Ophiolites, metasomatism and NZ geology

Question 1

Ophiolites

Answer 1

Sections of oceanic crust and underlying upper mantle uplifted and exposed above sea level, often obducted on to the continental crust that formed in back-arc basins near subduction zones.

Question 2

Ophiolite sequences

Answer 2

Pelagic sediments rest on top
Basaltic pillow lavas (interfingering tubes, glassy rinds and highest quench rates).
Sheeted dike complex (extensional regime results in repetitive intrusion of dikes).
Gabbros
-isotropic gabbros - fractionation of magma chamber
-foliated gabbro - ductile deformation
-layered gabbro - minerals settling out of the magma chamber
Peridotites (ultramafic mantle rocks)
-cumulate wehrlite and dunite
-harzburgite residuum of basaltic melt

Question 3

Peridotites

Answer 3

Dominant rock type in the Earth’s upper mantle

• Iherzolite - unaltered mangle (olivine, orthopyroxene and clinopyroxene)
• dunite - olivine
• harburgite - olivine and orthopyroxene
• wehrlite - olivine and clinopyroxene

Question 4

Metasomatism

Answer 4

Circulation of hot fluids causing hydrothermal metamorphism. Loss and or gain of material and big change in overall rock composition.
Cold water seeps into the crust and is heated by magma below. Buoyant hot fluids rise and interact with cold seawater to form black smokers.
White smokers form at slightly cooler T and do not precipitate metals.
Diffuse flows occurs in even cooler fluids, but still above boiling T on the seafloor. Minerals precipitate below seafloor.

Question 5

Metasomatism - rocks

Answer 5

Super-heated ocean water changes rock composition. High T and low P rocks form (minerals dissolve or react with seawater). Alteration of Me-Fe rich hydrous minerals. Large amounts of water absorbed, increasing the rock volume and destroying earlier structure.
Pillow basalts -> sulphide minerals, quartz, clay and micas.
Dikes and gabbro -> hydrothermal chlorite formation
Zeolite - greenschist

Question 6

Serpentinisation

Answer 6

Olivine and pyroxene in peridotites and gabbros hydrate to serpentine. Large amount of H2O, loss of Mg and addition of Si, T 100-600 degrees.
Polymorphs
-antigorite -high T
-chrysotile (asbestos)
-lizadite -low T
Serpentinisation fixes water and fluid-mobile elements from the ocean into the oceanic crust, transporting them to the subduction zone, releasing them from serpentine and driving melting.

Question 7

Carbon Sequestation

Answer 7

Adding CO2 and Fe produced methane and hydrogen (support ecosystems).

Question 8

Rodingite

Answer 8

Dyke-like bodies, occur next to serpentines. Ca-rich pyroxenes and Ca-rich garnet. By product of peridotite hydrating to serpentinisation.

Question 9

Skarns

Answer 9

Metasomatised carbonates. Hot fluids derived from granitic magma are rich in Si, Fe, Al, Mg and dissolve carbonate in contact zone, releasing CO2, react to form wide variety of Ca-silicate minerals.

Question 10

Greisen

Answer 10

Formed from gas- and water-rich phases expelled from granites at the last stage of crystallisation (formed from unwanted things). Fluid/gas are forced into interstitial spaces of the granite.

Question 11

Terrane

Answer 11

Fault-bounded slice of regional extent, with its own distinctive geologic history.

Question 12

NZ geologic history

Answer 12

Early sedimentation - ~545-370 Ma
Tuhua orogeny - ~360-330 Ma
NZ geosyncline - ~330-142 Ma
Rangitata orogeny - 142-100 Ma
Gondwana break up - 100-24 Ma
Kaikoura orogeny - 24 Ma -now

Question 13

Cobb Valley ultramafics (Nelson)

Answer 13

Oldest rocks in NZ (~545-370 Ma), early sedimentation (sediments and volcanics related to Gondwana).
Metasomatism -> serpentinisation
Has cumulate textures - indicates ultramafic magma chamber
Formed at the active margin of Gondwana in the Tuhua basin
Metamorphic ore minerals – chromite, talc, magnesite and asbestos (serpentine)

Question 14

Tuhua orogeny

Answer 14

~545-370Ma
Only rocks in west of South Island affected (rest of NZ didn’t exist).
Sediments/ultramafics pushed up onto land during subduction of the Pacific margin (Tuhua basin) under Gondwana - regional metamorphism
Lots of faulting.
Limestone metamorphosed to marble, sediment metamorphised to schist and metamorphism of ultramafics.
Riwaka intrusion and Karamea batholith - intrusions related to subduction, causing contact metamorphism of older sediments.

