Graveyard Geology of Chicago, Part 4: Close-up of an Entrance-Pylon Ashlar Block, Graceland Cemetery, Uptown Neighborhood (1896)
A sinistral digital appendage of an older Homo sapiens specimen is here provided for scale. It measures a little more than 9 in (23 cm) from wrist to middle fingertip, and gives some indication of the size of the beastly feldspars in this rock-faced block of Waupaca Granite. These took a very long time to crystallize out of their source magma.
And so we have our first up-close look at this remarkable if regrettably rare igneous-intrusive stone selection. The big brick-red chunks are alkali feldspars—primarily microcline and orthoclase. They're set in a matrix of hornblende and biotite mica, both blackish, and glassy gray quartz. What's not apparent, though, is the Waupaca's rapakivi texture that features white plagioclase haloes around the ruddy phenocrysts. To see that, we really need a nice polished section. And that will be provided later.
The Waupaca Granite formed from partly melted lower continetal crust in the Mesoproterozoic era, and has been radiometrically dated in the range of 1.484±2 to 1.468±4 Ga. It's one part of a major magmatic feature in central Wisconsin geologists call the Wolf River Batholith (WRB).
The origin of the WRB remains a matter of debate because it apparently did not occur in association with a subducting plate, but rather in what even then was continental interior. Suffice it to say that something rather weird may have been happening down in the Earth's mantle that had an effect on the crust above it.
To learn much more about this site and 200 others in the Windy City, make it your top personal priority to get a copy of my book Chicago in Stone and Clay, described at www.cornellpress.cornell.edu/book/9781501765063/chicago-i...
And to peruse the other photos and descriptions in this series, pay a visit to my Graveyard Geology of Chicago album.
Graveyard Geology of Chicago, Part 4: Close-up of an Entrance-Pylon Ashlar Block, Graceland Cemetery, Uptown Neighborhood (1896)
A sinistral digital appendage of an older Homo sapiens specimen is here provided for scale. It measures a little more than 9 in (23 cm) from wrist to middle fingertip, and gives some indication of the size of the beastly feldspars in this rock-faced block of Waupaca Granite. These took a very long time to crystallize out of their source magma.
And so we have our first up-close look at this remarkable if regrettably rare igneous-intrusive stone selection. The big brick-red chunks are alkali feldspars—primarily microcline and orthoclase. They're set in a matrix of hornblende and biotite mica, both blackish, and glassy gray quartz. What's not apparent, though, is the Waupaca's rapakivi texture that features white plagioclase haloes around the ruddy phenocrysts. To see that, we really need a nice polished section. And that will be provided later.
The Waupaca Granite formed from partly melted lower continetal crust in the Mesoproterozoic era, and has been radiometrically dated in the range of 1.484±2 to 1.468±4 Ga. It's one part of a major magmatic feature in central Wisconsin geologists call the Wolf River Batholith (WRB).
The origin of the WRB remains a matter of debate because it apparently did not occur in association with a subducting plate, but rather in what even then was continental interior. Suffice it to say that something rather weird may have been happening down in the Earth's mantle that had an effect on the crust above it.
To learn much more about this site and 200 others in the Windy City, make it your top personal priority to get a copy of my book Chicago in Stone and Clay, described at www.cornellpress.cornell.edu/book/9781501765063/chicago-i...
And to peruse the other photos and descriptions in this series, pay a visit to my Graveyard Geology of Chicago album.