Showing papers in "Geological Journal in 2018"

Diagenesis and reservoir quality in tight gas sandstones: The fourth member of the Upper Triassic Xujiahe Formation, Central Sichuan Basin, Southwest China

Abstract: 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

Nature and origin of dolomitization associated with sulphide mineralization: new insights from the Tappehsorkh Zn-Pb (-Ag-Ba) deposit, Irankuh Mining District, Iran

Abstract: Several types of dolomitization are observed in the Lower Cretaceous carbonate sequence of the Tappehsorkh deposit, Irankuh Mining District, Iran: 1) regional and extensive early diagenetic dolomitization (D1), and 2) localized hydrothermal dolomitization (D2 and D3). D1 regional dolomite, which partly to completely replace limestone, is characterized by fine-grained euhedral to subhedral dolomite rhombs. There is no evidence for Zn-Pb sulphide mineralization associated with this type of dolomite. Medium to coarse-grained D2 and D3 hydrothermal dolomites occur along the syn-sedimentary Gushfil-Baghabrisham normal fault, about 40 m above the Lower Cretaceous sequence within black siltstone, dolostone, and crystal lithic tuff and lava rocks. Hydrothermal dolomite cross-cuts and brecciates the host rocks. This dolomite was replaced by quartz and sulphide minerals of the main ore stage. D1 regional dolomite has average values of -7.37‰ and 2.20‰ for δ18O and δ13C, respectively. The δ13C values of this dolomite fall well within the range of the Lower Cretaceous carbonates. D2 hydrothermal dolomite has average δ18O and δ13C values of -7.92‰ and 2.93‰, respectively. The respective δ18O and δ13C values of D3 hydrothermal dolomite are -12.27‰ and 2.39‰. The δ18O values of D2 and D3 dolomites are more negative than those of D1 regional dolomite; this can be due to their hydrothermal origin. Fluid inclusion studies on D3 dolomite and quartz show temperatures of 170-260 °C. The concentrations of Fe and Mn in D1 regional dolomite are very similar to those of limestone, suggesting that they likely precipitated from the same fluid (seawater), whereas those of D2 and D3 dolomites are relatively high, in agreement with the hydrothermal origin for these dolomites. Precipitation of gypsum, which is ubiquitous in the study area, could have lowered the Ca/Mg ratio of seawater. While the extensive regional dolomitization formed from low-temperature (evolved) seawater during the early diagenesis, D2 and D3 dolomites associated with the main stage of sulphide mineralization were formed by high-temperature hydrothermal fluids moving along the Gushfil-Baghabrisham Fault. Copyright © 2016 John Wiley & Sons, Ltd.

Mineral chemistry, bulk rock geochemistry, and S‐isotope signature of lode‐gold mineralization in the Bétaré Oya gold district, south‐east Cameroon

Abstract: Lode‐gold mineralization in the Betare Oya gold district is related to a series of quartz–sulphide veins that define a steeply dipping NNE–SSW‐trending brittle–ductile shear zone. The veins transect metasedimentary sequences in the vicinity of small granitic intrusions. Quartz veins range from barren (massive quartz) to mineralized with textures that vary from foliated, sheared, vuggy, fractured, and sugary to brecciated with inclusions of altered wall rock. They contain gold, pyrite, and galena in addition to chalcopyrite and both hypogene and supergene haematite and covellite. Parameters that consistently define primary gold mineralization in the area include silicification, sulphidation, sericitization, K‐feldspar alteration, haematitization, and carbonatization. Gold grains recovered from heavy mineral concentrate and grains that occur as inclusions in pyrite were characterized by microchemical methods. The gold grains are zoned and exhibit a simple internal structure of partly deformed and leached rims and high‐purity intragranular veinlets. Gold is alloyed with Ag; gold fineness varies between 859 and 877 in the core and from 958 to 997 in the leached rims. Inclusions of gold in pyrite cluster between 875 and 900. Pyrite has up to 0.59 wt% Au, and other trace elements occur in low quantities but for Pb (0.13 wt%), Ag (1.36 wt%), and Sb (1.63 wt%). The bulk trace element signature of the veins is characterized by Au, Ag, As, Bi, Ba, Cr, Cu, Mn, Pb, Sb, Sr, V, W, Zn, and Zr. This depicts a geochemical association of Au ± Cu ± Bi ± As ± Pb. The occurrence of Au as well as elevated Pb and Zn could suggest a granitic source for the ore‐bearing fluid. The auriferous quartz veins are characterized by a widespread in δ34S from 2.8 to 14.9‰ pointing to multiple sources of sulphur in the system, perhaps a mixture of sulphur from the metasedimentary and the granitic rocks.

