granodiorite - teaching hand specimen of a subduction-related granitic rock

Description

If making multiple purchases, please read the note following the
red text
below!
Teaching hand specimen of granodiorite - a subduction-related igneous rock
This granitic rock differs from granite, since it normally has very little orthoclase, and with plagioclase being the predominant feldspar.  In granite, orthoclase is the predominant feldspar and it is this feldspar that gives many granites their pink color. Quartz makes up more than 20% of the composition of both granite and granodiorite and both rocks can have the dark minerals biotite and hornblende, though these dark minerals make up a greater percentage of the composition of granodiorite.  This granodiorite has abundant biotite, some hornblende and has a surprisingly large amount of orthoclase feldspar. Its quartz content is about 15%.
Granodiorite typically originates above a subduction zone from the fractional melting of the iron and magnesium-rich mantle wedge above the subducting plate. It forms large batholiths covering thousands of square miles. This granodiorite is from the Sierra Nevada batholith in California, which was intruded during the Jurassic and subsequently uplifted over five miles. The top two miles have been stripped off by erosion and weathering to expose this rock.
When a granodiorite magma reaches the surface it forms dacite, with more quartz than the andesite lavas of composite volcanos such as Mount St. Helens, Vesuvius and Mt. Fuji, though dacite and andesite are similar in appearance and are often difficult to distinguish in the field.
The irregular black mineral in these specimens is biotite mica. The rectangular black crystals are hornblende. For beginning geology or grade school students, recognizing this as a "granitic rock," in the family of granite, is an acceptable outcome,  even though it's intermediate in composition. They should be able to tell the biotite from hornblende. Geology majors in a petrology course should be able to come up with the exact name.
The extrusive equivalent of granodiorite is dacite.
The second QAPF diagram shows the composition of dacite. Notice how it overlaps the compositions of granodiorite and tonalite. To compare, select one diagram. Click to enlarge. Hover over it to get left and right arrows and toggle back and forth.
The field photos show a granodiorite boulder with an intruded dike, and weathering boulders of granite which intruded the granodiorite of the Sierra Nevada batholith nearby. Notice how much darker the granodiorite is than the granite. Both photos were taken at Sand Canyon in Inyo County, California.
Select a specimen:
When more than one specimen is shown, you can select a particular specimen by telling us what is in front of it - a black mechanical pencil, a silver and black pen, a silver and blue pen, or one of those plus some number of coins, or you can let us make the selection.
Making multiple purchases?
Just add items to your cart. When finished shopping, click on
“request total”
at upper right in the Shopping Cart window.
Do not proceed to checkout.
We will send an eBay invoice with the correct combined shipping.
The combined shipping will often be less than eBay shows. Please wait for that invoice before paying.
There is no obligation to purchase. You can decide after seeing what shipping will cost.
Shipping:
Ships in a Priority Mail small flat rate box unless combined with other purchases. We always combine shipping to get your purchase to you at the lowest cost. We do not charge for handling. We refund any overpayments on shipping beyond our actual cost.
International customers:
We sell and ship internationally. If eBay’s page does not let you enter your country name to determine the shipping cost for something you are interested in, please contact us.  We can tell you what the shipping would cost and how to make the purchase. You can then decide if you wish to complete the transaction. Use the
Contact Seller
link in the Seller Information box on the right side of this page.
Pay securely with your debit or credit card through PayPal! You do not need a Pay Pal account to do this. Yes, we do combine shipping!