Geology of Kennesaw Mountain, migmatite, McConnell & Abrams, 1984
Crop of Plate I, McConnell & Abrams
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Crop of Figure 9, page 18, Vincent et al.
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McConnell, K.I. & C.E. Abrams. 1984. Geology of the Greater Atlanta region. Georgia Geologic Survey Bulletin 29. Atlanta, Georgia.
Vincent, H.R., K.I. McConnell & P.C. Perley. 1990. Geology of selected mafic and ultramafic rocks of Georgia: a review. Georgia Geologic Survey Information Circular 82, Atlanta, Georgia.
Laura Lake Mafic Complex
llu migmatitic garnet amphibolite of the Laura Lake Mafic Complex undifferentiated
llg pyroxene (relict)-bearing metagabbro
lld meta-quartz diorite
pfu Powers Ferry undifferentiated unit of the Sandy Springs Group-intercalated biotite gneiss, mica schist and amphibolite
unmapped metaultramafic rock and banded iron formation.
Magnetite occurs as common porphyoblasts in amphibolite and coarse-grained amphibole-quartz-plagioclase rock is common neosome.
The area west of the Chattahoochee Fault marked amp/hgn/fgn includes undifferentiated amphibolite, hornblende gneiss and felsic gneiss.
The red rectangle on the right map is an approximate indication of the cropped area of the Higgins et al 2003 map.
Vincent, H.R., K.I. McConnell & P.C. Perley. 1990. Geology of selected mafic and ultramafic rocks of Georgia: a review. Georgia Geologic Survey Information Circular 82, Atlanta, Georgia.
Laura Lake Mafic Complex
llu migmatitic garnet amphibolite of the Laura Lake Mafic Complex undifferentiated
llg pyroxene (relict)-bearing metagabbro
lld meta-quartz diorite
pfu Powers Ferry undifferentiated unit of the Sandy Springs Group-intercalated biotite gneiss, mica schist and amphibolite
unmapped metaultramafic rock and banded iron formation.
Magnetite occurs as common porphyoblasts in amphibolite and coarse-grained amphibole-quartz-plagioclase rock is common neosome.
The area west of the Chattahoochee Fault marked amp/hgn/fgn includes undifferentiated amphibolite, hornblende gneiss and felsic gneiss.
The red rectangle on the right map is an approximate indication of the cropped area of the Higgins et al 2003 map.
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Years ago, when I visited the now long-defunct Georgia Geologic Survey book store in Atlanta, I found McConnell & Abrams with accompanying map plates when it was a brand new publication. For two decades this was the definitive word on Atlanta geology. It greatly informed and influenced both my observations and mental constructs.
By this I mean that I studied the book and its maps and learned the names of the geologic formations. As I encounter them in the field, I read the technical description of the rock and match it with what I see. At this time I was utterly unaware of the metamorphic history of the area and the very concept of Barrovian metamorphism and the incredible story plate tectonics has to tell in the Atlanta area. I was looking mostly at the rocks and just wondering why this mountain is here and where all this gneiss came from.
In 1989 I stumbled upon the fascinating career testing buildings for radon gas. Over the next decade I made well over 5,000 measurements throughout the greater Atlanta region. Early on I recognized that radon is geologically controlled and began recording all of my test locations and results on the McConnell & Abrams map.
This extensive travel and correlation with the map helped me to become very conversant about the various rocks, their location, composition and names, and their propensity for producing radon outgassing. So much so that two papers were accepted on the relation of radon to geology in Atlanta:
Ranger, L.S. 1993. Geologic control of radon in the greater Atlanta region, Georgia. In The 1993 International Radon Conference pre-prints, American Association of Radon Scientists and Technologists, p.IVP-2.
Ranger, L.S. 1995. Radon and geology in the Atlanta area. In the 1995 International Radon Symposium pre-prints, American Association of Radon Scientists and Technologists, p.IIIP-1.1.
By this I mean that I studied the book and its maps and learned the names of the geologic formations. As I encounter them in the field, I read the technical description of the rock and match it with what I see. At this time I was utterly unaware of the metamorphic history of the area and the very concept of Barrovian metamorphism and the incredible story plate tectonics has to tell in the Atlanta area. I was looking mostly at the rocks and just wondering why this mountain is here and where all this gneiss came from.
