Petrology and Geochemical Features of Crystalline Rocks in Ora-Ekiti,Southwestern Nigeria

This research investigates and reports on the petrology and geochemical characteristics of crystalline basement rocks in Ora-Ekiti,Southwestern Nigeria.Exhaustive geological investigation reveals migmatite,banded gneiss,gran­ite gneiss and biotite gneiss underlie the area.In reducing order of abundance,petrographic examination reveals that migmatite contains quartz,muscovite and opaque minerals.Banded geniuses contain quartz,biotite,plagioclase,and opaque minerals.Granite geniuses contain quartz,plagioclase,biotite,microcline and opaque;while biotite geniuses contain biotite,plagioclase,opaque minerals,and quartz.Silica contents in migmatite(69.50%-72.66%;ca.71.23%),banded gneiss(71.66%-77.1%;ca.75.23%),biotite gneiss(72.32%-76.18%;ca.73.83%)and granite gneiss(69.82%-73.15%;ca.71.95%)indicate the rocks are siliceous.High alumina contents in migmatite(12.18%),banded gneiss(10.28%),biotite gneiss(11.46%)and granite gneiss(9.97%)are comparable to similar rocks in the basement com­plex.All the rocks show Ba,Sr and Rb enrichment.Harker diagrams of Al_(2)O_(3)versus SiO_(2)and CaO versus SiO_(2)show negative trends while Na_(2)O versus SiO_(2),K_(2)O versus SiO_(2)and TiO_(2)versus SiO_(2)plots showed positive trends.This var­iation probably depicts extensive crystal fractionation in the magmatic systems that produced the rocks prior to meta­morphism or partial melting of the precursor rock.SiO_(2)versus(Na_(2)O+K_(2)O)classifies the rocks as granite to granodi­orite.The rocks are high K-calc-alkaline and calc-alkalic on SiO_(2)-K_(2)O plot.This shows the rocks are potassic meaning that they are formed from a potassium-rich source.The plot of Al_(2)O_(3)/(Na_(2)O+K_(2)O)versus Al_(2)O_(3)/(CaO+Na_(2)O+K_(2)O)reveals the crystalline rocks are orogenic and originated from granitoid with meta luminous affinity.The rocks consist of gneisses of no economic minerals,but the petrology reveals them as common rocks typical of metamorphic terrains and geochemical features of the rocks reveal they are felsic and of granitic composition.

Basement interface structural characteristics beneath Jiashi strong earthquake swarm area in Xinjiang, China

The seismic data obtained from high resolution seismic refraction profile in Jiashi strong earthquake swarm area in Xinjiang, China were further processed with ray hit analysis method and more complete basement interface structural characteristics beneath Jiashi strong earthquake swarm area were determined. The results show that there are two clear basement interfaces at the upper crust in Jiashi strong earthquake swarm area. The first one with buried depth ranging from 2.6 km to 3.3 km presents integral and continuous structure, and it appears an inclined plane interface and smoothly rises up toward Tianshan Mountain. The second basement interface with buried depth from 8.5 km to 11.8 km, is the antiquated crystalline basement of Tarim basin. Near the post number of 37 km, the buried depth of the crystalline basement changed abruptly by 2.5 km, which maybe result from an ultra crystalline basement fault. If taking this fault as a boundary, the crystalline basement could be divided into two parts, i.e. the southwestern segment with buried depth about 11.5 km, and the northeastern segment with buried depth approxi-mately from 8.5 km to 9.0 km. That is to say, in each segment, the buried depth changes not too much. The northeast segment rises up as a whole and upheaves slightly from southwest to northeast, which reflects the upper crustal deformation characteristics under the special tectonic background at the northwestern edge of Tarim basin.

Study on the basement structure in the southeastern North China by blasting seismic

Since the Mesozoic, southeastern North China Craton has experienced intense crustal thinning and lithosphere destruction. Some of the responses of the deep activity in the upper crust crystalline basement have been retained in a series of tectonic evolution. The study of the upper crust velocity structure,especially the properties of the basement interface, is of great significance for studying the tectonic evolution and seismic hazard in the southeastern part of North China. In this study, we selected Pg waves of the blasting seismic data in the southeastern part of North China in recent years, which reflect the west Shandong uplift, offshore sedimentary basins and the Tanlu Fault zone and the Sulu orogenic transition zone, to study the structural and seismological characteristics of basement in North China Craton. The results of this study showed as follows: First, the obvious lag of Pg wave arrival time in Dongying depression and North Jiangsu basin reveals the thick sedimentary, low velocity and unstable basement structure. Second, the advance Pg wave arrival time with high apparent velocity, which reflects the basement structure of the west Shandong uplift, indicates the thin sediments and the shallow basement. Third, combined with many geophysical phenomena, such as electrical structure, density structure and terrestrial heat flow, we hold that the Tanlu tectonic belt and the Sulu orogenic belt have experienced great lithosphere destruction and there is shallow crust and the thinnest lithosphere in the vicinity of the Tanlu fault zone.

