Michael A. Dungan

Partial assimilative recycling of the mafic plutonic roots of arc volcanoes: An example from the Chilean Andes

At long-lived arc volcanic centers, open-system interactions between new magma and the plutonic roots of the arc will increase with time relative to interactions with older crust. Assimilative recycling of mafic plutonic arc roots will have different isotopic consequences than incorporation of aged continental crust. In the Tatara–San Pedro complex, 30 comagmatic basaltic lavas are characterized by xenocrysts and microxenoliths of olivine, augite, and plagioclase derived from solidified cumulates, wide but poorly correlated ranges of incompatible and compatible major and trace elements, and limited isotopic variability. This mineralogical-chemical signature reflects assimilation wherein grain-boundary melting and disaggregation of xenoliths led to blending of incompatible element– enriched melts derived mainly from hornblende, phlogopite, and plagioclase in combination with variable retention of xenocrysts, hence variable but high compatible element concentrations.

Short time scales of magmatic assimilation from diffusion modeling of multiple elements in olivine

Open-system processes have a large capacity to modify magma compositions during differentiation. Obtaining the rates of such processes is essential to understanding and constraining the evolution of magmatic systems. Here we quantify the time scales for magmatic assimilation of hydrous mafic to ultramafic cumulates by ascending basalts using the zoning patterns of olivine xenocrysts. Robust diffusion modeling results have been obtained by treating multiple compositional profiles for multiple elements (Fe-Mg, Ni, Mn, Ca) on multiple crystals from multiple flows. We find that the time between assimilation and eruption ranges from a few months to ∼25 yr, although 80% of the results are <10 yr. These ranges are shorter than either magma transport times from the mantle to the surface or typical repose periods of arc volcanoes. Thus, modification of the geochemical and mineralogical features of basalts by assimilation of plutonic rocks is a fast and probably unavoidable magmatic process.

Temperature-induced Al-zoning in hornblendes of the Fish Canyon magma, Colorado

Abstract An extensive electron microprobe survey of amphibole compositions in the Fish Canyon magma (2146 analyses), more than 80% of which are from high-resolution (<10 μm steps) core-to-rim traverses across large euhedral phenocrysts, provides: (1) temporal constraints on the immediately pre-eruptive P-T-fH₂O evolution of the magma, and (2) a means of evaluating recent calibrations of the Al-inhornblende barometer (Anderson and Smith 1995; hereafter AS1995) and thermometers (Blundy and Holland 1990; thermometers A and B of Holland and Blundy 1994; hereafter BH1990, HB1994TA, and HB1994TB). Hornblende phenocrysts are variable for most major elements (e.g., 5-9 wt% Al2O3 and 44-50 wt% SiO2). This compositional range is controlled by two major temperature-sensitive coupled substitutions. Approximately 50% of the total Al variation (~0.8 atoms per formula unit = apfu) is due to the edenite exchange [TSi + A■ = TAl + A(Na + K)] and another 25-30% is the consequence of a Ti- Tschermak exchange (TSi + M1-M3 Mn = TAl + M1-M3Ti). In contrast, the pressure-sensitive Al-Tschermak substitution (TSi + M1-M3 Mg = TAl + M1-M3Al) did not play a significant role, as M1-M3Al does not correlate with TAl and is always <0.2 apfu. In order to constrain the ranges of absolute P and T over which these hornblendes crystallized and to assess the sensitivity of the recent thermo-barometric algorithms of BH1990, HB1994TA (requiring silica saturation), HB1994TB (not requiring silica saturation) and AS1995, we have calculated pressures and temperatures for two selected populations of analyses wherein Al2O3 contents are within analytical error (5.95 to 6.05 wt% Al2O3, N = 78 and 7.7 to 7.8 wt% Al2O3, N = 40). The barometric formulation of AS1995 gives a mean pressure of 2.24 ± 0.05 for the high-Al population at 760 °C, which is indistinguishable from the 2.4 ± 0.5 kbar estimate of Johnson and Rutherford (1989a). A high sensitivity to temperature at low P is suggested by the geologically implausibly shallow depths calculated for the low-Al population (<1 kbar at 760 °C). The three thermometric formulations give reasonable results between 706 and 814 °C, but the HB1994TA calibration gives a mean temperature higher by ~50 °C and is more sensitive to small analytical differences (~100 °C spread for each population). HB1994TB is considered the most reliable calibration of the Al-in-hornblende thermometer as it most precisely reproduces T estimates determined by independent methods. Nine out of 14 traverses across large phenocrysts from the Fish Canyon magma display rimward increases in TAl, A(Na + K), and M1-M3Ti, compensated by decreases in TSi, and M1-M3Mn. Using the HB1994TB algorithm, the low-Al population, typical of near-core compositions, gives a mean temperature of ~715 °C, which is ~35-45 °C above the water-saturated granite solidus at 2-2.5 kbar. The high-Al population, representing the average rim composition, gives a value around 760 °C, which is indistinguishable from independent T determinations using coexisting Fe-Ti oxides and Qtz-Mag oxygen isotope thermometry. These profiles suggest that Fish Canyon hornblendes crystallized during near-isobaric reheating over a temperature range of ~40 °C, which is consistent with our model of rejuvenation and remobilization of a pre-existing near-solidus crystal mush of batholithic dimensions via shallow intrusion of more mafic magma (Bachmann et al. 2002). Crystallization of hornblende from a high-SiO2, low-MgO melt during reheating requires an open system, in which both heat and mass, in particular volatiles, are transferred from the underlying mafic magma.

