西秦岭地区晚中生代基性火山岩地球化学特征及构造意义

本文对甘肃省夏河县红墙地区火山岩进行了系统的元素-同位素地球化学研究。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(45.32%~46.83%),低MgO含量高,变化较大(7.68%~9.48%),具有较高的Cr(228~441μg/g)和Ni(140~217μg/g)含量。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=9.9~22.9,重稀土弱分异,(Gd/Yb)CN=2.6~3.0,Eu异常不明显,Eu/Eu*=1.04~1.08。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)=0.702766~0.702920,εNd(t)=8.42~10.15,t=110Ma),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,可能是含软流圈地幔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,早期深俯冲再循环扬子地壳对源区也有一定贡献,其起源可能与构造走滑作用导致岩石圈伸展碱薄和软流圈地幔上涌有关,这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。

GEOCHEMICAL CHARACTERISTICS OF LATE MESOZOIC MAFIC VOLCANIC ROCKS FROM WESTERN QINLING AND THEIR TECTONIC IMPLICATIONS

This paper presents major and trace elements and Sr-Nd isotopic data of Mesozoic volcanic rocks from the Hongqiang area, Xiahe County, Gansu Province. These volcanic rocks belong to calc-alkaline and high-k calc-alkaline basalts, with low SiO2 (45.32%~46.83%), high MgO (7.68%~9.48%), and Cr(228~441μg/g), Ni(140~217μg/g). All samples exhibit LREE-enriched right-declined REE patterns with weak Eu anomaly (La/Yb)CN=9.9~22.9,(Gd/Yb)CN=2.6~3.0, Eu/Eu=1.04~1.08). On a primitive mantle-normalized diagram, they show no depletion in Ba, Nb-Ta, Th-U and relative depletion in Rb, K with positive Sr anomaly. The Hongqiang basalts have unradiogenic Sr (87Sr/86Sr(t)=0.702766~0.702920) and highly radiogenic Nd (εNd(t) =8.42~10.15,t=110Ma) compositions,analogous to Hawaiin OIB escaping significant crustal contamination. Combined major, trace element and Sr-Nd isotope data indicate that these basalts were derived from lower degree partial melting of asthenosphere mantle in response to lithospheric extension/thinning related to the post-collisional lithospheric evolution in the Qinling-Dabie Orogen.