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Bulletin of the Geological Society of America
Journal Prestige (SJR): 2.329  Citation Impact (citeScore): 4 Number of Followers: 34  Hybrid journal (It can contain Open Access articles)ISSN (Print) 0016-7606 - ISSN (Online) 1943-2674 Published by Geological Society of America  [4 journals]
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- Cambrian–Ordovician magmatic flare-up in NE Gondwana: A silicic
large igneous province'-
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Abstract: The origin of the Cambrian–Ordovician tectono-magmatic events affecting NE Gondwana and the adjacent peri-Gondwanan terranes (e.g., Himalaya, Lhasa, Southern Qiangtang, Baoshan, Tengchong, Sibumasu, Helmand, and Karakorum) is controversial. Although its volume is poorly constrained, we propose that an extensive belt of granitic rocks that formed in various pulses between ca. 510 Ma and 460 Ma may represent the remains of a potential silicic large igneous province (LIP), which is referred to here as the Pinghe silicic LIP, with an areal extent of ∼2.5 Mkm2. The putative Pinghe silicic LIP is composed predominantly of S-type granites with subordinate A-type granites and minor intraplate mafic rocks. The recognition of this belt of granitic rocks aids in the refinement of tectonic reconstructions of Gondwana and of models for the rifting of terranes from its northern margin. The Pinghe silicic LIP broadly coincides with the adjacent 511 Ma Kalkarindji LIP in northern Australia, and the plume or mantle upwelling that triggered the Kalkarindji LIP may have been responsible for driving crustal melting that generated the granitic rocks, in a manner analogous to the Karoo–Chon Aike association.
PubDate: Tue, 25 Oct 2022 00:00:00 GMT
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- Cretaceous–Cenozoic cooling history of the Qiangtang terrane and
implications for Central Tibet formation-
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Abstract: Reconstructing the growth process of the Qiangtang terrane in space and time is crucial for understanding the geological evolution of Central Tibet. However, its growth process and dynamic mechanism remain unclear. Here, we present new fission track data obtained along a N-S transect near the Puruo Kangri Mountain in the central zone of the Northern Qiangtang terrane. The completely reset apatite fission track ages of detrital samples range from 65.1 to 89.6 Ma, which show a northward younging trend. The thermal history modeling results indicate that this region underwent northward-propagating exhumation during the Late Cretaceous (ca. 92–65 Ma). Our data, combined with previously reported low-temperature thermochronology data for the Qiangtang terrane, suggest that the Qiangtang terrane experienced three main stages of cooling: ca. 120–65, ca. 55–35, and <25–0 Ma. The first stage (ca. 120–65 Ma) displays an outward-propagating cooling pattern from the Central Qiangtang terrane, which was related to the crustal shortening and thickening driven by the Lhasa-Qiangtang collision. The low exhumation rate, flat lavas, and paleoaltimetry studies imply that the central and southern zones of the Northern Qiangtang terrane and Central Qiangtang terrane may form plateau landscape by 65–55 Ma. The second stage (ca. 55–35 Ma) of cooling is mainly documented in the Southern Qiangtang terrane, and the northern zone of the Northern Qiangtang terrane. This cooling event was caused by the crustal deformation and shortening driven by intracontinental subduction related to ongoing convergence of the Indian and Asian plates. Subsequently, both the transition to low erosion rates (<0.05 mm/yr) and paleoaltimetry data indicate that the Qiangtang terrane became a primitive plateau by ca. 35 Ma. The final stage (<25–0 Ma) of cooling was linked to the E-W extension in the Qiangtang terrane.
PubDate: Fri, 21 Oct 2022 00:00:00 GMT
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- Late Cenozoic deepening of Yosemite Valley, USA
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Abstract: Although Yosemite Valley, USA, catalyzed the modern environmental movement and fueled foundational debates in geomorphology, a century of investigation has failed to definitively determine when it formed. The non-depositional nature of the landscape and homogeneous bedrock have prevented direct geological assessments. Indirect assumptions about the age of downcutting have ranged from pre-Eocene to Pleistocene. Clarity on this issue would not only satisfy public interest but also provide a new constraint for contentious debates about the Cenozoic tectonic and geomorphologic history of the Sierra Nevada in California. Here we use thermochronometric analysis of radiogenic helium in apatite crystals, coupled with numerical models of crustal temperatures beneath evolving topography, to demonstrate significant late Cenozoic deepening of Tenaya Canyon, Yosemite’s northeastern branch. Approximately 40%–90% of the current relief has developed since 10 Ma and most likely since 5 Ma. This coincides with renewed regional tectonism, which is a long-hypothesized but much debated driver of Sierran canyon development. Pleistocene glaciation caused spatially variable incision and valley widening in Yosemite Valley, whereas little contemporaneous erosion occurred in the adjacent upper Tuolumne watershed. Such variations probably arise from glacial erosion’s dependence on topographic focusing of ice discharge into zones of rapid flow, and on the abundance of pre-existing fractures in the substrate. All available data, including those from our study, are consistent with a moderately high and slowly eroding mid-Cenozoic Sierra Nevada followed by significant late Cenozoic incision of some, but not all, west-side canyons. A likely driver of this event was range-crest uplift accompanied by fault-induced beheading of some major drainages, although other mechanisms such as drainage reorganization following volcanic deposition are plausible.
PubDate: Wed, 19 Oct 2022 00:00:00 GMT
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- Geological architecture and history of the Antigua volcano and carbonate
platform: Was there an Oligo–Miocene lull in Lesser Antilles arc
magmatism'-
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Abstract: Since the acceptance of plate tectonics, the presence of calc-alkaline magmatic rocks has been recognized as evidence of subduction. But under specific geodynamic circumstances, subduction may occur without generating magmas. Here, we investigate the Cenozoic northern Lesser Antilles arc where, from sparsely exposed magmatic records, Eocene–Oligocene and Pliocene magmatic flare-ups and a Miocene lull were postulated. Nevertheless, most of the arc is submarine, so it is challenging to discern lulls and flare-ups from sampling bias. We review the magmatic evidence exposed onshore in the Lesser Antilles and investigate in detail the island of Antigua, which exposes an Eocene to Miocene volcanic sequence and platform carbonate series that coincide with the postulated lull. By combining lithostratigraphic analysis, structural mapping, 40Ar/39Ar geochronology, and biostratigraphy, we refine the magmatic history of the island and date the arrest of extensive arc magmatism at 35 Ma, with minor activity until 27 Ma. No magmatic products are interleaved with the platform sequence until the latest Oligocene, which confirms a lull in northern Lesser Antilles arc magmatism that may have lasted ca. 20 Ma. Flare-up of magmatic activity contributed to crustal thickening and land emersion, whereas magmatic lulls and related thermal cooling induced subsidence/submersion. Thus, we propose that the paleo-(bio)-geographical evolution of the eastern Caribbean region has been partly controlled by magmatic activity.Fault kinematic analysis, along with anisotropy of magnetic susceptibility, suggest that, at the island scale, magmatic arrest is not associated with a change in stress field during the Oligocene. We speculate that slab flattening triggered by progressive curvature played a role in the temporal shutdown of the northern Lesser Antilles arc.
