New GSA Bulletin Articles Published Ahead of Print in May
Boulder, Colo., USA: The Geological Society of America regularly publishes
articles online ahead of print. GSA Bulletin topics studied this
month include the nature and dynamics of China and Tibet. Other locations
include the Colorado Plateau, New Mexico, Arizona, the Arctic, and the
Pampean Plains of Argentina, and the Lake Superior region. You can find
these articles at
https://bulletin.geoscienceworld.org/content/early/recent
.
Sedimentology of the latest Permian to Early Triassic in the
terrestrial settings of the North China Basin: Low-latitude climate
change during a warming-driven crisis
Kaixuan Ji; Paul B. Wignall; Jinnan Tong; Yingyue Yu; Wenwei Guo ...
Abstract:
In terrestrial settings, the Permo-Triassic mass extinction is commonly
linked to major changes in sedimentological and climatic conditions that
include a switch from meandering to braided fluvial systems and increased
aridity. We examined the predominantly terrestrial strata of North China to
reveal that, on the contrary, there was little substantial sedimentological
change during the Permo-Triassic mass extinction. The crisis level occurs
in the upper Sunjiagou Formation, where a range of environments, including
low-sinuosity rivers, distally terminating alluvial fans, and mud-dominated
coastal plains are recorded. A major sedimentological change occurred
slightly higher, within the lowest Triassic, and involved a switch to
braided fluvial and shallow lacustrine/fluviodeltaic conditions of the
Liujiagou Formation. Later in the Early Triassic, fine-grained, playa lake,
and alluvial plain facies became widespread (Heshanggou Formation).
Paleosols are present throughout the basin fill and record a transition
from Aridisols and Vertisols in the Late Permian to Inceptisols in the
Early Triassic followed by gleyed Aridisols, Vertisols, and Inceptisols
later in the Early Triassic. Previously reported aeolian facies from Lower
Triassic strata were not encountered in this study and were unlikely to
have occurred given the prevalence of lacustrine and fluvial conditions at
this time. Overall, the North China Basin experienced an increase in
seasonal rainfall in the latest Permian and a base-level rise that
introduced marine conditions into the southwest of the basin. During the
Early Triassic, seasonal or interannual rainfall was frequent, with strong
evaporation in a hot climate. Overall, substantial sedimentological changes
occurred in the terrestrial Permo-Triassic environments of North China, but
despite earlier claims from studies in North China and elsewhere, there was
no abrupt transition in fluvial styles around the Permo-Triassic mass
extinction.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36260.1/614067/Sedimentology-of-the-latest-Permian-to-Early
Detached structural model of the Keweenaw fault system, Lake Superior
region, North America: Implications for its origin and relationship to
the Midcontinent Rift System
James M. DeGraff; Brad T. Carter
Abstract:
The Keweenaw fault system along Lake Superior’s south shore in Michigan,
USA is one of the most significant fault systems associated with North
America’s Midcontinent Rift System. Reverse slip has thrust Portage Lake
Volcanics (ca. 1.1 Ga) southeastward over Jacobsville Sandstone (ca. 1.0
Ga). Growing consensus in the 1970s about a major rift beneath Lake
Superior led to the idea that reverse movement on the Keweenaw fault
involved inversion of a rift-bounding normal fault. To test this idea, we
integrated bedrock geology maps, subsurface mining data, and geophysical
data to build a well-constrained cross-section from the Minnesota,
USA-Ontario, Canada border to central Upper Michigan. The cross-section
shows the Keweenaw and Hancock faults to be parts of a reverse, listric
system detached deep within the layered volcanic section. Correlation of
offshore seismic units with onshore stratigraphic units defines a gradual
change in stratal dip from 55° NW near the faults to horizontal near the
center of Lake Superior. This stratal geometry implies that onshore listric
fault geometry continues offshore and that the fault system soles into a
basal detachment at 12−15 km depth. Absence of second-order folds northwest
of the peninsula indicates that the fault system does not ramp downward
from the basal detachment through the lower crust. The observations and
model preclude ideas that the Keweenaw fault was a rift-bounding normal
fault inverted by post-rift crustal shortening, or that it formed by upward
propagation of a deeper normal fault undergoing inversion. Instead, our
results imply that the fault initiated as a detached thrust in relatively
brittle upper crust and that shortening of relatively ductile lower crust
occurred on other structures.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36186.1/614023/Detached-structural-model-of-the-Keweenaw-fault
Terrestrial paleoclimate transition associated with continental
weathering and drift during the Aptian−Albian of East Asia
Jianliang Jia; Changsheng Miao; Wenquan Xie
Abstract:
Reconstruction of Aptian−Albian paleoclimate obtained from marine records
remains a challenging topic, but studies on coeval terrestrial paleoclimate
and trigger mechanisms have lagged substantially. In this study, new
multiproxy data from mudrocks in the Fuxin Basin of NE China provide a
high-resolution terrestrial climate record from East Asia. Here, we
demonstrate the occurrence of terrestrial climate cooling during the late
Aptian (118−113 Ma), which interrupted the mid-Cretaceous warming shown in
global records. Nearly uniform long-term global climate trends attributable
to tectonism, volcanism, and weathering occur in Early Cretaceous
terrestrial and marine records. In the Fuxin Basin, the long-term
terrestrial climate was characterized by increasing temperatures during the
late early Aptian, gradual cooling during the late Aptian, and subsequent
enhanced warming during the early Albian. Moreover, chemical weathering and
humidity during these intervals were low, moderate to high, and then
moderate, respectively. A markedly reduced high-elevation
paleogeomorphology under strong continental weathering during the late
Aptian increased the variability in chemical weathering fluxes as the
Eurasian plate in NE China drifted SE during the Early Cretaceous and then
NE during the Late Cretaceous. We suggest that a combination of enhanced
continental weathering and weakened plate drift induced changes in
atmospheric CO2, while the geographic setting ultimately led to
cooling in the Fuxin Basin during the late Aptian. Our results illustrate
the importance of exploring long-term tectonic-climatic-biotic feedbacks to
improve our understanding of tectonic processes and ecological transitions
across various spatiotemporal scales.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36253.1/614024/Terrestrial-paleoclimate-transition-associated
Environmental conditions and mechanisms restricting microbial
methanogenesis in the Miquan region of the southern Junggar Basin, NW
China
Haijiao Fu; Yueguo Li; Xianbo Su; Detian Yan; Shuguang Yang ...
