云南澜沧老厂花岗斑岩中锆石标型特征及地质意义

Typomorphic characteristics and geological significance of the zircon from the Laochang granite porphyry in Lancang,Yunnan Province

运用Pupin的锆石标型研究方法,通过对锆石晶形的鉴定和统计来探讨云南省澜沧老厂花岗斑岩的成因信息。研究表明,本区花岗斑岩中的锆石有20种亚型,主要由{110}、{100}柱面和{101}、{211}锥面构成,其中柱面以{110}最为发育,锥面{101}和{211}都发育,但{101}较{211}更为发育;T.E.T曲线比较短,且主要分布在演化趋势图的右上角,表明锆石主要在低温过碱环境中形成,结晶速度较快,结晶温度范围750～600℃。锆石的结晶标型显示,本区隐伏花岗斑岩是以壳源为主的壳幔源混合成因的花岗斑岩。

Petrogenesis of Laochang granite-porphyry is discussed in this paper based on evaluating and counting zircon crystal forms. In this study, Pupin＇s method for zircon typomorphic research was employed.The result shows that the growth of zircon cone was controlled by the ratio of aluminum-alkali to silicon-aluminum, whereas the crystallization temperature was the dominant factor controlling the growth of zircon cylinder. If the zircon was formed in an aluminum or low-alkali environment, {211} cone is more developed than {101} cone, and sometimes only {211} cone is developed, with no {101}cone. Where the zircon was produced in an alkali or low aluminum environment, {101} cone is more developed than {211} cone, and the zircon in alkali granite only develops {101} cone and does not have {211} cone. T. E. T has a long evolutionary line when magmatic crystallization was slow and the time-domain of zircon crystallization was long. On the contrary, if the magma rapidly crystallized or the zircon was quickly crystallized from the magma, the T. E. T line is relatively short, the zircon crystal form is relatively unitary and does not show remarkable variation. Statistics show that zircon crystals in Laochang granite-porphyry assume mainly short or long cylindrical, tetragonal bipyramidal and ditetragonal dipyramidal forms and subordinately sharpened pencil-like or equigranular forms, pale yellow or colorless in color. The crystal size varies in the range of 42 μm×35.5 μm～302 μm×115 μm, and the ratio of length to width is mainly between 2∶1 and 3.5∶1 and subordinately between 1.5∶1 and 4∶1. Crystal inclusions are rarely seen, and cracks are not developed. Twenty zircon crystal forms were identified in the Laochang granite-porphyry, and G1, L4, S3, S4, S5, S6, S7, S8, S9, S12, S13, S14 crystal forms are relatively numerous. The zircon crystals are composed of cylinder{110}, {100} and cone {101}, {211}. With {100} cylinder and {101} and {211}cone being very well developed, and {101} more developed than {211}. T.E.T line is short and is mainly distributed at the top right corner of the evolution trend map. The zircon typomorphic characteristics suggest that the zircon in Laochang granite-porphyry was formed in a low-temperature and peraalkaline environment characterized by rapid crystallization, with the crystallization temperature ranging between 750℃ and 600℃. A comprehensive analysis of chemical composition, chronology and zircon typomorphic features of Laochang granite-porphyry shows that the Laochang concealed granite-porphyry was formed mainly by crust source petrogenesis with the addition of mantle source composition, and the rich alkali, rich water and rich ore-forming materials in this granite-porphyry constituted a favorable ore-forming magma system.