Crustal age domains and metamorphic reworking of the deep crust in Northern-Central Tanzania: a U/Pb zircon and monazite age study

Abstract

Geochronological data are presented from Northern Tanzania, where deep-crustal terranes of different age are exposed. Stacking of these terranes was diachronous with one peak around 640 Ma, defined as East African Orogeny, and final consolidation at 550–580 Ma, that is defined as Kuunga Orogeny. This later event is predominant in the Western Granulite Belt of northern Tanzania and related to thrusting onto the Tanzanian Craton. The Tanzania Craton itself experienced a polycyclic history; age domains around 2.64 Ga prevail in the studied samples. There is no evidence of the Paleoproterozoic Usagaran Belt in northern Tanzania. Here the gneisses contain relicts of reworked Archean basement and are therefore considered part of the Western Granulites. Inliers of the Western Granulites are also found in the cores of marble antiforms that are part of the upper, sedimentary sequence of the Eastern Granulites. Those inliers formed during the Kuungan orogenic phase when the Eastern Granulites have taken their final position and were folded together with the Western Granulites.

Appendix

Analytical procedure for geochronology

The first step in zircon/monazite separation was crushing and sieving to obtain mineral fractions between 63 and 180 μm. Further separation of the heavy minerals was performed with the heavy liquid Na-Polytungstate (SOMETU) and the magnetic heavy minerals were removed by the FRANTZ magnetic separator. Handpicking of the grains was done under the binocular. The mounts were put into epoxy resin together with reference zircon crystal, dried and polished to get a section through the grains centre. For information about the growth structure of the zircons CL and BSE images were taken at the JEOL 6310 SEM in Graz at conditions of 5 nA and 15 kV. One measuring campaign was performed at the Center for Earth Sciences, University of Vienna. There, Zircon/Monazite ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²⁰⁶Pb ages were determined with a multi-collector LA-ICP-MS (Nu Instruments HR) coupled to a 193 nm solid state Nd-YAG laser (NewWave UP193-SS) at the Center for Earth Sciences, University of Vienna. Zircons were ablated in a Helium atmosphere, either spot- or raster-wise, and the zonation was derived from CL imaging of each grain. The diameter of spot analyses was 20–50 μm, whereas when scanning, the line widths were 10–35 μm with a rastering speed of 5 μm/s. Energy densities were 5–8 J/cm² with a laser repetition rate of 10 Hz. The carrier gas (Helium) was mixed with the Argon carrier gas flow prior to the plasma torch. Ablation duration was 60 to 120 s with a 30 s gas- and Hg-blank count rate measurement before ablation started. Ablation count rates were corrected accordingly offline; remaining counts on mass ²⁰⁴ were interpreted to be common ²⁰⁴Pb. Static mass spectrometer analysis was as follows: ²³⁸U was analysed with a Faraday detector, ²⁰⁷Pb, ²⁰⁶Pb, and ²⁰⁴(Pb+Hg) were analysed with ion counter detectors. ²⁰⁸Pb was not detected and ²³⁵U was calculated by the assumption that the ratio ²³⁸U/²³⁵U is 137.88 (Steiger and Jäger 1977). An integration time of 0.2 s was used for all measurements. The ion counter (Faraday and inter-ion counter) gain factors were determined before the analytical session using standard zircon 91500 (Wiedenbeck et al., 1995). Sensitivity on standard zircon 91500 was 30000 cps per ppm for ²⁰⁶Pb and 35000 cps for ²³⁸U, respectively. Mass and elemental bias and mass spectrometer drift of both U/Pb and Pb/Pb ratios, respectively, were corrected using a multi-step approach: first-order mass bias is corrected using a dried ²³³U-²⁰⁵Tl-²⁰³Tl spike solution which is aspirated continuously in Ar and mixed to the He carrier gas coming from the laser before entering the plasma. This corrects for bias effects stemming from the mass spectrometer. The strongly time-dependent elemental fractionation coming from the ablation process itself is then corrected for using the “intercept method” of Sylvester and Ghaderi (1997). The calculated ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²⁰⁶Pb intercept values are corrected for mass discrimination from analyses of standard 91500 measured during the analytical session using a standard bracketing method. The correction utilises regression of standard measurements by a quadratic function.

A second campaign was performed using a CAMECA IMS1270 ion microprobe at the Swedish Museum of Natural History, Stockholm. Analyses and data reduction procedures follow those outlined by Whitehouse et al. (1997) and references therein. In brief, a ca. 5 nA O2- primary beam was used in aperture illumination mode to sample nominal c. 25 μm elliptical areas. The mass spectrometer was operated in monocollector mode at a mass resolution of c. 5000, with secondary ions detected using an ion counting electron multiplier. Pb/U ratios, calibrated to 91500 include an error component propagated from the standard analyses in a particular session while Pb-isotope ratio errors are counting statistic based or observed errors. Correction for common lead uses measured ²⁰⁴Pb and assumes a present day composition from the Stacey and Kramers (1975) model. In many of the samples analysed with very low Pb concentrations, ²⁰⁴Pb count rates were statistically indistinguishable from background and, in these cases, we have not made a correction. All data reduction was carried out using the routines of Isoplot (Ludwig, 2001).