Forearc density structure of the overriding plate in the northern area of the giant 1960 Valdivia earthquake

The objective of this work is to analyse the density structure of the continental forearc in the northern segment of the 1960 Mw 9.6 Valdivia earthquake. Regional 2D and local 3D density models have been obtained from available gravity data in the area, complemented by new gravimetric stations. Models are constrained by independent geophysical and geological information and new TEM and MT soundings. The results show a segmentation of the continental wedge along and perpendicular to the margin, highlighting a high-density anomaly, below the onshore forearc basin, that limits the late Paleozoic–early Mesozoic metamorphic basement in the region where Chaitenia terrane has been proposed. A progressive landward shift of this anomaly correlates with the high slip patch of the giant 1960 Mw 9.6 Valdivia earthquake. Based on these results, we propose that the horizontal extension of the less rigid basement units conforming the marine wedge and Coastal Cordillera domain could modify the process of stress loading during the interseismic periods, and also that changes in position and extension of the late Paleozoic–early Mesozoic accretionary complex could be linked with the frictional properties of the interplate boundary. This analysis provides new evidence of the role of the overriding plate structure in the seismotectonic process in subduction zones.

Geochemical and Geochronological Constraints on a Granitoid Containing the Largest Indosinian Tungsten (W) Deposit in South China (SC): Petrogenesis and Implications

Chuankou tungsten (W) ore field, with an estimated WO3 reserve exceeding 300,000 tonnes, is so far the largest Indosinian (Triassic) granite-related W ore field in South China. However, the precise emplacement ages, sources of granitoids, and their relationship with W mineralization are still not well understood. In this research, four main magmatic stages (G-1 to G-4) have been identified in the Chuankou ore field, including G-1 (phase I, biotite monzogranite), G-2 (phase II, two-mica monzogranite), G-3 (phase III, fine-grained granite), and G-4 (phase IV, granite porphyry). LA-ICP-MS U-Pb dating of zircon grains from granitoids of the Chuankou W ore field yields emplacement ages of 230.8 ± 1.6 Ma, 222.1 ± 0.56 Ma, 203.1 ± 1.6 Ma, and 135.5 ± 2.4 Ma, respectively. Granitoids from the Chuankou ore field contain a large amount of peraluminous minerals such as biotite, musvite, garnet and tourmaline. Geochemically, the granitoids have high Si and Al (A/CNK > 1.1) content but low alkali, Fe, Mg, Mn, and Ca content. Moreover, there is enrichment of Rb, Zr, Hf, Th, and U, but depletions of Ba, Sr, P, and Ti. The granitoids have especially low Zr + Nb + Ce + Y and high Rb/Ba ratios, further indicating a highly fractionated S-type granite affinity with a significant crystal fractionation process in regard to K-feldspar, plagioclase, biotite, Ti-bearing minerals (except rutile), zircon, apatite, allanite, and monazite. Whole-rock εNd(t) and TDM2 values are −10.77 and 2090 Ma for G-1, −9.09 to −7.47 and 1764–1684 Ma for G-2, −10.07 to −6.53 and 1669–1471 Ma for G-3, respectively, indicating that the Chuankou granitoids were derived from two episodes of partial melting of the Paleoproterozoic to Mesoproterozoic metamorphic basement. Trace elements within the zircons and whole-rock geochemistry yielded evidence of the close relationship between W mineralization and G-1 and G-2 granitoids of the Chuankou ore field. The batholith of the Chuankou ore field was formed 20–10 Ma later than the peak age of the collisions orogeny and formed in a post-collisional setting.

The enriched Variscan lithosphere of NE Iberia: data from postcollisional Permian calc-alkaline lamprophyre dykes of Les Guilleries

Post-collisional mafic dykes crosscut the Paleozoic metamorphic basement and late-Variscan plutons in Les Guilleries massif (Catalan Coastal Ranges, NE Iberia). The predominance of mafic phenocrysts, porphyritic texture, abundant amphibole, high MgO and volatile content, together with crustal-like trace-element patterns indicate that the dykes correspond to calc-alkaline lamprophyres, mainly spessartites. Their enrichment in LILE, HFSE and REE and initial Sr-Nd isotopic compositions (87Sr/86Sri between 0.70851 and 0.71127, epsilon Ndi between -5.23 and -4.63) are consistent with an enriched subcontinental lithospheric mantle source. U-Pb ages of matrix titanite crystals yield concordia ages of 262±7Ma, congruent with crosscutting relationships. Postmagmatic processes are evidenced by intense chloritization and albitization of the lamprophyres, together with systematic variations of Na2O vs SiO2, K2O, CaO, Ba, Rb, Cs, Pb, Sr, Tl, and Zn, and possibly the removal of F. The geochemical and geochronological data support an orogenic geochemical affinity, in accordance with the transitional tectonic regime between Variscan compression/transpression and post-collisional transtension/extension, related to the fragmentation of Pangea and thinning of the lithosphere. The lamprophyre dykes studied could represent the youngest pulse of Variscan orogenic magmatism and, therefore, mark its end in NE Iberia before the onset of the generalized Triassic extension.

