24. BASALTS OF THE MIDDLE AMERICA TRENCH (GUATEMALA TRANSECT) AND THEIR CORRELATION WITH OCEANIC AND ISLAND-ARC THOLEIITES 1

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1 . BASALTS F THE MIDDLE AMERICA TRENCH (GUATEMALA TRANSECT) AND THEIR CRRELATIN WITH CEANIC AND ISLAND-ARC THLEIITES Yur I. Dmtrev, Insttute of Geology of re Deposts, Petrography, Mneralogy, and Geochemstry of the U.S.S.R. Academy of Scences, Moscow, U.S.S.R. ABSTRACT Durng the Leg drllng of the Mddle Amerca Trench (Guatemala transect), basalts were reached n Hole 9, km seaward from the Trench axsj n Holes 00 and 00B at the foot of the contnental slope, and at four other holes not sampled for ths study. nly olvne-plagoclase phyrc basalts are present n Hole 9, whereas n Holes 00 and 00B these rocks are assocated wth plagoclase phyrc hgh-alumna basalts. As llustrated by the content of T, A, and the K K + Na rato, some of the Mddle Amerca Trench basalts do not dffer essentally from oceanc tholetes, but others have a composton transtonal to sland-arc tholetc basalts. It s suggested that basalts transtonal from oceanc to sland-arc tholetes are typcal manfestatons of magmatsm n zones of convergence of the oceanc and contnental or sland-arc crust. INTRDUCTIN In the Guatemala transect of the Mddle Amerca Trench, drlled durng Leg, basalts were recovered n Holes 9, 00, and 00B (Fg. ), and at four other holes hot sampled for ths study. Hole 9 ( 9.'N, 9 0.'W) was drlled on the oceanc sde of the Trench km from ts axs and 9 meters above the floor of the Trench, on the horstlke rdge. The depth of the ocean at the ste of drllng was 0 meters. Basalts were reached under upper Mocene nannofossl-foramnferal chalk, n the nterval from. to. meters below the seafloor. Hole 00 (.3'N, 90.9'W) was drlled n the Trench axs at the foot of the contnental slope at a water depth of 09 meters. Basalts, cored n the nterval. to. meters below the seafloor, underle upper Mocene nannofossl and foramnferal sedments. Hole 00B ( 'N, 90.'W) was drlled at a water depth of 090 meters and s also located n the Trench axs, but seaward from Hole 00 (see Ste 00 report for locaton map). Basalts were recovered n the nterval to 33. meters below the seafloor, under Quaternary datomaceous mud. Accordng to sesmc reflecton profles, basalts located n the Guatemala transect of the Mddle Amerca Trench belong to the oceanc crust of the Cocos Lthospherc Plate, subductng under the contnental Carbbean Plate (von Huene et al., 90). n the bass of petrographc and petrochemcal peculartes, basalts of the Mddle Amerca Trench (Guatemala transect) are subdvded nto () olvne-plagoclase phyrc normal basalts and () plagoclase phyrc hgh-alumna basalts. Auboun, J., von Huene, R., et al., Int. Repts. DSDP, : Washngton (U.S. Govt. Prntng ffce). PETRGRAPHY F BASALTS lvne-plagoclase Phyrc Basalts Accordng to avalable data, olvne-plagoclase phyrc basalts are the only gneous rocks recovered n Hole 9; however, n Hole 00 they overle plagoclase phyrc basalts. lvne-plagoclase phyrc basalts consst of a glassy or fne-graned matrx wth small crystals of plagoclase, pyroxene, and ore mnerals, as well as phenocrysts of plagoclase, olvne, and spnel. The content of phenocrysts changes from several grans to to % (Table ). The texture of basalts s usually hyalopltc or ntersertal> sometmes wth trachytc features caused by subparallel orentaton of plagoclase mcrolaths. Vescles comprse as much as.