Geochemistry of effusive rocks of Rift zones

Geochemistry of Effusive Rocks of Rift Zones V. I. GERASIMOVSKY V. 1. Vermadsky Institute of Geochemistry and Analytical Chemistry, U.S.S.R. Academy of Sciences, Moscow, U.S.S.R.

ABSTRACT The following effusive rock series within the rift zones are distinguished: tholeiitic basaltsandesites--rhyolites; basalts--trachytesand phonolites; Na-alkaline series: olivine melanonephelinitesnephelinites and phonolites and K-alkaline one: katungites--ugandites--leueitites--leueiticbasanites. The earbonatitesare geneticallyrelated with two latter seriesand their separationfrom alkalinemeltsis due to liquation. The formation of the effusiverock series is consideredas the result of evolution of the initial (mantle) melt which was probably formed within the upper mantle during riftogenesis.The effusiverock series (excludingK-alkaline)are characterizedby increaseof the rare lithophylicelementcontent and by decreaseof content of the marietypomorphieelementsfrom the initial to the end members of magmatic differentiation. Alkaline varieties of basalts and other rock types are enriched in the rare lithophylic elements, fluorine and some maric typomorphic elements (Sr,Ti,P) in comparison with less alkaline varieties. Iceland is located within the region of the Mid-Atlantic ridge. The iceland is mostly comprised of basalts. Only 10% of rocks is represented with acidic effusives, intrusive rocks (gabbro, granophyres) and sedimentary-pyroclastic series (Palmason and Saemundsson, 1974). The most ancient rocks in Iceland are dated approximately about 16 m.y. and are located commonly within the marginal zones of the Iceland. The Quaternary rocks (basic and acidic effusives) are localized mostly within the submeridional (NNE-SSW) rift zone. The effusive paroxysms were observed recently and several times (1970---volcano Hticla; 1971--iceland Heimaey; 1975, 1977--northern part of the rift). The effusive series of rocks contain all varieties from basalts to rhyolites. Two rock series are distinctly identified: tholeiitic basalts-andesitic basalts-andesites-rhyodacites-rhyolites and alkaline basaltsoversaturated alkaline effusives (rhyodacites and rhyolites) (Gerasimovsky et al., 1974). The chemical composition of effusive rocks of Iceland is markedly variable (Jakobsson, 1974; Sigvaldason, 1974; Bailey and Noe-Nygaard, 1974; Wood et al., 1976). In East Africa, east rift zone and West rift zone are distinguished. The East zone (rift Gregory) is localized in Kenya, Northern Tanzania and Eastern Uganda. Western rift zone (Tanganyika zone) is coincident with the chain of lakes (Alberta, Edward, Kivu and Tanganyika). The tormation of the rift zones was accompanied by the Neogene-Quarternary volcanism. The associations of effusive rocks within these zones are extremely variable in their composition and chemistry. Three rock series are distinguished (Gerasimovsky and Polyakov, 1974). The alkaline basaltic rock series (picritic basalts, basalts, olivine basalts, trachy-basalts, andesitic basalts, trachytes and phonolites) is abundant in the Eastern rift and in minor quantities within the Western rift. The Na-alkaline rock series (olivine melilitites, olivine melanephelinites, nephelinites and phonolites) are widely represented 361

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within the Eastern rift whereas the K-alkaline rock series are frequent members of the Western rift zone rock series (katungites, mafurites, ugandites, leucitites, leucite basanites, etc.). The alkaline rocks of the Eastern rift are characterized by predominant role of sodium in relation to potassium; the Western rift is characterized by the inverse relations. The main rock-forming mineral of the alkaline series of the Eastern rift is represented by nepheline while the leucite is the typomorphic mineral of the Western rift alkaline series. The effusive carbonatites are observed in the association with both alkaline rock series. The oversaturated per alkaline effusives (comendites, pantellerites) are found in subordinate quantities within the Eastern rift (near Lake Naivasha, Kenya). Furthermore, the intrusive alkaline rocks (nepheline syenites, urtite-ijolite association, carbonatites) are identified among the effusive rocks of the Eastern rift zone (Eastern Uganda, etc.). All effusive rock series (excluding K-alkaline) are characterized by increase of Na, K, SiO 2 content, rare lithophylic elements (Li, Be, REE, Zr, Nb, Th, etc.), F and by decrease of mafic typomorphic elements (Ca, Mg, Fe, Ti, V, Cr, Ni, Co, Sc, P) from the initial to the end members of magrnatic differentiation. Tables 1 and 2 show these concentrations in two effusive rock series: basalt-andesite-rhyolitic (Iceland) and Na-alkaline (Eastern rift of East Africa). Table 3 represents the regular change of ratio values of coherent elements tor

