Download Granites and collisions

Igneous petrology
Part III – Important igneous
associations
• Granites (and convergence/collision)
• Ophiolites (oceanic crust) and MORB
(Mid-ocean ridge basalts)
• Layered igneous complexes (intra-plate,
economic importance)
• Oceanic island basalts (OIB) (intraplate)
• Continental alkali series (intraplate)
• Andesites (active subductions)
• Continental arcs (active subductions)
• TTG (Archaean)
• Komatiites (Archaean)
Granites and collisions
Exemple of the Himalaya
• Granites are typically associated to
convergent plate boundaries
• Different types form at different moments
of the convergence
• Example of an active collision zone : the
Himalaya
Subducting oceanic
lithosphere deforms
sediment at edge of
continental plate
Collision – welding
together of continental
crust
Post-collision: two
continental plates are
welded together, mountain
stands where once was
ocean
Rifting of continental crust to form a new ocean basin
The Himalayas: geodynamic
context
• India-Eurasia convergence
• Destruction of the Tethys ocean
• Subduction stage (> 100 Ma – 25 Ma =
Cretaceous-Oligocene)
• Collision stage (25 Ma – present =
Miocene and Pliocene)
• Post-collision stage (present)
Himalayan collision
5 cm/an
10 cm/an
18 cm/an
Remontée
de
l ’Inde
et collision
à
55 Ma
C. 70 Ma
30°
M
ak
ra
n
Asian Margin
O
m
KohistanLadakh
an
F u tu
Arabia
0°
re IT
SZ
Tibet
W
S.L.
F
Fr u t u r
ac e
tu O w
re e
zo n
ne
Africa
S.T.
?
30°
India
Intra-oceanic arc
Active continental
margin
The subduction stage
Les témoins de la subduction de l ’Inde sous l ’Asie
Zanskar
Indian crust
MHT
The
Les témoins de la
collisioncollision
stagecontinentale
Std
Ky
Bt
Mu
Zanskar
Indian crust
MHT
400
T°C
500
700
600
2
4
L
50 Ma
M2
18-25 Ma
6
8
M1
10
45 Ma
37-32 Ma
SSW
NNE
Spontang
Tethys -Himalaya
MHT
Higher Himalaya
Tibet
Ladakh
Moho
55 Ma
N
eo
t
eth
Shikar Beh
Higher Himalaya
Tethys -Himalaya
MHT
Ladakh
Tibet
50 Ma
55-40 Ma : > 400 km > 2.6 cm/y
Sub-Himalaya
Tethys -Himalaya
Lesser Himalaya
Ladakh
Higher Himalaya
MHT
Tibet
40 Ma
ISZ
STDS
MCT
0
Sub-Himalaya
30
60
km
Less
er
H im
Tethys -Himalaya
alaya
Tibet
Moho
MHT
0
30
60km
40-20 Ma : ~ 360 km , ~ 1.8 cm/y
20 Ma
ys
Erosion
200
400
600
200°C
10
400
20
600
30
800
40
50
km
800
0
100
200 km
The « late to post » collision stage
Successive magmatic associations
(mostly granites!)
150
125
100
75
50
25
0
tps (Ma)
Subduction stage
• Trans-Himalayan batholith
• Cretaceous-Oligocene
• Similar to Andean or Cordileran
(California, British Columbia, Japan…)
plutons
• I-types (Andean)
Diorites
Tonalites
Granodiorites
Granites
Hornblende granodiorite
Hbl-Biotite granodiorite
Cpx
Hbl
Bt
Major elements
• .. See assignment
Trace elements
Isotopes
Mixed sources
(mantle +
some crust ?)
Origin
• Will be discussed during the
« subduction » lectures
Successive magmatic associations
(mostly granites!)
150
125
100
75
50
25
0
tps (Ma)
Collision stage
• High Himalaya leucogranites
• Miocene
• S-type
Granites
± Alk. Granites
± Granodiorites
2 micas granites
Tourmaline granite
Bt
Kfs
Ms
Pl
•
•
•
•
•
Biotite
Muscovite
Tourmaline
Garnet
(Cordierite)
Major elements
• .. See assignment
Trace elements
Isotopes
Very « crustal »
Origin
1. Lesser Himalaya
2. Formation I (Greywackes et métapélites)
3. Formation II (Gneiss calciques)
4. Formation III (Orthogneiss)
5. Sédiments tibétains
6. Leucogranite du Manaslu
7. Dykes
Dalle du Tibet
Les granites syncollisionels du Haut Himalaya
Migmatites de la formation I
Successive magmatic associations
(mostly granites!)
150
125
100
75
50
25
0
tps (Ma)
Late to post-collision stage
• Syenites and alkali granites
• Miocene to present
• A-type
• N.B. Some « sub-alkali », « Mg-K » I-types (cf.
Vredenburg pluton as seen in Paternoster) are
also emplaced at this stage
Le magmatisme « post-collisionel » himalayen
Cas du magmatisme Néogène du Sud Karakorum
Syenites
Qtz. Syenites
Granites
Alk. granites
Cpx, Fe-rich
Sometimes
Na-Cpx or
Amph
Little/no plag
(Riebeckite, Aegyrine
Ardfersonite)
Major elements
• .. See assignment
Trace elements
Isotopes
10 Asthénosphère
5
eNd
0
-5
Composite (mantle + crust),
with some mantle-derived
units and some crustal units
-10
-15
leucogranites
du Haut Himlaya, 20 Myr
-20
0.70 0.71 0.72 0.73 0.74 0.75 0.76 0.77
86Sr/87Sr
Sud Karakorum
Hemasil, 8 Myr
Hunza, 4 -25 Myr
Baltoro, 17-25 Myr
Sud Tibet, 10-16 Myr
Sud Tibet, 17 Myr
Sud Tibet, 18-23 Myr
Sud Tibet, 23 - 25 Myr
Origin
• Shear heating
• Slab breakoff
« Shear heating » ?
Chaleur de frottement
Karakorum
Kunlun
BALTORO
MKT
N
K2
HEMASIL
MMT
Lamprophyres
South Tibet
neogenous
magmatism
North Tibet
neogenous
magmatism
Himalaya
Tibetan plateau
MCT
200km
MBT
ITSZ
« Slab breakoff »
Conclusion: a succession of granite
types
• Subduction (pre-collision): I « andean »
• Syn-collision: S-type leucogranites
• Post-collision : A (and I « Mg-K »)
This is, of course, a very simplified view !
Trace elements are markers…
• Of the different types of magmas
• Of their origin
• Ba = fluid mobile element
• Rb = strongly incomp.element
• Zr: fluid immobile,relatively depelted in the
crust but not in the mantle…
Assignment for this week:
1. Read from J.D. Winter, Chapter 18, pages 343—361
2. Material available for discussion
– Thin sections and samples of Cape Granite Suite I, A and S granites, to
be used as examples;
– Your field observations, photos and notes from 1 and 2 April;
– An excel table with composition of examples of I, S and A granites;
– This lecture, for additional examples.
3. Assignment
Propose a comparison table for I, S and A granites, including:
–
–
–
–
–
–
Field characteristics;
Mineralogy & texture;
Chemical and/or normative composition;
Possible sources and evolution of the magmas;
Typical geodynamical context or contexts;
And any other interesting features you can think of !
Obviously, you will need either a very big table, or a lot of attached
explanatory notes, graphs, sketches, etc…