Magmatic evolution of zoned and unzoned ignimbrites: evidence for a complex crustal architecture feeding four rapid-sequence, caldera-forming eruptions in the San Juan Mountains, Colorado

2021 ◽  
Author(s):  
Adam Curry ◽  
Luca Caricchi ◽  
Peter W Lipman
Keyword(s):  
Trace Element ◽  
The Other ◽  
San Juan ◽  
Volcanic Complex ◽  
Accessory Zircon ◽  
San Juan Mountains ◽  
Reservoir Architecture ◽  
Magma Reservoirs ◽  
Magmatic Body ◽  
Mineral Assemblages

Abstract The last four caldera-forming ignimbrites in the central San Juan caldera cluster, Colorado, erupted 1,400 km3 in ≤ 80 k.y. and alternated between zoned crystal-poor rhyolite to crystal-rich dacite and unzoned, crystal-rich dacite. The zoned 150 km3 Rat Creek Tuff (26.91 Ma), unzoned 250 km3 Cebolla Creek Tuff, and zoned 500 km3 Nelson Mountain Tuff (26.90 Ma) formed the nested San Luis caldera complex with slightly offset calderas, and the unzoned 500 km3 Snowshoe Mountain Tuff (26.87 Ma) formed the Creede caldera to the south. The Rat Creek Tuff, Nelson Mountain Tuff, and Snowshoe Mountain Tuff have similar mineral assemblages of plagioclase, sanidine, quartz, biotite, hornblende, clinopyroxene, Fe-Ti oxides, and accessory zircon, titanite, and apatite. The Cebolla Creek Tuff differs from the other three ignimbrites with more abundant hornblende and lack of quartz and sanidine. Trace element compositions of interstitial glass are unique to each ignimbrite, correlating with mineral assemblages and inferred crystallization depths. Glass, feldspar, hornblende, and clinopyroxene thermobarometry calculations provide evidence for vertically extensive crustal magma reservoirs with a common magmatic zone at ∼435-470 MPa (∼16-17 km) transitioning into shallow pre-eruptive reservoirs between ∼110-340 MPa (∼4-13 km), similar to the estimated magma reservoir architecture of the Altiplano Puna Volcanic Complex. The upper portions of the eruptible parts of the magma reservoirs of the Rat Creek Tuff (215 ± 50 MPa/∼810-820 °C), Cebolla Creek Tuff (340 ± 20 MPa/∼860-880° C), Nelson Mountain Tuff (215 ± 20 MPa/∼745-800 °C), and Snowshoe Mountain Tuff (110 ± 40 MPa/825 ± 10 °C) occupied shallow levels in the crust similar to other magma reservoirs of the central San Juan caldera complex. Trace element modelling correlates with a deep crystallization signature in the unzoned Cebolla Creek Tuff and Snowshoe Mountain Tuff, typified by a flat trend in Ba versus Sr whole-rock and glass chemistry. The zoned Rat Creek Tuff and Nelson Mountain Tuff are typified by a steep trend in Ba versus Sr chemistry interpreted as a shallower crystallization signature. Similarly, the unzoned Cebolla Creek Tuff and Snowshoe Mountain Tuff have flatter slopes in FeO versus An space of plagioclase chemistry interpreted as more abundant deep plagioclase crystallization and a difficulty to physically mix with Fe-rich mafic recharge magma due to higher viscosity. The zoned Rat Creek Tuff and Nelson Mountain Tuff have higher slopes in FeO versus An space of plagioclase chemistry interpreted as more abundant shallow plagioclase crystallization and more feasible mixing with Fe-rich mafic recharge magma due to lower viscosity. The eruption of the Rat Creek Tuff was likely triggered by mafic injection, but the other three ignimbrites lack mingling textures in pumice, suggesting that other mechanisms were important in causing such large eruptions. After a prolonged period of mantle-derived magma injection and crustal heating (∼25,000 km3 Conejos Formation erupted during ∼35-29 Ma), the San Juan magmatic body became a robust and versatile producer of diverse eruptible magmas in short time periods during its Oligocene ignimbrite flare-up.

Trace element partition coefficients and petrogenesis of the 154 ka dacitic Haramul Mic lava dome (Ciomadul, Romania)

2021 ◽  
Author(s):  
Emese Pánczél ◽  
Maurizio Petrelli ◽  
Réka Lukács ◽  
Szabolcs Harangi
Keyword(s):  
Trace Element ◽  
Partition Coefficients ◽  
Lava Dome ◽  
Bulk Composition ◽  
Volcanic Complex ◽  
Elevated Concentration ◽  
Positive Anomaly ◽  
Accessory Zircon ◽  
Eruption Age ◽  
Li Content

