# Appinite

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Type of igneous rock

**Appinite** is an [amphibole](/source/Amphibole)-rich [plutonic rock](/source/Plutonic_rock) of high [geochemical variability](/source/Geochemistry). Appinites are therefore regarded as a *rock series* comprising [hornblendites](/source/Hornblendite), meladiorites, [diorites](/source/Diorites), but also [granodiorites](/source/Granodiorite) and [granites](/source/Granite). Appinites have formed from [magmas](/source/Magma) very rich in water. They occur in very different [geological environments](/source/Geology). The ultimate source region of these peculiar rocks is the [upper mantle](/source/Upper_mantle), which was altered [metasomatically](/source/Metasomatism) and geochemically before [melting](/source/Melt_(geology)).

## Etymology

Countryside near the type locality Appin

The rock appinite was named after its [type locality](/source/Type_locality_(geology)) [Appin](/source/Appin) near [Ballachulish](/source/Ballachulish) in [Scotland](/source/Scotland). Appin was originally called *An Appain* in [Scottish Gaelic](/source/Scottish_Gaelic). This is derived from [Middle Irish](/source/Middle_Irish) *apdain* or from [Old Irish](/source/Old_Irish) *aibit* with the meaning of [abbey](/source/Abbey) – referring to the ancient abbey on the neighbouring island [Lismore](/source/Lismore%2C_Scotland).[*[citation needed](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)*]

## Definition

Bailey and Maufe (1916) defined appinite originally as

a medium- to coarse-grained, meso- to melanocratic igneous rock, that stands out by conspicuous crystals of [hornblende](/source/Hornblende), which are enclosed by a [matrix](/source/Matrix_(geology)) of [plagioclase](/source/Plagioclase) ([oligoclase](/source/Oligoclase) to [andesine](/source/Andesine)) and/or [orthoclase](/source/Orthoclase). [Quartz](/source/Quartz) often is present, but can also be absent.

Generally, appinites are plutonic equivalents of [calc-alkaline](/source/Calc-alkaline_magma_series) [lamprophyres](/source/Lamprophyre) such as [vogesite](/source/Vogesite) and [spessartite](/source/Spessartite).[1]

## Introduction

Appinites – often synonymously used for *hornblende diorites* – are a coeval rock suite of plutonic or subvolcanic igneous rocks with variable chemical compositions, covering [ultramafic](/source/Ultramafic_rock) to [felsic](/source/Felsic) igneous rocks. They are characterized in all their lithologies by [euhedral](/source/Euhedral_and_anhedral) hornblende crystals as the dominant mafic mineral. Hornblende mainly appears as big [prismatic](/source/Prism_(optics)) [phenocrysts](/source/Phenocryst), but can also be found in the [groundmass](/source/Groundmass).[*[citation needed](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)*]

On top appinites have very different [textures](/source/Texture_(geology)) – featuring planar and linear magmatic fabrics, [cumulate textures](/source/Cumulate_texture), intercumulate textures and also [poikilitic](/source/Poikilitic) fabrics. They also can occur as mafic [pegmatites](/source/Pegmatite) and show common *mixing* and *mingling* between coeval mafic and felsic magmas. Often they are variably contaminated by the country rocks.[*[citation needed](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)*]

Most appinites crystallize from an important [gas](/source/Gas) phase. This implies an anomalously water-rich magma including both mantle components and [meteoric](/source/Meteoric_water) components.[2] The appinite suite therefore offers a unique occasion to study the role of water in the production and in the [crystallization](/source/Crystallization) history of mafic to felsic magmas, but also more generally in intrusional processes.[*[citation needed](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)*]

Appinitic [intrusions](/source/Igneous_intrusion) possess a whole gamut of differing plutonic bodies and show very different ways of emplacement. Most of the appinites precede granitic intrusions, but can appear also at the same time. This can be perfectly observed at the [Ardara pluton](https://en.wikipedia.org/w/index.php?title=Ardara_pluton&action=edit&redlink=1) in [Donegal](/source/County_Donegal). Their emplacement is usually directed by [tectonics](/source/Tectonics) – especially by important [shear zones](/source/Shear_zone), who potentially facilitate the rising of the magmas through the [crust](/source/Crust_(geology)).[3]

## General remarks

In general, appinites appear as relatively small, rather flat intrusional bodies in the crust. Their diameter never exceeds more than two kilometers – like for instance the defining appinites in Scotland. Appinites rose along the periphery of granitic plutons and usually are associated with important, deep reaching [faults](/source/Fault_(geology)) along which they ascended into higher crustal levels.

