Metamorphism and Metasedimentary Rocks Dr Stephen Crabtree October

Metamorphism and Metasedimentary Rocks Dr. Stephen Crabtree October 5, 2018

So far we’ve talked about: Igneous Rocks • Lavas and Plutons • Formerly molten rocks • Crystallized from melt Sedimentary Rocks • Clastics, Biochemical, and Chemical • Deposited bits and pieces

The last major rock type: Metamorphic • “New” rocks formed from other rocks due to changes in three factors – Heat – Pressure – Fluids • NO MELTING INVOLVED! • NOT WEATHERING • Time is another huge factor – Different results depending on how long the above factors are able to take effect

Difference from Old to New • Depending on the impact of each factor, the new rock may have completely different textures and minerals from the original rock Shale Gneiss • Not always different minerals though…

New Terminology • Protolith: What the rock used to be before it was metamorphosed • Metamorphic Grade: How strong/how long was the deformation period? – Described as Low, Medium or High – Also referred to as “Facies” (usually with respect to some index mineral, e. g. garnet facies)

New Terminology • Porphyroblast: a large crystal in the rock – Equivalent to ‘phenocryst’ from igneous rocks

New Terminology • Index Mineral: a mineral that indicates the metamorphic grade – Mineral only grows from a given protolith under certain specific temperature/pressure conditions

Metamorphic Textures • Foliation – Flat minerals (i. e. clays, micas) align in broad sheets, often resembling sedimentary bedding (it has nothing to do with bedding) • Lineation – Segregation of light and dark prismatic minerals into aligned lines – not sheets or lenses • Relict textures – Any sedimentary or igneous texture may survive metamorphism as a relict texture (e. g. relict bedding)

Foliation Resulting from Directed Stress

Metamorphism Methods • Contact Metamorphism – Source of heat changing the rock at low pressures • ex: an ascending magma chamber – Strong heating of nearby rock material – Small-scale phenomena

Contact Metamorphism

Metamorphism Methods • Contact Metamorphism – Source of heat changing the rock at low pressures • ex: an ascending magma chamber – Strong heating of nearby rock material – Small-scale phenomena • Regional Metamorphism – Large-scale process – Typically associated with tectonics/ mountain-building

Regional Metamorphism

Classifying Metamorphic Rocks

Three Major Sedimentary Protoliths • Sandstone • Shale • Limestone

Two have Simple Products • Sandstone • Shale • Limestone

Metamorphism of Sandstone • Sandstone – Major minerals: Quartz, Feldspars – Arenite sandstones nothing but quartz – Apply heat and pressure to remold/recrystallize these minerals

Quartzite – Major mineral: Quartz (and maybe feldspars) – Melting point of quartz up to ~1800 °C • Mid to high-grade metamorphic rock

Metamorphism of Limestone • Limestone – Major mineral: Calcite (or other carbonates) – “Dirty” vs. “Clean” limestones – Apply mild heat and pressure to remold and recrystallize these minerals

Marble – Major mineral: Calcite (and maybe other trace) – Calcite has no true “melting” point, but rather a breakdown temperature: ~700 °C • Low to mid-grade metamorphic rock

Marble – aka “metacarbonate” – Marble does have a series of four “index minerals” (talc tremolite forsterite spinel) – But it’s still marble, and still a relatively low to mid-grade rock in all cases

Marble – aka “metacarbonate” Incr easi ng M etam orph ic G r ade Talc Mg 3 Si 4 O 10 (OH)2 Incr easi Tremolite Ca 2 Mg 5 Si 8 O 22(OH)2 ng T emp e ratu re a nd P ress Forsterite Mg 2 Si. O 4 ure Spinel Mg. Al 2 O 4

Shale has a Series of Products • Sandstone • Shale • Limestone Referred to collectively as “metapelites”

Depends on Degree of Impacting Factors • • Heat Pressure Fluids Time All Metapelites contain Quartz, Plagioclase, and Muscovite Can use Index Minerals Chlorite Biotite Garnet Staurolite Kyanite Sillimanite

Common Theme Among Index Minerals • • • Chlorite: (Mg, Fe)3(Si, Al)4 O 10(OH)2 · (Mg, Fe)3(OH)6 Biotite: K(Mg, Fe)3(Al. Si 3 O 10)(OH)2 Garnet: (Mg, Fe)3 Al 2(Si. O 4)3 Staurolite: Fe 2 Al 9 O 6(Si. O 4)4(O, OH)2 Kyanite: Al 2 Si. O 5 Sillimanite: Al 2 Si. O 5 Aluminum-rich Silicates in all cases Protolith byproduct of weathering feldspar to clays

How can clays turn into something else? Example: Al 2 Si 2 O 5(OH)4 Al 2 Si. O 5 + Si. O 2 + 2 H 2 O kaolinite andalusite + quartz + H 2 O (clay)

