CONTAMINATION MECHANISMS OF ALKALINE-GABBROID INTRUSIONS IN THE SOUTHERN FRAMING OF THE SIBERIAN PLATFORM - EVIDENCE FROM STRONTIUM AND OXYGEN ISOTOPIC COMPOSITIONS

Wide variations in the O and Sr isotopic composition of rocks in alkal ine-gabbroid intrusions of the Kuznetsk Alatau, northern Mongolia, and Vitim magmatic provinces provide evidence for the origin of these roc ks in open systems and active interaction between the magmas and host sedimentary rocks. The average Sr-87/Sr-86 ratios and delta(18)O value s of intrusions in the Kuznetsk Alatau Province, which were studied mo st exhaustively, increase systematically in the succession gabbro (0.7 046 +/- 0.0005 and 7.7 +/- 0.3, respectively)-theralite (0.7050 +/- 0. 0003 and 8.8 +/- 0.8)-feldspar urtite and ijolite-urtite (0.7054 +/- 0 .0003 and 9.1 +/- 1.0)-urtite (0.7058 +/- 0.0005 and 9.6 +/- 0.3)-neph eline syenite (0.7066 +/- 0.0013 and 10.4 +/- 0.9) as a function of th e involvement of crustal material increasing from earlier to later mag matic phases. The isotopic signatures of intrusions in northern Mongol ia are similar to those of the northern Kuznetsk Alatau. The delta(18) O of the urtite from the Mukhal Massif attains 12.1 in whole-rock samp les and 11.8 in pyroxene, and the (Sr-87/Sr-86)(0) ratios are as high as 0.7080. In terms of the content of the ''crustal'' O-18 isotope, th ese rocks have no analogues among alkaline rocks examined so far and c an be correlated only with granites of the S type. The character of th e dependencies between the isotopic compositions of O and Sr and betwe en the isotopic and elementary compositions suggest that the magmas we re contaminated as a result of the interaction with highly concentrate d solutions from the host rocks but not by the assimilation of solid m aterial.