U-Pb zircon, geochemical, and Sr-Nd-Hf isotopic data for late Mesozoic volcanic rocks along the Tan-Lu fault zone of Shandong Province, eastern China: constraints on magma genesis and lithospheric thinning

摘要

A combined study of whole-rock major and trace elements, Sr-Nd-Hf isotopes, and zircon U-Pb geochronology was conducted on late Mesozoic volcanic rocks along the Tan-Lu fault zone (the Anqiu, Gaoqiao, Juxian, and Junan regions) of Shandong Province, eastern China. The results constrain the petrogenesis of the volcanic rocks and provide important insights into the significance of the Tan-Lu fault zone for lithospheric thinning. The volcanic rocks are composed of basaltic trachyandesite, trachyandesite, trachydacite, andesite, dacite, and rhyolite, which yielded zircon U-Pb ages of 125.4 +/- 0.9, 126.2 +/- 1, 125.7 +/- 1, 127.1 +/- 1.2, 120 +/- 0.65, and 122.8 +/- 1 Ma, respectively, indicating eruption in the Early Cretaceous. A large amount of Palaeoproterozoic and Archaean inherited zircons exist in Anqiu and Gaoqiao volcanic rocks, but Neoproterozoic inherited zircons in Juxian and Junan volcanic rocks, in agreement with the age spectrum of the basement rocks of the North China Craton (NCC) and Neoproterozoic protolith ages of ultrahigh-pressure metaigneous rocks in Sulu orogenic belt, respectively. The volcanic rocks have high Na2O + K2O concentrations (5.94-11.33 wt.%), indicating that they are alkaline. All the rocks are characterized by enriched in light rare earth elements and large ion lithophile elements (e.g. Rb, Th, U, and K) but are depleted in high field strength elements (e.g. Nb, Ta, and Ti). Anqiu and Gaoqiao volcanic rocks have wide initial Sr-87/Sr-86 ratios (0.707678-0.711259), variable negative epsilon(Nd)(t) values (-15.2 to -11) and epsilon(Hf)(t) values (-19.5 to -2.2). The varied isotopic compositions and the abundance inherited zircons, together with mineral disequilibrium features, suggest that the Anqiu and Gaoqiao volcanic rocks were produced by the mixing of magmas from enriched lithospheric mantle and lower crust. The enriched lithospheric mantle originated from the ancient lithospheric mantle beneath the NCC, which was modified by the subduction of Yangtze crustal material. Juxian and Junan voalcanic rocks have initial Sr-87/Sr-86 ratios of 0.707502-0.709238, low epsilon(Nd)(t) values of -20.5 to -16.1, and negative zircon epsilon(Hf)(t) values of -24.6 to -16.4, similar to granitoids in Sulu orogenic belt, suggesting that they derived from similar crustal sources. Combining the existence of the Neoproterozoic inherited zircons and the isotope characteristics, we propose that the Juxian and Junan volcanic rocks were dominantly derived from partial melting of the Subducted Yangtze crust but with contributions from the mantle. The volcanic rocks along the Tan-Lu fault zone were produced under an extensional regime linked with lithospheric thinning in the eastern NCC during the Early Cretaceous. According to the characteristic spatial and temporal distributions of late Mesozoic volcanic activity along the Tan-Lu fault zone, combined with the distributions of regional adakitic rocks and the evolutionary history of the fault, we suggested that the Tan-Lu Fault played an important role in the initiation of lithospheric thinning and provide a favourable channel and beneficial conditions for magma activity.