Geochemistry, sediment provenance, and the Holocene paleoenvironmental evolution of the Southern Po River coastal plain (Italy) were examined based on the geochemical analysis of 279 sediment samples from 35 cores. The Holocene succession is subdivided into four major facies associations: back‐barrier, transgressive barrier, shallow‐marine, and beach‐ridge deposits. Bulk chemical composition shows remarkable differences between facies associations, and the poly‐modal behavior in the related frequency curves suggests distinct sources of sediment. Using MgO and Ni as geochemical tracers, changes in sediment provenance were assessed with the aim to reconstruct the Holocene paleoenvironmental evolution of the area. The Po River and the Apenninic rivers represented the main sources of sediment during the early stages of transgression and highstand sea‐level conditions, whereas transgressive barrier and early shallow‐marine facies associations exhibit a clear North Adriatic river (eastern Alpine) provenance. The Holocene paleoenvironmental evolution of the study area, between about 10 ky and the present, indicates the presence of a bypass zone in front of the Po River mouth between 9 and 6 ky BP. At that time, sediment supplied by the North Adriatic rivers was transported southward by the longshore drift, bypassed the weak Po River influx and accumulated close to the Apennines. Comparison between Holocene facies associations and current back‐barrier and shallow‐marine sediments confirms that the present‐day basin circulation is similar to the one that acted during the last episode of sea‐level rise.
The Jinchang Au deposit is a large‐sized deposit in Heilongjiang Province, and mineralization is closely related to the Early Cretaceous granitoids. The orebodies occurred within granite, granite porphyry, granodiorite, and diorite. Gold mineralization type consists of crypto‐explosive breccia, vein, and veinlet‐disseminated types. In this paper, we present U–Pb age as well as major and trace element geochemistry of intrusive bodies from the Jinchang deposit. Our study indicates that granite, granite porphyry, and granodiorite from the Jinchang deposit are calc‐alkaline to high‐K calc‐alkaline with geochemical characteristics of typical island‐arc environment. Diorite and diorite porphyrite show calc‐alkaline affinities with geochemical characteristics of adakite or adakitic rock. Concordant zircon U–Pb ages of 184.69 ± 0.98 Ma and 203.62 ± 0.86 Ma are obtained from granites; 190.4 ± 4.7 Ma, 205.88 ± 0.95 Ma, and 109.0 ± 2.4 Ma are obtained from granite porphyry, granodiorite, and quartz diorite. Combined with previous geochronological data, the diagenetic ages for granitoids in the Jinchang deposit are mainly focused on 2 episodes: 110 ~ 120 Ma and 185 ~ 200 Ma. The corresponding tectonic settings for the 2 episodes of magmatic activities are subduction of the Izanagi Plate towards the Eurasian Plate, and post‐collision system after the collision of North China Craton and Jiamusi‐Khanka Massif, respectively. We suggest that the magmatism (110 ~ 120 Ma) formed adakitic diorite and diorite porphyrite and closely associated with mineralization. Upwelling mantle results in partial melting of the lower crust and extract and transfer metals to the upper crust for further concentration. It also reveals that Jinchang Au deposit is an adakite‐associated deposit.
The Southeast Trans‐Baikal region in Russia hosts large and super large Mesozoic polymetallic mineral deposits including chalcophile and lithophile elements, rare metals, rare earth elements, and uranium. Here, we analyse the salient features of this metallogenic belt in terms of their distribution, mineral speciation, and relation to regional structures. The deep mantle architecture beneath this region is analyzed from published geophysical data including seismic tomography. The results show that the highly productive ores are largely located above the axial line of an undulating Moho that projects from the apex region of a subducted slab stagnant in the mantle transition zone. We propose that thermal and fluid flux from the asthenospheric mantle, metasomatism of the overlying shallow mantle, and mantle–crust interaction intensified the magmatism associated with metallogeny. Our study provides an example for deep‐seated geodynamics influencing the formation and distribution of giant ore belts.
Results of analyses of a shallow marine sediment vibrocore from the Atlantic seafloor just west of the Aran Islands, western Ireland, are presented. The sediments range from ~16,000 to younger than 9,000 cal BP in age, based on 14C dating of foraminifers and molluscs. One sample within the core is dated at 12804 ± 125 cal BP and lies a very short distance stratigraphically below a conspicuous horizon containing isolated pebbles and sand‐size clasts, in what is an otherwise fine‐grained sequence of sediment. These outsized clasts are interpreted to be ice‐rafted debris. Approximately coeval with this horizon, the diversity and abundance of foraminifers decrease significantly and the foraminifer Nonionellina labradorica makes its first appearance in the core sediment. Stable isotope analysis shows a significant increase in values of δ18O approaching the horizon containing ice‐rafted debris. These results show the effects of a relatively sudden cooling of this part of the Atlantic Ocean at middle latitudes coincident with, or just after, the onset of the Younger Dryas event and demonstrate the extent of floating icebergs and probable later pack ice as a result of this temperature decrease.
