PETROGRAPHY OF ALKALINE VOLCANIC-SUBVOLCANIC ROCKS FROM THE BRAZILIAN FERNANDO DE NORONHA ARCHIPELAGO, SOUTHERN ATLANTIC OCEAN
M.N,C .U1brich
KEY-WORDS: oceanic is!ands, petrography of alkaline rocks, Fernando de Noronha, dikes and flows
ULBRlCH, M .N.C. (1993) Petrography of alkaline volcanic-subvolcanic roeu from the Brazilian Fernando de Noronha ArthipeLago, Southern AtLantic Ocean, 8Q/. IG-VSi'. Sér.Cient.. 24:77-94
ABSTRACT
A classification schcmc for lhe volcanie-subvolcanie rock.s of the Fernando de Noronha Arthipelago ispresenlCd,followingthe ruGS recommendations, basedon lhe petrographiestudyofa large number of 5aDlples. In Fernando de Noronha 1\0>0 main volcanie events were defined by Almeida (19jj): an clcier Remédios Formation (12 to 8 Ma old) composed of domes, plugs and dilr::es of tephrilie-basanitic to trachytie anil phonolitie compositions, \\ith essexite porphyries, limburgites and alkaline lamprophyres intrud ing basal pyroclastic rods, anil a younger Quixaba FormaLion (3 to 2 Ma), made up mostIy by flows of nephelinitie composítion (ankarauiles). The lamprophyres wen: separalCd in two groups: tephritie or me1anocratic lamprophyres anil typicallamprophyres, the fim oncs belonging to a suile eharacterizcd by coo tinoow iDCrease in amphibole content. MOS! of lhe roeu of lhe Remédios Formation appear lO belong lO lWO distinctpetrographiescries,oncreprescnlcdbyanundersaturalcdsodicba!;anite-tcphrile(csscxile)-poonolile trend and the olher by a poussic alkali basall-trachyandesite-trachyte scries, while the limburgites and \ampro phyresareofuncenainancestry Some ankaratrite and olivine nephelinitc dikes lhat only cul the Remédios roeu are also anributcd 10 the Quixaba Formation. The basanitie flow with mantle xenoLiths ar the São José lsland. COIISidered b)' Almei~ (19.5S) to form lhe )'ouogcst São José FormaLion, is Ilere tentatively interpreted as represcnting lhe waning stages of lhe Quixaba volcanism.
RESUMO
Apresenta-se neste trabalho um esquema de classificação para as rochas ~ulcãnieas do Ar chipclago de Fernando de Noronha, com base no CSlUdo de um grande nUmero de amostras Dois eventos vulcânicos principais foram definidos em Fernando de Noronha por Almeida (1IlSS). A FormaçãoRl:médios é a mais antiga (12 a 8 Ma), composta pordom~ · plugs" e diques e pordepOs'_ tos piroc\ásticos buais por eles cortado!;, que apresentam composiç('lcs variando de tefrilos-balianilos a UlIqllÍlOS c fonolitos, junto oom cssexitos pórfiros, limbwgitos c Iamprofiros ak:alinor;. A formaçlo mais nova, a de Qui xaba, ~ oonstituida principalmente por derrames de ncrclinitos (ankaratrilOS). Os lamprofiros foram separados em dois grupos: lamprófiros tcfrlticos ou mclanocliticos c lamprofiros tlpicos. os primeiros pertencendo a uma suitc caracterizada pelo aumento oontinuo no contCÍldo modal de anfibólio. As rochas da FollUaÇAo Remédios parecem pcrtcncer a duas séries pctroglificas, uma subsaturada, dc tcndência s6dica. representada porbasanj tos-tcfritos(esscxitos)-fooolitoscaootradctcndénciapotássicaconstituidaporalkalibasaltos-traquiandesitos traqultos. Os lamprófiros c Umburgitos do de origem duvidosa. Algun5 diques que cortam a Fonnaçio Remédios (antaratritos c ncfclinito) foram atribul do5 6 FormaçAo Quixaba. Os derrames de basanitos com xcn6litos mant~licos da Ilha de São José. que Almej da (19.5.5) atribul aurnaformaçAo mais nova, adc SIoJosé,são aqui considcrados como pertcnccntes aos esté gios finais do vulcanismo oc:fclinltico da Formaç;lo Quixaba.
INTRODUcnON
In recent years there has been an in new rock names, or used rather loosely a ereasing interest in the geochemistry of simplified IUGS c1assifieation of volcanie ocean island rocks with lhe intention of rocks without proper description and loca obtaining more information about their tion of samples, thus eonfusing the ocea mantle sources as well as the compositional sional reader. variations of the oceanie mantle as a whole lt is the aim of tbis contribution to (Sun & McDonough, 1989; Weaver, 1991, present the results of petrographic stuilies and references therein). on a collection of rock samples trom Fer Recent review papers and detailed nando de Noronha, discussing the strati geochemieal and isotopie work on the graphie setting and petrography of the roco of several Atlantie oceanie islands are rather bewildering variety of roek types based on the foundations of many previous found in Fernando de Noronha, at the same petrographic and geologie studies (i.e., time addressing the issue of a proper pet Chaffey et al., 1989; Davies et al., 1989; rographie nomenclature. Geochemical as Schmineke, 1982). pects were discussed in Ulbrieh et aJo The on1y comprehensive work deal (1994a). ing with lhe geology and petrography of lhe Fernando de Noronha Archipelago is GEOWGICAL SETTING, STRA TI the ciassie paper by Almeida (1955) whieh GRAPHY AND K-Ar AGES presents, along with the geologie map and some outstanding petrographie descrip The Arehipelago of Fernando de tions, the now commonly used stratigraphic Noronha, 345 km 0tI' the northeastem division. A summary of the work of Almei Brazilian coast, at latitude )051'S and da and a historical review with comments longitude 32"25W, consists of a main is on petrography and geochemical trends is land and about 20 islets with a total area of given by Mitchell-Thomé (1970). 18.4 km 2 (Fig. l) Almeida (1955) used many varietal It is composed of volcanic and sub names to eharacterize rock types, following volcanic rocks constituting the exposed a common practice in earlier years, particu upper part of a large volcanic strueture that larly in relation to alkaline roch Subse emerges, as the westemmost expression of quent work (Gunn & Watkins, 1976; a submerged volcame chain, from a depth Comeu, 1986; Weaver, 1990) introduced of about 4000 m (Bryan et al., 1972) Figure I _ Qc:ologicaJ map of lhe Fernando de Noronha Archipelago (simplified after Almeida, 1955), showing appcoxi.male sample localities. Sampled rock typeS are idcntiritd as: A"'ankamuile: Ab--alkali basalt: B-1Juanile: Eooessexile; P-PhonoLite: T-trachytc; Ta-.:rachyandesite. O indicales lhe Location of ullrabasic basic dtkcs. For detaikd informalion see TabLe I. Abreviations: &-baía, E-cnsc:ada. l'*ilha, M-morro, Pta"'POnla.
