Genesis of the Eastern Adamello Plutons (Northern Italy): Inferences for the Alpine Geodynamics

The Corno Alto–Monte Ospedale magmatic complex crops out at the eastern border of the Adamello batholith, west of the South Giudicarie Fault (NE Italy). This complex includes tonalites, trondhjemites, granodiorites, granites and diorites exhibiting an unfoliated structure suggesting passive intrusion under extensional-to-transtensional conditions. Major, minor elements, REE and isotopic analyses and geochemical and thermodynamic modelling have been performed to reconstruct the genesis of this complex. Geochemical analyses unravel a marked heterogeneity with a lack of intermediate terms. Samples from different crust sections were considered as possible contaminants of a parental melt, with the European crust of the Serre basement delivering the best fit. The results of the thermodynamic modelling show that crustal melts were produced in the lower crust. Results of the geochemical modelling display how Corno Alto felsic rocks are not reproduced by fractional crystallization nor by partial melting alone: their compositions are intermediate between anatectic melts and melts produced by fractional crystallization. The tectonic scenario which favored the intrusion of this complex was characterized by extensional faults, active in the Southalpine domain during Eocene. This extensional scenario is related to the subduction of the Alpine Tethys in the Eastern Alps starting at Late Cretaceous time.

Geochemistry, zircon U-Pb geochronology and Sr-Nd-Hf isotopic composition of the Cha Val plutonic rocks in central Vietnam: Implications for Permian-Triassic Paleo-Tethys subduction-related magmatism

together with abundant Permian-Triassic magmatic rocks. This magmatic complex provides important information to reconstruct the tectonic evolution of the Indochina block and surrounding areas. The Cha Val plutonic rocks mainly comprise diorite, quartz diorite, and granodiorite. Geochemically, they are metaluminous with low A/CNK (0.49 to 1.16 with an average of 0.85), medium to high K, low to medium SiO2, and Na2O/K2O>1. Trace and rare earth element compositions display enrichment in Cs, U, Pb, and Nd, but depletion in Ba, Nb, Ta, P, Eu, and Ti, similar to those of continental arc-related magmas. Rock-forming minerals of the Cha Val plutonic rocks are characterized by abundant hornblende. All observed petrographical and geochemical characteristics suggest that the Cha Val plutonic rocks are typical for I-type affinity generated from a subduction regime. LA-ICP-MS U-Pb zircon analyses of three representative samples yielded their crystallization ages between 258.0 Ma and 248.9 Ma, temporally coeval with Late Permian-Early Triassic magmatism previously reported in the Truong Son belt. The (87Sr/86Sr)i ratios (0.7081 to 0.7244), negative whole-rock εNd(t) values (-4.5 to -2.9), zircon εHf(t) values (-1.04 to 2.71), and whole-rock Nd and zircon Hf model ages (TDM2) (1394 Ma to 1111 Ma) indicate that the Cha Val plutonic rocks are derived from melting of Mesoproterozoic crustal materials with a minor contribution of mantle-derived melt. Together with other Permian-Triassic magmatic complexes along the Song Ma suture zone and the Truong Son Belt, the Cha Val plutonic rocks are a representative of magmatism associated with the subduction-collision that amalgamated the South China and Indochina blocks after the closure of a branch of Paleo-Tethys along the Song Ma suture zone during the Late Permian-Early Triassic Indosinian orogeny.

Aspects of the petrology and geochemistry of the Huckleberry Ridge Tuff, Yellowstone

