Magmatic inclusions in olivine leucitites from NE China: implications for evolution of potassic magmas

Ming ZHANG, Norm Pearson and Suzanne Y. O'Reilly (GEMOC, Macquarie Univ., Australia)

Small (² 5cm) magmatic inclusions characteristic of various highly evolved alkaline mineral assemblages (mela-phonolite and aegirine-augite syenite) are found in olivine leucitites from Keluo province, Northeast China. According to their mineral phases, these inclusions can be divided into two groups: feldspathoid-bearing and feldspathoid-free. The feldspathoid-bearing inclusions contain nepheline, leucite and sodalite. Some of them also have zoned hyalophane with BaO decreasing from 10.6-2.5 wt% (20.2-4.6 mol% celsian) toward the rim. Clinopyroxene, barian titanian phlogopite (²10 wt% BaO & ²13 wt% TiO2), and aenigmatite (as overgrowths on clinopyroxene) are the mafic phases and titanomagnetite, ilmenite, and apatite are accessory minerals. Pyroxenes are mostly titanian augite - titanian aegirine (aegirine - neptunite solid solution) series (²10 wt% TiO2, ² 6.0 wt% Al2O3, and 0.6-13 wt% Na2O) and often contain augite cores with hourglass texture and complex normal, reverse, and oscillatory zoning patterns in terms of TiO2 and Al2O3 and aegirine-rich rims. Titanomagnetite contain ilmenite and pseudobrookite lamellae and are sometimes replaced by a Ba-rich Ti- and Fe-oxide ((Ba, K)(Fe3+, Ti, Mg)12O19) of magnetoplumbite group. Apatite is rich in halogens (2.7-3.1 wt% F and 0.2-0.6 wt% Cl). The felsic phase in the feldspathoid-free inclusions is alkaline feldspar containing ²6 mol% hyalophane component (Or66-55Ab32-40An0-0.3Cn0-5.8). Replacing phlogopite as a mafic phase, amphibole, belonging to sodic-calcic and alkali amphibole groups (potassian- titanian-magnesio katophorite, potassian- titanian-richterite, and titanian- (calcian-) eckermannite), is characterised by high TiO2 (2.4-5.6 wt%), Na2O (5.8-9.2 wt%), and K2O (1.5-1.7 wt%) and coexists with clinopyroxene in the feldspathoid-free inclusions.

Both nepheline and alkaline feldspar are homogeneous in terms of K/Na ratios (~Ne80Ks20 and Or35Ab65 respectively). This is consistent with the experimentally determined temperature minimums for the Ne-Ks-Q system, thus suggesting low crystallisation temperatures (²700°C) at volatile-rich conditions for both types of inclusions. The presence of leucite and aenigmatite suggests a low crystallisation pressure (²1kb?) for the feldspathoid-bearing ones. Compositional and textural characteristics of Ti- and Fe-oxides also indicate a highly oxidised environment, whereas the presence of aenigmatite surrounding aegirine-augite may reflect decreasing fO2 (below FMQ buffer) at the latest stage of crystallisation when halogens became the dominant volatile components at the expense of water in forming hydrous phases.

The diversity in mineral assemblages, the heterogeneities in composition of pyroxenes and in Ba-zoning patterns in alkali feldspars suggest that these inclusions were formed under non-equilibrium conditions from vanishingly small volumes of residual liquid pockets. The residual liquids should be enriched in Ba, alkaline metals, and volatile components of water, chlorine and fluorine. The studies of mineral chemistry and paragenesis of the inclusions and potassic rocks (including olivine leucitites and leucite basanites) in the region demonstrate that the feldspathoid-bearing inclusions are likely to be products of the latest-stage crystallisation from their host leucititic magma, whereas the feldspathoid-free ones may have leucite basanites as their parental magma.