Structural, stratigraphic, and petrological clues for a Cretaceous–Paleogene abortive rift in the southern Adria domain (southern Apennines, Italy)

Abstract: This study provides new structural, stratigraphic, and geochemical data and a literature review of the Cretaceous–Paleogene stratigraphy, biostratigraphy, tectonics, and magmatism in the southern Apennines belt, Italy, with the aim to demonstrate the occurrence of an Albian to Eocene abortive rifting stage in the southern Adria domain. During this time, the tectono-stratigraphic evolution of the Adria domain is characterized by episodes of coeval uplift and drowning. Different sectors of the Apennine and Apulian platforms were so characterized by changes in the paleoenvironments, leading to different stratigraphic records (from shallow-water to slope and basin), as well as the development of thick bauxitic levels. Contemporaneously, a large amount of calciclastic sediments supply from the emerging sectors was deposited in the basins surrounding the carbonate platforms (i.e., Ligurian and Lagonegro–Molise basins). The Albian–Eocene interval was also characterized by the occurrence of anorogenic magmatism and synsedimentary extensional faulting that, along with the changed sedimentary facies distribution, points out for a crustal-scale extensional tectonics. We suggest that such tectonics is the result of a rifting episode, characterized by limited anorogenic magmatism, starting in the Albian and reaching its climax in the uppermost Cretaceous–Eocene times. In this scenario, the extensional tectonics recorded in the Adria domain was the product during an event of a single abortive rift system, which extended toward the south, from the southern margin of the Ligurian Ocean to the Hyblean (Sicily), Pelagian (Tunisia), and Sirte Basin Province Rift (Libya).

Mineralization processes at the giant Jinding Zn–Pb deposit, Lanping Basin, Sanjiang Tethys Orogen: Evidence from in situ trace element analysis of pyrite and marcasite

Abstract: The giant Jinding Zn–Pb deposit is in the Mesozoic–Cenozoic Lanping Basin of southern China and hosted by sandstone in the Early Cretaceous Jingxing Formation and limestone breccia and sandstone and gypsum in the Palaeocene Yunlong Formation. Mineral associations at the deposit are early (Stage 1) marcasite–sphalerite(–pyrite) and galena–quartz; through (Stage 2) pyrite–sphalerite–galena(–arsenopyrite) and marcasite–celestite–carbonate–gypsum; and the late (Stage 3) galena–sphalerite–pyrite–sulfate–carbonate(–celestite), gypsum, and barite. Pyrite and marcasite have higher Pb, Zn, Ag, Bi, As, Se, and Tl and lower V, Cr, Co, Ni, Cu, and Sn assays in orebodies hosted by sandstone and brecciated limestone. The concentration of these elements progresses from the early to late stages with increasing Cu, Zn, Ag, Bi, V, Cr, Co, Ni, and Se; decreasing Tl and As; and constant Pb, Sn, and Sb. This chemical characteristic of the pyrite and marcasite indicates that their source is basinal brine with minor contribution from the host rocks. During the early and middle stages, pyrite and marcasite contain higher concentration of Pb, Tl, and As, which is related to mineralized basinal brine and H2S-rich fluids at a low pH during relatively higher temperature conditions. During the late stage mineralization, pyrite and marcasite contain the highest concentrations of Cu, Zn, Ag, Sb, Pb, Bi, and Se, which is associated with metal-rich basinal brine and meteoric water at a near-neutral pH and lower temperature conditions.

