Felsic Source Rocks Research Articles

Petrochemistry of sandstones from Neoproterozoic basins of the Bastar craton, Central Indian Shield: Implications for paleoweathering, provenance and tectonic history

AbstractPetrographic analysis and chemical analysis of major and trace elements including rare earth elements of the Neoproterozoic sandstones from the Chandarpur Group and the Tiratgarh Formation have been carried out to determine their provenance, tectonic setting and weathering conditions. All sandstone samples are highly enriched in quartz but very poor in feldspar and lithic fragments. Petrographically and geochemically these sandstones are classified as subarkose, sublitharenite and arenite. The Chemical Index of Alteration (CIA mean 68) and Th/U ratios (mean 4.2) for these sandstones suggest their moderate weathering nature. Generally, all sandstone samples are strongly depleted in major elements (except SiO2), trace elements (except Zr) and REE in comparison with Post Archean Australian Shale (PAAS) and Upper Continental Crust (UCC). Their mineralogy and mean of elemental ratios suitable for determination of provenance and tectonic setting, e.g. Al2O3/SiO2 (0.02), K2O/Na2O (10), Eu/Eu* (0.67), (La/Lu)n (10.4), La/Sc (3), Th/Sc (1.2), La/Co (0.22), Th/Co (0.08), and Cr/Th (7.2), support a felsic source and a passive margin tectonic setting for these sandstones. Also these key elemental ratios do not show much variation over a range of SiO2. Thus we attest their significance in determining source rock characteristics of quartz rich sandstones. Chondrite‐normalized REE patterns with LREE enrichment and a strong negative Eu anomaly are also attributed to felsic source rock characteristics for these sandstones. The source rocks identified are granite and gneiss of the Bastar craton. Minor amounts may have been derived from older supracrustals of the Bastar craton. However, the major element data of the Paleoproterozoic Sakoli schists when compared with those of the Neoproterozoic sandstones indicate that the schists were derived from a mafic source and deposited in an active continental margin tectonic setting. There is, however, little difference in CIA values between the Paleoproterozoic Sakoli schists and Neoproterozoic sandstones, indicating prevailing of similar (moderate‐intense) weathering conditions throughout the Proterozoic in the Bastar craton. Our study also suggests a change in the provenance and tectonic setting of deposition of sediments from dominantly a mafic source and an active continental margin in the Paleoproterozoic to dominantly granite and gneiss (felsic source) and a passive continental margin in the Neoproterozoic in the Bastar craton.

Rare earth element (REE) geochemistry and genetic implications of the Mortaş bauxite deposit (Seydişehir/Konya – Southern Turkey)

The Mediterranean-type karst-bauxite deposit of Mortaş, south Turkey, placed unconformably between Cenomanian and Senonian shallow marine limestones is built of massive (MB), oolithic (OB), breccia-bearing (BB) and earthy (EB) bauxite horizons, from top to bottom. The MB layer is enriched in Al and REE (except Ce) due to loss of Si, Na, K, Mg and P. REE are accumulated in the BB but depleted in the EB layers. The ferruginous OB lost LREE and gained in HREE probable due to scavenging by authigenic heavy minerals like rutile, anatase and titanite. Total REE contents in the bauxite profile display an increasing trend from bottom to top, while negative and maximum positive Ce anomalies characterize the upper and the lower parts of the profile, respectively. This unusual REE behavior is explicable by assuming mobilization of Ce(IV) either under reducing condition or chemical complexation under alkaline conditions in the top layer and scavenging of Ce by Al–Mg hydrosilicates and Ti-oxides and/or precipitation with authigenic REE minerals, especially of the bastnäsite group near the bedrock limestones. Similarity in chondrite normalized-REE patterns of the Seydişehir phyllites, bauxite and terra rossa samples and the presence of tridymite (?) in bauxites makes a felsic source rock most likely and reveal a close genetic relationship between the Seydişehir phyllites and the recent terra rossa occurrences. The REE patterns of the bauxites resemble those of the Katrangediği limestone despite variations in ΣREE. Field observations and geochemical data together with mass-balance calculations suggest that the Mortaş deposit was derived from the Seydişehir phyllites and argillic phase within the Katrangediği limestone which in turn have Precambrian (?) felsic, probably granitic precursors.

