Showing papers on "Petrography published in 2014"

Provenance of Passive-Margin Sand (Southern Africa)

Abstract: This study investigates the petrographic, mineralogical, geochronological, and geochemical signatures of river sands across southern Africa. We single out the several factors that control sand generation, including weathering and recycling, and monitor the compositional changes caused by chemical and physical processes during fluvial transport from cratonic sources to passive-margin sinks. Passive-margin sands have two first-cycle sources. Quartz and feldspars with amphibole, epidote, garnet, staurolite, and kyanite are derived from crystalline basements exposed at the core of ancient orogens and cratonic blocks (dissected continental block provenance). Volcanic rock fragments, plagioclase, and clinopyroxene are derived from flood basalts erupted during the initial phases of rifting (volcanic rift provenance). First-cycle detritus mixes invariably with quartzose detritus recycled from ancient sedimentary successions (undissected continental block provenance) or recent siliciclastic deposits (e.g.,...

Sem Petrography of Eastern Mediterranean Sapropels: Analogue Data For Assessing Organic Matter In Oil and Gas Shales

Abstract: Examination of organic-matter-hosted pore systems in unconventional reservoir rocks has drawn attention to the appearance of sedimentary organic matter (OM) as seen in high-resolution SEM images. Field-emission SEM imaging of eastern Mediterranean sapropels (Pliocene–Pleistocene age from Ocean Drilling Program drill sites) was performed on samples prepared by Ar-ion cross-section polishing. The immaturity of marine kerogen in these rocks allows inspection of petrographic textures without overprints related to thermal maturation (hydrocarbon generation). Kerogen in these samples includes discrete particles that in some cases contain primary intragranular pores. The most abundant organic matter is in the form of microns-thick flaky or stringy material, largely nonporous and internally amorphous, and lacking the well-defined shapes of discrete particles. This dominant OM has behaved in a highly ductile manner in compaction and is now highly pervaded into intergranular spaces between silt- and clay-size grains. A portion of the larger mineral-hosted pores remain open, unfilled by ductile OM. Minor nanometer-scale pores of uncertain origin occur in the dominant OM. Silt- and clay-size mineral crystals are distributed in organic matter of most types, and it is difficult to fully ascertain if the OM/mineral mixing has occurred during compaction versus during aggregation during gravity settling (marine-snow sedimentation) or through sediment accumulation in OM-rich microbial mats. Results indicate that sediments rich in marine kerogen are subject to substantial compactional porosity loss in early burial. These observations of marine detrital organic matter in its early diagenetic state provide a useful baseline for interpretation of petrographic features in more mature OM-rich mudrocks. It is clear that petrographic discrimination of diagenetically generated organic matter (bitumen and related OM types) from ductile dispersed amorphous detrital OM (kerogen) using SEM data will be challenging in the absence of compositional information.

Characterization of granitoid and gneissic weathering profiles of the Mucone River basin (Calabria, southern Italy)

Abstract: The paper deals with the development of a multidisciplinary research on weathering profiles of granitoid and gneissic rocks related to tectonic and landscape evolution of the western Sila Grande Massif (Calabria, southern Italy). Field scale observations and petrographical and mineralogical features are used to characterize in detail the weathering processes. The weathering profiles of the granitoid cut slopes are generally simple showing a progressive weathering increase toward the top of the slopes, whereas the weathering profiles of the gneissic cut slopes are generally complex with irregularities in the spatial distribution of weathered horizons. The microfabric and petrographic analyses show that gneissic samples (classes V–VI of weathering grade) are characterized by higher percentage of altered minerals and microfracture and void rather than granitoid samples (classes V–VI of weathering grade). The main mineralogical changes concern the partial transformation of biotite and the partial destruction of feldspars (mainly plagioclases), associated with the neoformation of secondary minerals (clay minerals and Fe-oxides) and with a substitution of the original rock fabric. Neoformed clay minerals and ferruginous products replaced feldspars and biotite during the most advanced weathering stage. Referred as physical changes coupled with chemical variations, the final results of weathering process are a soil-like material characterized by sand–gravel grain-size fraction for the granitoid rocks and by both silt–clay and sand–silt grain-size fraction for the gneissic rocks. This generally produces a greater value of the SGI (Sand Generation Index) for granitoids and explains the great productivity in sandy deposits of this lithology.

