Triassic orthogneisses of the Antarctic Peninsula provide evidence for the Palaeozoic and Mesozoic geological evolution of southern Gondwana within Pangaea. These rocks are sporadically exposed in southeastern Graham Land and northwestern Palmer Land, although reliable geochronological, geochemical and isotopic data are sparse. We combine new geochronological (LA-ICP-MS zircon U-Pb), geochemical, and zircon (Hf, O) and whole rock isotopic (Nd, Sr and Pb) data to constrain the age and Triassic – Palaeozoic tectonic setting of these rocks. Zircon cores record a Palaeozoic magmatic arc between 252.5 ± 2 and 527.8 ± 6.2 Ma, which was mainly located to the west of the Eastern Palmer Land Shear Zone (Central Domain; Vaughan and Storey, 2000). The arc is considered to be an extension of contemporaneous Palaeozoic arcs that have been identified along the Pacific margin of South America and the Thurston Island Block. Regions to the east of the Palmer Land Shear Zone (Eastern Domain, Vaughan and Storey, 2000) were located distal from the Terra Australis Margin, and possibly resided within Sunsas-aged belts within Pangaea. Triassic continental arc, calc-alkaline magmatism during 223.4 – 203.3 Ma modified the crust of the Antarctic Peninsula on both sides of the Eastern Palmer Land Shear Zone. Magmatic sources included igneous and sedimentary crustal material, which formed by crustal reworking during Sunsas- and Braziliano-aged orogenesis, and Palaeozoic arc magmatism. Arc magmatism accompanied sinistral extension which brought both domains into the arc and resulted in steady oceanward migration of the Triassic arc during the Middle – Late Triassic. We conclude that the Eastern Palmer Land Shear Zone formed in the Triassic, and that both the Eastern and Central Domains are autochthonous to Gondwana.
Silicic large igneous provinces (SLIPs) are periods of particularly voluminous felsic volcanism in the geologic record. Previous work has suggested an overall long lifespan for SLIPs of 20 to 40 Myr, commonly punctuated by shorter-lived 'flare-ups' of higher volcanic productivity (1–5 Myr), but detailed studies of individual flare-up events are lacking. The Jurassic Chon Aike SLIP (CASP) is the product of an exceptional geological event wherein voluminous felsic volcanism (ca. 219,000 km3) was generated predominately via crustal anatexis over 45 Myr. We focus on the Late Jurassic El Quemado Complex (EQC), which marked the final stages of felsic volcanism for the CASP. In-situ U-Pb ages for the EQC previously suggested a duration of ∼5 Myr; however, new high-precision CA-ID-TIMS ages indicate the total durations of ignimbrite successions were shorter than 350 kyr. A compilation of zircon U-Pb ages for the eight CASP formations in Patagonia and the Antarctic Peninsula reveals changes in volcanic duration between formations deposited in the intraplate relative to the continental margin, suggesting a spatial control over the magmatic lifespans of geographically restricted systems. We suggest that the rate of magmatism in the CASP was primarily controlled by heterogeneities in the crust, largely between Proterozoic igneous crust and younger, metasedimentary crust that was recently accreted relative to the timing of Jurassic volcanism. The observed duration of volcanism was modulated by these differences in crustal properties from the injection of mafic magmas into the lower crust during extension and mantle upwelling, following rollback of the subducting oceanic slab towards the Paleo-Pacific margin of Gondwana. The short duration in the EQC is the product of unique overlapping conditions that favored crustal melting and resulted in the voluminous ignimbrite flareup (104 km3) of some of the Earth's highest δ18O magmas measured.
