THE AUREOLE OF THE MAIN DONEGAL GRANITE
Authors: PITCHER WALLACE SPENCER , READ HERBERT HAROLD 
Published: October 2009

The Main Donegal Granite, intruded into Dalradian metasediments a n d metadolerites under conditions that combined high temperature and strong directed pressure, has produced a wide aureole of contact schists that are like many regionally metamorphosed rocks. The aureole rocks were primarily of low metamorphic grade and, towards the granite contact, intense new structures and high-grade mineral assemblages have replaced the regional characters.

Metadolerites occur throughout the aureole and can be used for metamorphic correlation of the diversified metasedimentary contact schists. The regional metadolerites are assemblages of albite, zoisite, actinolite and quartz, with relict pyroxene ; ophitic textures are preserved. Towards the granite contact, these rocks pass into coarse plagioclase-amphibolites with strong foliation and lineation.

Mineralogical and structural transformations in the aureole are interconnected. Calc-pelites, outside the aureole showing calcite, quartz and phlogopitic mica, develop tremolite within the aureole with diopside at the immediate contact. Pelitic rocks develop kyanite, staurolite, garnet, andalusite, chloritoid, plagioclase, muscovite, biotite, with fibrolite near the contact. Structural effects are recorded farther out than mineralogical reconstruction. Small open folds appear and become tightened and steeper towards the granite; strain-slip cleavage first appearing in the outer aureole becomes the powerful schistosity of the inner aureole. Foliation, lineation, mullioning and boudinage of the rocks of the inner aureole are shared by the marginal parts of the granite.

Certain minerals, such as staurolite, kyanite, plagioclase and garnet, formed during the early stages of deformation, were rotated and ruptured during a later stage in which other minerals continued to grow to make the sehistosity. A final episode is marked by great cataclastic ruptural belts that form most of the external edge of the aureole. The conditions in the aureole began with a brittle state, passed to plastic deformation and returned to brittleness.

In the rafts enclosed in the granite, stability is reached by the formation of granoblastic hornfels-like rocks of simple character. Pelites give sillimanite- or andalusitegneisses, metadolerites give granoblastic hornblende-andesine-quartz rocks.

The question of the controls of aluminium silicate formation is discussed. Fibrolite is considered to be metasomatic. Kyanite and andalusite are not replacements of one another and are coeval with the staurolite, garnet and plagioclase. Andalusite tends to occur in the outer aureole, kyanite in the inner. This suggests that andalusite forms at lower temperature and stress, kyanite at higher temperature and stress. But their development is not that of simple isochemical zoning, since andalusite seems to occur in rocks of special composition––pelites rich in divalent cations, Fe and Mg, the concentration of which may control the species of aluminium silicate formed.

The mineral composition, the texture and structure of the aureole rocks agree with the interpretation of the emplacement of the Main Donegal Granite previously put forward by the authors.