August 2015 LIP of the Month

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The Madagascar Large Igneous Province

Ciro Cucciniello, Leone Melluso, Vincenzo Morra

Dipartimento di Scienze della Terra, dell’Ambiente e delle Risorse (DiSTAR), Università di Napoli Federico II, via Mezzocannone 8, 80134 Napoli (Naples), Italy;;


The Madagascar large igneous province (LIP) is one of the largest magmatic events in the Late

Cretaceous. It consists of lava flows, dykes, sills and intrusions (e.g., Storey et al., 1997; Melluso et al., 1997, 2001, 2002, 2003, 2005, 2006, 2009; Mahoney et al., 1991, 2008; Dostal et al., 1992). Remnants of this igneous province (the original pre-erosion extent of the province is difficult to estimate, although it probably exceeded 1 × 106 km2 including also the Madagascar Plateau and Conrad Rise; Storey et al. 1995) crop out along the rifted margin of the eastern coast, in the Mahajanga and Morondava basins of western Madagascar as well as the high plateau of the continental interior (Fig. 1), and in western India (cf. Melluso et al. 2009). The igneous rocks are most voluminous in the Volcan de l’Androy complex (southern Madagascar; where more than 2000 m of lava flows are present) and the Mahajanga Basin (northwestern Madagascar; Fig. 2). Significant outcrops, in the form of intrusive complexes, dykes and lava flows are in the Morondava Basin, the Maningoza area, and in two districts along the east coast: Sambava-Cap Masoala and Mahanoro-Manambondro. The main dyke trends in each region form a radiating pattern which converges near the estimated 88 Ma location of the Marion hotspot (Ernst & Buchan, 1997). Rare, isolated outcrops are also known in the high plateau (tampoketsa) of the continental interior of Kamoro, Beveromay, and Analamaitso (Fig. 1). The Antampombato-Ambatovy intrusion (Melluso et al. 2005) is rich in ultramafic rocks, and its lateritic soil is currently exploited for transition elements such as Ni and Co. The Ambohiby alkali granites cross-cut the central-western part of the Madagascan basement. Most of the volcanic successions are less than 200 m thick. The Madagascar LIP is dominated by mafic rocks; silicic rocks represent only a small volume of the province, overall (Melluso et al., 2001, 2005, 2009; Mahoney et al., 2008). The mafic rocks are mostly tholeitiic basalts and basaltic andesites (Fig. 3). Detailed geochemical and petrological studies of the vulcanic successions of northwestern margin of Madagascar were reported by Melluso et al. (1997, 2001, 2003, 2006) and Cucciniello et al. (2010, 2013).

Figure 1: Simplified geological map of Madagascar, showing the outcrops of Cretaceous igneous rocks (black and dark grey areas) and 40Ar/39Ar (grey circles) and U-Pb (black circles) age determinations with analytical uncertainties (2σ). The data are classified from north (top) to south (bottom), the y axis represents the exact latitude.

Figure 2: Spectacular tholeiitic lava flow (group C) from Antanimena plateau (western Mahajanga basin).

Figure 3: Total alkali vs SiO2 (T.A.S.) classification diagram for the Cretaceous igneous rocks of Madagascar LIP.

Current models of Madagascar flood basalt volcanism suggest that the Madagascar province occurred in response to rifting between Madagascar and Greater India and upwelling of Marion mantle plume (Storey et al., 1995; Torsvik et al., 1998). To date, there is no clear evidence of a chemical input of a plume component to the source of the basaltic rocks of Madagascar LIP.