Abstract:
Early microbial gas has been sealed in reservoirs in the Miquan region of
NW China, with little or no supply of current microbial gas. To date, the
environmental conditions and mechanisms restricting microbial
methanogenesis are still unclear in the Miquan region. Thus, in this study,
a series of gas and water samples from coalbed methane (CBM) exploitation
wells and in situ coal samples were collected and analyzed to determine the
potential for current microbial gas generation, the methanogenic pathways,
the source of nutrients, the influence of the environmental conditions on
in situ microbial communities and their methanogenesis, and the mechanisms
restricting microbial methanogenesis. The gas-production simulation
experiments revealed that the existing microbes in the coalbed water were
less efficient at converting coal into methane under the approximate in
situ conditions, which further verified that there was little or no supply
of current microbial gas. The stable isotope compositions of the gas
samples suggested that carbon dioxide (CO2) reduction was the
dominant metabolic pathway for generating CBM, whereas the methanogenic
communities contained a mixture of acetoclastic and methylotrophic
methanogens in local areas. The nutrients available for the microbes mainly
included the total dissolved nitrogen (TDN) and total dissolved carbon
(TDOC), and the in situ dissolution of the coals was a significant source
of the TDN, whereas the TDOC was mainly supplied by surface water. The
microbes in the different tectonic settings were significantly controlled
by different combinations of environmental factors, and there was no single
environmental factor that completely dominated the spatial variability of
the microbial communities. The gradual stagnation of the water environment
led to an increase in salinity and a decrease in nutrients, which were
likely the main factors restricting microbial methanogenesis under in situ
conditions. Combined with the results of the rate-limiting stages of the
anaerobic fermentation, the mechanisms restricting microbial methanogenesis
can be finally determined in the Miquan region. These discoveries presented
in this case study provide a significant supplement to the geological
theory of CBM accumulation, and have a guiding significance for CBM
development in the Miquan region.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36272.1/614025/Environmental-conditions-and-mechanisms
Diatom evidence for a groundwater divide that limited the extent of
Lake Estancia, New Mexico, USA, highstands during the Last Glacial
Maximum
Kirsten M. Menking; Rebecca J. Bixby; Savannah M. Cutler
Abstract:
A high-resolution time series of diatoms from the Estancia Basin of central
New Mexico, USA, reveals decadal to millennial changes in water chemistry
during the Last Glacial Maximum (LGM) and suggests that groundwater leakage
limited the size of Lake Estancia highstands. Previous work showed that the
lake expanded at least five times during the LGM, with each highstand
reaching the same elevation; numerical modeling experiments showed that
highstands required increases in precipitation of 1.5−2 times that of
modern times but also demonstrated that these conditions could have lasted
for only a few decades or the lake would have overflowed the basin’s sill.
The fact that the lake returned to the same elevation suggests that
highstands were climatologically similar. Here we present an alternative
scenario, that once the lake reached a particular threshold volume,
groundwater leakage prevented further expansion, which means that
precipitation increases may have been larger than previously thought. The
LGM diatom sequence is dominated by freshwater planktonic species in the Lindavia ocellata complex, the freshwater to brackish littoral
species Pseudostaurosira brevistriata, and the brackish to saline
benthic species Diploneis cf. smithii, all of which show
decadal to centennial oscillations that were likely driven by solar cycles.
Canonical correspondence analyses using mineralogical time series as
proxies for wetter and drier conditions reveal that diatom assemblages
varied with salinity and pH/alkalinity and suggest that groundwater outflow
led to variable water chemistry for different highstands of the same
magnitude. Nonmetric multidimensional scaling confirms the uniqueness of
each highstand in terms of the diatom assemblages. The diatom data
presented here are among only a handful of such records for Pleistocene
pluvial lakes in the American West and reveal that saline alkaline lakes
can yield important information despite valve preservation issues.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36283.1/613858/Diatom-evidence-for-a-groundwater-divide-that
Detrital zircon provenance and transport pathways of
Pleistocene-Holocene eolian sediment in the Pampean Plains, Argentina
Austin Bruner; Andrew L. Leier; David L. Barbeau Jr.; Alex Pullen; Mary
Kate Fidler ...
