Younger Granite Province Research Articles

Termination of anorogenic alkaline magmatism in Nigerian Younger Granite province: insights from Afu A-type granite complex

Termination of anorogenic alkaline magmatism in Nigerian Younger Granite province: insights from Afu A-type granite complex

Provenance Studies of Alluvial Tin Deposits in Parts of Ropp Younger Granite Complex, Northcentral Nigeria

The study area in this research is within the Ropp Complex, part of the Nigerian Mesozoic Younger Granite province. The study aims to interpret the depositional environment and establish the provenance of the alluvial cassiterite deposits in the study area. Boreholes/mining pits were logged for this study, and stanniferous sandstone samples were collected, which were used for textural and mineralogical studies. The mineral assemblages documented in the samples include ilmenite (3% to 27%). Cassiterite (2 to 14%), Zircon (2 to 16%), magnetite (0 to 17%), tourmaline (5 to 11%), rutile (2 to 8%) and monazite (2 to 7%).The ZTR Index calculated from the result of heavy minerals analysis for the selected pieces is 59%. Mineralogical studies revealed that quartz is the most dominant detrital mineral averaging about 93-99%, indicating that the stanniferous sandstones are compositionally matured and have experienced a high degree of chemical weathering. The quartz grains have grain sizes ranging from coarse to very coarse. They are poorly sorted, sub-angular to sub-rounded, with low sphericity. This also indicates a closeness to the source and textural immaturity. The occurrence of relatively very few feldspar grains suggests a slow sedimentation rate, very high rate of chemical weathering and composition maturity. The bivariate plots, univariate grain size parameters and probability plots, and the absence of fossils and trace fossils suggest deposition in a fluvial environment. The results of the granulometric analysis indicate that the study area's stanniferous sandstone was deposited in a fluvial environment by a low-energy fluvial (river) depositional system and the deposition in proximal (close to the source). This study suggests that the Basement complex and Younger Granite are the sources of the stanniferous placer deposits.

New geochronological and geochemical constraints on the Banke, Dutsen Wai, Guraka, and Zuku alkaline granite complexes from the northcentral Nigerian younger granite province

ABSTRACT An integrated study was carried out on four anorogenic syenite-granite complexes (i.e. Banke, Dutsen Wai, Guraka, and Zuku) from the Nigerian Younger Granite (NaYG) province, in a bid to constrain their magma sources, rare metal mineralization potentials, and tectonic implications. These four complexes primarily consist of quartz syenite, granite porphyry, arfvedsonite aegirine granite, and biotite granite. The studied rocks are chemically characterized by high alkalis, HFSEs, Ga/Al ratios, and zircon saturation temperature (776–1127°C, average = 932°C), typically of A1-type granites. The zircon εHf(t) values of the arfvedsonite aegirine granite (−6.0 to −0.79) are higher than those of the granite porphyry (−9.57 to −7.67) and biotite granite (−9.49 to −5.86) suggesting moderate crustal contamination of mantle-derived melts for the origin of the peralkaline granites. Generation of the biotite granite likely involved inputs of both crustal- and mantle-derived melts and is evidenced by the presence of inherited Pan-African (600–610 Ma) zircons and low whole rock Nb/U and Nb/Ta ratios. Furthermore, input of crustal material likely increased the metal budget of the residual melt and enhanced Nb, Sn, and sulphide mineralization in the Banke granite porphyry and biotite granite and Dutsen Wai biotite granite. Zircon U-Pb dating results constrain the emplacement ages to early Mesozoic era, from the Banke granite porphyry of 180 ± 1 Ma, Dutsen Wai biotite granite of 177 ± 3 Ma, Guraka arfvedsonite aegirine granite of 227 ± 2 Ma and Zuku arfvedsonite granite of 186 ± 2 Ma. These age data indicate a local E-W age migration pattern for the alkaline magma emplacement in northcentral NaYG province. Integrated with other geological evidence, we favour a formation model for the anorogenic NaYG province likely driven by the reactivation of ancient shear zones and structures.