PubDate: Wed, 19 Oct 2022 00:00:00 GMT
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- East Antarctic Ice Sheet variability during the middle Miocene Climate
Transition captured in drill cores from the Friis Hills, Transantarctic
Mountains-
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Abstract: This study describes 16 well-dated, terrestrial glacial sedimentary cycles deposited during astronomically paced climate cycles from the termination of the Miocene Climatic Optimum (MCO) through the middle Miocene Climate Transition (MMCT) (15.1–13.8 Ma) in the Friis Hills, Transantarctic Mountains, Antarctica. Three locations were continuously cored (79% recovery) to a maximum depth of 50.48 m through a succession of interbedded till sheets and fossil-bearing, fluvio-lacustrine sediments. A composite chronostratigraphic framework is presented for the cores based on the previous mapping, a seismic refraction survey that defines basin geometry, and a new, integrated age model based on paleomagnetic stratigraphy that is constrained by radioisotopic 40Ar/39Ar numeric ages on two newly identified silicic tephra.The paleoecologic and sedimentologic characteristics of organic-rich lithologies are relatively consistent up-section, which implies that successively younger interglacial deposits during the MMCT represented broadly similar environmental and climatic conditions. During these interglacials, the Friis Hills hinterland was likely ice-free. Major disconformities in the section suggest a transition to colder climates, and after ca. 14.6 Ma, thicker, more extensive and erosive ice cover occurred across the Friis Hills during glacial episodes. Diamictites in the upper three cycles suggest that climate cooled and became drier after ca. 14.2 Ma. However, cyclical retreat of the ice and a return to warm climate conditions during interglacials continued through ca. 13.9 Ma. These direct records reflect a highly variable East Antarctic Ice Sheet margin but show that the ice margin became progressively more extensive during successive glacial intervals, which is consistent with a cooling trend toward more glacial values in the far-field benthic foraminifera δ18O proxy ice volume and temperature record.Age constraints show that glacial-interglacial variability at the terrestrial margin of the East Antarctic Ice Sheet was primarily paced by astronomical precession (∼23 k.y.) through the onset of the MMCT (15–14.7 Ma). Precession-driven cycles are modulated by short-period (∼100 k.y.) eccentricity cycles. Intervals of maximum eccentricity (high seasonality) coincide with sedimentary cycles comprising thin diamictites and relatively thick interglacial sandstone and mudstone units. Intervals of minimum eccentricity (low seasonality) coincide with sedimentary cycles comprising thick diamictites and relatively thin interglacial sedimentary deposits. Major disconformities in the Friis Hills succession that span more than ∼100 k.y. reflect episodes of expansion of erosive ice across, and well beyond, the Transantarctic Mountains and coincide with nodes in eccentricity (∼400 k.y.). These relationships suggest that during relatively warm intervals in the middle Miocene, the East Antarctic Ice Sheet expanded and contracted over 100 k.y. cycles, while its margins continued to fluctuate at higher (∼23 k.y.) frequency. After 14.5 Ma, obliquity is the dominant frequency in δ18O records, marking a period during which large regions of the Antarctic Ice Sheet grounded in marine environments.
PubDate: Wed, 19 Oct 2022 00:00:00 GMT
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- New insights into deep carbon recycling and formation of nepheline-bearing
alkaline rocks from Sr-Nd-Mg isotope compositions-
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Abstract: Magnesium isotopes are widely used to trace recycled carbonates in the mantle source. We recently recognized extremely light Mg isotope values (δ26Mg = −0.50 to −0.62‰) in nepheline syenites in the Tarim large igneous province (TLIP), NW China. To evaluate the significance of the light Mg isotopes, we conducted petrological, mineral chemical, zircon U-Pb ages, and geochemical and isotopes (Sr-Nd-Mg) analyses on the nepheline syenite to understand its petrogenesis. Laser ablation–inductively coupled plasma–mass spectrometry zircon U-Pb dating yields an age of 272.5 ± 1.4 Ma for the nepheline syenite. Petrographic and geochemical studies show that the nepheline syenite and nephelinite in the TLIP display similar mineral assemblages, clinopyroxene Sr isotope compositions and bulk-rock Sr and Nd isotope compositions (87Sr/86Sr(i) = 0.70364–0.70399, εNd(t) = +3.51 to +4.49 versus 87Sr/86Sr(i) = 0.70348–0.70371, εNd(t) = +3.28 to +3.88 for nepheline syenite and nephelinite, respectively), indicating they are possibly co-magmatic. Rhyolite-MELTS modeling shows that the nepheline syenite formed from nephelinite by fractional crystallization of spinel, olivine, clinopyroxene, apatite, and biotite. In combination with information from previous studies, we correlated the extremely light magnesium isotopes of nepheline syenite to “genetic genes” of nephelinite (δ26Mg = −0.35 to −0.55‰) which were produced by the reaction between peridotite and carbonated silicate melt derived from the carbonated eclogite. We invoke a three-stage model for the genesis of the nepheline syenite in the TLIP. Initially, the subduction of oceanic crust delivered the sedimentary carbonate rocks into the deep mantle and formed carbonated eclogite. The carbonated silicate melt derived by the melting of the carbonated eclogite reacted with ambient peridotite to form primary nephelinitic magma. Finally, fractional crystallization of nephelinitic melt during ascent produced the nepheline syenite. Our study provides insights into the implication of light magnesium isotopes for deep carbon recycling in the origin of alkaline rocks.
PubDate: Wed, 19 Oct 2022 00:00:00 GMT
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- Guide for interpreting and reporting luminescence dating results
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Abstract: The development and application of luminescence dating and dosimetry techniques have grown exponentially in the last several decades. Luminescence methods provide age control for a broad range of geological and archaeological contexts and can characterize mineral and glass properties linked to geologic origin, Earth-surface processes, and past exposure to light, heat, and ionizing radiation. The applicable age range for luminescence methods spans the last 500,000 years or more, which covers the period of modern human evolution, and provides context for rates and magnitudes of geological processes, hazards, and climate change. Given the growth in applications and publications of luminescence data, there is a need for unified, community-driven guidance regarding the publication and interpretation of luminescence results.This paper presents a guide to the essential information necessary for publishing and archiving luminescence ages as well as supporting data that is transportable and expandable for different research objectives and publication outlets. We outline the information needed for the interpretation of luminescence data sets, including data associated with equivalent dose, dose rate, age models, and stratigraphic context. A brief review of the fundamentals of luminescence techniques and applications, including guidance on sample collection and insight into laboratory processing and analysis steps, is presented to provide context for publishing and data archiving.
PubDate: Thu, 29 Sep 2022 00:00:00 GMT
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- Mid–late Neoproterozoic tectonic evolution of the northern margin of the
Yangtze Block, South China, as constrained from the detrital zircon record
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Abstract: The northern margin of the Yangtze Block (NYB) experienced distinct tectonomagmatic evolution during the mid- and late-Neoproterozoic, as compared with the northwestern margins of the Yangtze Block (NWYB), although the geodynamic mechanism remains controversial. The South Qinling Belt (SQB), which forms the northern-most NYB, is a significant target for studying the Neoproterozoic tectonic evolution of the NYB. Here we present U-Pb geochronology and Lu-Hf isotopes of detrital zircons from clastic rocks belonging to the Yaolinghe and Wudangshan Groups in the SQB. The U-Pb ages reveal that the maximum depositional ages are respectively ca. 630 Ma and ca. 680 Ma for the Yaolinghe and Wudangshan Groups in the Ankang and Wudang Uplifts. Based on the compilation of U-Pb ages and Hf–Nd–O isotopic data as well as multidimensional scaling (MDS) analysis, clastic rocks of the Yaolinghe Group are suggested to be sourced mainly from the NWYB, whereas those of the Wudangshan Group were derived mostly from the NWYB and the NYB. The data compiled also indicate ca. 800–680 Ma northeastward migration of magmatism in response to slab rollback and ca. 680–600 Ma eastward migration of magmatism resulting from slab tearing. A tectonic model is proposed that envisages an arcuate subduction system around the Yangtze Block during the mid–late Neoproterozoic. The continuous slab rollback and slab tearing underneath the NYB resulted in an extensional environment at that time. The rocks in the NWYB underwent uplift and erosion, and the detritus was transported to the NYB. The new model can well account for the two distinct groups of volcanic rocks in the Yaolinghe Group.
