Characteristics of Rock-Forming Minerals of Analcime Phonolite in the Damxung Area, Qinghai-Tibet Plateau: Evidence for Primary Analcime
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摘要: 方沸石响岩是一种罕见的碱性火山岩.采用电子探针、粉晶X射线衍射、扫描电镜、拉曼光谱等研究了青藏高原当雄地区方沸石响岩的主要造岩矿物种属、共生关系和结晶顺序.研究表明, 斑晶由方沸石和长石组成.方沸石为岩浆结晶晚期形成的原生矿物; 长石均发育“次生边”结构, 中央相为斜长石, 边缘相为碱性长石.基质由碱性长石、次透辉石、钛磁铁矿和褐铁矿组成.原生矿物的结晶顺序是: 斑晶长石的中央相→斑晶方沸石+斑晶长石边缘相富钾长石→基质长石→次透辉石→钛磁铁矿和褐铁矿.利用方沸石-熔体平衡估算出方沸石结晶时岩浆的温度和压力条件分别为600~640 ℃和(5~13) ×108 Pa, 考虑到青藏高原当时已形成巨厚地壳, 认为岩浆房存在于地壳深部.Abstract: Analcime phonolite is a rare alkaline volcanic rock.Using the electron microprobe, X-ray powder diffraction, scanning electron microscope, and Raman spectroscopy, this paper studies characteristics of rock-forming minerals, coexisting relations and crystallization sequences of the analcime phonolite from the Damxung area, Qinghai-Tibet plateau.Phenocrysts are euhedral analcime and feldspar, while the groundmass is composed of K-feldspar, salite, titanomagnetite, and limonite.Analcime is primary and crystallizes in the late magmatic stage.Phenocrystal feldspars have secondary rims of K-feldspar, and their cores are plagioclases.Crystallization sequences are the plagioclase core of feldspar phenocrysts, analcime and K-feldspar in the rim of feldspar phenocrysts, K-feldspar in the groundmass, salite, titanomagnetite and limonite.The balance between analcime and melt places constraints on the P/T regime (5-13) ×108 Pa and 600-640 ℃ respectively, implying depths of origin in the deep crust.
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Key words:
- primary analcime /
- feldspar /
- phonolite /
- mineralogical characteristics /
- Lhasa block
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图 1 鱼鳞山组火山岩的TAS图解(据Le Maitre, 1989)
▲本文数据; ▽引自廖思平等(2002); □引自刘焱等(2003);○引自李佑国等(2005); Pc.苦橄玄武岩; B.玄武岩; O1.玄武安山岩; O2.安山岩; O3.英安岩; S1.粗面玄武岩; S2.玄武粗安岩; S3.粗安岩; T.粗面岩/粗面英安岩; R.流纹岩; U1.碱玄岩/碧玄岩; U2.响岩质碱玄岩; U3.碱玄质响岩; Ph.响岩; F.副长石岩
Fig. 1. TAS diagram of volcanic rocks of the Yulinshan Formation
图 3 方沸石的粉晶X射线图图谱(a)和二次电子扫描电镜图(b)
(a)分析由中国地质大学(武汉)地质过程与矿产资源国家重点实验室完成.仪器型号: D/MAX-M B粉晶術射仪: 靶: Cu; 滤波片; Ni;狭缝: 0.15 mm: 管电压/电流: 40 kV/40 mA; 起始角; 30°; 终止角: 65°; 扫描速度: 2°/min; 温度: 20 ℃; 湿度: 60%
Fig. 3. X-ray powder diffraction pattern (a) and secondary electron SEM image (b) of the analcime in analcime phonolite
图 5 方沸石响岩中长石成分投影图(a) 和拉曼图谱(b) (据赵珊茸等, 2003)
(b) 图分析由中国地质大学(武汉) 地质过程与矿产资源国家重点实验室激光拉曼室完成.仪器型号为RM-1000型, 实验条件为氩离子激光器, 波长为514.5 nm, 狭缝宽度为25 μm, 叠加次数为3次; (A) TM48-2-c; (B) TM48-2-r; (C) TM50-2-r; (D) TM50-2-c
Fig. 5. Projection drawing of feldspars' composition (a) and Raman spectrum (b) of feldspars in analcime phonolite
表 1 方沸石响岩主要矿物的电子探针分析结果(%)
Table 1. Electron microprobe analyses results of major minerals in analcime phonolite
表 2 方沸石响岩各矿物晶体化学式
Table 2. Crystallochemical formulas of minerals in analcime phonolite
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