Petrology of Metaluminous A-Type Rhyolite Discovered from Hadjer el Hamis Volcanoes (Lake Chad Basin)

Metaluminous (P.I. > 1) rhyolite from Hadjer el Hamis consisted of quartz, alkali feldspar, clinopyroxene (hedenbergite), amphibole (F-arfvedsonite) and oxides-hydroxides (ilmenite, magnetite, limonite) phenocrysts is characterized by the negative Eu, Ba, P, Sr and Ti anomalies. This metaluminous rhyolite and the early discovered peralkaline rhyolites in Hadjer el Hamis volcanoes derive likely from the same source, according to their coexistence on the same sector and their similar Zr/Nb ratios. The causes of magma heterogeneity are likely linked to varying amounts of extraction of an earlier melt phase or tectonic juxtaposition or a sudden increasing of fO2 in silicic magmas, triggered from a hydrothermal process, associated with F- and alkali-bearing fluids influx, which promoted the enrichment of Na in the hedenbergite rims and the crystallization of arfvedsonite.

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.

Petrogenesis and Geochemical Characterization of the Granitoids of the Magba Shear Zone West Cameroon Central Africa

Magba granitoids are made up of granites, orthogneiss, migmatites, metagabbro, mafic dykes and mylonites with respectively porphyritic, porphyroblastic, grano-lepido-porphyroblastic, and cataclastic texture. Mafic dykes and metagabbro occur as intrusives into the mylonitic and granitic rocks. Magba rocks were subjected to whole rock geochemistry analyses and results show that those rocks have the chemical composition of gabbro, monzodiorite, monzonite syenite, quartz-monzonite, granodiorite and diorite. The rocks are metaluminous, display high-K, calc-alkaline to shoshonitic affinities and plot on the field of volcanic arc granites and are formed by differentiation of I-type magma. They are largely situated within the syn-collision to within plate fields, show a subduction—to collision-related magmatism, and suggest their emplacement during the syn—to post-collisional phase of the Pan-African orogeny.

Geology and Geochemistry of Iroko-Granites Southwestern Nigeria:Implication on Provenance

The geology,geochemistry of Iroko-granites have been studied and reported in this paper.The study area has been described as of Archaean-Early Proterozoic terrain underlain by migmatite-gneiss-quartzite complex with supracustal rocks.Large number of granites which outcrops in Iroko-Ekiti represent a typical occurrence of granitoids sporadically distributed in the basement and are known to belong to the Older Granite suites,which are attributable to the Pan-African Orogeny(750±150Ma).The rocks occur,mostly as flat and low lying within sparse vegetation.Structures common on and around the outcrops include quartz vein,veinlets,pegmatite dykes which trend North-south,discrete exfoliated surfaces and xenoliths of older rocks.This study reveals that the granites belong to calc-alkalic suites,demonstrate metaluminous nature,and exhibit characteristics of I-type granites.The granite is a distinctive type in that it is relatively highly potassic,has high FeO/(FeO+MgO)ratio,and high average Zr(299.75ppm)concentration with other high field strength elements.The trace elements study implicates pronounced fractional crystallization during evolution of the granites and thus petrogenetically discriminates as Syn-collision provenance.

Geochemical Study of Almoqlaq Intrusive Body in North of Asadabad in Hamedan Province, Iran

The study area is located in the west of Iran and in the north of Asadabad. This is the range between Eastern lengths of 00', 48° to 15', 48°, and northern latitudes of 45', 34° to 00', 35°. This range is limited from north to the asphalt road of Hamadan Qorveh, Sanandaj, and from the East and Southeast, to Asadabad of Kermanshah-Hamedan road, and from the West to Chahardoli plain and from the south to the plains and the town of Asadabad. According to the geological map of the study area, intrusive outcropping of diorite-gabbro of Almoqlaq, and quartz syenite is at Almoqlaq mountains. Intrusive rocks of the region, based on the total Alkali oxide, against silica rocks of study area fall within the quartz monzonite, gabbro, granodiorite, granite, and syenite between quartz and alkali granite. And granite in the study area (due to aluminum saturation index), is in the range of metaluminous, and metaluminous to peraluminous. Based on normalized spider diagram with the upper crust, the typical study area belongs to this model, and most of the elements are placed near or on standard line of 1.

Petrology of Intrusive Bodies of Azizabad-Tinamo Area in the South of Songour, Iran

In terms of structural divisions of the Iran’s earth crust, the studied area is considered as a part of the Sanandaj-Sirjan structural zone and Zagros Thrust. In this area, there are two gabbroic-dioritic and syenitic igneous intrusive bodies. The host rocks of these intrusive bodies are upper Cretaceous limestones and formed by contactmetamorphism at the contact zone between hornfels rocks and skarn. Field and geochemical datasets represent that all various rocks within these intrusive bodies have been originated from the same magma. The process of fractional crystallization and probably a small amount of contamination and assimilation has also played a role in the evolution of parent magma. Generally, these magmas are sub-alkaline and lie in themagmatic range of tholeiitic and calc-alkaline series. Samples investigated are metaluminous and peralkaline.

Diversity of Mesozoic tin-bearing granites in the Nanling and adjacent regions,South China:Distinctive mineralogical patterns

The Nanling and adjacent regions of South China host a series of tin deposits related to Mesozoic granites with diverse petrological characteristics. The rocks are amphibole-bearing biotite granites, or （topaz-） albite-lepidolite （zinnwaldite） granites, and geochemically correspond to mealuminous and peraluminous types, respectively. Mineralogical studies demonstrate highly distinctive and critical patterns for each type of granites. In mealuminous tin granites amphibole, biotite and perthite are the typical rock-forming mineral association; titanite and magnetite are typical accessory minerals, indicating highjO2 magmatic conditions; cassiterite, biotite and titanite are the principal Sn-bearing minerals; and pure cassiterite has low trace-element contents. However, in peraluminous tin granites zirmwaldite-lepidolite, K-feldspar and albite are typical rock-forming minerals; topaz is a common accessory phase, indicative of high peraluminity of this type of granites; cassiterite is present as a uniquely important tin mineral, typically rich in Nb and Ta. Mineralogical distinction between the two types of tin granites is largely controlled by redox state, volatile content and differentiation of magmatic melts. In oxidized metaluminous granitic melts, Sn4＋ is readily concentrated in Ti-bearing rock-forming and accessory minerals. Such Sn-bearing minerals are typical of oxidized tin granites, and are enriched in granites at the late fractionation stage. In relatively reduced peraluminous granitic melts, Sn2＋ is not readily incorporated into rock-forming and accessory minerals, except for cassiterite at fractionation stage of granite magma, which serves as an indicator of tin mineralization associated with this type of granites. The nature of magma and the geochemical behavior of tin in the two types of granites thus result in the formation of different types of tin deposits. Metaluminous granites host disseminated tin mineralization, and are locally related to deposits of the chlorite quartz-vein, greisen, and skarn types. Greisen, skarn, and quartz-vein tin deposits can occur related to peraluminous granites, but disseminated mineralization of cassiterite is more typical.