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1. Petrogenesis and Geodynamic Evolution in the Northern Westfjords, Iceland, Elucidated by Iceland's Oldest Silicic Rocks

   https://doi.org/10.1029/2022JB026265  

   Banik, Tenley J. ; Coble, Matthew A. ; Miller, Calvin F. ; Fisher, Christopher M. ; Jordan, Brennan T. ; Vervoort, Jeffrey D. ; Carley, Tamara L. ( June 2023 , Journal of Geophysical Research: Solid Earth)

   Iceland's oldest silicic rocks provide unique insight into the island's early crustal evolution. We present new zircon U‐Pb ages bolstered with zircon trace element and isotopic compositions, and whole rock Nd, Hf, and Pb isotope compositions, from three silicic magmatic centers—Hrafnsfjörður, Árnes, and Kaldalón—to understand the petrogenesis of large silicic volcanic centers in the northern Westfjords, Iceland. Our data confirm Hrafnsfjörður as the oldest known central volcano in Iceland (∼14 Ma) and establish an older age for Árnes (∼13 Ma) than previously estimated. We also report the first U‐Pb zircon dates from Kaldalón (∼13.5 Ma). Zircon oxygen isotope compositions range from δ¹⁸O∼+2 to +4‰ and indicate involvement of a low‐¹⁸O component in their source magmas. Hrafnsfjörður zircon Hf (mean sampleε_Hf∼ +15.3–16.0) and whole rock Hf and Nd (ε_Hf = +14.5 to +15;ε_Nd = +7.9 to +8.1) isotopic compositions are more radiogenic than those from Árnes (zircon sampleε_Hf∼ +11.8–13; whole rockε_Hf = +12.8 to +15.1;ε_Nd = +7.3 to +7.7), but Hrafnsfjörður whole rock Pb isotope compositions (^208/204Pb = 37.95–37.96;^206/204Pb = 18.33–18.35) are less radiogenic than those from Árnes (^208/204Pb = 38.34–38.48;^206/204Pb = 18.64–18.78). Kaldalón has zircon Hf isotope compositions ofε_Hf∼+14.8 and 15.5 (sample means). These age and isotopic differences suggest that interaction of rift and plume, and thus the geodynamic evolution of the Westfjords, is complex. Isotopic compositions of Hrafnsfjörður and Árnes support involvement of an enriched mantle (EM)‐like mantle component associated with a pulsing plume that resulted in variable spreading rates and magma fluxes and highlight the heterogeneity of the Icelandic mantle.

    

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2. Constraining the timescales of mafic magmatism of the Central Karoo Large Igneous Province using high precision U-Pb zircon geochronology

   https://doi.org/10.25131/sajg.125.0009  

   Muedi, T. ; MacLennan, S. ; Szymanowski, D. ; Schoene, B. ; Ramezani, J. ; Oalmann, J. ; Linol, B. ( March 2022 , South African Journal of Geology)

   Abstract Recent U-Pb high-precision geochronological studies have shown rapid emplacement of the intrusive doleritic component of the Karoo Large Igneous Province (KLIP) in Southern Africa. However, these studies focused on a relatively small geographic and altitudinal region of the KLIP. Additionally, the timing of initiation of extrusive volcanism, preserved in the Drakensberg-Lesotho highlands and its relationship to the intrusive suite, has only been imprecisely constrained by Ar-Ar dates. Here, we present new high-resolution U-Pb zircon ages on dolerite sills and dykes from across the central eastern Karoo Basin (South Africa) at elevations between mean sea level and 1 560 m, as well as U-Pb detrital zircon data that can be used to estimate the maximum age of volcaniclastic deposition near the base of the extrusive component of the KLIP. Dolerite samples were taken across two areas: (1) thick dykes exposed along the coast of the Indian Ocean to ~1 600 m flanking the Drakensberg Escarpment in the Eastern Cape; and (2) sills between 20 and 220 m below surface, in a borehole core within the interior of the Karoo Basin, 400 km hinterland from the coastline. Our estimated dolerite emplacement ages span a range of ca. 80 thousand years (Kyr), between 183.122 ± 0.029/-0.061 and 183.042 ± 0.042/-0.072 million years ago (Ma), and fall within the 331 +60/-54 Kyr age range previously established for magmatism related to the KLIP, despite the marked increase in sampling coverage in terms of area and altitude in this study. Therefore, KLIP geochronology is consistent with other LIPS such as the Siberian and Deccan Traps that supports the hypothesis of rapid emplacement timescales (<1 Myr). Additionally, these data are consistent with, but better delineate that the KLIP in southern Africa appears to be ca. 500 Kyr older than the main phase of magmatism in the Ferrar LIP of Antarctica. Detrital zircons from the basal volcanic sequence of the Drakensberg Group exhibit age peaks at ca. 1 and 0.5 Ga, typical of the surrounding Namaqua-Natal and Pan-African basement rocks, as well as younger peaks at ca. 260 and 200 Ma that likely relate to source provenances from south-western Gondwana and reworking of the Karoo Supergroup sedimentary rocks. High-precision U-Pb dates of the youngest zircon grains result in a maximum depositional age for the basal pyroclastics of 185.25 ± 0.25 Ma, allowing for a ca. 2 Myr offset with the intrusive Karoo dolerite suite. 

