The Micron Scale Pb Isotopic Systematics of the Lower Continental Crust

Abstract

The thermochronology, genetic relationships, and U-Th-Pb history of the deep crust in the North China Craton, the Tanzanian Craton, and the Mozambique Belt have been interrogated using the Pb isotopic system in feldspars from lower crustal xenoliths measured via a novel LA-MC-ICP-MS method developed at the University of Maryland, College Park.

Individual feldspars analyzed from the lower crust beneath the Tanzanian Craton show a large spread in Pb isotopic compositions. The spread in isotopic data reflects radiogenic Pb that accumulated in a closed feldspar system that was below 600°C since 2.4 Ga. The entire suite of feldspar Pb isotopic compositions overlaps the common Pb measurements from the present day surface of the Tanzanian Craton. The combined surface and lower crust Pb isotopic compositions indicate a mantle extraction age of 2.7 Ga. The feldspar thermochronometry of the lower crust, along with other geochemical data, indicate that the crust of the Tanzanian Craton was extracted from the mantle ca. 2.7 Ga, in an arc setting, and cooled to a temperature below 600°C by 2.4 Ga, and has remained undisturbed since that time.

Common Pb isotopic compositions in feldspars analyzed from the surface and deep crust of the Mozambique belt, which sits adjacent to the Tanzanian Craton show that common Pb isotopic compositions can be used to determine provenance of granulite belts at depth. Common Pb modeling of the lower, middle, and upper crust of the Western Granulites indicate a 2.5x enrichment of U in the upper crust occurred around the time of mantle extraction ca. 2.7 Ga. Additionally, the <super>238</super>U/<super>204</super>Pb in the cratonic lower crust underlying the Mozambique Belt increases with distance away from the surface craton margin. The enrichment of U in the basement away from the surface margin of the craton, could be the reason that the Mozambique belt has been continually re-worked since the Archean, while the craton has been stable.

The Pb isotopic compositions of feldspars analyzed in granulite xenoliths from two different volcanic provinces in the North China Craton (Hannuoba and Nushan) reveal that there was a widespread crustal generation event ca. 2.5 Ga, followed by metamorphism at 1.8 Ga, as well as basaltic underplating ca. 0.15 Ga. The basaltic underplates show a composition that is a mixture between new basalt and ancient material. Felsic granulites, were extracted from the mantle ca. 2.9 Ga, and were emplaced in the crust ca. 1.8 Ga, and re-equilibrated ca. 0.15 Ga. Intermediate granulites, which were the product of the large 2.5 Ga crustal generation event, have internal feldspar isochrons of 1.7 Ga, which indicates while the lower crust was largely overprinted by the Mesozoic underplating, the lower crust was not elevated above 600°.