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Russian Geology and Geophysics

2012 year, number 3


A.A. Shchipanskya, L.I. Khodorevskayab, and A.I. Slabunovc
aGeological Institute of the Russian Academy of Sciences, Pyzhevskii per. 7, Moscow, 198017, Russia
bInstitute of Experimental Mineralogy, Chernogolovka, Moscow Region, 142432, Russia
cInstitute of Geology, Karelian Scientific Center of the Russian Academy of Sciences, ul. Pushkinskaya 11, Petrozavodsk, 185910, Russia
Keywords: Eclogites, TTG gneisses, oceanic crust, isotope dating, SHRIMP, oceanic zircon, subduction, megamelange belt, Archean
Pages: 262-280


We present results of geochemical studies and isotope dating of eclogites and associated rocks from the Kuru-Vaara quarry, Belomorian Belt, Northeastern Baltic Shield. The southern and northern eclogites are similar in geochemical features. Their protoliths were primitive, mainly high-Mg basalts of oceanic genesis derived from a primitive mantle source rather than from a depleted mantle source characteristic of modern MORB. The post-eclogitic intrusive rocks show obvious evidence for crustal contamination. The eclogite-hosting tonalite-trondhjemite-granodiorite (TTG) gneisses form a coherent series including high-Al and low-Al varieties. The trace-element data show that the TTG series formed through the hydrous partial melting of the southern eclogites in the presence of garnet and amphibole in the field of the rutile stability (>15 kbar).
Zircons from the southern eclogites exhibit features of their strong re-equilibration by coupled dissolution-reprecipitation processes but have locally preserved patches with a primary magmatic zoning. The geochemistry of the patches points to the oceanic provenance of protolithic zircons; their isotope dating (SHRIMP-II) yielded a concordant age of 2821 ± 21 Ma. Zircons from the trondhemite gneiss with geochemical features of Archean adakite were dated at 2805 ± 11 Ma, which evidences the syn-eclogitic origin of theTTG melts. The concordant age of high-pressure zircons from the northern eclogites is 2722 ± 21 Ma, close to the age of the earlier studied Gridino eclogites.
The overview of the isotopically dated eclogite bodies show the presence of at least three temporally distinct groups of eclogites in the Belomorian Belt, ~2.86-2.87, ~2.82-2.80, and ~2.72 Ga, which is in good accordance with the known isotopic age for majer crust-forming events in the belt. This, in turn, implies a close genetic relationship between the eclogites and the TTG origin, which might be consistent with the model of the short intermitted events of subduction of the thickened Archean oceanic crust.
The presence of HP/UHP elcogites of different ages and the structural style of the Belomorian Belt permit it to be assigned to megamelange belts.