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

The Inagli complex, one of several Mesozoic intrusive complexes of the Aldan Shield (Siberian Platform), exhibits a concentric structure com prising several alkaline ultramafic rock-types. A central dunite body is surrounded by olivine- and phlogopite-clinopyroxenites forming an i nner rim. The outer rim consists of different shonkinitic and malignit ic rocks. The K-Ar ages obtained for the whole complex vary around 132 Ma. The dunites and clinopyroxenites are characterized by cumulate te xtures. With increasing modal abundances of clinopyroxene and subordin ate phlogopite, the rocks develop to olivine-clinopyroxenite, shonkini te, and malignite with intercumulus potassium feldspar. Mineralogical characterization of the rocks suggests they evolved by fractional crys tallization. The highly forsteritic olivines (Fo up to 95) require a m elt as magnesian as mg# 87.1, representing +/-26 wt.% MgO. The parenta l melt is likely to be an olivine-, H2O- and K2O-rich picritic liquid of shoshonitic character. Major and trace element systematics show hig h LILE/LREE and LREE/HFSE ratios indicating the involvement of a subdu ction zone component in the genesis of these rocks.