关键词: 骑田岭花岗岩基, 侵位年龄, 侵位-结晶时差, 印支造山运动, 岩浆动力学

Abstract:

Although many  factors can  influence  the duration of cooling-crystallization process of a granitic melt, such as  the
initial temperature of the granite melt, the crystallization temperature, the temperature and thermal conductivity of country rocks,
the depth of emplacement, the volume of granite pluton, the radiogenic heat of the granite and other thermophysical parameters,
the calculated results showed that the volume of granite body is the most important factor. For example, using the same parameters
as  the Qitianling granite batholith, but with different volumes,  the △tECTD values  for granite plutons with various outcrop areas are caculated as follows: 42.1 Ma for Qitainling granite batholith of 520 km2
； 0.7 Ma for granite pluton of 50 km2
； 0.05 Ma for
granite pluton of 4 km2
. Using tabular model and in combination with zircon U-Pb age (161 Ma), the emplacement age ( tE) of the
Qitainling granite batholith is estimated to be 206.Ma, while the tE is 203 Ma for cubic model . This suggests that the emplacement
of the Qitainling granite magma was in the Indosinian Period. Using least squares regression procedure, a best regression equation
( tRb= 0.9928×t Zr+2.158) with high correlation coefficient  (R=0.997)  is  fitted  for 205 pairs of zircon U-Pb ages and whole  rock
Rb-Sr ages of granite plutons. The frequency analysis of 205 individual values of difference between each pair (Δt= t Zr-tRb ) for
granite plutons shows symmetrical normal distribution  (skewness CSK=-0.148; kutrocess CKU=6.771) with  the Δt  median of 0.0
Ma and  the Δt mode of 2.0 Ma. These statistical characteristics  indicate  that,  for granites as a whole, both zircon U-Pb dating
age and whole-rock Rb-Sr  isochron ages are consistent within error and  thus  the zircon U-Pb dating age cannot  represent  the
emplacement age of a granite batholith.   Therefore, we recognized  that  the conclusion of “U-Pb closure  temperature >900 ℃”
drawn from experiment of U-Pb diffusion in zircon by heating zircon crystals by Lee et al (1997) and Cherniak et al (2000) may
only be applied  to understanding and explaining  the behaviour of U-Pb system of  inherited zircons within granites during  the
heating and partial melting process  in source  regions of  the granites, but can not be applied  to  the zircon crystallized  from  the
granitic melts. The time difference of the whole rock Rb-Sr isochron ages (ΔtRb-Rb=15.7~32 Ma) between syntexis type granites
(Longtang granodiorite, Changtai granodiarite) and its co-magmatic volcanic rocks, and the two stages of U-Pb dating results (with
age difference of 24~50 Ma) of zircons  from some granite plutons  in South China also support existence of  large emplacement-
crystallization time difference (ECTD) in granite batholiths.

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