Reuniting the Ötztal Nappe: the tectonic evolution of the Schneeberg Complex

The Ötztal Nappe in the Eastern Alps is a thrust sheet of Variscan metamorphic basement rocks and their Mesozoic sediment cover. It has been argued that the main part of the Ötztal Nappe and its southeastern part, the Texel Complex, belong to two different Austroalpine nappe systems and are separated by a major tectonic contact. Different locations have been proposed for this boundary. We use microprobe mapping of garnet and structural field geology to test the hypothesis of such a tectonic separation. The Pre-Mesozoic rocks in the area include several lithotectonic units: Ötztal Complex s.str., Texel Complex, Laas Complex, Schneeberg Complex, and Schneeberg Frame Zone. With the exception of the Schneeberg Complex which contains only single-phased (Eoalpine, i.e. Late Cretaceous) garnet, all these units have two-phased garnet with Variscan cores and Eoalpine rims. The Schneeberg Complex represents Paleozoic sediments with only low-grade (sub-garnet-grade) Variscan metamorphism which was thrust over the other units and their Mesozoic cover (Brenner Mesozoic) during an early stage of the Eoalpine orogeny, before the peak of Eoalpine metamorphism and garnet growth. Folding of the thrust later modified the structural setting so that the Schneeberg Thrust was locally inverted and the Schneeberg Complex came to lie under the Ötztal Complex s.str. The hypothesized Ötztal/Texel boundaries of earlier authors either cut across undisturbed lithological layering or are unsupported by any structural evidence. Our results support the existence of one coherent Ötztal Nappe, including the Texel Complex, and showing a southeastward increase of Eoalpine metamorphism which resulted from southeastward subduction.

Paragenetic diversity of the composition garnets from kimberlites Nakyn field (Yakutia)

Studied the composition of garnets from three kimberlite bodies of Nakyn field (Nyurbinskaya and Botuobinskaya pipes, Markhinskaya dyke). Paragenetic specialization of garnets was determined on the basis of their chemical composition with use of the original MineralogicalAnalyse program. This program was specially created for paragenetic classification of kimberlite minerals and is currently capable of recognizing 41 paragenesis by the composition of garnets. The set of paragenetic groups determined by the program most fully characterizes paragenetic diversity of the composition garnets from kimberlites and associated placers to date. A significant difference was established between the kimberlite bodies of Nakyn field, both in the set of different garnet paragenesis and in their percentage. A distinctive feature of garnets this field is the increased content of eclogite paragenesis among them. Xenoliths of metamorphic basement rocks did not significantly affect the paragenetic association of garnets from studied kimberlite bodies. Using of the Mineralogical Analyse program allows one to judge both paragenetic affiliation of garnets and the deep sources of diamonds of specific kimberlite bodies.

Formation of ultrapotassic magma via crustal contamination and hybridization of mafic magma: an example from the Stomanovo monzonite, Central Rhodope Massif, Bulgaria

Generally all orogenic ultrapotassic rocks are formed after melting of metasomatized sub-continental lithospheric mantle via subducted crustal mica-bearing lithologies. Here we present another possible model, based on the study of the small Stomanovo ultrapotassic monzonite porphyry intrusion in the Central Rhodope Massif, Bulgaria. The monzonite dated at 30.50 ± 0.46 Ma is intruded into the voluminous Oligocene (31.63 ± 0.40 Ma) Bratsigovo–Dospat ignimbrite. The monzonite hosts both normally and reversely zoned clinopyroxene phenocrysts. The normally zoned clinopyroxene is characterized by gradually diminishing core-to-rim Mg no. (89–74), whereas the reversely zoned clinopyroxene has green Fe-rich cores (Mg no. 71–55) mantled by normally zoned clinopyroxene (Mg no. 87–74). Neither the core of the normally zoned clinopyroxene nor the Fe-rich green cores are in equilibrium with the host monzonite. This ultrapotassic monzonite shows more radiogenic Sr isotopes ((87Sr/86Sr)i = 0.71066) and ϵNd(t) = −7.8 to −8.0 that are distinct from the host ignimbrites with (87Sr/86Sr)i = 0.70917–0.70927 and ϵNd(t) = −4.6 to −6.5. The Sr–Nd isotopic data and the presence of copious zircon xenocrysts from the underlying metamorphic basement suggest extensive crustal assimilation. Our observations indicate that the Stomanovo ultrapotassic monzonite formed after extensive lower or middle crustal fractional crystallization from an evolved magma producing cumulates. The process was followed by hybridization with primitive mantle-derived magma and subsequent continuous crustal contamination. We suggest that instead of inheriting their high K2O and large-ion lithophile element enrichments from slab-derived/metasomatic fluids, the Stomanovo ultrapotassic monzonite may owe some of its unusually high alkalinity to the assimilation of potassium-rich phases from the Rhodope Massif basement rocks.