% of the volume of the basalts and are flled wth smectte and calcte. Phenocrysts of the plagoclase are represented by bytownte An 0 _ (Table ), sometmes contan nclusons of Cr-spnel (Table 3), and form glomerpporphyrc aggregates wth phenocrysts of olvne-chrysolte Fo _ (Table ). The abundance of plagoclase phenocrysts n olvne-plagoclase phyrc basalts s rather constant and does not depend on the degree of ther crystallzaton. Expermental homogenzaton of melt nclusons n plagoclase phenocrysts from Sample 9--, 3 cm ndcated ther crystallzaton at a temperature of about 0 to 0 C (analyss by I. P. Solovova, Insttute of Geology of re Deposts, Petrography, Mneralogy, and Geochemstry of the U.S.S.R. Academy of Scences, Moscow). Phenocrysts of spnel wth the rato CrCr + Al = were formed under nearly the same condtons, that s 30 to 0 C (Fst and Bence, 90). Among groundmass mnerals, plagoclase mcroltes (bytownte, An _ 0 are the most domorphc crystallne phase (Table ). The content of plagoclase crystals n

2 Y. I. DMITRIEV N A KEY Quaternary volcanoes -W ' 9 90 Fgure. Locaton map of holes from whch basalts were recovered on Legs and. Table. Modal composton of olvne-plagoclase phyrc and plagoclase phyrc basalts (vol.%). Sample a lvne-plagoclase Phyrc Basalts 9--, 9^-, 3 9--, 9--, , 3 B-3-, Plagoclase Phyrc Basalts 00-9-, , Sub-bottom Depth (m) Phenocrysts Plagoclase 3. 3, lvne + b c Groundmass of the Rock Plagoclase re Mneral.0 In matrx d In matrx. Matrx a The sample number ndcates hole-core-secton, and dstance from the begnnng of the secton n cm. b + ndcates less than 0. vol.%. c ndcates absence. " In matrx ndcates tny nclusons were counted together wth glassy matrx. the groundmass of basalts correlates drectly wth the degree of ther crystallzaton. Clnopyroxenes are present n the groundmass of basalts n the form of ncpent crystals and ther aggregates (augte, En33_ Wθ_3Fs _) (Table ). re mnerals of olvne-plagoclase phyrc basalts are represented by ttanomagnette and lmente (Table ), whch are more abundant n better-crystallzed rocks. Plagoclase Phyrc Basalts Plagoclase phyrc basalts underle olvne-plagoclase phyrc basalts n Hole 00. Absence of olvne phenocrysts dstngushes the uppermost rocks. (Table ). The composton of rock-formng mnerals n olvneplagoclase phyrc basalts and n plagoclase phyrc basalts s, n general, smlar (Tables, 3, and ), wth a somewhat hgher content of Ca n plagoclases and clnopyroxenes as well as of A n spnel n plagoclase phyrc basalts. The crystallzaton temperature of plagoclase phenocrysts n Sample 00-9-, 9 cm s 3 to C (analyss by I. P. Solovova), whch s practcally the same as the crystallzaton temperature of plagoclase phenocrysts n olvne-plagoclase phyrc basalts. The lower rato CrCr + Al =.0 n spnel from plagoclase phyrc basalts possbly ndcates crystallzaton at a lower temperature than crystallzaton of spnel n olvne-plagoclase phyrc basalts. PETRCHEMISTRY AND GECHEMISTRY F BASALTS The relatve abundances of S and K + Na ndcate that basalts of the Mddle Amerca Trench (Guatemala transect) belong to dervatves of tholetc basaltc magma.

