ill. (some color), maps; Thesis (Ph.D.)--Pennsylvania State University; Includes bibliographic references (p. 118-124);
iii ABSTRACT Andesites and other rock types of the calc-alkaline volcanic series associated with the Circum-Pacific orogenic zone commonly contain polymineralic crystal clots. Polymineralic crystal clots and host rock have been investigated petrographically and geochemically from a stratigraphic sequence of eight cone-building andesitic flows from Crater Lake, Oregon. In these andesites, the clots consist of essentially the same phases that occur as phenocrysts in the rocks - plagioclase, orthopyroxene, clinopyroxene, magnetite, ilmenite, and a small amount of apatite -in a tightly interlocking microgranitic texture. The origin of these clots has been attributed to 1) random accumulations of phenocrysts, 2) phenocryst clusters formed by synneusis, 3) cumulate material that has been disrupted and reincorporated into the magma, 4) low pressure anhydrous products formed by the breakdown of high pressure amphiboles formed during the early crystallization history of a basaltic parent melt, 5) accidental microxenoliths, and 6) refractory residual material carried up from a zone of partial melting, possibly in the lower crust. Thus the origin of these clots has a bearing on the more general problem of the origin of calc-alkaline andesites. Electron probe results indicate that minerals occurring as both phenocryst and crystal clot constituents have virtually identical chemical compositions. Slight variations in chemistry iv indicate that the clots formed at slightly higher temperatures and at a slightly slower rate than did the phenocrysts. Petrographic results indicate that relative settling velocities of constituent phases of the clots can explain differences in modes between the phenocrysts and clots. The variations in chemical composition observed in the stratigraphic sequence of flows cannot be accounted for by using phases occurring as phenocrysts in previous flows as crystal extracts, nor can the sequence be derived from a single, differentiating parent using only the phenocrysts present in the flows as crystal extracts. The compositions of all eight flows can be derived from a "unique11 parent composition (similar in composition to basaltic andesites of the Crater Lake area) by fractional crystallization and settling of phases which occur as phenocrysts (in the proper proportions) in the flows. Thus, the existence of the polymineralic crystal clots in these rocks is best explained as being the result of reincorporation of disrupted cumulate material into the magma upon eruption. These results indicate that fractional crystallization and settling of crystals may have played a large role in the geochemical evolution of calc-alkaline andesitic magmas at Crater Lake, Oregon.
iii ABSTRACT Andesites and other rock types of the calc-alkaline volcanic series associated with the Circum-Pacific orogenic zone commonly contain polymineralic crystal clots. Polymineralic crystal clots and host rock have been investigated petrographically and geochemically from a stratigraphic sequence of eight cone-building andesitic flows from Crater Lake, Oregon. In these andesites, the clots consist of essentially the same phases that occur as phenocrysts in the rocks - plagioclase, orthopyroxene, clinopyroxene, magnetite, ilmenite, and a small amount of apatite -in a tightly interlocking microgranitic texture. The origin of these clots has been attributed to 1) random accumulations of phenocrysts, 2) phenocryst clusters formed by synneusis, 3) cumulate material that has been disrupted and reincorporated into the magma, 4) low pressure anhydrous products formed by the breakdown of high pressure amphiboles formed during the early crystallization history of a basaltic parent melt, 5) accidental microxenoliths, and 6) refractory residual material carried up from a zone of partial melting, possibly in the lower crust. Thus the origin of these clots has a bearing on the more general problem of the origin of calc-alkaline andesites. Electron probe results indicate that minerals occurring as both phenocryst and crystal clot constituents have virtually identical chemical compositions. Slight variations in chemistry iv indicate that the clots formed at slightly higher temperatures and at a slightly slower rate than did the phenocrysts. Petrographic results indicate that relative settling velocities of constituent phases of the clots can explain differences in modes between the phenocrysts and clots. The variations in chemical composition observed in the stratigraphic sequence of flows cannot be accounted for by using phases occurring as phenocrysts in previous flows as crystal extracts, nor can the sequence be derived from a single, differentiating parent using only the phenocrysts present in the flows as crystal extracts. The compositions of all eight flows can be derived from a "unique11 parent composition (similar in composition to basaltic andesites of the Crater Lake area) by fractional crystallization and settling of phases which occur as phenocrysts (in the proper proportions) in the flows. Thus, the existence of the polymineralic crystal clots in these rocks is best explained as being the result of reincorporation of disrupted cumulate material into the magma upon eruption. These results indicate that fractional crystallization and settling of crystals may have played a large role in the geochemical evolution of calc-alkaline andesitic magmas at Crater Lake, Oregon.