Forearc density structure of the overriding plate in the northern area of the giant 1960 Valdivia earthquake

The objective of this work is to analyse the density structure of the continental forearc in the northern segment of the 1960 Mw 9.6 Valdivia earthquake. Regional 2D and local 3D density models have been obtained from available gravity data in the area, complemented with new gravimetric stations. Models are constrained by independent geophysical/geological information and new TEM and MT soundings. The results show a segmentation of the continental wedge along and perpendicular to the margin, highlighting a high density anomaly, below the onshore forearc basin, that limits the Late Paleozoic-Early Mesozoic metamorphic basement in the region where Chaitenia terrain has been proposed. A progressive landward shift of this anomaly correlates with the high slip patch of the giant 1960 Mw 9.6 Valdivia earthquake. Based on these results, we propose that the horizontal extension of the less rigid basement units conforming the marine wedge and Coastal Cordillera domain could modify the process of stress loading during the interseismic periods, and also that changes in position and extension of the Late Paleozoic-Early Mesozoic accretionary complex could be linked with the frictional properties of the interplate boundary. This analysis provides new evidence of the role of the overriding plate structure on the seismotectonic process in subduction zones.

Results of 40Ar/39Ar dating of subvolcanic body in the vent tunnelKukurtly volcano

Эльбрус изучается с 1852 г., однако вопросы о времени его образования и временных этапах эволюции до сих пор остаются дискуссионными. По мере развития новых методов датирования геологических событий исследователями Эльбру- са применялись все более современные методы и методики. Так, на самых ранних этапах исследований время образования вулкана приблизительно оценивалось только по геологическим данным, затем стал очень широко применяться геолого-гео- морфологический метод относительной геохронологии, а с конца прошлого столетия – калий-аргоновый (K-Ar), аргон-арго- новый (40Ar/39Ar), иониевый (Io/234U) и радиоуглеродный (14C) методы датирования. В полученных вышеуказанными методами датировках (цифрах возрастов) по одному и тому же объекту часто имелись существенные различия. При 40Ar/39Ar датировании биотита из туфов риолитового состава раннего этапа кальдерной стадии было доказано, что при эксплозивных извержениях, приведших к формированию этих туфов и ассоциирующих с ними игнимбритов, был захвачен дезинтегрированный материал древнего гранитно-метаморфического основания Эльбруса, который после аэрального пере- носа стал составной частью туфов и игнимбритов. Следовательно, все изотопные датировки этого пирокластического материала (по биотиту, пироксену, плагиоклазу) не отражают время образования рассматриваемых пород, так как они со- держат мельчайшие обломки древнего гранитно-метаморфического фундамента и слагающего их породообразующего био- тита, что, естественно, искажает изотопный возраст пирокластики в неизвестном масштабе. Устойчивое плато при ступенчатом нагревании пробы № 542 мегакристалла санидина однозначно свидетельствует об от- сутствии избыточного аргона, а полученный аргон-аргоновый возраст (620.3±3.3 тыс. лет) отражает время остывания кри- сталла санидина при температуре закрытия системы ~ 300о С и свидетельствует об отсутствии значительного перерыва во времени между формированием пород раннего и позднего этапов кальдерной стадии. Ключевые слова: вулкан Эльбрус, 40Ar/39Ar датирования, субвулканическое тело дацитового состава, жерло вулкана Кюкюрт- ли, удревнение или омоложение изотопного возраста. Elbrus is under study since 1852 year, however the questions about time of it origin and temporal stage of evolution, up to now are remains as a discussional.So far as development of a new method for dating of geological events, investigators of Elbrus using all more current status of methods and procedures.So, on the very early stages of investigation, time of volcano origin approximately assessed by geological data only, then -geologo-geomorphological method of relative geochronology became in very regular use and from last century - potassium-argon (K-Ar), argon-argonian(40Ar/39Ar), ionic (Io/234U) and radio-carbon(14C) methods of dating. Inobtainedbytheabovementionedmethodsofdating (fi gureofage) oncommon poles object very often there was an fundamental dissimilarities. As a result of 40Ar/39Ar dating of biotite from tuff of the rhyolite composition of early period of calderian stage, it was proved, that during explosive eruption, lead to forming of these tuff and associated ignimbrites, the old disintegrated material of granite-metamorphic basement of Elbrus was encroach and which after aeral transportation became as a constituent part of tuff and ignimbrites. Consequently, all isotope age determination of this pyroclastic material (on biotite, pyroxene, plagioclase) do not take account of time origin of covered rocks, as they contain a very small debris of old granite-metamorphic basement and rock-building biotite, that natural, to distort of isotope age of pyroclastics in unknown scale. The tolerant plateau at scalariform heating sample № 542 - megacrystal of sanidine is uniquely testifi es about absence of excess argon and argon-argon age (620.3±3.3Ka) obtained is accounts time of cooling down ofsanidinemegacrystal at the temperature of clousing of the system - 3000 C and testifi es about absence of signifi cant interruption in time between rock forming of early and late periods of calderian stage

