Mineralogy, Geochemistry, Petrology and Petrogenesis of the Meydan-Zilan (Ercis-Van, Turkey) Area Volcanic Rocks

[摘要] The stratigraphy in the area starts with the Upper Miocene Kizildere formation containing carbonates, sandstone, tuff and tuffite, followed by Pliocene-Quaternary volcanic rocks. The sediments have been studied in some detail. The volcanic rocks are divided into three main suites; (1) a calcalkaline suite consisting of Kizildere lavas, andesite, trachyandesite, dacite, agglomerate, ignimbrite, rhyolite dome and dykes, pumice and the Gurgurbaba lava; (2) the Meydan alkaline suite containing alkali olivine basalt, hawaiite, mugearite, benmoreite and trachyte; (3) a transitional suite containing the Aladag lava and the Zilan lava. All of these suites have characteristic mineralogical and chemical compositions. (1) In the calcalkaline suite, andesite, trachyandesite and dacite show a variety of petrographical disequilibrium textures reflecting contamination and magma mixing processes. The crystallization conditions obtained from different mineralogical assemblages range 786-1074°C and fO2=l.56-l.98 for andesite, 1015-1089°C for trachyandesite, 724-828(+/-75)°C, fO2=l-71-1.76 and P=5.08-8.90(+/-0.5) kbar for dacite. Ignimbrites indicate high temperature emplacement with moderate to strongly welded textures, and crystallization conditions of 750-921°C and fO2=1.42-2.29. A rhyolite dome and dykes also show evidence of disequilibrium textures. Mineral thermobarometers for the rhyolite dome and dykes gave 750-850°C, fO2=2.21-2.29 and P=3-3.5 kbar. Pumice tephra formed as pyroclastic fall and surge by mainly Plinian- and Phreatoplinian-type explosive productions. (2) The Meydan alkaline lavas show evolution from alkaline olivine basalt to trachyte, with lack of ilmenite and alkali feldspar indicating relatively high oxygen fugacity. Mugearite and trachyte show disequilibrium textures as a result of magma mixing and disequilibrium crystallization. Olivine-liquid thermometers yielded crystallization temperatures of 1142-1248(+/-40) for alkali olivine basalt, 1025-1141 (+/-40) for hawaiite, 1037-1085(+/-40) for mugearite, and 913-964(+/-40) for benmoreite. Coexisting magnetite and ilmenite in trachyte gave values of 825-1056°C and fO2 = l-28-1.38 relative FMQ. (3) In the transitional suite, the Aladag lava formed under low and falling oxygen fugacity, and at probably 869-1017°C and 14 kbar. Crystallization temperature in the Zilan lava ranges between 1073 and 1095°C. Plagioclase delta18O values(7-9‰) in the Meydan alkaline lavas suggest a contribution from 18O-rich continental crust during the evolution of the rocks. The degree of contamination is large for the evolved rocks. Furthermore, the variation of 87Sr/86Sr(0.7048-0.7056) in the Meydan suite is evidence of open-system processes. 87Sr/86Sr values from the most primitive samples are close to those characterizing an enriched mantle source. The 87Sr/86Sr (~0.7050) of the basaltic magma suggest that it may have undergone some interaction with crust prior to its emplacement into a crustal magma chamber. Small Sr-isotopic variations in the Meydan suite suggest fractionation proceeded with assimilation of crustal rocks. Although isotopic data contradicts with an AFC process in the evolution of the calcalkaline suite, minor amounts of contamination have occurred in differentiated rocks. O- and Sr-isotope covariations suggest a two stage process of (a) source contamination controlled by hydrous flux from a subducted slab which gives rise to a large range of Sr isotopes, and (b) followed by crustal contamination and fractional crystallization. In the Meydan alkaline suite, mugearite, benmoreite and trachyte magmas were modified by open-system fractionation and replenishment with increasing fractional crystallization as a result of a long period in the magma chamber. Crystal fractionation was the main factor controlling the variations in bulk compositions. Hawaiites have been derived from an alkali olivine basalt parent by fractionation of an augite-dominated assemblage at high pressure. Olivine and titanomagnetite fractionations are also evident. Plagioclase and apatite fractionations were significant in the more evolved rocks. Generally, alkali olivine basalt represent fractionated liquids derived from reservoirs at upper mantle levels.

[发布机构] University:University of Glasgow

[关键词] Geology, Petrology, Mineralogy, Geochemistry