The geochronologic investigations in Yakutia are part of a multidisciplinary collaborative investigation which includes paleomagnetism, seismology and structural geology which are used to gain an understanding of the tectonic history of the Sakha Republic (Yakutia). These other aspects of the project are discussed under "Paleomagnetism" which outlines the history and directions of our collaborative research.
The geochronologic investigations to date have focused on determining the timing of major events that affected the Chersky Range in eastern Yakutia. We have collected over 300 samples for geochronologic and geochemical analysis. Two areas of focus in 1993 and 1994 were (i) the description and chronology of ophiolites in the region (ii) the timing of granitic intrusion in the region.
Ophiolites in the Chersky Range
Mafic and ultramafic rocks are found in isolated complexes along the Chersky Range, eastern Sakha Republic (Yakutia), Russia. Five of the six exposures are located on the eastern side of a Devonian-Ordovician carbonate platform and appear to be thrust over it; the sixth, the Debin fragment, is located to the west. On the basis of lithology, geochemical composition and structural position, the ultramafic rocks and their associated mafic volcanics and deep-marine sedimentary rocks are interpreted as ophiolites or ophiolite fragments derived from back-arc or oceanic crust that was located to the east of the Chersky Range. Faunal and radiometric dating indicate that the oceanic crust has an age of about 370-430 Ma, Early to Middle Devonian. The ages of metamorphic minerals suggest that the ophiolites were obducted and metamorphosed during the amalgamation of the Kolyma-Omolon superterrane, about 170-174 Ma (Callovian, late Middle Jurassic), at which time olistostromes containing fragments of the ophiolite were formed. This age predates the formation of the Uyandina-Yasachnaya volcanic arc, believed to represent the final closure of the ocean basin between the Kolyma-Omolon superterrane and the North Asian craton. It is possible that the southernmost ophiolite, the Debin fragment, has a different history from the other Chersky Range ophiolites and was emplaced as a result of the accretion of the Kolyma-Omolon superterrane.
Granitic Rocks in the Chersky Range
Our geochronologic investigations of plutonic rocks from the 'Kolymian Structural Loop' (KSL) are intended to investigate whether KSL formed in response to a single tectonic event or to discrete events. Precise ages were obtained from 40Ar/39Ar step-heating of biotite and, in some cases, hornblende, muscovite or feldspar. Plutons from the main northwest-southeast trending axis of the Chersky range show no significant age variations along strike, however, there is a decrease in age across the belt from 142 Ma in the east to 135 Ma in the west. These ages are significantly older than ages from granites of the east-west trending Polousney range north of the Chersky range. These plutons also show a decrease in age across the belt from 127 Ma in the south to 120 Ma along the northern edge of the belt. Thus, our preliminary data suggest discrete tectonic events are responsible for the formation of the Chersky (western KSL) and Polousney (northern KSL) ranges. Linear granitic belts which radiate away from the main KSL tend to be significantly younger than the KSL granites. A 106 Ma north trending transverse belt extending from the Polousney range north to Bolshoi Lyakhov island shows no age progression. A southwest trending belt into the Verkhoyansk foldbelt shows a much clearer age progression. The ages range from 132 Ma near the Adycha-Taryn fault and decrease westward to 124 Ma. These results indicate that various parts of the KSL and other structures in eastern Yakutia formed over a protracted time in the early Cretaceous.
In addition to these Cretaceous intrusions, we have confirmed the presence of at least one Carboniferous intrusive complex in the region, the Tommot Massif which is composed of alkaline-ultrabasic rocks, alkaline and subalkaline gabbroids, and alkaline and quartz syenites. It is located to the east of the Chersky Range. It appears that this complex region has escaped regional Cretaceous thermal overprinting events which seem to have reset the paleomagnetic signature and radiometric ages in volcanics and other rocks.
Funded by NSF Grant OPP92-24-29.
Scientific personnel:
Paul Layer,
David Stone
and Daniel Hackett;
Kazuya Fujita
(Michigan State University);
Leonid Parfenov and Vladimir Oxman (Yakutsk)