Time Of Emplacement Research Articles

Limited thermochemical sulfate reduction in hot, anhydritic, sour gas carbonate reservoirs: The Upper Jurassic Arab Formation, United Arab Emirates

Abstract Limited thermochemical sulfate reduction (TSR) in hot (130–160°C) and anhydrite-rich sour gas reservoir carbonates of the Arab Formation (Upper Jurassic) is manifested by rare calcitization of anhydrite with slightly lower δ13CVPDB values (−3.2 to −0.1‰) than calcite precipitated in equilibrium with Late Jurassic seawater. Fluid inclusion microthermometry of calcite that has replaced anhydrite indicates that TSR occurred between 130°C and 160°C. The lack of evidence for extensive TSR, despite the suitable current temperatures and abundant sulfates in the gas reservoir, coupled with the relatively more common TSR-related calcite in the flanks (water zone) than crest (gas zone), indicate that: (1) gas emplacement while the reservoir was buried at shallower depth slowed down or inhibited TSR in the crest even when it subsequently reached depths where extensive TSR would occur, and (2) H2S (up to 38 vol%) has migrated from the underlying Permo-Triassic and/or Jurassic sulfate-carbonate deposits. This study demonstrates that constraining the timing of hydrocarbon emplacement within the context of burial-thermal history is crucial for a better understanding of the origin and distribution of H2S in hot, anhydrite-rich, sour gas reservoirs.

A-type granites in the western margin of the Siberian Craton: Implications for breakup of the Precambrian supercontinents Columbia/Nuna and Rodinia

The tectonic evolution of the Siberian Cratonic margins offers important clues for global paleogeographic reconstructions, within the complex continental collage of Central Asia. The Yenisey Ridge fold–and–thrust belt at the western margin of the Siberian Craton forms part of the Central Asian Orogenic Belt (CAOB) and is a key to understand the Precambrian tectonic evolution of the Siberian Craton and crustal growth in the CAOB, the world's largest Phanerozoic accretionary orogenic belt. Here we report the occurrence of A-type granites with geochemical features indicating intraplate setting from the Yenisey Ridge and provide evidence for rift-related magmatism. Zircon SHRIMP U–Pb analyses coupled with in situ U–Th–Pb geochronology of monazite constrain the timing of emplacement of the rift–related granitoids and suggest two consequential breakup events. The magmatic events at 1380 Ma and 800–720 Ma along the western margin of the Siberian Craton and other continental blocks can be associated with the breakup of the Precambrian supercontinents Nuna-Columbia (1.8–1.3 Ga) and Rodinia (1.2–0.7 Ga). These pre-Grenville and post-Grenville episodes of regional crustal evolution are correlated with the synchronous successions and similar style of rocks along the Arctic margin of Nuna–Columbia and Rodinia and supports the spatial proximity of Siberia and North Atlantic cratons (Laurentia and Baltica) over the long period 1.38–0.72 Ga. Our data confirm the proposed Neoproterozoic paleogeographic reconstructions of Columbia and Rodinia as constrained from the large igneous province (LIP) record.

Timing and mechanism of Bangong-Nujiang ophiolite emplacement in the Gerze area of central Tibet

The Bangong-Nujiang suture zone (BNSZ) separates the Lhasa terrane from the Qiangtang terrane and contains remnants of the Bangong-Nujiang oceanic lithosphere (ophiolites). Despite decades of research, when and how the Bangong-Nujiang ophiolites were emplaced remains enigmatic. In the Gerze area (western segment of the BNSZ), the geochemistry and provenance discrimination of chromian spinels (Cr-spinels) from the pre-collisional subduction complex (Mugagangri Group) and syn-collisional peripheral foreland basin succession (Wuga Formation) can help us solve this fundamental problem in the BNSZ evolution. This study compares the geochemistry of Cr-spinels from the Mugagangri Group and Wuga Formation with those from the Bangong-Nujiang ophiolites. Cr-spinels in the Bangong-Nujiang ophiolites have either low TiO2 (0.01–0.15%) and low Al2O3 (11.74–26.76%), indicating an SSZ peridotite origin, or high Al2O3 (45.28–49.15%), indicating a MORB peridotite origin. Cr-spinels from the ultramafic fragments within the Mugagangri Group have extremely low TiO2 (<0.06%) and geochemically overlap with those from the Dong Co ophiolite, suggesting that these ultramafic fragments were sourced from the Dong Co ophiolite above the subduction zone rather than off-scrapped remnants from the subducting oceanic lithosphere. Compositional fingerprints of detrital Cr-spinels from the Wuga Formation indicate provenance either derived from the Bangong-Nujiang ophiolites or recycled from the Mugagangri Group in the north, with minor input possibly from the Lhasa terrane in the south, consistent with the depositional pattern of a peripheral foreland basin. Provenance data reveals that the Bangong-Nujiang ophiolites in the Gerze area had been emplaced and exposed to erosion during northward oceanic subduction prior to the Lhasa-Qiangtang collision. Contrasting the Tethyan-type Yarlung-Zangbo ophiolites in southern Tibet, the Bangong-Nujiang ophiolites in central Tibet are Cordilleran-type in terms of emplacement mechanism, which were uplifted above sea-level by progressive growth of the subduction complex structurally beneath ophiolite. The emplacement of the Cordilleran-type ophiolites in the western segment of the BNSZ is divided into two stages: (1) intra-oceanic subduction initiation at ~177–179 Ma based mainly on zircon U-Pb dating of plagiogranite from the SSZ-type Laguo Co ophiolite; (2) accretionary emplacement of the ophiolites at ~151–168 Ma constrained by the depositional age of the Mugagangri subduction complex. Final closure of the Bangong-Nujiang Tethyan Ocean may convert the ophiolite emplacement mechanism from “accretionary” to “collisional” at ~150–152 Ma, evidenced by the first development of a peripheral foreland basin.

