P e t r o c h e m i s t r y o f V o l c a n i c R o c k s i n R e l a t i o n t o T h e F o r m a t i o n o f I s l a n d Arcs(*) G . S . G O R S H K O V ( * * ) R i c e v u t o il 7 dicembre 1960. As it is known the volcanic rocks of island arcs have a pronounced calc-alkali character. They vary in the content from olivine or nonolivine basalts to quartz-bearing dacites and zhyolites. The most widespread group of rocks in the series are pyroxene-andesite and transitional var- ieties — andesite-basalt and dacite. Characteristic feature of this series is a very basic plagioclase even in acid varieties and as well as frequent presence of quartz. Among the dare coloured minerals there occur rhom- bic pyroxenes or poor in lime clinopyroxenes of pigeonite type, often clinopypoxene is represented by diopside augite (especially in more basic rocks). I n acid rocks pyroxene is replaced by hornblende and in exep- tionally acid members of this series biotite and orthoclase appear. Closer to t h e continents from island arc a gradual growth of alcali- nity of rocks is encountered. The changing of petrochemical character- istics is noticable already at the distance of 10-12 km across the pitch of the arc. Numerous examples of such an alteration occur in Kamchatka, Kurile Islands and in J a p a n . At the distance of scores of kilometers it is possible to observe the changes in the mineral composition of rocks. At first these changes are observed in acid rocks only, the plagioclase becoming more acid; almost always biotite or orthoclase occur and some- times both minerals occur together. Pyroxene is more often represented by diopside augite. Still closer to the continent orthoclase acquires greater importance, it appears even in andesites often forming margins around plagioclase. Rhombic pyroxene disappear giving way to hornblende and diopside augite. As to the chemical composition the role of potassium in relation to sodium becomes greater. (*) P a p e r r e a d a t t h e Helsinky Assembly of t h e I . U . G . G . 1960. (**) L a b o r a t o r y of Vulcanology, Acad, of Sci. of t h e U . S . S . R . Moscow. 1 3 8 G. S . G O R S I I K O V Intercontinental volcanoes now issue purely alkalic lavas distinctly having pure alkalic composition characteristic both for acid and basic kinds of rocks (from limburgite basanites and trachybasalts to t r a c h y t e s and phonolites). The mineral composition reflects this phenomena b y the presence potassium-sodium feldspars, aegirine-augites, titano-augites, alkaline ampliiboles and a t last feldspathoides-leucite and nepheline. Potassium rocks often dominate. I n the direction f r o m island arcs to t h e ocean a sharp substitution of calc-alkalic rocks by alkaline lavas characteristic for all oceanic islands (a series of rocks from picrite-basalts and nepheline basalts to trachytes) takes place. Olivine basalts with more acid plagioclase (than in basalts of island arcs) dominate here with pigeonite in the main mass. I n ande- sites of this series plagioclase is represented b y andesine and oligloclase, then alkalic amphiboles appear and in t h e main mass biotite occurs. I t is not always easy to distinguish the rocks of different series (oceanic, island arcs and continental) by their mineral composition b u t by their petrochemical features t h e y can be distinguished guite readily. Petrochemical calculations and comparison are possible to perform b y different methods ( H A R K E R , N I G G L I , C.I.P.W., etc.) b u t all these are rather complicated and permit t o compare visually not more t h a n two rock series simultaneously. I m a d e use of A. N. Zavaritsky's method which permits to draw a q u a n t i t y of analyses on a diagram and compare a n y number of rocks series. The petrochemical m e t h o d b y Zavaritsky allows to express on a plane diagram for each rock a t t h e same time: the ratio between salic and femic constituents, the ratio between alkaline and calcareous alu- mosilicates, t h e n a t u r e of alkaline alumosilicates (ratio Na to K) and the n a t u r e of the femic constituents (ratio Fe, Ca, Mg and Al). This method is considered in details in a book " The Introduction into t h e Petrochemistry of Igneous Rocks " (in Russian). As f a r as the a u t h o r is informed it is translated only into German of all t h e Western-Euro- pean languages. One can found all details in " Einfiirung in die Petro- chemie der Eruptivgesteine " by A. N. Zawaritzki, Berlin, Akademie- Yerlag, 1954. On the graph (Pig. 1) variation curves for the series of oceanic, con- tinental and island arcs lavas are drawn according to A. N. Zavaritsky. To make the graph more simple figurative points of rocks and their vectors are omitted. Upon considering the graph it becomes obvious t h a t as a m a t t e r of f a c t there are two classes of completely different volcanic rocks dif- P E T R O C H E M I S T R Y O F V O L C A N I C R O C K S I N R E L A T I O N , E T C . 