摘要:
拉萨地体南缘桑日地区中酸性火山岩出露广泛,多期次岩浆活动的叠加使得区内岩浆岩成分复杂、形成时代跨度大。为深入研究该地区火山岩的形成时代、构造背景和成因机制,本文选取桑日县西侧塔木村附近的中酸性火山岩为研究对象,开展锆石U-Pb定年、全岩主微量及Sr-Nd同位素分析。锆石U-Pb年龄表明桑日地区塔木村火山岩形成于91.7～90.8 Ma,属于晚白垩世。桑日地区晚白垩世火山岩SiO2含量为55.79%～74.26%,MgO含量为1.31%～4.34%,同时具有高Sr(平均含量为779×10–6)、低Y(8.36×10–6～15.85×10–6)、Yb(0.92×10–6～1.38×10–6)含量及高Sr/Y比值（36.28～105.59）的特征,表明桑日晚白垩世火山岩为埃达克质岩石。同时该地区晚白垩世火山岩富集大离子亲石元素Rb、Sr和Pb,亏损高场强元素Nb、Ta和Ti,具有较低的87Sr/86Sr初始值（0.704152～0.704515）和较高的143Nd/144Nd初始值（0.512676～0.512 750）。综合岩石地球化学特征和区域地质资料,桑日地区晚白垩世火山岩形成于新特提斯洋壳北向俯冲消减有关的活动大陆边缘环境,是新特提斯洋壳部分熔融的产物,洋壳熔体上升过程中伴有地幔楔物质的加入并经历了分离结晶作用。
Abstract:
Intermediate-acid volcanic rocks are widely exposed in the Sangri area of the southern margin of Lhasa terrane. The superposition of multiple magmatic activities over a long formation time has made the composition of magmatic rocks in the area complex. To study the formation age, tectonic setting, and genetic mechanism of volcanic rocks in this area, we selected the intermediate-acid volcanic rocks near Tamu Village in the west of Sangri County as the research object, conducted zircon U-Pb dating, and analyzed whole rock major and trace elements and Sr-Nd isotopes. The SiO2 and MgO contents in Late Cretaceous volcanic rocks in the Sangri area were 55.79%～74.26% and 1.31%～4.34%, respectively. In addition, they have high Sr(average 779×10–6), low Y（8.36～15.85）×10–6 and Yb（0.92～ –1.38）×10–6, and a high Sr/Y ratio（36.28～05.59）. Geochemical characteristics showed that the Late Cretaceous volcanic rocks in the Sangri area are adakite rocks. Moreover, the Late Cretaceous Sangri volcanic rocks are enriched in large ion lithophile elements（LILE）, such as Rb, Sr, and Pb, and depleted in high field strength elements（HFSE）, such as Nb, Ta, and Ti, with low initial values of 87Sr/86Sr（0.704 152～0.704 515） and high initial values of 143Nd/144Nd（0.512 676～0.512 750）. Based on petrogeochemical characteristics and regional geological data, the Late Cretaceous volcanic rocks in the Sangri area formed in an active continental margin environment related to the northward subduction of the Neo-Tethyan oceanic crust and were produced by the partial melting of the Neo-Tethyan oceanic crust. The rising process of ocean crust melting was accompanied by the addition of mantle wedge material, which led to fractional crystallization.
下载: