Provenance and tectonic setting of the Early and Middle Devonian Xueshan Formation, the North Qilian Belt, China

The North Qilian orogenic belt is the key in the exploration of the formation and assembly of Asia. The Early–Middle Devonian Xueshan Formation, which is deposited in the north area of North Qilian Orogenic Belt, is the most important for revealing the evolution basin–mountain transition of the North Qilian Belt. In order to reveal the tectonic evolution of the central orogenic belt in China and Paleozoic global plate reconstruction, it is necessary to perform provenance analysis of the Xueshan Formation. Eighteen samples were collected from Yumen and Yongchang profiles in the west of North Qilian Belt (WN), and 16 samples were obtained from Gulang and Jingtai profiles in the east of North Qilian Belt (EN). The compositions of clastic conglomerates and the sandstones suggested that felsic volcanic was the main source of the WN Xueshan Formation deposits and that the sedimentary and metamorphic rocks were the main source of the EN sediments. According to major geochemistry indexes, such as Al2O3/SiO2 ratio ranging from 0.08 to 0.37 and the chemical index of alteration less than 80, the source rocks were moderately weathered and of moderately matured. The samples from the EN were relatively enriched in Cr, Ni, Sc, and V elements and depleted in Hf element. However, those from the WN were enriched in Hf without the enrichment of Cr, Ni, Sc, or V. The samples from WN showed a strong negative Eu anomaly, but the samples from EN showed a weak negative Eu anomaly or a positive Eu anomaly. All the above geochemical characteristics suggested the prominent input of felsic clasts with granitic rocks into the WN. Meanwhile, the discrimination diagrams of trace elements implied that WN rocks were derived from continental island arc and that the EN rocks were derived from orogenic belt and the passive continental margin. The North China Plate subducted southwards and formed subduction‐related arc magmatism along the southern margin of the North Qilian Terrane during the Early–Middle Devonian. In conclusion, the WN is a fore‐arc basin, but EN is a foreland basin.

Geochronology, geochemistry, and petrogenesis of the late Mesozoic Luoyang volcanics: Implications for the geodynamic evolution of the Zhejiang–Fujian region, SE China

The Zhejiang–Fujian region, situated along the southeastern coast of China and in the eastern part of the Cathaysia Block, is characterized by large volumes of late Mesozoic granitoid volcanics. We present new zircon U–Pb ages, whole‐rock geochemistry, and Sr─Nd─Hf‐isotope data for 3 rhyolite porphyries from the Luoyang region (LY‐1, LY‐2, and LY‐3). We identify 2 magmatic events. In Stage 1 (146 Ma), rhyolite porphyry LY‐1 was produced by the partial melting of Proterozoic Mayuan Group amphibolites and metasediments. In Stage 2 (130–126 Ma), rhyolite porphyries LY‐2 and LY‐3 were generated by the continued melting of amphibolites but with only a minor contribution from metasedimentary rocks. The fractionation of plagioclase, biotite, titanite, and allanite (degree of fractionation 0.7–0.5) from an LY‐2 parent magma produced LY‐3. Our new data, together with other published data, indicate that minor volcanism, related to the initial northwestward subduction of the Paleo‐Pacific Plate beneath SE China, occurred in a continental arc setting in the Zhejiang–Fujian coastal region from 160 to 140 Ma but that the major period of volcanism took place from 140 to 120 Ma in a back‐arc setting and occurred because of rollback of the subducting slab.

Origin of ooids in ooidal‐muddy laminites: A case study of the lower Cambrian Qingxudong Formation in the Sichuan Basin, South China

Ooids have been considered the most intriguing component of carbonate rocks, but the formation mechanism of carbonate ooids remains a matter of intense discussion. The lower Cambrian Qingxudong Formation in the southeastern Sichuan Basin contains abundant ooidal‐muddy laminites. Petrographic and sedimentological analyses revealed that these laminites formed in a shallow‐marine environment (preferably along the windward side of the reef‐shoal). Changing energy conditions caused variable supplies of grainy and muddy accumulation. A wide variety of ooids developed in grainy intervals. The normal ooids (2 mm in diameter) in the upper portion of the laminites developed in a deepening and moderate‐energy subtidal environment but were influenced by episodic hydrodynamic events. Petrographic and geochemical analyses suggest that microbes played an important role in ooid formation. High carbonate‐supersaturated and alkaline conditions that were mediated by microbial activity (e.g., bacterial sulfate reduction and photosynthesis) and agitated seawater conditions were the most important factors that mediated the formation of the Qingxudong ooids. This study provides new insights into paleooceanographic conditions during the Early Cambrian and offers a perspective on the formation of ancient normal and giant ooids.

