Lunar polymict breccia 14321 - A petrographic study
Lunar polymict breccia 14321 - A petrographic study
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月球多角砾岩 14321 - 岩相学研究
DOI：
10.1016/0016-7037(75)90193-3
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发表时间：
1975
影响因子：
5
通讯作者：
D. Weill
中科院分区：
文献类型：
--
作者：
R. Grieve;G. Mckay;H. Smith;D. Weill
月球岩石样本14321的7个岩石薄片，“Big Bertha”，已经被检查。这是一种包含多种岩屑和单晶碎片的复杂岩石，代表了非均质Fra Mauro地层的样本，作者认为这是来自Imbrium撞击事件的岩屑。电子探针显微分析和微观结构研究揭示了角砾岩的组装历史，这反过来又可以解释前雨期地壳的性质和雨期撞击的影响以及随后运往阿波罗14号地点的影响。现今的多晶角砾岩14321是由源自单一或密切相关的一套熔岩冷却单元破碎的玄武岩碎屑、一套被称为微角砾岩3（本身为多晶微角砾岩）的碎片碎屑和一个浅色基质组成的，该基质通过胶结两大类碎屑形成岩石14321。浅色基质物质来源于玄武岩与微角砾岩3的破碎互磨作用。在结构关系一致的基础上，在微角砾岩3内鉴定出两套较老的微角砾岩。微角砾岩1碎屑圆润，颜色较浅，呈北褐红色。它们总是完全封闭在微角砾岩3中，最常见的是形成圆形增生石状结构的中心核，我们称之为微角砾岩2。微角砾岩1、2、3和大角砾岩14321代表了一系列的破碎和岩化事件。从矿物学和结构标准来看，每一个事件都涉及一些热变质和/或冲击变质作用，角砾岩形成的时间顺序也与相关热效应的强度递减相对应。本文详细介绍了14321的岩石学和矿物学特征。uncanet al.（1975a）给出了对岩石学和化学综合数据的更一般的解释。
Seven petrographic thin sections of lunar rock sample 14321, ‘Big Bertha’, have been examined. It is a complex rock incorporating diverse lithic and single crystal fragments and represents a sampling of the heterogeneous Fra Mauro formation, considered by the writers to be lithified debris from the Imbrium impact event. Electron probe microanalysis and microscopic study of textures reveal the assembly history of this breccia which in turn allows some interpretation of the nature of the pre-Imbrium crust and the effect of the Imbrium impact and the subsequent transportation to the Apollo 14 site. The present-day polymict breccia 14321 is composed of basaltic clasts originating from the fragmentation of a single or closely related set of lava cooling units, a set of fragmental clasts designated as microbreccia 3 (themselves polymict microbreccias), and a light colored matrix which formed rock 14321 by cementing the two major groups of clasts. The light colored matrix material is derived from the fragmentation and mutual abrasion of the basalt and microbreccia 3. On the basis of consistent textural relations two older sets of microbreccias have been identified within microbreccia 3. Microbreccia 1 clasts are well-rounded, relatively light colored, and noritic. They are always completely enclosed within microbreccia 3, most often forming the central cores of rounded accretionary lapilli structures which we have designated as microbreccia 2. Microbreccias 1, 2, 3, and macrobreccia 14321 represent a chronological series of fragmentation and lithification events. Each of these events involved some thermal and/or shock metamorphism as evidenced by mineralogical and textural criteria, and the chronological order of formation of the breccias also corresponds to a decreasing intensity of associated thermal effects. The petrology and mineralogy of 14321 are described in detail in this paper. A more general interpretation of the combined petrographic and chemical data is given inDuncanet al.(1975a).