U-Pb dating of zircons from the Four-Girl Mountain Pluton in the Songpan-Garzê Terrane, and the relationship between the pluton and the Wenchuan Ms 8.0 Earthquakeiof 2008

Abstract：The granitoid intrusions in the eastern part of the Songpan-Garzê terrane were mainly formed in Late Indo-Chinese-Early Yenshanian period, and the Four-Girl Mountain Granite（FGMG）is one of these intrusions. Zircons from FGMG commonly have characteristics of magma zircons. LA-ICP-MS zircon U-Pb dating of 23 zircon grains from biotite granodiorite of the FGMG yielded a weighted 206Pb/238U mean age of (191±1)Ma (Early Yenshanian), which is considered to be the magmtic crystallization age of FGMG. This age has great significance in understanding the initial formation time of the Longmenshan fault belt. The NW-trending Jingchuan-Lixian tectono-magmatic belt defined during the previous regional geological survey can’t represent the product of the collision between he Yangtze massif and the Songpan-Garzê terrane, although the major axes and mineral directions of most single granitic masses assume parallel NW-trending arrangement in the Songpan-Garzê terrane. Therefore, the NE-trending Douhu-Jingchuan-Xiaojing-Heishui tectono-magmatic belt should be reasonably established. The Four-Girl Mountain Granite（FGMG）pluton is a representative granitic mass and represents the product of partial fusing in the low velocity layer existent in the middle-lower crust of the Songpan-Garzê terrane when the Yangtze massif was wedged into the Songpan-Garzê terrane along the Longmenshan structural belt. Deep geophysical exploration shows that the Four-Girl Mountain area has “roots of mountain”, which are made up of many great granitic plutons. These roots of mountain did not cause the Wenchuan Earthquake, but might have weaken the damage influence during the northwestward seismic wave propagation towards the Tibetan Plateau, and probably prevented the southwestward expansion of the Longmenshan fault belt when the Wenchuan Earthequake occurred. In other words, the probability of aftershocks that occurred along the southwest section of the Longmenshan fault belt was reduced by the roots of mountain.