• 作者： 楊宏儀
• 作者服務機構： 國立成功大學
• 中文摘要： 本文選擇了臺灣四種不同產狀之含橄欖石火成岩，用電子微探分析儀，分析其中之橄欖石，以求了解其成分變化與結晶環境之關係，並討論這些火成岩的成因。 臺灣東部海岸山脈之基性及超基性深成岩中之橄欖石為粗粒且均質者，其含鐵量，從橄欖岩至橄欖輝長岩逐漸增加。關西地區之玄武岩質岩石含有很多純橄欖岩，尖晶石一斜方輝橄岩，尖晶石一二輝橄欖岩，等之超基性結核。斜方輝橄岩與二輝橄欖岩中之橄欖石成份均勻，大致是Fo91, Fa9。 偶而在這些橄欖石中，可見一或二組排列整齊的板狀包裹物，可能為從橄欖石分凝出來的尖晶石類之礦物。純橄欖岩結核中橄欖石顆粒之邊綠顯示中度鐵質富集，表示為從原來橄欖石晶體之周圍再附著生成者。 關西一竹東地區玄武岩質岩石中橄欖石成份大致在F087.3Fa,16.3至F060.4Fa39.6之間，並且可和主要岩石類型對比。電子微探分析顯示此區岩石中之橄欖石常有成份分帶現象，由橄欖石之微探數據推知，矽質玄武岩是由母岩漿分離早期之方沸橄欖石玄武岩之後結晶形成的。橄欖石在正常岩漿分化中之鐵質富集，可從四條連續之玄武岩流及一條厚的玄武岩床得到證明。 烘爐山地區高鋁玄武岩中橄欖石成份從F075,3Fa24.7至F0e6.3 Fa33.7且FeO成份在邊綠區比中心多4州，成正常分帶現象。觀音山地區高鋁玄武岩中橄欖石成份，從F079.4Fa2O.5至Fo75.8Fa24.z其正常成份分帶現象之程度甚小。 橄欖石中鈣之含量亦可作為結晶環境的正確對比，在火山噴出岩石中橄欖石之鈣含量較在深成岩者為高，橄欖石中錳含量和鐵含量成正比一即錳含量隨鐵含量之增加而增加。橄欖石中之鎳含量隨鎂之增加，可能與結晶環境有關。
• 英文摘要： Olivines in the olivine-bearing igneous rocks selected from four differenttypes of occurrence in Taiwan have been analyzed with an electron micro-probe in order to understand the compositional variations with the environ-ment of crystallization and to discuss the petrogenesis of these igneousrocks. Olivines in the basic and ultramafic plutonic rocks from the CoastalRange of Eastern Taiwan are coarse-grained and homogeneous, and showmoderate iron-enrichment from peridotites to olivine gabbro． The basaltic rocks in the vicinity of Kuanshi contain abundant ultra-mafic nodules of dunite, spinel harzburgite, and spinel lherzolite. Olivinesin harzburgite and lherzolite are homogeneous in composition and theircompositions cluster around Fo9,Fas. One or two sets of oriented plateinclusions can occasionally be found in these olivines. They may be somekind of spinel exsolved from olivines. Olivine grains in some dunite nodulesexhibit moderate iron-enrichment in the marginal area, suggestive of anadcumulus growth on the olivine cumulus. The compositions of olivine in the basaltic rocks from Kuanshi-Chutungarea generally fall in the range from Fo33.7Fa,e.3 to Foso.aFa3s.s and can becorrelated with the major rock types. The microprobe analysis also showsthat olivines in these rocks often exhibit compositional zonings. The micro·probe data on olivines strongly suggest that tholeiitic basalt was formedthrough the early separation of analcite olivine basalt from a parentalmagma. Iron-enrichment in olivine in a normal magmatic differentiationis well demonstrated by a series of four successive basaltic flows and alsoby a thick basaltic sill. Olivines in the high-alumina basalt from Hunglushan range fromFo7b.3Fa2,.7 to Fow9Fan.7 in compositicn and are slightly normally zonedwith an FcO content about 4% more in the marginal area than in the The composition of olivines in the high-alumina basalt from Kuan-yinshan ranges from Fo，，aFaZO.s to Fo7s.8Faaa.z, and the normal composi-tional zoning is also minimal. The Ca content in olivines is found to correlate closely with theenvironment of crystallization, being much higher in olivines from volcanicrocks than from the plutonic rocks. Positive correlation exists between theMn content and the fayalite component for olivines, i.e. Mn concentra-tion increases with the increasing Fe. Finally, the Ni content in olivinestends to increase with the increasing Mg and may probably be dependenton the environment of crystallization.
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