Litho-Structural Analysis of the Sara-Fier Ring Complex, Northcentral Nigeria

The Sara-Fier igneous Complex is an excellent example of high level, anorogenic granite suites of North-central Nigeria. It is part of the larger Younger granite province, which has been traced to the central HoggarProvince. The Sara-Fier Complex is characterised by different variety of intrusive and extrusive granitic rocks. Although the area is interesting in terms of geologic-tectonic setting and potentials, detailed

Structural Analysis of Selected Ring Complexes in Some Parts of the Nigerian Younger Granite Provinces

The study area falls within the basement complex of northcentral Nigeria and covers four adjacent quarter degree sheets of 126 (Ririwai), 127 (Kalatu), 147 (Lere) and 148 (Toro) in some parts of Kaduna, Bauchi and Plateau States, Nigeria. This paper is aimed at interpreting the Aeromagnetic data to delineate structures in some parts of the Nigerian younger granite province. This was achieved by determining depth to basement, and developing a 2D model of the shape, location and depth of structures in some parts of the younger granite province. The anomalies on the aeromagnetic map were defined by fitting a first order polynomial to the total fields, by the method of least squares to obtain the residual field data. First vertical derivative and analytic signal computed, defined distinct pattern of the magnetic signatures. Depths to the source of the geologic structures where obtained from Werner and Euler deconvolution solutions which gives an average depths range of 231.2 m to 1040 m, with very few solutions having depths less than 200 m, the most prominent structure particularly the Ririwai ring complex have a depth range of 337.5 m to 465.5 m. The depth to basement for Werner solution ranged from 60 m to 420 m and the depth to basement of the contact model is shallow with depth of (60 to 420m) as compared to the dike model (200 to 420 m).

Onset of the anorogenic alkaline magmatism in the Nigerian Younger Granite province: Constraints from the Daura and Dutse complexes

The Younger Granite province of Niger and Nigeria exemplifies a classical anorogenic alkaline ring granite belt with southward age migration from ca. 470 Ma to 140 Ma. However, the precise age transition from the older Nigerien Younger Granite (NrYG) to the younger Nigerian Younger Granite (NaYG) is poorly constrained. The Daura and Dutse complexes, located at the northernmost tip of the NaYG province, represent a temporal and spatial link between the NrYG and NaYG provinces. The Daura complex is composed of amphibole syenite and biotite granite, while the Dutse complex consists of fayalite syenite and arfvedsonite aegirine granite. Similar to other alkaline rocks of the NrYG and NaYG provinces, the Daura and Dutse samples all chemically show typical A1-type granite features, such as high alkalis (K2O + N2O = 8.33–11.27 wt%.), Ga/Al ratios, Zr + Nb + Ce + Y contents (527–2345 ppm) and FeOt/(FeOt+MgO) ratios (0.03–0.70) as well as elevated HFSEs. The NdHf and Pb isotopic systematics show that all the syenite-granites yield εNd (t) = −2.9 to +1.5, ɛHf (t) = −3.8 to +16.7, and 206Pb/204Pb = 17.818 to 19.555, suggestive of magmas derived from a slightly depleted mantle source with OIB imprints. A reduced to slightly oxidized redox state is invoked for the parental melt based on estimations from mineral chemistry. The Daura amphibole syenite is composed of >60% alkali feldspar, show high Eu/Eu* (>1), Ba/Zr (>3.5) values and cumulate textures, resembling an alkali feldspar cumulate rock. The other alkaline granitic rocks were likely generated from variable fractional crystallization of the residual melt with distinct crustal contaminations. Zircon UPb dating reveal that the Daura amphibole syenite and biotite granite were emplaced at 324 ± 1 Ma and 321 ± 3 Ma, respectively, whereas the Dutse granitoids showed episodic intrusive periods of ca. 264 Ma and 232 Ma. All these results predate other anorogenic alkaline complexes of the NaYG and update the oldest age (ca. 214 Ma in previous references) recorded in the alkaline granite province. To the north, it connects well with ages from the Zinder complex (~330 Ma) in the southernmost NrYG province, reflecting a significant link between the NrYG and NaYG provinces. From a broad view, a linear age migration from north to south in the Niger-Nigeria Younger Granite province remains well preserved, although temporal and spatial intervals exist in local region. This emplacement pattern can be traced to the periodic and repeated reactivation of lithospheric shear zones and transcurrent faults.