3 BASALTS F THE MIDDLE AMERICA TRENCH Table. Composton of plagoclase n olvne-plagoclase phyrc and plagoclase phyrc basalts (wt.%). S T A Fe Mg Ca Na K S T Al Fe Mg- Ca Na K % Ab?o An Name Phenocrysts lvne-plagoclase Phyrc Basalts a Mcroltes Catons on oxygens Bytownte Labradorte- Bytownte Plagoclase Phyrc Basalts b Phenocrysts Bytownte Mcroltes Bytownte a = Sample 9--, 93 cm from begnnng of secton. = Sample 00--, 3 cm from begnnng of secton. " Sample 00-9-, 9 cm from begnnng of secton. Table 3. Composton of spnel n olvne-plagoclase phyrc and plagoclase phyrc basalts (wt.%). S A Fe Mn Mg Ca N Cr S T Al Fe Mn Mg Ca N Cr CrCr + Al FeFe + Mg lvne-plagoclase Phyrc Basalts (Sample 9--, 93 cm) Catons on oxygens Plagoclase Phyrc Basalts (Sample 00-9-, 9 cm) lvne-plagoclase phyrc and plagoclase phyrc basalts that are dstngushed by petrographcal features also have dfferent chemcal compostons (Table ). The dfference n content of A s the most obvous and constant. All the plagoclase phyrc basalts are dstnct from the olvne-plagoclase phyrc basalts, as Table. Composton of olvne n olvne-plagoclase phyrc basalts (wt.%). Sample 9--, 93 cm Sample 00--, 3 cm A Fe Mn Mg Ca N S Al Fe Mn Mg Ca N % Fo Name Catons on oxygens Chrysolte Chrysolte Table. Composton of clnopyroxene n olvne-plagoclase phyrc and plagoclase phyrc basalts (wt.%). S T A Fe Mn Mg Ca K S T Al Fe Mn Mg Ca Na K Io En % Wo Wo Fs Name lvne-plagoclase Phyrc Basalts Sample 9--, 93 cm Sample 00--, 3 cm Augte Catons on oxygens Augte Plagoclase Phyrc Basalts Sample 00-9-, 9 cm Augte shown by an Al Fe θ3 + Fe + Mg rato of more than. Normal and hgh-alumna basalts can be connected wth oceanc as well as wth sland-arc magmatsm, therefore to determne formatonal type and petrochemcal peculartes of Mddle Amerca Trench basalts, t s necessary to compare them wth the same knd of basalts from oceanc and sland-arc envronments. It should be remembered that accordng to the concepts of plate tectoncs (Le Pchon et al., 93), basalts of the outer zones of deep-sea trenches belong to a for- 9

4 Y. I. DMITRIEV Table. Composton of ore mnerals n olvneplagoclase phyrc basalts (wt.%). S Tθ A Fe Mn Mg Ca Name lvne-plagoclase Phyrc Basalts (Sample 9--, 93 cm) Ttanomagnette Ilmente matonal typeoceanc spreadng zone basalts. These basalts were formed n axal zones of md-oceanc rdges and then mechancally transported to convergent margns of lthospherc plates, where oceanc crust began to be subducted under sland arcs and contnents. Accordng to ths concept, deep-sea trenches are consdered amagmatc morphostructures, behnd whch, at a dstance controlled by the nclnaton of Benoff zone, volcanoes of sland arcs and margnal contnental cordlleras occur. ther scentsts who have analyzed oceanc and contnental magmatsm from the standpont of fxsm (a concept based on the predomnance of vertcal over horzontal tectonc movements) also beleve that deep-sea trenches mark a boundary between oceanc and contnental (sland-arc) crust, where there "must be dscontnuty of all materal and geophyscal parameters" (Lutz, 90). In the last few years, dscoveres have been made of volcanc vents on margnal swells and oceanc slopes of deep-sea trenches (Avdeko et al., 90) and of basalts on the oceanc slope of the Yap Trench that are essentally younger than those of adjacent areas of the Pacfc floor (Sharaskn et al., 90). These dscoveres evdently ndcate relatvely recent volcansm n the outer zone of deep-sea trenches, about whch lttle more than nothng s known. The geodynamc regme n regons of magmatc actvty should leave a mark on the composton of gneous rock assocatons. Deep-sea trenches represent morphostructural zones where regmes of spreadng, typcal