Sedimentary Geology, 52 (1987) Department of Earth Sciences, Nagoya Universi(~, Chikusa, Nagoya 464 (Japan)

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1 Sedimentary Gelgy, (987) Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands GEOCHEMICAL CHARACTERISTICS AND DEPOSITIONAL ENVIRONMENTS OF CHERTS AND ASSOCIATED ROCKS IN THE FRANCISCAN AND SHIMANTO TERRANES KOSHI YAMAMOTO * Department f Earth Sciences, Nagya Universi(~, Chikusa, Nagya 464 (Japan) (Received Nvember 8, 98; revised and accepted July 4, 986) ABSTRACT Yamamt, K., 987. Gechemical characteristics and depsitinal envirnments f cherts and assciated rcks in the Franciscan and Shimant Terranes. Sediment. Gel., : 6-8. Analyses f Jurassic-Cretaceus rck samples frm the Franciscan Terrane, Califrnia, and 4 Cretaceus samples frm the Shimant Terrane, suthwest Japan, shw that the cherts and assciated rcks accmpanied by pillw basalts are enriched in Fe and Mn which may have been derived frm hydrthermal activities. The results f the fractin analyses fr terrigenus, basaltic, hydrthermal, and bigenic end-members in the Franciscan rcks shw that hydrthermal emanatins caused irn and manganese enrichment in the rck samples and played an imprtant rle in the frmatin f siliceus rcks. Vertical variatins f the hydrthermal fractin and TiO-nrmalized values fr hydrthermal elements such as Fe, Mn, Ni, Zn, Y and Pb shw a pssibility that the bedded cherts f the Franciscan Terrane were depsited in a hydrthermal field n r near sme active ceanic ridge, fr mre than Ma and were a remnant slice f ceanic crust added t the Nrth American cntinent. On the ther hand, frm the field ccurrences and vertical variatins f TiO-nrmalized values f hydrthermal elements, the Shimant rcks are inferred t have been depsited in a hemipelagic envirnment such as marginal and inter-arc basins. INTRODUCTION The rigin and the depsitinal envirnments f bedded cherts in rgenic belts have been widely debated. Bedded cherts ccur nt nly lying ver philites (Barrett, 98) but als intercalated in clastic rcks such as shales (e.g. Adachi, 976). Sugisaki et al. (98) examined Triassic bedded cherts f the latter type frm the Min Terrane, central Japan, and cncluded as fllws; () the bedded cherts are cmpsed mainly f remains f siliceus rganisms and cntain small amunts * Present address: Department f Earth Sciences, Kbe University, Nada, Kbe 67, Japan. 7-78/87/$. 987 Elsevier Science Publishers B.V.

2 I 66 f detrital materials; and () the cherts were depsited in a hemipelagic envirnment such as a cntinental slpe r marginal basin. Barrett (98) pinted ut that the Jurassic bedded cherts verlying philites in the nrth Apennines, Italy, cntain SausaLit =m 6! i! i. *W 4"W U.S.A. -6 i- i - - IF =~--~ z ~= - - (m) Bedded Chert JA - t ~ / Kin. "E ;4"E () t i grn - - i~ Shale ~ Chert --(m) ~ BasaltLimest ne Fig.. Sampling lcatin (circle) and clumnar sectin. Hrizntal bar n the left side f the clumnar sectin indicates sampling hrizn fr shale, and that n the right fr chert and basal Basalt.

3 67 hydrthermal cmpnents and presumably were depsited in a pelagic envirnment. The examinatin f samples recvered by the Deep Sea Drilling Prject (DSDP) demnstrated that metalliferus depsits smetimes ccur n cean-flr basalts arund cean ridges such as the East Pacific Rise and the Mid-Atlantic Ridge (e.g. Piper, 97). These metalliferus depsits are accepted t be frmed by hydrthermal activities alng active spreading centers. These hydrthermal depsits are remved frm the cean ridge with the mving ceanic plate and buried under pelagic clays and/r remains f siliceus rganisms (e.g. Crnan, 976). A series f sediments frmed by this prcess is ften cmpared with bedded cherts accmpanied by metalliferus depsits verlying philites in rgenic belts. Sme researchers assumed, n the basis f the gelgical and petrlgical similarity f bth sediments, that philites are bducted fragments f ld ceanic crust erupted at an cean ridge (e.g. Bnatti et al., 976). Althugh the gechemical study f the rigin f philites has mstly been fcused n basalts (e.g. Miyashir, 97) r metalliferus depsits (e.g. Crerar et al., 98), the chemical characteristics and depsitinal envirnments f siliceus rcks verlying philites must als be an imprtant key t the nature and emplacement envirnment f the philites. In this study, I have analyzed cherts and shales accmpanied by pillw basalts in the Franciscan Terrane, Califrnia, and in the Shimant Terrane, suthwest Japan (Fig. ). The gechemical features f the siliceus rcks assciated with basalt and the hydrthermal cntributin t their frmatin are discussed in this paper. Mrever, the depsitinal envirnment f the bedded cherts was examined by means f chemical cmparisn f the bedded cherts frm the Franciscan Terrane with marine sediments and hydrthermal depsits f varius envirnments. The Franciscan and the Shimant Terranes ccur n bth sides f the Pacific Ocean, and the gelgic structures f bth have ften been cmpared with each ther. Nevertheless, their frmatin prcesses and the gechemical characteristics f cherts in these terranes are still very prly understd. In this study, I discuss the frmatin prcesses f bth terranes n the basis f gechemical data frm siliceus rcks. SAMPLING LOCATION AND SAMPLE DESCRIPTION The Franciscan Terrane The Franciscan Terrane is ne f the majr gelgic units in the Califrnia Cast Ranges f western Nrth America; its gelgy has been studied extensively by many wrkers. Accrding t the current pinin (Blake and Jnes, 974), the Franciscan assemblage is divided int three nrth-suth-trending belts, namely Castal, Central, and Ylla Blly, which are separated by east-dipping thrust faults. The Castal belt is the westernmst unit and is cmpsed mainly f flysch-like graywacke sandstne. The Central belt is cmpsed primarily f graywacke sandstne, shale, minr