Often appinites – and likewise the Scottish appinites – get tied up with active [subduction](/source/Subduction), the formation of granitoids and also the termination of subduction by *slab breakoff*. In the case of the Scottish appinites it is believed that they only formed once the [Iapetus Ocean](/source/Iapetus_Ocean) was closed by continental collision between the southern continental margin of [Laurentia](/source/Laurentia) and the northwestern side of [Eastern Avalonia](/source/Avalonia) and that the subduction within Iapetus had stopped.

Yet newer [geochronological studies](/source/Geochronology) seem to indicate, that the relation between subduction, appinite formation and granite magmatism involves a rather lengthy process.

It is also believed that the mafic component of appinites only was able to form once the subducting plate had broken off enabling hot [asthenospheric material](/source/Asthenosphere) to flow in through the gap. The asthenospheric extra [heat](/source/Heat) initiated magmas containing juvenile mantle components, but also components of [Subcontinental Lithospheric Mantle](/source/Subcontinental_Lithospheric_Mantle) (*SCLM*). Furthermore, the magmas show affinities to [Shoshonites](/source/Shoshonite). The felsic components of appinites are connected to big [batholiths](/source/Batholith) with [fractional crystallization](/source/Fractional_crystallization_(geology)) being the main petrogenetic process. The assimilation of country rocks was of hardly any importance.

## Occurrences and ages

 Colonsay – Kiloran Bay as seen from the west – with two little appinite outcrops along the northern side of the bay

Appinites occur more or less worldwide. Temporally, the oldest appinites are 2700 million years old (the [Neoarchaean](/source/Neoarchaean) Era); the youngest are of [Holocene](/source/Holocene) age. The Neoarchaean appinites are associated genetically with coeval [sanukitoids](/source/Sanukitoid). This is often taken as proof for [plate tectonics](/source/Plate_tectonics) going back that far in time.

Besides the [type locality](/source/Type_locality_(geology)) in the Scottish [caledonides](/source/Caledonides) (within the [Central Highlands terrane](/source/Central_Highlands_terrane) or *Grampian terrane*) appinites also occur in [Ireland](/source/Ireland) within and in the vicinity of the [Donegal batholith](/source/Donegal_batholith) – especially in association with the Ardara pluton – but also within the [Leinster granite](https://en.wikipedia.org/w/index.php?title=Leinster_granite&action=edit&redlink=1)[4] and within the [Galway granite batholith](https://en.wikipedia.org/w/index.php?title=Galway_granite_batholith&action=edit&redlink=1).[5]

All these appinites have [Silurian](/source/Silurian) ages. Further occurrences in Scotland are found near [Loch Lomond](/source/Loch_Lomond) and in central [Sutherland](/source/Sutherland), which already belongs to the [Northern Highlands terrane](https://en.wikipedia.org/w/index.php?title=Northern_Highlands_terrane&action=edit&redlink=1). The appinites in the Northern Highlands terrane are mainly associated with the [Ratagain complex](https://en.wikipedia.org/w/index.php?title=Ratagain_complex&action=edit&redlink=1), the [Rogart granite](https://en.wikipedia.org/w/index.php?title=Rogart_granite&action=edit&redlink=1) and the [Strontian granite](https://en.wikipedia.org/w/index.php?title=Strontian_granite&action=edit&redlink=1).[6] The appinites from the Rogart granite and from the Strontian granite also have Silurian ages and are between 425 and 420 million years old.

