Geophysical interpretation of the crustal and upper mantle structure in the Wiborg rapakivi granite area, southeastern Finland

Abstract

Abstract The rapakivis in southeastern Finland and Russian Karelia cover a total area of more than 20,000 km 2 and comprise the Proterozoic (1.6-1.3 Ga) anorogenic Wiborg, Ahvenisto, Suomenniemi, and Onas batholiths, and associated mafic rocks. A model of the crustal and upper mantle structure in this area based on gravity, aeromagnetic, deep seismic sounding, and petrophysical data is presented. The average density (2625 kg/m 3 ) of the batholiths is lower than that of the surrounding Svecofennian rocks by about 80 kg/m 3 . The density histogram of the rapakivi granites is divided into two normally distributed components with mean densities of 2625 kg/m 3 (major) and 2690 kg/m 3 (minor), respectively. There are related local gravity anomalies. The susceptibility histogram of the rapakivi granites is decomposed into five log-normally distributed components. The two major components have mean susceptibilities of (165 and 1305) × 10 −6 SI units. The former component (40%) is virtually paramagnetic, and the latter (60%) is weakly ferromagnetic. The two components with mean susceptibilities of (6000 and 13,5000) × 10 −6 SI units constitute only 3% of all the samples. Associated gabbro-anorthosites also have high susceptibilities (1450-30,370) × 10 −6 SI units. These ferrimagnetic rocks cause noticeable local aeromagnetic anomalies. An extensive Bouguer anomaly low is associated with the batholiths. A pronounced regional high surrounds this low. The range of values is nearly 60 mGal. The anomaly pattern is primarily the result of superposition of the gravity low, which is due to the relatively low-density upper and middle crust, onto the broader gravity high, that is due to the uplifted Moho. According to the published deep seismic sounding results, the crust is 41 km thick in the rapakivi region. Thus it is 6–20 km thinner than to the west, north and south of the rapakivi batholiths. The refraction model also implies a 6 km thick high-velocity body at a depth of 10 km. This body has a high v P v S -ratio and is interpreted as gabbro-anorthosite. The magnetic anomaly values in the rapakivi area are slightly higher than those in the surrounding schist areas but lower than the values in the metavolcanic belt to the west. Within the Wiborg batholith there is a zone of positive regional magnetic anomalies with a separate, well defined magnetic high of about 400 nT in the western part. This anomaly is interpreted as a pipe-like magnetic body with an abnormally strong magnetization. The upper surface of the body lies at depths between 12 and 15 km. The body extends from the upper to the lower crust. The model derived from geophysical data supports a Proterozoic mantle upwelling, which provided excess heat to cause uplift and partial melting of both the upper mantle and the lower crust. Partial melting of the upper mantle produced intrusions of gabbro-anorthosites and diabases, while extensive partial melting of the lower crust produced rapakivi magmas and caused their emplacement into the upper crust.