The Magnificent Morton Gneiss, Part 5: The Ancient and the Very Ancient | St. Mark's Catholic Church Auditorium & School, Kenosha, Wisconsin, USA (1968)
This close-up shows just one portion of a cladding panel of the entrance-portico wall depicted in Part 3 of this series.
To get the most out of the following discussion of the constituent rock types visible in this image, take a moment to review the description in Part 1.
My hand provides scale. From tip of the longest finger to the wrist is 9 in = 22.9 cm.
Before we discuss the details of this photo, keep in mind that the Morton Gneiss is a classic example of that offbeat metamorphic rock type known as migmatite. As such, it's experienced different episodes of tectonic activity, metamorphism, and injection by younger granitoid magmas. That's why it's sometimes described, by me and others, as a "composite rock type."
While a migmatite's later ingredients may or may not be subsequently metamorphosed themselves, it's always best to classify the overall migmatite as metamorphic rather than igneous. After all, it's characteristic of very highly deformed and partially melted rock bodies found in ancient metamorphic terranes.
So please note that I've never said, in either my Flickr posts or my books, that the Morton should be classified as igneous.
That understood, this photo shows two of the Morton's several constituents. The gray gneiss to the right of my hand is what I take to be the oldest granitoid-derived type. Originally granodiorite and tonalite, it has been assigned an age by the very sophisticated SHRIMP ("sensitive high-resolution ion micro-probe") technique of 3524 ± 9 Ma, or about 3.524 Ga. That makes it three-quarters the age of our planet. The black amphibolite shown in Part 4 may be as old or older, but to my knowledge it has not been dated yet.
In contrast, the pink gneiss at left and bottom center is one of the more youthful (but by our puny standards still ancient) intrusions, which have been dated from 3.37 Ga (3370 Ma) to 2.619 Ga (2619 Ma).
To see the other photos and descriptions in this set, visit my Magnificent Morton Gneiss album.
The Magnificent Morton Gneiss, Part 5: The Ancient and the Very Ancient | St. Mark's Catholic Church Auditorium & School, Kenosha, Wisconsin, USA (1968)
This close-up shows just one portion of a cladding panel of the entrance-portico wall depicted in Part 3 of this series.
To get the most out of the following discussion of the constituent rock types visible in this image, take a moment to review the description in Part 1.
My hand provides scale. From tip of the longest finger to the wrist is 9 in = 22.9 cm.
Before we discuss the details of this photo, keep in mind that the Morton Gneiss is a classic example of that offbeat metamorphic rock type known as migmatite. As such, it's experienced different episodes of tectonic activity, metamorphism, and injection by younger granitoid magmas. That's why it's sometimes described, by me and others, as a "composite rock type."
While a migmatite's later ingredients may or may not be subsequently metamorphosed themselves, it's always best to classify the overall migmatite as metamorphic rather than igneous. After all, it's characteristic of very highly deformed and partially melted rock bodies found in ancient metamorphic terranes.
So please note that I've never said, in either my Flickr posts or my books, that the Morton should be classified as igneous.
That understood, this photo shows two of the Morton's several constituents. The gray gneiss to the right of my hand is what I take to be the oldest granitoid-derived type. Originally granodiorite and tonalite, it has been assigned an age by the very sophisticated SHRIMP ("sensitive high-resolution ion micro-probe") technique of 3524 ± 9 Ma, or about 3.524 Ga. That makes it three-quarters the age of our planet. The black amphibolite shown in Part 4 may be as old or older, but to my knowledge it has not been dated yet.
In contrast, the pink gneiss at left and bottom center is one of the more youthful (but by our puny standards still ancient) intrusions, which have been dated from 3.37 Ga (3370 Ma) to 2.619 Ga (2619 Ma).
To see the other photos and descriptions in this set, visit my Magnificent Morton Gneiss album.