The Mesoarchaean Tartoq Group suprasubduction zone ophiolite, SW Greenland

The Tartoq Group comprises supracrustal rocks including: pillow lavas, sills/dykes, gabbros, and serpentinites. Metamorphic conditions range from greenschist to granulite facies. LA-ICP-MS U-Pb zircon age dating of an intrusive TTG sheet yields a minimum age of 2986 ±4 Ma for the supracrustal rocks. The mafic metavolcanic rocks are dominated by tholeiitic basaltic compositions (MgO = 4.2-16.5 wt.%, Fe2O3T = 5.7-17.3 wt.% and TiO2 = 0.3-1.6). They possess negative primitive mantle-normalized Nb-anomalies (Nb/Nb* = 0.26-1.31 and NbN/LaN from 0.3-0.9), positive Pb, K2O and Sr anomalies, and flat chondrite-normalized REE patterns (LaCN/SmCN = 0.67-1.96). La, Y, and Nb contents are similar to those of modern back-arc basalts and Th/Yb vs. Nb/Yb also indicates a subduction zone component in the source of the volcanic sequence. The serpentinites have major- and trace element contents consistent with them being mantle harzburgites and their PGE pattern also strongly supports a mantle origin. However, these ultramafic rocks have rather high Fe2O3T (10.5-18.8 wt.%). The similar flat trace element patterns of the different mafic lithologies are consistent with a co-magmatic assemblage of rocks that represents an ophiolite sequence. Combined metamorphic and structural data show horizontal fabrics and P-T conditions, which support a convergent margin setting that have undergone collision tectonics. Thus, we propose that the Tartoq Group represents a slab of now dismembered Mesoarchaean suprasubduction zone oceanic crust.