4 68 amunts f chert and assciated basalt, gabbr, and serpentinite. The Ylla Blly belt is cmpsed mainly f graywacke and metagraywacke with minr chert and basalt (e.g. Blake and Jnes, 974). Cherts and assciated rcks were sampled in cuttings alng a rad t Sausalit,. km nrth f the Glden Gate Bridge (Fig. ) in the Central belt, where red and pale-green bedded cherts are well expsed. The bedded cherts are bserved at tw utcrps, 4 m apart. The lwer cherts lie cnfrmably n basalt, are 7 m thick, and are chclate brwn t reddish brwn in clr. The upper cherts, 8 m thick, are nt assciated with basalt and are pale green in clr. Chert layers are generally frm t cm thick and average abut cm; the shale interlayers are generally much thinner and average less than cm. Eighty-eight chert, 8 shale partings, and 6 underlying basalt samples were cllected frm these utcrps. The basal pillw basalt cnsists essentially f medium t carse-grained pyrxene and plagiclase, with minr amunts f secndary pumpellyite and chlrite. It shws well-develped phitic texture. The chert is cmpsed largely f micrcrystalline quartz with lesser amunts f hematite and clay minerals, whereas shale parting is cmpsed mstly f clay minerals, hematite, and micrcrystalline quartz. Hematite tends t be mre abundantly disseminated in samples f the lwer hrizns. Bth chert and shale cmmnly cntain varying amunts f radilarian remains, but three samples f basal chert (ns. 9-) cntain n radilarians. These cherts have an exceptinally large amunt f hematite and are veined by megaquartz. Radilarian remains are cmpsed mainly f chalcednic quartz. Chclate-brwn cherts abut m abve the basement (ns. 7 and 84) yield radilarians f the Lwer-Middle Jurassic such as Parahsuum sp. cf. P simplum (Ya, 98) and Pachyncus? sp. On the cntrary, pale-green cherts abut 6 m abve the basement (ns. and ) cntain Lwer Cretaceus radilarians such as Sethcapsa sp. and Napra sp. f Pessagn (977). Radilarians were identified by H. Yshida f the Department f Earth Sciences, Nagya University. The Shimant Terrane The Shimant Terrane extends frm suthern Kant t suthern Kyushu, alng the Pacific cast f suthwest Japan. In the Kii Peninsula f suthwest Japan, the Shimant Terrane can be divided int three belts frm nrth t suth, namely, the Cretaceus Hidakagawa, the Ecene (?) Otnashigawa, and the Oligcene-early Micene Mur belts. The Hidakagawa belt is the nrthernmst unit and is cmpsed mainly f sandstne, shale, alternatin f sandstne and shale, and minr amunts f chert and basalt. The latter tw belts are cmpsed largely f flysch-like alternatins f sandstne and mudstne, sandstne, mudstne, and cnglmerate (Nakazawa et al., 98). Twelve chert, 7 shale, and basalt samples were cllected frm the Idani area f the Hidakagawa Belt, km sutheast f Wakayama City (Fig. ). Chert and shale

5 69 verlying pillw basalt d nt shw a bedded structure, but are massive and intercalate a limestne-basalt layer f m thick. The basalt is cmpsed mainly f fine-grained plagiclase and pyrxene, and rarely shws sub-phitic texture. It als cntains minr amunts f secndary calcite, prehnite, and pumpellyite as veinlets. Calcite pseudmrphs after livine are als bserved. Chert is chclate brwn and is cmpsed mainly f micrcrystalline quartz, paque minerals, and clays with a number f radilarian tests filled with chalcedny. Shale is cmpsed predminantly f clay minerals, varius amunts f radilarian remains, and small amunts f paque minerals. Bth chert and shale are cmmnly veined by calcite, chlrite, and/r quartz. Althugh few micrfssils indicating gelgic ages are preserved in the Shimant rcks f this area, Nakazawa et al. (98) reprted that they were depsited in the Cretaceus. ANALYTICAL METHOD Samples were grund int < mesh with an agate mrtar. Majr and minr cmpnents were analyzed n an autmatic X-ray flurescence spectrmeter, JEOL JSX-S, accrding t the methd by Sugisaki et al. (977, 98). Ferrus irn, CO, and H were als determined by the methd f Sugisaki (98a). The analytical results are listed in Appendi~es and fr the samples frm the Franciscan Terrane and the Shimant Terrane, respectively. The analytical data were recalculated n water, carbnate-, and thers-free basis. The recalculated values are used in the fllwing discussin. GENERAL CHARACTERISTICS OF CHERT OVERLYING BASALT Cherts are generally classified int thse assciated with basalts and thse assciated with ther rcks. The average chemical cmpsitins f cherts frm the Franciscan and the Shimant Terranes analyzed in the present wrk are given in Table, and thse frm DSDP Leg (Adachi et al., 986), DSDP Leg 6 (Hein et al., 98), and the Min Terrane, central Japan (Sugisaki et al., 98) are als listed fr cmparisn. The first three cherts are resting n basalt and the last tw are nt. Chemical cmpsitins ther than SiO were used fr the cmparisn, because the chert is cmpsed predminantly f SiO z. A is mst prminent, except fr SiO, in the cherts nt assciated with basalt (the DSDP Leg 6 and the Min Terrane), whereas in the cherts resting n basalt (the Franciscan Terrane, the Shimant Terrane, and DSDP Leg ) Fe ~ (ttal irn as Fe) is twice as high as A, and much MnO is cntained. Thugh the minr-element cmpsitins f bth grups are apprximately similar t each ther, the cherts assciated with basalt are enriched in Ni, Zn, Y and Pb. Chemical cmpsitins f the shale clsely assciated with the chert are imprtant fr the examinatin f the chemical characteristics f the chert (Sugisaki et

6 7 TABLE Average chemical cmpsitins f cherts frm the Franciscan Terrane, the Shimant Terrane, the DSDP Leg 6, and the Min Terrane and f cherts and prcetlanites frm the DSDP Leg, with their standard deviatins (water-, carbnate-, and thers-free basis) Franciscan Shimant DSDP Leg DSDP Leg 6 Min SiO 9.6_ _ _ TiO _.%.7._+, AI _ , Fe* _-4-.7 MnO.8-+.7,78_ MgO._+.7.9_ CaO.-+.6._ Na.6± _+.4.6_+.9 K O _+.. ±.4 PzO~ _+. FeO.6_+..7_+.46,6_+.4.7_+. Fe.67_+.7.±..68_+.9._+.8 Cr 9. _ _+ 4.. _+. C. ± _+ 4.8 Ni 6. _ _+ 4. 6, _ _+. Zn _ _+. Rb 9. _ _+ 7,. _+ 4. Sr. _+. 4. ± 6.. _+ 7.. _+. Y 9. ±.. _+., _+. Zr Nb. -+.,7 ±..7 _+.4 M Pb ± 6. Th , Ba _+. 94, _4-, 9. _+4. Ga _ N. f samples 9.9±.46.±..97±,.9± :.. ±.9, ~.4.6 ±.8.±.9.6 ±.8 4+ ±.. ± :. 9. ± ± f 8.4. ±..7 4~,9 g. ± ±.. 4. ± 8.. ±. 7 Elements in ppm; cmpunds in %. al., 98). The average chemical cmpsitins f the shale partings in bedded cherts in the Franciscan and the Min Terranes and f massive shales in the Shimant Terrane are shwn in Table. Average shale published by Turekian and Wedephl (96) is als shwn in the table. The shale partings frm the Franciscan Terrane have SiO cntents similar t that f average shale. Hwever, the SiO cntents f the shales frm the Shimant and the Min Terranes are 6-% higher than thse f the Franciscan shales and average shale; this is explained by the difference in the chemical cmpsitin f detrital materials cntained in them and/r by the additin f SiO derived frm

7 7 TABLE Average chemical cmpsitins f shales frm the Franciscan, the Shimant, and the Min Terrane and f average shale with their standard deviatins (water-, carbnate-, and thers-free basis) Franciscan Shimant Min Shale SiO ± _ TiO ±.89.7_ AI ± ±.7. Fe ~ MnO.7+.7.±.9.8±.7. MgO.+.6. ± CaO.9±.4.47_+..6_+.4.9 Na _ K P FeO Fe 4.69± _ Cr ±. 68, C Ni 7. ± Zn _ Rb ± Sr 4. ± Y ± Zr ± Nb ±.4. M _..6 Pb ± Th 4. +_ ± Ba Ga 9, _ N. f samples Cmpunds in %; elements in ppm. radilarian tests. Shales frm the Franciscan and the Shimant Terranes are als enriched in Fe ~' and MnO, and are depleted in A. Enrichment f Fe ~' and MnO, therefre, seems t be an intrinsic character f cherts and shales resting n basalt. The shales assciated with basalt in the Franciscan Terrane are als characterized by enrichment f Pb and Ba. Cmpsitins f the cherts and shales frm the Franciscan and the Shimant Terranes are pltted n the SiOz-AlzO and the SiO-Fe ~' diagrams (Fig. ). As reference data, samples frm several lcatins--dsdp Leg, the Min Terrane, and averages f mdem argillaceus sediments frm varius envirnments such as nearshre, marginal, and pelagic--are pltted.

