Deep-seated Rocks Research Articles

Identification of Subsurface Geological Structures from Gravity Data by Modelling Techniques in Kalak-Bardarash Area, West of Erbil City Iraqi Kurdistan Region

A quantitative interpretation of gravity anomalies in the Kalak-Bardarash area, west of Erbil City in the Kurdistan Region of Iraq, was performed using two-dimensional analysis techniques. These techniques modeled geological structures identified from a previously created Bouguer anomaly map of the area. Five profiles were selected for the analysis, resulting in eight models constructed for both deep and shallow-seated rock units. The deep-seated rocks are represented by the basement rocks, with models constructed in the NNE-SSW, WNW-ESE, and NW-SE directions using the regional anomaly component. The depth of the basement rocks ranges from five to nine kilometers and is affected by numerous faults in the form of graben and step structures. The basement rock surface slopes towards the NE and ESE at a gradient of 110 m/km. The shallow-seated rocks are represented by the sedimentary cover, which includes Neogene, Paleogene, and Cretaceous-aged groups of formations. The residual component of the anomalies of the five profiles were interpreted. Three profiles, oriented NNE-SSW and nearly perpendicular to the main anticlinal structure, revealed that the Bardarash anticline is influenced by two faults striking both limbs and extending in the NNW-SSE direction, with an estimated throw of 200 meters on each side. The southern fault is believed to extend down to the basement rocks. The other two profiles, oriented NW-SE and nearly perpendicular to the Zab River, indicated the presence of two additional faults with a throw of approximately 600 meters in the deeper parts striking the basement rocks, dipping towards the southeast. The throw of these faults gradually decreases upward in the sedimentary cover, reaching about 200 meters. These faults directly influence the course of the Zab River, directing it in a NE-SW direction at this district.

Imaging flow focusing and isolation of aqueous fluids in synthetic quartzite: implications for permeability and retained fluid fraction in deep-seated rocks

The microstructure of realistic fluid–rock systems evolves to minimize the overall interfacial energy, enabling local variations in fluid geometry beyond ideal models. Consequently, the permeability–porosity relationship and fluid distribution in these systems may deviate from theoretical expectations. Here, we aimed to better understand the permeability development and fluid retention in deep-seated rocks at low fluid fractions by using a combined approach of high-resolution synchrotron radiation X-ray computed microtomography imaging of synthesized rocks and numerical permeability computation. We first synthesized quartzite using a piston-cylinder apparatus at different fluid fractions and wetting properties (wetting and non-wetting systems with dihedral angles of 52° and 61°–71°, respectively) under conditions of efficient grain growth. Although all fluids should be connected along grain edges and tubules in the homogeneous isotropic wetting fluid–rock system enabling segregation by gravitational compaction in natural settings, the fluid connectivity rapidly decreased to ~ 0 when the total fluid fraction decreased to 0.030–0.037, as the non-ideality of quartzite, including the interfacial energy anisotropy (i.e., grain faceting), became critical. In non-wetting systems, where the minimum energy fluid fraction based solely on the dihedral angle is ~ 0.015–0.035, the isolated (disconnected) fractions was 0.048–0.062. A streamline computation in the non-wetting system revealed that with decreasing total porosity, flow focusing into fewer channels maintained permeability, allowing the effective segregation of the connected fluids. These results provide insight into how non-wetting fluids segregate from rocks and exemplify the fraction of retained fluids in non-wetting systems. Thus, the findings suggest a potential way for wetting system fluids to be transported into the deep Earth's interior, and the amount of fluids dragged down to the Earth’s interior could be higher than what was previously estimated.

T-t Evolution of the Early Proterozoic Rocks in the Northern Ladoga Region from the Data on U-Pb, Rb-Sr and Sm-Nd Systems in Minerals

This paper presents the results of a study of isotopic systems in minerals and rocks in southern margin of the epi-Archean Karelian craton in the zone of its junction with the Svecofennian mobile belt. U-Pb, Sm-Nd and Rb-Sr mineral ages of metamorphic rocks allowed reconstructing a T-t trend during ~1.88–1.61 Ga, which reflects a wide-ranging cooling history of metamorphic rocks from the peak values of about 650–700 °C at 1.88–1.79 Ga (U-Pb age of monazites and apparent oldest Sm-Nd age of amphiboles) to 300–400 °C at 1.61 Ga (model Rb-Sr age of biotites) in zones of low- and medium-temperature metamorphism. The specificity of removal of deep-seated rocks to the present-day erosion surface and the reconstructed T-t trend comply with the development of thrust-nappe structures during the exhumation of the Svecofennids. It is also assumed that differential vertical block movements played a significant role during the post-orogenic extensional collapse and neorifting.

