Evolution of the mantle: Geochemical evidence from alkali basalt

Transcription

1 Evlutin f the mantle: Gechemical evidence frm alkali basalt ABSTRACT A careful cnsideratin f the hetergeneities in the trace-element cncentratins and lead and strntium istpe ratis fr primary alkali basalt and nephelinite shws that the istpic hetergeneities reflect real gechemical hetergeneities f their mantle surce that have existed variusly frm abut 1,000 t 3,000 m.y. ag. It is suggested that at present there is nt mantle-wide cnvectin and that the mantle surce fr alkali basalt is deeper than the presently cnvecting mantle. During Archean time, mantle cnvectin was much deeper than at the present time, and this deeper cnvectin led t ht-line tectnics that may have determined the tectnic style fund in Archean greenstne belts. Shine Sn Sun, Gilbert N. Hansn Department f Earth and Space Sciences State University f New Yrk at Stny Brk Stny Brk, New Yrk INTRODUCTION A systematic cmparisn f the majr-element cmpsitins f alkali basalt by Schwarzer and Rgers (1974) shws that alkali basalt in cean islands, n cntinents, and back f island arcs is a primary type f magma with a similar parentage regardless f its tectnic envirnment. This des nt include alkali basalt and high-ptassic basalt typically fund in the east African rift valleys r the Rhine graben, as these are gechemically distinct (S. Kessn, 1974, ral cmmun.). Cmpared t the surces fr cean-ridge basalt, the surces fr alkali basalt are mre enriched in the incmpatible elements, that is, K, Ba, Rb, and light rare-earth elements, and the alkali basalt appears t have been derived frm a less depleted mantle than the cean-ridge basalt (Gast, 1968). Kay and Gast (1973) studied the cncentratins f the rare-earth elements, Ba, Rb, and Sr in alkali basalt and nephelinite and cncluded that they result frm several percent r less f partial melting f a garnet-bearing mantle with a chndritic rare-earth element (REE) pattern and REE cncentratins f 2 t 5 times chndrite at depths greater than 60 km and that the chemical differences between nephelinite and alkali basalt can be explained by nephelinite having frmed frm the same mantle surce but frm lwer extents f partial melting. We (S. Sun and G. Hansn, in prep.) have reevaluated the evidence fr the REE cmpsitin f the mantle surce fr alkali basalt and nephelinite and cncluded that alkali basalt results frm 7 t 1 5 percent melting, and nephelinite results frm 3 t 7 percent melting, f a garnet-bearing mantle with a light REE-enriched pattern relative t chndrite, in which La is abut 14 times chndrite and Yb is abut 3 times chndrite. Lead and strntium istpes frm alkali basalt and thleiite n cean islands are generally mre variable and mre radigenic than thse frm cean-ridge basalt (Gast and thers, 1964; Gast, 1967; Tatsumt, 1966a, 1966b; Cper and Richards, 1966; De, 1968;Oversby and Gast, 1970; Hedge and Peterman, 1970). The istpic variatins in the alkali basalt n cean islands have been explained t be a result f cntaminatin by pelagic sediment, frequent additin f cntinental crust material t the mantle, mixing f alkali-basalt surce with cean-ridge basalt surce, istpic disequilibrium during partial melting, r real hetergeneities within the mantle surce that have existed fr at least many hundreds f millins f years (Gast and thers, 1964; Cper and Richards, 1966; Tatsumt, 1966b; Gast, 1967; Armstrng, 1968; Oversby and Gast, 1970; Armstrng and Hein, 1973; O'Nins and Pankhurst, 1973). The surces f alkali basalt and nephelinite may be attributed t melting f mantle in the lw-velcity zne, deepmantle plumes (Mrgan, 1972), r less depleted prtins f present-day cnvectin cells. This paper is an utgrwth f ur study f the alkali basalt at Rss Island, Antarctica, as part f the Dry Valley Drilling Prject (S. Sun and G. Hansn, in prep.) and the study f lead istpes frm ceanic vlcanic rcks (S. Sun, in prep.). New ways f lking at the gechemical data resulted when we asked the questin, "Hw similar in trace elements and Sr and Pb istpes is the surce f the vlcanic rcks that ccur n a cntinental crust t similar vlcanic rcks n cean islands?" With recent