High-alumina Olivine Tholeiite Research Articles

Primitive arc magma diversity: New geochemical insights in the Cascade Arc

The origin of the large range of compositional diversity encompassed by primitive arc magmas and the respective roles of mantle heterogeneity versus slab-derived contributions are the center of ongoing debate. The Cascade Arc of western North America is a global endmember hot subduction zone and an ideal setting in which to examine fundamental questions about the origin of arc magmas. Cascade Arc primitive magmas define several distinct compositional endmembers, the most widespread of which are high alumina olivine tholeiite (HAOT), calc-alkaline basalt (CAB), and intraplate-type basalt (IPB). New high precision Sr-Nd-Hf-Pb isotopic and trace element data for 49 of the most primitive magmas from seven major High Cascades volcanic centers are used to assess whether the basalt groups are derived from geochemically distinct mantle sources. Along with recently published data for the Garibaldi Volcanic Belt (GVB), the northern and other major segment of the Cascade Arc, and Mt. Rainier, the new data expand the high-precision isotopic coverage to seventeen volcanic centers spanning the ~1300km length of the arc. In all investigated isotopic systems, the new High Cascades data allow for finer resolution with much less scatter than previous data. HAOT mantle sources are systematically more depleted in incompatible elements than CAB sources, yet High Cascades CABs and HAOTs are isotopically indistinguishable, defining a remarkable linear Pb isotopic array consistent with a single mantle endmember that is similar to the source of Juan de Fuca mid-ocean ridge basalts. This mantle has been variably modified by contributions from a single subducting sediment endmember that is a close isotopic match to average Northern Cascadia basin sediment. Although Astoria Fan sediment was previously used as a proxy for the subducting sediment composition in the Cascadia subduction zone, the isotope ratios of Cascades basalts record no input from Astoria Fan sediment. High Cascades CABs and HAOTs both contain subducting sediment input in the form of a melt, but CABs also contain melts of subducting oceanic crust which is most apparent in the southern Cascades high-Sr/P suite and in the GVB. The least sediment input is recorded by GVB basalts, which also become progressively lower in 208Pb*/206Pb* and Hf isotope ratios to the north, reflecting the influx of an isotopically distinct enriched mantle component that generates IPBs at the northern slab edge. Enriched mantle with a composition trending towards HIMU is also recorded by a distinct isotopic array that is defined by IPBs from the Mt. Adams-Simcoe back arc region. We propose that slab gaps permit the influx of sub-slab asthenospheric mantle in both the northern GVB and Mt. Adams-Simcoe back-arc. Otherwise, western North America is underlain predominantly by isotopically homogeneous, MORB-type depleted mantle from which both the CAB and HAOT basalt groups are derived. Our results from the Cascades imply that most of the compositional heterogeneity encompassed by primitive arc magmas is not related to the presence of multiple mantle components but to variability in the composition and quantity of slab contributions. This study highlights the importance of high precision Sr-Nd-Hf-Pb isotope and trace element data in obtaining new insights on the petrogenesis of arc magmas.

Isotopic compositions of intrusive rocks from the Wallowa and Olds Ferry arc terranes of northeastern Oregon and western Idaho: Implications for Cordilleran evolution, lithospheric structure, and Miocene magmatism

Late Paleozoic and Mesozoic intrusive rocks from the Wallowa and Olds Ferry arc terranes of the Blue Mountains Province, Oregon-Idaho, provide constraints on the paleogeographic and tectonic setting of magmatism preserved in both arcs. Sr, Nd, and Pb isotopic data show that the Wallowa terrane represents an isotopically depleted, juvenile intra-oceanic island arc. By contrast, isotopic data for intrusive rocks of the Olds Ferry arc are more isotopically enriched, and thereby establish a clear distinction between the two arcs. This distinction strengthens paleogeographic interpretation of the Olds Ferry terrane as a fringing continental arc, and it provides a basis for correlation to other inboard Cordilleran arc terranes including Quesnellia and Stikinia. The Wallowa terrane is by contrast more similar geologically and isotopically to the outboard Insular terranes. These isotopic data also constrain interpretations of regional lithospheric architecture. Isotopic profiles generated orthogonal to the inferred Wallowa–Olds Ferry terrane boundary and the western Idaho shear zone show abrupt increases in initial 87 Sr/ 86 Sr that mark the transitions between three geochemically distinct lithospheric columns. West-to-east spatial variability in the isotopic compositions of Neogene volcanic rocks is explained by the partial melting of these three geochemically distinct mantle reservoirs coupled to their respective crustal columns since the early Mesozoic, rather than alternative models of lithosphere-scale decollement offset during Sevier shortening. The inherited arc-related mantle of the Olds Ferry arc may also have played a primary role in the petrogenesis of distinctive Neogene low-K, high-alumina olivine tholeiites of the High Lava Plains.

