The Cabugi Peak, 7 km west of Lages, State of Rio Grande do Norte, NE Brazil, is one of the many necks in the Tertiary alkalic basaltic province of Rio Grande do Norte. It records the youngest (19.7 Ma) continental magmatism in Brazil. It consists of ankaratrites, basanites and olivine-basalts with basanitic or tholeiitic affinities, mainly composed of olivine, titanoaugite, iron oxide minerals, plagioclase, apatite and occasional glass. Spinel lherzolite nodules, although small, are common inclusions, composed of olivine, orthopyroxene, chrome diopside and spinel. The basalts are moderately to strongly silica undersaturated, SiO₂ contents varying from 39% to 45%, with almost all samples being nepheline-normative, and exhibit high concentrations of some incompatible elements such as Ti, K, Sr, and Ba. Two nodules show initial ⁸⁷Sr/⁸⁶Sr values of 0.70575 and 0.7042, higher than those for the host basalts, interpreted as a single source for the basaltic magmas or sources more homogeneous than the portion of the lithospheric mantle above it that provided the nodule suite. These results reveal isotopic heterogeneity and ⁸⁷Sr-enrichment in the mantle of the northeastern Brazil. Whole rock d ¹⁸O values for basalts of this suite vary from +6.7 to +7.9‰_SMOW, and values for pyroxenes from nodules vary from +6.5 to +7.2‰_SMOW, confirming that these are primary oxygen isotope values. The emplacement of this Tertiary basaltic suite is related either to internal readjustments within the South American plate during its westward displacement, or to the Tertiary pressure release of arched zones formed in the Upper Mesozoic during the opening of the South Atlantic ocean.

The Tertiary volcanic suite of Rio Grande do Norte, northeastern Brazil, is composed of ankaratrites, basanites and olivine-basalts with basanitic or tholeiitic affinities. They form plugs, necks, flows and dikes, which cut Proterozoic basement and Cretaceous sedimentary rocks of the Apodi Basin, in a N-S trend, which extends about 120 km and has a width of about 25 km (Sial, 1976).

The Cabugi peak (also known as Itaretama Hill, which means "region of many stones" in the old Tupi language) is one of the many necks of the suite, being the one in which a well-formed conical shape is better preserved, standing out about 500 m above the surrounding level, constituting one of the highest topographic points in Rio Grande do Norte. It records the youngest (19.7 Ma; Cordani, 1970) continental magmatism in Brazil.

The Tertiary alkalic basaltic suite occurs mostly concentrated in the State of Rio Grande do Norte, but there are some occurrences in the State of Paraíba and one in the State of Pernambuco. The Cabugi Peak is located 7 km west of Lajes town, in the valley of the Assu and Piranhas rivers, about 120 km from Natal, capital of Rio Grande do Norte; the center of its peak in the coordinates 36^o19’24"W and 05^o42’17"S. The access to the peak is via the paved BR 304 Federal road, from Natal (Fig. 1). The peak is easily seen from the road, once it stands high above the regional level, its black rocks forming a typical conical shape. It is part of the divisor between the basins of the Ceará-Mirim and Salgado rivers, the latter a tributary of the Assu River. It is located in a semi-arid region known in northeastern Brazil as sertão, and its talus is covered with a vegetation typical of this dry, hot climate. The alkalic basalts of the Cabugi Peak cut Paleoproterozoic gneisses of the Caicó Complex.

Figure 1. Location of the Cabugi Peak, 7 km west of Lajes, State of Rio Grande do Norte, northeastern Brazil.

Figure 2. Panoramic view of the Cabugi Peak, as seen from the road Lajes-Angicos, its basaltic rocks standing out around 500 m above the surrounding regional level. The talus of the Peak is covered by vegetation typical of dry and hot climate.

One of the first reports on the existence of the Cabugi Peak was provided by Moraes (1920), who recognized the Peak as an extinct volcano. Rolff (1965) considered it as a neck, and informed on the existence of similar necks in the region. The work by Leonardos and Araújo (1968) emphasized the petrography of peridotite nodules, abundant in this basaltic plug. Petrological work on these rocks involving geochemical approaches, including mineral chemistry were provided by several authors (e.g. Comin-Chiaramonti et al., 1986; Princivale et al., 1989; Sial et al., 1991), but a detailed work on the petrology and tectonic significance of this Tertiary alkaline province, as well as Paleozoic toleiitic basalts of the region was performed by Sial (1974), who studied also the peridotite nodules in several necks and plugs of the region. Geochronological work on some of the plugs and necks of the Tertiary basaltic suite of Rio Grande do Norte was provided by Ebert and Brochini (1968), Cordani (1970) and Ebert and Rodrigues (1973).

