THE ROLE OF QUARTZ CRYSTALLIZATION IN THE DEVELOPMENT AND PRESERVATION OF IGNEOUS TEXTURE IN GRANITIC-ROCKS - EXPERIMENTAL-EVIDENCE AT 1-KBAR

Crystal growth experiments with haplogranodiorite compositions demonstrate that the quartz textures produced are dependent not only on degree of initial undercooling (DELTA-T(i)) but also on duration of the crystallization experiment (t). Eight granitic compositions, H2O-undersaturated to slightly oversaturated, were crystallized isothermally between 850-degrees-C and 597-degrees-C at 1 kbar for times ranging from 24 to 4320 h, producing a variety of crystal habits. Quartz crystallizes as euhedral hexagonal dipyramids, skeletal crystals, anhedral crystals, granophyric intergrowths, micropoikilitic quartz, and as a component of spherulites. Hexagonal dipyramids crystallize at small DELTA-T(i) and are replaced by skeletal quartz at moderate to large DELTA-T(i). Both tend to develop modified shapes during prolonged isothermal crystallization, characterized by a reduction in the degree of faceting and, for skeletal quartz, by deep reentrants left in partially filled crystal faces. Quartz-bearing spherulites crystallize later than single crystals of quartz at moderate DELTA-T(i) and predominate at larger DELTA-T(i). At moderate DELTA-T(i) in some melt compositions, these spherulites may enclose single skeletal crystals of quartz, forming granophyric intergrowths. At large DELTA-T(i), granophyric intergrowths are absent but micropoikilitic quartz develops around and interstitial to the spherulites. From these results, we propose that complex granitic textures can form from sequential crystallization of two or more quartz morphologies under conditions of continuous nonequilibrium cooling and that the specific sequences vary with cooling rate as illustrated on TTM diagrams. Quartz is less susceptible than fibrous feldspars to recrystallization during the experiments and probably also during normal cooling of an igneous body. Quartz therefore provides more reliable textural information than feldspars about the cooling histories of granitic melts.