Radiometric dating of late Quaternary loess in the northern piedmont of South Tianshan Mountains: Implications for reliable dating

Abstract: Reliable chronologies are prerequisites when interpreting proxy records in terrestrial archives of Quaternary climate and environmental change. Optically stimulated luminescence (OSL) dating and accelerator mass spectrometry radiocarbon dating (AMS C-14) are commonly used to date late Quaternary loess deposits in the Chinese Loess Plateau, but the range and reliability of the two methods in Central Asia are still debated. In this study, we investigate both fine-grained quartz OSL and AMS C-14 dating of a late Quaternary loess section located at the northern piedmont of the South Tianshan Mountains in Central Asia and discuss the reliability of the two radiometric dating methods. The results show that the OSL and AMS C-14 ages are generally consistent with the stratigraphic sequence when the ages are younger than 25cal ka BP, which means that both can be used to establish a reliable chronology in the Ili Basin. But beyond this age, the OSL dating method seems to be a more reliable approach. The results also supported previous dates based on medium-grained quartz OSL dating of the Ili loess in the southern piedmont of the North Tianshan Mountains. Radiocarbon ages older than 25cal ka BP should be treated with caution, and attention must be paid to the influence of pedoturbation on OSL signals in the Central Asian loess. Multiple dating approaches for mutual authentication and exploring new dating materials are suggested in further loess chronological research. These findings will be helpful in establishing a reliable timescale and in reconstructing high-resolution environmental change in Central Asia.

Paleoenvironmental, diagenetic, and eustatic controls on the Permo–Triassic carbonate–evaporite reservoir quality, Upper Dalan and Kangan formations, Lavan Gas Field, Zagros Basin

Abstract: Permo–Triassic (P–T) successions in Iran are known as one of the most extensive carbonate–evaporite sequences in the world, holding prolific hydrocarbon accumulations, such as the gas fields in the Zagros Basin. This study addresses the impact of paleoenvironmental changes across the P–T extinction boundary, variations in eustatic level and diagenesis on the carbonate reservoir quality of the Permo–Triassic Upper Dalan and Kangan formations in the Lavan Gas Field (offshore Zagros). A total of eight lithofacies have been defined and interpreted, representing the shallower parts (inner ramp to shoal) of a carbonate homoclinal ramp. The diagenetic features show the mixing influence of marine, meteoric and burial processes, with prevalence of multistage dolomitization and late meteoric-related imprints. The main dolomitization processes are interpreted to be penecontemporaneous and associated with continuous seawater reflux and high evaporation within the intertidal environment. Combined analysis of facies and petrophysical properties has been utilized to define six rock types, including RT4 (grainstones from oolitic shoals) as the best reservoir, whereas RT1 (supratidal anhydrite) is acting as the main cap rock within the studied carbonate–evaporite sequence. Multiscale characterization of lithological and petrophysical properties suggest that the Upper Dalan Formation has a single reservoir unit (D1), whereas the Kangan Formation may be divided into two reservoir units (K1 and K2). The best reservoir quality is promoted by dissolution and dolomitization associated with late Transgressive Sequence Tracts (TST) to early Highstand Sequence Tracts (HST) sequences, while low reservoir quality corresponds to late HST sequences accompanied by pore-occluded cements. Petrophysical analysis suggest better porosity-permability for the Upper Dalan than for the Kangan Formation. This observation is closely associated with the different nature of lithofacies, where the Kangan Formation contains more fine-grained carbonate rocks, and the higher intensity of porosity-grain dolomitization in the Upper Dalan Formation. This study provides a better understanding on reservoir quality prediction across the P–T boundary and the effect of eustatic fluctuations and diagenesis in controlling porosity-permeability evolution with similar settings elsewhere.

