Boulder, Colo., USA: The Geological Society of America regularly publishes articles online ahead of print. GSA Bulletin topics include multiple articles about the dynamics of China and Tibet; the end-Permian terrestrial extinction paradigm in South Africa; prehistoric lava flows from the urban district of Catania (Etna volcano, Italy); the debated origins of granite, and “a tale of two Tweefonteins.” You can find these articles at https://bulletin.geoscienceworld.org/content/early/recent .
Authigenic berthierine and incipient chloritization in shallowly buried
sandstone reservoirs: Key role of the source-to-sink context
Maxime Virolle; Benjamin Brigaud; Daniel Beaufort; Patricia Patrier; Eid
Abdelrahman ...
Abstract:
Chlorite is recognized worldwide as a key mineral that inhibits the
development of quartz cement in deeply buried sandstone reservoirs.
Iron-rich chlorite is mainly formed by the transformation of a precursor
clay mineral; however, few studies have focused on the early stages before
the crystallization of chlorite. This study analyzed shallowly buried
(400−1000 m) coastal sandstones from within the Wealden Group of the Paris
Basin. Berthierine, a 7 Å trioctahedral clay mineral belonging to the
serpentine group, approximatively with same chemistry as chlorite but a
different crystal structure, has been identified in a 900-m-deep well but
not in a 400−600-m-deep well. Berthierine has mainly been observed as clay
coatings around detrital grains with a typical honeycomb texture.
Nanopetrographic observations suggest that the honeycomb textural
organization of the clay particles found in deeper buried sandstone
reservoirs (>1500 m) is acquired from a berthierine precursor at shallow
depths. However, small amounts of quartz overgrowths are observed on the
surface of detrital grains at shallow depths and low temperature (below 40
°C), and it is believed that precursor berthierine coatings are primarily
responsible for the inhibition of quartz overgrowths before Fe-rich
chlorite is formed. This suggests that the key mineral primarily
controlling the reservoir quality of deeply buried sandstone reservoirs is
berthierine rather than iron-rich chlorite, which challenges the commonly
accepted assertion that chlorite coating is the main process that inhibits
quartz overgrowths. The source-to-sink context of the Paris Basin during
the Early Cretaceous was decisive with respect to the supply of sands and
berthierine clay precursors (in particular kaolinite and iron-rich,
hydroxy-interlayered clay minerals) to the center of the basin.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35865.1/605739/Authigenic-berthierine-and-incipient
Subduction initiation-induced rapid emplacement of garnet-bearing
peridotites at a nascent forearc: Petrological and Os-Li isotopic
evidence from the Purang ophiolite, Tibet
Xiao-Han Gong; Ji-Feng Xu; Ren-Deng Shi; Ben-Xun Su; Qi-Shuai Huang ...
Abstract:
Garnet-bearing peridotites commonly occur in the deeper parts of mature or
thickened oceanic lithosphere, and are rarely exhumed and emplaced onto the
seafloor. The Purang ophiolitic peridotites in south Tibet contain rare
symplectite pseudomorphs after garnet, offering a unique window into the
still poorly understood evolution of the deep oceanic lithosphere. Here,
integrated petrologic and Os-Li isotopic data are used to constrain the
evolution and dynamics of emplacement for these garnet peridotite
protoliths. The Purang peridotites show wide variations of chemical
compositions (spinel Cr#: 0.2−0.8) and Os model ages (up to 2.0 Ga), thus
representing a piece of heterogeneous oceanic mantle lithosphere. Dunite
channels show two distinctive groups of Cr# of spinels and Os-isotope
compositions, with the low- to medium-Cr# (0.2−0.6) and high-Cr# (0.7−0.8)
dunites reflecting the reaction of host lherzolites/harzburgites with
percolating mid-ocean ridge basalt−like and boninitic melts, respectively.