INTERPRETATION OF AEROMAGNETIC DATA AND LANDSAT IMAGERY OVER THE NIGERIAN YOUNGER GRANITES IN AND AROUND KAFANCHAN AREA, NORTH-CENTRAL NIGERIA

In this research the lineaments of the Kafanchan area in North-central Nigeria were investigated in order to explore the mineralization zones of the area. Aeromagnetic data over Kafanchan and environs within the Younger Granite Province, in the North-Central Nigeria were collated and analyzed. The aeromagnetic map of the area was interpreted both qualitatively and quantitatively so as to identify the nature of the magnetic sources and the trends direction in the study area. The trend of the Total Magnetic Intensity (TMI) map is predominantly in NE-SW. The First Vertical Derivative (FVD) Lineaments Map was also correlated with LADSAT lineaments map and both maps agreed in most areas. The study area is characterized by predominant magnetic lineament trend in NE-SW direction and subordinate E-W direction. The result also shows that the most significant structural trends affecting the distribution of these magnetic anomalies in the study area is in NE-SW direction. The TMI map indicates that there are three major mineralization zones in the study area. The high magnetization contrast in the NE and SE parts of the study area correlates with the migmatite-gneiss, biotite-granites, granites and basalts which are associated with high magnetic contrasts. Also, the high magnetization contrast in the NW part of the area correlates with basalt and the biotite-granite. However, the predominant low magnetization contrast observed in the western half does not correlate with the basic igneous rock

Contrasting Neoproterozoic and Mesozoic granitoids in Zaranda complex (Nigeria): insights into the distinct origins, tectonic settings and mineralization potential

Combined whole-rock geochemistry, zircon U–Pb dating, and Sr–Nd–Pb isotopic analyses were carried out to constrain the petrogenesis and tectonic implications of the “Zaranda Older Granites” (ZOG) and “Younger Granites” (ZYG) in north-central Nigeria. Zircon U–Pb dating yielded 633 ± 3 Ma for the ZOG and 203 ± 1 Ma for the ZYG, corresponding to Late Neoproterozoic and Late Triassic, respectively. The ZOG consists dominantly of biotite granite, which displays a whole-rock ɛNd(t) values of − 3.48 to − 5.00 and Nd model ages of 1620–1720 Ma. Most ZOG samples exhibit elevated Sr/Y (107–128) and La/Yb ratios (36–40), with a positive Eu anomaly (Eu/Eu* = 2.2–2.58), interpreted to reflect magma derivation through partial melting of a thickened lower crust. The ZOG is geochemically and geochronologically similar to the Pan-African granites from Togo–Benin (West Africa) and Northeast Borborema, Sao Paulo (Brazil), suggesting that their emplacement probably marked the initial phase of the Pan-African orogeny during which the Gondwana was amalgamated. The ZYG consists of compositionally diverse alkaline granitoids including aegirine syenites, quartz syenite, and fayalite granite porphyry. These rocks are strongly ferroan, alkalic and show petrographic and geochemical features of classical A-type granites. Substantial depletion in Ba, Sr, P, Ti, and Eu signifies fractionation of felsic and mafic minerals such as feldspar, biotite, amphibole, and Ti-rich augite. The low Y/Nb and Yb/Ta ratios (≤ 2) indicate that the ZYG samples display geochemical features typical of OIB-derived A-type granites. The ɛNd(t) values in the ZYG range from + 4.16 to − 2.76 and the Nd model ages range from 0.6 to 1.2 Ga. Based on these results, we propose that the ZYG samples and by extension, other alkaline granitoids from the Nigerian Younger Granite (NYG) province with similar geochemical compositions were likely generated through extensive differentiation of OIB-type mantle-derived magma. The concentrations of Nb and other HFSEs are much higher in ZYG than ZOG, indicating that the former has higher rare metal mineralization potential.