Comagmatic granophyric granite in the Fish Canyon Tuff, Colorado: Implications for magma-chamber processes during a large ash-flow eruption

The 27.8 Ma Fish Canyon Tuff, a vast ash-flow sheet (∼ 5000 km 3 ) of uniform phenocryst-rich dacite, is representative of “monotonous intermediate” eruptions from a magma chamber that lacked compositional gradients. Sparse small fragments of comagmatic granophyre in late-erupted tuff and postcaldera lava, having mineral compositions indistinguishable from phenocrysts in the tuff and precaldera lava-like rocks, record complex events in the Fish Canyon chamber just prior to eruption. Sanidine phenocrysts in the granophyre preserve zoning evidence of mingling with andesitic magma, then shattering by decompression and volatile loss accompanying early Fish Canyon eruptions before overgrowth by granophyre. The textural and chemical disequilibria indicate that the eruption resulted from batholith-scale remobilization of a shallow subvolcanic chamber, contrary to previous interpretations of magma storage and phenocryst growth in the lower crust.

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.

Inherited argon in a Pleistocene andesite lava: 40Ar/39Ar incremental-heating and laser-fusion analyses of plagioclase

By using 40 Ar/ 39 Ar incremental-heating and laser-fusion techniques, xenocrystic plagioclase was discovered in a late Pleistocene andesitic lava that erupted through the Andean Cordillera. Inherited argon in the xenocrysts is as much as ∼450 times older than the host lava, the age of which is independently known, and is an obstacle to dating the lava by using a whole-rock sample. The xenocrysts are impossible to identify from petrography or chemical parameters such as their K/Ca ratios. Holocrystalline groundmass, carefully separated from plagioclase and other phenocrysts, gives an accurate 40 Ar/ 39 Ar age for the lava. The xenocrysts could not have been degassed for more than several days in the magma and probably were assimilated from Paleozoic rocks buried under kilometers of Mesozoic and Tertiary arc rocks composing the Cordillera in central Chile. The required magma ascent velocity, on the order of kilometers/day, is extraordinarily high compared to the 10 −4 km/day minimum implied by the 226 Ra excess in continental arc lavas. These data permit magma migration and storage in the mantle and lower crust for as much as thousands of years, followed abruptly by rapid ascent to the surface.

