Appinite is anamphibole-richplutonic rock of highgeochemical variability. Appinites are therefore regarded as arock series comprisinghornblendites, meladiorites,diorites, but alsogranodiorites andgranites. Appinites have formed frommagmas very rich in water. They occur in very differentgeological environments. The ultimate source region of these peculiar rocks is theupper mantle, which was alteredmetasomatically and geochemically beforemelting.

The rock appinite was named after itstype localityAppin nearBallachulish inScotland. Appin was originally calledAn Appain inScottish Gaelic. This is derived fromMiddle Irishapdain or fromOld Irishaibit with the meaning ofabbey – referring to the ancient abbey on the neighbouring islandLismore.^{[citation needed]}

Bailey and Maufe (1916) defined appinite originally as

a medium- to coarse-grained, meso- to melanocratic igneous rock, that stands out by conspicuous crystals ofhornblende, which are enclosed by amatrix ofplagioclase (oligoclase toandesine) and/ororthoclase.Quartz often is present, but can also be absent.

Generally, appinites are plutonic equivalents ofcalc-alkalinelamprophyres such asvogesite andspessartite.^[1]

Appinites – often synonymously used forhornblende diorites – are a coeval rock suite of plutonic or subvolcanic igneous rocks with variable chemical compositions, coveringultramafic tofelsic igneous rocks. They are characterized in all their lithologies byeuhedral hornblende crystals as the dominant mafic mineral. Hornblende mainly appears as bigprismaticphenocrysts, but can also be found in thegroundmass.^{[citation needed]}

On top appinites have very differenttextures – featuring planar and linear magmatic fabrics,cumulate textures, intercumulate textures and alsopoikilitic fabrics. They also can occur as maficpegmatites and show commonmixing andmingling between coeval mafic and felsic magmas. Often they are variably contaminated by the country rocks.^{[citation needed]}

Most appinites crystallize from an importantgas phase. This implies an anomalously water-rich magma including both mantle components andmeteoric components.^[2] The appinite suite therefore offers a unique occasion to study the role of water in the production and in thecrystallization history of mafic to felsic magmas, but also more generally in intrusional processes.^{[citation needed]}

Appiniticintrusions 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 theArdara pluton inDonegal. Their emplacement is usually directed bytectonics – especially by importantshear zones, who potentially facilitate the rising of the magmas through thecrust.^[3]

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 reachingfaults along which they ascended into higher crustal levels.

Often appinites – and likewise the Scottish appinites – get tied up with activesubduction, the formation of granitoids and also the termination of subduction byslab breakoff. In the case of the Scottish appinites it is believed that they only formed once theIapetus Ocean was closed by continental collision between the southern continental margin ofLaurentia and the northwestern side ofEastern Avalonia and that the subduction within Iapetus had stopped.

Yet newergeochronological studies 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 hotasthenospheric material to flow in through the gap. The asthenospheric extraheat initiated magmas containing juvenile mantle components, but also components ofSubcontinental Lithospheric Mantle (SCLM). Furthermore, the magmas show affinities toShoshonites. The felsic components of appinites are connected to bigbatholiths withfractional crystallization being the main petrogenetic process. The assimilation of country rocks was of hardly any importance.

Appinites occur more or less worldwide. Temporally, the oldest appinites are 2700 million years old (theNeoarchaean Era); the youngest are ofHolocene age. The Neoarchaean appinites are associated genetically with coevalsanukitoids. This is often taken as proof forplate tectonics going back that far in time.

Besides thetype locality in the Scottishcaledonides (within theCentral Highlands terrane orGrampian terrane) appinites also occur inIreland within and in the vicinity of theDonegal batholith – especially in association with the Ardara pluton – but also within theLeinster granite^[4] and within theGalway granite batholith.^[5]

All these appinites haveSilurian ages. Further occurrences in Scotland are found nearLoch Lomond and in centralSutherland, which already belongs to theNorthern Highlands terrane. The appinites in the Northern Highlands terrane are mainly associated with theRatagain complex, theRogart granite and theStrontian granite.^[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 northernMichigan. They go back in time roughly 2700 million years and belong to theNorthern Complex – agreenstone belt along the southern edge of theSuperior craton.^[7]

Fairly old appinites are reported fromCanada, for instance from theFrog Lake hornblende gabbro situated within the lateneoproterozoicAvalon terrane inNova Scotia.^[8] TheWamsutta diorite in theWhite Mountains ofNew Hampshire also has similarities with appinites. The diorite is 408 million years old and belongs to theAcadian Orogeny.^[9]

Younger appinites from theCarboniferous appear nearPuebla de Sanabria in theVariscides of northwesternSpain.^[10] They are also found in theAvila batholith.^[11] Amongst Variscan occurrences appinites often carry local names likeDurbachites (in theBlack Forest),Redwitzites (in theFichtelgebirge),Vaugnerites (in the FrenchMassif Central),^[12] and sometimes they also hide under the headerHigh Ba Sr Granitoids (an example being the Rogart Granite in Scotland).

