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Geology 200A - Lab 2 (Ophiolites)

Vocabulary: talc serpentine olivine bronzite clinopyroxene spinel chromite magnetite plagioclase amphibole epidote chlorite lherzolite harzburgite dunite wehrlite websterite anorthosite troctolite gabbro trondhjemite gabbro basalt chert cumulate intercumulus tectonite segregation pegmatite breccia pillow veins xenocryst restite foliation honeycomb effusive aphyric microlites phenocryst lythophysae variolites chilled .
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Crystals v glass: holocrystalline -> hypocrystalline -> holohyaline; microlites, crystallites.

Grain size: cryptocrystalline; aphanitic; phaneritic; pegmatitic. Fine <1 mm; coarse > 5 mm; very coarse > 3 cm.

Crystal shape: euhedral; subhedral; anhedral

000 2-2a-1: serpentinite

001 2-2a-2: talc serpentine schist: grey-beige, soft, waxy, phyllitic sheen from basal cleavage; hydrated ultramafic rock formed during low temperature; shear propagation along the basal slip zone of an obducted ophiolite.

002 2-2b: orange/brown weathered lherzolite with metamorphic segregation layers of opx, emerald green cpx (colour due to the presence of chromium) and black aluminous spinel (MgAl2O4); from the type locality of lherzolite, Lherz, French Pyrenees; primitive mantle material.

003 2-2c: orange/brown rough weathered sample of fresh lherzolite containing olivine, orthopyroxene, clinopyroxene and Al,Fe3+, Cr-spinel; from the Atlin (Horst) ophiolite of Northern British Columbia; part of the mantle section of obducted late Paleozoic oceanic crust of the Canadian Cordillera.

004 2-2e1 2-2e2 3 samples: orange/brown weathered serpentinized knobbly harzburgite composed of serpentinized olivine, orthopyroxene, and black spinel; this sample represents partially melted mantle lherzolite from which all clinopyroxene has been removed into the basaltic melt; it is a mantle restite.

005 2-2d: orange/brown weathering, smooth surfaced, serpentinized (matte black) dunite with black grains of chromite; the rock, in as much as it is composed of olivine and chromite only, represents the residue remaining from the high temperature melting removal of all clino- and ortho-pyroxene from the primitive mantle lherzolite represented by sample 2-2c;.

006 2-3a: coarse grained orthopyroxenite composed of greeny-bronze coloured orthopyroxene and disseminated, anhedral grains of black chromite; vein of mantle melt material derived from the high temperature melting in the mantle of harzburgite.

007 2-4-1-a: slab of fresh crystalline, cumulate dunite with cross-bedded cumulate chromite layers;.

008 2-4-1-c: fresh chromite-bearing dunite interlayered with boudinaged layers of massive chromite; likely chromitite cumulate.

009 2-4-1-d: thin chromite rich layers in dunite; likely of cumulate origin.

010 2-4-2 2 samples: black, submetallic texture, dense, medium grained chromitite; cumulate chromite and intercumulus, white weathering serpentinized olivine.

011 2-4-1-b: graded 'grape shot' chromite in a matrix of serpentinized olivine; chromitite cumulate within the lower ultramafic sequence of an ophiolite body.

012 2-3b: ultramafic rock composed of serpentinized olivine and fresh coarse bronzite; note large bladed crystal of orthopyroxene; ultramafic cumulate.

013 2-5-1-a 2 samples: orange/brown, serpentinized honeycomb textured harzburgite composed of cumulate olivine and intercumulus orthopyroxene; olivine now black serpentine; ophiolitic ultramafic cumulate rock.

014 2-5-1-b: serpentinized layered harzburgite cumulates composed of dunite and orthopyroxenite layers; ultramafic cumulate rock.

015 2-5-2-c: troctolite composed of cumulate olivine and intercumulus plagioclase; crystallization sequence is olivine followed by plagioclase.

016 2-5-2-a: layered troctolitic cumulate sequence composed of layers of black, submetallic chromite, black serpentinized olivine, and white feldspar; the low density anorthositic material forms the uppmost unit of a chromite-dunite-anorthosite cumulate cycle, whereas the chromitite and olivine layers form the lowermost unit of a similar overlying cumulate cycle; the dunite layer is therefore the youngest unit in this specimen, the plagioclase unit the oldest. Sample 'gone missing'.

017 2-5-2-b: layered troctolitic cumulate sequence composed of layers of black serpentinized olivine, and white feldspar; the low density anorthositic material forms the uppmost unit of a dunite-anorthosite cumulate cycle, whereas the chromitite and olivine layers form the lowermost unit of a similar overlying cumulate cycle; the dunite layer is therefore the youngest unit in this specimen, the plagioclase unit the oldest.