<p>Silicic (i.e. dacitic-rhyolitic) magmatic systems have the potential to generate large, explosive caldera-forming eruptions which have global effects and consequences. How, and over what timescale, magma accumulates and is stored in the upper crust are key aspects in understanding such systems and their associated hazards. The absence of such eruptions in the historical record, however, has forced understanding of these systems to be developed through numerical models or the study of the deposits in the geological record. Numerical models primarily focus on the long-term generation but instantaneous eruption of single magma (i.e. melt-dominant) bodies. In contrast, the stratigraphic and geochemical nature of eruption deposits often show features more consistent with complex magmatic systems comprising multiple melt-dominant bodies that may have formed rapidly but erupted episodically. Further studies of past eruption deposits are valuable, therefore, in reconstructing silicic magmatic systems and highlighting the nature of melt-dominant body generation and storage. To this end, this thesis examines the 2.08 Ma, ∼2,500 km³ Huckleberry Ridge Tuff (HRT), Yellowstone Plateau volcanic field (YPVF), U.S.A, the deposit of the first and largest of three caldera-forming eruptions in the YPVF. The HRT comprises an initial fall deposit followed by three ignimbrite members (A, B and C) with a second fall deposit between members B and C. Despite emanating from an archetypal silicic volcanic field, minimal previous work has been undertaken on the geochemical nature of the HRT but it is thought to conform to traditional, unitary magma body ideas. A revised stratigraphic framework, detailing an episodic and prolonged initial fall deposit, identification of a weeks-months time gap between members A and B, and a similar but longer years-decades hiatus in activity between members B and C provides the context for this geochemical investigation. A large sample suite representative of the diverse range of physical characteristics of clasts and material found in the HRT was analysed. In situ micro-analysis of matrix glass (major and trace elements) and crystals (major elements) in the initial fall deposit are coupled with major and trace element, and isotopic compositions of single silicic clasts (i.e. pumice/fiamme) from all three ignimbrite members, supplemented by in situ analysis of their crystals and groundmass glass. These data are used to reconstruct the silicic magmatic system. Furthermore, major and trace element, andisotopic compositions of rare mafic (i.e. basaltic to andesitic) material found in members A and B provide an insight into the thermal and chemical drivers of HRT silicic volcanism. This macro- and micro-analytical investigation using multiple techniques reveals remarkable complexity within the large-scale HRT magmatic complex. Four geochemically distinct magmatic systems are differentiated on single clast elemental and isotopic characteristics that are further reflected in crystal and glass compositions. Two of these systems (1 and 2) were active in the initial fall deposit and member A. Magmatic system 1 is volumetrically dominant in the HRT and is characterised by moderate-high Ba single clast (450-3540 ppm) and glass (100-3360 ppm) compositions, in contrast to the distinctly low-Ba (≤250 ppm single clast, <65 ppm glass Ba contents) magmatic system 2. Both these magmatic systems exhibit clustered glass compositions, indicating multiple, laterally-adjacent melt-dominant bodies were present, and shared moderate isotopic compositions (e.g. ⁸⁷Sr/⁸⁶SrAC = 0.70950-0.71191) are explicable by a multi-stage partial melting-fractional crystallisation petrogenesis. The time break between members A and B is associated with mixing and mingling within magmatic system 1, related to a renewed influx of mafic material, and a cessation of activity of system 2, which is absent from member B. The time break between members B and C reflects significant changes within the magmatic complex. Magmatic system 2 is rejuvenated and melt-dominant bodies associated with two new magmatic systems (3 and 4) are formed, with at least system 3 comprising multiple bodies. These latter two magmatic systems strongly differ in their elemental characteristics (system 3: high SiO₂ [75-78 wt% SiO₂]; system 4: dacite-rhyolite [66-75 wt% SiO₂]). Despite this, they have similar and highly radiogenic (e.g. ⁸⁷Sr/⁸⁶SrAC = 0.72462-0.72962) isotopic compositions indicating shared extensive incorporation of Archean crust. They also contrast in their relation to mafic compositions, with system 4 associated with olivine tholeiitic compositions erupted prior to and following the HRT in the YPVF. In contrast, system 3, like systems 1 and 2, is associated with high-Ba, high-Zr mafic compositions found co-erupted in HRT members A and B. These compositions are similar to lava flows erupted further west at the Craters of the Moon field, and are interpreted as representing partial melts from regions in the lithospheric mantle enriched by high-T, P fluids emanating from the subducted Farallon slab. Overall, the HRT magmatic complex was remarkably heterogeneous. Two contemporaneous mafic root zones drove four silicic magmatic systems, episodically active throughout the eruption. At least three of these systems comprised multiple laterally-adjacent melt-dominant bodies. Intra-eruption time breaks are associated with broad-scale reorganisation of the magmatic complex. This complexity highlights the utility of a detailed, systematic, multi-technique geochemical investigation, within a stratigraphic framework, of the deposits of large silicic caldera-forming eruptions, and breaks new ground in the understanding of such systems.</p>

Aspects of the petrology and geochemistry of the Huckleberry Ridge Tuff, Yellowstone

