IGPET LECTURE 9 OPHIOLITES igneous petrology notes

Igneous petrology notes

1. OPHIOLITES
Lecture M.Sc Tech (I year) 2008-2009
Dr. N.V.CHALAPATHI RAO
READER
DEPARTMENT OF GEOLOGY
BANARAS HINDU UNIVERSITY
VARANASI-221005 (U.P)
[email protected]
http://www.nvcraobravehost.com
http://www.bhu.ac.in/Geology/STAFF/NVC%20Rao.htm

2. What are Ophiolites?
•Plate tectonic theory provides an explanation for transience of
ocean floors , being derived from mantle and returning to it
within a time scale of ~200 Ma.
•Occasionally some portions of oceanic floor ESCAPE
destruction at subduction zones & during final stages of
oceanic closure gets obducted to to one of the colliding
continental forelands to form a OPHIOLITE.
•Name was first coined by STEINMANN (1927)
•Ophiolite is not a SINGLE ROCK but an association of various
litho-units.
•Ophiolite association represent SLICES OF OCEANIC
CRUST that has been THRUST upon the continent.

3. What are ophiolites?
•An ophiolite is a unique assemblage of volcanic, mafic cumulate and
ultramafic rocks.
•Because of the vagaries of emplacement, ophiolites are generally
represented as dismembered parts or small blocks along fault
systems.
•Other evidences for ophiolites as oceanic crust :
•(1) Range in thickness of internal units of well exposed
ophiolites matches well with the range of thickness of
oceanic crust layers obtained by seismic studies.
•(2) Dredging of oceanic studies, heat flow studies &
petrological deductions (mantle models) are compatible
with ophiolite model.

4. OBDUCTION of OCEANIC CRUST

5. Ophiolite stratigraphy (from study of world ophiolites)
•1. Deformed (tectonized ) peridotite: (Bottom most layer) and is
essentially made of harzburgite, dunite (rarely lherzolite is also present) and
serpentine. Peridotite layer is considered to be Residual (depleted) or restite
layer after removal of partial melt products.
•2. Layered Cumulate rock, with olivine chromite layers at the base giving
way to typical gabbroic—tronjhematic (plagiogranites), anorthositic,
troctolitic rocks (differentiated products of gabbro) at the top. Sometimes,
a non-cumulate gabbro (isotropic gabbro) may also found at the top.
•3. Sheeted Dyke complex: Dike swarms with 100% dykes (vertical dykes);
they are severely affected by secondary hydrothermal processes.
•4. Basaltic volcanic member composed of hydrothermally altered pillow
lava (sometimes spilitic).
•5. All the above igneous associations are capped by a thin zone of
radiolarian cherts & / or other marine sediments. This zone definetely
links ophiolites to marine environments.

6. Idealized Ophiolite sequence
•Please note that all members are seldom present in all ophiolite complexes!

7. Specific Terminology of Ophiolite complexes
•OLISTOSTROME : The base of peridotite is mylonitized (highly
shared and cataclastically deformed) Consists of greenschists. This
is called OLISTOSTROME (or tectonic melange).
•Residual heat from ophiolite and / or frictional heating during
thrusting is believed to be responsible for this metamorphism.
•OBDUCTION: Slicing of oceanic crust thrusted onto continental
plate during continental collisions or subduction.
•SEISMIC MOHO: Seismic distinction between oceanic crust &
upper mantle occurs where gabbro grade down towards layered
peridotites.
•PETROLOGICAL MOHO: Layered peridotites (produced by crystal
settling in magma chambers) change to massive peridotites in upper
mantle.
•STEINMANN TRINITY: another name for Ophiolite complexes!
(ultramafic, mafic & pelagic sediments)

8. Samail ophiolite, Oman
•It extends to over 650 kms.
•It has 30,000 sq km of outcrop
•Stratigraphic thickness of 18 km.
•All members of ophiolite sequence are
well exposed.
•Contacts can be well studied.

9. Ophiolite Sequence at Sumail, Oman

10. Serpentinites (weathered peridotites)

11. PILLOW LAVAS IN SUMAIL OPHIOLITE, OMAN

12. Pillow Lavas – a closer look

13. LAYERED GABBRO in SUMAIL OPHIOLITE, OMAN

14. Plagiogranite in SUMAIL OPHIOLITE, OMAN

15. SHEETED DYKES IN SUMAIL OPHIOLITE, OMAN

16. World & Indian ophiolites
•Alpine ophiolites are world famous.
•Ophiolites of Himalaya
•Part of a continental Ophiolite zone passes from
Baluchistan arc-Kashmir syntaxis- Tsang-Po- Naga &
Manipur hills- Burma – Andamans- Indonesia.
•Various names;
•Indus-ophiolite belt
•Shyok ophiolite belt
•Malla Johar ophiolite belt of Uttaranchal/UP

17. Distribution of European Ophiolites
•European ophiolites are related to the collision of Europe with
Africa.
•They represent remnants of the Jurassic Tethyan Ocean
•Ophiolites provide yet another evidence for Plate tectonics!!!