西藏冈底斯带则许地区日穷啊晚白垩世花岗闪长岩年代学、地球化学特征及成因

Geochronology, Geochemistry and Genesis of Late Cretaceous Riqionga Granodiorite in the Zexu Area, Gangdese Belt, Tibet

西藏则许地区日穷啊花岗闪长岩体产出于冈底斯弧背断隆带。LA-ICP-MS锆石U-Pb测年结果显示,日穷啊花岗闪长岩的形成时代为86.4±0.6 Ma(MSWD=0.52),属晚白垩世。岩石具高钾钙碱性特征,SiO_(2)含量为62.42~64.93 wt%,Na_(2)O/K_(2)O=0.87~1.35,略富钠,铝饱和指数(A/CNK)为0.83-1.06,Al_(2)O_(3)含量为14.86~16 wt%,为准铝质岩石。稀土元素总量为164.87×10^(-6)-233.44×10^(-6),(La/Yb)_(N)为11.46-18.27,负铕异常变化较大(δEu=0.17-0.86),稀土元素配分模式为轻稀土元素强烈富集的右倾型。岩石富集Rb、Ba、Sr而亏损Nb、Zr等元素,具有弧岩浆岩的地球化学特征。日穷啊花岗闪长岩体地球化学成分具有明显I型花岗岩特征,源区物质具有壳幔混合成因的特点,是晚白垩世雅鲁藏布江洋板片向北俯冲于拉萨地块之下的产物。

The Riqionga granodiorite of the Zexu area,Tibet,crops out in the Gangdese Retroarc Uplift Belt.Its LA-ICP-MS zircon U-Pb is dated at 86.4±0.6 Ma(MSWD=0.52).The intrusion is characterized by high-K calc-alkaline,with SiO_(2) of 64.42-64.93 wt%,Na_(2)O/K_(2)O of 0.87-1.35,A/CNK of 0.83-1.06,and Al_(2)O_(3) of 14.86-16.00 wt%.TheΣREE is 164.87-233.44μg/g,(LA/Yb)_(N) is 11.46-18.27,the negative Eu anomaly changes greatly(δEu=0.17-0.86),and the REE distribution pattern is right-leaning type with strong LREE enrichment.The rocks are enriched in Rb,Ba and Sr,but depleted in Nb and Zr,and have geochemical characteristics of island arc magmatic rocks.The Riqionga granodiorite has obvious geochemical characteristics of Ⅰtype granite,and a source of crust-mantle mixing,which shows that the granite is the product of the northward subduction of the Yarlung Zangbo oceanic plate beneath the Lhasa block in the late Cretaceous.