ROCKS FROM OPHIOLITES
Ophiolites are fragments of oceanic lithosphere (basaltic crust + uppermost
mantle) that have been metamorphosed and plastered onto the edges of
continental lithospheric plates by obduction (the opposite of
subduction). Shown below are samples from ophiolites in Austria, the
Shetland Islands, Quebec, Cyprus, Newfoundland, Tasmania, and Maryland.
KRAUBAUTH COMPLEX
Chromitic serpentinite (7.9 cm across) from the Kraubath Complex (Speik
Complex) in the Eastern Alps of Austria. The Kraubauth Complex is an
ultramafic massif dominated by metamorphosed dunites & harzburgites.
The original rock (protolith) was a Proterozoic-Early Paleozoic upper mantle
dunite peridotite that has been multiply metamorphosed during the Devonian,
Permian, and Mesozoic. Greenish = serpentine; black =
chromite (FeCr2O4 - iron chromium oxide).
Locality:
Sommergraben, Murz River Valley area, Muralpen, Eastern Alps, central Styria
Province, Austria.
THETFORD MINES OPHIOLITE
Serpentinite (10.0 cm across) from Quebec's Thetford Mines Ophiolite Complex,
representing ancient oceanic lithospheric peridotite that was metamorphosed
during the Taconic Orogeny (Ordovician).
Locality:
Thetford Mines area, southeastern Mgantic County, southern Quebec, Chaudiere
Appalachians, south of the St. Lawrence River, southeastern Canada.
SHETLAND OPHIOLITE
These next three rocks are from the island of Unst
in the northeastern Shetland Islands (in the North Sea, between Scotland &
Norway). They are from the Shetland Ophiolite Complex (aka
Unst Ophiolite Complex), representing ~basal crustal rocks metamorphosed during
the Caledonian Orogeny (~425-500 million years ago). Published research
indicates that the original rocks were from a late Neoproterozoic-Cambrian
intracontinental, ocean-floored rift basin.
Chromitic talcose serpentinite (6.4 cm across), originally a ~basal crustal dunite
peridotite; from abandoned quarry on island of Unst, northeastern Shetland
Islands, North Sea.
Metapyroxenite (8.0 cm across) composed of chromian diopside
pyroxene ((Ca,Cr)MgSi2O6 - calcium chromium magnesium
silicate), from the upper pyroxenite layer of the Unst Ophiolite Complex;
collected from an outcrop at Skeo Taing, shores of Balta Sound, eastern margin
of island of Unst, northeastern Shetland Islands, North Sea. Since this
pyroxenite (a clinopyroxenite, really) is all diopside, a better rock name is diopsidite.
Chromitite (5.2 cm across) with chromite (black) & talc and/or antigorite
(whitish); from abandoned chromite mine at Nikka Vord, island of Unst,
northeastern Shetland Islands, North Sea.
TROODOS OPHIOLITE
The classic place on Earth for seeing ophiolite rocks
is the island of Cyprus in the eastern Mediterranean Basin. Cyprus
has nice outcrops showing upper mantle rocks, basal crustal rocks, and the
mantle-crust transition (the Moho). The ophiolite rocks on Cyprus are
called the Troodos Complex (aka Troodos Ophiolite, Troodos
Ophiolite Complex). These rocks represent Late Cretaceous oceanic
lithosphere that has been obducted & is now sitting above sea level.
The sample depicted below is an olivine chromitite
from near-basal crustal rocks at Kokkinorotsos Mountain, west-central
Cyprus. There, chromitites are mined for their chromium. The
chromitites at Kokkinorotsos Mt. occur in large, dunite-enveloped,
irregularly-shaped, vertically elongated pods, all within a harzburgite matrix.
Olivine chromitite (7.3 cm across), composed of chromite (metallic
black) and some olivine (dirty yellowish green).
Some of the above info. about Cyprus chromitite is
from:
Constantinides et al. (1980) - The occurrence of
platinum group minerals in the chromitites of the Kokkinorotsos chrome mine,
Cyprus. pp. 93-101 in Ophiolites, Proceedings,
International Ophiolite Symposium, Cyprus 1979.
LITTLE PORT COMPLEX
Another classic place for seeing mantle rocks in an
ophiolite is western Newfoundland. Here's a couple rocks from the upper
mantle portion of the late Middle Cambrian-aged (505 m.y.) Little Port
Complex (Lewis Hills Massif, Bay of Islands Ophiolite).
The 1st rock shown below was originally a chromitic
cumulate dunite hosted by harzburgite, but it's been metamorphosed. It's now
a chromitic serpentinite (the olivine crystals have been altered to
serpentine). The dirty yellowish portions of the rock are the
serpentine. The black portions are chromite. These mantle rocks
have been inferred to represent the lower lithosphere of an ancient island arc.
