1. EVIDENCE FROM THE EASTERN FENNOSCANDIAN SHIELD
HIGH-GRADE COMPLEXES OF THE EARLY PRECAMBRIAN:
EVIDENCE FROM THE EASTERN FENNOSCANDIAN SHIELD
E.V. Sharkov and A.V. Chistyakov
Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry (IGEM) RAS, Moscow, Russia

2. Three types of high-grade rocks are known in the Early Precambrian
Three types of high-grade rocks are known in the Early Precambrian. Two of them occurred in form of granulite belts and considered with tectonic processes; the third type has areal expansion and forms the lower continental crust.
We discuss peculiarities of these occurrences on example of the eastern Fennoscandian Shield, especially in the Kola region.
During my talk I dwell on geological features of these occurrences, their tectonic settings, and give views on their origin.

3. Archean type of granulite belts: evidence from Neoarchean Kola-Norwegian Granulite Belt
Despite the fact that eastern Fennoscandian Shield underwent by powerful tectono-meta-morphic reworking in Mid-Paleoproterozoic, general scheme of Neoarchean structure of the region is survived. Like on the other Precambrian shields, two major types of simultaneously developed structural domains are distinguished here:
(1) Karelian and Murmansk granite-greenstone terranes (GGTs), which represented areas of prevailing extension, uplifting and erosion,
(2) divided them Kola-Norwegian Granulite (granulite-gneissic) Belt (KNGB) - area of prevailing compression, subsidence, and sedimentation.
Belomorian Mobile Belt (BMB) occurs between them; it made up of a system of tectonic slices and represented by zone of gentle tectonic flowage to granulite belt direction, occurred under the amphibolite-facies conditions.
(1) Granite-greenstone domains: Karelian GGT and M, Murmansk block; (2) greenstone belts; (3) greenstone belts reconstructed by Rybakov; (4) Belomorian mobile belt (BMB): (a) proved and (b) inferred (5) Central-Kola GB:; (6) Keivy structure; (7) Svecofennides; (8) boundaries.

4. The close situation was established in South Africa where Limpopo granulite Belt with transitional marginal zones occurs between Zimbabwe and Kaapwaal cratons (Van Reenen et al., 1992). Two next papers will be devoted to this belt and to clear up its features.
From all these data follow that development of aforementioned interrelated tectonic domains were accompanied by appearance of regional structural-metamorphic zoning: from predominate greenschist facies on GGTs (cratons) via amphibolite facies in transitional zones to granulite facies in the belts.

5. How such a situation can be explained?
We suggest that formation of GGTs in Archean was considered with ascending of mantle superplumes. GGTs were formed above their extended heads and were characterized by mantle derived magmatism in network of greenstone belts (protorift structures).
Between GGTs, on places of descending mantle currents, large sedimentary basins were formed; this material gradually transformed during submersion into granulites. Such sagduction structures were, probably, prototypes of future subduction zones in the Phanerozoic.

6. Instead of the Phanerozoic mantle plumes, superplumes of the first generation (early Precambrian type) were formed by depleted ultra-mafic material. Partial melting of their heads was led to formation of:
1) mantle derived high-Mg komatiite-basaltic series, which basaltic differentiates are close in composition to MORB,
2) mantle-crustal derived continental siliceous high-Mg (boninite-like) series close in composition to Phanerozoic island-arc rocks.
Such geochemical features of the early Precambrian volcanics usually serve the major arguments for followers of idea of plate tectonics prolongation to the Early Precambrian.
However, judging on existence among komatiites Al-depleted and Al-enriched varieties, extention of these superplumes heads occurred at depths km and more, and could not lead to ruptures of ancient continental crust. It became possible from the Mid-Paleoproterozoic ( Ga) because appearance of mantle plumes of the second generation (thermochemical, or Phanerozoic type) which formed ar core-mantle boundary and enriched in fluid components and can reach more shallow depths (Sharkov, Bogatikov, 2010). Extention of their heads was already accompanied by intense interaction with ancient lithosphere and led to its disruptions, appearance lithospheric plates, oceanic spreading, etc., i.e. to plate tectonics existence.

7. Mid-Paleoproterozoic type of granulite belts
It was started from Ga ago and marked by appearance of a new, high-pressure type of granulite metamorphism, which was coeval with changing of archaic Early Precambrian plume tectonics to the plate-tectonics. Then Svecofennian orogen began to develop in the central part of the Fennoscandian Shield and related Lapland-Kola Collision on its NE periphery. Collision made by a system of tectonic blocks separated by regional deep faults; the axial structure of this collision was the Main Lapland Thrust (Priyatkina, Sharkov, 1979).
It represented by huge (more than 700 km long) belt of high-pressure granulite metamorphism. This metamorphism developed as large shear zone upon intrusive and metasedimentary rocks.