Geochemical and isotopic constraints on the origin of Paleoproterozoic red shales of the Gamagara/Mapedi Formation, Postmasburg Group, South Africa

The occurrence of red shales in Precambrian sedimentary rocks has been used to infer the presence of free O 2 in the atmosphere. Red shale samples from the ~2.2 Ga Gamagara/Mapedi Formation of the Postmasburg Group, South Africa were analyzed for mineralogy, geochemistry, and stable isotope composition (Fe and organic C) to constrain the timing and mechanism of red shale formation. These data suggest that the red shales were detrital accumulations of lateritic soils that were produced from moderate to intense chemical weathering of intermediate to felsic source rocks. Such weathering occurred under stable climatic conditions in low-latitude (sub)tropical regions. The data also suggest that the red shales were potentially influenced by hydrothermal activity. Hematite, a pigmenting agent of the red shales, was probably formed during the lateritic weathering of the source rocks ~2.2 Ga ago, rather than by the modern oxidation/weathering of pre-existing Fe 2+ -bearing minerals in the shales. Dissolution, remobilization, and reprecipitation of Fe and U during the diagenetic stage of shales are suggested from variations in the Fe 3+ /Ti, Fe 2+ /Ti, Fe isotope compositions, and U/Th ratios. Leaching of Fe 2+ -bearing silicate and/or reductive dissolution of Fe 3+ -oxide minerals, probably by organic acids, resulted in the discoloration of several red shales (to pale-green/white) during red shale diagenesis. Oxidation of Fe 2+ on the seafloor that was supplied by hydrothermal discharge and migration (upward diffusion) of leached Fe 2+ in sediments was additional source of Fe 3+ for red shale formation. Geochemical evidence thus supports the existence of an oxygenated atmosphere and shallow ocean system at least ~2.2 Ga ago.

Relationship between detrital zircon age-spectra and the tectonic evolution of the Late Archaean Witwatersrand Basin, South Africa

Abstract SHRIMP U–Pb detrital zircon age-spectra for 14 samples from the West Rand and Central Rand Groups of the Witwatersrand Basin indicate that felsic source rocks mostly formed at ∼3090–3060 and ∼3000–2870 Ma, with a smaller proportion formed prior to 3100 Ma. Provenance evaluation indicates that source rocks have largely vanished through erosion, such that greenstone terrains extending from the Amalia–Kraaipan Belt in the southwest to the Murchison Belt in the northeast are models of, but not the actual, source areas. Analysis of detrital zircon age-modes reveals that the West Rand and Central Rand Groups record different basins. The West Rand Group is characterised by age-subpopulations with a much-smaller range and less complex distribution than those in the Central Rand Group. Furthermore, detrital zircon age-spectra for the West Rand Group generally decrease in complexity up-section, whereas they become increasingly more-complex in the Central Rand Group. It is concluded that the source area for the West Rand Group is consistent with a passive-margin setting, with the decrease in complexity up-section of zircon age-spectra being the consequence of increasing maturity of the hinterland. Conversely, the source area of the Central Rand Group was far-more dynamic, with an increasing age-range of source rocks being exposed through time. Pulsed tectonic rejuvenation of the source area is implied. Contemporaneous emplacement, uplift and erosion of granitoids supports a magmatic fold-thrust belt as the source, and a complementary retroarc basin setting. Detrital xenotime grains record the ages of high-U granitoids emplaced in the fold-thrust belt during deposition of the Central Rand Group, and are much closer to depositional ages than are detrital zircon grains. Change from a passive margin to a retroarc basin across the West Rand-Central Rand boundary is emphasised by change from simple to increasingly complex detrital zircon age-spectra. This study establishes that detrital zircon geochronology can overcome the problem of overly quartzose compositions in Archaean sandstones due to severe chemical weathering and authigenic alteration, thereby enabling distinction between passive-margin and retroarc-basin stages through systematic changes in the age complexity of the source area.

