Chronology of Late Paleozoic and Mesozoic events in the Udokan Ridge: Ar-40/Ar-39 dating of primary and secondary minerals from intrusive rocks

Stepwise-heating Ar-40/Ar-39 dating experiments were performed on quartz di orite mineral separates from an intrusive massif of uncertain age. The mass if is located in the Udokan Ridge (Northern Transbaikalia) between the Earl y Proterozoic metamorphosed sediments of the Kodar-Udokan foredeep and Late Paleozoic granites of the Ingamakit complex. Interpretation of Ar-Ar spect ra has shown that quartz diorite underwent two stages of thermal impact dur ing the emplacement of the Ingamakit pluton. The onset of plutonism may dat e back to about 297.3+/-2.9 Ma BP, as attested by an apparent step age that accounts for 40% of the released argon obtained on separation of transpare nt plagioclase from quartz diorite. The emplacement of granitic intrusions led to partial recrystallization of quartz diorite and formation of biotite and amphibole. The biotite yields a plateau age of 281.6+/-1.1 Ma. The typ e of argon release from biotite on stepwise heating, as well as calculated diffusion parameters, suggest abrupt cooling of the sample after biotite an d amphibole had crystallized to below 130-140 degrees C. The quartz diorite sample must have existed under temperatures above 50 degrees C for over 80 Ma, as indicated by its diffusion parameters and by the age 197.6+/-0.8 Ma of the subplateau obtained on a milky-white (altered) plagioclase separate . The sample was rapidly drawn up to the subsurface zone during the Mesozoi c tectonic activity.