This confirms recent subduction initiation-related melt percolation in the
Purang peridotites. Coexisting olivines and pyroxenes in the peridotites
show systematic Li elemental and isotopic disequilibrium, suggesting fast
cooling of the peridotites to Li closure temperature shortly after the melt
percolations, likely during exhumation of the peridotites onto the
seafloor. This supports a close link between subduction initiation and
tectonic emplacement of the Purang peridotites. Combined with other
geological evidence, we suggest the Purang peridotites may originate from
the deep part of old, thick oceanic lithosphere of the Neo-Tethys. This
thick oceanic lithosphere was progressively weakened and thinned likely
during widespread plume-lithosphere interaction, triggering the
transformation of garnet peridotite protoliths to spinel peridotites.
Subsequently, initiation of a new subduction zone along the lithospheric
weakness caused rapid ascent and emplacement of the Purang peridotites at a
nascent forearc.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35960.1/604591/Subduction-initiation-induced-rapid-emplacement-of
A tale of two Tweefonteins: What physical correlation, geochronology,
magnetic polarity stratigraphy, and palynology reveal about the
end-Permian terrestrial extinction paradigm in South Africa
Robert A. Gastaldo; Johann Neveling; John W. Geissman; Sandra L. Kamo;
Cindy V. Looy
Abstract:
The contact between the Daptocephalus to Lystrosaurus declivis (previously Lystrosaurus)
Assemblage Zones (AZs) described from continental deposits of the Karoo
Basin was commonly interpreted to represent an extinction crisis associated
with the end-Permian mass-extinction event at ca. 251.901 ± 0.024 Ma. This
terrestrial extinction model is based on several sections in the Eastern
Cape and Free State Provinces of South Africa. Here, new stratigraphic and
paleontologic data are presented for the Eastern Cape Province, in
geochronologic and magnetostratigraphic context, wherein lithologic and
biologic changes are assessed over a physically correlated stratigraphy
exceeding 4.5 km in distance. Spatial variation in lithofacies demonstrates
the gradational nature of lithostratigraphic boundaries and depositional
trends. This pattern is mimicked by the distribution of vertebrates
assigned to the Daptocephalus and L. declivis AZs where
diagnostic taxa of each co-occur as lateral equivalents in landscapes
dominated by a Glossopteris flora. High-precision U-Pb zircon
(chemical abrasion-isotope dilution-thermal ionization mass spectrometry)
age results indicate maximum Changhsingian depositional dates that can be
used as approximate tie points in our stratigraphic framework, which is
supported by a magnetic polarity stratigraphy. The coeval nature of
diagnostic pre- and post-extinction vertebrate taxa demonstrates that the L. declivis AZ did not replace the Daptocephalus AZ
stratigraphically, that a biotic crisis and turnover likely is absent, and
a reevaluation is required for the utilization of these biozones here and
globally. Based on our data set, we propose a multidisciplinary approach to
correlate the classic Upper Permian localities of the Eastern Cape Province
with the Free State Province localities, which demonstrates their
time-transgressive nature.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35830.1/604557/A-tale-of-two-Tweefonteins-What-physical
Paleomagnetic dating of prehistoric lava flows from the urban district
of Catania (Etna volcano, Italy)
Andrea Magli; Stefano Branca; Fabio Speranza; Gilda Risica; Gaia Siravo ...
Abstract:
Determining the ages of past eruptions of active volcanoes whose slopes
were historically inhabited is vitally important for investigating the
relationships between eruptive phenomena and human settlements. During its
almost three-millennia-long history, Catania—the biggest city lying at the
toe of Etna volcano—was directly impacted only once by the huge lava flow
emplaced during the A.D. 1669 Etna flank eruption. However, other lava
flows reached the present-day Catania urban district in prehistoric ages
before the founding of the city in Greek times (729/728 B.C., i.e.,
2679/2678 yr B.P.). In this work, the Holocene lava flows of Barriera del
Bosco, Larmisi, and San Giovanni Galermo, which are exposed in the Catania
urban district, were paleomagnetically investigated at 12 sites (120
oriented cores). Paleomagnetic dating was obtained by comparing flow-mean
paleomagnetic directions to updated geomagnetic reference models for the
Holocene. The Barriera del Bosco flow turns out to represent the oldest
eruptive event and is paleomagnetically dated to the 11,234−10,941 yr B.P.