of an oceanc envronment, are perhaps replaced by predomnantly pressure regmes of destructve plate boundares. The transtonal tectonc poston of deep-sea trenches between areas wth oceanc magmatsm and regons wth sland-arc-type magmatsm could mprnt the composton of trench-zone basalts and brng about ther ntermedate nature between abyssal and sland-arc tholetes. The petrochemstry and geochemstry of the Mddle Amerca Trench (Guatemala transect) basalts confrm ths hypothess. As mentoned earler, the outer slope of the Mddle Amerca Trench and adjacent oceanc floor belong to the Cocos Plate. Insde ths fragment of the Pacfc lthosphere basaltc magmatsm s recognzed n the axal zone of the East Pacfc Rse, n the Galapagos Spreadng Center, and, possbly, n such transform faults as Squeros. Basc tholetc rocks that occur there are represented by normal, hgh alumna, and ferrobasalts (Table ). If Mddle Amerca Trench basalts had been formed together wth other basalts of the Cocos Plate n spreadng or fault zones of ths part of the Pacfc, ther petrographc, petrochemcal, and geochemcal features should be the same (Dmtrev, 90). Mddle Amerca Trench basalts have been compared wth sland-arc-type basalts, such as Cenozoc basalts of the Central Amerca and South Amerca Cordllera, the formaton of whch s connected wth processes n the zone of convergence of the Cocos, Nazca, Carbbean, and Amercan lthospherc plates (Pchler and Weyl, Table. Chemcal composton of olvne-plagoclase phyrc and plagoclase phyrc basalts (wt.%). lvne-plagoclase Phyrc Basalts 3 Plagoclase Phyrc Basalts S T A Fe Fe Mn Mg Ca Na K H ~ H + P C S F Cl Cr V L Rb Cs Total , Note: = Sample 9--, 3 cm from begnnng of secton; = Sample 9--, 93 cm; 3 = Sample 00--, 3 cm; = Sample B-3-, cm; = average from frst four analyses. = Sample 00-9-, 9 cm; = Sample 00-9-, cm; = average from last two analyses; ndcates that analyss for ths component was not carred out; and ndcates that component s absent. 0

5 BASALTS F THE MIDDLE AMERICA TRENCH Table. Chemcal analyses of Pacfc oceanc floor and sland arc basalts (wt.%). S T A3 Fe3 Fe Mn Mg Ca Na K H + P Total xdes No. of analyses Normal Hgh-lumna Ferrobasalts Note:,, = Squeros Transform Fault (Batza et al., 9);,, = Galapagos Spreadng Center (Anderson et al., 9, 9; Dmtrev. n press; Engel and Engel, 9: Rudnk, 9); 3, 9 - East Pacfc Rse (Dmtrev, 90; Yeats et al., 93); = Galapagos Islands (McBrney and Wllams, 99); = not determned. 93; Stober and Carr, 9). In Table 9 these basalts are also compared wth those of Pacfc sland arcs representng dfferent stages of evoluton (Marana, Tonga, New Hebrdes, Aleutan, and Kamchatka-Kurl). To reveal and confrm typcal features of deep-sea trench basalts, analyses of basalts from the Phlppne and Yap trenches (outer slopes) were used along wth analyses of Mddle Amerca Trench basalts (Table 0). Petrochemcal studes show that specfc features of basalts erupted fferent oceanc tectonc settngs are most clearly expressed n concentratons and relatons of such elements as Fe, Mg, Al, T, and the alkales. Accordngly, to contrast and compare basalts of the Mddle Amerca Trench wth basalts of other morphostructures of the Pacfc, ratos such as 00 FeVFe' + Mg (Fe' s total Fe as Fe), Al Fe + Fe + Mg, T Al, K K + Na, and KRb were used n ths study. By these parameters, normal tholetc basalts of the Mddle Amerca Trench (Guatemala transect) resemble basalts of oceanc spreadng zones, whch, however, have essentally a hgher KRb rato and a lower Al Fe + Fe + Mg rato (Table ). By the latter parameter, Mddle Amerca Trench basalts are correlated wth basalts of the Central Amerca Cordllera. The correlaton of Mddle Amerca Trench (Guatemala transect) hgh-alumna