Monazite and zircon U–(Th–)Pb dating reveals multiple episodes of HT metamorphism in the Cima Lunga unit (Central Alps): implications for the exhumation of high‐pressure rocks

High- to ultrahigh-pressure (HP–UHP) rocks recording high-temperature (HT) > 700 °C are well exposed in the Central Alps, making it an ideal region to study the timing of metamorphic stages and the mechanisms of deep-seated rocks exhumation. Here, we report an integrated dataset of petrological and U–(Th–)Pb dating of metapelites surrounding ultramafic lenses from the Cima Lunga unit. At the interface with ultramafics preserving (U)HP–HT assemblages (1.5–3.1 GPa, 650–850 °C), metapelites record higher P‒T values (1.3–2.7 GPa, 700–850 °C) and traces of partial melting, whereas the rest of the unit is dominated by amphibolite-facies conditions. U–Th–Pb dating on zircon and monazite from migmatites indicates that partial melting was episodic involving at least two stages at ~38 to 35 Ma and 33–30 Ma, respectively. While the 38–35 Ma stage matches the HP conditions (> 1.5 GPa) and it is recorded around only one lens with scarce volumes of melt, partial melting at 33–30 Ma is witnessed at lower pressure (~1 GPa) and more widely distributed around the lenses, as within the major shear zones. Far from the ultramafics, zircon from the amphibolite-facies metasedimentary rocks record inherited pre-Variscan ages, while monazite ages at ~22 Ma document mineral growth during the Barrovian cooling. Field and petro-chronological evidence highlight that multiple episodes of partial melting locally developed at the rheological interface promoted by the interplay of fluids extracted from the ultramafic lenses associated with shear heating. New evidence suggests that local variation of P‒T equilibria play a significant role during the exhumation history.Graphical

Experimental Modeling of the Interaction between Garnets of Mantle Parageneses and CO2 Fluid at 6.3 GPa and 950–1550 °C

Abstract —Experimental modeling of the interaction of eclogitic and lherzolitic garnets with CO2 fluid was carried out on a multianvil high-pressure apparatus of the “split-sphere” type (BARS) in platinum ampoules with inner graphite capsules, using a buffered high-pressure cell with a hematite container, at a pressure of 6.3 GPa and in the temperature range 950–1550 °C. It has been established that the main interaction processes at 6.3 GPa and 950–1250 °C are partial dissolution, recrystallization, and carbonation of garnet which lead to the formation of magnesian carbonate, kyanite, and coesite, a decrease in Mg contents in the recrystallized garnet, and the formation of carbonate, silicate, and oxide inclusions in it. Under these conditions, crystallization of metastable graphite and growth of diamond on the seed at ≥1250 °C were observed. In the temperature range 1350–1550 °C, the garnet underwent partial dissolution and recrystallization in CO2 fluid; no carbonation took place. These processes were accompanied by a decrease in the portion of the grossular component in the garnet and by the enrichment of the fluid phase with calcium. We have established the indicative characteristics of garnet that interacted with CO2 fluid: zoning, with low contents of CaO and MgO in the rims of crystals relative to the cores, and the presence of carbonate, kyanite, coesite, and CO2 inclusions. The compositions of the produced garnet and carbonates are consistent with the data on these minerals in mantle peridotite and eclogite parageneses and in inclusions in diamonds, which suggests a significant role of metasomatism with the participation of CO2 fluid in the evolution of deep-seated rocks and in the diamond formation. In this experimental research, we have first studied the processes of diamond crystallization and determined the boundary conditions for diamond growth in the system silicate–carbonate–CO2, which simulates natural diamond formation media. In general, the established regularities can be regarded as potential indicators of mantle metasomatism and mineral formation with the participation of CO2 fluid.

