New GSA Bulletin Articles Published Online Ahead of Print
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; a possible
sea-level fall trigger for the youngest rejuvenated volcanism in Hawaiʻi;
and porphyry copper mineralization. You can find these articles at
https://bulletin.geoscienceworld.org/content/early/recent.
Stratigraphy of the Cretaceous/Paleogene (K/Pg) boundary at the
Global Stratotype Section and Point (GSSP) in El Kef, Tunisia: New
insights from the El Kef Coring Project
Heather L. Jones; Thomas Westerhold; Heather Birch; Pincelli Hull; M. Hédi
Negra ...
The Cretaceous/Paleogene (K/Pg) boundary is marked by one of the largest
mass extinctions in Earth’s history, with geological evidence for this
event being expressed in hundreds of locations worldwide. An extensively
studied section located near El Kef, northwestern Tunisia, is characterized
by the classic iridium-rich K/Pg boundary layer, abundant and
well-preserved microfossils, and apparently continuous sedimentation
throughout the early Danian with no previously described structural
complication. These features led to its designation in 1991 as the Global
Stratigraphic Section and Point (GSSP) for the base of the Danian (i.e.,
the K/Pg boundary). However, the outcrop section has become weathered, and
the “golden spike” marking the GSSP is difficult to locate. Therefore, the
El Kef Coring Project aimed to provide a continuous record of unweathered
sediments across the K/Pg transition in cores recovered from five
rotary-drilled holes located close to the El Kef GSSP. Here, we present
new, high-resolution lithologic, biostratigraphic, and geochemical data
from these cores. The recovered stratigraphic successions of each hole (all
drilled within ∼75 m of one another) are unexpectedly different, and we
identified a formerly unknown unconformity within planktic foraminiferal
biozone P1b. Our results provide evidence that sedimentation at El Kef was
not as continuous or free from structural complication as previously
thought. Despite these challenges, we present a new composite section from
the five El Kef holes and an age model correlated to the orbitally tuned
record at Walvis Ridge, South Atlantic Ocean, which is critical in placing
the paleoenvironmental and paleoecological records from El Kef in a global
context.
Implications of an integrated late Ediacaran to early Cambrian
stratigraphy of the Siberian Platform, Russia
Fred T. Bowyer; Andrey Yu Zhuravlev; Rachel Wood; Fangchen Zhao; Sergei S.
Sukhov ...
The transition from the terminal Ediacaran to early Cambrian (ca. 550−530
Ma) witnessed both the decline of Ediacaran-type soft-bodied and skeletal
biota and the rapid diversification of Cambrian-type skeletal biota, which
dominate the Terreneuvian (ca. 538.8−521 Ma) fossil record. This interval
hosts globally widespread positive and negative δ13Ccarb excursions, including a negative δ13C carb excursion near the Ediacaran-Cambrian boundary termed the
1n/BACE. Efforts to produce a global composite chemostratigraphic and
biostratigraphic correlation through this interval are complicated by
stratigraphic incompleteness and a dearth of radiometric ages with which to
constrain δ13Ccarb chemostratigraphy. Extensive and
richly fossiliferous open-marine carbonates of the Siberian Platform were
deposited from the terminal Ediacaran to beyond Cambrian Series 2, and they
offer a unique archive to refine this chemostratigraphic and
biostratigraphic framework. Here, we present new δ13C carb data from two sections of the southeastern Siberian
Platform, and we synthesize these with published δ13C carb data from multiple sections throughout the Siberian
Platform that record near-continuous carbonate deposition from the latest
Ediacaran to Cambrian Series 2. This compilation allowed the construction
of two possible chemostratigraphic age models that conform to a coherent
framework of lithostratigraphic correlation and platformwide stratal
stacking patterns. These age models were then used to test alternative
calibrations of fossil first appearances and the spatiotemporal evolution
of carbonate deposition on the Siberian Platform. Both models support a
pre-1n/BACE appearance of anabaritids in the most distal open-marine
sections, and they confirm a transitional Ediacaran-Cambrian biotic
assemblage that consisted of co-occurring cloudinids and anabaritids.
Sedimentologic and sequence stratigraphic analysis on the Siberian Platform
also provides strong evidence to indicate that the 1n/BACE marks the onset
of a gradual, pulsed rise in relative sea level that was sustained
throughout the Terreneuvian and Series 2 of the Cambrian.
