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.