--The retrogression of HP metamorphic parageneses
to greenschists-facies conditions occurs in relation to the exhumation of the ChMC.
Based upon the mineral parageneses
and chemical compositions of minerals (wet chemical analysis of garnet, biotite, and amphibole monomineral fractions) and the use of different geothermometers and geobarometers, the peak metamorphic conditions in main structural zones of the Estonian basement were estimated (Koppelmaa et al.
Liu, J.C., Zhang, R., Ernst, W.G., Liu, J., and McLimans, R.: 1998, Mineral parageneses
in the Piampaludo eclogitic body.
In this scheme the two most commonly occurring cleft parageneses
(with subtypes, ignored here) embrace most of the finest mineralized clefts of the central massifs; Parker (1973) calls these parageneses
Fundortgruppe 4 (Locality Group 4, dominant in the western and central parts of the region) and Fundortgruppe 3 (Locality Group 3, dominant in the east).
In some parageneses
it appears to have formed in place of fluorapatite.
Hematite is found, both as iron roses and other habits, in approximately half of the 30 distinct parageneses
in Switzerland's Alpine clefts that have been classified in detail by Parker (1973), but the finest and most abundant iron roses are found in only one of these parageneses
(to be described later).
By studying the compositions, crystal morphologies and parageneses
of the minerals in these deposits, and by making comparisons with similar deposits in the upper Mississippi Valley region whose origins are relatively well established, it is possible to reach some general conclusions about the environments of formation of the minerals.
The basal gneiss unit of the massif has an apparent monoclinal structure with a slight northerly dip, and can be subdivided into three types defined by their structural features and their parageneses
. From base to top, these are (1) migmatitic gneiss, (2) augen gneiss characterized by a fairly constant blastomylonitic texture probably related to shear movements, and (3) intermediate gneiss that has preserved the augen structure but has apparently undergone slight migmatization.