Mull's Tertiary Granite "enigma":

During Mull’s Tertiary volcanic history when the North Atlantic was first opening an active magma chamber at the top of a mantle plume underlay the crust. Not only did this magmatic centre of intrusive igneous activity shift gradually towards the north-west over time but some anachronistic igneous rocks were formed. The Earth’s mantle is composed of  ultra-basic material which, when it partially melts produces magma of ultra-basic or basic composition. The enigma with Mull is that its magma chamber produced acid igneous rocks such as granites, as well as basic igneous rocks such as gabbros. The sequence of  intrusion was first acid, then basic, followed by acid, then basic again. 40% of  Mull’s present Tertiary surface is granitic in chemistry. There are two theories which help explain this situation, Assimilation and Differentiation. Mull also has a very rare example of a mixed acid and basic intrusion, the Loch Ba Felsite.

1. Assimilation of country rock:

If Mull’s country rocks overlying the mantle plume were assimilated then there is no problem over space. The country rocks (basement rocks underlying the lavas) are the Lewisian, Torridonian, Moine schists and psammites. All of these, if melted during assimilation would produce an acid magma. Isotope work on these Tertiary granites at first produced very surprising results. They are known by all the field evidence to be about 60 MY old yet they gave much earlier dates. This may well be because they assimilated much older rock and the earlier radiometric date was retained. There is also the problem as to how a rising cylinder of magma made enough space for itself in the crust as it rose. The answer is provided by the phenomenom of assimilation.

2. Differentiation by crystal fractionation:

Two famous geologists, Bailey and Bowen worked closely together. Bowen worked out the details of a reaction series amongst the silicate minerals and he realised that highest temperature silicate minerals would always crystallise out first, the other silicate minerals at successively lower temperatures, and quartz last of all. As crystallisation of these specific minerals occurred they would deplete the magma in those elements taken into their crystal lattice, leaving a depleted ‘store’ of ‘ingredients’ for the other silicate minerals to form from. In this way the first crystal accumulate would form ultra-basic rock, next basic rock, then intermediate rock, and finally acid rock. The sequence of crystallisation is known as Bowen’s Reaction Series. Whilst there is evidence that this happened within Mull’s magma chamber, there is far too much granite (rhyolites, aplites and granophyres) on Mull for this to be the only process.  Mull may have an example of differentiation provided by a large irregular intrusion in Cruach Choireadail, Glen More, where the rock is basic at the base, intermediate halfway up, and acid at the top. Crystal fractionation also provides the mechanism whereby the layered gabbros of Ben Buie and Corra Bhein formed, with olivine rich bands passing upwards into feldspar rich rock.

3. The Loch Ba Felsite:

When the famous Loch Ba ring dyke was formed, overlying crust cataclysmically dropped down into the depleted magma chamber below. This magma chamber contained acid magma floating on top of  basic magma. With the crustal collapse and the explosive release of the chamber’s confined gas which pneumatically blasted and widened the ring fracture, this magma was violently intruded into the cylindrical space, acid and basic material together. Here the mix cooled and solidified, the acid material as a porphyritic felsite, the basic as twisted ‘toffee-like’ inclusions, creating this rare example of a mix intrusion. Late stage meteoric water, heated by the Central Igneous complex altered the basalt lavas adjacent to the intrusion complex.