Search for provenance of Oligocene Barail sandstones in and around Jotsoma, Kohima, Nagaland
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Keywords:
Staurolite
Heavy mineral
Arenite
Sillimanite
Lithic fragment
Andalusite
Tourmaline
The Guaritas Group comprises sediments from alluvial fan, fluvial and eolian depositional systems. This study aimed with the heavy mineral analysis of sandstone samples from Varzinha, Pedra Pintada, Pedra da Arara and Serra do Apertado formations, units from the middle and upper portions of the Guaritas Group. For the heavy mineral analysis, it was used the very fine sand fraction of 29 samples collected in outcrops. This analysis included the identification and counting of the non-opaque and non-micaceous heavy minerals as well as the determination of the ZTi (zircon/(tourmaline+zircon)) and ZRi (zircon/(rutile+zircon) indexes and morphological types of zircon grains. The studied samples correspond to subarkoses, sublitharenites and litharenites. The identified heavy minerals were: zircon (82-13%), apatite (64-0%), tourmaline (41-3%), anatase (22-0%), garnet (29-0%), rutile (10-0%), sillimanite (5-0%), monazite (4-0%), epidote (2-0%), kyanite (1-0%) and fluorite (1-0%). There were recognized four morphological types of zircon grains: very well/well rounded short prism (47-6%), subrounded/subeuhedral short prism (92-42%), euhedral short prims (13-0%) and euhedral long prims (3-0%). The huge quantity of zircon, apatite and tourmaline grains (common accessory minerals in nearby granitic rocks) and the reduced proportion or absence of typically metamorphic heavy minerals like rutile, sillimanite, kyanite, staurolite, amphiboles and pyroxenes, are compatible with source area dominated by igneous acid rocks (granites). Very well rounded grains of zircon still indicate sediments from ancient sedimentary rocks. Similar values of the ZRi (0.92-0.96) and ZTi (0.71-0.82) indexes suggest, respectively, maintenance of the source area configuration and minor variation of the sedimentary reworking degree during the deposition of the middle and upper portions of the Guaritas Group. The Serra do Apertado Formation slightly differs from the other units by its relative higher ZTi and proportion of euhedral zircon grains. This is indicative of higher degree of sedimentary reworking and increasing contribution of sediments derived from igneous rocks during the final phase of sedimentary fill of the Camaqua basin.
Tourmaline
Sillimanite
Heavy mineral
Andalusite
Staurolite
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The study which involves forty-one (41) sandstone samples' collected from the area delimited by longitudes 7°00'E and 8°30'E and latitudes 5°00'N and 6°30'N, entailed evaluating and characterizing these sediments. Detailed field studies to establish the lithostratigraphy was carried out and fresh samples were subjected to petrographic, granulometric and heavy mineral analyses. Field observations revealed that the sandstones of the Awi Formation are angular to subangular, feldspathic, and generally fines upward and Amaseri are calcareous, indurated and laterally extensive. Agbani Sandstone consists of thick vertical sequences of ferruginized and indurated sandstones. Granulometric study shows medium to coarse-grained (0.78-1.30?), poorly to moderately sorted (0.87-1.19?), coarse to strongly fine skewed (-0.17-0.57?) and very platykurtic to leptokurtic (0.66-1.31?). This indicates mostly high energy condition and this is characteristic of fluvial environments. Petrographic studies for Awi samples indicate that they are texturally immature to submature, subarkosic arenites from a continental block provenance, while Amaseri Sandstones are texturally submature sublitharenites from a recycled orogen provenance. Agbani Sandstones are texturally immature to mature subarkosic arenites, sublitharenites and quartz arenites. Heavy mineral studies revealed zircon, tourmaline, rutile and hornblende to be common to all formations, sillimanite, kyanite, garnet, and staurolite are mostly common to Awi and Amaseri Sandstones while Agbani Sandstone has a dominance apatite with spinel. This suite of minerals suggests an acid igneous rock to typical dynamothermal metamorphic rock sources. The ZTR indices (74.0-94.4%) indicate mineralogically mature sediments. The study established that the pre- Santonian siliciclastic sequences of the Lower Benue Trough are texturally immature to mature and compositionally submature to supermature products of a continental block and recycled orogenic provenance under humid climates and deposited in fluvial to tidally influenced fluvial environments with high to moderate energy conditions humid climates which are deposited under high to moderate energy conditions.
