THE KALGAN VOLCANIC COMPLEX AND THE OPTICAL PROPERTIES OF ITS ALKALI FELDSPAR PHENOCRYSTS

The Mesozoic Kalgan volcanic complex is distributed around the city of Kalgan inan area about 1000 to 1200 km～2,and with its sequence as below: (?) The volcanic complex contains a great variety of rocks,among which are basalts,augite-dacites,quartz-trachytes,quartz porphyry,rhyolite and various kinds of glassyrocks,tuffs and other volcanic ejectamenta.It is these diverse forms of volcanic rocksthat impart the splendid existing geomorphological features. 17 chemical analysis of main varieties of these rocks are taken and plotted accordingto projected the Niggli's petrochemical diagrams which show that the process of eruptionof the acidic magma was proceeding early from the normal series into the super saturatedwith aluminum series later,and from relatively rich in alkali(high Na_2O)into rich in SiO_2and Al_2O_3,and high K_2O in alkali. Six extinct craters were found by the auther in the field; thcy were all of the centraltype and the positions of eruptive centres were shifted from the southeast part of the areain the early period to the northwest part later. The alkali feldspar phenocrysts from these volcanic rocks are monoclinic or pseudo-monoclinic,their compositions are in the intermidiate range of the K-Na feldspar series,as Or45—Or55.The refractive index Ng=1.526—1.535,Nm=1.524—1.534,Np=1.519—1.529,which are projected into tuttle's diagram showing the majority close to thehigh temperature curves,but a few from the middle parts of volcanic bodies have a ten-dency close to the low temperature curves.Birefringence Ng—Np-0.005—0.007. The values of optical angles of 50 feldspar crystals from one specimen of obsidianhave been determined.(-)2V=27.5°—46°.When using thedispersive degree-2Vdand theconcentrative degrec-2Vc of the optical angle to show the variation features,(-)2Vd=46°—27.5°=18.5°;(-)2Vc=36°—29°=7°. Two vertical sections of volcanic bodies of quartz porphyry and rhyolite are chosen tostudy precisely the optical angles of feldspars from each specimen.The results show thatthere are different variation features of 2V between the basal,intermidiate and topportions of volcanic bodies.The values of 2V are smaller and 2Vd ate.greater in thebasal and top portions than those in the intermidiate portions. One crystal of Na-anorthoclase from tuff is chosen to determine its 2V at 90 pointson it,crystal surface(see table).(-)2Vd=45°-31°=14°,(-)2Vc-41°-36°=5°.From the contour line of 2V in the enlarged figure we see that the values of 2V are not uni-form,being smaller in the central part,increasing in outer zone of crystal.The contourlines appear zonal and elongated,approximately corresponding to the outline of crystal,but at the four corners of this crystal,the contours open out as harbors and the values of2V dicrease slightly. The only main variation of optical orientation of the alkali feldspar phenocryst is theangle of Np∧a,when this value increases the main value of 2V increases also. The dispersion of alkali feldspar phenocryst has been determined: rv,2Vr—2Vv=4.5° to 0.5°,dispersion of optical plane from 3° to 4.5°. The principle conclusions: 1.There are two main varieties of alkali feldspar in the Kalgan volcanic rocks.Oneis high temperature variety—low sanidine,(—)2V=26° to 40°,distributed mainly in therhyolite and glassy rocks; another is the intermediate temperature variety—high orthoclase,(—)2V=40° to 62°,distributed mainly in the quartz porphyry.But their compositionsare all in the intermediate range of K-Na feldspar series. 2.The variations of the optical angles of alkali feldspar in the Kalgan complexhave been proved to be: 2V is a sensitive optical constant of petrogenesis.Its variationis not only shown in one and the same extrusive or intrusive body,but also in one andthe same specimen of rocks or crystal with some definite regulations.