Metamorphic rocks and the rock cycle

What is metamorphism? Differentiate it from diagenesis and melting, stating the range of conditions in which it takes place.

Name the three agents of metamorphism and explain how each acts on rock minerals.

What is the recrystallisation of minerals during metamorphism and how does it differ from the melting of the rock?

With an average geothermal gradient of $30\ \text{°C/km}$, estimate at what depth the $300\ \text{°C}$ typical of low-grade metamorphism are reached, and comment on the limitations of that calculation (variable gradients, fluid circulation).

In the axial zones of collisional orogens (such as the Iberian Central System), migmatite complexes with stromatic structures crop out, where light millimeter-scale bands (quartz-feldspathic leucosomes with granular texture) alternate with dark bands (foliated melanosomes rich in biotite, cordierite, and sillimanite). A technical report concludes that "the leucosome formed by the injection of mantle basaltic magma veins through fractures in a pre-existing gneiss."

1. a) Refute the report's conclusion: justify, based on mica dehydration reactions and the concept of eutectic minimum in the quartz-feldspar system, why leucosome and melanosome are the complementary result of an "in situ" anatexis process at the expense of a felsic crustal protolith.
2. b) If the rate of partial melting increases from 15% to 40% due to prolonged crustal thickening, predict the change in rheological behavior that the rock will experience and describe how this phenomenon leads to the segregation and ascent of peraluminous granitic magmas toward higher crustal levels.
3. c) Deduce what mineralogical and refractory characteristics the unmelted residue (restites) will exhibit, which remains retained in the deep crust or engulfed as xenoliths in the resulting granitic plutons.

What is foliation and which type of pressure (lithostatic or directed) produces it? Give an example of a foliated rock.

Distinguish slate and schist by their appearance and their metamorphic grade. What happens to the size of the micas when passing from one to the other?

Order the following series in increasing metamorphic grade from a mudstone: gneiss, phyllite, slate, and schist. Explain what textural and mineralogical changes accompany the increase in grade.

Which metamorphic rock comes from the transformation of granite and which from limestone? Why does marble rarely display foliation even though it forms under directed pressure?

Distinguish slaty cleavage and schistosity: at what metamorphic grade each one appears, which minerals define them and how they are recognised in hand specimen.

Within the same tectonically coalescent geological unit, two rocks A and B were certainly subjected to the same peak metamorphic conditions (P ≈ 0,7 GPa, T ≈ 500 °C). However, petrographic analysis reveals completely different mineral assemblages:

• Rock A: actinolite + epidote + chlorite + albite.

• Rock B: quartz + muscovite + biotite + garnet (almandine) + incipient staurolite.

1. a) Drawing upon Eskola's concept of metamorphic facies, determine which facies both rocks belong to and justify how two rocks with completely disparate mineral assemblages can belong to the same metamorphic facies.
2. b) Deduce the lithological nature of the protolith for each rock (choose and justify between: tholeiitic basalt, aluminum-rich pelitic shale, and pure limestone), explaining which major chemical elements present in each rock controlled the development of their diagnostic minerals.
3. c) If an increase in regional deformation raises conditions to 1,2 GPa and 650 °C, predict what new diagnostic mineral assemblage will replace Rock A (amphibolite facies / transition to eclogite) and reason which of the two protoliths will better record the conditions of the pressure peak through microanalysis of zoned garnets.

In the same regional metamorphic series affected by the Variscan orogeny, two adjacent 1 m thick sedimentary layers have experienced identical medium-grade conditions (T ≈ 550 °C, P ≈ 0,5 GPa):

• Layer 1: derived from a pelitic shale (rich in Al2O3 and K2O, very poor in CaO).

• Layer 2: derived from a marl (rich in CaO, CO2, and SiO2).

1. a) Predict which index minerals will appear in Layer 1 and which in Layer 2; justify why minerals typical of regional metamorphism such as staurolite or almandine garnet will not develop in Layer 2 despite sharing the same metamorphic grade.
2. b) To date the metamorphic event, a geochronological study is performed on Layer 1: U-Pb dating on zircon cores yields an age of 520 Ma, whereas 40Ar/39Ar dating on muscovites from the same sample yields 315 Ma. Interpret this discrepancy relying on the concepts of "isotopic closure temperature" and "inherited mineral", deducing which of the two dates is related to the orogenic metamorphism and whether that date corresponds exactly to its climax.
3. c) Design an analytical strategy of petrographic and microchemical observation (for example, with an electron microprobe on polished thin sections) to test whether the rims of the zircons underwent metamorphic overgrowth during the 315 Ma event.

What is contact metamorphism and where does it occur? What structure does it form around an igneous intrusion?

What are hornfels and what characteristics do they show (fine grain, hardness, absence of foliation)? From which rocks are they generated?

What is a mylonite and in what type of zones does it form? Which deformation process produces it?

Distinguish regional, contact, and dynamic metamorphism, and relate each to a specific plate tectonic setting.

Regional metamorphism accompanies orogenies. Explain why compression and the burial of large thicknesses of rocks raise pressure and temperature at the same time.

In the aureole of a granite, slates pass into hornfels with andalusite and, near the contact, with cordierite. Explain the concentric zone arrangement of the metamorphic aureole and what each mineral represents.

Serpentinites come from the hydration of ultramafic rocks such as peridotites. Explain what ultramafic rocks are, where they originally form and what geological context their presence at the surface indicates.

