ABSTRACT

Zircons from the granitic part of the Lauterbrunnen Crystalline Complex are a mixture of detrital zircons and predominantly clear euhedral types which occasionally have detrital cores. The clear euhedral crystals contain old radiogenic lead which has probably been generated over approximately 2b.y. The zircons from gneissic xenoliths, which are inherent parts of the Lauterbrunnen Crystalline Complex, are all detrital rounded types and their upper projection with ‘concordia’ is also approximately 2b.y. Uranium lead data for zircons from the granitic and transition rocks define a curve rather than a straight line. The ‘reverse’ discordancy pattern (a feature observed in other possible anatectic granites) and incompatibility of the data to any event as defined by Rb-Sr mica ages are consistent with the conclusions drawn from detailed mapping, petrography and geochemistry that the Lauterbrunnen Crystalline Complex was formed in situ from former sediments rather than intrusion from depth. Fission track registration shows the uranium to be evenly distributed throughout the zircons from the gneiss, transition rock and final granitic product, although some crystals from the transition granitic rock have uranium-rich rims.