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Gel 201 lecture 13

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Information about Gel 201 lecture 13
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Published on March 8, 2014

Author: jasondloxton

Source: slideshare.net

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Precambrian Geology

Step 1: Earth, Sun, and planets form out of collapsing, spinning, gassy nebula

Step 2: Solar wind “blows” away close most gases; protoplanets 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 Mantle & core by 4.5 Ga differentiate

Step 5: Outgassing: 2O, etc. H released producing primitive atmosphere/ ocean

Zircon Step 5: Outgassing: 2O, etc. H released producing primitive atmosphere/ ocean

Water by 4.4 Ga Step 5: Outgassing: 2O, etc. H released producing primitive atmosphere/ ocean

Cooling not linear • Earth cools: rapid loss of short half-life isotopes, slowing of impacts, etc.

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

Ages worked out through Stenonian principles C B E D A Cross-section: North Shore Lake Superior

Ages worked out through Stenonian principles C B E D A Cross-section: North Shore Lake Superior

Radiometric dating allowed correlation within NA/world Cross-section: North Shore Lake Superior

Orogeny = Mountain building (‘oros’ = ‘mountain’ + ‘genesis’ = ‘creation’)

Orogeny = Mountain building Orogenic belt = Linear/arcshaped 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

Plus… Thick, widespread flood basalt: ca. 1.2 Ga

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)

A collage Story? Joining of independent continental & oceanic blocks

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)

Proterozoic Process continues: Approx. 1.5 Ga, Archean frags & Proterzoic terranes arrive Proterozoic

Then… approx. 1.2 Ga the craton rifts, forms flood basalts

Finally: Approx. 1 Ga Grenville arrives, Laurentia assembled

Voila! Ancient North America! Simple! Finally: Approx. 1 Ga Grenville arrives, Laurentia assembled

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?

All the smart and famous scientists June, 2006: Wyoming

Estimates? June, 2006: Wyoming

Estimates? 4 Ga to 1 Ga June, 2006: Wyoming

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

Pseudotectonics and accretion

Pseudotectonics and accretion

• ~3 Ga modernstyle 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

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