SYN-TECTONIC INJECTION OF ~3.65 GA GRANITOID MAGMAS INTO A DEVELOPING GNEISS COMPLEX, ISUKASIA REGION, INNER GODTHÅBSFJORD, GREENLAND: MAGNITUDE, POSSIBLE MECHANISMS OF EMPLACEMENT, AND LACK OF CAUSAL RELATIONSHIP TO SUBSEQUENT DOMING

The “central gneiss” of the Isukasia region, an Early Archean gneiss complex enclosed by the Isua Greenstone Belt (IGB), in southern West Greenland, forms a classic Archean granitoid dome. However, emplacement of the granitoids and doming appear to have been completely separate events. Further SW (outer Godthåbsfjord), the Early Archean gneiss is known (notorious?) for extensive tectonic and igneous reworking at ~3.65 Ga, producing granulite-facies metamorphism and extensive migmatization. In the Isukasia region, though, events at ~3.65 Ga were weaker, allowing the untangling of the relative contributions of deformation and magmatism, as well as the fundamental components of the central gneiss.

The dominant protoliths of the central gneiss are ~3.7 Ga tonalite, quartz diorite and granite that intrude the overlying supracrustal rocks of the IGB. At ~3.65 Ga, however, voluminous granite and pegmatite were intruded, approximately coeval with formation of the predominant S2 fabric in the gneiss, and of major shear zones within the IGB. The map distribution of the late granitoids parallels S2, reflecting both control of emplacement by S2 and subsequent flattening. Because the granitoids intrude the tonalite gneiss at sub-map scales, the gneiss was mapped according to their percentage: i.e. <30%, 30-60%, and >60% late granite and pegmatite. Thus the ~3.65 Ga granitoids make up a median value of 20% of the total gneiss outcrop area (max. 35%, min. 4%). Supracrustal xenoliths within the gneiss were metamorphosed to no more than lower-amphibolite facies as a result.

Heterogeneity of D2 strain shows no symmetrical relationship with the dome structure defined by S2, requiring that doming postdates D2. Thus, D2 tectonics—formation of S2, granitoid intrusion, formation of IGB shear zones—were sub-horizontal. Less deformed exposures in the central gneiss preserve details of the late granitoid emplacement: both thick subvertical dikes and pervasive lit-par-lit intrusion of the earlier gneiss were common. The granitoids may have been forcibly injected in pulses in the midst of ongoing D2 flattening or shearing. The doming (within the following 100 m.y. or so) was not aided by the small amounts of heat advected by the ~3.65 Ga intrusions, but possibly by more regional thermal and directed-stress fields.