1 3 9 fering in the decline of the variation curves to coordinate axes or other- wise in the sp?ed of alkaline growth in the process of differentiation. One of these classes includes lavas of oceanic islands (lines 1-4, in fig. 1), t h e other embraces lavas of island arcs (lines 5-7) together with alkaline rocks of continental volcanoes (lines 8-10). C 5 10 20 30 A III1!;! 10 6 7 6 9 10 /,«' ; / • •• / / / / ' V ' Hi i N: I W 1 III 1 f t hi L 1 1 I k •• / .•• / / // > / / ' V ' Hi i N: I W 1 III 1 f t hi L 1 1 I hi/ 7 /,••/ / - /••• / »/ ,•• / // V eli io i / / / •• ki / / F 7 ;••" " .. / / '/ / 5 \ \' 6 7 '•1-4 10 sk| 7f i 2 '4 B Fig. 1 - T h e d i a g r a m of t h e t y p e s of t h e n a t u r a l associations of volcanic rocks (by. A. Z a v a r i t s k y ) . The order numbers of lines given in fig. 1, are in conformity with certain petrochemical types of rocks: 1) volcanoes of t h e Hawaiian archipelago; 2) the lavas of the Tahiti Islands; 3) and 4) very alkaline types of rocks, which occur in some islands of the Pacific, t h e Indian and the Atlantic; 5) " Pelee t y p e " according to B u r n , including lavas of E a s t e r n K a m c h a t k a , J a p a n volcanoes etc.; 6) " S a n Francisco t y p e " according to Burri; 7) " Yellowstone P a r k t y p e " according to Burri; 8) intermidiate t y p e separating calc-alkaline rocks from pure alkaline (" E t n a t y p e " according to Zavaritsky); 9) " Eosita Hills t y p e " to which lavas of alkaline province of Eastern Asia belong; 10) very alkaline con- tinental lavas (Uyun Kholdong in N E China, Bufimbira in Africa and others). 138 G. S. G O R S I I K O V The transition from calc-alkali lavas of island arcs to t h e lavas of continental volcanoes takes place not only in space b u t also in time. I n m a n y places (NE Asia, the Carpatians etc.) there are examples t h a t old lavas which correspond to t h e geosyncline stage of development (or island arc stage) being calc-alkaline ones and more young lavas (of con- tinental stage) being alkaline ones. Thus, the class of rocks embracing calc-alkali rocks of island arcs and alkaline lavas of continents belongs to a continuous class and the rocks have aconsanguinity. The primary magma of this class is calc-alkalic. magma and in t h e course of geological time the growth ni alkalinity takes place. The relations between calc-alkalic and oceanic lavas are not so clear a t first sight. The intermediate varieties are not known and t h e change of one kind of rocks by other is very sharp. A t t h e same time t h e fields of basic rocks of both classes (in fig. 1) partially cover each other and as B a r t h stated (*) it is enough to have a relatively small change in composition of primary melt for process of cristallization to go absolu- tely different ways: in one case to t h e formation of quartz dacites and ryolites and to t h e formation of alkaline nepheline rocks in t h e other. Fig. 2 borrowed f r o m B a r t h ' s work illustrates t h e above s t a t e m e n t . Thus, a relatively small change in composition of primary m a g m a m a y greatly alter the character of differentiation and transfer one t y p e of rocks into another. The mentioned petrochemical features as the uniformity of rock types in all island arcs, gradual growth of alkalinity towards the conti- nents and sharp difference in rock types of island arcs and oceanic islands are of general importance. T h a t t h e lavas belong to one or t h e other t y p e of rocks depends on oceanic, continental or intermediate (island arc) location or, otherwise, on t h e continental or oceanic t y p e of the E a r t h ' s crust and does not depend a t all on the composition and structure of upper layers of the crust (with a few exeptions). The unifor- mity of lavas of oceanic islands or island arcs is well known. The con- tinental volcanoes also issue lavas uniform enough (in t h e petrochemical sence) irrespective of the location of volcaneos in the regions of Mesozoic or Tertiary folding (the Carpatians), the rift zone (Africa) or on the old platform (NE China). I t had been long proposed on the basis of general theoretical postul- ates t h a t the feeding sources of volcanoes are situated a t great depths (*) BARTH, TOM. F . W . , Theoretical Petrology, 1952. P E T R O C H E M I S T R Y OF V O L C A N I C R O C K S I N R E L A T I O N , E T C . 