felsic content Archives - Geology North https://www.geologynorth.uk/tag/felsic-content/ No vestige of a beginning, - no prospect of an end Sat, 19 May 2018 23:03:09 +0000 en-GB hourly 1 https://wordpress.org/?v=6.3 https://www.geologynorth.uk/wp-content/uploads/2022/03/Icon-53x53.jpeg felsic content Archives - Geology North https://www.geologynorth.uk/tag/felsic-content/ 32 32 Dunmoor Hill https://www.geologynorth.uk/dunmoor-hill/ Sat, 03 May 2014 22:15:28 +0000 http://www.geologynorth.uk/?p=16875 We go to Dunmoor Hill in search of Al-Hafdh’s hitherto elusive chilled margins. Dunmoor Hill is a particularly good site as exposure, unlike in many areas of the Cheviot hills, is plentiful. We are finding Al-Hafdh’s thesis on the Cheviot pluton immensely stimulating. It has provided clear direction to our own research, although we are … Continue reading Dunmoor Hill →

The post Dunmoor Hill appeared first on Geology North.

]]> We go to Dunmoor Hill in search of Al-Hafdh’s hitherto elusive chilled margins. Dunmoor Hill is a particularly good site as exposure, unlike in many areas of the Cheviot hills, is plentiful. We are finding Al-Hafdh’s thesis on the Cheviot pluton immensely stimulating. It has provided clear direction to our own research, although we are beginning to doubt his proposal that the Cheviot pluton consisted of a series of ring dykes.
The area below Cunyon Crags has plenty of small outcrops where we find a bewildering mixture of felsic and mafic fine-grained material. At first, we think that the mafic rock is altered andesite. However, thin sections show very well-developed granophyric texture in the felsic rock which imply late intrusion into already established plutonic bodies. The mafic rock has a very high biotite/opaque iron oxide content, and appears to be restite.
Subsequently, we are able to trace the junction between the pink rock (Al-Hafdh’s Dunmoor type) and the more mafic Marginal rocks right across the south slope of Dunmoor Hill. The junction doesn’t show a clear cut boundary but it does present frequent inter-penetration of the two types. The implication is that neither type was fully consolidated when the intrusion took place.
We are still unable to find any chilled margins to confirm an intrusion sequence. Finer rocks always turn out to be aplite veins or small dykes.

The post Dunmoor Hill appeared first on Geology North.

]]>