Magmatic-Orogenic cycles
508-2015_109
N. American Cordillera scale
•
•
•
•
•
No depth bias;
Mostly upper plate-derived magmas;
Significant pre-existing crust involved;
Flare-ups are compressional;
Fluxes vary by a factor of 10.
Regional age-depth
12
10
Depth (km)
8
6
4
2
0
0
50
100
150
200
250
Age (Ma)
300
350
400
450
Quartz delta 18O- all N. American Cordilera-
Average quartz ~ 9.4 ‰ SMOW
Nd isotopes in Cordilleran batholiths
Average North American Cordillera nd(i)= -4.5
Predominant input of North American lithosphere
Magmatic fluxes
• Average (Reymer and Schubert, 1984)
– 20-40 km3/kmMa
• Lulls and flare-ups.
– Need detailed info on individual pluton surface
area, depth of emplacement and age. Lots of data
everywhere, very little synthesis work has been
done. Enough data for Sierra, Coastal batholith of
Peru, maybe a couple more worldwide;
– 10-25 km3/kmMa, baseline
– Up to 500 km3/kmMa flare-ups
Barton et al., 1988
Barton, 1996
Idaho Batholith - Strontium
IDB - Age vs. Sr(i)
0.74
Sr(i)
0.73
Idaho
0.72
Montana
0.71
0.7
0
20
40
60
80
100
Age (Ma)
120
140
160
180
N Peninsular Ranges Batholith (K only)
0
65
75
85
95
-5
eNd(t)
-10
-15
-20
-25
Age (Ma)
105
115
125
Epsilon Nd vs time
California arc
Coast Mountains - Including Alaska
Flare-ups are marked by increased N-American input- these are
compressional arcs.
What drives high flux events?
plate kinematics
upper plate deformation
Other?
Plate kinematics and fluxes
• Data available only for magmatic arcs
– Early work on the Coast Mountains
batholith (Armstrong, 1988) proposed they
may be correlated: faster convergence higher fluxes.
California arc - no correlation (see next
slide).
Apparent intrusive flux vs. time and plate motions
Ducea, 2001
Magmatic flare-ups
• Short, high flux events separated by lulls
• Baseline fluxes coincide with steady state
island arcs (10-30 km3/km Ma). Flare-ups
generate 10 times more magma within short
(5-15 My) periods. Most of the continental
arcs are made in flare-ups.
• Don’t know what ignites the high flux events
– We do know they are compressional events.
Some Relevant Facts:
Shortening in some thrust belts amounts to 300-700 km (Andes,
North American Cordillera, Himalaya)
Rocks involved are almost exclusively upper crustal
Requires disposal of large volumes of continental lower crust and
lithosphere beneath orogenic belts
Some Questions:
What are the spatial-temporal relationships between shortening,
magmatism, and mantle processes?
Can we track these through time?
How much continental crust is involved and where does it go?
Implications for mantle chemistry and Earth evolution?
The concept of a tectono-magmatic cycle in major
orogenic belts, with emphasis on subduction of
continental crust and lithosphere on the foreland
side: Cordilleran but can be extended to collisional
Age of ductile thrusting
predates flare-ups by some
15-35 my
DeCelles, Ducea, Zandt,Kapp 2009
What is not in the model
• Forearc and trench contributions,
known to be a factor, in some arcs
• Possible storage and release
mechanisms in the lower crust
• Need better constraints on plate
kinematics to fully rule those effects out.
•>65% of the volume of magmatic rocks
in the coastal batholiths of North America
are formed in short, 5-15 My flare-up (hih
flux) events;
•These episodes are dominated by upper
plate mass, mantle and crust;
•They are not demonstrably related to
plate kinematic parameters in the few
areas where enough data is available;
•Oxygen isotopes are clearly showing
tens of % of mass is recycled crustal;
•One mechanism suggested here is
retroarc thickening. It reconciles geologic
constraints.