The geological features and lhe $lra abundant pyroclastic rocks (aggIomerates, ligraphic re1ationships were characteriz.ed breccias, tuffs) intruded by subvolcanic by Almeida (1955), who recogniz.ed two domes of phonolites, plugs of trachytes, a main eruptive events of alkaline volcanism, silJ-like body of essexite porphyry and nu the older Remédios Formation and the merous dikes of ultrabasic (ankaratrites., younger Quixaba Formation, separaled by limburgites., basanites and tephrites) to ba an erosional hiatus (Fig. 1). sic-intennediate compositions (tephrites, The Remédios Formation, present phonolitic tephrites, tephritic phonolites, mostly in the central part of the main island phonolites, alkali basalts. trachyandes.ites, and in the coastal arcas, is composed of trachytes and allcaline lamprophyres). As pointed out by Almeida (1955), (Fig. I). Almeida (1955) also mentions a lhe explosive volcanie activity in the Arehi phonolitie dike inlruding essexite porphy pelago, now represented by the pyroclastie ries in the Ponta do Atalaia and Iwo dikes rodu, may have just preceeded the intru of Iraehyandesites eutting a small body of sion of the domes and plugs of trachytes alkali basall in lhe Enseada do Abreu. and phonolites, as part of a continuous vol The older roeks in the Remédios canie episode. The matnx of tutfs and ag Formation are lhe alkali basalls, essexites, glomerates is trachytie-phonolitie; the lapilli phonolites and trachyandesites--Irachytes, and rounded blocks ofthe aggIomerates are foUowed by tephritie and lamprophyrie also mostly constituted by trachytes and rocks; lhe mutual reJationship hetween the phonolites, togethet with essexites, some lasl two types is not yel ascertained. The tephritie phonolites, phonolitie tephrites tannbuschiles of Almeida (1955; ankaratrile and olivine-free lamprophyres. Some of lhe dikes, see below), nol inlruded by any of blocks within the aggIomeratea are me the other rock types in the Enseada das dium-grained phaneritie roeks not found as Caieiras, are here con-elated with lhe ank outcrops in the island (see also Almeida, aratrite flows ofthe Quixaba Formation. 1955), sueh as nepheline syenites, amphi Whole roek K-Ar ages of the bole gabbros. nepheline-bearing mon Remédios Formalion vary between 12.3 zogabbros and monzosyeniles, some clin and 8 Ma (Cordani, 1970). The older ages opyroxene-magnetite and elinopyroxene are yielded by the alkali basalts of the En amphibole cumulates and other types with seada do Abreu (12.32±O.37 Ma), while unusual compositions. Some of these rocks ages ofphonolites vary fiom 11.l5±O.34 10 are a1so found as enelaves in the above 8.02±O.24 Ma, overlapping those of mentioned-dikes. trachytes (l0.76±O.32±O.29 Ma). Two The intrusion sequence of ultrabasie other lithological varieties were anaIized: and basle dikes is difficult to determine an olivine-free lamprophyre (-fourchite-) since most of these rocks are almost uni with a K-Ar age of 8.80±0.26 Ma and an fonnly grey-black, with clinopyroxene ± ultrabasie dike rock with an age of 2.62± oüvine phenocrysts and an aphanilie ma 0.26 Ma. The data obtained by Cordani teix. Even so, some observations are pre (1970) broadly confirm the field observa senled here. The phonoütie domes, contam tions aboul lhe slratigraphie position of ing rare dm-sized enclaves of nepheline some of the ultrabasie dikes of Remédios syenite, are cut by tcphritie and alkaline Formation, and include these dikes in lhe lamprophyrie dikes, whieh commonly pre later volcanie event whieh produced the la sent irregularly shaped enelaves of vas ofthe Quixaba Fonnalion (soe below). nepheline syenites and isolated erystals of Ihe Quixaba Formation, deposited feldspar and nephe1ine; lamprophyrie brec following a period of quiescenee, is mainly das with abundanl rounded clasts of fcid eonstituled by extensive flows of ullrabasie spar have also been found. In lhe Enseada lavas, now exposed in lhe western and das Caieiras ofthe main island (Fig. 1; Ia eastem parts of the main island and in some ble I), where a dike complex IS more islets (Fig. I). In some places, lhe lavas are dearty exposed, trachytes and trachyan more than 200 m thiek. A careful Sludy of desites are cut by ultrabasic-basie and lhe thiekness, extension and dip of the lava lamprophyr1e dikes (see also Almeida, flows in lhe westem part of lhe island al 1955, p. 28). A smaU trachyandesite pipe lowed Almeida (1955) to pinpoint the ex.is intrudes phonolites in the Baia do Sueste tence of voleanie vents, now submerged, 8oI.lG-USP, Sk.Ci."r., 24:77-94
Table I • Location of sampled ultrabasic4lasic dikes Locatioa DÜI.e rock type WalI rodr.
c-w~
Enseada da Caieira Ankanatrites, PyrocLuticrocb limburgite5, teplvitic Jamprophyres, ,tcphriticlamprophyres
P...... Nephelinite Ankaratrileflow
Enseada do Abreu AoIwoIriIa, Pyrocluticrocks, teplvites,basanitcs, phonolite dome tephritic lamprophyres
Ponta do Atalaia Tephrites, Euexite porphyry sill, tcphriticlamprophyres pyrocluticrocks typical1amprophyres
aliado Sueste Limburgites Phonolite dome
T ephritic lamprophyra Morro do Forte typicaIlam~ Phonolite dome
Vilado.Rernédios Ankaratrite Ankarauiteflow
Ponta do Capim Açú Ankanatrite Ankaratriteflow
Morro do Meio Teplvites, . Pbonolite dome. typicallamptopbyres pyroclasticrocb
EssexiteporphyrysiU Teplvites, Essexile porphyry sill, tepbritic lamprophyra pyroclasticrocb
Close to the Morro do Pico Typicallamprophyres, Pyroclasticrocks olivinenephelinite along the northem coast in the Enseada do dearly a ... oleano, as shown by shape and Carreiro de Pedra; he also stressed that outline and the presence on its slopes of la anbratritic pipes, sueh as Ihose exposed in ... as and abundant oxidized scoria, and is lhe Peninsula de Santo Antonio (dose to certainly the ... ent that expelled flows of lhe Ponta de Santo Antonio, sce Fig. I), ankaratrites present in the vicinity and, acted as additional '1mts responsibJe for possibly, in the whole eastem part of the lhe extrusion of uJtramafic la... as. The mainisland. Morro do Francês, on lhe othel" hand, is Ankaratrites are dominam among lhe Ulbrlçh. M.N.C. P.UOII,~hyof.k.i .... voIc.ni.,.lIUbvolç.nic,och...