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3. Anatectic melting or extreme fractionation? Chemical and temporal evolution of Quaternary small-volume, high-silica rhyolites from Mineral Mountains, Utah, USA

   https://doi.org/10.46427/gold2022.8923  

   Rivera, Tiffany ; Jicha, Brian ( June 2022 , Goldschmidt 2022)

   The origins and evolution of small-volume, high-silica intercontinental rhyolites have been attributed to numerous processes such as derivation from granitic partial melts or small melt fractions remaining from fractional crystallization. Investigations into the thermo-chemical-temporal evolution of these rhyolites has provided insights into the storage and differentiation mechanisms of small volume magmas. In the Mineral Mountains, Utah, high-silica rhyolites erupted through Miocene granitoids between ca. 0.8 and 0.5 Ma, and produced numerous domes, obsidian flows, and pyroclastic deposits. Temporally equivalent basalts erupted in the valleys north and east of the Mineral Mountains, hinting at a potential relationship between mafic and felsic volcanic activity. Here we test competing hypotheses. Are the rhyolites products of extreme fractionation of the coeval basalts? Or do they represent anatectic melts of the granitoids through which they erupted? We address these questions through modeling with new whole rock geochemical data and zircon trace element chemistry, thermometry, and U/Pb LA-ICPMS dates. We couple these data with new 40Ar/39Ar eruption ages to improve upon the volcanic stratigraphy and address the recurrence interval for the most evolved rhyolites. Geochemical data from zircon crystals extracted from six domes suggest increasing differentiation with age and eruptive location, however there is minimal evidence for recycling of earlier crystallized zircon. These data suggest that magma batches were isolated from one another and zircon nucleation and crystallization occurred close to the eruption, thus limiting the residence time of the magmas. These data also perhaps suggest that the magmas were generated in small batches within each of the granitoids rather than from a large crystal mush body underlying the region, as seen at large silicic systems. Our preliminary geochemical models and zircon petrochronology eliminate extreme fractionation and favor local anatectic melting of different granitoids as a mechanism to produce chemical signatures observed in the Quaternary rhyolites in the Mineral Mountains. 

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4. Identifying crustal contributions in the Patagonian Chon Aike Silicic Large Igneous Province

   https://doi.org/10.1007/s00410-023-02065-1  

   Foley, Michelle L. ; Putlitz, Benita ; Baumgartner, Lukas P. ; Renda, Emiliano M. ; Ulianov, Alexey ; Siron, Guillaume ; Chiaradia, Massimo ( October 2023 , Contributions to Mineralogy and Petrology)

