Petrology and geochemistry of Shetland granites

• Main Author: Gamil, Ali Saif
• Published: University of Liverpool 1991
• Subjects: 551; Geology
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.316890

This study is concerned with the distribution, age, chemistry and geotectonic setting of the 21 granites in Shetland. The granites range in size from a few metres to 8 Km across and in age from Pre-Moine to early Carboniferous. The granites occupy a unique position in being placed approximately half way between Scotland and Norway Caledonides and about 320 Km east of southeast coast of Greenland, if Greenland is restored to its pre-continental Drift position as predicted by the best fit of Greenland and Europe. The Shetland granites have been subdivided into 5 groups according to their location to the east or west of Walls Boundary Fault ( a continuation of Great Glen Fault), mineralogical content and their occurrence in situ as granitic pebbles, as follows; Granites to the east of WBF include (1) hornblende-bearing granites (2) hornblende-free granites and (3) Granitic pebbles a) the Rova Head conglomerate & b) the Funzie conglomerate. Granites to the west of WBF include (4) Ronas Hill granite and its satellites and (5) Sandsting & Bixter granites which are grouped together because of their proximity and because the Bixter granite seems to be the acidic end product of the Sandsting granitoid. A detailed petrological and geochemical study has been made of each group. Samples from each occurrence have been examined for mineralogical content and analysed by a variety of methods for major and trace elements (XRF, INAA, RNAA). This infonnation has been used to derive the mechanisms which may have been responsible for the observed differentiation trends. To the east of the Walls Boundary Fault (WBF) are hornblende-bearing granites, hornblende-free granites and Funzie and Rova Head granitic pebbles. The hornblende-bearing granites are truncated by the WBF and characterized by high Sr and Ba values. The Spiggie granite within this hornblende-bearing group also contains considerable amounts of primary epidote. The hornblende-free granites are a miscellaneous group of granitic rocks ranging between two-mica granites and garnet bearing granites, albite keratophyre and trondhjemite dykes. To the west of WBF are Ronas Hill granophyre and its satellites (with drusy cavities containing crystals of stilpnomelane, quartz and epidote), Sandsting complex and Bixter granites. Major element modelling suggests that a plagioclase, biotite and hornblende fractionation process is appropriate for hornblende-bearing granites and Sandsting granite. Graphical and trace element modelling do not conflict with this too, but rare earth element modelling requires extraction of a quartz-feldspar phase. The classification of Shetland granitoids on the basis of ages and petrology has not been previously attempted. According to the Read classification, the Caledonian hornblende-free granites could be classified as Pretectonic and Syntectonic intrusions equivalent to his older granites. In contrast to the hornblende-bearing granites to the east ofWBF (Graven, Brae complexes and the Spiggie granite), the Graven and Brae appear to belong to the Appinite Suite while the Spiggie granite seems to be a forceful Newer Granite. According to Read the forceful Newer Granites were emplaced just after the Appinite Suite but in Shetland the Graven and Spiggie granites appear to be the same age (400 Ma). The Ronas Hill granite and its satellites, Sandsting and Bixter granites to the west of WBF are Upper Devonian granites and equivalent to the permitted last intrusions of Read, but do not appear to be of the caulderon or ring complex type typical of those in Scotland In terms of ages the hornblende-bearing granites have given K-Ar ages of about 400-430 Ma. The Ronas Hill granite and its satellites, Sandsting complex and Bixter granite give K-Ar ages about 360 Ma. Consideration of typology indicates that the hornblende-bearing granites, Ronas Hill & its satellites, the Sandsting complex and Bixter granite are I-Caledonian type whereas some of the hornblende-free granites are close to S-type. On the De La Roche classification system the Shetland granites are high-K calc-alkaline (except trondhjemite dykes and keratophyre in the hornblende-free granites group) and mostly plotted in the shoshonitic trend The geochemical comparison of the the post Devonian Ronas Hill granite and its satellites on the one hand and the closely associated late Devonian Shetland volcanics on the other, reveals that there is no link between them. A comparison of the Shetland and Scottish granitoids in terms of major, trace and rare earth elements reveals very close similarities between these two Caledonian regions. Both show the same high K-calc-alkaline (peccerillo & Taylor, 1976), the high alumina and alkali (Kuno, 1966) character, also show a general alkali-calcic character (Peacock, 1930) and they also have very good correlation in terms of some trace element variation diagrams such as Sr and Ba