Transcript Tectonic-Igneous Associations   Associations on a larger scale than the petrogenetic provinces An attempt to address global patterns of igneous activity by grouping provinces based upon.

Tectonic-Igneous Associations


Associations on a larger scale than the
petrogenetic provinces
An attempt to address global patterns
of igneous activity by grouping
provinces based upon similarities in
occurrence and genesis
Tectonic-Igneous Associations
Mid-Ocean Ridge Volcanism
 Ocean Intra-plate (Island) volcanism
 Continental Plateau Basalts
 Subduction-related volcanism and plutonism
 Island Arcs
 Continental Arcs
 Granites (not a true T-I Association)
 Mostly alkaline igneous processes of stable
craton interiors
 Anorthosite Massifs

Chapter 13: Mid-Ocean Rifts
The Mid-Ocean Ridge System
Figure 13.1. After Minster et al. (1974) Geophys. J. Roy. Astr. Soc., 36, 541-576.
Ridge Segments and Spreading Rates
• Slow-spreading ridges:
< 3 cm/a
• Fast-spreading ridges:
> 4 cm/a are considered
• Temporal variations are
also known
Ridge Segments and Spreading Rates
Hierarchy of ridge segmentation
Deval
OSC
OSC = overlapping spreading center
Deval = deviation from axial linearity
Figure 13.3. S1-S4 refer to ridge segments of first- to fourth-order and D1-D4 refer to discontinuities between corresponding
segments. After Macdonald (1998).
Oceanic Crust and Upper Mantle Structure




4 layers distinguished via seismic velocities
Deep Sea Drilling Program
Dredging of fracture zone scarps
Ophiolites
Oceanic Crust and
Upper Mantle Structure
Typical Ophiolite
Figure 13.4. Lithology and thickness of
a typical ophiolite sequence, based on
the Samial Ophiolite in Oman. After
Boudier and Nicolas (1985) Earth
Planet. Sci. Lett., 76, 84-92.
Oceanic Crust and Upper Mantle Structure
Layer 1
A thin layer
of pelagic
sediment
Figure 13.5. Modified after
Brown and Mussett (1993) The
Inaccessible Earth: An
Integrated View of Its Structure
and Composition. Chapman &
Hall. London.
Oceanic Crust and Upper Mantle Structure
Layer 2 is basaltic
Subdivided into
two sub-layers
Layer 2A & B =
pillow basalts
Layer 2C = vertical
sheeted dikes
Figure 13.5. Modified after
Brown and Mussett (1993) The
Inaccessible Earth: An
Integrated View of Its Structure
and Composition. Chapman &
Hall. London.
Layer 3 more complex and controversial
Believed to be mostly gabbros, crystallized from a shallow axial
magma chamber (feeds the dikes and basalts)
Layer 3A = upper
isotropic and
lower, somewhat
foliated
(“transitional”)
gabbros
Layer 3B is more
layered, & may
exhibit cumulate
textures
Oceanic Crust and
Upper Mantle
Structure
Discontinuous diorite
and tonalite
(“plagiogranite”)
bodies = late
differentiated liquids
Figure 13.4. Lithology and thickness of
a typical ophiolite sequence, based on
the Samial Ophiolite in Oman. After
Boudier and Nicolas (1985) Earth
Planet. Sci. Lett., 76, 84-92.
Layer 4 = ultramafic rocks
Ophiolites: base of 3B
grades into layered
cumulate wehrlite &
gabbro
Wehrlite intruded into
layered gabbros
Below  cumulate dunite
with harzburgite xenoliths
Below this is a tectonite
harzburgite and dunite
(unmelted residuum of the
original mantle)
Elevation of ridge reduces with time as plate cools
Petrography and Major Element Chemistry
A “typical” MORB is an olivine tholeiite with
low K2O (< 0.2%) and low TiO2 (< 2.0%)
Only glass is certain to represent liquid
compositions
The common crystallization sequence is: olivine ( MgCr spinel), olivine + plagioclase ( Mg-Cr spinel),
olivine + plagioclase + clinopyroxene
Figure 7.2. After Bowen
(1915), A. J. Sci., and
Morse (1994), Basalts and
Phase Diagrams. Krieger
Publishers.

