Granulitic xenoliths vs. terrains in an Archean Craton: a case study of lower crust beneath north China

Abstract

Granulite xenoliths in Mesozoic and Cenozoic volcanic rocks have been believed to be direct samples of the lower crust. The question has been raised for a long time whether granulite xenoliths or granulite terrains are representative samples for lower crust. To address this problem, a case study should be conducted for comparative research between xenoliths and terrains in the same region. Granulite xenoliths from several localities have been reported in eastern China recently. Occurences of such xenoliths in Archean cratons are rare. In the North China Block, the granulitic terrain is well-known to have an Archean age. The pyroxenitic and granulitic xenoliths include garnet pyroxenite (GP), garnet plagioclase pyroxenite (GPP), plagioclase two pyroxene granulite (PPG), garnet two-feldspar granulite (GFG) and two-feldspar granulite (FG). The traditional T-P estimation gives results of 1085-1171šC and 17.7-19 kbar for GP, 1037šC and 16.6 kbar for GPP, and 862-941šC and 6-10 kbar for PPG, respectively. Major element chemistry results show that the composition of granulite xenoliths is dominated by a mafic component, however, compositions cover a wide range and many are felsic. Systematic major element, trace element and Sr, Nd, Pb isotope data (87Sr/86Sr =0.70288-0.74472, 143Nd/144Nd =0.51135-0.51314, and 206Pb/204Pb =15.82-18.68), suggest that mafic granulite xenoliths could be product of basaltic underplating, with modification to different degrees. This implies interaction between basaltic components and existing crust. Moreover, these xenoliths would come from the greatest depth. The study of the local granulite terrain demonstrates that a mafic component also dominates the lithologies, and equilibration T-P conditions are 770-830šC and 0.9-1.0 GPa. The initial isotopic composition of the terrain has a depleted signature. The terrains equilibrated are from the middle crust to the upper part of the lower crust. Combined with available geophysical, mantle xenolith and heat flow data, a three dimensional stratified profile of the lithosphere in the region has been proposed. Further study is required to clarify and quantify the genesis and evolution path of these xenoliths and to construct a 4D model.