THE PETROLOGY AND GEOCHEMISTRY OF HIGH CASCADE VOLCANICS IN SOUTHERN WASHINGTON: MOUNT ST. HELENS VOLCANO AND THE INDIAN HEAVEN BASALT FIELD (TRACE ELEMENT, ISOTOPES, MINERALOGY)

[摘要] Mount St. Helens volcano (Washington, USA) has been characterized by four eruptive periods during the last 2200 years. Eruptive products include a wide spectrum of rock types including basaltic to andesitic lavas, andesitic to dacitic pyroclastic flows and tephra, and dacite domes. The major and trace element compositions of some andesites and dacites are broadly consistent with their derivation from a basaltic andesite parental magma by fractional crystallization processes involving the observed phenocryst assemblages. However, the strontium and oxygen isotopic compositions of representative samples of the Mount St. Helens suite indicate that closed system processes cannot explain the isotopic variations. The isotopic ratios are positively correlated with one another and with bulk composition (SiO(,2), Mg number, etc.). The isotopic variations and trace element data support an origin of some intermediate and silicic rock types by combined processes of assimilation and fractional crystallization. The vents of the nearby Indian Heaven Quaternary volcanic field erupted several basalt types which can be defined on the basis of major and trace element composition--calcalkaline (low and high TiO(,2) varieties), transitional, and tholeiitic. Several of these basalt types occur at Mount St. Helens as well, but Indian Heaven lavas are generally more primitive as indicated by higher Mg/(Mg + Fe) ratios. Based on trace element abundances of estimated primary magma compositions, their genesis seemingly involves melting of heterogeneous upper mantle sources. The distribution of volcanic rock types in relation to local structures in the Cascade Range of southern Washington and northern Oregon suggests that crustal structure may influence the degree of evolution of specific volcanic fields. Two associations are observed: (1) Significant amounts of dacitic, as well as basaltic and andesitic, magmas are erupted from centers apparently associated with local compressional strain (e.g. Mount St. Helens). (2) Relatively undifferentiated basalts and lesser amounts of andesite are erupted in areas of local extensional strain. Dacites are scarce and tholeiitic basalts appear to be restricted to these areas (e.g. Indian Heaven). An extensional environment apparently favors the ascent of primitive basaltic magmas whereas compression favors stagnation within the crust, prolonged differentiation, and interaction with crustal material. The relationship between crustal strain and related volcanism in the Cascade arc suggests that volcanic arc magma evolution does not necessarily produce a continuous sequence from tholeiitic to calcalkaline rocks in time or space.

[发布机构] Rice University