8 7 Clear negative crrelatins between SiO and A and between SiO and Fe ~ are fund n the plts fr the cherts and shales frm the Min Terrane, which are nt assciated with ba~ts. The plts cluster alng a line cnnecting the pints f average marine sediments and that f pure SiO. This fact and micrscpic bservatin shw that the chert is regarded as a mixture f bigenic SiO and detrital fragments represented by marine argillaceus sediments (Sugisaki et al., 98; Yamamt, 98); namely the chert can be cnsidered t be bigenic. On the ther hand, pints fr the chert and shale resting n basalt (the Franciscan Terrane, the Shimant Terrane, and DSDP Leg ) fall int a field different frm that fr the Min Terrane, and the relatins between SiO and Al and between SiO and Fe~O~ are nt clear. This might shw that detrital materials cntained in the chert and shale n basalt are chemically different frm thse nt accmpanied by basalt. As a surce material f the chert and shale n basalt, elastic materials riginating frm the underlying basalt shuld be taken int accunt; that is, the siliceus rcks may have been frmed by the mixing f basalt-elastics and bigenic silica. Nevertheless, the data pints fr the samples resting n basalt deviate smewhat dwnward fr A and upward fr Fe ~ frm the mixing line (( AI l ~ z (%) \. \ 4 & A A "x \, O~ O \, ~, \ \ O & %a " /., (%)

9 7 (b ( Fe ~ z (%). 8 ~ A 6,~, O~C 8 ~ 8 A ~i A ii A & z,, ('/,) Fig.. Plts f SiO against A and ttal irn as FeOa(Fe~). Sample symbls: big slid circles = Franciscan cherts; big pen circles = Franciscan shales; slid triangles = Shimant cherts; pen triangles = Shimant shales. Other data pints: slid and pen squares = cherts and prcellanites frm the DSDP Leg Site 4, respectively (Adachi et al., 986); small slid and pen circles = cherts and shales frm the Min Terrane, respectively (Yamamt, 98); stars = average mdern marine sediment -- = the Nishitsugaru Basin in the Japan Sea, 8 samples (Sugisaki, in press); = the Yanlat Bank in the Japan Sea, samples (Sugisaki, 979); = the Japan Trench, DSDP Legs 6 and 7, samples (Sugisaki, 98); 4 and = arund the Izu-Ogasawara Trench, 4 and samples, respectively (Sugisaki and Kinshita, 98); 6 = the nrthern Central Pacific Basin, samples (Sugisaki, 98b); 7= the central Pacific transect, Wake t Tahiti, 6 samples (Sugisaki and Kinshita, 98). B represents the average value f basalt (Turekian and Wedephl, 96). Regressin lines are calculated by the methd f least squares fr Min cherts and shales. cnnecting the pure silica with the basaltic cmpsitin (Turekian and Wedephl, 96). This suggests that these samples were derived nt nly frm bigenic silica and basalt-clastics, but als frm ther surces. HYDROTHERMAL CONTRIBUTION TO CHERT AND SHALE The mst prblable surce f sme elements fr these samples resting n basalt is hydrthermal emanatin. Sedimentary rcks verlying basalt are extremely enriched

10 74 AI./ \ ',\ Fe " " Mn AI (b),~ N.,4 N.~ \\ r /,OO: O:O. /. : I*. / "...," "%.. -. N -a[ " Mn ~.... ~ Fe Fig.. A-Fe-Mn diagram shwing the effect f hydrthermal emanatins. The cncentratin f each crner is nt xide but element. Sample symbls: big slid circles = Franciscan cherts; big pen circles = Franciscan shales: slid triangles = Shimam cherts; pen triangles = Shimant shales. Other data pints: small slid circles = nn-hydrthermal cherts and shales frm the Min Terrane (Yamamt. 98); stars=averaged value f nn-hydrthermal mdern marine sediments--/= the Nishitsugaru Basin in the Japan sea, 8 samples (Sugisaki, in press); = the Yamat Bank in the Japan Sea, samples (Sugisaki, 979); = the Japan Trench. DSDP Legs 6 and 7. samples (Sugisaki, 98); 4 and = arund the Izu-Ogasawara Trench, 4 and samples, respectively (Sugisald and Kinshita. 98); 6 = the nrthern Central Pacific Basin, samples (Sugisaki, 98b); 7= the central Pacific transect, Wake t Tahiti, 6 samples (Sugisald and Kinshita, 98); asterisks = averaged values f hydrthermal depsits arund active cean ridge l = East Pacific Rise depsit, grup b, 9 samples (Bstr~m and Petersn, 969); = East Pacific Rise depsit, grup a, samples (BsttOm and Petersn. 969); = East Pacific Rise depsit, 8 samples (Piper, 97); 4 = nntrnites in Galapags Munds sediment, DSDP Leg 7, 88 samples (Mrby and Crnan. 979); = sediment frm the TAG field.

11 7 in Fe ~ and MnO (Tables and ). Adachi et al. (986) cncluded that the DSDP Leg chert, just n basalt, was derived frm hydrthermal fluids. This chert is cmpsed mainly f hematite (partly mre than %) and megaquartz cntaining many fluid inclusins and is markedly enriched in Fe ~" and MnO. Adachi et al. (986) named the chert "Hydrthermal Chert", and its chemical characteristics appear t have affinity with thse f the cherts frm the Franciscan and the Shimant Terranes. The cherts recvered by DSDP Leg are used as reference data in the fllwing discussin. The effect f hydrthermal emanatin was examined n the A-Fe-Mn diagram (Fig. ), which was prpsed by BstriSm and Petersn (969). Als pltted n this diagram are hydrthermal depsits arund the East Pacific Rise (BstriSm and Petersn, 969; Piper, 97), the Mid-Atlantic Ridge (Shearme et al., 98), and the Galapags Munds (Mrby and Crnan, 979). Pints fr the cherts and shales frm the Min Terrane and fr mdern marine sediments which are free frm hydrthermal effects fall int the small Al-rich and Mn-pr field. Pelagic sediments are generally enriched in hydrgenus elements such as Mn, C, and Ni (Sugisaki, 984) and this enrichment can be attributed t micr-manganese ndules (Ohashi, 98). The enrichment f Mn in pelagic sediments, hwever, is nt salient n this figure. On the ther hand, the cherts and shale partings frm the Franciscan Terrane, ccupy the field rich in Fe and Mn. Especially the cherts just n the basalt (ns. 9-) are remarkably enriched in Fe and clse t the Fe-apex. As the cherts are separated vertically frm the basalt, the Fe and Mn cntents becme less. Cherts frm the uppermst hrizn (ns., 4, and 6) fall int the same field as that f the mdern marine sediments. Representative hydrthermal depsits frm the East Pacific Rise, the Mid-Atlantic Ridge, and the Galapags Munds, which are interpreted as direct precipitates frm hydrthermal slutin, are als generally enriched in Fe and Mn. The Canadian American Seamunt Expeditin (98) fund amrphus silica f hydrthermal rigin arund the Juan de Fuca Ridge. All these facts shw that the cherts and shales frm the Franciscan Terrane have been subjected t hydrthermal activities in relatin t the underlying basalt. The same hlds true fr the cherts frm the Shimant Terrane. FRACTION ANALYSES OF THE FRANCISCAN BEDDED CHERTS In additin t the hydrthermal cnstituents, the cherts and shales frm the Franciscan and the Shimant Terranes cntain remains f siliceus rganisms and clastic detritus, as mentined in the sample descriptin. If we regard these three cnstituents as end-member cmpnents and pstulate their chemical cmpsitins, the fractin f each cnstituent in each sample can be calculated. Fractins were analyzed accrding t the fllwing equatin with the least squares methd.