ultrabasic lavas, with scarce leveis of meli· broad contemporaneity with the roelcs of lite ankaratrites intercalated with breccias the Remêdios Formarion; these ages may and tuffs. The flows are eommonly intruded be misleading since there is a possibility of by dikes of the same composition contamination of the basanite samples by (ankaratrites, nephelinites). Recently, an incompletely degassified mande xenoliths intrusive basanitic pipe was recognized in and xenocrysts (Cordani, 1970). the Baía do Sancho (Ulbrich & Ruberti, Petrographic characteristics (cf. de 1992). scriptions below; see also Ulbrich & Five whole rock K·Ar ages (Cordani, Ruberti, 1992) strongly suggest Ihat the 1970) of ankaratrite flow samples vary S10 José basanites actually belong to the !Tom 3.33±O.11 to 1.8I±O.13 Ma; a single Quixaba voleanic cycle. sarople (a bloek found on the beaeh, dose to the Morro Branco) is 6.64±O.20 Ma PETROGRAPHY old. Of the five saroples, three are !Tom the eastem flows (Morro do Francês, 2 saro· The described roclcs wiU be separated pies, 3.33±O.ll - 2.96±O.52 Ma; Ilha Rata, according to stratigraphic setting and .as· 1.81±O.13 Ma) and two from the westem soeiation in the field, and grouped on ac· flows (ponta do Capim Açu, 2.49±0.09 Ma count of their petrographic characteristics. and Morro dos Dois Abraços, 2.99±0.27 Seclion A, B anel C will deal wilh the roek.s Ma). attributed lo the Remêdios and Quixaba The stratigraphic record and the age Formations. and a short final description detenninations clearly 51ate the contempo depicts some of the features of the basan· raneity of the ultrabasic volcanism in both ites of the Ilha de São José (section Dl. the eastem and westem pans of the Arehi Table 2 shows a summary of lhe stmi· pelago. graphie attribution orthe severa! rock types Besides the Remédios and the Qui. and is a guide for the roUowing descrip xaba fonnations, Almeida (1955) also de tions. The pyroclastic roeks of the scribed basarutic flows carrying em to dm RemCdios Formation are generally very sized mande xenoliths (dunites, peridotites) poorly exposed and most1y altered, covered whieh form the islets Slo José, CUSCU! and by vegetation and converted to soil!. They de Fora (Fig. I). The volcanie basanites are will not be described here. and lhe inler only found in these islets and there are no csted reader is referrect to Almeida's ac visible conlacts between Ihem and lhe eount(1955). roeks of either the Remédios or the Qui. .) SubvolCl.Dic plup .nd domes or xaba Formations. However, lhe NNE dip lbe Remédâo. Fonu.lioD .nd ~iated or the ankaratrite lavas along the coas! of dikes. the main island in the vieinity of the I - PhonoIitic lephrites 10 lephritic basanitic outerops suggests that lhese lavas phonolites anti their coarser-grained 110· underlje the basanites. Almeida (1955) thw rielies. The roeks are mostly holocrystalline suggested that the basanites are the and rorm a smaII body anel severa1 dikes. In younger roeles of lhe ArchipeJago and general, pyroxene Is lhe main mafic phase grouped them into a separate unit, lhe S10 with subordinate olivine (6-8%), about 5% José Formation. of opaque grains and 500/. or more or felsic Whole roek K-Ar dating gave ages components. Some roeles or this group of around 9.5 Ma on selected samples of show glassy groundmass and are seen as lhe SIo José basanites, indicating instead a narrowdik.es. Table 2 - Stratigraphy and roek types in Fernando de Noronha
StnlicnPllk Varietal ••doQerulllft K-Aralet(2) a«rib.tioa(!) uttdilltllelitenture (Ma)
Slo José baSlnileflowb
Baia do Sancho buanile piJX
Quixaba Formation Anlwatrite dikes TaMboschiles(l)
Nepbelinitedikn
Ankaratriteps
Tephrile,bu&nitedikes Pyroxene fourcbiles (I) 8.8 Olivina tescbcnites (I)
Typieallamprophyresdikes Camptonites(l)
Amphibole-richtephrite, FourdIites,noteansannaites. tephritic Iampropbyre dikes melanoa.uc monchiquites: (I)
TrachyteplJlp,dikes AIkali tncltytes (I) 10.7
Remêdios Formation Trachyandesitephtgs, dikes KaJj-puteiles (I), tristanitCl (3), beramoreites(4) Pbonolite ao.es 11.1 lO 8.0
Euaiteporpbyrysill 9.2109.7
A1kali bualtplug Hawaiites(3) 12,3
Lapillilutfs,brec:ciu, .... """"'" ._PyrodMllcroclulPdukanuileflowlareu..Clldearqi-.t_iIo"RcmédioIIPdQuixaberonroaliollS.~ tberestollhe~JlIqIICIICei5uDdc:arIPdOlllylCDlati...,.SCIe_for~II·AIlIIcida(l"$)COIIIidenu nx:kI uput oftbe)'OUlllel"SIoJolt fonnllllon. bultbey lU)' IlQUalJybcLoq 10 tbeQui:Qba vokaaisnI ai. CIdmiaalift&pbNoo (lO"lUcll .-,.abobeabo tbecuel"ortbeBaladoSuocloaMaDiu..) R~fo~ I) A1.... (19~~); (2) CordMi (1970); (l)G\UlA Wad<ÚoI;(I976); (") Cotner(19I6); (') Sch..ab A BJoc:II(lH~); 'lmpnlpCl"-'(6)Weaver(I990) ___
The coarse-grained roeks are por clase (labradorite to andesine), K-feldspar, pbyritic with aphaJútic matri", but finer and nepheJ.ine or anaIcime, in variable pro gnined equigranular phaneritic roeks are portions. The most abund.nt variety of this also found. The felsic minerais are plagio- group are the essexile porpIryries nephciine monzogabbros to monzodiorites), wruch islets and also occur as dikes. The roeks form a sill·like body (Fig. I). Other types vary from almost aphyric to high1y por are less common and occur as dikes. AI· phyritic types; usually, one of the two tex· meida (1955) mentioM glenmuiriles lures predominates in some areas or in in· (analcime monzogabbros) and al/ochetites dividual domes, as clearly shown by AI· (nepheline monzosyenites). The only varie· meida', mapping (Fig. I). tal name that is worth maintaining is Aphyric phonolites are greenish essexite porphyry, not only because of its roda wilh conchoidal fractures. They have geological importance and rustorical record <1 to 3% of smaU phenocrysts of alkali in the literature on the Archipelago bot also feldspar and idiomorpruc nepheline, with because -essexite- is commonly used for minor isotropic feldspathoid (sodalite?) similar rocks in other oceanic islands (e.g. filled with tiny opaque incJusions, titanile Cabo Verde; Fumes & Stillman, 1987). and kaersutitic amprubole, the lasl one ai· The essexite porphyries ways bordered by aegirine-augite and (inequigranular, coarse--grained equivalems opaque grains. The very tine-grained matrix of phonolitic tephrites), are slightly to consisls of flow-oriented alkali feldspar strongly porphyritic with subidiomorpruc laths and tiny aegirine prisms, eilher iso· phenocrysts of yellow.pink to greenish lated or gathered in divergent sheafs, plus pyroxene (titanaugite to salitic? pyroxene) idiomorpruc nepheline and isotropic feld· and resorbed irregular olivine grains (5· spathoid (sodalile?). 