Siem Reap has 98km of new roads and new sidewalks much of which has been paved with interesting felsic intrusive rocks from quarries in Shandong Province in China. The composition ranges from granodiorite to tonalite and is locally granophyric and pegmatitic. The intrusive consists dominantly of plagioclase, quartz, pyroxene and hornblende. Ovoid structures known as Miarolitic Cavities are evident in the sidewalks to the observant and indicates that the parental magma was hydrous. While no cavities have been observed in the sidewalks of Siem Reap it is quite possible that miarolitic cavities which retained cavities were not processed into saleable dimension stone. When the location of the quarry in Shandon Province is identified maybe a field trip is warranted.
The term miarolitic comes from the Italian miarole in reference to the mineral-rich pegmatite region of Baveno and Cuasso al Monte in northern Italy.
There are two types of miarolitic cavities observed. One has a coarsely pegmatitic core of intergrown quartz and plagioclase surrounded by a conspicuous pyroxene rim and the core may contain coarse tourmaline. The second type has a conspicuous leucocratic zone and a complex core of intergrown quartz and tourmaline. Where there are sufficient sections these features have a long axis that that is maybe 5 times the sectional axis.
In some of the miarolitic cavities late stage UST textures are evident with euhedral quartz crystals vectored into the cavity. All of the granophyric textures are observed within or around these structures. It is suggested that these features are the result of segregation of a hydrous melt phase late in the crystallization history of the host intrusion. The granophyric textures visible in hand specimen likely resulted from the simultaneous crystallization of quartz and feldspar from the hydrous melt. How much water was in this residual phase – possibly as much as 5%. Elsewhere similar features have open crystal filled cavities and often contain minerals containing elements that are incompatible with typical silicate granitic mineralogy. Minerals containing lithium, rubidium, beryllium, boron, niobium, tantalum, tin, bismuth, fluorine and rare-earth elements can often be found in miarolitic cavities.
While the miarolitic cavities are spectacular the intrusive shows rapid and widespread textural variation with pyroxene rich wispy layers and feldspar rich aggregates as shown below.
In conclusion these pegmatoidal features are mariolitic cavities which have been completely infilled by late stage minerals which would have incorporated much of the water which was incompatible with the bulk of the crystalizing magma. There is also a possibility that at least some of this hydrous phase escaped from the intrusion into the surrounding host rocks. The elongate habit of these features suggests that they may have acted as conduits for late stage hydrous melts and fluids.
