Download - Gel 201 lecture 13
Precambrian Geology
Step 1: Earth, Sun, and planets form out of collapsing, spinning,
gassy nebula
Step 2: Solar wind
“blows” away close most
gases; proto-planets form by accretion
Step 3: Planets grow,
collapse under own gravity producing
heat
Step 4: Contraction, plus impacts and fission
melt planet; materials
differentiate
Step 4: Contraction, plus impacts and fission
melt planet; materials
differentiateMantle & core by 4.5 Ga
Step 5: Outgassing:
H2O, etc. released
producing primitive
atmosphere/ocean
Zircon Step 5: Outgassing:
H2O, etc. released
producing primitive
atmosphere/ocean
Water by 4.4 Ga
Step 5: Outgassing:
H2O, etc. released
producing primitive
atmosphere/ocean
• Earth cools: rapid loss of short half-life isotopes, slowing of impacts, etc.
Cooling not linear
When does plate tectonics start?
When does plate tectonics start?
What is plate tectonics?
What is plate tectonics?
?
What is plate tectonics?
1) Rigid plates (lithosphere)2) Sea floor spreading (move apart
& create new crust)3) Subduction (slide beneath, get
recycled)
What is plate tectonics?
1) Rigid plates (lithosphere)2) Sea floor spreading (move
apart, create new crust)3) Subduction (slide beneath, get
recycled)
Precambrian explorers circa 1885
Cross-section: North Shore Lake Superior
Ages worked out through Stenonian principles
BE
C
D A
Cross-section: North Shore Lake Superior
Ages worked out through Stenonian principles
BE
C
D A
Cross-section: North Shore Lake Superior
Radiometric dating allowed correlation within NA/world
Orogeny = Mountain building
(‘oros’ = ‘mountain’ + ‘genesis’ = ‘creation’)
Orogeny = Mountain building
Orogenic belt = Linear/arc-shaped zones of deformation/ volcanism
Orogeny = Mountain building
Orogenic belt = Collision zones!(Mountain ranges)
Stable interior of NA
= craton
Geological provinces
(defined by radiometric dates)
Dates = Archean to late Protereozoic;Concentrically arranged by age
OLDEST
Dates = Archean to late Protereozoic;Concentrically arranged by age
What are the provinces made of?
X-bedded sandstone
X-bedded sandstone
Graded, immature seds (i.e., volcanics eroded/deposited near source)
X-bedded sandstone
Also: LOTS of granite, volcanics, etc., often
very metamorphosed
Thick, widespread flood basalt: ca. 1.2 Ga
Plus…
Oldest rocks = two kinds: Greenstones and Gneiss belts
Greenstone belts: named for green minerals produced when ultra mafic volcanic rocks metamorphosed
Komatiite
Suggest oceanic volcanism, with associated sediments
Komatiite
Suggest oceanic volcanism, with associated sediments
Komatiite:Only form in Archean—hot process
Gneiss belts: granites, volcanics, & seds
Continental-style material
Oldest zircons (4.4
Ga) and oldest rocks (>4.03 Ga)
Story? Joining of independent continental & oceanic blocks
A collage
Greenstone belts = “captured” basins
Greenstone belts = “captured” basins
Greenstone belts = “captured” basins
A Timeline
Approx. 2 billion years ago, Archean terranes collide (Superior, etc.,
stitched by Trans-Hudson Belt)
Process continues: Approx. 1.5 Ga, Archean
frags & Proterzoic terranes arrive
Proterozoic
Proterozoic
Then… approx. 1.2 Ga the craton rifts,
forms flood basalts
Finally: Approx. 1 Ga Grenville arrives, Laurentia
assembled
Finally: Approx. 1 Ga Grenville arrives, Laurentia
assembled
Voila! Ancient North America! Simple!
More realistic depiction
Whitmeyer and Karlstrom, 2007
So, do we have tectonics?
So, do we have tectonics?
When?
What is plate tectonics?
1) Rigid plates (lithosphere)2) Sea floor spreading (move apart
& create new crust)3) Subduction (slide beneath, get
recycled)
What is plate tectonics?
1) Rigid plates (lithosphere)2) Sea floor spreading (move apart
& create new crust)3) Subduction (slide beneath, get
recycled)
What is plate tectonics?
1) Rigid plates (lithosphere)2) Sea floor spreading (move apart
& create new crust)3) Subduction (slide beneath, get
recycled)
What is plate tectonics?
1) Rigid plates (lithosphere)2) Sea floor spreading (move apart
& create new crust)3) Subduction (slide beneath, get
recycled)
Are you sure?
June, 2006: Wyoming
All the smart and famous scientists
June, 2006: Wyoming
Estimates?
June, 2006: Wyoming
Estimates? 4 Ga to 1 Ga
4 Ga to 1 Ga—Why?
4 Ga to 1 Ga—Why?
Conclusive proof: Ophiolite suites (scooped up oceanic crust) and blueschist (metamorphic rocks assoc. with high
pressures). None till ~1 Ga
3 Ga to 4 Ga
End of meeting
Final resolution?
Diamonds!
Kimberlites: Mantle derived
Kimberlites: 125-175 km
Mineral inclusions
>3 Ga peridotite minerals
>3 Ga peridotite minerals
<3 Ga eclogite minerals
>3 Ga peridotite minerals
<3 Ga eclogite minerals
<100 km
Big picture
Earth forms
Differentiation
Water and initial crust
Pseudo-tectonics and
accretion
Pseudo-tectonics and
accretion
• ~3 Ga modern-style plate tectonics
• ~2.5-2 Ga large, stable cratons
Microplates, plumes, direct down-going
>3 Ga
Analogy for early Archean
Analogy for early Archean
Analogy for early Archean
Microplates, plumes, direct downgoing
<3 Ga
2.5-2 Ga continents
formed(Wilson
cycle begun)
Next class:Proterozoic