The mafic metavolcanic rocks (blueschists and greenschists) of
the
Rýchory Mountains crystalline
complex (West Sudetes) experienced sea-floor hydrothermal alteration
(spilitization?) prior to
regional metamorphism. The metabasite geochemistry (namely trace element
and
REE abundances) indicates
that the protolith was comparable in composition with (1) tholeiitic to
transitional ocean-floor basalts
and (2) transitional and alkaline intra-oceanic island basalts. Two main
metamorphic events affected the
Rýchory Mountains metabasites. In an earlier high pressure–low
temperature metamorphic event, the rocks
experienced blueschist facies metamorphism. The results of
40Ar–39Ar geochronology studies on phengites
from the mafic blueschists date the end of the earlier metamorphism to
360 Ma.
The greenschist metamorphic
overprint followed around 340 Ma. The elongated bodies of mafic metavolcanic
rocks are situated
within the prominent NNE–SSW Leszczyniec shear zone following the
trend
of the Rýchory Mountains and
the Rudawy Janowickie Mountains. Both the geochemical affinities and the
blueschist facies metamorphism
of the metabasites suggest that this shear zone evolved from the Variscan
suture dividing western and central
terranes of the West Sudetes. According to the radiometric age for the
end
of the high pressure–low temperature
metamorphism, the terranes accreted during the Famennian. A considerable
time-span between the formation
of the metabasite protolith and the blueschist metamorphism may indicate
long-lasting subduction
of a large oceanic plate between Gondwana and Laurussia, possibly accompanied
by terrane accretion, prior to the Variscan orogeny.