Mineral chemistry constraints on the evolution of the 1.88-1.87 Ga post-kinematic granite plutons in the Central Finland Granitoid Complex

A suite of post-kinematic, 1.88-1.87 Ga, silicic plutons crosscut 1.89-1.88 Ga synkinematic granitoids in the Central Finland Granitoid Complex (CFGC) in south-central Finland. The plutons range from biotite +/- hornblende qu artz monzonite to syenogranite and include pyroxene- and olivine-bearing va rieties. Mineral chemical data on feldspars, biotite, amphibole, pyroxenes, olivine, and oxides of the post-kinematic plutons are presented. The data are interpreted to show that these plutons register (1) a considerable rang e in pressure from 2-4 kbar (amphibole barometry) to 5-7 kbar (olivine-pyro xene barometry), (2) temperatures mostly reflecting resetting during coolin g (450-800 degrees C; QUILF thermometry), and (3) low fO(2) (log fO(2) Delt a FMQ - 0.3 to - 1.5; QUIlF equilibria). In particular, plutons with olivin e- and pyroxene-bearing margins and amphibole-dominated central parts recor d progressive oxidation and hydration upon cooling, shifting from the QUIlF equilibrium toward KUIlB. The post-kinematic granites can be considered po st-collisional in regard to compressional events in the CFGC and display ma ny of the characteristics of the anorogenic 1.6 Ga rapakivi granites furthe r south. They were presumably derived from a deep and dry crustal source, l ike the rapakivi granites. (C) 1998 Elsevier Science B.V. All rights reserv ed.