MODEL P-T CONDITIONS OF HIGH-MAGNESIA MAGMA GENERATION IN THE PRECAMBRIAN OF THE FENNOSCANDIAN SHIELD

Abstract

Highly magnesian volcanics (komatiites, picrites, and Mg-rich basalts) are most abundant in Precambrian complexes. Their parental magmas were generated through high to maximum degrees of melting of the mantle. Hence, it is interesting to analyze the conditions of magma generation in the upper mantle using data on high-magnesia volcanics. Our calculations were based on the data obtained during experimental data on the P –T conditions of magma generation as a function of its composition. The research subjects were highmagnesia volcanics from the Archean greenstone belts and Proterozoic structures of Karelia, Finland, and Kola Peninsula dated by Sm–Nd and U–Pb methods. Data on basalts from the Early Proterozoic ophiolitic complexes of the Kola Peninsula and central Finland were used for comparison. The calculations were made with 165 analyses of chill zones of lava flows and subvolcanic intrusions in 21 Early and Late Archean and Early Proterozoic structures. Their results show that the Archean stage of magma generation (3.4–2.5 Ga) occurred under the highest temperatures (1700–1850°C), and temperature decreased considerably (down to 1300°C) during the Early Proterozoic stage (2.5–1.9 Ga). The difference between the generation and eruption temperatures was higher for komatiitic than for picritic magmas, which explains their different subsequent differentiation. According to our calculations, the pressure of mantle melting decreased from 7–6 GPa at the Archean stage to 1–2 GPa at the Early Proterozoic stage. The generation of Archean komatiitic magmas occurred at depths of 220–180 km, and Early Proterozoic komatiitic magmas were derived from depths of 180–160 km. Basaltic magmas were generated at depths of 30–50 km, which is consistent with experimental data. Compared with the model presented by Richter [1], our temperature values for the Archean and Early Proterozoic upper-mantle magma generation are higher by 200–300 and 100 °C, respectively.