Abstract:
The Pampas of Argentina contain a broad distribution of Pleistocene to
Holocene loessic sediments and eolian dune deposits. Models describing the
sediment provenance of this eolian system have, at times, conflicted. We
address the provenance of these deposits through U-Pb detrital-zircon
geochronology. Our results indicate broad similarity in age distributions
between samples, with a dominant Permian-Triassic mode, and widespread but
lesser Cenozoic, Devonian-Mississippian, Ediacaran-Cambrian, and
Mesoproterozoic modes. These data are inconsistent with a large
contribution of detritus from Patagonia as previously suggested. These data
are consistent with very limited contribution of first cycle volcanogenic
zircon to the Pampean eolian system, but abundances of older Neogene zircon
indicate proto-sources in the Andes. The ríos Desaguadero, Colorado, and
Negro contain populations that were likely within the dust production
pathways of most of the loess, paleosol, and eolian dune deposits, but the
derivation of the zircon ages in these sediments cannot be explained solely
by these river systems. One statistical outlier, a loess sample from the
Atlantic coast of the Pampa region, indicates quantitative similarity to
the age spectra from the ríos Colorado and Negro, consistent with
derivation from these subparallel rivers systems during subaerial exposure
of the continental shelf under high global ice-volume. Another statistical
outlier, a paleosol sample from the Río Paraná delta region, has zircon
ages more closely associated with sediments in the Paraná region than in
rivers south of the Pampa region. Collectively, these data point to the
complexity of the Pampean eolian system and substantial spatial-temporal
variation in this Pleistocene−Holocene eolian system.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36267.1/613859/Detrital-zircon-provenance-and-transport-pathways
Late Cretaceous time-transgressive onset of Laramide arch exhumation
and basin subsidence across northern Arizona−New Mexico, USA, and the
role of a dehydrating Farallon flat slab
Jacob O. Thacker; Karl E. Karlstrom; Shari A. Kelley; Ryan S. Crow; Jerry
J. Kendall
Abstract:
Spatiotemporal constraints for Late Cretaceous tectonism across the
Colorado Plateau and southern Rocky Mountains (northern Arizona−New Mexico,
USA) are interpreted in regards to Laramide orogenic mechanisms. Onset of
Laramide arch development is estimated from cooling recorded in
representative thermochronologic samples in a three-step process of initial
forward models, secondary HeFTy inverse models with informed constraint
boxes, and a custom script to statistically estimate timing of rapid
cooling from inverse model results. Onset of Laramide basin development is
interpreted from increased rates of tectonic subsidence. Onset estimates
are compared to published estimates for Laramide timing, and together
suggest tectonism commenced ca. 90 Ma in northwestern Arizona and
progressed eastward with later onset in north-central New Mexico by ca.
75−70 Ma. The interpreted sweep of onset progressed at a rate of ∼50
km/m.y. and was approximately half the 100−150 km/m.y. rate estimated for
Late Cretaceous Farallon-North America convergence during the same
timeframe. Previous suggestions that the Laramide tectonic front progressed
at a rate similar to convergence via basal traction are not supported by
our results. We thereby suggest that (1) a plate margin end load
established far field compression and that (2) sequential Laramide-style
strain was facilitated by progressive weakening of North American
lithosphere from the dehydrating Farallon flat slab. Results are compared
to models of sweeping tectonism and magmatism in other parts of the
Laramide foreland. Discussions of the utility of the custom script and the
potential for stratigraphic constraints to represent only minimum onset
estimates are also presented.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36245.1/613740/Late-Cretaceous-time-transgressive-onset-of
Cenozoic deformation in the eastern domain of the North Qaidam thrust
belt, northern Tibetan Plateau
Bing Li; Yongchao Wang; Andrew V. Zuza; Xuanhua Chen; Zhaogang Shao ...
Abstract:
The present topography of the northern Tibetan Plateau is characterized by
the northwest-trending Eastern Kunlun Range, Qaidam Basin, and Qilian Shan,
which figure importantly into the evolution and mechanism of Tibetan
plateau development during Cenozoic Indo-Asian convergence. Understanding
the Cenozoic deformation history and the source-to-sink relationship
through time has significant implications for deciphering the growth
history of the northern Tibetan Plateau. Despite decades of study, the
timing, pattern, and mechanisms of deformation across the northern Tibetan
Plateau are still vigorously debated. The North Qaidam thrust belt, located
between the Qaidam Basin and Qilian Shan thrust belt, provides a valuable
record of Cenozoic deformation in the northern Tibetan Plateau. Here, we
present the results of new geologic mapping, structural and sedimentology
analysis, and apatite fission track thermochronology to constrain the
Cenozoic evolution history and reconstruct the paleogeomorphology of the
eastern domain of the North Qaidam thrust belt and its foreland, the Wulan
Basin. Our analyses reveal the North Qaidam thrust belt experienced
multi-phase exhumation since the Cretaceous. A period of Eocene localized
thrust-related uplift of the North Qaidam thrust belt initiated shortly
after India-Asia collision, and lower erosion rates in the Oligocene
allowed the thrust belt to expand along-strike eastward. Local uplift shed
sediments to the southwest, directly into the Qaidam Basin. Reactivation of
the proximal thrust faults and initiation of the northwest-striking
right-slip Elashan fault at ca. 15−10 Ma drove the final accelerated
mid-Miocene cooling and denudation to the surface. This phase of
deformation established the overall framework morphology of the
northeastern margin of the Tibetan Plateau, including the overall structure
of the basins and ranges.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36215.1/613631/Cenozoic-deformation-in-the-eastern-domain-of-the
Ediacaran magmatism and rifting along the northern margin of the Tarim
craton: Implications for the late Neoproterozoic Rodinia configuration
and breakup
Hong-Xiang Wu; Yildirim Dilek; Feng-Qi Zhang; Han-Lin Chen; Huan Chen ...
