Chronological, Petrological And Geochemical Characteristics Of The Paleo-Mesoproterozoic Alkali-rich Intrusive Rocks Along The Southern Part Of The North China Craton

This paper systematically studied the geochemical, REE and trace elements characteristics and isotopes characteristics of the Paleo-Mesoproterozoic alkaline rocks in the south area of the NCC. Zircon U-Pb dating by SHRIMP are employed to determine their ages. We discussed temporal and spatial distribution, magma sources, tectonic environment of these rocks and the results provide evidences for people to study the tectonic evolution of CNN and polymerization and pyrolysis of Columbia supercontinent.The alkaline rocks are limited between 33.5°and 34.5°N. They are distributed in Xionger triple-junction aulacogen system. SHRIMP Zircon U-Pb ages of these alkaline rocks are during the period of 1840-1598Ma.Representative rocks include Jialu aegirine syenite body, Zhuyangcha amphibolite quartz monzonite body, Yanyaozhai subvolcanics body, Longwangzhuang alkaline granite body, Maping alkaline syenite body.According to the bulk compositions, those Paleo-Mesoproterozoic alkaline rocks in the south area of the NCC are dominant with mid-acid rocks with over-saturated aluminum, with few basic rocks. The rocks are characterized by enrichment of alkalis(average value of w(Na2O+K2O) =8.61%); high K; low Ca, Mg, Fe. Total REE concentrations of the rocks are in the range of 85.646～2943.20μg/g, with mean value 485.97μg/g. They are strongly depleted in HREE, with the average LREE/HREE value 15.16, and the Eu are generally negative anomaly. The PM-normalized spider diagrams exhibit LILE and LREE enrichment with strong Sr, Ti negative and Gd positive anomalies.The 176Hf/177Hf values of the studied alkaline rocks are 0.281415～0.281646, and theεHf(t) value are-11.264～-3.102, with an average of-6.587.The initial 143Nd/144Nd values of the rocks have a range from 0.509531 to 0.510104; with an average of 0.509895.TheirεNd(t) values are between-15.7 and -3.7, with an average of -8.11.All these major elements, rare earth elements, trace elements and isotopes results suggest that Paleo-Mesoproterozoic alkaline rocks in the south area of the NCC are formed under post-collision tectonic settings, and indicate that their magma source is related to the enriched lithospheric mantle and suffered contamination of crustal components.Alkaline magmatism during 1.84-1.60Ga in Xionger triple-junction aulacogen system in the south area of the NCC mean that this triple-junction aulacogen extended at least 1.84Ga ago. In view of alkaline magmatism during 1.87-1.70Ga in the north area of the NCC, we infer that the NCC was in extensional environment 1.84Ga ago. So, "Lvliang orogeny", the collision of eastern block and western block and the aggregation event of Columbia supercontinent probably occurred before 1.85Ga.The studied alkaline rocks are controlled by Xionger triple-junction aulacogen system. So they have the geodynamics same to the triple-junction aulacogen and relate to mantle plumes.