So far the oldest known appinites come from northern [Michigan](/source/Michigan). They go back in time roughly 2700 million years and belong to the *Northern Complex* – a [greenstone belt](/source/Greenstone_belt) along the southern edge of the [Superior craton](/source/Superior_craton).[7]

Fairly old appinites are reported from [Canada](/source/Canada), for instance from the [Frog Lake hornblende gabbro](https://en.wikipedia.org/w/index.php?title=Frog_Lake_hornblende_gabbro&action=edit&redlink=1) situated within the late [neoproterozoic](/source/Neoproterozoic) *Avalon terrane* in [Nova Scotia](/source/Nova_Scotia).[8] The [Wamsutta diorite](https://en.wikipedia.org/w/index.php?title=Wamsutta_diorite&action=edit&redlink=1) in the [White Mountains](/source/White_Mountains_(New_Hampshire)) of [New Hampshire](/source/New_Hampshire) also has similarities with appinites. The diorite is 408 million years old and belongs to the [Acadian Orogeny](/source/Acadian_Orogeny).[9]

Younger appinites from the [Carboniferous](/source/Carboniferous) appear near [Puebla de Sanabria](/source/Puebla_de_Sanabria) in the [Variscides](/source/Variscides) of northwestern [Spain](/source/Spain).[10] They are also found in the [Avila batholith](https://en.wikipedia.org/w/index.php?title=Avila_batholith&action=edit&redlink=1).[11] Amongst Variscan occurrences appinites often carry local names like [Durbachites](https://en.wikipedia.org/w/index.php?title=Durbachite&action=edit&redlink=1) (in the [Black Forest](/source/Black_Forest)), [Redwitzites](https://en.wikipedia.org/w/index.php?title=Redwitzite&action=edit&redlink=1) (in the [Fichtelgebirge](/source/Fichtelgebirge)), [Vaugnerites](https://en.wikipedia.org/w/index.php?title=Vaugnerite&action=edit&redlink=1) (in the French [Massif Central](/source/Massif_Central)),[12] and sometimes they also hide under the header *High Ba Sr Granitoids* (an example being the Rogart Granite in Scotland).

Variscan appinites can also be found in the [Southern Alps](/source/Southern_Alps) of [Northern Italy](/source/Northern_Italy). They are associated here with the [permian](/source/Permian) *Serie dei Laghi* – a rock series of gabbros and granites.[13] The age of these Italian appinites is about 285 million years.

In [Asia](/source/Asia) appinites are known to occur in [China](/source/China) and in [Tibet](/source/Tibet).

In China appinites appear in the [Upper Ordovician](/source/Upper_Ordovician) (495 - 452 million years) [Datong Pluton](https://en.wikipedia.org/w/index.php?title=Datong_Pluton&action=edit&redlink=1) of the [Western Kunlun](/source/Kunlun).[14] and again in the [Triassic](/source/Triassic) [Laocheng Pluton](https://en.wikipedia.org/w/index.php?title=Laocheng_Pluton&action=edit&redlink=1) of the [Qinling](/source/Qinling)[15] During the [Upper Permian](/source/Upper_Permian) appinites formed along the northern edge of the [North China Craton](/source/North_China_craton) (in northwestern [Liaoning](/source/Liaoning)) and during the Triassic in [Heilongjiang](/source/Heilongjiang) (near [Duobaoshan](https://en.wikipedia.org/w/index.php?title=Duobaoshan&action=edit&redlink=1)), also belonging to the North China Craton.

In the Tibetan [Himalaya](/source/Himalaya) Appinite-cumulates are found in the [Gangdese batholith](/source/Gangdese_batholith) of the [Lhasa terrane](/source/Lhasa_terrane). These appinites formed during the Upper Triassic and are 220 to 213 million years old.[16] Another appinite association in Tibet occurs near [Pengcuolin](https://en.wikipedia.org/w/index.php?title=Pengcuolin&action=edit&redlink=1) northwest of [Xigazê](/source/Xigaz%C3%AA). It belongs to the southern Lhasa terrane and is only 51 million years old i.e. [Ypresian](/source/Ypresian) ([Eocene](/source/Eocene)).[17]

Very young examples of appinites come from [Iran](/source/Iran), like appinites from the [Baneh Pluton](https://en.wikipedia.org/w/index.php?title=Baneh_Pluton&action=edit&redlink=1) in the [Zagros](/source/Zagros). These appinites are 40 million years old and stem from the Middle Eocene. They mark the *Zagros Suture Zone*.[18] At about the same time appinites also formed near [Sardasht](/source/Sardasht%2C_West_Azerbaijan) more to the northwest.[19]