and 8395−8236 yr B.P. age intervals. The mean paleomagnetic directions from
the San Giovanni Galermo and Larmisi flows overlap when statistical
uncertainties are considered. This datum, along with geologic, geochemical,
and petrologic evidence, implies that the two lava flows can be considered
as parts of a single lava field that erupted in a narrow time window
between 5494 yr B.P. and 5387 yr B.P. The emplacement of such a huge lava
flow field may have buried several Neolithic settlements, which would thus
explain the scarce occurrence of archaeological sites of that age found
below the town of Catania.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36026.1/602750/Paleomagnetic-dating-of-prehistoric-lava-flows
Migmatite and leucogranite in a continental-scale exhumed strike-slip
shear zone: Implications for tectonic evolution and initiation of
shearing
Junyu Li; Shuyun Cao; Xuemei Cheng; Franz Neubauer; Haobo Wang ...
Abstract:
Plutons within continental strike-slip shear zones bear important
geological processes on late-stage plate transpression and
continent-continent collision and associated lateral block extrusion.
Where, when, and how intrusions and shearing along transpressional
strike-slip shear zones respond to plate interactions, however, are often
debated. In this study, we investigated migmatite associated leucogranite
and pegmatite from the exhumed >1000-km-long Ailao Shan–Red River
left-lateral strike-slip shear zone in Southeast Asia that was active
during India-Eurasia plate convergence. Most zircons from the migmatites
and leucogranitic intrusions present inherited core-rim structure. The
depletion of rare earth element patterns and positive Eu anomalies suggest
that leucosomes and leucogranites are the result of crustal anatexis.
Zircon rims from the foliated migmatites and leucogranites record U-Pb ages
of 41−28 Ma, revealing the timing of the Cenozoic crustal anatexis event
along this strike-slip shear zone. Ages of the magmatic zircons from the
unfoliated pegmatites provide the timing of the termination of a
high-temperature tectono-thermal event and ductile left-lateral shearing at
26−23 Ma. The Cenozoic crustal anatexis along the Ailao Shan–Red River
strike-slip shear zone indicates that thickened crust underneath the shear
zone involved previously subducted crust. We propose that the Cenozoic
thermal state has an important effect on the crustal anatexis and thus on
the rheological behavior of the lithosphere by thermal weakening, which
plays an essential role in localizing the initiation of the deep-seated
lower-crustal shear zone.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35988.1/602751/Migmatite-and-leucogranite-in-a-continental-scale
The impact of a tear in the subducted Indian plate on the Miocene
geology of the Himalayan-Tibetan orogen
Rui Wang; Roberto F. Weinberg; Di-Cheng Zhu; Zeng-Qian Hou; Zhi-Ming Yang
Abstract:
The Yadong-Gulu Rift, cutting across the Gangdese belt and Himalayan
terranes, is currently associated with a thermal anomaly in the mantle and
crustal melting at 15−20 km depth. The rift follows the trace of a tear in
the underthrusted Indian continental lithospheric slab recognized by high
resolution geophysical methods. The Miocene evolution of a 400-km-wide band
following the trace of the tear and the rift, records differences
interpreted as indicative of a higher heat flow than its surroundings. In
the Gangdese belt, this band is characterized by high-Sr/Y granitic
magmatism that lasted 5 m.y. longer than elsewhere and by the highest
values of εHf(i) and association with the largest porphyry Cu-Mo
deposits in the Gangdese belt. Anomalously young magmatic rocks continue
south along the rift in the Tethyan and Higher Himalayas. Here, a
300-km-wide belt includes some of the youngest Miocene Himalayan
leucogranites; the only occurrence of mantle-derived mafic enclaves in a
leucogranite; young mantle-derived lamprophyre dikes; and the youngest and
hottest migmatites in the Higher Himalayas. These migmatites record a
history of rapid exhumation contemporaneous with the exhumation of Miocene
mafic eclogite blocks, which are unique to this region and which were both
heated to >800 °C at ca. 15−13 Ma, followed by isothermal decompression.
