Chronology, Geochemical Characterisation and Tectonic Background of Early Cretaceous Volcanic Rocks in the Lengjimanda Plate, Da Hinggan Mountains

Abstract

The Lengjimanda Plate in the central Da Hinggan Mountains is mainly composed of a series of acid volcanic rocks such as rhyolites and rhyolitic crystal tuffs of the Early Cretaceous Baiyingaolao and Guanghua formations. LA-ICP-MS zircon U–Pb dating indicates that rhyolites of the Baiyingaolao Formation formed between 140 and 137 Ma, while rhyolites of the Guanghua Formation formed between 129 and 128 Ma. Rocks of the Baiyingaolao Formation exhibit the characteristics of high silica contents, alkali enrichment and aluminium depletion, thus resembling A-type granites. They show significant fractionation of light and heavy rare earth elements with a pronounced negative Eu anomaly. They zircon ¹⁷⁶Hf/¹⁷⁷Hf ratios range from 0.282932 to 0.252800, with an average ε _Hf(t) value of +1.28; (⁸⁷Sr/⁸⁶Sr) _i ranges from 0.68848 to 0.70068, with ε_Nd(t) ranging from +1.4 to +3.7; and t _DM2 ranges from 629 to 817 Ma, indicating magma derived from newly formed young crustal material after the closure of the Mongolian-Okhotsk Ocean. The volcanic rocks of the Guanghua Formation have characteristics that are similar to I-Type granites, with silica and alkali enrichment and Ca and Mg depletion. They also show a right-skewed “V”-shaped pattern in their REE distribution curves, with a strong negative Eu anomaly. The zircon ¹⁷⁶Hf/¹⁷⁷Hf ratios range from 0.282887 to 0.282986, with an average ε _Hf(t) value of +8.78; (⁸⁷Sr/⁸⁶Sr)_i ranges from 0.70060 to 0.70096, with ε _Nd(t) ranging from +3.0 to +3.8; and t _DM2 ranges from 616 to 677 Ma, indicating the partial melting of depleted mantle-derived mafic lower crustal material, influenced by the closure of the Mongolian-Okhotsk Ocean and subduction of the ancient Pacific Plate.