In this Tertiary province, flows and dikes are mostly olivine basalts, whereas plugs and necks are mainly basanites. In the nepheline-free olivine basalts in the plugs and necks, including the Cabugi Peak, olivine, titanaugite, iron oxide minerals, plagioclase, apatite and glass are the main constituent phases. Olivine occurs as euhedral to subhedral grains, which partially underwent reactions with the liquid (Sial, 1976). They are believed to be xenocrysts as they exhibit deformation, kink bands and wavy extinction. Plagioclase occurs as long laths, which are included by augite. Euhedral titanaugite occurs as phenocrysts as well as in the matrix, the larger grains showing sector zoning and herringbone structures.

In these rocks, crystallization began with olivine followed in sequence by plagioclase, pyroxene and iron oxides and some glass.

Peridotite nodules are common inclusions in this Tertiary suite, although they are rather small (average grain size is 3mm). Ultramafic nodules are mostly spinel lherzolites, but subordinate harzbugite also occurs at the nearby Serra Aguda plug (Sial et al., 1991). The lherzolite nodules present same mineral phases (olivine, orthopyroxene, chrome diopside and spinel), but show a pronounced modal variation that suggests an inhomogeneous or layered distribution of phases in the source mantle peridotite (Sial, 1974). Most lherzolite xenoliths is equigranular, the largest grains are usually olivine or orthopyroxene, which are anhedral, sometimes showing deformation. Reaction margins around orthopyroxene with small olivine grains and glass were interpreted by Sial (1974) as resultant from the interaction with the host basaltic magma.

Chrome diopside is less abundant than olivine and orthopyroxene, and is often smaller and interstitial to other minerals. Chromium spinel varies in abundance (1-7%), size, chemical composition, texture and degree of alteration in different nodules. It is anhedral, brown, or less frequently, green in color, with dark rims produced by reactions with basaltic magma. Often spinel forms graphic intergrowth with orthopyroxene or olivine, which in this case form large porphyroclasts. Spinel xenocrysts are found in the host basalts, with dark reaction rims and sometimes replaced by magnetite (Sial, 1974).

According to Sial (1976), there was little iron enrichment during the fractionation of the basaltic magma, which mostly crystallized under high oxygen fugacity (around –10), as indicated by the magnetite-ilmenite mineral chemistry. The crystallization initiated with magnesian olivine and was followed by magnetite with a long interval of crystallization. The chemistry of the oxide minerals together with the Ni partitioning between olivine and pyroxene or between one of these two phases and the groundmass, indicate crystallization temperature around 1100^oC, which is interpreted as the temperature under which the groundmass was formed (Sial, 1974). The temperatures for the lherzolite nodules are around 900^oC, much lower than the temperature found for the basaltic liquid when it reached the surface (Sial, 1976), which could be explained if it is assumed that the nodules represent refractory pieces of mantle rocks, brought up to the surface not in equilibrium with the basaltic liquid.

All the Tertiary suite was assumed by Sial (1976) to be related to a single magmatic chamber at a depth ³ 64km, as determined from the alumina content of orthopyroxene of the nodules, using the petrogenetic grid proposed by McGregor (1974). Sial (op cit) suggested that the first liquid produced in the magma chamber, derived from the partial melting of a spinel-lherzolite under a pressure of at least 20 kbar, was of alkali basaltic composition, which became picritic as the melting increased, giving rise to the rest of the basalts in this suite.

The basalts are moderately to strongly silica undersaturated, SiO₂ contents in the plugs and necks varying from 39% to 45%, with almost all samples being nepheline-normative (Sial et al., 1981). K₂O and Na₂O are higher in samples with lower silica content. Usually the basalts in the flows and dikes are richer in silica, alumina and lower in K₂O and Na₂O than in the plugs and necks (Sial, 1978).

These alkali and olivine basalts exhibit a very high concentration of some incompatible elements such as Ti, K, Sr (up to 6000 ppm) and Ba (1000-1300ppm) (Sial, 1978). Chondrite-normalized multi-elemental patterns for these basalts are negatively inclined and show enrichment in La and Ce, and a negative K anomaly, with total REE in xenoliths from the Cabugi Peak varying from 16 to 23 ppm (Sial et al., 1991). Chondrite-normalized REE patterns are all LREE-enriched, with steep negative slopes common in alkali basalts (Sial et al., op cit.).

