硷性玄武岩中包体型金云母矿物学特征

MINERALOGICAL CHARACTERISTICS OF INCLUSIONTYPE PHLOGOPITE IN ALKALI BASALTS

一、概述蒙古高原南缘,中国河北省张家口市以北,分布着约1700平方公里的汉诺坝玄武岩。K—Ar 年龄测定结果为13.56±0.57m.y.至24.52±0.87m.y.,属中新世产物。汉诺坝玄武岩以裂隙式溢流型广阔熔岩被为其主体,不整合于太古界、侏罗系、白垩系等古老岩石之上,其间有多层河湖相沉积夹层。

The well known Hannuoba Basalts spread over about 1700 KM^2 at sourth margin of Mongolia Plateau,N.China,the K-Ar age determined as 13.56±0.57 to 24.52±0.87 M.Y.,with maximum thickness of 400M, and comprising both alkali series and tholeiilite series,both of them alternate repeadedly to form about ten liathological cycles,the evolution tendenee showing earlier basalts mainly alkali series the later tholeiilite series.Most particularly,basalts of alkali series (especially basanites) all contain in large amount of deep—sourced rock inclusions(or'rock bails') and gigantic crystals of high pressure. Inclusions:mainly Cr—spinel lherzolites(92%),proxenites(2%), gabbros(0.5%)with rare other ultrabasic and basic rocks.Gigantic crystals:a small amount of glassy augite and anorthoclase with rare Ti—magnetite,garnet and phlogopite.Inclusions also occur in mafic— ultramafic“rock—ball cumulates”in very grent amount.The greatest “cumulate body”,now known as No.1 cumulate of Tamaping,about 500M long,100M in width,with greatest thickness 60M,in which rock—balls marked by varieties of compositions,complex textures,different sizes and various shapes. Geological features of phlogopite—bearing inclusions:Phl—bearing deep—sourced inclusions here are found firstly by the authors in 1980. Main characters shown in table. In various inclusions,all phlogopites formed apparently some later than other minerals(as Fe-Sp,Am,Pyx,O1)and appear as irregular flakes or flake-aggregates filling in intergranular spaces and penetrating or enclosing ether minerals. Mineralogical characteristics or phlogopite: Phlogopites in Phl-bearing inclusions brown—yellow—brown red colored,sp.gr.2.342—2.940(4 samples);Ng≈Nm=1.5951,Np=1.5486,-2v =3°,with strong pleochroism,Ng=red brown,Nm=red brownish,Np= Kinds of Phlogopite—beaying Inclusions (?) yellowish;formular calculated is(K_(0.947) Na_(0.059) Ca_(0.045) Ba_(0.026))_(1.077) (Mg_(2.179) Fe^(2+) Ni_(0.005) Al_(0.372) Fe_(0.172) Cr_(0.054) Ti_(0.157) Mn_(0.003))_3.113 [Ali_(1.238)Si_(2.764)]_4·O_(10)·[OH_(0.507)F_(0.478)O_(1.022)]_(2.00) DTA suggests the cur- ve rather genterly flattened and only a heat-absorption peak at 1270℃,wh- ile the weight-lose apparent.Data from X-ray diffraction and wissenberg method,a=5.3297,b=9.2508(?),c=9.9093(?),β=100°,thus defined as phlogopite of IM type.K-Ar age of d_(10-10) are 26.09±0.56 M.Y.and 26.76± 0.23 M.Y.respectively. SOME RECOGNIZABLE CONCEPTIONS:1.Discovery of phlogopite- bearing inclusions in basanite is the first geological information of our country.In the light of all characters mentioned above it is believed that phlogopite is formed later than all other rock-forming minerals, however it is yet original mineral belonging:to rock-forming epoch.2.On calculation,Phl-bearing inclusions are formed at temperature 977—1200℃, at pressure around 20kb,nearly same as those of Cr-spinel lherzolite enclosed in alkali basalt,i.e.1029—1266℃ and 19.43—21.86kb.The woho surface here is about 33KM in depth.It may be believed that Phl-bearing inclusions,just as Cr-spinel lherzolite inclusions,both are origenated from upper part of upper mantle.3.K-Ar age determination for phlogopite suggests that rock-forming epoch of these inclusions is near to that of the carrier basalts,or i.e.both of them are closely related genetically.4.Hannuoba basalt plays a most important part in widespreading miocene basalt belt,E.China and thus here is the most favorable area for research field on basalt,ultramafic and upper mantle rocks.