We suggest that the prominent tear in the Indian lithosphere, sub-parallel
to the rift, is the most likely source for these tectono-thermal anomalies
since the Miocene.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36023.1/602752/The-impact-of-a-tear-in-the-subducted-Indian-plate
The timeline of prolonged accretionary processes in eastern Central
Asian Orogenic Belt: Insights from episodic Paleozoic intrusions in
central Inner Mongolia, North China
Lingling Yuan; Xiaohui Zhang; Zhili Yang
Abstract:
Updating magmatic profile in crucial constituent terranes across the
Central Asian Orogenic Belt presents a key to chronicling the timeline of
prolonged accretionary processes and termination of the Paleo-Asian Ocean
in the northern China−southern Mongolia tract. Here we performed a
systematic geochronological and geochemical study on a spectrum of
Paleozoic intrusions from the Erenhot region in central Inner Mongolia,
North China, within the hinterland of the tract, with four distinct
magmatic episodes unraveled. Combining these episodes with the previously
documented events from contiguous regions defines two major
tectono-magmatic cycles. The early Paleozoic cycle (500−450 Ma) evolved
from initial fluid-fluxed tholeiitic and calc-alkaline granitoids to
melt-fertilized mafic-intermediate magmatism. It appears to experience the
initiation and maturation of a Western Pacific-type intra-oceanic arc
system that culminated in ridge subduction. The late Paleozoic cycle
expanded in magmatic expression from sporadic Late Devonian (373−365 Ma)
calc-alkaline intermediate-felsic pulses through Early-Middle Carboniferous
(356−320 Ma) medium to high-K calc-alkaline flare-up to a Late
Carboniferous−Early Permian (310−277 Ma) province of diverse lithologies.
These magmatic episodes seem to encompass a complete active continental
arc−back-arc system that spanned from resuming oceanic plate subduction
through slab rollback and backarc rifting to ridge-trench collision and
backarc basin closure. Featuring a Rodinia-aged terrane affinity and a
representative Paleozoic magmatic profile, the Erenhot region provides an
optimal site for correlating the evolution of mosaic terranes in southern
Mongolia and northern China, and for evaluating the coupled evolution of
shifting tectonic regimes and plural crustal generation mechanisms within a
retreating accretionary orogen.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35907.1/602753/The-timeline-of-prolonged-accretionary-processes
Pure sediment-derived granites in a subduction zone
Jian Xu; Xiao-Ping Xia; Qiang Wang; Christopher J. Spencer; Chun-Kit Lai
...
Abstract:
The Earth is unique in the Solar System due to significant volumes of
granite in the lithosphere. However, the origins of granites are still
highly debated, especially sediment-derived granites, which are often
treated as a geochemical end-member of the continental crust. In the Yunnan
region of South China, we identify the occurrence of pure sediment-derived
granite in a subduction system. The suite of strongly peraluminous granite
reported herein is interpreted to represent pure metasedimentary melts
based on their whole-rock elemental and Sr-Nd-B and zircon Hf-O isotopic
compositions. These Late Permian−Early Triassic (ca. 254−248 Ma) granites
are characterized by radiogenically enriched Sr, Nd, and Hf isotopic
signatures. They show δ11B and δ18O signatures akin
to those of continental shales. Geochemical modeling indicates no
contributions from the mantle that can be detected. Considering the
regional tectonic evolution, these granites are suggested to be formed in a
subduction zone by decompression melting of rapidly exhumed back-arc
sediments. We posit that decompression melting was triggered by widespread
extension and thinning of the crust prompted by rollback of the subducting
oceanic crust. These granites thus provide evidence that granite formation
in subduction zones does not necessarily contribute to crustal growth.