7%) bordered by tlakes of biotile and Porphyriric phono/ites are grayish, opaque grains. Labradorite appears as phe porphyritic rocks with variable amounts of nocrysts and microphenocrysts mantled by phenocrysts (from ±20 up to ±40"/.). The alkali feldspar. Some larger idiomorpruc felsic phenocrysl assemblage is similar lo opaques and feldspalhoids (nosean, that of lhe aphyric rocks excepl for some hauyne?) are also seen. The matrix is Iextura! features. In the porphyritic roeles, formed by unorienled plagioclase lalhs wilh some alkali feldspar laths bordered by small alkali feldspar, idiomorphic nepheline and opaque inclusions show discontinuous apatite, opaque grains and small flakes of overgrowths of clean feldspar; also, reddish, rughly pleochroic biotite. glomerules of nepheline and sodalite(?) are A rock with analcime inSlead of common. The mafic assemblage consisu of nepheline appears as a block in the Enseada kaersutitic amphibole. clinopyroxene. dasCaieiras. The anaJcime also substitules opaque grains and titanite. The zoned for alkali feldspar; graphic intergrowths idiomorphic to rounded amphibole crystals between alkali feldspar and analcime are are partially substituted by opaque material ubiquitous. and totally sUITounded by aegirine-augite A glassy tephritic phonolite dike has prisms and opaque grains. The pyroxenes fresh amphibole in lhe phenocryst assem may OCUIT as isolated or grouped prisms blage logether with clinopyroxene, .patite. and almoSl always present an irregular yel. a mineral of lhe sodalite family and scarce lowish slightly pleochroic (augitic?) core opaque grains immersed in fresh fluidal bordered by aegirine-augite with externai glass (7oe/e of the rock) with needles of idiomorphic shapes. The matrix shows a apalite. trachytic texture produced by oriented 2 • Domes of pIronolites. The pho abundant alkali feldspar laths; other matrix nolites constitute lhe domes that appear in minerais are nepheHne. rods of aegirinic lhe central part ofthe main island and some pyroxene and inlerstitial analcime. 3 - DUres and plugs 01alkali basalts, 1989), with the addition of lhe prefix ~ kaJi" trachybasaJts, basa/tic trachyandesites, (kaIigauteites) to register the absence or traclryontksites and trachy/es. A1tered aI rarity of sodic plagioclase. Comen (1986) kaJj basalts were described by Almeida preterred the name tristanite (cf. Le (1955) in the Baia do Sueste; frtsh mate Maitre, 1989) to point out some similarities rial, studied in this work, was found in the with the mildly potassic trachyandesites of same small body s.ampled by Almeida Iristan da Cunha and Gough Island. In the (1955), located in the Enseada do Abreu IUGS classification the K-rich trachyan (Fig.l, Table 2). More corrunon are varie desites would be named /al/les. ties of trachyandesites and trachytes, which Confusion arises from the use of dif ocurr as dikes and also form small pipes ferent c1assification criteria by the different and plugs intruding pyroclastic roeks and, authors who define a rock name as a result rardy, also phonolites. A dome oftrachytes ofthe total a!kali content irrespective ofthe constitutes the Morro Branco (Fig. I). K:zOlNa20 ratios. Cox et al. (1979), for Basic to intermediate rocks such as instance, designate ali alkali-rich rocks of these are commonly found in oceanic is the trachybasalHrachyande5ite group as lands. usually separated into sodic or hawaiite5, mugearites and benmoreites, potasaic series, characterized in previous thereby !umping together both lhe Na02- classifiGations by .severa! chcmical parame rich and the K:zO-rich varieties. ters, particularly the K:zOlNa:zO ratio In our view, the nomenclature pro (equal to, or less than. 1:2 for the sodic posed by the IUGS Subcomission on the types; Wilkinson, 1974). Differences in this Systematics of 19neous Rocks (cf. Le ratio are the only criterion maintained in Te Maitre, 1989) should be used, with suffi cem c1assification schemes for these rocks cient alIowance for the distinction of Na- or (Le Maitre, 1989). K-enriched series. Ihe literature on Fernando de AllraJi basa/ts. These are porphyritic Noronha registers severa! varieta1 names roeks with phenoerysts of irregularly used to define roeles of the sodic series, shaped olivine anel idiomorphic yeUowish $Uch as nawa;Ue (trachybasalt; Comen, augite-titanaugite set in a fine-grained in 1986), mugearite (basaItic trachyandesite; tergranular matrix composed of zoned lab Gerlach et al.. 1987) and benmoreite radorite, clinopyroxene and opaques. A (trachyandesite; Weaver, 1990). The avail sample studied in this work contains 9"/0 able chenUcaI data of the more basic types olivine. and corresponds to an olivine-rich in Fernando de Noronha do not show a aIkaIi basalt. Ihe rocks described by Al strong Na enrichment, as is the case for the meida (1955) are olivine-free and have !ab type rocks of Hawaii or the Hebrides radorite (zaned to andesine) and some (Wilkinson, 1974). In the case of the more spots of analcime in the matrix. Comen abundant dikes of trachyandesites. the (1986) denominated these roeks as ha· situation seems to be the opposite. most w&ütes, but they are actuaIly trachybasalts samples being actually mildly (to strongly) according to the IUGS nomenclature. Beth potassic (cf. mbrich et al., 1994&; also the our sample and that described by Almeida -benmoreite" ana1ized by Weaver, 1990, come from the same locality (a plug?) and with 2.19"/0 Na:zO and 7.27% K20). The present uneven concentrations of olivine. potassic character of these trachyandesites possibly as a result of internal fractionation was recognized by Almeida (1955) who by crystal settling. For that reason, only the narned them gauleiks (cf. Le Maitre. name a/lrali basalt will be here used for theserocJes. oligoclase and bordered by alkali feldspar. Basa/tic trachyandesite.