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Petrography of Alkaline Volcanic-Subvolcanic Rocks from the Brazilian Fernando De Noronha Archipelago, Southern Atlantic Ocean

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Petrography of Alkaline Volcanic-Subvolcanic Rocks from the Brazilian Fernando De Noronha Archipelago, Southern Atlantic Ocean 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,
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