Reconstructed Characteristics of the Initial Melt of the Kaalamo Multiphase Clinopyroxenite–Gabbronorite–Diorite Intrusion, Northern Ladoga Area, Southern Karelia

The Kaalamo early orogenic Paleoproterozoic massif is located in the southeastern part of the Raahe–Ladoga zone, a junction zone between the Archean Karelian craton and the Proterozoic Svecofennian orogen. The massif consists of three intrusive phases: the first phase is peridotites, olivine clinopyroxenites, and gabbro; the second one is gabbro-norites and gabbro-diorites; and the third phase is diorites, tonalites, and plagiogranites. The paper presents newly obtained data on the petrochemistry and geochemistry of metaperidotites, metapyroxenites, and gabbroids from the Kaalamo complex and on the compositions of their rock-forming minerals. These data were used in the COMAGMAT-3.75 program for thermodynamic calculations of the equilibrium crystallization trajectories of representative rocks and average compositions of the first and second intrusive phases. The calculation results were processed using the geochemical thermometry method, which made it possible to estimate the temperature (∼1220°C) and the probable composition of the initial high-Mg melt (∼9.5 wt % MgO, olivine with 84 mol % forsterite). Comparison of this primitive melt with the model compositions of derivatives and petro- and geochemical characteristics confirms that the rocks of the first and second intrusive phases were derived from a single source, which is consistent with the REE patterns of these rocks and data on their Sm–Nd isotope system and indicate that the rocks have a common mantle source. It was found out that the calculated crystallization sequences of the rocks are in good agreement with the abundance of peridotites and olivine pyroxenites, indicating that the primitive melt was in equilibrium with clinopyroxene and orthopyroxene, which are in peritectic relationships with the olivine. The most differentiated rocks of the first phase are characterized by the crystallization of cotectic plagioclase containing about 80 mol % anorthite. The rocks of the second phase are more differentiated material, corresponding to the gabbronorite association of cumulus phases without olivine and with the late crystallization of titanomagnetite. Comparison of the model and real mineral compositions shows a systematic shift of the natural compositions of the olivine and pyroxenes toward the iron richer from early to late phases, which is a consequence of the interaction of the early cumulus minerals with residual intercumulus melt. The rocks are similar to intrusions of the nickel-bearing belt of Finland, which opens prospects for correlations of magmatic events on an interregional scale.