蛇绿岩2、岛弧.ppt

地幔部分熔融 地幔橄榄岩的矿物组成 地幔橄榄岩的主要组成矿物： 橄榄石　(Fe, Mg)2SiO4 斜方辉石　(Fe, Mg)2Si2O6 单斜辉石　Ca(Fe,Mg)Si2O6 含Al的矿物相： 斜长石：0-30 km 尖晶石：30-80 km 石榴石：80 km 地幔部分熔融 部分熔融的条件 升温 加挥发份 减压 第四章 岛弧和活动陆缘火成岩共生组合及其形成机制 Ocean-ocean ? Island Arc (IA) Ocean-continent ? Continental Arc or Active Continental Margin (ACM) Structure of an Island Arc Island Arc Magmatism Activity along arcuate volcanic island chains along subduction zones Distinctly different from the mainly basaltic provinces thus far Composition more diverse and silicic Basalt generally occurs in subordinate quantities Also more explosive than the quiescent basalts Strato-volcanoes are the most common volcanic landform Volcanic Rocks of Island Arcs Complex tectonic situation and broad spectrum High proportion of basaltic andesite and andesite Most andesites occur in subduction zone settings Petrography of island-arc volcanic rocks Petrography of island-arc volcanic rocks Petrography of island-arc volcanic rocks Alkaline and Subalkaline Rock Suites Tholeiitic vs. Calc-alkaline Trends（AFM wt％) Major Elements and Magma Series a. Alkali vs. silica b. AFM c. FeO*/MgO vs. silica diagrams for 1946 analyses from ~ 30 island and continental arcs with emphasis on the more primitive volcanics K2O is an important discriminator Spatial “K-h”: low-K tholeiite near trench ? C-A ? alkaline as depth to seismic zone increases Temporal Early tholeiitic ? later C-A and often latest alkaline is common Sub-series of Calc-Alkaline K2O is an important discriminator ? 3 subseries 6 sub-series if combine tholeiite and C-A (some are rare) Tholeiitic vs. Calc-alkaline differentiation Tholeiitic vs. Calc-alkaline differentiation C-A shows continually increasing SiO2 and lacks dramatic Fe enrichment Calc-alkaline differentiation Early crystallization of an Fe-Ti oxide phase Probably related to the high water content of calc-alkaline magmas in arcs High water pressure also depresses the plagioclase liquidus and ? more An-rich As hydrous magma rises, plagi