<p>Silicic (i.e. dacitic-rhyolitic) magmatic systems have the potential to generate large, explosive caldera-forming eruptions which have global effects and consequences. How, and over what timescale, magma accumulates and is stored in the upper crust are key aspects in understanding such systems and their associated hazards. The absence of such eruptions in the historical record, however, has forced understanding of these systems to be developed through numerical models or the study of the deposits in the geological record. Numerical models primarily focus on the long-term generation but instantaneous eruption of single magma (i.e. melt-dominant) bodies. In contrast, the stratigraphic and geochemical nature of eruption deposits often show features more consistent with complex magmatic systems comprising multiple melt-dominant bodies that may have formed rapidly but erupted episodically. Further studies of past eruption deposits are valuable, therefore, in reconstructing silicic magmatic systems and highlighting the nature of melt-dominant body generation and storage. To this end, this thesis examines the 2.08 Ma, ∼2,500 km³ Huckleberry Ridge Tuff (HRT), Yellowstone Plateau volcanic field (YPVF), U.S.A, the deposit of the first and largest of three caldera-forming eruptions in the YPVF. The HRT comprises an initial fall deposit followed by three ignimbrite members (A, B and C) with a second fall deposit between members B and C. Despite emanating from an archetypal silicic volcanic field, minimal previous work has been undertaken on the geochemical nature of the HRT but it is thought to conform to traditional, unitary magma body ideas. A revised stratigraphic framework, detailing an episodic and prolonged initial fall deposit, identification of a weeks-months time gap between members A and B, and a similar but longer years-decades hiatus in activity between members B and C provides the context for this geochemical investigation. A large sample suite representative of the diverse range of physical characteristics of clasts and material found in the HRT was analysed. In situ micro-analysis of matrix glass (major and trace elements) and crystals (major elements) in the initial fall deposit are coupled with major and trace element, and isotopic compositions of single silicic clasts (i.e. pumice/fiamme) from all three ignimbrite members, supplemented by in situ analysis of their crystals and groundmass glass. These data are used to reconstruct the silicic magmatic system. Furthermore, major and trace element, andisotopic compositions of rare mafic (i.e. basaltic to andesitic) material found in members A and B provide an insight into the thermal and chemical drivers of HRT silicic volcanism. This macro- and micro-analytical investigation using multiple techniques reveals remarkable complexity within the large-scale HRT magmatic complex. Four geochemically distinct magmatic systems are differentiated on single clast elemental and isotopic characteristics that are further reflected in crystal and glass compositions. Two of these systems (1 and 2) were active in the initial fall deposit and member A. Magmatic system 1 is volumetrically dominant in the HRT and is characterised by moderate-high Ba single clast (450-3540 ppm) and glass (100-3360 ppm) compositions, in contrast to the distinctly low-Ba (≤250 ppm single clast, <65 ppm glass Ba contents) magmatic system 2. Both these magmatic systems exhibit clustered glass compositions, indicating multiple, laterally-adjacent melt-dominant bodies were present, and shared moderate isotopic compositions (e.g. ⁸⁷Sr/⁸⁶SrAC = 0.70950-0.71191) are explicable by a multi-stage partial melting-fractional crystallisation petrogenesis. The time break between members A and B is associated with mixing and mingling within magmatic system 1, related to a renewed influx of mafic material, and a cessation of activity of system 2, which is absent from member B. The time break between members B and C reflects significant changes within the magmatic complex. Magmatic system 2 is rejuvenated and melt-dominant bodies associated with two new magmatic systems (3 and 4) are formed, with at least system 3 comprising multiple bodies. These latter two magmatic systems strongly differ in their elemental characteristics (system 3: high SiO₂ [75-78 wt% SiO₂]; system 4: dacite-rhyolite [66-75 wt% SiO₂]). Despite this, they have similar and highly radiogenic (e.g. ⁸⁷Sr/⁸⁶SrAC = 0.72462-0.72962) isotopic compositions indicating shared extensive incorporation of Archean crust. They also contrast in their relation to mafic compositions, with system 4 associated with olivine tholeiitic compositions erupted prior to and following the HRT in the YPVF. In contrast, system 3, like systems 1 and 2, is associated with high-Ba, high-Zr mafic compositions found co-erupted in HRT members A and B. These compositions are similar to lava flows erupted further west at the Craters of the Moon field, and are interpreted as representing partial melts from regions in the lithospheric mantle enriched by high-T, P fluids emanating from the subducted Farallon slab. Overall, the HRT magmatic complex was remarkably heterogeneous. Two contemporaneous mafic root zones drove four silicic magmatic systems, episodically active throughout the eruption. At least three of these systems comprised multiple laterally-adjacent melt-dominant bodies. Intra-eruption time breaks are associated with broad-scale reorganisation of the magmatic complex. This complexity highlights the utility of a detailed, systematic, multi-technique geochemical investigation, within a stratigraphic framework, of the deposits of large silicic caldera-forming eruptions, and breaks new ground in the understanding of such systems.</p>

