青藏高原当雄地区方沸石响岩的主要造岩矿物特征: 原生方沸石的证据

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. 

Characteristics of Rock-Forming Minerals of Analcime Phonolite in the Damxung Area,Qinghai-Tibet Plateau:Evidence for Primary Analcime

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.