Geomorphological Classification of Volcanic Cones in the Itasy Volcanic Field, Central Madagascar

The Itasy Volcanic Field (IVF) is a monogenetic volcanic field located in the central highlands of Madagascar and is characterized by numerous volcanic cones displaying various geomorphological landforms. The IVF was a result of Pliocene to Quaternary volcanic eruptions and mainly composed of basaltic volcanic cones and trachyte domes. This paper aims to describe and to establish a geomorphological classification of basaltic volcanic cones within the IVF, based on their morphometric characteristics such as shape and diameter of the cone base; slope of the flanks of the edifice, and height of the crater relative to the cone base. Digital Elevation Model (DEM) data from Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and Shuttle Radar Topography Mission (STRM) were used, coupled with remote sensing data from Google Earth Pro and field investigation. DEM and imagery analyses and processing were performed using ArcGIS and Global Mapper. Five types of major basaltic volcanic landforms were identified within the IVF: semi-circular cones (e.g. Kasigie cone), representing 8.39% of the identified cones; symmetric and asymmetric horseshoes-shaped edifices (e.g. Mananasy and Fasia cones), which are the most dominant 74.8% of the cones; fissure cones (e.g. Mandetika cone) forming 5.34% of the cones; multiple clustered -shaped breached cones forming 6.1% of the volcanic cones (e.g. Ambohitritainerina cone) and single cone with multiple craters (e.g. Ambohitromby cone), which represent 6.1% of the cone. This contribution enables the characterization of morphological types of the volcanic cones within the IVF

Rapid conversion of an oceanic spreading center to a subduction zone inferred from high-precision geochronology

Where and how subduction zones initiate is a fundamental tectonic problem, yet there are few well-constrained geologic tests that address the tectonic settings and dynamics of the process. Numerical modeling has shown that oceanic spreading centers are some of the weakest parts of the plate tectonic system [Gurnis M, Hall C, Lavier L (2004) Geochem Geophys Geosys 5:Q07001], but previous studies have not favored them for subduction initiation because of the positive buoyancy of young lithosphere. Instead, other weak zones, such as fracture zones, have been invoked. Because these models differ in terms of the ages of crust that are juxtaposed at the site of subduction initiation, they can be tested by dating the protoliths of metamorphosed oceanic crust that is formed by underthrusting at the beginning of subduction and comparing that age with the age of the overlying lithosphere and the timing of subduction initiation itself. In the western Philippines, we find that oceanic crust was less than ∼1My old when it was underthrust andmetamorphosed at the onset of subduction in Palawan, Philippines, implying forced subduction initiation at a spreading center. This result shows that young and positively buoyant, butweak, lithosphere was the preferred site for subduction nucleation despite the proximity of other potential weak zones with older, denser lithosphere and that plate motion rapidly changed from divergence to convergence

Chemical determination of the Jorullo volcano in the Michoacán Guanajuato Volcanic Field (MGVF), Mexico

The 1759-1774 eruption of the Jorullo volcano in the Michoacán Guanajuato Volcanic Field (MGVF), Mexico, produced lavas that range in composition from basalt to basaltic andesite. We have conducted new major and trace element and isotopic studies (whole rock Sr-Nd-Pb-Hf-Os, and O isotopes in olivine separates) of the Jorullo lavas and tephras spanning the duration and compositional range of the eruption, to further constrain the potential roles of mantle source heterogeneity, subduction-related metasomatism, and crustal assimilation in the petrogenesis of the Jorullo magmas. This study presents the first Hf, high precision Pb and comprehensive oxygen isotope measurements for Jorullo volcanic rocks. All samples have arc-like trace element patterns with enrichments in large ion lithophile elements (e.g. Ba, Rb, and Pb) and depletions in fluid immobile elements (e.g. Nb, Ta). In addition, the samples show variations in 87Sr/86Sr (0.7038-0.7040), 143Nd/144Nd (0.51280-0.51285), 176Hf/177Hf (0.28297-0.28300), 206Pb/204Pb (18.62-18.66), 207Pb/204Pb (15.57-15.59) and 208Pb/204Pb (38.34-38.43). Osmium isotope signatures are, with one exception, more radiogenic than the depleted and primitive mantle (187Os/188Os = 0.1231-0.1616). Oxygen isotope ratios of olivine phenocrysts (d18.OSMOW = 5.70-6.02 per mil) show limited variation, but are isotopically heavier than normal mantle olivine. The samples define two geochemical groups: high-MgO samples with higher 87Sr/86Sr, lower 143Nd/144Nd and 176Hf/177Hf, and a positive correlation of Sr and Pb isotopes; and low-MgO samples displaying lower 87Sr/86Sr but higher 143Nd/144Nd and 176Hf/177Hf than the former group, and a negative correlation of Sr and Pb isotopes. The high-MgO group comprises most of the early tephra and lavas, whereas the low-MgO group includes most of the late tephra and lavas. These compositional variations are inconsistent with shallow level contamination, but rather are interpreted to reflect mantle source heterogeneity. Trace element and isotopic signatures are consistent with North Mexican Extensional Province (NMEP) mantle metasomatised by subduction components composed of sediment- and oceanic crust-derived hydrous fluid. The temporal-compositional variations observed in Jorullo magmas are inferred to result from a combination of variable degrees of fractional crystallization of magmas produced by tapping a progressively less metasomatised mantle source that is vertically and/or laterally heterogeneous