Petrographical and mineral chemistry characteristics of mafic dykes in Nhi Ha area, Ninh Thuan and its meaning

Abstract

The NE-SW and NW-SE mafic dyke rocks in Nhi Ha area intruded through Cretaceous granitoid Dinh Quan or overlapped by Quaternary sediment materials. It has block type and blue grey in color in the field. Under microscope, based on minerals and textures, divided into 2 types of mafic dyke rock, such as diabase and porphyritic diabase. Minerals chemical analysis by EPMA method: plagioclase almost are labradorite (An 60.80%), oligoclase (An from 12.31 to 14.29%), some grain had albitation (An from 6.75 to 8.46%); pyroxene with augite (Wo_41,80 En_44,22 Fs_13,98 to Wo_43,40 En_37,50 Fs_19,10) and diopsite composition (Wo_45,45 En_42,48 Fs_12,07 to Wo_48,16 En_40,02 Fs_11,81); amphiboles in basalt are magnesio-hastingsite (Ca-amphibole group). Popular alteration minerals are chlorite, epidote and sericite. Chlorite is pycnochlorite type, ratio Fe/(Fe+Mg) <0,6 show that it formed by hydrothermal alteration process with temperature from 150 to 325⁰C. Opaque minerals are pyrite, chalcopyrite, ilmenite, magnetite, titanomagnetite, titanite (sphene) and a little bit of rutile.