High water contents of magmas and extensive fluid exsolution during the formation of the Yulong porphyry Cu-Mo deposit, eastern Tibet

The Yulong porphyry Cu-Mo ore district in eastern Tibet features a series of Eocene felsic porphyry intrusions, only one of which is intensively mineralized. In this study, zircon U-Pb ages, and major and trace element compositions of whole rock samples and in-situ accessory minerals (zircon, apatite and titanite) from these various intrusions were analyzed to determine the factors controlling Cu mineralization. New zircon U-Pb dating, together with published zircon U-Pb ages, suggest that these porphyry intrusions were emplaced over a relatively short period of time (43.9 ± 0.6 Ma to 40.9 ± 0.3 Ma), with the mineralized Yulong intrusion emplaced at a late stage of the magmatic activity (41.1 ± 0.3 Ma to 40.9 ± 0.3 Ma). These intrusions have indistinguishable whole-rock major and trace element compositions, but the mineralized Yulong intrusion has higher apatite and titanite La/Yb ratios (average apatite La/Yb = 115.1 ± 28.5, n = 52; average titanite La/Yb = 22.4 ± 4.9, n = 38) than the earlier subeconomic intrusions (average apatite La/Yb = 50.4 ± 12.3, n = 72; average titanite La/Yb = 12.4 ± 3.5, n = 58), suggesting more fractionation of amphibole from the magmas that sourced it. Additionally, a negative relationship between La/Yb and CeN/CeN* ratios in apatite and titanite are interpreted to reflect increasing oxidation states through magma evolution.Indistinguishable zircon EuN/EuN* values and apatite core SO3 contents from the mineralized (zircon EuN/EuN* = 0.69 ± 0.06, n = 25; apatite SO3 = 0.65 ± 0.24 wt%, n = 21) and subeconomic intrusions (zircon EuN/EuN* = 0.62 ± 0.05, n = 64; apatite SO3 = 0.61 ± 0.27 wt%, n = 15) suggest that the entire magmatic suite was relatively oxidized, hydrous and sulfur-rich, and was therefore fertile for ore-formation. However, apatite crystals from the mineralized Yulong intrusion have significantly lower Cl contents (0.08 ± 0.03 wt%, n = 72) and higher F/Cl ratios (47.80 ± 21.97, n = 72) than those from the subeconomic intrusions (Cl = 0.29 ± 0.29 wt%; F/Cl = 12.87 ± 5.16; n = 78). These data may reflect more extensive fluid exsolution from the parental magma chamber at the time of emplacement of the mineralizing Yulong magmas. Sudden and voluminous release of volatiles coeval with emplacement of the Yulong intrusion was triggered by an as yet unidentified process, but possibly including magmatic recharge.

First 36Cl cosmogenic moraine geochronology of the Dinaric mountain karst: Velež and Crvanj Mountains of Bosnia and Herzegovina

This article presents the first attempt to date moraines in the Dinaric mountain karst using cosmogenic 36Cl surface exposure dating technique. Twenty samples were collected from moraine boulders from two sets of the lowest and largest lateral moraines on the Velež (1965 m asl) and Crvanj mountains (1920 m asl) in Bosnia and Herzegovina. The dated lateral-terminal moraine complexes, spanning elevations from ∼980 to 1350 m asl, are up to 2.7 km long and rise more than 100 m above the valley floor. The moraine boulders yielded 36Cl ages spanning from Oldest Dryas for Velež (14.9 ± 1.1 ka) to Younger Dryas for Crvanj (11.9 ± 0.9 ka), considering the average age of the two oldest samples from each lateral moraine as the most representative time of moraine emplacement. The dated moraines mark the largest extent of glaciers in both study areas, which have been reconstructed to ∼28 km2 for Velež and ∼24 km2 for Crvanj, having a mean equilibrium line altitude at 1388 m and 1541 m, respectively. Under modern precipitation values, which account for ∼2000 mm, the temperature depression between 8 and 10 °C is required to sustain the palaeoglaciers with reconstructed equilibrium line altitudes. Glaciers of similar size with such low equilibrium line altitudes during the Lateglacial have not been reported until now for the Balkan Peninsula. It is very likely that the boulder ages reflect complex exhumation and denudation histories, which at this point do not allow obtaining more precise moraine chronologies for the study areas. Nevertheless, this article delivers new data on the extent and timing of Quaternary glaciations in the Mediterranean mountains, where records of glacier fluctuations seem to be asynchronous amongst different areas. It is clear that dating moraines with cosmogenic 36Cl surface exposure dating in carbonate lithologies in areas of high precipitation like the Dinaric karst, remains challenging.

