Juvenile granite in the Sanandaj–Sirjan Zone,NW Iran: Late Jurassic–Early Cretaceous arc–continent collision

The Sanandaj–Sirjan Zone (SSZ) of western Iran is characterized by numerous granitoids of mainly calc-alkaline affinities. Several leucogranite and monzonite bodies crop out in the eastern Sanandaj. Whole-rock Rb–Sr isochrons demonstrate that the Mobarak Abad monzonite (MAM) formed in two phases at 185 and 131 Ma. Low ⁸⁷Sr/⁸⁶Sr(i) (i represents initial) and high ¹⁴³Nd/¹⁴⁴Nd(i) ratios, resulting in positive ?^t _Nd, imply that the source magma originated from a depleted mantle; large ion lithophile element (LILE) and light rare earth element (LREE) enrichments imply that slab fluid was involved in the evolution of the parent magma. Geochemical characteristics of the MAM rocks show an affinity with I- and A-type granites, and the positive values of ?^t _Nd (+2 to +6), confirm that the MAM represents juvenile granite. Therefore, the MAM rocks are different from Himalayan, Hercynian, and Caledonian granites. Based on the geology of granitic host rocks that form the protoliths of metamorphic rocks, it is likely that the mafic part of the MAM formed in an island arc setting on Neo-Tethyan oceanic crust during Early to Middle Jurassic time. Subsequent collision of the island arc with the western part of the SSZ occurred in the Late Jurassic to Early Cretaceous. Metamorphism, accompanied by partial melting, occurred during collision. Finally, leucogranite magmas of the young Mobarak Abad dikes and the Suffi Abad body were generated in this collision zone. This new model suggests a Late Jurassic–Early Cretaceous arc–continental collision before final closing of the Neo-Tethys.     

Boundary Between Upper Cretaceous and Lower Paleocene in the Zagros Mountain Ranges of Southwestern Iran

In the present study, over 3000?m of Upper Cretaceous sediments (Tarbur Formation) in seven stratigraphic, columnar sections were studied. The area is located in the Zagros mountain ranges of southwestern Iran, attributed to the converging continental Arabian Shield, and is interpreted as the result of subduction and collision. Based on foraminiferal studies of the Tarbur Formation in the sections, we have established many new biozones in the stratigraphic sections. However, investigations of the biozones indicate that there is no lithostratigraphic variation between the Upper Cretaceous and Lower Paleocene sediments in some of the studied sections. The concept of widespread formations and lithologic correlations is not applicable in this area, probably as a result of the tectonic unrest at that time. The biostratigraphic boundary between the Cretaceous and the Lower Paleocene is nevertheless well defined by the intraformational boundary in the sections.     

Characterising the main karst aquifers of the Alvand basin, northwest of Zagros, Iran, by a hydrogeochemical approach

The Alvand River basin, situated in the northwest of the Zagros mountain range, Iran, drains carbonate aquifers through some important karst springs. The physical, chemical and isotopic characteristics of spring water were studied for two years in order to assess the origin of groundwater and determine the factors driving the geochemical composition. Principal components analysis was used to identify the main factors controlling the water chemistry. Two groups of springs were identified: (1) low mineralisation, ion concentration, especially sulphate, low temperature, light isotope composition and high elevation of the recharge area, and (2) moderate to high mineralisation, especially sulphate, higher temperature, heavy isotope composition and low altitude of the recharge area. The main factors controlling the groundwater composition and its seasonal variations are the geology, because of the presence of evaporite formations, the elevation and the rate of karst development. In both groups, the carbonate chemistry is diagnostic of the effect of karst development. The supersaturation with respect to calcite indicates CO₂ degassing, occurring either inside the aquifer in open conduits, or at the outlet in reservoirs. The undersaturation with respect to calcite shows the existence of fast flow and short residence-time conditions inside the aquifer. A PCA analysis showed that, contrary to most developed karst systems, where dilution occurs during the wet season, leaching of the gypsum-bearing Gachsaran Formation by rainwater produced higher mineralisation.