high-quality data frm the literature (S. Sun and G. Hansn, in prep.; S. Sun, in prep.), it is pssible t evaluate the prpsed mdels fr the gechemical hetergeneities fund in alkali basalt and nephelinite. We find the mst viable mdel t be ne in which the istpic hetergeneities are in the surce; this has implicatins fr presentday cnvectin in the mantle as well as fr cnvectin during Precambrian time. The depth f cnvectin may have had an effect n cntrlling the style f Archean and later tectnics. TRACE-ELEMENT AND ISOTOPE DATA We will assume that during melting the majr and trace elements in the melt are in at least lcal equilibrium with the residual minerals. This seems reasnable, as Kay and Gast (1973) have explained the enrichment in light REE and incmpatible elements and the relative depletin f the heavy REE by using GEOLOGY 297

2 mineral-melt distributin cefficients fr an experimentally determined garnet peridtite mineralgy necessary t prduce alkali basalt and nephelinite (Green, 1973). Als, if there were large and varying extents f disequilibrium, it wuld be difficult t explain why the majr and trace elements in alkali basalt and nephelinite are s similar. Figure 1 cmpares REE fr primary 1 alkali basalt frm Rss Island, Antarctica, with highest precisin REE data frm ther primary alkali basalt frm cean islands and cntinents. The ttal variatin fr an REE fr primary alkali basalt is less than a factr f 2.5 and may reflect variatins in (1) REE abundances f mantle surce, (2) mineralgic cmpsitin f mantle surce, r (3) extents f partial melting. 1 T reduce the effect f fractinal crystallizatin, primary alkali basalt and nephelinite are cnsidered t be thse with Mg/(Mg+Fe +2 ) (atmic) >0.64 and nickel cntents >100 ppm. Irving (1971) shwed that primary basalt equilibrated with mantle livine (F088-92) shuld have Mg/(Mg+Fe ) ratis f 0.68 t Since livine has a mineralmelt distributin cefficient fr Ni f abut 10 (Gast, 1968), primary melts f a mantle with abut 2,000 ppm Ni (a cmmn value fr peridtite ndules) shuld have abut 300 ppm Ni. Fr 10-percent fractinal crystallizatin f equal amunt f livine and clinpyrxene, the liquidus minerals fr alkali basalt will reduce the Mg/(Mg+Fe +2 ) rati in the melt by abut 5 percent f the riginal rati and the Ni cntent by abut 40 percent f the riginal Ni cntent. Thus the Ni cncentratin in particular is very sensitive fr indicating small percentages f fractinal crystallizatin f livine and clinpyrxene. cr z 0 1 V UJ 200 O ST. PAUL ROCK NEW SOUTH WALES x SAMOA A KAUAI + OAHU Figure 2 presents the lead data fr cean-island vlcanic rcks and the cntinental Rss Island vlcanic rcks and ceanridge basalt. The cean-ridge basalt ccupies a restricted field n the diagram, whereas the cean-island vlcanic rcks vary linearly, and the lines fr each island r island grup are subparallel. The linear relatin fr the cean-island vlcanic rcks may be interpreted in tw ways: (1) a result f mixing, such as an alkali-basalt cmpnent mixing with an cean-ridge basalt r a sediment cmpnent either befre r after melting, r (2) the alkali basalt surces having a histry f islatin, with lng-term mantle hetergeneity in the 238 U/ 204 Pb rati, the hetergeneity having existed fr apprximately the last 2,000 m.y. Data fr a precise 238 U/ 204 Pb versus 206 Pb/ 204 Pb ischrn age are sparse; hwever, the available data plt with very large scatter abut an ischrn f apprximately 1,000 m.y. Figure 3 is a 87 Sr/ 86 Sr versus 87 Rb/ 86 Sr plt f primary alkali basalt and nephelinite. Fr cmparisn purpses, a 4,600-m.y.-ld ischrn with an initial rati f and a 2,000-m.y.