Event Report: Carrock Fell Igneous Complex- Revisited 14 May 2017

Carrock Fell Igneous Complex- Revisited 14 May 2017

Led by Alan Wise

On a bright clear morning 8 members set off from Mosedale to consider the Carrock Fell Complex by traversing the eastern side.

The leader had first mapped Carrock Fell as part of his thesis in the mid ‘70s. As well as distributing some written notes and diagrams, he set the scene by showing us his hand drawn map of the area and explaining that the Carrock Fell Igneous complex is made up of 12 different rock types between the Skiddaw slates to the South and the Eycott volcanics to the North, this includes 5 different bands of gabbro. He noted that there were xenoliths of Skiddaw slates and of Eycott volcanics in the gabbros.

The area was last surveyed by the BGS in 1928/32 and published in 1968, when it was interpreted as differentiation in situ. However, understanding and interpretation of gabbros had advanced over the last 80 years.

The party set off up Carrock Fell broadly along the contact of the Skiddaw slates and the gabbro and paused to examine a contact. It was noted that the slates were hard but no metamorphic aureole associated with Carrock Fell was observed. The leader suggested that the magma had been injected in a series of pulses as a composite sill, with cooling between each pulse, then turned so it was now almost vertical. He noted that there was evidence of chilled margins in the gabbros so that ruled out magma differentiation.

He demonstrated that the magnetite and ilmenite in the gabbros meant that compass readings were unreliable, a real problem in the 1970’s.

At the next exposed junctions of the slates and gabbros we observed some folding in the slates and looked in vain for definitive evidence that the Skiddaw slates had been overturned.

Noticing the change of slope we moved up and away from the slate and onto the gabbro. The Marginal gabbro was initially fine grained, then as we moved north, courser grained with bigger crystals and some pods of crystals.

Then we came to a fine grained andesite outcrop of some 100 meters and examined the contact at the north end with the gabbro. No evidence was known of mixing between the andesite and the gabbros. The wide andesite exposure was interpreted as the roof of the Eycott lavas which had collapsed into the gabbros.

Continuing the traverse of the slope in a northerly direction we examined an outcrop of Leucogabbro which had a fabric and crystal sedimentation then an outcrop of the Buck Kirk gabbro which had no fabric and was darker and finer grained, so was postulated as being intruded at a lower temperature as a crystal mush. An andesite dyke some 4 meters wide had been intruded later. Further exposures of light and dark bands and pegmatites were considered before crossing Further Gill Syke.

North of Further Gill Syke we moved from the gabbro through 6 types of granophyre. These were interpreted as later pulses from the same magma source as the gabbros, which had become more granitic.

We then descended to the road, avoiding the steeper parts of Further Gill Syke. As we walked to the cars the leader pointed out white cumulates with a black base dipping 60 degrees to the North in the cliff.

The leader summarised his interpretation as Marginal gabbro was intruded between the Skiddaw Slates and the Eycott volcanics. The junction was dipping at about 35 degrees to the north. Four further pulses of gabbro were intruded along contraction joints. Three of these magmas were at a higher temperature so allowed for crystal sedimentation under gravity forming layers of cumulate with a preferred orientation. The Buck Kirk gabbro was intruded as a crustal mush at a lower temperature. Granophyre and Diabase were intruded along the northern margin of the complex. Skiddaw granite was intruded to the South of the Complex and rotates the complex to the North by some 60 degrees.

Although the ground was rough and steep the party benefitted that the day was dry and bright, the bracken had not yet shot up, the leader had brought his brushes to clean the various exposures and provided fresh specimens to examine.

When the party reached the car park they thanked Alan Wise for sharing his research and knowledge and for a very interesting and challenging day and encouraged him to write up his interpretation in the proceedings.