Chemical compaction and fluid segregation in piston cylinder experiments

Fluid segregation is a ubiquitous process in deep-seated rocks, mainly driven by buoyancy and gravitational compaction, which occur in drained systems until permeability decreases and the system become practically undrained. The mechanism and rate of fluid segregation following this stage are poorly constrained, despite the importance of these factors for fluid distribution and the physical and chemical properties in the Earth. To this end, we performed sintering experiments of quartzite, with H2O–CO2 fluids of 1.9%–18.0% added volume fraction using a piston-cylinder apparatus at 900°C under nominally isotropic pressure of 1 GPa. The subsequent chemical redistribution of silica and fluids resulted in capsule-scale fluid segregation (CFS) and the formation of dense quartzite (∼0.3%) within 192 h in pure H2O systems. Comparative experiments showed that dissolution/precipitation was not caused by the temperature gradient. Instead, we considered the fluid pressure difference within the experimental capsule between fluid-rich and fluid-poor domains caused by the different response against small pressure oscillation during experiments. Fluid-rich rock is elastically “soft” (i.e., having larger Skempton's coefficient) compared to fluid-poor rock to increase pore fluid pressure under a compressional stress change. Therefore, more silica dissolves in fluid-rich domains with higher solubility, while it precipitates in fluid-poor domains through diffusive transport, expanding porosity contrast. This chemical compaction in the capsule scale is effective as long as elasticity dominates matrix viscosity because the pressure difference relaxes with time. The calculated relaxation time of quartzite at the experimental condition was comparable to the intervals of the oil pressure addition which produced stress change. A one-dimensional model for porosity evolution showed that time-averaged SiO2 diffusivity during CFS was much larger than the grain boundary diffusivity of tightly sintered quartz aggregates, but was comparable to the diffusivity of SiO2 in aqueous fluid with ∼0.1 vol% fluid fraction. During metamorphism where the fluid pressure difference may be maintained for longer duration, the spontaneous silica cementation may play a role for formation of syn-metamorphic veins and control of the amount of pore fluids transported to deep Earth.

Discovery of Stishovite in the Prismatine-Bearing Granulite from Waldheim, Germany: A Possible Role of Supercritical Fluids of Ultrahigh-Pressure Origin

For the first time in the sixty years since the synthesis of stishovite, we report unambiguous evidence of stishovite formed in the deep Earth. A minimum pressure of about 7.5 GPa at 1000 °C is necessary for the formation of stishovite, corresponding to a depth of about 230 km. In this manuscript we report the identification of stishovite along with coesite as inclusions in mineral grains from the Waldheim granulite. This implies that the stishovite was transported upwards, probably very rapidly to a depth of about 130 km, corresponding to the highest pressure indicated by newly identified coesite in the prismatine of the Waldheim granulite, and continuing up to the depth of emplacement of the Waldheim prismatine granulite. The analysis of the Raman spectra obtained from a metastable trapped stishovite micro-crystal show that all the diagnostic Raman bands are present. However, given the metastability of the stishovite at room temperatures and pressures, this mineral breaks down step-by-step into stable polymorphs, first coesite and then quartz and cristobalite, during the Raman stimulation. The rare coesite crystals in prismatine have also resulted from the irreversible transformation from stishovite. Although the Waldheim occurrence may be unique, we suggest that Raman analysis of co-trapped crystals in similar deep-seated rocks, an area of limited previous research, may prove an important innovation in the study of mantle processes.

Long-Lasting Boiling-Wells: Geochemical Windows into the Tectonic Activity of the Maodong Fault (China)

The Maodong Fault (China) was mainly active during the Late Pleistocene. However, in the past century, numerous destructive earthquakes have occurred along the fault zone, indicating its continuing activity. Therefore, refined monitoring of the tectonic activity along the fault is required. Boiling-Wells located in the Maodong Fault Zone were selected for this purpose. The parameters, including the rare earth elements (REE) and gas components, such as CO2, Rn, and Total Volatile Organic Compounds (TVOC), in the wells were analyzed. By combining field observations with the analytical data, we constrained the relationships between the anomalies of the hydrochemical composition and the gas composition in the Boiling-Wells and the Maodong Fault: (1) CO2 and TVOC in the Boiling-Wells originated from Cenozoic magmatism and associated intrusive rocks. High concentrations of Rn are closely linked to tectonic activities of the Maodong Fault. CO2, TVOC, and Rn are all transported to the Boiling-Wells along the Maodong Fault, with CO2 acting as a carrier gas for Rn. (2) REE in the Boiling-Wells was mainly sourced from CO2 fluids that originated from deep-seated Cenozoic magmas and intrusive rocks. The concentrations of the REE and their distribution patterns were controlled by the input of CO2 fluids and by epigenetic processes. (3) The abnormally high contents of Ca2+, HCO3−, Pb2+, and Al3+ in the Boiling-Wells are attributed to the migration of externally-derived (deep) CO2 fluids through the Maodong Fault. (4) The anomalies of the gaseous (Rn, CO2, and TVOC) and hydrochemical components (Ca2+, HCO3−, Pb2+, Al3+, ∑REE, and REE patterns) in the Boiling-Wells are closely related to the tectonic activity of the Maodong Fault. Therefore, the long-lasting Boiling-Wells provide an excellent geochemical window into the evolution of the Maodong Fault. Our study documents that the contents and variations of specific hydrochemical and gaseous components of Boiling-Wells are well-suited geochemical tracers to identify and characterize the tectonic activity of the Maodong Fault. This method is also applicable for the monitoring of tectonic activities of major faults zones with comparable preconditions worldwide.