-ld ischrn with an initial rati f are pltted. The primary alkali basalt and nephelinite can be seen t plt abut the 2,000-m.y. ischrn. Fr bth the lead and strntium data (Figs. 2, 3), individual vlcanic grups have slpes suggesting a hetergeneity f ages frm abut 1,000 t 3,000 m.y. but with a mde f abut 2,000 m.y. The apprximate 2,000-m.y. age fr the Pb and Sr istpe data wuld suggest that inhmgeneities in the 238 U/ 204 Pb and Rb/Sr ratis were impsed abut 2,000 m.y. ag if they are nt a result f recent prcesses. In rder t determine if mixing may be a viable mdel, 206 Pb/ 204 Pb is pltted against 87 Sr/ 86 Sr in Figure 4. If there is any crrelatin between 206 Pb/ 204 Pb and 87 Sr/ 86 Sr, it is an inverse relatinship, and the data d nt lie n a mixing line between ceanridge basalt and a primitive alkali basalt. Fr Iceland, hwever, where there is mixing with cean-ridge basalt magma (Schilling, 1973; O'Nins and Pankhurst, 1973; Sun and thers, in prep.), the data fr a suite f vlcanic rcks plt alng a line between the cean-ridge basalt and alkali basalt. Gast and thers (1964) have shwn that sedimentary cntaminatin sufficient t N er Cape Verde Reunin I Buv Guadalupe St. Helena Sm Eu Gd Yb Lu Figure 1. Cmparisn f REE fr primary alkali basalt cvering the ttal range in REE. The shaded field is fr Rss Island, Antarctica. Data are nrmalized t Leedey chndrite (Masuda and thers, 1973). REE data are frm Frey (1970), Kay and Gast (1973), Hubbard (1971), and S. Sun and G. Hansn (in prep.) Pb/ 2 Figure 2. Plt f 207 Pb/ 204 Pb versus 206 Pb/ 204 Pb fields fr ceanisland vlcanic rcks nt restricted t primary alkali basalt r nephelinite and fr cean-ridge basalt. Data frm S. Sun (in prep.), S. Sun and G. Hansn (in prep.), Oversby (1972), and Tatsumt (1966a). 298 JUNE 1975

3 change significantly the 87 Sr/ 86 Sr rati fr alkali basalt requires s much sediment that it wuld greatly change the majrelement chemistry and mineralgy and wuld cmpletely dminate the lead-istpe cmpsitin. O'Nins and Pankhurst (1973) have suggested that the variatins in the strntium istpe ratis might be explained by istpic disequilibrium during partial melting f a hmgeneus mantle. They suggest that during melting the minerals that melt at lw temperature, presumably amphible, phlgpite, and apatite, give their istpic character t the melt; that is, the melt is nt in istpic equilibrium with the ttal residue. This leads t melts with a mre radigenic istpic cmpsitin than the ttal parent. Assuming that amphible, phlgpite, and apatite in the mantle can retain a radigenic istpic cmpsitin fr hundreds f millins f years at mantle temperatures, the lead data in Figure 2 suggest that the mantle surce has maintained at least mineral-istpic hetergeneities fr abut 2,000 m.y. In the melting prcess presumably mre radigenic lead wuld first be remved frm a mineral such as apatite, and as melting prceeded the less radigenic lead frm phlgpite r amphible wuld be added, r vice versa. In either case, the lead-istpic data wuld be alng an ischrn defining the time when the minerals were last istpically hmgenized. If disequilibrium were an imprtant prcess, the strntium-istpe ratis in melts frm ne regin shuld shw an inverse relatin t the extent f partial melting. In the Hawaiian Islands, hwever, thleiite as well as alkali basalt, and nephelinite frmed by very different extents f partial melting (Kay and Gast, 1973; Hubbard, 1969), give essentially the same 87 Sr/ 86 Sr rati. Thus we cannt seriusly cnsider that istpic disequilibrium during partial melting is the main surce f the istpic variatins fund in alkali basalt and nephelinite. Rather, the variatins in the 238 U/ 204 Pb and Rb/Sr ratis seem t be in the mantle surce and have existed fr 1,000 t 3,000 m.y. Trace-element data have been used by Kay and Gast (1973) t cnfirm that alkali basalt and nephelinite are derived by lw percentages f partial melting f garnet peridtite. It shuld als be pssible t test fr the presence f the istpically imprtant minerals apatite and phlgpite in the residue at the time the melt left the residue. The cerium cntent can be used t determine the extent f partial melting if the mantle surce fr alkali basalt is relatively hmgeneus, if there is n apatite in the residue, and if the numerical value fr the fractin f melting is greater than the bulk distributin cefficient fr the mantle. 