The Chagan Sag is a small Early Cretaceous intracontinental passive rift subbasin in the Yingen‐Ejinaqi Basin Group generated on the Tianshan‐Xingmeng Orogenic Belt. The Lower Cretaceous strata contain the Bayingebi, Suhongtu, and Yingen formations deposited during the syn‐rift phase. The Wuliji Structural Zone bounded by a high‐dip syn‐sedimentary fault is an intensively studied area with an extent of ~300 km2 having abundant available subsurface dataset and great hydrocarbon potential. According to well‐ and seismic‐based sequence stratigraphic study, the Upper Bayingebi Member generally corresponds to a complete third‐order transgressive–regressive sequence in the Wuliji Structural Zone. The transgressive system tract gradually thins towards the north, but the regressive system tract covers the entire study area in this third‐order sequence. Further investigations reveal that tectonic subsidence and climate change are two predominant allogenic factors for controlling stratigraphic development in the Wuliji Structural Zone.
During the deposition of the Upper Bayingebi Member, transverse gravity flow dominated fan‐delta complexes sourced from the Maodun Subuplift are well developed in the Wuliji Structural Zone. The retrogradational conical fan deltas are mainly located in the southern sector during the transgressive system tract stage, but the progradational Gilbert‐type fan deltas are widely developed during the regressive system tract stage. The sedimentary evolution and stratigraphic architecture are directly linked with preexisting topography, accommodation, and sediment supply. In addition, climate change and vegetation types are thought to be critical controls for fluid properties originated from feeder drainage systems and depositional processes of the construction of fan‐delta complexes in the distinct system tracts.
The Beishan area, located in the southernmost part of the Central Asia Orogenic Belt, is crucial for understanding the tectonic evolution associated with the closure of the Paleo‐Asian Ocean. One intermediate and two granitic plutons from the eastern Beishan area between Mingshui–Shibanjing–Xiaohuangshan (MSX) and Niujuanzi–Xichangjing (NX) ophiolitic belts, including Tongchangkou gabbroic diorite, Northern Tongchangkou and Sandaomingshui plutons, are dated at 426 ± 3, 408 ± 2, and 365 ± 2 Ma by SIMS U–Pb zircon analyses. The Middle Silurian Tongchangkou gabbroic diorites with medium‐K calc‐alkaline metaluminous characteristics represent volcanic arc rocks. The Early Devonian Northern Tongchangkou pluton, a highly evolved calc‐alkaline mylonitic monzogranite with low εNd(t), −3.5 and −2.4, and relatively low initial 87Sr/86Sr (0.707095 and 0.707249), was mainly derived from Mesoproterozoic mafic to intermediate igneous protolith and shows both volcanic arc and postcollisional characteristics. The Late Devonian Sandaomingshui pluton, a calc‐alkaline granite with slightly positive εNd(t), 0.6 and 0.9, and low (87Sr/86Sr)t, 0.705543 and 0.705851, is likely from a more depleted source and represents a volcanic arc granite. Combined with previous studies of Late Paleozoic granites, we find that the widely distributed Permian granites in Central Asia Orogenic Belt are absent between MSX and NX ophiolitic belts. Given the regional geology, we conclude that the back‐arc basin represented by the NX ophiolitic belt closed during Early Devonian and afterward, flat subduction of the oceanic basin represented by the MSX ophiolitic belt occurred. It also indicates that the oceanic crust was still being produced in the Paleo‐Asian Ocean during the early Carboniferous.
Eocene–Oligocene reefs have been reported in Europe largely from the circum‐Mediterranean region. In this paper, small coral reefs from the northwestern Transylvanian Basin (Romania) are described for the first time. They developed near the Eocene–Oligocene boundary, most probably during the Priabonian, and belong to northernmost outposts of the reef belt developed during this time in Europe. The studied sedimentary successions in Letca and Băbeni–Cuciulat (Sălaj County), up to 55 m thick, belong to the Cozla Formation. The reefs occur within a shallow‐water succession composed mostly of bedded limestones, dominated by bioclastic (coralline) packstones. Low‐relief (constratal) reefs, locally up to 10–15 m in thickness, are spaced cluster (matrix‐supported) reefs. Scleractinian corals are common but poorly diversified (10 species and 8 genera). Branching ramose colonies, branching low‐integrated phaceloid, and sheet‐like (foliaceous) corals dominate. Neither lateral zonation nor vertical succession of reefs was recognized. Corals co‐occur with encrusting and geniculate red algae, but they are of subordinate significance for a nonrigid reef framework. Branching corals baffled or trapped suspended carbonate mud that contributed to the reef growth and ongoing development of topographic relief. Associated fossils are of low to moderate diversity. A relatively low‐energy environment, moderate to high sedimentation rate, and increased turbidity are inferred from carbonate muddy and fine‐grained matrix, dominance of sediment‐resistant corals, their morphology, common occurrence in growth position, as well as low to moderate degree of bioerosion and encrustation. Transylvanian reefs in terms of poor coral diversity, matrix‐supported texture, and turbid‐water sedimentary setting show similarities with many coeval reefs from the circum‐Tethyan area.
