Petrogenesis Of The Early Cretaceous Alkaline Intrusive Rocks In The Central North China Block: Constraints From Geochronology And Sr-Nd-Hf Isotopes

This paper reports the mineralogical, petrological, whole rock geochemical and Sr-Nd isotopic data of the Early Cretaceous alkaline intrusive rocks in the central North China Block(NCB), including Erfengshan aegirine-augite monzonite in south part of the study area, Huanglongnao aegirine-augite syenite and Shuiye aegirine-augite nepheline syenite in central part of the study area, and Huyanshan quartz monzonite in north part of the study area. In addition, the LA-ICP-MS zircon U-Pb dating and Hf isotopic data were also studied to ascertain their formation age, nature of magma source, petrogenesis, and the tectonic setting. This study further reveals the nature of the Early Cretaceous lithosphere mantle and the deep dynamic process of the NCB.1. The forming age of the Early Cretaceous alkaline intrusive rocks in the central NCB The LA-ICP-MS zircon U-Pb dating analyses have been determined for Erfengshan aegirine-augite monzonite, Huanglongnao aegirine-augite syenite, and Huyanshan quartz monzonite. The dating results yield 206Pb/238 U ages in the range of 128 Ma ~ 132 Ma、124 Ma ~ 136 Ma and 123 Ma ~ 139 Ma, respectively, with weighted mean 206Pb/238 U ages of 130 Ma, 127 Ma, and 130 Ma, respectively, suggesting that these rocks were emplaced in the Early Cretaceous.2. Geochemisty of the Early Cretaceous alkaline intrusive rocks in the central NCB Whole rock geochemical data from the Early Cretaceous alkaline intrusive rocks in the central NCB indicate that these intrusive rocks have low silica and high alkali, and belong to high-K cal-alkaline series with Si O2 = 59.40 % ~ 63.10 %, Mg O = 0.21 % ~ 0.37 % and Na2O+K2O = 8.20 % ~ 14.38 %. Furthermore, they are characterized by enriching in large ion lithophile elements(LILEs, e.g., Rb, Ba, and Sr) and light rare earth elements(LREE), depleting in high field strength elements(HFSEs, e.g., Nb, Ta, and Ti) and heavy rare earth elements(HREE), and displaying prominent positive Pb anomalies and weak Eu anomalies with ∑REE = 74.6×10-6 ~ 188×10-6, ∑LREE = 67.6×10-6 ~ 170×10-6, ∑HREE = 3.12×10-6 ~ 17.8×10-6, ∑LREE/∑HREE = 8.52 ~ 26.7, and(La/Yb) N = 9.10 ~ 26.1.3. Sr-N d-Hf isotopic compositions of the Early Cretaceous alkaline intrusive rocks in the central NCB Initial 87Sr/86 Sr radios of the Early Cretaceous alkaline intrusive rocks are relatively scattered, while the Nd isotopic compositions are relatively concentrated. The εNd(t) values from all samples range from –20.07 to –13.31, with a mean value of-16.30. Initial 87Sr/86 Sr value of the Erfengshan aegirine-augite monzonite is 0.706344 and εNd(t) value is-16.88; The Huanglongnao aegirine-augite syenite has(87Sr/86Sr)i = 0.705060 ~ 0.705243 and εNd(t) =-18.17 ~-16.38; The Shuiye aegirine-augite nepheline syenite is characterized by εNd(t) value of-20.07; Initial 87Sr/86 Sr radio of the Huyanshan quartz monzonite has relatively low(87Sr/86Sr)i = 0.706654, and its εNd(t) value is-13.31. The Early Cretaceous alkaline intrusive rocks in the central NCB have similar zircon Hf isotope compositions, which fall between the enriched lithospheric mantle and the low crust of the North China Craton(NCC). Zircons from the Erfengshan aegirine-augite monzonite have 176Hf/177 Hf ratios ranging from 0.281926 to 0.282119, εHf(t) values ranging from-27.69 to-20.67, and their two stage model(TDM2) ages ranging from 2490 Ma to 2924 Ma. The Huanglongnao aegirine-augite syenite exhibits 176Hf/177 Hf ratios of 0.282006 ~ 0.282165, εHf(t) values of-24.38 ~-18.8, and TDM2 ages of 2372 Ma ~ 2723 Ma; The Huyanshan quartz monzonite shows their 176Hf/177 Hf ratios of 0.281905 ~ 0.282219, εHf(t) values of-28.42 ~-17.36, and their TDM2 ages of 2280 Ma ~ 2970 Ma.4. Petrogenesis and tectonic setting of the Early Cretaceous alkaline intrusive rocks in the central NCB The Early Cretaceous alkaline intrusive rocks in the central NCB have low Si O2(59.40 % ~ 63.10 %) and high Na concentrations, and are enriched in large LILEs(e.g., Rb, Ba, and Sr) and LREE(67.6×10-6 ~ 170×10-6), suggesting their parent magma are likely derived from an enriched lithosphere mantle. However, the relatively high initial 87Sr/86 Sr values(0.7051 ~ 0.7067) and low εNd(t) values(-20.07 ~-13.31), zircon εHf(t) values(-28.4 ~-17.4), depletion in HFSEs(e.g., Nb, Ta, and Ti) and prominent positive Pb anomalies, imply that the magma of the Early Cretaceous alkaline intrusive rocks shows the characteristics of lower continental crust of the NCB. Combined with the presence of Paleoproterozoic inherited zircons, we conclude that the Early Cretaceous alkaline intrusive rocks were derived from partial melting of the enriched lithospheric mantle modified by the delaminated lower continental crust. The alkaline intrusive rocks in the central NCB were emplaced in the Early Cretaceous, which is consistent with the time of the Mesozoic strongest magmatic event in the NCC and even eastern C hina, and the formation time of the NNE large extensional structures and pull-apart basins. Combined with the presence of the Early Cretaceous bimodal volcanic rock in the eastern C hina, the alkaline intrusive rocks in the central NCB formed in an extensional environment related to the westward subduction of the Paleo-Pacific plate.