12 76 l Y'. a(i, j).b(j, k)= c(i, k) j=l i=,... n; k=l,... m where a(i, j) represents the cntributin f the jth end member in the ith samples, b(j, k) is the cncentratin f the kth dement in the jth end member, and c(i, k) is the cncentratin f kth dement in the ith sample, n is the number f samples, m the number f dements and the number f end members. The chemical cmpsitin f each end member was estimated by the fllwing prcedure: () The A/(AI + Fe + Mn) rati is an indicatin f a hydrthermal cntributin t sediments (BstriJm and Petersn, 969). Fr example, precipitates frm hydrthermal emanatins ccurring n the East Pacific Rise are characterized by ratis as lw as. (Bstr~Sm and Petersn, 969), whereas the average shale (Turekian and Wedephl, 96) and the bedded cherts f the Min Terrane (Sugisaki et al., 98; Yamamt, 98) shw the ratis f.6 and.6, respectively. That is, the A/(AI + Fe + Mn) rati in sediments decreases with increasing hydrthermal input t the sediments. The A/(AI + Fe + Mn) rati f the Franciscan bedded cherts increases upward frm the lwest hrizn (Fig. 4). This shws that the stratigraphically lwer samples had received an intense hydrthermal input. The basal chert (n. ) resting just n the basalt shws a rati as tw as.4, which is clse t the value fr the metalliferus depsit n the East Pacific Rise. Micrscpic bservatin shws that the basal Franciscan chert is cmpsed mainly f hematite and megaquartz and includes n radilarian remains. These characteristics are the same as thse f the hydrthermal chert described by Adachi et al. (986). N. basal chert can be assumed t be cmpsed exclusively f hydrthermal cnstituents. Thus, I have used it as a hydrthermal end-member. () Radilarians extracted frm the recent radilarian ze in the central Pacific cmprise nearly pure silica (Yumamt, 98). I assumed that the bigenic endmember is pure silica (SiO = %). () In rder t estimate the cmpsitin f the detrital end-member, the At/TiO rati was pltted against the vertical distance frm the basal basalt (Fig. 4). Since the AI and TiO cntents f the hydrthermal end-member are nly. and.4%, respectively, and they are nt cntained in the bigenic end-member, the A/TIO rati f each sample depends mainly n the cmpsitin f elastic detritus in it. If the cmpsitin f clastic detritus in each sample is unifrm, the AlzO/TiO rati des nt fluctuate. Fr example, the AI/TiO rati f the bedded cherts frm the Min Terrane des nt fluctuate much stratisxaphieauy and the average (. +.) is clsely similar t the average shale. This shws invariance f the elastic cmpsitin thrughut the st ratigraphic sectin (Sugisaki et al., 98). On the cntrary, the A/TiO rati f the

13 77 AII(AI FeMn)..~.6 6., ~ 6,,d~, t, ALTi de~ I O Fractin O, im ~!.~. n m t I ~ 6 ~ a' / ',n / / / A,' / / U - lid ~ I. O.~.. e~e ;' ~t,' # "~e ~'~ E Tf ~, (m) J g... h / &/ 4L," 7" : Fig. 4. Vertical variatins f AI/(AI+ Fe + Mn) and Al//TiO ratis fr cherts and shales and thse f detrital, bigenic, and hydrthermal fractins fr cherts frm the Franciscan Terrane. Clsed and pen circles represent chert and shale, respectively. The asterisks represent average value f basal basalt. If an end-member in a sample calculated frm the fractin analysis shws a rate less than -., the result fr the sample is nt used. If an end-member shws a rate less than and mre than -., the cntent f the end-member is assumed t be and the ther tw are recalculated. Pltted pints f each fractin are traced by a dtted line, fr cnvenience, and each area divided by the lines represents degree f the cntributin f the end-member. Franciscan bedded cherts increases upward frm the lwest hrizn (Fig. 4). The lwest and the uppermst cherts shw ratis f - and -, respectively. This suggests that the cmpsitin f clastic detritus in the bedded cherts is nt unifrm but varies with the hrizn, and the underlying basalt is a pssible surce f the clastics. The average shale (Turekian and Wedephl, 96) and the Franciscan underlying basalt have AIOa/TiO ratis f.9 and 4., respectively. The A/TIO rati f the average shale is equivalent t that f chert frm the upper hrizn, whereas that f the basalt is equal t that f the lwest hrizn. This shws that clastics in the bedded cherts f the lwest hrizn were derived mainly frm the basal basalts and thse f the uppermst hrizn were frm cmmn terrigenus

14 78 materials. Thus, the basaltic detritus and terrigenus materials were added t the end members fr fractin analyses. The average cmpsitin f basal basalts (ns. -) was used as basaltic end-member and the cmpsitin f n. 9 shale (the uppermst shale) was adpted as terrigenus end-member. This shale shws an AEO/TiO rati f 7., which is a little lwer than that f chert frm the uppermst hrizn. N. 9 shale lies in a hrizn abut 7 m abve the basal basalt and is inferred t cntain n basaltic detritus. Under the micrscpe, this shale is cmpsed mainly f clay minerals and cntains n hydrthermal hematite and quartz; detectable radilarian remains are few, if any. The hydrgenus cnstituents were nt used fr fractin analyses, because the cntributin f hydrgenus cmpnent t marine sediments seems t be small (Fig. ). Accrdingly, the fur end members; hydrthermal, bigenic, basaltic and terrigenus, were emplyed in fractin analyses. This calculatin methd is simplified frm that prpsed by Dymnd et al. (984). The number f elements, n, in the calculatin is 4, invlving majr and 4 minr elements. The calculated results f fractin analyses are shwn fr chert alne in Fig. 4. Vertical variatin f the Al//TiO rati shws that the basaltic fractin decreases upward. Since the cntributins f the basaltic and terrigenus detritus are s small, the ttal f bth is shwn as detritial fractin in Fig. 4. The vertical trend f each fractin can be recgnized, althugh each fractin shws a relatively large fluctuatin. Pltted pints f each fractin are traced by a dtted line, fr cnvenience, and each area divided by the lines represents the degree f the cntributin f each end member. The detrital fractin increases frm the lwer t the upper hrizn. It is at mst. and the clastic detritals played nly a minr rle in the frmatin f the cherts. The bigenic fractin increases remarkably upward. It amunts t.4-.6 in the lwer (frm t m abve the basalt) cmpared t.8 in the uppermst hrizn. Siliceus rganisms played the mst imprtant rle in the frmatin f cherts, as a whle. The cherts mre than 6 m abve the basal basalt are cmpsed exclusively f bigenic materials with small amunts f clastic detritus. These cherts appear t be f the same genesis as thse frm the Min Terrane (Suglsaki et al., 98), which are nt assciated with basalt. The hydrthermal prtin amunts t mre than % within a hrizn 6 m thick frm the basal basalt, and it shws marked upward decrease. In the lwer prtin f the hrizn, the hydrthermal fractin cntributes t the chert frmatin mre significantly than des the detrital fractin. The hydrthermal fractin, althugh small, is recgnized up t the 6 m hrizn. 4 m-thick layers between them are nt expsed. Fr cmparisn, fractins were anal~ fr the cherts and prceltanites frm DSDP Leg Site 4 and the results are shwn in Fig.. Analyses f the basal