Provenance of eolian deposits in the Xorkol Basin: Implications for
Eocene dust-transport pattern in western China
Feng Pan; Jianxing Li; Junsheng Nie; Yong Xu; Leping Yue
Atmospheric circulation in western China significantly affects the inland
climate of Asia. Constraining the provenance of eolian sequences can
provide robust evidence for dust-transport pathways, and thus the evolution
of paleo-atmospheric circulation. Western China has been dominated by the
westerly wind regime since the late Oligocene, as demonstrated by
provenance studies of eolian deposits. However, the wind regime responsible
for the accumulation of eolian deposits during the Eocene is unclear due to
a lack of corresponding provenance studies. Here, we present the first
provenance analysis for an Eocene (ca. 51−40 Ma) eolian Red Clay sequence
in the Xishuigou Formation of the Xorkol Basin in western China, based on
multiproxy geochemical and geochronological data. Comparison of zircon U−Pb
ages from the Xishuigou sequence with ages from potential source regions
reveals that the northern piedmont of the West Kunlun Range was a
persistent source, although sediments from the nearby Altun-Xorkol region
were an important dust source for the lower part of the Xishuigou sequence
which were deposited between ca. 51 Ma and 46.5 Ma. These results reveal
that the westerly dust-transport pattern has been dominant in western China
since at least ca. 51 Ma. The similarity of provenance between the Eocene
Red Clay of the Xishuigou Formation and the Neogene eolian sequence of the
overlying Caihonggou Formation in the same area implies that this
dust-transport pattern was stable from the Eocene to the Neogene, despite
dramatic changes in paleogeography, paleotopography, and global climate.
Origin of Late Cretaceous, enclave-bearing granitoids in southern
Tibet: Implications for magma recharge and crustal thickening
Ding-Jun Wen; Xiumian Hu; Jin-Hai Yu; Xiao-Lei Wang; Timothy Chapman ...
Exposures of enclave-bearing granitoids can provide rare opportunities to
directly evaluate the connection between compositional variability and the
depth of origin of arc magmatic rocks. The ∼1000 km long Gangdese batholith
is a composite batholith with composition from mafic to felsic; SiO 2 ranges from 51 wt% to 70 wt%. New zircon U−Pb dating of the
Nyemo plutons, Renbu plutons, and Xigaze plutons in the Gangdese batholith
is consistent with their emplacement and crystallization in the Late
Cretaceous (ca. 90−85 Ma). Mafic magmatic enclaves (MMEs) in the
plutons are characterized by low SiO2 (50.9−56.0 wt%) and Nb/U,
Ce/Pb, and Nb/La ratios coupled with enrichment in light rare earth
elements and large ion lithophile elements and depletion in high field
strength elements. These geochemical features, combined with depleted
whole-rock εNd(t) (+4.2 to +4.7) and zircon ε Hf(t) (+9.0 to +13.8), suggest that they were derived
by partial melting of a depleted mantle source associated with
subduction-related fluids. The granitoids with high SiO2
(55.6−66.9 wt%) display adakitic geochemical characteristics, such as low Y
and Yb contents, and high Sr/Y and La/Yb ratios. Their positive whole-rock
εNd(t) (+4.0 to +5.5) and zircon εHf( t) (+6.9 to +14.3) values, as well as enrichment of incompatible
elements, indicate that the granitoids were derived from partial melting of
the juvenile lower crust. Geochemical modeling suggests that the
compositional diversities of MMEs and adakitic granitoids were inherited
from heterogeneous sources. This genetic relationship indicates that the
underplated basaltic magmas could have supplied sufficient heat to trigger
the melting of the thickened crust and thus formation of the
enclave-bearing granitoid. In this regard, the origin of arc rocks can
mirror the evolution of crustal thickness. Our results reveal that the
crust was thickened to ∼50 km during the Late Cretaceous (90−85 Ma) and
provide a magmatic record of crustal thickening prior to the Cenozoic
Indo-Asia collision.
Late Holocene volcanic stratigraphy and eruption chronology of the
dacitic Young Doña Juana volcano, Colombia
Natalia Pardo; Roberto Sulpizio; Federico Lucchi; Guido Giordano; Shane
Cronin ...