Timing and significance of gabbro emplacement within two distinct plutonic domains of the Peninsular Ranges batholith, southern and Baja California

Evaluating the crucial role postulated for mantle-derived mafic magmas in the formation of silicic batholiths requires well-determined igneous crystallization ages from mafic rocks as well as petrologic data reflecting possible mantle source heterogeneities and/or variations in subcrustal processes. The abundant mafic plutons that crop out within the mid-Cretaceous Peninsular Ranges batholith (PRB, southern and Baja California) provide an important opportunity to investigate the relationship of mafic and silicic magmatism in Cordilleran batholiths. Zircon U-Pb laser ablation inductively coupled plasma mass spectrometry ages are reported for 24 samples of PRB gabbro collected throughout its 1400-km-long extent. The samples span the entire compositional spectrum of PRB gabbro from cumulate olivine gabbro with <10 ppm whole rock Zr content, to gabbronorite, to more fractionated hornblende gabbro. Eighteen of the samples are from the western zone “gabbro belt” of the batholith including the type locality for San Marcos Gabbro, while six are from the eastern zone where gabbro is rare, composing <1% of the outcrop area. The morphology of zircon in PRB gabbro ranges from anhedral fragments to euhedral grains, and this zircon has distinctly lower uranium concentrations and higher Th/U ratios than zircon from PRB granitoids. Most samples exhibit simple U-Pb systematics, but several contain inherited or entrained zircons, and two eastern gabbro samples contain distinctly bimodal age populations. Weighted mean 206Pb/238U zircon ages are interpreted as crystallization ages and range from 130.5 ± 1.5 Ma to 100.2 ± 3.0 Ma for western zone samples. The ages demonstrate that gabbros were emplaced throughout the ∼30 m.y. interval during which the bulk of the western zone was constructed, and conflict with the earlier interpretation of western gabbros as mafic forerunners that preceded emplacement of granitoid plutons. Eastern zone gabbros, in contrast, yield U-Pb ages that range mostly from 109.4 ± 2.9 Ma to 97.8 ± 2.3 Ma, narrowly predating the ca. 92–98 Ma eastern zone tonalite-trondhjemite-granodiorite batholith flareup, and thus may have played a role in crustal thickening and triggering of the flareup. Despite their partially cumulate character, whole rock major and trace element compositions of PRB gabbro closely match high-alumina olivine tholeiite and arc basalt and basaltic andesite from modern arcs. Gabbros from the west and east sides of the batholith, transverse to regional strike, have similar nearly flat rare earth element patterns that reflect derivation from a common partially contaminated depleted mantle source. The apparent across-strike petrologic uniformity of the gabbros contrasts strongly with PRB granitoids, which are well known for across-strike asymmetries in trace elements, initial 87Sr/86Sr, and δ18O, reflecting progressively deeper partial melting of mafic source rocks from west to east with additional input from a high-18O, high-87Sr end member on the east side of the batholith. The western PRB gabbros represent a plausible deep crustal source composition for derivation of western granitoids by partial melting or fractional crystallization. For the eastern batholith, the mafic source region requires modification by input of accretionary complex supracrustal rocks delivered via forearc subduction erosion underthrusting.

Volcanic signature of Basin and Range extension on the shrinking Cascade arc, Klamath Falls‐Keno area, Oregon

AbstractDetailed geologic mapping of the Klamath Falls‐Keno area revealed the complex relationship between subduction, crustal extension, and magmatic composition of the southern Oregon Cascade volcanic arc. Volcanism in the study area at ~7–4 Ma consisted of calc‐alkaline basaltic andesite and andesite lava flowing over a relatively flat landscape. Local angular unconformities are evidence that Basin and Range extension began at by at least ~4 Ma and continues today with fault blocks tilting at a long‐term rate of ~2°/Ma to 3°/Ma. Minimum NW‐SE extension is ~1.5 km over ~28 km (~5%). High‐alumina olivine tholeiite (HAOT) or low‐K, low‐Ti transitional high‐alumina olivine tholeiite (LKLT) erupted within and adjacent to the back edge of the calc‐alkaline arc as the edge receded westward at a rate of ~10 km/Ma at 2.7–0.45 Ma. The volcanic front migrated east much slower than the back arc migrated west: ~0 km/Ma for 6–0.4 Ma calc‐alkaline rocks; ~0.7 km/Ma, if ~6 Ma HAOT‐LKLT is included; and ~1 km/Ma, if highly differentiated 17–30 Ma volcanic rocks of the early Western Cascades are included. Declining convergence probably decreased asthenospheric corner flow, decreasing width of calc‐alkaline and HAOT‐LKLT volcanism and the associated heat flow anomaly, the margins of which focused on Basin and Range extension and leakage of HAOT‐LKLT magma to the surface. This declining corner flow combined with steepening slab dip shifted the back arc west. Compensation of extension by volcanic intrusion and extrusion allowed growth of imposing range‐front fault scarps only behind the trailing edge of the shrinking arc.

