Petrology of granulite-facies metapelites of the Highland Complex, Sri Lanka: implications for the metamorphic zonation and the P-T path

Abstract

In the Pan-African granulite-facies Highland Complex of Sri Lanka, the metamorphic grade increases from the central northwestern part towards boththe east and the southeast. In metapelites, the following prograde changes in metamorphic assemblage are recognized: biotite-ilmenite-garnet (or co cordierite)-sillimanite-alkali feldspar-quartz in the central northwestern part give way to garnet-rutile-sillimanite-ilmenite (or biotite)-alkali feldspar-quartz in the E and SE and are replaced by gar garnet-cordierite-biotite-magnetite-ilmenite-alkali feldspar-quartz-bearing assemblages in the southwest. Cordierite coexisting with garnet is restricted to the western Highland Complex where metamorphic pressures of 5–7 kbar havebeen determined. In the eastern Highland Complex where garnet-clinopyroxene-quartz is stable in metabasic rocks, metamorphic pressures of 8–10 kbar have been obtained from barometers on metapelites. Phase relations of metapelites as well as geobarometry point to a continuous metamorphic transition between the eastern and the western part of the Highland Complex. A clockwise P-T path for the whole Highland Complex is indicated by the succession kyanite-sillimanite-andalusite. An even earlier stage of the prograde metamorphism, documented by sillimanite needles older than kyanite-inclusions in garnet,hhave been found in two samples of the eastern Highland Complex. This points to a strong pressure increase at amphibolite facies conditions probably due to tectonic thickening of the crust during prograde metamorphism. Other inclusion assemblages in garnet and reaction textures indicate the breakdown of staurolite to kyanite-spinel-garnet and subsequent continued garnet growth in the sillimanite field at the expense of biotite, sillimanite, and quartz during further heating. Ternary feldspars in metapelites bear evidence of extreme metamorphic temperatures of about 900°C, previously only indicated by coexisting pyroxenes in metabasic and charnockitic rocks. Within a few garnets, inclusions of staurolite which are Ti-rich (1.34 wt% TiO2) and richer in Fe (XFe=0.745) than the enclosing garnet have survived the very high-grade metamorphism. The high-grade stage of the prograde metamorphism was accompanied by strong flattening and a general N-S stretching which is many metapelites ceased after peak temperatures but proceeded in others. Tectonic uplift of the whole Highland Complex occurred only after a phase of near-isobaric cooling in the lower crust. Zircon (∼610 Ma) and biotite cooling ages (∼460 Ma, Hölzl et al., 1991) as well as retrograde P-T paths which passed the andalusite stability field, are similar in the eastern and western part of the Highland Complex. This suggests that the lower crustal cross-section exposed in the Highland Complex was already tilted during cooling.