FLUID-FLOW VS SCALE OF SHEAR ZONES IN THE LOWER CONTINENTAL-CRUST ANDTHE GRANULITE PARADOX

The external or internal origin of CO2 and H2O in marbles and metabasi tes from a large-scale granulite section is related to their structura l setting: outside shear zones, within major shear zones (approximate to 25 km wide by >340 km long), or within minor shear zones (<10 km wi de by <150 km long). Outside shear zones, marbles have isotopic compos itions similar to their protolith values and metabasites record mantle delta(13)C signatures. Marbles from major shear zones show little var iation of delta(18)O but are depleted in C-13 down to -3 parts per tho usand due to exchange with mantle-derived carbon, In addition to an ea rlier input of mantle carbon related to mafic magmatism, major shear z ones act as conduits for mantle carbon influx, They are probably mantl e rooted. Oxygen was buffered by the crust, Marbles and their related skarns from minor shear zones were subjected to devolatilization react ions and water-rich infiltration from crustal sources with no evidence for a mantle carbon contribution. Large isotopic variations at the me tre scale in the shear zones reflect the heterogeneous distribution of fluid flow associated with heterogeneous deformation and contrasting petrologic features and permeabilities. These geochemical-structural r elationships reconcile the conflicting arguments on the origin of flui ds during granulite genesis.