V. A. Forearc basin evolution during the transition from collision to subduction: Neogene tectonostratigraphy and sedimentary provenance of the northwesternmost Andes.

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Introduction. Transtension is a system of stresses that tends to cause oblique extension, i.e. combined extension and strike slip. Syn-volcanic transtensional deformations of the lithosphere may provide two possible scenarios for control of magmatic processes. One scenario assumes ascending sub-lithospheric melts that mark the permeable lithosphere in a transtension area without melting of the lithospheric material; products of volcanic eruptions in such a zone show only the sub-lithospheric mantle material; components of magmatic liquids do not reveal any connection to the lithospheric structure. Another scenario yields a direct control of melting in lithospheric sources in an evolving transtensional structure. In this case, spatial-temporal changes of lithospheric and sub-lithospheric components are a direct indication of the evolving transtensional zone. In this paper, we present arguments in favor of the transtensional origin of the lithosphere-derived melting anomaly along the Wudalianchi volcanic zone, which are based on the study of components in the rocks sampled from the volcanic field of the same name.Analytical methods. Trace elements were determined by ICP-MS using a mass-spectrometer Agilent 7500ce and isotopes using a mass-spectrometer Finnigan MAT 262. The methods used were described in the previous papers by Rasskazov et al. [2011] and Yasnygina et al. [2015]. Major oxides were measured by "wet chemistry".Structural setting of the Wudalianchi zone. This zone extends north-south for 230 km at the northern circuit of the Songliao basin, subsided in the Late Mesozoic -Early Cenozoic (Fig. 1).Timing of volcanism and variations of K2O contents in rocks from the Wudalianchi zone. Rocks, dated back to the Pliocene and Quaternary, show the stepwise increasing K2O content interval along the Wudalianchi zone from the southernmost Erkeshan volcanic field (5.6-5.8 wt %) to the northernmost Xiaogulihe-Menlu volcanic field (2.0-9.5 wt %) (Fig. 2).Spatial-temporal clustering of volcanoes in the Wudalianchi field. In terms of the general Quaternary evolution of volcanism in Asia [Rasskazov et al., 2012], spatial-temporal distribution and compositional variations of volcanic products, we distinguish three time intervals of the volcanic evolution: (1) 2.5-2.0 Ma, (2) 1.3-0.8 Ma, and (3) <0.6 Ma. The Central group of volcanoes showed persistent shifting of eruptions from Wohushan (1.33-0.42 Ma) to to Laoheishan (1720-1721, possibly earlier) to Huoshaoshan (1721) (Figs 3, 4, 5). No spatial-temporal regularity of eruptions in volcanoes of the Erkeshan field and Western and Eastern groups of the Wudalianchi field reflected background activity.Sampling. Representative sampling of rocks from the Wohushan-Huoshaoshan volcanic line was aimed to identify changing geochemical signatures along the whole volcanic line and in the course of eruptions in each volcano (Figs 3, 6, 7). For comparisons, the background volcanoes were also sampled.Silica and alkalis oxides. On the total alkalis-silica (TAS) diagram (Fig. ...

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[1] Postglacial basalts from Theistareykir (younger than 10,000 years), northern Iceland, define the depleted end of the spectrum of chemical and isotopic compositions observed in Icelandic volcanics but extend to some of the most enriched chemical and isotopic compositions found in Icelandic tholeiites. A spectacular feature of these basalts is the impressive correlations observed between radiogenic isotope ratios (Sr, Nd, Hf, and Pb) and almost the entire spectrum of major and trace element concentrations and ratios. The radiogenic isotope and major and trace element compositions are little affected by crystal fractionation and are essentially unaffected by interaction with the preexisting crust. The Theistareykir basalts must therefore be relatively close in composition to primary melts from the mantle. Consequently, their chemical and isotopic compositions provide a unique opportunity to investigate the nature of melting beneath Iceland and the geochemical character and origin of the mantle source. Large variations in incompatible element abundances require source heterogeneity, as well as variable extents of melting, to be important factors in determining the final chemical composition of the melts. Melting integrates over a large pressure range and is dominated by melting a depleted peridotite similar to the ambient depleted North Atlantic mantle. The isotopically enriched component is of relatively minor abundance and probably has a lower solidus temperature compared to the depleted component. More than one isotopically enriched component must be involved, but it is difficult to identify the end-member compositions using those of the lavas because of preeruptive averaging and damping of the enriched isotopic signals by mixing with the ambient depleted mantle or melts thereof, suggesting that the isotopic signals in Icelandic melts represent a somewhat muted isotopic signal of the enriched component(s) in the Icelandic source mantle. Comparison of the isotopic arrays of Icelandic basalts with those of global OIB suggests that the dominant enriched

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