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.
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 Mégantic County, southern Quebec, Chaudiere Appalachians, south of the St. Lawrence River, southeastern Canada.
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 (a.k.a. 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.
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 (a.k.a. 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. 48º 47’ 32” North, 58º 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 peridotite. The rock 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)16·4H2O). 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.