Petrochronology of metapelitic rocks: a review of key concepts

Metapelitic rocks are key petrochronological markers, not only for their varied and sensitive mineral paragenesis and presence of dat-able phases but also due to their broad and continuous distribution along terrains, thus supporting integrated and detailed studies of the metamorphism and events that took place in different regions. Therefore, the present work aimed to synthesize relevant aspects to the petrological and geochronological characterization of metapelites for medium pressure regimes. Considering the KFMASH (K₂O, FeO, MgO, Al₂O₃, SiO₂, H₂O) system, minerals such as chlorite, muscovite, chloritoid, biotite, staurolite, garnet, cordierite, andalusite, kyanite, sillimanite, and potassium feldspar are distinctive of these rocks paragenesis in low to medium pressure regimes, as long as there is chemical availability and sufficient P-T conditions for their formation. With the increase in P-T conditions and the appearance of the respective index-minerals, the indicative metamorphic zones are as follow: chlorite, biotite, garnet, staurolite, kyanite, silliman-ite, and sillimanite + orthoclase. For the petrogenetic and metamorphic conditions of characterization of such lithotypes, macro- and microstructural studies, associated with mineral chemistry and bulk-rock rock analysis, favor the application of different thermobaro-metric methods, from conventional ones through optimized and isochemical phase-diagrams, to single-element thermometers, each one with its particularities and application possibilities. To promote a petrochronological study, mineral phases such as zircon, garnet, monazite, and rutile, using their respective isotopic systems, allow assigning ages to these metamorphic events and, with the integration and interpretation of the data obtained, constraining the P-T-t-d trajectory of rocks formation. A systematic approach must be employed, in accordance with the particularities of the studied rock, hence guaranteeing the accuracy of the results obtained. (author)