These subduction-related pure sediment-derived granites have different
elemental ratios and contents (e.g., Al2O3/TiO 2 and Yb) from the Himalayan leucogranites. Considering their
source compositions (e.g., pelitic rocks), which are similar to those of
the Himalayan leucogranites, these differences are likely due to their
higher formation temperature and lower pressure despite a great similarity
in isotopic compositions. Identification of pure sediment-derived, strongly
peraluminous granites (SPGs) in subduction systems provides an important
geodynamic mechanism for crustal anatexis, which can both geochemically and
tectonically complement their collisional counterparts identified in the
Himalayas.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36016.1/602698/Pure-sediment-derived-granites-in-a-subduction
Mafic-ultramafic intrusion formed by multi-stage evolution of hydrous
basaltic melts
Qi-Wei Li; Jun-Hong Zhao; Mei-Fu Zhou; Jian-Feng Gao
Abstract:
The magmatic processes beneath the active continental margins are very
complicated and affect structures and compositions of the arc roots.
Neoproterozoic igneous rocks are widely distributed around the margins of
the Tarim Block in NW China. The Xingdier mafic-ultramafic intrusion is a
composite body, located at the northern margin of the Tarim Block, and
consists of gabbro, pyroxenite, and peridotite units. The gabbro unit has a
secondary ion mass spectrometry zircon U-Pb age of 727 ± 5 Ma. Rocks from
the Xingdier intrusion have a large range of MgO (12.9−32.8 wt%) and SiO 2 (43.0−57.9 wt%), and low K2O+Na2O
(0.11−2.25 wt%) contents. They have right inclined chondrite-normalized
rare earth element patterns with (La/Yb)N ratios of 2.2−8.6.
Their primitive mantle normalized trace element patterns show arc-affinity
geochemical features characterized by enrichment in Rb, Ba, Th, U, and Pb
and depletion in Nb, Ta, and Ti. They have variable initial 87
Sr/86Sr ratios (0.7063−0.7093), εNd(t) values (−2.9 to −7.8),206Pb/204Pb (17.08−17.80), 207Pb/ 204Pb (15.42−15.49), and 208Pb/204Pb
ratios (37.48−38.05), forming an evolution trend from the peridotite unit
to the gabbro and pyroxenite units. Clinopyroxene in the three units is
chemically similar to those formed in hydrous magmas. The spinel inclusions
in olivine from the peridotite unit show unmixing texture and have high Al
contents and oxygen fugacity of ∼FMQ+1. Therefore, the parental magma was
probably derived from a lithospheric mantle enriched by slab-derived
fluids. Rocks from the gabbro and peridotite units are proposed to have
been derived from olivine-normative melts, whereas rocks from the
pyroxenite unit are cumulates from the quartz-normative melts. Such
contrasting parental magmas resulted from variable degrees of crustal
contamination and fractional crystallization in the arc root.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35920.1/601081/Mafic-ultramafic-intrusion-formed-by-multi-stage
Origin of syn-collisional granitoids in the Gangdese orogen: Reworking
of the juvenile arc crust and the ancient continental crust
Yu-Wei Tang; Long Chen; Zi-Fu Zhao; Yong-Fei Zheng
Abstract:
Granitoids at convergent plate boundaries can be produced either by partial
melting of crustal rocks (either continental or oceanic) or by fractional
crystallization of mantle-derived mafic magmas. Whereas granitoid formation
through partial melting of the continental crust results in reworking of
the pre-existing continental crust, granitoid formation through either
partial melting of the oceanic crust or fractional crystallization of the
mafic magmas leads to growth of the continental crust. This category is
primarily based on the radiogenic Nd isotope compositions of crustal rocks;
positive εNd(t) values indicate juvenile crust whereas negative
εNd(t) values indicate ancient crust. Positive εNd(t)
values are common for syn-collisional granitoids in southern Tibet, which
leads to the hypothesis that continental collision zones are important
sites for the net growth of continental crust. This hypothesis is examined
through an integrated study of in situ zircon U-Pb ages and Hf isotopes,
whole-rock major trace elements, and Sr-Nd-Hf isotopes as well as mineral O
isotopes for felsic igneous rocks of Eocene ages from the Gangdese orogen
in southern Tibet. The results show that these rocks can be divided into
two groups according to their emplacement ages and geochemical features.