basalts wth the same type basalts of other morphostructures of the Pacfc (Table ) reveals that by ther petrochemcal features these rocks more closely resemble hgh-alumna sland-arc basalts. By the K K + Na rato, Mddle Amerca Trench hgh-alumna basalts closely resemble the hghalumna basalts of the South Amerca Cordllera and of such prmtve sland arcs as Tonga and Marana. The comparson of partcular and especally average analyses of gneous rocks cannot provde a comprehensve noton of ther genetc relatons, because chemcal peculartes, whch reflect the stage of the parental magma dfferentaton, are not fully consdered n such an approach. Therefore, to decde whether manfestatons of magmatsm of, say, ths or that oceanc morphostructure resemble each other or not, t s more nformatve to compare the composton not of separate types of rocks that occur n these morphostructures but of seres of these rocks, formed durng the evoluton of correspondng parental magmas. The poston of rocks n the sequence of dfferentates of the tholetc basaltc magma can be determned qute relably by the value of the FeVFe' + Mg rato, calculated on the bass of bulk chemcal analyses of the rocks. Thus varatons of concentratons of rockformng oxdes n sequences of basaltc rocks wth ncreasng values of the Fe 'Fe' + Mg rato reflect basc regulartes of evoluton of these rock parental magmas. Unformty or fluctuatons n varaton trends provde the possblty of speakng about unque or multple parental magmas. Let us analyze the poston of Mddle Amerca Trench (Guatemala transect) basalts as well as basalts of other Pacfc morphostructures on T -00 FeVFe' + Mg, Al -00 FeVFe' + Mg, and K -K + Na dagrams (Fgs. -). The concentraton of T s one of the most senstve and relable ndcators of basaltc rock formatonal type (Chayes, 9); ndeed, on the T -00 FeV Fe' + Mg dagram (Fg. ),tholetc basalts of the Galapagos Islands, the Squeros Transform Fault, and sland arcs form varaton trends dsconnected n space. From the same dagram t s qute obvous that East Table 9. Chemcal analyses of basalts of Pacfc sland arcs (wt.%). xdes Normal 3 Hgh-alumna 9 Ferrobasalts 0 S T A3 Fe3 Fe Mn Mg Ca Na K H~ H + P Total No. of analyses _ , _ _ _ Note:, 9, 0 = Marana (Larson, 9; Stark, 93);, = Kamchatka (Leonova et al., 9); 3 = Central Amerca Cordllera, volcano Irazu (Robn and Tournon, 9); = New Hebrdes (Gorton, 9); = Tonga (Bryan et al.. 9); = South Amerca Cordllera, Casablanca volcano (Lopes-Escobar et al., 9); and = Aleutan (Marsh, 9); = not determned; = absent.

6 Y. I. DMITRIEV Table 0. Chemcal analyses of deep-sea trench (outer slope) basalts of the Pacfc (wt.%). xdes Yap (Sharaskn et al., 90) Normal Mddle Amerca Trench (South Mexco transect) (Dmtrev, n press) Phlppne Trench (Dmtrev, 99) Mddle Amerca Trench (South Mexco transect) (Dmtrev, n press) Hgh-alumna Yap (Sharaskn et al., 90). S T A Fe Fe Mn Mg Ca Naθ K H ~ H + P C Total No. of Analyses Table. Some petrochemcal and geochemcal parameters of normal tholetc basalts of Pacfc morphostructures. 3 Deep-sea Trenches 9 ceanc Floor 0 Island Arcs 3 ceanc Islands 00 Fe'Fe' + Mg Al θ3fβθ3 + Fe + Mg T Al θ3 K K + Na KRb No data avalable No data avalable No data avalable Note: Mddle Amerca Trench, Guatemala transect:, Sample B-3-, cm;. Sample 9--, 93 cm; 3. Sample 00--, 3 cm;. Sample 9--, 3 cm;. average from frst four analyses;. South Mexco transect;. Yap Trench;. Phlppne Trench; 9. Squeros Fault; 0. Galapagos Spreadng Center;. East Pacfc Rse;. Marana sland arc; 3. Kamchatka;. Central Amerca Cordllera;. New Hebrdes;. Galapagos Islands. Parameters were calculated on the bass of analyses n Tables, 9, and 0. Table. Some petrochemcal and geochemcal parameters