Along a 40 km transect in an orogenic belt, a geology team maps the first appearance of several index minerals in metamorphosed shales in an east-west direction: chlorite (km 0), biotite (km 11), almandine garnet (km 21), staurolite (km 29), kyanite (km 34), and sillimanite (km 38).

1. a) Interpret the type of metamorphism and the pressure and temperature (P/T) gradient represented; deduce in which direction tectonic burial was progressing and the thermal polarity of the orogen.
2. b) At km 32, intercalated between staurolite-bearing schists, a thick quartzite layer appears, composed of 98% quartz and trace amounts of muscovite. A student concludes that this layer corresponds to a very low-grade metamorphic zone because it lacks staurolite and garnet. Refute their conclusion by relying on the concepts of metamorphic facies and chemical composition of the protolith.
3. c) If at km 39 it is observed under the microscope that sillimanite occurs as an overgrowth and direct replacement of kyanite prisms, deduce the P-T path (clockwise with decompression and continued heating or isobaric cooling) experienced by that crustal sector during orogenesis.

At mid-ocean ridges, hydrothermal circulation alters basalts and subvolcanic dykes at temperatures of 200 °C to 400 °C and pressures below 0,2 GPa. Deep ocean floor drilling reveals that the rocks undergo massive hydration (> 3,0 wt% structural water) and replacement of their primary minerals by the assemblage actinolite + chlorite + epidote + albite.

1. a) Justify why this greenschist facies mineral paragenesis develops under such low confining pressure conditions, highlighting the chemical and thermal role of heated seawater compared to orogenic regional metamorphism.
2. b) In the fluids of hydrothermal vents ("black smokers"), dissolved magnesium drops to virtually zero, while calcium, iron, manganese, and silicon increase drastically relative to normal seawater. Interpret this ion exchange and explain why seafloor hydrothermal metamorphism is an essential geochemical sink for the global salt balance in the oceans.
3. c) Predict the consequences that the progressive dehydration of this altered oceanic crust will have when it subducts millions of years later beneath a continental plate, relating it to seismicity along the Benioff zone and partial melting in the overlying mantle wedge.

In Alpine-type mountain ranges, blueschist belts appear (with glaucophane, epidote, and phengite) locally associated with eclogites containing pyrope and omphacite.

1. a) Justify what geothermal gradient (temperature/depth) characterizes the blueschist forming environment and reason why active subduction zones are the only terrestrial geodynamic setting capable of generating pressures greater than 1,2 GPa at temperatures below 400 °C.
2. b) Under the petrographic microscope, the glaucophane crystals in these schists show perimetric alteration rims composed of actinolite and chlorite. Deduce what P-T-t (pressure-temperature-time) path explains this texture and argue why the presence of well-preserved blueschists at the surface requires rapid tectonic exhumation mechanisms rather than simple slow erosional denudation.
3. c) Why is the absence of blueschists in Archean-age shields (> 2500 Ma) interpreted as evidence that the geodynamic regime of the early Earth was significantly different from modern plate tectonics with cold subduction?

A geology student makes the following claim while looking at the rock cycle diagram: "In order for a metamorphic rock (3) to form, it is essential that the rock first pass through the sedimentation stage (2) starting from an igneous rock (1); furthermore, every magma (4) originates directly from the melting of a prior metamorphic rock (3)".

1. a) Based on plate dynamics in a collisional orogen, reasonably refute both claims by designing "shortcuts" or alternative pathways between the lithological reservoirs that demonstrate that the cycle is neither a closed nor a unidirectional sequence.
2. b) If metamorphic rock 3 originates directly from igneous rock 1 without passing through sedimentary state 2 (ortho-derived rock), what petrological, geochemical, or fabric criteria would allow you to distinguish it in the field from a para-derived rock (originated from 2)?
3. c) In the transition from 3 to 4 by crustal anatexis, the melt generated fails to reach the surface and crystallizes at depth, giving rise to 1. Predict the resulting plutonic rock if the protolith was an aluminum-rich metapelite and explain what texture it will develop based on its cooling rate.

The rock cycle is not a closed, balanced loop, but a dissipative system coupled to Earth's thermal and tectonic engine. Consider the following global observations:

1) Basaltic oceanic crust has densities of 2,9 to 3,0 g/cm³ and is almost completely recycled into the mantle, never exceeding 200 Ma in age.

2) Continental crust has an average density of 2,7 g/cm³ and preserves rocks older than 3800 Ma.

3) In active collisional belts, the rate of erosional denudation can reach 2 to 5 mm/year.

1. a) Based on these data, justify why continental crust acts as a cumulative reservoir and undergoes irreversible geochemical differentiation toward compositions richer in silica and incompatible elements over geological time, instead of being destroyed in the mantle.
2. b) Model the coupled feedback mechanism between orogenesis, climate, and the rock cycle: explain how the chemical weathering of silicates in young orogens alters global atmospheric composition and how superficial erosional unloading triggers isostatic rebound and the exhumation of deep crustal rocks.
3. c) In a pure continental transform fault (with no convergent or divergent component), predict which lithological transformations of the cycle will be dominant (in terms of metamorphic type and rock deformation) and which will be practically absent compared to a convergent boundary, justifying your answer according to the prevailing thermal and stress regime.