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 δ¹³C_carb 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 δ¹³C_carb 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 ε_Nd(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 ε_Nd(t) (+4.0 to +5.5) and zircon ε_Hf( 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 https://bulletin.geoscienceworld.org/content/early/recent. Representatives of the media may obtain complimentary copies of articles by contacting Kea Giles. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to The Geological Society of America Bulletin in articles published. Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.

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