~ to trachy The amphibole, origina11y idiomorphic, is andesites. These roeles are mesocratic to almost totaJly converted to an aggregate of leucocratic with aIkali feldspar and feld aegirine-augite and opaque grains. The spathoids and decreasing amounts of pla idiomorphic feldspathoid is also totally aJ gioclase. The mafic phenocrysts are clin tered to feathery natrolite plus cancrinite opyroxene and amphlbole. The pyroxene and caleite in most samples. The matrix is varies from yellowish augite-titanaugite in trachytic with predominance of flow-ori basaltic trachyandesites to greenish (sodic?) ented aJkaJi feldspars plus idiomorphic, aI grains with augitic cores in trachyandesites. tered feldspathoid, roels of aegirine-augite, The amphlbole is partially altered brown opaques and rare apatite. kaersutite, usually substituted by green di 4 - Ultrabasic to basic di/ces nopyroxene (soda augite?) and opaque (tephrites. basanites and lamprophyres). grains along the borders. The felsic phenoc These dikes intrude me pyroclastic rocks rysts are fresh sanidine, isotropic altered and cut the trachytic and phonolitic dikes, feldspathoid (sodalite?) either as isolated domes and plugs of the Remédios Forma idiomorphlc crystals or as corroded aggre tion (Fig. I) and also each other (see Al gates and, rarely, some andesine laths meida, 1955, p. 28), except for the ank and/or g10merules of aIkali feldspar and aratritic dikes (see below), which appear to oligoclase-andesine. The amount of phe be the youngest intrusions. nocrysts varies from ±3 to 20% in different The rocks, black to gray in colour, samples. are porphyritic with variable quantities The matrix has a flow-oriented (usually more than 15%) of mafic pheno trachytic texture, particularly in the basaltic crysts and microphenocrysts (olivine and/or trachyandesites; in these rocks the cores of clinopyroxene, and sometimes amphibole). alkali feldspar laths are substituted by In some cases they also have minor cryptocrystalline material and carbonate, amounts of a feldspathoid of the sodalite possibly representing fonner plagioclase, famiIy (hauyne, nosean). The matrix is also while more typica1 trachyandesites have mostly composed of mafic minerais and fresh homogeneous alkali feldspars. Other opaque grains. The fe1sic components are components of the matrix are idiomorphlc variable amounts of plagioclase, isotropic feldspathoids of the sodalite fam "anorthoclase", aIkaIi feldspar and feld i[y, titanite, green clinopyroxene, scarce spathoid, aJthough partially or totally glassy amphibole, apatite and devitrified glass. mesostasis is a common feature. Most Some samples present also rare flakes of dikes show abundaot vugs fil1ed with caI strongly pleochroic brówn biotite. cite and/or zeolites and chlorites. Trachytes. Alkali feldspar is the most These dikes are here separated in abundant phenocryst and matrix mineral four groups, according to total amount of and OCCUfS as independent lãths or in ag mafic minerals, characteristic mafic pheno gregates together with plagioclase. Other cryst assemblage, texture and composition phenocrysts are plagioclase, aegirine ofthe felsic minerais in the matrix. augite, amphlbole, altered feldspathoid, ti Group I, limburgitic dilces. In these tanite and opaques. The plagioclase, whlch rocks, the predominant phenocrysts are foons the larger phenocrysts (up to ± l em) idio- to subidiomorphic titanaugitic clin and also microphenocrysts in some sam opyroxene, appearing either as isolated pies, is common1y zoned from andesme 10 crystals or in glomerules together with magnetite, accompanied by rounded, fresh, with greenish areas or patches. to pink smaIIer olivine grains. In some samples, the violet strongly zoned titanaugites; the phenocryst assemblage is set in a fresh mineral may occur as isolated idiomorphic g1usy base with microlites of mafie miner grains or appear in cJusters. together with als and needles of apatite. In others, lhe magnetite. Some of these clinopyroxenes glass is partially altered or devitrified anel are actuaJly xenoerysts in Ihese roeks lhe matrix cJinopyroxene and opaque grains (Ulbrich et al., 1994b). The groundmass is (sometimes associated with biotite) show almost always partly g1assy (with fresh or larger me and idiomorphie outlines. altered, devitrified or zeolitized glass) and The name limburgite, very I;Ommon panly crystalline, consisting of labradorite in the literature on oceanic island rocks, is laths and some analcime or showing only mainly a textural denomination for high1y slender poikilitie laths of alkali feldspar (in mafic alkalic rocks composed of olivine, part, twinned -anorthoclase-), with or clinopyroxene and magnetile phenocrysts without analeime with a glusy groundmass containing mi This is a heterogeneous group of crontes of the same minerais and no feld roeks whieh, however, do not deserve spe spars (Le Maitre. 1989). As the composi dai names, except perhaps for the anaIci tion of the felsic matrix cannot be ascer me--rieh varieties (analdme tephrites) tained under lhe microsco~ the general Some ofthe pyrtlXene fOltrchites mentioned use of this denomination as a Fernando de by Almeida (1955) will fali into this group Noronha "roek type- may be misleading. (olivine-free roeks, with only small Nevenheleu, the name is here used for amounts of matrix amphiboles). Also, lhe high1y mafic roeks with a g1assy base (see ho1ocrystalline olivine teschenites also AIme;da. 1955), but the term will be (Almeida, 1955) with 8-11% olivine plus e:xtended to cover rocks with devitritied labradorite, some alkali feldspar and anal g1asa and lhe above mcntioned phenocryst cime in lhe matrix, can be defined as the aasemblage, without amphibole. coarser-grained equivalents of basaniles Graup li, lephritic anel basanitic ar analcime basanites. dilres. These denominations are employed More groundmass-rich roeks. with here to include aJJ dikes with clinopyroxene leu caleie plagioelase andlor alkali feld as lhe dominant phenocryst phase and in spar, will grade inlo phonolitie tephrites which more or leu altered olivine consti and tephritie phonolites. tutes less than 100ft of the rocks Group J/I, amphibok-rich lephrite lo (occasionally, sIightly more Ihan 100ft), ac me/anocralie ar lephrilic lamprophyre companied sometimes by magnetite grains. dilres. Th.is group includes amphibole AJso included are olivine- free tephritic bearing rocks having the main mineralogi dikes. The matrix is composed main1y of cal and textura! featores similar to those of clinopyroxene and magnetite with variable the tephrites described in Group 11. The contents (up to 20-35%) of felsic ground amphibole is a brown to reddish variety mass. They may have some amphibole in (possibly iron and titanium rieh) appearing the matrix (up to 2~.) as evenly distributed, as slender prisms in the matrix and some slender or skeleta1 prisms. Some roeks are times also as pbenoel)'5ts. There are no rich in apatite needles. clear petrographie limits between lhe roeks The cJinopyroxene phenocrysts vary of Group 11 and rn from yellowish, almost non-plechroic and In some lephriles. lhe amphibole is sIightly zoned saJjtic pyroxene, sometimes unevenly distributed in the matrix of the P. Uogf"Phyof ..UIi .... YClc.nic-wbYCIc ....c rock ••••
