Previously, the West Estonian zone, with geophysical fields characteristic of the BBGH but with prevailing amphibolite facies of rock metamorphism, was somehow separated from the SSW terrain (Puura et al.
The main tectonic boundary in the Estonian basement, PPZ, has a transitional position between the Belarus-Baltic assemblage of prevailing granulitic terrains in the SSW and amphibolite facies zones in the NNE.
However, in many places retrograde assemblages have formed in the conditions of the amphibolite facies
This lower amphibolite facies
metamorphism overprints, in many samples, a pre-existing epidote-hornfels or greenschist assemblage containing epidote + variably sericitised plagioclase + actinolite [+ or -] biotite.
Drill core F-265 consists of biotite-plagioclase and pyroxene amphibolites of amphibolite facies
, likely of igneous origin.
The formation is metamorphosed to lower amphibolite facies
and is deformed.
The first one corresponds to amphibolite facies
(Figure 3e), defined by hornblende + plagioclase [+ or -] garnet [+ or -] quartz.
The dominant lithology encountered during the drilling was a coarse-grained metamorphic amphibolite facies
The other missing sample (12) cannot be plotted on these diagrams because it is at amphibolite facies
as a result of contact metamorphism by a Devonian intrusion, as discussed below.
The supracrustal rocks are generally metamorphosed to amphibolite facies
, less commonly to granulite facies, and rarely to greenschist facies.
On the basis of geophysical and petrological studies, the Estonian basement can be divided into two major geological units--North Estonian amphibolite facies
and South Estonian granulite facies complexes, which are separated from each other by a tectonical boundary (Paldiski-Pskov shear zone) and include six petrological-structural zones: the Tallinn, Alutaguse, Johvi, West Estonian, Tapa, and South Estonian zones (Fig.
From our studies, we determined that the most common assemblage at Dark Ridge is olivine + talc + tremolite + chlorite + chromite, which indicates metamorphism in the amphibolite facies
It contains terrigenous sedimentary rocks, mafic to ultramafic volcanic rocks and plutonic rocks metamorphosed from the greenschist facies to the lower amphibolite facies
The Project is underlain by rocks of the Yukon-Tanana Terrane consisting of Paleozoic metavolcanic and metasedimentary rocks of the Cleary Sequence and Fairbanks Schist, which are in thrust contact and interleaved with eclogite and amphibolite facies
rocks of the Chatanika Terrane.
Romer & Wright (1992) investigated one columbite sample in the contact aureole of a Paleoproterozoic granite that was subjected to a Caledonian upper greenschist to lower amphibolite facies
regional metamorphic overprint and this columbite displayed only partial resetting during Caledonian metamorphism.