Zircon U-Pb Age,Geochemical, and Sr-Nd-Pb-Hf Isotopic Constraints on the Time Frame and Origin of Early Cretaceous Mafic Dykes in the Wuling Mountain Gravity Lineament,South China

In view of the importance of mafic dyke swarms and their contribution to current scientific problems relating to South China, herein, we present the findings of studies on twenty–five representative mafic dykes cropping out in Hunan Province and Guangxi Zhuang Autonomous Region, within the southern Wuling Mountain gravity lineament, China. These results include new zircon LA-ICP-MS U-Pb age, whole rock geochemical, Sr-Nd-Pb isotopic, and zircon Hf isotopic data for these dykes. The dykes formed between 131.5 ± 1.2 and 121.6 ± 1.1 Ma, and have typical doleritic textures. They fall into the alkaline and shoshonitic series, are enriched in light rare earth elements(LREE), some large ion lithophile elements(LILE; e.g., Rb, Ba, and Sr), Th, U, and Pb, and are depleted in Nb, Ta, Hf, and Ti. Moreover, the dolerites have high initial87 Sr/86 Sr ratios(0.7055–0.7057), negative εNd(t) and zircon εHf(t) values(-14.8 to-11.9,-30.4 to-14.9), and relatively constant initial Pb isotopic ratios(that are EM1-like, 16.77–16.94, 15.43–15.47, and 36.84–36.92 for 206 Pb/204 Pb,207 Pb/204 Pb, and 208 Pb/204 Pb, respectively). These results indicate that the dykes were likely derived from magma generated through low-degree partial melting(1.0%–10%) of an EM1-like garnet–lherzolite mantle source. The parental magmas fractionated olivine, clinopyroxene, plagioclase, and Ti-bearing phases with negligible crustal contamination, during ascent and dyke emplacement. Several possible models have been proposed to explain the origin of Mesozoic magmatism along the Wuling Mountain gravity lineament. Herein we propose a reasonable model for the origin of these mafic dykes,involving the collision between the paleo-Pacific Plate and South China, which led to subsequent lithospheric extension and asthenosphere upwelling, resulting in partial melting the underlying mantle lithosphere in the Early Cretaceous, to form the parental magmas to the WMGL mafic dykes, as studied.     

Characterization of mesostasis regions in lunar basalts: Understanding late‐stage melt evolution and its influence on apatite formation

Recent studies geared toward understanding the volatile abundances of the lunar interior have focused on the volatile‐bearing accessory mineral apatite. Translating measurements of volatile abundances in lunar apatite into the volatile inventory of the silicate melts from which they crystallized, and ultimately of the mantle source regions of lunar magmas, however, has proved more difficult than initially thought. In this contribution, we report a detailed characterization of mesostasis regions in four Apollo mare basalts (10044, 12064, 15058, and 70035) in order to ascertain the compositions of the melts from which apatite crystallized. The texture, modal mineralogy, and reconstructed bulk composition of these mesostasis regions vary greatly within and between samples. There is no clear relationship between bulk‐rock basaltic composition and that of bulk‐mesostasis regions, indicating that bulk‐rock composition may have little influence on mesostasis compositions. The development of individual melt pockets, combined with the occurrence of silicate liquid immiscibility, exerts greater control on the composition and texture of mesostasis regions. In general, the reconstructed late‐stage lunar melts have roughly andesitic to dacitic compositions with low alkali contents, displaying much higher SiO₂ abundances than the bulk compositions of their host magmatic rocks. Relevant partition coefficients for apatite‐melt volatile partitioning under lunar conditions should, therefore, be derived from experiments conducted using intermediate compositions instead of compositions representing mare basalts.     

