Pb-Sr-Nd Isotopic Characteristics Of The Early Cretaceous Mafic Rocks From Eastern China: The Contribution Of The Lower Crust To The Mantle Source

The geochronology and Sr-Nd-Pb isotopic compositions of the Early Cretaceous mafic rocks from different tectonic units of the eastern China, including western Shandong province in North China, Dabie orogenic belt and northeast margin of the lower Yangtze region, together with the pyroxene megacrysts within Late Cretaceous basalts from Jiaozhou, are studied to deduce the characteristics of the Early Cretaceous subcontinental lithospheric mantle (SCLM) of eastern China.The isotopic characteristics of the upper and lower crusts of these tectonic units are constrained based on the data from the literatures. The isotopic compositions of the granulites from the Qianxi Group and the granitic gneisses from the Taishan Group are used to represent those of the lower and mid-upper crusts of North China, respectively. The isotopic characteristics of the Paleozoic mantle of North China before thinning are represented by those of Paleozoic kimberlite inclusions. For the Yangtze block, Sr-Nd isotopic compositions of its lower crust are represented by those of the granitic gneisses of the Kongling Group, and Pb isotopic compositions are approximated by those of Mesozoic granites from the Tongling district. While Sr-Nd isotopic compositions of the Yangtze mid-upper crust are represented by those of the gneisses of the Dongling Group and Proterozoic metamorphic rocks from southern Anhui, Pb isotopic compositions are represented by those of Phanerozoic sedimentary rocks along the Yangtze River in Anhui province and those of the Banxi Group. The isotopic characteristics of Dabie lower crust are represented by those of the gneisses from the North Dabie, and those of the upper crust are represented by those of the gneiss from the South Dabie.The samples from Jinan and Zouping intrusions contain gabbro and gabbroic-diorite. Zircons separated from the gabbroic-diorite at Zouping yield a SHRIMP U-Pb age of 128.2±2.0 Ma, which is interpreted as its formation time. Gabbros of both localities belong to tholeiitic series, most of gabbroic samples show significant positive Rb, Ba, Pb and Sr anomalies and negative Nb, Ta and Ti anomalies and are highly enriched in LREEs with positive Eu anomalies. The initial magma of the gabbros from Jinan and Zouping has been little contaminated by curstal materials during the magmas ascending based on geochemical, isotopic and geochronological evidences. Therefore, the isotopic characteristics of the gabbros from the two locations can be used to approximate those of their mantle source. Gabbros from Jinan and Zouping show the range of the initial isotopic ratios as follows: ⁸⁷Sr/⁸⁶Sr (t) = 0.7041～0.7055,ε_Nd(t) = -6～-18.7, ²⁰⁶Pb/²⁰⁴Pb(t) = 16.55～17.00, ²⁰⁷Pb/²⁰⁴Pb(t) = 15.22～15.35 and ²⁰⁸Pb/²⁰⁴Pb(t) = 36.29～36.95, suggesting that the mantle source of the gabbros possesses an enriched isotopic characteristics. Although it is general considered that the Mesozoic subcontinental mantle of the North China shows an isotopic characteristic which is similar to the EMI end member, isotopic character of the Mesozoic mantle of North China is different from the typical EMI end member defined by oceanic basalts. The lower Nd and Pb isotopic ratios suggest a contribution of the lower crust. It is inferred that the mantle source of the gabbros from Jinan and Zouping may be a mixture of the Paleozoic lithospheric mantle with an isotopic character similar to that of the EMII end member and the lower crust of North China.The Luzong volcanoic rocks, located at lower Yangtze regions, the northeastern margin of the Yangtze block, are a suite of shoshonitic rocks, which include trachybasalt-basaltic trachyandesite and trachyte. Both basalts of the Shuangmiao Formation and trachytes of the Fushan Formation show high K₂O, Al₂O₃ and Fe₂O₃, but low MgO and TiO₂, are enriched in LILEs, such as Th, U, K and LREEs, but with no negative Eu anomalies. The zircons separated from the basalt yield an U-Pb age of 129.5±2.3 Ma, which