A Noncognate Relationship between Megacrysts and Host Basalts from the Tuoyun Basin, Chinese Tian Shan

Abstract

Abstract Abundant kaersutite and alkali feldspar megacrysts occur together with mantle-derived and lower crustal xenoliths, including kaersutite hornblendite and syenite, in the Early Cretaceous alkaline basalts of the Tuoyun basin in the westernmost Chinese Tian Shan. Particularly, several kaersutite hornblendite cumulate xenoliths have the assemblage of kaersutite + alkali feldspar + titanian mica ± titanomagnetite ± clinopyroxene, in which kaersutite occurs as an early-stage cumulus phase crystallized from a parental magma and alkali feldspar occurs as a late-stage intercumulus phase from the evolved magma between accumulating kaersutite grains. The megacrystic and xenolithic kaersutite has much higher Mg# values (0.70 –0.96) than the host basalt (0.47–0.60). The kaersutite and alkali feldspar megacrysts have similar initial ratios (0.7032–0.7033 and 0.7035–0.7038, respectively), significantly different from their host basalts (0.7047–0.7054). Xenolithic petrography and megacrystic Sr isotope geochemis...