Question 15

NZ geosyncline (deposition)

Answer 15

~330-142 Ma
Enormous thickness of sediments derived from the uplift of Gondwana margin accumulated.
Murihiku Terrane (West) - shallower water, continental shelf/slope deposits
-Southern Syncline - thick sediment pile, low-grade burial metamorphism (slates with slaty cleavage), zeolite facies
Torlesse rocks and Haast schist (east) - deeper water

Question 16

Rangitata orogeny

Answer 16

~142-100 Ma
Sediments deposited in geosyncline smeared on to Gondwana.
West - not so intensely deformed (burial)
East - closer to collision, slightly more deformation (orogenic)

Question 17

Otago schist

Answer 17

Orogenic metamorphism ~200-120 Ma, mostly low grade (except near Alpine fault), greenschist facies.

Question 18

Torlesse rocks

Answer 18

Orogenic metamorphism ~200-120 Ma, even lower grade, prehnite-pumpellyite facies

Question 19

Caples Terrance

Answer 19

Protolith - volcanics, chert, volcaniclastics, mudstones and sandstones
Suffered at least 2 periods of metamorphism
-M1 - high P/T
-M2 - high T/P (overprints M1 in area next to Alpine fault)

Question 20

Dun Mountain ophiolite belt

Answer 20

~280 Ma oceanic crust (ophiolite complex) has undergone metasomatism. Overprinted by later tectonic and burial effects. Mafic and ultramafic rocks.
Rangitata orogeny ~142-100Ma, oceanic crust between western and eastern terranes incorporated as the basin between was smeared onto the continent and obducted.
Split up by alpine fault in Kaikoura orogeny (Nelson and Otago). Little vegetation due to ultramafic concentrations of Ni and Mg.

Question 21

Western Fiordland orthogneiss

Answer 21

Composes a large chunk of Fiordland. Deep equivalent of granites (same chemistry and age) found in Nelson. Metamorphosed ~118Ma due to deep burial in Rangitata orogeny. High-grade, granulite facies.
Some of the hardest rocks in NZ. Resistant to erosion, interlocking crystals, granoblastic textures. Sheer, steep cliffs.

Question 22

Gondwana break up

Answer 22

~100-24 Ma. Pounamu terrane (ophiolite and sediment metamorphosed) on western side of Rakaia terrane indicates period of subduction from west (change from east), possible volcanic arc. Subduction zone where the alpine fault is now, smeared metamorphosed oceanic crust onto the west of the eastern province.
Gondwana starts to break up.
~83Ma Tasman sea spreading commences
~55 Ma Tasma sea stops spreading
~45Ma Emerald basin starts to open
~23 Ma new plate boundary develops
Today Alpine fault is active (sinistral then dextral movement).

Question 23

Paparoa core complex

Answer 23

Developed as the crust thinned ~98Ma. Tasman sea spreading commences and NZ rifts away from Australia and Antarctica.
Form during major continental crustal extension. Ductile middle-lower crustal rocks dragged out from beneath fracturing and extending brittle upper crustal rock. Juxtaposes brittle rocks against ductilely deformed rocks along detachment faults (mylonites!). Due to tectonic denudation, the weight of the upper crust gets less, middle-lower crust further uplifted due to isostatic rebound. Normal faulting results in middle-lower crustal rocks at the surface.
Rifting resulted in a lot of extensional tectonics and normal fault formation. Mylonites formed at depth adjacent to detachment faults that occurred at depth where ductile deformation occurs. No brecciated metamorphics found as they have been eroded off the top.
Shear sense indicators and lineations found.

Question 24

Zealandia

Answer 24

Result of extension.
7th geologic continent
-elevation relative to oceanic crust
-siliceous basement geology
-thicker crust and lower seismic velocity
-size >1 million km^2

Question 25

Kaikoura orogeny

Answer 25

~55-24 Ma quiet sedimentation post-rifting (limestones forming). Oligocene drowning.
~24Ma-now Kaikoura orogeny, Alpine fault activates and Southern alps uplifted.

Question 26

Alpine fault

Answer 26

Localised dynamic metamorphism. At surface cataclastics (brittle) and at deep mylonites (ductile) are faulted to the surface.
Highest grade rocks next to AF. Most rapid uplift and erosion (youngest rocks). Regional metamorphism