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

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.

The origin of high-Mg magmas in Mt Shasta and Medicine Lake volcanoes, Cascade Arc (California): higher and lower than mantle oxygen isotope signatures attributed to current and past subduction

We report the oxygen isotope composition of olivine and orthopyroxene phenocrysts in lavas from the main magma types at Mt Shasta and Medicine Lake Vol- canoes: primitive high-alumina olivine tholeiite (HAOT), basaltic andesites (BA), primitive magnesian andesites (PMA), and dacites. The most primitive HAOT (MgO ( 9 wt%) from Mt. Shasta has olivine d 18 O( d 18 OOl) values of 5.9-6.1%, which are about 1% higher than those observed in olivine from normal mantle-derived magmas. In con- trast, HAOT lavas from Medicine Lake have d 18 OOl values ranging from 4.7 to 5.5%, which are similar to or lower than values for olivine in equilibrium with mantle-derived magmas. Other magma types from both volcanoes show intermediate d 18 OOl values. The oxygen isotope composi- tion of the most magnesian lavas cannot be explained by crustal contamination and the trace element composition of olivine phenocrysts precludes a pyroxenitic mantle source. Therefore, the high and variable d 18 OOl signature of the most magnesian samples studied (HAOT and BA) comes from the peridotitic mantle wedge itself. As HAOT magma is generated by anhydrous adiabatic partial melting of the shallow mantle, its 1.4% range in d 18 OOl reflects a heter- ogeneous composition of the shallow mantle source that has been influenced by subduction fluids and/or melts sometime in the past. Magmas generated in the mantle wedge by flux melting due to modern subduction fluids, as exemplified by BA and probably PMA, display more homogeneous composition with only 0.5% variation. The high-d 18 O values observed in magnesian lavas, and prin- cipally in the HAOT, are difficult to explain by a single- stage flux-melting process in the mantle wedge above the modern subduction zone and require a mantle source enriched in 18 O. It is here explained by flow of older, pre- enriched portions of the mantle through the slab window beneath the South Cascades.

Eruptive history and tectonic setting of Medicine Lake Volcano, a large rear-arc volcano in the southern Cascades

Medicine Lake Volcano (MLV), located in the southern Cascades ∼ 55 km east-northeast of contemporaneous Mount Shasta, has been found by exploratory geothermal drilling to have a surprisingly silicic core mantled by mafic lavas. This unexpected result is very different from the long-held view derived from previous mapping of exposed geology that MLV is a dominantly basaltic shield volcano. Detailed mapping shows that < 6% of the ∼ 2000 km 2 of mapped MLV lavas on this southern Cascade Range shield-shaped edifice are rhyolitic and dacitic, but drill holes on the edifice penetrated more than 30% silicic lava. Argon dating yields ages in the range ∼ 475 to 300 ka for early rhyolites. Dates on the stratigraphically lowest mafic lavas at MLV fall into this time frame as well, indicating that volcanism at MLV began about half a million years ago. Mafic compositions apparently did not dominate until ∼ 300 ka. Rhyolite eruptions were scarce post-300 ka until late Holocene time. However, a dacite episode at ∼ 200 to ∼ 180 ka included the volcano's only ash-flow tuff, which was erupted from within the summit caldera. At ∼ 100 ka, compositionally distinctive high-Na andesite and minor dacite built most of the present caldera rim. Eruption of these lavas was followed soon after by several large basalt flows, such that the combined area covered by eruptions between 100 ka and postglacial time amounts to nearly two-thirds of the volcano's area. Postglacial eruptive activity was strongly episodic and also covered a disproportionate amount of area. The volcano has erupted 9 times in the past 5200 years, one of the highest rates of late Holocene eruptive activity in the Cascades. Estimated volume of MLV is ∼ 600 km 3, giving an overall effusion rate of ∼ 1.2 km 3 per thousand years, although the rate for the past 100 kyr may be only half that. During much of the volcano's history, both dry HAOT (high-alumina olivine tholeiite) and hydrous calcalkaline basalts erupted together in close temporal and spatial proximity. Petrologic studies indicate that the HAOT magmas were derived by dry melting of spinel peridotite mantle near the crust mantle boundary. Subduction-derived H 2O-rich fluids played an important role in the generation of calcalkaline magmas. Petrology, geochemistry and proximity indicate that MLV is part of the Cascades magmatic arc and not a Basin and Range volcano, although Basin and Range extension impinges on the volcano and strongly influences its eruptive style. MLV may be analogous to Mount Adams in southern Washington, but not, as sometimes proposed, to the older distributed back-arc Simcoe Mountains volcanic field.

