K-rich brine and chemical modification of the crust during continent–continent collision, Nagssugtoqidian Orogen, West Greenland

The Nordre Strømfjord Shear Zone (NSSZ) in West Greenland is the northernmost zone of intense deformation within the 1.7–2.0 Gya Nagssugtoqidian Orogen (NO). The NO preserves a record of continent–continent collision and crustal-scale shearing associated with the assembly of the Columbia super-continent. Also preserved in the NO is a complex of calc-alkaline rocks that formed in the root zone of an arc system during the pre-collision period when ocean floor subduction was occurring. Chemical analysis of the calc-alkaline rocks provide strong evidence that large volumes of hypersaline fluids metasomatically altered the pre-existing rocks as fluid flowed along preferential pathways within the NSSZ. The minimum integrated fluid volume flux required to accomplish the observed chemical modification is 4.66 × 104 m3/m2. The composition of the fluid was such that it resulted in highly enriching the rock in K, Rb, Th, P, Zr, Pb, LREE, Nb and Ba. The likely fluid source was devolatilization reactions in hydrated, subducted ocean crust and mantle, the underthrust continental mass, and the release of trapped pore fluids. The extent and magnitude of the mass transfer from deep crustal levels to intermediate and shallow levels represents a significant mechanism for modification of the compositional features of deep and intermediate continental crust. Hence, the processes recorded in the NSSZ may provide an important mechanism that would influence compositional stratification of the continental crust.