LAYERING IN THE SOUTHERN SAKUN HIGH-POTASSIUM ALKALINE MASSIF, ALDAN SHIELD, RUSSIA

A comprehensive petrological, mineralogical, and geochemical investiga tion was carried for the first time on various types of layering in th e potassium alkaline rocks of the Southern Sakun Massif (western Aldan Shield, Russia). The massif is composed of the following two rock uni ts: a lower (outer) mesocratic and an upper (inner) leucocratic unit, each of which is about 1300 m thick, The layering in the massif has a steep dip, which is believed to be related to later tectonic deformati ons, The layering in the mesocratic unit consists of intercalating mic rorhythms composed of shonkinites and pulaskites with upward gradation to more leucocratic rocks. Several tens of microrhythms form a macror hythm. The section of the unit consists of several macrorhythms. Evide nce of magmatic erosion is often shown at the boundaries of the macror hythms, The cryptic layering within the mesocratic unit is pronounced in the enrichment of the main rock-forming minerals (Cpx, Kfs, and Bt( 1)) in lower temperature components upward across the unit. The leucoc ratic unit is generally composed of PsLct syenite and synnyrite, The p seudoleucite aggregates are subdivided into two types: with Kls and wi th Ne, which are always separated in space and never occur in the same rock. We suggested that the Nepseudoleucite replaces potassium-rich a nalcite, whereas the Kls-pseudoleucite is formed after leucite. Thus, the Ne-PsLct syenite was K-Anl + Kfs cumulate, Kls-PsLct syenite was L et + Kfs cumulate, and synnyrites was Let cumulate. Indications of mul tiple magma intrusion into the shallow-level magma chamber (mingling, disturbance of cumulus stratigraphy, evidence of erosion of cumulates by magma currents etc.) show that this chamber was an open system duri ng its crystallization, and magma differentiation took place both in t he shallow-level and intermediate magma chambers.