Caledonian high-pressure metamorphism in central western Spitsbergen

One of the oldest high-pressure metamorphic rocks, from the northernmost locality (Motalafjella in central western Spitsbergen, 73° 25’ N and 13°E), is described, and an attempt is made to construct a model of Caledonian subduction from fragmentary evidence. Glaucophane schist and eclogites in Spitsbergen have been known since 1957. These rocks have been studied by British and Norwegian geologists; recently Japanese penologists have been involved in this study. Three important new discoveries have been made in Motalafjella and adjacent areas: 1) An unconformity has been found at the base of an Upper Ordovician-Silurian succession (the Bulltinden Formation). The basal conglomerate contains pebbles of stratigraphically-underlying, low-grade—high-pressure metamorphic rocks (Lower Unit of the Vestgötabreen Formation) and fossils in the matrix. 2) Lawsonite has been widely found in the low-grade metabasites of the Lower Unit, and jadeite-quartz-albite-bearing assemblages have been identified in the high-grade cherty rocks associated with eclogites. Thus, the Vestgötabreen Formation contains low- and high-grade rocks of the jadeite-glaucophane type metamorphic-facies series. 3) An upper [late] Riphean succession with shallow-marine-reef sedimentary rocks (the calc-argillo-volcanic formation) is intruded by metadiabase-gabbros, which have strong chemical similarities to oceanic basic rocks. From 1) it can be inferred that the high-pressure metamorphism, i.e., the early Caledonian metamorphism, is definitely older than Upper Ordovician-Silurian beds, and discovery 2) provides evidence for a high-pressure metamorphic-facies series. The combination of oceanic basic rocks and the sedimentary rocks of shallow-marine-reef facies (3) is explained in terms of a change of tectonic domain, i.e., from that of a continental shelf during sedimentation to an active oceanic condition, e.g., an oceanic accretion zone and its related tectonic zones, by the time of basic intrusion. This change implies an active plate boundary between inferred continent and ocean plates and that this consuming boundary is responsible for the high-pressure metamorphism in an early Caledonian phase.