Ajhil: /* Rocks associated with the Huntly and Knock plutons */

2019-12-03T11:23:22Z

Rocks associated with the Huntly and Knock plutons

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	==Hornfelsed and migmatitic rocks ==		==Hornfelsed and migmatitic rocks ==
	===Rocks associated with the Huntly and Knock plutons===		===Rocks associated with the Huntly and Knock plutons===
	Associated with the large mafic-ultramafic plutons and related intrusions are contaminated mafic rocks, and migmatitic and xenolithic rocks that include numerous hornfelsed metasedimentary enclaves. Ashworth (1975<ref name="Ashworth 1975">~~ASHWORTH, J R. 1975. The sillimanite zones of the Huntly–Portsoy area in the north-east Dalradian, Scotland. ''Geological Magazine'', Vol. 112, 113–136.~~ </ref>, 1976<ref name="Ashworth 1976">ASHWORTH, J R. 1976. Petrogenesis of migmatites in the Huntly – Portsoy area, north-east Scotland. ''Mineralogical Magazine'', Vol. 40, 661–682. </ref>) ascribed these high grade rocks that lie within and immediately east of the Huntly, Knock and Portsoy intrusions, and the nearby migmatitic gneisses of the Cowhythe Psammite Formation, to the sillimanite-potash feldspar zone (upper amphibolite grade). Peak metamorphic temperatures estimated for these rocks range between 700° and 950°C, with pressures ranging from 4 to 5 kb (Ashworth and Chinner, 1978<ref name="Ashworth 1978">ASHWORTH J R, and CHINNER, G A. 1978. Coexisting garnet and cordierite in migmatites from the Scottish Caledonides. ''Contributions to Mineralogy and Petrology'', Vol. 65, 379–394 </ref>; Droop and Charnley, 1985<ref name="Droop 1985"></ref>; Droop et al., 2003<ref name="Droop 2003"></ref>). Many of the rocks are migmatites and it can be difficult to separate migmatitic and partially melted metasedimentary rocks from contaminated mafic igneous rocks, both in the field and petrographically. Indeed, the distinction between hybrid igneous rocks and partially melted pelitic rocks may be gradational in some areas. The rocks are generally ‘noritic’ and contain upper-amphibolite facies mineral assemblages that typically include cordierite, hypersthene, sillimanite, garnet, biotite, K-feldspar and hercynite (spinel) (Read, 1923<ref name="Read 1923"></ref>; Ashworth, 1976<ref name="Ashworth 1976"></ref>; Droop and Charnley, 1985<ref name="Droop 1985"></ref>; Droop et al., 2003<ref name="Droop 2003"></ref>; see also discussion of the Huntly and Knock plutons, [[OR/15/026 Intrusive igneous rocks #Emplacement model\|Emplacement model to comparison with pretectonic mafic-ultramafic intrusions]]).		Associated with the large mafic-ultramafic plutons and related intrusions are contaminated mafic rocks, and migmatitic and xenolithic rocks that include numerous hornfelsed metasedimentary enclaves. Ashworth (1975<ref name="Ashworth 1975"></ref>, 1976<ref name="Ashworth 1976">ASHWORTH, J R. 1976. Petrogenesis of migmatites in the Huntly – Portsoy area, north-east Scotland. ''Mineralogical Magazine'', Vol. 40, 661–682. </ref>) ascribed these high grade rocks that lie within and immediately east of the Huntly, Knock and Portsoy intrusions, and the nearby migmatitic gneisses of the Cowhythe Psammite Formation, to the sillimanite-potash feldspar zone (upper amphibolite grade). Peak metamorphic temperatures estimated for these rocks range between 700° and 950°C, with pressures ranging from 4 to 5 kb (Ashworth and Chinner, 1978<ref name="Ashworth 1978">ASHWORTH J R, and CHINNER, G A. 1978. Coexisting garnet and cordierite in migmatites from the Scottish Caledonides. ''Contributions to Mineralogy and Petrology'', Vol. 65, 379–394 </ref>; Droop and Charnley, 1985<ref name="Droop 1985"></ref>; Droop et al., 2003<ref name="Droop 2003"></ref>). Many of the rocks are migmatites and it can be difficult to separate migmatitic and partially melted metasedimentary rocks from contaminated mafic igneous rocks, both in the field and petrographically. Indeed, the distinction between hybrid igneous rocks and partially melted pelitic rocks may be gradational in some areas. The rocks are generally ‘noritic’ and contain upper-amphibolite facies mineral assemblages that typically include cordierite, hypersthene, sillimanite, garnet, biotite, K-feldspar and hercynite (spinel) (Read, 1923<ref name="Read 1923"></ref>; Ashworth, 1976<ref name="Ashworth 1976"></ref>; Droop and Charnley, 1985<ref name="Droop 1985"></ref>; Droop et al., 2003<ref name="Droop 2003"></ref>; see also discussion of the Huntly and Knock plutons, [[OR/15/026 Intrusive igneous rocks #Emplacement model\|Emplacement model to comparison with pretectonic mafic-ultramafic intrusions]]).

