Glomeroporphyritic metabasite (amphibolite)

--- at China University of Geosciences- Beijing, China

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Figures 1-2. A spectacular porphyritic igneous rock in which aggregates of centimetre-sized plagioclase feldspar phenocrysts ("glomerocrysts") occur within a fine-grained groundmass. The latter appears to be rich in biotite and amphibole, and to have a maximum grain size of about 1 mm. Translated into English as "amphibolite" in the label of the smaller sample, located in the nearby museum on the campus. Another outdoor "mudan stone", perhaps half the size of the CUGB example, but the identical lithology, can be seen in the garden of the Geological Museum of China, next to Exit D (southwest) of the Xisi (line 4) subway station.


"Rock of the Month #178, posted for April 2016" ---

Glomeroporphyritic metabasite

in two large specimens at the China University of Geosciences - Beijing. The multi-tonne polished block is on the grounds of the campus, about 100 metres south of the Research Building. The smaller, labelled sample is in the museum galleries (systematic collections of minerals, rocks and ores) on the 12th floor of the nearby Yifu Building. The museum holdings are well worth viewing, as are the grounds and buildings. The campus is an oasis of calm in north Beijing, some 10 km north of Tiananmen Square. The streets and gardens contain at least two dozen species of trees, including magnolias and others which flower in late March and early April. The buildings, such as the Geoscience Conference Centre (built for the 30th International Geological Congress in 1996) feature assorted, visually undeformed granitic and dioritic plutonic rocks, used in diverse ways as polished building stone.

This month's samples are evidently metamorphosed but do not show obvious deformation. The igneous textures, with the intriguing flower-like aggregates of plagioclase phenocrysts, are consequently preserved beautifully. The label (see Figure 6) lists the source province as Henan, a region of east-central China. I looked at the fine grain size of the matrix and took the rock to be of basaltic volcanic origin. A geologically young age would be consistent with the evident lack of penetrative deformation of the rock. However, the rock has been identified as a diorite porphyry, so maybe this was a shallow intrusive rock, later exposed by uplift and shaped by weathering and erosion into a spectacular boulder, later well-polished by admiring humans! The age of the rock is stated to be 1750-1800 Ma, mid-Proterozoic.

The traditional Chinese interest in the aesthetic properties of geological specimens differs from the Western emphasis on crystal form and mineral species. In China the interest is on large, pleasing stones for gardens and parks, or on rocks such as this month's boulder, with the phenocrysts presenting a biological analogy to flowers (Ottens, 2005, pp.20-21). Small rounded pebbles, with radiating aggregates of celestine (strontium sulphate) prisms in a dark matrix, are called chrysanthemum stones, and are a kind of "worry stone" in China, with alleged healing properties. These are also known as chrysanthemum palms, in reference to the frond-like appearance of both tree and crystals. These generally fit easily in the palm of the hand, weight in the 50-200 gram range. Yangtze River Yuhua agate is known as rain flower stone. All these are unrelated to our featured rock, except for the visual allusions to plants. In some cases the term "chrysanthemum stone" is also extended to clusters of clear "rock crystal" quartz prisms. The name seems natural, in that east Asia and especially China are said to be the source of the chrysanthemum family.

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Figures 3-6. Here are some additional close-ups of the textures, and a photo of the label of the museum display sample, number 10Y062. The label affirms that the essential mineralogy is plagioclase feldspar in a hornblende matrix, and that the source province is Henan. An artistic translation of the texture is "peony stone", another reference to the flower-like plagioclase feldspar crystal aggregates. More than that, this name refers also to the "City of Peony" - Luoyang City, Henan province, roughly 700 km S.S.W. of Beijing.


Acknowledgements Thanks to Dr Patrick Wen for original translations, and to Dr Jun Meng for additional insights into the nature, age and source of the rock.

Reference

Ottens,B (2005) Mineral dealing in China. Mineralogical Record 36 no.1, 17-23. January issue.

Graham Wilson, 03,04,08,10 April 2016, last update 17 May 2016.

See other samples from CHINA, such as these tektites from Guangdong

or one of the many superb granite building stones in China, a rapakivi granite,

or colourful arsenic sulphides realgar and orpiment,

or a remarkable late Triassic crinoid, Traumatocrinus from Guizhou, China,

or coarse prisms of green beryl from China ,

or visit the Turnstone "Rock of the Month" Archives!