## Mineralogy

Appinites consist mainly of [amphibole](/source/Amphibole) (hornblende) taking up between 50 and 80 volume percent. [Anorthite](/source/Anorthite)-rich plagioclase with An50-70 reaches about 20 vol. %. The rest is made up of [clinopyroxene](/source/Clinopyroxene) (5 to 15 vol. %) and [olivine](/source/Olivine) (5 to 10 vol. %). Some [biotite](/source/Biotite) and occasional [phlogopite](/source/Phlogopite) are also encountered. In more felsic appinites appear [alkali feldspar](/source/Alkali_feldspar) and [quartz](/source/Quartz). Represented amongst the [accessory minerals](/source/Accessory_mineral) are [sphene](/source/Sphene), [ilmenite](/source/Ilmenite), [zircon](/source/Zircon) and [apatite](/source/Apatite). [Allanite](/source/Allanite) can be found in more felsic members.

A special occurrence is [myrmekite](/source/Myrmekite) found in an appinite of the Italian *Serie dei Laghi* – indicating metasomatic alterations.

Amongst the amphiboles (mainly brown amphiboles, but also some greenish amphiboles) two populations with high and low [aluminium](/source/Aluminium) content can be differentiated. [Tschermakite](/source/Tschermakite) and [magnesiohastingsite](https://en.wikipedia.org/w/index.php?title=Hastingsite&action=edit&redlink=1) are rich in aluminium, whereas [magnesiohornblende](/source/Magnesiohornblende) contains much less. Plagioclase can also be subdivided into two groups – one anorthite-rich with An80-88 and the other anorthite-poor with An36-52. Plagioclase with a high anorthite component is surrounded by amphiboles or mantled by plagioclases with a low anorthite component. Therefore, it can be assumed, that plagioclase crystallized before amphibole. The [grain size](/source/Grain_size) of amphiboles varies from 2 millimeters to several centimeters.

Plagioclase, olivine and clinopyroxene settled as [cumulates](/source/Cumulate), whereas amphiboles grew afterwards as intercumulate crystals which also can show [corona textures](https://en.wikipedia.org/w/index.php?title=Corona_texture&action=edit&redlink=1).

## Petrology

### Major elements

Amongst the [major elements](/source/Abundance_of_the_chemical_elements) the SiO2 contents of the appinite suite usually vary between 42 and 61 weight %. The rocks are therefore ultramafic, mafic and intermediate in their geochemical composition. Felsic end members can reach up to 72.1 weight % SiO2. The SiO2 contents correspond with the rock types [cortlandtite](https://en.wikipedia.org/w/index.php?title=Cortlandtite&action=edit&redlink=1) (a melagabbro), hornblendite, hornblende diorite, meladiorite and diorite, the felsic end members with granodiorite till granite.

The Al2O3 contents vary between 13 and 22 weight %. Appinites are [metaluminous](/source/Metaluminous) with A/NK > 1 and A/CNK < 1. The contents of MgO fall between 5 and 16 weight % and the [magnesium numbers](https://en.wikipedia.org/w/index.php?title=Magnesium_number&action=edit&redlink=1) generally oscillate between 0.22 and 0.57 (or between 22 and 57). Appinites are *magnesian* rocks (and not *ferroan*), because in the relation SiO2 plotted against Fe2O3tot/(Fe2O3tot + MgO) their values are always lower than 0.66. Their [magnesium](/source/Magnesium) contents are higher than what can be expected from melting of metabasalts and they approach sanukitoids of modern [island arcs](/source/Island_arc). The K2O contents vary between 0.5 and 4.0 weight %, appinites are thus calc-alkaline (middle K and high K). Strongly differentiated samples can even touch into the shoshonitic field. With a value of 0.3 weight % K2O the appinite from *Kilrean* has not been differentiated at all and represents an island arc [tholeiite](/source/Tholeiite). The ratio Na2O/K2O is rather high in appinites (right up to 5.43) and is similar to [Cenozoic](/source/Cenozoic) [adakites](/source/Adakite), which were produced by the melting of subducted [oceanic crust](/source/Oceanic_crust). Accordingly, appinites are a rock suite dominated by [sodium](/source/Sodium).

