10: rocks from melts (igneous activity in a plat tectonic context)

melting due to addition of volatiles (flux melting)

- volatiles help to break chemical bonds - adding volatiles decreases a rock's melting temperature - water most important volatile - happens in mantle above subduction crust - see graph

hot-spot volcanism

- basaltic extrusives - basaltic intrusives

how does it relate to plate tectonics?

- both style of igneous activity and composition of igneous rocks relate strongly to plate tectonic setting

lower melting temperatures

- decreased pressure - increased water content - more felsic

fractional crystalization

- different minerals crystallise at different temperatures - separation of crystak fraction from remaining liquid (ex gravitional setting) - residual melt will be different from parent melt

major types of magma

- four major types based on SiO2 content - the 4 types differ in their chemistry and their physical properties (ex how easily they flow and freeze)

magmas form locally by melting of pre-existing solid rock

- heating (not usually the most important cause of magmas) - pressure decrease: important at divergent boundaries (extension) and mantle plumes - addition of volatiles: important at convergent boundaries (compression/squeezing) - original composition

what is magma?

- hot, mainly liquid solutions of silicate rock-forming elements of volatiles - form in the crust and upper mantle, within 2000km (most much shallower) of the surface - crust and mantle solid at these depths

assimilation

- incorporation of wall rock through which magma passes as lumps that dissolve - elements may migrate out of wall into magma without wholesale melting of wall

factors affecting melting tmeperatures: higher melting temperatures

- increased pressure - decreased water content - mroe mafic

why do magmas rise? - what does viscosity depend on? (3)

- less dense: buoyant - weight of overlying rock creates pressure at depth: magma squeezed upwards - less viscous magmas flow more easily - viscosity depends on temperature, volatile content and silica content

continental plate subduction

- mafic to felsic intrusives - mafic to felsic extrusives

island arc plate subduction

- mafic to intermediate intrusives (plutonism) - mafic to intermediate extrusives (volcanism)

variation in source rock composition

- mantle (ultramafic) - ocean crust (mafic) - continental crust (variable: mafic to intermediate to felsic) *** note how variation will relate to plate tectonic context

large igneous provinces (LIPS)

- mantle plumes at the base of the lithosphere - huge volumes of low-viscocity mafic magmas - persist for an extended interval of time - emitted as floods basalts - associated with gas emissions (ex SO2): possible role in mass extinction events

examples chosen involve only oceanic crust

- more complicated scenarios when continental crust involved but general rule would be that spectrum of magmas will include more felsic examples: a) fractional crystallization b) assimilation from crust (remember the average composition of continental crust)

melting due to decrease in pressure (decompression melting)

- occurs where material from asthenosphere rises to shallower depths - mantle plumes - divergent margins: importance of extension - mantle plume, continental rift, mid-ocean rift diagrams

exception...

- pegmatite: very coarse-grained (10mm + common) - typically occurs as veins or dykes intruded into cool / solid rock

melting as a result of heat transfer from rising magmas

- rising magma heats surrounding rocks - surrounding crustal rock may melt - very hot magma from mantle stalls at base of crust or in crust and transfers heat onto the crust

partial melting

-most rocks contain variety of minerals - melting of each not identical - initial magma to form will not have same composition as parent rock - a smelting progresses progressively less silica remains

order of low to high silica

1) ultramafic: peridotite 2) mafic: gabbro 3) intermediate: diorite 4) felsic: granite

volatiles

- substances dissolved in the magma - don't go into minerals as the magma solidifies - released in the form of liquids or gasses, H2O (most abundant), CO2, SO2, and H2S

magma mixing

- two or more different magmas mixed

pressure and temperatures interact to determine if a rock stays solid or begins to melt

- solidus: P/T conditions at which rock starts to melt - liquidus: P/T conditions at which rock melts completely - see graphs

steps more detailed

1) hot mantle rock rises, decompresses, and melts to a mush of crystals and basaltic magma 2) a thin dike erupts, spilling lava on the ocean floor in characteristic pillows 3) as the basal mush cools, dikes intrude to form sheeted dikes (remnants of the spreading center move away laterally) 4) sediments are deposited on the spreading seafloor 5) a gabbro layer is formed adjacent to the magma chamber 6) in the magma chamber, crystals settle out of the magma, forming the peridotite

spreading centres as magma factories (mid ocean ridges): sequence of events

1) input: periodotie from asthenosphere 2) plates diverge 3) pressure reduced 4) periodotite rises to fill space 5) drop in pressure (ie decompression melting) 6) preferential melting of certain minerals 7) parent ultramafic but magma mafic (> Si and Fe) 8) melt buoyant + rises 9) residual peridotite layer at base

subduction zones as magma factories: ocean-ocean convergence

1) input: variable (depends on precisely what is subducted), composition will be different from composition at mid-ocean ridges 2) water source from intergranular spaces in rocks and by alteration of hydrous minerals is released 3) water rises 4) composition broadly mafic, but variable - causes melting of descending plate and overlying plate 5) fractional crystalization can occur here, producing more felsic magmas - magma accumulates in magma chambers 6) line of volcanoes parallel to subduction zone: eruptions at surface as volcanoes

viscocity affects melt movement

1) low viscocity: hotter and less SiO2 (mafic) 2) high viscocity: cooler nad more SiO2 (felsic) - viscocity depends on temperature, volatile content, and percent of silica content

steps of melting

1) partial melting of country rock creates a magma of a particular composition 2) cooling causes minerals to crystallize and settle 3) a basaltic magma chamber breaks through, causing turbulent flow 4) mixing of two magmas results in andesitic magma 5) crystals formed in the mixed magma have a different composition, and may accumulate on the sides and roof of the magma chambers due to turbulence

layers of ophiolite suite

1) sediment layers 2) pillow lavas with sheeted dike complex 3) thin section of gabbro 4) thin section of peridotite

structures produced bottom to top

1) slow cooling coarse-grained gabbro (magma chamber) 2) propagates upwards cools in fractures (sheeted dykes) 3) erupts onto surface via fractures (pillow lavas)

why are volcanoes not found directly above the subduction zone?

1) subducting oceanic crust carries sediments with it, water remains trapped between sediment granis 2) the trapped water, as well as chemically bound water, is release as the temperature increases 3) causes the sedimentary rocks to melt at lower temperatures than surrounding dry mantle rocks 4) the mantle and molten sediments move upward and melt parts of the overlying plate 5) the resulting magmas accumulate in magma chambers 6) these can then erupt to form volcanoes

why is there such a great variability in the composition of magmas? (5)

1) variation in source rock composition 2) partial melting 3) fractional crystalization 4) magma mixing 5) assimilation

major silicate rock forming elements

O, Si, Al, Fe, Mg, K, Na, Ca

plate divergence

basaltic extrusives - basaltic intrusives

so why so coarse grained?

crystals can grow very rapidly due to water-rich melt

magma to rock: how fast does it cool - depends on (4)

depend son: 1) depth of intrusion 2) shape of magma body 3) size of magma body 4) presence of circulating groundwater - see diagrams