TABLE 1. ELEMENT CONTENT IN THE EFFUSIVE ROCKS OF ICELAND IN THE ALKALINE BASALT OF THE EASTERN RIFT OF EAST AFRICA

Iceland

Element

Basalt

Africa

Andesite Rhyolite

Basalt

Na,% K,% Li ppm Rb ppm Ba ppm Y ppm Zr ppm Nb ppm F ppm

1.97 0.39 7.4 8.7 134 33 166 17.5 545

2.96 1.59 15 40 380 71 420 59 1170

3.35 2.85 24 77 730 109 620 73 1340

2.41 1.12 7.9 21 440 26 300 57 940

Carlo Mg,% Fe,% ,o, Tl,/o V ppm Cr ppm Ni ppm Co ppm Sc ppm Sr ppm P ppm

8.16 4.02 9.87 1.49 374 166 82 49 32 255 1040

3.83 0.90 6.87 0.92 113 46 25 22 19 160 1100

0.85 0.06 1.82 0.16 22 16 11 8 5 80 170

6.60 4.82 9.77 1.55 263 322 135 51 23 735 2100

The following methods were used for the determination of the elements concentration: speetrocbemical analysis (Cr, Ni, Co, V, Cu, Sr, Ba, F, P), flame photometry (Na, K, Li, Rb), gamma spectrometry (Th, U, K), X-ray spectrometry (REE, Zr), mass spectrometry (oxygen,

carbon isotopic analysis), wet chemistry silicate analysis (petrogenic elements). The determinations has been made mainly in the laboratories of V.I. Vernadsky Institute of Geochemistry and Analytical Chemistry of U.S.S.R, Academy of Sciences, Moscow.

Geochemistry of Effusive Rocks of Rift Zones

363

Iceland effusive rocks series (basalts-andesite-basalts-andesites--dacites-rhyodacites-rhyolites). Geochemical history of minor rare elements is determined by the history of petrogenic elements primarily of Na, K, Ca, Mg, Fe and Si. Usually the content of rare lithophylic elements is found to increase with the increase of alkalies (Na and K) and SiO z in certain series of rocks (excluding K-alkaline one). The best correlation is found between the rare lithophylic elements and potassium. Abundances of titanium, phosphorous and other mafic typomorphic elements (V, Cr, Ni, Co and Cs) decreases with the depletion in calcium, magnesium and iron. The mafic typomorphic elements are concentrated predominantly in olivine and pyroxenes substituting isomorphously iron and magnesium. The rocks of each effusive rock series are genetically interdependent. This feature is resulted in gradual increase or decrease of several element concentrations (Tables 1 and 2) from the initial to the end derivatives (from basalt to rhyolite or from melilitite to phonolite). The analogous regularities are displayed by chemical composition and crystallization temperature of phenocrysts (Polyakov et al., 1976). For example, Iceland effusive rock series (basalts-andesites-rhyolites) is characterized by depletion in anorthite component of the plagioclases (70-80 An for basalt, 60-62 An for andesite = dacite, 37-38 An for rhyolite), by depletion of enstatite and enrichment in ferrosilite in clinopyroxenes (54 En and 6 Fs for basalt; 43 En and 14 Fs for andesite-dacite; 15 En and 45 for rhyolite) and by depletion in fayalite component (1 0-13 Fa for basalt, 84 Fa for rhyolite) with increase of SiO 2 content in the mentioned rocks. The most probable crystallization temperature of plagioclase phenocrysts is about 1150-1250°C for basalts, 1000-1150°C for andesites and 920-980°C for dacites. Isotopic composition of helium (Mamyrin et al., 1974), oxygen (Muehlenbachs, 1973), strontium and lead (Brooks and Jakobsson, 1974) reveals mantle source of melts, which give rise to Iceland TABLE 2. ELEMENTCONTENTINTHENa--ALKALINEROCKSERIESOFTHEEASTERNRIFTANDIN K--ALKALINEROCK SERIESOFTHEWESTERNRIFTOF EASTAFRICA Na-alkaline rocks

