Forward modeling corona growth in a partially eclogitized leucogabbro,Bourbon Island,North-East Greenland

摘要

Coronitic textures are common in partially eclogitized igneous bodies,such as gabbros,leucogabbros,and anorthosites,east of the Germania Land Deformation Zone in North-East Greenland.Coronas formed by prograde metamorphic processes that transformed the gabbroic bodies to eclogite facies,and record frozen stages of the prograde metamorphic evolution of these rocks.A metaleucogabbro-norite body on Bourbon Island in jokelbugt is characterized by three concentric areas:a coronitic core,a mottled inner rim with areas of completely eclogitized material surrounded by a matrix of coronitic metaleucogabbro,and an outer rim of strongly foliated and completely retrogressed amphibolite.The Bourbon body preserves four stages of the prograde metamorphic history:Stage I,Stage II,Stage III,and Eclogite Stage.Stage I coronas are found only in the core of the body,which is the least reacted part of the leucogabbro-norite and the closest to the protolith,and is characterized by the corona sequence Pl_(rim)/ Grt+Kfs+Amp/Grt+Amp/Cpx_(rim).The typical corona sequence for Stage II is Pl_(rim)/Grt+Pl+Zo/Cpx_(rim)/Amp_(rim),.Stage HI samples show a Pl_(rim)+Ky+Scp/Grt+Pl+Qtz/Qtz+Pl sequence,with the relict clinopyroxene being replaced in part by microcrystalline aggregates of Cpx+Amp+Pl.The Eclogite Stage shows relict Pl completely replaced by Grt,and the relict Cpx completely replaced by aggregates of Omp+Pl+Kfs+Amp.We tested open-system grain boundary diffusion(OSGBD)theories to model the prograde Stage I symplectitic coronas.The observed ratio of the thickness of the different layers is Pl_(rim):Grt+Kfs+Amp:Grt+ Amp:Cpx_(rim)equal to 3:1.3:0.95:0.5.These ratios are very close to the modeled ones of 2.7:1.1:1:0.5.Furthermore,subtle textural changes within the Grt+Kfs+Amp corona were also reproduced by the model.The model gave us insight into the conditions of the metamorphic system in which the coronas formed.The sequence Pl_(rim)/Grt+Kfs+Amp/Grt+Amp/Cpx_(rim)formed by diffusion driven reactions in an open system involving gain of Fe,K,and Na,and loss of Ca and Mg at the original clinopyroxene-plagioclase boundary.Relative mobilities of the different components within the corona layers were L_(MgMg)>L_(AlAl)>L_(SiSi)>L_(CaCa)>L(KK)>L_(FeFe)>L_(NaNa).Fluid circulation was active to some degree during the transformation to eclogite.The differences between core,inner rim,and the two domains within the inner rim of the metaleucogabbro-norite can be explained by different degrees of fluid circulation in different portions of the rock.The presence of phases containing Cl and P,such as scapolite,in completely eclogitized samples supports the presence of fluid circulation in the system.Another possible explanation for the mottled appearance of the inner rim is protolith heterogeneity.