Archean crustal evolution of the Aldan Shield, Siberia: geochemical and isotopic constraints

• Published in: Precambrian research Vol. 91; no. 3; pp. 333 - 363
• Main Authors: Jahn, B.-M., Gruau, G., Capdevila, R., Cornichet, J., Nemchin, A., Pidgeon, R., Rudnik, V.A.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 31.08.1998
• Subjects: Aldan shield; Archean crustal evolution; Granulite Komatiite; REE patterns; Sm-Nd dating; TTG (Tonalite-Trondhjemite-Granodiorite); Zircon U-Pb dating; TTG (Tonalite-Trondhjemite-Granodiorite); Archean crustal evolution; Aldan shield; Zircon U-Pb dating; Granulite Komatiite; Sm-Nd dating; REE patterns
• ISSN: 0301-9268; 1872-7433
• DOI: 10.1016/S0301-9268(98)00057-6

The relatively unknown Aldan Shield in eastern Siberia is made up of Archean granite–greenstone and high-grade gneiss terrains as commonly observed in many Precambrian shield areas. New results of geochemical and isotopic study on two terrains of contrasting metamorphic grades (Olekma Granite–Greenstone and West Aldan Granulite Gneiss terrains) are present with the following principal conclusions being reached: (1) the 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 retrograded granulite enclave, but the most important crust-forming and tectonothermal event is firmly established at ca 3.0 Ga. (2) New SHRIMP and Pb evaporation analyses on zircons have confirmed the presence of crustal rocks older than 3 Ga. However, the zircon U–Pb systems appear to have been disturbed by later thermal effects, and no components of ≥3.5 Ga have been identified. (3) Komatiitic and basaltic amphibolites form important supracrustal sequences in the Olekma terrain. Both Group I and II komatiites occur and garnet (or majorite) fractionation in mantle melting processes is likely to have taken place during the genesis of group II komatiites. Overall, depleted mantle sources were involved in the geneses of basic and ultrabasic magmas as evidenced from LREE depletion and Nd isotopic compositions [ ϵ Nd( T)=+2]. Crustal contamination was not significant as suggested 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 that separation of garnet and amphibole has occurred during melting of their mafic sources. (5) Although the Olekma and West Aldan terrains have undergone different metamorphic evolution, there is no substantial difference in the time of major crust-forming events at ∼3.0 Ga and a strong Proterozoic thermal disturbance at ∼2 Ga. Furthermore, from the geochemical comparison of the two terrains it is concluded 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 restites of intracrustal melting.