КОЛЕКТОРСЬКІ ВЛАСТИВОСТІ ГЛИБОКОЗАНУРЕНИХ УЩІЛЬНЕНИХ ПОРІД КАРБОНУ ЦЕНТРАЛЬНОГО ГРАБЕНА ДДЗ

Paper concerns the researches of porosity and permeability properties of deep-seated consolidated rocks (siltstones, poor-porous sandstones) of Carboniferous period of the central graben of the Dnieper-Donetsk depression. The purpose of the research was to study the petrophysical parameters of the consolidated reservoir rocks, as the basis of the integrated analysis of their physical properties. Such reservoir parameters as the open porosity factor and void factor, permeability coefficient and residual water saturation factor have been studied. The article presents the limits of changes and the average values of porosity and permeability properties of rocks, and the classification of their reservoir properties. On the basis of capillarimetric research, an evaluation of the structure of the void space of rocks was made. The correlation analysis has allowed establishing a series of empirical relationships between the reservoir parameters (density, porosity coefficient, effective porosity factor and residual water saturation factor) and, also, determining correlation dependences between porosity coefficients measured in atmospheric and reservoir conditions. These relationships can be used in the data interpretation of geophysical studies of wells and in the modeling of porosity and permeability properties of consolidated rocks of the central graben of the Dnieper-Donetsk depression.

Изменчивость состава глубинных ксенолитов шпинелевых перидотитов из позднекайнозойских базальтов на вулканах Шилийн-Богд и Ачагийн-Душ Юго-Восточной Монголии: реконструкция процессов в континентальной литосферной мантии

From major oxides and trace elements in deep-seated xenoliths of garnet-free peridotites from a basalt pedestal and cinder-basalt edifice of the Shiliin-Bogd volcano, rocks are subdivided into groups according to the degree of depletion–enrichment and the effect of garnet on their compositions. Processes of depletion of peridotites with the restite formation, metasomatism with enrichment of rocks in light rare earth elements (REE), and mixing of various components of peridotites with the fluid one of protomantle are established. Limited compositions of peridotite xenoliths from the volcanic edifice associated with anorthoclase and clinopyroxene megacrysts is determined. The data obtained are compared with those on deep-seated peridotite xenoliths from cinder of the Achagiin-Dush volcano located 6 km from the Shiliin-Bogd one. It is inferred that compositions of deep-seated rocks under the volcanoes show different variability. A sequence of generation of the continental lithospheric mantle is reconstructed.

Carboniferous-Permian tectono-metamorphic evolution of the Pelvoux Massif (External Crystalline Massif, Western Alps), with discussion on flow kinematics of the Eastern-Variscan Shear Zone

Based on new structural, petrological and U-Th-Pb geochronological data, a reappraisal of the Variscan tectono-metamorphic history of the Pelvoux Massif (External Crystalline Massif, French Alps) is proposed with the aim to understand the flow pattern and kinematics of the Variscan partially molten crust and the Eastern Variscan Shear Zone. The Pelvoux Massif consists of high-grade metamorphic rocks of middle to lower crust, mostly migmatites, that record a prominent syn-metamorphic deformation event (D2) characterized by a pervasive NE-SW striking, steeply dipping, S2 foliation, and a network of anastomosed NS and NW-SE trending shear zones, the kinematics of which indicates a sinistral transpression. Relics of an early syn-metamorphic event (D1/M1) related to crustal thickening and top-to-the-east nappe stacking are also reported. Both the D1 and D2 features are interpreted as reflecting a NW-SE shortening event, firstly marked by dominant nappe stacking, and secondly overprinted by a sinistral transpression that started at peak metamorphism with the onset of crustal partial melting at ca. 650 °C during the late Visean (ca. 335–330 Ma). Ongoing sinistral D2 transpression in the partially molten middle-lower crust of the Pelvoux involved strain partitioning between C and C’ shear zones and horizontal longitudinal flow in the range 330–300 Ma. Along the anatectic front, vertical shortening and top-to-the-NW shearing (D3) is coeval with D2 and argue for southeastward motion of the partially molten crust. The contemporaneity between NW-SE directed transpressional flow and vertical shortening is supported by our radiometric data of D2 and D3 and attests for strain partitioning between the suprastructure and infrastructure during horizontal crustal flow under transpressive regime. The exhumation of deep-seated rocks during sinistral transpression followed a near isothermal (ca. 700 °C) evolution down to pressure of ca. 0.5 GPa in the period 325–306 Ma. The sinistral transpression recorded in the Pelvoux Massif might corresponds to an antithetic shear zone coeval with the dextral East-Variscan Shear Zone, proposed for this part of the Variscan orogen.

