Early Cretaceous Adakitic Rocks in the Northern Great Xing’an Range, NE China: Implications for the Final Closure of Mongol-Okhotsk Ocean and Regional Extensional Setting

A large amount of igneous rocks in NE China formed in an extensional setting during Late Mesozoic. However, there is still controversy about how the Mongol-Okhotsk Ocean and the Paleo-Pacific Ocean effected the lithosphere in NE China. In this paper, we carried out a comprehensive study for andesites from the Keyihe area using LA-ICP-MS zircon UPb dating and geochemical and Hf isotopic analysis to investigate the petrogenesis and tectonic setting of these andesites. The U-Pb dating yields an Early Cretaceous crystallization age of 128.3±0.4 Ma. Geochemically, the andesites contain high Sr(686–930 ppm) and HREE contents, low Y(11.9–19.8 ppm) and Yb(1.08–1.52 ppm) contents, and they therefore have high Sr/Y(42–63) and La/Yb(24–36) ratios, showing the characteristics of adakitic rocks. Moreover, they exhibit high K_2O/Na_2O ratios(0.57–0.81), low Mg O contents(0.77–3.06 wt%), low Mg# value(17–49) and negative εHf(t) values(-1.7 to-8.5) with no negative Eu anomalies, indicating that they are not related to the oceanic plate subduction. Based on the geochemical and isotopic data provided in this paper and regional geological data, it can be concluded that the Keyihe adakitic rocks were affected by the Mongol-Okhotsk tectonic regime, forming in a transition setting from crustal thickening to regional extension thinning. They were derived from the partial melting of the thickened lower crust. The closure of the Mongol-Okhotsk Ocean may finish in early Early Cretaceous, followed by the collisional orogenic process. The southern part region of its suture belt was in a post-orogenic extensional setting in the late Early Cretaceous.     

From a river valley to a ria: Evolution of an incised valley (Ría de Ferrol,north‐west Spain) since the Last Glacial Maximum

The evolution of incised valleys is an important area of research due to the invaluable data it provides on sea‐level variations and depositional environments. In this article the sedimentary evolution of the Ría de Ferrol (north‐west Spain) from the Last Glacial Maximum to the present is reconstructed using a multidisciplinary approach, combining seismic and sedimentary facies, and supported by radiocarbon data and geochemical proxies to distinguish the elements of sedimentary architecture within the ria infill. The main objectives are: (i) to analyse the ria environment as a type of incised valley to evaluate the response of the system to the different drivers; (ii) to investigate the major controlling factors; and (iii) to explore the differentiation between rias and estuaries. As a consequence of the sea‐level rise subsequent to the Last Glacial Maximum (ca 20 kyr bp ), an extensive basin, drained by a braided palaeoriver, evolved into a tide‐dominated estuary and finally into a ria environment. Late Pleistocene and Holocene high‐frequency sea‐level variations were major factors that modulated the type of depositional environments and their evolution. Another major modulating factor was the antecedent morphology of the ria, with a rock‐incised narrow channel in the middle of the basin (the Ferrol Strait), which influenced the evolution of the ria as it became flooded during Holocene transgression. The strait acted as a rock‐bounded ‘tidal inlet’ enhancing the tidal erosion and deposition at both ends, i.e. with an ebb‐tidal delta in the outer sector and tidal sandbanks in the inner sector. The final step in the evolution of the incised valley into the modern‐defined ria system was driven by the last relative sea‐level rise (after 4 kyr bp ) when the river mouths retreated landward and a single palaeoriver was converted into minor rivers and streams with scattered mouths in an extensive coastal area.     

Effect of pedogenesis on the stable isotopic composition of calcretes and n‐alkanes: Implications for palaeoenvironmental reconstruction

In a fluvial system, depending on sub‐aerial exposure, non‐pedogenic pond calcretes can be modified into pedogenic calcretes. The present study attempts to understand the effect of sub‐aerial exposure and pedogenesis on calcretes using carbon and oxygen isotopic composition. For this purpose, two profiles (profile‐A and profile‐B) from the same stratigraphic level in Rayka from the western part of India were selected. The profiles are separated by a distance of 500 m and showed differences in calcrete characteristics. In profile‐A, the calcretes showed pedogenic features (root traces and void filling spar) whereas calcretes in profile‐B showed non‐pedogenic characteristics (fine laminations). However, some of the calcretes in profile‐A exhibited remnants of fine laminations suggesting that initially the calcretes had a non‐pedogenic origin but were modified due to pedogenesis. In profile‐A, the carbon and oxygen isotope values of pedogenic calcrete (δ¹³C_PC and δ¹⁸O_PC) showed more variation compared with non‐pedogenic pond calcretes (δ¹³C_SPC and δ¹⁸O_SPC) in profile‐B. The δ¹³C_PC and δ¹³C_SPC values exhibited a spread of 3·0‰ and 1·3‰, respectively, and δ¹⁸O_PC and δ¹⁸O_SPC values showed a spread of 2·3‰ and 1·3‰, respectively. The differences in the isotopic composition between the two profiles suggest that pedogenesis controlled the isotopic inheritance in calcretes. In addition, the carbon isotopic composition of organic matter (δ¹³C_OM) and n‐alkanes (δ¹³C_n‐alk) that forms the basis of palaeovegetational reconstruction have also been measured to understand the effect of pedogenesis on organic matter in both of the profiles. The average δ¹³C_OM values in profile‐A and profile‐B are ?23·4‰ and ?21·1‰, respectively. The disparity in δ¹³C_OM values is a result of the difference in the sources and preservation of organic matter. However, the δ¹³C_n‐alk values show a similar trend in profile‐A and profile‐B, indicating that sources of n‐alkanes are the same in both of the profiles and δ¹³C_n‐alk values are unaffected by the pedogenic modifications.     

