辽西中侏罗世高Sr低Y型火山岩的成因及其地质意义

Genesis and geological significance for the middle Jurassic high Sr and low Y type volcanic rocks in Fuxin area of west Liaoning, northeastern China.

通过对辽西阜新地区中侏罗世蓝旗组火山岩的岩石地球化学研究,燕山地区存在着中侏罗世高Sr低Y型火山岩。这些高Sr低Y型火山岩为高铝高钠低镁(Al2O3>15％,Na2O≥3.37％,MgO≤1.33％,Mg＃≤0.35)的安山岩-英安岩,高Sr(≥541μg/g)低Y、Yb(Y≤19μg/g,Yb≤1.35μg/g)含量及高Sr/Y(≥40)比值,强烈的稀土元素分馏((La/Yb)N>17,(No/Yb)N>1.2)),Eu异常不明显(0.82～0.96),高场强元素(如Nb、Ta)相对亏损。这些特征与太古代英云闪长岩-奥长花岗岩-花岗闪长岩(TTG)岩套相似。我们认为这些火山岩起源于古老的下地壳玄武质岩石的部分熔融,其形成可能与玄武质岩浆的底侵作用有关。辽西中侏罗世蓝旗组高Sr低Y型火山岩对探讨燕山造山带岩浆起源及地球动力学具有重要意义。

The middle Jurassic high-Sr and low-Y type volcanic rocks have recognized in Yanshan area based on thegeochemical data for the volcanic rocks of Lanqi Formation in Fuxin area of West Liaoning, northeastern China. The volcanic rocks are high-Al (A12O3>15%)、high-Na(Na2O≥3. 37%, Na2O/ K2O >1)and low Mg (MgO≤1. 33%, ≤0. 35 in Mg#) andesites and dacites (SiO2≥60. 86%), which have high Sr (≥541 μg/g) and low Y and Yb(Y≤19μg/g, Yb≤1. 35 μg/g) contents, as well as high Sr/Y ratios(≥40), and are characterized by strongly fractionated REE((La/Yb)N>17, (Ho/Yb)N >1. 2) and absence of Eu negative anomaly(0. 82-0. 96), and relative depletion in the high field strength elements (such as Nb, Ta), resembling Archean trondhjemite- tonalite- graniodiorite suites (TTG). We think that the volcanic rocks were derived from the partial melting of ancient basaltic rocks in the lower crust and related to the underplating of basic magma. The high Sr and low Y volcanic rocks have an important geological significance in discussion on magma derivation and geodynamics in Yanshan orogenic belt.