幔源角闪石巨晶中硫化物熔融包裹体研究

Melt Inclusions of Sulfide in Mantle-Driven Amphibole Megacrysts

硫化物熔融包裹体研究是认识硫化物矿床成矿元素来源和演化的重要手段,由于硫化物熔融包裹体的体积较小(粒径仅为10～20μm),其详细化学元素组分的难以获得一直是制约进一步研究的瓶颈。笔者在前人研究的基础上,借助于扫描电镜、电镜能谱和二次飞行时间离子探针(Tof-SIMS)对产于铜陵地区角闪石巨晶中的硫化物熔融包裹体进行了详细的研究,首次获得了一套精确的矿物化学资料和元素分布图。矿物学研究表明,角闪石巨晶在上地幔和下地壳均有结晶,温压区间分别为T:850～900℃(温度),P:0.70×109～0.82×109Pa(压力),对应深度D:23.10～27.06km;和T:900～950℃,P:1.09×109～1.17×109Pa,D:35.97～38.61km。元素分布图显示,硫化物熔融包裹体主要有两种元素组成体系:S-Fe-Mn-Ni-Rb-Sr-Ba和S-Fe-Cu-Sr,幔源硫化物体系中Mn、Ni、Rb、Ba等元素具有相似的性质特征可共溶,与Cu则表现出不混溶。在铜陵地区,上地幔的部分熔融形成了一套碱性玄武岩浆,后受岩浆底侵作用和壳幔相互作用影响,底侵进入下地壳深位岩浆房,发生结晶分异和同化混染作用,形成一套轻度演化的玄武岩浆,可能为辉长质。上地幔和下地壳的角闪石巨晶分别是由上地幔碱性玄武岩浆和下地壳轻度演化的玄武岩浆(辉长质)高压下结晶的产物。当上地幔碱性玄武岩浆上侵到下地壳深位岩浆房以后,发生结晶分异作用,又由于地壳硅镁层的混染作用,使得玄武岩浆中硫溶解度降低,促其熔离,从而释放大量的硫(S,以及Ni、Cu、Cr)。角闪石巨晶中的硫化物熔融包裹体正是在下地壳深位岩浆房中,由正在结晶的角闪石巨晶在结晶分异和轻度演化的玄武质岩浆中捕获的不混溶硫化物熔融液滴形成的。铜陵地区在中生代经历了一个长期的大规模的岩浆底侵作用和壳幔相互作用过程,由于下地壳硅镁层混染作用使得轻度演化的玄武岩浆释放大量硫,必然会在莫霍面附近形成大规模高浓度的硫富集区,这些组分在岩浆上侵作用、地壳减薄作用或者裂谷作用的影响下很容易再活化,进入区域岩浆-热液流体系统,最终参与形成区域大规模的硫化物矿床。

Sulphide deposits are of considerable economic importance but the origin of metals and sulphur in these deposits is commonly ambiguous and raises questions. Much attention has been focused on a primary mantle origin for sulphides after extensive research on sulphide melt inclusions （SMI） in mantle-derived xenoliths in basaltic and kimberlitic rocks. Most studies treated the evolution of magma and SMI separately by numerical simulations based on isotopic geochemistry or petrochemistry, few have attempted to use detailed chemical analysis and petrographic evidence for the origin of the sulphide, let alone describing how the SMI evolve in mantle-derived magmas. Using ToF-SIMS, we have analyzed the detailed element compositions, obtaining positive and negative ion maps of the SMI and surrounding host amphibole megacrysts Amp-M. SMI enclosed in mantle-derived Amp-M that represents melt trapped in the minerals provide important clues as to the behavior of immiscible sulphide liquids during the evolution of magmas and the formation of sulphide deposits. Temperature and pressure during formation of the Amp-M were estimated using the TiO2-Al2O3 geothermometer and geobarometer formula has two main interval distribution： The upper-mantle Amp-M： T： 850~900℃（temperature）,P： 0.70×109~0.82×109Pa（pressure）, corresponding to a depth of D： 23.10~27.06 km; and the lower-crust Amp-M： T： 900~950℃, P： 1.09×109~1.17×109Pa, D： 35.97~38.61 km. Observations and researches indicate that the upper-mantle Amp-M was formed from the crystallization of the upper-mantle alkaline basaltic magma, which resulted from previous partial melting of the upper mantle, and the lower-crust Amp-M were formed from the fractional crystallization and evolution basaltic magma in magma chambers at the base of the continental crust. At the mantle-crust boundary, basaltic magma upwelled and pooled in the lower crust. During fractional crystallization and the sima contamination, S （and Ni, Cu and Cr） were released and were eventually trapped as droplets within growing amphibole megacrysts. As part of the general process of mantle metasomatism, this could lead to a significant concentration of sulphur and chalcophile elements in the lowermost crust, making these components available for remobilization during episodes of crustal thinning at 140Ma, resulting in the MLYR large-scale mineralization belt. Our study suggests that this process is more common than previous thought.