Geophysical survey of Precambrian terranes in SW Lleyn, Caernarvonshire, Wales
✍ Scribed by R. J. E. Lodge; M. A. Moore; A. D. Evans; N. R. Goulty
- Book ID
- 101281691
- Publisher
- John Wiley and Sons
- Year
- 1997
- Tongue
- English
- Weight
- 288 KB
- Volume
- 32
- Category
- Article
- ISSN
- 0072-1050
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✦ Synopsis
The geology of southwest Lleyn comprises two Late Precambrian terranes: the Gwna Me lange in the west and the Sarn Complex in the east, separated by the Lleyn Shear Zone. The location of the terrane boundary is poorly constrained due to the limited exposure. We undertook a ground magnetic survey and also measured four gravity pro®les with the original intention of investigating the cause of a positive aeromagnetic anomaly previously recorded close to the terrane boundary at grid reference [SH 2200 3000]. This Sarn anomaly' appears to be associated with a shallow body within the Sarn Complex, which is known to be a heterogeneous plutonic igneous unit. Of greater interest was the identi®cation of a much larger-amplitude magnetic anomaly striking roughly N±S for over 7 km close to the mapped position of the Lleyn Shear Zone. It can be modelled as a near-vertical discontinuous body of overall dyke-like form, and is too narrow to have been resolved by the aeromagnetic survey. We discovered a previously unreported outcrop of gabbro at Brynhunog Bach [2100 3127] located on this high-amplitude Brynhunog anomaly'. It seems likely that the whole anomaly is due to a gabbroic body which is an original constituent of the Sarn Complex, but an alternative possibility is that it is a later intrusion along the Lleyn Shear Zone.