Mineralogy and geochemistry of sands along the Manzanillo and El Carrizal beach areas, southern Mexico: implications for palaeoweathering, provenance and tectonic setting

The provenance of sands from the Manzanillo (MN) and El Carrizal (CAR) beach areas along the Mexican Pacific coast (southern Mexico) was investigated based on their modal composition, mineralogy and geochemical data. The average quartz, feldspar and lithic fragment (QFL) ratios revealed that the proportion of quartz is higher in CAR sands than in MN sands. In comparison with MN, the CAR sands are enriched in SiO2 content with higher SiO2/Al2O3 ratio indicating that sediment recycling is greatest for the CAR sands. The variation in K2O/Al2O3 and Na2O/K2O ratios between the MN and CAR sands specified a difference in the relative proportion of plagioclase. The chemical index of alteration (CIA) and Plagioclase Index of Alteration (PIA) values revealed a moderate weathering intensity in the source areas. The MN sands are characterized by a low degree of rare earth element (REE) fractionation (LaCN/YbCN = ~6–7), whereas the CAR sands define fractionated REE patterns (LaCN/YbCN = ~8.8–16.8). The fractionated REE pattern with elevated total low REE (∑LREE) content of the CAR sands suggested that sediment sorting is higher for the CAR than MN sands. Small negative and positive Eu anomalies (Eu/Eu* = ~0.91–1.23) of MN sands indicated the derivation of sediments from felsic‐intermediate source rocks. The CAR sands showed higher concentration of ∑LREE than total heavy REE (∑HREE) and characterized by a significant negative Eu anomaly (Eu/Eu* = ~0.46–0.78), suggested that the source rock is felsic type. The result obtained by the comparison of REE patterns of the beach sands to the probable source rocks is also consistent with this observation. The compositional differences identified between the MN and CAR beach areas suggested that longshore current in the mixing and homogenization of sands is not significant. The new tectonic discriminant‐function‐based multidimensional diagrams showed an arc setting (active) for the MN sands and a rift setting (passive) for the CAR sands, which is consistent with the regional geology of southern Mexico. Copyright © 2016 John Wiley & Sons, Ltd.

Vesicle distribution in acidic lava flow units in the rift basins of NE China: implications for petroleum reservoir exploration

Acidic lava flows comprise a large proportion of the total volcanic succession in the rift basins of northeast China. The objective of this study was to explore the vesicle distribution in the acidic lava flows in those basins. Two representative areas with acidic lava flow outcrops in NE China were selected for study. These lava flows share similar magma sources and chemistries. The types and inner structures of the acidic lava flows were determined in detail by measuring the shape of the flows (thickness and lateral extent) and the vertical sequences in the outcrops and shallow coring well Y1D1 in the two outcrop areas. Furthermore, the interpretation of deep borehole data and 3D seismic data from the Changling Graben of the Songliao Basin provided additional thickness and geometry data as a robust complement to the outcrop data. This study revealed that the shapes of the acidic lava flows transition from dome to tabular to braided with decreasing size. Furthermore, the vertical sequences can be classified into three zones: the upper vesicle zone, the massive core zone and the basal zone. The borehole thickness data indicate a trend in the maximum proportions occupied by the upper vesicle zone: thick flows have a relatively low proportion occupied by upper vesicle zones, whereas thin flows display a wide range in the proportion represented by upper vesicle zones. Therefore, braided acidic lava flows are more likely to have a high proportion of upper vesicle zones. Upper vesicle zones usually have high porosity and permeability and represent the primary effective gas reservoir system. The greatest lateral extents of gas reservoirs are usually in the tabular acidic lava flows, but the largest proportions occupied by gas reservoirs are usually in stacked braided lava flows. This study can help us to better understand the distribution of volcanic petroleum reservoirs in acidic lava successions where hydrocarbons are encountered but borehole data are limited. Furthermore, this study is useful for interpreting acidic lava flows in logs and seismic data from basins. Copyright © 2016 John Wiley & Sons, Ltd.

History of moisture change derived from slope sediments of the eastern Gonghe Basin (northeastern Qinghai–Tibetan Plateau) during the last 17 ka

