Tectonothermal evolution of the Western Aldan shield, Siberia

Abstract

Precambrian terrains of different ages and exposed in the Western Aldan shield provide clues to the nature of the Precambrian continental crust and reveal major stages in its development, i.e. cycles of magmatic and metamorphic activity. Tonalitic and trondhjemitic gneisses found in the Aldan River Valley and Olekma area probably constitute a basement to the supracrustal Iengra group. There is evidence to suggest an extensive 3.5 Ga tonalitic and trondhjemitic basement for the whole Aldan shield. The first magmatic cycle involved two geotectonic environments in the Western Aldan shield, forming the oldest granite-greenstone terrain in the Olekma region, and the Aldan granulite-gneiss terrain, which are probably coeval in their development. A tectonic suture, which was subsequently transformed into a complicated tectonic zone developed in early Archaean times. This first cycle is documented by relics of greenstone complexes dated at 3.3–3.5 Ga in the Olekma River Valley and granulite assemblages from the westernmost Aldan shield with an age for granulite metamorphism at about 3.3 Ga. The second cycle resulted in the transformation of the Olekma granite-greenstone terrain into 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 cycle resulted in the stabilization of the Olekma area to form a granite-greenstone terrain. A network of upright linear belts and domal structures in the 2.96-2.70 Ga age range formed there. The Aldan shield at that time was not affected by significant magmatic activity but was rather 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 Aldan terrain, local structural reworking of Archaean rocks, superposed metamorphism of amphibolite facies and intrusion of alaskitic K-rich granites occurred during this event.