Igneous, Metamorphic and Sedimentary Petrology

Igneous rock classification

Igneous rocks are named with the IUGS scheme, which uses modal mineral proportions for coarse-grained rocks and chemical composition for fine-grained or glassy rocks. The first division is by silica content and colour index. Ultramafic rocks have very low silica, below about 45 wt% SiO_2, and are dominated by olivine and pyroxene. Mafic rocks have about 45 to 52 wt% SiO_2, are rich in Mg and Fe, and consist mainly of plagioclase and pyroxene. Intermediate rocks such as dacite, with roughly 63 to 68 wt% SiO_2, lie above the mafic range, and rhyolite, with more than about 68 wt% SiO_2, is the most silicic common volcanic rock.

• Basalt — the fine-grained extrusive equivalent of gabbro and the most common mafic lava, containing plagioclase and pyroxene with or without olivine. Example: a dark, fine-grained lava rich in Fe and Mg with low silica is basalt, whereas a pale lava with about 65 wt% SiO_2 is intermediate.
• Trachyte — an alkali-rich felsic to intermediate volcanic rock dominated by alkali feldspar, not a mafic rock.

Felsicity describes the proportion of light-coloured minerals, quartz and feldspars, in a rock, whereas mafic minerals are olivine, pyroxene, amphibole and biotite. A rock with no mafic minerals is wholly felsic, and a rock made only of olivine and pyroxene has no felsic content. When comparing rocks, compare their felsic percentages: a rock with 20 % biotite and 80 % quartz plus feldspar is less felsic than a rock with no mafic minerals at all.

Dunite is the ultramafic plutonic rock made of more than 90 % olivine, while the peridotite group contains about 40 to 90 % olivine. Within peridotite, the relative amounts of orthopyroxene and clinopyroxene decide the name. Harzburgite is dominated by orthopyroxene, wehrlite contains clinopyroxene with almost no orthopyroxene, and lherzolite carries both pyroxenes in substantial amounts.

On the IUGS olivine-orthopyroxene-clinopyroxene ternary, the field of a point is read from its three proportions. A point near the centre with roughly equal pyroxenes and moderate olivine plots as lherzolite, whereas a point with orthopyroxene dominant and a small olivine content plots as olivine websterite.

For coarse-grained felsic plutonic rocks, the IUGS QAP diagram uses normalised quartz (Q) and the ratio A/(A+P) of alkali feldspar to total feldspar. Rocks with Q between 5 and 20 % and A/(A+P) between about 65 and 90 % fall in the quartz syenite field, while rocks with Q between 20 and 60 % and P/(A+P) between about 65 and 90 % are granodiorite. Example: a rock with Q 10 %, A 70 % and P 20 % has A/(A+P) of about 0.78 and plots as quartz syenite.

Extrusive rocks cool quickly at the surface and show fine-grained or glassy textures, whereas intrusive rocks cool slowly at depth and are coarse-grained. Most plutonic rocks have a volcanic counterpart of similar composition, such as granite and rhyolite, or syenite and trachyte. Syenite is therefore a plutonic rock and not an extrusive one, while komatiite is an ultramafic lava and dacite and trachyte are volcanic.

Trace element fractional crystallization

Trace elements are present in such small amounts that they behave in accordance with Henry's Law, and their partitioning between a crystal and the melt is described by the partition coefficient D = C_{s}/C_{l}. Compatible elements have D > 1 and enter early-formed crystals such as olivine, whereas incompatible elements have D < 1 and remain in the melt. During fractional crystallization the melt becomes steadily depleted in a compatible element as crystals form, while an incompatible element becomes progressively enriched in the residual liquid.

