广州增城地质公园安山质-流纹质火山岩地球化学特征及Rb-Sr年龄测定

广州市增城地质公园发育有大量的燕山期安山岩和流纹岩,由于缺少详细的岩石地球化学研究,这些火山岩的成因和所代表的大地构造意义一直未明确。文章对上述火山岩进行了较为系统的全岩地球化学以及同位素地球化学分析。研究结果显示,安山岩具有安第斯型火山岩特点,显示Nb、Ta、Sr和Ti的亏损,Isr值介于0.70332~0.7144,平均值0.7092,岩石稀土总量较低(ΣREE=158.9×10^-6~215.0×10^-6),平均值186.8×10^-6,轻重稀土元素分异较弱((La/Yb)N5.06~9.87),平均值7.01,Eu负异常不明显(δEu=0.80~1.38),平均值δEu=0.94。流纹岩具有高钾特点,有明显的Ba、Sr、P、Eu、Ti负异常和Pb、Yb正异常,其Isr值介于0.71393~0.73650,平均值0.72615,岩石稀土总量较低(ΣREE=93.4×10^-6~481.5×10^-6),平均值285.7×10^-6,轻重稀土元素分异弱((La/Yb)N=0.65~9.51),平均值4.35,Eu负异常很明显(δEu=0.01~0.03),平均值δEu=0.02,全岩Rb-Sr同位素年龄为(112±12)Ma。综合的地球化学研究表明,增城地质公园安山质-流纹质火山岩均属壳幔混合成因,其中安山岩以幔源为主,而流纹岩则以壳源为主,分别形成于早侏罗世和早白垩世太平洋板块俯冲碰撞挤压的构造背景下。这对华南地区中生代构造演化的深入认识具有重要的地质意义。

Rb-Sr dating and geochemistry of andesitic-rhyolitic volcanics in the Zengcheng Geopark, Guangzhou, Guangdong Province

A large number of Yanshanian andesite and rhyolite rocks are developed in the Zengcheng Geopark in Guangzhou.Due to the lack of detailed petrogeochemical studies,their genesis and the tectonic significance have not been clarified.Therefore,a more systematic analysis was conducted for the whole-rock geochemistry and isotopic geochemistry of these volcanic rocks.The results show that the andesitic rocks are somewhat analogous to the Andes-type volcanic ones,with obviously depleted Nb,Ta,Sr and Ti and low Isr values(0.70332-0.7144,averaging 0.7092),lower rare earthΣREE(ΣREE=158.9×10^-6-215.0×10^-6,averaging 186.8×10^-6),with obvious LREE and HREE differentiation((La/Yb)N5.06-9.87,averaging7.01)and negative Eu anomalies(δEu=0.80-1.38,averaging 0.94)in the chondrite-normalized REE distribution patterns.The rhyolitic rocks have high potassium contents with significant negative Ba,Sr,P,Eu,Ti anomalies and positive Pb and Yb anomalies.They are characterized by the Isr values ranging from 0.71393 to 0.73650(averaging 0.72615),lower rare earthΣREE(ΣREE=93.4×10^-6-481.5×10^-6,averaging 285.7×10^-6)with obvious LREE and HREE fractionation((La/Yb)N 0.65~9.51,averaging 4.35)and negative Eu anomalies(δEu=0.01-0.03,averaging 0.02)in the chondrite-normalized REE distribution patterns.The whole rock Rb-Sr Isotopic age is 112±12 ma.The integrated geochemical studies demonstrate that both the andesitic and rhyolitic magmas are of crust-mantle mixing origin,of which the andesitic magma was mainly originated from the mantle and the rhyolitic one mostly from the crust.The andesite rocks and rhyolitic volcanics were formed under the tectonic background of Pacific plate subduction and collision in early Jurassic and early Cretaceous respectively.