The behaviour of zircon, monazite and their U-Pb isotopic systems during the high-grade metamorphism of quartzo-feldspathic sedimentary rocks

Full text: Evidence of reactions between the rock-forming minerals can be used to define, with reasonable confidence, the sequence of P-T conditions a particular suite of rocks has experienced during a particular metamorphic event. Placing that P-T path accurately within a numerical time frame can be difficult however. Many of the minerals used for radioisotope geochronology, for example amphiboles, micas and feldspars, have closure temperatures below the peak temperatures reached at high metamorphic grades. Those minerals provide valuable information on the retrograde path, but they retain little or no record of the prograde path. Recently, particularly following the advent of SHRIMP, extensive use has been made of minerals with higher closure temperatures, such as zircon and monazite, to provide information on the timing of near-peak metamorphic conditions. Zircon has proved especially valuable, the closure temperature of its U-Pb isotopic system (∼ 900 deg C) being above even the temperature of many magmas. Much remains to be learned, however, about how these minerals behave during metamorphism and, for different rock types, what points on the P-T path they might record. In the Reynolds Range, northern Arunta Block, late Palaeoproterozoic metasediments of the Reynolds Range Group overlie a basement of earlier Palaeoproterozoic granites and metasediments. The Reynolds Range Group has been subject to low to medium pressure (4-5 kbar) regional metamorphism that in the SE reaches granulite facies (∼750 deg C), producing local partial melting and, within the marbles and metapelites of the Upper Calcsilicate Unit, extensive zones of retrogression resulting from infiltration of hot (650-700 deg C), water-rich metamorphic fluids. In places these zones are intruded by pegmatites and coarse-grained, quartz-rich, cordierite and sillimanite-bearing segregations up to several metres wide. Zircon from a granulite grade semipelite away from zones of obvious retrogression yields a range of Palaeoproterozoic ages reflecting the provenance of the metasediment. Most grains have narrow overgrowths of new zircon, characteristically low in Th/U, which yield an age of 1594 ± 6 Ma, presumably reflecting the granulite grade event. Monazite from the same rock appears to be wholly metamorphic, all analysed grains yielding the significantly younger age of 1576 ±8 Ma. Zircon from one semi-concordant metamorphic segregation from a retrograde zone is, in contrast, dominantly metamorphic. Thick overgrowths of two contrasting chemical compositions yield indistinguishable ages of 1589 ± 8 Ma and 1582 ± 8 Ma-the small zircon cores range in age from Palaeoproterozoic to late Archean. The monazite once again appears wholly metamorphic, yielding 1576 ± 12 Ma. A second, structurally latest segregation contains sillimanite-perforated metamorphic zircon megacrysts which yield 1568 ± 4 Ma. Temperatures high enough for metamorphic zircon and monazite growth were sustained for ∼25 Ma, but nevertheless the detrital zircon, and therefore presumably also the early-formed metamorphic zircon, were not isotopically reset. At Cooma, in SE Australia, a small area of early Palaeozoic quartz-rich turbidites has been regionally metamorphosed at low pressure (∼4 kb) to the point of partial melting. Over a distance of about 8 km, the grade rises progressively from chlorite to biotite to andalusite to sillimanite grade, thence through a zone of migmatites and partial melting to a small body of low-temperature peraluminous granite. The granite appears to be a consequence, not the cause, of the metamorphism. The individual detrital zircon and monazite grains in the biotitegrade metasediment yield a wide range of ages reflecting the ages of the various components in the sediment's source region. With rising grade the monazite grains become increasingly corroded, although still retaining their age range, until eventually they disappear. At sillimanite grade and above monazite again is present, but yields only the age of the Silurian metamorphism, ∼430 Ma. In contrast, zircon appears to be unaffected by the metamorphism either morphologically or isotopically until zircon overgrowths begin to form at sillimanite grade. The amount of new growth increases with grade, peaking in the migmatite leucosomes. The age of the zircon overgrowths matches that of the metamorphic monazite-the ages of the zircon cores, even in the granite, match closely the ages of the detrital zircons. The granite has formed from the deeper equivalents of the exposed sedimentary rocks, and the inherited zircon cores in the granite have survived temperatures in excess of 750 deg C, preserving an accurate, unbiased record of the ages of the detrital zircons in that sediment. In contrast, all the monazite in the migmatite and granite appears to be newly grown