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Plate 8.3 Quartz feldspar porphyry from the volcanic pile

The porphyry contains white, cream or pink feldspar phenocrysts and glassy quartz crystals set in a microcrystalline groundmass

Plate 8.4 Quartz and feldspar phenocrysts in thin section

Table 8.2 Phenocryst population in the volcanics and volcanic feeder dyke. volcanics feeder SS116 117/4 SS157 SS158 SS160 SS161 SS162 SS163 SS167 SS175 Quartz 8.4 9.9 15.9 14.8 15.1 9.9 9.6 10.3 12.8 7.8 Feldspar 36.0 24.5 22.7 28.9 33.6 30.9 33.4 24.4 28.9 16.7 Groundmass 55.5 65.3 61.4 55.6 51.3 58.9 56.8 65.3 58.3 75.5 Total Phenocryst 42.4 34.4 38.6 43.7 48.7 40.8 43.0 34.7 41.7 24.5 Population

A recount of SS160 to check precision gave quartz (15.1%), feldspar (34.1%) and groundmass (50.8%).

The quartz phenocrysts which vary in size from 2-4mm in diameter, show both rounded and bi-pyramidal forms. The stubby hexagonal bipyramidal habit is of high p quartz which has spontaneously inverted to low alpha quartz at 570°C but which has retained the high quartz structure. Often these quartz phenocrysts are embayed or skeletal in form, indicating rapid crystallisation (Plate 8.4). Resorption features have also been noted. Sometimes also the phenocrysts are fractured and broken, which is interpreted as due to explosive eruption and volatile loss (Plate 8.5). The quartz phenocrysts are clear, unstrained and evenly distributed. Occasionally, groundmass crystals occur within the quartz phenocrysts or their embayments. These are interpreted as having filled voids in the phenocrysts at the same time as growth of myrmekitic rims to feldspar phenocrysts, during groundmass crystallisation. There are abundant fluid inclusions, some are gas rich, others are fluid-rich with moderate-sized vapour bubble and some contain birefringent daughter minerals.

The feldspar phenocrysts vary from 2-7mm in length, are regularly distributed throughout the rock, and are euhedral in shape. The phenocrysts of feldspar in hand specimen are sometimes glassy with small cracks generated by thermal shock on cooling. These cracks are characterised by the development of white or pink turbid areas. They may show Carlsbad twinning, sometimes they are broken and embayed. In thin sections SS116 and S117/4 from the southwest margin, rare feldspar phenocrysts have clear cores and turbid margins traversed by cloudy zones in which faint microperthite lamellae can be distinguished (Plate 8.6).

Plate 8.5 SS157 Fractured quartz phenocryst

set in a fine-grained groundmass with clots of biotite. Quartz feldspar porphyry. x50

Plate 8.6 SS117/4 Orthoclase phenocryst with turbid zones of

microcline set in the fine-grained groundmass of the quartz feldspar porphyry. x50

shock on cooling. Residual fluids have exploited these cracks, producing

reaction zones in which microperthite has developed. Feldspar in the groundmass has also reacted with the fluid and its increased ordering is reflected in its turbidity. More commonly however in the samples examined, the whole feldspar population is perthitic. Initially, the perthite contains regular alternations of lamellae of probable exsolution origin. In slightly altered samples, the lamellae gradually coarsen and then break down into irregular domains of intermediate microcline (dark grey) and albite (white), to give a patch perthite (Plate 8.7). Film perthite develops from the patch perthite in more hydrothermally affected specimens from the Dawa area. Myrmekitic intergrowths of quartz occur along phenocryst margins. As this overgrowth occurs even in fragmental phenocrysts, and as it is in continuity with the groundmass, it is assumed that the overgrowth was contemporaneous with groundmass crystallisation. By analogy with the work of Ike (1979) on the feldspars of the quartz porphyry of the Tibchi complex, it is believed that the transparent feldspar is orthoclase cryptoperthite and the turbid zones with exsolved albite consist of a three-phase assemblage of a dominant orthoclase component with subordinate amounts of intermediate microcline and low albite.

The matrix is variable depending on the location:­

- In samples collected at Dawa, the matrix consists of quartz and alkali feldspar. The recrystallisation of the groundmass has resulted in the formation of clots of sieve-textured, khaki-coloured biotite - poikilitically enclosing rounded quartz (Plate 8.8) - accompanied by occasional streaks, lenses and blebs of fluorite, crystals of zircon, prismatic apatite and irregular patches of opaque iron-titanium oxides, probably representing recrystallised earlier mafic minerals. In addition to the biotite clusters there are rounded granules of biotite in the groundmass. Usually these are isolated, but they sometimes form beaded chains. These granules are greeny brown to tan in colour, pleochroic to straw yellow. Rare euhedral flakes of greeny brown mica, less than 0.1mm in size and also pleochroic to pale straw colour also occur in the groundmass. These flakes are most abundant in SS160 which also has the coarsest groundmass texture.

The accessory mineral assemblage is quite varied. Allanite, zircon and apatite

occur in groundmass mica clusters. Cassiterite and sphalerite may occur in grains up to 0.5mm across, opaque ores - ilmenite, magnetite and haematite - are scattered in blebs through the groundmass with ilmenite laths up to 2mm recorded.

- In samples from the Ladini area, the groundmass mineralogy is rather different. The matrix which shows less recrystallisation than at Dawa is finer grained. It consists of quartz, feldspar and arfvedsonite with minor amounts of aegirine and no biotite. In sample SS116, the original mafic mineralogy has been completely destroyed and the only remains are clusters of opaques and rarely chlorite. In SS117/4 however, there is an anphibole whose probable composition is in the range richterite to arfvedsonite. It occurs as minute rounded discrete granules which are extremely abundant in the groundmass, less common spongy clusters up to 0.3mm in size, which are largely altered, prismatic laths, <0.004mm long in the groundmass, larger laths which are replaced by iron oxides, and rare skeletal, sieve-textured crystals up to 0.3mm

Plate 8.7 Lamellar perthite altering to patch perthite in the quartz feldspar porphyry. SS162, XPL x50

Plate 8.8 Recrystallised groundmass in SS157 with a clot of

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Plate 8.9 Arfvedsonite in the quartz feldspar porphyry

SS117/4 Skeletal, sieve-textured arfvedsonite with small granules of aegirine xl25

Plate 8.10 Quartz feldspar porphyry feeder dyke

Aggregates of patch perthite are set in a

groundmass which is coarser textured than that of the volcanic pile. x25