WANG Gang, WANG Zong-qi, ZHANG Ying-li, WANG Kun-ming
(MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, CAGS, Beijing 100037, China)
Abstract: Trachyte from the Ziyang area is located in the North Daba Mountains tectonic belt containing widely developed biotite. This paper discusses the conditions of formation for biotite from trachyte and its petrological significance through systematic mineral chemistry analysis from two typical profiles of the rock. Electron microprobe analysis reveals that the biotite from two sections of the trachyte has a uniform Fe2+/(Fe2++Mg) value with low Ca content, indicating the biotite is of magmatic origin. Biotite from a cross-section of the Haoping trachyte is Mg-rich biotite and displays varying Fe-rich content moving from the lower to the upper section of the rock formation. However, biotite from the Jinzhudian cross-section is Fe-rich biotite, with decreasing average MF and MgO values for biotite 0.598→0.533→0.350 and 14.40→12.14→7.61 respectively, but with increasing average FeOT/(FeOT+MgO) and Al2O3 values 0.55→0.61→0.77 and 12.19→13.43→15.00 respectively. The results suggest that the magma is from a crust-mantle mixing source to a crustal source, along with the different stages of magmatic evolution, and may have formed in the orogenic background environment. Mineral chemistry analysis reveals that the crystallization temperature for biotite in early phases is 542－675 ℃ and 575 ℃ in later phases, and the oxygen fugacity is about from -10－-11.9 to -11.7－-13.5 (even decreasing to -15.5－-16.1), indicating that the biotite formed in a medium temperature and medium oxygen fugacity environment, and the diagenetic condition began from the early relative oxidation environment changing to a relative reducing environment as magmatic conditions evolved.
Key words: trachyte; biotite; mineral chemistry; Daba Mountains
ACTA MINERALOGICA SINICA Vol. 34, No. 3, 2014, Page 343-350
