Av. Ivanov et al., Chronology of Late Paleozoic and Mesozoic events in the Udokan Ridge: Ar-40/Ar-39 dating of primary and secondary minerals from intrusive rocks, GEOL GEOFIZ, 41(5), 2000, pp. 686-695
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.