Age of Madagascar LIP

Very few reliable 40Ar/39Ar ages are available for the Madagascar Cretaceous flood basalt province. Carefully statistically filtered 40Ar/39Ar age compilations were proposed by Cucciniello et al. (2013) following the χ2 statistical test approach (see Baksi, 2007a,b; Nomade et al. 2007; Jourdan et al. 2007b). The published data was reviewed for accuracy, precision and filtered in order to limit problematic artifacts such as the presence of non-atmospheric 40Ar (excess argon), significant sample disturbance or alteration. As a consequence, only 16 ages ranging from 92 to 85 Ma are available. In southern Madagascar, 40Ar/39Ar ages on feldspars from basaltic and rhyolitic lava flows and dykes (Volcan de l’Androy, Ejeda-Bekily, Mananjary) give a time span between 85 and 88 Ma(cf. filtered compilation by Cucciniello et al., 2013). In central and northern Madagascar, previous 40Ar/39Ar on whole-rock and plagioclase from basaltic lava flows and dykes (Vatomandry, Tamatave, Sambava, Antanimena and Bongolava-Manasamody) show older ages from 88 to 92 Ma (cf. filtered compilation by Cucciniello et al., 2013). Zircon U-Pb dating on rhyodacitic lava flows (Mailaka; Cucciniello et al. 2010), syenite (Antampombato; Melluso et al. 2005) and gabbro (Ananalava; Torsvik et al. 1998) intrusions range from ~90 to 92 Ma. The 40Ar/39Ar and U-Pb ages indicate that the Cretaceous magmatism first ceased in the northern part of the island.

Petrographic features and geochemical affinities of magma types

In the central part of the province (Mailaka area), the lavas and associated mafic dykes and sills include alkaline (or transitional) and subalkaline types. The subalkaline rocks can be classified as picrite basalt, tholeiitic basalt, basaltic andesite, andesite, dacite and rhyodacite. Rocks belonging to the alkaline type are picrite basalt and basalt, and have nepheline or few percent of hypersthene in their norms. The chemical and isotopic composition of the dykes and sills are identical to that of the basaltic flows. The picrite basalts have olivine phenocrysts set in a fine-grained groundmass of olivine, plagioclase, augite and Fe-Ti oxides. Basalts, basaltic andesites and andesites are aphyric to sparsely phyric, and contain phenocrysts of plagioclase and clinopyroxene with or without olivine. Orthopyroxene occurs as microphenocryst in a few (tholeiitic) samples, and pigeonite is mostly found in the groundmass. Dacites are aphyric rocks, with feldspar, pyroxenes and oxide microlites, whereas rhyodacites are porphyritic rocks, with plagioclase, orthopyroxene, fayalite, cordierite, Fe-Ti oxides as phenocrysts in a silicic glass (Melluso et al., 2001, 2006).

The transitional basalts have low abundances of Nb, Ta, Zr, and Hf. In the primitive mantle-normalized patterns (Fig. 4) the abundance of many elements are almost indistinguishable from MORB except for Rb, Ba, and Sr; also, the patterns are steeper from Gd to Lu than found in most Indian (or other) MORB. The selective enrichment of Rb, Ba, and Sr in the transitional basalts could indicate secondary alteration processes. The tholeiitic basalts have low TiO2 (<1.43 wt.%) and Nb contents and high La/Nb, are relatively LREE enriched and have peaks at Ba and Sr in the primitive- mantle-normalized patterns (Fig. 4). The transitional basalts show a relatively large range in (87Sr/86Sr)i (0.70298–0.70545; where i indicates corrected to 90 Ma), over a restricted range of (143Nd/144Nd)i (0.51273–0.51291; εNdi = +4.1 to +7.8). The large range in (87Sr/86Sr)i is very likely due to the influence of seawater (see Melluso et al. 2001). Their Pb isotope ratios exhibit small variation in 206Pb/204Pb (17.606–18.195), 207Pb/204Pb (15.437–15.492), and 208Pb/204Pb (37.594–37.951). The tholeiitic basalts display similar radiogenic (87Sr/86Sr)i (0.70357–0.70502) but less radiogenic 143Nd/144Ndi (0.51235– 0.51247; εNdi = -1.1 to -3.5; Fig. 5) than the transitional basalts.

Figure 4: Primitive mantle normalized patterns for mafic rocks of Madagascar LIP. Primitive mantle values are from Lyubetskaya & Korenaga (2007). Average normal mid-ocean ridge basalt (N-MORB) is from Niu & O’Hara (2003). Marion hotspot data are from Mahoney et al. (1992).

Figure 5: Sr, Nd and Pb isotopic compositions of Cretaceous Madagascar mafic rocks. The data of the Madagascar igneous province are from Cucciniello et al. (2010, 2013), Melluso et al. (2001, 2002, 2003, 2005), Storey et al. (1997), Mahoney et al. (1991, 2008). The high TiO2 (> 3wt %) basalts of Mananjary define Trend I. The Sambava and low TiO2 (< 2 wt %) Mananjary basalts (including the high Mg-Ti basalts), and the Tamatave-SainteMarie dyke swarm form Trend II. Fields for modern Marion hotspot and Southwest Indian Ridge (SWIR) MORB data are from are from Mahoney et al. (1992), Janney et al. (2005) and Meyzen et al. (2005). In 207Pb/204Pb vs 206Pb/204Pb diagram (b), the Northern Hemisphere Reference Line (NHRL) and the Geochron at 4.55 Ga are also shown.