Characteristics and Deposit Stratigraphy of Submarine-Erupted Silicic Ash, Havre Volcano, Kermadec Arc, New Zealand

Submarine eruptions dominate volcanism on Earth, but few are observed or even identified. Knowledge of how they operate is largely based on inference from ancient deposits, lagging by a decade or more our understanding of subaerial eruptions. In 2012, the largest wholly deep-subaqueous silicic eruption with any observational record occurred 700–1220 m below sea level at Havre volcano, Kermadec Arc, New Zealand. Pre- and post-eruption shipboard bathymetry surveys, acquisition by autonomous underwater vehicle of meter-scale-resolution bathymetry, and sampling by remote-operated vehicle revealed 14 seafloor lavas and three major seafloor clastic deposits. Here we analyze one of these clastic deposits, an Ash with Lapilli (AL) unit, which drapes the Havre caldera, and interpret the fragmentation and dispersal processes that produced it. Seafloor images of the unit reveal multiple subunits, all ash-dominated. Sampling destroyed layering in all but two samples, but by combining seafloor imagery with granulometry and componentry, we were able to determine the subunits’ stratigraphy and spatial extents throughout the study area. Five subunits are distinguished; from the base these are Subunit 1, Subunit 2a, Subunit 3, Subunit 4 (comprising the coeval Subunit 4 west and Subunit 4 east), and Subunit 2b. The stratigraphic relationships of the four AL unit subunits to other seafloor products of the 2012 Havre eruption, coupled with the wealth of remote-operated vehicle observations and detailed AUV bathymetry, allow us to infer the overall order of events through the eruption. Ash formed by explosive fragmentation of a glassy vesicular magma and was dispersed by a buoyant thermal plume and dilute density currents from which Subunits 1 and 2 were deposited. Following a time break (days/weeks?), effusion of lava along the southern caldera rim led to additional ash generation; first by syn-extrusive ash venting, quenching, brecciation, and comminution (S3 and S4e) and then by gravitational collapse of a dome (S4w). Slow deposition of extremely fine ash sustained S2 deposition across the times of S3 and S4 emplacement, so that S2 ash was the last deposited. These thin ash deposits hold information critical for interpretation of the overall eruption, even though they are small in volume and bathymetrically unimpressive. Ash deposits formed during other submarine eruptions are similarly likely to offer new perspectives on associated lavas and coarse pumice beds, both modern and ancient, and on the eruptions that formed them. Submarine ash is widely dispersed prior to deposition, and tuff is likely to be the first product of eruption identified in reconnaissance exploration; it is the start of the trail to vent hydrothermal systems and associated mineralized deposits of submarine volcanoes, as well as a sensitive indicator of submarine eruptive processes.

Tectonic setting and mineralization potential of the Zefreh porphyry Cu-Mo deposit, central Iran: Constraints from petrographic and geochemical data

The Zefreh porphyry Cu-Mo deposit is located in the central part of the Cenozoic Urumieh-Dokhtar Magmatic Arc (UDMA), 75 km NE of Isfahan city, central Iran. The Zefreh rocks, ranging from diorite to granodiorite in composition, formed in a subduction-related arc setting, and likely in a pre-plate collisional environment. They are enriched in Th, U, Rb, Pb, light rare earth elements (LREE), but depleted in Ti, Nb, Ta, Ba, Sr, and P. Negative to slightly positive Eu anomalies (0.5–1.1), low to moderate Sr contents, (189–567 ppm), low to moderate Sr/Y ratios (5–23), and low La/Yb ratios (3–8) in the Zefreh magmatic suite rule out considerable amounts of fractionated amphibole from a hydrous magma in the lower crust. Arc crustal thickness in the Zefreh area was probably <40 km at the time of emplacement of the granitoids, under which amphibole did not fractionate considerably from the primitive magma. Melting of garnet-free lower-crustal amphibolites, together with fractional crystallization and crustal assimilation processes may be the main source of the Zefreh granitoids. On the basis of geochemical characteristics of the granitoids (e.g., low to moderate Sr contents, moderate to high contents of Y and Yb, low to moderate ratios of Sr/Y and Dy/Yb, low La/Yb ratios, negative to slightly positive Eu anomalies, and negative anomalies of Sr) and immature arc with thin crust, the Zefreh deposit is likely to be a small, sub-economic porphyry Cu-Mo deposit.

Deep-crustal metasedimentary rocks support Late Cretaceous “Mojave-BC” translation

