Abstract
Detrital zircon grains with U-Pb ages >3.8 Ga from the Tanzania Craton represent a rare, and so far understudied, record of early crust formation in Africa. Here we report microbeam-based isotopic (U-Th-Pb, Lu-Hf, O) and trace element (REE, U, Th, Ti) compositions for Eoarchean to Hadean grains from Tanzania and discuss this record in the context of early crust formation. Zircon, extracted from a pre-3.2 Ga quartzite sequence in the Simba-Nguru Hills of the Dodoma Basement Complex, has Pb-207/Pb-206 ages of 4013 to 3482 Ma. The most concordant analyses are spread between 4.0 and 3.6 Ga. Oscillatory-zoned zircons preserve magmatic REE patterns, and Ti-in-zircon model temperatures of 818 +/- 33 degrees C suggest crystallisation from TTG-like magmas. Estimates of oxygen fugacity (fO(2)) derived from zircon REE systematics (-3.4 to +1.8 Delta FMQ) match both modern and Archean mantle values. Zircon delta O-18 largely overlaps the 'mantle' range through the entire 400 Ma age span and initial epsilon(Hf) values are near-chondritic, becoming slightly more juvenile with time. These signatures are difficult to reconcile with zircon parent rocks derived from strongly depleted (high-epsilon(Hf)) mantle-like sources, from old and evolved continental crust, or from strongly hydrated sources with a low-temperature, high-delta O-18 signature. Thus, the data add to a growing body of evidence that the early Earth crust was formed from chondritic mantle. We propose zircon to have crystallised in TTG magmas that formed through reaction of underplated basalt with mafic granulite at the Moho of the proto-continental crust, likely in an intraplate setting.