In 1989 I stumbled upon the fascinating career testing buildings for radon gas. Over the next decade I made well over 5,000 measurements throughout the greater Atlanta region. Early on I recognized that radon is geologically controlled and began recording all of my test locations and results on the McConnell & Abrams map.
This extensive travel and correlation with the map helped me to become very conversant about the various rocks, their location, composition and names, and their propensity for producing radon outgassing. So much so that two papers were accepted on the relation of radon to geology in Atlanta:
Ranger, L.S. 1993. Geologic control of radon in the greater Atlanta region, Georgia. In The 1993 International Radon Conference pre-prints, American Association of Radon Scientists and Technologists, p.IVP-2.
Ranger, L.S. 1995. Radon and geology in the Atlanta area. In the 1995 International Radon Symposium pre-prints, American Association of Radon Scientists and Technologists, p.IIIP-1.1.
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This results in me being very conversant and visually familiar with the rocks of the Atlanta area. Since the summit of Kennesaw Mountain is only 4.1 km (2.6 mi) from my house, I spend a good bit of time looking at and hiking on the mountain—more for botany than geology—but it is here that the two disciplines meet and make me very curious about how they interact with each other.
I learn from McConnell & Abrams that the mountain is mafic and made of "migmatitic garnet amphibolite" and that the Vulcan quarry 4.6 km (2.7 mi) north is producing "meta-quartz diorite". It's 0.441 sq km (0.17 sq mi), 43 m (140 ft) deep hole is visible in the middle of this photo taken from the top of Kennesaw Mountain. From this I needed to learn two new words, mafic and migmatite, and reacquaint myself with the diorite I learned in the west. |
View north from the summit of Kennesaw Mountain including the Vulcan quarry
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Mafic
adj. [Geology] Pertaining to minerals or igneous rocks composed of minerals that are rich in iron and magnesium, dense, and typically dark in color. The term comes from the words magnesium and ferric. Common mafic minerals are olivine and pyroxene. Basalt is a mafic igneous rock. (Compare with felsic.) [The Oilfield Glossary]
McConnell & Abrams call the dark rock of Kennesaw Mountain migmatitic garnet amphibolite (see my photo of the garnets here). The first clue to understand the story of the dark rock of Kennesaw Mountain is in the basalitc origin of many mafic rocks and is the protolith of what is now called the Ropes Creek metabasalt of Kennesaw Mountain. (See my notes on amphibolite.)
When I learn that mafic rocks contain far more calcium* than the typical gneiss and schist of the Piedmont, it tweaks my interest in the effects this might have on the flora. It turns out there are quite a number of plants on Kennesaw Mountain that are more at home in the basic limestone areas of the Valley and Ridge than the acidic Piedmont. If you click here and head to the bottom of the page you can download my checklist of the flora of Kennesaw Mountain with many introductory notes on the effects of the geology on the flora.
*Vincent et al found calcium levels from 10.80 to 16.20% in five samples of Laura Lake mafic complex rock, almost triple the amount in granite and granite gneiss.
Watch out for the dark rocks!
There will be much confusion if you just look for dark rocks. The rocks of Kennesaw Mountain have been subject to weathering for nearly 200 million years. That's a lot of time for the exposed surface of the rock to be modified chemically or biologically. There are probably a hundred or more species of lichen that grow on the rock, some are dark, some are light. The manganese in the rock is just soluble enough to come out of the rock and coat the surface with eons of wet and dry periods. This makes the rock very dark. The only way to really tell if the rock is light or dark is to break a piece and see an unweathered surface. Since this is a National Park, this is illegal, thus requiring more work at finding naturally exposed bare rock surfaces.
adj. [Geology] Pertaining to minerals or igneous rocks composed of minerals that are rich in iron and magnesium, dense, and typically dark in color. The term comes from the words magnesium and ferric. Common mafic minerals are olivine and pyroxene. Basalt is a mafic igneous rock. (Compare with felsic.) [The Oilfield Glossary]
McConnell & Abrams call the dark rock of Kennesaw Mountain migmatitic garnet amphibolite (see my photo of the garnets here). The first clue to understand the story of the dark rock of Kennesaw Mountain is in the basalitc origin of many mafic rocks and is the protolith of what is now called the Ropes Creek metabasalt of Kennesaw Mountain. (See my notes on amphibolite.)
