Martin J. Streck

High-Magnesian Andesite from Mount Shasta: A product of Magma Mixing and Contamination, not a Primitive Mantle Melt

It has been proposed that high-Mg andesites (HMAs) from the Mount Shasta area may represent near-primary mantle melts, carrying signatures of slab melt interaction with the Cascadia mantle wedge. We present strong evidence that their formation involved mixing of dacitic and basaltic magmas and entrainment of ultramafic crystal material, and thus they cannot represent primitive magmas. The rocks contain (1) low-Mg# (65–72) clinopyroxene (cpx) and orthopyroxene (opx) phenocryst cores containing dacitic melt inclusions, and (2) high-Mg# opx and olivine xenocrysts, all of which are rimmed by euhedral overgrowths of cpx or opx similar in Mg# (87) to skeletal olivine phenocrysts. Textural relations indicate that ultramafic xenocrysts reacted with dacitic liquid, after which the contaminated magma mixed with basaltic liquid to produce a hybrid HMA bulk composition. High Mg, Cr, and Ni derive from the latter inputs, whereas high Sr/Y and overall adakite affinity is inherited from the dacite end member, which is arguably crustal in origin. We suggest that open system processes may be more important in the petrogenesis of HMAs than generally recognized, and that their magnesian compositions do not necessarily imply that they are primitive mantle melts.

Crystallization and welding variations in a widespread ignimbrite sheet; the Rattlesnake Tuff, eastern oregon, USA

The 7.05 Ma Rattlesnake Tuff covers ca. 9000 km2, but the reconstructed original coverage was between 30000 and 40000 km2. Thicknesses are remarkably uniform, ranging between 15 and 30 m for the most complete sections. Only 13% of the area is covered with tuff thicker than 30 m, to a maximum of 70 m. The present day estimated tuff volume is 130 km3 and the reconstructed magma volume of the outflow is 280 km3 DRE (dense rock equivalent). The source area of the tuff is inferred to be in the western Harney Basin, near the center of the tuff distribution, based mainly on a radial exponential decrease in average pumice size, and is consistent with a general radial decrease in welding and degree of post-emplacement crystallization. Rheomorphic tuff is found to a radius of 40–60 km from the inferred source.Four facies of welding and four of post-emplacement crystallization are distinguishable. They are: non-welded, incipiently welded, partially welded and densely welded zones; and vapor phase, pervasively devitrified, spherulite and lithophysae zones. The vapor phase, pervasively devitrified and lithophysae zones are divided into macroscopically distinguishable subzones. At constant thickness (20±3 m), and over a distance of 1–3 km, nonrheomorphic sections can cary between two extremes: (a) entirely vitric sections grading from nonwelded to incipiently welded; and (b) highly zoned sections. Highly zoned sections have a basal non- to densely welded vitric tuff overlain by a spherulite zone that grades upward through a lithophysae-dominated zone to a zone of pervasive devitrification, which, in turn, is overlain by a zone of vapor-phase crystallization and is capped by partially welded vitric tuff. A three-dimensional welding and crystallization model has been developed based on integrating local and regional variations of 85 measured sections.Strong local variations are interpreted to be the result of threshold-governed welding and crystallization controlled by residence time above a critical temperature, which is achieved through differences in thickness and accumulation rate.

Anhydrite, pyrrhotite, and sulfur-rich apatite: tracing the sulfur evolution of an Oligocene andesite (Eagle Mountain, CO, USA)

Abstract The oldest known occurrence of magmatic anhydrite in a shallow sub-volcanic rock is at the Oligocene andesitic Eagle Mountain volcanic center (Huerto Formation, San Juan volcanic field), which immediately postdates the pyrrhotite-bearing Fish Canyon Tuff. Estimates of intensive parameters derived from the compositions of coexisting minerals (hornblende+plagioclase+anhydrite+pyrrhotite+S-rich apatite+augite+hypersthene+FeTi-oxides) indicate that the Eagle Mountain magma was wet, oxidized, and S-rich. It was much like K-rich andesitic and dacitic anhydrite-bearing magmas recently erupted from the subduction-related volcanoes El Chichon, Mt. Pinatubo, and Lascar. Although the Oligocene geodynamic setting of the San Juan volcanic field is poorly constrained, and its enormous volume is not easily reconciled with an arc setting, the Eagle Mountain occurrence emphasizes how closely some of its magmas resemble those from active continental margin arcs. S-rich magmas similar to the Eagle Mountain andesite also may have contributed to the formation of some hydrothermal ore deposits of the San Juan volcanic field.