K-alkaline rocks

Melilite

Melanonephelinite

Nephelinite

Na,% K,% Li ppm Rb ppm REE ppm Zr ppm Nb ppm Th ppm F ppm

2.04 1.54 17 48 340 285 120 I 1.0 1650

3.82 2.03 21 63 322 270 110 8.2 1200

6.43 3.39 26 I 16 414 710 200 17.3 2600

6.53 3.86 20 130 390 860 180 24.6 1530

1.03-2.49 3.26-7.59 8.5-11.3 124--485 410-955 320-410 120-190 11.0-27.9 800-1400

Ca,% Mg,% Fe,% Ti,% V ppm Cr ppm Ni ppm Sc ppm P ppm

9.95 6.72 10.77 1.77 212 150 75 23 ! 900

8.0 3.67 9.06 1.63 250 50 38 16 1400

5.67 1.39 6.85 0.89 140 16 18 3.5 1450

2.20 0.63 4.59 0.64 42 22 12 3.5 1100

6.32-10.70 3.38-10.22 8.29-9.82 1.99-2.79 201-290 307-1180 47-326 20-45 1200-2300

Element

Phonolite

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V. 1. Gerasimovsky TABLE 3, RATIO VALUES OF COHERENT ELEMENTS IN ICELAND EFFUSIVE ROCKS SERIES

Na/K K/Rb Ca/Sr Sr/Y Sr/Ba Ti/Nb Ni/Co Ca/Mg Fe/Mg F/P

Basalt

Andesitebasalt

5.05 448 316 7.8 1.90 851 1.64 2.01 2.50 0.52

2.35 446 223 5.0 0.72 290 1.26 2.43 4.90 0.59

Andesite 1.86 397 192 3.0 0.53 156 1.23 4.26 7.80 1.06

Dacite

Rhyolitedacite

Rhyolite

1.43 383 146 2.2 0.29 67 1.22 3.65 7.20 3,30

i .25 372 124 1.08 0.29 32 1.20 I 1.2 27.5 4.98

l. 18 370 114 0.73 0.11 22 0.80 14.2 30.3 5.18

effusive rocks. High values of He3/He4 ratio for basalt (1 × 10 " s) and rhyolite-dacite (1.5 × 10-s) are characteristic of basalt layer of the earth's crust. Iceland basalt are closely related to the basalts of the-Mid Atlantic ridge as concerning their value of SrST/'Srs° ratio (0.703040 for the former and 0.702650 for the latter) and average values of lead isotopic ratios (value of Pb2°6/pb 2°4 ratio is equal to 18.82 and 18.672 correspondingly, that of PbZ°7/pb2°4 to 15.59 and 15.543, pb2°S/pb 2°° to 38.68 and 38.307). The regularities of element distribution in both the effusive rock series of the East Africa rift zones (Na-alkaline and K-alkaline~ are not the same. The latter in comparison with the Na-alkaline series (Table 2) is characterized by considerable predominance of potassium over sodium (Na/K = 0.20-0.44), high concentration of a number of mafic typomorphic elements (Ca, Mg, Fe, V, Cr, Ni, Co, Sc, Ti, P), some elements (Cr, Ni, Sr, Ti) being more abundant than in alkaline basalts of the East Africa rift zones (Table 1). At the same time the rocks of K-alkaline series are enriched in many rare lithophylic elements: Pb (124-385 ppm), Ba (1300-2380 ppm), REE (410-955 ppm), Nb (120-190 ppm), Ta (9.2-18 ppm) and Th (11-27.9 ppm). It is difficult to explain the genesis of the potassium alkaline rocks of the Western rift by means of assimilation hypothesis taking into account the concentration of mafic typomorphic elements in these rocks. Assimilation of sialic material of the earth's crust process by magmatic melt would rather promote depletion in these rock typomorphic elements. The effusive rocks of the K-alkaline series are presumably originated from the mantle melts initially enriched in potassium. It is also worth noting the homogenization temperature of phenocryst inclusions from the rocks of the K-alkaline series varies about 1080-1400'~C. This is also a contradiction with the assimilation hypothesis because homogenization temperatures of phenocryst inclusions from other rock types (Na-alkaline series and alkaline basalts) is somewhat lower (Naumov et al., 1972). These data support the assumption that the primary melts of the K-alkaline rock series are considered as the derivatives of the initial (juvenile) ultrabasic potassium alkaline magma. Alkaline rock varieties in comparison with less alkaline varieties are enriched in rare lithophylic elements, fluorine and some basic rock typomorphic elements: titanium, strontium and phosphorous and are depleted in marie typomorphic elements (excluding Ti, Sr and P). The data concerned with the concentration of some elements in Iceland basalts: toleiitic varieties of the Terstareykyre area (northern part of the rift zone) (1) and alkaline varieties from the island Heimaey (2) are predicted:

