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1.1 Troodos Ophiolite (Upper) (Not simple fractional crystallisation…
1.1
Troodos Ophiolite (Upper)
Not simple fractional crystallisation
Zoopigi to Ag Theodoros Rd
Primitive dykes crosscut more evolved dyke
Primitive dykes
Dark& heavily weathered - suggests primitive mafic comp
A) Reddy brown with wavy contacts with B & C
B) Blueish grey, very crumbly
Evolved dykes
Cream, rich in plagioclase & quartz
Unlikely to get such high SiO2 conc via fractional crystallisaton
Plagiogranite has > 60% wt SiO2
Dating
≈ 90-92 Ma
Zircon forms when high degree crystallisation
Plagiogranite
U/Pb age ≈ 90.3 - 92.4 Ma
Agrees w ophiolites in Turkey in S. Tethys Ocean
Dykes
trend E-W
Contrasting overal N-S trend in Cyprus
Near Arakapas Transform Zone
∴ dyke orientation swung E-W due to transpression
This is not typical ophiolite area
Dyke interp
Normal fract cryst does not apply
SiO2 plagiogranite too high
Would not lead to primitive rock crosscutting evolved
Mafic & felsic rocks would not be beside eachother
Vertical structures ---> dykes?
Not normal MOR spreading
MOR - Basalt injected from magma chamber
Magma in chamber gets progressively more evolved with basalt production
Multiple magma chambers
Multiple, small shifting magma chambers invoked
Magma gets very evolved in some chambers
Evolved chambers crosscut by later magma chambers
Plagiogranite bodies - small bodies (tens/hundreds m)
Sheeted Dyke Terrain
On E903 road
Terrain made of 100% sheeted dykes = typical of spreading zones
Extension entirely accom by dyke precipitation
∴ no country rock here
Terrain = very rugged & steep
Sheeted dykes = toughest rock type in Cyprus
Resistant to erosion ∴ supports v steep slopes
Dykes dipping NE
Lower magma supply --> extension has greater effect
∴ Dykes more likely to be tilted
Overall trending N-S
Sheeted dyke interp
100% sheeted dykes w no host rock
--> suggests fossil spreading axis/ridge (Gass interp)
Will only ever see axis in ophiolite complexes if on land
So, why dykes not vertical ?
Dykes often tilted (up to 45°)
due to extensional forces in spreading region
Spreading irreg / strike slip faulting
lead to domains of dykes w different dip
Can assume intruded vertically
since seawater = lowest density density direction
Landscape Evolution
Transient landscape incising as baselevel falls due to uplift
Landscape @ transition btwn debris flow & fluvial zone
Expect dyke
material downstream
as v resistant to erosion
∴ retained pref over weaker mafic minerals
Will it evolve toward smooth hillslopes?
Dykes probs = too resistant to form thick soils
Where was spreading ridge ?
Spreading centre = where magma injected vertically into space
Chilled margins
Form either side of new intrusions
as fresh lava chilled by older, colder adjacent dyke
Glassy, fine-grained & dark
As spreading progresses, each dyke = halved & separated
--> to make space for new dyke
Spreading ridge indicator
statistical switch from cms being on LHS to RHS of dyke
∴ asymmetric cms either side
Analyses support MOR spreading
CY14 Borehole
Lower half ophiolite = med grained ultramafic
Magma chamber discovered (slowly cooled)
Lower layers = ol-rich
Less ol moving upwards
Base of each cumulate
= marked by compositional change
Troodos Ophiolite
inverted
∴ deepest ophiolite unit / highest elevation