Coexisting A1 and A2 granites of Kudaru Complex: implications for genetic and tectonic diversity of A-type granite in the Younger Granite province, north-central Nigeria

The Nigerian alkaline ring complexes have long been regarded as typical anorogenic A-type granite. However, uncertainty still surrounds many aspects of their petrogenesis and tectonic implication. In this paper, a combined study of whole rock geochemistry, Nd-Pb isotope and zircon U-Pb dating was carried out on the Kudaru ring complex from north-central Nigeria. The complex consists of mildly alkaline fayalite granite porphyry, peralkaline arfvedsonite granite and metaluminous biotite granite, which were emplaced between 176.9 ± 2.5 and 180.55 ± 0.6 Ma. These rocks are characterized by variably high alkalis, HFSEs, Ga/Al ratios, and zircon saturation temperature, typically of A-type granites. Notably, the popular subdivision scheme of A-type granite permits the discrimination of the alkaline-peralkaline granites as typical “A1-type” granites and the metaluminous granite as “A2-type”. These co-existing A1 and A2 granites show distinct isotopic compositions with less negative eNd (t) values and higher 206Pb/204Pb ratios for the former, indicating that the magma which gave rise to the A1 and A2 suites was derived from an enriched mantle source but modified to different extent during ascent. Notably, the coexistence of A1-A2 granites do not indicate discrete tectonic regimes (intraplate setting for A1 and post-orogenic setting for A2), but rather point to a significant role of crustal contamination, as seen also from the differences in Y/Nb ratios.

Preliminary interpretation of isostatic residual gravity anomalies within the central portion of the Equatorial Atlantic African region

Airy’s isostatic residual gravity anomaly map and profiles were interpreted for the central portion of the Equatorial Atlantic African region with a view to understanding the lithospheric dynamics, hence imminent vertical crustal movements and thus predict the future outlook of evolving landscape within the study area. In order to compute the isostatic residual gravity anomaly, the AIRYROOT algorithm of the United States Geological Survey was used and isostatically overcompensated, undercompensated and completely compensated portions were identified on the isostatic residual gravity anomaly map and profiles. The Airy’s isostatic residual gravity anomaly map showed three distinct parts which were the oceanic south-western part characterised by negative isostatic residual gravity anomaly values (− 11 to − 200 mGal), the volcanic eastern and extreme northcentral parts characterised by positive isostatic residual gravity anomaly values (+ 10 to + 90 mGal) and the central part having a network of sedimentary basins, characterised by isostatic residual gravity anomaly values that approach zero (− 10 to + 9 mGal). It was discovered that the oceanic south-western part was isostatically overcompensated, the uplifted eastern part and the Younger Granite Province of the extreme northcentral part were undercompensated and the central part consisting of the Niger Delta, Benue Trough and the Mid-Niger (Nupe) Trough was completely compensated. It was consequently predicted that vertical uplift is imminent in the isostatically overcompensated oceanic region while subsidence is expected over the region of the eastern upland and the Younger Granite Province which were associated with isostatic undercompensation. The sedimentary basins within the study area are expected to remain stable.

