Geochemistry and Mantle Source Characteristics of the Itasy Volcanic Field: Implications for the Petrogenesis of Basaltic Magmas in Intra-Continental-Rifts

Abstract We present new major and trace element and Sr, Nd, Pb, Hf, and Os isotope data for Quaternary basaltic lavas and tephra from the Itasy volcanic field, in order to assess the origin and sources of recent volcanism in the central highlands of Madagascar. Our age determinations provide further evidence for Pleistocene volcanism on the island, with ages between ∼31 to 104 ka. Mafic magmas from Itasy exhibit significant geochemical heterogeneity, and they are chemically distinct from those of the neighboring Ankaratra volcanic complex. Trace element signatures of the Itasy volcanic field are very similar to those of ocean island basalts (OIB), with significant enrichment in high field strength elements (e.g. Nb and Ta) and depletion in large ion lithophile elements (e.g. Rb, Cs, and K), and relatively high Nb/U (>45) and Ce/Pb (>25) ratios. Significant intra-volcanic field isotopic heterogeneity is observed, with well-defined negative correlations of 87Sr/86Sr with 206Pb/204Pb and 143Nd/144Nd. These correlations, along with highly radiogenic Os isotopic signatures in many Itasy samples, could be attributed to assimilation of the Precambrian basement. However, with the exception of Os, the elemental and isotopic characteristics of the Itasy magmas are inconsistent with significant crustal assimilation, and instead are interpreted to represent features of their mantle sources. The Itasy ΔeHf and Δ7/4 values fall within the range of MORB and OIB, and indicate derivation primarily from an asthenospheric mantle source. The absence of significantly negative ΔƐHf in the Itasy magmas argues against the involvement of ancient recycled oceanic crust or stranded lower crust in their sources, but the presence of a low µ (LOMU) component in some Itasy magmas can be attributed to pollution of the asthenosphere by delaminated subcontinental lithospheric mantle. A simple two-stage, three-component mixing model indicates that a mixture of mantle containing 90% depleted asthenosphere (DMM) and 10% foundered SCLM, which is further mixed with 30 to 50 % of a plume-type mantle component, can reproduce the isotopic compositions of the Itasy magmas. Based on our model, it is possible that easily fusible superplume materials might be widely distributed in the upper mantle underneath the EARS, from the Indian Ocean to the Red Sea-Gulf of Aden, and mixed with shallow asthenosphere that has itself incorporated eroded SCLM. These components are heterogeneously distributed at a small spatial scale, similar to that observed recently in other mafic monogenetic volcanic fields in both intraplate and subduction settings.