Influence of slab thermal structure on basalt source regions and melting conditions: REE and HFSE constraints from the Garibaldi volcanic belt, northern Cascadia subduction system

Garibaldi volcanic belt (GVB) basalts were erupted above the relatively young (≤ 24 Ma) Juan de Fuca plate, which comprises the subducted oceanic lithosphere that becomes progressively younger (22–13 Ma), and presumably hotter, northward along the northern Cascadia convergent margin. Primitive and near-primitive mafic lavas of the 15-km-wide volcanic belt change from high-alumina olivine tholeiites and magnesian andesites near Glacier Peak, northwestern Washington, through transitional basalts to alkali-olivine basalts and basanites in the Bridge River-Salal Glacier areas, southwestern British Columbia. The distribution of different basalt types is consistent with varied source conditions imposed by differences in the thermal structure of the underlying subducted plate. Significant arc-parallel variations characterize REE and HFSE contents in GVB basalts and suggest that source enrichment processes and melting conditions vary within the mantle wedge as the age and thermal state of the underlying subducted plate changes. More northerly GVB basaltic suites tend to have higher TiO 2, Nb, Ta, total REE, La, Sm / Yb, Nb / Yb, Ti / V, Y / Sc and Zr / Yb and lower Th / U, Zr / Ti and Zr / Nb than their southern counterparts. The basalts have sub-chondritic to chondritic Nb / Ta (6–21) and super-chondritic Zr / Hf (up to 55.90) ratios that exhibit positive correlation. Only Mount Baker and Glacier Peak basalts exhibit the distinctive negative Nb–Ta anomalies associated with arc lavas. Inter-HFSE and REE fractionations (including La / Yb, La / Nb and Ce / Pb) show significant correlations with the inferred age of the underlying subducted plate. Proportions of slab-derived HFSE-REE components (SC) transferred to basalt sources in the Cascadia mantle wedge appear to vary from negligible (Ti, Nb, Ta, Zr, Hf, Y, Sm, Eu and Tb: less than 15% SC) to perceptible (Nd: up to 35% SC) through moderate (La: up to 75% SC) to substantial (U, Th and Pb: up to 95% SC). Arc-parallel HFSE-REE variations in primitive GVB basalts cannot be explained by variable degrees of depletion produced during prior episodes of melt generation in the mantle wedge. Instead, these differences in basalt chemistry probably reflect different extents of melting of a regionally homogeneous, locally heterogeneous, mantle wedge under conditions influenced by the thermal structure of the underlying subduction zone. Phase relationships and REE systematics of the primitive to near-primitive basalts argue that conditions of magma generation beneath the Bridge River, Salal Glacier, Meager Mountain and Cheakamus Valley lava fields involved lower degrees of melting, higher pressures, and mantle sources richer in garnet than those beneath Mount Baker and Glacier Peak. In addition, Nb / Ta in the Glacier Peak basalts exhibits slight positive correlations with Nb, Ta, La / Yb, and Th / Yb but not Nb / La or Nb / Th, implying that a residual mineral, most likely rutile, controlled extremely low HFSE partitioning into subduction-related fluids that equilibrated with mantle source regions above older, colder portions of the subducted plate.