Abstract:
The Tarim craton in modern Central Asia was an important component of the
supercontinent Rodinia in the Neoproterozoic, although its paleogeography
in Rodinia during that era is still controversial. Here, we present new
stratigraphic, geochemical, and geochronological data from the
Neoproterozoic sedimentary and volcanic rock successions along the
northwestern margin of the Tarim craton and discuss the significance of
these data and our interpretations for its tectonic evolution and
paleogeographic position within Rodinia. The Lower Ediacaran sedimentary
sequence (Sugetbrak Formation) in northwest Tarim includes terrestrial and
shallow-marine clastic rocks intercalated with two discrete basaltic lava
flows near the top. The Upper Ediacaran sedimentary sequence conformably
overlying the volcanic and clastic rocks consists mainly of stromatolitic
dolomite (Chigebrak Formation), representing a transgressive shallow-marine
environment. Previous U-Pb zircon dating of the basaltic lava flows has
constrained the timing of their eruption in the early Ediacaran (615 Ma).
Detrital zircon U-Pb dating of a feldspar-quartz-sandstone unit situated
between the two lava flows revealed an oldest age of 2517 ± 18 Ma and a
youngest age of 612 ± 6 Ma, with a majority of zircon grains (n =
42) dated at 891−754 Ma (Tonian). A quartz-sandstone unit above the upper
lava flow revealed an oldest age of 2724 ± 15 Ma and a youngest age of 607
± 8 Ma, with a missing age group of 891−800 Ma. These data and observations
indicate: (1) a major switch in the depositional setting from a terrestrial
(synrifting) to shallow-marine environment following the eruption of the
upper lava unit; and (2) an abrupt disappearance of the source rocks of the
891−800 Ma zircons and sediments from the provenance of the post-615 Ma
(postrifting) sedimentary sequence. The basaltic rocks have low SiO2 and MgO but high total Fe2O3 and TiO 2 contents (2.34−3.19 wt%), analogous to high-Ti basalts and
continental flood basalts. Their Ti/V ratios (65−88), low Th/Nb ratios
(∼0.1), and high TiO2/Yb ratios (∼1.1) are similar to those of
ocean-island basalt (OIB). Combined with their Sm/Yb and La/Sm ratios and
Sr-Nd-Pb-Hf isotope values, we infer that magmas of the Sugetbrak basalts
were likely derived from partial melting of an enriched mantle source (EM
I) in a transitional spinel-garnet lherzolite field. This petrogenetic
evolution was a result of mantle plume−influenced rift magmatism during the
Ediacaran breakup of Rodinia. The Central Tianshan terrane, which was
attached to the northern Tarim craton until the Ediacaran Period, was the
provenance of 891−800 Ma (Tonian) zircons in the synrift sedimentary
succession. As the Central Tianshan terrane broke away from the Tarim
craton after ca. 615 Ma, Tonian-aged zircons were no longer available to
the depocenter of the postrift sedimentary sequence. The transition from
rifting to drifting between the Tarim craton and the Central Tianshan
terrane marked the final breakup of Rodinia, a global event that was
possibly driven by mantle plume activities coeval with the development of
the Central Iapetus magmatic province in Laurentia during the Ediacaran.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36305.1/613632/Ediacaran-magmatism-and-rifting-along-the-northern
Featured Neoarchean granitoid association in the central North China
Craton: An indicator of warm plate subduction
Guozheng Sun; Yalu Hu; Shuwen Liu; Sanzhong Li; Jinghao Fu ...
Abstract:
Diverse Neoarchean granitoid assemblages, which generally include
tonalites−trondhjemites−granodiorites (TTGs) and various K-rich granitoids,
are prevalent in most basement terranes of the North China Craton. However,
the Hengshan terrane is an exceptional case in the North China Craton; it
is dominated by late Neoarchean sodic diorite-TTGs (DTTGs) and sanukitoids.
These sanukitoids are the only high-K granitoids and show Mg-rich chemical
features. The late Neoarchean DTTGs and sanukitoids were generated at ca.
