Review on Age of Magmatism and Crust Formation in Sri Lanka: U–Pb and Lu–Hf Isotopic Perspectives

Abstract

Recent studies on zircons at high resolution separated from
metamorphosed rocks of igneous origin of the four tectonic domains of
Sri Lanka by Lu–Hf and U–Pb isotope systematics have shed light upon
its tectono-magmatic history. While providing better insights, the new
dataset is not contradictory with Nd-model ages and U–Pb zircon dates
discovered from Sri Lanka in early 90s. These new data suggest that
both Wanni (WC) and Vijayan (VC) Complexes were magmatic arcs with
the former being relatively older than the latter, and the Kadugannawa
Complex (KC) is a marginal arc magmatic suite in the vicinity of WC. The
oldest Highland Complex (HC) has been derived of Mesoproterozoic
to Archean crustal and subducted sedimentary components. The Lu–
Hf systematics of rocks from the HC record the oldest Hf-crustal model
age of ~ 3.5 Ga and highly negative εHf(t)(t) values (up to − 30) inferring
contribution of older subducted sediments and/or crustal components.
In the WC and KC tectonic units, variable εHf(t) of zircon from negative
to positive values from gneisses indicate the involvement of both juvenile
mantle components and older continental materials in the generation of
the arc-related magma with Hf-model ages from ~ 700 to 2800 Ma. On
the other hand, the meta igneous rocks of the VC have distinct positive
εHf(t) data with Hf-modal ages in the range of ~ 700–1600 Ma supporting
entirely a juvenile origin. During the Neoproterozoic to Cambrian (ca.
700–500 Ma), the HC has predominantly served as a suture zone for the
collision of the WC and VC arcs. This suggests that the Sri Lankan terrains
were juxtaposed at an active continental margin setting associated
with two-staged subduction during the Gondwana amalgamation. The
metamorphism took place during the Neoproterozoic in the entire basement
up to the granulite facies conditions reaching intermittently ultrahigh
temperature (UHT) conditions.