Abstract:

Trachyte is one of the important deep-gases reservoirs among volcanic rocks in Yingcheng Formation of Xujiaweizi fault depression, Songliao basin. Core, drill and seismic data exhibit that trachyte always erupt at higher location and flow to low-lying areas. In general, the thickness increases toward crater. Major element, trace element and isotopic geochemistry studies indicate that the trachyte belongs to shoshonitic series. The trachyte are prominently enriched strongly incompatible elements such as Rb, Ba, Th, U, REE, and have low initial ⁸⁷Sr/⁸⁶Sr (0.704321-0.705395) ratios. All ε_Nd(t) values of the trachyte are all positive and have a narrow range (3.36-3.83), but they have an analogical radiogenic Pb. The mean of (²⁰⁶Pb/²⁰⁴Pb）_i,（²⁰⁷Pb/²⁰⁴Pb）_i，（²⁰⁸Pb/²⁰⁴Pb）i are 18.43，15.51 and 38.23, respectively. The features of geochemical data indicate that the trachyte magma are derived from partial melting juvenile crust following fractional crystallization without crustal contamination. Hydrothermal gases continuously gather into trachyte after many times eruptions. The trachytes would be crypto-explosive when the pressure is not large enough to bear. The crypto-explosive breccias are formed when the autobreccias are cemented by hydrothermal fluids, which can creat many cracks and inter-granular space.

Key words: Songliao basin, Xujiaweizi fault depression, trachyte, petrogenesis, crypto-explosive breccia, volcanic rocks