Ore geology and fluid inclusion study of the Donggebi giant porphyry Mo deposit, Eastern Tianshan, NW China

Abstract: The Donggebi giant porphyry Mo deposit in Hami city, Xinjiang Province, NW China, was formed in the Triassic in the Jueluotage metallogenic belt of Eastern Tianshan Mo mineralization is associated with the Donggebi porphyritic granite, occurring as numerous stockworks in the altered wall rocks The hydrothermal ore-forming process can be divided into four stages, from early to late, characterized by veinlets composed of (1) quartz + K-feldspar ± beryl ± pyrite ± tourmaline; (2) quartz + molybdenite ± K-feldspar ± tourmaline ± fluorite ± beryl; (3) quartz + polymetallic sulphides ± fluorite; and (4) quartz + calcite ± fluorite Three types of fluid inclusions (FIs) are distinguished in this study based on petrographic and microthermometric criteria, ie carbonic (C-type), aqueous (W-type) and solid-bearing fluid inclusions (S-type) All three types of FIs can be observed in the quartz, beryl and fluorite formed in stages 1, 2 and 3, while the stage 4 minerals only contain aqueous inclusions The FIs in minerals of stages 1, 2, 3 and 4 yield homogenization temperatures of 261–443, 210–385, 152–304 and 159–263 °C, respectively, and salinities of ≤93 wt% NaCl equiv, without consideration of the contribution from daughter minerals The maximum estimated pressure decrease from 193 MPa in stage 1, through 182 MPa in stage 2, to 129 MPa in stage 3, suggesting the maximum mineralizing depths decreased from 84, through 79, to 56 km, and possibly occurred at 56–79 km Therefore, the ore-forming fluids at Donggebi are characterized by high-temperature, CO2-bearing and NaCl-poor and then progressively evolved to CO2-poor and lower-temperature Copyright © 2014 John Wiley & Sons, Ltd

Dissolution-precipitation processes governing the carbonation and silicification of the serpentinite sole of the New Caledonia ophiolite

Abstract: The weathering of mantle peridotite tectoni-cally exposed to the atmosphere leads commonly to natural carbonation processes. Extensive cryptocrystalline mag-nesite veins and stock-work are widespread in the ser-pentinite sole of the New Caledonia ophiolite. Silica is systematically associated with magnesite. It is commonly admitted that Mg and Si are released during the laterization of overlying peridotites. Thus, the occurrence of these veins is generally attributed to a per descensum mechanism that involves the infiltration of meteoric waters enriched in dissolved atmospheric CO 2. In this study, we investigate serpentinite carbonation processes, and related silicifica-tion, based on a detailed petrographic and crystal chemical study of serpentinites. The relationships between serpen-tine and alteration products are described using an original method for the analysis of micro-X-ray fluorescence images performed at the centimeter scale. Our investigations highlight a carbonation mechanism, together with precipitation of amorphous silica and sepiolite, based on a dis-solution–precipitation process. In contrast with the per descensum Mg/Si-enrichment model that is mainly concentrated in rock fractures, dissolution–precipitation process is much more pervasive. Thus, although the texture of rocks remains relatively preserved, this process extends more widely into the rock and may represent a major part of total carbonation of the ophiolite.