Geospatial Distribution and Analysis of Trace Element Geochemistry along the lower Reaches of Kassa River System Younger Granite Province, North Central Nigeria

A study was conducted to investigate the Geospatial distribution of trace elements including Iron along the lower reaches of theKassa river system within the Younger granite province of North Central Nigeria. A total of thirty sediment sample were collectedto investigate the concentration and potential environmental effects of trace elements within the area of study. The X-Ray fluorescence spectroscopy was used to determine the concentration of the trace elements including Iron while geographicinformation system technique of interpolation was used to interpolate not the element present but their ambient concentration as well. Results showed that the relative concentration of trace elements in the lower reaches of the Kassa river sediments is in the order Fe˃Ze˃Pb˃Th˃Rb˃Nb. Pollution Load Index values of studied area ranged from 0.596-1.476ppm which testify that theriver sediments are mostly unpolluted except for two of the sampling sites. However, the Igeo values indicate that anthropogenicactivities in the study area contribute to the distribution and concentration of the trace elements within the area of study.

Improved Mapping of the Structural Disposition of Some Younger Granite Ring Complexes of Nigeria Using High Resolution Aeromagnetic Data

Surface and subsurface structural interpretation over a part of the Younger Granite province of Northern Nigeria using Aeromagnetic data was carried out to demonstrate the use of recently available high resolution aeromagnetic data in mapping precisely the Younger Granite. State of the art software packages were used in data reduction, processing/analysis and interpretation. Results of the aeromagnetic study show that the distinct magnetic anomalies clearly revealed in the area coincide spatially with outcropping ring complexes. Apart from outcropping ring complexes, some ring complexes without surface exposures (unexposed ring complexes) were also delineated. One of such ring complexes was studied in greater details and its lateral dimensions with that of the adjoining ring complexes were better resolved. The unexposed ring complex was found to have a surface area extent of approximately 169.5 km2 which is similar to that of Ririwai (180 km2), Banke (128 km2), Kudaru (174 km2) and Zuku (121 km2). The results of the 3D Euler solutions indicate that the depths of the ring complexes range from outcrop to 1800 m. For the inferred unexposed ring complex, the depth ranges from 5 to 1200 m. The Younger Granites show low Source Parameter Imaging (SPI) depth values of about 4 km below the sea level indicating deeper magnetic sources. Three (3) profiles drawn along the NESW, NNW-SSE and E-W directions on the Analytic Signal map of the study area gave an overburden thickness of less than 20 m that masked the unexposed ring complex with its mappable root situated at approximately 200 m below the sea level. The Ririwai, Tibchi, Banke, Amo, Kudaru and Zuku ring complexes have outcrops of approximately 1300 m, 950 m, 920 m, 1400 m, 1150 m and 1220 m respectively above mean sea level with their mappable roots situated at approximately 400 m, 180 m, 400 m, 800 m, 10 m and 300 m respectively below the sea level. It delineated numerous lineaments as structural features whose major trend is along the NE-SW direction. It was equally observed that the ring complexes of Northern Nigeria display diverse surface plans/shapes and exhibit both inward and outward dipping structural orientations with depth. The likely reason for the unexposed nature of the newly proposed ring complex could be as a result of partial subsidence of the enclosed block on which the extrusive edifice of the central volcano originally rested probably occasioned by excess space (much subsurface accommodation) for continuously rising magma within the country rock. Its geometry is supportive of this assertion as it revealed majorly outward dipping trend with depth along different axis. Another reason could be due to incomplete fluidization along the already formed ring fracture and poor assimilation of the country rock by the fluidizing agent amidst a slow/near quiescent piece meal stopping process during the central granite intrusion phase which usually marks the end of ring complex formation. Finally, this research has demonstrated the efficacy of aeromagnetic data in delineating both outcropping ring complexes as well as those without surface exposures (outcrops).