In the Antanimena plateau (western Mahajanga basin; northern Madagascar) two main basalt groups (A and C) have been identified by Melluso et al. (1997). Both groups are tholeiitic and range from picritic basalt to basaltic andesite (MgO ranges from 13.2 to 3.3 wt.%). The basalts and basaltic andesites are nearly aphyric and contain plagioclase, augite, pigeonite, Fe-Ti oxide and glass. Plagioclase and augite are the dominant phases, both as microphenocrysts and microlites in the groundmass. Some of the Fe-rich fine-grained basalts show evidence of liquid immiscibility. The picritic basalts have olivine phenocrysts set in an ophitic matrix of olivine, plagioclase, clinopyroxene and opaque oxides (Melluso et al., 1997, 2006). The group A magma type is characterized by low TiO2 (< 1.5 wt.%), Nb and Zr concentration. Primitive mantle-normalized patterns (Fig. 4) are characterized by peaks in Ba and Pb and troughs in Nb and Ta. Group C rocks exhibit a narrow range in MgO (3.3-5.0 wt.%) and moderate TiO2 (2.1-2.6 wt.%) and Zr concentration. In the primitive mantle-normalized diagram (Fig. 4), the rocks of group C show troughs at Nb and Ta and peaks at Ba and Pb similar to those observed in group A. The basalts of groups A and C cover a wide range of (87Sr/86Sr)i and 143Nd/144Ndi (corrected to 90 Ma), from 0.70331 to 0.70837 and from 0.51253 to 0.51197 (εNdi = +0.1 to –10.8), respectively (Fig. 5) and the values correlate roughly with degree of LREE enrichment.

The basalts from Bongolava-Manasamody plateau (eastern Mahajanga basin; northern Madagascar) and Tampoketsa Kamoreen area were subdivided in two groups (B and D; Melluso et al., 1997). They belong to tholeiitic series (from basalt to basaltic andesite) and are mostly aphyric with rare plagioclase and augite microphenocrysts in a matrix of plagioclase, augite, orthopyroxene and/or pigeonite and Fe-Ti oxides (Melluso et al., 1997, 2006). Microlites of olivine are present only in the group B and in Tampoketsa Kamoreen basalts. The basalts of group B have moderate TiO2 (2.2-2.5 wt.%) and Zr contents. In the primitive-mantle normalized incompatible element patterns (Fig. 4), the basalts of group B have peaks at Ba and Nb, troughs at K, and smoothly decreasing normalized abundances from Nb to Lu. The group D is characterized by high TiO2 (3.3-4.9 wt.%), Nb and Zr contents. The incompatible element patterns (Fig. 4) of group D are the most enriched of the four magma groups. The basalts of groups B and D display less radiogenic (87Sr/86Sr)i (0.70377-0.70533) and more radiogenic εNdi (+1.0 to +4.0; Fig. 5), than the basaltic rocks of groups A and C. The basaltic rocks of groups A and C show low 206Pb/204Pb (15.283-16.325), 207Pb/204Pb (15.058-15.269) and 208Pb/204Pb (35.483-36.547). The basaltic rocks of group B have a restricted range in 206Pb/204Pb (17.327-17.355), 207Pb/204Pb (15.404) and 208Pb/204Pb (37.880-37.899) than the basalts of group D (206Pb/204Pb =16.518-17.075; 207Pb/204Pb = 15.086-15.279; 208Pb/204Pb = 37.511-38.009). Relative to basaltic rocks of groups A and C, all samples of groups B and D have higher Pb isotopic compositions (Fig. 5).

