The West Elk Breccia has been studied since the late 1800’s with many interpretations regarding its origin. One unrecognized possibility is that parts of it are debris-avalanche deposits. This study has recognized evidence for this interpretation at three scales: volcano scale, outcrop scale, and intra-outcrop scale. At the volcano scale, a scarp in the old volcano reveals underlying Mesozoic bedrock, suggesting sector collapse. At the outcrop scale, megablocks of the original edifice, up to hundreds of meters in length, have atypical orientations and are surrounded by a gravel matrix. At the intra-outcrop scale, jigsaw-fit fracturing and rip-up clasts are common in distal deposits, which are documented in analogous debris-avalanche deposits. Similar to the debris-avalanche deposit at Mt. Shasta, medial-to-distal-matrix volcaniclast content decreases by 23%; Paleozoic and Mesozoic clasts increase by 5%; and the size of megablocks decreases. The geochemical and petrographic signatures reveal breccia blocks composed of pyroxene-andesite, a more silicic matrix facies, and the andesitic-to-dacitic East Elk Creek Tuff, all compositions that corroborate previous work on this northern extension of the San Juan volcanic field. Measured sections in the 100-km² study area allow for an estimation of total formation volume of approximately 8.5 km3.

Primitive magmas can provide important details about the mantle from which they are derived. In this thesis, petrographic observations and new mineral chemistry analyses of the Sälskär breccia from the Alnö alkaline complex, central Sweden, are reported. The rock is a volcanic breccia in which three main constituents were identified: spheroidal melilititic lapilli, angular sövite fragments and a carbonatedominated matrix. The mineralogy of the juvenile lapilli (i.e., with olivine, phlogopite, diopside, titanomagnetite, Cr-spinel, monticellite, perovskite, apatite and calcite) are consistent with what is expected to form by small degrees of partial melting of a volatile-rich, metasomatized, mantle source (indicated by the presence of phlogopite and primary calcite). The melilititic lapilli are frequently cored by a large crystal of olivine or phlogopite with concentrically aligned lath-shaped pseudomorphs after melilitite. During ascent the melilititic magma entrained mantle debris and traveled rapidly through the crust without significant fractionation.  The pyroclast textures indicate an explosive eruption, similar to those that produce pelletal lapilli in kimberlite-type volcanism. The sövite fragments and carbonate-dominated matrix were explosively fragmented during the eruption, which also dispersed the melilite mineralogy throughout the matrix. During the eruption a diatreme formed of within the uppermost 1.5 kilometers of the crust. A mismatch between the model of diatreme formation and a previous erosional model of the area leads to the conclusion that the Sälskär diatreme is either: (i) very deep, or (ii) that the diatreme volcanism took place significantly later than the rest of the complex / Primitiva magmor kan ge viktiga detaljer om manteln som de härstammar ifrån. I denna avhandling redovisas petrografiska observationer och nya mineralkemiska analyser av Sälskärsbreccian från Alnö alkalina område, Mellansverige. Bergarten är en vulkanisk breccia där tre huvudbeståndsdelar identifierades: sfäroidala melilitiska lapilli, kantiga sövitfragment och en karbonatdominerad matrix. Mineralogin hos de juvenila lapilli (dvs med olivin, flogopit, diopsid, titanomagnetit, Cr-spinel, monticellit, perovskit, apatit och kalcit) överensstämmer med vad som förväntas att det bildas av små grader av partiell smältning av en volatilrik, metasomatiserad, mantelkälla (indikeras av närvaron av flogopit och primär kalcit). De melilitiska lapilli är ofta kärnade av en stor kristall av olivin eller flogopit med koncentriskt riktade list-formade pseudomorfer efter melilitit. Under uppstigningen tog den melilitiska magman med sig mantelfragment och färdades snabbt genom skorpan utan betydande fraktionering. Pyroklastexturer indikerar ett explosivt utbrott, liknande de som producerar pelletala lapilli i kimberlitvulkansim. Sövitfragmenten och den karbonatdominerade matrixen fragmenterades explosivt under utbrottet, vilket också spred melilit mineralogin genom matrixen. Under utbrottet bildades en diatrem i de översta 1,5 kilometerna av skorpan. Detta betyder att modellen för diatrem-bildning och den tidigare erosionsmodellen av området inte överensstämmer. Detta leder till slutsatsen att Sälskärsdiatremen är antingen: (i) mycket djup, eller (ii) att diatrem-vulkanismen ägde rum betydligt senare än den sopm bildade resten av området.