A basanitic lava flow bearing deep-seated inclusions erupted about 10 Ma ag
o south of the Tunka basin in the Zun-Murin River valley on the northern sl
ope of the Khamar-Daban ridge. The inclusions involve uppermost mantle rock
s, namely Cr-diopside-bearing spinel lherzolites ("green" series) and clino
pyroxenites, wehrlites, and gabbros with augite-salite-fassaite pyroxenes (
"black" series), Clinopyroxenes and spinels show broadly varying ferric-fer
rous ratios. Spinel lherzolites underwent metasomatism, which resulted In o
xidation of Iron In Cr-diopsides accompanied by an increase in Na and a dec
rease in Cr contents and partial melting manifested in reduction of iron an
d lower concentrations of Na.
The deep inclusions in the Tunka alkali basalts formed in the transitive a
crust-mantle zone at the suture between the Riphean Tuva-Mongolian and Cale
donian Dzhida terranes. The depth of uptake of crustal xenoliths by the asc
ending alkali-basaltic melts may have been controlled by the major fault th
at bounds the northern steep side of the rift valley and reaches the crust-
mantle boundary just beneath the Zun-Murin site. North of the major fault,
inclusions from alkali basalts are olivine-clinopyroxene to plagioclase-amp
hibole cumulates crystallized From basaltic melts trapped at crustal depths
.