LEAD MOBILIZATION DURING TECTONIC REACTIVATION OF THE WESTERN BALTIC SHIELD

Lead isotope data from sulfide deposits of the western part of the Bal tic Shield define mixing lines in the Pb-206/Pb-204-Pb-207/Pb-204 diag ram. Lead from two types of sulfide deposits have been investigated: ( 1) Exhalative and volcanogenic deposits that are syngenetic with their host rocks; and (2) vein deposits. The syngenetic deposits locally sh ow a very wide range of lead isotopic compositions that reflect a vari able addition of highly radiogenic lead, while the vein deposits, alth ough they have radiogenic lead isotopic compositions, exhibit only lim ited isotopic variations. In different provinces of the shield, both t ypes of deposits fall on the same lead mixing array. The slope of the lead mixing lines varies as a function of the age of basement rocks an d the age of the tectonic event which produced the lead mobilization a nd therefore relates the source rock age with the age of lead mobiliza tion. Calculated mixing ages fall into several short time periods that correspond either to orogenic events or to major phases of continenta l rifting. The orogenic events are the ca 360-430 Ma Caledonian, ca 90 0-1100 Ma Sveconorwegian, and the ca 1800-1900 Ma Svecofennian orogeni c cycles. The rifting events correspond to the formation of the ca 280 Ma Oslo rift and the Ordovician (ca 450 Ma) graben system in the area of the present Gulf of Bothnia. Each mixing age indicates that lead w as mobilized, probably as a consequence of mild thermal disturbances, and that the crust was permeable to lead migration. The data show that the geographic distribution of sulfide deposits with highly radiogeni c lead isotopic compositions coincides with old graben systems, orogen ic belts, and orogenic forelands on the Baltic Shield. The ages of vei n deposits and their geographic distribution demonstrate multiple tect onic reactivation of the interior of the Baltic Shield in response to orogenic events at its margin.