TY - JOUR A1 - Läger, Kathrin A1 - Halama, Ralf A1 - Hansteen, Thor A1 - Savov, Ivan P. A1 - Murcia, Hugo F. A1 - Cortes, Gloria P. A1 - Garbe-Schönberg, Dieter T1 - Crystallization conditions and petrogenesis of the lava dome from the similar to 900 years BP eruption of Cerro Machin Volcano, Colombia JF - Journal of South American earth sciences N2 - The last known eruption at Cerro Machin Volcano (CMV) in the Central Cordillera of Colombia occurred 900 years BP and ended with the formation of a dacitic lava dome. The dome rocks contain both normally and reversely zoned plagioclase (An(24-54)), unzoned and reversely zoned amphiboles of dominantly tschermakite and pargasite/magnesio-hastingsite composition and olivine xenocrysts (Fo = 85-88) with amphibole/clinopyroxene overgrowth, all suggesting interaction with mafic magma at depth. Plagioclase additionally exhibits complex oscillatory zoning patterns reflecting repeated replenishment, fractionation and changes in intrinsic conditions in the magma reservoir. Unzoned amphiboles and cores of the reversely zoned amphiboles give identical crystallization conditions of 910 +/- 30 degrees C and 360 +/- 70 MPa, corresponding to a depth of about 13 +/- 2 km, at moderately oxidized conditions (f(O2) = +0.5 +/- 0.2 Delta NNO). The water content in the melt, calculated based on amphibole chemistry, is 7.1 +/- 0.4 wt.%. Rims of the reversely zoned amphiboles are relatively enriched in MgO and yield higher crystallization temperatures (T = 970 +/- 25 degrees C), slightly lower melt H2O contents (6.1 +/- 0.7 wt.%) and overlapping pressures (410 +/- 100 MPa). We suggest that these rims crystallized following an influx of mafic melt into a resident magma reservoir at mid-crustal depths, further supported by the occurrence of xenocrystic olivine. Crystallization of biotite, albite-rich plagioclase and quartz occurred at comparatively low temperatures (probably <800 degrees C) during early stages of ascent or storage at shallower levels. Based on amphibole mineral chemistry, the felsic resident melt had a rhyolitic composition (71 +/- 2 wt.% SiO2), whereas the hybrid magma, from which the amphibole rims crystallized, was dacitic (64 +/- 3 wt.% SiO2). The bulk rock chemistry of the CMV lava dome dacites is homogenous. They have elevated (La/Nb)(N) ratios of 3.8-4.5, typical for convergent margin magmas, and display several geochemical characteristics, of adakites. Both Sr and Nd isotope compositions (Sr-87/Sr-86 similar to 0.70497, Nd-143/Nd-144 similar to 0.51267) are among the most radiogenic observed for the Northern Volcanic Zone of the Andes. They are distinct from oceanic crust that has been subducted in the region, pointing to a continental crustal control on the isotope composition and hence the adakitic signature, possibly in a crustal "hot zone". KW - Colombian Andes KW - Cerro Machin Volcano KW - Magma mixing KW - Amphibole geothermobarometry KW - Trace element geochemistry Y1 - 2013 U6 - https://doi.org/10.1016/j.jsames.2013.09.009 SN - 0895-9811 VL - 48 IS - 12 SP - 193 EP - 208 PB - Elsevier CY - Oxford ER - TY - JOUR A1 - Sokol, Krzysztof A1 - Halama, Ralf A1 - Meliksetian, Khachatur A1 - Savov, Ivan P. A1 - Navasardyan, Gevorg A1 - Sudo, Masafumi T1 - Alkaline magmas in zones of continental convergence BT - the Tezhsar volcano-intrusive ring complex, Armenia JF - Lithos : an international journal of mineralogy, petrology, and geochemistry N2 - Alkaline igneous rocks are relatively rare in settings of tectonic convergence and little is known about their petrogenesis in these settings. This study aims to contribute to a better understanding of the formation of alkaline igneous rocks by an investigation of the Tezhsar volcano-intrusive alkaline ring complex (TAC) in the Armenian Lesser Caucasus, which is located between the converging Eurasian and Arabian plates. We present new petrological, geochemical and Sr Nd isotope data for the TAC to constrain magma genesis and magma source characteristics. Moreover, we provide a new Ar-40/Ar-39 age of 41.0 +/- 0.5 Ma on amphibole from a nepheline syenite that is integrated into the regional context of ongoing regional convergence and widespread magmatism. The TAC is spatially concentric and measures similar to 10 km in diameter representing the relatively shallow plumbing system of a major stratovolcano juxtaposed by ring faulting with its extrusive products. The plutonic units comprise syenites and nepheline syenites, whereas the extrusive units are dominated by trachytic-phonolitic rocks. The characteristic feature of the TAC is the development of pseudomorphs after leucite in all types of the volcanic, subvolcanic and intrusive alkaline rocks. Whole-rock major element data show a metaluminous (Alkalinity Index = 0-0.1), alkalic and silica-undersaturated (Feldspathoid Silica-Saturation Index <0) character of the TAC. The general trace element enrichment and strong fractionation of REEs (La-N/Yb-N up to 70) indicate a relatively enriched magma source and small degrees of partial melting. All TAC rocks show a negative Nb Ta anomalies typical of subduction zone settings. The initial 87Sr/85Sr ratios (0.704-0.705) and positive sNd values (+3 to +5) indicate an isotopically depleted upper mantle and lack of significant crustal influence, which in turn suggests the TAC magma has formed via differentiation from lithospheric mantle melts. KW - Alkaline igneous rocks KW - Ring complex KW - Armenia KW - Geochemistry KW - Ar-40/Ar-39 dating KW - Pseudoleucite Y1 - 2018 U6 - https://doi.org/10.1016/j.lithos.2018.08.028 SN - 0024-4937 SN - 1872-6143 VL - 320 SP - 172 EP - 191 PB - Elsevier CY - Amsterdam ER -