High Heat Producing Mesoproterozoic Granitoids and Their Impact on the Geothermal Field in Lithuania, Baltic Basin

The Palaeoproterozoic crystalline basement is overlain by the Baltic Basin. Lithuania is situated in the shallow eastern periphery and grades into the deep part of the basin, which comprises a number oil fields; the thickness of the sedimentary cover varies from 0.2 to 2.3 km. The Mesoproterozoic granitoid intrusions of different scales were discovered in the crystalline basement. In total, thirteen intrusions were defined on the gravity and magnetic maps and studied by abundant deep boreholes drilled in Lithuania. The recent dating revealed several phases of magmatic activity ranging from 1625 to 1445 Ma. No systematic lateral and temporal distribution of intrusions was noticed. The intrusions comprise sub-alkaline I-type diorites and quartz monzodiorites, granodiorites, and granites. The radiogenic granitoids are characterized by anomalous heat production ranging from 2.8 to 18.2 μW/m3 (average 7.26 μW/m3). The shoshonitic series correlates with high heat production. Furthermore, the Th series is documented in west Lithuanian (WLD) intrusions, while Th-U-enriched granitoids show high heat production in east Lithuania (LBB) domains. The high iron (magnetite) content of the Mesoproterozoic magmatic rocks accounts for specific high magnetic field anomalies. The most voluminous intrusions are mapped in the West Lithuanian Geothermal Anomaly, which is the most spectacular geothermal feature recognized in the East European Platform.