Fe-Ti oxides are restricted to the groundmass, and
thus form late in the MORB sequence
Figure 8.2. AFM diagram for
Crater Lake volcanics,
Oregon Cascades. Data
compiled by Rick Conrey
(personal communication).
The major element chemistry of MORBs
Originally considered to be extremely uniform,
interpreted as a simple petrogenesis
More extensive sampling has shown that they
display a (restricted) range of compositions
Table 13-2. Average Analyses and CIPW Norms of MORBs
(BVTP Table 1.2.5.2)
The major element
chemistry of MORBs
Oxide (wt%)
SiO2
TiO2
Al2O3
FeO*
MgO
CaO
Na2O
K2O
P2O5
Total
All
50.5
1.56
15.3
10.5
7.47
11.5
2.62
0.16
0.13
99.74
MAR
50.7
1.49
15.6
9.85
7.69
11.4
2.66
0.17
0.12
99.68
EPR
50.2
1.77
14.9
11.3
7.10
11.4
2.66
0.16
0.14
99.63
IOR
50.9
1.19
15.2
10.3
7.69
11.8
2.32
0.14
0.10
99.64
Norm
q
or
ab
an
di
hy
ol
mt
il
ap
0.94
0.95
22.17
29.44
21.62
17.19
0.0
4.44
2.96
0.30
0.76
1.0
22.51
30.13
20.84
17.32
0.0
4.34
2.83
0.28
0.93
0.95
22.51
28.14
22.5
16.53
0.0
4.74
3.36
0.32
1.60
0.83
19.64
30.53
22.38
18.62
0.0
3.90
2.26
0.23
All: Ave of glasses from Atlantic, Pacific and Indian Ocean ridges.
MAR: Ave. of MAR glasses. EPR: Ave. of EPR glasses.
IOR: Ave. of Indian Ocean ridge glasses.
The major element
chemistry of MORBs
Figure 13.6. “Fenner-type” variation
diagrams for basaltic glasses from the
Afar region of the MAR. Note different
ordinate scales. From Stakes et al.
(1984) J. Geophys. Res., 89, 6995-7028.
Conclusions about MORBs, and the processes
beneath mid-ocean ridges
 MORBs are not such completely uniform
magmas
 Chemical trends consistent with
fractional crystallization of olivine,
plagioclase, and perhaps clinopyroxene
 MORBs cannot be primary magmas, but
are derivative magmas resulting from
fractional crystallization (up to ~ 60%)


Fast ridge segments
(EPR)  a broader range
of compositions and a
larger proportion of
evolved liquids
(magmas erupted slightly
off the axis of ridges are
more evolved than those
at the axis itself)
Figure 13.9. Histograms of over 1600 glass
compositions from slow and fast midocean ridges. After Sinton and Detrick
(1992) J. Geophys. Res., 97, 197-216.

For constant Mg# considerable variation is still apparent.
Figure 13.10. Data from Schilling et
al. (1983) Amer. J. Sci., 283, 510-586.
Incompatible-rich and incompatible-poor mantle source
regions for MORB magmas
 N-MORB (normal MORB) taps the depleted upper
mantle source
 Mg# > 65: K2O < 0.10 TiO2 < 1.0
 E-MORB (enriched MORB, also called P-MORB for
plume) taps the (deeper) fertile mantle
 Mg# > 65: K2O > 0.10 TiO2 > 1.0
Trace Element and Isotope Chemistry
REE diagram for MORBs
Figure 13.11. Data
from Schilling et
al. (1983) Amer. J.
Sci., 283, 510-586.
E-MORBs are enriched over N-MORBs: regardless of Mg#
Lack of a distinct break suggests three MORB types
 E-MORBs
La/Sm > 1.8
 N-MORBs
La/Sm < 0.7
 T-MORBs (transitional)
intermediate values
Figure 13.12. Data from
Schilling et al. (1983) Amer.
J. Sci., 283, 510-586.


N-MORBs: 87Sr/86Sr < 0.7035 and 143Nd/144Nd >
0.5030,  depleted mantle source
E-MORBs extend to more enriched values 
stronger support distinct mantle reservoirs for Ntype and E-type MORBs
Figure 13.13. Data from Ito
et al. (1987) Chemical
Geology, 62, 157-176; and
LeRoex et al. (1983) J.
Petrol., 24, 267-318.
Conclusions:

MORBs have > 1 source region

The mantle beneath the ocean basins is not
homogeneous
N-MORBs tap an upper, depleted mantle
 E-MORBs tap a deeper enriched source
 T-MORBs = mixing of N- and E- magmas
during ascent and/or in shallow chambers