Variscan appinites can also be found in theSouthern Alps ofNorthern Italy. They are associated here with thepermianSerie dei Laghi – a rock series of gabbros and granites.^[13] The age of these Italian appinites is about 285 million years.

InAsia appinites are known to occur inChina and inTibet.

In China appinites appear in theUpper Ordovician (495 - 452 million years)Datong Pluton of theWestern Kunlun.^[14] and again in theTriassicLaocheng Pluton of theQinling^[15] During theUpper Permian appinites formed along the northern edge of theNorth China Craton (in northwesternLiaoning) and during the Triassic inHeilongjiang (nearDuobaoshan), also belonging to the North China Craton.

In the TibetanHimalaya Appinite-cumulates are found in theGangdese batholith of theLhasa terrane. These appinites formed during the Upper Triassic and are 220 to 213 million years old.^[16] Another appinite association in Tibet occurs nearPengcuolin northwest ofXigazê. It belongs to the southern Lhasa terrane and is only 51 million years old i.e.Ypresian (Eocene).^[17]

Very young examples of appinites come fromIran, like appinites from theBaneh Pluton in theZagros. These appinites are 40 million years old and stem from the Middle Eocene. They mark theZagros Suture Zone.^[18] At about the same time appinites also formed nearSardasht more to the northwest.^[19]

Appinites consist mainly ofamphibole (hornblende) taking up between 50 and 80 volume percent.Anorthite-rich plagioclase with An_50-70 reaches about 20 vol. %. The rest is made up ofclinopyroxene (5 to 15 vol. %) andolivine (5 to 10 vol. %). Somebiotite and occasionalphlogopite are also encountered. In more felsic appinites appearalkali feldspar andquartz. Represented amongst theaccessory minerals aresphene,ilmenite,zircon andapatite.Allanite can be found in more felsic members.

A special occurrence ismyrmekite found in an appinite of the ItalianSerie dei Laghi – indicating metasomatic alterations.

Amongst the amphiboles (mainly brown amphiboles, but also some greenish amphiboles) two populations with high and lowaluminium content can be differentiated.Tschermakite andmagnesiohastingsite are rich in aluminium, whereasmagnesiohornblende contains much less. Plagioclase can also be subdivided into two groups – one anorthite-rich with An_80-88 and the other anorthite-poor with An_36-52. 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. Thegrain size of amphiboles varies from 2 millimeters to several centimeters.

Plagioclase, olivine and clinopyroxene settled ascumulates, whereas amphiboles grew afterwards as intercumulate crystals which also can showcorona textures.

Amongst themajor 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 typescortlandtite (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 aremetaluminous with A/NK > 1 and A/CNK < 1. The contents of MgO fall between 5 and 16 weight % and themagnesium numbers generally oscillate between 0.22 and 0.57 (or between 22 and 57). Appinites aremagnesian rocks (and notferroan), because in the relation SiO₂ plotted against Fe₂O₃tot/(Fe₂O₃tot + MgO) their values are always lower than 0.66. Theirmagnesium contents are higher than what can be expected from melting of metabasalts and they approach sanukitoids of modernisland arcs. 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 fromKilrean has not been differentiated at all and represents an island arctholeiite. The ratio Na₂O/K₂O is rather high in appinites (right up to 5.43) and is similar toCenozoicadakites, which were produced by the melting of subductedoceanic crust. Accordingly, appinites are a rock suite dominated bysodium.

In theTAS diagram appinites appear mainly in thesubalcaline field, but they can extend into thealcaline field. They plot in the fields ofbasalt, basaltic andesite and andesite, but touch as well the fields ofbasanite,trachybasalt, basaltic trachyandesite andtrachyandesite. The magmatic equivalents are gabbro, gabbroic diorite and diorite, extending towards peridotgabbro, foidgabbro, monzogabbro and monzodiorite.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]}

