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?


?


1) Rigid plates (lithosphere)2) Sea floor spreading (move apart

& create new crust)3) Subduction (slide beneath, get

recycled)


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

Radiometric dating allowed correlation within NA/world

Orogeny = Mountain building

(‘oros’ = ‘mountain’ + ‘genesis’ = ‘creation’)


Orogenic belt = Linear/arc-shaped zones of deformation/ volcanism


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

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?




recycled)




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 eclogite minerals


<3 Ga eclogite minerals

<100 km

Big picture

Earth forms

Differentiation

Water and initial crust

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

Microplates, plumes, direct downgoing

<3 Ga

2.5-2 Ga continents

formed(Wilson

cycle begun)

Next class:Proterozoic

Gel 201 lecture 13

Education

Transcript of Gel 201 lecture 13