15 79 AI/(AI+Fe+Mn) ld..~.6 AI/TiO Fractin,. I 8 l O O [ / L 8 E u - 6O D~P ~ rrn 'L I C.9 / J "I "7-6 /. u elm it,' / (m) Fig.. Vertical variatins f A/(A + Fe + Mn) and A/TiO ratis fr the cherts, prcellanites, and shales and thse f detrital, bigenic, and hydrthermal fractins fr cherts and prcellanites frm the DSDP Leg Site 4. Slid and pen circles, and pen squares represent chert, shale, and prcellanite, respectively. The methd f calculatin is the same as that used fr Fig. 4. chert and uppermst shale (ns. 6 and 8 in Adachi et al., 986) and basal basalt (Marshall, 97) were used as the hydrthermal, terrigenus, and basaltic end-member, respectively. The number f elements used fr the calculatin was 6; the elements are cmmn t the present wrk and the wrks by Marshall (97) and Adachi et al. (986). The variatin f the hydrthermal fractin fr samples frm Site 4 (Fig. ) is smewhat different frm that in the Franciscan cherts; that is, the hydrthermal fractin decreases upward but increases arund the 4 m hrizn. Similarly, the detrital fractin als increases slightly arund this hrizn but the A/(A + Fe + Mn) rati, as an indicatin f hydrthermal activity, des nt change significantly. These facts suggest that the increase f the hydrthermal fractin arund the 4 m hrizn resulted frm a relative decrease f bigenic activities. The hydrthermal fractin in samples frm t 9 m abve the basal basalt amunts t abut., suggesting participatin in the chert frmatin. The cherts frm the Shirnant Terrane shw A/(A + Fe + Mn) ratis as large

16 8 as ver.9 (Fig. 9), and the hydrthermal end-member cannt be assumed. Fractin analysis is, therefre, impssible. CHEMICAL CHARACTERISTICS OF HYDROTHERMAL COMPONENT The fractin analysis shws that the siliceus rcks f the Franciscan Terrane cntain significant amunts f hydrthermal cnstituents characterized by the enrichment f Fe and Mn. Varius elements ther than Fe and Mn may be cncentrated in the hydrthermal cmpnents. The characteristics f the hydrthermal cmpnents were examined n the basis f the data fr the samples within m frm the basal basalt, which were subjected t intense hydrthermal activity. As shwn in Table, Fe ~ crrelates psitively with varius elements ther than SiO, CaO, and Na; n the ther hand MnO crrelates with thse except fr SIO, CaO, Na, FeO, Rb, and Sr. Varius elements, therefre, seem t behave, with Fe and/r Mn, as hydrthermal cmpnents. On the ther hand, SiO crrelates negatively with almst all elements. On the SiO-A and the SiO - Fe ~' diagrams (Fig. ), the pltted pints f the Franciscan cherts appear t cnverge apprximately at SiO = %. The feature is attributed t a prcess in which clastics and hydrtherrnal cmpnents are mixed with bigenic silica. Accrdingly, these apparent crrelatins between elements may have resulted frm the difference in the mixing rati. Thus, the cntributin f silica derived frm siliceus rganisms shuld be excluded, and hence the TiO,nrmalized values fr varius elements were used fr this purpse. The TiO-nrmalized values are useful indicatins f the chemical characteristics and depsitinal envirnments f siliceus rcks (Sugisaki, 984). With these values we can examine the chemical cmpsitin f detrital and hydrthermal cmpnents, excluding the bigenic cntributin. In rder t cmpare sedimentary rcks cntaining hydrthermal cmpnents with thse withut the cmpnents, averages f TiO-nrmalized values fr siliceus rcks frm varius lcatins and average shale are listed in Table 4 with their standard deviatins. The TiO-nrmalized values fr majr elements f cherts frm the Min Terrane and DSDP Leg 6 are similar t thse f average shale, except fr the SiO/TiO: rati. Since elements ther than SiO are derived mainly frm clasties (Sugisaki et al., 98), the elastics in cherts frm the Min Terrane and DSDP Leg 6 are chemically similar t average shale. As fr the TiO-nrmalized values fr minr elemnts, the Min cherts have Cr, Zn, M, and Ba values larger than the average shale. Cmpared with the Min Terrane, the Franciscan cherts and shales have larger TiO:-nrmalized values fr F~O~', MnO, Ni, Zn, Y, Pb and Ba. Tw alternative explanatins may be pssible. First, elastics cntained in the Franciscan samples differ chemically frm thse in the Min cherts and average shale. Secndly, since the hydrthermal cmpnent is enriched in these elements and depleted in TiO, the

17 SiO z TiO A Fe ~" MnO MgO CaO Na K P FeO Fe Cr Fe ~ , MnO ,67 -, , ,9.78 SiO , ,964 C Ni Zn Rb Sr Y Zr Nb M Pb Th Ba Fe ~' , ,6.6, MnO SiO Ga.8,, ,967 TABLE Crrelatin cefficients between Fe ~, MnO, and SiO and ther cmpnents in Franciscan cherts and shales within m frm the basal basalt

18 8 TABLE 4 TiO-nrmalized values fr Franciscan cherts and shales within m frm the basal basalt, the Min cherts, the DSDP Leg 6 cherts, and average shale, with their standard deviatins Franciscan Min DSDP Leg 6 shale SiO 7. _ _+. 7. A._+.6._+. 9._+.. Fe ~ 9._ _ MnO _+.7. MgO CaO _+.6 7._ Na._ _+.7.7 K._+.7.6_ P. -+., Cr _+9 C 4 + _ Ni Zn _ _+ 8 Rb _+ 8 Sr _ 4 9 Y Zr 4 _ Nb _ M _+ Pb 4 _ _+ 7 6 Th.%_+ 4 8 _+ 6 Ba _+ 7 7 _+ 4 7 Ga _+ 7 _4_ N. f samples TiO-nrmalized values fr these elements f Franciscan siliceus rcks are inevitably enhanced. If an element is abundant in the hydrthermal cmpnent, the TiO:-nrmalized value f the element crrelates with that f Fe ~ r MnO; as described earlier, the hydrthermal cmpnent is enriched in Fe and Mn, and the large TiO-nrmalized values f Fe ff and MnO result frm the cntributin f hydrthermal cmpnents. As shwn in Table, the crrelatin cefficients between Fe~/TiO and TiO-nrmalized values fr SiO, Cr. C, Zn, M and Pb and thse between MnO/TiO and TiO-nrmalized values fr Ni, Sr and Y are all greater than.z Amng these elements, the TiO-nrrnalized values fr SIO, Cr, C, M and Sr f the Franciscan cherts and shales are nearly the same as thse f the Min bigenic cherts (Table 4); accrdingly, it des nt necessarily fllw that hydrthermal cmpnents are enriched in these elements. On the ther hand, the TiO-nrmalized