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Resumen La cuenca del río Alvand, situada en el noreste de la cadena montañosa Zagros drena acuíferos compuestos por carbonatos a través de manantiales kársticos. Durante dos años se estudiaron las características físicas, químicas e isotópicas del agua de manantial con el objetivo de determinar el origen del agua subterránea y de los factores que controlan su composición geoquímica. Se utililizó el análisis de componentes principales para identificar los factores principales que controlan la química del agua. Se identificaron dos grupos de mantaniales: (1) de baja mineralización, concentración de iones especialmente sulfatos, y baja temperatura, composición de isotopos livianos y elevación alta del área de recarga, y (2) mineralización moderada y alta, especialmente, sulfatos, y temperatura más alta, composición de isotopos pesados y altitud más baja del agua de recarga. Los factores principales que controlan la composición del agua subterránea y sus variaciones estacionales son la geología debido a la presencia de formaciones evaporitas, la elevación y la tasa a la que se desarrolla el karst. En ambos grupos la química de los carbonatos es un diagnóstico del efecto del desarollo de la estructura kárstica. La supersaturación con respecto a la calcita indica presencia de CO₂ que está en proceso de degasificación que ocurre dentro de acuífero en conductos abiertos o a la salida en reservorios. La subsaturación en lo referente a la calcita muestra la existencia de flujo rápido y de tiempos cortos de residencia al interior del acuífero. El análisis de componentes principales muestra que a diferencia de la mayor parte de sistemas kársticos en donde la dilución ocurre durante la temporada de lluvias, la lixiviación de la formación Gachsaran que contiene yeso a causa de lluvia produce mineralizaciones más altas.

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Résumé Le bassin de la rivière Alvand, situé dans le nord-ouest de la chaîne de montagne Zagros, draine des aquifères carbonatés par le biais dun important réseau de sources karstiques. Les caractéristiques physiques, chimiques et isotopiques de leau provenant des sources ont été étudiées pendant deux ans afin de déterminer lorigine de leau souterraine et les facteurs responsables de sa composition chimique actuelle. Lanalyse des éléments chimiques majeurs a été utilisée pour identifier les processus qui contrôlent la composition chimique de leau. Deux groupes différents de sources ont été identifiés: (1) faible minéralisation et concentration en ions, spécialement en sulfates, faible température, composition isotopique légère et haute élévation de la zone de recharge, et (2) minéralisation moyenne à élevée, particulièrement en sulfates, température élevée, composition isotopique lourde et plus basse altitude de la zone de recharge. Les facteurs principaux qui contrôlent la composition et la variation saisonnière de leau souterraine sont la géologie, en raison de la présence dévaporites, lélévation et le taux de développement des karsts. Dans les deux groupes, la teneur en carbonates est un diagnostic de leffet du développement de la structure des karsts. La sursaturation par rapport à la calcite indique un dégazage en CO₂ qui a lieu soit à lintérieur de laquifère, dans les conduits ouverts, ou à sa sortie, dans les réservoirs. La sous-saturation par rapport à la calcite indique lexistence dun écoulement rapide et un faible temps de résidence dans laquifère. Une analyse des éléments chimiques majeurs a démontré que contrairement aux systèmes de karst les mieux développés où la dilution a lieu lors de la saison humide, le lessivage du gypse de la formation de Gachsaran par leau de pluie produit une minéralisation plus élevée.

     

Organic geochemistry and petroleum potential of Early Cretaceous Garau Formation in central part of Lurestan zone,northwest of Zagros,Iran