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Research Article| September 01, 1997 Nucleation-dominated crystallization of forsterite in the Ubehebe Peak contact aureole, California Gregory T. Roselle; Gregory T. Roselle 1Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 Search for other works by this author on: GSW Google Scholar Lukas P. Baumgartner; Lukas P. Baumgartner 1Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 Search for other works by this author on: GSW Google Scholar John A. Chapman John A. Chapman 1Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 Search for other works by this author on: GSW Google Scholar Author and Article Information Gregory T. Roselle 1Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 Lukas P. Baumgartner 1Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 John A. Chapman 1Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1997) 25 (9): 823–826. https://doi.org/10.1130/0091-7613(1997)025<0823:NDCOFI>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Gregory T. Roselle, Lukas P. Baumgartner, John A. Chapman; Nucleation-dominated crystallization of forsterite in the Ubehebe Peak contact aureole, California. Geology 1997;; 25 (9): 823–826. doi: https://doi.org/10.1130/0091-7613(1997)025<0823:NDCOFI>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The morphology and number of forsterite crystals in the Ubehebe Peak, California, contact aureole vary systematically as a function of metamorphic grade. From their first appearance to as close as ∼150 m from the intrusive contact, forsterite crystals are large (5–20 mm) and have a tabular habit (a ≈ c ≫ b). In contrast, forsterite near the contact is equigranular and much smaller (<1 mm in diameter). The number of crystals per mole of forsterite increases from 3.5 × 104 at the forsterite-in isograd to more than 1.5 × 108 near the contact. This trend is interpreted to result from an increase in the ratio of nucleation rates with respect to growth rates with proximity to the intrusion. The change in morphology from tabular to equigranular is explained by kinetic surface roughening. The nucleation and growth information gained from this study highlights the important role of nucleation kinetics in the crystallization of forsterite at the Ubehebe Peak aureole. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Abstract 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 have 18 O-enriched values (δ 18 O from 7 to 9.5‰) with correspondingly negative initial εHf values (− 2.0 to − 8.0). The WCA zircon have δ 18 O values between 6 and 7‰ and εHf values ranging between − 4.0 and + 1.5. Binary δ 18 O-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.
Concrete is a most important geomaterial as it is used for a large part in our buildings and infrastructures. According to Planetoscope (2012) its production is about 6 billion m3 per year (190 m3 each second) which makes it the most used manufactured material in the world. The alteration of concrete, tightly associated with the durability and security of our infrastructures, depends on its primary composition but also on a wide range of environmental factors (mechanical solicitations, freezing-thawing cycles, alkali-aggregate reactions, etc.). Hence, the contribution of geological or geological-related analysis to understand concrete alteration processes is required because this material is mainly composed of aggregates made of different minerals and rocks, the type and quality of which are important due to their influence on concrete behaviour. In this study, high resolution X-ray Computed tomography (X-ray micro-CT) was used to image 3D different types of concrete cores in order to characterize their respective state of alteration. The global alteration index (GAI) developed by Christe et al. (2010) for natural cataclastic rocks was applied to the segmented X-ray CT images of these concrete cores and compared to the results of microstructural analysis on thin slices and of compression tests. Also, an internal attack of concrete by hydrochloric acid was carried out in laboratory to simulate artificial alteration through carbonate dissolution and its process was monitored along time by X-ray micro-CT imaging (4D monitoring). Our first results show that X-ray CT imagery enables, without any destruction of the specimens, to characterize concrete internal features in terms of macroporosity, highly microporous cement paste, standard cement paste or aggregates. In particular, initial entrapped porosity as well as cracks are easily detected, characterised and quantified before and after mechanical testing. Petrographic analyses on thin sections enabled to verify the physical meaning of the X-ray CT-based detected features, such as the highly microporous cement paste, the opening of the microcracks and the detachment halos around aggregates which all act as weakening parameters. Despite the limited number of samples, a coherent relation between the GAI and the compressive strength of the concrete specimens is observed as the concrete compressive strength clearly decreases when the GAI increases. In this case, it logically means that the concrete with a higher porosity is less resistant. Moreover, the 4D monitoring of the acidic attack test led to dynamically show how the carbonated structure of the concrete was progressively altered. These preliminary results demonstrate that GAI, based on XRCT imagery analysis, can be used to evaluate the degree of alteration of concrete and could thus lead to estimate its strength. In more general terms, X-ray CT analysis opens new perspectives to relate the quality of concrete with its mechanical properties. This method could probably be further applied to a wide range of problems related to the inspection and maintenance of concrete infrastructures.