The Antampombato-Ambatovy complex is the largest intrusion in the central-eastern part of Madagascar LIP. The core of intrusion consists of  dunites, wehrlites and clinopyroxenites. Dunites (variably serpentinized) have cumulus olivine with interstitial spinel. Wehrlite and clinopyroxenite have cumulus clinopyroxene. In the clinopyroxenites, olivine is rare and interstitial. The mafic rocks (olivine gabbros and gabbros) have plagioclase and clinopyroxene as main cumulus phases. Olivine is a cumulus mineral only in some Mg-rich samples. Amphiboles and oxides are principally confined to the interstices between cumulus minerals or along the rims of clinopyroxenes. Accessory minerals as biotite, apatite, titanite and zircon are common (Melluso et al., 2005). The Antampombato dykes range in composition from Mg-rich basalts to rhyolites. The basaltic dykes show a pseudo-ophitic to intersertal texture and contain plagioclase, clinopyroxene, olivine and opaques. The Mg-rich basaltic dykes are strongly porphyritic with olivine phenocrysts set in an altered groundmass rich in secondary amphibole (Melluso et al. 2005). The ultramafic rocks are characterized by variable TiO2 (0.05-0.77 wt.%), Ni (558-2200 ppm) and Cr (1440-4095 ppm) contents. The abundances of incompatible elements are always low (e.g. Nb 0-3 ppm, Zr <30 ppm). The gabbroic rocks are characterized by higher TiO2 (0.42-1.44 wt.%) and Zr (17-74 ppm) and lower Ni (437-211 ppm) and Cr (843-210 ppm) contents than ultramafic rocks. The Antampombato dykes are weakly alkaline (nepheline normative or slightly hyperstene normative) and are characterized by wide range of MgO (24-6 wt.%), Nb (3.8-16.2 ppm), Zr (52-135 ppm) and Ba (46-382 ppm) contents. The Antampombato dykes are moderately LREE enriched and show no Nb anomalies in the primitive mantle normalized element patterns (Fig. 4) . The mafic and ultramafic rocks of Antampombato-Ambatovy complex have a moderate range in 87Sr/86Sri (0.70301-0.70367), for a restricted range of 143Nd/144Ndi  (0.51287-0.51293; εNdi = +6.7 to +8.0; Fig. 5).

In southern Madagascar, four groups of rocks were identified (1) tholeiitic basalts in the southeastern coastal area with low contents of incompatible elements, relatively high initial εNdi (+3.3 to +5.8) and low (87Sr/86Sr)i; (2) tholeiitic basalts and basaltic andesites in the southwestern area which have low εNdi (−5.0 to −17.4) and high (87Sr/86Sr)i (0.7114–0.7213), 207Pb/204Pb and 208Pb/204Pb for their 206Pb/204Pb, and variable TiO2 concentration (1.5–3.4 wt %); (3) dykes of alkaline basalts and basanites (Ejeda–Bekily dyke swarm) with εNdi from −2.4 to +4.9, (87Sr/86Sr)i from 0.7037 to 0.7056, low 206Pb/204Pb (17.4–18.1), and generally low TiO2 (1.0–2.3 wt %) and Nb (10–30 ppm; Mahoney et al. 1991); and (4) tholeiitic and andesitic basalts (group B1), transitional basalts (group B2) and rhyolites (groups R1 and R2) in the Androy complex (Mahoney et al., 2008). Mahoney et al. (1991) pointed out that the southwestern tholeiitic basalts were variably contaminated by ancient, low εNd and high 87Sr/86Sr continental material, whereas the Ejeda–Bekily alkaline rocks do not seem to have been contaminated by continental material. Three different mantle sources (Marion hotspot, a N-MORB-like component and an unusual low 206Pb/204Pb, low-εNd source) were invoked to explain the different geochemical features observed.

