| Early Cretaceous volcanic rocks, located in the eastern part of the North China Craton(NCC), were formed during the magmatic climax corresponding to the destruction of the NCC, and thus provide vital constraints on the thermochemical state of the subcontinental lithospheric mantle(SCLM) during the cratonic destruction. Studies of these volcanic rocks can help better understand the spatial and temporal evolution of the SCLM and the associated deep-Earth geodynamics for the cratonic destruction. Targeting on the Early Cretaceous mafic to intermediate lavas in the Shandong Province and the Liaodong Peninsula in eastern NCC, this study aims to precisely determine their eruption ages and tempo-spatial distribution patterns using 40Ar-39 Ar dating techniques, and to unravel mineral compositions, water contents and metasomatism of melting sources. All these information will collectively be applied to understand the geodynamic processes for the destruction of the NCC.40~Ar-~39 Ar dating results show that the lavas of the Qingshan Formation in the Luxi region were emplaced from 129 to 115 Ma, whereas their counterparts in the Jiaodong Peninsula erupted at 126 to 91 Ma, suggesting that the Luxi lavas were formed earlier than the Jiaodong lavas. The lavas of the Xiaoling Formation in the Liaodong Peninsula were formed at ~110 Ma, much younger than those of the Qingshan and Yixian formations in the Shandong and western Liaoning provinces respectively. These therefore imply that the Early Cretaceous magmatism are spatially and temporally diachronous.The lavas of the Qingshan Formation in the Zouping area in the Luxi region are composed of basaltic andesite and andesite, along with minor basalts. Phenocrysts in the Zouping andesites are dominated by plagioclase, high-Ca/low-Ca augite and tschermakite. The compositions of amphibole indicate that the temperatures of magma are from 916 to 943 °C, and water contents of the equilibrated melt(H2Omelt) are between 4.2 and 5.9 wt%. Samples displaying Si O2 of 50.66~58.76 wt% and Mg O of 1.68~10.52 wt% are characterized by LILE enrichment, and HFSE depletion. The studied Zouping andesites have variable εNd(t) values(+0.1~-7.96), but uniform initial 87Sr/86Sr(0.7043~0.7060). It is proposed that the Zouping lavas may have been generated as a result of the interaction between asthenosphere-derived melts and ancient SCLM.Basalts of the Qingshan Formation in the Mengyin area are dominated by diopside and augite phenocrysts, along with plagioclase. The pyroxenes have clear oscillatory zoning, indicating multiple extraction and replenishment of melts during partial melting. In contrast, andesitic samples mainly consist of augite, tschermakite, magnesiohastingsite and plagioclase. According to the amphibole compositions, the temperatures of magma are from 880 to 973 °C, and H2 Omelt are of 2.6 to 5.4 wt%, lower than those of the Zouping andesites. The basaltic samples have Si O2 of 50.4~51.8 wt% and Mg# of 60.3~62.1, showing similar Ti O2, Fe2O3 t, Ca O, Mn O and Al2O3 contents to those of the Fangcheng basalts, whereas the andesitic samples have higher Si O2(61.2~63.4 wt%) but lower Mg#(50.3~56.1). Basaltic and andesitic samples have high initial 87Sr/86 Sr of 0.7093 and 0.7110, but low εNd(t) of-10.5 and-14.1 respectively, falling into the EM2 field. All analyzed samples display enriched LILE, depleted HFSE, and were interpreted to be derived from partial melts of the SCLM, metasomatized by melts or fluids released from subducted Yangtze slabs.Lavas of the Qingshan Formation in the Laiyang area mainly consist of basaltic andesite and andesite, comprising clinopyroxene and plagioclase phenocrysts. The clinopyroxene show clear oscillatory zoning, with normal zoning from the core to the mantle and reverse zoning from the mantle to the rim. Their cores have Mg# of as high as 86, demonstrating that they were in equilibrium with the primary magma. The complete cycle of normal and reverse zoning in the Cpx indicates multiple extraction and replenishment of melts during partial melting. The studied Laiyang andesite have varied Si O2(55.5~61.6 wt%) and Mg O(3.5~5.7 wt%), and are featured by the enrichment of LILE, the depletion of HFSE, and weak Eu negative anomalies. The Laiyang andesites display EM2-like Sr-Nd compositions(εNd(t) =-16.6~-17.6, 87Sr/86 Sri = 0.7087~0.7110). Lavas of the Qingshan Formation in the Jimo area mainly comprise basalt, andesite and dacite, showing a bimodal feature. The studied dacitic samples have high Si O2(62.5~66.6 wt%), low Mg O(1.5~3.0 wt%), and varied Mg#(37~61). All samples are characterized by the enrichment of LILE, the depletion of HFSE and weak Eu negative anomalies. The εNd(t)(-16.7~-17.2) and initial 87Sr/86 Sr ratios(~0.7084) of the Jimo lavas are similar to those of the Laiyang andesites. It is thus suggested that the Laiyang and Jimo lavas may have been derived from partial melting of the SCLM metasomatized by melts or fluids released from subducted Yangtze slabs.Lavas of the Xiaoling Formation in the Liaodong Peninsula comprise basalt, andesite and dacite, with phenocrysts of olivine, diopside, Mg-augite, plagioclase and magnesiohastingsite. The calculated temperatures of magma are from 902 to 968 °C, and H2 Omelt are from 3.9 to 6.5 wt%. The Xiaoling lavas have a wide range of Si O2(45~62 wt%), Mg O(1.2~9.2 wt%) and Ca O(1.8~10 wt%), and are characterized by depletion of HFSE, enrichment of LILE, and EM1-like Sr-Nd compositions(εNd(t) =-8.7~-16.0, 87Sr/86 Sri = 0.7046~0.7054), consistent with a derivation from the SCLM. With the exception of Ti O2, the studied samples have major element compositions similar to those of experimentally determined partial melts of volatile-free MORB-like eclogite at 3~5 GPa, but differ from anhydrous peridotite-derived melts. The olivine phenocrysts have high Ni and Fe/Mn, and low Ca contents, which are typical of the olivines crystallized from melts derived from garnet pyroxenitic mantle source. This suggests a pyroxenitic mantle source, which may have been formed by the solid-state reaction between recycled crustal materials and their surrounding peridotite during the Late Archean to Paleoproterozoic.The age patterns of the Late Triassic to Early Cretaceous magmatism in the eastern NCC suggest that the lithospheric thinning and cratonic destruction are temporally and spatially diachronous. The occurrence of Late Triassic magmatism could have been caused by post-orogenic extension following the collision between the Yangtze and North China plate. The Late Triassic magmatism marks the initiation of the destruction of the NCC along its eastern margin. The Early to Middle Jurassic magmatism were interpreted to be related to the subduction of the Pacific plate beneath the eastern margin of the eastern China, whereas the Early Cretaceous magmatism were trigged by the abrupt change of subduction direction of the Pacific plate and subsequent slab tearing and roll back. The presence of abundant amphibole phenocrysts in the Early Cretaceous lavas in the eastern NCC suggests a highly hydrated SCLM in this region. The genesis of the Early Cretaceous lavas in the eastern NCC therefore highlights the important role of water and Pacific subduction in the destruction of the North China Craton. |
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