Dynamic Earth Lecture 7: Orogenic Granites

Question 1

Granitoid composition

Answer 1

Siliceous feldspathic rocks

Essential quartz, plagioclase and alkali feldspar

Granite = albite, oligoclase & orthoclase and microcline

Diorites and granodiorites have variable felsic compositions but lots of mafic minerals such as pyroxenes, hornblende, micas, plus reasonably abundant titanite, apatite etc (these only found in granites intrace amounts)

Question 2

Sensu stricto?

Answer 2

In the strict sense/ Using the most precise, literal, or limited definition of a term

Question 3

accessory minerals from hydrothermal processes in granitoids

Answer 3

Tourmaline (B), topaz (F); almandine (Fe); spessartine (Mn); epidote, allanite.

Sulphides and oxides of base and occasionally precious metals:
Cu-Ni-Fe-Zn-Pb-Sn-As-Au-Ag etc.

Iron oxide depending on ƒO2:
Magnetite, hematite.

Others:
Fluorite, barite. OPX in charnockite (high-T anhydrous granite)

Question 4

Granitoid textures

Answer 4

Megacrystic (aligned) granite
Igneous layering in granodiorite

Rapakivi texture: plagioclase over k-spar (- coarse, early oligoclase mantle around orthoclase core)
-> solid solution becomes unstable.
- The plagioclase component (oligoclase) unmixes first because it is the least soluble at high T.

Mafic enclaves in granodiorite (mafic blobs in otherwise felsic matrix)

Question 5

Granophyric texture

Answer 5

fine-grained, microscopic igneous rock texture characterized by the intimate, often radiating, or angular intergrowth of quartz and alkali feldspar

Looks like zebra stripes but stripes disconnected

Question 6

Quartz resoprtion

Answer 6

quartz phenocrysts in magma are partially dissolved or corroded by the surrounding melt (change environmental conditions, such as temperature, pressure, or chemical composition)

Question 7

Granitoid textures in thin section?

Answer 7

Quartz resorption
Granophytic texture

Question 8

Crystallisation order in granitoids

Answer 8

Not easy to determine

accessory phases appear first: zircon, apatite, titanite ~650-700oC

Followed by plag + mafics: hornblende, biotite.

Quartz & alk feldspar as late interstitial phases

Variables: bulk comp, P, T, O2%, H2O%
H2O faciliitates

Question 9

Nucleation

Answer 9

Nucleation = the formation of new crystal seeds

melt contains a lot of dissolved H₂O:
-> melt structure becomes more depolymerised (more “fluid” at atomic scale)
-> energy barrier for forming a stable crystal nucleus increases.
-> fewer crystals nucleate, and they nucleate later.

why pegmatites don’t have millions of tiny crystals—they don’t nucleate readily

Question 10

H2O impact on crystal nucleation and growth

Answer 10

melt contains a lot of dissolved H₂O:
-> melt structure becomes more depolymerised (more “fluid” at atomic scale)
-> energy barrier for forming a stable crystal nucleus increases.
-> fewer crystals nucleate, and they nucleate later.

why pegmatites don’t have millions of tiny crystals—they don’t nucleate readily

Once crystal nucleus does form, H₂O:
- It dramatically increases ion mobility.
- It lowers melt viscosity.
- It enhances diffusion of Si, Al, Na, K, Ca.

So nuclei that exist can grow extremely fast and to very large sizes.

Question 11

More in depth IUGS silica classification

Answer 11

INTERMEDIATE: 52 – 63 wt %.
52 – 57 wt % gabbroids (basaltic andesite)
57 – 63 wt % diorites (andesites)

ACID: 63 - >77 wt %.
63 – 77 wt % granodiorites (dacites)
>77 wt % granites (rhyolites)

Question 12

Aliminium saturation of granitoids

Answer 12

Granitoid rocks also classified through Aluminum proportions relative to CaO, K2O & Na2O (these feldspar elements are classed as one group)

Groups are Peraluminous, Metaluminous & Peralkaline

Question 13

Specify Peraluminous, Metaluminous & Peralkaline

Answer 13

Peraluminous = Greater Al% than feldspar elements
Biotite,Muscovite,cordierite,Andalusite,Garnet

Metaluminous = Similar Al% to feldspar elements
Pyroxene, hornblende, biotite

Peralkaline = Lesser Al% than feldspar elements
Mostly Alkali amphiboles and pyroxenes (Aegirine, Riebeckite & Afvredsonite)

Question 14

Occurance and distribution

Question 15

Granitoid origins

Answer 15

Composition implies derivation from crustal anatexis (partial melting)

Composition of crust varies

Question 16

f means fugacity

Answer 16

Meaning availability of element to react in solution

Answer 17

crystallisation order impact on euhedral/sub/anhedral

As more crystals form there is a fight for space resulting in other minerals either losing space and becoming sub-anhedral rather than euthedral

Answer 18

Cant distinguish between igneous and mantle rocks down the microscope so in exam say I/M

Question 19

Pneumatolysis

Answer 19

Pneumatolysis is the alteration of rocks or magmas by high temperature volatile rich fluids -e.g. gaseous phases released from cooling magma

Greisenisation: Feldspar replaced by quartz and muscovite.

Tourmalinisation: Essentially converting granite to quartz and tourmaline.