K-Ar dating,whole-rock and Sr-Nd isotope geochemistry of calc-alkaline volcanic rocks around the Gümüşhane area: implications for post-collisional volcanism in the Eastern Pontides,Northeast Turkey

Volcanic rocks from the Gümü?hane area in the southern part of the Eastern Pontides (NE Turkey) consist mainly of andesitic lava flows associated with tuffs, and rare basaltic dykes. The K-Ar whole-rock dating of these rocks range from 37.62?±?3.33 Ma (Middle Eocene) to 30.02?±?2.84 Ma (Early Oligocene) for the andesitic lava flows, but are 15.80?±?1.71 Ma (Middle Miocene) for the basaltic dykes. Petrochemically, the volcanic rocks are dominantly medium-K calc-alkaline in composition and show enrichment of large ion lithophile elements, as well as depletion of high field strength elements, thus revealing that volcanic rocks evolved from a parental magmas derived from an enriched mantle source. Chondrite-normalized rare-earth element patterns of the volcanic rocks are concave upwards with low- to-medium enrichment (La_CN/Lu_CN?=?3.39 to 12.56), thereby revealing clinopyroxene- and hornblende-dominated fractionations for andesitic-basaltic rocks and tuffs, respectively. The volcanic rocks have low initial ⁸⁷Sr/⁸⁶Sr ratios (0.70464 to 0.70494) and εNd_(i) values (+1.11 to +3.08), with Nd-model ages (T_DM) of 0.68 to 1.02 Ga, suggesting an enriched lithospheric mantle source of Proterozoic age. Trace element and isotopic data, as well as the modelling results, show that fractional crystallization and minor assimilation played an important role in the evolution of the volcanic rocks studied. The Eocene to Miocene volcanism in the region has resulted from lithospheric delamination and the associated convective thinning of the mantle, which led to the partial melting of the subduction-metasomatized lithospheric mantle.     

The Petrological and Geochemical Evolution of Ediacaran Rare-Metal Bearing A-type Granites from the Jabal Aja Complex, Northern Arabian Shield, Saudi Arabia

New fieldwork, mineralogical and geochemical data and interpretations are presented for the rare-metal bearing A-type granites of the Aja intrusive complex(AIC) in the northern segment of the Arabian Shield. This complex is characterized by discontinuous ring-shaped outcrops cut by later faulting. The A-type rocks of the AIC are late Neoproterozoic post-collisional granites, including alkali feldspar granite, alkaline granite and peralkaline granite. They represent the outer zones of the AIC, surrounding a core of older rocks including monzogranite, syenogranite and granophyre granite. The sharp contacts between A-type granites of the outer zone and the different granitic rocks of the inner zone suggest that the AIC was emplaced as different phases over a time interval, following complete crystallization of earlier batches. The A-type granites represent the late intrusive phases of the AIC, which were emplaced during tectonic extension, as shown by the emplacement of dykes synchronous with the granite emplacement and the presence of cataclastic features. The A-type granites consist of K-feldspars, quartz, albite, amphiboles and sodic pyroxene with a wide variety of accessory minerals, including Fe-Ti oxides, zircon, allanite, fluorite, monazite, titanite, apatite, columbite, xenotime and epidote. They are highly evolved(71.3–75.8 wt% SiO_2) and display the typical geochemical characteristics of post-collisional, within-plate granites. They are rare-metal granites enriched in total alkalis, Nb, Zr, Y, Ga, Ta, REE with low CaO, MgO, Ba, and Sr. Eu-negative anomalies(Eu/Eu* = 0.17–0.37) of the A-type granites reflect extreme magmatic fractionation and perhaps the effects of late fluid-rock interactions. The chemical characteristics indicate that the A-type granites of the AIC represent products of extreme fractional crystallization involving alkali feldspar, quartz and, to a lesser extent, ferromagnesian minerals. The parent magma was derived from the partial melting of a juvenile crustal protolith with a mantle contribution. Accumulation of residual volatile-rich melt and exsolved fluids in the late stage of the magma evolution produced pegmatite and quartz veins that cut the peripheries of the AIC. Post-magmatic alteration related to the final stages of the evolution of the A-type granitic magma, indicated by alterations of sodic amphibole and sodic pyroxene, hematitization and partial albitization.     

Hydrothermal alteration of plagioclase in granitic rocks from Proterozoic basement of SE Sweden

Petrographic examinations and electron microprobe analyses of Proterozoic granitic rocks, SE Sweden aimed to characterize and unravel the mechanisms and conditions of plagioclase alterations. These alterations include saussuritization, albitization and replacement of plagioclase by K‐feldspar. The hydrothermal alterations, which are inferred to have occurred at ca. 250–400°C, resulted in concomitant formation of Al‐rich titanite, epidote, calcite, pumpellyite, prehnite and iron oxides. Replacement of plagioclase by K‐feldspar occurs in red‐stained zones, which have developed close to thin fractures owing to the precipitation of tiny Fe‐oxide pigment particles within the altered plagioclase, whereas saussuritized plagioclase has less systematic spatial relationships to these fractures. Albitization of plagioclase occurred in rocks that are poor in biotite compared to rocks that suffered extensive saussuritization. The chemical and textural characterization of various types of plagioclase alterations allows elucidation of the granitic hydrothermal systems. Features of feldspar alteration in the granitic rocks are similar to those encountered in feldspathic sandstones and should hence be considered in studies on diagenetic changes of siliciclastic successions during basin evolution. Copyright © 2009 John Wiley & Sons, Ltd.