2 If apatite is a minr phase in the mantle during melting, it will cntrl the Ce cntent f the melt, as apatite has a mineral-melt distributin cefficient fr Ce f abut 5 (inferred frm Frey and Green, 1974), whereas the ther presumed mantle minerals have a bulk distributin cefficient f less than 0.01 (Kay and Gast, 1973). If there is apatite in the residue, the P2O5 cncentratins f the melts wuld be independent f the extent f melting, because P is a stichimetric element in apatite, whereas the Ce cncentratin wuld be inversely prprtinal t the extent f melting and percent f apatite present. Hwever, if there is n phsphate mineral in the residue, P2O5 and Ce shuld be cvariant because 2 The cncentratin f trace elements in the magma relative t the riginal mantle (Ci/C 0 ) as a result f simple partial melting is given by Shaw (1970) as CJC 0 = F where D is the bulk distribu- D(\-R) + R tin cefficient f the residue at the mment f remval f liquid frm " T the residue. D = "EXQK^ where X A is the weight fractin f each phase and K^ is the mineral-melt distributin cefficient. If D is very large fr small percentages f melting, C1 /C 0 = 1 ID. If D appraches zer, Ci/C 0 = L/F. If zne refining is an imprtant prcess, it will increase the cncentratin f the trace elements fr alkali basalt t the limit OICJCO - 1 ID (Harris, 1974). In mst cases the effect f zne refining cannt be distinguished frm the effects f small extents f partial melting. Whether zne refining ccurs r nt has little effect n_the cnclusins derived regarding the mantle surce fr alkali basalt r nephelinite. GOUGH- TRISTAN- DISCOVERY TABLE MOUNT t -I t ;l EAST SAMOA ST. HELENA 8 7 Rb/ 8 6 S, Figure 3. Plt f 87 Sr/ 86 Sr versus 87 Rb/ 86 Sr fr primary alkali basalt and nephelinite fr cean islands. 1, Sama; 2, Kerguelen; 3, Gugh; 4, Tristan de Cunha; 5, Tahiti; 6, Reunin; 7, Amsterdam; 8, Crzet; 9, Eniwitk; 10, Kilauea, Hawaii (thleiite); 11, Oahu, Hawaii; 12, St. Helena; 13, Galapags; 14, Guadalupe. Data frm Stuckless (1974, ral cmmun.), Hedge and Peterman (1970), Hedge and thers (1973), Hedge (1974, written cmmun.), McBirney and Williams (1969), Peterman and Hedge (1971), Hedge and thers (1972), Hart (1973), S. Sun and G. Hansn (in prep.), and O'Nins and Pankhurst (1974). 206ph/204 p Figure 4. Plt f 87 Sr/ 86 Sr versus 206 Pb/ 204 Pb fr vlcanic rcks (nt restricted t primary vlcanics) frm cean islands and Rss Island. Data frm references in Figure 2 and Figure 3 plus S. Sun (in prep.), Swainbank (1967), O'Nins and Pankhurst (1974), Sun and thers (in prep.), McDugall and Cmpstn (1965), Stuckless (1974, ral cmmun.), Grant and thers (1973), and Klerkx and thers (1974). GEOLOGY 299

4 Figure 6. Plt f K 2 0 versus cerium fr primary alkali basalt and nephelinite. Data frm Philptts and thers (1972) and frm references in Figure 5. the cncentratin f bth will be inversely prprtinal t the extent f partial melting as a result f dilutin. Figure 5 is a plt f P 2 O s versus Ce shwing that bth are rughly cvariant fr bth alkali basalt and nephelinite. Much f the variatin abut the line in Figure 5 may be a result f relatively large analytical uncertainties, particularly fr P Hwever, frm these data it can be suggested that the P 2 O s /Ce rati in the mantle surce is 75 ± 15. We cnclude that apatite is prbably nt a mineral phase in the residue at the time f magma separatin and that it shuld be pssible t use Ce cncentratin as an indicatr f the extent f partial melting, assuming alkali basalt and nephelinite were prduced frm relatively hmgeneus mantle surces. Figure 6 is a plt f K 2 0 versus Ce fr primary alkali basalt and nephelinite. The mineral-melt distributin cefficients fr K are lw fr all pssible mantle minerals except phlgpite, fr which K is stichimetric, and perhaps amphible. Thus if phlgpite is absent and amphible is minr, there shuld be an inverse relatinship between K 2 0 and the extent f melting, leading t a cvariance f K 2 0 and Ce. This is the case fr the alkali basalt. The nephelinite data, hwever, are mre scattered, suggesting that K is either much mre variable in the mantle surce fr the nephelinite r that a phase cntrlling K cntent, fr example, phlgpite (Yder and Kushir, 1969), is a residual mineral in the mantle fr nephelinite melts. The suggestin that nephelinite is a result f lwer degrees f partial melting than is the alkali basalt (Green, 1973) is in agreement with the higher Ce cncentratin fr nephelinite. Fr the nephelinite we interpret the K 2 0-versus-Ce plt t indicate that a K-rich mineral such as phlgpite is in