Investigation of the bedrock (metamorphic basement) geometry of Santorini island using single-station ambient noise data

<p>We investigate the geometry of the metamorphic basement of the Santorini volcanic island using ambient noise data to determine the pre-Alpine/pre-volcanic bedrock structure. The geometry of pre-volcanic Santorini is important in order to constrain the recent volcanic history of the island and also to study the site-effect of the volcanic formations on seismic motions. Santorini is the most active volcano of the Southern Aegean Volcanic Arc and is the southernmost island of the Cyclades islands metamorphic complex. As a result, the volcanic material that has accumulated during the last 600+ Kyrs has been superimposed on the pre-volcanic Santorini (Cycladic) island. To map the thickness of volcanic material, we have performed a large number (>200) of single-station noise measurements in the Santorini area.  Measurements were mainly performed using conventional acquisition systems (Guralp-40T 30sec seismometer and Reftek-130A digitizer). We also employed additional single-station noise data from several previous studies (Dimitriadis et al. 2006, PROTEUS Project 2015), as well as permanent stations from the Hellenic Seismological Network in the same region. HVSR curves were calculated using single-station noise data and were used to estimate the fundamental frequency, f<sub>0</sub>, as well as the corresponding maximum HVSR amplitude, A<sub>0</sub><sup>HVSR</sup>. The majority of HVSR curves showed prominent peaks (A<sub>0</sub><sup>HVSR</sup> locally larger than 7-8), indicating a clear impedance contrast between volcanics and metamorphic formations. To map the bedrock depth, we estimated the thickness of the upper volcanic formations using the quarter-wavelength approximation for each site. For this assessment, the average shear-wave velocity (Vs) of the volcanic formations was estimated from the inversion of several passive ambient noise array data, as well as additional constraints from selected MASW measurements. Where possible, the reliability of the spatial variation of volcanic formation thickness was checked with independent geological information. Using the digital elevation model and the volcanic formation thickness for each site of the single-station noise data, we estimated the spatial distribution of the pre-Alpine, metamorphic bedrock depth. The resulting geometry of the pre-volcanic Santorini island shows very deep basins (now filled with volcanic formations) around the pre-Alpine bedrock outcrop in the southern part of Santorini (Profitis Ilias), increasing to 100+ meters in the Kamari-Perissa basin area (southeastern Santorini) and to more than 400+ in the central (Fira-Imerovigli) and the north Santorini areas (Oia), in agreement with recent larger-scale tomographic results (Heath et al., 2019). The results are also in very good agreement with the pre-Alpine bedrock geometry independently inferred from gravity data inversion (Tzanis et al., 2019.)</p><p><strong>This research is co-financed by Greece and the European Union (European Social Fund- ESF) through the Operational Programme «Human Resources Development, Education and Lifelong Learning» in the context of the project “Strengthening Human Resources Research Potential via Doctorate Research” (MIS-5000432), implemented by the State Scholarships Foundation (ΙΚΥ), the Hellenic Foundation for Research and Innovation (HFRI) under the “First Call for HFRI Research Projects to support Faculty members and Researchers and the procurement of high-cost research equipment grant” (Project Number: 2924) and the Institute for the Study and Monitoring Of the SAntorini Volcano (ISMOSAV). </strong></p>