Experimental data: parent was multiply saturated with
olivine, cpx, and opx  P range = 0.8 - 1.2 GPa (25-35 km)
Figure 13.11. Data
from Schilling et
al. (1983) Amer. J.
Sci., 283, 510-586.
Implications of shallow P range from major element data:
 MORB magmas = partial melting of mantle lherzolite in a
rising solid diapir
 Melting must take place over a range of pressures
 P of multiple saturation = point at which melt was last in
equilibrium with solid mantle
Trace element and isotopic characteristics of melt reflect
equilibrium distribution between melt and source reservoir
(deeper for E-MORB)
The major element (and hence mineralogical) character
controlled by equilibrium between melt and residual mantle
during rise until melt separates as a system with its own distinct
character (shallow)
MORB Petrogenesis
Generation




Separation of plates
Upward motion of mantle
material into extended zone
Decompression partial
melting associated with
near-adiabatic rise
N-MORB melting initiated
~ 60-80 km depth in upper
depleted mantle where it
inherits depleted trace
element and isotopic char.
Figure 13.14. After Zindler et al. (1984) Earth
Planet. Sci. Lett., 70, 175-195. and Wilson (1989)
Igneous Petrogenesis, Kluwer.
Generation


Region of melting
Melt blobs separate at about
25-35 km
Figure 13.14. After Zindler et al. (1984) Earth
Planet. Sci. Lett., 70, 175-195. and Wilson (1989)
Igneous Petrogenesis, Kluwer.


Lower enriched
mantle reservoir may
also be tapped by an
E-MORB plume
initiated near the
core-mantle
boundary
Some ridge segments
may be drawn to
vigorous plumes
(e.g. Iceland)
Figure 13.14. After Zindler et al.
(1984) Earth Planet. Sci. Lett., 70,
175-195. and Wilson (1989) Igneous
Petrogenesis, Kluwer.
Langmuir “corner flow” model for rising and diverging
mantle passing through a triangular melting region
Hotter plume (deeper origin at a) creates larger melt triangle
than cooler mantle (shallower origin at b)
Mantle rising nearer axis of plume traverses greater portion
of triangle and thus melts more extensively
Figure 13.15. After Langmuir et al. (1992). AGU.
Project
FAMOUS
(MAR)
From Ballard and Van Andel
(1977) GSA Bull., 88, 495-506.
The Axial Magma Chamber
Original Model




Semi-permanent
Fractional crystallization
 derivative MORB
magmas
Periodic reinjection of
fresh, primitive MORB
Dikes upward through
extending/faulting roof
Figure 13.16. From Byran and Moore (1977)
Geol. Soc. Amer. Bull., 88, 556-570.
Hekinian et al. (1976)
Contr. Min. Pet. 58, 107.




Crystallization at top and sides
 successive layers of gabbro
(layer 3) “infinite onion”
Dense olivine and pyroxene
crystals  ultramafic
cumulates (layer 4)
Layering in lower gabbros
(layer 3B) from density
currents flowing down the
sloping walls and floor?
Moho?? Seismic vs. Petrologic
Figure 13.16. From Byran and Moore (1977)
Geol. Soc. Amer. Bull., 88, 556-570.
A modern concept of the axial
magma chamber beneath a fastspreading ridge
Figure 13-15. After Perfit et al.
(1994) Geology, 22, 375-379.
The crystal mush zone
contains perhaps 30%
melt and constitutes
an excellent boundary
layer for the in situ
crystallization process
proposed by Langmuir
Figure 11.12 From Winter
(2001) An Introduction to
Igneous and Metamorphic
Petrology. Prentice Hall
Attempts to reconcile the lack of a large permanent
magma chamber with the apparent cumulate textures and
layered appearance of lower cumulates.
Figure 13.18 “Gabbro glacier” model of ductile flow imparting a tectonic
foliation to the lower gabbros. From Phipps Morgan et al. (1994).
Attempts to reconcile the lack of a large permanent
magma chamber with the apparent cumulate textures and
layered appearance of lower cumulates.
Figure 13.19. “Sheeted sill” model in which shallow melt lens feeds into only a
minor fraction of upper gabbros. From Kelemen et al. (1997).
Attempts to reconcile the lack of a large permanent
magma chamber with the apparent cumulate textures and
layered appearance of lower cumulates.
Figure 13.20. Hybrid models for development of oceanic lithosphere at a fast-spreading ridge
(arrows represent material flow-lines). a. Ductile flow model incorporating a second melt lens at
the base of the crust (e.g. Schouten and Denham, 1995). b. Ductile flow with two melt lenses and
off-axis sills (e.g. Boudier et al., 1996). c. Sheeted-sill hybrid model in which lower sills are fed
from above by descending dense cumulate slurries from the upper melt lens (Rayleigh-Taylor
instabilities) into the lower mush region (Buck, 2000).