	Droop et al. (2003)<ref name="Droop 2003"></ref> also used THERMOCALC in combination with garnet-biotite, garnet-cordierite and garnet-orthopyroxene Fe, Mg exchange geothermometers, the orthopyroxene-garnet geothermometer, and the garnet-orthopyroxene-plagioclase-quartz geobarometer to obtain P-T values on both nonmigmatitic and migmatitic hornfelsed rocks that occur within and marginal to the Huntly pluton. A nonmigmatitic cordierite-sillimanite-bearing hornfelsed pelite, was sampled from a large loose block on the east side of the River Deveron, north of Dunbennan Hill at [NJ 499 422]. It contains scattered subhedral garnet porphyroblasts up to 3 mm across, and randomly orientated ‘bundles’ of prismatic sillimanite, typically about 8 x 2 x 2 mm, interpreted as pseudomorphs after andalusite. The porphyroblasts lie in a fine-grained granoblastic matrix of biotite, cordierite, plagioclase, quartz and ilmenite. The cordierites are locally replaced by chlorite + andalusite. P-T values calculated for this rock were 5.1 ± 0.8 kb and 628 ± 75°C respectively, taking an arbitrary ''a''H<sub>2</sub>O value of 1.0. Droop and Charnley (1985)<ref name="Droop 1985"></ref> describe hybrid mineral assemblages from the western side of the Huntly intrusion at Cuttle Hill [NJ 494 474]; in thin section, sector-twinned cordierites, 0.5 to 1 cm across, are surrounded by staurolite and gedrite clusters, set in a matrix of biotite, plagioclase, cordierite and quartz with minor garnet and sillimanite.		Droop et al. (2003)<ref name="Droop 2003"></ref> also used THERMOCALC in combination with garnet-biotite, garnet-cordierite and garnet-orthopyroxene Fe, Mg exchange geothermometers, the orthopyroxene-garnet geothermometer, and the garnet-orthopyroxene-plagioclase-quartz geobarometer to obtain P-T values on both nonmigmatitic and migmatitic hornfelsed rocks that occur within and marginal to the Huntly pluton. A nonmigmatitic cordierite-sillimanite-bearing hornfelsed pelite, was sampled from a large loose block on the east side of the River Deveron, north of Dunbennan Hill at [NJ 499 422]. It contains scattered subhedral garnet porphyroblasts up to 3 mm across, and randomly orientated ‘bundles’ of prismatic sillimanite, typically about 8 x 2 x 2 mm, interpreted as pseudomorphs after andalusite. The porphyroblasts lie in a fine-grained granoblastic matrix of biotite, cordierite, plagioclase, quartz and ilmenite. The cordierites are locally replaced by chlorite + andalusite. P-T values calculated for this rock were 5.1 ± 0.8 kb and 628 ± 75°C respectively, taking an arbitrary ''a''H<sub>2</sub>O value of 1.0. Droop and Charnley (1985)<ref name="Droop 1985"></ref> describe hybrid mineral assemblages from the western side of the Huntly intrusion at Cuttle Hill [NJ 494 474]; in thin section, sector-twinned cordierites, 0.5 to 1 cm across, are surrounded by staurolite and gedrite clusters, set in a matrix of biotite, plagioclase, cordierite and quartz with minor garnet and sillimanite.

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Ajhil: /* Rocks associated with the Huntly and Knock plutons */

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