Research Article| January 04, 2019 Deep-crustal metasedimentary rocks support Late Cretaceous “Mojave-BC” translation Kirsten B. Sauer; Kirsten B. Sauer 1Department of Geological Sciences, University of Nevada–Reno, Reno, Nevada 89557, USA Search for other works by this author on: GSW Google Scholar Stacia M. Gordon; Stacia M. Gordon 1Department of Geological Sciences, University of Nevada–Reno, Reno, Nevada 89557, USA Search for other works by this author on: GSW Google Scholar Robert B. Miller; Robert B. Miller 2Department of Geology, San Jose State University, San Jose, California 95112, USA Search for other works by this author on: GSW Google Scholar Carl E. Jacobson; Carl E. Jacobson 3Department of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa 50011, USA4Department of Earth and Space Sciences, West Chester University, West Chester, Pennsylvania 19383, USA Search for other works by this author on: GSW Google Scholar Marty Grove; Marty Grove 5Department of Geological Sciences, Stanford University, Stanford, California 94305, USA Search for other works by this author on: GSW Google Scholar Jeffrey D. Vervoort; Jeffrey D. Vervoort 6School of the Environment, Washington State University, Pullman, Washington 99163, USA Search for other works by this author on: GSW Google Scholar Christopher M. Fisher Christopher M. Fisher 6School of the Environment, Washington State University, Pullman, Washington 99163, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Kirsten B. Sauer 1Department of Geological Sciences, University of Nevada–Reno, Reno, Nevada 89557, USA Stacia M. Gordon 1Department of Geological Sciences, University of Nevada–Reno, Reno, Nevada 89557, USA Robert B. Miller 2Department of Geology, San Jose State University, San Jose, California 95112, USA Carl E. Jacobson 3Department of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa 50011, USA4Department of Earth and Space Sciences, West Chester University, West Chester, Pennsylvania 19383, USA Marty Grove 5Department of Geological Sciences, Stanford University, Stanford, California 94305, USA Jeffrey D. Vervoort 6School of the Environment, Washington State University, Pullman, Washington 99163, USA Christopher M. Fisher 6School of the Environment, Washington State University, Pullman, Washington 99163, USA Publisher: Geological Society of America Received: 04 Sep 2018 Revision Received: 11 Nov 2018 Accepted: 27 Nov 2018 First Online: 04 Jan 2019 Online Issn: 1943-2682 Print Issn: 0091-7613 © 2019 Geological Society of America Geology (2019) 47 (2): 99–102. https://doi.org/10.1130/G45554.1 Article history Received: 04 Sep 2018 Revision Received: 11 Nov 2018 Accepted: 27 Nov 2018 First Online: 04 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 Kirsten B. Sauer, Stacia M. Gordon, Robert B. Miller, Carl E. Jacobson, Marty Grove, Jeffrey D. Vervoort, Christopher M. Fisher; Deep-crustal metasedimentary rocks support Late Cretaceous “Mojave-BC” translation. Geology 2019;; 47 (2): 99–102. doi: https://doi.org/10.1130/G45554.1 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 SocietyGeology Search Advanced Search Abstract Metasedimentary rocks in the mid- to lower crust of exhumed continental arcs preserve information about both sedimentary provenance and metamorphic pressure-temperature paths, providing a unique perspective on the tectonic history of an arc, including margin-parallel translation and sediment-burial mechanisms. The Swakane Biotite Gneiss (Washington State, USA) and the Pelona, Orocopia, Rand, Sierra de Salinas, and related schists (PORS) (California and Arizona) represent metasedimentary rocks that were incorporated into the deep levels of North American Cordilleran arc systems. We evaluated the provenances for the sedimentary protoliths of these units by detrital zircon U-Pb and Hf-isotope analyses to assess paleogeographic reconstructions for western North America. All samples have similar Mesozoic peaks, and Proterozoic age populations (1.38 Ga and 1.6–1.8 Ga) are present in samples with maximum depositional ages younger than ca. 86 Ma. Zircon Hf-isotope results are similar for these Proterozoic populations, but the Swakane Mesozoic zircon samples reveal slight differences in comparison to PORS samples. Similar detrital zircon patterns, timing of emplacement, lithology, and structural setting suggest that these two distinct units formed at the same latitude and are consistent with recent interpretations of paleomagnetic data that indicate “moderate” (∼1600 km) northward translation for the North Cascades arc. This study provides an example of how sediment provenance signatures combined with interpreted metamorphic histories can be used to track vertical displacement and horizontal translation within a complex, long-lived, convergent-margin system. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Variation in style of magmatism and emplacement mechanism induced by changes in basin environments and stress fields (Pannonian Basin, Central Europe)

AbstractThe development of high‐resolution 3D seismic cubes has permitted recognition of variable subvolcanic features mostly located in passive continental margins. Our study area is situated in a different tectonic setting, in the extensional Pannonian Basin system (central Europe) where the lithospheric extension was associated with a wide variety of magmatic suites during the Miocene. Our primary objective is to map the buried magmatic bodies, to better understand the temporal and spatial variation in the style of magmatism and emplacement mechanism within the first order Mid‐Hungarian Fault Zone (MHFZ) along which the substantial Miocene displacement took place. The combination of seismic, borehole and log data interpretation enabled us to delineate various previously unknown subvolcanic‐volcanic features. In addition, a new approach of neural network analysis on log data was applied to detect and quantitatively characterise hydrothermal mounds that are hard to interpret solely from seismic data. The volcanic activity started in the Middle Miocene and induced the development of extrusive volcanic mounds south of the NE‐SW trending, continuous strike‐slip fault zone (Hajdú Fault Zone). In the earliest Late Miocene (11.6–9.78 Ma), the style of magmatic activity changed resulting in emplacement of intrusions and development of hydrothermal mounds. Sill emplacement occurred from south‐east to north‐west based on primary flow‐emplacement structures. The time of sill emplacement and the development of hydrothermal mounds can be bracketed by onlapped forced folds and mounds. This time coincided with the acceleration of sedimentation producing poorly consolidated, water‐saturated sediments preventing magma from flowing to the paleosurface. The change in extensional direction resulted in change in fault pattern, thus the formerly continuous basin‐bounding strike‐slip fault became segmented which could facilitate the magma flow toward the basin centre.

