北山南部玻基纯橄岩的发现及其岩石学和地球化学特征

Discovery of Vitric Dunite(Meimechite) in Southern Beishan Mountain,and Its Petrological and Geochemical Characteristics

北山南部地区,二叠纪闪长岩岩体的西北部,发现玻基纯橄岩[Vitric Dunite;Meimechite,Меймечит;中文亦有音译为麦美奇岩]和蚀变玻基纯橄岩。①玻基纯橄岩样品呈致密块状,浅灰绿色,表面可见清楚的粒状斑晶矿物和黑色胶结物,具发丝一样的小条带。蚀变玻基纯橄岩样品呈致密块状,深黑色,局部具细小条带。显微镜下,前者主要矿物橄榄石为斑状结构;后者主要矿物橄榄石多蚀变为蛇纹石,表现为变余斑状结构,斑晶是蛇纹石化的橄榄石假像。基质均为玻璃质结构。②X射线衍射分析表明:橄榄石为镁橄榄石[forsterite,(Mg_(1.8)Fe_(0.2))(SiO_4)],蛇纹石为利蛇纹石{lizardite,(Mg,Al)_3[(Si,Fe)_2O_5](OH)_4}。③电子探针检测,磁铁矿呈细条纹状,基质矿物有绿泥石、绿帘石、黝帘石、角闪石、钛铁矿、褐铁矿、镍黄铁矿、斜长石、尖晶石、金云母等。④样品岩石化学分析,主元素氧化物的含量分别为,SiO_2为36.40%～37.29%,Al_2O_3为2.61%～4.70%,TiO_2为0.058%～0.22%,MgO为33.30%～35.36%,Fe_2O_3+FeO为10.51%～12.89%,Na_2O+K_2O为0.03%～0.14%,为超铁镁质超基性喷出岩。Mg#为83.5～86.5,δSr为1.14～4.61,Sr/Y为5.70～42.74。稀土元素∑REE为2.18～9.71,δEu为0.68～2.43,La/Yb为0.90～10.00。微量元素和稀土元素具有原始地幔的岩石化学特征。⑤原始岩浆可能经历了结晶分异过程,并可能在裂谷构造环境形成。⑥本文发现的玻基纯橄岩,类似于典型的西伯利亚麦美奇河流域和勘察加半岛的低钛"麦美奇岩"。玻基纯橄岩在该地区的发现,为该地区岩浆的形成和演化研究及铜镍矿的找矿和勘探工作,提供了重要的岩石学依据。

Objectives： The ultramafic vitric dunite,that is meimechite,also known as a igneous rock,rarely appear all over the world,has been found in Permian diorite（ δP） for the first time in southern Beishan Mountain,China.The vitric dunite sample shows as dense mass and celandine green with weakly magnetism,and has clear granular porphyry minerals and black cement with small hair-like bands on the surface.While,the alteration vitric dunite samples show as dense mass and deep darker black with weakly magnetism and small vein material.However,the petrology and geochemistry of this important rock,as well as the formation of the environment,are not well understood.Based on a lot of results of lab identification,testing,analysis,we focus on the every characteristics and formation mechanism of this rock.Methods： Laboratory,Xi＇an Institute of Geology and Mineral Resources.① Microscopical identification and Mineral porphyry contrast.② X-ray diffraction analysis in D/max-2500 instrument.③ The detection of electron probe in JEOL JXA-8230 instrument and equipment.④ Petrochemical analysis： FeO analysis use volumetric method,LOI and H2O＋analysis use gravimetric method,the other main elements analysis use Xrf-Xrd Spectrometry,Trace elements and REE elements analysis use Plasma Mass Spectrometer ICP-MS method.⑤ Trace elements and REE elements standardization,tectonic environment discrimination.Results：① Under the microscopical identification,the main minerals of the vitric dunite,they contain only large phenocrysts of olivine,are porphyritic texture.Yet not,the alteration rock,the main minerals,they contain abundant large phenocrysts of olivine and serpentine,are blastoporphyritic texture.The phenocrysts are serpentinized olivine illusion.Groundmass of the each sort is glass structure.② Results of X-ray diffraction analysis,olivine is forsterite [（ Mg（1.8）Fe（0.2））（ SiO4） ],serpentine is lizardite{（ Mg,Al）3[（ Si,Fe）2O5]（OH）4 }.③ By the detection of electron probe,the meimechite samples comprise of these minerals.Serpentinized veins show as deep colour across the main phenocrysts and the matrix,Larger bright fine stripe magnetites are found in the olivine,while the fine-grained groundmass contains chlorite,epidote,zoisite,hornblende,ilmenite,limonite,pentlandite,plagioclase,spinel,phlogopite, and serpentinized glass, besides the main minerals olivine and serpentine.④ Chemical compositions data on the rock from this area are reported.They are low in SiO2（ 36.40% ～ 37.29%） and Al2O3（ 2.61%～4.70%） and TiO2（ 0.058% ～ 0.22%） and Na2O ＋ K2O（ 0.03% ～ 0.14%）; Relatively,high in MgO（ 33.30% ～35.36%） and Fe2O3＋FeO（ 10.51% ～ 12.89%）.It is a especial ultramafic extrusive rock.Mg#= 83.5 ～86.5,δSr = 1.14 ～ 4.61,Sr/Y = 5.70 ～ 42.74.∑ REE = 2.18 ～ 9.71,δEu = 0.68 ～ 2.43,La/Yb = 0.90 ～ 10.00.Trace elements and REE elements have the lithologic characteristics of the primitive mantle.Conclusions：① The original magma may undergo the process of crystallization and differentiation,and also,which is supposed to be formed in the rift valley tectonic environment.② We propose that the rocks are compositionally similar to type Siberian meimechites and closely resemble the ＂Meimechite ＂– low in TiO2 eruptives of Meimechite from Maimecha—Kotui Province and Kamchatka peninsula.The finding of the rocks in this region,has extremely important significance for the study of the formation and evolution of magma,and the prospecting and exploration of copper and nickel ore deposit.