Granitoid building stone (a Rapakivi Granite)

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

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1-2. Two photographs of the rock, which takes an excellent polish. It appears to be composed largely of pink orthoclase (potassium [K] feldspar, or alkali feldspar), granular grey quartz and a greenish plagioclase feldspar. The latter colour is possibly due to microscopic inclusions of iron oxides. Traces of irregular, shiny pyrite crystals may also occur in the dark groundmass.


"Rock of the Month #179, posted for May 2016" ---

A granitoid rock with spectacular igneous textures

lines the entrance and steps of the Geoscience Conference Centre and hotel, on the campus of the China University of Geosciences - Beijing. The centre was built for the 30th International Geological Congress, held here in August 1996. The outdoor patio and steps provide a huge and highly-polished "outcrop" sample, with an area on the order of 1,000 square metres, on which to examine some interesting igneous textures.

The orthoclase, forming all the largest crystals in the rock, presumably crystallized first (Figs. 1-6). Note that the presence of this coarse orthoclase in a plagioclase-bearing matrix runs contrary to the general sequence of magmatic crystallization in Bowen's reaction series. This, the abundant inclusions, and the evident rims on some K-feldspar crystals all imply a state of disequilibrium in the granitic magma. The orthoclase megacrysts, in particular, show evidence of multiple phases of growth, trapping concentric bands of the finer-grained host-rock minerals. They are often rounded rather than tabular, and may have undergone substantial resorption by the surrounding magma. Some further observations follow:

  • The coarse, pink K-feldspar appears to be perthitic orthoclase;
  • The visually estimated average QAP mode of the rock is 25-55-20, consistent with classification as granite (syenogranite - the QAP mode refers to Streckeisen's classification using the average proportions of quartz, alkali feldspar and plagioclase in a rock);
  • No mica seen, the melt seems relatively anhydrous;
  • No ferromagnesian phases noted, except that there may be relict grains; abundant wispy hematite can be seen in plagioclase and in indeterminate grains in matrix; the rock does not seem to be appreciably magnetic;
  • Most of the well-rounded phenocrysts / megacrysts are 1.5 to 6 cm in diameter, extreme 7 cm; their form appears to be spherical to slightly ovoid; they are often twinned, as can often be seen when one oriented individual "catches the light" and reflects brightly (e.g., Figs. 1,6);
  • The green rims, that appear to be plagioclase rather than orthopyroxene, are usually 2-3 mm thick; they may be mantled by renewed K-feldspar growth; a few examples are as wide as 6 mm;
  • Quartz is generally smoky, equant and rarely subhedral, commonly 1-2 mm in diameter, occasionally to 8 mm; it is mostly in the granular matrix;
  • Some of the smaller K-feldspar phenocrysts, circa 2 cm wide, appear to be deformed against one another, suggesting that the greenish plagioclase rinds formed on the orthoclase while the rock was still in a hot, plastic state; and lastly,
  • One single instance was noted of a 7x5-cm rounded mass of unusually coarse granophyre (quartz and K-feldspar intimately mingled on a scale of circa 1-2 mm), with a partial rim of greenish plagioclase on one side (Fig. 7). This is possibly a rare example of a rounded megacryst that has remelted, affirming that the remarkable textures formed in some degree near and above the liquidus of the late-stage melt.

INTERPRETATION: This distinctive fabric, with large ovoids of K-feldspar, often mantled by rims of sodic plagioclase (typically oligoclase) is referred to as rapakivi texture (see, e.g., Muller, 2007). It is part of the phenomenon of anorogenic magmatism (hence, A-type granitoids) found worldwide. Occurrences are known in, e.g., Finland, Russia (Karelia), Greenland, Quebec, Nevada, Brazil, Australia, China and Namibia.

The presence of the plagioclase rims in many examples has resulted in a varietal name for such rocks, vyborgite or wiborgite. The type region is Finland, and, while rapakivi granites of all ages occur, they are especially common in the Proterozoic. This is true of examples in the Beijing region, dated at circa 1700 Ma. Proterozoic, anorogenic rapakivi granites and related potassic alkaline volcanics are present in the region of eastern Beijing (see Deng, 1996; Ma and Bai, 1998). One example is the Shachang massif. A possible setting for the example used to such beautiful effect in the CUGB building would be a modest sill emplaced at shallow to intermediate depth. It is suggested that magma mixing of a dry granitic melt with a reheated mafic melt might cause the changes in temperature (pressure, composition) that could explain the disequilibrium noted in the foregoing description.

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Figures 3-4. More photographs of the textures, including the striking, equant, typically 2 to 6-cm-wide K-feldspar crystals with numerous small, granular inclusions, often mantled by plagioclase feldspar, that characterize the rapakivi texture.

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Figures 5-6. Two further close-up views of the textures described in the bullet points, above.

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Figure 7. A close-up of a 7x5-cm bleb of granophyre, the only such example noted in many casual strolls across the polished "exposure" on the west end of the conference centre at CUGB.

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Figures 8-9. A uniquely large, 17x9-cm inclusion in a rock where the largest complex megacryst is circa 7x6 cm in plan. This is the largest of three such inclusions on a single large tile, possibly quarried from near the original roof, side wall or feeder conduit to the magma chamber. On the right: the granite steps and patio to the entrance of the hotel and conference centre at CUGB.


References

Deng,N (editor) (1996) Field Trip Guide, Volume 6 (Beijing and its Adjacent Areas). 30th International Geological Congress, Field Trip Guide 6. Geological Publishing House, Beijing.

Ma,X and Bai,J (1998) Precambrian Crustal Evolution of China. Springer / Geological Publishing House, 331pp.

Muller,A (2007) Rapakivi granites. Geology Today 23 no.3, 114-120.

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Figure 10. Another tile shows three K-feldspar masses being squeezed together, with a single plagioclase rind surrounding all three, indicating that the rock was in a plastic, hot state when these mantles were formed.

Graham Wilson, 14-17,26 April 2016, last update 09 June 2016.

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