ACTIVE SKARN FORMATION BENEATH LASCAR-VOLCANO, NORTHERN CHILE - A PETROGRAPHIC AND GEOCHEMICAL STUDY OF XENOLITHS IN ERUPTION PRODUCTS

Calcsilicate xenoliths occur in large numbers in some lavas and pyrocl astic flows of Lascar Volcano. Their whole-rock major element and REE compositions indicate that the protolith was the Upper Cretaceous Yaco raite Formation, which crops out extensively in NW Argentina. The whol e-rock major element compositions of the xenoliths fall into specific groups suggesting a strong geochemical zonation in the skarn zone. Thr ee geochemical zones have been identified; (1)an outer metamorphic zon e rich in wollastonite; (2) a middle zone rich in pyroxene and garnet; (3) an inner zone rich in pyroxene and magnetite. The two innermost z ones have developed from the wollastonite zone by infiltration of meta somatic fluids rich in Fe, Mn, Mg, Ti and Al. Whole-rock REE patterns have not changed significantly during prograde metamorphism and metaso matism, indicating REE immobility in the altering fluids. Retrograde a lteration by acid-sulphate fluids produced anhydrite skarns and second ary calcite and wilkeite veins in the wollastonite zone. The carbon an d oxygen isotopic compositions of this calcite indicate that it formed by Rayleigh crystallization from a low-temperature (<200 degrees C) f luid containing dissolved H2CO3. The calculated delta(18)O of the wate r in this fluid suggests a magmatic origin whereas the calculated delt a(13)C Of the dissolved carbonate is consistent with derivation from r ocks of the Yacoraite Formation at 350 degrees C. It is suggested that the magmatic acid-sulphate fluid was responsible for leaching carbona te from the surrounding carbonate rocks and redepositing it in the ska rn zone. REEs were mobilized during the retrograde acid-sulphate and a cid-carbonate alteration. A negative Ce anomaly associated with this c arbonate and sulphate indicates high oxygen fugacities in the minerali zing fluids.