PREVIOUS   NEXT   CONTENTS   HOME 

5. Petrographic Descriptions of Eight Rocks and Artefacts from the Wong Tei Tung Site

Here, the rock sample, rock core and thin section are described macroscopically, and the thin section is then described microscopically.

Number 1 Debitage from the seashore: coarse-grained siltstone

Macroscopic description

Rock and core

The rock is laminated, indurated, fine-grained sediment with a pronounced conchoidal fracture. The weathered surface is predominantly medium light grey (N6 on the GSA rock-colour chart) but locally is bluish-white (5B 9/1) A 30mm thick, greyish-orange (10YR 7/4) coarser siltstone/fine-grained sandstone band contrasts with most of the sediment where laminae are 0.1 to 1.0mm in width and vary from green-grey (light bluish-grey 5B 7/1) and 1mm wide to thin medium-light grey (N6). Clasts are 0.2 to 0.5mm in diameter.

Thin section

A 3mm thick coarse siltstone/fine-grained sandstone is pale yellowish-orange (10YR 8/6) and carries 0.5-1mm diameter clasts that are matrix supported. In the finer lithology the planar laminae vary from 0.1 to 0.2mm in width and are subparallel to each other although there are suggestions of disconformities and cross-bedding. A few laminae are slightly coarse grained and up to 1mm in thickness.

Microscopic description

A thin layer comprises a matrix-supported siltstone/fine-grained sandstone with subangular clasts of rounded quartz, zoned sodic plagioclase, alkali feldspar including perthite itself surrounding a 120 µm diameter chalcopyrite inclusion, plus small, 20-60 µm diameter, zoned zircon. Other clasts now comprise fine-grained, polycrystalline albite aggregates.

The majority of the section is a fine-grained mudstone with small quartz clasts and feldspar microliths within a matrix that includes much low birefringent chlorite/epidote(?). The mudstone has a fabric with the long axes of the small clasts lying along the laminae. Fine-grained brown carbonaceous matter 5-60 µm in length and locally associated with pale-coloured TiO2 and very rare 4 x 1 µm size graphite often lies at right angles to the laminae. Pale-coloured TiO2 aggregates, 50-200 µm diameter, comprise 1-5 µm sized crystals; elsewhere discrete TiO2 crystals are 10-50 µm in length or form thin veinlets cross-cutting the sediment. Sulphides occur in trace amounts and include 2-5 µm diameter pyrite and 10-60 µm diameter orange sphalerite. Patches of brown carbonate 100-200 µm diameter are present and are associated with 1-10 µm diameter, very rare galena(?).

Number 2 Debitage from the seashore: a planar laminated siltstone

Macroscopic description

Rock and core

The rock is laminated, indurated, fine-grained, quite homogeneous sediment. The weathered surface is predominantly very light grey (N8 on the GSA rock-colour chart) with small 0.4mm diameter pits. The cut surface is uniform with 0.2 to 0.5mm diameter pale spots. Most of the lithology is a medium bluish-grey (5b 4/1) but 4mm wide, green (greenish-grey 5G 6/1) laminae are present.

Thin section

A light grey (very light grey N8) laminated siltstone with paler laminae up to 1mm wide amid other laminae 0.2mm thick. The laminae are subparallel to each other and vary in thickness.

Microscopic description

A laminated, fine-grained, indurated siltstone/mudstone. The rock is a fine-grained siltstone/mudstone with small quartz clasts, plagioclase and feldspar microliths showing simple twinning within a matrix that includes quartz and much low birefringent chlorite. Small void spaces are infilled with albite and later chlorite. Elsewhere, small polycrystalline albite or albite-chlorite ± carbonate segregations may be pseudomorphs, void infilling or metamorphic 'clots'.

Fine-grained brown carbonaceous matter is 2-20 µm in length and is locally associated with pale coloured TiO2, sphalerite and very rare 2-5 x 1 µm size graphite or is present within 20-40 µm diameter patches of 2-5 µm long flecks. Pale-coloured TiO2 aggregates, 10-40 µm diameter, comprise 1-2 µm sized crystals; elsewhere discrete, pale-coloured TiO2 crystals are 10-30 µm in length or form thin veinlets up to 5 µm wide cross-cutting the sediment. Sulphides occur in trace amounts and include 2 µm diameter pyrite and 5-30 µm diameter yellow to orange sphalerite. Trace amounts of 5 µm diameter haematite(?) occur. Patches of carbonate are associated with sulphides and fine-grained(?) haematite and carbonate also form thin cross-cutting veinlets.

