The Magnificent Morton Gneiss, Part 15: Split Personality | Morton, Minnesota, USA
A close-up of the same roadcut featured in the four previous posts of this series. It's situated on the southwestern edge of the Morton Outcrops Scientific and Natural Area, and near the northern end of the access drive leading from US Route 71 to the gas station at the corner of 71 and Minnesota 19.
To get an overall view of this outcrop, see Part 12 of this series.
After meditation and intense self-scrutiny I have decided to reveal an awful truth about myself. I am decidedly maficentric. I know it isn't right, but I have trouble treating felsic igneous rocks (granitoids, etc.) with the same level of respect that I shamelessly lavish upon mafics and ultramafics of all sorts—your gabbros, your basalts, your komatiites and peridotites, and their metamorphic derivatives, the amphibolites.
There. It's out in the open. I'm not proud of it, but it's who I am.
Still, now you can understand why I took this photo. Of course it wasn't all the light-colored granitoid gneiss or pegmatite I wanted to capture. It was that black and weakly foliated amphibolite clast, which at some point in its immensely long existence has been split by a vein of felsic magma during some now long-forgotten bit of crustal compression and mountain-building.
To my knowledge, the amphibolite, unlike the other components of the Paleoarchean-to-Neoarchean Morton Gneiss migmatite, has not been assigned an age (or ages) of origin. Could it be that geochronologists are felsicentrists? Perhaps, but one source, Goldich and Wooden 1980, mentions that the rock contains radiometric markers indicating that "the [already extant] amphibolites were involved in a high-grade metamorphic event approximately 3,000 m.y. ago as well as in the 2,600-m.y. B.P. [before present]"
The implication in that and other papers I've read is that the amphibolite is either somewhat older, the same age, or somewhat younger than the oldest tonalitic gneiss assigned an age of 3,524 ± 9 Ma = 3.524 Ga. Or it may have had more than one origin, and hence more than one age. Accordingly, I just call it "very old."
To see the other photos and descriptions in this set, visit my Magnificent Morton Gneiss album.
The Magnificent Morton Gneiss, Part 15: Split Personality | Morton, Minnesota, USA
A close-up of the same roadcut featured in the four previous posts of this series. It's situated on the southwestern edge of the Morton Outcrops Scientific and Natural Area, and near the northern end of the access drive leading from US Route 71 to the gas station at the corner of 71 and Minnesota 19.
To get an overall view of this outcrop, see Part 12 of this series.
After meditation and intense self-scrutiny I have decided to reveal an awful truth about myself. I am decidedly maficentric. I know it isn't right, but I have trouble treating felsic igneous rocks (granitoids, etc.) with the same level of respect that I shamelessly lavish upon mafics and ultramafics of all sorts—your gabbros, your basalts, your komatiites and peridotites, and their metamorphic derivatives, the amphibolites.
There. It's out in the open. I'm not proud of it, but it's who I am.
Still, now you can understand why I took this photo. Of course it wasn't all the light-colored granitoid gneiss or pegmatite I wanted to capture. It was that black and weakly foliated amphibolite clast, which at some point in its immensely long existence has been split by a vein of felsic magma during some now long-forgotten bit of crustal compression and mountain-building.
To my knowledge, the amphibolite, unlike the other components of the Paleoarchean-to-Neoarchean Morton Gneiss migmatite, has not been assigned an age (or ages) of origin. Could it be that geochronologists are felsicentrists? Perhaps, but one source, Goldich and Wooden 1980, mentions that the rock contains radiometric markers indicating that "the [already extant] amphibolites were involved in a high-grade metamorphic event approximately 3,000 m.y. ago as well as in the 2,600-m.y. B.P. [before present]"
The implication in that and other papers I've read is that the amphibolite is either somewhat older, the same age, or somewhat younger than the oldest tonalitic gneiss assigned an age of 3,524 ± 9 Ma = 3.524 Ga. Or it may have had more than one origin, and hence more than one age. Accordingly, I just call it "very old."
To see the other photos and descriptions in this set, visit my Magnificent Morton Gneiss album.