The genesis of the Renjiatan—Qiaomaishan syenite porphyry in Luonan County and its indication for the late Paleoproterozoic Breakup——Petrochemistry,zircon U-Pb age and Lu—Hf isotope characteristics
Objectives:In the southern margin of North China Craton,a series of alkaline rocks of 1.8～1.6 Ga were found in Luonan,Shaanxi Province,and the west area of Henan Province.The study of these alkaline rocks can facilitate the understanding of the Paleoproterozoic tectonic environment of the southern margin of the North China Craton.Methods:This paper systematically studied the alkaline intrusive rocks in the Renjiatan—Qiaomaishan area in Luonan County,Shaanxi Province in terms of petrology,LA-ICP-MS zircon U-Pb dating,Lu—Hf isotope analysis,and petrogeochemistry,and discussed their genetic mechanism and tectonic setting.Results:The results show that the main lithology of the Renjiatan—Qiaomaishan pluton is syenite porphyry,with a weighted average age of 1689±20 Ma(MSWD=0.12),which indicates that the emplacement of the pluton happened in the Paleoproterozoic.The syenite porphyry is rich in alkali(Na2O+K2O=7.95％～9.5％)and high in potassium(K2O=4.67％～8.51％).The total amount of rare earth elements is high(496.2×10-6～650.2 ×10-6,average 543.1×10-6),and the fractionation of light(LREE=454×10-6～605×10-6)and heavy rare earth elements is obvious(HREE=39.3×10-6～45.6×10-6).The light rare earth elements are relatively rich,while the heavy rare earth elements are deficient,and there is a weak negative europium anomaly.In addition,K,Rb,Ba,Th is rich,while Nb,Ta,P,Ti is deficient,and there is an obvious negative Sr anomaly.The value ofεHf(t)is less than 0,the value of TDM1 is 2038～2218 Ma and TDM2 is 2250～2516 Ma,indicating that the materials in the magmatic source area are related to the enriched mantle or contaminated by the crustal materials.Conclusions:Therefore,the syenite porphyry in the Renjiatan—Qiaomaishan area is a melt formed by mixing the basaltic magma of the mantle with the medium—acid granitic magma formed by the Taihua complex TTG crust.It was formed in the tectonic environment of an intracontinental rift through strong crystallization differentiation,reflecting a response of the southern margin of the North China Craton to the cracking event of the Columbia Supercontinent.
south margin of North China Cratonalkaline rockssyenite porphyryColombian supercontinentfragmentation
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