Stratigraphic evolution of the nearshore to fluvial plain of the Upper Cuyo Group,Neuquén,Argentina

The upper portion of the Cuyo Group in the Zapala region, south‐eastern Neuquén Basin (Western Argentina), encompasses marine and transitional deposits (Lajas Formation) overlain by alluvial rocks (Challacó Formation). The Challacó Formation is covered by the Mendoza Group above a second‐order sequence boundary. The present study presents the stratigraphic framework and palaeophysiographic evolution of this Bajocian to Eo‐Calovian interval. The studied succession comprises the following genetic facies associations: (i) offshore and lower shoreface–offshore transition; (ii) lower shoreface; (iii) upper shoreface; iv) intertidal–subtidal; (v) supratidal–intertidal; (vi) braided fluvial to delta plain; (vii) meandering river; and (viii) braided river. The stratigraphic framework embraces four third‐order depositional sequences (C1 to C4) whose boundaries are characterized by the abrupt superposition of proximal over distal facies associations. Sequences C1 to C3 comprise mostly littoral deposits and display well‐defined, small‐scale transgressive–regressive cycles associated with fourth‐order depositional sequences. Such high‐frequency cycles are usually bounded by ravinement surfaces associated with transgressive lags. At last, the depositional sequence C4 delineates an important tectonic reorganization probably associated with an uplift of the Huincul Ridge. This is suggested by an inversion of the transport trend, north‐westward during the deposition of C1 to C3 depositional sequences (Lajas Formation) to a south‐west trend during the deposition of the braided fluvial strata related to the C4 depositional sequence (Challacó Formation).     

Deep‐water dunes on drowned isolated carbonate terraces (Mozambique Channel,south‐west Indian Ocean)

Subaqueous sand dunes are common bedforms on continental shelves dominated by tidal and geostrophic currents. However, much less is known about sand dunes in deep‐marine settings that are affected by strong bottom currents. In this study, dune fields were identified on drowned isolated carbonate platforms in the Mozambique Channel (south‐west Indian Ocean). The acquired data include multibeam bathymetry, multi‐channel high‐resolution seismic reflection data, sea floor imagery, a sediment sample and current measurements from a moored current meter and hull‐mounted acoustic Doppler current profiler. The dunes are located at water depths ranging from 200 to 600 m on the slope terraces of a modern atoll (Bassas da India Atoll) and within small depressions formed during tectonic deformation of drowned carbonate platforms (Sakalaves Seamount and Jaguar Bank). Dunes are composed of bioclastic medium size sand, and are large to very large, with wavelengths of 40 to 350 m and heights of 0·9 to 9·0 m. Dune migration seems to be unidirectional in each dune field, suggesting a continuous import and export of bioclastic sand, with little sand being recycled. Oceanic currents are very intense in the Mozambique Channel and may be able to erode submerged carbonates, generating carbonate sand at great depths. A mooring located at 463 m water depth on the Hall Bank (30 km west of the Jaguar Bank) showed vigorous bottom currents, with mean speeds of 14 cm sec^?1 and maximum speeds of 57 cm sec^?1, compatible with sand dune formation. The intensity of currents is highly variable and is related to tidal processes (high‐frequency variability) and to anticyclonic eddies near the seamounts (low‐frequency variability). This study contributes to a better understanding of the formation of dunes in deep‐marine settings and provides valuable information about carbonate preservation after drowning, and the impact of bottom currents on sediment distribution and sea floor morphology.     