Phenocryst-poor rhyolites of bimodal, tholeiitic provinces: the Rattlesnake Tuff and implications for mush extraction models

We consider the origin of rhyolites associated with tholeiitic basalt in bimodal provinces, as exemplified by the Rattlesnake Tuff of the High Lava Plains of eastern Oregon, in comparison to rhyolites associated with calcalkaline suites in light of recent models of extraction of rhyolite from crystal mush (Hildreth, J Volcanol Geotherm Res, 136:169–198, 2004; Bachmann and Bergantz, J Petrol, 45:1565–1582, 2004). The High Lava Plains encompass a strongly bimodal, tholeiite-rhyolite suite, spatially and compositionally related to the Snake River Plain and Yellowstone Plateau. In our assessment we draw the distinction between fractionation dominated processes to make rhyolites from rhyolites and processes required to make the parental rhyolite melt. New isotopic data and compositional zoning profiles in phenocrysts confirm that crystal fractionation dominated the generation of progressively more evolved, discrete rhyolites in the zoned Rattlesnake Tuff and are consistent with an origin of the least evolved high-silica rhyolites by partial melting of a mafic crust. While the most evolved rhyolites are compositionally virtually indistinguishable from those of calcalkaline suites, the parental rhyolites from bimodal suites are more Fe-rich than their calcalkaline counterparts. Oxygen isotope thermometry yields pre-eruptive temperatures of 860°C, in keeping with 800–880°C zircon saturation temperatures. High magmatic temperatures are common among rhyolites of bimodal suites, distinguishing them from cooler rhyolites of calcalkaline suites. Extraction of interstitial melt from a granodioritic mush cannot produce compositions of the Rattlesnake Tuff on the basis of major and trace element arguments (especially Fe, Ba, Sr, and Eu) and on the basis of temperature considerations. Chemically viable parental crystal mushes are syenite and alkali (A-type) granites for the production of all more evolved Rattlesnake Tuff rhyolites; ferro-dacitic mush is required for production of the least-evolved, parental Rattlesnake Tuff rhyolite. Paucity of such ferro-dacitic compositions in tholeiitic bimodal suites, especially compared to the abundance of dacitic (granodioritic) compositions in calcalkaline suites, argues against the mush extraction model for the parental rhyolite. Furthermore, rhyolites of bimodal suites lack associated voluminous eruptions of crystal-rich ignimbrite that might represent a parental mush, as exemplified by the “monotonous intermediate” Fish Canyon Tuff in calcalkaline suites. We conclude that extensive fractionation is common among rhyolites and may obscure their ancestry. Fe-rich parental rhyolites common in bimodal tholeiitic suites, as represented by Rattlesnake Tuff, may often be the result of partial melting of mafic to intermediate crust, in contrast to calcalkaline high-silica rhyolites that are related to voluminous suites of intermediate intrusive rocks where the pre-plutonic mush-extraction model works better.

