藏南确当地区高Sr/Y比值二云母花岗岩的形成机制及其构造动力学意义

High Sr/Y two-mica granite from Quedang area,southern Tibet,China:Formation mechanism and tectonic implications

最近的研究表明,喜马拉雅造山带普遍经历了大于30Ma的高温变质和部分熔融作用,形成了不同规模的混合岩或花岗岩。厘定这些地壳深熔岩浆的地球化学性质及其形成机理对于深化认识大型碰撞造山带的岩浆作用及其构造物理效应具有重要意义。在雅拉香波穹隆核部及其南部发育一系列类似的二云母花岗岩岩体,其中确当花岗岩侵入到二叠纪(?)复理石地层内。全岩地球化学测试结果表明,确当花岗岩具有与雅拉香波花岗岩、打拉花岗岩类似的特征,即①高SiO2(68.2%～69.3%)和铝(Al2O3>15.14%),低铁(TFeO<2.0%)和镁(MgO<1.5%),为富钠过铝质花岗岩;②富集LREE,亏损HREE,平坦的Ho到Lu稀土元素分布样式(Ho/Yb)N=1.2～1.4),无或微弱Eu负异常(Eu/Eu*=0.9～1.0);③较低的Y(<8.1×10-6)和Yb(<0.7×10-6)含量,较高的Sr/Y(>37.5)和La/Yb(>29.3)比值。这些特征表明,确当花岗岩是以角闪岩为主和变泥质岩为辅组成的源区发生部分熔融的结果。

Recent studies indicate the Himalayan orogen has commonly experienced 〉30 Ma episodes of high temperature metamor- phism and partial melting that resulted in the formation of migmadte and granites. Characterizing the geochemical nature of and mechanisms for these anatectic events is critical to understand geochemical and tectonophysical effects of magmatic processes in large colli- sion belts. A series of similar two-mica granites occur in the core and to the south of the Yardoi gneiss dome. The Quedang pluton intruded into the Permian flysch formation. Similar to Yardoi and Dala two-mica granites, whole rock geochemical data of the Quedang granites show the following characteristics： （1） high SiO2 （68.2-69.3 wt%） and Al2O3（〉15.1 wt%）, low TFeO（〈2.0 wt%） and MgO（〈1.5 wt%）; （2） enriched in LREE, depleted in HREE, flat HREE patterns for elements from Ho to Yb with （Ho/Yb）N=1.2- 1.4, and weak or no Eu anomalies with Eu/Eu＊ =0.9-1.0; （3） relatively low contents of Y（〈8.1 × 10^-6） and Yb （〈0.7 ×10^-6）, high St/Y（〉37.5） and La/Yb（〉29.3） ratios. These data indicate that the Quedang granites were derived from partial melting of a source region dominated by amphibolite with subordinate metapelite.