Geochemistry of Upper Miocene Kudankulam Limestones, Southern India

Concentrations of major, trace, and rare-earth elements (REE) were measured in shallow marine limestone samples of the upper Miocene Kudankulam Formation, southern India, in order to investigate the geochemical variations among various litho-units. The CaCO3 content is higher in algal limestone (AL; 92 ± 1, n = 3) and clastic limestone (CL; 90 ± 2, n = 3) than sandy shell limestone (SSL; 81 ± 1, n = 3). All trace elements exhibit lower concentrations than post-Archean Australian Shale (PAAS) values, except one SSL sample. Large variations in ΣREE content are observed among CL, SSL, and AL (~14-142, ~68-124, and ~38-98, respectively). Almost all limestone samples analyzed from the Kudankulam Formation show a small negative cerium anomaly (Ce/Ce* ~0.8-0.9), except one AL sample, which lacks this cerium anomaly (Ce/Ce* ~1.04). Variations in Ce anomalies and ΣREE contents in Kudankulam limestone samples are mainly controlled by the amount of terrigenous sediments and diagenetic behavior. Shale-normalized REE patterns and (La/Yb)s, La/Sc, La/Th, and Th/Sc ratios suggest that the detrital sediments present in the limestones were probably derived from felsic source rocks. The observed low content of U (0.9 ± 0.5, n = 9) and U/Th (0.2 ± 0.1, n = 9) ratio in these limestones are probably related to an oxygen-rich environment.

Active or passive continental margin? Geochemical and Nd isotope constraints of metasediments in the backstop of a pre-Andean accretionary wedge in southernmost Chile (46°30′-48°30′S)

Abstract Provenance analysis of siliciclastic sedimentary rocks gives indications of the tectonic evolution and setting of source regions and the rocks contained in them. The composition of sedimentary rocks ideally reflects the nature of these regions, and only indirectly the tectonic setting of the basin where the erosional debris is deposited. This makes it possible to interpret Late Devonian to Early Carboniferous metasedimentary basement rocks of the Andes in southernmost Chile as having been deposited at a passive margin, despite geochemical indications of an active margin setting for the source rocks, and the position of the metasediments in the backstop of an accretionary wedge. Major and trace elements point to felsic source rocks from an active margin environment. The Nd model ages of 1170–1490 Ma indicate that the source rocks were part of an old continental crust in the Late Palaeozoic. The ɛ Nd (T) values range between −7 and −2. These characteristics, in combination with the regional geology, suggest that the geochemical signal is dominated by rocks formed at an active margin, which later acted as feeders for the sediments deposited in a passive-margin environment. If corroborated by research in progress this emphasizes the problem of deducing the tectonic setting of a depositional basin from provenance data.

Geochemistry and provenance of metasedimentary rocks from the Archean Golden Pond sequence (Casa Berardi mining district, Abitibi subprovince)

The Archean Golden Pond sequence is made up of deformed and metamorphosed conglomerates, greywackes, and mafic volcanic rocks, and is overlain by ferrugineous metasedimentary rocks of the North iron formation. The clastic rocks were derived mainly from a volcanic source that had undergone weak chemical weathering. Their source area was dominated by the presence of 60–80% high-Al2O3 felsic volcanics having strongly fractionated [La/Sm]N (= 3.7 ± 0.3) and very low Ta/Th ratios (= 0.09 ± 0.02), with lesser proportions of basaltic (10–30%) and ultramafic volcanic rocks (1–10%). The ferrugineous metasedimentary rocks can be modelled by mixing 20–40% siliciclastic material, of the composition of the average Golden Pond greywacke, with an Fe- and Si-rich precipitate (molecular Fe/Si = 0.6 ± 0.2). The high-Al2O3 felsic source rocks were most likely produced by subduction processes within an oceanic arc environment, but the mafic and ultramafic volcanic rocks were derived by different processes from an asthenospheric mantle source, possibly in an oceanic rift environment. Therefore, it is suggested that the ultramafic, mafic, and felsic volcanic rocks were brought to the same erosional level by dissection of the arc system and rapid exhumation of the felsic arc lithologies and the deeper ocean floor. Intrabasinal hydrothermal activity associated with contemporaneous mafic volcanism and (or) graben development may have also been responsible for the local production of the Fe-rich precipitates of the North iron formation.