PubDate: Thu, 29 Sep 2022 00:00:00 GMT
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- Filling critical gaps in the space-time record of High Lava Plains and
co-Columbia River Basalt Group rhyolite volcanism-
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Abstract: Miocene rhyolitic volcanism of eastern Oregon, USA, can be divided into two main episodes. Mantle plume upwelling is thought to have generated Columbia River Basalt Group (CRBG) lavas and coeval >16.5–15 Ma silicic volcanism trending north–south from northeast Oregon to northern Nevada. Rhyolite volcanism of the 12–0 Ma High Lava Plains province has been ascribed to either buoyancy-driven westward plume spreading or to slab rollback and mantle convection spanning from southeast Oregon to Newberry Volcano to the west. The apparent ca. 15–12 Ma eruptive hiatus suggests that rhyolites of these provinces were a product of separate processes, yet this gap was based on incomplete data. The lack of data on ∼33 of the total ∼50 rhyolitic eruptive centers in the area where the two provinces overlap (117–119°W, 43–44°N) yields only tenuous relationships between these two provinces.We acquired 40Ar/39Ar ages for 29 previously unanalyzed rhyolite centers that confirm the existence of a rhyolitic eruptive episode concurrent with CRBG flood basalt volcanism. Rhyolite eruptions gradually initiated at ca. 17.5 Ma, and our new ages indicate that peak intensity of the first eruptive episode occurred between 16.3 Ma and 14.4 Ma. We refine the ca. 15–12 Ma rhyolitic eruptive hiatus to 14.4–12.1 Ma, where strong recommencement of rhyolite eruptions began with Beatys Butte at 12.05 Ma. We find two prominent fluxes in rhyolitic eruptive activity after 12.1 Ma as opposed to one continuous, age-progressive trend, at 12.1–9.6 Ma and 7.7–5.1 Ma, which are separated by an ∼2 m.y. period of decreased rhyolite volcanism. Rhyolite eruptions were scarce after 5.1 Ma, at which point most eruptions were associated with Newberry Volcano. Periodicity of rhyolite volcanism along the High Lava Plains demands more punctuated basalt inputs than what continuous partial melting from west-spreading plume material should generate.Our new data suggest that regional rhyolite eruptions are a series of episodic events related to the arrival and storage of mafic mantle magmas. Paucity in rhyolite eruptions from 14.4 Ma to 12.1 Ma is related to decreased flux of CRBG flood basalt magmas at ca. 15 Ma. Strong recommencement of rhyolite volcanism at 12.1 Ma is related to continued Northwest Basin and Range extension and a peak rotation rate of Siletzia affecting regional lithosphere weakened by CRBG volcanism. Waning rhyolitic eruptive activity from ca. 9.6 Ma to 7.7 Ma reflects a regional transition in the primary mode of accommodation of extension from Northwest Basin and Range normal faulting to extension and shearing of the Brothers Fault Zone. Rhyolite volcanism between 7.7 Ma and 5.1 Ma was driven by continued regional extension in an area less affected by CRBG magmatism. Post-5.1 Ma rhyolite eruptions occurred within crust not influenced by CRBG magmatism but impacted by both regional extension and the Cascadia subduction zone.
PubDate: Fri, 23 Sep 2022 00:00:00 GMT
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- Timing and spatial variation of deformation along the Kanggur-Huangshan
shear zone in the Chinese Tianshan: Implications for regional differential
uplift and mineralization-
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Abstract: The Chinese Tianshan experienced largescale transcurrent tectonics, synkinematic emplacement of ultramafic to felsic intrusions, and the formation of various mineral deposits during late Paleozoic accretionary orogenesis. The relationships among the spatial variation of deformation, the distribution of Permian orogenic Au and magmatic Ni-Cu sulfide deposits, and the kinematic evolution of crustal-scale shear zones, however, remain ambiguous. To address these ambiguities, the spatial variation in the degree of deformation in the Kanggur-Huangshan shear zone in the Chinese Tianshan was characterized using detailed structural measurements and zircon U-Pb and muscovite 40Ar/39Ar age data. The new structural data indicate that a prominent spatial variation exists in the style of deformation throughout the Kanggur-Huangshan shear zone; intense ductile deformation structures are dominant in the east, while brittle structures become progressively more dominant toward to the west. Zircon U-Pb and muscovite 40Ar/39Ar age data for syn- and postkinematic intrusions along the Kanggur-Huangshan shear zone indicate that dextral strike-slip shearing occurred between 279 Ma and 249 Ma. The spatial variation in the degree of deformation and exhumation along the Kanggur-Huangshan shear zone was potentially caused by regional differential uplift induced by the collision of the Tianshan and Beishan regions; this was likely responsible for the predominant occurrence of magmatic Ni-Cu sulfide deposits in the eastern portion of the Kanggur-Huangshan shear zone and orogenic Au deposits in the western portion. The identified spatio-temporal relationship between deformation and distribution of orogenic Au and magmatic Ni-Cu sulfide deposits is crucial to the future success of mineral exploration in the Central Asian orogenic belt.
PubDate: Fri, 23 Sep 2022 00:00:00 GMT
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- A 50 m.y. melting model for the rare metal–rich Koktokay pegmatite in
the Chinese Altai: Implications from a newly identified Jurassic granite-
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Abstract: The petrogenesis of the world famous Koktokay No. 3 pegmatite in the Chinese Altai Mountains is still enigmatic due to its superenrichment of rare metals and the apparent absence of a parental granite. We present results from a granite apophysis that was recently discovered in the No. 3 pegmatite open pit. Results show that it has low K/ Rb (32.35–38.76), Zr/Hf (13.78–23.30), Nb/ Ta (1.00–6.02), and extremely low K2O/Na2O (0.12–0.20) ratios, which, together with its mica composition and the occurrences of garnet, indicate that it is a highly evolved muscovite albite granite. Columbite and apatite from the granite apophysis yielded U-Pb ages of 182.3 ± 1.0 Ma and 184.9 ± 4.3 Ma, respectively, which are younger than the Triassic ages (ca. 210 Ma) of the main magmatic stage but fall into the age range of the No. 3 pegmatite series (220–175 Ma). Both the granite and the apatite grains within it share nearly identical rare earth element patterns with the magmatic stage of the No. 3 pegmatite. The whole-rock εHf(t) values (t = 183 Ma) range from –0.11 to +1.32, consistent with those of the No. 3 pegmatite and indicating a similar source. We propose that the Jurassic granite represents a late pulse of magma injected as apophyses from a deep-seated magma chamber (perhaps from the lower crust), which overlapped with the early pegmatite and promoted the rare metal mineralization. The No. 3 pegmatite and the Jurassic granite may represent a continuous magmatic system with a 50 m.y. melting process, generating this unusual giant pegmatite intrusion with abundant rare metals.
