The Paleozoic very low to low-grade metamorphic rocks of the
Bukulja-Lazarevac Unit designated as Drina, Golija and Birac formations are
contact metamorphosed by the intrusion of the Tertiary Brajkovac granodiorite
into spotted slates and hornfelses. In some parts, they are slightly
migmatized at the contact. In addition to their outcrops found at the
western, eastern and northern parts of the formation, these rocks are also
found in boreholes near Dudovica at about 8 km south-west from the pluton.
There, at a depth of 110 m, the spotted slates comprise oval to ellipsoid
pinite-rich spots which can be regarded as incipient cordierite
porphyroblasts (up to 5 mm in diameter) overgrowing the existing regional
foliation. They are composed of cryptocrystalline mixture of a very fine
sericitic material ? light glassy orange ?film? (some kind of an amorphous
gel-like material often mixed with limonite matter) and are abundant in
inclusions: minute quartz and dusty ore minerals (magnetite) prevail. In
addition, within some spots an increased number of xenotime and monazite
inclusions are noted. Minute flakes of neobiotite are formed at the expense
of quartz-sericite-chlorite matrix. The secondary chlorite occurring as
overgrowths on pinite-cordierite spots shows variable composition
(brunsvigite to diabandite). The Mg/Fe+Mg ratio of cryptocrystalline pinitic
mixture ranges from 0.14-0.67. The Si vs AlIV+AlVI relations deviate from the
ideal muscovite-phengite join due to Tschermak substitution towards chloritic
composition or a more complex mixture, including clay minerals (which
reflected a decrease of Altot and Si with increase of Fe2+). Obtained data
indicates that the cordierite-pinite spots can be related to contact
metamorphic processes that occurred within the temperature range 300-450?C.
CLAY MINERALS IN THE MIOCENE LOW-GRADE METAMORPHIC ROCKS, TANZAWA MOUNTAINS, KANAGAWA PREFECTURE, CENTRAL JAPAN
