The 7 ka old Qixiangzhan lava flow(QXZ,Tianchi volcano)represents the last eruptive event before the 946 CE,caldera-forming‘Millennium’eruption(ME).Petrographic,whole rock,mineral composition,Sr-Nd isotopic data on ...The 7 ka old Qixiangzhan lava flow(QXZ,Tianchi volcano)represents the last eruptive event before the 946 CE,caldera-forming‘Millennium’eruption(ME).Petrographic,whole rock,mineral composition,Sr-Nd isotopic data on QXZ show that:(a)the lava consists of two components,constituted by comenditic obsidian fragments immersed in a continuous,aphanitic component;(b)both components have the same geochemical and isotopic variations of the ME magma.The QXZ and ME comendites result from fractional crystallization and crustal assimilation processes.The temperature of the QXZ magma was about 790℃ and the depth of the magma reservoir around 7 km,the same values as estimated for ME.QXZ had a viscosity of 10^(5.5)-10^(9) Pa s and a velocity of 3-10 km/yr.The emplacement time was 0.5-1.6yr and the flow rate 0.48-1.50 m^(3)/s.These values lie within the range estimated for other rhyolitic flows worldwide.The QXZ lava originated through a mixed explosive-effusive activity with the obsidian resulting from the ascent of undercooling,degassing and the fragmentation of magma along the conduit walls,whereas the aphanitic component testifies to the less undercooled and segregated flow at the center of the conduit.The QXZ lava demonstrates the extensive history of the ME magma chamber.展开更多
西藏冈底斯成矿带分布着大面积古近纪（70—40Ma）林子宗群火山岩。但如此强烈的火山-岩浆作用，与安第斯成矿带相比（如马力昆带（FranjadeMaricunga）、印地-帕斯瓜带（Franja El Indio—Pascua）），除了碰撞伸展阶段形成的驱龙、甲...西藏冈底斯成矿带分布着大面积古近纪（70—40Ma）林子宗群火山岩。但如此强烈的火山-岩浆作用，与安第斯成矿带相比（如马力昆带（FranjadeMaricunga）、印地-帕斯瓜带（Franja El Indio—Pascua）），除了碰撞伸展阶段形成的驱龙、甲玛等斑岩-矽卡岩型铜多金属矿床外（23—13Ma），“缺位”资源规模大、经济价值高的浅成低温热液型矿床成矿亚系列。该类矿床是剥蚀了，还是没有发现？本文在前人工作基础上，通过详细的地质勘探、地质填图、地质编录、镜下鉴定、能谱和电子探针分析，在南木林盆地斯弄多地区林子宗群陆相火山岩中识别出低硫化浅成低温热液型银铅锌（铟镉金）矿床。矿体由产于流纹斑岩中的隐爆角砾岩型银铅锌矿体、火山机构旁侧的热液脉型银铅锌矿体及断裂上盘的银（铅锌）矿体组成，目前控制Pb＋Zn资源量超过30万吨（331＋332为主）@Pb＋Zn〉5％，Ag资源量超过400吨@Ag〉50g／t。主要金属矿物为方铅矿、闪锌矿、辉银矿、硫砷铜银矿、黄铁矿，微量黄铜矿，主要蚀变组合为：石英一玉髓一碧玉，重晶石．萤石，冰长石．伊利石．绢云母，菱铁矿一菱锰矿；矿石构造以脉状、角砾状、网脉状、条带状、层纹状、皮壳状、块状、浸染状等，发育结晶作用和交代作用形成的矿石结构；近地表发育古热泉喷口，堆积条带状、层纹状硅质沉积物。综合上述地质信息，确定该矿床为典型低硫化浅成低温热液型银多金属矿床。这一重要矿床类型的发现和确定在冈底斯成矿带乃至西藏特提斯成矿省尚属首例，对冈底斯成矿带广泛发育形成于70～40Ma的林子宗群火山岩地区区域找矿具有积极和重要的指导意义，其重要性不容低估。展开更多
基金funded by the National Natural Science Foundation of China(Grant Nos.41972313 and 41790453)the Engineering Research Center of Geothermal Resources Development Technology and Equipment,Ministry of Education,Jilin University。
文摘The 7 ka old Qixiangzhan lava flow(QXZ,Tianchi volcano)represents the last eruptive event before the 946 CE,caldera-forming‘Millennium’eruption(ME).Petrographic,whole rock,mineral composition,Sr-Nd isotopic data on QXZ show that:(a)the lava consists of two components,constituted by comenditic obsidian fragments immersed in a continuous,aphanitic component;(b)both components have the same geochemical and isotopic variations of the ME magma.The QXZ and ME comendites result from fractional crystallization and crustal assimilation processes.The temperature of the QXZ magma was about 790℃ and the depth of the magma reservoir around 7 km,the same values as estimated for ME.QXZ had a viscosity of 10^(5.5)-10^(9) Pa s and a velocity of 3-10 km/yr.The emplacement time was 0.5-1.6yr and the flow rate 0.48-1.50 m^(3)/s.These values lie within the range estimated for other rhyolitic flows worldwide.The QXZ lava originated through a mixed explosive-effusive activity with the obsidian resulting from the ascent of undercooling,degassing and the fragmentation of magma along the conduit walls,whereas the aphanitic component testifies to the less undercooled and segregated flow at the center of the conduit.The QXZ lava demonstrates the extensive history of the ME magma chamber.
文摘西藏冈底斯成矿带分布着大面积古近纪（70—40Ma）林子宗群火山岩。但如此强烈的火山-岩浆作用，与安第斯成矿带相比（如马力昆带（FranjadeMaricunga）、印地-帕斯瓜带（Franja El Indio—Pascua）），除了碰撞伸展阶段形成的驱龙、甲玛等斑岩-矽卡岩型铜多金属矿床外（23—13Ma），“缺位”资源规模大、经济价值高的浅成低温热液型矿床成矿亚系列。该类矿床是剥蚀了，还是没有发现？本文在前人工作基础上，通过详细的地质勘探、地质填图、地质编录、镜下鉴定、能谱和电子探针分析，在南木林盆地斯弄多地区林子宗群陆相火山岩中识别出低硫化浅成低温热液型银铅锌（铟镉金）矿床。矿体由产于流纹斑岩中的隐爆角砾岩型银铅锌矿体、火山机构旁侧的热液脉型银铅锌矿体及断裂上盘的银（铅锌）矿体组成，目前控制Pb＋Zn资源量超过30万吨（331＋332为主）@Pb＋Zn〉5％，Ag资源量超过400吨@Ag〉50g／t。主要金属矿物为方铅矿、闪锌矿、辉银矿、硫砷铜银矿、黄铁矿，微量黄铜矿，主要蚀变组合为：石英一玉髓一碧玉，重晶石．萤石，冰长石．伊利石．绢云母，菱铁矿一菱锰矿；矿石构造以脉状、角砾状、网脉状、条带状、层纹状、皮壳状、块状、浸染状等，发育结晶作用和交代作用形成的矿石结构；近地表发育古热泉喷口，堆积条带状、层纹状硅质沉积物。综合上述地质信息，确定该矿床为典型低硫化浅成低温热液型银多金属矿床。这一重要矿床类型的发现和确定在冈底斯成矿带乃至西藏特提斯成矿省尚属首例，对冈底斯成矿带广泛发育形成于70～40Ma的林子宗群火山岩地区区域找矿具有积极和重要的指导意义，其重要性不容低估。