Existence of a possible subsurface ring complex in the northwestern part of the Nigerian Younger Granite province: a contribution from aeromagnetic and remote sensing data

The study area lies within the north-western part of the Nigerian Younger Granite province. It is bounded by latitude 10°30′N and 11°00′N and longitude 8°00′E and 9°00′E. Previous geological and geophysical studies that were carried out in the study area focused mainly on the ring complexes with surface exposure while nothing is known about those without the surface exposure. This has resulted in gap in geo-scientific knowledge which this study aimed to fulfill. First vertical derivative (FVD) and Euler deconvolution of the aeromagnetic data along with processed DEM were used to achieve this purpose. Results of the interpretation revealed a magnetic anomaly, almost circular in shape, with characteristics that are similar to the anomalies that spatially coincide with the prominent ring complexes but without the outcrops that are associated with the complexes. This anomaly is interpreted to be an unexposed ring. Lineaments revealed by the results are mainly oriented in the E–W, NE–SW and NW–SE. The lineaments in the NE–SW and its conjugate are the most developed lineament system. Presence of ring complexes on and along the lineaments favours the opinion that the emplacement sites of the complexes are influenced by the reactivated Pan-African shear zones and transcurrent faults. In particular, the cross-over of major lineaments at the non-outcropping anomaly may suggest a very weak zone that probably allowed the volcanic eruption that initiated the emplacement of the unexposed ring complex. Results of the Euler deconvolution suggest a cylindrical-like body for the unexposed ring complex, which is consistent with pluton geometry, and a depth that range from outcrop to 1,200 m for it.

An Interpretation of Structures from the Aeromagnetic Field over a Region in the Nigerian Younger Granite Province

An interpretation of high resolution aeromagnetic data was carried out to provide new information about the study area. Located between latitude 10°30'N and 11°00'N and longitude 8°30'E and 9°00'E, the interpretation revealed previously unknown folds and magnetic anomaly with a signature that is similar to those associated with ring complexes in the study area but without the surface manifestation that is associated with the complexes. Qualitatively interpreted as unexposed ring complex, the magnetic anomaly is underlain by cross-over of major lineaments; implying presence of weak zone that probably allowed the volcanic eruption that initiated the emplacement of the source of the anomaly. A depth that ranges from outcrop to 1200 m and from outcrop to 1800 m is respectively suggested for the unexposed ring complex and folds by the Euler deconvolution results.

Highlights of some environmental problems of geomedical significance in Nigeria

This paper attempts to discuss the links between the geochemical composition of rocks and minerals and the geographical distribution of diseases in human beings in Nigeria. We know that the natural composition of elements in our environment (in the bedrock, soils, water, and vegetation) may be the major cause of enrichment or depletion in these elements and may become a direct risk to human health. Similarly, anthropogenic activities such as mining and mineral processes, industrial waste disposal, agriculture, etc., could distort the natural geochemical equilibrium of the environment. Thus, the enrichment or depletion of geochemical elements in the environment are controlled either by natural and/or anthropogenic processes. The increased ingestion of toxic trace elements such as As, Cd, Hg, Pb, and F, whether directly or indirectly, adversely affects human health. Of these, Cd has most dangerous long-term effect on human health. Environmental exposure to As and Hg is a causal factor in human carcinogenesis and numerous cancer health disorders. Available information on iodine deficiency disorder (IDD) in Nigeria indicates goiter prevalence rates of between 15% and 59% in several affected areas. There have been reported cases of dental fluorosis resulting from intake of water with fluoride content >1.5 ppm. Dental caries among children shows an overall prevalence rate of 39.9%. Within the Younger Granite province in central Nigeria, cases of cancer and miscarriages in pregnant women have been linked to natural radiation These examples and a number of others from the existing literature underscore the pressing need for the development of collaborative research to increase our understanding of the relationship between the geographical distribution of human and animal diseases in Nigeria and environmental factors. We submit that such knowledge is essential for the control and management of these diseases.