When I learn that mafic rocks contain far more calcium* than the typical gneiss and schist of the Piedmont, it tweaks my interest in the effects this might have on the flora. It turns out there are quite a number of plants on Kennesaw Mountain that are more at home in the basic limestone areas of the Valley and Ridge than the acidic Piedmont. If you click here and head to the bottom of the page you can download my checklist of the flora of Kennesaw Mountain with many introductory notes on the effects of the geology on the flora.
*Vincent et al found calcium levels from 10.80 to 16.20% in five samples of Laura Lake mafic complex rock, almost triple the amount in granite and granite gneiss.
Watch out for the dark rocks!
There will be much confusion if you just look for dark rocks. The rocks of Kennesaw Mountain have been subject to weathering for nearly 200 million years. That's a lot of time for the exposed surface of the rock to be modified chemically or biologically. There are probably a hundred or more species of lichen that grow on the rock, some are dark, some are light. The manganese in the rock is just soluble enough to come out of the rock and coat the surface with eons of wet and dry periods. This makes the rock very dark. The only way to really tell if the rock is light or dark is to break a piece and see an unweathered surface. Since this is a National Park, this is illegal, thus requiring more work at finding naturally exposed bare rock surfaces.
Migmatite
Migmatitic metatrondhjemite gneiss with ptygmatic folds on Pigeon Hill
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Migmatitic metatrondhjemite gneiss with ptygmatic folds on Pigeon Hill
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Here is a thorough, if tortured, description of migmatite, with words not defined in the text asterisked and defined below:
"A migmatite is a banded, granular metamorphic rock that contains light coloured bands with evidence for partial melting. Migmatites comprise leucosome* bands, comprised largely of quartz and feldspar, which crystallised from partial melts or segregated metamorphic fluids, and mesosomes* and melanosomes* that represent the residues of partial melting and are often dominated by mafic components such as amphibole, pyroxene and biotite and are often finer-grained than leucosomes. The terms neosome and palaeosome are used to denote components of the migmatite that have experience significant and little partial melting respectively.
"Migmatites have usually experienced significant ductile deformation and exhibit incoherent folding with ptygmatic folds restricted to individual layers and extensive boudinage. Clast-like bodies can occur within migmatites and are composed of restite, solid blocks of partial melt residue, that do not undergo ductile deformation. Often leucosomes in migmatites have rims of mafic minerals known as selvages that form by reaction with the melt or a coexisting fluid. Migmatites are sometimes sub-divided into metatexites, that have experienced low degrees of partial melting, or diatexites, that have experienced high degrees. Metatexites can also often be termed hornfels or gniess and are prefixed with "migmatised".
"Migmatites form by high temperature regional and/or thermal metamorphism of protolith rocks where temperatures are sufficient to cause partial melting. They are often associated with granulites and gneisses, and often are spatially associated to granitic intrusions. The partial melting of crustal rocks (or anatexis), which results in migmatites, is responsible for the generation of many granite magmas. Partial melting within migmatites leads to the formation of a banded rock by segregation of viscous partial melts and rigid resistite due to differential stress." [Imperial College Rock Library]
*leucosomes are the lightest colored parts of a migmatite
*mesosomes are intermediate between the dark and light colored parts of a migmatite and usually an unmodified remnant of the protolith; synonymous with paleosome.
*melanosomes are the darkest parts of a migmatite
"A migmatite is a banded, granular metamorphic rock that contains light coloured bands with evidence for partial melting. Migmatites comprise leucosome* bands, comprised largely of quartz and feldspar, which crystallised from partial melts or segregated metamorphic fluids, and mesosomes* and melanosomes* that represent the residues of partial melting and are often dominated by mafic components such as amphibole, pyroxene and biotite and are often finer-grained than leucosomes. The terms neosome and palaeosome are used to denote components of the migmatite that have experience significant and little partial melting respectively.
