花岗岩中宇宙尘的发现及其地质意义

DISCOVERY OF COSMIC DUST IN GRANITE AND ITS GEOLOGICAL SIGNIFICANCE

昌黎-阜新构造岩浆钼金成矿带,沿NE向断裂带分布有中生代花岗岩体27处,以重熔岩浆花岗岩为主,交代花岗岩次之,包括Ⅰ、A和S型三种花岗岩,分四期侵入,第三期花岗岩伴随有钼金多金属成矿作用,其中的11个不同期次不同类型的花岗岩体中发现有宇宙尘,可分为铁质空心球、铁质实心球和硅酸玻璃球状三种宇宙尘,组成矿物有磁铁矿、蔷薇辉石、石英、刚玉和锥纹石。花岗岩中宇宙尘的发现进一步证明该区花岗岩是壳源产物,而且这种源岩曾经出现在地表,接受宇宙降落物质,而后下沉重熔或交代改造生成花岗岩。

The Changli-Fuxin gold- and molybdenum-bearing granite belt is located in the NE-trending fracture belt between the Shanhaiguan-Yiwulu uplift and the Yanshan-Liaoning subsidence belt, which possesses 27 granite bodies (>5 km^2), including I-type, S-type and A-type. Although they belong to crustal granite, anatectic granite is dominant and metasomatic granite is subordinate. Four phases may be distinguished from the middle-later Indosinian stage to late Yanshanian stage. Molybdenum in the granites was derived from the Archean basement, while lead and zinc occurring in the contact belt were derived from the country rocks of the Sinian.Cosmic dust has been discovered in eleven granite bodies of different stages and different types at Honglaoshan, Jinxi, Yangjiazhangzi, Jianchang, Yiwulu etc. There are three types of cosmic dust: silicate glass ball, ferruginous hollow balland ferruginous solid ball. Cosmic dust is spherical, elliptical, spindle, milkdrop, dumbbell etc. in shape. The diameter of the cosmic dust ranges from 0.12 to 0.45 mm and the thickness of the crust varies from 0.01 to 0.07 mm. The surface of the cosmic dust is smooth with anasphaltic-semimetallic luster and strong magnetism, and is brittle. According to the electron microprobe, X-ray and microscopic analyses, the crust is composed mainly of magnetite and rhodonite with minor quartz and corundum. The grains of the magnetite are extremely fine (<0.004 mm) and are arranged regularly in the shapes of snowflake-like, feather-like, imbricate and pilotaxitic arabesquitic crystallites. The core is composed of kamacite (α-Fe).Obviously, the cosmic dust in the granites was derived from the basement source rocks, and such rocks were bound to have been exposed at the surface once. only thus could the cosmic dust in the air fall on it at that time, and then it descended to depths to undergo anataxis or metasomatism to form the granite. In this way, the cosmic dust has been kept as a refractory constituent in the granite. Therefore the existence of the cosmic dust in granite is the most direct and important evidence for the crustal granite.