PubDate: Fri, 23 Sep 2022 00:00:00 GMT
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- The Ediacaran–Cambrian transition in the southern Great Basin,
United States-
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Abstract: The Ediacaran–Cambrian boundary strata in the Great Basin of the southwestern United States record biological, geochemical, and tectonic change during the transformative interval of Earth history in which metazoans diversified. Here, we integrate new and compiled chemostratigraphic, paleontological, sedimentological, and stratigraphic data sets from the Death Valley region, the White-Inyo Ranges, and Esmeralda County in Nevada and California and evaluate these data within a regional geologic framework. A large negative carbon isotope (δ13C) excursion—also known as the Basal Cambrian Excursion, or BACE—is regionally reproducible, despite lateral changes in sedimentary facies and dolomitization across ∼250 km, consistent with a primary marine origin for this perturbation. Across the southern Great Basin, Ediacaran body fossils are preserved in a variety of taphonomic modes, including cast and mold preservation, two-dimensional compressional preservation, two-dimensional and three-dimensional pyritization, and calcification. The stratigraphic framework of these occurrences is used to consider the relationships among taphonomic modes for fossil preservation and paleoenvironmental settings within this basin. In this region, Ediacaran-type fossils occur below the nadir of the BACE, while Cambrian-type trace fossils occur above. Sedimentological features that include giant ooids, stromatolites, and textured organic surfaces are widespread and abundant within the interval that records biotic turnover and coincide with basaltic volcanism and the BACE. We hypothesize that the prevalence of these sedimentological features, the BACE, and the disappearance of some Ediacaran clades were caused by environmental perturbation at the Ediacaran–Cambrian boundary.
PubDate: Thu, 22 Sep 2022 00:00:00 GMT
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- Plio-Pleistocene cooling of the northeastern Tibetan Plateau due to global
climate change and surface uplift-
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Abstract: It has been proposed that the northeastern Tibetan Plateau (NETP) was the most recent plateau sub-region to gain elevation, leading to profound environmental changes in northern East Asia around the mid-Pliocene (ca. 3.6 Ma). Alternatively, environmental changes in the region have been linked to ongoing global cooling and glacial intensification after 3.3 Ma. Here, we test these hypotheses using Plio-Pleistocene estimates of paleotemperatures and paleoelevations in the NETP derived from oxygen and carbon stable isotopic composition (n = 792) and clumped isotope measurements (n = 32) of carbonates from a Lake Qinghai Basin sediment core, eastern NETP. From 5 to 2 Ma, basinal mean annual air temperatures (MAATs) decreased by 4.9 ± 2.8 °C at rates of 1.6 °C ± 0.5 °C/myr, concurrently with regional and global cooling. However, the largest MAAT decline occurs between ca. 4.8–3.7 Ma (4.1 ± 3.2 °C) and ca. 3.4–2.0 Ma (−0.3 ± 2.8 °C) and may correspond with an elevation change of 1.0 ± 1.0 km at 3.6 Ma, coincident with tectonic activity in the Lake Qinghai Basin and several other basins in the NETP and its foreland. Taken together, these results suggest a combination of global cooling and a small magnitude of surface uplift (<1 km) at ca. 3.6 Ma may have contributed to the regional evolution of climate in the Lake Qinghai Basin.
PubDate: Fri, 16 Sep 2022 00:00:00 GMT
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- Petrogenesis of arsenic and platinum-group minerals from a partially
serpentinized dunite in East Dover, Vermont, USA-
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Abstract: In the Appalachian Mountains of Vermont, USA, variably serpentinized ultramafic rocks mark the Ordovician Taconic orogenic suture zone. These ultramafic rocks provide evidence for several alteration events that occurred during Appalachian orogenesis. The largest of these ultramafic bodies occurs as a partially serpentinized meta-dunite located in East Dover, Vermont. Whole-rock X-ray fluorescence spectroscopy and electron microprobe data on variably serpentinized meta-dunite samples are interpreted with respect to several processes including fluid/melt-rock interaction in the mantle, serpentinization, and subsequent regional metamorphism. We report the first discovery of nickel arsenide minerals hosted in this meta-dunite, as well as rare occurrences of platinum-group mineral inclusions in chromitite. Although the platinum-group minerals and chromitite are rare, their occurrence and chemistry suggest that they formed by fluid/ melt-rock interaction during partial melting events that produced the dunite, likely in a supra-subduction zone setting. Arsenic minerals are rare in un-serpentinized samples but are ubiquitous in highly serpentinized samples, which suggests that most of the arsenic was introduced into the ultramafic rocks during serpentinization. Whole-rock geochemical analyses also indicate that highly serpentinized samples contain the highest concentrations of arsenic. The discovery of arsenic minerals identifies a potential source to explain elevated arsenic in groundwater in Vermont, which is a serious health concern in places where wells have been drilled in serpentinite bedrock.
PubDate: Fri, 16 Sep 2022 00:00:00 GMT
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- How headward erosion breaches upstream paleolakes: Insights from dated
longitudinal fluvial terrace correlations within the Sanmen Gorge, Yellow
River-
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Abstract: Headward erosion breaching a formerly closed paleolake has been interpreted as an integration process between individual drainage networks. However, the rarity of well-documented cases of this process makes it difficult to explore the fluvial response or its mechanism. Fortunately, dated deposits from a former paleolake in the Fenwei Basin, coupled with fluvial terraces along the Sanmen Gorge of the Yellow River, provide ample opportunities to replicate the upstream integration process and associated landform response. Herein, we applied terrace correlation and age constraints to construct dated longitudinal profiles throughout the Fenwei Basin and the downstream Sanmen Gorge. We identified two age models in longitudinal profiles: (1) relatively high diachronous terraces (Terrace series A) aged headward from ca. 621 ka to 336 ka, localized in the Sanmen Gorge; and (2) relatively low isochronous terraces (Terrace series B) capped by S2 paleosol (ca. 245–190 ka) developed throughout the basin and the gorge. This study hypothesized these two terrace series to have emerged as a mid-Pleistocene bottom-up integration event. In this event, headward incision initiated at least at ca. 621 ka, progressed upstream at a rate of 79.8 mm/a, and breached the Fenwei Basin at ca. 245 ka. This event is likely ascribed to tectonic subsidence of the North China Plain, and may be the latest integration process between the middle and lower reach of the Yellow River. We synthesize comprehensive models to describe terrace genesis and responses in tectonic, climatic, and bottom-up integration processes, which could widen our understanding of long-term large river behaviors.
PubDate: Fri, 16 Sep 2022 00:00:00 GMT
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- Drainage initiation, expansion, and channel-head arrest in heterogenous
bedrock landscapes of the Colorado Plateau-
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Abstract: The transition between hillslopes and channels defines landscape characteristics such as drainage density, ridge spacing, and hillslope length. Previous studies of the location of this transition have demonstrated that soil advection and diffusion work either individually or in concert to set the limits of channelization in largely soil-mantled landscapes. However, no such model explaining the limits of channelization in bedrock landscapes has been established. In this study, we explore the morphologic signatures and processes of channel initiation, headward elongation, and eventual arrest in the bedrock landscape of the Raplee Ridge monocline, Utah, USA. The monocline provides an opportunity to test whether a threshold shear stress controls the initiation of channels through inspection of slope and upstream drainage area at varyingly incised portions of the landscape. Using a combination of lidar and structure-from-motion data, we find that incised and unincised channels occupy overlapping but separable portions of slope-upstream area space, while geomorphic mapping and field-based observations allow for further distinction between erosional processes with threshold shear stress values that range from ∼60 Pa to 160 Pa. We develop a conceptual model of the initiation and expansion of these bedrock channels using field observations, in which channels first initiate by vertical plucking of blocks, disaggregate limestone by block sliding, elongate upslope by knickpoint retreat in variably erosive lithologies, and eventually arrest at predictable landscape positions due to block buttressing. These results suggest that shear stress-controlled processes play a fundamental role in setting the degree of channelization in bedrock landscapes; however, the thresholds for channel erosion can change in response to local geologic factors such as lithology and structural geometry. This implies that detailed geologic knowledge may be necessary for the interpretation and modeling of fluvial channels in terrestrial bedrock landscapes and those on other planets.
