quartz diorite - teaching student specimens - UNIT OF 5 SPECIMENS
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.
Secondary students should identify this and any similar rocks as "granitic," a silica rich rock, in the same general family as granite. The tendency is to call anything that looks similar to this "granite," though granite is not particularly common. The exact composition is determined under a hand lens or microscope, or by using a thin section with a petrographic microscope. In a college petrology course, students should be able to make the identification. To read the QAPF diagram, click, then click again to enlarge even more. See how this rock fits into the classification of intrusive igneous rocks.
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