Ancient and Modern Subduction Zone Contributions to the Mantle Sources of Lavas from the Lassen Region of California Inferred from Lu-Hf Isotopic Systematics

Hafnium isotopic compositions have been determined on a suite of INTRODUCTION calc-alkaline and high-alumina-olivine tholeiitic lavas from the The geochemical and isotopic variability of mantle wedge Lassen region of California and are used, in conjunction with peridotites beneath continental volcanic arcs is poorly previously published mineralogical, geochemical, and isotopic data, constrained. This is a result of the fact that most of what to constrain their petrogenesis. Positive correlation between Hf is known about the geochemistry of the mantle wedge is values and geochemical indices of the modern subduction component inferred from the composition of basaltic arc lavas, which, indicates that the isotopic compositions of the calc-alkaline lavas although derived from the wedge, are affected by variable record addition of radiogenic Hf from the subducted slab. However, contributions from the subducting slab and continental the addition of the modern subduction component increases the Hf crust (e.g. Gill, 1981; Tatsumi et al., 1986; Hawkesworth values of most calc-alkaline lavas by <0·5 units over estimates of et al., 1993, 1994). Despite these limitations there is non-subduction enriched peridotites of the mantle wedge. The Lu–Hf significant evidence to suggest that mantle wedge periisotopic systematics of the Lassen lavas suggest that the calc-alkaline dotites are heterogeneous in composition and mineralogy. magmas have equilibrated with garnet at some point in their history, For example, it has been postulated that the compositions whereas the tholeiitic magmas have not. These observations require of mantle peridotites beneath the Cascade arc vary from the two lava types to be derived from different sources. The isotopic compositions that are similar to sources for mid-ocean variability of the Lassen lavas cannot be produced by mixing mantle ridge basalt (MORB), Hawaiian ocean-island basalt sources inferred to be present in the eastern–central Pacific and (OIB), and high-alumina-olivine tholeiites (HAOT) from western USA with a modern subduction component. Instead, the the Basin and Range province (Hughes, 1990; Leeman isotopic variability is consistent with mixing of a depleted mantle et al., 1990; Bacon et al., 1997; Borg et al., 1997; Clynne source, a more fertile mantle source enriched by an ancient subduction & Borg, 1997). The restite mineralogy of the source component, and a modern subduction component. regions for Cascade magmas is poorly constrained as well. For example, experiments conducted by Baker et al. (1994) found that calc-alkaline basalts from the Mt. Shasta region of California did not have garnet on their liquidus, and Reiners et al. (2000) were able to model

Hot, shallow mantle melting under the Cascades volcanic arc

Melting occurs at progressively greater depths and higher temperatures from west to east across the Cascades volcanic arc in northern California, as demonstrated by compositional variations observed in high-alumina olivine tholeiites. The lavas studied erupted from seven vents defining a 75-km-long, east-west transect across the arc, from near Mount Shasta to east of Medicine Lake volcano. The increase in melting depth across the arc parallels modeled isotherms in the mantle wedge and does not parallel the inferred dip of the slab. The depth of mantle melting at which the high-alumina olivine tholeiites were created is ∼36 km at the western end of the transect and 66 km at the eastern end. The very high temperatures of dry melting so close to the crust indicate a transitory condition of the mantle.

A comparison of basaltic volcanism in the Cascades and western Mexico: compositional diversity in continental arcs

Many physical similarities between the Cascades and western Mexican subduction zones would lead one to believe that volcanism in these two arcs should be similar. Despite the fact that there are four basaltic lava series [calc-alkaline basalt (CAB), intraplate alkaline (IA) basalt, high-alumina olivine tholeiite (HAOT) and high K (lamprophyric and relatively dry potassic)] represented in these arcs, there are significant differences which remain difficult to explain. First is the occurrence of lamprophyric lavas in western Mexico but not the Cascades. Second is the occurrence of HAOT in the Cascades but not western Mexico. The presence of lamprophyric lavas in western Mexico, but not in the Cascades may be due to the presence of older metasomatized upper mantle beneath Mexico. Many explanations have been proposed for the origin of the IA- and HAOT-series including melting of depleted lithospheric mantle, both shallow and deep melting of asthenosphere, and melting of heterogeneous mantle. However, the most compelling and successful idea is that these series result from decompression melting of depleted and/or enriched asthenosphere. Models for the genesis of calc-alkaline basalt series are similar for both arcs — melting of a subduction modified mantle at depths near the base of the crust. Future studies must concentrate on unravelling the importance of source contamination (due to slab dehydration) and crustal contamination.