19 Crrelatin cefficients between TiO-nrmalizcd values fr Fe ~" and MnO and thse fr ther cmpnents in Franciscan cherts and shales within m SiO AI Fe ~ MnO MgO CaO Na K P FeO Fe.~ Cr Fe ~' MnO C Ni Zn Rb Sr Y Zr Nb M Pb Th Ba Ga Fe~ MnO , ,8 -.7 TABLE frm the basal basalt

20 84 % C x I--- N g % x j-. Z % A A & A ~ A 8" ~,^& A % x I-- % x & e %. Q. & tp A i Fe~/Ti ~,6 MnO/Ti Fig. 6. Plts f TiO-nrmalized values fr Zn, Pb, Ni and Y against Fe~'/TiO r MnO/TiO ratis. Sample symbls: slid circles = Franciscan cherts and shales within m frm basal basalt; pen circles - Shimant cherts and shales; triangles = DSDP Leg Site 4 cherts and prcellanites. values fr Zn, Ni, Y and Pb f the Franciscan rcks are much larger than thse f the Min rcks (Table 4). This suggests that hydrthermal cmpnents are enriched in these fur elements. Figure 6 shws the plts f TiO-nrmalized values fr these elements against the Fe~'/TiO r MnO/TiO ratis. Fr the Franciscan rcks and DSDP rcks, the TiO-nrmali~,exi values fr Zn and Pb increase with increasing Fe~'/TiO rati. Likewise, the plts shwing the Franciscan and Shimant rcks shw similar trends n the MnO/TiO-Ni/TiO and the MnO/TiO-Y/TiO diagrams. These results shw that Zn and Pb behave with Fe, while Ni and Y fllw Mn. The Pb/TiO ratis f the Shimant rcks, hwever, are generally smaller than thse f the Franciscan and DSDP rcks at a certain Fe~'/TiO rati. The TiO-nrmalize~ values f Ni and Y f DSDP rcks are generally larger than thse f the thers at a certain MnO/TiO rati. This may be ascribed t sme difference

21 8 in hydrthermal activity such as the Eh-pH cnditin when the hydrthermal cmpnent was depsited frm the hydrthermal fluid, as described later. VERTICAL VARIATION OF TiO-NORMALIZED VALUES The vertical variatins f TiO-nrmalized values fr Fe~', MnO, Ni, Zn, Y and Pb are pltted and the variatins are cmpared with that f the hydrthermal fractin. If the stratigraphic trend f the TiO-nrmalized values fr these elements is cncrdant with that f the hydrthermal fractin, the result f fractin analyses can be reasnable, and the elements such as Ni, Zn, Y and Pb may behave as hydrthermal elements. Vertical variatin f the Fe~'/TiO rati fr the Franciscan bedded cherts Shws a trend similar t that f the hydrthermal fractin. That is, the largest Fe~'/TiO rati is fund in the chert lying immediately n the basalt, and the rati decreases upward. Thugh the TiOE-nrmalized values fr Ni, Zn and Y are relatively small within m frm the basal basalt, each decreases upward frm the m-hrizn and shws a trend similar t that f the Fe~'/TiO rati (Fig. 7). This shws the pssibility that Ni, Zn and Y are cntained mainly in an Fe-rich phase (hematite). The MnO/TiO and Pb/TiO ratis d nt shw such a clear trend as Fe~'/TiO. The vertical trend f Pb/TiO, hwever, is in gd accrdance with that f MnO/TiO, and Pb seems t be cncentrated in a Mn-rich phase. These results are apparently incnsistent with thse btained in the frmer sectin. This may be caused by the difference in the samples used fr the discussin. That is, Franciscan rcks with intense hydrthermal inputs within m frm the basal basalt were used in the frmer sectin, whereas in this sectin all samples were used fr the discussin. Fractinatin between Fe and Mn in hydrthermal depsits is bserved in varius regins. Accrding t Tth (98), the chemical stability limits f xides f Fe and Mn indicate that irn will precipitate befre manganese in an envirnment f increasing ph and xidatin ptential, and decreasing temperature, as wuld be encuntered upn mixing a hydrthermal fluid with xygenated sea water. The lw TiO-nrmalized values fr Ni, Zn and Y just upn the basal basalt may als result frm a similar prcess. The features f TiO-nrmalized values shw that the hydrthermal activity had cntinued up t the hrizn 6 m abve the basal basalt. This result is nt incnsistent with that f the fractin analysis. The TiOE-nrmalized values fr Fe ~' and Ni f DSDP rcks (Fig. 8) decrease gradually frm the lwer t the upper hrizn. The hydrthermal fractin frm the fractin analysis shws a distinct increase arund 4 m abve the basal basalt. This is ascribed t the relative decrease f rganic activity. The TiOE-nrmalized values fr Fe~", MnO, and Ni indicate the gradual and unifrm decrease f the hydrthermal cntributin frm lwer t upper hrizn. Thugh the fractinatin between Fe and Mn is recgnized in the lwer hrizn, the TiO-nrmalized values

22 86 8 r, FeTi MnOITi NilTiO xlo t 6C!$! i i m E.e.- I [ ~. '.-.. K t: U ~' -A,. <~8==* ee 8 t'~ p ee ~ ~e 6e lie % ~ g O O (ml 6 ~ Zn/TiOxl m YITi xlo PblTiOxl z e~e 6 "" ~eb ~ ;,.,,,. e ~ " 8 ; ; e -* " e e ;.. e eedp Z " :" Fig. 7. Vertical variatins f TiO-nrmatized values fr hydrthermal elements f cherts and shales frm the Franciscan Terrane. Clsed and pen circles represent chert and shale, respectively.

23 87 FeTi MnO/Ti lo Nil TiO xlo n~ Ull l 8C % = m a ul E 6C = m m C t~ OCD a ~ U L,- (m) Zn/Ti xlo 6 YITi xlo Pb/TiO xlo a a a 8c m [] a m 6 D OI D 4c ~ COO mi~= c Fig. 8. Vertical variatins f TiO-nrmalized values fr hydrthermal elements f cherts, prcellanites, and shales frm the DSDP Leg Site 4. Clsed and pen circles, and pen squares represent chert, shale and prcellanite, respectively.