Chromitic serpentinite (8.8 cm tall) from Newfoundland's Cambrian-aged
Little Port Complex. Locality: Springer's Hill outcrop, Lewis
Hills, western Newfoundland. 48¼ 47' 32" North, 58¼ 26' 38' West.
The 2nd rock is a finely-crystalline dunite
(olivine-rich peridotite) havine a "dun"-colored weathering
rind. The dunite has a very coarsely-crystalline "vein" of clinopyroxenite
(the nearly up-and-down, medium gray-green colored structure in the middle), a
clinopyroxene-rich ultramafic igneous rock. The specimen also has a patch
of coarsely-crystalline wehrlite (upper & upper right portions of
photo), which is a peridotite variety dominated by olivine + clinopyroxene, but
having <10% orthopyroxene.
Dunite
with clinopyroxenite and wehrlite (wet, cut surface; field of
view ~5.8 cm across) and brownish weathering rind. From Newfoundland's
Cambrian-aged Little Port Complex. Locality: Springers Hill
Chromite showing, Lewis Hills, western Newfoundland.
Some info. from:
- Tony Peterson (pers. comm.)
- Geological Association of Canada/Mineralogical
Association of Canada 1988 Field Trip Guidebook B8, day 1, stop 1.
- Suhr & Cawood (2001) - Geological Society of
America Bulletin 113(8): 1025-1038.
DUNDAS ULTRAMAFIC COMPLEX
Stichtitic serpentinite (8.6 cm across) from Tasmania. Greenish
= serpentine; purplish = stichtite; black crust near center =
magnetite; tiny black spots within stichtite masses = chromite.
This is the prettiest ophiolite rock I've yet encountered. This is a
serpentinite with nice, purplish- to lilac-colored masses of stichtite,
a rare hydrous magnesium chromium hydroxy-carbonate mineral (Mg6Cr2CO3(OH)16į4H2O).
This rock comes from Stichtite Hill in western Tasmania, the type locality for
stichtite. The host rock is chromitic-magnetitic serpentinite from the Dundas
Ultramafic Complex, a Cambrian-aged ophiolite that's been cut up by thrust
faults. Before metamorphism, the serpentinite was a chromitic dunite (as
were several of the above rocks).
The origin of stichtite is not yet completely
resolved. The stichtite masses have tiny black specks of chromite, which
was a component of the original dunite. What's seems to be clear is that
the stichtite is derived from the chromite by hydrothermal
metamorphism. The details of the alteration process are still discussed
in the literature. The timing of stichtite formation is also
unclear. Several orogenic and thermal events affected the rocks of
western Tasmania during the Devonian, Jurassic, Cretaceous, and Cenozoic.
Stichtite formation could have accompanied any one of those events.
Stichtite info. synthesized from:
Ashwal & Cairncross (1997) - Mineralogy and origin
of stichtite in chromite-bearing serpentinites. Contributions to
Mineralogy and Petrology 127: 75-86.
Bottrill & Graham (2006) - The nature and origin of
stichtite from western Tasmania. Abstracts of the Australian Earth
Sciences Convention 2006. 3 pp.
BALTIMORE MAFIC COMPLEX
Chromitic serpentinite (5.2 cm across at its widest) from Maryland,
USA. This rock is from the Soldier's Delight Ultramafite, and represents
metamorphosed upper mantle rock exposed in an ophiolite in the Maryland-Pennsylvania-Delaware
Piedmont along America's eastern seaboard.
Unit & age: Soldier's Delight Ultramafite, lower Baltimore Mafic
Complex, probably Neoproterozoic to Cambrian oceanic lithosphere, metamorphosed
during the Taconic Orogeny (Late Ordovician-Silurian).
Locality:
unrecorded old chromite mine in the Bare Hills, north of Baltimore city limits,
Baltimore County, northern Maryland, USA.
Chromitic serpentinite (5.3 cm across at its widest) from Pennsylvania, USA.
Light green - serpentine. Black - magnesiochromite ((Mg,Fe)Cr2O4).
This rock is also from the Soldier's Delight Ultramafite.
It represents metamorphosed chromitic dunite from the upper mantle exposed in
an ophiolite.
Unit & age: Soldier's Delight Ultramafite, lower Baltimore Mafic
Complex, probably Neoproterozoic to Cambrian oceanic lithosphere (one published
igneous date is 490 m.y. - near-latest Cambrian), metamorphosed during the
Taconic Orogeny (Late Ordovician-Silurian).
Locality:
Red Pit Mine, ~0.3 miles northeast of milepost 21 along Mason-Dixon Road,
southern Fulton Township, far-southern Lancaster County, State Line
Serpentinite District, southeastern Pennsylvania,
Maryland-Pennsylvania-Delaware Piedmont, USA.