242-19 Was Rapid Intermediate-T Cooling at the end of High-T Metamorphism in Sri Lanka Related to Thermal Relaxation after Gondwana Construction or Orogenic Collapse?

Abstract:

The Neoproterozoic crustal block of Sri Lanka is well suited for investigating how volcanic arcs are transformed tectonically via accretion/collisions, UHT metamorphism, intense deformation, and ultimately stabilized into continental crust. Abundant zirconology over the past five decades has greatly enhanced our understanding of the tectono-metamorphic history of Sri Lanka during its construction, yet a dearth of information exists on the post-polymetamorphic intermediate temperature (500-300 C) thermal history. Recent concordant hornblende and biotite plateau Ar/Ar ages from near the eastern suture zone in southern Sri Lanka suggest rapid cooling through intermediate mid-crustal temperatures in the early Ordovician (490-480 Ma), similar to prior Rb-Sr biotite ages and one hornblende age clustering ~470 Ma from the central Highland Complex and ~450 Ma from the bounding arc terranes. The Ar/Ar ages are synchronous with the cessation of high-T zircon and garnet growth, and with late pegmatite crystallization and hydrothermal gold formation. We are evaluating whether this rapid cooling might be related 1) to the end of the construction phase of Gondwana, perhaps marking the cessation of thrusting and the onset of thermal relaxation, or 2) to post-thrusting orogenic extensional collapse associated with isostatic re-equilibration. The temporal linkage between the end of long duration (600-500 Ma) UHT metamorphism followed shortly by rapid intermediate-T cooling is consistent with thermal relaxation following thrust cessation. Alternatively, recent work in terranes formerly peripheral to Sri Lanka (East Antarctica and India) are identifying late extensional structures which post-date Gondwana contractional features. Similarly, in Sri Lanka, doubly plunging dome-and keel-type structures in the western Central Highlands were recently reinterpreted to have formed during post-orogenic intraplate extension, although their timing is poorly constrained. To test between thermal relaxation and extensional collapse induced rapid cooling, we have collected samples from a wider geographical area, including from the Vijayan Complex in the south, and from the Central Highland and Wanni Complex to the north. The absence of intermediate temperature ages younger than ~450 Ma from both Rb-Sr and Ar-Ar systematics is consistent with a lack of petrologic evidence for low-grade metamorphic overprinting in Sri Lanka, making it likely that more thermochronologic mineral ages will yield robust post-Gondwana primary (and not younger reset) cooling ages. 

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-19423

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