Number 3 Debitage from the seashore: a planar laminated siltstone.

Macroscopic description

Rock and core

The rock is laminated, indurated, fine-grained, quite homogeneous sediment. The weathered surface is locally a very light grey (N8 on the GSA rock-colour chart) but mainly medium bluish-grey (5B 4/1). Approximately 1mm wide, green (greenish-grey 5G 6/1) laminae are present and laminae are 0.5 to 1mm in thickness. Thin veinlets cut the laminae at right angles.

Thin section

The siltstone is mainly light grey (N7) but with 1mm wide, light greenish-grey (5GY 8/1) bands. It shows strong planar lamination with many 0.1mm thick laminae, some are quite dark. Cross-cutting veinlets, 0.5mm wide, cut the laminae at right angles.

Microscopic description

A laminated, fine-grained, indurated siltstone/mudstone. The rock is a fine-grained siltstone/mudstone with small quartz clasts, plagioclase and feldspar microliths showing simple twinning within a fine-grained matrix that includes quartz and much low birefringent chlorite. Some laminae are darker and more chlorite-rich. Small void spaces are infilled with albite and later, with chlorite or rarely white mica/muscovite. Elsewhere, small polycrystalline albite or albite-chlorite ± carbonate segregations may be pseudomorphs, void infilling or metamorphic 'clots'. Chlorite-carbonate segregations also occur.

Fine-grained brown carbonaceous matter is 5-40 µm in length and is locally associated with sphalerite; 40 µm diameter, zoned zircon is also present. Pale-coloured TiO2 aggregates, 40-120 µm in diameter, comprise 2-5 µm sized crystals; elsewhere discrete, pale-coloured TiO2 crystals are 5-20 µm in length or form thin veinlets up to 5 µm wide cross-cutting the sediment. Sulphides occur in trace amounts and include 10 µm diameter chalcopyrite and 5-50 µm diameter yellow to orange sphalerite enclosing 2-5 µm diameter galena/haematite. Trace amounts of 10 µm diameter haematite(?) occur associated with pyrite.

Fine-grained brown carbonate with euhedral rhombic margins (dolomite/ankerite?) forms thin cross-cutting veinlets, some veinlets also carry pale-coloured TiO2. A larger carbonate veinlet has polished away.

Number 4 Debitage from the seashore: an indurated fine-grained siltstone/mudstone.

Macroscopic description

Rock and core

The natural surface is a medium bluish-grey (5B 4/1 on the GSA rock-colour chart) and displays 0.5 µm pale-coloured lensoidal patches. The rock is fine-grained and homogeneous.

Thin section

The rock is a very light grey (N8). A very fine-grained, 10mm thick mudstone(?) has few spots. Most of the section comprises a fine-grained siltstone with 0.5mm diameter angular quartz and TiO2(?) spots. There is a faint fabric.

Microscopic description

A banded, fine-grained, indurated mudstone(?).

The rock is a fine-grained metasediments/volcaniclastic with small quartz, plagioclase and potassium feldspar clasts within a fine-grained matrix that includes quartz, feldspar microliths showing simple twinning and much low birefringent chlorite. Some simply twinned potassium feldspar (including perthite) is altered to fine-grained mixtures of white mica ± carbonate. Small void spaces are infilled with 20 µm long, euhedral albite or similar-sized euhedral TiO2 crystals and both are associated with later chlorite. Elsewhere, small polycrystalline albite or brown and green chlorite or albite-chlorite-quartz segregations occur.

Fine-grained, brown carbonaceous matter is 20-40 µm in diameter or is present as 1-2 µm laths in cluster up to 40 µm in diameter. Pale-coloured TiO2 aggregates, approximately 200 µm in diameter, comprise 2-5 µm sized crystals; elsewhere discrete, pale blue-white TiO2 crystals are 10-60 µm in length. Sulphides occur in trace amounts and include 10-40 µm diameter, orange sphalerite enclosing 1-2 µm diameter galena/haematite or have suffered very fine-grained chalcopyrite disease. Trace amounts of 1-2 µm long galena/haematite occur associated in rare 40 µm diameter aggregates. This has some differences from 1 3(2).

