科研之友
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摘要
The East Greenland Caledonides formed in the overriding plate as Baltica was subducted westward beneath Laurentia from 460 to 360 Ma, and offer a unique opportunity to investigate lower crustal deformation in the overriding plates of continental collisions. Field work and new zircon geochronology from gneisses in southern Liverpool Land, exposed in the hinterland similar to 100 km east of the nearest Caledonian gneisses, define three tectonostratigraphic units that are, from the bottom up, the eclogite + peridotite-bearing Tvaerdal complex and the granulite-facies Jaettedal complex in the footwall of the top-N Gubbedalen shear zone, and the Hurry Inlet granite and associated paragneiss screens in its immediate hangingwall. Zircons from Tvaerdal complex gneisses yield metamorphic rims that cluster in age from 409 to 401 Ma and overgrow magmatic cores of 1674 and 1665 Ma in two samples, and range from similar to 1800-1000 Ma in a third sample. In contrast, zircons from three samples in the Jaettedal complex and two samples in the paragneiss screens of the Hurry Inlet granite yield metamorphic rims that cluster in age from 438 to 417 Ma with Archean-Early Neoproterozoic detrital cores. A cross-cutting granitic dike in the Jaettedal complex yields an age of 394 Ma. Archean-Early Neoproterozoic detrital zircons associated with similar to 440-420 Ma metamorphism in the Liverpool Land paragneisses suggests correlation with the Krummedal sequence and the Hagar Bjerg thrust sheet of Laurentian affinity. 1670 Ma cores in the Tvaerdal complex, and similar to 400 Ma eclogite-facies metamorphism, allow correlation of the Tvaerdal complex with the Western Gneiss Region in Norway, and it may therefore be of Baltican affinity. Furthermore, the contact between the older Jaettedal complex with the younger Tvaerdal complex requires the existence of a structure, named the Ittoqqortoormiit shear zone herein, which juxtaposed these rocks prior to the initiation of normal-sense slip along the Gubbedalen shear zone. This work provides geochronologic evidence for continental underplating of the overriding plate by the subducting plate during orogenesis, and supports models for high-pressure exhumation in continental collisional settings that identify separate structures associated with initial emplacement in the lower-middle crust and subsequent upper-crustal exhumation.
- 出版日期2010-9-1