"Migmatites have usually experienced significant ductile deformation and exhibit incoherent folding with ptygmatic folds restricted to individual layers and extensive boudinage. Clast-like bodies can occur within migmatites and are composed of restite, solid blocks of partial melt residue, that do not undergo ductile deformation. Often leucosomes in migmatites have rims of mafic minerals known as selvages that form by reaction with the melt or a coexisting fluid. Migmatites are sometimes sub-divided into metatexites, that have experienced low degrees of partial melting, or diatexites, that have experienced high degrees. Metatexites can also often be termed hornfels or gniess and are prefixed with "migmatised".
"Migmatites form by high temperature regional and/or thermal metamorphism of protolith rocks where temperatures are sufficient to cause partial melting. They are often associated with granulites and gneisses, and often are spatially associated to granitic intrusions. The partial melting of crustal rocks (or anatexis), which results in migmatites, is responsible for the generation of many granite magmas. Partial melting within migmatites leads to the formation of a banded rock by segregation of viscous partial melts and rigid resistite due to differential stress." [Imperial College Rock Library]
*leucosomes are the lightest colored parts of a migmatite
*mesosomes are intermediate between the dark and light colored parts of a migmatite and usually an unmodified remnant of the protolith; synonymous with paleosome.
*melanosomes are the darkest parts of a migmatite
Migmatitic Ropes Creek metabasalt with ptygmatic folds and boudinage on Camp Brumby Trail
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Migmatitic Ropes Creek metabasalt with ptygmatic folds on Camp Brumby Trail
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Finnish petrologist Jakob Johannes Sederholm (1863 – 1934) studied the strange mix of metamorphic and granitic rocks in the Precambrian Baltic Shield and coined the word from the Greek μιγμα migma, a mixture.
Migmatites form where the pressure and temperature regime tease with the melting point of the rock; sometimes above, sometimes below. The temperature must be at least 800°C but less than ~1,200°C. This simplifies a very complex world as all the minerals in the rock don't have the same melting point. Some will completely melt, others become plastic and yet others still completely solid and undergoing solid solution metamorphism. With viscous meeting solid and liquid, very strange patterns of minerals form, especially when the viscous are forced into the liquid or fractures in the solid and the resistance creates very odd shapes of ptygmatic folds and boudins.
Where does this sort of environment exist? Plate tectonics gives us the answer. When continents collide, especially large continents like Laurasia and Gondwanaland, massive amounts of pressure develop in many directions as the rocks pile on top of each other and fold from incredible lateral force. Migmatites form fairly deep in the earth under these gigantic collisions, usually at a depth of at least three kilometers, often much greater. But they can't be deep enough where the temperature and pressure will cause all the rock to melt. It is the strange world of "almost melting". Perhaps a poor analogy would be very slowly heating a wax candle until it is soft enough to bend, then pushing on it from several directions. If two different colors melt at different temperatures, one would melt and one would remain solid. Imagine what this would look like if you pushed on the mix from several directions at once.
Diorite
"Diorite is a grey to dark grey intermediate intrusive igneous rock composed principally of plagioclase feldspar (typically andesine), hornblende, and/or pyroxene. Varieties deficient in hornblende and other dark minerals are called leucodiorite. It is often described as “salt and pepper” when composed largely of light-colored minerals randomly interspersed with dark minerals. When olivine and more iron-rich augite are present, the rock grades into ferrodiorite, which is transitional to gabbro*. The presence of quartz makes the rock type quartz-diorite or tonalite, and if orthoclase (potassium feldspar) is present the rock type grades into granodiorite.
"Diorites may be associated with either granite or gabbro intrusions, into which they may subtly merge. Diorite results from partial melting of a mafic rock above a subduction* zone. It is commonly produced in volcanic arcs, and in cordilleran* mountain building (subduction along the edge of a continent, such as with the Andes Mountains). It appears in thousands of square miles of large batholiths (mass of intrusive igneous rock believed to have solidified deep within the earth). The extrusive volcanic equivalent is andesite.