018 2-6-1 2 samples: fresh surface dark green, weathered surface dirty brown; medium grained serpentinized olivine clinopyroxenite; typically occurs at the base of one class of ophiolite gabbro.

019 2-6-2-b: dark (mela-) clinopyroxenite - gabbro; base of the gabbro unit; clinopyroxene is the cumulate phase, plagioclase is intercumulus.

020 2-7a: dark grey, compositionally layered and foliated gabbro composed of clinopyroxene and plagioclase; basal cumulate of an ophiolite gabbro layer; this rock has a primary high temperature 'plastic' foliation induced during sea-floor spreading.

021 2-7b-1 2 samples: coarse grained gabbro; clinoproxene is converted to amphibole by reaction with convecting sea-water.

022 2-7b-2a 2-7b-2b 2 samples: medium grey, medium grained, plagioclase-phyric leucogabbro: plag>cpx; note that one sample has a large xenocryst of orthopyroxene with a dark reaction rim; upper gabbro layer of the ophiolite.

023 2-7b-4: pegmatoidal pyroxenite/gabbro; base of the gabbro unit; crystallization order - cumulate pyroxene first, plagioclase as intercumulus.

024 2-7c: salt and pepper textured, medium grained gabbro; cpx about 10 % in multicrystalline aggregates, some ophitic plag and cpx intergrowths; note how the earthy alteration of clinopyroxene and the chalky white nature of feldspar on the weathered surface makes it easier to differentiate the two minerals on the weathered surface than on the fresh surface; upper part of the roof gabbro unit of an ophiolite.

025 2-7d; 2 samples: tan to grey weathered, fine grained trondhjemite; rock is composed of finely intergrown plagioclase and quartz (quartz not visible in hand-specimen); uppermost part of the ophiolite gabbro unit; this rock represents the remainder of a highly fractionated MORB basaltic liquid; contains crystals of zircon, and therefore can be used to date the sea floor represented by the ophiolite.

026 6-11: epidosite composed largely of pistachio green, ferric iron-rich epidote; formed as a result of the hydrothermal alteration (addition of H2O and O2) of basalt by sea-water conveyed by mid-cean ridge hydrous convection cells.

027 M59: diabase that has been hydrothermally altered by a sea-water convection cell, as evidenced by the presence of epidote.

028 2-8a: dull green, fine grained (aphanitic = no visible crystals) diabase dike intruding a coarser diabase; note the chilled margin of the finer grained diabase; ophiolite sheeted dike complex.

029 2-8b: breccia composed of diabase fragments; the unsorted and angular character of the fragments may indicate that the brecciation resulted from steam explosion related to sea-water convection within a sheeted dike complex.

030 2-8c: very fine grained, essentially aphanitic diabase with a relatively small number of microlitic feldspar crystals; rapidly cooled diabase rock within a sheeted diabase unit of an ophiolite.

031 2-8d: feldspar porphyric diabase dike; feldspars occur as both large crystals (phenocrysts), and as microlitic laths; sheeted dike unit of an ophiolite.

032 2-8e: feldspar porphyritic (plagiocalse-phyric) diabase dike; feldspars occur as large 'floating' crystals (phenocrysts).

033 2-9a: dark green, aphanitic pillow basalt with calcite-bearing amygdules; effusive volcanic rock representing the uppermost igneous unit of ocean crust.

034 2-9b1: 2-9b2 dull green, aphanitic small pillow basalt with spherical aggregates of feldspar and pyroxene microlites called lithophysae (this texture is known as variolitic texture); rapid nucleation from chilling of the hot lava by cold seawater; uppermost effusive unit of the ophiolite.

035 2-9c: part of a pillow exhibiting an outer rim of epidote bearing material; the epidote forms by reaction of plagioclase with downwards circulating oxygenated sea water; the fluid has totally removed all base metals, gold and sulphur from the epidotitized portion of the pillow, and consequently the fluid is progressively reduced as it passes downwards into the ophiolite.

036 2-10: black, amorphous, dense, brittle, subconchoidal, mottled to featureless 'radiolarioan' chert; milky quartz veinlets and patches; represents the capping pelagic, sedimentary sequence of oceanic crust; formed below the carbonate compensation depth in an area too far from the continent to receive any influx of coarse-grained clastic sediment; low energy environment with no sediment reworking; composed of the remains of silicic skeletal radiolaria.


The Bay of Islands ophiolite.


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