2486−2537 Ma and show an intimate spatial association. The granitoid
assemblages of the DTTGs and sanukitoids are characterized by high Mg# [100
× Mg/(Mg + Fetotal)] values (43−65) and enriched in light rare
earth elements, large ion lithophile elements, heterogeneous zircon Lu−Hf
(εHf = −1.6 to +7.4), and whole-rock Sm−Nd (εNd = +0.9 to +4.2) isotopic
components, which indicates that they may be derived from varying degrees
of interactions between mantle peridotite and subduction-related materials.
Combined with the relatively high apparent geothermal gradient (∼17 ± 2
°C/km) and the relatively low basal heat flow of continental crust (∼25 ± 5
mW m−2), the crust-mantle interaction process indicates that the
occurrence of late Neoarchean high-Mg magmatism was closely related to warm
oceanic slab subduction in the Hengshan terrane, and the featured
lithological association of DTTGs and sanukitoids most likely developed in
the active continental margin at the end of the Archean.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36410.1/613633/Featured-Neoarchean-granitoid-association-in-the
Geochemical evidence for incorporation of subducting sediment-derived
melts into the mantle source of Paleozoic high-Mg andesites from
northwestern Tianshan in western China
Li-Tao Ma; Li-Qun Dai; Yong-Fei Zheng; Zi-Fu Zhao; Wei Fang ...
Abstract:
Although high-Mg andesites (HMA) have attracted increasing attention due to
their unique geochemical composition and important geological significance,
there is no consensus on their petrogenesis. The present study indicates
that the subducting terrigenous sediment-derived hydrous melts were
incorporated into the mantle source of Paleozoic HMA in northwestern
Tianshan, western China. These HMA are composed of basaltic andesite and
andesite. They generally exhibit arc-type trace element distribution
patterns and weakly enriched Sr-Nd-Hf isotope compositions. Some of them
show remarkably higher Ba/La, Ba/Th, Rb/Nb, and U/Th ratios than normal
mid-ocean ridge basalts (MORB). Furthermore, most of them are characterized
by higher Th/Nb, Th/Yb, and Th/Nd ratios but lower Nb/U ratios relative to
normal MORB, similar to those of terrigenous sediments. Whole-rock Nd
isotopes covary with Rb/Nb and Th/Yb ratios, indicating contributions from
both oceanic crust-derived aqueous solutions and terrigenous
sediment-derived hydrous melts. Together with their high zircon δ 18O values, it appears that the mantle source of the target HMA
contains terrigenous sediment-derived hydrous melts in addition to
subducting oceanic crust-derived aqueous solutions. In the studied HMA, the
andesite generally exhibits higher contents of large-ion lithophile
elements and light rare earth elements and more enrichments in Sr-Nd-Hf
isotopes than the basaltic andesite. These differences indicate that the
mantle source of andesite would contain more subducting sediment-derived
hydrous melts than that of basaltic andesite. As a consequence, relatively
Si-rich and Si-poor pyroxenite sources were respectively generated as the
mantle sources of the andesite and basaltic andesite. This qualitative
interpretation is verified by quantitative modeling of the geochemical
transfer from subducting oceanic crust into the mantle wedge. Model
calculations indicate that the addition of ∼3% oceanic crust-derived
aqueous solutions and 4%−12% terrigenous sediment-derived hydrous melts
into the mantle wedge peridotite can account for the geochemical
compositions of the target HMA. Therefore, the HMA in northwestern Tianshan
provide the geochemical evidence for the crust-mantle interaction during
the oceanic subduction in the Paleozoic. As such, the subducting
terrigenous sediment-derived hydrous melts play a dominant role in the
composition of the lithochemically fertile, geochemically enriched mantle
sources and thus in the origin of HMA above oceanic subduction zones.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36341.1/613634/Geochemical-evidence-for-incorporation-of
Correlation between South China and India and development of double
rift systems in the South China−India Duo during late Neoproterozoic
time
Bingbing Liu; Touping Peng; Weiming Fan; Guochun Zhao; Jianfeng Gao ...
Abstract:
South China, India, and their derivative blocks preserve many similar
magmatic and sedimentary records related to the tectonic transition from
Rodinia to Gondwana. They provide crucial insights into not only the
paleogeographic correlation between them but also the geodynamic mechanism
for such a transition. Our new results, combined with published data from
these blocks, reveal that South China remained linked with India at least
from ca. 830 Ma to ca. 510 Ma and formed the South China−India Duo, which
is located at the western margin of Rodinia. The identical magmatism and
sedimentation reflect that double late Neoproterozoic rift systems in the
South China−India Duo developed owing to the rollback of subducting oceanic
slab beneath them. For example, an intracontinental rift developed along
the Jiangnan−Aravalli−Delhi fold belt, which separated the Yangtze-Marwar
block from the Cathaysia-Bundelkhand block. Another intra-arc rift
developed contemporaneously along the northern and western margins of the
Yangtze block, through the Marwar terrane of western India, and then into
the Seychelles and Madagascar terranes. Such an intra-arc rift is the most
feasible explanation for the common development of coeval arc-like and
extension-related magmatic rocks and extensional sedimentary sequences on
the western margin of the South China−India Duo, in Seychelles and
Madagascar, and even at other subduction zones. South China was finally
separated from Indian Gondwana at ca. 510 Ma due to the opening of the
Proto-Tethys Ocean.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36397.1/613635/Correlation-between-South-China-and-India-and
Fingerprinting the metal source and cycling of the world’s largest
antimony deposit in Xikuangshan, China
Zheng-Yu Long; Kun-Feng Qiu; M. Santosh; Hao-Cheng Yu; Xiang-Yong Jiang ...