The Strawberry Volcanics: generation of ‘orogenic’ andesites from tholeiite within an intra-continental volcanic suite centred on the Columbia River flood basalt province, USA

Abstract The widely distributed, mid-Miocene lavas of the Strawberry Volcanics of NE Oregon are compositionally diverse, ranging from basalt to rhyolite. They are composed mainly of calc-alkaline and mildly tholeiitic basaltic andesite and andesite. Ar–Ar dating and stratigraphic relationships indicate that the volcanic field was active from >16 Ma to c. 12 Ma ago and thus is coeval for the first 1–2 Ma with strongly tholeiitic flood basalts of the Columbia River Province that encircle the Strawberry Volcanics. Tholeiitic and calc-alkaline compositions develop subtle but noticeable differences towards higher silica contents. At silica contents of <55 wt% SiO2, calc-alkaline and tholeiitic lavas are essentially indistinguishable. Trace element constraints among Strawberry Volcanics and crustal rocks indicate that open-system processes, such as assimilation or magma mixing, are responsible for evolution along a calc-alkaline trend leading to ‘orogenic’ andesites from tholeiite. Exclusively tholeiitic basalts carrying evidence for a metasomatized mantle source erupted during the mid-Miocene of eastern Oregon. Consequently, tholeiite imparted the ‘subduction signals’ and crustal processing generated the calc-alkaline character to end up with compositions of typical ‘orogenic’ andesites at the Strawberry Volcanics. No primitive calc-alkaline basalt from the mantle is needed as parental magma here and possibly at other similar intra-continental calc-alkaline suites.

Temporal and crustal effects on differentiation of tholeiite to calcalkaline and ferro‐trachytic suites, High Lava Plains, Oregon, USA

[1] Strongly bimodal, basalt-rhyolite volcanism of the High Lava Plains Province of Oregon followed the Middle Miocene flood basalts of the Pacific Northwest and extends to recent time. During the 8 m.y. of volcanism recorded in the central High Lava Plains, in western Harney Basin, three distinct mafic magmatic trends originate from primitive high-alumina olivine tholeiites (HAOT); they are tholeiitic, calcalkaline and ferro-trachytic. Tholeiitic basalts occur throughout the history and their compositions are derived by crystal fractionation while traversing the crust and mixing with evolved mafic magmas. Scavenging of apatite from crustal rocks and minor contamination with felsic melts accounts for P, incompatible element enrichments and increasing tilts of incompatible element patterns with differentiation. The calcalkaline mafic suite occurs in temporal association with abundant silicic volcanism and is the only suite with Fe decreasing with Mg. Calcalkaline compositions are derived from evolved tholeiitic basalt by crystal fractionation coupled with assimilation of felsic crust or crustal melts. The ferro-trachytic suite occurs mainly late, is highly enriched in incompatible element with patterns parallel to tholeiites from which it is derived by protracted fractionation and recharge. The three suites primarily reflect changes in magma flux and crustal interactions in time. High magma flux promotes crustal melting and contamination of tholeiite to make the calcalkaline suite. On the other hand, ferrotrachytic magmas erupted mainly late in the sequence, during magmatic waning and after significant basaltification of the crust.

Large, persistent rhyolitic magma reservoirs above Columbia River Basalt storage sites: The Dinner Creek Tuff Eruptive Center, eastern Oregon

Our understanding of the Yellowstone hotspot and its connection to flood basalts of the Columbia River Basalt province (western and northwestern USA) has grown tremendously over the past decades since the model was first proposed in 1972. Despite strong support for a plume origin of the entire Yellowstone–Columbia River Basalt magmatic province, new non-plume models have emerged to explain early flood basalt volcanism. Unresolved issues of the early flood basalt stage include the location of crustal magma reservoirs feeding these voluminous eruptions and to what extent these were associated with contemporaneous silicic reservoirs. This study focuses on the newly defined ca. 16–15 Ma Dinner Creek Tuff Eruptive Center that overlaps in time and space with flood basalt volcanism of the Columbia River Basalt Group. New work on distribution, lithologic variations, geochemical compositions, and eruption ages indicate that the extensive Dinner Creek Welded Tuff (herein Dinner Creek Tuff) and associated mapped and unmapped ignimbrites include a minimum of 4 discrete cooling units that spread out over an area of ∼25,000 km2. Widespread fallout deposits in northeast Oregon and the neighboring states of Nevada, Idaho, and Washington have now been compositionally correlated with the redefined Dinner Creek Tuff. Compositional coherence between the ignimbrite sheets and fallout deposits indicate a common source, herein referred to as the Dinner Creek Tuff eruptive center (DITEC). Cognate mafic components (glass shards, pumice shards, and mafic globules) that range from dacite (∼68 wt% SiO2) to Fe-rich basaltic andesite (∼56 wt% SiO2) in composition are found in two of the cooling units. Major and trace element compositions of the more mafic components match the compositions of nearby Grande Ronde Basalt flows and dikes. Compositional similarities between cognate mafic components and Grande Ronde Basalt flows are direct evidence for coeval mafic and silicic magmatism linking DITEC and Grande Ronde Basalt eruptions. Furthermore, finding Grande Ronde Basalt magmas as coeruptive component in Dinner Creek Tuff suggests that Grande Ronde Basalt magmas were stored beneath Dinner Creek Tuff rhyolites, thereby providing the first direct evidence for the location of a storage site of Columbia River Basalt magmas. Shallow crustal rhyolitic reservoirs active during ca. 16–15 Ma that yielded tuffs of the DITEC and other surrounding contemporaneous and widespread rhyolites of the area likely imposed control on timing and place of eruption of Columbia River Basalt Group lava flows.