PubDate: Thu, 15 Sep 2022 00:00:00 GMT
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- Late Permian-Triassic ridge subduction beneath the northeastern margin of
the North China Craton: Evidence from the Kaiyuan appinitic to high-Mg
intermediate intrusions in northern Liaoning, North China-
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Abstract: Recognizing terminal ridge subduction is of first-order importance in chronicling the tectonic timeline of any accretionary orogens, but remains elusive in the final suturing zone of the Paleo-Asian Ocean along the Solonker–Xar Moron–Changchun–Yanji swath. Here we report two generations of ultramaficmafic to intermediate intrusions from northern Liaoning, North China, linking eastern Central Asian Orogenic Belt and northern North China Craton. The ca. 259 Ma appinites show variable enrichment in large-ion lithophile elements (LILEs) but depletion in high-field strength elements (HFSEs), and variably enriched isotopic compositions with εNd(t) of −11.8 to −2.4 and zircon εHf(t) of –15.7 to +5.0. These petro-geochemical traits argue for their derivation from partial melting of subduction-modified lithospheric mantle. By contrast, the high-Mg diorites of 226–223 Ma display decoupling features between LILEs and HFSEs and juvenile isotopic signatures with εNd(t) of –0.3 to +2.6 and zircon εHf(t) of +5.4 to +11.4. These characters are consistent with a magmatic origin from partial fusion of sediment-fertilized asthenospheric mantle. Synthesizing the two mantle-melting events with regional coeval igneous activities leads to the identification of Late Permian (265–250 Ma) and Middle-Late Triassic (241–218 Ma) magmatic flare-ups with a 10–15 m.y. lull along the northern North China Craton. This magmatic tempo tends to record a salient mantle replacement from hydrous metasomatized lithosphere to anhydrous asthenosphere. In combination with ensuing progressive crustal growth, simultaneous metamorphism, and structural activities, these diverse manifestations make a compelling case of ridge subduction and concomitant slab window formation during the eventual closure of the Paleo-Asian Ocean along the Solonker–Xar Moron–Changchun–Yanji swath.
PubDate: Wed, 07 Sep 2022 00:00:00 GMT
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- Migration and localization of faulting near the intersection of the Dead
Sea Fault and the Carmel–Gilboa–Faria Fault System-
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Abstract: Crustal deformation and seismicity in the Levant region are mainly related to the plate-boundary Dead Sea Fault (DSF) and the intraplate Carmel–Gilboa–Faria Fault System (CGFS). The intersection between these two major fault systems is generally treated as an ∼35-km-wide deformation belt stretched between the Faria and Gilboa Faults. Here, we present spatial and temporal analysis of faulting near this intersection. Our analysis is based on new geological mapping, new high-resolution airborne light detection and ranging (LiDAR) data, and seismic reflection profiles and indicates northward migration and localization of the intersection over time since the early Miocene. We discovered and mapped outcrops of Miocene, Pliocene, and Pleistocene rock units as well as faults and reconstructed the evolution of deformation. Three main tectonic phases were identified in this area covering the following periods: the early–middle Miocene, the late Miocene–Pliocene, and the Quaternary. During the first phase, the DSF and the CGFS developed, and the CGFS faulted along a series of subparallel grabens and elongated NW–SE, between the southernmost Faria and the northernmost Gilboa faults, over a belt width of ∼35 km. During the second phase, deformation along the CGFS migrated northward and concentrated at an ∼6-km-wide zone in the northern Faria Anticline. During the third stage, small-scale northward migration and localization of the deformation to a width zone of ∼1–2 km at the southern boundary of the Beit She’an Valley occurred. Faults from the third phase reveal both sinistral and normal faulting. We propose that the currently active intersection between the DSF and the CGFS is located east of this localized deformation zone, near a right step of the DSF and the uplifted area of Tel Al-Qarn in the eastern Jordan Valley. We suggest that the northward migration and localization of this intersection are related to regional tectonic changes, spatial variations in the Sinai-Arabia Euler pole, and the localization of deformation along the DSF.
PubDate: Wed, 07 Sep 2022 00:00:00 GMT
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- Tracing remnants of Ediacaran S-type granitoids from beach placers in SE
Madagascar-
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Abstract: Southeast Madagascar hosts several major deposits of beach placers, the provenance of which is little understood. To redress this imbalance, we present new laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb ages, and trace elements of zircons and monazites, as well as Hf isotopes of zircons from heavy mineral beach sands at Taolagnaro (Fort Dauphin) in SE Madagascar. The ages of detrital zircon grains range from 650 Ma to 450 Ma and from 1900 Ma to 1700 Ma (one grain has an age of 2633 Ma), and detrital monazite ages range from 574 Ma to 484 Ma. The Ediacaran–Cambrian zircons (650–450 Ma) and monazites (574–484 Ma) record a major Pan-African orogenic event in SE Madagascar. Geologically, the 650–450 Ma ages correlate well with coeval granitic gneisses in southern Madagascar, and a few 1900–1700 Ma ages can be linked to putative Paleoproterozoic protoliths of metasedimentary rocks from the Anosyen Domain and from the Itremo Group of central Madagascar. The zircon mineral chemistry indicates that some of the grains (P > 25 μmol g–1) were sourced from S-type granitic rocks. The monazite chemistry points toward a metamorphic provenance from garnet-bearing amphibolite facies rocks, such as migmatitic gneisses in southern Madagascar. The Hf isotopic compositions of detrital zircons indicate reworking of their probable Paleoproterozoic–Archean source rocks. We conclude that the beach sand zircons and monazites were largely sourced from the uplifted and eroded Anosyen and Androyan Domains in SE Madagascar, and with minor contributions from central Madagascar, which were all transported by rivers to the ambient ocean to the southeast.
PubDate: Fri, 02 Sep 2022 00:00:00 GMT
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- Deoxygenation and organic carbon sequestration in the Tethyan realm
associated with the middle Eocene climatic optimum-
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Abstract: The middle Eocene climatic optimum (ca. 40 Ma) stands out as a transient global warming phase of ∼400 k.y. duration that interrupted long-term Eocene cooling; it has been associated with a rise in atmospheric CO2 concentrations that has been linked to a flare-up in Arabia-Eurasia continental arc volcanism. Increased organic carbon burial in the Tethys Ocean has been proposed as a carbon sequestration mechanism to bring the middle Eocene climatic optimum to an end. To further test these hypotheses, we assessed the sedimentary and geochemical expression of the middle Eocene climatic optimum in the northern Peri-Tethys, specifically, the organic-rich Kuma Formation of the Belaya River section, located on the edge of the Scythian Platform in the North Caucasus, Russia. We constructed an age-depth model using nannofossil chronobiostratigraphy. Throughout the studied middle Eocene interval (41.2–39.9 Ma), we documented sea-surface temperatures of 32–36 °C based on the tetraether index of tetraethers consisting of 86 carbons (TEX86), depending on proxy calibration, and during the early middle Eocene climatic optimum, we observed sea-surface warming of 2–3 °C. Despite the proximity of the section to the Arabia-Eurasia volcanic arc, the hypothesized source of volcanic CO2, we found no evidence for enhanced regional volcanism in sedimentary mercury concentrations. Sedimentary trace-element concentrations and iron speciation indicate reducing bottom waters throughout the middle Eocene, but the most reducing, even euxinic, conditions were reached during late middle Eocene climatic optimum cooling. This apparent regional decoupling between ocean warming and deoxygenation hints at a role for regional tectonics in causing basin restriction and anoxia. Associated excess organic carbon burial, extrapolated to the entire regional Kuma Formation, may have been ∼8.1 Tg C yr–1, comprising ∼450 Pg C over this ∼55 k.y. interval. Combined with evidence for enhanced organic carbon drawdown in the western Peri-Tethys, this supports a quantitatively significant role for the basin in the termination of the middle Eocene climatic optimum by acting as a large organic carbon sink, and these results collectively illustrate that the closing Tethys Ocean might have affected global Paleogene climate. Moreover, this study highlights the importance of the interplay between global climate and regional oceanic gateway evolution in determining local climate and oceanographic change.