Early Miocene Exhumation of High-Pressure Rocks in the Himalaya: A Response to Reduced India-Asia Convergence Velocity

Low-viscosity channel flow, originating from a melt-weakened mid-crustal layer, is one of the most popular tectonic models to explain the exhumation of deep-seated rocks in the Greater Himalayan Sequence (GHS). The driving mechanism of such channel flow, generally attributed to focused erosion in the mountain front, is still debated, and yet to be resolved. Moreover, the channel flow model cannot explain eclogites in the GHS. In this study, we present a new two-dimensional thermo-mechanical numerical model, based on lubrication dynamics to demonstrate the exhumation process of deep crustal rocks in GHS. The model suggests that a dynamic-pressure drop in the Himalayan wedge, following a large reduction in the India-Asia convergence velocity (15 cm/yr at 50 Ma to nearly 5 cm/yr at ∼22 Ma) localized a fully developed extrusion zone, which controlled the pressure-temperature-time (P-T-t) path of GHS rocks. We show that the wedge extrusion, originated in the lower crust (∼60 km), was initially bounded by two oppositely directed ductile shear zones: the South Tibetan Detachment systems (STDS) at the top and the Higher Himalayan Discontinuity (HHD) at the bottom. With time the bottom shear boundary of the extrusion zone underwent a southward migration, forming the Main Central Thrust (MCT) at ∼17 Ma. Our model successfully reproduces two apparently major paradoxical observations in the Himalaya: syn-convergence extension and inverted metamorphic isograds. Model peak P (10–17 kb) and T (700–820°C) and the exhumation P-T-t path estimated from several Lagrangian points, traveling through the extrusion zone, are largely compatible with the petrological observations in the GHS. The model results account for the observed asymmetric P-T distribution between the MCT and STDS, showing peak P-T values close to the MCT. The lubrication dynamics proposed in this article sheds light on the fast exhumation event (>1 cm/yr) in the most active phase of crustal extrusion (22-17 Ma), followed by a slowed-down event. Finally, our model explains why the extrusion zone became weak in the last 8-10 Ma in the history of India-Asia collision.

New data on the composition of organic matter of deep-seated rocks of Bashkortostan by NMR in a low magnetic field

The study of the organic matter of Riphean-Vendian deposits, the features of its distribution are the subject of research by many authors. The problem of hydrocarbon of these deposits remains relevant. We have conducted studies of deep-seated rocks of the Lower Riphean of the kaltasinskaya suite of the Eastern-Askinskaya area, with an increased content of organic matter in individual intervals. In addition to standard research methods, the method of nuclear magnetic relaxation in a low magnetic field was used. This method allows determining the phase state of organic matter in the original rock without any impact and after isothermal heating of the rock at 350°C. The free induction signal and the type of solid-phase signal, which corresponds to the waveform for the asphaltene component of the oil, were analyzed. The predominant part of the liquid-phase signal - about 35%, is characterized by a transverse relaxation time of 540 ¼s, corresponding to the resinous component of organic matter. The quantitative ratio of asphaltenes before and after heat treatment was also analyzed. A significant decrease in its amount was recorded. During thermal exposure, more than 55% of light, more mobile hydrocarbon molecules are formed in the liquid-phase part of organic matter.

Large-scale structure of the Doldrums multi-fault transform system (7-8ºN Equatorial Atlantic): preliminary results from the 45th expedition of the R/V A.N. Strakhov

The Equatorial portion of the Mid Atlantic Ridge is displaced by a series of large offset oceanic transforms, also called “megatransforms”. These transformdomains are characterized by a wide zone of deformation that may include different conjugated fault systems and intra-transform spreading centers (ITRs).Among these megatransforms, the Doldrums system (7-8oN) is arguably the less studied, although it may be considered the most magmatically active. Newgeophysical data and rock samples were recently collected during the 45th expedition of the R/V Akademik Nikolaj Strakhov. Preliminary cruise results allowto reconstruct the large-scale structure and the tectonic evolution of this poorly-known feature of the Equatorial Atlantic. Swath bathymetry data, coupled withextensive dredging, were collected along the entire megatransform domain, covering an area of approximately 29,000 km2. The new data clearly indicate thatthe Doldrums is an extremely complex transform system that includes 4 active ITRs bounded by 5 fracture zones. Although the axial depth decreases toward thecentral part of the system, recent volcanism is significantly more abundant in the central ITRs when compared to that of the peripheral ITRs. Our preliminaryinterpretation is that a region of intense mantle melting is located in the central part of the Doldrums system as consequence of either a general transtensive regimeor the occurrence of a more fertile mantle domain. Large regions of basement exposure characterize the transform valleys and the ridge-transform intersections.We speculate that different mechanisms may be responsible for the exposure of basement rocks. These include the uplift of slivers of oceanic lithosphereby tectonic tilting (median and transverse ridges formation), the denudation of deformed gabbro and peridotite by detachment faulting at inner corner highs, andthe exposure of deep-seated rocks at the footwall of high-angle normal faults at the intersection of mid-ocean ridges with transform valleys