The first group is less granitic with lower SiO2 contents of
59.82−64.41 wt%, and it was emplaced at 50−48 Ma in the early Eocene. The
second group is more granitic with higher SiO2 contents of
63.93−68.81 wt%, and it was emplaced at 42 Ma in the late Eocene. The early
Eocene granitoids exhibit relatively depleted whole-rock Sr-Nd-Hf isotope
compositions with low (87Sr/86Sr)i ratios
of 0.7044−0.7048, positive εNd(t) values of 0.6−3.9, ε Hf(t) values of 6.5−10.5, zircon εHf(t) values of
1.6−12.1, and zircon δ18O values of 5.28−6.26‰. These isotopic
characteristics are quite similar to those of Late Cretaceous mafic arc
igneous rocks in the Gangdese orogen, which indicates their derivation from
partial melting of the juvenile mafic arc crust. In comparison, the late
Eocene granitoids have relatively lower MgO, Fe2O3,
Al2O3, and heavy rare earth element (HREE) contents
but higher K2O, Rb, Sr, Th, U, Pb contents, Sr/Y, and (La/Yb) N ratios. They also exhibit more enriched whole-rock Sr-Nd-Hf
isotope compositions with high (87Sr/86Sr) i ratios of 0.7070−0.7085, negative εNd(t) values of
−5.2 to −3.9 and neutral εHf(t) values of 0.9−2.3, and
relatively lower zircon εHf(t) values of −2.8−8.0 and slightly
higher zircon δ18O values of 6.25−6.68‰. An integrated
interpretation of these geochemical features is that both the juvenile arc
crust and the ancient continental crust partially melted to produce the
late Eocene granitoids. In this regard, the compositional evolution of
syn-collisional granitoids from the early to late Eocene indicates a
temporal change of their magma sources from the complete juvenile arc crust
to a mixture of the juvenile and ancient crust. In either case, the
syn-collisional granitoids in the Gangdese orogen are the reworking
products of the pre-existing continental crust. Therefore, they do not
contribute to crustal growth in the continental collision zone.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35928.1/601082/Origin-of-syn-collisional-granitoids-in-the
Mantle source of tephritic porphyry in the Tarim Large Igneous Province
constrained from Mg, Zn, Sr, and Nd isotope systematics: Implications
for deep carbon cycling
Weiliang Kong; Zhaochong Zhang; Zhiguo Cheng; Bingxiang Liu; M. Santosh ...
Abstract:
The nature and source of magmatism associated with large igneous provinces
(LIPs) remain disputed. Here we investigate the role of recycled materials
that contributed to mantle heterogeneity in the Tarim Large Igneous
Province (TLIP) in China through integrated Zn−Mg−Sr−Nd isotopes of a rare
tephritic rock suite. The Sr−Nd isotopes [(87Sr/86Sr)i = 0.70368−0.70629; εNd(t) = −0.25−4.64] and δ 26Mg values (−0.23‰ to −0.34‰) of the tephritic porphyries are
consistent with a normal mantle origin. In contrast, the whole rock and
pyroxene phenocrysts yield δ66Zn values of +0.28‰ to +0.46‰ and
+0.30‰ to +0.39‰, which are slightly heavier than those of the terrestrial
mantle (+0.16 ± 0.06‰) and mid-oceanic-ridge basalts (MORBs) (+0.27 ±
0.05‰). We exclude the possibility that the heavy Zn isotopes of the
Wajilitag tephritic porphyries are caused by magmatic processes such as
fractional crystallization and partial melting and correlate the isotopic
features to the role of altered oceanic crust along with magnesite in the
mantle source. The Wajilitag tephritic porphyry displays trace-element
patterns similar to those of the melts from natural hornblendite,
especially those of hornblendite + peridotite. Additionally, the
geochemical characteristics such as enrichment in Nb and Ta, depletion in
K, high TiO2, and constant Na2O/K2O ratios
also suggest that the tephritic porphyries were derived from an
amphibole-bearing source contributed by altered oceanic crust along with
sedimentary carbonates. Our study provides insight into the contribution of
subducted materials to the mantle heterogeneity beneath the TLIP and
highlights the role of subduction in the deep carbon cycle and subducted
slab-lithosphere-plume interaction in the generation of LIPs.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35902.1/600886/Mantle-source-of-tephritic-porphyry-in-the-Tarim
Sedimentology, geochronology, and provenance of the late Cenozoic
“Yangtze Gravel”: Implications for Lower Yangtze River reorganization
and tectonic evolution in southeast China
Ping Wang; Hongbo Zheng; Yongdong Wang; Xiaochun Wei; Lingyu Tang ...
