安徽铜陵凤凰山花岗闪长岩岩石学和矿物学特征及其地质意义

PETROGRAPHIC AND MINERALOGICAL CHARACTERISTICS OF THE FENGHUANGSHAN GRANODIORITE IN THE TONGLING AREA,ANHUI PROVINCE AND ITS GEOLOGICAL SIGNIFICANCE

凤凰山花岗闪长岩与凤凰山铜矿床在成因上关系密切,通过对未蚀变花岗闪长岩中的角闪石和斜长石进行岩石学观察及详细的矿物成分分析,并利用角闪石-斜长石矿物平衡温压计反演角闪石、斜长石结晶时的温压条件,进一步探讨花岗闪长岩的形成条件及其成岩成矿意义。电子探针分析结果显示,角闪石分子中Si=6.37～7.16p.f.u.(p.f.u.为每单位晶胞),Mg/(Fe2++Mg)=0.63～0.76,Al(Ⅳ)T=0.84～1.63p.f.u.,属于镁角闪石和镁钙闪石。斜长石牌号An变化范围为:23～36,主要为更长石和中长石。通过角闪石-斜长石温压计的计算,得出早期角闪石结晶压力为446MPa～474MPa,对应的岩体侵位深度为14.72km～15.63km,结晶温度为1 066.39℃～1 070.55℃;晚期角闪石结晶压力为100MPa～191MPa,对应的岩体侵位深度为3.32km～6.32km,结晶温度为784.26℃～822.7℃。综合分析表明凤凰山花岗闪长岩是来自于深部幔源的岩浆沿断裂上侵至中下地壳(约15km),形成深部岩浆房,并发生结晶分异和同化混染作用,随后在构造扰动下,深部岩浆房中的岩浆沿构造薄弱面再次上侵至近地表(3km～6km),经冷凝固结形成的。矿物温压计算结果和区域地层学资料显示,自141 Ma以来,凤凰山矿区遭受剥蚀的主要是地层,岩体以及矿床的剥蚀程度较小,属于保存条件较为完好的成矿系统,而当前凤凰山铜矿勘探和开采深度不足1km,因此,深部具有较好的找矿前景。

The Fenghuangshan copper deposit has a close relationship with the Fenghuangshan granodiorite in the cause of formation. In this paper, the petrographic and mineralogical character- istics of amphibole and plagioclase in the Fenghuangshan granodiorite are investigated. Pressure and temperature （P-T） conditions of the crystallization of amphibole and plagioclase have been studied based on hornblende-plagioclase geothermobarometers. According to the pressure and temperature analysis, the conditions and significance for petrogenesis and mineralization of Feng- huangshan granodiorite can be revealed. Electron microprobe analysis （EPMA） shows that Si= 6.37-7.16 p f u,Mg/（Fe2＋ ＋Mg）=0.63-0.76,A1 （iv）T=o. 84-1. 63 p f U. The amphiboles in the rocks belong to magnesio-hornblende and calcium-hornblende. The plagioclases consist of andesine and oligoclase,and the An ranges from 23.02 to 36.15. The early stage amphiboles were formed at pressure of 446 MPa-474 MPa,at depth of 14.72 kraal5.63 km and at temperature of 1 066.39℃ -1 070.55℃ and the late stage amphiboles were formed at pressure of 100 MPa- 191 MPa,at depth of 3.32 km-6.32 km and at temperature of 784.26℃-822.7℃. Comprehen- sive analysis indicates that the magma derives from the deep mantle emplaced along the tectonic crack to the middle-lower crust （about 15 kin） and formed the deep magma chamber accompanied by fractionation crystallization and assimilation. Then under the influence of geological structure, the magma in the deep magma chamber emplaced again along the weak tectonic crack to the shal- low part of earthrs surface （3 km to 6 kin） and eventually formed the Fenghuangshan granodior- ire. The results of the geothermobarometry and the data of regional stratigraphy show that strata are denudated mostly and there is slight denudation on the Fenghuangshan granodiorite and cop- per deposit from Late Yanshanian,so the metallogenic system is well preserved. Considering that the current exploration depth and mining is less than 1 kin, the exploration and prospecting in deep space is promising.