Abstract:
The Xikuangshan antimony (Sb) deposit is the largest Sb deposit in the
world; however, the metal source and cycling and a holistic understanding
of the deposit genesis remain equivocal. Sulfur isotope signatures offer a
means of fingerprinting different sources in a hydrothermal deposit,
although one must be careful to rule out subsequent isotope fractionation
during fluid ascent, mixing, and ore precipitation. Here, we investigated
the sulfur isotope composition of stibnite occurring at depth in the
Xikuangshan deposit to distinguish the isotopic signals from the source and
the superimposed imprint from near-surface−derived sulfur mixing or
isotopic fractionation. All stibnites from the deep orebodies displayed
limited δ34S variation from +6.8‰ to +8.4‰, despite their widely
varying depths. These results provide direct evidence that δ34S
values measured in deep orebodies are representative of the isotopic
composition of initial fluids. The most likely factor controlling the
variation of the sulfur isotopes in shallow stibnites (+3.5‰ to +16.3‰) is
a series of hydrothermal processes, including Rayleigh fractionation during
ore precipitation, fluid boiling induced by pressure release, and/or local
input of pyrite from wall rocks via fluid-rock interaction. Accordingly, we
conclude that the Neoproterozoic basement served as the metal source. We
propose a holistic genetic model wherein we envisage that Sb and S were
leached from the basement rocks, and the ore-bearing fluids ascended along
a deep fault and eventually precipitated beneath the Devonian shale cap.
Thus, sulfur isotopic systematics represent a powerful repository for
interrogating the metal source and cycling in the hydrothermal ore system.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36377.1/613531/Fingerprinting-the-metal-source-and-cycling-of-the
Strontium isotope stratigraphy and LA-ICP-MS U-Pb carbonate age
constraints on the Cenozoic tectonic evolution of the southern South
China Sea
Gang Li; Weihai Xu; Yun Luo; Jianguo Liu; Jianxin Zhao ...
Abstract:
The tectonic evolution of the South China Sea is closely associated with
multiple subduction-collision processes in Southeast Asia. When the
collision of the Dangerous Grounds terrane with Sabah-Palawan Islands
terminated is debated due to poor age constraints at the southern margin of
South China Sea. A deep well drilled on Meiji Atoll penetrates Cenozoic
carbonate strata in central Dangerous Grounds. Robust strontium isotope
ages and laser ablation−inductively coupled plasma−mass spectrometry
(LA-ICP-MS) U-Pb dates provide critical chronological constraints on the
Cenozoic evolution of the southern South China Sea. A middle Miocene hiatus
spanning 9 m.y. on Meiji Atoll is thought to be mainly caused by tectonic
uplift in the central Dangerous Grounds. The uplift in the central
Dangerous Grounds was accompanied by under-thrusting beneath the southern
Palawan margin and orogenic uplifting in north Borneo during the middle
Miocene. Data interpretation indicates an active collision in the southern
South China Sea during the middle Miocene. The regrowth of the Meiji Atoll
above the middle Miocene hiatus represents the end of this collision event
in the southern South China Sea at ca. 11 Ma, after the cessation of
seafloor spreading, which occurred at ca. 15 Ma.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36365.1/613532/Strontium-isotope-stratigraphy-and-LA-ICP-MS-U-Pb
Compositional and metamorphic controls on tectonic erosion along a
continental subduction-collision zone: Implications from mafic
granulites in the northern Sulu orogen
Lishuang Liu; Fulai Liu; Matthew J. Kohn; Jinghui Guo
Abstract:
Subduction erosion has been reported recently in continental
subduction-collision zones, and the response of the upper plate remains
enigmatic. The Dabie-Sulu orogen is considered to have formed by deep
northward subduction of the Yangtze block (lower plate) beneath the North
China block (upper plate). However, within the northern Sulu
ultrahigh-pressure (UHP) belt, the Haiyangsuo complex has intriguingly been
accepted as Neoarchean−Paleoproterozoic metamorphic basement from the North
China block, implying subduction-erosion could have occurred. Here, we
examined new petrographic, mineral chemical, and published geochronological
data from the Haiyangsuo mafic granulites to decipher their multiphase
metamorphic evolution. Pressure-temperature (P-T) estimates from
pseudosection modeling and geothermobarometry show two major metamorphic
events: a medium-pressure granulite-facies event (7.3−8.3 kbar and 830−895
°C) and a later high-pressure granulite-facies event (12.2−16.6 kbar and
800−875 °C). Geochronology and mineral geochemistry link these events to
Paleoproterozoic and Triassic metamorphism, respectively. Peak Triassic
high-pressure granulite-facies metamorphism was followed by near-isothermal
decompression and then near-isobaric cooling. These rocks derive from the
upper plate and were metamorphosed at the same time as UHP eclogites, but
maximum pressures were substantially lower, and P-T paths did not
converge until late-stage exhumation. Evidently, metamorphic basement from
the North China block was dragged to lower-crustal to upper-mantle depths
(50−60 km) while that from the Yangtze block was subducted deeply, and then
they were tectonically juxtaposed during exhumation. These data corroborate
thermal-mechanical models that predict tectonic erosion as a major process
during continental subduction and collision.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36287.1/613511/Compositional-and-metamorphic-controls-on-tectonic
Spatially averaged stratigraphic data to inform watershed sediment
routing: An example from the Mid-Atlantic United States
J.E. Pizzuto; K.J. Skalak; A. Benthem; S.A. Mahan; M. Sherif ...
