Multi-Stage Formation of Apatite in the Early Proterozoic Metagabbroids of the Velimyaki Massif (the Northern Ladoga Region)

The Early Proterozoic Velimyaki gabbroid massif of the Northern Ladoga region was formed in two intrusive phases. The rocks of the first and second intrusive phases contain apatite, which forms ore clusters together with magnetite and ilmenite in pyroxenites and monzogabbros of the first phase or in monzodiorites and monzonites of the second phase. Modeling of magmatic mineral formation suggests that the rocks of phases I and II originated from a single parent melt through fractional crystallization. The results of metamorphic modeling are consistent with the presence of two stages of metamorphic alteration of the rocks: an earlier low-temperature stage (replacement of clinopyroxene by actinolite) and a later medium-temperature stage (formation of green hornblende rims). Further study of apatite in the rocks of the second phase (monzodiorites and monzonites) shows that the U–Pb apatite system demonstrates stages of postmagmatic mineral formation during 1.80–1.81 Ga regional metamorphism. The ages and genesis of the apatite are consistent with its morphology and paragenetic relationships with titanite, actinolite, green hornblende, acid plagioclase and quartz, which formed during regional metamorphism. Sm–Nd isotope analysis of hornblende and titanite from the host amphibolites also indicates the manifestation of regional metamorphism about 1.84–1.80 Ga ago.