New GSA Bulletin Articles Published Ahead of Print in September
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; guidance
regarding the publication and interpretation of luminescence results; the
Columbia River Basalt Group; the southern Great Basin of the United States;
and arsenic and platinum-group minerals in East Dover, Vermont. You can
find these articles at
https://bulletin.geoscienceworld.org/content/early/recent
.
Mid−late Neoproterozoic tectonic evolution of the northern margin of
the Yangtze Block, South China, as constrained from the detrital zircon
record
Ruirui Wang; Zhiqin Xu; M. Santosh
Abstract:
The northern margin of the Yangtze Block (NYB) experienced distinct
tectono-magmatic 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 northernmost 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.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36419.1/617921/Mid-late-Neoproterozoic-tectonic-evolution-of-the
Guide for interpreting and reporting luminescence dating results
Shannon A. Mahan; Tammy M. Rittenour; Michelle S. Nelson; Nina Ataee;
Nathan Brown ...
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.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36404.1/617922/Guide-for-interpreting-and-reporting-luminescence
A 50 m.y. melting model for the rare metal−rich Koktokay pegmatite in
the Chinese Altai: Implications from a newly identified Jurassic
granite
Jinsheng Han; Zhenhua Zhao; Pete Hollings; Huayong Chen
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/Na 2O (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.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36513.1/617781/A-50-m-y-melting-model-for-the-rare-metal-rich
Timing and spatial variation of deformation along the Kanggur-Huangshan
shear zone in the Chinese Tianshan: Implications for regional
differential uplift and mineralization
M.N. Muhtar; Chang-Zhi Wu; Matthew J. Brzozowski; M. Santosh; Rong-Song
Tian ...
Abstract:
The Chinese Tianshan experienced large-scale 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.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36221.1/617782/Timing-and-spatial-variation-of-deformation-along
Filling critical gaps in the space-time record of High Lava Plains and
co-Columbia River Basalt Group rhyolite volcanism
Vanessa M. Swenton; Martin J. Streck; Daniel P. Miggins; William C.
McIntosh
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.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36346.1/617783/Filling-critical-gaps-in-the-space-time-record-of
The Ediacaran−Cambrian transition in the southern Great Basin, United
States
Emily F. Smith; Lyle L. Nelson; Nizhoni O’Connell; Athena Eyster; Mary C.
Lonsdale
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 (δ 13C) 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.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36401.1/617772/The-Ediacaran-Cambrian-transition-in-the-southern
How headward erosion breaches upstream paleolakes: Insights from dated
longitudinal fluvial terrace correlations within the Sanmen Gorge,
Yellow River
Hao Liang; Ke Zhang; Zhongyun Li; Jianli Fu; Zhangxin Yu ...
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 S 2 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.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36537.1/617108/How-headward-erosion-breaches-upstream-paleolakes
Petrogenesis of arsenic and platinum-group minerals from a partially
serpentinized dunite in East Dover, Vermont, USA
John M. Brigham; Suzanne L. Baldwin
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.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36388.1/617712/Petrogenesis-of-arsenic-and-platinum-group
Plio-Pleistocene cooling of the northeastern Tibetan Plateau due to
global climate change and surface uplift
Fabiana Richter; Carmala N. Garzione; Weiguo Liu; Xiaoke Qiang; Hong Chang
...
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.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36302.1/617714/Plio-Pleistocene-cooling-of-the-northeastern
Drainage initiation, expansion, and channel-head arrest in heterogenous
bedrock landscapes of the Colorado Plateau
A.T. Steelquist; M.G.A. Lapôtre; G.E. Hilley
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.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36375.1/617671/Drainage-initiation-expansion-and-channel-head
Migration and localization of faulting near the intersection of the
Dead Sea Fault and the Carmel−Gilboa−Faria Fault System
Yariv Hamiel; Oded Katz; Yoav Avni
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 a ∼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.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36361.1/617107/Migration-and-localization-of-faulting-near-the
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
Lingling Yuan; Xiaohui Zhang; Zhili Yang
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
ultramafic-mafic 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.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36437.1/617109/Late-Permian-Triassic-ridge-subduction-beneath-the
Tracing remnants of Ediacaran S-type granitoids from beach placers in
SE Madagascar
P.G. Athira; K. Sajeev; P.M. George; M.G. Zhai; L.G. Zhou ...
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.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36176.1/616569/Tracing-remnants-of-Ediacaran-S-type-granitoids
Structure, geochronology, and petrogenesis of Permian peraluminous
granite dykes in the southern Chinese Altai as indicators of Altai−East
Junggar convergence
Tan Shu; Yingde Jiang; Karel Schulmann; Yang Yu; Chao Yuan ...
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/86
Srinitial: 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.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36408.1/616562/Structure-geochronology-and-petrogenesis-of
Deoxygenation and organic carbon sequestration in the Tethyan realm
associated with the middle Eocene climatic optimum
Margot J. Cramwinckel; Robin van der Ploeg; Niels A.G.M. van Helmond; Niels
Waarlo; Claudia Agnini ...
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 CO 2 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.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36280.1/616563/Deoxygenation-and-organic-carbon-sequestration-in
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