ARCHEAN CRUSTAL EVOLUTION OF THE ALDAN SHIELD, SIBERIA - GEOCHEMICAL AND ISOTOPIC CONSTRAINTS

The relatively unknown Aldan Shield in eastern Siberia is made up of A rchean granite-greenstone and high-grade gneiss terrains as commonly o bserved in many Precambrian shield areas. New results of geochemical a nd isotopic study on two terrains of contrasting metamorphic grades (O lekma Granite-Greenstone and West Aldan Granulite Gneiss terrains) are present with the following principal conclusions being reached: (I) t he crustal evolution of the Aldan Shield started as early as 3.5 Ga as suggested by the T-DM model ages of a few high-grade gneisses and a r etrograded granulite enclave, but the most important crust-forming and tectonothermal event is firmly established at ca 3.0 Ga. (2) New SHRI MP and Pb evaporation analyses on zircons have confirmed the presence of crustal rocks older than 3 Ga. However, the zircon U-Pb systems app ear to have been disturbed by later thermal effects, and no components of greater than or equal to 3.5 Ca have been identified. (3) Komatiit ic and basaltic amphibolites form important supracrustal sequences in the Olekma terrain. Both Group I and II komatiites occur and garnet (o r majorite) fractionation in mantle melting processes is likely to hav e taken place during the genesis of group II komatiites. Overall, depl eted mantle sources were involved in the geneses of basic and ultrabas ic magmas as evidenced from LREE depletion and Nd isotopic composition s [epsilon(Nd)(T) = +2] Crustal contamination was not significant as s uggested by the low La/Nb ratios (=1) in komatiites and metabasalts. ( 4) As common to most Archean tonalite-trondhjemite-granodiorite (TTG) rocks, the Aldan granitic gneisses also show highly fractionated rare earth element (REE) patterns with heavy REE depletions, suggesting tha t separation of garnet and amphibole has occurred during melting of th eir mafic sources. (5) Although the Olekma and West Aldan terrains hav e undergone different metamorphic evolution, there is no substantial d ifference in the time of major crust-forming events at similar to 3.0 Ga and a strong Proterozoic thermal disturbance at similar to 2 Ga. Fu rthermore, from the geochemical comparison of the two terrains it is c oncluded that the nature of mantle sources for basic-ultrabasic magmas and the process of TTG magma generation are similar in both terrains and that the granulite facies rocks cannot be considered as the restit es of intracrustal melting. (C) 1998 Elsevier Science B.V.