Geochemistry of Nevado de Longaví Volcano (36.2°S): a compositionally atypical arc volcano in the Southern Volcanic Zone of the Andes

The Quaternary Nevado de Longavi volcano of the Andean Southern Volcanic Zone (SVZ) has erupted magmas that range in composition from basalt to low-silica dacite, although andesites are the dominant erupted magma type. Amphibole is a common phenocryst phase in andesites throughout the volcano, and it is the dominant mafic phase in Holocene dacites and their included mafic enclaves. Compositions of magmas erupted at Longavi volcano define arrays that diverge from trends delineated by neighboring frontal-arc volcanoes. Although mafic compositions at Longavi are broadly similar to basalts at other SVZ centers, Longavi intermediate and evolved magmas have systematically lower abundances of incompatible major (K 2 O, P 2 O 5 ) and trace elements (Rb, Zr, Nb, REE, Th, etc), as well as high Ba/Th, Sr/Y, and La/Yb ratios. Longavi volcano magmas define two differentiation series with regard to enrichments of Rb (and other incompatible elements) with increasing silica. A high-Rb series that includes the oldest units of the volcano comprises basalts to andesites dominated by anhydrous mineral assemblages with chemical compositions similar to other SVZ magmatic series. The series with low Rb, on the other hand, includes the Holocene units that evolved from basaltic andesites to dacites by means of fractional crystallization wherein amphibole and calcic plagioclase dominate the mineral assemblage. Magmas parental to low-Rb series are interpreted to be high-degree mantle melts, highly hydrous and oxidized, formed as a response to high fluid inputs into the subarc mantle. Enhanced water transport to the subarc mantle is a plausible effect of the subduction of the oceanic Mocha Fracture Zone that projects beneath Nevado de Longavi. Volcanoes located over oceanic fracture zones further south along the SVZ have erupted hornblende-bearing magmas that share some chemical similarities with Longavi volcano magmas.

Tectonic response of the central Chilean margin (35–38°S) to the collision and subduction of heterogeneous oceanic crust: a thermochronological study

Along-strike geological segmentation in the Andean chain has been recognized at various scales and is usually attributed to changes in plate motion vectors, as well as the upper-plate expression of differing subducted slab age, strength and composition. We present new multi-phase 40Ar/39Ar, apatite fission-track and zircon and apatite (U–Th)/He data from a north–south-oriented traverse between 35 and 38°S along the Principal Andean Cordillera of Chile that reveal (1) rapid cooling at 18 and 15 Ma, which can be attributed to both thermal relaxation following magmatic intrusion and regional-scale exhumation, and (2) along-strike differences in the extent of exhumation since 7.5 Ma that may be a consequence of the subduction of the Juan Fernandez Ridge above the flat-slab segment at 32°, since 10 Ma. A comparison of the response of the South American Plate to the collision of the Carnegie, Nazca and Juan Fernandez ridges suggests that slab flattening is not the dominant driving force that exhumes the upper plate in these settings. Rather, the extent of exhumation is controlled by pre-existing structural weaknesses, the time duration of the dynamically supported topography, and climate.

Magmatic Na-rich phlogopite in a suite of gabbroic crustal xenoliths from Volcan San Pedro, Chilean Andes: Evidence for a solvus relation between phlogopite and aspidolite

Abstract Magmatic Na-rich phlogopite (1-5 wt% Na2O) is present as a late-crystallizing mineral in two groups of texturally and mineralogically distinct gabbroic xenoliths at Volcán San Pedro (36°S, Chile), an Andean arc volcano. Phlogopites are characterized by high 100·Mg/(Mg + Fe) (up to 83) and high Cr2O3 contents (up to 0.4 wt%), and they are always found surrounding variably resorbed olivine, pyroxenes, Cr-spinel, and in some cases, plagioclase. We interpret these micas as the result of open-system processes involving infiltration of water-rich evolved melts [with high Na/(Na + K)] and reaction with refractory minerals. The highest 100·Na/(Na + K) (~70) and Na2O concentrations (~5 wt%) in phlogopite appear to require reaction with liquids of unrealistically high Na/(Na + K) if no other factor is considered. This, together with the observation that phlogopites consist of alternating Na-rich and Na-poor cleavage-parallel bands, can be best interpreted by the presence of a solvus between the aspidolite (Na) and phlogopite (K) end-members. The high proportions (up to 15 vol%) of Na-rich phlogopite in two different groups of gabbroic xenoliths suggest that it might be a more common and abundant mineral than has been previously recognized, and that it may be used as an indicator of open-system processes.