the residue and that the variatin in the K cntent in nephelinite may be a functin f mantle and melt cmpsitin (particularly vlatiles such as F, CI, and H 2 0) as well as temperature and ttal pressure, which wuld cntrl the stability f phlgpite and thus the cncentratin f ptassium in the melt. MANTLE EVOLUTION We suggest that magma mixing, sediment cntaminatin, r istpic disequilibrium during partial melting are nt cntrlling factrs in determining the istpic cmpsitin f Pb and Sr fr alkali basalt and nephelinite. Rather they are cntrlled by hetergeneities in the mineralgy and chemistry f the mantle and histry f the mantle. It is als suggested that alkali basalt and nephelinite have a mantle surce which is similar t and independent f the envirnments in which they are fund. The cean-ridge basalt, derived presumably frm the cnvecting mantle, has Rb/Sr ratis t lw t supprt its 87 Sr/ 86 Sr ratis, lwer Ba/Sr ratis relative t alkali basalt, depleted light REE cncentratins relative t heavy REE, and high K/Rb ratis relative t alkali basalt (Gast, 1968). These gechemical prperties suggest that the mantle surces fr cean-ridge basalt have undergne mre extensive melting than have the surces fr alkali basalt and nephelinite. The surces f alkali basalt and cean-ridge basalt must differ, and mantle cnvectin must have a gemetry that islates these surces. Figure 7 is a graphical representatin shwing pssible mantle activity and its surficial vlcanic expressin. One pssible surce fr the alkali basalt is near the middle f a cnvectin cell shwn by plume 1, which may nt have as cmplicated a histry as the uter parts f the cell. This surce wuld restrict alkali basalt t regins away frm plate margins and therefre is nt a gd surce fr all alkali basalt and nephelinite. Anther pssible surce is the lw-velcity zne. Leeds and thers (1974) suggested that the lid f the lwvelcity zne cmes essentially t the base f the ceanic crust near the ridge and becmes deeper away frm the ridge. Many cean islands with alkali basalt ccur near ridges (surce 2 in Fig. 7), where the lw-velcity zne wuld be t shallw t have garnet present as a stable phase (Ringwd, 1969) and where a depleted mantle wuld be invlved. The lw-velcity 300 JUNE 1975

5 INTER ARC ISLAND OCEANIC OCEANIC BASIN ARC ISLAND RIDGE Figure 7. Diagrammatic representatin f present-day mantle. 1 is pssible surce f alkali basalt and nephelinite in center f cnvectin cell, 2 is pssible surce in lw-velcity zne, and 3 is mst prbable surce in mantle belw present-day cnvectin. zne, therefre, is prbably nt the surce. Unless there is sme way t maintain an islated clsed system fr lng perids f time within a large part f the cnvecting mantle, we suggest that a mantle plume (Mrgan, 1972; Wilsn, 1973; 3 in Fig. 7) riginating belw the presently cnvecting mantle is the nly viable alternative fr a relatively similar surce available under all tectnic envirnments. If the alkali basalt is a result f mantle plumes, then this deeper mantle was invlved in magma generatin prir t sme 1,000 t 3,000 m.y. ag, suggesting deeper cnvectin and steeper gethermal gradients. Presumably, then, in early Precambrian time the cnvectin was much deeper, and high percentages f melted materials frm this less depleted mantle were the surce f basalt in the Archean greenstne belts. This suggestin is supprted by the trace-element gechemistry f thleiite in Archean greenstne belts, whse mantle surces have strng affinities t the mantle surces f present-day alkali basalt (Hart and thers, 1970). Richter (1973) suggested that with steeper gethermal gradients a secnd-rder (r maybe even a higher rder) cnvectin may ccur, frming hrizntal rlls nrmal t the majr cnvectin. This results in ht lines, which may be capable f breaking up a cntinent, rather than ht spts. Tectnically this might be similar in sme ways t present-day rift valleys r interarc basins n cntinents. The ht lines wuld have assciated with them massive amunts f thleiitic magma resulting frm high percentages f melting f the mantle. Basalt wuld be frming alng ht lines n the ceanic crust as well as n the cntinental crust. If at the same time there was an ceanic crust acting