Geology and petrology of the Nordeste volcanic complex, Sao Miguel, Azores: Summary LOUIS A. FERNANDEZ Department of Earth Sciences, University of New Orleans, New Orleans, Louisiana 70122 INTRODUCTION m. The rock types of this sequence grade upward from slightly porphyritic olivine alkali basalt to aphyric trachybasalt. A K-Ar The Azores archipelago is comprised of nine islands trending ob- date of 1.86 m.y. on the basal unit of the upper basalts and 4.01 liquely across the Mid-Atlantic Ridge between latitudes 37°N and m.y. for the lowermost unit of the lower basalts gives a time span of 40°N; it marks the western terminus of the Eurasian-African plate 2.15 m.y., during which most of the exposed Nordeste complex boundary (Laughton and Whitmarsh, 1975). The islands straddle was emplaced. the Mid-Atlantic rift from east-southeast to west-northwest with an The two youngest volcanic sequences, the tristanites and the eastern group — Sao Miguel, Santa Maria, and the Formigas Bank; trachytes, constitute the thinnest units and have the least areal ex- a middle group — Faial, Terceira, Sao Jorge, Graciosa, and Pico; tent. The tristanites consist of short, stubby flows as much as 10 m and a western group — Corvo and Flo res, which lie just west of the thick. The trachytes consist of flows which are similar to the trista- median-rift of the Mid-Atlantic Ridge. The Nordeste volcanic nites and also appear as small, plug-like intrusives. Cutting the complex, underlying an 85-km2 portion of northeastern Sao Miguel complex are a multitude of dikes, both simple and multiple, varying Island, is one of the oldest exposed alkaline basalt provinces in the in composition from ankaramite to alkali basalt to hornblende- eastern Azores. Field mapping at 1:16,000 of the Nordeste complex bearing trachybasalt. The three youngest isotopic dates, 1.28, 1.23, (Fernandez, 1969), which consists predominantly of alkali basalt and 0.95 m.y., were obtained on a trachyte plug, a trachybasalt and trachybasalt with minor tristanite and trachyte flows and small dike, and a tristanite flow, respectively. The dates are consistent trachyte plugs, has provided the framework for a detailed stratig- with the field interpretations and support the hypothesis, based on raphic, structural, and petrochemical analysis (Fig. 1). the mapping, that different portions of the complex were fed by magma chambers which were in different stages of differentiation. STRATIGRAPHY AND STRUCTURE PETROGRAPHY The 1,300-m-thick exposed volcanic rocks which make up the bulk of the Nordeste complex have been subdivided into five The Nordeste volcanics constitute a K-rich, alkali basalt- stratigraphic sequences, from oldest to youngest: lower basalts, trachybasalt-tristanite-trachyte series. Modal quartz and Nordeste ankaramites, upper basalts, tristanites, and trachytes. The feldspathoids are absent, but alkali feldspar patches, interstitial to lower basalts (~ 1,000 m thick) are composed of a generally con- groundmass minerals, are common. In addition to their essentially formable sequence of lavas varying from ankaramites to alkali silica-saturated chemistry, the mafic rocks of the complex, which basalts to trachybasalts which does not show any simple relation- account for more than about 90% of the rock series, are strikingly ship between the compositions of the flows and their sequence of rich in phenocrysts of olivine and clinopyroxene. extrusion. The only exception to this is at the base of this sequence, More than one-half of the analyzed rocks are alkali basalts. They best exposed along the eastern coastline, where a thin series of five are simple mineralogically but extremely variable in texture. to ten flows overlies some small scoria cones of similar chemistry. Aphanitic textures are the least common, and porphyritic and These lowermost units are the oldest exposed rocks on Sao Miguel, strongly porphyritic (ankaramitic) textures are extremely common. 4.01 m.y. old (Abdel-Monem and others, 1975), and are charac- A typical porphyritic alkali basalt is characterized by abundant, terized chemically by exceptionally low K20, Na20, and Ti02 rela- strongly zoned titanaugite and olivine phenocrysts, and plagioclase tive to the rest of the complex. These lowermost units are termed microphenocrysts set in a mesotasis of plagioclase, pyroxene and "transitional" basalts because their chemistry and mineralogy is in- minor olivine, iron-oxides, biotite, alkali feldspar, and apatite. The termediate to those of oceanic tholeiites and alkali basalts. ankaramites differ in being very rich in phenocrysts (30% to Unconformably overlying the lower basalts is the Nordeste an- ~50%) and in having complexly zoned and mantled pyroxenes karamites sequence, the