The palaeosol–loess–aeolian sand sequence in the eastern Gonghe Basin, which is located at the convergence of the Asian summer monsoon, winter monsoon and the westerlies, reveals detailed moisture changes in the northeastern Qinghai–Tibetan Plateau (NETP) during the last 17 ka. Analysis of magnetic susceptibility (MS), total organic carbon, carbonate content, grain size and geochemical parameters indicate that the region was dominated by an extremely dry climate during ~17.0–15.82 ka, accompanying large‐scale desert expansion in the last glacial maximum (LGM). Subsequently, the climate became generally dry during 15.82–14.6 ka, an interval in which loess rapidly accumulated. Enhanced humidity occurred at 14.6 ka, probably associated with increased strength of the East Asian summer monsoon. The region experienced slightly decreased moisture at around 6.5 and 5.8 ka. After 2.7 ka, the climate became wetter. This interpretation correlates with climatic records from lacustrine and aeolian deposits in the NETP during the LGM, the last deglaciation and the early–middle Holocene. High climatic moisture in the Late Holocene facilitated the formation of a well‐developed palaeosol in the NETP. The effective moisture change in the Gonghe Basin is not simply ascribed to the influences of Asian summer monsoons; instead, it was possibly influenced by the interaction of evaporation and monsoonal precipitation forced by solar insolation variation. The balance between these variables was very influential on the effective moisture change in the closed inland basin of the NETP. Copyright © 2016 John Wiley & Sons, Ltd.

Facies controls on the distribution of diagenetic alterations in fan delta deposits: a case study from the Lower Cretaceous sandstone, Chagan Sag, Inner Mongolia, China

By integrating diagenesis and sedimentary facies, the distribution of diagenetic alterations and their impacts on reservoir quality were investigated within a lacustrine fan delta depositional environment in the Lower Cretaceous of the Western sub‐sag in the Chagan Sag, Yin‐E Basin, northern China. Core observation and analyses of petrography and geochemistry of the sandstones revealed that the eogenetic alterations display spatial and temporal distribution patterns associated with sedimentary facies, including distributary channels of a fan delta front (DC), distributary bays (DB) and sheet sands (SS) and mouth bar (MB) (SS and MB are collectively referred to as S). Percolation of meteoric waters occurring in high permeability DC sandstones resulted in leaching of feldspar, the formation of kaolinite, as well as mechanical infiltrated clays around detrital grains. Conversely, the DB deposits containing abundant ductile lithic fragments were subjected to mechanical compaction and thus the development of a pseudomatrix. During a long burial residence time (the Early Cretaceous period) in a lacustrine environment, abundant early carbonate cements were precipitated, especially in the distal S sandstones. The sandstones lacking eogenetic cements were subjected to stronger mechanical compaction. Additionally, eogenetic alterations have an important impact on the distribution of the mesogenetic alterations. Sandstones containing few eogenetic cements or thin or discontinuous infiltrated clay rims around the detrital grains were subjected to quartz cementation. However, during the hydrocarbon generation stage, the most efficient percolation of meteoric waters and organic acid dissolved feldspar minerals and carbonate cements, resulted in DC sandstones having more intragranular and intergranular porosity. Owing to high matrix contents and early calcite cement contents, the DB and S sandstones had lower permeability and were rarely dissolved. During the late mesogenetic stage, late carbonate cements occurred in all sedimentary facies and iron and magnesium ions were released from the transformation from kaolinite to illite or to chlorite. The results from this study show possible diagenetic evolutionary pathways in the reservoir sandstones within the fan delta depositional environment, which in turn provides some insights into the controls on reservoir potential. Copyright © 2016 John Wiley & Sons, Ltd.

U–Pb Zircon geochronology and geochemistry of Late Palaeozoic and Early Mesozoic igneous rocks of the Bujinhei area: implications for the tectonic evolution of south Great Xing’an Range

The Bujinhei region, situated in the southern reach of the Great Xing’an Range, has experienced multistage tectonic‐magmatic activities, which have led to the formation of different stages and types of magmatic rocks. LA‐ICP‐MS zircon U–Pb dating was performed, and geochemical data were obtained for magmatic rocks samples from the Bujinhei area to analyse the times of magmatic rocks and their tectonic setting. Zircon U–Pb dating indicates that Middle Permian (269.8 Ma) magmatic event formed two‐mica granite, and Middle Jurassic (158–170 Ma) magmatic events formed monzogranite, quartz monzonite‐porphyry and granite porphyry. Chemical analyses indicate that the Middle Permian two‐mica granite is S‐type granite, rich in SiO2, Al2O3 and K2O and low in Na2O, A/CNK > 1. Middle Jurassic monzogranite is a metaluminous I‐type granite, rich in SiO2, Al2O3 and K2O and low in Na2O and CaO. Quartz monzonite porphyry and granite porphyry are high in K2O and peraluminous and derived from a granitic liquid as a result of strong interaction with hydrothermal fluid during the late magmatic stage; they have high Zr saturation temperature and belong to A‐type granites. Geochemically, two‐mica granite formed in a compressional tectonic regime related to collision, whereas Middle Jurassic intrusive rocks have affinities with intrusive rocks from the extensional environment influenced by post‐collision. Therefore, the southern Great Xing’an Range has experienced the collisional orogenesis of the Central Asian Orogenic Belt in the Middle Permian, whereas in the Middle Jurassic, the southern Great Xing’an Range turned into a post‐collision extensional setting influenced by the Mongol–Okhotsk tectonic regime. Copyright © 2016 John Wiley & Sons, Ltd.