P-T conditions, fluid evolution, and time of formation of staurolite-containing parageneses in the rocks of the metamorphic complex of the Northern Ladoga region
DOI:
https://doi.org/10.17308/geology/1609-0691/2024/1/74-88Keywords:
staurolite, paragenesis, fluid regime, metamorphismAbstract
Introduction: This paper presents the results of a study of the staurolite-bearing rocks of the Northern Ladoga metamorphic complex, including P-T conditions and fluid regime features of their formation, supplemented by data on the age of metamorphism of these rocks using the U-Pb isotope dating method of monazite.
Methods: The work used multi-equilibrium mineral geothermobarometry of rocks (winTWQ), Raman spectrometry of fluid inclusions from synmetamorphic quartz veins, U-Pb (ID-TIMS) dating of monazite from staurolite-bearing rocks.
Results and discussion: Within the staurolite zone of the Northern Ladoga region, three blocks (subzones) are distinguished, which differ in the P-T conditions of rock formation: Western block: T = 585–615°C, P = 3.7–5.2 kbar, Central: T = 645–650°C, P = 5.3–6.0 kbar and Eastern: T = 510–592°C, P = 3.7–6.5 kbar. The composition of the gas part of fluid inclusions in synmetamorphic quartz veins of the staurolite zone is determined mainly by CO2 and CH4. In the Eastern block the composition often contains the gas phase N2 with impurities CH4 and H2. The density of CO2 fluid inclusions (0.58–0.82 g/cm3) and estimates of metamorphic pressure from them (1–3 kbar) are lower than those obtained from thermobarometric data of rocks (4–6 kbar). The U-Pb age of monazite from staurolite-bearing schists is 1787 ± 5 Ma.
Conclusion: The revealed P-T parameters of rock metamorphism in different parts of the staurolite zone of the Northern Ladoga region and the fluid composition features in them reflect lateral heterogeneity: higher P-T parameters were recorded for the rocks of the Central block, which may indicate a greater erosion of this block with the opening of underlying more high-temperature and high-pressure rocks. The composition of fluid inclusions in quartz veins reflects the specific fluid regime at the late regressive stages of rock metamorphism. The time of staurolite parageneses formation (~ 1.79 Ga) corresponds to the stage of completion of orogenic events in the region.
