Geochronological evolution of HP metamorphic rocks of the Adula nappe, Central Alps, in pre-Alpine and Alpine subduction cycles
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SHRIMP dating of zircon from eclogites, their country rock gneisses and quartz veins from the northern and middle Adula nappe, Central Alps, accompanied by cathodoluminescence imaging and REE geochemistry of these dated zircon crystals revealed that the Adula nappe has been affected by two pre-Alpine high-pressure (HP) metamorphic events (northern: c. 330–340 Ma; middle: c. 370 Ma). In the northern Adula nappe, loss of radiogenic Pb of the c. 330–340 Ma zircon domains in combination with previous geochronological data argue for an overprinting Alpine HP event. In the middle Adula nappe, metamorphic zircon domains record in addition a 33–32 Ma event, interpreted to reflect HT metamorphism, already identified in the southern part of the nappe, where such SHRIMP ages have been related to HT (Lepontine) metamorphism. The Alpine HP event indicated by previous SHRIMP data in the southern Adula nappe at c. 35 Ma is not recorded in the zircons of the rocks dated herein. The absence in this dataset is probably because either no HP (c. 35 Ma) zircon domains were formed or such domains were completely reset during the 33–32 Ma HT event. Protolith ages of orthogneisses and eclogites are 470–457 Ma and 595–587 Ma, respectively, and maximum sedimentation ages of paragneisses 500–460 Ma. The lack of Hercynian protolith zircon ages implies pre-Hercynian formation for the precursors of the dated HP rocks. Our data suggest that the Adula nappe contains various Hercynian basement slivers that underwent subduction to HP conditions at c. 330–340 Ma and c. 370 Ma prior to the Alpine orogeny, similar to many other parts of the Hercynian basement in Central and Southern Europe.