Comparison of U-spatial statistics method with Classical Statistics results in the determination of geochemical anomalies of Epithermal Gold in Khoshnameh area, Hashtjin, Iran

In this study, methods based on the distribution model (with and without personal opinion) were used for the separation of anomalous zones, which include two different methods of U-spatial statistics and mean plus values of standard deviation (). The primary purpose is to compare the results of these methods with each other. To increase the accuracy of comparison, regional geochemical data were used where occurrences and mineralization zones of epithermal gold have been introduced. The study area is part of the Hashtjin geological map, which is structurally part of the folded and thrust belt and part of the Alborz Tertiary magmatic complex. Samples were taken from secondary lithogeochemical environments. Au element data concerning epithermal gold reserves were used to investigate the efficacy of these two methods. In the U- spatial statistics method, and criteria were used to determine the threshold, and in the method, the element enrichment index of the region rock units was obtained with grouping these units. The anomalous areas were identified by, and criteria. Comparison of methods was made considering the position of discovered occurrences and the occurrences obtained from these methods, the flexibility of the methods in separating the anomalous zones, and the two-dimensional spatial correlation of the three elements As, Pb, and Ag with Au element. The ability of two methods to identify potential areas is acceptable. Among these methods, it seems the method with criteria has a high degree of flexibility in separating anomalous regions in the case of epithermal type gold deposits.

Numerical simulation of the Late Jurassic closure of the Vardar Tethys

&lt;p&gt;We report updated results of our ongoing research on constraining geodynamic conditions associated with the final closure of the Vardar branch of the Tethys Ocean by means of application of numerical simulations (previous interim results reported in EGU2020-5919).&lt;/p&gt;&lt;p&gt;The aim of our numerical study is to test the hypothesis that a single eastward subduction in the Jurassic is a valid explanation for the occurrence of three major, presently observed geological entities that are left behind after the closure of the Vardar Tethys. These include: ophiolite-like igneous rocks of the Sava-Vardar zone and presumably subduction related Timok Magmatic Complex, both Late Cretaceous in age as well as Jurassic ophiolites obducted onto the Adriatic margin. In our simulations we initiate an intraoceanic subduction in the Early/Middle Jurassic, which eventually transitions into an oceanic closure and subsequent continental collision processes.&lt;/p&gt;&lt;p&gt;In the scope of our study numerical simulations are performed by solving a set of partial differential equations: the continuity equation, the Navier-Stokes equations and the temperature equation. To this end we used I2VIS thermo-mechanical code which utilizes marker in cell approach with finite difference discretization of equations on a staggered grid [Gerya et al., 2000; Gerya&amp;Yuen, 2003].&lt;/p&gt;&lt;p&gt;The 2D model consists of two continental plates separated by two oceanic slabs connected at a mid-oceanic ridge. Intraoceanic subduction is initiated along the ridge by assigning a weak zone beneath the ridge. Time-dependent boundary conditions for velocity are imposed on the simulation in order to model a transient spreading period. The change of sign in plate velocities is found to be useful for both obtaining obduction / ophiolite emplacement [Duretz et al., 2016] and causing back-arc extension. Changes in velocities are linear in time. Simulations follow a three-phase evolution of velocity boundary conditions consisting of two convergent phases separated by a single divergent phase where spreading regime is dominant. Effect of duration and magnitude of the second phase on model evolution is also explored.&lt;/p&gt;&lt;p&gt;Our so far obtained simulations were able to reproduce the westward obduction and certain extension processes along the active (European) margin, which match the existing geological relationships. However, the simulations involve an unreasonably short geodynamic event (cca 15-20 My) and we are working on solving this problem with new simulations.&amp;#160;&lt;/p&gt;

Amphibole CPO in retrograded gabbro from the Lyngen Magmatic Complex (Northern Scandinavian Caledonides, Norway).

