A review on the origin of micrographic granites (Masanites) in the southern Kyongsang Basin, Korea

Abstract

The origin of micrographic granites (formerly called Masanites) in the southern Kyongsang Basin is discussed on the basis of the petrological, mineralogical, and geochemical characteristics along with comparisons with experimental references. The micrographic granite in the Masan area occurs as several dikes of minor quantity in a hornblende-biotite granite, whereas those in the Kimhae and Busan areas are gradational to the porphyritic biotite (±hornblende) granites, forming discrete stocks together with the accompanying granites. Geochemical properties of the micrographic granites are largely dependent upon those of the accompanying granites. This suggests that the individual micrographic granites are not independent intrusive bodies. The most important factor for the formation of micrographic intergrowths was probably the degree of undercooling during the decompression under a kinetically driven, nonequilibrium condition. The micrographic intergrowth of quartz was probably formed at moderate degree (at least 100°C) of undercooling from a H2O-saturated magma emplaced into the shallow crustal levels. Common occurrence of miarolitic cavities in the micrographic granites indicates that vapor exsolution would enhance the degree of undercooling of the melt. Chemical compositions of magmas were another important factor for controlling the sequence of liquidus phases. Quartz should have higher liquidus temperature than alkali feldspar under the condition of moderate degree of undercooling. The bulk composition of magmas might not correspond to the eutectic composition, so as to form micrographic intergrowths, given the fact that the composition of the Kimhae micrographic granite is incompatible with any known eutectic one. The spatial distribution of the micrographic granites was largely controlled by penecontemporaneous faults and/or fractures. This suggests that the micrographic intergrowths were formed in specific parts within a pluton where decompression as well as vapor exsolution was rapid enough to result in moderate degree of undercooling.