Petrogenesis of the late Early Palaeozoic adakitic granitoids in the southern margin of the Songliao Basin, NE China: Implications for the subduction of the Palaeo‐Asian Ocean

Many data have been reported from the Early Palaeozoic subduction complex belt and accretionary arc to the south of the Solonker‐Xar Moron‐Changchun‐Yanji Suture (SXCYS) zone. However, as the suture is covered to the east by the Mesozoic–Cenozoic Songliao Basin, there have yet been no related reports about the Early Palaeozoic igneous rocks in this area. The authors discovered an Early Palaeozoic granitoid intrusion in the Fangjiatun area, northern Liaoning, and performed petrologic, geochemical, zircon U–Pb dating and Hf isotopic analyses on it. Zircon SHRIMP U–Pb dating results show that the intrusion formed at 437–432 Ma, that is, in the late stage of the Early Silurian. Petrogeochemical analysis indicates that the intrusion records 63.14%–66.45% SiO2, with Na2O/K2O ratios of 1.01–6.12 and magnesian index (Mg#) values of 32.42–40.82. The REEs show a certain degree of LREE/HREE fractionation and slightly lower ΣREE contents. The trace elements are relatively enriched in Rb, Ba, Th, K, and Sr (large‐ion lithophile elements [LILEs]) and depleted in Nb, Ta, P, and Ti (high‐field‐strength elements [HFSEs]), displaying continental magmatic arc characteristics. The rocks have high Sr mass fractions, low Y and Yb mass fractions, and high Sr/Y ratios, which are consistent with the geochemical signatures of typical adakites. Zircon Hf isotopic analyses indicate that their εHf(t) values range from 0.29–6.56, reflecting the relatively depleted signatures of the source area. Based on its occurrence location and emplacement time, the intrusion is interpreted to have been produced by tectonic thermal events during the subduction of the Palaeo‐Asian oceanic crust beneath the northern margin of the North China Craton (NCC), thus reflecting the existence of an Early Palaeozoic continental marginal accretionary arc in the eastern segment of the NCC along the south of the SXCYS and provide new evidence for the tectonic evolution of the northern margin of the NCC in the Early Palaeozoic.

Zircon U–Pb ages and Lu–Hf isotopes of metagranitoids from the Subansiri region, Eastern Himalaya: implications for crustal evolution along the northern Indian passive margin in the early Paleozoic

Abstract We studied the zircon U–Pb ages, Hf isotopes, and whole-rock and mineral chemistry of metagranitoids from the Subansiri region of the Eastern Himalaya to constrain their emplacement age, origin and geodynamic evolution. The investigated metagranitoids have high SiO 2 , Na 2 O + K 2 O, Rb, Zr and low Fe 2 O 3 , Nb, Ga/Al ratios with fractionated rare earth element patterns [(Ce/Yb) N = 6.46–42.15] and strong negative Eu anomalies (Eu/Eu* = 0.16–0.44). They are peraluminous (molar A/CNK = 1.04–1.27) and calc-alkaline in nature, with normative corundum (1.04–3.61) and relatively high FeO t /MgO ratios in biotite ( c. 3.38), indicating their affinity with S-type granites. The time of emplacement of the Subansiri metagranitoids is constrained by zircon U–Pb ages between 516 and 486 Ma. The zircon grains have negative ε Hf ( t ) values ranging from −1.4 to −12.7 and yield crustal Hf model ages from 1.5 to 2.2 Ga, suggesting the occurrence of a major crustal growth event in the Proterozoic and re-melting of the crust during the early Paleozoic. The geochemical data in conjunction with the U–Pb ages and Hf isotope data suggest that the Subansiri metagranitoids were produced by partial melting of older metasedimentary rocks in the Indian passive margin. Supplementary material: Hf isotope results for the Mud Tank zircon standard acquired during the experimental period are available at https://doi.org/10.6084/m9.figshare.c.4299830

Late Carboniferous–Early Permian high- and low-Sr/Y granitoids of the Xing'an Block, northeastern China: Implications for the late Paleozoic tectonic evolution of the eastern Central Asian Orogenic Belt

This study presents new zircon U-Pb geochronological, whole-rock geochemical, and zircon Hf isotopic data along with regional geological observations for five late Paleozoic granitic plutons within the Xing'an Block of northeastern China. These data provide insights into the petrogenesis of these granitoids and the late Paleozoic tectonic evolution of the region. The geochronological and geochemical data indicate that the intrusions were emplaced during two separate periods, with the resulting granites having distinct geochemical features. The early Late Carboniferous (321–310 Ma) granitic Lizishan (LZS), Xiaonangou (XNG), and Dabeigou (DBG) plutons have high Sr/Y values (33.15–126.73), with low concentrations of Y and heavy rare earth elements (HREE), indicating adakitic affinities. However, the XNG granodiorites contain higher concentrations of MgO, Cr, and Ni, and have higher Mg# values (57–59) than the other plutons. The latest Carboniferous–Early Permian (301–296 Ma) granitic Weilegesi (WLGS) and Wuyi (WY) plutons are high-K calc-alkaline, with low Sr/Y ratios (0.18–24.15), and show petrological and geochemical characteristics of I-type granites. In addition, the magmatic zircon crystals within these plutons yield positive εHf(t) values (+4.1 to +12.9) and juvenile two-stage model (TDM2) ages (1056–499 Ma). The timing of emplacement and geochemical and isotopic compositions of the LZS and DBG granitoids are indicative of derivation from magmas generated by the partial melting of thickened lower crustal material, whereas the XNG granitoids were most likely formed from magmas generated by the partial melting of delaminated lower crustal material that subsequently interacted with mantle material during ascent. The latest Carboniferous–Early Permian low-Sr/Y granitoids may have formed from magmas generated by the partial melting of a dominantly juvenile amphibolite-facies crustal source under relatively low-pressure conditions, with these magmas subsequently undergoing variable degrees of fractional crystallization. Combining these new data with the results of previous research indicates that the terminal collision between the Xing'an and Songliao blocks occurred between the late Early and early Late Carboniferous, and the voluminous Late Carboniferous to Early Permian granitoids in the Xing'an Block are most likely related to post-collisional delamination.

