松潘甘孜地块四姑娘山花岗岩锆石U-Pb年代学证据及与汶川地震的关系

松潘甘孜地体东部的花岗岩主要形成于印支晚期至燕山初期,其中四姑娘山花岗岩体的锆石普遍具有岩浆锆石的特征。通过四姑娘山花岗岩体的黑云母花岗闪长岩中的23粒锆石的锆石激光探针U-Pb定年,确定其岩浆结晶年龄为燕山早期(191±1)Ma,此年龄对解决龙门山断裂带形成的初始时间有重要意义。虽然在松潘甘孜地体多数岩体长轴走向和矿物定向均呈北西向,但过去区调中划分的北西向的金川—理县构造岩浆带不能代表这种楔入作用的产物,而应重新划分为北东向的道孚—金川—小金—黑水构造岩浆带,四姑娘山岩体是这个北东向构造岩浆带中的典型。这些花岗岩主要是扬子地块沿龙门山构造带向松潘甘孜地体内楔入导致松潘甘孜地体中下地壳低速层发生部分熔融的结果。地球物理资料显示,四姑娘山地区是有"山根"的,这些"山根"主要由巨大的花岗岩基组成,它们不是引起汶川地震的原因,但是减弱了汶川地震的地震波向西北方向的青藏高原传递,并对汶川地震时的龙门山断裂带向西南段的扩张起了一定的阻挡作用,即降低了汶川地震的大量余震在龙门山断裂带西南段发生的概率。

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

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.