撞击玻璃与酸性火山玻璃的判别——以一块疑似芒农型撞击玻璃为例  

Distinction between tektites and acid volcanic glass:a case study of a suspectable Muong Nong-type tektites

作　　者：樊聪 陈宏毅 谢兰芳[1,2,3] 张晋瑜 殷一曼[1,2,3] 赵妍菲 黄延凤 白令安 FAN Cong;CHEN Hongyi;XIE Lanfang;ZHANG Jinyu;YIN Yiman;ZHAO Yanfei;HUANG Yanfeng;BAI Ling'an(Key Laboratory of Planetary Geological Evolution of Guangxi Provincial Universities,Guilin 541006,China;Guangxi Key Laboratory of Hidden Metallic Ore Deposits Exploration,Guilin 541006,China;Collaborative Innovation Center for Exploration of Nonferrous Metal Deposits and Efficient Utilization of Resources in Guangxi,Guilin 541006,China;Guangxi Nature Museum for Earth Memory,Lipu 546600,China)

机构地区：[1]桂林理工大学行星地质演化广西高校重点实验室,广西桂林541006 [2]桂林理工大学广西隐伏金属矿产勘查重点实验室,广西桂林541006 [3]桂林理工大学有色金属矿产勘查与资源高效利用省部共建协同创新中心,广西桂林541006 [4]广西地球记忆自然博物馆,广西荔浦546600

出　　处：《桂林理工大学学报》2024年第4期576-588,共13页Journal of Guilin University of Technology

基　　金：国家自然科学基金项目(42473051);广西自然科学基金项目(2021GXNSFAA220027);广西科技基地与人才计划项目(AD23026339,AD23026084)。

摘　　要：撞击玻璃与酸性火山玻璃特征相似,常不易区分。通过对一块疑似芒农型撞击玻璃样品开展岩石学、矿物学和地球化学特征研究,确认其属于过铝质钙碱性酸性火山玻璃类的北美黑曜岩系列,主要判别依据:(1)玻璃基质中含有自形斜长石、磁铁矿、黑云母等矿物,无冲击变质特征,矿物颗粒呈层状定向分布,气泡较少;(2)玻璃基质主微量元素成分在毫米尺度上均匀,斜长石具有清晰的岩浆分异结晶成分环带,黑云母属于原生(岩浆型)镁质黑云母,玻璃基质FeO、TiO_(2)、MgO平均含量(分别为0.56%、0.09%、0.07%)低于撞击玻璃,而K_(2)O含量(3.70%)高于撞击玻璃,A/CNK值为2.15~2.69,A/NK值为2.41~3.13,σ43值为0.45~0.79;(3)玻璃基质富集Rb、U、Th等大离子亲石元素,亏损Cr、Co、Ni等亲铁元素,Eu强烈负异常(δEu值为0.47~0.60)。以上特征类似于北美黑曜岩,而不同于撞击玻璃。玻璃基质SiO_(2)、Al_(2)O_(3)、CaO和Na_(2)O平均含量分别为80.21%、13.13%、0.69%和1.14%,稀土元素配分模式为右倾型(ΣLREE/ΣHREE值为8.17~8.96),与北美群撞击玻璃和黑曜岩均相似。通过样品的矿物类型、冲击变质特征、主微量元素、δEu值、Fe^(3+)/Fe^(2+)值等综合指标分析可有效区分撞击玻璃与酸性火山玻璃。Tektites and acid volcanic glass have similar characteristics and are therefore often difficult to distinguish.According to the petrological,mineralogical,and geochemical studies of a suspectable Muong Nong-type tektite,it is considered to belong to the North American obsidian series of peraluminous calc alkaline acid volcanic glass.(1)Glass matrix contains mineral inclusions such as euhedral plagioclase,magnetite,biotite,no impact metamorphism,and these inclusions are distributed in layered orientation with fewer vesicles.(2)The composition of major and trace elements of the glass matrix are uniform on the millimetre scale.Plagioclase has a clear composition zone for magma differentiation crystallization.Biotite belongs to the primary magmatic type,and the glass matrix FeO,TiO_(2)and MgO contents(0.56%,0.09%,0.07%,respectively)are lower than those of tektites,whereas K_(2)O content(3.70%)is higher than that of tektites.A/CNK values range from 2.15 to 2.69.A/NK values range from 2.41 to 3.13,andσ43 values range from 0.45 to 0.79.(3)Glass matrix enriched in large ionic lithophile elements such as Rb,U and Th,depleted in siderophile elements such as Cr,Co and Ni,and with a strong negative Eu anomaly(δEu values range from 0.47 to 0.60).In contrast to tektites,the characteristics of the glass sample in this study are similar to those of North American obsidian.The average glass matrix SiO_(2),Al_(2)O_(3),CaO,and Na_(2)O contents are 80.21%,13.13%,0.69%,and 1.14%,respectively,and the rare earth elements distribution pattern is right-dipping(ΣLREE/ΣHREE values range from 8.17 to 8.96).These features are similar to those of North American group of tekties and obsidian and cannot be used as indicators to distinguish them.Through comprehensive analysis on mineral inclusions type,impact metamorphic characteristics,the characteristics of main and trace elements,δEu,Fe^(3+)/Fe^(2+)values of the samples,these indicators can efficiently distinguish tektites from acidic volcanic glass.

关 键 词：地球化学 撞击玻璃 火山玻璃 黑曜岩 

分 类 号：P185.3[天文地球—天文学] P185.83