ce/Pb (<4.3) and Nb/U (<4.8) and moderate (Gd/Yb)N (2.8-3.9). U-Pb dating of zircons yields concordant and lower-intercept ages of～220Ma, indicating that they erupted during the late Triassic. Concordant grains and an upper intercept age of～2.50Ga suggest that Neoarchean materials may have been involved in their petrogenesis. The relatively low initial87Sr/86Sr (0.70529to0.70540) and negative εNd(t)(-3.9to-9.9) of the these rocks, and the negative εHf(t)(-8.6to-1.1) of their zircons, suggest that the magmas were derived by partial melting of the cratonic lower crust, induced by continuous magmatic underplating under an extensional regime following the southward subduction of the Palaeo-Asian Ocean. Their high Sr/Y is inherited from their source, and does not necessarily imply melting at great depths (e.g., garnet-bearing lower crust). We suggest that partial melting of the ancient lower crust may be important for the petrogenesis of "adakitic" magmas in a continental extensional setting.The Haifanggou volcanic rocks consist of basaltic andesite, trachy-andesite, trachyte, dacite and pyroclastic rocks. The zircon U-Pb age of the Haifanggou trachyte yield a middle Jurassic age (173.4±3.1Ma), indicating the origin of angiosperms could be as early as～173Ma and the time of phase A of Yanshanian movement is middle Jurassic. The Haifanggou volcanic rocks with variable SiO2(51.7-66.8wt.%), high Al2O3(14.8-18.4wt.%), moderate MgO (0.6-4.4wt.%) and low Mg#(19-50), TiO2(0.57-1.03wt.%), Ni (13-33ppm) and Cr (7.2-61.8ppm). The lavas with high Sr (≥695ppm) and low HREE (Yb≤1.65ppm), also show enrichment in LREE and LILEs (e.g., Rb, Ba and Pb), and depletion in HFSEs (e.g., Nb, Ta and Ti). The EMI type Sr-Nd isotopic compositions (initial87Sr/86Sr=0.70491to0.70542, εNd(t)=-16.3to-7.3) of these rocks, and the evolved Hf isotopic compositions (εHf(t)=-21.7to-12.7, Tcrust=2.02to2.58Ga) of their zircons, suggest the magmas were derived by partial melting of an ancient lower crust, induced by continuous magmatic underplating. They erupted in a switched tectonic regime from within-plate extension to orogeny, which is responded to the closure of Mongolo-Okhotsk Sea and following collision of an amalgamated North China-Mongolian plate with Siberian plate.The Lanqi volcanic rocks consist of basalt, basaltic andesite, trachy-andesite, trachyte, dacite, rhyolite and pyroclastic rocks. The zircon U-Pb age (159.4±3.4Ma) of the andesitic breccia lava indicates that they erupted during the late Jurassic. Combined with the pervious geochronology data and the relationship of strata, we propose the Lanqi Fm./Tiaojishan Fm. are formed between166Ma to152Ma. The Lanqi lavas can be subdivided into four group base on their geochemical compositions. Group1basalt and basaltic andesite, with low SiO2(<56wt.%), LREE (La<35ppm) and LaN/YbN (<14), were probably generated by fractionation of olivine, clinopyroxene and Fe-Ti oxide and subsequent interaction with lower crust, of a primary magma derived from partial melting of enriched lithospheric mantle. Group2andesite and trachy-andesite, with moderate SiO2(55.9-58.0%), high REE (ΣREE=273-308ppm) and low LaN/YbN, were derived from intermediate granulites in the lower crust, induced by basaltic magma underplating. Group3intermediate-felsic lavas, with high LaN/YbN (>18) and Sr/Y (>50), were derivation from mafic lower continental crust with involvement of minor mantle materials. Group4high SiO2(>65%) dacite and rhyolite, with low Sr/Y and negative Eu anomalies (δEu=0.60to0.79), were produced by amphibole and plagioclase fractionation of melt derived from partial melting of felsic granulites and gneisses of the lower crust. The Lanqi volcanic rocks are the products of crust-mantle interaction, induced by rapid and oblique subduction of the Izanagi plate.The Mesozoic igneous rocks in Yanshan fold and thrust belt provide evidence for the continued reworking of lower crust of the northern part of NCC in Mesozoic time, which caused by episodic widespread magma underplating (-220Ma,160-152Ma,130-80Ma). Their petrogenesis also suggest that the igneous rocks with high Sr/Y "adakitic" signatures can be formed by partial melting of an ancient lower continental crust of normal thickness, without relationship to the melting of eclogite. Thus, they could not be used as an indicator of delamination of lower continental crust and the presence of Mesozoic plateau in the eastern NCC. The asthenospheric material involved in the generation of early Mesozoic magmatic rocks in the Yanshan belt, suggest the onset of the lithospheric destruction in the northern NCC may have occurred as early as the late Triassic, inducing by closure of Palaeo-Asian Ocean. The closure of Mongolo-Okhotsk Sea and following collision of an amalgamated North China-Mongolian plate with Siberian plate only caused the Mesozoic deformation without significantly modification of the lithospheric mantle beneath the North China craton. Pacific plate subduction resulted in extensive Jurassic magmatism and NE-to NNE-striking sinistral faults represented by the Tan-Lu fault zone which could be as channel of the upwelling asthenosphere. A shift from oblique shallow subduction of the Izanagi plate to orthogonal, steep subduction of the Pacific plate in early Cretaceous induced more intensive upwelling of asthenosphere and destruction of the North China craton. Thus, we propose that the destruction of the NCC lithospheric mantle was induced by subduction and collision of adjacent blocks in Phanerozoic. |
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