is interpreted as the formation age of the Luzong volcanic rocks. The initial magma of the Luzong volcanic rocks has been little contaminated by curstal materials during ascending based on the results of geochemistry, isotope and chronology, thus, the isotopic characteristics of the volcanic rocks can be reasonably used to indicate their mantle source. Luzong volcanic rocks show the range of the initial isotopic ratios as follows: ⁸⁷Sr/⁸⁶Sr(t) = 0.7057～0.7065,ε_Nd(t) = -3.9～-6.2, ²⁰⁶Pb/²⁰⁴Pb(t) = 17.88～18.08, ²⁰⁷Pb/²⁰⁴Pb(t) = 15.50～15.55 and ²⁰⁸Pb/²⁰⁴Pb(t) = 37.93～38.18. These values are located between the DMM and EMII end members. Furthermore, the isotopic compositions also suggest the contribution of the Yangtze lower crust to the mantle source.The mafic rocks from Jiaoziyan and Shacun. including gabbro and pyroxenite. mostly belong to the tholeiitic series, while a few are plotted in the field of the alkali series. The samples are highly enriched in LREEs. with small negative Eu anomalies and show distinct negative anomalies in Nb, Ti and Zr. and positive anomalies in Rb. Ba and Pb. Similar to the gabbros from Jinan and Zouping and the Luzong volcanic rocks, the initial mafic magma has also been little contaminated by the curstal materials during intrusion. The initial Sr-Nd-Pb isotope compositions of the North Dabie mafic rocks are in the ranges as follows: ⁸⁷Sr/⁸⁶Sr(t) = 0.7067～0.7085,ε_Nd(t) = -6.5～-19.1, ²⁰⁶Pb/²⁰⁴Pb(t)=16.37～17.40, ²⁰⁷Pb/²⁰⁴Pb(t)=15.31～15.45 and ²⁰⁸Pb/²⁰⁴Pb(t) = 37.11～37.98. Comparing with the mafic rocks from the North China, those from the North Dabie show higher ⁸⁷Sr/⁸⁶Sr (t). but lowerε_Nd (t); higher ²⁰⁷Pb/²⁰⁴Pb (t) and ²⁰⁸Pb/²⁰⁴Pb (t) at the same ²⁰⁶Pb/²⁰⁴Pb, which suggest that the mantle source of North Dabie has higherμvalue and Th/U ratio than that of the North China Block. In the Pb isotopic evolution diagram, data of the mafic intrusions from the North Dabie located close to EMI, while in ⁸⁷Sr/⁸⁶Sr(t) vs.ε_Nd(t) plot, these samples lie between EMI and EMII end members. On the other hand, in both isotopic diagrams, the mafic intrusions and the gneisses from the North Dabie share the same range, suggesting that the lower crust defined by the gneisses of the North Dabie has the significant contribution to the mantle source.In summary, the isotopic characteristics of Early Cretaceous subcontinental lithospheric mantle of eastern China, including North China Block, Dabie orogenic belt and the lower Yangtze region, suggest the contribution of the corresponding lower crustal material to the mantle source. The involvement of different lower crustal material is one of the main processes which resulted in different isotopic characteristics in the mantle sources of different tectonic units in Eastern China at the Late Mesozoic. When the contamination of the mantle-derived magma by the curstal material during intrusion is eliminated, the source mixing between the crustal and mantle materials is the primary process. The lithosphere delamination may be a possible dynamic model for explaining the incooperation of the lower crustal materials into the mantle source, but metasomatism model cannot be ruled out.The pyroxene megacrysts hosted by basalt (73 Ma) at Daxizhuang, Jiaozhou. contain two types: clinopyroxene megacrysts and orthopyroxene megacrysts based on major element analyses. Although their major element compositions are different, their isotopic compositions are highly homogeneous. Megacrysts and host basalts show uniform isotopic compositions, ⁸⁷Sr/⁸⁶Sr(t)=0.7033～0.7041 andε_Nd(t)= +4.5～+7.6, respectively, which suggest that they were derived from the same mantle source, and the megacrysts were crystallized from the host basaltic magma under high pressure. Moreover, the 73 Ma Daxizhuang basalts and hosted megacrysts show the similar depleted isotopic characteristics with Cenozoic basalts from Eastern China, implying that the transformation of the isotopic characters of the lithospheric mantle from enrichment to depletion in Jiaozhou, and also the lithosphere thinning was completed in Late Cretaceous period (73 Ma).