Geological mapping using fractal technique

In this work the use of fractal scaling exponents for geological mapping was first investigated using theoretical models, and results from the analysis showed that the scaling exponents mapped isolated bodies but did not properly resolve bodies close to each other. However application on real data (the Mamfe basin, the Gongola basin and the Younger granite province in Nigeria) showed good correlation with the geological maps of the areas. The results also indicated that basement rocks can generally be represented by scaling exponents with values ranging between -3.0 and -2.0. Keywords : Fractal, dimension, susceptibility, spectra, scaling exponent Nigerian Journal of Physics Vol. 18 (2) 2006: pp. 277-286

Comparative use of TIMS and SHRIMP for U–Pb zircon dating of A-type granites and mafic tholeiitic layered complexes and dykes from the Corsican Batholith (France)

The Corsica-Sardinia batholith in the southern realm of the Hercynian belt of Europe shows evidence for gravitational collapse of this part of the mountain belt, causing major felsic and mafic magmatism. The latest intrusions are composed of leucomonzogranite and late metaluminous and alkaline granite, associated with tholeiitic layered complexes and dykes. Three dating methods on zircon (Pb-evaporation, ID-TIMS and SHRIMP) were used to unravel the chronology of these felsic and mafic rocks. Dating of zircons by the conventional U–Pb method, using TIMS after zircon dissolution, achieved an analytical uncertainty of 1 Ma for favourable cases. The TIMS Pb-evaporation technique resulted in ages with an uncertainty range of 4 to 8 Ma. After 15 to 20 analyses with the SHRIMP method, a precision ranging from 2 to 5 Ma was obtained (all at 2 σ). The three methods applied to the same zircon population extracted from four A-type granites, show that the uncertainty ranges within 2–5 Ma according to the sample. This error seems to correspond to the real geochronological uncertainty that can be achieved. The results obtained show that all six tested alkaline granites were emplaced during a very short interval of about 3–5 Ma at about 288 Ma, almost contemporaneous with the leucomonzogranite emplacement (291–287 Ma) that ended the batholith formation. In addition, there is no significant gap with the age of emplacement of the mafic tholeiitic magmatism (around 286 Ma) crosscutting the “A-type” granites. The late alkaline granites definitely do not show up here as precursors of the Tethyan rifting that began at about 170 Ma, i.e. some 100 Ma after their emplacement. It is thus necessary to examine if alternative hypotheses to the anorogenic model of the A-type Younger Granite province better fit the new geochronological data. A model involving depleted continental-crust derived magma should be compatible with the timing and geodynamical constraints as far as isotopic data are concerned.

Magma source and evolution of Late Neoproterozoic granitoids in the Gabal El-Urf area, Eastern Desert, Egypt: geochemical and Sr–Nd isotopic constraints

Granitoids in the Gabal El-Urf area in Eastern Egypt consist of a monzogranite pluton, belonging to the Younger Granite province, emplaced in granodioritic rocks. Whole rock Rb–Sr dating indicate ages of 650±95 Ma and 600±11 Ma for the granodiorites and monzogranites, respectively. The granodiorites (65–70% SiO2) are calc-alkaline and metaluminous with low Rb/Sr, Th and Nb contents, moderate enrichment in the LILE (K2O, Rb, and Ba) and display most of the chemical and field characteristics of syn-to late-tectonic I-type granitoids described elsewhere in the Arabian–Nubian Shield. The monzogranites (72–77% SiO2) are metaluminous to mildly peraluminous, highly fractionated and depleted in Al2O3, MgO, CaO, TiO2, Sr and Ba with corresponding enrichment in Rb, Nb, Zr, and Y. They can be correlated with the undeformed post-orogenic granites in the Arabian–Nubian Shield that chemically resemble A-type granites emplaced in extensional settings. The mineralogical and chemical variations within the granodiorites and monzogranites are consistent with their evolution by fractional crystallization. The granodiorites have a low initial 87Sr/86Sr ratio (0.7024) and high ∈Nd values (+6.9–+7.3) and are significantly different from those (initial 87Sr/86Sr ratio=0.7029, ∈Nd values=+5.2–+5.8) of the monzogranites. These data suggest a predominant mantle derivation for both granite types and demonstrate that they originated from different source materials.The granodiorite melt was most probably generated through vapour-saturated partial melting of an early Neoproterozoic depleted mafic lower-crust reservoir due to crustal thickening associated with orogenic compression and/or arc magma underplating. The mineralogical and geochemical data of the A-type monzogranites are consistent with their derivation as a residual granitic liquid from a LILE-enriched mafic magma through crystal-liquid fractionation of plagioclase, amphibole, Fe–Ti oxides and apatite. The parental mafic magma was originated in the upper mantle due to crustal thinning associated with extension in the late stage of the Neoproterozoic crustal evolution of north-eastern Egypt.