24 88 AII(AI.Fe*Mn) ~..6 Fe/Ti MnO/TiO Nil TiO xlo ---7 I 'l" E e u e a O Q I " (m) g g g ZnlTiOxlO YITiO xl Pbl TiO xl ' [ 4 O 8 O Fig. 9. Vertical variatins f AI/(AI + Fe+ Mn) rati and TiO-nrmalized values fr hydrthermal elements f cherts and shales frm the Shimant Terrane. Clsed and pen circles represent chert and shale, respectively. suggest that the hydrthermal cntributin extended t the chert 84 m abve the basalt. The vrtical variatin f AI/(A + Fe + Mn) rati and TiO-nrm~ed values in the sedimentary rcks f the Shimant Terrane are given in Fig. 9. Scarcely any vertical trend is fund in these ratis. The Shimant rcks, n the whle, have lw AI/(A + Fe + Mn) ratis and high TiO-nrmalized values. The ratis f A/(A + Fe + Mn), Fe~'/TiO, and MnO/TiO are generally in the ranges f.-.,

25 89 4-9, and -, respectively. These ranges are equivalent t thse f the Franciscan rcks frm t m abve the basal basalt. This suggests that the Shimant rcks had als received an intense hydrthermal input, althugh hydrthermal chert fund at DSDP Site 4 and in the Franciscan Terrane was nt recgnized. In the hrizn abut m and -9 m abve the basalt, hwever, the shales have significantly higher A/(A + Fe + Mn) ratis and lwer TiO-nrmalized values than the cherts and shales frm ther hrizns. These lw TiO-nrmalized values agree with thse f average shale (Table 4). This shws that these shales cntain hardly any hydrthermal cnstituents. Accrdingly, it is inferred that all f the sedimentary rcks f the Shimant Terrane were nt piled up cnfrmably but had been disturbed after the sedimentatin. This may have resulted frm a cmplex repetitin f strata as discussed later. ENVIRONMENT OF ERUPTION OF PILLOW BASALT UNDERLYING CHERT Sedimentary rcks f the Franciscan and the Shimant Terranes are accmpanied by pillw basalt. The envirnment f eruptin f these pillw basalts may give imprtant infrmatin n the depsitinal envirnment f the cherts. The Ti-Zr-Y Ti xlo - Dpm I Zr pprn Y ppm Fig.. Ti-Zr-Y triangular diagram fr the basal basalt frm the Franciscan and the Shimant Terranes. H utlines the Hawaiian thleiites, O the cean-flr basalts, A the andesites and J Japanese island-arc thleiites (Pearce and Cann, 97). Sample symbls: slid circles = basal basalt frm the Franciscan Terrane; pen circles = basal basalt frm the Shimant Terrane; pen squares = inter-arc basin basalt published by Ridley et al. (974).

26 9 diagram, as a current criterin fr the discriminatin f the eruptin envirnment f basalt (Pearce and Cann, 97t), was applied t the present samples (Fig. ). Pearce and Cann (97) distinguished between cean-flr basalt, Hawaiian thleiite, Japanese island-arc thleiite, and Japanese andesite. The pints f the Franciscan and the Shimant basalt fall int the area f the cean-flr basalt field n the diagram (Fig. ). The results suggest a pssibility that the pillw basalts f the Franciscan and the Shimant Terranes had erupted in an envirnment similar t that f the recent cean-ridge and that the hydrthermal cmpnent cntained in the sedimentary rcks are derived frm the hydrthermal emanatins alng :an active spreading center. DISCUSSION The examinatin described abve shws that the Franciscan bedded cherts are chemically akin t the cherts and prcellanites f DSDP Leg Site 4, because hydrthermal activities are respnsible fr the rck frmatin in bth areas. Cncentratins f SiO in the hydrthermal cherts frm the Franciscan Terrane (n. ) and that f DSDP Leg (n. 6) are 89.9 and 87.9~ (n water- and thers-free basis), respectively. This suggests that relatively large munts f SiO are cntained in the hydrthermal cmpnent, and this hydrthermal silica as well as bigenic silica is embraced in the siliceus rcks. Inrganic precipitates f silica f this srt are cmmnly bserved as siliceus sinters in ht springs. The hydrthermal cmpnent can be recgnized in the chert up t 6 m abve the basal basalt in the Franciscan Terrane. The cherts f this hrizn (ns. and ) yield Lwer Cretaceus radilarians such as Sethcapsa sp. and Napra sp. On the ther-hand, radilarians indicating the Lwer-Middle(?) Jurassic such as Parahsuum sp. f. P. simplum and Pachyncus? sp. ccur in the cherts abut m abve the basalt (ns. 7 and 84). The fssil data shw that the bedded cherts f the Franciscan Terrane were depsited in the hydrthermal field fr at least Ma. On the ther hand, at Site 4 the hydrthermal cntributin up t the hrizn abut 8 m abve the basal basalt suggests that the cherts and prcellanites depsited frm the Hauterivian/Valanginian t Albian had undergne sme hydrthermal cntaminatin fr abut Ma (Adachi et al., 986). Surface sediments within abut km frm the suthern part f the East Pacific Rise are subjected t intense hydrthermal activity (Bstri~m and Petersn. 969; Leinen and Pisias, 984). If the spreading rate f the East Pacific Rise in the past has been the same as that at present (7.9 cm yr- calculated by the data f Minster and Jrdan, 978), the sediments cred at a lcatin km distant frm the spreading center have been subjected t hydrthermal cnditins fr mre than Ma. Except fr the cean ridge regin, such an intense hydrthermal activity f a lng duratin has nt been knwn. Analgically, the sedimentary rcks frm the Franciscan Terrane culd have frmed in the hydrthermal envirnment n r near

27 an active cean ridge. Sedimentary rcks f the DSDP Leg were als inferred t have been depsited in an envirnment near the Cretaceus active ridge (the Japanese spreading center: Lancelt and Larsn, 97; Adachi et al., 986). Larsn and Chase (97) assumed that the Faralln Plate was subducted under the Nrth American Plate abut Ma ag. Sediments depsited arund the cean ridge between the Pacific Plate and the Faralln Plate mved eastward with the plate mtin (Atwater, 97). This plate mtin suggests that the sediments n the FaraUn Hate were accreted t the Nrth American Plate. The bedded cherts f the Franciscan Terrane might be a remnant slice f the ceanic crust added t the Nrth American cntinent in this manner. If the abve inference is true, the basalt underlying the bedded cherts, an philite member, is assumed t be an bducted fragment f ld ceanic crust erupted at the cean ridge. These results were btained mainly by chemical cmparisn between the Franciscan cherts and deep sea cherts. Nevertheless, the ccurrences f the cherts f the Franciscan Terrane and f the DSDP Leg are entirely different frm each ther. The frmer are typical bedded (ribbn) cherts whereas the latter shw a lenticular r ndular shape. Such thick and stratigraphically cntinuus chert sequences as the Franciscan bedded cherts have never been reprted frm the present-day cean basin (Jenkyns and Winterer, 98). This shws that the abve deductin fr the envirnment f frmatin f philite is nt always cnclusive. Harper (98) suggested that the Jsephine philite cmplex (nrthwestern Califrnia) including cherts, appears t be ne fragment f a cmplex cntinental-margin arc, island-arc, and marginal basin system that develped alng part f western Nrth America during Jurassic time, prbably as a respnse t eastward subductin. On the ther hand, the cherts and shales f the Shimant Terrane seem t cntain cnsiderable amunts f hydrthermal cmpnents, in view f the vertical variatins f the A/(A + Fe + Mn) rati and the TiOE-nrmalized values fr the hydrthermal elements. Shales withut hydrthermal inputs, hwever, lie in the lwer hrizn near the basal basalt. Mrever, n Shimant rck in this study shws an A/(A + Fe Mn) rati as lw as the.-. bserved in the hydrthermal chert frm the DSDP Leg Site 4 and the Franciscan Terrane. All Shimant rcks shw ratis ver.9. These facts demnstrate that the hydrthermal activity assciated with the eruptin f the basal basalt in the Shimant Terrane was smewhat weaker than in the Franciscan Terrane and in the recent spreading ceanic ridge. The field ccurrence suggests that the stratigraphic relatin f the sedimentary sequences f the Shimant Terrane is nt simple but rather had been disturbed after sedimentatin; perhaps a cmplex repetitin f the strata has ccurred. Relating t this matter, the pillw basalt in the Shimant Terrane has an intimate affinity n the Ti-Zr-Y diagram (Fig. ) with cean-flr basalt erupted at the cean ridge. Hawkins (976), hwever, has claimed that the basalt ccurring in an inter-arc basin is chemically similar t cean-ridge basalt; Hart et al. (97) 9