Melt body  continuous reflector up to several
kilometers along the ridge crest, with gaps at fracture
zones, devals and OSCs
Large-scale chemical variations indicate poor mixing
along axis, and/or intermittent liquid magma lenses,
each fed by a source conduit
Figure 13.21 After Sinton
and Detrick (1992) J.
Geophys. Res., 97, 197-216.
Model for magma chamber beneath a slow-spreading
ridge, such as the Mid-Atlantic Ridge


Dike-like mush zone and a smaller transition zone beneath
well-developed rift valley
Most of body well below the liquidus temperature, so
convection and mixing is far less likely than at fast ridges
Depth (km)
2
Rift Valley
4
6
Moho
Figure 13.22 After
Sinton and
Detrick (1992) J.
Geophys. Res., 97,
197-216.
Transition
zone
Gabbro
Mush
8
10
5
0
Distance (km)
5
10


Nisbit and Fowler (1978) suggested that numerous, small,
ephemeral magma bodies occur at slow ridges (“infinite leek”)
Slow ridges are generally less differentiated than fast ridges
 No continuous liquid lenses, so magmas entering the axial
area are more likely to erupt directly to the surface (hence
more primitive), with some mixing of mush
Depth (km)
2
Rift Valley
4
6
Moho
Transition
zone
Gabbro
Mush
8
10
5
0
Distance (km)
5
Figure 13.22 After Sinton and Detrick (1992)
J. Geophys. Res., 97, 197-216.
10
Table 13.3 General Differences Between Fast (> ~5 cm/a) and Slow-Spreading Ridges
Fast-Spreading Ridge
Ophiolite example: Semial (Oman)
Axial magma chambers are more steady-state,
volcanism more frequent
Smoother flanks (less faulted)
Symmetric and less tectonically disrupted
Slow-Spreading Ridge
Ophiolite example: Troodos (Cyprus)
Axial magma chambers are more ephemeral and
scattered, volcanism less frequent
Rougher flanks (highly faulted)
Commonly asymmetric, more listric &
detachment faulting. Layering is less uniform.
Ridge typically higher (shallower)
Ridge typically lower (deeper)
Longer tectonic and magmatic segments
Shorter tectonic and magmatic segments
Narrow axial rise with small axial trough
Deep discontinuous axial valleys, uplifted flanks
Wider low seismic velocity (partial melt) zone
Narrower low velocity zone- melt lens rare
Narrow axial neovolcanic zone
Wider irregular axial neovolcanic zone with more
distributed local sources  hills, seamounts
Thicker lithosphere (lower heat flow)
Thinner lithosphere (higher heat flow)
Thinner, less uniform crust
Thicker, more uniform crust
Pillow lavas dominate extrusives
Extensive sheet lava flows
Slightly less evolved magmas (avg. Mg# = 57.1).
Slightly more evolved magmas (avg. Mg# = 52.8).
More compositional diversity within areas
Less compositional diversity within areas
Mantle upwelling more “three-dimensional”
Mantle upwelling more “two-dimensional”
Commonly exhibit “global” magmatic trends of Klein Commonly exhibit “local” magmatic trends of
Klein and Langmuir (1987, 1989).
and Langmuir (1987, 1989).
Figure 13.23. Topographic profiles.
From Macdonald (1998). AGU
Along-axis
Across-axis
Listric and detachment faulting at slow-spreading centers (MAR).
Note extensive exposure of ultramafic mantle rocks (source of
dredge samples?).
Figure 13.24 Interpretive cross-section across the slow-spreading Mid-Atlantic Ridge near the Kane
fracture zone. Tectonic extension results in series of normal faults and exhumation along a shallowdipping detachment surface, producing a disrupted and distinctly asymmetric architecture. From Thy
and Dilek (2000).
Figure 13.25 Geochemical systematics of Klein and
Langmuir (1987, 1989) using the global trend vs.
local trend scoring system of Niu and Batiza (1993).
Global trends predominate at spreading (half) rates
greater than 5 cm/a, whereas local trends are more
apparent at lesser rates. After Niu and Batiza (1993)
and Phipps Morgan et al. (1994).
Figures I don’t use in class
Figure 13.6. From Stakes
et al. (1984) J. Geophys.
Res., 89, 6995-7028.
Figures I don’t
use in class
Figure 13.7. Data from Schilling et
al. (1983) Amer. J. Sci., 283, 510-586.
Figures I don’t
use in class
Figures I don’t
use in class