TY - JOUR

T1 - U-Pb dating of zircon from the Svecofennian plagiogranite intrusion, Baltic Shield

T2 - Specific features of the Upper and Lower intercepts

AU - Baltybaev, Sh K.

AU - Levchenkov, O. A.

AU - Glebovitsky, V. A.

AU - Levskii, L. K.

AU - Matukov, D. I.

AU - Berezhnaya, N. G.

PY - 2005/6

Y1 - 2005/6

N2 - Svecofennian orogeny within the Baltic Shield occurred 2.0-1.7 Ga B.P. The beginning of this stage was marked by the initiation of oceanic and marine basins with large-scale accumulations of volcanosedimentary rocks. After a short time, sedimentation was replaced by plutonic and metamorphic activities. Processes of metamorphism were characterized by intense heat flows and the development of mineral assemblages of low- and moderate-pressure (up to the granulite facies). This is exemplified by the Ladoga andalusite-sillimanite type metamorphic complex. Similar Svecofennian complexes occur in the adjacent regions of Finland and are traced in Sweden. Further westward of Sweden, Svecofennides are overlain by Caledonian allochthon and are exposed only in tectonic windows in Norway. The intense endogenous processes mentioned above were responsib le for the formation of the Svecofennian zircon population and the recrystallization of the earlier (Archean) zircon. Most Svecofennian zircons with an upper intercept at ∼1.9-1.7 Ga have a near-zero lower intercept in the 235U/207Pb-238U/206Pb diagram. For such cases, the upper intercept is interpreted as the geological event (formation of magmatic or metamorphic zircon), while the lower intercept is interpreted as the loss under supergene conditions. The nonzero lower intercept for Svecofennian zircons is typically considered to have no geological significance. The present communication is devoted to the discussion of two cases of lower intercept for Svecofennian zircon - these cases are different and require individual consideration.

AB - Svecofennian orogeny within the Baltic Shield occurred 2.0-1.7 Ga B.P. The beginning of this stage was marked by the initiation of oceanic and marine basins with large-scale accumulations of volcanosedimentary rocks. After a short time, sedimentation was replaced by plutonic and metamorphic activities. Processes of metamorphism were characterized by intense heat flows and the development of mineral assemblages of low- and moderate-pressure (up to the granulite facies). This is exemplified by the Ladoga andalusite-sillimanite type metamorphic complex. Similar Svecofennian complexes occur in the adjacent regions of Finland and are traced in Sweden. Further westward of Sweden, Svecofennides are overlain by Caledonian allochthon and are exposed only in tectonic windows in Norway. The intense endogenous processes mentioned above were responsib le for the formation of the Svecofennian zircon population and the recrystallization of the earlier (Archean) zircon. Most Svecofennian zircons with an upper intercept at ∼1.9-1.7 Ga have a near-zero lower intercept in the 235U/207Pb-238U/206Pb diagram. For such cases, the upper intercept is interpreted as the geological event (formation of magmatic or metamorphic zircon), while the lower intercept is interpreted as the loss under supergene conditions. The nonzero lower intercept for Svecofennian zircons is typically considered to have no geological significance. The present communication is devoted to the discussion of two cases of lower intercept for Svecofennian zircon - these cases are different and require individual consideration.

UR - http://www.scopus.com/inward/record.url?scp=23944432152&partnerID=8YFLogxK

M3 - Article

AN - SCOPUS:23944432152

VL - 403

SP - 750

EP - 753

JO - Doklady Earth Sciences

JF - Doklady Earth Sciences

SN - 1028-334X

IS - 5

ER -