In the [TAS diagram](/source/TAS_diagram) appinites appear mainly in the [subalcaline field](https://en.wikipedia.org/w/index.php?title=Subalcaline_magma&action=edit&redlink=1), but they can extend into the [alcaline field](https://en.wikipedia.org/w/index.php?title=Alcaline_magma&action=edit&redlink=1). They plot in the fields of [basalt](/source/Basalt), basaltic andesite and andesite, but touch as well the fields of [basanite](/source/Basanite), [trachybasalt](/source/Trachybasalt), basaltic trachyandesite and [trachyandesite](/source/Trachyandesite). The magmatic equivalents are gabbro, gabbroic diorite and diorite, extending towards peridotgabbro, foidgabbro, monzogabbro and monzodiorite. [Monzonite](/source/Monzonite) hardly ever is realized.

The following table shows major element compositions of several appinites – in comparison with the lamprophyre from Narin-Portnoo:[*[citation needed](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)*]

Oxide weight % Appinite Meenalargan Appinite Narin-Portnoo Appinite Colonsay Appinite Serie dei Laghi 1 Appinite Serie dei Laghi 2 Laocheng Appinite 1 Laocheng Appinite 2 Appinite Pengcuolin Lamprophyre Narin-Portnoo SiO2 48.90 50.20 52.30 49.76 56.03 46.55 50.44 41.16–48.13 49.37 TiO2 1.65 1.00 0.72 1.64 1.02 2.33 0.73 0.79–2.22 3.15 Al2O3 15.51 14.30 15.23 17.01 15.36 15.59 12.18 16.20–18.26 13.42 Fe2O3tot 9.18 7.70 7.59 10.83 8.04 11.48 8.31 9.65–16.21 14.29 MnO 0.13 0.10 0.14 0.19 0.13 0.15 0.13 0.23 MgO 9.10 7.90 5.77 5.58 8.30 7.62 10.58 5.25–8.66 5.64 CaO 9.96 11.80 7.85 9.84 6.59 8.16 13.15 10.10–11.48 9.90 Na2O 2.60 2.80 2.16 2.74 2.74 3.61 1.89 1.86–2.79 2.57 K2O 1.20 1.00 3.00 2.03 1.56 2.37 0.91 0.49–0.90 0.51 P2O5 0.37 0.30 1.11 0.35 0.22 0.76 0.17 0.36 LOI 2.20 2.40 1.85 0.03 0.01 1.73 1.58 0.56 Mg# 0.35 0.41 0.62 0.50 0.67 0.60 0.74 0.39 – 0.61 0.46 Na/K 3.30 4.26 1.09 2.06 2.66 2.31 3.14 2.48 – 5.43 7.69 Al/K+Na 2.79 2.51 2.24 2.54 2.48 1.83 2.97 2.81 Al/K+Na+Ca 0.66 0.53 0.72 0.69 0.84 0.67 0.43 0.59

### Trace elements

Amongst the [trace elements](/source/Trace_element) the mafic members of appinites manifest high concentrations in transitional metals like [nickel](/source/Nickel) (98-288 [ppm](/source/Parts-per_notation)), [chromium](/source/Chromium) (100-810 ppm) and [vanadium](/source/Vanadium) (179-462 ppm). The [large-ion lithophile elements](/source/Large-ion_lithophile) (LILE), for example [rubidium](/source/Rubidium), potassium, [barium](/source/Barium) (253-528 ppm), [cesium](/source/Cesium) and [strontium](/source/Strontium) (415-813 ppm), also have elevated concentrations – and so do the light rare-earth elements ([LREE](/source/LREE)). Low in concentration are the heavy rare-earth elements ([HREE](/source/Rare-earth_element)) and also the [high field strength elements](/source/Incompatible_element) (HFSE) [niobium](/source/Niobium), [tantalium](/source/Tantalium), [zirconium](/source/Zirconium), [phosphorus](/source/Phosphorus), [titanium](/source/Titanium) and [thorium](/source/Thorium). Still the HFSE are higher concentrated than in the associated granodiorites and granites. Compared with [chondrites](/source/Chondrite) the LREE show an enrichment by factors 20-200. The HREE fractionation (expressed through the ratio GdN/YbN) shows values between 1.4 and 6.1. A positive [europium anomaly](/source/Europium_anomaly) is very weakly expressed and in more felsic appinites the anomaly turns slightly negative (0.96-0.70). The values for [yttrium](/source/Yttrium) are rather low (17-30 ppm).