Late Holocene hydrous mafic magmatism at the Paint Pot Crater and Callahan flows, Medicine Lake Volcano, N. California and the influence of H2O in the generation of silicic magmas

This paper characterizes late Holocene basalts and basaltic andesites at Medicine Lake volcano that contain high pre-eruptive H2O contents inherited from a subduction related hydrous component in the mantle. The basaltic andesite of Paint Pot Crater and the compositionally zoned basaltic to andesitic lavas of the Callahan flow erupted approximately 1000 14C years Before Present (14C years b.p.). Petrologic, geochemical and isotopic evidence indicates that this late Holocene mafic magmatism was characterized by H2O contents of 3 to 6 wt% H2O and elevated abundances of large ion lithophile elements (LILE). These hydrous mafic inputs contrast with the preceding episodes of mafic magmatism (from 10,600 to ∼3000 14C years b.p.) that was characterized by the eruption of primitive high alumina olivine tholeiite (HAOT) with low H2O (<0.2 wt%), lower LILE abundance and different isotopic characteristics. Thus, the mantle-derived inputs into the Medicine Lake system have not always been low H2O, primitive HAOT, but have alternated between HAOT and hydrous subduction related, calc-alkaline basalt. This influx of hydrous mafic magma coincides temporally and spatially with rhyolite eruption at Glass Mountain and Little Glass Mountain. The rhyolites contain quenched magmatic inclusions similar in character to the mafic lavas at Callahan and Paint Pot Crater. The influence of H2O on fractional crystallization of hydrous mafic magma and melting of pre-existing granite crust beneath the volcano combined to produce the rhyolite. Fractionation under hydrous conditions at upper crustal pressures leads to the early crystallization of Fe-Mg silicates and the suppression of plagioclase as an early crystallizing phase. In addition, H2O lowers the saturation temperature of Fe and Mg silicates, and brings the temperature of oxide crystallization closer to the liquidus. These combined effects generate SiO2-enrichment that leads to rhyodacitic differentiated lavas. In contrast, low H2O HAOT magmas at Medicine Lake differentiate to iron-rich basaltic liquids. When these Fe-enriched basalts mix with melted granitic crust, the result is an andesitic magma. Since mid-Holocene time, mafic volcanism has been dominated primarily by hydrous basaltic andesite and andesite at Medicine Lake Volcano. However, during the late Holocene, H2O-poor mafic magmas continued to be erupted along with hydrous mafic magmas, although in significantly smaller volumes.

Enrichment of basalt and mixing of dacite in the rootzone of a large rhyolite chamber: inclusions and pumices from the Rattlesnake Tuff, Oregon

A variety of cognate basalt to basaltic andesite inclusions and dacite pumices occur in the 7-Ma Rattlesnake Tuff of eastern Oregon. The tuff represents ∼280 km3 of high-silica rhyolite magma zoned from highly differentiated rhyolite near the roof to less evolved rhyolite at deeper levels. The mafic inclusions provide a window into the processes acting beneath a large silicic chamber. Quenched basaltic andesite inclusions are substantially enriched in incompatible trace elements compared to regional primitive high-alumina olivine tholeiite (HAOT) lavas, but continuous chemical and mineralogical trends indicate a genetic relationship between them. Basaltic andesite evolved from primitive basalt mainly through protracted crystal fractionation and multiple cycles (≥10) of mafic recharge, which enriched incompatible elements while maintaining a mafic bulk composition. The crystal fractionation history is partially preserved in the mineralogy of crystal-rich inclusions (olivine, plagioclase ± clinopyroxene) and the recharge history is supported by the presence of mafic inclusions containing olivines of Fo80. Small amounts of assimilation (∼2%) of high-silica rhyolite magma improves the calculated fit between observed and modeled enrichments in basaltic andesite and reduces the number of fractionation and recharge cycles needed. The composition of dacite pumices is consistent with mixing of equal proportions of basaltic andesite and least-evolved, high-silica rhyolite. In support of the mixing model, most dacite pumices have a bimodal mineral assemblage with crystals of rhyolitic and basaltic parentage. Equilibrium dacite phenocrysts are rare. Dacites are mainly the product of mingling of basaltic andesite and rhyolite before or during eruption and to a lesser extent of equilibration between the two. The Rattlesnake magma column illustrates the feedback between mafic and silicic magmas that drives differentiation in both. Low-density rhyolite traps basalts and induces extensive fractionation and recharge that causes incompatible element enrichment relative to the primitive input. The basaltic root zone, in turn, thermally maintains the rhyolitic magma chamber and promotes compositional zonation.