Mineralogy and chemical aspects of some ophiolitic metaultramafics, central Eastern Desert, Egypt: Evidences from chromites, sulphides and gangues

Abstract: Chromites and sulphides are the most common ore minerals in the ophiolitic metasomatized ultramafics (~790 Ma) of Um Halham, Fawakhir and Barramiya, central Eastern Desert, Egypt. These ultramafics exhibit variable degrees of alteration and metasomatism and include massive serpentinites (serpentinized peridotite and serpentinite), tremolite-talc rocks, talc-carbonate rocks, listwaenite-like rocks and typical listwaenite. The alteration of chromite to Cr-magnetite was accompanied by the formation of chloritic aureoles due to the release of Al and Mg from chromite. Textural and compositional features of the chromites suggest a greenschist up to lower amphibolite facies metamorphism (at 500–600 °C), which is facial with the host ultramafics. The chromites exhibit a boninitic affinity and a forearc suprasubduction setting of the present ophiolite assemblages. The variability of chromite chemistry indicates a melt-rock reaction, together with the water-bearing melt which is necessary for crystallizing chromite. Pentlandite, pyrrhotite, arsenopyrite, gersdorffite and pyrite are the most common sulphide minerals. Gold occurs either as an invisible phase associated with gersdorffite, arsenopyrite, and As-rich pyrite or as native nuggets in listwaenites of Barramiya area. Gangue metasomatized phases associated with these ore minerals are identified as sericite, carbonate and chlorite. The investigated sericite ranges in composition from mariposite (>0.5% Cr) to fuchsite (>1% Cr), which indicates typical listwaenites. Five varieties of carbonate minerals are identified; magnesite, breunnerite, dolomite, calcite and ankerite. The CO2-rich fluids are released during progressive decarbonation reactions in carbonate-bearing metasediments, whereas Ni and As-rich fluids are attributed to nearby granitic intrusions affected on serpentinites and leached Ni and precious metals like gold forming As-Ni-rich fluids. These metasomatic fluids are thought to reflect a range of melts derived from a compositionally evolving source during subduction initiation in a forearc environment.

Early Carboniferous tectonic evolution of the northern Heihe–Nenjiang–Hegenshan suture zone, NE China: Constraints from the mylonitized Nenjiang rhyolites and the Moguqi gabbros

Abstract: The Heihe–Nenjiang–Hegenshan suture zone has long been accepted as the major tectonic boundary between the Xing'an and Songliao blocks and extends through the Great Xing'an Range in NE China, but its location of the northern segment between the Moguqi and Nenjiang areas and its timing remain unclear. We address these issues by presenting zircon LA-ICP-MS U–Pb ages, Lu-Hf isotopes, bulk-rock major, and trace elemental data for mylonitized rhyolites collected from the Moergenhe Formation in the Nenjiang area and for gabbros of the Moguqi area, respectively. The mylonitized rhyolites, which display an arc-related geochemical affinity with enrichment in Th and U, and depletion of Nb, Ta, and Ti, and gently right-tilted rare earth element (REE) patterns (light REE [LREE]/heavy REE [HREE] =4.53–7.60), as well as the eHf (t) values (+6.4 to +11.8) of analyzed zircons, indicate an origin by partial melting of potentially young lower continental crust of a subducting slab. The zircon LA-ICP-MS U–Pb data show the formation age of the mylonitized rhyolites is 352.4 Ma. The analyzed gabbros with an emplacement age of 352.6 Ma have high concentrations of Th and U, slightly enriched LREE patterns and relative low LREE/HREE ratios (4.3 to 4.6). These features, together with their high positive eHf (t) values (+7.7 to +15.2), suggest that they were likely derived from the partial melting of a depleted mantle source that was metasomatized by subduction-related fluids. Combined with the geochemical features of the coeval igneous rocks from the northern Great Xing'an Range, these results reveal that the existence of an early Carboniferous NE-trending magmatic arc (ca. 350–330 Ma), extending along the west of the Heihe–Nenjiang–Hegenshan suture zone, gives more constraints on the amalgamation of the Xing'an and Songliao blocks along the Nenjiang–Moguqi areas and indicates that the amalgamation should have terminated by at least the end of the early Carboniferous.

