Rb–Sr isotope system and Rb–Sr partitioning in the rocks of the Ary-Bulak ongonite massif, formed with the participation of fluoride–silicate magmatic immiscibility

Abstract

The rocks of the Ary-Bulak ongonite massif (eastern Transbaikalia) have widely varying Rb (1499–4274 ppm) and Sr (10–2654 ppm) contents. In passing from porphyritic ongonites to aphyric rocks at the endocontact, Rb content increases by two to three times; Sr content, by two to three orders of magnitude. Feldspars and Fe-rich micas are the major Rb carriers and concentrators. The aphyric rocks accumulate Rb, because the weight percentage of sanidine in their total mineral balance increases relative to that in the porphyritic ongonites. In the latter, Sr is relatively uniformly distributed over the phenocrysts and groundmass. Prosopite and products of the quenching (glass) and partial crystallization of a calcium fluoride melt accumulate a considerable amount of Sr. The presence of these phases in the porphyritic and aphyric rocks abnormally increases their Sr content. The Rb content of silicate glass in melt inclusions (MI) varies much more widely (634 ppm—3.17 wt.%) than that of the massif rocks (1435–4309 ppm), especially in terms of the maximum value. Most of silicate glass in the MI is much poorer in Sr (<1–2 ppm) than the rocks. The highest Sr content (376–422 ppm) was detected in silicate glass from a MI with segregations of immiscible fluoride glass. Rubidium–strontium isotope dating confirmed the Early Cretaceous (141.6 ± 0.5 Ma) age of all the massif rocks, with ( 87Sr/ 86Sr)0 = 0.70817 ± 0.00025, that is, intermediate between mantle and typical crustal values. A near-linear dependence has been detected between 1/Sr and 87Sr/ 86Sr, which is usually interpreted as a false isochron and explained by models involving mixed components with different 87Sr/ 86Sr. According to calculations of the weight percentage of Sr isotopes in the massif rocks, this linear dependence is explained by Sr and Rb partitioning in ongonitic magma, which has the same initial Sr isotope ratio. Only fluoride–silicate liquid immiscibility determined the Rb and Sr partitioning in ongonitic magma and, correspondingly, the subsequent evolution of the Rb–Sr isotope system in the Ary-Bulak massif rocks. This immiscibility (liquation) in ongonitic magma might have created very special rocks, abnormally enriched in Ca, F, and Sr, even if the initial homogeneous silicate melt was relatively poor in these elements. The relatively small initial isotope ratio ( 87Sr/ 86Sr)0 ≈ 0.708 in the massif rocks does not contradict the hypothesis of the intense heating of a F-enriched crustal substrate and the formation of a rare-metal granitoid melt chamber (from which the residual ongonitic melt was produced) under the thermal effect of deep-seated basaltoid magma.