Deposition of organic rich black shales and dark gray limestones in the Berriasian-Turonian interval has been documented in many parts of the world. The Early Cretaceous Garau Formation is well exposed in Lurestan zone in Iran and is composed of organic-rich shales and argillaceous limestones. The present study focuses on organic matter characterization and source rock potential of the Garau Formations in central part of Lurestan zone. A total of 81 core samples from 12 exploratory wells were subjected to detailed geochemical analyses. These samples have been investigated to determine the type and origin of the organic matter as well as their petroleum-generation potential by using Rock-Eval/TOC pyrolysis, GC and GCMS techniques. The results showed that TOC content ranges from 0.5 to 4.95 percent, PI and Tmax values are in the range of 0.2 and 0.6, and 437 and 502 °C. Most organic matter is marine in origin with sub ordinary amounts of terrestrial input suggesting kerogen types II-III and III. Measured vitrinite reflectance (Rrandom%) values varying between 0.78 and 1.21% indicating that the Garau sediments are thermally mature and represent peak to late stage of hydrocarbon generation window. Hydrocarbon potentiality of this formation is assessed fair to very good capable of generating chiefly gas and some oil. Biomarker characteristics are used to provide information about source and maturity of organic matter input and depositional environment. The relevant data include normal alkane and acyclic isoprenoids, distribution of the terpane and sterane aliphatic biomarkers. The Garau Formation is characterized by low Pr/Ph ratio (<1.0), high concentrations of C27 regular steranes and the presence of tricyclic terpanes. These data indicated a carbonate/shale source rock containing a mixture of aquatic (algal and bacterial) organic matter with a minor terrigenous organic matter contribution that was deposited in a marine environment under reducing conditions. The results obtained from biomarker characteristics also suggest that the Garau Formation is thermally mature which is in agreement with the results of Rock-Eval pyrolysis.     

Mineralogical and geochemical investigations of the Mombi bauxite deposit,Zagros Mountains,Iran

The Mombi bauxite deposit is located in 165 km northwest of Dehdasht city, southwestern Iran. The deposit is situated in the Zagros Simply Fold Belt and developed as discontinuous stratified layers in Upper Cretaceous carbonates (Sarvak Formation). Outcrops of the bauxitic horizons occur in NW-SE trending Bangestan anticline and are situated between the marine neritic limestones of the Ilam and Sarvak Formations. From the bottom to top, the deposit is generally consisting of brown, gray, pink, pisolitic, red, and yellow bauxite horizons. Boehmite, diaspore, kaolinite, and hematite are the major mineral components, while gibbsite, goethite, anatase, rutile, pyrite, chlorite, quartz, as well as feldspar occur to a lesser extent. The Eh–pH conditions during bauxitization in the Mombi bauxite deposit show oxidizing to reducing conditions during the Upper Cretaceous. This feature seems to be general and had a significant effect on the mineral composition of Cretaceous bauxite deposits in the Zagros fold belt. Geochemical data show that Al₂O₃, SiO₂, Fe₂O₃ and TiO₂ are the main components in the bauxite ores at Mombi and immobile elements like Al, Ti, Nb, Zr, Hf, Cr, Ta, Y, and Th were enriched while Rb, Ba, K, Sr, and P were depleted during the bauxitization process. Chondrite-normalized REE pattern in the bauxite ores indicate REE enrichment (ΣREE = 162.8–755.28 ppm, ave. ∼399.36 ppm) relative to argillic limestone (ΣREE = 76.26–84.03 ppm, ave. ∼80.145 ppm) and Sarvak Formation (ΣREE = 40.15 ppm). The REE patterns also reflect enrichment in LREE relative to HREE. Both positive and negative Ce anomalies (0.48–2.0) are observed in the Mombi bauxite horizons. These anomalies are related to the change of oxidation state of Ce (from Ce³⁺ to Ce⁴⁺), ionic potential, and complexation of Ce⁴⁺ with carbonate compounds in the studied horizons. It seems that the variations in the chemistry of ore-forming solutions (e.g., Eh and pH), function of carbonate host rock as a geochemical barrier, and leaching degree of lanthanide-bearing minerals are the most important controlling factors in the distribution and concentration of REEs. Several lines of evidences such as Zr/Hf and Nb/Ta ratios as well as similarity in REE patterns indicate that the underlying marly limestone (Sarvak Formation) could be considered as the source of bauxite horizons. Based on mineralogical and geochemical data, it could be inferred that the Mombi deposit has been formed in a karstic environment during karstification and weathering of the Sarvak limy Formation.