The high concentrations in the elements Mg, Ni, Cr and Ba point towards a mantle source region.[20]

Compared with [MORBs](/source/MORB) the elements rubidium, barium, potassium and also [cerium](/source/Cerium) are strongly enriched, yet titanium, [ytterbium](/source/Ytterbium) and yttrium are depleted.

The following table shows trace elements of different appinites:[*[citation needed](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)*]

Trace elements ppm Appinite Meenalargan Appinite Narin-Portnoo Appinite Serie dei Laghi 1 Appinite Serie dei Laghi 2 Laocheng Appinite 1 Laocheng Appinite 2 Pb – 11.0 – – 4.90 4.94 Ni 95 35 22 128 127 125 Cr – – 93 374 650 677 V 271 230 – – 193 194 Zr 76 62 114 141 72.2 69.1 Y 30.0 18.0 33.0 24.0 17.1 17.5 Sr 813 415 401 370 635 596 Ba 336 – 125 294 347 332 Rb 37.0 31.0 72.0 70.0 58.6 38.7 Nb 4.0 4.0 11.0 9.0 4.17 4.21

### Isotopes

According to Harmon et al. (1984) appinites possess the following εNd-, εSr- and εHf values:[21]

- εNd varies between − 8 and + 2 (i. e. between 0.5123 and 0.51275 – in the *Serie dei Laghi* between 0.5119 and 0.5123 for 143Nd/144Nd)

- εSr varies between − 5 und + 10 (i. e. between 0.7044 and 0.711 for 87Sr/86Sr).

- εHf(t) in zircon varies between 3.3 and 7.9, but can descend to − 1.7.

Appinites prolong the [mantle array](https://en.wikipedia.org/w/index.php?title=Mantle_array&action=edit&redlink=1) into the field of negative εNd. Yet their mafic members plot very close to enriched MORB (EMORB) with εNd = + 2 and 87Sr/86Sr = 0.7048. Their εSr falls slightly above 0.

Whole rock analyses for δ18O delivered values of 6.7 ‰, yet for single minerals values from 4.3 to 6.1 ‰.[22]

The isotopic ratio 206Pb/204Pb varies between 17.9 and 18.4.

## Geochemistry

The [geochemical composition](/source/Geochemistry) of appinites is mainly calc-alkaline, sometimes shoshonitic and rarely tholeiitic. Therefore, appinites resemble shoshonites, shoshonitic [lamprophyres](/source/Lamprophyre),[23] but also magnesian andesites,[24] sanukitoids, adakites and [TTG rocks](/source/Tonalite%E2%80%93trondhjemite%E2%80%93granodiorite) ([tonalites](/source/Tonalite), [trondhjemites](/source/Trondhjemite) and granodiorites). The TTGs appear especially in the late [Archean](/source/Archean) and during the [Paleoproterozoic](/source/Paleoproterozoic).[25]

## Genesis

The appinites in western Scotland and in northwestern Ireland originated from a gas-rich basaltic magma. The occurrences near Ballachulish are calc-alkaline and belong to the high-K type. They are evolving towards more continental conditions. In contrast, the Ardara appinites show transitions from calc-alkaline towards tholeiitic, and were thus evolving towards island arc rocks. The Loch Lomond appinites are intermediate between the two, and they are common calc-alkaline rocks.

In the appinites from Ballachulish, [olivine](/source/Olivine) appears on the [liquidus](/source/Liquidus) at a depth of about 70 to 80 kilometers, from where they ascended into overlying crustal domains. Their ascent was impeded by structural complications caused by [folded rocks](/source/Fold_(geology)) of the [Dalradian Supergroup](/source/Dalradian_Supergroup). Further crystallizations then happened under falling temperatures and rather variable gas pressures, caused by explosions within [subvolcanic pipes](/source/Diatreme).