Time-space distribution of silicic plutonism in a gneiss dome of the Iberian Variscan Belt: The Évora Massif (Ossa-Morena Zone, Portugal)

In the Iberian Variscan Belt, polyphasic deformation has been recognized as comprising an early phase of crustal thickening, followed by an intermediate phase of crustal extension and doming, and a later phase of shortening. The Évora Massif is a gneiss dome of the westernmost domains of the Ossa-Morena Zone (SW Iberia), which provides a remarkable insight into the late Paleozoic deep crustal structure of the Variscan continental crust of northern Gondwana. In this study, we bring new structural and geochronological U-Pb data for the northern hanging-wall of the Évora Massif. We describe the existence of low-dipping D2 extensional shear zones associated with Buchan-type metamorphism (M2); this enables three tectono-metamorphic units to be distinguished: the Lower Gneiss Unit, the Intermediate Schist Unit, and the Upper Slate Unit. D2-M2 structures experienced subhorizontal shortening (D3) and were transposed by low-plunging folding, thrusting and strike-slip faulting. Zircon grains extracted from Pavia quartz-feldspathic gneiss of the Lower Gneiss Unit yielded a crystallization age of ca. 521 Ma (Cambrian Stage 2–3), which establishes a correlation with tectono-metamorphic units of the footwall and southern hanging-wall of the Évora Massif. U-Pb zircon dating of Divôr foliated quartz-diorite (339 ± 7 Ma), Malarranha weakly foliated biotite-rich granite (322 ± 3 Ma), and undeformed porphyritic granite of the Pavia pluton (314 ± 4 Ma) constrain the timing of emplacement of granitic magmas synchronously with doming. Carboniferous magmatism initiated with doming (ME1 - ca. 343–335 Ma), continued through D2-M2 (ME2 - ca. 328–319 Ma), and lasted until the waning stage of crustal extension (ME3 - ca. 317–313 Ma). The Évora Massif gneiss dome probably formed as result of the combined effect of gravitational collapse of the thickened crust and buoyancy-driven gravitational instability developed in the partially molten continental crust influenced by the transfer of heat from rising mantle-derived (i.e. dioritic-gabbroic) magmas rocks found in the footwall of the Évora Massif.

A-type Medina batholith and post-collisional anatexis in the Araçuaí orogen (SE Brazil)

The Medina batholith and its host granitic migmatites record intriguing plutonic processes in the northern Araçuaí orogen (SE Brazil). This orogen shows a long lasting (630–480 Ma) succession of granite production events from the earliest pre-collisional plutons to the latest post-collisional intrusions. The Medina batholith includes granite intrusions ascribed to the post-collisional stage. They show high alkali and halogen contents, low CaO (at SiO2 = 71%: Na2O + K2O = 7 to 9%; CaO = 1.6%), and high FeOt/(FeOt + MgO) ratios (0.78 to 0.92). The Medina granites are metaluminous to weakly peraluminous, with ASI (molecular ratio Al/(Ca-1.67P + Na_K)) values of 1.76 to 2.07, and have high concentrations of high field strength elements (Zr + Nb + Ce + Y > 700 ppm), as well as high Ga/Al ratios. Accordingly, the Medina intrusions are typical ferroan A-type granites. UPb ages from zircon (501 ± 2 Ma) and monazite (497 ± 2 Ma) constrain the emplacement timing of the Medina batholith. Surprisingly, all monazite ages from host rocks also cluster around 500 Ma, despite their nature and distance from the batholith, suggesting that they would have shared a same thermal process. The studied host rocks are granitic migmatites varying from patch metatexite to nebulitic diatexite, comprising paleosome of foliated sillimanite-garnet-biotite metagranite to gneiss, and non-foliated garnet-cordierite neosome poor to free of biotite. A metatexite (R14) located relatively far from the Medina batholith, and a diatexite (M26) found at the batholith contact were sampled for detailed studies. The paleosome of foliated metagranite (R14A) only shows zircon grains with igneous features and Th/U ratio from 1.64 to 0.26. Although the spreading of zircon spots, the main cluster yields a Concordia age at 556 ± 6 Ma, constraining the protolith magmatic crystallization. A minor cluster furnishes a Concordia age at 499 ± 7 Ma, in agreement with the UPb monazite age at 501 ± 2 Ma. Extracted from the same metatexite sample, the non-foliated garnet-cordierite neosome (R14B) shows both igneous and metamorphic zircon domains with Th/U ratios ranging from 1.47 to 0.00. Again, the UPb spots cluster at two distinct Concordia ages (562 ± 3 Ma and 499 ± 3 Ma). The youngest of them, fitting with the monazite age (495 ± 3 Ma), constrains melt crystallization, while the oldest age suggests paleosome inheritance. The nebulitic diatexite (M26) shows monazite (497 ± 2 Ma) and zircon (Th/U = 1.7 to 0.0; Concordia ages at 564 ± 2 Ma and 507 ± 3 Ma) populations similar to the metatexite neosome, also with the youngest ages bracketing the melt crystallization process around 500 Ma. Accordingly, all those ages at around 500 Ma disclose a partial melting episode coeval with the Medina batholith emplacement. Phase equilibrium modeling on a garnet-cordierite neosome furnished P-T conditions of 750–840 °C at 2.4–3.5 kbar for that post-collisional anatexis. Evidence for such a late thermal event are common in the Araçuaí orogen, even far from the post-collisional batholiths. Thus, a possible major heat source can be envisaged, like a mantle plume triggering crustal anatexis and regional fluid circulation during the gravitational collapse of the Araçuaí orogen.

