Triassic Volcanism In Eastern Heilongjiang And Jilin Provinces, Ne China: Chronology, Geochemistry, And Tectonic Implications

摘要

With the aim of constraining the Early Mesozoic tectonic evolution of the eastern section of the Central Asian Orogenic Belt (CAOB), we undertook zircon U-Pb dating and geochemical analyses (major and trace elements, Sr-Nd isotopes) of volcanic rocks of the Luoquanzhan Formation and Daxinggou Group in eastern Heilongjiang and Jilin provinces, China. The analyzed rocks consist mainly of dacite and rhyolite, with SiO_2 contents of 68.52-76.65 wt%. Three samples from the Luoquanzhan Formation and one from the Daxinggou Group were analyzed using laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb zircon techniques. Three zircons with well-defined oscillatory zoning yielded weighted mean ~(206)Pb/~(238)U ages of 217 ± 1, 214 ± 2, and 208 ± 1 Ma, and one zircon with oscillatory zoning yielded a weighted mean ~(206)Pb/~(238)U age of 201 ± 1 Ma. These ages are interpreted to represent the timing of eruption of the volcanic rocks. The Triassic volcanic rocks are characterized by high SiO_2 and low MgO concentrations, enrichment in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), depletion in high field strength elements (HFSEs) and heavy rare earth elements (HREEs), (~(87)Sr/~(86)Sr)_i = 0.7040-0.7050 (Luoquanzhan Formation) and 0.7163-0.7381 (Daxinggou Group), and ε_(Nd) (t)=1.89-3.94 (Luoquanzhan Formation) and 3.42-3.68 (Daxinggou Group). These geochemical features indicate an origin involving the partial melting of juvenile lower crust (Nd model ages (T_(DM2)) of 651-821 Ma) and that compositional variation among the volcanic rocks arose from mineral fractionation and minor assimilation. These volcanic rocks formed within an extensional environment following collision of the NCC and Jiamusi-Khanka Massif during the Late Paleozoic-Early Triassic.