INAA in the determination of the elemental constituents of a natural fluorite

A complementary use of the comparative and the semi-absolute variations of instrumental neutron activation analysis (INAA) has enabled us to determine 22 different elements in a sample of natural fluorite originating from the younger granite province of Nigeria. The mineral has a brownish purple appearance with some veinlets having a deep purple coloration. Of all the 22 elements measured, Fe (851.36 ppm), Sb (7.69 ppm), Ag (87.4 ppm), Hg (2.71 ppm) and Se (1.97 ppm) are found to be exclusive to the veinlets while Au (109.15 ppm), Co (2.15 ppm) and W (173.20 ppm) are concentrated in the main matrix of the fluorite. With the aid of the semi-absolute method, it was possible to measure the Au concentration in the IAEA Soil-7 reference material to be 56.83±5.87% ppm. A qualitative electron microprobe analysis (EMA) showed that the bulk of the matrix is composed mainly of Ca and F, as expected.

Post-collision, A-type granites of Homrit Waggat Complex, Egypt: petrological and geochemical constraints on its origin

The Homrit Waggat Complex (HWC) is a ring complex which belongs to the younger granite province of the Nubian Shield. It comprises a central biotite granite encircled by an intermediate ring of mylonitized pink granite. both of which have been emplaced by cauldron subsidence in a granodiorite body. The mylonitized pink granite is locally cut across by hypersolvus coarse-grained red granite. The mylonitized pink granite is characterized by fully-developed mylonitic fabrics and suffered inflation and flattening due to a ‘ballooning effect’ which suggests that it has experienced stronger ductile deformation than the granodiorite and biotite granite. The HW granites are mildly peraluminous, highly fractionated and enriched in Rb, Nb. Y, REE (except Eu) and F with corresponding depletion in Mg, Fe, Ti, Ca, Sr and Ba. REE fractionation patterns and Eu Eu∗ values decrease from granodiorite ( La Yb n = 6.24−6.69 ; Eu Eu∗ = 0.95−1.56 ) to biotite granite ( La Yb n = 2.48−4.68 ; Eu Eu∗ = 0.25−0.75 ). The mylonitized pink and red granites display flat patterns and large negative Eu anomalies ( La Yb n = 0.33−1.09 ; Eu Eu∗ = 0.01−0.07 , respectively). Although accessory minerals (e.g. apatite, allanite and fluorite) exerted some control on REE concentrations, volatile transfer largely controls REE abundances, particularly HREE (heavy rare-earth element). Geochemical data indicate that the granites of HWC are post-collision, A-types. The less fractionated granodiorite has an I-type and volcanic arc geochemical signature. Least-squares analyses of major elements and modelling of REEs indicate that the HW A-type granites were formed through two stages of differentiation. A first stage, crystalmelt fractionation, was dominated by the removal of plagioclase, biotite, quartz and apatite from the granodiorite magma to yield the biotite granite. Further crystal fractionation of the biotite granite liquid with K-feldspar as a dominant separating phase in addition to plagioclase, biotite and quartz gave rise to the mylonitized pink granite (second stage). Volatile phase transfer occurred during the second stage of fractionation in the magma chamber. The red granite represents the last crystallizing granite type from a residual, volatile-rich fluids which accumulated and emplaced as ring dykes in the solidified part of the mylonitized pink granite.