PubDate: Thu, 01 Sep 2022 00:00:00 GMT
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- Structure, geochronology, and petrogenesis of Permian peraluminous granite
dykes in the southern Chinese Altai as indicators of Altai–East Junggar
convergence-
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Abstract: How and at which thermal conditions the convergence between the Chinese Altai and East Junggar operated remain poorly understood. This issue is addressed in the current study by focusing on the timing and petrogenesis of syntectonic granite dykes from the representative areas of Fuyun (convergent front) and Kalasu-Aletai (Chinese Altai interior). It is shown that Fuyun and Kalasu-Aletai dykes are fractionated I- and S-type granites, with zircon and monazite U-Pb ages of 300–291 Ma and 281–265 Ma, respectively. Geochemically, the Fuyun dykes have lower contents of aluminous (ASI: 0.97–1.13) and light rare earth element-enriched features, while the Kalasu-Aletai dykes have ASI = 1.01–2.17 and show overall flat rare earth element patterns with tetrad effects. The Fuyun dykes exhibit less evolved Sr-Nd isotopic characteristics (87Sr/86Srinitial: 0.7039–0.7048, εNd(t): +5.7 to +6.1) with respect to those of the Kalasu-Aletai dykes (87Sr/86Srinitial: 0.6978–0.7183, εNd(t): −7.6 to +3.0). The Fuyun and Kalasu-Aletai dykes are geochemically compatible with isotopically less evolved East Junggar arc components and heterogeneous Ordovician wedge sediment of the Chinese Altai, respectively, implying genetic links. We propose that the late Paleozoic Chinese Altai–Junggar convergence created a local perturbation of weak mantle beneath the southern Chinese Altai, causing partial melting of the underthrusting East Junggar and the overriding Altai components successively. The resulting magmas were emplaced along northward propagating syn-tectonic tensional fractures perpendicular to the Chinese Altai–East Junggar deformation front that serves as an excellent indicator of the convergent-shortening process.
PubDate: Thu, 01 Sep 2022 00:00:00 GMT
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- Unconformity-bounded rift sequences in Terreneuvian–Miaolingian strata
of the Caledonian Highlands, Atlantic Canada-
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Abstract: The Cambrian syn-rift strata preserved in western Avalonia provide a distinctive example of how unconformity-bounded sequences are diachronous throughout proximal to marginal rift branches. Terreneuvian‒Miaolingian third-order sequences of the Caledonian Highlands, New Brunswick, Canada, reflect a complex interplay among syn-rift tectonic events, denudation pulses, and sea-level fluctuations. Unconformably overlying the early, rift-related volcanosedimentary Coldbrook Group (ca. 560‒550 Ma), the Ratcliffe Brook, Glen Falls, Hanford Brook, and Forest Hills Formations can be subdivided into two transgressive systems tract (TST)‒highstand systems tract (HST) sequences (each ∼10 m.y.) and an incomplete TST sequence that are separated by stratigraphic gaps. They reflect uplift and tilting events affecting the basement, transgressive and drowning surfaces, and condensed sections. Arid to semi-arid climatic episodes are supported by the excellent preservation of mafic to felsic volcanic clasts in non-marine breccias and conglomerates, which are derived from the Ediacaran basement, and the local precipitation of marine gypsum through the evaporation of pore fluids. Early Miaolingian episodes of microbial/shelly carbonate production preserved precipitates of coeval evaporite (gypsum pseudomorphs after drusy mosaics of calcite) and ikaite (glendonitic, star-shaped aggregates and crusts). Both minerals, traditionally considered to be indicators of contrasting climate conditions, potentially co-occur in temperate-water substrates recording high rates of microbial activity. The early rift phases preserved in the western Avalonian rift transect comprise stepwise uplift and unroofing of rift shoulders, which are related to diachronous, angular discordances and paraconformities bounded by syntectonic slope-apron deposits. Facies homogenization was attained during Miaolingian times as a result of generalized flooding, sealing of paleotopographies, and blanketing with monotonous offshore-dominant shales.
PubDate: Tue, 30 Aug 2022 00:00:00 GMT
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- Constraint on the temperature of A-type magma from contact metamorphic
aureole, Biesituobie batholith, West Junggar in NW China, Central Asian
Orogenic Belt-
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Abstract: The Biesituobie A-type batholith in West Junggar in NW China of the Central Asian Orogenic Belt contains metapelite xenoliths derived from the contact metamorphic aureole. These xenoliths could be divided into two types: biotite-muscovite-andalusite hornfels and cordierite-alkaline feldspar-sillimanite-corundum gneiss, indicating prograde metamorphism. The phase equilibrium modeling on the cordierite-alkaline feldspar-sillimanite-corundum gneiss sample yielded a peak condition of P = 3.0–4.0 kbar, T = 760–800 °C. Similarly, the Na-in-cordierite geothermometer yielded a peak condition of T = 771–780 °C. The temperature of the peak metamorphism could be considered as a lower limit of the temperature of the Biesituobie A-type batholith. On the contrary, the Ti-in-zircon geothermometer applied to the Biesituobie A-type batholith zircons yields a mean temperature of 672 ± 22 °C. It suggested that the application of Ti-in-zircon geothermometer on A-type magma may involve a >100 °C underestimation. Zircon U-Pb dating indicates that the age of the contact metamorphism is between 263 and 286 Ma, consistent with the age of the pluton at 274 Ma. The result of 760–800 °C calculated from the contact metamorphic aureole of the Biesituobie batholith put a lower limit on considering the temperature condition of the A-type granite series from a metamorphic constraint.
PubDate: Mon, 29 Aug 2022 00:00:00 GMT
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- Fault zone architecture and lithology-dependent deformation mechanisms of
the Himalayan frontal fold-thrust belt: Insights from the Nahan Thrust,
India-
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Abstract: Brittle shallow crustal faults typically develop a complex fault zone architecture with distinct structural domains that display diverse microstructures, mineralogy, and deformation mechanisms. The development of such domains is typically controlled by the strength and composition of the protoliths, physical conditions of deformation, fluid ingress, and diachronous fault growth in response to stress accumulation and co-seismic slip. Herein, we studied the microstructure-mineralogy-kinematics of fault rocks in the Nahan Thrust, in the vicinity of the Main Frontal Thrust that represents a tectonically active zone in the Himalayan orogen. The Nahan Thrust is characterized by alternating red and gray gouge layers, and a single black gouge layer. Our results from electron microscopy and X-ray diffractometry indicate that the protolith of the red gouge layers is argillaceous sandstone, whereas that of the gray and black gouge layers is sandstone. Microstructures suggest an initially distributed deformation (aseismic creep), followed by a protracted brittle deformation event, and a later aseismic creep stage. The brittle stage is marked by progressive localization of stress, fracture development, cataclasis, frictional sliding, and seismic slips. The black gouge layer acted as the principal slip zone and exhibited ultrafine bands of micrometerscale slip zones with vapor escape structures and clay clast aggregates, indicating seismic faulting and frictional heating during seismic slips. The preferential seismic rupture nucleation in the black gouge layer indicates a strong lithological dependence on seismic slip in the Nahan Thrust. We also conclude that heterogeneity within the Nahan Thrust resulted from primary lithological variations of the protoliths.