High-magnesian andesite from Mount Shasta: A product of magma mixing and contamination, not a primitive melt: COMMENT AND REPLY

The comment by [Kelemen and Yogodzinski (2007)][1] criticizes extrapolation of our results for Mt. Shasta high-Mg andesite (S-HMA) to interpretations of other occurrences of HMA. Our intent was to encourage the petrologic community to re-examine all HMAs to better understand their petrogenesis on an

High-magnesian andesite from Mount Shasta: A product of magma mixing and contamination, not a primitive melt: COMMENT AND REPLY REPLY

The comment by [Kelemen and Yogodzinski (2007)][1] criticizes extrapolation of our results for Mt. Shasta high-Mg andesite (S-HMA) to interpretations of other occurrences of HMA. Our intent was to encourage the petrologic community to re-examine all HMAs to better understand their petrogenesis on an

Petrology of “Mt. Shasta” high-magnesian andesite (HMA): A product of multi-stage crustal assembly

Abstract Occurrences of high-Mg andesite (HMA) in modern volcanic arcs raise the possibility that significant volumes of continental crust could be directly derived from Earth’s mantle. Such rocks are commonly associated with subduction of young, warm oceanic lithosphere or occur in areas heated by mantle convection. A relatively rare occurrence near Mt. Shasta in the Cascades volcanic arc has been considered to represent one such primary mantle-derived magma type, from which more evolved andesitic and dacitic magmas are derived. Recognition that the Shasta area HMA is actually a hybrid mixed magma, calls into question this notion as well as the criteria upon which it is based. We report new chemical and mineralogical data for samples of the Shasta HMA that bear on the components and processes involved in its formation. Several generations of pyroxenes and olivines are present along with different generations of oxide minerals and melt inclusions. The most magnesian olivines (Fo93) exhibit disequilibria textures, exotic melt inclusions, and reaction rims of Fo87 composition; these crystals along with spongy, ~Mg# 87 orthopyroxene crystals are interpreted to be xenocrystic and do not signify a primitive mantle derivation. The groundmass is andesitic with moderate MgO content, and melt inclusions of similar compositions are hosted by equilibrium olivine (~Fo87). The bulk magma (whole rock) is more magnesian, but primarily due to incorporation of mafic minerals and ultramafic xenolith debris. We propose that the exotic crystal and lithic debris in these rocks is derived from (1) dacitic magmas of possible crustal derivation, (2) prograded ultramafic rocks in the underlying crust, and (3) random lithic debris and crystals derived from conduit wall rocks and earlier intruded magmas within the feeder plexus beneath Shasta. The HMA is inferred to represent a mixture between evolved dacitic and primitive basaltic magmas as well as incorporation of xenolithic crystal cargo. There is no compelling evidence that HMA is present in large volumes, and it is not considered to be an important parental liquid to more evolved magmas at Shasta.