The role of contamination in the tightrope of Gran Canaria felsic magma differentiation

<p>Peralkaline magmatism is mostly sustained by extensive feldspar fractionation from mafic parents at shallow depths in intraplate settings. In this case, silica saturation is critical as it controls the differentiation trend that a peralkaline magma follows. SiO<sub>2</sub>-oversaturated parents fractionate towards rhyolites, and SiO<sub>2</sub>-undersaturated towards phonolitic compositions. The Miocene post-shield stage of Gran Canaria records both differentiation trends, which has previously been ascribed to changes in the mantle source. Such stage has been divided in the Mogan and Fataga Group based on silica saturation. Here, we propose that contamination plays a key role in the differentiation of Gran Canaria volcanics. This assumption is supported with new <sup>40</sup>Ar/<sup>39</sup>Ar geochronology, mineral, glass and juvenile clast chemistry (oxygen isotopes, major and trace elements) merged with a detailed stratigraphy. Two types of contaminants were identified, one being cogenetic feldspar-dominated cumulates and the second one being sediments within the island crust. We propose that barium-rich trachytic magmas with positive europium anomalies are linked to melting of the feldspar cumulates left after extensive fractional crystallisation. The chemistry of such trachytes does not follow a liquid line of descent and contains reverse-zoned alkali-feldspars. The shift in silica saturation took less than 1 Ma and is marked by an increase in peralkalinity from 0.9 to 1.5 and a decrease in oxygen isotopes ratio from 7.0 to 5.0 ‰. We interpret these observations as the consequence of maturation of the shallow magma reservoir towards less sediment contamination. Such assimilation of sediments is limited thermally, and compositionally because the system should remain alumina deficient. Crustal assimilation in Gran Canaria did not produce voluminous silicic melts by itself but allowed the deviation of the differentiation trend of a more primitive, initially SiO<sub>2</sub>-undersaturated magma. The tightrope of differentiation is represented by the thermal divide between the granite and phonolite minima (i.e. feldspar join in petrogeny’s residua system). Contamination by sediments produces a transient SiO<sub>2</sub>-oversaturated system (Mogan Group). Cogenetic assimilation of cumulates by thermal rejuvenation of the reservoir attracts the magma towards the thermal divide (ubiquitous during the peralkaline stage). Armouring against sediment assimilation through time relaxes the system back to the initial SiO<sub>2</sub>-undersaturated conditions (Fataga Group).</p>

Mantle Melting and Magmatic Processes Under La Picada Stratovolcano (CSVZ, Chile)

Where and how arc magmas are generated and differentiated are still debated and these questions are investigated in the context of part of the Andean arc (Chilean Southern Volcanic Zone) where the continental crust is thin. Results are presented for the La Picada stratovolcano (41°S) that belongs to the Central Southern Volcanic Zone (CSVZ) (38°S–41·5°S, Chile) which results from the subduction of the Nazca plate beneath the western margin of the South American continent. Forty-seven representative samples collected from different units of the volcano define a differentiation trend from basalt to basaltic andesite and dacite (50·9 to 65·6 wt % SiO2). This trend straddles the tholeiitic and calc-alkaline fields and displays a conspicuous compositional Daly Gap between 57·0 and 62·7 wt % SiO2. Interstitial, mostly dacitic, glass pockets extend the trend to 76·0 wt % SiO2. Mineral compositions and geochemical data indicate that differentiation from the basaltic parent magmas to the dacites occurred in the upper crust (∼0·2 GPa) with no sign of an intermediate fractionation stage in the lower crust. However, we have currently no precise constraint on the depth of differentiation from the primary magmas to the basaltic parent magmas. Stalling of the basaltic parent magmas in the upper crust could have been controlled by the occurrence of a major crustal discontinuity, by vapor saturation that induced volatile exsolution resulting in an increase of melt viscosity, or by both processes acting concomitantly. The observed Daly Gap thus results from upper crustal magmatic processes. Samples from both sides of the Daly Gap show contrasting textures: basalts and basaltic andesites, found as lavas, are rich in macrocrysts, whereas dacites, only observed in crosscutting dykes, are very poor in macrocrysts. Moreover, modelling of the fractional crystallization process indicates a total fractionation of 43% to reach the most evolved basaltic andesites. The Daly Gap is thus interpreted as resulting from critical crystallinity that was reached in the basaltic andesites within the main storage region, precluding eruption of more evolved lavas. Some interstitial dacitic melt was extracted from the crystal mush and emplaced as dykes, possibly connected to small dacitic domes, now eroded away. In addition to the overall differentiation trend, the basalts to basaltic andesites display variable MgO, Cr and Ni contents at a given SiO2. Crystal accumulation and high pressure fractionation fail to predict this geochemical variability which is interpreted as resulting from variable extents of fractional crystallization. Geothermobarometry using recalculated primary magmas indicates last equilibration at about 1·3–1·5 GPa and at a temperature higher than the anhydrous peridotite solidus, pointing to a potential role of decompression melting. However, because the basalts are enriched in slab components and H2O compared to N-MORB, wet melting is highly likely.

