COMPARATIVE PETROLOGY OF DIAMOND-BEARING COMPLEXES


PERCHUK L., YAPASKURT V., OKAY A.

PETROLOGY, vol.3, no.3, pp.238-277, 1995 (Journal Indexed in SCI) identifier

  • Publication Type: Article / Article
  • Volume: 3 Issue: 3
  • Publication Date: 1995
  • Title of Journal : PETROLOGY
  • Page Numbers: pp.238-277

Abstract

Diamond-bearing metamorphic rocks have been detected in only two complexes: the Kokchetav Median Massif, northern Kazakhstan, and the Dabie Mountains, eastern China. Both of them consist of rocks that are compositionally very similar, whose geologic settings are controlled by the V-shaped intersections of large regional faults. The bulk of the high-pressure mineral assemblages are restricted to these faults. The great majority of gneisses, calc-silicate, and pyroxene-garnet rocks formed within the temperature interval 400 - 900 degrees C, whereas the temperature for the eclogites ranged from 500 to 1200 degrees C. Geobarometric studies of the Cpx + Grt + Pl + Qtz assemblages in the rocks of both complexes demonstrated that the complete metamorphic evolution reflected in them proceeded at a temperature decline from 1200 +/- 20 degrees C/25 +/- 3 kbar to the normal geotherm 500 +/- 100 degrees C/13 +/- 2 kbar. This trend was satisfactorily correlated with the trends of subisobaric cooling of some granulite complexes and cataclazed garnet Iherzolites from South African and Yakutian kimberlite pipes. Later, the rocks were affected by regional metamorphism at eclogite facies conditions. It was established that the protolith of the Kokchetav Massif(gneisses and eclogites) formed in the Late Proterozoic and was later affected by at least two metamorphic events: (1) high-pressure retrograde metamorphism in the Middle Cambrian (530 Ma) at a pressure of 18 - 12 kbar and (2) inverted metamorphism with a prograde P-T path from 650 degrees C/14 kbar to 860 degrees C/16.5 kbar, with a subsequent decline to 800 degrees C/15 kbar. The age of the protolith of the Dabie Shan Complex has not been determined, but its retrograde metamorphism was dated at approximately 244 Ma. It was hypothesized that the inverted metamorphism at eclogite facies conditions occurred at 212 Ma (U-Pb isochron data). Its prograde P-T path from 600 degrees C/13.5 kbar to 800 degrees C/17 kbar graded into a retrograde trajectory to 500 degrees C/10.5 kbar. The relict ultrahigh-pressure mineral assemblages (diamond, coesite, high-alumina sphene in assemblage with rutile, etc.), which occur mainly as inclusions in the garnet and zircon, implied an initial crystallization depth of 120 - 150 km at approximately 530 Ma in the Kokchetav Massif and 244 Ma in the Dabie Shan Complex. No relations between these assemblages and the eclogite bodies (whose Sm-Nd model age is 1.5 Ga) were confirmed for the Kokchetav Massif, but such relationships were supposed for the Dabie Shan Complex. The rocks of both complexes commonly bear calc-silicate melange, in which diamond ubiquitously occurs in assemblage with carbonates. Without disregarding the possibilities that the genesis of the ultrahigh-pressure assemblages was collision- or subduction-related, we regarded the hypothesis of their mantle origin under the effect of intratelluric plumes, which intruded in the form of carbonatites or kimberlites (rocks of the lower crust) and were melanged, together with these rocks, during the squeezing-up of this material toward the surfase of the Earth.