Number 5 Debitage from the seashore: an indurated fine-grained volcaniclastic.

Macroscopic description

Rock and core

The natural surface is mainly a moderate bluish-grey (5B 4/1 on the GSA rock-colour chart) but locally has a very pale surface (very light grey N8). The rock is unlaminated, homogeneous and flinty.

Thin section

The rock is a very light grey (N8). A 3mm wide very fine-grained sediment/tuff(?) with small TiO2 patches has an irregular contact surface with most of the rock, which carries many TiO2 patches and spots up to 0.5mm in diameter. The rock is homogeneous.

Microscopic description

A fine-grained, banded volcaniclastic/sediment.

The matrix is very fine grained and comprises small angular sherds of quartz and feldspar plus rare 10-40 µm diameter zircon together with slightly larger quartz, plagioclase, some with slight zoning and tabular potassium(?) feldspar. Locally rounded patches of brown carbonate associated with sulphides; 100 µm diameter chlorite associated with 5-10 µm haematite(?) and 80-100 µm diameter, fine-grained, 1-5 µm size TiO2 occur.

Sulphides include 5-40 µm diameter pentagonal dodecahedral pyrite carrying 1-2 µm diameter pyrrhotite and galena and discrete 10-40 µm diameter orange sphalerite with very fine-grained chalcopyrite-disease and thin 1 µm wide galena rims. Patches of a 10-60 µm long hard pale phase are visually identified as haematite.

Organic matter was not recognised.

Not same as 1-3

Number 6 Debitage from quarry site: a porphyritic rhyolite.

Macroscopic description

Rock and core

A 7mm thick pale orange (10YR 8/2 on the GSA rock-colour chart) weathered surface is separated from the light grey (N7) unweathered rhyolite by a 2mm wide, light brown (5YR 7/4) limonitic zone. Phenocrysts include 2mm long feldspar, 1-2mm quartz and up to 4mm long altered feldspars.

Thin section

A 1mm wide greyish-orange (10YR 7/4) limonite-stained band separates unaltered very light grey (N8) rhyolite from the yellowish-grey (5Y 8/1) outer weathered zone. Angular clear quartz phenocrysts are up to 3mm in diameter and cloudy feldspars are up to 2mm long.

Microscopic description

The rock is a slightly altered porphyritic acid volcanic, namely a porphyritic rhyolite.

Discrete phenocrysts comprise euhedral to rounded and 'resorbed' low quartz(?); laths of multiple twinned, sodic plagioclase showing a little alteration to fine-grained white mica; orange-stained muscovite laths with 1-20 µm long TiO2 minerals along cleavage planes; potassium feldspar showing simple twinning and altering along cleavage planes to fine-grained white mica and limonite; and perthite. Patch perthite is more abundant than sodic plagioclase and is altered to fine-grained white mica and limonite. Very rarely potassium feldspar may be microcline and a trace of green amphibole(?) is present in altered alkali feldspar. Glomerocrysts comprising quartz-potassium feldspar/perthite ± sodic plagioclase ± muscovite show a well-defined granophyric texture between quartz and the feldspars and are a characteristic granophyre. Euhedral, unzoned zircon up to 40 µm in diameter is rare. The phenocrysts are enclosed within a fine 'mottled' matrix showing a sub-perlitic texture (especially where the matrix is altered) comprising alkali feldspar and quartz(?). Small, 10-20 µm diameter magnetite altering to haematite; 20-60 µm diameter euhedral, zoned zircon; and 200 µm diameter TiO2 pseudomorphs after original FeTiO minerals occur in the matrix.

The rock shows marked alteration to fine-grained, pale green white mica ('sericite')/chlorite(?), local quartz mosaics and much limonite.

Opaque minerals are minor in amount. TiO2 minerals lie along the muscovite cleavage or form discrete euhedral, pale-coloured crystals 10-50 µm in size No sulphides were recognised but some orange to brown TiO2 may be sphalerite and 20-40 euhedral limonite grains are probably pseudomorphs after pyrite.

