285-2 The metamorphic signature of lithospheric mantle delamination: Insights from the Naxos metamorphic core complex, Greece

Session: Rock Deformation and the Dynamics of Mountain Building: A Session Honoring the Scientific Contributions of John P. Platt, Part II

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The transition from horizontal contraction to extension characterizes the late evolution of orogenic belts, and has been linked with rapid exhumation of rock, surface uplift, metamorphism, renewed phases of magmatism, and the formation of mineral deposits. Yet, the causes of the change in strain-rate regime remain difficult to ascertain. Here we combine cross-cutting field relationships with monazite U-Th-Pb petrochronology and petrological modelling results from kyanite and sillimanite grade gneisses and migmatites from the deepest structural levels from the Naxos metamorphic core complex in the Cyclades in the central Aegean, Greece. Monazite show increasing Dy/Yb between ca. 34-18 Ma associated with garnet growth from 6-10 kbar and 580-700°C during prograde metamorphism associated with continued burial and contractional tectonism after overthrusting of the Cycladic Blueschist Unit at ca. 50-40 Ma. However, monazite Dy/Yb sharply decreases from ca. 18-13 Ma, and is associated with Y-rich monazite rims. This is consistent with monazite crystallization during garnet breakdown under isothermal decompression from ca. 10-5 kbar, which is associated with partial melting and sillimanite overprinting kyanite. This phase of decompression was accommodated on purely ductile extensional top-to-NNE shear zones exposed within the core complex. A U-Pb rutile age of ca. 12.7 Ma constrains the timing of cooling through ~520°C isotherm in a regime of surface-breaking normal faulting that cross-cuts the earlier ductile structures, after the initial phase of decompression. Therefore, the change from contraction to extension on Naxos occurred between ca. 18-13 Ma. At the same time crustal derived granitoids intruded across the Cyclades implying rapid heating and melting of the lower crust. These observations are consistent with the metamorphic, structural, and igneous consequences of convective removal of the lower lithospheric mantle beneath the central Aegean. This process, as well as rapidly heating the remaining lithosphere and lower crust, would have rapidly increased gravitational potential energy of the region, initiating crustal thinning and accounting for the almost isothermal decompression in the latest phase of metamorphism.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8715

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