摘要
岩浆混合作用是造成火成岩多样性的主要原因之一,也是诱发火山喷发的重要机制。以往的研究多集中于基性和酸性岩浆之间的混合作用,但近年来酸性岩浆之间的混合作用受到越来越多的关注和研究。本文报道了浙东小雄破火山一个次级火山口内粗面质和流纹质两种酸性岩浆之间的混合现象。野外调查及岩相学研究显示,粗面质岩浆多呈大小不一的条带状以及透镜体状分布于流纹质岩浆内,局部发生扩散,粗面岩中斑晶大多为粗大的正长石斑晶,强烈熔蚀且聚斑结构普遍;在副矿物聚晶(由钛磁铁矿+磷灰石+锆石组成)的周围常可见反应边结构。流纹岩的斑晶主要由正长石、透长石及石英组成,晶体粒径较小,且熔蚀现象不发育。全岩主、微量元素特征及其他地质证据均显示,两种酸性岩浆之间以机械混合为主,其地球化学成分变化趋势主要受结晶分异过程控制。粗面质及流纹质岩浆在矿物组成、结构等方面的差异表明两者来源于同一层状岩浆房内的不同部位,其中粗面质岩浆应代表岩浆房底部及边部富晶体、贫熔体的粥状层部分(正长石+磁钛铁矿+锆石+磷灰石);而分异程度较高的流纹质岩浆则聚集于岩浆房上部形成富熔体、贫晶体的部分。两种酸性岩浆的混合现象是它们在地壳浅部层状岩浆房内自混合的结果,这一过程可能受岩浆房底部基性岩浆的聚集作用所控制,当更热、更基性的岩浆聚集时,岩浆房下部晶粥区内的粗面质岩浆迅速升温、活化,从而向上运移并与上部富熔体贫晶体的流纹质岩浆发生自混合作用。这一发现为我们理解中国东南沿海地区晚中生代大规模酸性火山喷发及岩浆演化机制、岩浆房结构提供了重要的参考,同时也为认识地壳浅部岩浆房内岩浆之间的自混合作用提供了可靠的例证。
Magma mingling(mixing) is one of the main mechanism for the diversity of igneous rock,as well,it is a very important trigger to volcanic eruption.Previous studies have been focused on the magma mixing between mafic and silicic magmas.However,in recent years,more and more petrologists realized that the mingling between silicic magmas could be more common and important.In this paper,we study the phenomenon of magma mingling between trachytic and rhyolitic magmas in a small crater of the Xiaoxiong Caldera,eastern Zhejiang Province.On the outcrops,trachytic lumps scatter in the rhyolite,showing various sizes and with band,lense or balloon shapes.The phenocrysts of trachyte are composed of orthoclase+titanomagnetite+apatite+zircon and often aggregated to a glomerocryst.We can find the reaction texture in the titanomagnetite+apatite+zircon aggregate,which are surrounded by biotites.It is noted that the orthoclase phenocrysts are generally large and heavily resorbed.In contrast,phenocrysts in rhyolite are mainly orthoclase and quartz,they are usually small and the resorptions of feldspar are rare.Major and trace elements characters as well as other geological evidences show that both silicic magmas are originated from a common crust source,and have experienced mechanically mingling.Their geochemical variations are mainly controlled by fractional crystallization.The differences of whole-rock compositions,mineral compositions,and structures between trachytic and rhyolitic magmas indicate that both magmas should be derived from different parts of one chamber.The trachytic magma could represent the crystal mush(orthoclase+titanomagnetite+zircon+apatite) in bottom or wall of the chamber,while the more evolved rhyolitic magma could be stored at the upper part of the chamber,where is rich in melt and poor in crystals.We believe that the mingling between two silicic magmas is the result of self-mingling in a shallow magma chamber.This process may be controlled by the ponding of the malic magma at the bottom of the silicic magma chamber.That is,when hot mafic magma ponds at the base of magma chamber,the trachytic magma in the bottom of the chamber is rapidly heated up and rejuvenated,and then it moves up and mingles with the upper rhyolitic magma This study may provide an important reference for-understanding the large-scale silicic volcanic eruptions,magma evolution and magma chamber structures in the southeastern coastal area of China,as well give a reliable case of magma self-mingling effect in shallow magma chamber.
作者
高丽
杨祝良
余明刚
GAO Li;YANG ZhuLiang;YU MingGang(Chinese Academy of Geological Sciences,Beijing 100037,China;Nanjing Center of China Geological Survey,Nanjing 210016,China)
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2020年第4期1015-1029,共15页
Acta Petrologica Sinica
基金
国家重点研发计划项目(2016YFC0600203)
国家自然科学基金项目(41702061)
中国地质调查局项目(DD20179384)联合资助.
关键词
酸性岩浆
岩浆房过程
自混合
小雄破火山
白垩纪
浙东
Silicic magma
Magma chamber process
Self-mingling
Xiaoxiong Caldera
Cretaceous
Eastern Zhejiang
作者简介
第一作者:高丽,女,1995年生,硕士生,矿物学、岩石学、矿床学专业,E-mail:glkorea16@163.com;通讯作者:杨祝良,男,1967年生,研究员,长期从事中国东南部中生代火山岩调查研究,E-mail:180818878@qq.com。