Experimental evidence for high-temperature felsic melts formed during basaltic intrusion of the deep crust

Basaltic intrusion into the continental crust was simulated with melting experiments on layered charges of high-alumina olivine tholeiite (HAOT) and metapelite, at 11 kbar. Rapid diffusion of CaO and Na 2 O into the metapelite and H 2 O, K 2 O, SiO 2 , and Al 2 O 3 into the basalt resulted in spectacular changes in the phase assemblage. Garnet megacrysts crystallized profusely across the basalt layer, cumulate assemblages closely resembled rocks found in underplating localities, and glass compositions graded from dacite to rhyolite, even at temperatures as high as 1150 °C. Silicic melts can be generated during interaction of basalt with metapelite, at temperatures well above the H 2 O-saturated granite minimum. Calculations based on these results and comparison with natural occurrences suggest that high-temperature felsic melts with 2 O can form and segregate at rates of ∼1 cm/yr.

The igneous petrology and magmatic evolution of the Midcontinent rift system

The 1100 Ma Midcontinent rift system of North America is one of the world's great igneous provinces, containing over one million km 3 of mafic volcanic and plutonic rocks. Volcanic rocks range from picrite to rhyolite, although tholeiitic basalt predominates. Plutonic rocks range from peridotite to granite, with plagioclase-rich gabbros and troctolites being most abundant. The igneous history of the rift can be divided into early and late stages; flows of the early stage form a trend having low-alumina, high-magnesia picrite as the primitive endmember, whereas flows from the late stage form a trend having high-alumina olivine tholeiite as the primitive endmember. The early basalts have enriched Nd isotopic compositions and were derived largely from old, heterogeneous subcontinental lithosphère. The late basalts have chondritic to depleted Nd isotopic compositions and were derived from large degree partial melts of a mantle plume that had mixed with isotopically depleted asthenospheric mantle. Apparently, melting began initially in the subcontinental lithosphere as it was stretched. Subsequently, a lower mantle plume, which had entrained asthenospheric mantle during its ascent, rose beneath the rift and melted at large degrees to yield the voluminous later basalts.

Petrology of monogenetic volcanoes, Mount Bailey area, Cascade Range, Oregon

The Mt. Bailey area encompasses the contact between Western Cascade and High Cascade volcanic rocks in southern Oregon. Western Cascade rocks are represented by a thick stack of magnetically reversed lavas and intercalated laharic deposits (the “Devils Canyon sequence”) that range in composition from calc-alkaline basalt to andesite. The oldest (magnetically reversed) rocks of the High Cascade sequence formed two small andesitic shield volcanoes and, in the northern part of the area, several intracanyon flows of high-alumina olivine tholeiite (HAOT). These eruptions were followed by formation of a basaltic shield volcano (Sherwood Butte) and then by the eruption of andesitic lavas to form Mt. Bailey, and basaltic andesite which formed the adjacent cone of Garwood Butte. The Devils Canyon lavas are enriched in total Fe, total alkalis, K 2O, P 2O 5, and TiO 2 relative to the High Cascade lavas. High Cascade lavas display calc-alkaline affinities (e.g., enrichment of large-ion lithophile element and light rare earth elements) except for the intracanyon HAOT, which has trace-element characteristics typical of MORB. Rb/Zr ratios suggest that calc-alkaline lavas from each of the High Cascade volcanoes represent distinct episodes of partial melting. The source of the calc-alkaline magmas was probably mantle enriched by fluids derived from subducted oceanic crust, whereas the source of the HOAT was depleted mantle similar to the source of MORB.

Common parent magma for Miocene to Holocene mafic volcanism in the northwestern United States

Primitive high-alumina olivine tholeiites have been documented from a wide area of Cenozoic volcanism in the northwestern United States. Several new localities extend the geographic range of these rocks to the Cascade Range of northern Oregon, the Picture Gorge region of north-central Oregon, and the Powder River volcanic field of northeastern Oregon. These new sites also extend the age range over which these primitive magmas were generated in the northwestern United States to between 16 Ma and the present. All of the high-alumina olivine tholeiites are of remarkably uniform chemical and mineralogical composition and are similar to primitive ocean-ridge and island-arc tholeiites. The wide distribution in time and space of this magma type and uniform primitive composition suggest that a relatively homogeneous oceanic mantle source underlies much of the northwestern United States. Basalts from the different volcanic provinces exhibit varied evolutionary trends away from this parental composition, reflecting the distinct magmatic processes involved in each province. Trace element and rare earth element abundances in the primitive high-alumina olivine tholeiite lavas record the involvement of crustal material in their genesis. It is argued that this chemical signature was acquired by assimilation of crustal material by a primitive magma such as mid-ocean ridge basalt (MORB) en route to the surface.

