Olivine-garnet, olivine-spinel, and orthopyroxene metamorphic rocks of the Kokchetav Massif, northern Kazakhstan (vol 13, pg 514, 2005)
Reverdatto, VV; Selyatitskii, AYu; ,
Basic and ultrabasic rocks in high-and ultrahigh-pressure collision belts can be a source of important petrological information. Unusual deep rocks of this type were found at two sites in the well-known Kokchetav Massif in northern Kazakhstan. An irregularly shaped block of an olivine-garnet-cummingtonite rock was found in a eclogite sheet near diamond-bearing metapelites and metacarbonates in the vicinity of Kumdy-Kol' Lake in the western portion of the massif. Orthopyroxene, anthophyllite, and olivine-spinel mafic-ultramafic granulites closely associated with amphibolites and eclogites were found in the eastern part of the massif, near the village of Enbek-Berlyk. Both the garnet and the spinel ultramafic rocks from these two sites are very unusual and have similar chemical compositions. They are enriched in Al2O3 (18.2-21.5 wt %), FeO (12.2-14.5 wt %), TiO2 (1.2-2.6 wt %), P2O5 (0.2-0.5 wt %), Rb (0.6-5.9 ppm), Y (8.3-65 ppm), Zr (85-180 ppm), Nb, Sc, and REE but are depleted in SiO2 (31.5-39.5 wt %), MgO (20-26 wt %), Cr (106-142 ppm), and Co (33-43 ppm) compared to typical mantle Alpine-type peridotites. The orthopyroxene mafic granulites (often containing garnet) from the eastern part of the massif resemble basaltic rocks but are poorer in CaO and richer in Al2O3, and their MgO and FeO contents vary from 6-10 to 20 wt %. These rocks also contain 46-51.5 wt % SiO2, 1-2 wt % TiO2, 3-23 ppm Rb, 45-88 ppm Zr, 42-59 ppm Sc, 56-226 ppm Cr, and 40-57 ppm Co. The pyroxenites are enriched in LREE, whereas the ultrabasic rocks are usually enriched in HREE. The olivine- arnet, olivine- spinel, and orthopyroxene rocks seem to have been produced by the metamorphism of near-surface intrusive basalt bodies that had been metasomatized (variably chloritized) and had occurred at upper continental crustal levels before the collision. Metamorphism of the platelike shallow basic bodies that had not been metasomatized before the collision gave rise to eclogites and amphibolites. During the Middle Cambrian collision, the basic-ultrabasic protoliths and their enclosing continental crust were subducted, with the western part of the Kokchetav Massif brought to a greater depth than its eastern part. The protoliths of the olivine-garnet rocks were metamorphosed under the same P-T conditions as those of the diamond-bearing mineral assemblages, i.e., T > 900 degrees C and P > 40 kbar. The precursor rocks of the orthopyroxene and olivine-spinel mafic-ultramafic granulites in the eastern part of the massif were metamorphosed at similar to 800 degrees C and 14-15 kbar. During the Late Cambrian exhumation, the crustal package underwent retrograde regional metamorphism (P = 6-13 kbar and T = 500-700 degrees C) and deformation, and, later, the Ordovician granite intrusion in the eastern part of the massif produced low-temperature contact metamorphism of the pyroxenites (P = 1-2 kbar and T = 450-500 degrees C) bounded by a narrow zone.