Zoning of eclogitic garnet cores - a key pattern demonstrating the dominance of tectonic erosion as part of the burial process of worldwide occurring eclogites

Eclogites are witnesses of geodynamic processes that are commonly related to subduction of oceanic crust. Information on the part of these processes that refers to the burial of this rock type is rarely published but stored in the eclogitic garnet core and inclusions therein. To better understand general aspects of the burial process, a literature search on the chemical characteristics of garnet in worldwide occurrences of eclogite was undertaken. In most cases extended garnet cores show either a prograde growth zoning with increasing Mg, starting at a few percent of pyrope component, and decreasing Mn contents (type I eclogite) or a (nearly) constant chemical composition frequently with pyrope contents significantly above 10 percent (eclogites of types II and III). Only in minor cases, it is difficult to assign the reported garnet core to an eclogite type. The growth zoning of garnet was thermodynamically modelled for the chemical composition of a basalt following different burial paths. These paths are characterized either by a trajectory along a low geothermal gradient (type I eclogite), as expected for the subducting upper portion of oceanic crust, or a one characterized by nearly isothermal burial at temperatures above 500 degrees C reaching peak pressures up to 2.1 GPa (type III eclogite), as possibly due to crustal thickening during continent-continent collision, or more (type II eclogite) when basic rocks are tectonically eroded from the overriding continental plate before deep subduction. In addition, diffusion modelling was undertaken on mm-sized garnet demonstrating that the characteristics of the core zoning are not fully obliterated even during residence at temperatures of 800-850 degrees C within 10 million years. The scrutiny of more than 200 eclogites reported in the literature led to the following result: about half of them are type II eclogites; a third and a sixth can be related to type I and type III, respectively. Among type III are almost all of the few Proterozoic eclogites considered. To demonstrate the benefit of our study, we link the core zoning of eclogitic garnet from various (ultra)high-pressure terranes in Phanerozoic orogenic belts to the geodynamics shaping corresponding orogens. The eclogites in these belts are dominated by type II. Thus, we propose that some of the material of the lower portion of the overriding continental crust was tectonically eroded by a subducted oceanic plate and brought to great depth. Afterwards, this material was exhumed first in a deep subduction channel and then in an exhumation channel during continent-continent collision where a contact with the upper continental plate was re-established. Furthermore, we suggest that type II eclogite can also occur in extrusion wedges as far as oblique subduction took place.