roda. It is eoncentrated in irregular areas needles and tiny idiomorphic grains of accompanying the other minerais and may apatite were observed in roeks with elear also fill vugs 10gether wilh some magnetite, g1assy groundmass. The amphibol~rieh zeolites and chloritie materiais. In other roeu of this group conunonly contain rodes, lhe matrix is richer in slender amphi xenoliths of ultramafic phaneritic roeks bole prisms and the mineral appears more (e.g., clinopyroxene plus reddish, strongly evenly distributed. pleochroic biotite). Xenoerysts of alkali The phenoerystie amphibole is seen feldspar and, more rarely, nepheline were as rounded grains, partially or even tOla1ly a1so found in some of these lamprophyres substituted by minute opaque grains, (Ulbrich et al ., 1994b). sometimes accompanied by another gen The rocks of this group are classified eration of fresh brown amphibole crystals as amphibole-rich lephriles when the aro ofvariable size (up to I em). The develop ph.ibole is mainly confined to the matrix of ment of amphibole in individual dikes is, on the roeu The denomination of the whole, highly variable, resulting in three melanocralic or lephritic Jamprophyres is textura! variations: used for roeu with abundant amphibole a) roeks with unevenly distributed phenocrysts; these roela correspond mostly amphibole in the matrix,. with scarce altered to the me/amonchiquites of Almeida amphibole phenocrysts, and with or wilh (1955) bUl a1so to some of hisfourchiles out fresh amphibole phenoaysts. and maybe some of his nosean sannailes b) almoSl aphyrie roeu, with abun (Almeid~ \955). dant amphibole in the matrix anel no aro Group IV, typicallamprophyres. The phibole phenoerysts. rocu have panidiomorphie, porphyritic c) roeu with scarce amphibole in the g1omeroporphyritic textures with pheno matrix and abundam a1tered amphibole crysts of strongly pleochroic brown-red phenocrysts, with or withoul fresh pheno kaersutite, pink-violet titanaugite some crystie amphibole. times showing greerush (salitic?) irregular The inerease in amphibole content is arcas, magnetite and, eventually, very accompanied, in all roeks, by other minera- scarce serpentine pseudomorphs of olivine, 10gica1 ehanges. Qlivine erystals, when pre in a matrix composed of prisms of clinopy sent in roeu with amphibole, are a11ered to roxene anel amphibole, grains of magnetite serpentine minerals with some carbonate; and apalite needles, with 50"10 or more of the alteration is complete in lhe roeks with felsie groundmass. In most rocks, and~ larger amounts of amphibole. Sodalite oligoclase is the main felsic mineral with group minerais (2-4%) appear in roeu with variable amounts of interstitial anaIcime abundant amphibole, particulariy in those and tibrous zeolites. Some samples present 'Nith plenty of amphibole phenocrysts; in a devitri6ed matrix composed of a feathery some roeks, the feldspathoid (probably unidentified felsic mineral. The roeu are nosean) is colourless with a purplish hue usually fresh, and contam abundant vugs anel filled with tiny black. inclusions, while and g10bular structures filled mainly with in olhers it is bluish clean hauyne. The feld carbonate and also with zeolites and chio spathoids appear as idiomorphic to slightly ritic material. The camptoniles described by rounded, partially resorbed micropheno Almeida belong to this group. crysts, occasionally present within b) Dika .ttributed to tbe Quinb. pyroxene g1omerules, and are also present Fona.tioa in the malrix of some roeu. Abundant Ankaratrilic dilu!s. These roeks are very rich in mafic minerais, with o[ivine as was found intruding the luffs of the Remé the dominant phenoeryst phase ([5-2:5%) dios Fonnation (see also Almeida, 195:5), accompanied by :5-8% clinopyroxene phe The rock is porphyritic with a phaneritic nocrysts and having less than 10% of felsic intergranular matrix, showing clusters of groudmass, with nepheJine as the conuuon strongly pleochroic titanaugite crystals of febic mineral. more than I cm. The clinopyroxene forms Qlivine appears as irregularly shaped also smaller prisms (totalHng 10-15%) to individuais, sometimes showing remnants gether with rounded partially altered olivine of idiomorphic outlines; clusters of evenly (6-8%), abundant equant nepheline (>35%) sized grains are conunon in some samples. and opaque grains (±IO%). The nepheline, The grains (isolated or in clusters) are frtsh with some analcime and zeolite as altera and altered only along the borders and tion products, presents inc\usions of ali lhe fractures to carbonate, some serpentine and above mentioned minerais_ lnterstitiai areas talc; a yellow-brown spinel inc\usion Wa$ with analcime (and g1ass?) contain idio identified in one olivine crystal, The clin morphic apatite and opaque grains, some opyroxene is a slightly zoned, yellow to times with assoeiated biotite, alkali feldspar pink-violet titanaugite and occurs as idio and rare anorthoelase. Jt is here tentatively to subidiomorphic grains. Both mafic min assigned to the Quixaba Fonnation. erais show a seriate texture. c) Flows and associated dikes of The matrix is essentially composed of the Quis.aba Formation and basanites of clinopyroxene prisms and idiomorphic the Baia do Sancho, magnetite with scarce biotite flakes and in Ankaratrite flows. The massive ank terstitial poikilitc nepheline or analcime and aratrite flow roeks are predominant and are devitrified g1ass. The roeks contain sparse black-greyish types with small (up to 2 xenoerysts of orthopyroxene bordered by mm) mafic minerais in an aphanitic matrix; aggregates of small titanaugite grains. they present variable arnounts of arnyg The name "ankaratrite" is here pro du[es fLlled with carbonate or zeolites. posed for these dike rocks since their min Ankaratrites may appear also as reddish eralogy and texture are practically identical scoriaceous and highly amygdaloidal roc\cs to those shown by the ankaratrites lavas of (with calcite