Meso‐Cenozoic Evolution of Earth Surface System under the East Asian Tectonic Superconvergence

The East Asian geological setting has a long duration related to the superconvergence of the Paleo‐Asian, Tethyan and Paleo‐Pacific tectonic domains. The Triassic Indosinian Movement contributed to an unified passive continental margin in East Asia. The later ophiolites and I‐type granites associated with subduction of the Paleo‐Pacific Plate in the Late Triassic, suggest a transition from passive to active continental margins. With the presence of the ongoing westward migration of the Paleo‐Pacific Subduction Zone, the sinistral transpressional stress field could play an important role in the intraplate deformation in East Asia during the Late Triassic to Middle Jurassic, being characterized by the transition from the E‐W‐trending structural system controlled by the Tethys and Paleo‐Asian oceans to the NE‐trending structural system caused by the Paleo‐Pacific Ocean subduction. The continuously westward migration of the subduction zones resulted in the transpressional stress field in East Asia marked by the emergence of the Eastern North China Plateau and the formation of the Andean‐type active continental margin from late Late Jurassic to Early Cretaceous (160‐135 Ma), accompanied by the development of a small amount of adakites. In the Late Cretaceous (135‐90 Ma), due to the eastward retreat of the Paleo‐Pacific Subduction Zone, the regional stress field was replaced from sinistral transpression to transtension. Since a large amount of late‐stage adakites and metamorphic core complexes developed, the Andean‐type active continental margin was destroyed and the Eastern North China Plateau started to collapse. In the Late Cretaceous, the extension in East Asia gradually decreased the eastward retreat of the Paleo‐Pacific subduction zones. Futhermore, a significant topographic inversion had taken place during the Cenozoic that resulted from a rapid uplift of the Tibet Plateau resulting from the India‐Eurasian collision and the formation of the Bohai Bay Basin and other basins in the East Asian continental margin. The inversion caused a remarkable eastward migration of deformation, basin formation and magmatism. Meanwhile, the basins that mainly developed in the Paleogene resulted in a three‐step topography which typically appears to drop eastward in altitude. In the Neogene, the basins underwent a rapid subsidence in some depressions after basin‐controlled faulting, as well as the intracontinental extensional events in East Asia, and are likely to be a contribution to the uplift of the Tibetan Plateau.     

The Cretaceous Turn of Geological Evolution: Key Evidence from East Asia

this work focuses on one of the critical points of Earth’s history when the Solar System passed through the most distant point of its galactic orbit. During this event, Earth may have suffered from maximum extension, associated with its relative proximity to the Sun at that time, followed by long-term contraction related to its later distancing. This paper is based on generalized data on the Cretaceous evolution of the Earth as a whole and of East Asia in particular. The evidence suggests that major geological processes at this time may be interpreted as transitional changes in the state of Earth. A liquid nature of its core may have reacted to the gravitational and electromagnetic transformations. When the cosmic changes took place at 135-120 Ma, more turbulent flows in the outer core would have favoured the rise of voluminous magmatic plumes and associated fluid flows. These would substantially transform the mantle, crust, hydrosphere, biosphere and atmosphere. In particular, plume-related melting of overlying subducting slabs and lower continental crust could have initiated numerous adakitic melts that formed the East Asian Adakitic Province. These and associated juvenile events produced numerous metallic ore, coal, gas and oil deposits. The Cretaceous is one of the most significant resource-producing periods.     

The Impact Of Extreme Summer Melt on Net Accumulation of an Avalanche Fed Glacier,as Determined by Ground‐Penetrating Radar

Glacier mass balance is more sensitive to warming than cooling, but feedbacks related to the exposure of previously buried firn and ice in very warm years is not generally considered in sensitivity studies. A ground‐penetrating radar survey in the accumulation area of Rolleston Glacier, New Zealand shows that five years of previous net accumulation was removed by melt from parts of the glacier above the long‐term equilibrium line altitude during a single negative mass balance year. Rolleston Glacier receives a large amount of accumulation from snow avalanches, which may temporarily buffer it from climate warming by providing additional mass that has accumulated at higher elevations, effectively increasing the elevation range of the glacier. However, glaciers reliant on avalanche input may have high sensitivity to climatic variations because the extra mass is concentrated on a small part of the glacier, and small variations in avalanche input could have a large impact on overall glacier accumulation. Further research is needed to better estimate the amount and spatial distribution of accumulation by avalanche in order to quantify the climate sensitivity of small avalanche‐fed glaciers.     