Silica contents in the peridotite nodules varies from 43.6 to 45wt%; alumina and K₂O have a wider variation, from 0.7 up to 3.3wt%, for the former, and from 0.07 to 0.13wt% in the latter, which are normal values for mantle peridotites (Sial et al., 1991; Comin-Chiaramonti et al., 1986). Nodules from the Cabugi Peak are Ba (up to 90 times), K and LREE (around 10 times) enriched relative to primitive mantle compositions, and chondrite-normalized patterns are LREE enriched, La up to 30 times the chondrite abundance (Sial et al., 1991).

Apparent K-Ar ages of basaltic plugs and flows of this suite range from 13 to 42 Ma. (Sial et al., 1981). Clustering of data suggest that basaltic volcanism terminated about 13 m.y. ago, and that it commenced perhaps about 40 m.y. ago, but more likely as recently as 30 m.y. ago (Sial et al., 1991). Initial ⁸⁷Sr/⁸⁶Sr in four basaltic samples are between 0.7039 and 0.7044 (Sial et al., 1981), with and average value, calculated by weighting each datum inversely according to the size of its associated error, of 0.7042 ± 0.0012 (2s ) (Sial et al., 1991).

Two nodules from the Cabugi Peak show initial ⁸⁷Sr/⁸⁶Sr ratios of 0.70575 and 0.7042. These initial Sr ratios higher than those for the host basalts was interpreted by Sial et al (1991) as a single source for the basaltic magmas or sources more homogeneous than the portion of the lithospheric mantle above it that provided the nodule suite. These results reveal isotopic heterogeneity and ⁸⁷Sr-enrichment in the mantle of the northeastern Brazil (Sial et al., 1991).

Whole rock d ¹⁸O values for basalts of this suite vary from +6.7 to +7.9‰_SMOW, and values for pyroxenes from nodules vary from +6.5 to +7.2‰_SMOW. Although slightly higher than normal mantle values, results for pyroxenes confirm these are primary oxygen isotope values (Sial et al., 1991).

The emplacement of this Tertiary basaltic suite is related either to internal readjustments within the South American plate during its westward displacement, or to the Tertiary pressure release of arched zones formed in the Upper Mesozoic during the opening of the South Atlantic ocean (Sial, 1974).

The Cabugi Peak is one of the highest hills in the State of Rio Grande do Norte, its cone shape standing out about 500 m above surrounding level. As in most necks in this suite, the Cabugi Peak is composed of loose talus from the bottom to almost the top, formed by unsorted angular blocks, which resulted mostly from the collapse of primitive columns. Although most of the columnar structures are collapsed, horizontal columnar structures with pentagonal to tetragonal columns can still be observed. Basaltic bread-crust bombs, up to 40-cm diameter, can be found at half way to the top of the peak.

According to Sial (1974), the basaltic rocks in this peak are coarser grained than basalt (mainly basanite) in other plugs and necks in this province. They are composed of olivine, 10 to 20% of the volume of the rock, titanoaugite, iron oxide minerals, plagioclase, apatite and occasional glass. Olivine occurs as euhedral to subhedral grains both as phenocrysts and in the matrix, which partially underwent reaction with the magma. Titanoaugite occurs both in the matrix and as phenocrysts, in this case showing sector zoning and herringbone structure, and sometimes including plagioclase. Xenocrysts of olivine, orthopyroxene, clinopyroxene and spinel are common, assumed to be disaggregates from the lherzolite nodules. Potassium feldspar and quartz are less abundant xenocrysts. Anhedral olivine shows kink bands, wavy extinction and deformation lamellae and, sometimes, normal zoning. Many of the orthopyroxenes reacted with the magma, and show a rim of granular olivine, while around clinopyroxene a spongy material of indeterminate compositions is always present.

Peridotitic nodules are abundant in this nepheline-free olivine basalt, ranging in size from few cm up to 1 m (less common), and mainly located at the northeastern side of the neck. They are mostly equigranular spinel lherzolites and all have similar mineralogical composition (olivine, bronzite, chrome-diopside and spinel) with a pronounced modal variation. Olivine and orthopyroxene are by far the most abundant phases, whereas chrome-diopside tends to be smaller and interstitial to the other phases.

The Cabugi Peak is a State Ecologic Park (Parque Ecológico do Cabugi). The access to the hill, which is limited by barbed wire, can be done by car until the foot hill, and up hill by a walking path. It is suggested that the access to the hill as well as removing of any amount and size of material (basalts and nodules) should not be allowed without special permission. The few no-collapsed column structures could be destroyed and, eventually, the original cone-shape typical of the Peak could be modified, in the case of explotation of the rocks as building material. Signalization of the Peak could include geological informations, which would include, at least, rock type and age.