Yingmailai Granitic Intrusion in the Southern Tianshan：Magnetite-series or Ilmenite-series?

Abstract:

The Yingmailai granitic intrusion is located in the middle part of the southern Tianshan. It consists predominantly of biotite K-feldspar granite with minor two-mica K-feldspar granite. They have similar whole-rock geochemical characteristics, but distinct mineralogy. Opaque minerals in biotite K-feldspar granite are ilmenite, whereas they are magnetite in two-mica K-feldspar granite. Primary muscovite has been recognized in two-mica K-feldspar granite, which is characterized by high Mg/Fe in biotite, An contents of plagioclase and Ab contents of perthite. According to Ishihara’s classification(1977), biotite K-feldspar granite can undoubtedly be classified to ilmenite-series. For instance, opaque oxide minerals are less than 1 vol%, in which ilmenite is unique recognized; Fe-rich biotite (high FeO+Fe₂O₃) and low MgO, high FeO+Fe₂O₃/ FeO+Fe₂O₃+MgO ratio (0.957~0.980), low Mg numbers (<0.6), mostly Fe²⁺>1.1, and low Fe³⁺/（F^e3++Fe²⁺）. Although some characteristics, e.g., presence of magnetite and Mg-rich biotite, suggest magnetite-series, it should be noted that the magnetite in two-mica K-feldspar granite is formed by post magmatism. In combination with low Fe³⁺/（F^e3++Fe²⁺）ratio and presence of indicating mineral—muscovite, it can be inferred that it also belongs to ilmenite-series. The factors which control the appearance of secondary magnetite are sudden change of f_O2, pressure, temperature during magma emplacement rather than their source. In addition, Yingmailai granitic intrusion’s characteristics, such as mineralogy, CaO(wt%)<3.7, w(Na₂O)/w(K₂O)<1, high SiO₂ and (⁸⁷Sr/⁸⁶Sr)_t, low temperature during the process of forming, indicate S type granitoids, suggesting that ilmenite-series defined by Ishihara (1977) correspond to S-type granite in the south Tienshan. This research also suggests that the south Tianshan had not experienced within plate during early Permian.