Geochemistry of Dadeldhura granite and its mineral potential

Dadeldhura granite is a cordierite bearing two mica granite. It is S-type, leucocratic and rich in silica, alumina and potash and slightly low in soda. In this granite a remarkable evolutionary trend is marked by the general increase in SiO2, Al2O3 , K2O , Na2O , P2O5, Rb, Nb and depletion of CaO, MgO, Fe2O3, TiO2, Ba, Sr, Zr, Zn and V etc. towards later phase. Mineral chemistry of biotite and feldspar and plot of chemical data on variation diagrams and triangular diagrams show a moderate to fairly strong differentiation trend from (i) granite gneiss (GGN), (ii) biotite granite (BGR) to (iii) muscovite granite (MGR) and (iv) aplitic tourmaline granite (TGR). Many physical, mineralogical and chemical characteristics of Dadeldhura granite resemble with world's average tin granite.

The Inishdawros meta-peridotite, Callow, Ballyconneely, Connemara, Western Ireland

SummaryThe chemistry and petrography of a meta-ultrabasic lens containing peridotite with poikilitic pyroxenes and hornblende, both of probable igneous origin, are given. The body also contains feldspathic-bearing hornblende meta-gabbros and is a tectonic fragment in hornblende-plagioclase rock belonging to the same Errismore intrusion. Chemical analyses fail to discern any appreciable systematic differentiation trend across the body despite modal variation to hornblende plagioclase layers. The high Niggli mg near the structural bottom of the intrusion suggests that this part of the Errismore intrusion is right way up, contrary to a previous suggestion.

The petrogenesis and origin of the analcime in the volcanic rocks of the Crowsnest Formation, Alberta

The Crowsnest Formation consists of trachytes, analcime phonolites and blairmorites, metamorphosed to zeolite facies. The latter rocks contain large analcime phenocrysts variously suggested as being of primary igneous origin or due to transformation from original leucite by reaction of Na-rich fluids. Although neither field relationships or petrography provide convincing data favouring either hypothesis, the presence of primary undisrupted inclusion trails in the analcime tend to support the former hypothesis. Compositions of the analcimes differ from that of an analcime formed by transformation from leucite. The chemistry of the rocks and their constituent pyroxenes are consistent with a sodic rather than a potassic differentiation trend; feldspar and garnet analyses support this conclusion. Oxygen isotope values for the pyroxenes indicate no extensive exchange with a low temperature fluid. Thus it seems unlikely that leucite was ever a constituent of the Crowsnest suite as necessitated by the hypothesis of transformation from leucite. Geochemistry and known experimental data indicate that the analcime phonolites and blairmorites differentiated from a trachytic magma under restricted conditions at depths greater than 25 km by early sanidine and later analcime fractionation. The parental trachyte may be produced by partial fusion of crustal material at depths greater than 35 km.

The Fiskenæsset complex, West Greenland. Part III. Chemistry of silicate and oxide minerals from oxide-bearing rocks, mostly from Qeqertarssuatsiaq

The textures and chemistry of silicates and oxides were investigated for 63 rocks from the Fiskenæsset complex, West Greenland, by light microscopy and electron microprobe analysis. Emphasis was placed on those chemical distributions known to depend on temperature.Photomicrographs are given for the following intergrowths: spinel-magnetite, chromite-rutile, Cr-magnetite-Al-chromite. Magnetite occurs dispersed throughout Fe-bearing silicates, probably the result of oxidation. Chromite grains in chromitites commonly have a centre rich in inclusions of rutile and amphibole and a clean margin; partial recrystallization of the chromite followed by metamorphic equilibration to give a uniform composition from core to rim is indicated. Magnetite associated texturally with sulphides is Cr-free, and apparently results from oxidation.Coexisting spinels outline a solvus between (Fe, Mg)Fe2O4 and (Fe, Mg)Al204 which has a critical point near (Mg, Fe)(Fe0.6Al0.6 Cr0.8)O4. The wide tie-lines for Cr-poor specimens indicate equilibration below 500°C.Coexisting ilmenite and ferromagnesian silicates apparently equilibrated in most rocks near 650 ± ∽100°C, but in ones with late serpentine or tremolite, equilibration was probably below ∽425°C. Coexisting pyroxenes apparently equilibrated at higher temperatures, either near 700 or 850°C depending on which laboratory calibration is used. The Cr-rich spinels from the Fiskenæsset complex are lower in Mg than ones from dry basaltic stratiform complexes, and other chemical features lead to a distinctive composition range as proposed by Ghisler. Further data for the silicates confirm most of the observations and conclusions in Part II, but (a) five specimens were found to contain tremolite apparently in compositional equilibrium with hornblende, (b) highly calcic plagioclase does not necessarily coexist with chromite as was proposed in Part II, (c) some specimens with unusually sodic plagioclase and unusually Fe-rich silicates may represent a cryptic differentiation trend. Although many problems remain for further study, the silicates and oxides are interpreted as the variously metamorphosed products in the granulite, amphibolite or greenschist facies of a moderately wet AI-rich basaltic magma which underwent crystal-liquid differentiation under fairly oxidizing conditions. The original igneous assemblage apparently involved early precipitation of Mg-AI-Ti-Cr-magnetite, prolonged crystallization of highly calcic plagioclase and tschermakitic-magnesiohornblende, and late crystallization of high-Fe, medium-Cr spinel.