Orbicular granite

Abstract: The anorogenic Cretaceous Gross Spitzkoppe granite stock and associated bimodal dike swarms, are related to continental rifting, which led to separation of Africa from South America. The stock is composed of highly evolved, topaz-bearing, A-type granites. Subhorizontal (≤20m wide) aplite dikes and thinner aplite-pegmatite dikes cut the granites. The contact of the stock and the upper contacts of subhorizontal microgranite-aplite dikes are marked by "stockscheiders" composed of layered aplites and pegmatites. The layering is characterized by rhythmic variations in grain size, grain morphology, and mineral composition. Regular orientation of columnar alkali feldspar crystals, or plumose alkali feldspar-quartz intergrowths and dendritic biotite, indicate unidirectional crystal growth against the heat flow: in the marginal stockscheider toward the stock, in the upper parts of the subhorizontal aplite dikes downward, and in some steeply dipping aplite-pegmatite dikes from both margins inward. These structures and textures are interpreted to result from diffusion-controlled oscillatory nucleation and rapid growth from undercooled melt, coupled with changes in vapor pressure. Crystal growth from undercooled melt, with sparse alkali feldspar megacrysts acting as a substrate for the growing crystals, is indicated also by an irregular dike of orbicular granite cutting the marginal granite of the stock.

Abstract: The western end of the Galway granite batholith demonstrates the importance of stoping as a granite emplacement process, which is currently controversial, and also of space generation by uplift of the centre of a ring complex. The granite rings are shown (with a coloured 1:25 000 geological map) to be consanguineous, near coeval, and older than the 407–410 Ma late molybdenite mineralisation. A newly-recognised Mace–Ards granite, around and injected by the Aplitic Murvey-type granite of the ring core (both lacking hornblende and titanite), has biotite–muscovite–cordierite orbs and sulphide–granite orbs, showing separation of immiscible hydrous and sulphide fluids from the late magma which, with vugs, indicates a low pressure, near-roof site. The outer ring of the Errisbeg Townland granite (ETG, the main batholith granite with K-feldspar phenocrysts), was emplaced by progressive outward stoping of the country rock metagabbro, as shown by mapping, and by chemical fractionation of feldspars, biotites and bulk rocks, to the marginal, dry, fine-grained aphyric, in part garnetiferous, highly fractionated, siliceous Murvey granite. Stoping ceased when, after previously invading dense metagabbro, the outer ring complex reached the low-density Roundstone granite, which is shown for the first time to be older than the Galway batholith. This arresting of the batholith intrusion shows that stoping was such a significant process that emplacement ceased when stoping became impossible. The inside edge of the ETG grades into the slightly later, intrusive, aphyric Carna granite, which shows inward fractionation to the wet magma of the Mace–Ards granite. The ring complex core was injected by highly fractionated, dry, Aplitic Murvey-type granite, intensely hydrothermally altered by late magmatic water. The radially outward dipping, inclined igneous layering in the ETG shows that the original ETG centre was pushed upwards by the intruded Carna granite and eroded away. The Galway granite and its nearby magmatism matches the low Ba and Sr, high Th and Rb, Scottish Cairngorm Suite and similarly has few appinitic rocks associated with it. Magmatism extended over >45 Myr from ∼425 Ma to 380 Ma. It originated by slab breakoff and consequent rise of the asthenosphere, causing deep crustal melting.

Abstract: Orbicular granite gneisses occur as a xenolith within two-mica leucogranites, together with early Paleoproterozoic metasedimentary xenoliths, in Wangjeong-ri, Muju area. The whole-rock chemistries and SHRIMP zircon Pb/U ages of the leucogranites indicate that they are S-type granitoids formed in the continental tectonic setting at 1875±75 Ma. The SHRIMP age of monazites from the orbicular granite gneiss gives 1867±4 Ma as a metamorphic age which is similar to the intrusion age of the two-mica leucogranite within the error range. The similar ages between zircons and monazites represent that the orbicular granite gneisses formed by metamorphism during the intrusion of the two-mica leucogranite; the metasedimetary xenoliths which sank within the parent magma of leucogranites were metamorphosed into orbicular granite gneisses by thermal metamorphism (650-740C, 4-6.5 kbar) due to the heat supplied from surrounding magma. During the thermal metamorphism, the core of orbicular granite gneiss mainly consisting of cordierite formed, and in some orbicular granitic gneisses, the leucocratic melt formed by melting of quartz and plagioclase in the core, squeezed out from core and crystallized around the core forming outer rim. The hydrothermal fluid at the late stage of magma differentiation penetrated into the *Corresponding author Tel: 063-270-3397 E-mail: geopet2@daum.net