Abstract:
New and previously published stratigraphic data define Holocene to present
sediment storage time scales for Mid-Atlantic river corridors. Empirical
distributions of deposit ages and thicknesses were randomly sampled to
create synthetic age-depth records. Deposits predating European settlement
accumulated at a (median) rate of 0.06 cm yr−1, range from
∼18,000 to 225 yr old, and represent 39% (median) of the total
accumulation. Sediments deposited from 1750 to 1950 (“legacy sediments”)
accumulated at a (median) rate of 0.39 cm yr−1 and comprise 47%
(median) of the total, while “modern sediments” (1950−present) represent
11% of the total and accumulated at a (median) rate of 0.25 cm yr −1. Synthetic stratigraphic sequences, recast as age
distributions for the presettlement period, in 1900 A.D., and at present,
reflect rapid postsettlement alluviation, with enhanced preservation of
younger sediments related to postsettlement watershed disturbance. An
averaged present age distribution for vertically accreted sediment has
modal, median, and mean ages of 190, 230, and 630 yr, reflecting the
predominance of stored legacy sediments and the influence of relatively
few, much older early Holocene deposits. The present age distribution, if
represented by an exponential approximation (mean age ∼300 yr), and naively
assumed to represent steady-state conditions, implies median sediment
travel times on the order of centuries for travel distances greater than
∼100 km. The percentage of sediment reaching the watershed outlet in 30 yr
(a reasonable time horizon to achieve watershed restoration efficacy) is
∼60% for a distance of 50 km, but this decreases to <20% for distances
greater than 200 km.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36282.1/613512/Spatially-averaged-stratigraphic-data-to-inform
Geological history and supercontinent cycles of the Arctic
Christopher Harrison; Marc R. St-Onge
Abstract:
The geological history of the Arctic is constrained within the framework of
the assembly and breakup of three supercontinents. The first of these was
preceded by the crystallization of the oldest dated rocks on Earth and
consolidation of the Arctic region’s Archean cratons between 2.82 and 2.54
Ga. Following the emplacement of regional mafic dike swarms between 2.51
and 2.03 Ga, the cratons were amalgamated into the Nuna (Columbia)
supercontinent between 2.0 and 1.6 Ga, and the distribution of
low-thermal-gradient eclogite (indicative of continental subduction) and
ophiolite (indicative of obduction of oceanic crust onto a continental
margin) suggests that diagnostic plate-tectonic processes were well in
place by the early Paleoproterozoic. Basin formation, flood basalts, and
dike swarms are features of the partial(?) breakup of Nuna (Columbia) by
1.5−1.27 Ga. The extent to which specific dike swarms led to continental
breakup and a rift-to-drift transition remains unclear. Assembly of the
second supercontinent (Rodinia, 1.4−0.9 Ga) is recorded by a network of
Grenvillian and Sveconorwegian collisional orogenic belts. Prominent
features of Rodinia breakup (780−615 Ma) in the Arctic are extensive dike
swarms and regional-scale glacial-periglacial deposits associated with the
Sturtian (717−661 Ma) and Marinoan (ca. 645 ± 6 to ca. 635 Ma) snowball
Earth glaciations. Assembly of the third supercontinent, Pangea, between
600 Ma and ca. 250 Ma, was accomplished through stitching of four orogens
in the Arctic (Timan-Varanger, Caledonian, Ellesmerian, and Urals-Taymyr).
Pangea breakup (rifting since 250 Ma and oceanic spreading since the
Cretaceous) led to the emplacement of Cretaceous and Paleogene flood
basalts, new oceanic crust in the Labrador Sea, North Atlantic Ocean, and
Arctic Ocean, and orogens characterized by relatively small but
far-traveled accreted terranes with provenance in Laurentia, Baltica, and
Siberia. Paleogeographic similarities and geological correlations among
Laurentia, Baltica, Siberia, and the North China craton suggest that
Rodinia formed following incomplete breakup of Nuna (Columbia) and/or by
introversion, whereas unique paleogeographic traits for Pangea within the
Arctic region point to supercontinent formation by extroversion.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36398.1/613496/Geological-history-and-supercontinent-cycles-of
Modification of the lithospheric mantle induced by recycled crustal
components: Insights from Early Cretaceous appinites from the Liaodong
Peninsula, NE China
Xiang-Yu Yan; De-Bin Yang; Wen-Liang Xu; Hao-Tian Yang; Mao-Song Mu ...
