PETROGRAPHIC AND GEOCHEMICAL CHARACTERISTICS OF GLOBULAR GRANODIORITE IN HUANGLING AND ITS GLOBULAR FORMATION FAC-TOR
The globular granodiorite in Huangling,Hubei province,occurs within the Huangling batholith in the core of the Yangtze terrain and has limited exposure.The globular mass is composed of globule,interglobular matrix and main rock.The spherical body consists of a central spherical nucleus(including light-colored spherical nucleus or dark inclusion spherical nucleus,usually has a radial structure inside)and an outer spherical shell(including single-layer spherical shell or multi-layer rhythmic spherical shell),with plagioclase(mainly intermediate plagioclase)and hornblende as its main minerals.The interglobular matrix mainly contains plagioclase(primarily anorthite),quartz,and biotite,typically displaying a medium fine granitic texture.Compared to the globules,the interglobular matrix has higher SiO2 content,a greater total amount of rare earth elements(REE),a higher degree of heavy REE fraction-ation,and a higher abundance of strongly incompatible elements such as Rb,Th,Ta,Nb,Ce,Zr,and Hf.The SiO2 content in the light-colored spherical nucleus,spherical shell,and matrix is 55.02％to 58.05％,54.25％to 54.97％,and 60.00％to 69.50％,respectively.The total REE(∑REE)ranges from 37.23 ×10-6 to 53.13×10-6 in the light-colored nucleus,54.63× 10-6 to 73.4×10-6 in the spherical shell,and 94.58×10-6 to 246.03×10-6 in the matrix.The(La/Yb)N values range from 14.26 to 24.81 in the nucleus,9.52 to 15.95 in the shell,and 22.99 to 42.56 in the matrix.The δEu values are 1.66 to 2.10 for the nucleus,1.27 to 1.41 for the shell,and 0.78 to 1.29 for the matrix.The matrix has higher acidity compared to the spheroids and exhibits characteristics of late-stage magmat-ic crystallization,indicating that the matrix formed after the spheroids.Based on the structural characteristics and the truncation relationship between the spheroids,it is inferred that spheroids grow outward from the core.The radial growth and structure of the globular minerals likely result from non-uniform nucleation on the solid surface of metl under conditions of initial overheating followed by undercooling,leading to mineral crystallization around the center in the direction of heat transfer.The rhythmic development of the spherical shell may be caused by the spontaneous os-cillation of the excess saturation of the growth surface melt during the crystallization of minerals within the spheroids.
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