Download Sudbury Igneous Complex PowerPoint

Outline
• Economic Importance of Sudbury Complex
• Geological History favoring both sides of debate.
• Defense of large mantle-derived mafic intrusion.
– Layered mafic intrusion.
– Origion of unusual 𝑆𝑖𝑂2 content
– Re/Os ratios.
Economic Importance
• Host to world’s largest known Ni-Cu sulfide ore.
• Past production and reserves: 1.65𝑥109 tons.
• Average ore composition: 1.2% Ni, 1.02% Cu.
• Platinum Group Elements.
Geological History
Sudbury Igneous Complex
*1.85 G.a.
*1.8 G.a. Penkean
Orogeny
Canadian Shield
Superior Province
Southern Province
Greenville Province
Layered Mafic Intrusion
• Eu anomalies
Similiar to Bushveld, Stillwater,
Skargaard complexs
• Layers represent fractional
crystallization.
• Platinum group elements in
lower layer.
Figure from Kuo and Crocket: Showing layering similar to
layered mafic intrusion.
Origin of High 𝑆𝑖𝑂2
• Meteorite impact resulted in crustal melting.
• Active Convergent Margin explains higher 𝑆𝑖𝑂2 content.
• Composition of Proterozoic basalts from Superior Province in
area indicate a silica rich mantle.
• Xenoliths in sublayer indicate crustal-rich parental magma
Re/Os Ratio
• Re/Os can be sensitive tracer for meteoritic contamination.
• High levels of terrestrial Os in Sudbury favor endogenic
model.
• Meteorite Defense argues
instant vaporization.
Popagai and Chicxulub impact sites.
The Other Side:
Differentiated Impact Melt Sheet
Reasons Against Layered Mafic
Intrusion
• Strontium ratios for the various lithologies
are not compatible with mantle-derived
intrusions with a 1.85 Ga age
– Ratios would need a mixture of up to 80%
crustal material to achieve those levels
• Neodymium ratios are typical for average
upper continental crust at that time
– These Nd ages correspond well with U – Pb
zircon ages for local Archean rocks in the
Sudbury area
– Again incompatible with a significant
mantle-derived component in the complex
Reasons Against cont’d.
• Layering suggests nothing because
– compositional variations have been observed
within smaller impact melt sheets (Morokweng)
– average composition of the complex is similar to
other known impact melt sheets
Explanation on why layering isn’t
commonly found
• Not thick enough
– Morokweng – 800m
thick compared to
Sudbury 2.5km thick
• Probably too siliceous
and/or viscous
• Crystallized too quickly
for differentiation to
occur
Reasoning against contamination melt
• Crustal isotopic signature and the energy
required (~1022J) for the amount of
contamination to be unrealistic
• Proper mixes of major elements from the
target rocks have been found without
contamination
– One of these mixes is 93% average Archean Abitibi
granite-greenstone terrain and 7% Huronian
Lorrain arkose
Reasoning for impact melt sheet
• The basic
stratigraphic
relations at the
Sudbury structure
are nearly the
same as those
observed at other
large complex
impact structures
Reasons for cont’d.
• Primary Na-rich plagioclase are not common in
layered igneous complexes
• The coexistence of these unusual Na-rich
plagioclase crystals with the Ca-rich ones cannot
be achieved through normal processes of igneous
crystallization
- Plagioclases with reverse
zoning and complex twinning
have, however been
described in the
Manicouagan impact melt
rocks
Reasons for cont’d.
• Shatter cones and other
shock metamorphic
features such as
– planar deformation
features (PDFs), or shock
lamellae, in quartz within
lithic clasts of the
Onaping Formation
– as well as the occurrence
of high-pressure
polymorphs, such as
impact diamonds, in the
Onaping Formation
Comparisons to other large impact structures
Conclusion
• The debate on the origins of the Sudbury
Igneous Complex still rages
– Most recently, nearly all of the evidence has
pointed to the impact melt sheet model
– But still a few hold on to the notion of mantlederived layered mafic intrusion