甘肃西部红柳峡火山颈火山岩地球化学特征及意义

红柳峡火山颈是甘肃西部中生代以来唯一且保存完好的陆相火山口,颈内主要由玄武粗安岩充填,其w(SiO2)=51.79%～52.57%,w(Al2O3)=15.22%～16.38%,w(Na2O+K2O)=6.0%～7.00%,w(Na2O)>w(K2O),w(MgO)=5.02%～6.44%,Mg#=57～65,显示高铝、富镁及富钠贫钾之特征。区内火山岩形成于陆内伸展构造背景,源于过渡型地幔,Zr=254.2×10-6～290.2×10-6、Hf=5.595×10-6～6.313×10-6,Nb=30.85×10-6～34.52×10-6,Zr/Y=9.85～10.15,Hf/Th=0.98～1.04,Hf/Ta=3.05～3.20。Zr/Nb=8.24～8.63,接近于地壳平均值,表明在岩浆作用过程中可能发生过地壳混染作用。火山岩ΣREE含量高(294.49×10-6～334.04×10-6),并表现出轻稀土明显富集且轻稀土元素之间具明显的分馏作用,LREE/HREE=13.78～14.81,(La/Yb)N为18.99～21.56,重稀土元素之间分馏相对较弱,表现为重稀土分布曲线较为平坦,(Gd/Yb)N为2.58～2.73,无明显的Eu异常(δEu=0.93～0.99)和Ce异常(δCe=0.97～1.00),表明在岩浆演化过程中斜长石并未发生分离结晶作用以及上地壳物质的混染作用。可明显区分出早白垩世、晚白垩世及古近纪三期火山岩,表明火山颈形成时间为古近纪。红柳峡火山岩形成时代、构造属性及动力学背景的确定对于研究阿尔金断裂、北祁连北缘断裂有着十分重要的意义。

GEOCHEMICAL CHARACTERISTICS AND SIGNIFICANCE OF THE VOLCANIC ROCKS FROM VOLCANIC NECK OF HONGLIUXIA REGION IN WESTERN SECTION OF GANSU,CHINA

Volcanic neck at Hongliuxia filled mainly by basaltic-trachyandesites is the only well-preserved continental crater in the western Gansu since the Mesozoic era.The major（wt%） compositions of the volcanic rocks from Hongliuxia region is w（SiO₂）=51.79%～52.57%,w（Al₂O₃）=15.22%～16.38%,w（Na₂O）+w（K₂O）=6.0%～7.00%,w（Na₂O）>w（K₂O）,w（MgO）=5.02%～6.44%,Mg# = 57～65,which shows the high-alumina,magnesium-rich,sodium-rich but potassium-poor characteristics.Analyses of Zr=254.2×10-6～290.2×10-6,Hf=5.595×10-6 ～ 6.313×10-6,Nb=30.85×10-6～34.52×10-6,Zr/Y=9.85～10.15,Hf/Th=0.98～1.04,Hf/Ta = 3.05～3.20,indicate that the volcanic rocks formed in the tectonic setting of intra-continental extension and originated from transitional mantle.Zr/Nb=8.24～8.63,which is close to the crustal average,reveals that crustal contamination might occur in the process of magmatism.The ΣREE of the volcanic rocks is high（294.49×10-6～334.04×10-6）,light REE is rich,and there is not significant fractionation between the light REE.LREE/HREE=13.78～14.81,（La/Yb）_N=18.99～21.56,since the distribution plots of the heavy REE are flat,there is weak fractionation between the heavy REE.（Gd/Yb）_N=2.58～2.73,there is not obvious Eu anomaly（δEu=0.93～0.99） or Ce anomaly(δCe = 0.97 ～1.00,indicating that there is no plagioclase fractional crystallization but crustal contamination during the magmatic evolution.Three periods of volcanic rocks are recognized,which are formed in Early Cretaceous,Late Cretaceous and Paleogene respectively,and the volcanic neck was formed in Paleogene.The determination of formation age,tectonic attribute and geodynamic setting of the volcanic rocks from Hongliuxia region play very important role in the study of Altyn Tagh Fault and Northern marginal fault of Northern Qilian mountainous.