Amongst thetrace elements the mafic members of appinites manifest high concentrations in transitional metals likenickel (98-288ppm),chromium (100-810 ppm) andvanadium (179-462 ppm). Thelarge-ion lithophile elements (LILE), for examplerubidium, potassium,barium (253-528 ppm),cesium andstrontium (415-813 ppm), also have elevated concentrations – and so do the light rare-earth elements (LREE). Low in concentration are the heavy rare-earth elements (HREE) and also thehigh field strength elements (HFSE)niobium,tantalium,zirconium,phosphorus,titanium andthorium. Still the HFSE are higher concentrated than in the associated granodiorites and granites. Compared withchondrites the LREE show an enrichment by factors 20-200. The HREE fractionation (expressed through the ratio Gd_N/Yb_N) shows values between 1.4 and 6.1. A positiveeuropium anomaly is very weakly expressed and in more felsic appinites the anomaly turns slightly negative (0.96-0.70). The values foryttrium 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 withMORBs the elements rubidium, barium, potassium and alsocerium are strongly enriched, yet titanium,ytterbium and yttrium are depleted.

The following table shows trace elements of different appinites:^{[citation needed]}

According to Harmon et al. (1984) appinites possess the following ε_Nd-, ε_Sr- and ε_Hf values:^[21]

• ε_Nd varies between − 8 and + 2 (i. e. between 0.5123 and 0.51275 – in theSerie dei Laghi between 0.5119 and 0.5123 for¹⁴³Nd/¹⁴⁴Nd)
• ε_Sr varies between − 5 und + 10 (i. e. between 0.7044 and 0.711 for⁸⁷Sr/⁸⁶Sr).
• ε_Hf(t) in zircon varies between 3.3 and 7.9, but can descend to − 1.7.

Appinites prolong themantle array into the field of negative ε_Nd. Yet their mafic members plot very close to enriched MORB (EMORB) with ε_Nd = + 2 and⁸⁷Sr/⁸⁶Sr = 0.7048. Their ε_Sr 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 ratio²⁰⁶Pb/²⁰⁴Pb varies between 17.9 and 18.4.

Thegeochemical composition of appinites is mainly calc-alkaline, sometimes shoshonitic and rarely tholeiitic. Therefore, appinites resemble shoshonites, shoshoniticlamprophyres,^[23] but also magnesian andesites,^[24] sanukitoids, adakites andTTG rocks (tonalites,trondhjemites and granodiorites). The TTGs appear especially in the lateArchean and during thePaleoproterozoic.^[25]

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 appears on theliquidus 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 byfolded rocks of theDalradian Supergroup. Further crystallizations then happened under falling temperatures and rather variable gas pressures, caused by explosions withinsubvolcanic pipes.

Olivine crystallized first thenclinopyroxene, amphibole,mica and plagioclase, creating a progressive rock suite covering ultramafic to felsic compositions.^{[26][better source needed]}

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 meltviscosity, wherebyions can be transported more effectively to the sites of mineral growth.

The general source region of appinitic magmas is estimated to be situated at about 40 kilometers depth, just below the base of thecontinental 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 fromunderplated mafic sources with differing degrees of fractionation. They most likely resulted from subduction processes. From within the subcontinental lithospheric mantle they then rose into theMASH zone (abbreviation ofMelting, Assimilation, Storage and Homogenisation) just above theMOHO. 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 localstress 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 asrheological 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 graniticbatholiths – and more generally into the crustal growth process underneath island arcs.

The melting of appinites was triggered by the incursion of hot and less viscous asthenospheric material. The incursion was due toslab breakoff after the collision of terranes or after outright continental collision. Another possibility is the opening of aslab window, which is resulting from the collision of amid-ocean ridge with a subduction zone.

Mafic appinite magmas can contain a juvenile component.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 byanatexis – and not by fractional crystallization.

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 theNeotethys domain subducting northwards underneath theTibetan plateau, i.e.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 ofIndia and Eurasia.

The subcontinental mantle rocks were oflherzolithic composition, to be more specific anolivine lherzolite.

The temperatures were estimated at fairly low 800 °C due to the subducted oceanic crust. The overlying subcontinental lherzolite was fluxed byfluids 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 firstmagma 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 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 asmidwives of collisional granitoids.

• 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:2013ESRv..119...35M.doi:10.1016/j.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:10.12789/geocanj.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:2020IGRv...62..683M.doi:10.1080/00206814.2019.1630859.
• Murphy, J. Brendan; Collins, William J.; Archibald, Donnelly B. (2022)."Logan Medallist 7. Appinite Complexes, Granitoid Batholiths and Crustal Growth: A Conceptual Model".Geoscience Canada.49 (3–4):237–249.doi:10.12789/geocanj.2022.49.191.
• Pitcher, Wallace Spencer (1997).The nature and origin of granite. Chapman and Hall.ISBN 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.

• Mafic rocks
• Intermediate rocks
• Felsic rocks
• Subvolcanic rocks

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