Thermobaric structure of the Kokchetav ultrahigh-pressure-high-pressure massif deduced from a north-south transect in the Kulet and Saldat-Kol regions, northern Kazakhstan
T. Ota et al., Thermobaric structure of the Kokchetav ultrahigh-pressure-high-pressure massif deduced from a north-south transect in the Kulet and Saldat-Kol regions, northern Kazakhstan, ISL ARC, 9(3), 2000, pp. 328-357
To investigate the regional thermobaric structure of the diamondiferous Kok
chetav ultrahigh-pressure and high-pressure (UHP-HP) massif and adjacent un
its, eclogite and other metabasites in the Kulet and Saldat-Kol regions, no
rthern Kazakhstan, were examined. The UHP-HP massif is subdivided into four
units, bounded by subhorizontal faults. Unit I is situated at the lowest l
evel of the massif and consists of garnet-amphibolite and acidic gneiss wit
h minor pelitic schist and orthogneiss. Unit II, which structurally overlie
s Unit I, is composed mainly of pelitic schist and gneiss, and whiteschist
locally with abundant eclogite blocks. The primary minerals observed in Kul
et and Saldat-Kol eclogites are omphacite, sodic augite, garnet, quartz, ru
tile and minor barroisite, hornblende, zoisite, clinozoisite and phengite.
Rare kyanite occurs as inclusions in garnet. Coesite inclusions occur in ga
rnet porphyroblasts in whiteschist from Kulet, which are closely associated
with eclogite masses. Unit III consists of alternating orthogneiss and amp
hibolite with local eclogite masses. The structurally highest unit, Unit IV
, is composed of quartzitic schist with minor pelitic, calcareous, and basi
c schist intercalations. Mineral assemblages and compositions, and occurren
ces of polymorphs of SiO2 (quartz or coesite) in metabasites and associated
rocks in the Kulet and Saldat-Kol regions indicate that the metamorphic gr
ades correspond to epidote-amphibolite, through high-pressure amphibolite a
nd quartz-eclogite, to coesite-eclogite facies conditions. Based on estimat
ions by several geothermobarometers, eclogite from Unit II yielded the high
est peak pressure and temperature conditions in the UHP-HP massif, with met
amorphic pressure and temperature decreasing towards the upper and lower st
ructural units. The observed thermobaric structure is subhorizontal. The UH
P-HP massif is overlain by a weakly metamorphosed unit to the north and is
underlain by the low-pressure Daulet Suite to the south; boundaries are sub
horizontal faults. There is a distinct pressure gap across these boundaries
. These suggest that the highest grade unit, Unit II, has been selectively
extruded from the greatest depths within the UHP-HP unit during the exhumat
ion process, and that all of the UHP-HP unit has been tectonically intruded
and juxtaposed into the adjacent lower grade units at shallower depths of
about 10 km.