PETROLOGY OF GRANULITE-FACIES METAPELITES OF THE HIGHLAND COMPLEX, SRI-LANKA - IMPLICATIONS FOR THE METAMORPHIC ZONATION AND THE P-T PATH

In the Pan-African granulite-facies Highland Complex of Sri Lanka, the metamorphic grade increases from the central-northwestern part toward s both the east and the southeast. In metapelites, the following progr ade changes in metamorphic assemblage are recognized: biotite-ilmenite -garnet (or cordierite)-sillimanite-alkali feldspar-quartz in the cent ral-northwestern part give way to garnet-rutile-sillimanite-ilmenite ( or biotite)-alkali feldspar-quartz in the E and SE and are replaced by arnet-cordierite-biotite-magnetite-ilmenite-alkali feldspar-quartz-be aring assemblages in the southwest. Cordierite coexisting with garnet is restricted to the western Highland Complex where metamorphic pressu res of 57 kbar have been determined. In the eastern Highland Complex w here garnet-clinopyroxene-quartz is stable in metabasic rocks, metamor phic pressures of 8-10 kbar have been obtained from barometers on meta pelites. Phase relations of metapelites as well as geobarometry point to a continuous metamorphic transition between the eastern and the wes tern part of the Highland Complex. A clockwise P-T path for the whole Highland Complex is indicated by the succession kyanite-sillimanite-an dalusite. An even earlier stage of the prograde metamorphism, document ed by sillimanite needles older than kyanite inclusions in garnet, hav e been found in two samples of the eastern Highland Complex. This poin ts to a strong pressure increase at amphibolite facies conditions prob ably due to tectonic thickening of the crust during prograde metamorph ism. Other inclusion assemblages in garnet and reaction textures indic ate the breakdown of staurolite to kyanite-spinel-garnet and subsequen t continued garnet growth in the sillimanite field at the expense of b iotite, sillimanite, and quartz during further heating. Ternary feldsp ars in metapelites bear evidence of extreme metamorphic temperatures o f about 900-degrees-C, previously only indicated by coexisting pyroxen es in metabasic and charnockitic rocks. Within a few garnets, inclusio ns of staurolite which are Ti-rich (1.34 wt% TiO2) and richer in Fe (X (Fe) = 0.745) than the enclosing garnet have survived the very high-gr ade metamorphism. The high-grade stage of the prograde metamorphism wa s accompanied by strong flattening and a general N-S stretching which in many metapelites ceased after peak temperatures but proceeded in ot hers. Tectonic uplift of the whole Highland Complex occurred only afte r a phase of near-isobaric cooling in the lower crust. Zircon (approxi mately 610 Ma) and biotite cooling ages (approximately 460 Ma, Holzl e t al., 1991 ) as well as retrograde P-T paths which passed the andalus ite stability field, are similar in the eastern and western part of th e Highland Complex. This suggests that the lower crustal cross-section exposed in the Highland Complex was already tilted during cooling.