arnygdules), breccias, tuffs, the Quixaba Fonnation. Almeida (1955) and cogenetic subvertical dikes intruding recognized the similarity of these dikes the ankaratritic flows with the Quixaba ankaratrites but used the The more common roeks, which ap name "tannbuschites" to register the a» pear as flows (more rarely as dikes), show sence of biotite in the analyzed dike sam abundan! phenocrysts (10-20"10) of either pies and assigned them to the Remédios idiomorphic to irregularly shaped or re Fonnation. Biotite is, in any case, a minor sorbed olivine, in a matrix with 40 to 50% accessory phase in both these dikes and the of yeUowish salitic c\inopyroxene prisms, Quixaba ankaratrite flows, thus making it 15-20-;' of opaque grains anel 15 to 25% of unnecessary to use a new roek name. These nepheline. Small amounts of reddish, dikes do not cut Quixaba f1ows, but they strongly pleochroic biotite «I to 3%) and are considered an intrusive part of the apatite need[es appear in ali roeks, while Quixaba ankaratrites, on account of petrog perovskite is an occasional accessory min raphicsimilarities. eral, occurring mainly in roeks richer in Olivine nephelinite. Qnly one dike of olivine Melilite appears in the matrix of nephelinitic composition (>3:5% nepheline) some rocks (melilile ankaratrites). The olivine can be totally fresh or Roeks (te Maitre, 1989), since basalts are partially a1tered. The alteration minerals, by definition rodes with plagioclase. The located along the borders and fractures of mentioned .othor also divides the ankara the crystals. are either fibrous serpentine trites on chemical grounds into two groups and chlorite or reddish iron oxides. The denominated nephelinites and basanites pyroxene of the matrix is fresh and forms a (which appear to correspond to our A- and tight net of prisms together with subidio B-type ankaratrites, respectively), on ac morphic opaque grains. Nepheline, which caunt of the quantities of nollIl8tive may be partly substituted by analcime in nepheline. The name basanite, incidentally, some rocles, appears as poikilitic small in is inappropriate for holocrystalline roeles terstitial grains. the same habit commonly without plagioclase. shown by biotites. In our view, the rocks have to be This roek type will be here caIled called me/anocratic olivine nephelinites, type A ankaratrite, to distinguish it from although the varietal name ~ankaratrite· is the Iighter coloured ankaratrites found in proper and shou1d be maintained. Chemical the Alto da Bandeira and dose to the Ponta andlor mineralogical special features should do Capim Açú (Fig.l). Thi! second group, be added to a root name only as qualifiers, identitied as type. B ankaratrite, is com as proposed by the IUGS Subcomi»ion posed of roeks richer in clinopyroxene (± (Le Maitre, 1989). 60-65%) which appears as slightly zoned Nephelinite dilre. Almeida (1955) phenoerysts in the rodes. The amount of recognized one dike of nephelinite intrud olivine in both types is not very different ing ankaratrites in the Pontinha (Fig. I). It but the total amount of phenoerysts is was also the only one found by us during higher in the type B ankaratrites. sampling of the Quixaba tlows. It is a In the Iiterature, severa] names were phaneritic rock with pink-violet titan.ogite applied to these roeles. The name anlcara bordered by green aegirine-augite, almost trile was given by Almeida (1955) to char equant nepheline crystals, opaque grains, actcrize me1anoeratic olivine nephelinites apatite and rare titanite, perovskite and containing some biotite (see also Le biotite. The roek is porphyritic due to the Maitre, 1989). Gunn & Walkins (1976) and larger size of some titanaugite prism5 (> 1 Schwab & Bloeh (1985) denominated them cm). The pyroxene contains only apatite anJcaramiles (melanocratic basanites) be inclusions anel some opaque grains in the cause the rocks may have rare microlites of greenish rims. Nepheline, on the other plagioelase in the matrix; GuIUl & Watkins hanei, shows inclusions of pyroxene, apatite (1976) also propose the name limbllrgites and opaques. lnterstitia1 areas are filled for these rocks on account of their color with chloritic a1teration materiais contain indcx, ncar 67. Our study of28 samples of ing rounded to idiomorphic analcime ankaratrites trom different localities per greins. forrned dunng the present work did nOI re Basaniles of lhe BaÚl do Sancho. veal the presence of any feldspar, and lhe The basanitic rocks that fonn the subverti name limburgite is of course not approp cal pipe of lhe Baía do Sancho are por priate for roeles with a crystalline matrix. phyritic with 15-18% of subidiomorphic Weaver (1990) described the roeles as olivine grains partially or wholly converted nephe/ine basalts, a terminology consid mto iddingsite and subordinate fresh yel ered inapproppriate by the IUGS Sub lowish titanaugite phenocrysts in a matrix comission on lhe Systematics of Igneous constituted by clinopyroxene and about 20- 25% of large poikilitic irregularly shaped SUMMARY ANO CONCLUSIONS plagioclase (labradorite-andesine), opaque grains (±6%), scarce apatite and biotite and I. The phenocryst assemblage of che relatively abundant interstitial analcime. Fernando de Noronha rocks is rather sim Sampling alon8 a vertical cross-section of pie as is the case in most oceanic island (cf. the pipe shows that the amount of felsic Wilson, 1989). Predominam phenocrysts in mineraIs increases slight1y in the upper lev the more basic types are Ca-rich clinopy eis, accompanied by some increase in the roxene, Mg-rich olivine, magnetite and Ab content of the plagioclase (Ulbrich & sometimes plagioclase; magmatic (but nOI Ruberti, 1992). xenocrystic) orthopyroxene is absent. Ex d) BuaRites ofthe Uha de 810 José isting amphiboles (both as phenocrysts The basanitic flows are composed of andlor in the matrix) are brownish to red dark gray rocks with small mafic phenoc dish kaersutites. Groundmass mineralogy is rysts in an aphanitic matrix. Their most re usua11y more varied. Strongly pleochroic markable feature is that they bear abundant biotite is very scarce, but frequent in the rounded mande xenoliths of bottle-green matrix of some rocks (e.g., flows and dike dunites, blackish spinel peridotites ankaratrites; also in cssexite porphyries), ar (lherzolites and harzburgites) and possibly within coarse-grained enclaves found in also pyroxenites, with variable