Diagenesis of tight oil sand reservoirs: Upper Triassic tight sandstones of Yanchang Formation in Ordos Basin, China

Abstract: Upper Triassic sandstones in the Ordos Basin, northern-central China, comprise tight oil reservoirs. Using a combination of thin sections, SEM, BSE, EDS, XRD, and fluid inclusion analyses, 24 core samples from 13 wells were collected to study the petrology, paragenesis, and diagenetic processes and implications for reservoir quality. Quartz cement usually occurs as overgrowths or euhedral quartz. Extensive dissolution and albitization of K-feldspar can be observed. Five types of carbonate cements, ferrocalcite, ankerite, dolomite, calcite, and siderite, occur during different diagenetic stages. Two main types of illite and 5 main habits of chlorite are observed in this study. Kaolinite mainly occurs as booklets and vermicular aggregates. Diagenetic illite, chlorite, biotite, mixed-layer illite/smectite (I/S), and other minor minerals are also observed. The diagenetic processes include compaction, alteration of volcanic materials and mica, clay mineral transformation, cementation (silica, aluminosilicate, and carbonate), and dissolution of feldspars and rock fragments. Compaction was a significant porosity-reducing agent, and the presence of carbonate cement exerts a dominant impact on the reduction of porosity. Quartz cement and authigenic clays are less important; however, it is worth mentioning that pore-lining clays are conducive to porosity preservation. In this study, most of the porosity variation is caused by a combination of compaction, carbonate cements, quartz cement, and authigenic clays. This study gives insights into diagenetic alterations within tight sandstones and has implications for reservoir quality prediction in similar settings.

Sediment composition, provenance, and Holocene paleoenvironmental evolution of the Southern Po River coastal plain (Italy)

Abstract: 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.

Selective dissolution of alkali feldspars and its effect on Lower Triassic sandy conglomerate reservoirs in the Junggar Basin, northwestern China

Abstract: Feldspar dissolution resulting from fluid–rock reactions in clastic rocks is common in petroliferous sedimentary basins and significantly affects the quality of reservoirs. The objective of this study is a case study of this phenomenon in the sandy conglomerate reservoirs, based on the Lower Triassic Baikouquan Formation in the Mahu Sag at the northwestern margin of the Junggar Basin in northwestern China, which has obtained exploration breakthroughs recently. In these rocks, orthoclase has undergone dissolution and the dissolution intensity decreases gradually from the basal member (T1b1) to the upper member (T1b3) of the formation, whereas albite remained stable or has even overgrown crystals, displaying a selective dissolution of alkali feldspars. This is interpreted to be mainly controlled by two factors. First, illitization of mixed-layer illite/smectite in the sandy conglomerates consumed K, thereby increasing Na/K ratios in the formation waters and leading to orthoclase dissolution and albite overgrowths on perthite and microcline. Second, the charge of acidic, hydrocarbon-related fluids further promoted orthoclase dissolution. These relationships are confirmed by the fact that hydrocarbon charging intensity shows a gradual upward reduction from member T1b1 to T1b3, as does the scale of orthoclase dissolution. This selective dissolution resulted from the influence of different mineral components on diagenesis, the acidic nature of reservoir fluids, and the extent of water–rock interaction. The dissolution significantly improved porosity and permeability of the reservoirs by generating substantial secondary porosity. The results indicate a favorable exploration prospect located in the up-dip area of the Ma 18 and Aihu 2 wells on the western slope of the Mahu Sag.

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)

Abstract: 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.

Mineralogy, zircon U–Pb–Hf isotopes, and whole‐rock geochemistry of Late Cretaceous–Eocene granites from the Tengchong terrane, western Yunnan, China: Record of the closure of the Neo‐Tethyan Ocean