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The study area represents a part of the South Shillong Shelf, Meghalaya, where sandstones belonging to the Barail Group (Eocene-Oligocene) are well exposed. Thin section petrography and heavy mineral investigations have been undertaken to assess the geological significance of the sandstones. Framework grains of the Barail sediments comprise of quartz (56.98 to 71.91%), feldspar (1.87 to 7.35%), mica (1.07 to 9.72%), lithic fragments (2.20 to 8.41%) and matrix (9.20 to 20.43%). The detrital composition of the sandstones comprises primarily of quartz grains, angular to sub-rounded, fine to medium grained and moderately sorted in nature. The sandstones are classified as subarkose, feldspathic graywacke and lithic graywacke. Provenance discrimination triangular plots of QFL and QmFLt reflect that the detritus were mostly derived from cratonic interior sources. The Diamond diagram indicates that the Barail sandstones were derived from middle and upper rank metamorphic source. Heavy mineral study of these sandstones indicates the presence of zircon, tourmaline, rutile, sillimanite, kyanite, staurolite, garnet, epidote, sphene, hypersthene, hornblende, chlorite, chloritoid, andalusite, apatite and opaque minerals. The percentages of most stable heavy minerals zircon, tourmaline and rutile vary from 6.11to 21.61, 2.60 to 6.56 and 2.42 to 7.57 respectively. The ZTR maturity index varies from 15.15 to 34.37, which reflects that the sandstones are mineralogically immature. The petrographic and heavy mineral study of the sandstones suggest that the detritus was probably derived from Shillong Massif where Precambrian metamorphic rocks of pelitic and arenaceous composition with plutonic bodies were exposed around the shelf margin. As the Shillong Massif comprises of Precambrian metamorphic rocks of pelitic and semitic (arenaceous) composition with intrusive plutonic bodies.
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Sillimanite
Detritus
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Andalusite
Tourmaline
Hornblende
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Geostatistical and multivariate statistical analyses were applied to heavy mineral data from an Amazonian fluvial-lake system near the Tapajós River mouth to investigate the spatial distribution and source-area of sediments. Twenty-one points were investigated, and the physical characteristics of the Green Lake deepest point were determined. Sand accumulates in the lake margins and mud quantity increases towards the lake center. Heavy mineral assemblage is composed of zircon, tourmaline, kyanite, rutile, staurolite, anatase, sillimanite, garnet, and spinel. Tourmaline, staurolite, and spinel are more abundant in the southeast area of the lake, while kyanite is dominant in the north area and zircon is in the whole lake except in its southeast area. Zircon - tourmaline and zircon - staurolite pairs are negatively correlated (r= -0.947 and -0.775, respectively), while tourmaline - staurolite and sillimanite - anatase pairs have a positive correlation (r= 0.628 and 0.675, respectively) which indicate different source rock types. Geostatistical analysis grouped the heavy minerals in three grups: Group 1 (tourmaline – staurolite – spinel - kyanite) and Group 2 (garnet – rutile – sillimanite - anatase) related to metamorphic source rocks ranging from medium to high grade, and Group 3 (zircon) related to acid igneous source rocks. The heavy mineral assemblage of Green Lake is analogous to the assemblage of the Alter do Chão Formation, indicating that this formation is the source of sediments of Green Lake.
