Translithospheric magmatic plumbing system of a late Early Cretaceous intraplate volcano in NE China: Insights from geochemistry and phenocryst composition

The magmatic plumbing systems of intraplate volcanoes are key to understanding the structure of intraplate continental crust. A late Early Cretaceous volcano in NE China provides an opportunity to reconstruct the magmatic plumbing system of an intraplate paleo-volcano. Ar-Ar and zircon SIMS U-Pb dating results indicate that olivine basalts formed at 102.81 +/- 0.17 Ma and hornblende andesitic rocks formed at 101.1 +/- 1.1 Ma-102.84 +/- 1.2 Ma. Compared with coeval olivine basalts, the hornblende andesitic rocks have lower rare earth element (REE) abundance and parallel REE patterns, indicating that the basalts and andesitic rocks have independent origins. Olivine phenocrysts only occur in the basalts and are often zoned, with cores of Fo(78-86) and rims of Fo(62-68), the crystalline temperatures of olivine cores are 1204-1234 degree celsius. The plagioclases from the basalt groundmass have An = 62-63. There are two types of plagioclase phenocryst in the andesitic rocks. One has An = 62-50, the other has An = 27-34.The former is normally zoned crystals (An = 62-50 from the core to rim) with high-An rims (60-63), similar in composition to plagioclase in the groundmass of the coeval basalt. They formed at temperatures of 1140-1162 degree celsius and pressures of 4.7-7.0 kbar, implying that they were crystallized from basaltic melt with H2O contents of 0.60-0.82 wt%. The latter is low-An phenocrysts (27-34) with high-An (similar to 44) rims, suggesting that they were crystallized from felsic melt and that the injection of mafic magma had happened. The orthopyroxene phenocrysts only occur in andesitic rocks and have Mg# of 80-82, and are estimated to have crystallized at 6.7-8.5 kbar, and 1206-1222 degree celsius, implying that they were crystallized from basaltic melt. The hornblende phenocrysts in andesitic rocks are pargasite, tschermakite, magnesio-hornblende, and magnesio-hastingsite and formed at 2.4-3.5 kbar and 883-957 degree celsius. The estimated pressure and temperature of orthopyroxene and plagioclase (An>55) phenocrysts indicate that mafic magma chambers was located at depths of over 31.0 km, whereas those of hornblende phenocrysts show that felsic magma chamber was located at 8.9-12.8 km and has H2O content of 5.8-7.3 wt%. Using these data, we construct a new model of translithospheric magmatic plumbing system for intraplate volcanoes. Mantle-derived mafic magma chamber was emplaced at the crust-mantle boundary and causes partial melting of the lower crust, which in turn produces felsic magma that would be stored in a magma chamber in the upper crust. The mafic magma chamber at the crust-mantle boundary does not crystallize in-situ, as mafic magma could occasionally rise and be repeatedly injected and mix into the felsic magma chamber in the upper crust.