TECTONOTHERMAL EVOLUTION OF THE WESTERN ALDAN SHIELD, SIBERIA

Precambrian terrains of different ages and exposed in the Western Alda n shield provide clues to the nature of the Precambrian continental cr ust and reveal major stages in its development, i.e. cycles of magmati c and metamorphic activity. Tonalitic and trondhjemitic gneisses found in the Aldan River Valley and Olekma area probably constitute a basem ent to the supracrustal Iengra group. There is evidence to suggest an extensive 3.5 Ga tonalitic and trondhjemitic basement for the whole Al dan shield. The first magmatic cycle involved two geotectonic environm ents in the Western Aldan shield, forming the oldest granite-greenston e terrain in the Olekma region, and the Aldan granulite-gneiss terrain , which are probably coeval in their development. A tectonic suture, w hich was subsequently transformed into a complicated tectonic zone dev eloped in early Archaean times. This first cycle is documented by reli cs of greenstone complexes dated at 3.3-3.5 Ga in the Olekma River Val ley and granulite assemblages from the westernmost Aldan shield with a n age for granulite metamorphism at about 3.3 Ga. The second cycle res ulted in the transformation of the Olekma granite-greenstone terrain i nto a mobile zone, producing the Olekma group, high-grade metamorphism and granites and occurred under conditions of the amphibolite facies. The age of this metamorphism is estimated to be 3.1 Ga. The third cyc le resulted in the stabilization of the Olekma area to form a granite- greenstone terrain. A network of upright linear belts and domal struct ures in the 2.96-2.70 Ga age range formed there. The Aldan shield at t hat time was not affected by significant magmatic activity but was rat her stable. The fourth stage is Palaeoproterozoic in age (2.3-1.8 Ga) and resulted in the consolidation of continental crust in the Western Aldan shield. A miogeosynclinal terrigeneous succession in the Olekma region (Udokan group), accompanied by intrusion of remarkable volumes of hypabyssal porphyritic K-granites, formed at this time. In the Alda n terrain, local structural reworking of Archaean rocks, superposed me tamorphism of amphibolite facies and intrusion of alaskitic K-rich gra nites occurred during this event.