Studies Of Geochronology And Zircon Hf-O On The Volcanic Rocks From The Luzong Basin

Luzong Basin which located in the Middle-Lower Yangtze River Belt developed a series of unique olive latite shoshonite volcanic rocks. In this paper, the main and trace elements of four formations of volcanic rocks in Longmenyuan formation, Zhuanqiao, Shuangmiao formation and Fushan formation have been analyzed. The results shows that those four formations of volcanic rocks have relatively moderate content of SiO2, enrich K2O and high K2O/Na2O ratio, and with the features of high potassiμm-rich base. The distribution features and REE distribution patterns of the trace elements are similar to arc magmatic rocks with the characteristics of LILE, Pb and LREE enrichment, flat heavy rare earth elements depleted in high field strength elements (HFSE) and high Th, U and Pb.According to zircon LA-ICP MS U-Pb dating on the four formations of volcanic rocks, ages of volcanic rocks samples in Longmen formation are 134.5 ± 1.2Ma and 134.4 ± 1.3Ma respectively, ages of volcanic rocks samples in Zhuanqiao formation are 133.5 ± 1.4Ma and 132.9 ± 1.5Ma, ages of volcanic rocks samples in Shuangmiao formation are130.5±1.1Ma and131.1±1.2Ma, and ages of volcanic rocks samples in Fushan formation 127.3 ±1.3 Ma and 128.2 ± 1.4Ma respectively. Based on above-mentioned analysis, representative samples of zircon are tested under the εHf(t) and O isotope situ experiments. Volcanic zircon f (t) in Longmen formation is -9.70 ～-7.33, the weighted average of -8.44 ± 0.56,δ18O= 6.07‰～7.98‰ (average of 6.67‰), εHf(t) in Zhuanqiao formation is ～7.19 to-9.59, the weighted average of-8.04 ± 0.34,δ18O= 6.11‰～7.06‰ (average of 6.60‰), volcanic zircon εHf(t) in Shuangmiao formation is -7.90 -10.36 -weighted average was -8.48 ± 0.45,δ18O= 6.36～7.22‰(average of 6.729‰), While the volcanic zircon εHf(t) in Fushan formation is significantly different from the above-mentioned three formations, which is -2.29 to 0.59, a weighted average of -0.78 ± 0.59,δ18O= 7.05～7.86‰(average of 7.55‰).Comprehensive analysis showed that the Luzong volcanic basin rocks are the second phase products of Yangtze River magmatism. The beginning and ending ages of volcanic magma activities are 135～127Ma. The volcanic rocks of Fushan formation are originated from the enriched mantle which deposited by the subducted slab fluid/melt metasomatism. The volcanic rocks in Longmenyuan formation, Zhuanqiao formation and Shuangmiao formation are derived from this account lithospheric mantle partial melting, but they are contaminated with lower Archean crust during magma rising process.