COMPARATIVE PETROLOGY OF DIAMOND-BEARING COMPLEXES

Diamond-bearing metamorphic rocks have been detected in only two compl exes: the Kokchetav Median Massif, northern Kazakhstan, and the Dabie Mountains, eastern China. Both of them consist of rocks that are compo sitionally 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 grea t majority of gneisses, calc-silicate, and pyroxene-garnet rocks forme d within the temperature interval 400 - 900 degrees C, whereas the tem perature for the eclogites ranged from 500 to 1200 degrees C. Geobarom etric studies of the Cpx + Grt + Pl + Qtz assemblages in the rocks of both complexes demonstrated that the complete metamorphic evolution re flected in them proceeded at a temperature decline from 1200 +/- 20 de grees C/25 +/- 3 kbar to the normal geotherm 500 +/- 100 degrees C/13 +/- 2 kbar. This trend was satisfactorily correlated with the trends o f subisobaric cooling of some granulite complexes and cataclazed garne t Iherzolites from South African and Yakutian kimberlite pipes. Later, the rocks were affected by regional metamorphism at eclogite facies c onditions. It was established that the protolith of the Kokchetav Mass if(gneisses and eclogites) formed in the Late Proterozoic and was late r affected by at least two metamorphic events: (1) high-pressure retro grade 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 d ecline 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 inver ted metamorphism at eclogite facies conditions occurred at 212 Ma (U-P b isochron data). Its prograde P-T path from 600 degrees C/13.5 kbar t o 800 degrees C/17 kbar graded into a retrograde trajectory to 500 deg rees C/10.5 kbar. The relict ultrahigh-pressure mineral assemblages (d iamond, 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 rela tions between these assemblages and the eclogite bodies (whose Sm-Nd m odel age is 1.5 Ga) were confirmed for the Kokchetav Massif, but such relationships were supposed for the Dabie Shan Complex. The rocks of b oth complexes commonly bear calc-silicate melange, in which diamond ub iquitously occurs in assemblage with carbonates. Without disregarding the possibilities that the genesis of the ultrahigh-pressure assemblag es was collision- or subduction-related, we regarded the hypothesis of their mantle origin under the effect of intratelluric plumes, which i ntruded in the form of carbonatites or kimberlites (rocks of the lower crust) and were melanged, together with these rocks, during the squee zing-up of this material toward the surfase of the Earth.