中文摘要：

对青海省同仁县多禾茂地区火山岩进行系统的元素同位素地球化学研究表明：该地区白垩纪地层中发育的中基性火山岩，属于碱性火山岩系，岩石类型主要为玄武岩，该套玄武岩具有低且变化较小的SiO2质量分数（44.98%-48.20%），低MgO的质量分数高，变化较大（8.15%-10.98%），具有较高的Cr（208×10^-6-418×10^-6）和Ni（166×10^-6-231×10^-6）质量分数。所有样品都表现出轻稀土富集的右倾平滑分布模式，轻重稀土分异中等，（La/Yb）CN=10.09-27.2，重稀土弱分异，（Gd/Yb）CN=2.92-4.67，Eu异常不明显，Eu＊/Eu=0.99-1.04。Ba，Nb-Ta和Th-U不亏损，Rb，K相对亏损，Sr正异常。该区火山岩具有亏损的Sr-Nd同位素组成，Sr-Nd同位素组成变化范围不大[87Sr/86Sr（t）=0.7033-0.7039，εNd（t）=6.13-8.03，t=110 Ma]，与Hawaii-OIB的Sr-Nd同位素组成相似。其所有地球化学特征表明该火山岩具有类似OIB的特征，可能是含石榴子石橄榄岩低度部分熔融的产物。结合对西秦岭地质构造背景和演化历史的分析，暗示该地区晚中生代岩浆源区来源于软流圈，其起源可能与岩石圈拆沉作用，软流圈地幔上涌和岩石圈伸展减薄有关，可为秦岭大别造山带中生代岩石圈演化提供了有利的直接证据。

英文摘要：

This paper studied the major and trace elements and Sr-Nd isotopic dating of Mesozoic volcanic rocks from Duohemao area, in the Tongren County, Qinghai Province. These volcanic rocks are close to alkaline series, which belong to basalt. They have low SiO2 （44. 98%-48.20%）,but have relatively high MgO（8.15%-10.98%）,Cr（208× 10^-6 ～418×10^-6） and Ni （166× 10^-6～231 × 10^-6 ）. All samples exhibit LREE-rich right-oriental pattern with weak Eu anomaly of （La/Yb）CN=10.09-27.2, （Gd/Yb）CN =2.92-4.67, Eu＊/Eu = 0.99-1.04. On primitive mantle-normalized diagram,volcanic rocks show no depletion in Ba, Nb-Ta, Th-U and relative depletion in Rb,K and with positive Sr anomaly. These volcanic rocks have depletion Sr- Nd isotopic compositions[^87 Sr/86 Sr（t） = 0. 703 3 -0. 703 9 ,εNd （t） = 6.13 - 8.03, t = 110 Ma], analogous to Hawaii-OIB without significant crustal contamination. It is indicated that these volcanic rocks may be derived from lower degree partial melting of a garnet-bearing peridotite in response to lithospheric delamination/detachment during the upwelling of asthenosphere and extension of lithosphere. The study provides direct evidence for the lithospheric evolution in Qinling- Dubie orogenic .belt.