GEOLOGICAL SETTING AND PETROGENESIS OF SYMMETRICALLY ZONED, MIAROLITIC GRANITIC PEGMATITES AT STAK NALA, NANGA PARBAT HARAMOSH MASSIF, NORTHERN PAKISTAN

Miarolitic granitic pegmatites in the Stak valley in the northeast par t of the Nanga Parbat - Haramosh Massif, in northern Pakistan, locally contain economic quantities of bi- and tricolored tourmaline. The peg matites form flat-lying sills that range from less than 1 m to more th an 3 m thick and show symmetrical internal zonation. A narrow outer or border zone of medium- to coarse-grained oligoclase - K-feldspar - qu artz grades inward to a very coarse-grained wall zone characterized by K-feldspar - oligoclase - quartz - schorl tourmaline. Radiating spray s of schorl and flaring megacrysts of K-feldspar (intermediate microcl ine) point inward, indicating progressive crystallization toward the c ore. The core zone consists of variable mixtures of blocky K-feldspar (intermediate microcline), oligoclase, quartz, and sparse schorl or el baite, with local bodies of sodic aplite and miarolitic cavities of '' pockets''. The pockets contain well-formed crystals of albite, quartz, K-feldspar (maximum microcline +/- orthoclase overgrowths), schorl-el baite tourmaline, muscovite or lepidolite, topaz, and small amounts of other minerals. Elbaite is color-zoned from core to rim: green (Fe2+- and Mn2+-bearing), colorless (Mn2+-bearing), and light pink (trace Mn 2+). Within similar to 10 cm of the pegmatites, the granitic gneiss wa llrock is bleached owing to conversion of biotite to muscovite, with l ocal quartz and albite added. Schorl is disseminated through the alter ed gneiss, and veins of schorl with bleached selvages locally traverse the wallrock up to 1 m from the pegmatite contact. The schorl veins c an be traced into the outer part of the wall zone, which suggests that they formed from aqueous fluids derived during early saturation of th e pegmatite-forming leucogranitic magma rich in H2O, F, B, and Li. Pro gressive crystallization resulted in a late-stage sodic magma and abun dant aqueous fluids. Two late stages of volatile escape are recognized : the first stage caused pressure-quenching of the last magma, which p roduced aplite and caused albitization (An(3) to An(8)) of earlier cry stallized K-feldspar and oligoclase, The second stage, released during the rupture of miarolitic cavities, produced platy albite (''cleavela ndite,'' An(1)) locally associated with F-rich muscovite and elbaite. Albitization is likely due to cooling of alkali-fluoride-dominated flu ids at less than 2 kbar pressure. The pegmatites are derived from Hima layan leucogranitic magma emplaced prior to 5 Ma into granulitic gneis s that was at 300 degrees to 550 degrees C and 1.5 to 2 kbar. The pegm atites were emplaced during uplift of the Haramosh Massif, since they cross-cut ductile normal faults but are cut by brittle normal faults. Economically important pink tourmaline mineralization formed in pocket s concentrated near the crest of a broad antiform, as a result of trap ping of late magmatic aqueous fluids that had become Fe-poor owing to the prior crystallization of schorl.