Four major Metamorphic Products from Shale Chlorite Biotite Garnet Staurolite Kyanite Sillimanite Slate Phyllite Schist Gneiss ~150 °C ~700 °C Low Grade High Grade

Index Minerals in Metapelites • Chlorite – (Mg, Fe)3(Si, Al)4 O 10(OH)2 · (Mg, Fe)3(OH)6 --- hydrous! – Soft, green mica/clay mineral – phyllosilicate – May appear in original shale, not just slate/phyllite

Index Minerals in Metapelites • Biotite – K(Mg, Fe)3(Al. Si 3 O 10)(OH)2 -- hydrous! – Soft, brown mica mineral – phyllosilicate – May appear in original shale, not just slate/phyllite – Also a “common” igneous mineral

Index Minerals in Metapelites • Garnet – (Mg, Fe)3 Al 2(Si. O 4)3 -- anhydrous! – Hard, red-brown isometric mineral – nesosilicate – Will NOT appear in original shale

Index Minerals in Metapelites • Staurolite – Fe 2 Al 9 O 6(Si. O 4)4(O, OH)2 -- anhydrous! – Hard, brown prismatic mineral – nesosilicate – Will NOT appear in original shale

Index Minerals in Metapelites • Kyanite – Al 2 Si. O 5 -- anhydrous! – Hard and soft blue prismatic mineral – nesosilicate – Will NOT appear in original shale

Index Minerals in Metapelites • Sillimanite – Al 2 Si. O 5 -- anhydrous! Same formula as Kyanite – Hard tan-white needle mineral – nesosilicate – Will NOT appear in original shale

Al 2 Si. O 5 phase diagram

Four major Metamorphic Products from Shale Chlorite Biotite Garnet Staurolite Kyanite Sillimanite Slate Phyllite Schist Gneiss ~150 °C ~700 °C Low Grade High Grade

Four Major Metapelites from Shale • Slate – Lowest-grade metapelite – Contact and regional Metamorphism – Very fine-grained chlorite, biotite index minerals – Also contains quartz, plagioclase, muscovite – Very strongly foliated

Four Major Metapelites from Shale • Phyllite – Middle-grade metapelite – Contact and Regional Metamorphism – Less fine-grained chlorite, biotite index minerals – Also contains quartz, plagioclase, muscovite – Strongly foliated, but layers commonly folded – Larger-grained muscovite gives strong reflectivity

Four Major Metapelites from Shale • Schist – Middle-grade metapelite – Regional Metamorphism – “Coarse”-grained biotite, garnet, maybe staurolite index minerals – Also contains quartz, plagioclase, muscovite – Strongly foliated, but layers commonly folded around garnet/staurolite porphyroblasts – Larger-grained muscovite gives strong reflectivity

Four Major Metapelites from Shale • Gneiss – High-grade metapelite – Regional Metamorphism – “Coarse”-grained biotite thru sillimanite index minerals – Also contains quartz, plagioclase, muscovite – Strongly foliated and often lineated – The only distinctly lineated metamorphic rock we’ll talk about

Morton Block, Southwestern MN • Mostly made of Morton “Rainbow “Gneiss – Dated to 3. 6 to 3. 0 billion years ago – oldest in US! – Strongly metamorphosed at great depth – Severe folding and faulting of rocks

Quetico Subprovince, Northern MN • Low-grade metamorphism on fringes • Moderate to high-grade metamorphism into schist and gneiss in center – Strongly layered, strongly deformed

Review metamorphism basics • “New” rocks formed from other rocks due to changes in three factors – Heat – Pressure – Fluids • NO MELTING INVOLVED! • NOT WEATHERING • Time is another huge factor – Different results depending on how long the above factors are able to take effect

Review new metamorphism terms • Protolith: What the rock used to be before it was metamorphosed • Metamorphic Grade: How strong/how long was the deformation period? • Porphyroblast: a large crystal in the rock • Index Mineral: a mineral that indicates the metamorphic grade

Review Metamorphism Methods • Contact Metamorphism – Source of heat changing the rock at low pressures – Strong heating of nearby rock material – Small-scale phenomena • Regional Metamorphism – Large-scale process – Typically associated with tectonics/mountain-building

Review major metasedimentary rocks • Sandstone Quartzite (non-foliated) • Limestone Marble (non-foliated) • Shale Slate (very strongly foliated, fine-grained) Phyllite (strongly foliated, med-grained) Schist (strongly foliated, coarse-grained) Gneiss (strongly lineated, med-coarse-grained) But Wait… There’s more!! Meta-igneous rocks!! Meta-Metamorphic rocks!!

Next Time… • Metaigneous Rocks • Re-Metamorphosed Metamorphic Rocks • Other Metamorphic Environments