Chemical zonation and relative timing of growth sections in garnets from eclogites of the Leaota Massif, South Carpathians

摘要

We have investigated the chemical zoning of garnet in three texturally different eclogite types from the Leaota Massif, Romania, to test the various zonation patterns of Ca, Fe, Mg and Mn as indices of time-lines in porphyroblastic garnets. Mn zonation was found to mark various growth stages very well. It indicates in each of our three case studies a complex growth history dominated by multiple nucleation and coalescence, but also interrupted by partial resorption, as shown by morphological characteristics. Chemical variations of the main elements in garnet during prograde high-pressure metamorphism define time-lines other than those of Mn which decrease, highlighting the stages of porphyroblastic growth. In particular, the Ca/Mn, Mg/Mn and Mg/Fe ratios are useful in late growth stages, when low Mn contents and radial gradients do not allow a satisfactory resolution of Mn time-lines. These ratio-based time-lines further substantiate the potential of Mn time-lines in revealing non-concentric porphyroblastic growth. This is important in order to correctly relate garnet growth stages to stable mineral assemblages, a prerequisite for a reliable derivation of the metamorphic history of the host rock.