• Henry's Law — at low concentrations the concentration of a trace element in a phase is proportional to its concentration in the coexisting melt, so D is constant. Example: if a crystal holds half as much of an element per unit melt at every concentration, D stays the same over the whole crystallization path.
• Rayleigh fractionation — the residual melt concentration is C_L = C_0 F^{D-1}, where F is the fraction of melt remaining. Example: with C_0 = 100 ppm, F = 0.5 and D = 0.1, the residual liquid contains about 187 ppm.
• Europium anomaly — Eu^{2+} substitutes for Ca^{2+} and Sr^{2+} in plagioclase, so plagioclase that separates from the melt removes Eu and leaves the residual liquid depleted. Granites and granodiorites, as end-products of differentiation, therefore show negative Eu anomalies.

Cross-stratification paleocurrent indicators

Cross-stratification forms by migration of bedforms, and its geometry records the palaeocurrent. Trough cross-stratification forms in scour-and-fill channels with a concave-up erosional base, and the trough axis, seen on the truncated bedding surface of an undeformed bed, lies parallel to the flow direction. Herringbone cross-stratification contains foresets dipping in two opposite directions and so records reversing currents, while hummocky cross-stratification records oscillatory storm flow and gives no single direction.

On a ripple, the lee side is the slip face on the downcurrent side of the crest, where foresets accumulate. Foresets dip in the direction of the current that formed them, so the dip direction of the lee face gives the flow direction, while the dip angle itself is not the indicator.

A reactivation surface is an erosional surface inside a single cross-bed set that truncates the foresets at an angle, formed when the flow strength or direction changes and the bedform is reworked. It differs from a foreset, which is a depositional surface, and from a scoured channel base or an angular unconformity, which are larger-scale contacts.

Igneous textures and definitions

Igneous textures are defined by crystal size, shape and mutual relations, and each term has a specific meaning in petrology.

• Vitrophyre — a porphyritic rock with phenocrysts set in a glassy groundmass, typically an obsidian-like volcanic rock. Example: large feldspar crystals in black glass describe a vitrophyre.
• Rapakivi texture — an alkali feldspar crystal rimmed by plagioclase, commonly seen in some granites.
• Ocelli — radially arranged, needle-like aggregates of plagioclase, with or without clinopyroxene, forming rounded structures in a rock.
• Spinifex texture — sub-parallel, skeletal, platy olivine or pyroxene crystals, characteristic of komatiite.
• Ophitic texture — plagioclase laths enclosed by pyroxene, typical of gabbro and dolerite. Example: laths of plagioclase surrounded by a single pyroxene grain indicate ophitic texture.
• Porphyritic and porphyroblastic — porphyritic is an igneous term for phenocrysts in a finer groundmass, whereas porphyroblastic is a metamorphic term for large crystals grown in a finer matrix. Basalt is therefore porphyritic and not porphyroblastic.
• Granoblastic texture — equant, interlocking grains of similar size, the characteristic metamorphic texture of marble.
• Simultaneous intergrowths — granophyric texture (quartz and alkali feldspar) and myrmekite (vermicular quartz in plagioclase) both form when two minerals crystallize together.
• Reaction textures and cumulates — a corona of orthopyroxene around olivine forms by reaction with melt, and interstitial plagioclase in a pyroxene cumulate crystallizes later, so neither is simultaneous growth.

Binary eutectic phase diagram crystallization

In a binary system at fixed pressure, the liquidus gives the temperature at which the first crystals appear, and the solidus marks the end of crystallization. For a bulk composition on one side of the eutectic, the phase in excess of the eutectic composition crystallizes first. For example, a melt richer in component B than the eutectic begins by crystallizing pure B.

The lever rule gives the mass proportions of two coexisting phases. The fraction of crystals equals the distance from the liquid composition to the bulk composition divided by the distance between the liquid and solid compositions, and the liquid fraction is its complement. Example: if the bulk composition is 75 wt% B, the liquid is 60 wt% B and the solid is pure B, the crystals form (75-60)/(100-60), which is 37.5 %.

At the eutectic, the remaining liquid solidifies as both pure end-member phases at once, so the bulk composition of the final rock is the sum of those solids. In equilibrium crystallization the bulk composition is conserved throughout, so the final solid assemblage retains the composition of the initial liquid. Where two solid solutions are present, the lever rule applied to the solvus gives the ratio of the two solid phases.

