石榴子石反应边及其地质意义--以大兴安岭第四纪火山岩为例

Reaction rims of garnet and its geological significant:A case study on Quaternary volcanics in Da Hinggan Mountains

大兴安岭第四纪火山岩(包括诺敏河火山区和哈拉哈河-绰尔河火山区)地幔包体中含有少量石榴子石,普遍发育矿物反应边。根据显微照片和BSE图像特征,石榴子石反应边可分为3类:(1)冠冕状石榴子石反应边,包裹在石榴子石矿物外部,具有一期或者多期反应的特征,厚度通常为0.1~1 mm,反应边矿物组合为Opx+Glass、Cpx+Glass、Grt+Sp等;(2)脉状石榴子石反应边,深入矿物内部,呈脉状分布特征,宽度为几百微米,长度为几毫米,通常为一期反应的产物,反应边矿物组合为Opx+Glass;(3)微裂隙石榴子石反应边,石榴子石内部发育极细的网脉状分布的微裂隙,沿着裂隙发育几微米到十几微米的薄层反应边,反应边矿物组合为Sp+Glass。结合电子探针分析研究,将这些石榴子石反应边的成因机制归为两类:(1)岩浆/熔体与石榴子石反应成因。石榴子石反应边的化学成分与石榴子石明显不同,K_(2)O、Na_(2)O等元素含量比石榴子石高一两个数量级,暗示地幔硅酸盐熔体或者碱性岩浆参与石榴子石分解反应,石榴子石外面包裹的冠冕状反应边大多属于该类成因;(2)等化学分解成因(isochemical breakdown)。石榴子石反应边的总体成分与石榴子石相当,无明显的外源物质加入,暗示在封闭环境下石榴子石发生等化学分解,脉状石榴子石反应边和微裂隙石榴子石反应边通常属于该类成因,冠冕状石榴子石反应边是否属于该类成因有待进一步研究。大兴安岭第四纪地幔包体中发现两种不同成因类型的石榴子石反应边,记录了地幔熔体反应和富集的过程以及地幔温度变化和隆升历史,为深入研究地幔地球化学成分变化和热演化历史提供了重要证据。

Mantle xenoliths,with minor garnets,have been found in the Quaternary volcanic rocks in Da Hinggan Mountains.Reaction rims,documenting significant records of mantle dynamic evolution,are common in most of the garnets.Optical photomicrographs and BSE pictures demonstrate three types of reaction rims of garnets,(1)coronal rims,a thin layer of 0.1~1 mm enclosing the relic garnet,with a typical mineral assemblage of Opx+Glass,Cpx+Glass or Grt+Sp;(2)vein rims,vein-like strips in several 100μm in width and several millimeter in length,with a typical mineral assemblage of Opx+Glass;(3)microfracture rims,very thin reaction rims of several to several ten micrometers thick along microfractures in garnet,with a typical mineral assemblage of Sp+Glass.On the aspect of petrogenesis,these reaction rims were divided into two types,(1)origin of magma/melt reaction with garnet:Some reaction rims have quite different bulk chemical composition from the precursor garnets.For example,K_(2)O and Na_(2)O compositions in garnet reaction rims are one or two orders of magnitude higher than those in precursor garnets,indicating that compositions from mantle melts or alkaline magmas were induced to the mineral assemblages of reaction rims;(2)origin of isochemical breakdown of garnet:The other reaction rims have nearly the same bulk chemical compositions as the precursor garnet,indicating isochemical breakdown of garnet without any significant chemical compositions from mantle melts or other minerals,especially the vein rims and microfracture rims.Hence two types of garnet reaction rims with quiet different origins were found in Quaternary volcanic rocks in Da Hinggan Mountains.These garnet reaction rims provide evidences of K_(2)O-and Na_(2)O-rich melt reactions and mantle refertilizing processes,as well as evidences of evolution of mantle p-T conditions.Studies on garnet reaction rims may light on further understanding on chemical evolution and thermal history of lithospheric mantle in Da Hinggan Mountains.