365

Geochemistry of Effusive Rocks of Rift Zones 1 2

Na 1.44% 3.59

K 0.09% 0.85

Cr 413 ppm 29

Ni 133 ppm 20.4

Sc 36 ppm 15.5

Ti 0.54% 1.87

P 0.03% 0.176

1 2

Sr 65 ppm 323

Ba 30 ppm 625

Y 16 ppm 44

Nb 3.1 ppm 30

Zr 58 ppm 287

Be 1.1 ppm 5.9

100 ppm 720

F

Alkaline basalts of the East Africa (Eastern rift) are also enriched in rare lithophylic elements: Ba (440 ppm), REE (185 ppm), Zr (300 ppm), Nb (67 ppm), Ta (3.2 ppm), in F (940 ppm) and some marie typomorphic elements: Ti (1.55%), Sr (735 ppm) and P (2100 ppm) (Gerasimovsky and Polyakov, 1974). The carbonatites of the East Africa rift zones are very different in their geochemical features. Effusives carbonatites of the Eastern rift zone are subdivided into sodium-calcium and calcium ones (Dawson, 1966). The latter concerning their composition are analogous to the intrusive carbonatites of the same zone. Lavas of carbonate-silicate composition are found at the Western rift zone (Knorring and Bois, 1961). The carbonatites of the Eastern rift zone are spatially related to the Na-alkaline series of effusive rocks whereas the carbonatites of the Western zone to the K-alkaline effusive series. Typomorphic elements of the carbonatites are the following: calcium, carbon, fluorine, phosphorus, strontium, barium, rare-earth elements, titanium, zirconium and niobium. Many of them (CI, F, P and TR) have the same geochemical history as calcium being the components of the same minerals (calcite, apatite). Data on isotopic composition of strontium (values of Sr a7/Sr80 ratio are about 0.7034 for carbonatite lavas; Bell and Powell, 1970), carbon and oxygen (Vinogradov et al., 1970, 1971), homogenization temperature of inclusions in carbonatite calcite (640-690°C; Romanchev, 1972) and geological observations (repeated sequence of eruptions of carbonatite lava) reveal juvenile (mantle) source of carbonatite matter. According to data of Romanchev (1971) three-phase equilibrium: two liquids and gas was found in nepheline inclusions from effusive rocks (nephelinites). During cooling of one of the liquids (carbonate melt) calcite crystallized and another (silicate melt) was quenched into glass. These investigations imply that separation of carbonatite melt from the alkaline silicate one is due to liquation. Data on isotopic composition, geochemical and thermobarometric investigations showed that formation of the above-mentioned rock series could be considered as the result of differentiation of separate initial melts, which were originated within the upper mantle during riftogenesis. Melt composition was determined by the depth and relative melt volumes of upper mantle material. Differentiation of the initial melts occurred in the intermediate chambers. The deposits of rare and ore-forming elements are absent in the effusive rocks of the rift zones. The deposits of pyrochlore and apatite are related to the carbonatites of the Eastern rift zone of East Africa.

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