Open Session - SWISS GEOSCIENCE MEETINGs

88
Symposium 2: Mineralogy-Petrology-Geochemistry
2.1
Sr-Nd Isotopic Characteristics of Neogene volcanic rocks in Southeast of
Isfahan
Khodami Mahnaz *& Davoudian Ali Reza **
* Department of Geology, Islamic Azad, University, Mahallat Branch,Iran (mahnaz.khodami@gmail.com)
** Department of Natural Resources, Shahrekord University, Shahrekord, Iran
Neogene calc-alkaline volcanic rocks are exposed in southeast of Isfahan in the Urumieh Dokhtar magmatic belt in Central
Iran structural zone. These volcanic rocks have compositions ranging from basaltic andesites, andesites to dacites, which can
be find as lava flow and dome. Geochemical studies show these rocks are a medium to high K calc-alkaline suite and they
enriched from LILE and LREE in normalized multi-element patterns, and negative Nb, Ti, Ta and P. Condrite-normalized REE
patterns display a steep decrease from LREE to HREE without any Eu anomaly. These characteristics are consistent with ratios
obtained from subduction related volcanic rocks and in collision setting. Geochemical data show magma derived from a
heterogeneous source. Upper mantle lithosphere and lower crust are heterogeneous source but asthenosphere normally is
much more homogeneous(Seghedi, et al 2005). The mechanism of magma genesis is considered to be related to melting of
a heterogeneous source, which was enriched in incompatible elements situated at the upper continental lithospheric
mantle or lower crust. The geochemical characteristics of these volcanic rocks suggested that these volcanic rocks evolved
by magma-mixing contamination of a parental magma derived from metasomatized upper lithospheric mantle by partial
melting of the crust. These volcanic rocks carry a geochemical signature of continental crust through which they have
passed. These processes complicate distinguish of the source of parental. It is thought that dacitic rocks formed from a lower
crustal source and have less interaction with mantle. The mantle lithospheric derived magma evolved to basaltic andesite
as result of mixing with crustal melts. The crustal melts were generated contemporaneously, with a direct influence of the
mantle melts on crustal melting generation. Sr and Nd isotopic compositions (Table 1) are plotted in Fig. 2 along with regions
of known calcalkaline affinity. The data suggest that rocks plot at lower 87 Sr/ 86 Sr and 143 Nd/ 144 Nd ratios, around the Bulk
Silicate Earth reservoir, compared with normal calc-alkaline samples, which are much more scattered (Rosu, et al 2004). The
oldest measured rock (740) has higher 87 Sr/ 86 Sr and lower 143 Nd/ 144 Nd (Fig. 2) The most important petrogenetic process involved
in magma generation in this area is probably variable degrees of partial melting of a heterogeneous source, which was isotopically
depleted, but enriched in incompatible elements. The initial crustal contamination and insignificant fractional
crystallization processes is characteristic for an initial storage of the magmas in the crust.
REFERENCES
Rosu, E., Seghedi, I., Downes, H., Alderton, D.H.M., Szakács, A., Pécskay, Z., Panaiotu, C., Panaiotu, C. E. & Nedelcu, L., 2004:
Extension-related Miocene calc-alkaline magmatism in the Apuseni Mountains, Romania: Origin of magmas.
Schweizerische Mineralogische und Petrographische Mitteilungen, 84, 153–172.
Seghedi, I., Downes, H., Harangi, S., Mason P.R.D. & Pecskay, Z., 2005: Geochemical response of magmas to Neogene–
Quaternary continental collision in the Carpathian–Pannonian region: A review. Tectonophysics, 410, 485–499.
Table 1. Nd–Sr isotopic data for the volcanic rocks from southeast of Isfahan, Iran
Sample Rb Sr Nd Sm 87Sr/86Sr 143Nd/144Nd
Dacite with xenocrysts 59.1 596.8 20.7 3.1 0.705991 0.512578
Andesite 68.7 1039 18.7 3.8 0.704536 0.512613
Dacite 64 551.5 19.9 3.1 0.705598 0.512601
Basaltic andesite 33.3 613.4 21.4 4.6 0.706094 0.512615