Rock formation

The for­ma­tion of pla­giogran­ites is most often asso­ci­at­ed with the ocean­ic crust and ophi­o­lite com­plex­es. They are formed main­ly in two ways:

1. Par­tial melt­ing of the ocean­ic crust — this occurs when basaltic or gab­broid rocks in the low­er ocean­ic crust under­go par­tial melt­ing, form­ing a melt rich in sil­i­ca that crys­tal­lizes into pla­giogran­ite.

2. Mag­mat­ic dif­fer­en­ti­a­tion — this involves frac­tion­al crys­tal­liza­tion of maf­ic moth­er mag­ma, where ear­ly-crys­tal­lized maf­ic min­er­als (for exam­ple, olivine, pyrox­ene) are removed, result­ing in the con­cen­tra­tion of more acidic com­po­nents such as pla­gio­clase and quartz.

It is usu­al­ly asso­ci­at­ed with sheet­ed dike com­plex­es, gab­broic intru­sions, and pil­low basalts in ophi­o­lite sequences, reflect­ing its ori­gin in mid-ocean ridge envi­ron­ments and supra-sub­duc­tion zones.

Pla­giogran­ites have impor­tant geo­log­i­cal sig­nif­i­cance because they serve as indi­ca­tors of the evo­lu­tion of the ocean­ic crust and process­es of mag­mat­ic dif­fer­en­ti­a­tion. Their pres­ence in the com­po­si­tion of opi­o­lites indi­cates a late stage of crys­tal­liza­tion of mag­mat­ic sys­tems of mid-ocean ridges and supra-sub­duc­tion zones. In some cas­es, they reflect episodes of par­tial melt­ing in the upper man­tle or the low­er crust. The study of pla­giogran­ites makes it pos­si­ble to restore the con­di­tions of pres­sure, tem­per­a­ture and com­po­si­tion of flu­ids that exist­ed at the time of the for­ma­tion of the ocean­ic crust.

Chemical composition

Pla­giogran­ite (71–74 wt.% SiO₂) dif­fers from oth­er acidic rocks by high con­cen­tra­tions of CaO, Na₂O and low con­cen­tra­tions of K₂O, which is caused by the strong pre­dom­i­nance of pla­gio­clase over potassium–sodium feldspar. Low- and high-alu­mi­na vari­eties of pla­giogran­ites are dis­tin­guished. The for­mer con­tain less than 15 wt. % Al2O3 and is dis­tin­guished by a small amount of biotite in the rock, the appear­ance of amphi­bole. The sec­ond con­tain more than 15 wt. % Al2O3; the col­ored min­er­al in them is main­ly biotite.

The struc­ture is fine, medi­um — or coarse-grained, often por­phyrit­ic due to the pres­ence of large round­ed inclu­sions of quartz. The tex­ture is mas­sive or gneiss-like.

Physical properties of plagiogranites

Pla­giogran­ites are intru­sive igneous rocks belong­ing to the group of acidic deep rocks and char­ac­ter­ized by a high sil­i­ca con­tent (up to 70–75%). The main min­er­als that make up pla­giogran­ites are pla­gio­clase (usu­al­ly albite or oligo­clase), quartz, biotite, horn­blende, some­times amphi­bole, apatite, zir­con, titan­ite and iron oxides are present. Their min­er­alog­i­cal com­po­si­tion dif­fers from typ­i­cal gran­ites in that the con­tent of potas­si­um feldspar is reduced, and sodi­um-cal­ci­um pla­gio­clase is increased, which deter­mines their phys­i­cal prop­er­ties and appear­ance.

In terms of struc­ture, pla­giogran­ites are medi­um- or fine-grained, with a ful­ly crys­talline tex­ture. The col­or of the rock is usu­al­ly light gray or gray­ish-pink, more rarely slight­ly green­ish. Due to the high quartz con­tent, the rock has a lus­ter and a rather dense tex­ture. Its aver­age den­si­ty is 2.6–2.7 g/cm³, and its hard­ness is about 6–7 on the Mohs scale. The poros­i­ty of pla­giogran­ites is very low (up to 0.5%), so they have high com­pres­sive strength (100–250 MPa) and low water absorp­tion.

Pla­giogran­ites are char­ac­ter­ized by high frost resis­tance, resis­tance to weath­er­ing and chem­i­cal influ­ences. Due to these prop­er­ties, they per­sist even in dif­fi­cult cli­mat­ic con­di­tions and often form mas­sifs resis­tant to ero­sion. When weath­ered, the rock under­goes minor changes, main­ly sec­ondary clay min­er­als are formed from pla­gio­clase, but the gen­er­al struc­ture remains strong.