of hgh-alumna tholetc basalts of Pacfc morphostructures. Deep-sea Trenches 3 ceanc Floor 9 Island Arcs 0 00 Fe'Fe' + Mg Alθ3Fe θ3 + Fe + Mg T Al θ3 KK + Naθ KRb No data avalable No data avalable Note: Mddle Amerca Trench, Guatemala transect:. Sample 00-9-, cm;. Sample 00-9-, 9 cm; 3. average from frst two analyses.. South Mexco transect;. Yap Trench;. Galapagos Spreadng Center;. Squeros Transform Fault;. Tonga; 9. South Amerca Cordllera; 0. Kamchatka;. Aleutan;. Marana. Parameters arc calculated on the bass of analyses n Tables, 9, and 0. Pacfc Rse and Galapagos Spreadng Center basalts that erupted under a smlar geodynamc regme form practcally the sngle evolutonary trend. Basaltc seres of oceanc morphostructures dffer from those of sland-arc basaltc seres because of ther essentally hgher T concentratons and the prmtve nature of ther hgh-alumna basalts, whch plot at the begnnng of the evolutonary sequence (00 Fe Fe' + Mg < ); n contrast, sland-arc seres hghalumna basalts belong to late dfferentates (00 Fe Fe' + Mg > ). The dstrbuton of Mddle Amerca Trench basalts as well as basalts of other deep-sea trenches on the T - 00 FeFe' + Mg dagram shows that sgnfcant amounts of these rocks are transtonal between basalts of oceanc morphostructures and sland-arc basalts. Half of the Mddle Amerca Trench (Guatemala transect) basalts, the average of hgh-alumna basalts of ths transect, the average of normal and hgh-alumna basalts of the South Mexco transect of the Mddle Amerca Trench, and basalts of the Phlppne and Yap trenches plot n the sector between the evolutonary trend of East Pacfc Rse-Galapagos Spreadng Center basalts and the evolutonary trend of Marana sland-arc basalts. Dfferent behavor of alumnum n the course of the evoluton of oceanc and sland-arc tholetc basaltc magmas s clearly vsble on the Al -00 FeFe' + Mg dagram (Fg. 3). In basaltc seres of the Galapagos Spreadng Center-East Pacfc Rse, Squeros Transform Fault, and Galapagos slands, the content of A decreases as the 00 FeFe' + Mg rato ncreases. In sland-arc basaltc seres (Marana slands), however, alumnum does just the opposte. In basalts of the Guatemala transect of the Mddle Amerca Trench one can see both tendences of A. In basalts of Hole 9 drlled on the oceanc slope of the

7 BASALTS F THE MIDDLE AMERICA TRENCH o n ΔΛJ ^ S Gl 0EPR , ^ >^ 00--,3 MATG > *MATG * ^ p f ^9--,3 * ^ G 00B-3-. r ' " ^ ^ Δ K K A S^ ^ ^ 0 SACΔ. ^ ^ MATM _. ' < 30^ ^ " ANG MA-" ΔT FeθFe + Mg λ _.«" ΔA DGI 00-9-,90 \ _ _ΔM Fgure. T -00 FeVFe' + Mg dagram (normal ( ), hghalumna (), and ferrobasalts (0) of spreadng zones and transform faults [EPR = East Pacfc Rse, G = Galapagos Spreadng Center, S = Squeros Transform Fault]; normal ( ) and ferrobasalts (D) of oceanc slands [GI = Galapagos Islands]; normal (A), hgh-alumna (Δ), and ferrobasalts (Δ) of sland arcs [A = Aleutan, CAC = Central Amerca Cordllera, K = Kamchatka, M = Marana, NG = New Hebrdes, SAC = South Amerca Cordllera, T = Tonga]; normal ) and hgh-alumna basalts (0) of the deep-sea trench outer zones [MATG = Mddle Amerca Trench, Guatemala transect; MATM = Mddle Amerca Trench, South Mexco transect; Ph = Phlppne, Y = Yap]; and evolutonal trends of basalts). 