   The volcanic rocks of the Chon Aike Silicic Large Igneous Province (CASP) are recognized as magmas dominantly produced by crustal anatexis. Investigating the zircon of the CASP provides an opportunity to gain further insight into geochemical and isotopic differences of the potential magmatic sources (i.e., crust versus mantle), to identify crustal reservoirs that contributed to the felsic magmas during anatexis, and to quantify the contributions of the respective sources. We present a combined zircon oxygen and hafnium isotope and trace element dataset for 16 volcanic units of the two youngest volcanic phases in Patagonia, dated here with LA-ICP-MS U–Pb geochronology at ca. 148–153 Ma (El Quemado Complex, EQC) and ca. 159 Ma (western Chon Aike Formation, WCA). The EQC zircon have¹⁸O-enriched values (δ¹⁸O from 7 to 9.5‰) with correspondingly negative initial εHf values (− 2.0 to − 8.0). The WCA zircon have δ¹⁸O values between 6 and 7‰ and εHf values ranging between − 4.0 and + 1.5. Binary δ¹⁸O-εHf mixing models require an average of 70 and 60% melt derived from partial melting of isotopically distinct metasedimentary basements for the EQC and WCA, respectively. Zircon trace element compositions are consistent with anatexis of sedimentary protoliths derived from LIL-depleted upper continental crustal sources. The overlap between a high heat flux environment (i.e., widespread extension and lithospheric thinning) during supercontinental breakup and a fertile metasedimentary crust was key in producing voluminous felsic volcanism via anatexis following the injection and emplacement of basaltic magmas into the lower crust.

    

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5. Oxygen isotopic investigation of silicic magmatism in the Stillwater caldera complex, Nevada: Generation of large volume, low-δ18O rhyolitic tuffs and assessment of their regional context

   Watts KE, John DA ( April 2019 , Geological Society of America bulletin)

   ABSTRACT 18 Successive caldera-forming eruptions from ~30-25 Ma generated a large nested 19 caldera complex in western Nevada that was subsequently dissected by Basin and Range 20 extension, providing extraordinary cross-sectional views through a diverse range of 21 eruptive and intrusive products. A high-resolution oxygen isotopic study was conducted 22 on units that represent all major parts of the Job Canyon, Poco Canyon, Elevenmile 23 Canyon, and Louderback Mountains caldera cycles (29.2-25.1 Ma), and several 24 Cretaceous basement plutons that flank the Stillwater caldera complex. We also provide 2 25 new oxygen and strontium isotope data for regional caldera centers in the Great Basin, 26 which are synthesized with published oxygen and strontium isotope data for regional 27 Mesozoic basement rocks. Stillwater zircons span a large isotopic range (d18Ozircon of 3.6 28 to 8.2‰), and all caldera cycles possess low-d18O zircons. In some cases, they are a small 29 proportion of the total populations, and in others they dominate, such as in the low-d18O 30 rhyolitic tuffs of Job Canyon and Poco Canyon (d18Ozircon=4.0-4.3‰ and calculated d18O 31 magma=5.5-6‰). These are the first low-d18O rhyolites documented in middle Cenozoic 32 calderas of the Great Basin, adding to the global occurrence of these important magma 33 types that fingerprint recycling of shallow crust altered by low-d18O meteoric waters. The 34 appearance of low-d18O rhyolites in the Stillwater caldera complex is overprinted on a 35 Great Basin-wide trend of miogeoclinal sediment contribution to silicic magmas that 36 elevates d18O compositions, making identification of 18O depletions difficult. Oxygen and 37 strontium isotopic data and U-Pb zircon age inheritance points to derivation of the low38 d18O tuff of Job Canyon from melting and assimilation of hydrothermally altered 39 Cretaceous granitic basement. In contrast, oxygen and strontium isotopic modeling points 40 to derivation of the low-d18O tuff of Poco Canyon from melting and assimilation of 41 hydrothermally altered intracaldera Job Canyon rocks. Though not a nominally low-d18O 42 rhyolite, the tuff of Elevenmile Canyon possesses both low-d18O and high-d18O zircon 43 cores that are overgrown by homogenized zircon rims that approximate the bulk zircon 44 average, pointing to batch assembly of isotopically diverse upper crustal melts to 45 generate one of the most voluminous (2,500-5,000 km3) tuff eruptions in the Great Basin. 46 Low-d18O zircons in the tuff of Elevenmile Canyon are inherited from the Poco Canyon 47 cycle based on their unique trace and rare earth element chemistry. The tuffs of Poco and 3 48 Elevenmile Canyon both support a caldera cannibalization model in which 49 hydrothermally altered extrusive and intrusive rocks from previous caldera cycles are 50 recycled into later magmas. Though overlapping in space and time, each caldera-forming 51 cycle of the Stillwater complex had a unique oxygen isotope record as retained in single 52 zircons. Even plutons that were spatially and temporally coincident with calderas diverge 53 from the caldera-forming tuffs and cannot be their cogenetic remnants in most cases. 

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