139 of some scores of kilometers. Recently t h e depth of magmatic heart of Kliuchevsky Volcano in K a m c h a t k a was determined as being 50-70 k m . The independence of petrochemistry of lavas from t h e geological struc- t u r e and t h e structure of the upper layers of the crust proves once more t h e idea t h a t the feeding sources of the volcanoes are situated out of limit of the crust — in t h e upper p a r t of the E a r t h ' s m a n t l e where there is no sharp change in chemistry depending upon the concrete geological structure. Moreover, a striking uniformity of petrochemistry of volcanism over vast territories shows the relative role of assimilation of crust m a t - erial in t h e magma on its way to the surface. A well known opinion t h a t t h e " Pacific rocks " of island arcs are only derivatives of the " Atlantic magma " contaminated in the upper layers of t h e crust by sialitic material being brought f r o m the adjoining mainland to the dipping geosyncline is in contradiction to geophysical observations in the regions of island arcs. Modern observations proved t h a t in some arcs t h e crust of the continental t y p e is present only immed- iately beneath t h e arc and is absent not only f r o m t h e ocean side b u t also in the mariginal seas — f r o m the side of the continent. Naturally, in these conditions the supply of t h e sialitic material f r o m the surface cannot t a k e place. Thus, the source of contamination, if it does t a k e place, m a y be only of depth character. Summarizing t h e above said we come to t h e conclusion t h a t the two main types of magmas: oceanic and continental (including island arcs) are connected to the corresponding t y p e of the crust in space as well as genetically. The oceanic magma of the Hawaii t y p e (line 1 in fig. 1) is an ancestral m a g m a for all t h e other magmas (primitive m a g m a by Tilly, etc.). Under t h e conditions of gradual increase in alkalinity in t h e course of normal evolution this m a g m a becomes more and more alkaline (shift from line 1 to line 4 in fig. 1). During contamination of this magma by sialitic material t h e oceanic magma turns into t h e calc-alkalic one (the transition from point " N " to point " X " in fig. 2 or from line 1 to line 5 in fig. 1). This sharp change of magmatic t y p e is related to the upflow of sialitic material f r o m depths of the E a r t h ' s mantle. The island arcs indicate these narrow areas of the Globe where a t present the processes of depth differentiation and t h e rise of the sialitic material f r o m the depths of the m a n t l e to its surface t a k e place. This process leads to the thickening of the crust and a t t h e same time sharply changes the chemistry and the whole character of volcanism. F u r t h e r more, with t h e termination of the flow of portions of substance f r o m the depths of t h e m a n t l e the normal process of magmatic evolution 138 G. S . G O R S I I K O V in t h e direction of increase of alkalinity is renewed and t h e m a g m a gradually turns into the alkaline continental one. On its way to t h e E a r t h surface through t h e crust m a g m a in most cases does not undergo a notable contamination and does not change visibly t h e main features of its petrochemistry. The composition of primary magmas seems to correspond to basalts or more basic rocks. I t is necessary to note t h a t within the limits of t h e so called " andesine line " t h e basalts and andesite-basalts also have a wide occurence. Fig. 2 - Different possible r e s u l t s of t h e f r a c t i o n a l c r y s t a l l i z a t i o n of b a s a l t melts, (by T. B a r t h ) . More acid varieties of rocks are t h e products of differentiation of the corresponding more basic m a g m a s in a relatively small volumes of volcanic vents. A separate volcano or a group of neighbouring volcanoes m a y issue lavas different in their acidity b u t t h e y all will belong to one narrow t y p e and their figurative points will be situated along t h e same line in fig. 1. Somewhat noticeable shift across t h e curves (i.e. change in alkalinity) is not observed as a rule in short periods of time. Numerous cases of substitution of more acid lavas b y more basic ones during t h e same eruption are known; then a f t e r t h e cease of activity which is long enough for differentiation, there m a y appear acid lavas followed b y basic ones and so on. However a noticeable change in alkalinity (in petroclie- SiC P E T R O C H E M I S T R Y O F V O L C A N I C R O C K S I N R E L A T I O N , E T C . 