quartz diorite - teaching hand specimen of a subduction-related granitic rock
Quartz diorite is intermediate between gabbro and granite and is often associated with subduction and the emplacement of subduction-related batholiths. It has a composition similar to the andesite of volcanic arcs.
Commonly speckled black and white, quartz diorite is a member of a group of plagioclase feldspar-rich plutonic rocks, ranging from diorite with less than 5% quartz, to quartz diorite with 5% to 20% quartz to tonalite with over 20% quartz. The black mineral in this quartz diorite is biotite. The quartz diorite was emplaced in Late Jurassic as part of the Sierra Nevada batholith. These specimens were collected on the west side of the batholith, just upstream from the mouth of the Kern River gorge. When we showed it to other geologists, the first comment was, "What a pretty rock." It is a pretty rock.
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