The Oyotún Formation volcanic rocks are extremely widely distributed Mesozoic volcanic rocks in Northwestern Central Andes, and have important implications for understanding the Early Cretaceous crust-mantle interactions in Central Andes. The vol?canic rocks are exposed in a small area with thin thickness and uniform lithology in the west, and are outcropped extensively with large thickness and more complex lithofacies in the west. LA-ICP-MS zircon U-Pb dating shows that the Oyotún volcanic rocks are concentrated in the range of 139.7～131.0Ma. Geochemically, the Oyotún Formation volcanic rocks show the SiO2 range from 54.44％to 72.33％, medium enrichment of MgO (0.56％～1.55％), metaluminous-strong peraluminous nature (A/CNK=0.98～3.51), enrichment of Cs, Rb, Th, U, Pb and LREE with indistinct negative europium anomalies, and depletion of Nb, Ta, Ti, P, suggesting obvious characteristics of volcanic arc. The εHf(t) of Oyotún Formation volcanic rocks are between 3.37 and 8.42, with the εHf(t) values at the bottom significantly lower than the values at the top, indicating that the rocks possibly originated from the process of low degree partial melting of the juvenile basaltic crust through the action of plate fluid, with the contamination of Archean ancient continent materials. The degree of contamination decreased when the ages became younger. The juvenile basaltic crust was probably derived from depleted mantle in an extension setting on the active continental margin.%Oyotún组火山岩为中安第斯西北缘中生代出露范围最广的一套火山岩,对研究中安第斯早白垩世壳幔相互作用具有重要意义.研究区西部Oyotún组火山岩出露面积小,厚度薄,岩性单一,而东部出露范围大,厚度厚,岩性复杂.LA-ICP-MS锆石U-Pb定年表明,Oyotún组火山岩时代集中在139.7～131.0Ma之间.主量元素SiO2含量介于54.44％～72.33％之间,中等富MgO(0.56％～1.55％),偏铝-强过铝,A/CNK=0.98～3.51,富集Cs、Rb、Th、U、Pb和轻稀土元素,无明显Eu异常,亏损Nb、Ta、Ti、P等,具明显的火山弧特征.Oyotún组火山岩εHf(t)值在3.37～8.42之间,且底部εHf(t)值显著低于顶部,指示其可能形成于活动陆缘弧后伸展背景下,由亏损地幔衍生的新生玄武质下地壳,经板片流体作用而产生的低程度部分熔融过程,并受到太古宙古老陆壳物质的混染,其混染程度随时代变新而逐渐减小.
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