The Cretaceous igneous rocks that crop out along the eastern coast (Sambava, Tamatave, Sainte Marie island, Vatomandry and Mananjary; Fig. 1) have been studied by Storey et al. (1997), Melluso et al. (2002) and Cucciniello et al. (2011). Storey et al. (1997) identified high Fe–Ti (TiO2 >3 wt %), Fe–Ti (TiO2 <2 wt %) and high Mg-Ti basalts in the Mananjary district. The high Mg-Ti basalts are strongly enriched in the light REE similar to present-day Marion hotspot lavas and group D basalts (Fig. 4). In the primitive mantle-normalized incompatible element patterns (Fig. 4), the basalts from the Sambava, Tamatave and Sainte Marie show peaks at Ba, troughs at Sr, small negative or no Nb troughs with respect to La. Rb, Ba, and Th are slightly enriched relative to K. The less evolved Vatomandry basalts have low concentrations of incompatible elements (e.g. Nb = 5–9 ppm; Zr = 91–153 ppm) similar to those of Southwest Indian Ridge (SWIR) MORB and show no troughs at Nb, Sr and Ti on mantle-normalized incompatible element patterns (Fig. 4). The basalts of the Mananjary, Tamatave and Sambava transects form two isotopically distinct trends which correlate broadly with major element compositions (Fig. 5). Trend I appears to have been derived by mixing of normal mid- ocean-ridge-basalt (MORB)-like mantle and a Marion hotspot component, whereas Trend II is consistent with mixing of a low 206Pb/204Pb lithospheric mantle-derived component with a normal-MORB-like mantle component.


The Madagascan igneous rocks display a wide range of trace element and isotopic compositions suggesting that a number of distinct differentiation processes took place, involving partial melting, fractional crystallization and crustal contamination. Petrographic observations and major element behaviour indicate that the crystallization sequence of Madagascar mafic rocks is olivine ± plagioclase ± clinopyroxene. The sequence dunite-wehrlite-clinopyroxenite-gabbro at Antampompato indicates that more alkaline magmas filled that intrusion. Oxides and minor phase crystallization complete the paragenesis. The transitional basalts show evidence of fractionation of assemblages with higher clinopyroxene/plagioclase ratios. The tholeiitic lavas from Antanimena and Mailaka sectors are broadly compatible with the effects of fractional crystallization and low-pressure crustal contamination. The effects of crustal contamination are clear from trace element and isotopic data. The low εNdi and 206Pb/204Pb values indicate the involvement of ancient crustal material with low time-integrated Sm/Nd and U/Pb ratios. The tholeiitic basalts and dykes from southern Madagascar (Mahoney et al., 1991, 2008; Dostal et al., 1992) have trace element compositions similar to the Antanimena and Mailaka samples (Fig. 4), but their isotopic composition are different having higher 87Sr/86Sri, 206Pb/204Pbi, 207Pb/204Pbi and 208Pb/204Pbi ratios and lower εNdi values (Fig. 5). These characteristics are compatible with addition of crustal rocks with high time-integrated U/Pb and Th/Pb ratios.

Transitional basalts with moderate TiO2 contents crop out principally in the Mailaka district and in the Androy complex (B2 basalts) and occur as dykes and lava flows. The Mailaka transitional basalts have isotopic characteristics broadly similar to those of SWIR MORB (Mahoney et al., 1992). The limited Nd isotopic variations in the Antampombato and Mailaka transitional basalts exclude the involvement of crustal rocks in their genesis, thus confirming their primary feature of mantle source. Trace element and isotopic characteristics of Antampombato and Mailaka basalts support a greater involvement of MORB-like mantle in their genesis. The different Pb isotope ratios and trace element abundances between Mailaka and Antampombato transitional basalts and the present-day Marion hotspot lavas imply variable degrees of partial melting of distinct mantle sources. The slope of incompatible element patterns (from Gd to Lu) of Mailaka basalts suggests the presence of residual garnet in the mantle source.

Rocks with moderate to high TiO2 and Zr contents are abundant in the eastern coast (Tamatave, Sambava and Mananjary sectors) and in the eastern Mahajanga basin. These basaltic rocks appear to have been derived from an enriched mantle source.  Storey et al. (1997) noted that some basalts from eastern coast (Mananjary district) have a Sr-Nd-Pb isotopic compositions similar to those of the present-day Marion hotspot (Fig. 5) whereas, in the Tamatave and Sambava districts the contribution from lithospheric mantle become prevalent. The isotopic characteristics of eastern Mahajanga basin basalts indicate a derivation from an enriched lithospheric source with subsequently crustal contamination, as indicated by Pb isotope ratios.

The geochemical features observed in the basaltic rocks of Madagascan LIP suggest complex mixing relationships between partial melts of depleted mantle source and partial melts of enriched lithospheric mantle source (or Marion hotspot) plus assimilation of continental crust. The distribution of Madagascan igneous rocks reflects strong control by the lithospheric architecture.


We will always gratefully remember John J. Mahoney who was an inspiration for many geologists.


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