Identification of major sources controlling groundwater geochemistry in Mount Makabaï in the Far-North of Cameroon(the northernmost part of the Pan-African Belt)

The present work focuses on the volcanic basement rock geochemistry of Mount Makabaï(MarouaCameroon city)in relation to groundwater geochemistry.Investigations were centered on an approach involving an association of several methods such as geological cartography,hydrogeology,and geochemistry.Three rock samples,four disturbed soil samples,three undisturbed soil samples,and 20 samples of groundwater in 10 wells were collected during fieldwork and prepared for laboratory analyses.Bedrocks exhibit porphyritic,aphyric,and subaphiric textures.The phenocryst phases are mainly olivine,clinopyroxene,and plagioclase.Based on whole rock major element chemical compositions,the Makabaï samples,plotted on Na_(2)O+K_(2)O versus SiO_(2)diagram,are mugearite,hawaiite,and basalt.The ratio of(MgO+CaO)/(Na_(2)O+K_(2)O)exceeds unity and shows dominance of alkaline Earth metals(MgO:2.99-4.48%;CaO:6.19-9.80%)relatively to alkali metals(Na_(2)O:2.27-4.46%;K_(2)O:2.09-2.51%).Exchangeable base contents are high,mostly represented by Ca and Mg,whose contents vary from 14.6 to 35.28 cmol(+)kg^(-1)and from 3.28 to9.48 cmol(+)kg^(-1)respectively.The CECclayis likewise high,fluctuating between 116.80 and181.38 cmol(+)kg^(-1),values in line with the presence of 2:1 clay minerals represented in the study area by montmorillonite.In the water samples,the Ca2+and Na+contents are generally higher than K^(+)and Mg^(2+),reflecting the relative abundance of Na_(2)O and CaO oxides to K_(2)O and MgO oxides in rocks where different waters were sampled.The waters of Makabaï are acidic to neutral(6.3≤pH≤7.4)and weakly to highly mineralized(110.0≤EC≤1190.0μS/cm).Hydro-geochemical classification by using the Piper diagram revealed two water types:(1)CaMg-HCO_(3)(40%)and(2)CaMg-Cl or CaMg-SO4(60%).The average content of ions in all the analyzed samples was HCO_(3)^(-)>>NO_(3)^(-)>SO_(4)^(2-)>Cl-for anions and Ca^(2+)>>Mg^(2+)>Na^(+)>K^(+)for cations.Major mechanisms governing ionic constituents of groundwater in the study area are water-rock interactions,silicate weathering,and ion exchange.

Characteristic of Gravity and Magnetic Anomalies in the Daba Shan and the Sichuan basin, China: Implication for Architecture of the Daba Shan

The Dabashan nappe structural belt links the Hannan block to the west with the Huangling block to the east between Yangxian and Xiangfan. The Dabashan arc-shaped fold belt formed during late Jurassic and was superposed on earlier Triassic folds. To achieve an improved understanding of the deep tectonics of the Dabashan nappe structural belt, we processed and interpreted the gravity and magnetic data for this area using new deep reflection seismic and other geophysical data as constraints. The results show that the Sichuan basin and Daba Mountains lie between the Longmenshan and Wulingshan gravity gradient belts. The positive magnetic anomalies around Nanchong-Tongjiang-Wanyuan-Langao and around Shizhu result from the crystalline basement. Modeling of the gravity and magnetic anomalies in the Daba Mountains and the Sichuan basin shows that the crystalline basement around Nanchong-Tongjiang-Wanyuan-Langao extends to the northeast underneath the Wafangdian fault near Ziyang. The magnetic field boundary in the Zhenba-Wanyuan-Chengkou-Zhenping area is the major boundary of the Dabashan nappe thrusting above the Sichuan Basin. This boundary might be the demarcation between the south Dabashan and the north Dabashan structural elements. The low gravity anomaly between Tongjiang and Chengkou might be partly caused by thickened lower crust. The local low gravity anomaly to the south of Chengkou-Wanyuan might result from Mesozoic strata of low density in the Dabashan foreland depression area.

Geochemical and Nd-Hf Isotopic Constraints on the Petrogenesis of an Archean Granitoid in the Erguna Massif, NE China

Crystalline basement and Precambrian crustal growth of the continental massifs within the Central Asian Orogenic Belt(CAOB) are still pending problems. Our geological and geochemical investigations identified an Archean(2606 Ma) granitic pluton in the Biliya area of the Erguna Massif. The Neoarchean granitoids show high and positive in-situ zircon ε_(Ht)(t)(+0.3 to +10.0), whole-rock ε_(Nd)(t)(+4.8) and whole-rock ε_(Ht)(t) values(+2.1). They are characterized by high Y + Ce + Zr + Nb(> 220 ppm) and Zr contents and could be classified as A-type granites. These granitoids are characterized by high Zr saturation temperatures(T;)(796–836°C). They were derived from partial re-melting of juvenile mafic lower crust in an intracontinental back-arc extensional environment. This newly identified Neoarchean granitic pluton may represent the crystalline basement of the several continent massifs within the CAOB, and their high ε_(Ht)(t)–ε_(Nd)(t) values may also indicate the occurrence of lateral crustal growth events in these massifs during the Neoarchean.