28 9 and Ridley et al. (974) pinted ut the difficulty f chemical discriminatin f inter-arc basin basalt frm cean-ridge basalt. Pints n the Ti-Zr-Y diagram (Fig. ) f inter-arc basin basalt l~ablished by Ridley et al. (974) fall in the field f cean-flr basalt. Thus, the Shimartt basalt is nt necessarily akin t the ceanridge basalt but has an affinity with inter-arc basin basalt r marginal-basin basalt. Furthermre, thick acidic tufts ccurring in hrizns near thse f the present cherts and shales (Nakazawa et al., 98) shuld be taken int accunt. Thick acidic tufts intercalated in clastic rcks are generally regarded as prducts f acidic vlcanism in mbile belts and rarely ccur in pelagic sediments. All these lines f evidence n the $himant rcks suggest that the cherts and shales had accumulated in a hemipelagic envirnment such as a marginal basin and inter-arc basin, where the basalts had erupted. These results shw gd agreement with thse f Yanai (98). San (98) and Miyake (98) als btained similar cnclusins n the basis f gelgical and gechemical data. Mrever, the limestne layer intercalated in the sequence f cherts and shales f the Shimant Terrane shws that the depth f water was prbably shallwer than the carbnate cmpensatin depth (CCD). Frm the discussin abve, the bedded cherts frm the Franciscan Terrane are assumed t have been depsited in a spreading-ridge regin. On the ther hand, the Shimant rcks are inferred t have been depsited in a marginal basin r an inter-arc basin. Accrdingly, it des nt necessarily fllw that the siliceus rcks assciated with pillw basalt (a member f philite) in the wrld rgenic belts is clsely akin t the sediments arund the spreading ceanic ridge. ACKNOWLEDGEMENTS I thank the members f the Department f Earth Sciences, Nagya University. I especially wish t express my gratitude t Dr. R. Sugisaki wh cntinually cntributed with suggestins and criticism; t Dr. M. Adachi, wh gave help and cnstructive suggestins; t Mr. S. Yg, wh made thin sectins fr all samples; and t Mr. H. Yshida, wh identified n~erfssits. I am further indebted t Mrs. K. Taki and Miss A. Karnij fr their technical assistance.! als thank Dr. D.L. Jnes, U.S. Gelgical Survey, fr valuable infrmatin n the gelgy f the Franciscan Terrane.

29 APPENDIX Analyses f samples frm the Franciscan Terrane Sample n SiO , TiO A ,67, FeO Fe MnO MgO CaO Na K,6.8.6, P H/O( - ) H ( + ) Residuals ,4.48.9,69.46 C Ni Zn ii Rb Sr Y Zr Nb M Pb 6 6 Th Ba Ga ,

30 , I O APPENDIX (cntinued) Sample n SiO TiO.4.4 A,7.8 FeO..6 Fe..4 MnO.6.9 MgO.. CaO..8 Na, K..4 P. H( - ).4,4 H( + ).6.4 Residuals , , ~., C Ni 7 7 Zn Rb Sr 9 9 Y 6 Zr 9 Nb M Pb 8 6 Th Ba 6. 7 Ga O 7 7

31 Sample n SiO TiO A FeO Fe MnO MgO CaO Na K P H - ) H (+) Residuals C Ni Zn Rb Sr Y Zr Nb 4 M 6 Pb Th 9 Ba Ga

32 Sample n SiO TiO A FeO FezO MnO.7..4 ~ ,99,.7 MgO Ca Na K P....~) H( - ) H( + ).: Residuals C Ni Zn Rb 7 Sr Y Zr Nb 4 4 M 4 t Pb Th Ba Ga ,.9...4,

33 Sample n SiO TiO A FeO FezO MnO MgO CaO Na...4. K P ,7 H ( ) H ( + ) , Residuals , C Ni Zn Rb Sr Y Zr Nb 4 M 4 7 Pb Th Ba Ga

34 O O APPENDIX (cntinued) Sample n SiO TiO A FeO..,.4 Fe.4.9. MnO MgO CaO Na.6. K...7 POs... H~O(- ) H( + ) Residuals i C 6 9 Ni 8 Zn 4 8 Rb 4 Sr 4 8 Y Zr 9 4 Nb 4 M Pb 69 7 Th Ba Ga O

35 Sample n SiO TiO AI FeO Fe MnO MgO CaO Na K P H ( - ) H ( + ) Residuals C 7 Ni Zn Rb Sr 6 Y Zr Nb l M 4 Pb Th Ba Ga

36 APPENDIX (cntinued) Sample n. 8 Si 6.78 TiO.46 A 7. FeO.7 Fe.8 MnO.! MgO. CaO.9 Na. K. PO. H( - ). H ( + ) 4. Residuals.8 C Ni 4 Zn 9 Rb 74 Sr Y 8 Zr Nb M 6 Pb Th Ba 4 Ga , , ,

37 Sample n. SiO T A FeO Fe MnO MgO CaO Na K PO H zo( - ) I-I(+ ) Residuals C Ni Zn Rb Sr Y Zr Nb M Pb Th Ba Ga O , , , I O , ,...a

38 i , ,4..4. A APPENDIX (cntinued) Sample n SiO TiO.8.4 A.7. FeO.8.6 Fe.8. MnO.. MgO.9.6 CaO.. Na..7 K.9.4 P.. H( - )..9 H( + ).. Residuals ~ , C Ni 9 9 Zn Rb 6 Sr Y 9 Zr 6 Nb 6 4 M Pb Th Ba 8 Ga I O 7O

39 Sample n SiO TiO A FeO Fe MnO MgO CaO Na KzO POs HzO (-) H ( + ) Residuals ~ C Ni Zn Rb Sr Y Zr Nb M 6 4 Pb Th Ba Ga Residual materials are calculated by subtracting H frm ignitin lss. They may cntain sulfur, rganic materials and thers. Cmpunds in %; elements in ppm.

40 , , : O , :9.7,9.64, , APPENDIX Analyses f samples frm the Shimant Terrane Sample n, SiO TiO...,.8.6 AI ,9 FeO 4.9, Fe ,.97 MnO.7.4.8,,.97 MgO , CaO Na, K L.4 P.9...4,8.8 CaCO.77.4,..8.4 H( - ).67, H( + ) i Residuals :6.4 ~ :.7.4 C Ni Zn 9 Rb Sr 4 9 Y Zr Nb 9 6 M 7 7 Pb 9 Th Ba 4 8 Ga O O O O

41 Sample n SiO TiO A FeO...6..,, Fe ,.4.98, ,99 MnO,.88.6.,96...9,.68.. MgO.48., CaO ,,.47 Na , ,44.4, K ,9..7,8.,4.8 P CaCO ,6, H ( - ).8.48, , H (+) , Residuals, C Ni Zn Rb Sr Y Zr Nb 9 4 M Pb Th Ba Ga Residuals are calculated by subtracting H frm ignitin lss. They may cntain sulfur, rganic materials, and thers. Cmpunds in %; elements in ppm.

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