139 mical sence) in this case is not observed as all lavas belong to one type. The change in alkalinity and substitution of one t y p e of magma with another happens only in the course of geological periods of time. Thus, the consideration of petrochemistry of lavas lead us to the conclusion t h a t volcanism is not purely a surface or introcrustal process, b u t together with tectonic activity in general, represents t h e surface expression of the primary undercrustal processes t h a t govern all t h e course of the E a r t h ' s development. The formation of island arcs is one of the necessary and most essential links of this process. SUMMARY It ivas shown that there are tivo petrochemically sharply different classes of lavas: 1) alkali lavas of intraoceanic islands, and 2) calc-alkali lavas of island arcs together with alkali lavas of intracontinental volcanoes. The two main types of lavas: oceanic and continental (including islands arcs), are connected in space to the corresponding types of the Eearth's crust, but petrochemical peculiarities are independent of local geological features and structure of upper layers of the crust. Magmatic harts of volcanoes are located in the upper part of the mantle and petrochemical features reflect the changes in chemistry of subcrustal substance. Oceanic magma is an ancestral one and in the course of a normal evolution this magma becomes more and more alkaline. Calk-alkali magma of island arcs is derivative one and it is generated by the contamination of the oceanic magma with sialitic material upflowing from depths of the mantle (assimilation of crust material is insignificant). The island arcs indicate these narrow areas of the Globe where the process takes place. It leads to a thickening of the crust and chan- ges sharply the petrochemistry and the whole character of volcanism. Then in the course of the normal evolution the calc-alkali magma turns gradually into continental alkali one. Thus, volcanism is not purely a surface or intra- crustal process, but it represents the surface expression of primary subcrustal processes that govern all the course of the Earth's development. The formation of island arcs is one of the necessary and most important links of the process. RIASSUNTO Si e visto che ci sono due classi di lava lievemente diversa per la com- posizione petrochimica: 1) le lave ualkali'' delle isole oceaniche e 2) le lave "calc-alkali" degli archi insulari insieme con le lave 11 alkali" dei vulcani 138 G . S . G O R S I I K O V continentali. I due principali tipi di lava: oceanica e continentale (compreso gli archi insulari), sono connesse nello spazio ai corrispondenti tipi di cro- sta terrestre, ma le peculiarita petrochimiche non dipendono dalla locale composizione geologica e dalla struttura degli strati piii elevati della crosta. Gli " harts " di magma dei vulcani si trovano nella parte piii elevata del mantello e le composizioni petrochimiche riflettono i mutamenti chimici della sostanza della sottocrosta. II magma oceanico e ancestrale e nel corso di una evoluzione normale questo magma diventa sempre piii alcalino. II magma " calc-alkali " delle " island arcs " e un derivato ed e prodotto dalla contaminazione del magma oceanico con materiale sialitico che emerge dalle profondita del mantello (Vassimilazione del materiale della crosta e insigni- ficante). Gli archi insulari indicano queste ristrette zone del globo dove il processo ha luogo. Questo processo porta ad un ispessimento della crosta e muta lievemente la petrochimica e tutto il carattere del vulcanismo. Quindi nel corso della normale evoluzione il magma " calc-alkali " si cambia gra- dualmente in " alkali " continentale. Percio il vulcanismo non e semplice- mente una superficie o un processo intercrostale, ma rappresenta Vespressione della superficie dei processi subcrostali primari che regolano tutto il corso dello sviluppo della Terra. La formazione degli archi insulari e uno degli aspetti necessari e piii importanti del processo.