MINERAL CHEMISTRY AND GEOCHRONOLOGY OF THE POTASSIC ALKALINE ULTRAMAFIC INAGLI COMPLEX, ALDAN SHIELD, EASTERN SIBERIA

Abstract

The Inagli complex, one ot several Mesozoic intrusive complexes of the Aldan Shield (Siberian Platform), exhibits a concentric structure comprising several alkaline ultramafic rock-types. A central dunite body is surrounded by olivine- and phlogopite-clinopyroxenites forming an inner rim. The outer rim consists of different shonkinitic and malignitic rocks. The K-Ar ages obtained for the whole complex vary around 132 Ma. The dunites and clinopyroxenites are characterized by cumulate textures. With increasing modal abundances of clinopyroxene and subordinate phlogopite, the rocks develop to olivine-clinopyroxenite, shonkinite, and malignite with intercumulus potassium feldspar. Mineralogical characterization of the rocks suggests they evolved by fractional crystallization. The highly forsteritic olivines (Fo up to 95) require a melt as magnesian as mg# 87.1, representing ± 26 wt.% MgO. The parental melt is likely to be an olivine-, H2O- and K2-O-rich picritic liquid of shoshonitic character. Major and trace element systematics show high LILE/ LREE and LREE/HFSE ratios indicating the involvement of a subduction zone component in the genesis of these rocks.