In situ geochemistry of Lower Paleozoic dolomites in the northwestern Tarim basin: Implications for the nature, origin, and evolution of diagenetic fluids

Abstract: Lower Paleozoic sedimentary rocks in the northwestern Tarim basin were strongly altered by complicated geofluids, which resulted in the occurrence of various diagenetic minerals (e.g., dolomite). Here, in situmajor, trace, and rare earth element geochemistry of Lower Ordovician diagenetic dolomite grains as well as petrography were performed to unravel the geochemical features, the nature, and origin of the diagenetic fluids. The results indicate that different geochemical information can be detected within a single sample, even within a single dolomite grain. Five generations of diagenetic dolomite have been identified based on geochemical signatures, resulting from four distinct types of diagenetic fluids: (1) HREE enrichment (PAAS-normalized), low RREE, no Eu anomaly, low Mn, Ba, moderate Fe, and high Sr contents are probably due to early burial dolomitizing fluids; (2) MREE enrichment, high RREE, high Mn, Fe, and low Sr content are likely to be associated with Devonian deep-circulating crustal hydrothermal fluids; (3) flat or LREE enrichment pattern with obviously positive Eu anomaly is inferred to be linked to Permian magmatic hydrothermal fluids; and (4) flat REE pattern, moderate RREE, no Eu anomaly, low Mn, Ba, moderate Fe, and high Sr contents are probably due to late burial dolomitizing fluids. The significances of in situmethod demonstrated in this study, compared with the whole rock analysis, include not only contamination-free analysis but also unraveling the internal geochemical variation within a single sample or a mineral grain. Thus, for the geochemical study of complicated diagenetic process, in situmethod should be preferentially considered.

Petrography and geochemistry of Jumara Dome sediments, Kachchh Basin: Implications for provenance, tectonic setting and weathering intensity

Abstract: In the Kachchh Mainland, the Jumara Dome mixed carbonate-siliciclastic succession is represented by the Jhurio and Patcham formations and siliciclastic-dominating Chari Formation (Bathonian to Oxfordian). The Jumara Dome sediments were deposited during sea-level fluctuating, and were interrupted by storms in the shallow marine environment. The sandstones are generally medium-grained, moderately sorted, subangular to subrounded and of low sphericity. The sandstones are mineralogically mature and mainly composed of quartzarenite and subarkose. The plots of petrofacies in the Qt-F-L, Qm-F-Lt, Qp-Lv-Ls and Qm-P-K ternary diagrams suggest mainly the basement uplift source (craton interior) in rifted continental margin basin setting. The sandstones were cemented by carbonate, iron oxide and silica overgrowth. The Chemical Index of Alteration values (73% sandstone and 81% shale) indicate high weathering conditions in the source area. Overall study suggests that such strong chemical weathering conditions are of unconformity with worldwide humid and warm climates during the Jurassic period. Positive correlations between Al2O3 and Fe2O3, TiO2, Na2O, MgO, K2O are evident. A high correlation coefficient between Al2O3 and K2O in shale samples suggests that clay minerals control the major oxides. The analogous contents of Si, Al, Ti, LREE and TTE in the shale to PAAS with slightly depleted values of other elements ascribe a PAAS like source (granitic gneiss and minor mafics) to the present study. The petrographic and geochemical data strongly suggest that the studied sandstones/shales were deposited on a passive margin of the stable intracratonic basin.

Chemical and minero-petrographic features of Plio-Pleistocene fine-grained sediments in Calabria, southern Italy

Abstract: The composition of Plio-Pleistocene fine-grained sediments from different areas of Calabria (southern Italy), have been studied to unravel the interplay of provenance. The use of multivariate statistical methods (e.g., PCA) based on biplot, allows for the discrimination of different groups of sediments. The general chemical composition of the fine-grained samples reflects the mineralogical variation observed in the sediments, which are composed of phyllosilicates, quartz, calcite, dolomite and feldspars. The I-S mixed layers, 10 A-minerals (illite and micas) are the most abundant phyllosilicates, chlorite and kaolinite are present in variable amounts, and smectite and chlorite/smectite mixed layers are presents in trace amounts. These chemical and mineralogical variations are also confirmed by SEM analysis, which also demonstrates that phyllosilicates are characterized by open and folded structures with generation of neoformed clay minerals. Provenance proxies and element ratios testify greater input of felsic source rocks, with lack of a marked mafic-ultramafic detritus input for all studied sediments, and the minero-petrographical features of the studied fine-grained sediments. These chemical and mineralogical variations are also confirmed by SEM analysis showing abundant phyllosilicate minerals for Groups 1 (Crotone Basin) and 2 (Crati Graben) and high content of quartz and feldspars grains for Group 4 (Catanzaro Graben) samples, probably due to abundant siliciclastic input coming from plutonic-metamorphic source and associated Mesozoic to Miocene sedimentary sources (Sila and Serre Massifs), and abundant calcium-carbonate phases mainly related to the presence of planktonic calcareous microfossils (Globigerina and coccoliths) for Groups 3 (southern Ionian coast) and 5 (southern Tyrrhenian coast).Furthermore, paleogeographic reconstructions characterized by re-establishment of open-marine conditions in the Mediterranean following the Messinian salinity crisis at the beginning of the Pliocene, play an important role. A progressive increase in the connectivity of Mediterranean sub-basins with the Atlantic (e.g., subbasins at the south of the Catanzaro Strait, characterized by normal water supply) and Paratethys (e.g., sub-basins at the north of the Catanzaro Strait, characterized by freshwater supply) can be envisaged during the Neogene period; this paleogeographic reconstruction may also explain the different composition among the studied samples.

