Hydrothermal circulation during exhumation constrained by fluid inclusion analyses, zircon fission-track and monazite Th-Pb dating (Belledonne massif, external Alps).

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n the Lauziere granite (external crystalline massif of Belledonne, Alps), a multidisciplinary approach was usedto constrain the timing of hydrothermal activity and its influence on the geothermal gradient of the neighbouringrocks. The samples were collected within a metric cleft (a fissure partly filled with hydrothermally grown minerals)and within the granite host-rock, taken at different distances from the cleft. The cleft is mainly filled by quartz,albite, adularia, chlorite. It also contains numerous species of accessory minerals such as anatase, rutile, ilmenite,hematite, apatite, monazite or xenotime that can reach several millimeters in size. Clef monazite age was deter-mined by in-situ isotopic Th-Pb dating on different compositional domains of four grains. Within a single grain,monazite ages, obtained in domains with distinct compositions, overlap. This indicates that hydrothermal monazitegrain precipitates over a relatively short geological time period around a mean age of 12.4±0.1 Ma (MSWD =1.7; N= 86). In order to constrain the conditions of monazite precipitation, fluid inclusions analyses have beenattempted in quartz and monazite. Microthermometric data for fluid inclusions indicate that monazite precipitatedin the earliest stage of the vein formation from a hydrothermal fluid with a temperature probably >400◦C (basedon the isochor calculation at minimum pressure of 1.0 kbar). In the host-rock, exhumation age and the impact offluid circulation were investigated from zircon fission track (ZFT) dating in the host-rock. At the cleft contact,resetting of the ZFT ages indicates that hydrothermal fluid locally reheated the granite host-rock, which was al-ready exhumed and cooled down at temperature «