Mineralogical, petrographic, and geochemical analyses were performed to investigate the sandstone composition and texture, pore system, and diagenetic minerals of Upper Triassic sandstones in the Central Sichuan Basin, Southwest China. The results show that authigenic quartz, clay minerals, and carbonates are the main pore‐filling constituents. The pore systems consist of intergranular porosity, secondary porosity, microporosity, and microfractures. Loss of depositional porosity is greater due to compaction than to cementation. Eodiagenesis includes mechanical compaction, precipitation of early calcite cements, kaolinite, and smectite, and leaching of feldspars by meteoric water. Mesodiagenesis consists of compaction, framework grain dissolution, and subsequent precipitation of illite, quartz, and late carbonate cements. Six lithofacies were identified on the basis of petrographic analyses, namely, (a) argillaceous sandstones; (b) poorly sorted sandstones; (c) quartz‐cemented sandstones; (d) carbonate‐cemented sandstones; (e) clay‐mineral‐cemented sandstones, and (f) clean sandstones with abundant secondary porosity. The best reservoir‐quality rocks have high percentages of detrital quartz but low percentages of matrix, quartz, and carbonate cement. Differences in textural and compositional attributes of various lithofacies significantly affect porosity‐depth trends. The diagenetic evolution pathways and reservoir‐quality prediction models of various lithofacies are reconstructed by integrated petrographic data. This work provides insights into describing the different lithofacies by petrographic analysis and helps to investigate how detrital composition and texture influence the diagenesis and reservoir quality evolution in tight gas sandstones.
Yongxing Island is a coral reef island formed by the growth and accumulation of corals and other skeletal fragments on a reef platform. The reef flat is developed around the entire island. The southeast reef flat and the north and northwest reef flat are asymmetrical in proportions, showing significant differences in reef‐flat width, patterns of sedimentary and ecological zonation, fore‐reef slope gradients, and submarine geomorphic features. Through application of an underwater survey, measurements and statistics were recorded on the extensional pattern of the Yongxing reef platform, the reef pool, and the spur–groove systems incised on the reef crests and forereefs. It is demonstrated that the Yongxing reef platform is an irregular ovoid in form with a distinct NW‐SE elongation that includes gently and steeply sloping forereefs and fore‐reef slopes. A gradual transition to the deep sea basin occurs in the southeast part of the Yongxing reef platform, whereas a steep forereef and fore‐reef slope that rapidly transition to the deep‐water basin are more characteristic of the north and northwest sectors of the Yongxing reef platform. The width of the reef platform in the southeast direction is much greater than that in the northwest direction. Correspondingly, the developmental pattern of the north and northwest reef crest and forereef is different from that of the southeast reef crests and forereefs. The southeast reef crest and forereef are characterized by reef pools with various shapes, different water depths, and discontinuous tidal channels on the outer reef flat. In contrast, the north and northwest reef crest and forereef developed a typical spur‐and‐groove system. The former is related to coral reef growth and construction, whereas the latter is shaped by wave or tide flow scouring that contributes to substrate erosion. Similar geomorphic features are known from examples of Indian‐Pacific and Caribbean‐Atlantic reef crests and forereefs, but the occurrence of such features in different parts of the same reef platform provides a new example of reef crest and forereef development. The differences in slope gradient of forereef and fore‐reef slopes (gentle vs. steep in transition), the development of contrasting submarine platform widths in different directions, as well as other submarine geomorphic features on reef crests and forereefs around the reef flats of Yongxing Island may be related to the confluence of several factors. These include the tectonic uplift of reef platform, the island’s initial underwater topography, the windward and leeward positions, the direction and strength of winter and summer monsoon, the sea‐water dynamics, and the substrate characteristics related to coral reef growth.
The widespread tectonic and magmatic events in Southeast China during the Mesozoic were accompanied by large‐scale metallogenesis. Here, we investigate the petrology, geochemistry, and zircon U–Pb age of the porphyry dykes in relation to the gold mineralization at Dongyang deposit in the Fujian Province. The geochemical data show silica‐rich and Mg‐ and Ca‐poor A‐type granite affinity for the dykes. The rocks have high concentrations of Light Rare Earth Elements (LREE) and Large Ion Lithophile Elements (LILE) and relatively low contents of Heavy Rare Earth Elements (HREE) and High Field Strength Element (HFSE). They display (La/Yb)N and (La/Sm)N values in the range of 5.7 to 31.4 and 6.0 to 8.3, respectively, with negative anomalies of Ba, Nb, Ta, Sr, Zr, Eu, and Ti and positive Ce anomaly. The source magma for the dykes was derived from a mixture of partially melted Al‐rich crustal material, with an input of mantle components. We present sensitive high‐resolution ion microprobe zircon U–Pb data from the dykes, which show emplacement ages in the range of 158 to 162 Ma, coinciding with the onset of the Yanshanian magmatism in Southeast China. Integrating the data obtained from studies on geology, geochronology and petro‐geochemistry, we suggest that the dykes played role in controlling the gold mineralization, and the magmatism and gold mineralization are the response of continental lithosphere thinning in Southeast China triggered by the subduction of the ancient Pacific Ocean Plate.