Abstract:
The evolution of the Yangtze River, the longest river in Asia, provides a
spectacular example for understanding the Cenozoic interaction between
tectonics, climate, and surficial processes. The oldest Lower Yangtze
deposits, characterized by ∼100-m-thick sequences of unconsolidated
conglomerate, sandstone, and siltstone, referred to as “Yangtze Gravel,”
have been recently dated >23 Ma, indicating a pre-Miocene establishment
of a through-going river. However, the link between river integration and
tectonic evolution has never been established due to the limited study of
these sediments. Here, we report sedimentology, geochronology, and
provenance of the Yangtze Gravel based on 17 stratigraphic sections exposed
along the Lower Yangtze River. Our new chronostratigraphic results,
including 40Ar/39Ar ages from the overlying basalt and fossil-based
stratigraphic correlation, suggest an early-middle Miocene age for these
sediments. Detailed analysis of lithofacies reveals several sequences of
coarse-grained channel-belt deposits (channel fills and bars), indicating
braided alluvial deposition across the Jianghan Basin, North Jiangsu-South
Yellow Sea Basin, and East China Sea Shelf Basin. This ancient Lower
Yangtze River is further characterized by petrography and detrital zircon
U-Pb geochronology results which show similar provenance and erosion
pattern as the present-day Yangtze River. However, the ancient river in
early-middle Miocene is an alluvial, bedload-dominated braided river with
higher stream power and a more prolonged course flowing into the East China
Sea Shelf Basin. These differences between ancient and modern Lower Yangtze
River reflect varied climate and paleogeography in southeast China during
the late Cenozoic. Compared with the Paleogene red-colored, halite-bearing,
Ephedripite pollen-dominated, lacustrine deposits in Jianghan Basin and
North Jiangsu-South Yellow Sea Basin, the deposition of yellow to
green-colored, coarse-grained, arboreal pollen, and wood-dominated Yangtze
Gravel indicates a drainage reorganization from hydrologically closed lakes
to a through-going river system during late Oligocene to early Miocene.
