Timing and nature of multiple 3700–3600 Ma tectonic events in intrusive rocks north of the Isua greenstone belt, southern West Greenland

Abstract

One of the oldest well-preserved sequences of tectonic events on Earth is recorded in intrusive rocks that lie north of the Isua greenstone belt, southern West Greenland. Detailed mapping and U-Pb dating of zircon and titanite build upon previous work to better define the timing and nature of the 3700-3600 Ma tectonic history. Voluminous 3698 ′ 2 Ma tonalite was converted into gneiss during a deformation event (D 1 ) that predated intrusion of 3659 ′ 2 Ma diorite dikes. The dikes and the earliest gneissosity (S 1 ) are most abundant in a zone of low regional deformation (D 2 ) located 8 km north of the Isua greenstone belt. F 2 folding produced the dominant gneissosity (S 2 ) during injection of meter-wide granite and pegmatite sheets along S 2 at ca. 3650 Ma. D 3 shearing transposed S 2 into a narrow mylonite zone at the contact between the intrusive rocks and the Isua greenstone belt. Motion on this contact must have occurred in the Early Archean; mylonites are cut by dolerite dikes that are correlated with nearby Middle Archean dikes. Titanite from the low D 2 strain zone yielded concordant U-Pb dates of 3606 ′ 3 Ma, indicating that these rocks were little affected by Late Archean metamorphism. In contrast, significant Late Archean heating of rocks adjacent to the Isua greenstone belt is shown by titanite that lost considerable Pb and/or was largely recrystallized at 2915 ′ 32 Ma. Late Archean strain was also restricted to rocks adjacent to the belt. This sequence of overprinting deformation and metamorphism is similar to that in younger orogenic belts.