Topographical controls on small‐volume pyroclastic flows

Emplacement of small‐volume (<0·1 km³) pyroclastic flows is significantly influenced by topography. The Arico ignimbrite on Tenerife (Canary Islands) is a characteristic small‐volume pyroclastic flow deposit emplaced on high relief topography. The pyroclastic flow flowed down pre‐existing valleys on the southern slopes of the island. In proximal areas deep (up to 100 m) valleys acted as efficient conduits for the pyroclastic flow, which was mostly channelled; in this particular area the ignimbrite corresponds to a homogeneous, moderately welded deposit, consisting of flattened pumices in an abundant ashy matrix with a relatively low lithic fragment content. In intermediate zones significant changes occur in the steepness of the slope and, although still channelled, here the pyroclastic flow was influenced by hydraulic jumps. In this area, two different units can be clearly distinguished in the ignimbrite: the lower unit is composed of a lithic‐rich ground‐layer deposit that formed at the turbulent, highly concentrated head of the flow; the upper unit consists of a well welded pumice‐rich deposit that occasionally reveals a basal layer formed by shearing with the lower part. This division into two units is maintained as far as distal areas near the present‐day coastline, where the slope is very gentle or null and the ignimbrite is not channelled. The ground layer is not found in distal areas. The ignimbrite here only consists of the upper unit, which is occasionally repeated due to a surging process provoked by the lower flow speed, as the pyroclastic flow spread out of the channelled zone. A theoretical model on how topography controlled the deposition of the Arico ignimbrite is derived by interpreting the observed lithological and sedimentological variations in terms of changes in topography and bedrock morphology. This new model is of general applicability and will help to explain other deposits of similar characteristics.     

准饱和多孔介质中圈闭气体对渗透系数的影响

含圈闭气体的地下水流称为准饱和流,准饱和流中的圈闭气体对含水层渗透系数有重大影响。通过柱试验开展了粉砂、细砂、中砂和粗砂4种介质圈闭气体饱和度与准饱和渗透系数关系的研究。结果表明:圈闭气体饱和度明显受介质的粒径影响,在细粒介质中圈闭气体饱和度明显较大;4种介质圈闭气体饱和度在0~15%范围内,准饱和渗透系数与完全饱和相比减少了32.82%~56.38%,且准饱和渗透系数与圈闭气体饱和度之间可表达为一个负线性相关的经验公式;该公式与Faybishenko公式等效,但形式简单,参数较少,使用方便;准饱和渗透系数的变化规律可概化为圈闭气体占据了原有的有效孔隙,造成原有效孔隙度减少,从而使渗透系数减小。利用该理论,Kozeny-Carman方程能较准确地描述准饱和渗透系数的变化规律,而基于哈根-泊肃叶方程的渗透系数公式则存在较大误差,不适用于描述准饱和渗透系数;试验结果证明了室内测定饱和渗透系数时排除圈闭气体的必要性。     

Ore Genesis and Tectonic Significance of the Xiaojiashan Tungsten Deposit,Eastern Tianshan,Xinjiang, China: Evidence from Geology,Fluid Inclusions,and Stable Isotope Geochemistry

The Xiaojiashan tungsten deposit is located about 200 km northwest of Hami City, the Eastern Tianshan orogenic belt, Xinjiang, northwestern China, and is a quartz vein‐type tungsten deposit. Combined fluid inclusion microthermometry, host rock geochemistry, and H–O isotopic compositions are used to constrain the ore genesis and tectonic setting of the Xiaojiashan tungsten deposit. The orebodies occur in granite intrusions adjacent to the metamorphic crystal tuff, which consists of the second lithological section of the first Sub‐Formation of the Dananhu Formation (D₂d ₁²). Biotite granite is the most widely distributed intrusive bodies in the Xiaojiashan tungsten deposit. Altered diorite and metamorphic crystal tuff are the main surrounding rocks. The granite belongs to peraluminous A‐type granite with high potassic calc‐alkaline series, and all rocks show light Rare Earth Element (REE)‐enriched patterns. The trace element characters suggest that crystallization differentiation might even occur in the diagenetic process. The granite belongs to postcollisional extension granite, and the rocks formed in an extensional tectonic environment, which might result from magma activity in such an extensional tectonic environment. Tungsten‐bearing quartz veins are divided into gray quartz vein and white quartz veins. Based on petrography observation, fluid inclusions in both kinds of vein quartz are mainly aqueous inclusions. Microthermometry shows that gray quartz veins have 143–354°C of T_h, and white quartz veins have 154–312°C of T_h. The laser‐Raman test shows that CO₂ is found in fluid inclusions of the tungsten‐bearing quartz veins. Quadrupole mass spectrometry reveals that fluid inclusions contain major vapor‐phase contents of CO₂, H₂O. Meanwhile, fluid inclusions contain major liquid‐phase contents of Cl^?, Na⁺. It can be speculated that the ore‐forming fluid of the Xiaojiashan tungsten deposit is characterized by an H₂O–CO₂, low salinity, and H₂O–CO₂–NaCl system. The range of hydrogen and oxygen isotope compositions indicated that the ore‐forming fluids of the tungsten deposit were mainly magmatic water. The ore‐forming age of the Xiaojiashan deposit should to be ~227 Ma. During the ore‐forming process, the magmatic water had separated from magmatic intrusions, and the ore‐bearing complex was taken to a portion where tungsten‐bearing ores could be mineralized. The magmatic fluid was mixed by meteoric water in the late stage.