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Clino-amphibole, epidote, garnet, sillimanite, tourmaline, staurolite, andalusite, zircon, rutile, augite, ilmenite, hematite and magnetite are the heavy-minerals recorded in the Piranhas-Acu River. Six assemblages were identified: (i) Garnet-hornblende-tourmaline with sillimanite, when cutting rocks of the Serido Formation; (ii) Hornblende-garnet-zircon, when crossing rocks of the Caico gneiss-migmatite Complex; (Hi) Hornblende-zircon-epidote-staurolite, when in rocks of the Jucurutu Formation; (iv) Hornblende-zircon-epidote, when rocks of the Acu Formation; (v) Hornblende-zircon-staurolite, when the lowermost Agu River channel cuts limestones of the Jandaira Formation, and (vi) Zircon-tourmaline-staurolite in the Acu River mouth (Cenozoic rocks) where coastal dynamics dominate. Mineral ratios that reflect differences in grain shape, density, and selective chemical decomposition were used to isolate the effects of source and as controls of mineral variability. The reworking of the sediments was regionally effective in selective sorting; the more equant minerals (e.g. epidote) and heavier minerals (e.g. opaques) had a higher probability of being selected for permanent deposition during reworking. The process of selective decomposition dominates at the river mouth.
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Provenance and petro graphic studies of sandstones of the Bhadasar Formation have been ascertained by means of heavy mineral and petro graphic analysis. Petrography reveals an abundance of angular to sub-round mono-crystalline, at places poly-crystalline quartz followed by orthoclase, microcline and plagioclase feldspar. The overall composition of sandstone matches with those of calc-arenites. The heavy mineral crop of sandstones of the Bhadasar Formation shows Euhedral to sub-hedral as well as rounded to sub-rounded grains of tourmaline, zircon, garnet, rutile, apatite, epidote, staurolite, kyanite and hornblende with decreasing order of abundance. Heavy mineral suite coupled with petrography indicates a mixed provenance dominated by a sedimentary source which is derived from intra-cratonic basins.
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Staurolite
Arenite
Lithic fragment
Hornblende
Microcline
Tourmaline
Detritus
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The Guaritas Group comprises sediments from alluvial fan, fluvial and eolian depositional systems. This study aimed with the heavy mineral analysis of sandstone samples from Varzinha, Pedra Pintada, Pedra da Arara and Serra do Apertado formations, units from the middle and upper portions of the Guaritas Group. For the heavy mineral analysis, it was used the very fine sand fraction of 29 samples collected in outcrops. This analysis included the identification and counting of the non-opaque and non-micaceous heavy minerals as well as the determination of the ZTi (zircon/(tourmaline+zircon)) and ZRi (zircon/(rutile+zircon) indexes and morphological types of zircon grains. The studied samples correspond to subarkoses, sublitharenites and litharenites. The identified heavy minerals were: zircon (82-13%), apatite (64-0%), tourmaline (41-3%), anatase (22-0%), garnet (29-0%), rutile (10-0%), sillimanite (5-0%), monazite (4-0%), epidote (2-0%), kyanite (1-0%) and fluorite (1-0%). There were recognized four morphological types of zircon grains: very well/well rounded short prism (47-6%), subrounded/subeuhedral short prism (92-42%), euhedral short prims (13-0%) and euhedral long prims (3-0%). The huge quantity of zircon, apatite and tourmaline grains (common accessory minerals in nearby granitic rocks) and the reduced proportion or absence of typically metamorphic heavy minerals like rutile, sillimanite, kyanite, staurolite, amphiboles and pyroxenes, are compatible with source area dominated by igneous acid rocks (granites). Very well rounded grains of zircon still indicate sediments from ancient sedimentary rocks. Similar values of the ZRi (0.92-0.96) and ZTi (0.71-0.82) indexes suggest, respectively, maintenance of the source area configuration and minor variation of the sedimentary reworking degree during the deposition of the middle and upper portions of the Guaritas Group. The Serra do Apertado Formation slightly differs from the other units by its relative higher ZTi and proportion of euhedral zircon grains. This is indicative of higher degree of sedimentary reworking and increasing contribution of sediments derived from igneous rocks during the final phase of sedimentary fill of the Camaquã basin
Tourmaline
Sillimanite
Heavy mineral
Andalusite
Staurolite
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