Geochronological constraint on hydrothermal alteration process deduced from titanite U-Pb dating on Tono plutonic complex, Kitakami Mountains

Ogita, Yasuhiro  ; Niki, Sota*; Nagata, Mitsuhiro   ; Hirata, Takafumi*; Yuguchi, Takashi*

Alteration process in granitic rocks is important for mass transfer through rock-fluids interactions and the formation of microcracks as a pathway of ground water. Timescale of alteration in granitic rocks is enough unrevealed because limited methodologies are available. Focusing on biotite chroritization and following precipitation of titanite (sphene) during hydrothermal alteration in granitic rocks, hydrothermal titanite leads to geochronological constraint on timescale of alteration. This study reports U-Pb age of hydrothermal titanite from Tono Plutonic Complex (TPC), Kitakami Mountains, northeast Japan, and interprets its age comparing other geochronological data. Thinsection observation suggests that titanite from the TPC sample was secondary precipitated related to biotite chloritization during subsolidus stage. In thinsections, titanite is not enough to analyze using laser ablation-inductively coupled plasma-mass spectrometry. This study conducted U-Pb analysis for separated titanite and obtained age of 118.71.5 Ma. The occurrence of titanite shows that titanite was precipitated accompanied biotite chloritization during hydrothermal alteration in the TPC. Thus, the age of titanite is potentially able to constrain on hydrothermal activity during subsolidus stage of TPC. Moreover, the obtained age falls within zircon U-Pb age (1172 Ma) and biotite K-Ar age (122.62.7 Ma) reported from central facies of TPC. This indicates hydrothermal alteration process related to biotite chloritization has experienced short period during cooling on TPC.