PubDate: Mon, 29 Aug 2022 00:00:00 GMT
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- Thermal equation of state for zoisite: Implications for the transportation
of water into the upper mantle and the high-velocity anomaly in the
Farallon plate-
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Abstract: Seismic tomography studies have revealed a high-velocity anomaly at depths between 100 km and 300 km in the Farallon plate. However, the reasons for the high-velocity anomaly continue to be debated. An analysis of the mineral proportions of eclogites exhumed from the Farallon plate shows that the average amount of zoisite in eclogite is ∼16.0 vol%. Therefore, the presence of zoisite eclogite needs to be considered to explain the high-velocity anomaly of the Farallon plate. However, the thermal equation of state and stability of zoisite have not been fully investigated under high pressure–temperature (P-T) conditions. We investigated the high-pressure and high-temperature behavior of natural zoisite utilizing synchrotron single crystal-X-ray diffraction (XRD). The results indicate that zoisite is metastable up to 24.8 GPa and 700 K. We obtained the ambient unit-cell volume V0 = 901.26(3) Å3 by synchrotron single crystal-XRD measurement. We also fitted the pressure-volume-temperature data to a high-temperature Birch-Murnaghan equation of state and obtained the zero-pressure bulk modulus K0 = 134.7(8) GPa, the temperature derivative of the bulk modulus (∂K/∂T)P = –0.011(4) GPa/K, and the thermal expansion coefficients α0V = 1.9(7) × 10–5 K–1 and α1V = 3(2) × 10–8 K–2 when the pressure derivative of bulk modulus (K′0) is fixed at 4. By incorporating the results from previous studies, we calculated the density and bulk sound velocity profiles of zoisite eclogite along the Farallon plate geotherm. Finally, we infer that zoisite could carry water to depths of ∼300 km within cold subducting slabs and that the bulk sound velocity of typical zoisite eclogite with ∼61.0 vol% omphacite, ∼23.0 vol% garnet, and ∼16.0 vol% zoisite could cause the high-velocity anomalies at depths of 100–300 km in the Farallon plate.
PubDate: Thu, 25 Aug 2022 00:00:00 GMT
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- Intensified and apace bauxitization over the paleo-karstic surface linked
to volcanism-
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Abstract: Recent studies indicate that volcanism may be a potentially important factor in the formation of karstic bauxite, but more evidence is needed. Six billion tons of bauxite formed upon paleo-karstic terrain within a short time in the Late Carboniferous in the North China Basin (NCB) and in the Late Permian in the Youjiang Basin (YB) of China. The factors that trigger their apace formation remain unclear. Herein, we proposed that extensive volcanic eruptions have accounted for this large-scale bauxite formation based on the mineralogy, Hg isotopes, and Hg concentration enrichment proxies of the samples of bauxitic sequence in the NCB and YB. NCB bauxite generally has three layers, namely bottom Fe-bearing claystone, intermediate bauxite ore, and top claystone, while bottom Fe-bearing claystone is usually absent in YB bauxite, which directly covers carbonate rocks. The mineral assemblages of NCB and YB bauxite confirm that they were deposited in a superficial alkaline and reducing karstic environment. Strong Hg enrichment peaks with corresponding near-zero Δ199Hg, a signal of excess volcanogenic Hg, were discovered in the upper claystone of NCB bauxitic sequences, which overlaid the Ordovician carbonate substrate. It denotes volcanism occurred immediately subsequent to deposition of terrestrial weathered Al-rich remnants in paleo-karstic depressions during the Late Carboniferous. This volcanism, occurring in the northern margin of the NCB, is considered to have triggered the apace bauxitization in the NCB after its long exposure and weathering. In the YB, the remarkable Hg enrichment and near-zero Δ199Hg were observed in the entire Late Permian bauxitic sequences with Late Permian carbonate as the substrate. It denotes enhanced volcanogenic Hg inputs throughout the whole deposition process of bauxite during the Late Permian. This episode of volcanism associated with the Emeishan large igneous province and contemporaneous arc system resulted in the short term weathering of source materials and the apace bauxitization in YB. Volcanism is suggested to aggravate surface acidic and oxidizing conditions, in which rapid decomposition of source materials occurred to release Al3+ to precipitate diaspore above the physiochemical barrier of underlying carbonate.
PubDate: Thu, 25 Aug 2022 00:00:00 GMT
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- A case of Ampferer-type subduction beneath the New Caledonia arc: Evidence
for inefficient subduction of hydrated lithologies into the upper mantle-
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Abstract: New Caledonia is one of the world’s bestexposed subduction/obduction complexes and is central to understanding the geodynamic evolution of the southwest Pacific region. We present new geochemical and Ar/Ar age dates from the in situ eruptive sequences of the La Conception basaltic-andesite lavas (ca. 29.12 Ma) and correlate the generation of these lavas with the generation of the Saint Louis and Koum/Borindi Massifs (ca. 24 Ma) to provide information on the magmatic processes operating within the mantle wedge over time and to indicate the direction of the subducted slab during the Oligocene. The La Conception basaltic-andesite lavas were emplaced in an arc-to-trench direction to the southwest of New Caledonia due to the partial melting of metasomatized, amphibole-bearing garnet peridotites at the base of mantle wedge (∼112–118 km). However, both the Saint Louis and Koum/Borindi granodiorite massifs were derived from melting within the lower crust of an island arc overlying a mantle wedge. Such temporal and spatial variations of magmatism in New Caledonia are consistent with a northeast subduction zone during the Oligocene. The absence of voluminous arc magmatism related to the Oligocene subduction is a consequence of the low Tp temperatures of the mantle wedge, the old age and the high sinking velocity of the down-going slab, and the absence of stress within the overriding plate. The Oligocene subduction beneath New Caledonia, therefore, is an ideal example of Ampferer-type subduction, which is an amagmatic closure area due to the inefficient subduction of hydrated lithologies into the convective upper mantle. Once the Paleocene to Miocene contraction in the southwest Pacific region ceased, the subduction west of New Caledonia shut down, the system relaxed, and no more melting occurred during the Oligocene.
PubDate: Thu, 25 Aug 2022 00:00:00 GMT
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- Initial opening of the Neotethyan Ocean in SE Asia: Constraints from
Triassic magmatism and sedimentation-
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Abstract: How the Neotethyan Ocean evolved and extended southwards into Southeast Asia remains controversial. The paleographical correlation between India and the SE Asian blocks and/or terranes before the opening of the Neotethys and the initial opening time of the Neotethys is still unknown. The lack of this knowledge hampers our further understanding of the tectonic evolution of global Neotethys. Here we present a combined study on Triassic magmatism in the Tengchong Block and Triassic sedimentation in the Myitkyina area, located on the two sides of the Tagaung-Myitkyina Ophiolite Belt of northern Myanmar. Our results coupled with previous data demonstrate that a Triassic continental magmatic arc developed in the Tengchong Block and that the Triassic Myitkyina sedimentary sequence was part of the Tethyan Himalayan Langjiexue Group in northern India. Moreover, the Tengchong Triassic magmatic arc provided important detrital inputs to the whole Langjiexue Group of northern India. Such a provenance is the best explanation for the Permian–Triassic detrital zircons of the Langjuexue Group. Together, we propose that the Tagaung-Myitkyina Ophiolites in northern Myanmar are the relics of the Neotethyan Ocean rather than the Mesotethyan Ocean (Bangong-Nujiang Ocean) in SE Asia, and that the initial opening time of the Neotethys was the Early Jurassic of 200–190 Ma. Then, the earliest (185–165 Ma) intra-oceanic arc akin to the Izu-Bonin-Mariana arc in the West Pacific, developed soon after the Late Triassic opening of the Neotethys.