nodule di lamprophyres and other rocks, where it co mensions (from 1-2 mm up to 30-40 em). exists with varying proportions of other The basanites contain ±8 to 10010 of minerais (opaques, olivine, clinopyroxene, mostly fresh olivine and equivalent amounts amphibole, feldspar, etc.); the mineral is of yellowish, slight1y pleochroic titanaugite never seen as phenocrysts. Olivine, on the as phenocrysts., in a matrix composed of other hand, although present as phenoc clinopyroxene, opaque grains, plagioclase. rysts in more basic rocks, is absent in the apatite and interstitial poikilitic nepheline. groundmass; the exception is the alkali ba· The texture of the matrix is very in salt where the mineral ShOW5 a seriate tex homogenous on a microscopic scale, com ture. Melilite was found only in the matrix monly showing bands or lenses of different of one ankaratrite sample. The tenn grain size and concentration of felsic min "anorthoclasc", still a widely used mineral erals. The bands with larger grains contain name, has becn here applied to a finel y about 15% oflabradorite laths and 5-7% of twinned feldspar with low relief; the name nepheline representing the typical composi should probably be dropped (Smith & tion ofbasanitcs (Almeida, 1955) while the Brown, 1988) since it may represem a tine-grained areas appear to be almost free feldspar of ternary composition. Nepheline of felsic minerais: no plagioclasc is here and mcmbers ofthe sodaJite group (noscarr, seen and on1y tiny spots of glass (or anal hauyne) are common as microphenocrysts cime?) are observed. These finer-grained and groundmass minerais. areas resemble the matrix of some ankara 2. The rocks of the Fernando de trites. Noronha Archipelago were grouped into These basanites were grouped as the two formations by Almeida (1955). The São José Fonnation by Almeida (1955) but oldest one is the Remédios Fonnation (ages probably represent the culminadon of the about 12-8 Ma) with lapilli 1uffs, breccias Quixaba volcanism; they may be related, on and agglomerates as basal unit, invaded by pctrographic grounds, to the Baía do San a large petrographic variety of intrusive cho basanite pipe. domes, sills, plugs and dikes of alkali ba- salts, tephrites, phonolitie tephrites, here considered a part of the Quixaba vol tephritie phonolites, phonolites, trachytes, carnc event. The basanite flows of the Oba traehyandesites, basanites, severa! types of de São José are correlated with the basanite Iamprophyres and some of the intrusive pipe of Baía do Sancho, and tentatively in oquivalents of phonolites, tephrites and in· cluded in the Quixaba Formation. termediate roclo (foid monzogabbros and 5. The geologie features indicate that monzosyenites, essexite porphyries). The the main island represents a poorly exposed roeks of the Quixaba Formatian (ages ~central volcanic complex" (Fumes & aOOut 1-2 Ma) were deposited after an Stillman, 1987), capped by younger vol erosional hiatus. Predominant are ankara canie rocks. The rocks of the Remédios trite flOW5 and some associated dikes of Formation crystalli.zed and/or differentiated ankaratrites and nephelinites. The basanite in layered and/or compositionally inho flows ofthe Dha de São Jose were assigned mogeneous magma chambers. This ex· to the younger São Jose Formation by Al plains, on the one band, the appearance of meida (1955) but may represent the waning phonolitic domes composed almost entirely stages ofthe Quixaba voleanic event either by porphyritic or aphyric rocks 3. The Remédios Farmatian, petrog (Almeida, 1955), signalling that these in raphicaJly and strueturally rathel" complex, trusions were made up by magmas drawn shows a basal pyroclastic sequence and a from different leveis andlor different parts subsequent overlap and recurrence af in· ofa deeper-seated magma chamber. On the trusions of several rock lypeS. At least two other hand, the presenee of coarse-grained petrographic lineages are recognized, one rock5 - such as the esserite porphyry sheet made up by the potassic alkali basalt-trach and the blocks and enclaves of plutonic yandesite·trachyte series and the other by rocks in the agglomerates and dikes - the Na-enriched basanite-tephrite-phonolite clearly indicates that portions of these series (together with their coarser-grained magmas crystallized slowly at depth. counterparts sueh as essexite porphyries; 6. Enclaves of coarse-grained rocles cf also Almeida, 1955). Similar series are are found in many Remédios lamprophyre recognized in many oceanic islands (e.g. dikes, as weIl as xenocrysts af magmatic Wilson, 1989). Parenta! magmas for the origi.n (e.g. fragments of alkali feldspars or two series may be represented by alkali ba· nepheline coexisting with clinopyroxene salts and basanites for the K-rich and the and altered olivine in Group m dikes). Na-enhanced lineages., respectively. The Many of these enclaves present a mineral· Remédios Iimburgites (Group I dikes) and the typicallamprophyres (Group IV dikes) are ofuncertain ancestry. 4. The Quixaba rocks are structuraJ.ly and petrographically simpler than those of from different leveIs of the partly crystal the older F ormation, and are composed Iized deeper-seated magma chambers mainly by the A- and B-type ankaratrite ("cogenetic crystal-magma rnOOng"). On flows and some basanite intrusions., natural the other hand, these features may be an members of a sodic series. The ankaratrite indication that the different magmas corre dikes ("tannbuschites"; Almeida, 1955) sponding to both Na- and K·enhanced found as younger intrusions witmn the Remédios rock series actually coexisted, at Remédios rocles are petrographically simi least in part, and could roix with each lar to the Quixaba ankaratrite flows and are other. ACKNOWLEDGMENTS COX, K.G.; BELL, l .D.; PANKURST, RJ. (1979) The Interpretatioo of Ig. Y.Maringolo and specially E.Ruberti neous Roeu. London, Allen & Unwin, are thanked for help during sample collec 450p. tion. Discussions with Dr. D.Velde during DAVIES, G.R.; NORRY, M.1.; the field trip and widr V.Maringolo on the GERLACH, DC.; CLlFF. R.A. (1989) petrography and possible genetic relation A combinOO ehemieal and Pb-Sr isotopie ship ofthe dike rocks helped c1ear up some study of the Azores and Cape Verde obscure aspects. V.Claro Sanehes collabo hot-spots: the geodynamie implications rated with laboratory work. H. 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