"Diorite is a relatively rare rock; source localities include Sondrio, Italy; Thuringia and Sassonia in Germany; Finland; Romania; central Sweden; Scotland; the Andes Mountains; and the Basin and Range province and Minnesota in the USA." [Red Orbit]
My days at Crater Lake National Park gave me lots of experience with andesite and wanderings back to the Sierra Nevada batholith of California connected it to diorite. Nothing about the light rock of Kennesaw Mountain looks like either! While there is something of a mixture of "salt and pepper", the rock here is far, far lighter in color than any andesite or diorite in my experience. I'm now really confused about what I'm seeing here. The epiphany comes in 2003 with the publishing of the new Atlanta geology map by Higgins et al.
For the next chapter in the story, "putting it all together", click here.
*Gabbro is the mafic equivalent of granite, a mass of basalitc magama that cooled and solidified at depth.
*Subduction occurs when one piece of continental crust collides with another and slides underneath.
*A cordillera is a long, narrow mountain range such as the Cascades of the Pacific Northwest and the Ades of South America.
Migmatites form where the pressure and temperature regime tease with the melting point of the rock; sometimes above, sometimes below. The temperature must be at least 800°C but less than ~1,200°C. This simplifies a very complex world as all the minerals in the rock don't have the same melting point. Some will completely melt, others become plastic and yet others still completely solid and undergoing solid solution metamorphism. With viscous meeting solid and liquid, very strange patterns of minerals form, especially when the viscous are forced into the liquid or fractures in the solid and the resistance creates very odd shapes of ptygmatic folds and boudins.
Where does this sort of environment exist? Plate tectonics gives us the answer. When continents collide, especially large continents like Laurasia and Gondwanaland, massive amounts of pressure develop in many directions as the rocks pile on top of each other and fold from incredible lateral force. Migmatites form fairly deep in the earth under these gigantic collisions, usually at a depth of at least three kilometers, often much greater. But they can't be deep enough where the temperature and pressure will cause all the rock to melt. It is the strange world of "almost melting". Perhaps a poor analogy would be very slowly heating a wax candle until it is soft enough to bend, then pushing on it from several directions. If two different colors melt at different temperatures, one would melt and one would remain solid. Imagine what this would look like if you pushed on the mix from several directions at once.
Diorite
"Diorite is a grey to dark grey intermediate intrusive igneous rock composed principally of plagioclase feldspar (typically andesine), hornblende, and/or pyroxene. Varieties deficient in hornblende and other dark minerals are called leucodiorite. It is often described as “salt and pepper” when composed largely of light-colored minerals randomly interspersed with dark minerals. When olivine and more iron-rich augite are present, the rock grades into ferrodiorite, which is transitional to gabbro*. The presence of quartz makes the rock type quartz-diorite or tonalite, and if orthoclase (potassium feldspar) is present the rock type grades into granodiorite.
"Diorites may be associated with either granite or gabbro intrusions, into which they may subtly merge. Diorite results from partial melting of a mafic rock above a subduction* zone. It is commonly produced in volcanic arcs, and in cordilleran* mountain building (subduction along the edge of a continent, such as with the Andes Mountains). It appears in thousands of square miles of large batholiths (mass of intrusive igneous rock believed to have solidified deep within the earth). The extrusive volcanic equivalent is andesite.
"Diorite is a relatively rare rock; source localities include Sondrio, Italy; Thuringia and Sassonia in Germany; Finland; Romania; central Sweden; Scotland; the Andes Mountains; and the Basin and Range province and Minnesota in the USA." [Red Orbit]
My days at Crater Lake National Park gave me lots of experience with andesite and wanderings back to the Sierra Nevada batholith of California connected it to diorite. Nothing about the light rock of Kennesaw Mountain looks like either! While there is something of a mixture of "salt and pepper", the rock here is far, far lighter in color than any andesite or diorite in my experience. I'm now really confused about what I'm seeing here. The epiphany comes in 2003 with the publishing of the new Atlanta geology map by Higgins et al.
For the next chapter in the story, "putting it all together", click here.
*Gabbro is the mafic equivalent of granite, a mass of basalitc magama that cooled and solidified at depth.
*Subduction occurs when one piece of continental crust collides with another and slides underneath.
*A cordillera is a long, narrow mountain range such as the Cascades of the Pacific Northwest and the Ades of South America.