PubDate: Tue, 23 Aug 2022 00:00:00 GMT
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- Covering the Great Unconformity in southern Laurentia during Rodinia
breakup: Detrital zircon studies of provenance evolution during Cambrian
marine transgression (Llano Uplift, Texas)-
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Abstract: The Llano Uplift in central Texas, USA, exposes the southernmost expanse of Laurentian crystalline basement in North America and the overlying lower Paleozoic strata deposited on the Great Unconformity. Systematic detrital zircon (DZ) U-Pb provenance analysis of the Hickory Sandstone—the basal unit of the Cambrian Riley Formation that onlaps the Mesoproterozoic core of the Llano Uplift—yielded locally variable DZ U-Pb signatures with ages ranging from 1800 Ma to 485 Ma (n >1700). The Hickory Sandstone zircons are dominated by 1550–1300 Ma (50%) and 1300–1000 Ma (46%) ages. These two dominant Mesoproterozoic DZ age components likely were sourced from local Grenville metamorphic and igneous basement of the Llano Province (1300–1000 Ma) and from the extra-regional Granite-Rhyolite Province basement to the north and northwest of the Grenville Front, which suggests both local sourcing and a regional drainage system supplying sediment to the southern Laurentian margin during the Cambrian. This interpretation is supported by a minor component of 1800–1600 Ma zircons, sourced from the Yavapai-Mazatzal Province, and zircons with Early Cambrian ages that were likely sourced from the Wichita Igneous Province, which is located to the northwest and to the north of the Llano Uplift, respectively. Documented NNW-trending topographic ridges in the Mesoproterozoic basement surface of the Llano Uplift could have funneled aeolian and fluvial sand supply to the southern Laurentian coast that was reworked in the marginal marine environment.Samples from the western Llano Uplift are dominated by regionally sourced, early Mesoproterozoic Granite-Rhyolite Province DZ ages, whereas samples from the eastern Llano Uplift exhibit a dominant locally sourced Grenville signature. Hickory Sandstone samples also contain a small number of Neoproterozoic (850–600 Ma) and Cambrian (541–487 Ma) zircons. Sources for Neoproterozoic zircons likely were located along the Laurentian continental margins and derived from extension-related magmatism associated with the breakup of Rodinia. Cambrian zircons are most common in Hickory Sandstone samples on the southern and western flanks of the Llano Uplift. Some are reasonably sourced from the Wichita Igneous Province to the north, but the younger Cambrian zircons suggest sources to the west. Upper Hickory strata are dominated (>60%) by 1550–1300 Ma grains with up to 10% >1600 Ma Paleoproterozoic grains. Upper Cambrian sandstones of the Wilberns Formation also contain a significant contribution of >1600 Ma grains, which suggests a possible enlargement of the fluvial headwaters over time extending into older Laurentian provinces to the north and northwest. Differences among DZ populations over relatively limited distances may have been related to NW-oriented ridge and corridor landforms that developed on the Great Unconformity surface, which were products of the Proterozoic tectonic and lithologic architecture affected by aeolian and fluvial processes at this unusual stage in Earth history. Paleocurrent data and atypical detrital grain characteristics for a Hickory Sandstone locality in the southeastern Llano Uplift and its DZ age similarities with Cambrian sandstones of the Argentine Precordillera suggest a common provenance prior to or during Rodinia breakup.
PubDate: Tue, 23 Aug 2022 00:00:00 GMT
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- A combination of plume and subduction tectonics contributing to breakup of
northern Rodinia: Constraints from the Neoproterozoic magmatism in the
Dunhuang-Alxa Block, northwest China-
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Abstract: Neoproterozoic igneous rocks are widely distributed in the Dunhuang-Alxa Block, northwest China, and record geodynamic processes caused by the assembly and breakup of Rodinia. In this study, we present new petrological, geochemical, and zircon U-Pb-Hf isotope data for the Langshan gabbros in the northeastern Alxa Block. Langshan gabbros (827–819 Ma) have depleted whole-rock Nd (+3.2 to +4.9) and zircon Hf (+4 to +21) isotopic compositions and weak enrichments in large-ion lithophile elements. These rocks were probably derived by partial melting of a depleted mantle wedge metasomatized by fluids released from a subducted slab in a back-arc basin environment. Geochemical and zircon Hf isotope data for early Neoproterozoic igneous rocks are consistent with the presence of a long-lived (ca. 930–810 Ma) retreating subduction zone along the northern margin of the Dunhuang-Alxa Block, indicating it was located at the northern margin of Rodinia. Some mantle plume-related magmatism, including A1-type silicic volcanic rocks in the Langshan Group and Jinchuan ultramafic-mafic intrusions, occurred in the interior of the Dunhuang-Alxa Block, which was similar to early Neoproterozoic plumerelated magmatism (ca. 850–820 Ma) in the interior of northern Rodinia. The temporal link between subduction and extension suggests that this retreating subduction zone along the northern margin of Rodinia, in combination with a mantle plume beneath its interior, resulted in lithospheric extension and break-up of Rodinia during ca. 850–810 Ma.
PubDate: Mon, 22 Aug 2022 00:00:00 GMT
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- The long-lived transcrustal magmatic systems of Southeast China in
response to paleo-Pacific plate subduction, recorded by the Cretaceous
volcanic sequences in southeastern Zhejiang Province-
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Abstract: Processes taking place in subduction zones are highly debated. The Cretaceous volcanic rocks are voluminously distributed along the coastal area of southeastern (SE) China. To elucidate their petrogenesis and relationship with subduction, we use new zircon U-Pb ages and Hf-O isotopes for the representative Cretaceous volcanic sequences in Zhejiang Province, China. According to stratigraphic investigations, zircon U-Pb ages, and Hf-O isotopes, these volcanic rocks can be divided into different groups corresponding to three stages of volcanic activity: the early (Stage 1, 136–129 Ma), middle (Stage 2, 125–115 Ma), and late (Stage 3, 110–94 Ma) stages. Diverse zircon populations (including antecrysts, autocrysts, and xenocrysts) are recognized. Hf-O isotopes of autocrysts suggest different protoliths for the identified three stages of volcanic activity. Xenocrysts show obvious different compositions from autocrysts. Antecrysts share similar compositions with autocrysts, which favor similarities in the magmas from which they were generated. Our observations (very small age intervals between antecrysts and autocrysts, sources for volcanic rocks throughout the entire crust, and volcanic magmatism with long-term and discontinuous characteristics), were inconsistent with the traditional melt-dominated magma chamber model. Consequently, we propose that those volcanic rocks were derived from long-lived transcrustal magmatic systems (TCMS), dominated by crystal mush, instead of melt-dominated magma chambers, maintained and recharged by a discontinuous contribution of contemporaneous underplated mantle-derived magmas, triggered by paleo-Pacific plate subduction. We suggest the different stages of volcanic activity and corresponding long-lived TCMS were produced by the change of Pacific plate motion beneath SE China during the Cretaceous period.
PubDate: Wed, 03 Aug 2022 00:00:00 GMT
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