8. Main Lapland Thrust (Lapland high-P granulite belt) Sheared rocks of the early Paleoprotrozoic Kolvitsa anorthosite massif (southern part of LGB, Por’ya Guba Bay, the White Sea)

9. Sheared gabbro-anorthosites of the Kolvitsa massif, Por’ya Guba Bay (Photo by A.V. Chistyakov)

10. Metamorphic zoning in Lapland Granulite Belt
The belt shows an inverted metamorphic zoning, considered with stress deformations during the thrust activity (Priyatkina, Sharkov, 1979).
High-grade rocks pushed on Archean Belomorian granite-migmatite complex, metamorphosed under condition of amphibolite facies (P 6-7 kbar, T oC. Zone of garnet amphibolites, formed under T = oC and P = kbar, disposes higher. It developed upon early Paleopro-terozoic mafic volcanics of Kandalaksha complex below and lower part of the Kolvitsa anorthosite intrusion (blue).
Upper part of latter metamorphosed under conditions of high-pressure granulite metamorphism (P = 9-10, up to 12 kbar and T = oC). Isograd between these zones occurs in the middle part of the Kolvitsa anorthosite massif.

11. Problem of granulite metamorphism origin
Origin of high-grade metamorphism is usually considered with high density of heat flows. However, inverted metamorphic zoning of Lapland Belt cannot be explained such way because granulite facies zone stratigraphically higher amphibolite one. So, pressure does not reflect the depth of the process and heating from below could not occur.
The concept of heating from below is also not conform with absence of powerful source of heat beneath Archean granulite belts. In contrast to GGTs, which development accompanied by mantle magmatism, synkinematic magmatism in granulite belts represented only crustal-derived enderbite-charnockite melts.
These belts are elements of regional structural-metamorphic zoning: transition from GGTs to granulite belts occurred under strengthening of deformations and metamorphism grade from the greenschist via amphibolite to granulite facies.
All of these evidence that origin of granulite belts was not considered with heat flows from below and has another nature

12. It’s known, that transitions between metamorphic facies are considered mainly with increasing of temperature, not pressure. We suggest, that rising of temperature in the crust in such settings was caused heat generation in processes of deformations, which the most intense in granulite belts. According to (Molnar, Ingland, 1995), contribution of friction heat is оС, which can provide formation of the studied metamorphic zoning.

13. Garnet granulites of the lower crust
Precambrian high-grade rocks forms lower crust of the region and represented by xenoliths of garnet granulites in lamprophyre and kimberlite pipes. They found in the Kola Peninsula, in Arkhangelsk kimberlite province, in central Finland and Belarus, and are close in structure and composition (Downes et al., 2002).
Geochronological data and similarities in major and trace element geochemistry suggest that material of xenoliths was formed during the plume-related magmatic event - the early Paleoproterozoic large igneous province (LIP) of at Ga (Kempton et al., 2001).
This LIP is represented by rocks of aforementioned continental siliceous high-Mg series (Sharkov et al., 2005), So, these lower-crustal rocks are interpreted as high-grade metamorphic equivalents of these melts as a result of underplating.

14. The Kola suite of xenoliths includes mafic granulites (Ga + Cpx + Rut ± Pl ± Opx ± Phl ± Hbl) and felsic granulites (Pl + Ga + Cpx + Rut ± Qtz ± Ksp ± Phl ± Hbl), but mafic garnet granulites predominate. Metasediments are absent.
Metamorphism occurred under pressures of ca kbar at P 750°C, sometimes up to ca. 930°C at kbar.
Though PT-parameters of metamor-phism in the Lapland Trust are partly overlapped with metamorphism of the lower-crustal rocks, they have different origin: inverted zoning of the Lapland granulites evidences about their upper-crustal origin. Above, many xenoliths has rounded shape like kimberlite nodules and so derived from essential depths.
Xenolith of mafic garnet granulite from lamprophire pipe (Kola Peninsula, Elovy Island, Kandalaksha Bay, the White Sea). Collection of E.V. Sharkov.

15. CONCLUSIONS
So, three major types of high-grade Precambrian rocks occurrences occur on the eastern Fennoscandian Shield:
(1) the Early Precambrian moderate-pressure granulite belts are located between the Archean GGTs, being elements of regional structural-metamorphic zoning. It is suggested that synchrono-us formation of these structural domains was related to ascending of mantle superplumes with formation of GGTs above their extended heads. Zones of descending mantle flows, corresponded to large sedimentary basins, subsequently transformed into granulite belts during their submergence;
(2) the Mid-Paleoproterozoic high-pressure Lapland Granulite Belt was formed in process of stress deformation in suture zone of Main Lapland Thrust, inverted metamorphic zoning is characteristic for this belt;
(3) the lower-crustal garnet granulites have areal distribution; very likely, they resulted in process of underplating beneath the early Paleoproterozoic LIP of the siliceous high-Mg series, which developed Ga on all territory of the NE part of the East European Craton.

16. Thank you for attention