Petrology of the Early Proterozoic Burakovsky layered intrusion, southern Karelia, Russia: mineral and whole-rock major-element chemistry

Abstract

The Early Proterozoic (2449 ± 1.1 Ma) Burakovsky layered intrusion of southern Karelia, Russia, is the largest (700 km2) mafic pluton in the Fennoscandian Shield. The layered series contains two main parts: (i) an ultramafic series (85% dunite), and (ii) a mafic series (mostly gabbros). 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 Fo87 over the upper 1600 m). In contrast, the Mg# in pyroxenes from the gabbronorite zone varies from 85 to 58 over an interval of 500 m. On the basis of the current data, there are no apparent reversals in chemistry in the ultramafic zone, whereas there is evidence for several in the gabbronorite zone. Furthermore, there is a distinctive reversal to more primitive compositions in mineral and whole-rock chemistry at the top of the ultramafic zone. These differences indicate that either (i) the gabbronorite zone crystallized from the residue left from formation of the ultramafic zone and the subsequent "flushing" of the more evolved portions to form the Early Proterozoic Karelian volcanic fields, or (ii) there were two separate magmas. Plagioclase crystals from the gabbronorite zone are chemically zoned, providing evidence of the local processes affecting crystallization of this sequence. Plagioclase crystals from near the base commonly display relatively Na-rich cores that are believed to result from either supersaturation at the onset of nucleation or the prior crystallization of some other, Ca-rich phase (e.g., clinopyroxene). Many plagioclase grains also contain reversely zoned rims, indicative of either (i) exchange between trapped liquid and the main magma during times of replenishment; or (ii) an increase in the CIPW-normative plagioclase–liquid partition coefficient for Ca, due to an increase in the augite component of the melt. 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 only in the upper half of the intrusion.