Petrology and geochemistry of the early Proterozoic Burakovsky layered intrusion, southern Karelia, Russia

Author

Stefan John Higgins

Date of Award

5-1996

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Lawrence A. Taylor

Committee Members

Harry Y. McSween, Claudia I. Mora

Abstract

The early Proterozoic (2449 ± 1.1 Ma) 700 km² Burakovsky layered intrusion of Southern Karelia, Russia is the largest mafic pluton in the Baltic Shield. The intrusion has been divided into a layered series and a border group. The layered series is further subdivided into five zones: (1) an ultramafic (largely dunites); (2) a pyroxenite; (3) a gabbronorite; (4) a pigeonite-gabbronorite; and (5) a magnetite-gabbronorite. Mineral and whole-rock chemical variations in the ultramafic zone indicate little variation in chemistry over the entire sequence (e.g., in olivine, variations are from Fo₈₉ to Fo₈₂ over the upper 1700 meters). In contrast, the X_Mg values for orthopyroxenes from the gabbronorite zone vary from 0.85 to 0.58 over an interval of 500 meters. On the basis of the current data, there are no apparent reversals in chemistry in the ultramafic zone, while there is evidence for several in the gabbronorite zone. The monomineralic nature and absence of significant chemical variation in the ultramafic zone indicates a contrasting style of magmatism compared with the gabbronorite zone, which contains well-defined phase and cryptic layering. Moreover, the lack of evidence for abundant magma influxes in the ultramafic zone contrasts with the chemical reversals observed in the gabbronorite zone, suggesting that replenishment was a prevailing process in the upper half of the intrusion. Samples from the ultramafic zone are LREE-enriched indicating that they formed from a LREE-enriched parental magma. The LREE-enrichment in the parental magma suggests either that the source region was LREE-enriched or that the melt was contaminated by crust in route to the magma chamber. Samples from the gabbronorite zone are also LREE-enriched. Moreover, there are at least two parental magmas for the gabbronorite zone delineated by these REE characteristics. It is proposed here that the lower subzone of the gabbronorite zone be broken into two additional zones, denoted by Group I and Group II magmas. Group I magmas, from the lower part of the gabbronorite zone, have (Ce/Yb)_n ratios of 6.9 to 13.9 while Group II magmas, from the upper portion, have (Ce/Yb)_n ratios of 15.8 to 27.3. Volcanic rocks in Karelia that are coeval to the BLI and volcanic rocks of a similar age in other parts of the Baltic Shield are LREE-enriched. Furthermore, the BLI has an ε_ND value of -2.0, and other layered intrusions in the Baltic Shield of similar age have ε_ND values ranging from -2.3 to -2.0. All of these data are most consistent with a mantle that was LREE-enriched for a considerable length of time prior to the partial melting episode(s) which produced the magmatism in the Baltic Shield during the Archean-Proterozoic transition.

Recommended Citation

Higgins, Stefan John, "Petrology and geochemistry of the early Proterozoic Burakovsky layered intrusion, southern Karelia, Russia. " Master's Thesis, University of Tennessee, 1996.
https://trace.tennessee.edu/utk_gradthes/10845