Provenance Evolution During Arc–Continent Collision: Sedimentary Petrography of Miocene To Pleistocene Sediments In the Western Foreland Basin of Taiwan

Abstract: Taiwan is an example of an orogen involving a doubly vergent critical wedge. Critical-wedge theory predicts asymmetrical tectonic development of the pro- and retro- sides of an orogen. Whereas the consequences for exhumation, peak metamorphism, and erosion have been extensively investigated, the implications for sandstone petrography preserved on either side of an orogen have not been adequately documented. Plio-Pleistocene sandstones from the Western Foothills of Taiwan were deposited in a rapidly subsiding foreland basin recording the collision between a volcanic arc and the Asian passive margin. New data on the evolution of sedimentary petrography and clay minerals preserved in Miocene to Pleistocene sandstone of the western pro-foreland basin record the history of Taiwan's orogenesis. Sandstone petrography of the eastern basin on the retro-side of Taiwan's orogenic wedge had been studied previously. Comparison of the provenance record from the western and eastern basins illustrates the opposing signature of unroofing and recycling, held to be characteristic of an asymmetric orogenic wedge. Sandstone petrography can be used in ancient sedimentary basins to complement other indications of the polarity of subduction.

Petrology and geochemistry of Early Permian volcanic rocks in South Tian Shan, NW China: implications for the tectonic evolution and Phanerozoic continental growth

Abstract: Situated in the southwest of the Central Asian Orogenic Belt (CAOB), the South Tian Shan (STS) Block is a key area for understanding the final accretion of the CAOB. A suite of volcanic rocks interbedded with continental sediments from the Xiaotikanlike Formation lies along the southwestern edge of the Tian Shan orogen. Laser-ablation-inductively coupled plasma-mass spectrometer U–Pb dating provided a crystallization age of 295.0 ± 2.8 Ma (MSWD = 1.3), suggesting an Early Permian magmatic event. The volcanic rocks show a variable composition, with dominant rhyolites and dacites, subordinate basaltic andesites and few basalts. The felsic rocks are enriched in K and exhibit remarkably negative anomalies in Ba, Sr, Eu, P and Ti. These anomalies associated with their high negative !Nd(t) values and old Nd model ages suggest that they are most likely sourced from ancient lower crustal rocks. The mafic rocks are characterized by high Mg#, Cr, Ni contents, negative Nb, Ta anomalies and pronounced enrichment in light rare earth elements as well as mild enrichment in large-ion lithophile elements. The mafic rocks are thus inferred to derive from enriched subcontinental lithospheric mantle. The petrographic and geochemical characteristics of the Xiaotikanlike Formation volcanic rocks indicate that they were generated under a post-collisional regime. Therefore, the final collision between the Tarim Craton and the Kazakhstan– Yili terrane took place before Early Permian, most probably at Late Carboniferous. Differing from other tectonic units of the CAOB, the recycling of ancient lithospheric crust played a significant role in the continental growth of the STS Block.