Abstract:

From the geological-geochemical features of the Qitainling granite batholith, the depth of emplacement (5.5 km ),temperature of country rocks (196 ℃) and initial temperature of granite melt (950 ℃) during its emplacement are obtained.With these parameters a mathematic model calculating the emplacement age of the Qitainling granite batholith is established in this paper. Model calculation results suggest that the cooling time (Δt col ) for the Qitainling granite batholith from its initial temperature to crystallization temperature was 4.1 Ma, and the prolongation time (Δt L ) caused by latent heat generated from crystallization was 2.6 Ma. Since the radioactive elements of the Qitainling granite batholith (U，15.3×10-6；Th， 51.35×10-6；K2O， 5.02 % ) are as high as 2~3 times of those of the world average granite (U，5×10-6；Th， 20×10-6；K2O， 2.66 %) , the prolongation time (Δt A ) caused by radiogenic heat from the decay of the radioactive elements was 35.4 Ma , which is much longer than the Δt A value calculated for the world average granite (2.93 Ma). Therefore, the time elapsed from the magma emplacement to the crystallization-solidification (Δt ECTD), which should be the sum of Δt col , Δt L, and Δt A, was 42.1 Ma for the Qitainling granite batholith. In combination with zircon U-Pb age of the granite (161 Ma) representing the crystallization age, the emplacement age of the Qitainling granite batholith is estimated to be 203.1 Ma. This suggests that the emplacement of the Qitainling granite magma was in the Indo-Sinian Period.

Key words: Qitainling granite batholith, emplacement age, emplacement-crystallization time difference, Indo-Sinian orogeny;magma-dynamic evidence