Transcript Shallow subduction in a late Archean orogen: Evidence from 2.67

Diamonds in Archean Shoshonitic
Lamprophyres - Conclusions

The mixed peridotitic - eclogitic character of
diamond inclusions; low-P inclusions (plagioclase)
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Shoshonitic character of the host rocks
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Old-source Metasomatism in sub-arc style low-Al
websterites
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Orogenic Setting of host dikes and related plutonic
and volcanic rocks & Association with Lode Gold
--> All implicate a shallow diamond source that
requires a cold finger effect in the Archean Upper
Mantle
The P-T requirements can only be met
Subduction Tectonics
Diamond-Bearing Shoshonitic
Lamprophyres
In situ diamonds first
found in 2.7 Ga
lamprophyres near
Wawa, then in an
Abitibi occurrence.
Ongoing analysis by the
Ont Geol Survey
continues to find more
Abitibi occurrences
Timing, setting, host
rocks are all wellconstrained
Diamonds in Xenolithic Lamprophyre Dikes (2674 ± 8 Ma) and
Breccia Diatremes (2679.2 ± 2.1 Ma )
Wide range of diamond morphologies, inclusion types, complex N systematics
In diamond host lamprophyres the matrix is inherently primitive (14 wt% Mg) ).
A special magma? No, a spectrum of variably evolved 2674 Ma magmas exists
but longer crustal residence times guarantees no diamond preservation in
low-Mg examples. Abundant ultramafic xenoliths signifies rapid (± energetic)
ascent.
Metasomatised Low-Al Websterite Xenoliths: Higher
Rare Earth Content = “Old depleted mantle” Nd isotope
signature (previously melted)
T = 2674 Ma,
assuming REE
systematics are
re-set near the
time of dike
emplacement
2.7 Ga upper mantle
Hydrous fluids derived from an old source (100’s of Ma)
- probably linked to Wawa 3 Ga Events/Sources
Geodynamic Constraints
Any model must:

Be consistent with lamprophyre geochemistry (and that of related
shoshonitic syenites, trachytes, etc) that requires a subduction-style
modified mantle source

Meet diamond P-T requirements via a shallow mantle magma

Fit the upper crustal orogenic context associated with the time of
lamprophyre emplacement

Be consistent with the emplacement of Orogenic (vein) Gold
deposits

Accommodate arc-basement style (cumulate) websterites
overprinted by water-rich fluids that have “seen” old rocks
Subduction Diamonds: Griffin et al.
Model for NSW
500,000 carats
(placer diamonds)
Diamond-bearing slab punctured by deep lamproitic ( ) magma
Not consistent with shoshonitic lamprophyre depths ( ) of origin or
subduction-related chemistry
Another distinctive feature of the southern Superior
is the presence of “High-silica” adakites
(the adakites of Drummond & Defant)
Diamonds in shoshonitic lamprophyres can be
linked to subduction via models of adakite genesis
Not to be confused with pre-2.7 Ga flat subduction!
In the Gutscher et al model early slab melting and adakites are
followed by freezing of the mantle wedge
…..leads to a conceptual model for diamonds in and above a
“flat” subducting slab at depths appropriate for entrainment
in shoshonitic lamprophyres (Wyman et al.)
Ellipsis Modeling: Flat subduction & Orogeny
(O’Neil & Wyman)
a
b
c
•
As predicted, the mantle wedge freezes during flat subduction
•
The wedge enters the diamond stability field
•
At the start of orogeny, a continuous zone of diamond stability
extends from the slab through the wedge into the cratonic root
•
The wedge begins to reheat as the slab sinks after subduction
•
“Subduction-style” magmas ascend as the slab sinks (red arrow)
Flat Subduction Model: Multiple Diamond Histories
Conclusions
The association of diamondbearing shoshonitic
lamprophyres is a direct
consequence of flat
subduction prior to orogeny.
Moreover, flat subduction
after a widespread plume
event links komatiite
distribution with high silica
adakites, subduction
diamonds, and major gold
deposits…..