The AGM was held on 21st January, a cold evening with frequent snow flurries. The AGM was followed by one of Dr. Geoff Steel's very interesting talks. On this occasion his talk was entitled, “The Tyrone Ophiolite: ancient ocean floor on land in Northern Ireland”.
Geoff commenced the talk with an explanation of ophiolites in general. They are interpreted as fragments of ocean floor that have been sheared off from subducting crust, caught between colliding continents and then elevated to a continental location. A typical ophiolite suite can be defined as consisting of the following (from base up): unlayered serpentinised peridotite, layered gabbro, massive gabbro, sheeted dyke complex, pillow basalts, deep sea sediments. Although not all ophiolites contain all the layers. Ophiolites are important as they allow access to oceanic crust and upper mantle that otherwise would be unobtainable. The age of oceanic crust is dated to about 200my. Therefore, an ophiolite that is dated as much older allows study and understanding of much earlier events and provides evidence for the existence of old oceans and oceanic basins. The Tyrone ophiolite was obducted onto the continental margin of Laurentia during closure of the Iapetus Ocean during the Ordovician Period.
It was Geoff's intention to observe as many elements of the ophiolite as possible. On his tour he visited Beltonanean Mountain and Black Rock where he observed the gabbro. The gabbro here consisted of coarse to very coarse-grained varieties. The gabbros at Black rock are also cut by younger balsaltic and doleritic intrusions which are in the form of 1 to 2m wide dykes. These sheeted-dykes are also part of the ophiolite. They were also viewed at Carrickmore where they cut through Hornblende gabbro. Originally these dykes would have acted as feeders for the overlying sequence of pillow lavas. The latter were viewed at Creggan rock. The final location on the tour was the Carraghinault gold mine. This gold was formed as part of the orogenic phase of the closure of the Iapetus Ocean. As this is a working quarry it could only be viewed from a distance.
The talk ended with some of the members now thinking of a visit to Ireland!
The next meeting will be on Wednesday 18th February. Club Secretary, Bill Bagley will be giving a talk entitled: “Quartz and Other Forms of Silica.”