Olivine crystallized first then [clinopyroxene](/source/Clinopyroxene), amphibole, [mica](/source/Mica) and plagioclase, creating a progressive rock suite covering ultramafic to felsic compositions.[26][*[better source needed](https://en.wikipedia.org/wiki/Wikipedia:Verifiability#Questionable_sources)*]

Experimental and theoretical studies show that, with rising water pressure, the stability field of hornblende expands, restricting the stability fields of olivine and clinopyroxene. The characteristic textures of appinites point to rapid crystal growth. These studies also support the reduction of melt [viscosity](/source/Viscosity), whereby [ions](/source/Ion) can be transported more effectively to the sites of mineral growth.

## Source region

The general source region of appinitic magmas is estimated to be situated at about 40 kilometers depth, just below the base of the [continental crust](/source/Continental_crust). From there the magmas ascended and finally stalled at about 15 kilometers depth in upper crustal levels.

The water-bearing, basaltic appinitic magmas probably derive from [underplated](/source/Magmatic_underplating) mafic sources with differing degrees of fractionation. They most likely resulted from subduction processes. From within the subcontinental lithospheric mantle they then rose into the [MASH zone](https://en.wikipedia.org/w/index.php?title=MASH_zone&action=edit&redlink=1) (abbreviation of *Melting, Assimilation, Storage and Homogenisation*) just above the [MOHO](/source/Mohorovi%C4%8Di%C4%87_discontinuity). Here they engendered copious granitic magmas by partial melting processes.

It is assumed, that once the subduction came to an end water-bearing magmas rose from the underplated region into middle and upper crustal levels with 15 kilometers as upper intrusional depth level (corresponding to a pressure of 0.3 to 0.6 GPa or 3 to 6 kilobar). Here the magmas stalled, differentiated and crystallized under water-saturated conditions.

The granitic magmas also ascended in pulsating fashion and were making use of structures in the host rocks that were oriented to the local [stress](/source/Stress_(mechanics)) field in a favourable way – thus enabling the ascent. But later mafic pulses were hindered in their ascent by structurally higher, already crystallized granitic bodies – which functioned as [rheological](/source/Rheology) barriers. Still the appinite magmas were able to circumvent these barriers by using as ascent ways deep-reaching faults along the edges of the granitoids. According to this model appinites provide a direct link to mafic underplating. Their mafic members also offer insights into the formation of granitic [batholiths](/source/Batholith) – and more generally into the crustal growth process underneath island arcs.

## Melting

The melting of appinites was triggered by the incursion of hot and less viscous asthenospheric material. The incursion was due to *slab breakoff* after the collision of terranes or after outright continental collision. Another possibility is the opening of a *slab window*, which is resulting from the collision of a [mid-ocean ridge](/source/Mid-ocean_ridge) with a subduction zone.

Mafic appinite magmas can contain a juvenile component. [Neodymium](/source/Neodymium) isotopes show, however, that an additional SCLM-component was engaged. Quite often the SCLM-component had previously been metasomatized by hot fluids and magmas. This subcontinental lithospheric mantle component then was underplated by other mafics during subduction. Therefore, the composition of the mafic starting magmas can be quite variable for appinites. This explains, why certain appinite suites have calc-alkaline and others tholeiitic compositions – and therefore differ from the shoshonitic type locality.

Some felsic appinite magmas are thought to have formed by [anatexis](/source/Anatexis) – and not by fractional crystallization.

## Overview

The overview centers on the example of the Pengcuolin appinite in the Tibetan Lhasa terrane. In this case the source region is assumed to be directly above oceanic crust of the [Neotethys](/source/Neotethys) domain subducting northwards underneath the [Tibetan Plateau](/source/Tibetan_Plateau), i.e. [Eurasia](/source/Eurasia). The pressure in the source region is estimated at 3.6 GPa corresponding to a depth of 120 kilometers. This is quite deep considering the above-mentioned value of 80 kilometers. An explanation is of course overthickened crust caused by the continental collision of [India](/source/India) and Eurasia.

The subcontinental mantle rocks were of [lherzolithic composition](/source/Lherzolite), to be more specific an [olivine lherzolite](/source/Olivine_lherzolite).

