Step-heating 40Ar/39Ar dating experiments were performed on quartz diorite mineral separates from an intrusion of uncertain age located in the Udokan Ridge (Northern Transbaikalia) between the Early Proterozoic metamorphosed sediments of the Kodar- Udokan foredeep and Late Paleozoic granites of the Ingamakit complex. Interpretation of Ar-Ar age spectra showed that quartz diorite underwent two stages of thermal impact during the emplacement of the Ingamakit pluton. The onset of plutonism may date back to about 2973 ±2.9 Ma, as attested by an apparent step age that accounts for 40% of argon released from a transparent plagioclase separate. The emplacement of granitic intrusions led to partial recrystallization of quartz diorite and formation of biotite and amphibole. The biotite plateau age is 281.6±1.1 Ma. The step-heating Ar release pattern for biotite and calculated Ar diffusivity indicate abrupt cooling of the sample after biotite and amphibole had crystallized to below 130-140 °C. The quartz diorite sample must have existed under temperatures above 50 °C for over 80 Ma, as indicated by its diffusivity and by the 197.6±0.8 Ma subplateau age of milky-white (altered) plagioclase. The sample was rapidly drawn up to the subsurface conditions during the Mesozoic tectonic activity.

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