ABSTRACT
Variations of εNd(T) in mantle and crustal rocks (over 4,000 analyses) have been systematized andplotted on a εNd(T) – T diagram. The εNd(T) value shows a synchronous variation in geological time, reflecting a periodical change-over of epochs of maximum (L-megacycles) and minimum (S-megacycles) differentiation of magma-generating systems in the upper mantle. By the degree of variations of the εNd(T) parameter, duration of L- and S-megacycles has been determined. It corresponds to the intervals 0.85–0.70–0.55–0.40–0.35–0.15 Ga for L(l-6)-megacycles, and 0.28–0.35–0.3–0.3–0.25–0.15 Ga for S(l-6)-megacycles. Distinguished are four epochs during which depleted mantle magma-generating reservoirs with the following parameters appeared into existence: DM-1 εNd(T)=38.6 (= 0.9) - 9.01 T, f=.36; DM-2 εNd(T)=29.7 (±0.9) – 7.89 T, f=0.32; DM-3 0Nd(T)=13.7(±O.5) - 3.81 T, f=0.16; DM-4εNd(T)= 8.6(±0.5) – 2.01 T, f=0.08. The formation of DM mantle domains is confined to the following time intervals, respectively: 4280±60; 3400–3500; 3000–2800 and 4280 ? Ma. All the DM mantle domains formed in the Archaean time, which is therefore fundamentally different in the evolutionary nature from all younger periods of the planet life. By the degree of variations of the εNd(T) parameter, the Archaean can be subdivided into early (4.3–3.4 Ga), middle (3.4–3.15 Ga) and late (3.15–2.5 Ga) Archaean. At the early differentiation stages of the upper envelope of the Earth, simultaneously with the DM-I domains, a complementary, EM-1 enriched mantle domain was formed. It includes the protocrust and the underlying mantle zones enriched in incompatible elements with the following parameters: εNd(T)=−52 (±0.9)+12.6 T, f=-0.5, 4300–4130 Ma. At the interval of 3.4–0 Ga, secondary EM mantle domains were formed; these were grouped in the lithosphere part of the mantle (EM-2) and in deep mantle zones (EM-3). The EM-2 domairts are largely a result of mantle metasomatism, which was acting simultaneously with the generation of mantle magmas, as suggested by coinciding ages of mantle xenoliths and early stages of tectonic-magmatic cycles, based on the new detailed geochronologic scale of magmatic activity in the Baltic Shield. These stages were 40–60 Ma long and took place during early stages in the development of the cycles, the duration of which is 70–150 Ma. These stages resulted in the origination of mantle magmatic complexes, which were forming new crust in each tectonic-magmatic cycle, mostly from DM mantle domains. In rare cases, magmas appear to have been generated from the initial EM-1 domain that was preserved in the lithospheric zone of the mantle. Granulite metamorphism in the lower crust coincided with the intervals of mantle magmatism of the tectonic-magmatic cycles. As a result, in the vertical mantle-crust section, processes of magma-generation, mantle metasomatism and crustal metamorphism in each cycle were acting almost simultaneously. The secondary domains EM-3 resulted from Proterozoic subduction of crustal material into the mantle. K-enriched alkaline magmas are associated with these domains. The genesis of N- and E-types of MORB is related to DM-3 and DM-4 domains, respectively.
