METASOMATIZED PORPHYRITIC GRANITE
Here’s a porphyritic granite from the Jurassic of Nevada that has been significantly changed by chemical alteration. Intense chemical alteration (adding chemicals to a rock that weren’t there to begin with) is one of the key agents of metamorphism, and is called metasomatism.
The granite has been altered by the addition of sodium (Na) and calcium (Ca). Granites have to have a fair amount of K-feldspar, which is typically orangish- to pinkish- to salmon-colored. This rock lacks a pinkish K-feldspar component. The K-feldspar that used to be present is gone - it’s been altered to light grayish-colored plagioclase feldspar by the addition of Na & Ca to the rock.
The original quartz in the granite is still there (medium to dark gray, glassy-looking crystals) - it takes a lot to knock quartz out of a rock. The original mafic minerals in the granite (such as biotite mica or hornblende amphibole) have also been chemically altered - they are now actinolite (= small, pale greenish-gray patches).
The rock also has small, but noticeable, light brown crystals of sphene (a.k.a. titanite), a calcium titanium silicate mineral (CaTiO(SiO4)).
Geologic unit & age: metasomatized porphyritic granite dike intruding the McLeod Hill Quartz Monzodiorite of the Yerington Batholith, Bajocian Stage, Middle Jurasic, 168.5-169.4 million years.
Locality: ~0.3 miles west of Mickey Pass, ~crest of the Singatse Range, west of Yerington, northeast of Artesia Lake, central Lyon County, western Nevada, USA (= stop 8 of Dilles et al., 2000) (approximately 38° 58’ 51” North, 119° 15’ 02” West).
Sodic-calcic altered porphyritic granite (8.8 cm across) from the Middle Jurassic Yerington Batholith of western Nevada. Medium to dark gray = quartz; light gray = plagioclase feldspar; pale greenish gray = actinolite; small light brown crystals = sphene/titanite.
Most info. from:
Keith Wood (pers. comm.)
Dilles & Wright (1988) - The chronology of early Mesozoic arc magmatism in the Yerington District of western Nevada and its regional implications. Geological Society of America Bulletin 100: 644-652.
Dilles et al. (2000) - High-level sodic-calcic alteration. in Magmatic and hydrothermal features of the Yerington Batholith with emphasis on the porphyry Cu(Mo) deposits in the Ann Mason area. in Contrasting styles of intrusion-associated hydrothermal systems. Society of Economic Geologists Guidebook Series 32: 83-85.