Jurassic fossil hydrothermal alteration associated with volcano-sedimentary complex in North of Shahrekord (Sanandaj-Sirjan zone)

The volcano - sedimentary complex in the North of Shahrekord، center of Chahar mahal and bakhtiary province as a NW-SE trend belt has located in the central part of the Sanandaj- Sirjan structural zone is composed of volcanics (basalts، andesitic basalts and andesites)، subvolcanics (dolerites and microdiorites)، volcaniclastics (tuffs، lapilli tuffs، agglomerates، volcanic breccia and tuffites) belong to Upper Jurassic، Middle Kimmerian orogenic phase. These rocks with calc-alkaline to tholeiitic affinity were erupted in a subduction environment as island arc. The above mentioned lithological unites have been affected by hydrothermal fluids with pH=8-9، T=226◦C (in intermediate argillic zone) to 300◦C (in quartz- epidote veins) under 200 bar pressure. The secondary minerals have replaced primary rock components (selective pervasive) or have occurred as vein and vesicle fillings (non pervasive). There are three alteration zones in the study area including propylitic (chloritic)، sericitic and intermediate argillic based on variety amounts of the chlorite، epidote، clinozoisite، calcite and illite. Vesicles in basalts are filled by chlorite-epidote-clinozoisite-calcite- quartz assemblages too. The investigation of mass changes show that the volcanic rocks have lossed their mass as result of hydrothermal alteration partially in chloritic zone. CaO enrichment and Na2O depletion in chloritic zone، Na2O، K2O depletion and MgO enrichment in sericitic zone، FeO، Na2O، K2O enrichment in intermediate argillic zone are key indices to determination of fossil alteration zones.