Spread of plagiogranites

Pla­giogran­ites have an extreme­ly inter­est­ing and at the same time lim­it­ed dis­tri­b­u­tion in the world geo­log­i­cal struc­ture. They are main­ly asso­ci­at­ed with tec­ton­i­cal­ly active zones — places of for­ma­tion of new ocean­ic crust, sub­duc­tion zones, as well as ancient con­ti­nen­tal shields, where the remains of ancient geo­dy­nam­ic process­es have been pre­served.

From the point of view of struc­ture, pla­giogran­ites are most often part of ophi­o­lite com­plex­es — typ­i­cal geo­log­i­cal struc­tures that reflect a sec­tion of the ocean­ic crust. In such com­plex­es they form small bod­ies or lens-shaped inclu­sions among gab­bros, dia­bas­es, and basalts.Plagiogranites under these con­di­tions are prod­ucts of the late stage of crys­tal­liza­tion of ocean­ic mag­mas, when lighter, sil­i­ca-enriched frac­tions are released from the melt. Such rocks are often called ocean­ic pla­giogran­ites.

At the microstruc­tur­al lev­el, they are char­ac­ter­ized by a uni­form­ly grained or fine-grained crys­tal struc­ture, some­times with por­phyry inclu­sions of quartz or pla­gio­clase. In zones of tec­ton­ic defor­ma­tion, pla­giogran­ites may dis­play cat­a­clas­tic or migmatite-like struc­tures, which indi­cates their involve­ment in meta­mor­phic process­es and plas­tic defor­ma­tion under pres­sure.

Dis­tri­b­u­tion in the world:

• Mid-ocean ridges — pla­giogran­ites were found as part of ophi­o­lite com­plex­es of the Atlantic, Indi­an and Pacif­ic oceans.
• Ophi­o­lite belts of the Mediter­ranean region are known pla­giogran­ite deposits in Cyprus (Tro­do ophi­o­lite), in the moun­tains of Sul­ta­nia (Oman), in the Balka­ns (Ser­bia, Greece), in Turkey and in the Cau­ca­sus.
• North Amer­i­ca — pla­giogran­ites occur with­in the ophi­o­lites of Cal­i­for­nia (the Fran­cis­can Com­plex), in Alas­ka, and in New­found­land.
• Scan­di­na­vian shield — pla­giogran­ites here are asso­ci­at­ed with Archean and Pro­tero­zoic com­plex­es, which reflect the process­es of ear­ly for­ma­tion of the con­ti­nen­tal crust.
• Africa and Asia — pla­giogran­ites were found in Ethiopia, India, Tibet, Pak­istan, where they are part of the ancient meta­mor­phic belts, in par­tic­u­lar the Himalayan and Hin­du belts.

On the ter­ri­to­ry of Ukraine, pla­giogran­ites occur with­in the Ukrain­ian crys­talline shield — in par­tic­u­lar, in the Ingul, Azov and Mid­dle Dnieper megablocks. They form intru­sive mas­sifs of Archean and Low­er Pro­tero­zoic age, often accom­pa­nied by tonalites, dior­ites and gneiss­es.

Practical use

First of all, due to their phys­i­cal and mechan­i­cal prop­er­ties, pla­giogran­ites are used as a dec­o­ra­tive fac­ing stone. Due to its light gray, pink­ish or green­ish gray col­or, uni­form grain struc­ture and abil­i­ty to pol­ish well.

In con­struc­tion, pla­giogran­ites are used as a strong struc­tur­al mate­r­i­al. They are used for the pro­duc­tion of paving stones, curb­stones, and ele­ments of bridges and hydraulic engi­neer­ing struc­tures. Due to their high resis­tance to mechan­i­cal wear and frost action, pla­giogran­ite blocks are effec­tive­ly used in road paving and the improve­ment of urban squares.

In indus­try, crushed pla­giogran­ite is used as high-strength crushed stone, which is used for the man­u­fac­ture of con­crete, rein­forced con­crete struc­tures and road mix­tures. Due to its chem­i­cal inert­ness, the mate­r­i­al is resis­tant to cor­ro­sion, so it is used in aggres­sive envi­ron­ments — for exam­ple, in the con­struc­tion of hydrotech­ni­cal dams and sea embank­ments.