<p>Haramul Mic is a ~0.15 km<sup>3 </sup>volume, crystal-rich, homogeneous, high-K dacite lava dome, which is one of the oldest ones in the Ciomadul Volcanic Complex (Romania, eastern-central Europe). The eruption that formed the lava dome occurred after about 200.000 years of quiescence. Eruption age of the dome determined by (U-Th)/He dating on zircon gave 154 +/- 16 ka that is in agreement with the youngest zircon U-Th outer rim date (142 +18/-16 ka). The apparently continuous crystallization of zircon between the eruption age and the 306 +/- 37 ka oldest zircon core date records a long-living magmatic plumbing system.</p><p>The Haramul Mic lava dome rock has 35-40% average crystal content and consists of plagioclase, amphibole, biotite and accessory zircon, apatite, titanite and Fe-Ti oxides. The groundmass is mainly built up by perlitic glass with some microlites and sheared vesicles. The dacite contains sparse mafic enclaves with K-rich, shoshonitic bulk composition, composed of plagioclase and biotite in addition to less amount of amphibole. Felsic crystal clots are more common and they comprise plagioclase, amphibole, biotite and interstitial vesicular glass.</p><p>Trace element content of the mineral phases and the groundmass glass was determined by LA-ICP-MS. All of the studied phases show homogeneous trace element compositions and along with the textural characteristics these imply general equilibrium state in the magma storage system before the eruption. Amphibole-plagioclase geothermometer and geobarometer calculations result in 700-800 <sup>o</sup>C crystallization temperature and 200-300 MPa crystallization pressure.</p><p>In order to reveal the magma chamber processes that triggered the eruption and formed the Haramul Mic lava dome after long quiescence time, it is necessary to understand better the behaviour of trace elements as the most sensitive indicators of magma reservoir mechanisms. We determined mineral-liquid trace element partition coefficients and evaluated the result in the context of crystal lattice strain model. They show many similarities with those proposed for the Fish Canyon Tuff dacite except for Li and Sc. The anomalous behaviour of Sc is clearly expressed by the elevated concentration in the glass phase and many times, there are some zonation in Sc from crystal core to rim. This could be explained either by inherently higher Sc content of the melt reflecting the nature of the primary magmas or by partial remelting of biotite just before the eruption. Significant positive anomaly of Li content can be observed in biotite crystals of the mafic enclave compared with the dacitic host rock. Li content of plagioclase varies between 15-30 ppm with slight rimward depletion.</p><p>Eruption initiation cannot be explained by physical mixing of mafic recharge magma, but rather by volatile transfer or second boiling. The water-rich nature of the melt is reflected by the abundant vesicles in the glassy groundmass. Furthermore, the amphibole phenocrysts have sharp margin without resorption rim, which suggest hydrous melt phase and relatively fast magma ascent.</p><p>This research belongs to the NKFIH-OTKA K135179 project and was supported by the ÚNKP-19-1 New National Excellence Program of the Ministry for Innovation and Technology.</p>

scholarly journals Timescales and thermal evolution of large silicic magma reservoirs during an ignimbrite flare-up: perspectives from zircon

2021 ◽  
Vol 176 (12) ◽  
Author(s):  
Adam Curry ◽  
Sean P. Gaynor ◽  
J. H. F. L. Davies ◽  
Maria Ovtcharova ◽  
Guy Simpson ◽  
...  
Keyword(s):  
Thermal Evolution ◽  
San Juan ◽  
Whole Grain ◽  
Zircon Age ◽  
Magma Reservoirs ◽  
Magmatic Body ◽  
Icp Ms ◽  
Zircon Crystallization ◽  
Accretion Rates ◽  
River Plain

AbstractFour voluminous ignimbrites (150–500 km3) erupted in rapid succession at 27 Ma in the central San Juan caldera cluster, Colorado. To reconstruct the timescales and thermal evolution of these magma reservoirs, we used zircon ID-TIMS U–Pb geochronology, zircon LA-ICP-MS geochemistry, thermal modeling, and zircon age and crystallization modeling. Zircon geochronology reveals dispersed zircon age spectra in all ignimbrites, with decreasing age dispersion through time that we term a ‘chimney sweeping’ event. Zircon whole-grain age modeling suggests that 2σ zircon age spans represent approximately one-quarter of total zircon crystallization timescales due to the averaging effect of whole-grain, individual zircon ages, resulting in zircon crystallization timescales of 0.8–2.7 m.y. Thermal and zircon crystallization modeling combined with Ti-in-zircon temperatures indicates that magma reservoirs were built over millions of years at relatively low magmatic vertical accretion rates (VARs) of 2–5 × 10–3 m y−1 (2–5 × 10–6 km3 y−1 km−2), and we suggest that such low VARs were characteristic of the assembly of the greater San Juan magmatic body. Though we cannot unequivocally discern between dispersed zircon age spectra caused by inheritance (xenocrystic or antecrystic) versus prolonged crystallization from the same magma reservoir (autocrystic), our findings suggest that long-term magma input at relatively low VARs produced thermally mature upper crustal magma reservoirs resulting in protracted zircon crystallization timescales. Compiling all U–Pb ID-TIMS zircon ages of large ignimbrites, we interpret the longer timescales of subduction-related ignimbrites as a result of longer term, lower flux magmatism, and the shorter timescales of Snake River Plain ignimbrites as a result of shorter term, higher flux magmatism.

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