Green Tuff metamorphism of the Miocene basaltic rocks in the northern part of Northeast Japan.

日本东北部北部中新世玄武岩的绿色凝灰岩变质作用。

• 批准号: 63540639
• 负责人: ONUKI Hitoshi
• 金额: $ 0.9万
• 依托单位: HIROSAKI UNIVERSITY
• 依托单位国家: 日本
• 项目类别: Grant-in-Aid for General Scientific Research (C)
• 财政年份: 1988
• 资助国家: 日本
• 起止时间: 1988 至 1989
• 项目状态: 已结题
• 来源: https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-63540639/
• 关键词: Green Tuff hydrothermal alteration; Alteration of Miocene basaltic rocks; Zeolite to prehnite-pumpellyite facies assemblages.; Hydrothermal metamorphism; グリ-ンタフ熱水変質作用; 中新世玄武岩類の変質; 低温熱水変質作用; 中新世の低温; 低圧変成作用; サブ緑色片岩相; 低温変成鉱物の化学組成

This project was undertaken in order to understand better the secondary mineralization of the Miocene submarine lavas and pyroclastic rocks with basaltic to andesiiic compositions during the "Green Tuff alteration". The Hanaoka area of the Akita Prefecture, and the Okiura, Hakamagoshi-dake and Kanita areas of the Aosorl Prefecture were selected for this purpose in the northern part of the Northeast Japan. Drilling has confirmed that the Miocene basaltic piles developed in the Hanaoka and Okiura areas are at least 700 meters thick. Their thickness in the other areas are estimated to be more than a few hundred meters. The metabastes in these four areas well preserved their original textures and primary phases such as clinopyroxene and plagioclase, but they have been recrystallized to varying degrees at low temperatures, resulting in the formation of various secondary minerals.In the Hanaoka and Okiura areas, the metabastes are characterized by the assemblage of chlorite + epidote + pumpellyite + prehnite at the surface exposures, but chlorite + epidote + prehnite + actinolite in the deep drill core, in addition to albite and quartz. Zeolites occur only in veins and amygudules of the samples from the surface exposures. Ca-zeolites are laumontite and yugawaralite in the Hanaoka area and wairakite, laumontite and yugawaralite in the Okiura area. In the Hakamagoshi-dake and Kanita areas, chlorite, epidote, prehnite, and pumpellyite in the matrices and in pseudomorphs after plagioclase. Actinolite rarely occurs as overgrowth on relict clinopyroxene crystal in the Kanita area. Characteristic Ca-zeolites are laumontite, stilbite and heulandite in veins and amygudules in both areas.These Miocene rocks have been extensively subjected to the "Green Tuff" hydrothermal metamorphism. Their fossil geothermal gradients of more than 100ﾟC/km would be estimated in these four areas.

为了更好地了解中新世海底熔岩和玄武岩-安山岩组成的火山碎屑岩在“绿凝灰岩蚀变”期间的二次成矿作用，开展了该项目。日本东北部北部的秋田县花冈地区和青索县的大仓、袴谷、金田地区都是为了这个目的而选择的。钻探证实，花冈和Okiura地区发育的中新世玄武岩桩至少有700米厚。其他地区的冰层厚度估计超过几百米。这四个地区的变质岩较好地保存了其原始结构和斜辉石、斜长石等原生相，但在低温条件下发生了不同程度的再结晶，形成了多种次生矿物。在花冈和大浦地区，变质岩的表层表现为绿泥石+绿绿石+泵殖石+前褐石组合，而在深部岩心中则表现为绿泥石+绿泥石+前褐石+放光石组合，此外还有钠长石和石英。沸石只出现在表面暴露样品的静脉和淀粉体中。花冈地区的钙沸石主要为烟沸石和烟沸石，而大仓地区的钙沸石主要为白拉基石、烟沸石和烟沸石。在八谷桥和Kanita地区，绿泥石、绿帘石、前长石和泵状长石在基质和斜长石后呈假晶状。在卡尼塔地区，放线石很少生长在斜辉石残晶上。两个地区的钙沸石特征为脉状和淀粉状的湖沸石、静沸石和榴辉石。这些中新世岩石广泛受到“绿凝灰岩”热液变质作用的影响。这4个地区的地温梯度均大于100‰/km。