Abstract: New data from mineralogy, geochemistry, zircon U–Pb dating, and Hf isotopes have revealed Late Cretaceous to Eocene magmatic intrusions in the Tengchong terrane and constrained the origin, tectonic setting, and characteristics of tin-bearing and barren granitoids. We divide the studied granitoids into two groups: (a) the Late Cretaceous Zhinaxiang (70 Ma) and Early Paleocene Dazhupeng (65 Ma) S-type barren granites and (b) the Eocene Lailishan A2-type tin-bearing granite (50 Ma). All these granitoids display Si- and K-rich and calc-alkaline characteristics and have similar chondrite-normalized rare earth element patterns. However, geochemical differences do exist between the two groups of granites. The Zhinaxiang and Dazhupeng S-type granites are slightly peraluminous, enriched in large-ion lithophile elements (e.g., Rb and K), and depleted in high-field-strength elements (e.g., Nb, Ta, Zr, and Hf), whereas the Lailishan A2-type granite has higher TFe2O3, high-field-strength element content, and Ga/Al ratios. The geochemical and Hf isotopic data indicate that the granites in this study were generated by partial melting of Paleoproterozoic metasedimentary rocks. Due to the subduction of the Neo-Tethyan Ocean beneath the Eurasian plate, the Zhinaxiang and Dazhupeng S-type granites were formed in a thickened-crustal environment, whereas the Lailishan A2-type granite was emplaced in a post-collisional extensional setting attributed to slab break-off. By comparison between tin-bearing and barren granites, we propose that the Sn mineralization could be related to relatively high-temperature and low-pressure crystallization conditions.

Interplay of tectonism and eustasy during the Early Permian icehouse: Southern Sydney Basin, southeast Australia

Abstract: This investigation presents an outcrop-based integrated study of sedimentological analysis and sequence stratigraphy applied to the Lower Permian sedimentary succession in the southern Sydney Basin, Australia. This succession accumulated in several depositional environments and sub-environments that range from non-marine (fluvial) to marine (outer shelf), representing the fill of a sedimentary basin that resembles a fault-bounded, rift basin. The stratigraphic analysis indicates a deepening-upward trend, and the sequence stratigraphic approach has established that these sediments can be attributed to the lowstand, transgressive, and highstand systems tracts. Lowstand sediments can be defined by fluvial facies that are located between the subaerial unconformity and the maximum regressive surface. Transgressive facies correspond to estuarine, upper and lower shoreface, and inner and outer shelf depositional environments and are located between the maximum regressive and the maximum flooding surfaces. Highstand bottomset sediments are accumulated above the maximum flooding surface and are represented by outer shelf facies. The stratigraphic architecture indicates the development of an almost complete depositional sequence, mainly developed under the control of tectonically induced subsidence, but also influenced by the high sedimentation rates and the high gradient of the inherited topography. Eustatic sea-level fluctuations were of minor importance during the deposition of the examined sediments.

Petrogenesis and tectonic implication of the Late Mesozoic volcanic rocks in East Mongolia

Abstract: This paper presents new geochemical and geochronological data of the Late Mesozoic volcanic rocks in East Mongolia. These volcanic rocks belong to high-K calc-alkaline and shoshonitic series and exhibit features of bimodal rocks. The mafic rocks have elevated incompatible trace element concentrations and significantly negative Nb, Ta, and Ti anomalies, with Sr-87/Sr-86(i) and Nd-&(t) values of 0.70502-0.70572 and -1.72459 to +1.720736, respectively, which suggest that the mafic magma was derived from a lithospheric mantle source that might have been metasomatized by subduction-derived fluids, experienced fractional crystallization, and was contaminated by crustal materials. The felsic rocks have similar rare earth element patterns to the mafic rocks but show much more significantly negative Eu and Sr anomalies. The felsic rocks have higher Sr-87/Sr-86(i) values of 0.706496-0.71104 than the mafic rocks but similar Nd-&(t) values (-0.28003 to +2.928506) to the mafic ones. These data indicate that the felsic rocks originated from partial melting of a crustal source that is dominated by juvenile mafic rocks. Our new K-Ar dating, together with previous data, shows that the Late Mesozoic volcanism in East Mongolia took place during the Late Jurassic-Early Cretaceous between ca. 155 and 99 Ma. The model of back-arc extension, possibly induced by slab rollback of the westward subducted Pacific Plate and the subduction zone retreat, can explain the geodynamic setting and the eastward younging trend of the Late Mesozoic volcanism in East Mongolia and in adjacent NE China.