MAGMA DYNAMICS INFERRED FROM GEOCHEMICAL MODELING AND QUANTITATIVE TEXTURAL MEASUREMENTS OF IGNEOUS ROCKS FROM SE-BIRJAND (EAST OF IRAN)

Study area is located 120 km to SE of Birjand (East of Iran). Geologically, this region occurs in Sistan suture zone and is a wing of East Iran colored mélange. This study focused on the division of rocks younger than ophiolitic complex.

Textural analysis techniques and geochemical modeling were used to quantify textural and chemical diversities between shallow level intrusive rocks with quartz diorite - micro diorite and volcanic rocks with basaltic andesite composition. Some of the textural characterisitics such as presence of mafic enclaves, xenocrysts with reaction rims, oscillatory zoning and sieve texture of plagioclases, corroded and embayed quartz grains show evidence of magma mixing as recharge of more mafic and hot magma into open- system magma chamber. Results of major element modeling based on mass balance calculations and trace element modeling based on Rayleigh crystal fractionation models (FC), Assimilation and Fractional Crystallization (AFC), mixing combined with fractional crystallization (MFC) and mixing equations suggest that data trends is more consistent with magma mixing model. However, it seems that other processes also might have played important role in genesis of these rocks. These rocks represent geochemical signatures of adakites such as high Sr/Y and high Cr, Ni contents that suggest a subdaction source but it is possible that role of AFC with ultramafic rocks as assimillant caused this characterisitics.

Using quantitative textural measurements including crystal size distributions (CSD), Crystal Shape (S) and spatial distribution pattern (SDP) of plagioclases, some of the physical processes ongoing in the magma chamber modeled as Numerous repetitions of cycle of crystal nucleation and growth, injection of basic magma and following textural coarsening (Ostwald ripening) that finally ended by nucleation and growth of fine groundmass crystals but for basaltic andesites only affecting formation factor was kinetic processes of nucleation and growth.