Pyroxene oikocrysts in troctolitic cumulates ? evidence for supercooled crystallisation and postcumulus modification

Studies of pyroxene oikocrysts in plagioclase-olivine cumulates crystallised from high alumina olivine tholeiite magmas in layered intrusions inferred to represent periodically replenished magma chambers show that small plagioclases enclosed within the oikocrysts differ significantly from plagioclases in the host rock. Plagioclases within oikocrysts are very elongate with a high surface area, show extensive mutual attachment at high angles resembling skeletal growth, and tend to be randomly orientated. Some are rounded and embayed, are reversely zoned and more sodic by An 5–10 than some of the plagioclase in the host rock. Plagioclase crystal centres tend to be closer spaced in the oikocrysts than in the host rock. In contrast, plagioclases in the host rock are discrete, coarser grained, less elongate, strongly aligned, and locally polygonised.

Cyclic units in the Somerset Dam layered gabbro intrusion, southeastern Queensland, Australia

The well-preserved Somerset Dam intrusion probably represents a small, relatively shallow, subvolcanic magma chamber. The 500-m-thick exposed sequence consists of 22 macrolayers which are defined by sharp phase, modal and textural contacts. At least six cyclic units, 30–150 m thick, are exposed, and the sequence from the base to the top of a cyclic unit is inferred to be leucogabbro (plagioclase cumulate), troctolite (plagioclase-olivine cumulate), olivine gabbro (plagioclase-augite-olivine cumulate), and oxide gabbro (plagioclase-augite-(olivine)-magnetite-ilmenite cumulate). Mineral compositions in a typical cyclic unit show a reversed fractionation trend in the sequence leucogabbro-troctolite, and a normal fractionation trend from troctolite (the least fractionated rock type) to the oxide gabbro (the most fractionated rock type). The most sensitive parameters for defining the cryptic trends are An in plagioclase, Fo and Ni in olivine, and Cr in magnetite and augite. Whole-rock compositions also show marked changes, and Fe, Ti, V, S and Cu increase and Al, Mg Fe , Cr and Ni decrease from troctolite to oxide gabbro. Despite the remarkable similarity of successive cyclic units, significant differences exist between them in the sequences of layers, thicknesses of individual layers and of the cyclic units, mineral compositions and cryptic patterns, average level of fractionation and the size of the reversals. Unit 3 is particularly unusual because it is the least fractionated and consists of two incomplete subunits. Unit 1, the lowest exposed, is the most fractionated. These differences between the units cannot be explained in terms of a closed system, and are strong evidence for an open system involving periodic injections of magma. The formation of a cyclic unit appears to reflect the dominant control of the order of crystallisation from a batch of replenished magma, which is essentially plagioclase first, followed by olivine, augite, magnetite and ilmenite, and brown hornblende. The parent magma is inferred to have been a high-alumina olivine tholeiite. The gradual rather than sharp reversals in the cryptic trends in the lower part of each cyclic unit may reflect some magma mixing, though postcumulus processes could have been operative.

Areal distribution and age of low-K, high-alumina olivine tholeiite magmatism in the northwestern Great Basin

Field, petrographic, chemical, and geochronologic information has led to the identification and characterization of a widespread low-K, high-alumina olivine tholeiite (HAOT) magma type in the northwestern Great Basin. This basalt covers at least 22,000 km 2 and is estimated to represent a total volume of at least 650 km 3 . The time period over which HAOT lavas were erupted extends from late Miocene to Holocene (10.5–0 m.y. B.P.). This interval overlaps with the timing of Snake River, Cascade, and northwestern Basin and Range volcanism but distinguishes HAOT from the main pulse of Columbia River volcanism (∼15 m.y. B.P.). Furthermore, three major pulses of HAOT magmatism are suggested from the geochronology of this study: 0 to 2.5 m.y. B.P., 3.5 to 6 m.y. B.P., and 7 to 10 m.y. B.P. The distinctive holocrystalline, nonporphyritic, and diktytaxitic texture, the low incompatible-element concentrations, and the high MgO/FeO* of HAOT serve to distinguish this basalt from other basalts of the northwestern United States. The low incompatible-element signature accentuates the similarities between HAOT, mid-ocean–ridge basalts, and back-arc–basin basalts. These similarities, combined with the HAOT chronology, support the idea that the processes giving rise to extensional tectonism and HAOT magmatism in the northwestern Great Basin are similar to those acting in active back-arc–spreading regions.