@article{MAKHILLOJES20071225359,
    title = {Mineralogy and Geochemistry of the Neogene Andesites from Annaba`s Area(Northeast Algeria)-Calcic Amphibole Thermobarometry},
    journal = {Online Journal of Earth Sciences},
    volume = {1},
    number = {2},
    pages = {101-107},
    year = {2007},
    issn = {1991-7708},
    doi = {ojesci.2007.101.107},
    url = {https://makhillpublications.co/view-article.php?issn=1991-7708&doi=ojesci.2007.101.107},
    author = {Daif Menana and},
    keywords = {Algeria,neogene,calc-alkaline,potassic andesites,Ca-amphiboles,thermobarometry},
    abstract = {Neogene Andesites are part of an igneous series that outcrops in the Northeast of Algeria and that belongs to Maghreb`s magmatic margin belt. This series is composed of calc-alkaline rocks including essentially microgranites, rhyolites and diorites and well expressed andesites. Andesites consist of complete differentiated succession including scare basalts, small amounts of rhyolites and preponderant andesites and dacites. They are heterogeneous in texture, colour and structure but intimately related to each other in time, space, mineralogical and chemical features. They also have a particular mineralogy: olivine is lacking, orthopyroxene is restricted to basic andesites which are less common whereas plagioclases, clinopyroxenes and hydrous minerals are abundant, titanomagnetite, ilmenite and sphene are also present in small quantities. The salient geochemical characteristics of these rocks are high SiO<SUB>2</SUB>, K<SUB>2</SUB>O, Rb and low FeO<SUP>T</SUP>, MgO, CaO, Ba and Sr. They also show an enrichment in light Rare Earth Elements (REE) and a depletion in heavy REE.Whole rock <SUP>87</SUP>Sr / <SUP>86</SUP>Sr ratios are high and unequal (from 0.707-0.710). All these features testify that investigated andesites are mostly acid, medium to strongly potassic and are typical of calcalkaline suites of continental margins. The amphiboles represent an important ferromagnesian phase, it seems advantageous to examine more closely their composition. Detailed microprobe analysis shows that all amphiboles are calcic, silica poor, magnesian, weakly titaniferous and most of their aluminium is in tetraedrical site. They may be divided into two sub-groups: On the one hand the magnesiohastingsite, pargasite and kersutite group, on the other the tschernakite group (according to classification of Leake, 1997). Chemical composition of amphiboles allows thermobarometric investigations. The Aluminium in Hornblende (AH) barometer is used to estimate pressures (P<SUB>1</SUB>, P<SUB>2</SUB>) with two different calibrations (Hollister <I>et al.</I>, 1987 and Schmidt, 1992). Temperatures (T<SUB>1</SUB>, T<SUB>2</SUB>) are calculated using Ti in amphiboles thermometry (Otten, 1984 and Femenias <I>et al.,</I> 2006), compared to plagioclase + hornblende thermometer of Blundy and Holland (1990). In all studied rocks, amphiboles phenocrysts equilibrate at nearly constant pressure of 5±0.5 Kbar. But their temperatures of crystallization range from 800-950°C (T<SUB>1</SUB> from 850-950°C, T<SUB>2</SUB> from 800-900°C) with a slight decrease in dacites and rhyolites. These conditions correspond to a shallow crustal chamber and may explain most of andesite features: Zoning in phenocrysts, high SiO<SUB>2</SUB>, K<SUB>2</SUB>O and abundance of pyroclastic forms.}
    }