We present the late Holocene eruption history of the poorly known Doña
Juana volcanic complex, in SW Colombia, which last erupted in the twentieth
century. This represents a case study for potentially active volcanism in
the rural Northern Andes, where tropical climate conditions and a
fragmented social memory blur the record of dormant volcanoes. We
reconstructed the volcanic stratigraphy of the central-summit vent area by
integrating new mapping at 1:5000 scale with radiocarbon ages,
sedimentology analysis, and historical chronicles. Our results revealed
cyclic transitions from lava-dome growth phases and collapse to explosive
Vulcanian and possibly subplinian phases. Pyroclastic density currents were
generated by dome collapse producing block-and-ash flows or by pyroclastic
fountain/column collapse and were rapidly channelized into the deeply
incised fluvial valleys around the volcano summit. The pyroclastic density
currents were ∼4−10 × 106 m3 in volume and deposited
under granular flow− or fluid escape−dominated depositional regimes at high
clast concentrations. In places, more dilute upper portions reached a wider
areal distribution that affected the inhabited areas on high depositional
terraces. The coefficient of friction (ΔH/L) is higher
for block-and-ash flows and dense lava−bearing fountain/low-column-collapse
pyroclastic density currents compared to pumice-bearing, column-collapse
pyroclastic density currents. Associated mass-wasting processes included
syneruptive and intereruptive debris flows, with the last one documented in
1936 CE.
A possible sea-level fall trigger for the youngest rejuvenated
volcanism in Hawaiʻi
Brian R. Jicha; Michael O. Garcia; Charline Lormand
Many intraplate oceanic islands undergo “rejuvenated” volcanism following
the main edifice-building stage. Honolulu features Hawaiʻi’s most recent
rejuvenated volcanism. K-Ar dating of Honolulu volcanism suggests that it
started at ca. 750 ka and ended at <100 ka. Here, we present new 40Ar/39Ar ages and olivine diffusion modeling from
Koko Rift lavas to resolve when the most recent Honolulu eruptions occurred
and to evaluate possible mechanisms of rejuvenated volcanism and volcanic
hazards. Diffusion modeling of olivine zoning profiles in Koko Rift basalts
suggests that magmas were stored in the crust for many months prior to
eruption. Six new 40Ar/39Ar ages cluster at 67 ± 2 ka
(2σ), which demonstrates that Koko Rift is Hawaiʻi’s youngest known area of
rejuvenated volcanism. The timing of Koko Rift eruptions coincides with the
pronounced drop in global sea level (∼100 m) during Marine Isotope Stage 4.
This major sea-level fall may have triggered the eruptions of Koko Rift
magmas that were stored in the crust for months to years at < 15 km
depth. The proposed mechanism is similar to that at other volcanic islands,
which suggests that changes in global sea level may have significant
control on the magnitude and frequency of eruptions at ocean island
volcanoes.
Porphyry copper mineralization triggered by sulfate reduction and
alkali metasomatism: Constraints from an experimental investigation
Jianping Li; Weihua Liu; Long Su; Dengfeng Li; Shitao Zhang ...
The potassium silicate (K-silicate) alteration zone is the main ore
contributor in porphyry copper deposits worldwide. Knowledge of element
behaviors in the alteration and mineralization processes is essential for
an improved understanding of porphyry copper mineralization, but they are
still not well understood. In this study, we reacted synthetic Cl-rich
fluids, containing K, Na, Cu, Mo, Zn, etc., with andesite in a complex
experimental system to simulate the shallow porphyry copper mineralization
process. We aimed to bridge the gap between simple experimental studies and
complex natural systems and to evaluate the contribution of sulfate
reduction to porphyry ore formation and its relationship with early alkali
metasomatism. The results show that increasing temperature (from 300 to 500
°C) enhances the K-silicate alteration by promoting ion-exchange reactions,
and the K-feldspar is mainly formed by the transformation of plagioclase
via a dissolution-reprecipitation processes. The low-salinity vapor phase
has a stronger capacity for K-silicate alteration than the liquid phase at
similar temperatures. In addition, increasing temperature from 300 to 500
°C favors sulfate reduction to further enhance metal sulfide precipitation.
The limited availability of reduced sulfur in the fluid causes preferential
precipitation of Cu-(Mo) sulfides, while most of the Zn is soluble in the
fluid, and Cu precipitation as sulfides in the vapor is much more efficient
than in the coexisting liquid. The overlap between the K-silicate
alteration zone and the mineralization triggered by sulfate reduction in
porphyry copper deposits is controlled by several concomitant factors,
e.g., relatively high temperature (e.g., at 400−500 °C), vapor formation,
and decompression. Moreover, K-silicate alteration would further promote
mineralization by changing fluid compositions, e.g., removing K from the
fluid.
GSA BULLETIN articles published ahead of print are online at
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by contacting Kea Giles. Please discuss articles of interest with the
authors before publishing stories on their work, and please make reference
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