Glaciers,rock avalanches and the ‘buzzsaw’ in cirque development: Why mountain cirques are of mainly glacial origin

It has been proposed that most cirques are source-area depressions of large, deep-seated rock-slope failures. Yet the close relation between cirques and climate is convincing evidence of the dominance of glacial erosion, rather than rock-slope failure, in mountain cirque development and distribution. Cirque floor altitudes have a lower limit that varies with snowfall by 1000 m or more between windward and leeward sides of mountain systems. Glaciation levels and equilibrium line altitudes implied by cirques vary in parallel with those for modern glaciers. Cirques are often found mainly on the poleward or leeward slopes of individual mountain ranges, as are modern small glaciers (because of solar radiation and wind effects on ablation and accumulation). Most rock-slope failures (RSFs: rock slides, rock avalanches and gravitational deformations) do not involve the deep-seated rotational movement that would produce a cirque form. Although some deep-seated RSFs with arcuate head scars may be confused with cirques, identification as a glacial cirque is more confident as the floor is longer, wider and more gently sloping. Some scars from major RSFs may resemble poor or moderately developed cirques, but tend to have steeper floors, to be more scattered and closely related to geology, whereas glacial cirques develop on all rock types. Deep-seated RSFs high on slopes can be associated with seismic shaking, but cirques develop without relation to seismicity. Degree of cirque development can be related to duration of exposure to glaciation. Often RSFs are found adjacent to cirques, or in glacial transfluences; only a proportion are well situated to develop into glacial cirques. Valley-head cirques are continued down-valley by glacial troughs. The ‘overdeepening’ (rock basins with reversed slopes) found in a large minority of cirques is not due to rock avalanching, fluvial or periglacial erosion. The RSF proposal should therefore be rejected in favour of the traditional glacial explanation, without any nivation stage being necessary. Rock-slope failure is one of several possible ways of initiating hollows for glacier accumulation, as well as an ancillary process of cirque extension or widening through collapse of glacially oversteepened slopes. Headward extension of adjacent cirques on a ridge leads to displacement of the divide, sometimes by 2 km or more, lowering ridge and summit altitudes and producing the ‘glacial buzzsaw’ effect. Where a relatively lower snowline has led to cirque erosion on all sides of a mountain, cirque intersection lowers summits further. The buzzsaw hypothesis is not applicable, however, where remnants of a preglacial summit surface survive. © 2020 John Wiley & Sons, Ltd.     

Monitoring considerations for a dynamic dune restoration project: Pacific Rim National Park Reserve,British Columbia,Canada.

Historically, management of coastal dune systems has often involved artificial stabilization of active sand surfaces in order for coastal areas to be more easily controlled and modified for human benefit. In North America, the introduction of invasive grasses, namely European and American beach (marram) grasses (Ammophila spp.) has been one of the most successful strategies used for stabilizing active coastal dune sands. Recent research has demonstrated, however, that stabilization of coastal dunes often leads to reduced landform complexity and resilience, as well as declines in species diversity. More ‘dynamic’ restoration efforts have emerged over the past 20 years that encourage dune mobility and aeolian activity in order to provide a more resilient biogeomorphic system. In North America, there is generally little research relating restoration methods and outcomes to geomorphic responses despite the fundamental importance of sedimentary processes and dune morphodynamics in broader ecosystem function. This paper aims to better situate dynamic dune restoration within current geomorphic understanding. A brief review of key terms and concepts used in the emerging field of dynamic dune restoration is provided and expanded upon with respect to geomorphologic considerations. A case study of a recent dynamic restoration effort in Pacific Rim National Park Reserve, British Columbia, Canada is provide to demonstrate how these concepts are applied. Introduction of European marram at this site, coupled with a warming climate and increased precipitation in recent decades at this site, is thought to be associated with a rapid decline in aeolian activity, system stabilization and accelerated ecological succession. Preliminary results on the response of the dune system to mechanical removal of Ammophila are presented to provide the foundation for a research framework to guide the broader restoration project. Recommendations for improving treatment methodologies and monitoring protocols are provided to aid future restoration projects of this nature. Copyright © 2013 John Wiley & Sons, Ltd.     

