ARCHAEAN AGE AND PETROLOGY OF GRANITOIDS FROM GRIDINO AREA (BELOMORIAN ECLOGITE PROVINCE) AS LIMIT IN THE TIME OF HIGH-PRESSURE PROCESSES

Last years eclogites were found in the Belomorian mobile belt (location – Gridino and Salma). It is considered that in the Gridino eclogite-bearing zone (Northern Karelia) the eclogites have got two ages of formation: Archaean and Paleoproterozoic (Volodichev et al., 2004). Lenses of Archaean eclogite (2720.7±8 Ma) are in the felsic gneissic matrix as strongly deformed pods or layers and it is considered that they were formed in the enviroment of "warm" subduction of oceanic crust. Other type of “local” eclogite displayed in undeformed and deformed mafic dikes. Geochronology studies of mafic high-pressure metamorphic dykes gave Archaean dates coupled with Paleoproterozoic (2.35-2.42 Ga). Geochemical and petrological investigations of the both types of eclogites displayed a likeness in the chemical composition and the united step-by-step metamorphic P-T trend for all mafic rocks (from eclogite to amphibolite through HP-granulite). We dated Zircons (SHRIMP II) from granitoids which cross-cut the mafic postulate Paleoproterozoic dykes. New geochronology data permitted to determine the Archaean age of high-pressure metamorphic event.

The first sample was collected from a Phen-Fsp-Qtz leucosome developed at the continuation of the granulite gabbro dike, which was subjected here to deformation, amphibolization and migmatization. Petrological studies of the leucosome showed high-pressure magmatic relicts: Ba-bearing phengites (3.17 cations Si per 11 atoms O), K-Ba feldspar, mirmekite and near solidus symplectic intergrowths of Czo, Phe and Qtz. The conditions of leucosome crystallization are 16-20 kbar for 650 °C. Evidences of change the eclogite to granulite conditions are: Bt replaces the Phe; grossular Grt and Cpx replace the Czo-Qtz symplectites; Pl breaks down with antiperthite forming. The granulite stage was at 750-800 °Cƒn and 10-12 kbar. Last superposed amphibolitic metamorphic event fixes new mineral forming (600 °C at 8 kbar): Hbl replaces Cpx, new Bt and Czo with Ep forming. Zrns the Phen-Qtz-Fsp leucosome subdivide into some crystal faces. First routine Zrn group is brownish subhedral 100 x 200 µm grains, with clear core-rim structures in CL images. The cores display a relic oscillator zoning with underwent an alteration and a metamict. It is typical for magmatic Zrns with high-content of uranium (more 3000 ppm). One analysis of the least metamict core of the same Zrn gives concordant age 2713±6 Ma. The Zrns have a distinctive chemical composition characterized by low Th/U ratio (0.05), much enriched REE concentrations than others Zrns (summa REE=1205 ppm) and have smaller negative Eu (Eu/Eu*=0.57) and positive Ce anomalies (Ce/Ce*=1.62) at a relative flat chondrite-normalised REE pattern (Lu_N/Sm_N=21; LuN/SmN = 131). Second extended Zrn group is colourless ovoid 100x150 µm grains show core-rim zoning. In general Zrns are characterized a high-uranium (522-935 ppm) structureless cores. Some Zrns have a fir-tree zoning which is typical for high-pressure Zrns. The cores vary by Th/U ratio (0.11-0.32) and their age 2707±20 Ma. The REE pattern of such Zrn is characterized by a positive Ce anomaly, a negative Eu anomaly (Eu/Eu*=0.33), and low enrichment in HREE with respect to MREE (Lu_N/Sm_N=13, Lu_N/La_N=135). In rare instances the inner core display fine magmatic oscillatory zoning; external core has properties corresponding cores of second group. One point from inner oscillator core gave older age 2793±14 (²⁰⁷Pb/²⁰⁶Pb). All Zrns are surrounded by narrow structureless cracked low uranium (66-155 ppm) rims. Concordia age 1938±35 Ma and corresponds to the Svecofennian Orogeny, when imbricate thrusting brought deep-seated rocks of the Belomorian belt to shallower amphibolite levels (Bibikova et al., 2004). The rims are depleted in all the trace elements, except Hf and much low Th/U ratio (0.00-0.01). The REE pattern of rims is characterised by positive Ce anomaly, a negative Eu anomaly (Eu/Eu* = 0.41-0.51), and enrichment in HREE with respect to MREE and LREE (Lu_N/Sm_N=17-79, Lu_N/La_N=89-392).

Second geochronological sample was collected from enderbite vein cutting an eclogite gabbronorite dyke. Mineral composition of enderbite is Grt-Bt-Opx-Cpx-Pl-Qtz equilibrium paragenesis. There are no petrological evidences of amphibolite transformations in the enderbite. PT-path estimation of enderbite forming is 721 °C at 10 kbar. Zrns from the enderbite are subdivided into two groups. First group is subhedral grains with fine-scale oscillatory zoning, magmatic Th/U ratio (0.55-0.85) and ages about 3 Ga corresponding of the time of tonalite forming. Second Zrn group is subhedral structureless black in CL grains and rims around the core of the first type. They are characterized low Th/U ratio (0.03-0.04). Average weighted age (²⁰⁷Pb/²⁰⁶Pb) is 2719±39 Ga.

Thus the isochronous ages of granitoids are the upper age limit of Archean mafic dike intrusion and Archean high-pressure metamorphism. We believe that traditional interpretation of the age of mafic magmatism and emplacement of dikes (~2.4 Ga) is invalid. The high-pressure complexes of the Gridino zone were presumably formed during a continent–continent collision which took place about 2.71 Ga.