An island‐arc tectonic setting for the Neoarchean Sonakhan Greenstone Belt, Bastar Craton, Central India: Insights from the chromite mineral chemistry and geochemistry of the siliceous high‐Mg basalts (SHMB)

The Neoarchean Sonakhan Greenstone Belt, located in the northeastern fringes of Bastar Craton, Central India, is dominated by basalts, andesites, dacites, and rhyolites and also contains some basic rocks with very high MgO (up to 33.4 wt%). Chromite mineralization is present in these rocks along with the cumulates of olivine and clinopyoxenes. The rocks are classified as siliceous high‐magnesium basalts (SHMB) exhibiting enriched large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to the high field strength elements. Elevated Th/Yb ratios and negative Nb‐Ta‐Ti anomalies in the primitive mantle normalized multielement diagram indicates a significant role of subduction‐related melts/fluids in their genesis. The chromites in SHMB have high Cr# (0.67–0.75) and moderate Mg# (0.11–0.5) values. Parental melt calculations in the chromites indicate that they are crystallized from an SHMB magma in an island‐arc setting. A plausible model for the genesis of the rocks of Sonakhan Greenstone Belt includes initial subduction of an intraoceanic lithosphere followed by eruption of lava in an oceanic environment. Continued subduction of the slab followed by slab rollback followed by the generation of SHMB parental magma, which was introduced into the basal portions of the lithosphere in which cumulates of olivine and clinopyroxene have been developed and the chromite mineralization occurred in the inter cumulus space. Final emplacement of the magma took place in a forearc suprasubduction‐zone environment with SHMB signature carrying the cumulates, which were located in the lower part of the lithosphere.

Miocene depositional environments, processes, and depositional elements in the southern Gulf of Mexico

Detailed stratigraphic and facies analyses were conducted and combined with seismic facies and 3D seismic‐derived plan view images to interpret the depositional environments, processes and depositional elements during the Miocene for an area located in the southern Gulf of Mexico.
The results showed that deposition during the Miocene mainly occurred in a slope setting, with bathymetric changes associated with highs and mini‐basins related to salt features. From the sedimentological interpretation, 13 sedimentary facies were identified. The abundant lithofacies were structureless (massive) sandstone and massive mudstone. Ripple‐ , parallel and cross‐laminated sandstone and siltstone were found in minor proportions. The main depositional processes were related to turbidity currents, including high‐density and low‐density currents; debris flows (mud flows and grain flows) were of secondary importance, as was deposition from fallout of suspended hemipelagic mud particles.
The vertical and lateral distributions of facies revealed seven facies associations linked to depositional environments. These facies associations were the building blocks that were used to characterize the depositional elements recognized on seismic data. The main depositional elements identified were mass‐transport complexes, submarine channels, and frontal splays.
Finally, interpretations from different data sets enabled the conclusion that, during the Miocene in this area were submarine fans deposited on an irregular paleotopography, with topographic lows and highs controlled by salt tectonics.

Structural analysis of ductile shear zones in the North Qinling Orogen and its implications for the evolution of the Proto‐Tethys Ocean

The ductile shear zones in the North Qinling Orogen, such as the Luonan‐Luanchuan, Guanpo‐Qiaoduan, Zhuyangguan‐Xiaguan, and Shangdan shear zones, play an important role in the preclosure and postclosure evolution of the Proto‐Tethys Ocean. On the basis of detailed macrostructural/microstructural analysis and quartz C‐axis electron backscattered diffraction fabric analysis, the sense and formation environment of the 4 shear zones have been obtained. The Luonan‐Luanchuan and Guanpo‐Qiaoduan shear zones mainly present as dextral and sinistral shearing under low temperature, respectively. The Zhuyangguan‐Xiaguan shear zone mainly presents as dextral shearing under medium temperature. The Shangdan shear zone underwent an early dextral shearing under medium temperature and a late sinistral shearing under low temperature. The Qinling Group and the Taibai region were intensively affected by the late sinistral shearing of the Shangdan shear zone. According to published metamorphic ages, the 4 shear zones should have been formed during the collision between the North China Block and the South China Block. Due to the opening of the Mianlue and the Paleo‐Tethys oceans since ~380 Ma, the North China Block and the South China Block departed from northern Gondwanaland and drifted northward along transform faults, which led to the arrangement between the North China Block and the South China Block that transited from an east–west direction to a north–south direction. Since ~320 Ma, the scissors‐type closure between the North China Block and the South Qinling Terrane led to the westward extrusion of the North Qinling Orogen, and the dextral shearing of the Luonan‐Luanchuan and the Zhuyangguan‐Xiaguan shear zones and the sinistral shearing of the Guanpo‐Qiaoduan and the Shangdan shear zones.

Tiny ocean waves could make large ice shelves crumble (plus VIDEO)

Small ocean waves could play a bigger role in breaking up ice shelves than tsunamis or other large waves, a new study suggests. A new study examining vibrations in Antarctica’s Ross Ice Shelf finds small waves continuously impacting the ice shelf may create enough strain to extend existing cracks in the ice and potentially create new ones. An ocean wave of 1 centimeter (0.5 inches) in height can cause vibrations that repeatedly move the ice more than 20 centimeters (8 inches).