Part 6. Mafic Rocks

Solid Solutions and Exchange Vectors
Some minerals (e.g., quartz) have constant composition. However, most minerals are solid solutions that undergo compositional change in response to PTX (pressure, temperature, composition) changes. Garnet is an example of a relatively simple solid solution, with substitution occurring on only one site:

• almandine Fe₃Al₂Si₃O₁₂
• pyrope Mg₃Al₂Si₃O₁₂
• grossular Ca₃Al₂Si₃O₁₂
• spessartine Mn₃Al₂Si₃O₁₂

• mafic rocks have fewer minerals than pelitic rocks
• a given mineral in a mafic rock is stable over a broad PT range by virtue of its ability to shift composition in response to changes in P and T
• reactions in mafic rocks are continuous reactions that span a range of PT space over which the minerals change in composition and abundance

facies	minerals	notes
basalt and gabbro protoliths	plagioclase NaAlSi3O8-CaAl2Si2O8 + pyroxene CaSiO3-MgSiO3-FeSiO3	.
zeolite facies	common zeolites: laumontite, wairakite, analcime (Na,Ca)AlSiO(OH)x	zeolites: channel structure makes them useful for engineering common textures: in cracks, alteration of feldspar or glass
greenschist facies	albite + epidote + actinolite + chlorite + quartz albite NaAlSi3O8 takes up Na, Al, Si epidote CaAlSiO(OH)x takes up Ca actinolite Ca2(Fe,Mg)5Si8O22(OH)2 takes up Fe, Mg chlorite (Mg,Fe)(AlSi)O(OH)x	.
albite-epidote amphibolite facies	albite + epidote + hornblende + quartz albite NaAlSi3O8 takes up Na, Al, Si epidote CaAlSiO(OH)x takes up Ca hornblende (K,Na)(Ca,Fe,Mg)(AlSi)O22(OH)2	modal proportion of albite decreases, Na enters amphibole modal proportion of chlorite decreases, Fe + Mg enters amphibole
amphibolite facies	plagioclase + hornblende + quartz	epidote replaced by intermediate plagioclase remainder of elements in hornblende
granulite facies	orthopyroxene + clinopyroxene + plagioclase + quartz	must have 2 pyroxenes; can also have hornblende or garnet
blueschist facies	glaucophane Na2Mg3Al2Si8O22(OH)2 lawsonite or epidote CaAlSiO(OH)x	high P, low T
eclogite facies	omphacite (Na,Al,Mg,Fe)Si2O6 garnet (Ca,Mg,Fe,Mn)3Al2Si3O12	high P, high T
coesite-eclogite facies	coesite SiO2 diamond C majorite Mg4Si4O12	pressures >2.5 GPa

Facies Series
A series of facies at low, medium, or high pressure. Metamorphic rocks formed by crustal extension are characterized by a low-pressure facies series. Metamorphic rocks formed during continental collision are characterized by a medium-pressure facies series. Subduction-zone metamorphic rocks are characterized by a high-pressure facies series.

facies series	tectonic setting	minerals present	minerals absent
low P (Buchan)	crustal extension	andalusite, sillimanite, cordierite	staurolite, kyanite
crustal thickening	medium P (Barrovian)	kyanite, sillimanite, staurolite	cordierite
subduction	high P (Franciscan)	kyanite	staurolite