Coupled Zircon-Rutile U-Pb Chronology: LA ICP-MS Dating, Geological Significance and Applications to Sediment Provenance in the Eastern Himalayan-Indo-Burman Region

U-Pb dating by LA ICP-MS is one of the most popular and successful isotopic techniques available to the Earth Sciences to constrain timing and rates of geological processes thanks to its high spatial resolution, good precision (absolute U/Pb age resolution of ca. 2%, 2s), rapidity and relative affordability. The significant and continuous improvement of instrumentation and approaches has opened new fields of applications by extending the range of minerals that can be dated by this method. Following the development and distribution to the community of good quality reference materials in the last decade, rutile U-Pb thermochronology (with a precision only slightly worse than zircon) has become a commonly used method to track cooling of deep-seated rocks. Its sensitivity to mid- to low-crustal temperatures (~450 °C to 650 °C) is ideal to constrain exhumation in active and ancient orogens as well as thermal evolution of slow-cooled terranes. Recrystallization and secondary growth during metamorphism and the presence of grain boundary fluids can also affect the U-Pb isotopic system in rutile. A growing body of research focusing on U-Pb dating of rutile by LA ICP-MS is greatly improving our understanding of the behavior of this mineral with regards to retention of radiogenic Pb. This is key to fully exploit its potential as a tracker of geological processes. The latest developments in this field are reviewed in this contribution. The combined application of U-Pb zircon and rutile chronology in provenance studies, particularly when complemented by lower-T thermochronometry data, allows the isotopic characterization of the sources across a wide range of temperatures. The benefits of applying detrital zircon-rutile U-Pb chronology as a coupled provenance proxy are presented here, with a focus on the Eastern Himalayan-Indo-Burman region, where a growing number of successful studies employs such an approach to help constrain river drainage and basin evolution and to infer feedback relationships between erosion, tectonics and climate.

Detrital zircon double-dating of forearc basin strata reveals magmatic, exhumational, and thermal history of sediment source areas