&lt;p&gt;Deformation of natural mafic rocks by viscous deformation mechanisms can occur even at low temperature conditions. In such instances, crystal plastic mechanisms are not operative, as their activity is restricted to very high temperatures for amphiboles, pyroxenes, and plagioclase. Instead, simultaneous mineral reactions may facilitate deformation at low temperature conditions. The gabbro from the Lyngen Magmatic Complex (LMC) constitutes a good example of such processes, because it has experienced deformation at low temperatures of greenschist to lower amphibolite-facies conditions, and the rock has been transformed from gabbro to greenschist. This study focuses on detailed analysis of deformation processes, metamorphic reactions and fabric development in the LMC gabbro. Most samples are overprinted by epidote amphibolite and greenschist-facies mineral assemblages. Preliminary observations distinguish two different types of amphiboles, which have been interpreted as different generations. The predominant type defines the stretching lineation and shows long prismatic habits whereas the less abundant type crystallized in a sub- to anhedral manner. The metamorphic conditions of growth for each amphibole type is yet not well constrained. However, we initially interpret the former to grow during epidote amphibolite- or greenschist facies-conditions, whereas the latter could represent relict grains from the original magmatic assemblage or products generated at amphibolite- or epidote amphibolite-facies conditions. Further analysis will determine the orientation, geochemistry and metamorphic conditions during growth for both amphibole types. A recent model proposed for eclogites suggests that simultaneous mineral growth and deformation can result in new products growing in a preferred direction. Such preferential growth can generate a shape preferred orientation parallel to the lineation, which results in the formation of crystal preferred orientations (CPO). We aim to test if similar microstructural observations can be translated to the amphiboles of the LMC gabbro. In such case, amphibole CPO&amp;#8217;s would not be the product of crystal plasticity but of preferential growth. The large scale deformation of the LMC emphasizes the relevance of these results, as it would demonstrate that the interaction between mineral reactions and deformation can play a major role on regional deformation of large mafic bodies, such as the ocean floor.&lt;/p&gt;

Structure and shear-sense indicators of the Mesoproterozoic basement of the North Tianshan microcontinent: Example of granitoid gneisses of the Karadjilga pluton, NW Kyrgyzstan

&lt;p&gt;The Mesoproterozoic Karadjilga pluton is a poorly studied fragment of the North Tianshan microcontinent located in the western Central Asian Orogenic Belt. Metasedimentary rocks surrounding the pluton consist of marbles and mica schists of the Mesoproterozoic Ortotau Group. These rocks constitute a major west-northwest trending syncline with steep to subvertical limbs. The hinge of the fold is well expressed in the west part of the syncline and plunges east with 30-40&amp;#176; angle of plunge. Eastern termination of the syncline is cut by faults. Granitoid gneisses and granites of the Karadjilga pluton crop out in the core of the syncline. The contacts of the pluton are sub-parallel to bedding and schistosity in surrounding rocks. Primary magmatic contacts are locally reworked by reverse faults and thrusts. Our detailed mapping and structural study revealed inhomogeneous deformation of rocks of the Karadjilga pluton. The following rock types are identified: 1) undeformed granite 2) foliated granite 3) granite-gneiss and 4) mylonite. Undeformed granites form &lt;25-30% of total volume of the pluton and are most widespread in the northeast part of the pluton. On some geological maps they are shown as Ordovician or Devonian. However, U-Pb dating of 9 zircon grains by SHRIMP-II (VSEGEI, St. Petersburg, Russia) yielded a 1125&amp;#177;5 Ma concordant age. It agrees with previously reported U-Pb SHRIMP ages for deformed granites and gneisses (Degtyarev et al., 2011; Kr&amp;#246;ner et al., 2013) and indicates that undeformed granites belongs to the same Mesoproterozoic magmatic complex. Foliated granites and gneisses prevail and constitute up to 60-70% of total volume. They form west-northwest trending zones alternating with mylonites or undeformed granite. Mylonites are subordinate and occur mainly along the contacts of the pluton. Shear zones seem to be approximately parallel to the schistosity of deformed granites, but their geometry needs more study and mapping. Shear-sense indicators were studied in the oriented thin sections and are represented mainly by sigma and delta structures and oblique foliation with rare folds and other indicators. In all but one sample only strike-slip displacement has been identified. In the northern part of the pluton sinistral displacement predominates, whereas dextral displacement prevails in the southern part of the pluton. Shear zones are most widespread on the margins of the Karadjilga pluton, but locally also occur in the central part of the pluton, where they form narrow west-northwest trending zones. According to shear-sense indicators, displacement within the Karadjilga pluton occurred mainly in the approximately west-east direction that strongly differs from the north-south sense of displacement in the Paleozoic thrust and fold belts of Tianshan.&lt;/p&gt;&lt;p&gt;The study was supported by the RFBR project 20-05-00252.&lt;/p&gt;