balz@esc.cam.ac.uk
Modern geochronology can potentially resolve the time scale over which metamorphic processes occur. The bulk of speedometry studies is biased to the retrograde evolution of metamorphic rocks and implications for the peak- and prograde history remain equivocal. We report a multi-method study on one well characterised migmatite sample from the northernmost Archaean Limpopo Belt, with the aim of determining the duration of melting and a 0.4 GPa burial. This terrain experienced an anti-clockwise PT evolution reaching peak T of „800šC at low P (0.4 GPa) followed by a near isothermal P increase of ~0.4 GPa (Kamber and Biino, 1995). Our sample records initial melting by biotite+plagioclase breakdown to cordierite+granitic melt. As P increased, this changed to biotite breakdown producing orthopyroxene+granitic melt. Finally, the incomplete melt segregation allows us to observe initial cooling by the back-reaction of orthopyroxene+melt to biotite+quartz. An independent time bracket for the entire migmatisation is given by the youngest enderbite (2637±19 Ma, (Berger et al., 1995)) known to be migmatised and the oldest granite crosscutting the migmatites at 2591±4 Ma. Preservation of fresh cordierite testifies to minimal hydration of our rock during retrogression and final exhumation at 1.97 Ga. The presence of zircon, monazite and xenotime facilitates understanding of the time frame of the observed reaction history. All three accessories co-existed in the protolith. Hypidiomorphous zircon has a very low 208Pb/206Pb ratio of 0.01; slightly resorbed monazite and xenotime, are depleted in HREE and LREE, respectively. They are all found both interstitially and as inclusions in cordierite (abundant) and orthopyroxene (rare). Furthermore, tiny (¾2 mm) long-prismatic zircon inclusions are found in orthopyroxene. The ISOLAB 120 was used to explore age differences between host shielded and interstitial grains through 207Pb/206Pb dating at high spatial resolution. In addition, 3 size fractions of zircon were analysed by conventional miniaturised U/Pb chronology and orthopyroxene was dated using the U/Pb and step Pb/Pb leaching (Frei and Kamber, 1995) chronometers.
Zircon: Two SIMS analysis on a hypidiomorphous grain (25mm wide) fully armoured by cordierite held identical 207Pb/206Pb ages of 2671±10 Ma. Conventional U/Pb dating of 3 size fractions shows decreasing 207Pb/206Pb ages with decreasing size (60-80mm = 2898±25 Ma, discordant; 40-60mm = 2599±18 Ma, discordant; <40mm = 2554±25 Ma, concordant). Monazite and xenotime:
2 large monazite (300mm) and xenotime (200mm) grains fully armoured by cordierite show homogenous 207Pb/206Pb SIMS ages of 2612±8 and 2602±17 Ma. Interstitial grains are also homogenous, monazite holds 2600±5 but xenotime 2551±5 Ma. Orthopyroxene: U/Pb analysis resulted in a near concordant (99.5%) age of 2611±16 Ma. However, Pb-Pb step leaching of an aliquot shows that U (0.6 ppm) and Th are largely derived from microscopic inclusions. The 3 leachates and the residue define a 206Pb/206Pb spread of 1'080 to 32'000 but no isochron. Omitting the residual step with a much lower 208/206 Pb ratio (probably reflecting Pb contribution from microscopic zircon) results in an isochron age of 2596±5 Ma.
Zircon ages reflect partial protolith inheritance (2671±10 Ma SIMS, 2898±25 Ma conventional), total resetting during migmatisation (2599±18 Ma) and disturbance during cooling (2554±25 Ma). Without additional con-straints these ages could not be interpreted. Especially the concordant young age of 2554±25 Ma could erro-neously be seen as the age of peak metamorphism. However, the 2591±4 Ma age of granite crosscutting the migmatites disproves such an interpretation. Frei and Kamber (1995) obtained an age of 2532±35 Ma for a nearby skarn formation. We prefer to interpret the concordant 2554±25 Ma as resetting of the smallest zircons due to hydrothermal activ- ity. Zircon chronology is thus not appropriate to date migmatisation under low aH2O. Monazite and xenotime grains armoured by unaltered cordierite hold indistinguishable ages (2612±8 and 2602±17 Ma). They agree with additional age constraints (garnet Pb/Pb from similar leucosomes, 2601±5 Ma, (Frei and Blenkinsop, subm.)) and with the independent age bracket. Interstitial monazite holds an identical age (2600±5 Ma), but xenotime (2551±5 Ma) appears to have been reset during later hydrothermal activity. The spatial information allows us to propose that the U/Pb Tc for both phosphates is „800šC. The freshness of surrounding cordierite testifies that neither fluid infiltration nor re- crystallisation affected these grains. Only in this case does the concept of volume diffusion apply. As evidenced by the younger interstitial xenotime age, even minor infiltration of fluid along grain boundaries can reset the U/Pb system at a T significantly lower than Tc. Both the slightly discordant orthopyroxene U/Pb age (2611±16) and the step leach 207Pb/206Pb isochron age (2596±5 Ma) are within error of the monazite ages. However, step leaching provides a tool to investigate 'intracrystalline' Pb equilibrium (i.e. between host and microscopic inclusions) and in the case of our sample shows, that the U/Pb orthopyroxene age is an over-estimate, probably due to incorporated zircons with partial inheritance. The time frame for the PT loop recorded by the rock (e.g. a P increase of 0.4 GPa) cannot be resolved by the applied geochronometers. This is however not because of insufficient precision (e.g. monazite armoured by cordierite at 2612±8Ma@0.4 GPa vs. Pb step leach isochron of orthopyroxene at 2596±5 Ma@0.8 GPa) but due to insufficient accuracy of the age interpretation.
Berger et al., SMPM 75, 17 (1995).
Frei & Kamber, Earth Planet. Sci. Lett. 129, 261 (1995).
Frei & Blenkinsop, Econ. Geol. (subm.).
Kamber & Biino, SMPM 75, 427 (1995).