Halifax Pluton: Thin Sections

The following are thin sections from field trip stops 1, 3, and 8, the only ones from which I took samples. >NOTE: All images are plane-polarized light, which change to cross-polarized light when you put the cursor over them.

Stop 1. This is near the Halifax Pluton outer contact, which has numerous metamorphosed sedimentary xenoliths.

Small folded metasedimenary xenolith in the Halifax pluton. The mineralogy is prinicpally biotite, quartz, plagioclase, and K-feldspar, with minor muscovite, garnet, altered cordierite, and opaques. Field width is 8 mm.

Garnet in the small metasedimentary xenolith pictured above. Field width is 1.6 mm.

Altered cordierite in the small metasedimentary xenolith pictured above. Field width is 1.6 mm.

Halifax Pluton granite, host to the small metasedimentary xenolith pictured above. Mineralogy is K-feldspar, quartz, plagioclase, biotite, with lesser muscovite and opaques. There was no cordierite or altered cordierite seen in the granite part of this thin section. Field width is 8 mm.

Stop 3. This is in the pluton interior, and this thin section is from near the center of the large xenolith.

Large metasedimenary xenolith showing poikilitic texture. Lighter large crystals are andalusite, darker patches are mostly altered cordierite. In between is mostly quartz and muscovite with minor biotite and tourmaline. Field width is 8 mm.

Remnant fresh cordierite in the center, with altered cordierite around the rim. The alteration products appear to be a fine-grained mixture of muscovite and chlorite. Field width is 1.6 mm. Black specks are presumably graphite, but maybe not.

Closeup of an andalusite crystal, containing numerous inclusions of quartz, muscovite, opaques, and tourmaline. The andalusite is light-colored and clear, and looks like it is made of many grains but all are optically continuous. Altered cordierite is seen to the upper left and lower right.

Stop 8. Halifax Pluton granite at Stop 8, a cordierite-bearing locality.

Granite at this locality has weathered pits, including pits in large K-feldspar phenocrysts, that I assumed were weathered out altered cordierite. Though this sample does have altered cordierite (see below), I saw none in thin section in K-feldspar. At least many of the weathered out pits are probably altered calcic cores of plagioclase (like the one in the center). Field width is 8 mm.

Probable altered cordierite, now a patch of intergrown chlorite and muscovite. Field width is 1.6 mm.