火山岩岩石系列类型及形成环境

2025-03-16 12:40:54
推荐回答(1个)
回答1:

(一)火山岩岩石系列

在Winchester等(1977)提出的基于高场强元素对岩石分类的图解上,营城组火山岩基本落在碱性或偏碱性岩石范围内,如碱性玄武岩、粗安岩、粗面岩和碱流岩等,而火石岭组火山岩多位于靠近碱性或偏碱性岩石的投影区(图2-51)。Floyd& Winchester(1975)和Winchester & Floyd(1976)基于高场强元素的不活动特征,提出了一系列区分碱性和拉斑玄武岩系的图解,如P2O5—Zr、Nb/Y—Zr/(P2O5×104)、TiO2—Zr/(P2O5×104)、TiO2N/b关系图等,这些图解对于判别蚀变火山岩的岩系归属特别有效。长岭断陷及邻区营城组玄武质火山岩在上述图解上均投影在碱性玄武岩区,而火石岭组部分样品则落在拉斑玄武岩区,火石岭组玄武质岩石较之营城组玄武质岩石总体具有偏低的高场强元素含量。综合地球化学组成对长岭断陷及邻区深层火山岩岩系归属判别表明,营城组火山岩属碱性系列,而火石岭组火山岩的岩系归属介于碱性与钙碱性岩系之间。

(二)火山岩形成的大地构造环境及其地球动力学意义

对本区火山岩的大地构造背景分析采用在地质作用过程中相对稳定的高场强元素,对火石岭和营城两组地层中的玄武质岩石进行了构造环境综合判别。在有关构造判别图解上(图2-52),两组地层中玄武质岩石的投影点均落在板内环境,说明自晚侏罗世开始,松辽盆地应处于大陆裂谷的伸展引张环境。

图2-51 松南长岭凹陷及邻区火山岩Nb/Y—Zr/riO2岩石分类图

(底图据Winchester等,1977)

图2-52 长岭凹陷及邻区深层玄武岩Zr—Ti构造环境判别图

(底图据Pearce & Cann,1973)

对营城组流纹质火山岩的判别也得出类似结论,如在Pearce等(1984)提出的花岗质岩石构造判别图解上,样品点均落在板内区;在Maniar&Piccoli基于主量元素提出的构造环境判别图解上,样品点均落在后碰撞(POG)、或与裂谷有关(RRG)及与大陆造陆抬升有关花岗岩(CEUG)的伸展引张环境,而明显偏离各类与俯冲作用有关的花岗岩投影区,指示其形成与俯冲作用缺乏明显联系。

综合火山岩的岩石地球化学特征、中生代板块构造演化历史分析,松辽盆地是一有弧后背景(受古板块俯冲影响)的大陆裂陷盆地,火山岩是大陆裂陷作用的产物。

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