Anomalous concentrations of radionuclides in the groundwater of Ede area, southwestern Nigeria: a direct impact of geology

An assessment of the groundwater in areas underlain by pegmatite in Ede, southwestern Nigeria was carried out to determine the concentration of Potassium-40 (40K), Uranium-238 (238U) and Thorium-232 (232Th) radionuclides. In the earlier work, it was established that zircons in these pegmatites have suffered a high degree of metamictization that has enhanced continuous loss of some radionuclides since the time of emplacement of the pegmatite host rock to the present. The aim of this work is to determine whether or not there is corresponding increase in the concentration of radionuclides in groundwater in the study area. Fifteen groundwater samples were collected from both hand dug wells and boreholes in the area. Ten samples were from Ede town, two samples each from nearby communities of Iddo and Ekuro and one from Iwoye, where the bedrocks were not pegmatites. All the 15 samples were analyzed with the Sodium Iodide scintillator (NaI [Tl]) detector at the Centre for Energy Research and Development, Obafemi Awolowo University, Ile-Ife, Nigeria. The results showed varying concentrations of the radionuclides in the water samples. Activity concentrations of Potassium-40 (40K) indicated an average value of 17.149 Bq/L for samples from Ede, 9.265 Bq/L for Iddo, 6.6 Bq/L for Ekuro, while Iwoye has a value of 21.21 Bq/L. The Uranium-238 (238U) series had an average value of 13.64 Bq/L for Ede, 13.49 Bq/L for Ekuro, 11.685 Bq/L for Iddo and 12.04 Bq/L for the lone sample from Iwoye. Thorium-232 (232Th) series had an average value of 11.182 Bq/L for Ede, while average values of 7.79 Bq/L and 9.025 Bq/L, respectively, were recorded for Iddo and Ekuro and 12.25 Bq/L for Iwoye. The annual effective dose level of 40K is generally below the World Health Organization (WHO) recommended dosage of 0.1 mSv/y except for three locations, while those of 238U and 232Th are in excess of the standard values. The high radionuclides in the groundwater in the study area were not due to anthropogenic sources but directly due to geological processes which release the radioisotopes from rocks after weathering, continuous loss from metamict minerals and natural lateral mobility from regions of radionuclides’ release to other areas. There is thus the need to carry out systematic studies of the radionuclides concentrations on regional scale in the area which could lead to investigating on both short- and long-term health effects on organic species in these areas, and development of purification systems before utilizing water from the areas for domestic and industrial purposes.

Evidence for oxidation at the base of the nakhlite pile by reduction of sulfate salts at the time of lava emplacement

The assimilation of sulfate by Martian melts could explain the highly oxidized state of some Martian nakhlite meteorites, such as those paired with MIL 03346 (MIL 090030, MIL 090032, and MIL 090136). Here, a combination of new sulfur isotope data, mineral composition and abundance data, and consideration of mineral textures is used to link assimilation of surface-derived Martian sulfate to oxidation of nakhlite melts. The magnitudes of the mass independent sulfur isotope signatures (negative Δ33S) and the abundance of sulfide minerals accounts for much of the added oxygen implied by the occurrence of abundant skeletal titanomagnetite in the MIL pairs. Assimilation and reduction of sulfate amounts equivalent to that from 10′s of centimeters of Martian sediment are required to account for an oxidation front extending ∼1 m into the flow, a position previously proposed for MIL 03346, and implies a position at the bottom, rather than the top of a nakhlite flow. A similar positional and amount constraint is required for an alternative path for assimilation of sulfate from sulfate-rich brine. Assimilation and reduction of sulfate is therefore inferred to play a critical role in establishing both the enrichment in skeletal titanomagnetite within the lower portion of the nakhlite pile and the large Δ33S anomalies of the MIL pairs. Other nakhlites with smaller Δ33S anomalies and less titanomagnetite would occupy positions farther away from the source of sulfate.

The Agadir Slide offshore NW Africa: Morphology, emplacement dynamics, and potential contribution to the Moroccan Turbidite System

A newly identified large-scale submarine landslide on the NW African margin (Agadir Slide) is investigated in terms of its morphology, internal architecture, timing, and emplacement processes using high-resolution multibeam bathymetry data, 2D seismic profiles, and gravity cores. The Agadir Slide is located south of the Agadir Canyon at a water depth ranging from 500 m to 3,500 m, showing an estimated affected area of approximately 5,500 km2. The analysis of the Agadir Slide's complex morphology reveals the presence of two headwall areas and two slide fairways (the Western and Central slide fairways). Volume calculations indicate that ∼340 km3 of sediment were accumulated downslope along the slide fairways (∼270 km3) and inside the Agadir Canyon (∼70 km3). Stratigraphic correlations based on five gravity cores indicate an emplacement age of 142±1 ka for the Agadir Slide. However, its emplacement dynamics suggest that the slide was developed in two distinct, successive stages. The presence of two weak layers (glide planes) is a major preconditioning factor for the occurrence of slope instability in the study area, and local seismicity related to fault activity and halokinesis likely triggered the Agadir Slide. Importantly, the Agadir Slide neither disintegrated into sediment blocks nor was transformed into turbidity currents. The emplacement timing of the Agadir Slide does not correlate with any turbidites cored downslope across the Moroccan Turbidite System.