Research Article| January 30, 2019 Detrital zircon double-dating of forearc basin strata reveals magmatic, exhumational, and thermal history of sediment source areas Eva Enkelmann; Eva Enkelmann † 1University of Calgary, Department of Geoscience, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada †eva.enkelmann@ucalgary.ca Search for other works by this author on: GSW Google Scholar Sonia K. Sanchez Lohff; Sonia K. Sanchez Lohff 2University of Cincinnati, Department of Geology, 500 Geology Physics Building, Cincinnati, Ohio 45221-0013, USA Search for other works by this author on: GSW Google Scholar Emily S. Finzel Emily S. Finzel 3University of Iowa, Department of Earth and Environmental Science, 115 Trowbridge Hall, Iowa City, Iowa 52242, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Eva Enkelmann † 1University of Calgary, Department of Geoscience, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada Sonia K. Sanchez Lohff 2University of Cincinnati, Department of Geology, 500 Geology Physics Building, Cincinnati, Ohio 45221-0013, USA Emily S. Finzel 3University of Iowa, Department of Earth and Environmental Science, 115 Trowbridge Hall, Iowa City, Iowa 52242, USA †eva.enkelmann@ucalgary.ca Publisher: Geological Society of America Received: 22 May 2018 Revision Received: 04 Oct 2018 Accepted: 21 Nov 2018 First Online: 31 Jan 2019 Online Issn: 1943-2674 Print Issn: 0016-7606 © 2019 Geological Society of America GSA Bulletin (2019) 131 (7-8): 1364–1384. https://doi.org/10.1130/B35043.1 Article history Received: 22 May 2018 Revision Received: 04 Oct 2018 Accepted: 21 Nov 2018 First Online: 31 Jan 2019 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Eva Enkelmann, Sonia K. Sanchez Lohff, Emily S. Finzel; Detrital zircon double-dating of forearc basin strata reveals magmatic, exhumational, and thermal history of sediment source areas. GSA Bulletin 2019;; 131 (7-8): 1364–1384. doi: https://doi.org/10.1130/B35043.1 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 SocietyGSA Bulletin Search Advanced Search Abstract Subduction along the southern margin of Alaska, USA, has been ongoing since at least the Jurassic. However, the character of the subducting slab has changed through time and has included subduction of normal oceanic crust, a spreading ridge, and an oceanic plateau. The latter two resulted in a change of subduction mode by inducing a shallow subduction angle. Geologic processes and landscape evolution of the overriding plate were affected by these variations in subduction mode and are recorded in the forearc basin strata. We investigate the Cenozoic to modern sediment of the forearc Cook Inlet basin in south-central Alaska. Here we use a double-dating approach that combines fission track dating and U-Pb dating on individual detrital zircon grains. In total we analyzed more than 1700 zircons from Eocene to Pliocene strata and modern river sand that has eroded from the surrounding regions of the Cook Inlet basin. The double-dating approach combined with the existing knowledge of the regional geology allows us to discriminate between magmatic cooled grains of extrusive and shallow intrusive rocks, exhumational cooled grains, and thermal reset grains. We find that the erosion of both shallow and deep intrusive arc rocks dominate the detrital age signal, while syn-depositional extrusive grains are lacking. The erosion of rocks that have been thermally altered during the subduction of a spreading ridge dominates the fission track (FT) age signal. This pattern is particularly prominent in the accretionary prism where ages in the most inboard (older) portion have not been thermally reset, but thermal resetting is prevalent in the outboard (younger) portion located proximal to the Paleocene–Eocene near-trench intrusions. Thermal alteration is also evident in the region of the arc that was affected by the passage of the asthenospheric slab window. The erosional signal of the more inboard arc and backarc region (Alaska Range) is characterized by exhumational FT ages of deep-seated rocks, which currently provide material into the forearc basin. This age signal results from flat-slab subduction of the Yakutat microplate, which transfers stress far inboard and produces significant mountain building and deformation. This exhumational age signal, however, is not recorded in the late Cenozoic strata, suggesting that the modern landscape developed since <3 Ma. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Physicochemical Model of Formation of Gold-Bearing Magnetite-Chlorite-Carbonate Rocks at the Karabash Ultramafic Massif (Southern Urals, Russia)

We present a physicochemical model for the formation of magnetite-chlorite-carbonate rocks with copper gold in the Karabash ultramafic massif in the Southern Urals, Russia. The model was constructed based on the formation geotectonics of the Karabash massif, features of spatial distribution of metasomatically altered rocks in their central part, geochemical characteristics and mineral composition of altered ultramafic rocks, data on the pressure and temperature conditions of formation, and composition of the ore-forming fluids. Magnetite-chlorite-carbonate rocks were formed by the hydrothermal filling of the free space, whereas chloritolites were formed by the metasomatism of the serpentinites. As the source of the petrogenic and ore components, we considered rocks (serpentinites, gabbro, and limestones), deep magmatogenic fluids, probably mixed with metamorphogenic fluids released during dehydration and deserpentinization of rocks in the lower crust, and meteoric waters. The model supports the involvement of sodium chloride-carbon dioxide fluids extracting ore components (Au, Ag, and Cu) from deep-seated rocks and characterized by the ratio of ore elements corresponding to Clarke values in ultramafic rocks. The model calculations show that copper gold can also be deposited during serpentinization of deep-seated olivine-rich rocks and ore fluids raised by the tectonic flow to a higher hypsometric level. The results of our research allow predicting copper gold-rich ore occurrences in ultramafic massifs.

Comparison of the compositions of crystalline aluminosilicate rocks and their minerals in a planar triangular projection