Number 7 Debitage from quarry site: very weathered/altered porphyritic rhyolite.

Macroscopic description

Rock and core

The rhyolite has uniformly weathered/altered to a greyish-orange (10YR 7/4 on the GSA rock-colour chart). Very dark red (5R 2/6) 1-2mm veinlets and patches are present. Phenocrysts include 2-3mm diameter quartz.

Thin section

The rock is a uniform pale yellowish-orange (10YR 7/6). Clear quartz phenocrysts are up to 4mm in diameter and red-stained feldspars up to 1mm long.

Microscopic description

The rock is a highly altered porphyritic acid lava (rhyolite).

Discrete phenocrystsµcomprise low quartz, some showing 'resorption'; muscovite laths altering extensively to limonite and with 5-30 µm long TiO2 crystals lying along its cleavage planes and pseudomorphs after feldspars. No relict feldspar survives but tabular/lath-shaped ex-phenocrysts now comprise fine-grained white mica (sericite), limonite, fine-grained quartz, fine-grained haematite pigment and albite. No glomerocrysts were recognised. Zoned zircon 20-80 but up to 120 µm in length are widespread and ex-FeTiO minerals including magnetite 60-200 µm in size are now totally pseudomorphed by fine-grained white to yellow to orange TiO2. Rare white leucoxene up to 200 µm in diameter may be altered titanomagnetite or ilmenite. The matrix is completely altered and is now fine-grained quartz-chlorite(?) with local quartz mosaics and void spaces partially infilled with fine-grained white mica-albite-quartz. Locally an area of matrix carries small angular glass sherds and elsewhere relict perlitic textures occur. No sulphides are present but zoned, 10-20 µm diameter limonite pseudomorphs after pentagonal dodecahedral pyrite are common. Pale-coloured TiO2 crystals are widespread.

Number 8 Outcrop rock from quarry site: a slightly weathered porphyritic rhyolite

Macroscopic description

Rock and core

The rhyolite has a 2mm wide, pale yellowish-brown (10YR 6/2 on the GSA rock-colour chart) rim about a light grey (N7) unaltered core. Phenocrysts include 0.5-1mm diameter quartz, tabular feldspar, limonite-stained phenocrysts and 0.2-0.4mm diameter limonite patches.

Thin section

The rhyolite is very light grey (N8) with small, clear, 0.4-1mm diameter angular phenocrysts and cloudy phenocrysts up to 2mm in length. A 1mm wide pale orange-grey (10YR 8/4) rim encloses the unaltered rhyolite.

Microscopic description

The rock is a porphyritic acid volcanic, namely a porphyritic rhyolite.

Discrete phenocrysts comprise rounded and 'resorbed' low quartz(?), some showing strained extinction or are surrounded by a thin alkali feldspar rim; laths of multiple twinned, sodic plagioclase showing a little alteration to fine-grained white mica or carbonate; mainly unaltered muscovite laths up to 0.2mm in length but with 1-4 µm TiO2 minerals along cleavage planes; potassium feldspar showing simple twinning and altering along cleavage planes to fine-grained white mica and brown carbonate; and perthite. Patch perthite is more abundant than sodic plagioclase and shows slight 'resorption' and is altered to fine-grained white mica and carbonate and carries 0.2mm diameter pyrite. Glomerocrysts comprising quartz-potassium feldspar/perthite ± sodic plagioclase ± muscovite show a well-defined granophyric texture between quartz and the feldspars and are a characteristic granophyre. Euhedral, unzoned zircon up to 40 µm in diameter is rare. The phenocrysts are enclosed within a fine matrix showing a sub-perlitic texture or very locally a spherulitic texture comprising alkali feldspar and quartz(?).

The rock shows some alteration to fine-grained white mica ('sericite') carbonate and widespread limonite and a little chlorite may be present as are local patches of 1-5 µm long haematite pigment.


 PREVIOUS   NEXT   CONTENTS   HOME 

© Internet Archaeology/Author(s) URL: http://intarch.ac.uk/journal/issue26/8/5.html
Last updated: Mon Oct 5 2009