The temperatures were estimated at fairly low 800 °C due to the subducted oceanic crust. The overlying subcontinental lherzolite was fluxed by [fluids](/source/Fluid) rising from the slab, became hydrated and was therefore metasomatized. Incoming asthenospheric material additionally provided heat to the lherzolite which was slowly rising, mainly along deep-reaching tectonic fracture zones. At a pressure of 2.7 GPa or 90 kilometers depth the lherzolite had reached a temperature of 1329 °C and started to melt. The primary magma rose quite quickly along faults within the subcontinental mantle. Having traversed the MOHO and arrived at 27 kilometers depth (corresponding to a pressure of 0.8 GPa) the melt collected in a first [magma chamber](/source/Magma_chamber). Plagioclase rich in anorthite began crystallizing and olivine plus pyroxene fractionated. This anorthite-rich appinitic magma kept on rising through the lower crust and stagnated once more at 16 kilometers depth (or at 0.5 GPa). Meanwhile, it had cooled down to just above 800 °C and started to crystallize aluminium-rich amphibole and plagioclase depleted in anorthite. The final batch of appinitic magma then finally stalled in the upper crust at a depth of 10 kilometers (or 0.3 GPa). The last crystals to settle out then were aluminium-poor amphibole and anorthite-poor plagioclase.

[Heat](/source/Heat) and additional water contributed in the first magma chamber at 27 kilometers depth to produce felsic melts, which also rose into the upper crust and intruded as granitic plutons. The associated granitoids therefore owe their existence to the heat input of the appinites enabling lower crustal material to be melted anatectically. Consequently, appinites can be regarded as *midwives* of collisional granitoids.

## Literature

- Hamidullah, S. (1983). *Petrogenetic studies of the appinite suite of western Scotland*. Master's thesis University of Glasgow.

- Murphy, J. Brendan (2013). "Appinite suites: A record of the role of water in the genesis, transport, emplacement and crystallization of magma". *Earth-Science Reviews*. **119**: 35–59. [Bibcode](/source/Bibcode_(identifier)):[2013ESRv..119...35M](https://ui.adsabs.harvard.edu/abs/2013ESRv..119...35M). [doi](/source/Doi_(identifier)):[10.1016/j.earscirev.2013.02.002](https://doi.org/10.1016%2Fj.earscirev.2013.02.002).

- Murphy, J. Brendan; Nance, Damian; Gabler, Logan B.; Martell, Alexandra; Archibald, Douglas A. (2019). "Age, Geochemistry and Origin of the Ardara Appinite Plutons, Northwest Donegal, Ireland". *Geoscience Canada*. **46** (1): 31–48. [doi](/source/Doi_(identifier)):[10.12789/geocanj.2019.46.144](https://doi.org/10.12789%2Fgeocanj.2019.46.144).

- Murphy, J. Brendan (2020). "Appinite suites and their genetic relationship with coeval voluminous granitoid batholiths". *International Geology Review*. 62, n. 6 (6): 683–713. [Bibcode](/source/Bibcode_(identifier)):[2020IGRv...62..683M](https://ui.adsabs.harvard.edu/abs/2020IGRv...62..683M). [doi](/source/Doi_(identifier)):[10.1080/00206814.2019.1630859](https://doi.org/10.1080%2F00206814.2019.1630859).

- Murphy, J. Brendan; Collins, William J.; Archibald, Donnelly B. (2022). ["Logan Medallist 7. Appinite Complexes, Granitoid Batholiths and Crustal Growth: A Conceptual Model"](https://doi.org/10.12789%2Fgeocanj.2022.49.191). *Geoscience Canada*. **49** (3–4): 237–249. [doi](/source/Doi_(identifier)):[10.12789/geocanj.2022.49.191](https://doi.org/10.12789%2Fgeocanj.2022.49.191).

- Pitcher, Wallace Spencer (1997). *The nature and origin of granite*. Chapman and Hall. [ISBN](/source/ISBN_(identifier)) [0-412-75860-1](https://en.wikipedia.org/wiki/Special:BookSources/0-412-75860-1).

- Yarr, Timothy Roderick (1991). *A petrological study of the appinite suite associated with the Ardara Pluton, Co. Donegal, Ireland*. Master's thesis University of St Andrews.

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