Transient small boats as a long-distance coastal vector for dispersal of biofouling organisms

Alaska is at the northern end of an apparent latitudinal trend of decreasing coastal marine introductions on the West Coast of North America. Historical propagule supply may have played a role in forming this trend, but few studies have evaluated propagule supply to northern latitudes. Here, we examined the role of small boat traffic as a mechanism of long-distance spread for nonindigenous species (NIS) into coastal Alaska. We used a combination of public records, marina surveys, and boater interviews to characterize vessel traffic patterns and boater behaviors. In-water SCUBA sampling of recently arrived transient boats provided data on extent, richness, composition, and biogeography of biofouling incursions to Alaska from outside of the state. We documented a striking seasonality and directionality of vessel traffic, and most vessels were on voyages of >900 km. Most transient vessels sampled had few organisms, although one third had >100 organisms on submerged surfaces. Several NIS were recorded, including two that are not known to be established in Alaska (Watersipora subtorquata and Amphibalanus improvisus). The seasonal northward pulse of vessels and their cumulative biofouling species represent an important incursion mechanism for species yet to establish at the northern edge of a marine bioinvasion front in the northeastern Pacific. The low numbers of NIS sampled in this study coincide with the low number of marine NIS known from Alaska, which suggests that an opportunity remains to promote awareness and management of the vector to limit NIS influx to the region. This may be particularly relevant for future scenarios of increased vessel traffic and ocean warming, which are likely to interact to increase establishment success of invaders from the south.     

A depositional model for hydromagnesite–magnesite playas near Atlin,British Columbia,Canada

This study formulates a comprehensive depositional model for hydromagnesite–magnesite playas. Mineralogical, isotopic and hydrogeochemical data are coupled with electron microscopy and field observations of the hydromagnesite–magnesite playas near Atlin, British Columbia, Canada. Four surface environments are recognized: wetlands, grasslands, localized mounds (metre‐scale) and amalgamated mounds composed primarily of hydromagnesite [Mg₅(CO₃)₄(OH)₂·4H₂O], which are interpreted to represent stages in playa genesis. Water chemistry, precipitation kinetics and depositional environment are primary controls on sediment mineralogy. At depth (average ≈ 2 m), Ca–Mg‐carbonate sediments overlay early Holocene glaciolacustrine sediments indicating deposition within a lake post‐deglaciation. This mineralogical change corresponds to a shift from siliciclastic to chemical carbonate deposition as the supply of fresh surface water (for example, glacier meltwater) ceased and was replaced by alkaline groundwater. Weathering of ultramafic bedrock in the region produces Mg–HCO₃ groundwater that concentrates by evaporation upon discharging into closed basins, occupied by the playas. An uppermost unit of Mg‐carbonate sediments (hydromagnesite mounds) overlies the Ca–Mg‐carbonate sediments. This second mineralogical shift corresponds to a change in the depositional environment from subaqueous to subaerial, occurring once sediments ‘emerged’ from the water surface. Capillary action and evaporation draw Mg–HCO₃ water up towards the ground surface, precipitating Mg‐carbonate minerals. Evaporation at the water table causes precipitation of lansfordite [MgCO₃·5H₂O] which partially cements pre‐existing sediments forming a hardpan. As carbonate deposition continues, the weight of the overlying sediments causes compaction and minor lateral movement of the mounds leading to amalgamation of localized mounds. Radiocarbon dating of buried vegetation at the Ca–Mg‐carbonate boundary indicates that there has been ca 8000 years of continuous Mg‐carbonate deposition at a rate of 0·4 mm yr^?1. The depositional model accounts for the many sedimentological, mineralogical and geochemical processes that occur in the four surface environments; elucidating past and present carbonate deposition.