POTASSIC MAGMATISM OF THE ALDAN SHIELD, SOUTHEASTERN SIBERIA - AN INDICATOR OF THE MULTISTAGE EVOLUTION OF THE LITHOSPHERIC MANTLE

Potassic magmatism occurred on the Aldan Shield during the Paleozoic ( 288 Ma) and Mesozoic (155-135 Ma). The problems of the origin, composi tion, and evolution of rocks and their mantle sources were considered using precise major- and trace-element analyses and Sr-Nd isotopic dat a on a representative collection of rock samples from the Paleozoic po tassic Sakun Pluton and by comparing some indicative features of Paleo zoic and Mesozoic potassic rocks. In spite of the great diversity of r ock mineralogy and chemistry within the earlier intrusive phase of the Sakun Pluton (mica clinopyroxenites, shonkinites, pulaskites, pseudol eucite syenites, and synnyrites), all these rocks have similar primary isotope compositions of Sr (Sr-87/Sr-86 = 0.70398-0.70445) and Nd (Nd -143/Nd-144 = 0.51209-0.51192). This rock series probably formed durin g the fractional crystallization of a single mantle-derived magma. The role of another, possible lower crustal source is recognized in the c omposition of the of the monzonite and quartz syenite of the later int rusive phases of the Sakun Pluton (Sr-87/Sr-86=0.70517 and 0.70567, Nd -143/Nd-144=0.51159 and 0.51134, respectively). The Late Paleozoic roc ks of the Sakun Pluton are generally similar in trace-element concentr ations to Mesozoic rocks of the Central Aldan Province. Both have geoc hemical affinity with the rocks of island arcs and continental margins (they are depleted in Zr, Nb, Hf, and Ta). The rocks studied have hig h LILE (K, Ba, and Sr) concentrations. This is indicative of their for mation from mantle sources rich in these elements. According to the is otopic data, the alkaline rocks were derived from the lithospheric man tle of the EMI-type, which extended over a large area under the Aldan Shield. Data on the composition of the potassic rocks indicate a compl icated history of the lithosphere mantle of the Aldan Shield. The mant le evolution seems to have included several stages of depletion and en richment in some trace elements. The Nd model ages show that the enric hed source of the Paleozoic rocks wa formed at about 1.4-1.7 Ga, where as the sources of the Mesozoic potassic rocks are older and have ages of about 2.2-2.8 Ga.