Abstract:
The term “appinites” refers to amphibole-rich mafic rocks, as one of the
most typical products of partial melting of a metasomatic lithospheric
mantle. This paper presents results of integrated geochemical and isotopic
analyses for six Early Cretaceous (125−119 Ma) appinitic plutons from the
Liaodong Peninsula, northeastern North China Craton (NCC), to not only
investigate the properties of the metasomatic lithospheric mantle induced
by variable recycled crustal components but also to discuss a hydrous
mantle source. The appinites originated from partial melting of
sub-continental lithospheric mantle (SCLM), and can be geochemically
categorized into three groups distinguished by the input of different
recycled crustal components (e.g., terrigenous sediments and delaminated
lower continental crust) on the basis of two distinct Hf-O isotopic arrays.
Based on the occurrence of voluminous amphiboles (50−60 vol%) and minor
clinopyroxene remnants, a petrogenetic model was proposed to interpret the
formation of appinites. Asthenospheric upwelling caused by the subduction
and roll-back of the Paleo-Pacific plate firstly resulted in partial
melting of the SCLM to yield basaltic magmas. Subsequently, numerous
hydration reactions between early anhydrous minerals (e.g., olivine and
pyroxene) of basaltic magmas and hydrous melts derived from terrigenous
sediments and delaminated lower continental crust occurred to produce the
studied appinites. Ultimately, the recycled crustal materials can serve as
the metasomatic agent not only to transfer the enriched isotopic signatures
into the studied appinites but also to represent hydrous components to play
the part of hydrous reactions. The high abundance of amphibole in the
studied appinites is indicative of a hydrous lithospheric mantle beneath
the eastern NCC.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36352.1/613497/Modification-of-the-lithospheric-mantle-induced-by
Recognition of late Paleoproterozoic gold mineralization in the North
China craton: Evidence from multi-mineral U-Pb geochronology and stable
isotopes of the Shanggong deposit
Shao-Rui Zhao; Jian-Wei Li; Christopher R.M. McFarlane; Paul T. Robinson;
Zhan-Ke Li ...
Abstract:
The North China craton was stabilized in the late Paleoproterozoic but
experienced significant removal of ancient lithospheric keel in the late
Mesozoic that resulted in the formation of numerous world-class gold
deposits with combined reserves of more than 7000 t of gold. However, it
remains uncertain whether the North China craton contains older gold
deposits formed during generation and final stabilization of the craton.
Here, we show that the Shanggong gold deposit (105 t Au at 5.31 g/t) on the
southern margin of the North China craton formed in the late
Paleoproterozoic during the collision between the Eastern and Western
blocks that led to formation of the Trans−North China orogen and final
stabilization of the craton. The Shanggong deposit is hosted in
amphibolite-facies rocks of the Neoarchean to early Paleoproterozoic Taihua
Group and overlying volcanic rocks of the late Paleoproterozoic Xiong’er
Group. Gold mineralization is structurally controlled by NE-striking faults
and occurs in four segments: the Liuxiugou, Hugou, Shanggong, and Qiliping
segments. The ores consist mainly of quartz-ankerite-sulfide stockworks and
sulfide disseminations in hydrothermally altered wall rocks. Gold is mostly
contained in arsenian pyrite that is variably associated with minor
sphalerite, galena, and chalcopyrite. Ore-related alteration assemblages
comprise mainly quartz, ankerite, K-feldspar, sericite, and tourmaline.
Both the stockworks and mineralized alteration assemblages contain
hydrothermal accessory minerals, including monazite, apatite, and rutile.
Paragenetic relations and textural data show that these accessory phases
precipitated synchronously with gold-bearing sulfides. Laser
ablation−inductively coupled plasma−mass spectrometry spot analyses of
monazite and apatite from the Shanggong segment yielded reproducible U-Pb
dates of 1747 ± 20 Ma (2σ, mean square of weighted deviates [MSWD] = 0.46)
and 1788 ± 200 Ma (2σ, MSWD = 11.3), respectively. These dates are
indistinguishable within errors from an apatite U-Pb date of 1743 ± 79 Ma
(2σ, MSWD = 1.6) at the Liuxiugou segment and a rutile U-Pb date of 1804 ±
52 Ma (2σ, MSWD = 0.77) at the Hugou segment. These new dates suggest that
the Shanggong deposit formed at ca. 1.80−1.74 Ga, coeval with or
immediately after formation of the Trans−North China orogen and final
stabilization of the North China craton. Sulfides from the Shanggong gold
deposit have δ34S values ranging from −18.5‰ to −6.9‰, whereas
the coexisting ankerite has δ13CPDB of −6.81‰ to
−1.61‰ and δ18OSMOW of 15.70‰−17.62‰. The stable
isotope data are distinctively different from values of the Early
Cretaceous gold deposits in the southern North China craton, indicating
contrasting hydrothermal systems responsible for these two categories of
gold deposits. The results presented here, combined with independent
geologic evidence, allow Shanggong to be the first confirmed
Paleoproterozoic orogenic gold deposit in the North China craton.
Recognition of Paleoproterozoic orogenic gold mineralization provides
significant new insights into the gold metallogeny of the well-endowed
North China craton and has implications for future gold exploration along
the three Paleoproterozoic orogenic belts in this craton.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36281.1/613498/Recognition-of-late-Paleoproterozoic-gold
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