Garnet Systematics of Subducted Continental Margins

Abstract

Four case studies were performed on former continental margins and on a continental fragment inside an ophiolitic unit. The studies include Lu-Hf garnet dating, and garnet characterization by its chemical zoning. Lu zoning inside garnet was monitored by LA-ICPMS analyses in order to assign the obtained garnet ages to metamorphic stages of the individual subduction cycle.<br /> The Adula Nappe in the Central Alps represents a coherent part of pre-Mesozoic continental basement with minor amounts of Mesozoic cover rocks from the distal European continental margin. This unit was subducted to mantle depth during the Eocene orogenic cycle but already recorded Variscan high-pressure metamorphic conditions. It shows a continuous gradient of Alpine metamorphic conditions. Variscan eclogite-facies relics are well preserved in the in the North but mainly obliterated in the South. The Adula Nappe was subducted as a cohesive body in a single event between c. 38 to 36 Ma. Subduction was short-lived as the entire nappe pile was established before 32 Ma.<br /> The high grade metamorphic series of southeastern Pohorje in Slovenia was subjected to the highest pressure and temperature conditions during the Cretaceous Eo-Alpine metamorphic cycle. Eclogite-facies metamorphism in the ultramafic complex at the southeastern border of the Pohorje Mountains is contemporaneous with its surrounding country rocks. These rocks were subjected to eclogite facies conditions as a cohesive terrane in a single tectonic event between c. 97 and 90 Ma.<br /> The new Lu-Hf garnet age of 5.1 ± 1.7 Ma obtained from a blueschist-facies garnet-amphibole schist, reveals that the Yuli Belt of Taiwan’s Central Range was subducted after the beginning of arc-continent collision at about 6.5 Ma. Rocks from the Yuli belt remained at their maximum depth only for a very short period of time and were exhumed rapidly during ongoing convergence. Fast exhumation of these rocks can be explained by downward removal of the overlying forearc lithosphere and associated rise of deeply subducted material.<br /> The dated eclogite samples from the Theodul Glacier Unit in the western Central Alps revealed garnet growth ages of c. 57 Ma. The unit represents a continental outlier inside the ophiolotic Zermatt-Saas Zone. Subduction was controlled by gravity acting on the downward moving slab in a setting of rollback. The differing metamorphic ages of continental occurrences inside the Zermatt-Saas Zone reveal that subduction was continuous in this unit. Thus, the Zermatt-Saas Zone was not subducted in one coherent slab but rather represents slivers subducted at different times.<br /> The studies of the Adula Nappe, the Pohorje Nappe, and the Yuli Belt demonstrate that continental margins are subducted and exhumed to mid-crustal levels or even to the surface within short periods of time of clearly less than 10 Ma. In the case of the Adula Nappe this timeframe is evidently just 2 Ma. The results received from investigating the Adula Nappe and the high-grade metamorphic rocks of the Pohorje Mountains further demonstrate that these continental terranes underwent subduction cycles as coherent units. In contrast, garnet dating of a continental fragment from the Zermatt-Saas Zone and its comparison with literature data lead to the conclusion that this ophiolotic unit was assembled in a long-lasting subduction event.