Geochemical Constraints on the Petrogenesis of the High-K Rocks of the Southern Sakun Massif, Aldan Shield, Siberia

V. A. Kononova IGEM RAN, Staromonetny: 35, Moscow 109017, Russia

V. A. Pervov IGEM RAN, Staromonetny: 35, Moscow 109017, Russia

O. A. Bogatikov IGEM RAN, Staromonetny: 35, Moscow 109017, Russia

B. G. J. Upton Dept. of Geology & Geophysics, Edinburgh EH9 3JW, UK

P. Suddaby, Dept. of Geology, Imperial College, London SW7 2BP, UK

A. R. Woolley Dept. of Mineralogy, Natural History Museum, London, SW7 2BD, UK

The Southern Sakun massif (ca. 288 Ma), in which occur some of the world's most extreme potassic rocks, occurs in the Aldan shield at the SE border of the Siberian craton. It comprises two units: (1) an outer (lower) mesocratic unit (6km2) and
(2) an inner (upper) leucocratic unit (3km2), mainly composed of rocks rich in pseudoleucite, hauyne and K-feldspar. The
mesocratic unit consists of modally-layered cumulates seen as several macro-rhythms (15-20 m thick), each including several tens of micro-rhythms composed mainly of pulaskite and subordinate shonkinite. Ultramafic (biotite pyroxenite) also occurs in the outer unit. Whole-rock compositions show wide variance (SiO2, 36 - 59; Al2O3, 3 - 24; MgO, 0 - 13 and K2O, 1 - 20 wt.%), with excellent co-variance among both major and trace elements. The differentiation of rock-types is ascribed principally to crystal fractionation.

Characteristic geochemical signatures are richness in LILE and poverty in HFSE. Some trace element ratios are similar to those of OIB. An average of 10 of the more primitive samples has Ce/Pb 26, La/Sc 2.3, Th/Sc 0.16, La/Th 13 and Th/Ce 0.04. However, primitive mantle-normalised plots show distinct negative Nd, Zr and Hf anomalies and the rocks have high Ba/La, Th/Nd, Sr/Nd and low Ti/V and Nd/U, similar to island arc lavas.

Pyroxene and apatite separates (12 samples) have 87Sr/86Sri from 0.703978 - 0.704449 and 143Nd/144Ndi from 0.512096 - 0.511919.

It is concluded that the complex crystallised from one primary magma derived from a relatively homogeneous mantle source. The rocks have Sr isotope ratios similar to those of Bulk Earth and negative e Nd values indicative of derivation from an old enriched mantle source (EMI-type). Nd model age calculations suggest enrichment event(s) between 1.03 - 1.25 Ga. We suggest enrichment was related to Proterozoic subduction, consistent with the tectonic position of Sakun at the margin of the Aldan shield.