Geochemical Discrimination Between Magmatic Sources of a) Palaeozoic and b) Mesozoic, High-K Complexes in the Aldan Shield, Russia

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

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

V. A. Pervov 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 geochemistry of late Palaeozoic (ca. 288 Ma) and Mesozoic (ca. 155-135 Ma) high - K complexes of the Aldan shield, on the SE of the Siberian craton, has been investigated to assess the relative contribution of asthenosphere - and lithosphere-derived melts. The predominant rock-types are leucitites, phonolites, lamproites, phlogopite pyroxenites, shonkinites and feldspathoidal syenites.

In terms of major and trace element compositions, the Palaeozoic and Mesozoic suites are closely comparable. Most of the rocks are silica undersaturated with SiO2, 36 - 61 wt%. MgO and K2O contents range 0.5 - 25% and 0.5 - 20% respectively; most samples have K2O/Na2O wt% >1. Primitive mantle - normalised trace element plots are typical for supra - subduction zone magmatism with enrichment in LILE and LREE relative to HFSE and HREE, and with negative Nb, Ta, Ti (and sometimes Zr and Hf) anomalies.

However, whereas the late Palaeozoic rocks have some incompatible element ratios similar to OIB-source magmas (e.g. Th/La ~ 0.08; Ce/Pb ~ 26; Th/Ce ~ 0.04), those for the Mesozoic rocks are typical for island arc volcanics (0.21, 4.3 and 0.12 respectively). Th/Sc and La/Sc ratios for the Mesozoic suites (~ 0.2 and ~1.0) are, however, similar to those of OIB source suites and those of the Palaeozoic samples (~0.16 and ~2.3).

87Sr/86Sri and e Nd values are 0.703978 to 0.704449 and -3.56 to -6.80 for the Palaeozoic suites and 0.704990 to 0.707497 and -10.2 to -22.9 for the Mesozoic suites. Although all samples have negative e Nd values indicating derivation from old enriched mantle sources (EMI type), the age of enrichment (based on Nd model age calculations) was much older (1.89 - 2.05 Ga) for the Mesozoic rocks than for the late Palaeozoic rocks, (1.03 - 1.25 Ga). Source enrichment is attributed to Precambrian subduction events; the influence of asthenospheric mantle sources is much clearer for the late Palaeozoic suites.