During Paleogene, rift basins were filled by alluvial and
fluvial-lacustrine deposition with restricted flow distance and local
sources. From late Oligocene to early-middle Miocene, the post-rift
subsidence opens a path for the ancient Lower Yangtze River connecting the
Jianghan Basin, North Jiangsu-South Yellow Sea Basin, and East China Sea
Shelf Basin. We attribute the drainage reorganization of the Lower Yangtze
River to be a surficial response to Cenozoic tectonics, particularly the
western Pacific subduction, in southeast China. The deposition of the
widespread, coarse-grained Yangtze Gravel is probably due to the combined
effects of catchment expansion and strong monsoonal climate in East Asia.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35851.1/600887/Sedimentology-geochronology-and-provenance-of-the
Thermotectonic events recorded by U-Pb geochronology and Zr-in-rutile
thermometry of Ti oxides in basement rocks along the P2 fault, eastern
Athabasca Basin, Saskatchewan, Canada
E. Adlakha; K. Hattori
Abstract:
Basement rocks below the Athabasca Basin, Saskatchewan, have been intensely
altered through paleoweathering and multiple hydrothermal events, including
the formation of world-class unconformity-type uranium deposits. Here, we
demonstrate the utility of Ti-oxide thermochronology for identifying
thermotectonic events in these altered rocks leading to uranium
mineralization along basement structures. Rutile grains along the P2 fault,
a major fault in the eastern Athabasca Basin, exhibit 207Pb/ 206Pb ages of ca. 1850−1700 Ma, with a weighted mean of 1757 ± 6
Ma (mean square of weighted deviation [MSWD] = 1.4, n = 116). The
older ages (~1770 Ma) record regional metamorphism reaching a
temperature of 875 °C during the Trans-Hudson orogeny. Pb diffusion
modeling indicates that metamorphic rutile should exhibit cooling ages of
1760−1750 Ma. Rutile grains showing young ages, ~1750 Ma, reflect
isotopic resetting during regional asthenospheric upwelling between 1770
and 1730 Ma related to the emplacement of the Kivalliq igneous suite to the
north. This thermotectonic event (temperature ~550 °C) promoted
hydrothermal activity to produce silicified rocks, i.e., “quartzite,” along
the P2 fault, which later focused mineralizing fluids for unconformity-type
uranium deposits. The young rutile ages also indicate that the basement
rocks remained hot until 1700 Ma, providing the maximum age for the
deposition of the Athabasca sediments. Anatase yields a concordia age of
1569 ± 31 Ma (MSWD = 0.30, n = 5), which is within uncertainty of
the oldest ages for uraninite of the McArthur River deposit. This age
corresponds to the incursion of basinal fluids in the basement along the P2
fault during uranium mineralization.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35820.1/600747/Thermotectonic-events-recorded-by-U-Pb
Superposition of Cretaceous and Cenozoic deformation in northern Tibet:
A far-field response to the tectonic evolution of the Tethyan orogenic
system
Ye Wang; Xuanhua Chen; Yaoyao Zhang; Zheng Yin; Andrew V. Zuza ...
Abstract:
Although the Cenozoic Indo-Asian collision is largely responsible for the
formation of the Tibetan plateau, the role of pre-Cenozoic structures in
controlling the timing and development of Cenozoic deformation remains
poorly understood. In this study we address this problem by conducting an
integrated investigation in the northern foreland of the Tibetan plateau,
north of the Qilian Shan-Nan Shan thrust belt, NW China. The work involves
field mapping, U-Pb detrital-zircon dating of Cretaceous strata in the
northern foreland of the Tibetan plateau, examination of growth-strata
relationships, and construction and restoration of balanced cross sections.
Our field mapping reveals multiple phases of deformation in the area since
the Early Cretaceous, which was expressed by northwest-trending folding and
northwest-striking thrusting that occurred in the early stages of the Early
Cretaceous. The compressional event was followed immediately by extension
and kinematically linked right-slip faulting in the later stage of the
Early Cretaceous. The area underwent gentle northwest-trending folding
since the late Miocene. We estimate the magnitude of the Early Cretaceous
crustal shortening to be ∼35%, which we interpret to have resulted from a
far-field response to the collision between the Lhasa and the Qiangtang
terranes in the south. We suggest that the subsequent extension in the
Early Cretaceous was induced by orogenic collapse. U-Pb dating of detrital
zircons, sourced from Lower Cretaceous sedimentary clasts from the north
and the south, implies that the current foreland region of the Tibetan
plateau was a topographic depression between two highland regions in the
Early Cretaceous. Our work also shows that the Miocene strata in the
foreland region of the northern Tibetan plateau was dominantly sourced from
the north, which implies that the rise of the Qilian Shan did not impact
the sediment dispersal in the current foreland region of the Tibetan
plateau where this study was conducted.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35944.1/600737/Superposition-of-Cretaceous-and-Cenozoic
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Journal
ACM SIGSAM Bulletin