Crystallization of diopside removes CaO and MgO in roughly equal molar proportions and excludes Al_2O_3, so the residual melt becomes depleted in MgO and CaO relative to Al_2O_3. On a variation diagram this path moves toward lower MgO and lower CaO/Al_2O_3.

Clapeyron slope of metamorphic reaction

The slope of a univariant reaction in P–T space is given by the Clapeyron relation dP/dT = \Delta S_r/\Delta V_r, where \Delta S_r and \Delta V_r are the entropy and volume change of the reaction, calculated as products minus reactants. The sign of the slope follows the signs of both changes, and a steep slope means that the equilibrium temperature changes little with pressure. Example: for a reaction with \Delta S_r = 50 J K^{-1} and \Delta V_r = 2 J bar^{-1}, the slope is 25 bar K^{-1}.

A reaction is a good geothermometer when its equilibrium constant changes strongly with temperature but hardly depends on pressure. Because (\partial \ln K/\partial P)_T = -\Delta V_r/RT, a small \Delta V_r keeps the equilibrium insensitive to pressure, and a large \Delta S_r gives a clear temperature response along the equilibrium curve. The most suitable reaction therefore has small \Delta V_r and large \Delta S_r.

Contact metamorphism textures

Contact metamorphism occurs around an igneous intrusion, where heat drives recrystallization with little deformation. Rising temperature speeds up mineral reactions, because rate constants follow the Arrhenius relation, and it promotes grain growth. Grains therefore coarsen, and their volume-to-surface-area ratio increases, since volume scales with the cube of the radius while area scales with its square.

Dehydration reactions become favoured as temperature rises, so hydrous minerals such as micas and amphiboles break down at the contact and are less stable there than in regional metamorphism at similar pressure.

Porphyroblast growth textures

Porphyroblasts are dated relative to deformation by their inclusion trails, which preserve an earlier foliation, and by how the matrix foliation wraps or truncates them. Pre- or syn-kinematic porphyroblasts rotate during deformation, so their internal trails are sigmoidal or deflected, and the external foliation bends around them. Post-kinematic porphyroblasts grow after deformation has stopped, so their inclusion pattern is not rotated and the matrix foliation is not deflected around them.

• Post-kinematic indicators — randomly oriented aggregates that pseudomorph a porphyroblast, and an inclusion fabric that is discordant to the external foliation, both show static growth. Example: a garnet whose inclusions are straight and cut across the matrix foliation grew after the deformation.
• Relative growth sequence — a porphyroblast whose trails are only slightly deflected grew early in the development of the matrix foliation, a grain with strongly rotated trails grew later, and grains that cut the matrix foliation discordantly are the youngest.

Pelite metamorphic reactions

Pelitic rocks, derived from clay-rich sediments, record metamorphic grade through the progressive appearance of muscovite, chlorite, biotite, garnet, staurolite, kyanite, sillimanite, K-feldspar and cordierite. The sillimanite isograds are marked by reactions that consume staurolite or kyanite at moderate temperature, and by the breakdown of muscovite with quartz at higher temperature.

The second sillimanite isograd is marked by the dehydration breakdown of muscovite in the presence of quartz, which produces sillimanite, K-feldspar and H_2O. Example: a reaction of muscovite and quartz that yields sillimanite and K-feldspar marks the high-temperature limit of this isograd.

At granulite facies, dehydration melting of biotite in the presence of sillimanite and quartz gives cordierite, garnet, K-feldspar and melt. This assemblage is diagnostic of granulite facies pelites and is not produced by the lower-grade reactions of chlorite, staurolite or muscovite.

Bowen's reaction series

Bowen's reaction series describes the sequence in which minerals crystallize from a cooling mafic magma, and it has two branches. The discontinuous branch consists of mafic minerals whose structures change at each step, in the order olivine, pyroxene, amphibole and biotite. The continuous branch is plagioclase, which changes composition steadily from calcium-rich anorthite at high temperature to sodium-rich albite at low temperature, forming a complete solid-solution series.