0-9 S h < C XV S Q\ KA MATM o ΔT SACΔ y \ \ 00-9-,^ MATM^SΛ^NG^ F h \^M MA^ f \ 9 00B-3-,\ v AΔ ΔK^' I I x FeFe + Mg é ATG ΔM ΔM 9--,3 AM I ,9 0 0 Fgure 3. Al -00 FeVFe' + Mg dagram. (See Fg. capton for an explanaton of symbols and abbrevatons.) Trench km from the axs, A content decreases as the 00 FeVFe' + Mg rato ncreases; the evoluton of basaltc melts has the typcal oceanc pattern. In contrast, n basalts of the Trench axs (Holes 00, B), concentratons of A have a drect correlaton wth the value of the 00 FeVFe' + Mg rato; that s, parental basaltc magma of these rocks evolves as n an sland-arc envronment. In basalts of the oceanc slope of the Yap Trench, correlaton of A and the 00 FeVFe' + Mg rato s the same as n basalts of the Marana sland arc, although less clearly expressed. Tholetc basalts of sland arcs dffer from oceanc basalts because of the greater role of potassum; ths s qute obvous on the K -K + Na dagram (Fg. ). Also, on ths dagram as well as on the T -00 FeVFe' + Mg and Al -00 FeVFe' + Mg dagrams one can see that among basalts of the Mddle Amerca Trench and other deep-sea trenches there are rocks wth composton smlar to that of typcal oceanc tholetc basalts and rocks transtonal to sland-arc tholetc basalts. To ths ntermedate type of basalts, accordng to the K K + Na rato, belong basalts from Hole 00 and basalts of the Phlppne Trench. CNCLUSIN Analyss of petrochemcal peculartes of Mddle Amerca Trench (Guatemala transect) basalts reveals compostons typcal of oceanc tholetes and transtonal to sland-arc tholetes. Wth respect to degree of dfferentaton, transtonal-type basalts dffer from comparable rocks of oceanc and sland-arc seres by ntermedate values of T concentraton and the K K + Na rato. The common feature of transtonal basalts and sland-arc basalts s the advanced dfferentaton of hgh-alumna varetes n both types of rocks. Tholetc basalts of other deep-sea trenches, for example, Phlppne and Yap trenches, have analogous chemcal features. (wt. %) TΔ NGA MA^ MATM<^ 00B-3-, *S Y * S K + Na M>^ y T^ A MA KA KΔ 00-9-, 9, Φ00--, 3 SAC EPR 9--,3 y^k * M ATGg>s --, 9J S CACΔ Fgure. K -K + Na dagram. (See Fg. capton for an explanaton of symbols and abbrevatons.) AΔ GID - - 3

8 Y. I. DMITRIEV These petrochemcal analyses, together wth publshed data on the unexpectedly young age of some basalts from the seaward slopes of some deep-sea trenches, are perhaps evdences of local magmatsm connected wth the zone of convergence of oceanc and contnental (sland-arc) crust, where perhaps basalts transtonal between oceanc and sland-arc tholetes were erupted. ACKNWLEDGMENTS The author thanks the Leg scentfc party for provdng basalt samples for ths nvestgaton. Bulk chemcal analyses of these samples were carred out by Y. V. Dolnna and mcroprobe analyses of rockformng mnerals by G. N. Muravzkaja n laboratores of the Insttute of Geology of re Deposts, Petrography, Mneralogy, and Geochemstry of the U.S.S.R. Academy of Scences. The author s also grateful to N. A. Bogadnov and S. A. Tsvetkov, who revewed the manuscrpt. REFERENCES Anderson, R. N., Claque, D. A., Kltgord, K. D., Marshall, M., and Nshmor, R. K., 9. Magnetc and petrologc varatons along the Galapagos spreadng center and ther relaton to the Galapagos meltng anomaly. Geol. Soc. Am. Bull., :3-9. Anderson, R. N., Schllng, J. G., and Vogt, P., 9. Rare-earth, Fe and T varatons along the Galapagos spreadng center and ther relatonshp to the Galapagos mantle plume. Nature, (): 0-3. Avdeko, G. P., Gorshkov, A. P., Sapozhnkov, E. A., Selverstov, N. I., and Skrpko, K. A., 90. Underwater volcanoes of sland arcs and margnal seas. In Bogatkov,. A. (Ed.), Magmatc and Metamorphe Rocks of ceanc Floor and Ther Geness: Moscow (Academy of Scences of the U.S.S.R.), pp. -. (In Russan) Batza, K., Rozendahl, B. R., and Fsher, R. L., 9. Evoluton of oceanc crust. Part III. Petrology and chemstry of basalts from the East Pacfc Rse and Squeros Fracture Zone.. Geophys. Res., :-. Bryan, W. B., Stce, G. D., and Ewart, A., 9. Geology, petrography and geochemstry of the volcanc sland Tonga. J. Geophys. Res., :. Chayes, F., 9. 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