The problem of comparison of the composition of crystalline rocks with the composition of the constituent minerals of these rocks is considered. It is proposed to present the composition of rocks and the compositions of the constituent minerals in the form of a triangle on the plane. The experience of presentation of compositions in phase diagrams was taken as a basis for the construction. Analysis of the crystallochemical characteristics of clinopyroxenes and garnets has shown that three parameters are enough for depicting the compositions of these and other minerals. For this purpose, similar composition components of rocks and their minerals are summarized in molecular proportions and are plotted on the triangle DO-1/2(R2O3)-XO2, where DO = (MgO + CaO + FeO + MnO + NiO + ...) + 1/4(Na2O + Al2O3) + 1/4(K2O + Al2O3), 1/2(R2O3) = 1/2(Al2O3 + Fe2O3 + Cr2O3 + ...) - (1/4(Na2O + Al2O3) + 1/4(K2O + Al2O3)), and XO2 = SiO2 + TiO2. The compositions of minerals are expressed as the sums of their components: Ol = Fo + Fa + Lar + Neph + ..., Px = Di + En + Wol + Ged + Gip + Jd + Eg + ..., Ga = Pyr + Gross + Alm + Spe + Ski + Knr + Mj + ..., etc. A step-by-step calculation algorithm is proposed, which permits evaluation of the contents of bi- and trivalent iron during probe microanalyses of garnets and pyroxenes. Comparison of the compositions of deep-seated rocks and their minerals shows their good consistency. The proposed schematic projection permits a visual comparison of the compositions of rocks with low contents of carbonates and water, from ultrabasic (e.g., dunites) to acid (e.g., granites) ones.

Olivine-bearing rocks of the Lapland granulite belt (Baltic Shield)

Olivine-bearing varieties of garnet–clinopyroxene crystalline schists of the Lapland granulite belt have been studied in detail for the first time. Two types of olivine (iron mole fractions of 27 and 38%) are distinguished. Olivine with lower Fe content occurs as inclusions in clino- and orthopyroxene and in terms of СаО and Cr contents is close to magmatic minerals. Olivine with high Fe content presumably suffered highand moderate-temperature metamorphism. The olivine-bearing rocks contain several grains of omphacite with 30–37 mol % jadeite and garnet with 44–50 mol % pyrope, which can be regarded as relict assemblages of the early stage of eclogitization of a magmatic protolith. The presence of symplectites indicates their retrograde transformation during decompression. The protoliths of the studied rocks could be olivine gabbronorites and pyroxenites. It was found that the rocks contain high-alumina minerals: corundum, spinel, and sapphirine. In addition, Al2O3 content in some amphibole grains is as high as 19 wt %. This indicates that the ascent of the deep-seated rocks was accompanied by interaction with Al-rich fluid. The positive Eu anomaly in the olivine-bearing rocks and some of their minerals is indicative of the reducing character of fluid. Activation of fluid reworking leading to the formation and transformation of the olivine-bearing rocks, transfer of alumina and its precipitation at different depths are related to the processes at the base of the Paleoproterozoic rift system of Karelides.

Cenozoic landforms and post-orogenic landscape evolution of the Balkanide orogen: Evidence for alternatives to the tectonic denudation narrative in southern Bulgaria

Continental denudation is the mass transfer of rock from source areas to sedimentary depocentres, and is typically the result of Earth surface processes. However, a process known as tectonic denudation is also understood to expose deep-seated rocks in short periods of geological time by displacing large masses of continental crust along shallow-angle faults, and without requiring major contributions from surface erosion. Some parts of the world, such as the Basin and Range in the USA or the Aegean province in Europe, have been showcased for their Cenozoic tectonic denudation features, commonly described as metamorphic core-complexes or as supradetachment faults. Based on 22 new apatite fission-track (AFT) and 21 helium (AHe) cooling ages among rock samples collected widely from plateau summits and their adjacent valley floors, and elaborating on inconsistencies between the regional stratigraphic, topographic and denudational records, this study frames a revised perspective on the prevailing tectonic denudation narrative for southern Bulgaria. We conclude that conspicuous landforms in this region, such as erosion surfaces on basement-cored mountain ranges, are not primarily the result of Paleogene to Neogene core-complex formation. They result instead from “ordinary” erosion-driven, subaerial denudation. Rock cooling, each time suggesting at least 2km of crustal denudation, has exposed shallow Paleogene granitic plutons and documents a 3-stage wave of erosional denudation which progressed from north to south during the Middle Eocene, Oligocene, Early to Middle Miocene, and Late Miocene. Denudation initially prevailed during the Paleogene under a syn-orogenic compressional regime involving piggyback extensional basins (Phase 1), but subsequently migrated southward in response to post-orogenic upper-plate extension driven by trench rollback of the Hellenic subduction slab (Phase 2). Rare insight given by the denudation pattern indicates that trench rollback progressed at a mean velocity of 3 to 4km/Ma. The Neogene horst-and-graben mosaic that defines the modern landscape (Phase 3) has completely overprinted the earlier fabrics of Phases 1 and 2, and has been the prime focus of tectonic geomorphologists working in the region. The new narrative proposed here for linking the geodynamic evolution of SE Europe with surface landform assemblages raises issues in favour of better documenting the regional sedimentary record of existing Paleogene basins, which constitute a poorly documented missing link to the thermochronological evidence presented here.