most distinctive of the complex. This unit (diopside cores rimmed by titanaugite) and olivines (Fig. 2). consists of two widespread, thick flows (—200 m maximum thick- The trachybasalts differ from the alkali basalt group by being ness) of ankaramite and plagioclase-rich ankaramite. Conformably much poorer in ferromagnesian minerals and more variable in tex- overlying the Nordeste ankaramites is a sequence of relatively thin, ture and mineralogy. They vary from those which contain commonly aphyric, alkali basalts and trachybasalts totaling ~150 pyroxene, olivine, and plagioclase phenocrysts, set in an essentially The complete article, of which this is a summary, appears in Part II of the Bulletin, v. 91, no. 12, p. 2457—2557. Geological Society of America Bulletin, Part I, v. 91, p. 675-680, 4 figs., December 1980, Doc. no. S01201. 675 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/91/12/675/3444341/i0016-7606-91-12-675.pdf by guest on 25 September 2021 676 L. A. FERNANDEZ Figure 1. Geologic map of the Nordeste volcanic complex, Sao Miguel, Azores. feldspathic groundmass of plagioclase and minor olivine, pyroxene, MINERALOGY iron oxide, biotite, alkali feldspar, and apatite, to those in which plagioclase and hornblende are the dominant phenocrysts. Microprobe analysis was carried out extensively on all major Tristanite and trachyte are similar mineralogically, differing phases, with special emphasis being given to the olivines, mostly in texture and in different proportions of their constituent pyroxenes, and oxides because of the important roles they play in minerals. Both are characterized by plagioclase (more calcic in tris- the magmatic evolution of the complex. tanite than in trachyte) and kaersutite phenocrysts set in a domi- The over-all trend of olivine crystallization is one of limited iron nantly feldspathic matrix of alkali feldspar and minor clinopyrox- enrichment: Foss, average for zoned phenocrysts in basalt, to Fo40 ene, olivine, iron oxide, biotite, and apatite. for groundmass olivine in tristanite. No simple correlation could be Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/91/12/675/3444341/i0016-7606-91-12-675.pdf by guest on 25 September 2021 EXPLANATION J ® / Ql Alluvium Bt. Beach and Terrace •M / Clastic Sediments Pyroclastics sc; Scoria Cones Trachytes .An. Tristanites (Tristanite flows, plugs and minor trachybasolt flows) Utr Upper Basalts (Alkali basalt and trachybasolt flows) Ü \ iNa^ Nordeste Ankaramites Lb Lower Basalts (Alkali basalt and trachybasolt flows) Figure 1. (Continued). IO . Strike and dip of flow banding or foliation Generalized flow direction ( Petrography and structure of individual flows and groups of flows in each series cannot be unambiguously characterized into groups of rock types. See text for details of variation.) Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/91/12/675/3444341/i0016-7606-91-12-675.pdf by guest on 25 September 2021 678 L. A. FERNANDEZ made with major- or minor-element variations and the sequence of Biotite is a ubiquitous phase but never attains the status of an extrusion of the lavas (Boone and Fernandez, 1971). These obser- essential mineral. Its color and pleochroic formula indicate rela- vations, with corroborating data on pyroxenes, support the view tively high Ti4+ to Fe54 ratios. The amphiboles are restricted to in- that batches of cumulus crystals were frequently incorporated in termediate and late differentiates. Their pleochroism and 2VX val- successively differentiated liquids. ues indicate basaltic hornblendes or kaersutites. The one micro- Clinopyroxene, the most common ferromagnesian phase, ranges probe analysis is that of a low-Ti02 kaersutite. from diopsidic augite—> aluminous titanaugite —> augite—» aegirine augite. Comparison of the Nordeste clinopyroxene trend with the CHEMISTRY three pyroxene trends proposed by Gibb (1973) suggests that the Nordeste pyroxene trend is a variant of the strongly alkaline trend. The Nordeste volcanic rocks represent a potassic, mildly On the basis of the behavior of Al in the clinopyroxenes, the Nor- alkaline, mildly undersaturated to saturated alkali basalt —» deste lavas are divided into two groups: high-silica transitional trachybasalt—> tristanite —* trachyte series. All positive mineralogi- basalt with clinopyroxenes enriched in Si, Ca, and Cr, and a low- cal indicators of an alkali parentage are present: titanaugite, silica ankaramite —> trachyte series with pyroxenes enriched in Al groundmass olivine, and interstitial alkali feldspar. Plots of 60 and Ti. chemical analyses of Nordeste samples on a K20-versus-Na20 scat- ter diagram show that the Nordeste suite, in addition to the vol- canics from the younger provinces on Sao Miguel (central