smewhat like that f tday, with ridges, island arcs, and subductin znes, the vlcanic rcks assciated with the ht lines n the ceanic crust wuld be returned t the mantle, whereas thse vlcanic rcks frmed adjacent t r within the pre-existing cntinental crust wuld be preserved. This mdel might explain why the Archean terrane is made up f lw-grade greenstne belts and higher grade and smetimes lder gneiss belts (Hepwrth, 1971; Windley, 1973). The gneiss belts wuld represent the pre-existing cntinental crust derived by prcesses prbably unrelated t greenstne belts (Bridgewater and thers, 1974). The greenstne belts wuld represent the uplifted and rifted parts that are filled with vlcanics and mainly vlcangenic sediment. The later granitic rcks intrusive int the greenstnes are derived by partial melting f the deeper parts f the vlcanic and sedimentary pile r are derivatives f partial melting f material at mantle depths (Arth and Hansn, 1975). The lwered cntinental crust wuld be the basement fr sedimentary basins in which the preexisting cntinental crust and sediment are metamrphsed t granulite grade by the high heat flux. Due t the high heat flux, there may be melting f the lwer cntinental crust, resulting in synkinematic granitic intrusins at higher levels. In the later stages f the cycle, with reductin f the heat flux, lwer extents f melting wuld be represented in the greenstne belts by alkali basalt (Cke and Mrhuse, 1969; Ridler, 1970) and by late alkalic stcks (Anhauesser and thers, 1969; Arth and Hansn, 1975). At the end f the tectnic cycle, with isstatic readjustment, the rifted znes are lwered and nly slightly erded, leaving the relatively lw-grade metavlcanic and metasedimentary rcks in the greenstne belts. The cntinental crust wuld rebund and be mre deeply erded, expsing the high-grade metamrphic rcks f the gneiss belts. This mdel explains many f the majr features fund in an Archean terrane (Anhauesser and thers, 1969) and als explains the lack f sme features necessary fr island-arc analgies, fr example, the lack f blueschist-facies rcks, Alpine-type peridtite, and majr cmpressinal tectnic features in greenstne belts. Perhaps because f lss f heat due t extensive vlcanism during Archean time, the mantle cled, cnvectin became shallwer, and there was separatin f the present-day mantle surce f cean-ridge basalt and the mantle surce f alkali basalt. The Rb-Sr and Pb-Pb ages wuld suggest that sme parts f the mantle were islated nly fr the last 1,000 m.y. Perhaps the depth f cnvectin grew shallwer until at least that time. The 1,000- t 3,000-m.y. ages fr the present-day mantle surce fr alkali basalt and nephelinite may thus represent the last time f cnvectin r beginning f islatin f different parts f the Earth's mantle, which may r may nt be related t a time f special tectnic activity. In any case, it wuld appear that after Archean time, the style f cnvectin changed and became mre similar t that f the present. GEOLOGY 301

6 REFERENCES CITED Anhaeusser, C. R., Masn, R., Viljen, M. J., and Viljen, R. P., 1969, Reappraisal f sme aspects f Precambrian shield gelgy: Gel. Sc. America Bull., v. 80, p Armstrng, R. L., 1968, A mdel fr the evlutin f strntium and lead istpes in a dynamic earth: Rev. Gephysics, v. 6, p Armstrng, R. L., and Hein, S. M., 1973, Cmputer simulatin f Pb and Sr istpe evlutin f the Earth's crust and upper mantle: Gechim. et Csmchim. Acta, v. 37, p Arth, J. G., and Hansn, G. N., 1975, Gechemistry and rigin f the early Precambrian crust f nrtheastern Minnesta: Gechim. et Csmchim. Acta, v. 39, p Bridgewater, D., McGregr, V. R., and Myers, J. S., 1974, A hrizntal tectnic regime in the Archean f Greenland and its implicatins fr early crustal thickening: Precambrian Research, v. 1, p Cke, D. L., and Mrhuse, W. W., 1969, Timiskaming vlcanism in the Kirkland Lake area, Ontari, Canada: Canadian Jur. Earth Sci.,v. 6, p Cper, J. A., and Richards, J. 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Hdges prvided fruitful discussins, and S. S. Gldich supplied P2O5 analyses fr sme f the samples. MANUSCRIPT RECEIVED JANUARY 29, 1975 MANUSCRIPT ACCEPTED APRIL 2, PRINTED IN U.S.A. JUNE 1975