Abstract:
Ultra High Temperature (UHT) metamorphism, with temperature
>9000C, has been proposed and documented in very restricted localities in
the Highland Complex (HC) – Sri Lanka. We have started fresh sampling of
Al and Mg rich pelitic rocks (esp. sillimanite (sil), staurolite, sapphirine
(Spr), spinel (Spl) and corundum bearing) and essentially garnet (Grt) and
clinopyroxene (Cpx) bearing mafic rocks from localities in the HC, in order
to reconcile the spatial and temporal evolutionary path of the UHT
metamorphism. In this study we put forward our field and petrographic
observations at three sampling locations at Kotmale Ampitiya and Talatuoya
in the central Sri Lanka. Spr and Spl are found as closely associated rare
inclusions in the core and mantle areas of porphyroblastic Grt. This thin
layered pelitic rock close to Kotmale reservoir contains Grt, orthopyroxene
(Opx), Biotite (Bt) and Sil. There is no co-existence of Spr and Spl was
observed. Instead, there is clear evidence showing some Spr and Spl
independently co-existing with a mineral which is too tiny to be optically
identified. This unidentifiable mineral may be quartz (Qtz) and/or
Plagioclase (Plag). Fine to medium Sil needles could be found as inclusions
within the mantle area of Grt. Dry mafic rocks, at Ampitiya occur as blocks
within marble while as lenses within quartzo-feldspathic granulite at
Talatuoya. Rocks at both localities contain Grt, Cpx, Opx, Plag, Qtz and iron
ore together with plenty of textural features indicating multistage P-T
evolution. Co-existing Cpx-Plag and isolated Qtz inclusions within Grt
imply that Grt formation may have taken place via reaction
CPx+Plag1=Grt+Plag2+Qtz. Plag moat around Cpx occur as inclusions
within the rim area of Grt may indicate early decompression event. Cpx and
Plag inclusions towards the margin of same Grt indicate that further growth
of Grt followed early decompression event. Existence of Grt+Cpx+Qtz as the
peak metamorphic assemblage indicates UHT conditions, which can be
inferred based on abundant occurrence of Opx and Plag symplectite and
moat in rocks at Ampitiya and moats in rocks at Talatuoya. Therefore, these
Sap bearing pelitic granulites and Grt–Cpx–Qtz bearing mafic granulites
bear rare but important evidence to support UHT metamorphism in the HC
of Sri Lanka.