Geochemistry and origin of the Mirny field kimberlites, Siberia

Here we present new data from a systematic Sr, Nd, O, C isotope and geochemical study of kimberlites of Devonian age Mirny field that are located in the southernmost part of the Siberian diamondiferous province. Major and trace element compositions of the Mirny field kimberlites show a significant compositional variability both between pipes and within one diatreme. They are enriched in incompatible trace elements with La/Yb ratios in the range of (65–300). Initial Nd isotope ratios calculated back to the time of the Mirny field kimberlite emplacement (t = 360 ma) are depleted relative to the chondritic uniform reservoir (CHUR) model being 4 up to 6 ɛNd(t) units, suggesting an asthenospheric source for incompatible elements in kimberlites. Initial Sr isotope ratios are significantly variable, being in the range 0.70387–0.70845, indicating a complex source history and a strong influence of post-magmatic alteration. Four samples have almost identical initial Nd and Sr isotope compositions that are similar to the prevalent mantle (PREMA) reservoir. We propose that the source of the proto-kimberlite melt of the Mirny field kimberlites is the same as that for the majority of ocean island basalts (OIB). The source of the Mirny field kimberlites must possess three main features: It should be enriched with incompatible elements, be depleted in the major elements (Si, Al, Fe and Ti) and heavy rare earth elements (REE) and it should retain the asthenospheric Nd isotope composition. A two-stage model of kimberlite melt formation can fulfil those requirements. The intrusion of small bodies of this proto-kimberlite melt into lithospheric mantle forms a veined heterogeneously enriched source through fractional crystallization and metasomatism of adjacent peridotites. Re-melting of this source shortly after it was metasomatically enriched produced the kimberlite melt. The chemistry, mineralogy and diamond grade of each particular kimberlite are strongly dependent on the character of the heterogeneous source part from which they melted and ascended.

Middle Permian–early Triassic magmatism in the Western Pontides, NW Turkey: Geodynamic significance for the evolution of the Paleo-Tethys

Permian-Triassic magmatic rocks are widely exposed in different tectonic terranes and continental blocks in the Aegean region. This study is focussed on the İstanbul Zone and the Istranca (Strandja) Massif, which are located in the Western Pontides of NW Turkey. The Kırklareli and the Tepecik plutons intruded into the metamorphic basement rocks of the Istranca Massif, and the Sancaktepe pluton is emplaced into the İstanbul Paleozoic sedimentary sequences. We present new geochemical data for these plutons, in order to interpret their petrogenesis and time of emplacement and use this information for geodynamic interpretation. The zircon U–Pb crystallization ages are 268.3 ± 2.1 Ma for the Kırklareli pluton, 249.4 ± 1.5 Ma for the Tepecik pluton and 257.3 ± 1.5 Ma and 253.7 ± 1.75 Ma for the the Sancaktepe pluton. They are high-K, calc-alkaline to shoshonitic granitoids with aluminum saturation index (ASI) values between 0.9 and 1.3, which define them as peraluminous and slightly metaluminous. 87Sr/86Sr(i) values for the studied plutons range between 0.701123 and 0.707704. 143Nd/144Nd(i) ratios calculated for crystallization ages are between 0.512052 and 0.512431, and εNd(i) values vary from −4.71 to 2.22. TDM model ages range between 0.83 and 1.41 Ga. These plutons are enriched in LILEs and LREE and depleted in HFSEs with negative Eu anomalies, indicating that the melts were derived from mafic crustal sources. Our new data suggest that the Kırklareli pluton was generated in a subduction-related magmatic arc, and the highly fractionated Sancaktepe and Tepecik plutons were formed in a back-arc setting in response to the northward subduction of the Paleo-Tethys Ocean during the Mid Permian–Early Triassic.

Provenance history of detrital diamond deposits, West Coast of Namaqualand, South Africa

The West Coast of Namaqualand in South Africa hosts extensive detrital diamond deposits, but considerable debate exists as to the provenance of these diamonds. Some researchers have suggested derivation of the diamonds from Cretaceous-Jurassic kimberlites (also termed Group I kimberlites) and orangeites (also termed Group II kimberlites) located on the Kaapvaal Craton. However, others favour erosion of diamonds from the ca.300 Ma Dwyka Group sediments, with older, pre-Karoo kimberlites being the original source(s). Previous work has demonstrated that 40Ar/39Ar analyses of clinopyroxene inclusions, extracted from diamonds, yield ages approaching the time(s) of source kimberlite emplacement, which can be used to constrain the provenance of placer diamond deposits. In the current study, 40Ar/39Ar analyses were conducted on clinopyroxene inclusions from two similar batches of Namaqualand detrital diamonds, yielding (maximum) ages ranging from 117.5 ± 43.6 Ma to 3684 ± 191 Ma (2σ) and 120.6 ± 15.4 Ma to 688.8 ± 4.9 Ma (2σ), respectively. The vast majority of inclusions (88%) produced ages younger than 500 Ma, indicating that most Namaqualand diamonds originated from Cretaceous-Jurassic kimberlites/orangeites, with few, if any, derived from the Dwyka tillites. The provenance of the Namaqualand diamonds from ca.115–200 Ma orangeites is consistent with Late Cretaceous paleo-drainage reconstructions, as these localities could have been sampled by the ‘paleo-Karoo’ River and transported to the West Coast via an outlet close to the current Olifants River mouth. At ca.90 Ma, this drainage system appears to have been captured by the ‘paleo-Kalahari’ River, a precursor to the modern Orange River system. This latter drainage is considered to have transported diamonds eroded from both ca.80–90 Ma kimberlites and ca.115–200 Ma orangeites to the West Coast, which were subsequently reworked along the Namibian coast, forming additional placer deposits.