The retrograde P–T–t path for low‐pressure granulites from the Reynolds Range, central Australia: petrological constraints and implications for low‐P/high‐T metamorphism

Abstract

Several aspects of the petrogenesis of low-pressure granulite facies rocks from the Reynolds Range (central Australia) are contentious, including: (a) the shape of the retrograde P-T -time path, and whether it is an artefact of repeated thermal events at different P-T conditions; (b) the type of regional metamorphism; and (c) the causes of metamorphism. Granulite facies rocks from the Reynolds Range Group experienced three major periods of mineralogical equilibration. Metapelitic rocks underwent dehydration-melting reactions to form migmatites under peak M2 P-T conditions of c. 5.0-5.3 kbar and c. 750-800°C. Metapsammitic rocks that did not melt during M2 show spectacular garnet-orthopyroxene intergrowths that developed at c. 3.5-3.7 kbar and c. 700-750°C after penetrative regional deformation, but prior to amphibolite facies rehydration in discrete strike-parallel zones. Rehydration occurred within the sillimanite stability field at P-T conditions close to the granite solidus (c. 3.2-3.4 kbar and 650-700°C). Subsequently the terrane cooled into the andalusite stability field. Geochronological constraints suggest that: (a) peak-M2 conditions were reached at c. 1594 Ma; (b) the garnet-orthopyroxene intergrowths in unmelted metapsammites probably developed between c. 1594 Ma and c. 1586 Ma; and (c) upper amphibolite facies rehydration occurred between c. 1586 Ma and 1568 Ma. The lack of petrological evidence for multiple dehydration and rehydration of the rocks suggests that the three episodes of mineralogical recrystallization can be linked to yield a single continuous retrograde P-T -t path of minor initial decompression (c. 1.5 kbar) from the M2 peak, followed by cooling (c. 100°C) to the granite solidus over a period of c. 26 Ma. Late kyanite-bearing shear zones that dissect the terrane are unrelated to this event and formed during the c. 300-400 Ma Alice Springs Orogeny. The shape of the P-T -t path and the duration of M2 metamorphism suggests that advective heating was not the major cause of high-grade metamorphism, and that some other, longer lived heat source, such as the burial of anomalously radiogenic, pre-tectonic granites, is required.

LA-MC-ICPMS Pb–Pb dating of rutile from slowly cooled granulites: Confirmation of the high closure temperature for Pb diffusion in rutile

“…30 Myr over the period 1587-1557 Ma during very slow cooling at ca. 3°C/Myr, in a stable high geothermal regime (Vry and Cartwright, 1994;Williams et al, 1996;Buick et al, 1998;Buick et al, 1999;Rubatto et al, 2001). As discussed later in this paper, the results of the present study support this interpretation and suggest that despite the shallow crustal setting slow cooling at 2-3°C/Myr and could have prevailed for up to 200 Myr.…”

“…3°C/Myr for ca. 26-30 Myr; Williams et al, 1996;Buick et al, 1998;Rubatto et al, 2001). This extended period of cooling should enable precise determination of the T c for Pb diffusion in rutile.…”

“…(3) The metamorphic P-T-t path, from just prior to and during the peak of granulite-facies metamorphism, through to the subsequent early retrogression at high temperatures ( Fig. 2), has been carefully defined for this sample collection site (Vry and Cartwright, 1994) and subsequently confirmed for other highgrade parts of the Reynolds Range (Buick et al, 1998). The overall cooling rate for the early retrogression at high temperatures is well-defined and slow (ca.…”

“…In order to help resolve the current uncertainty as to the T c of Pb in natural rutile, we have carried out a new empirical determination of the T c using natural rutile from a site for which the timing of granulite-facies metamorphism (ca. 1584 Ma), the high-temperature metamorphic pressure-temperature (P-T) history, and the subsequent slow cooling (3°C/Myr) are all well-constrained (Vry and Cartwright, 1994;Vry et al, 1996;Williams et al, 1996;Buick et al, 1998;Buick et al, 1999;Rubatto et al, 2001). Even though a lower-temperature metamorphic event did affect the area much later during the 400-300 Ma Alice Springs orogeny (Teyssier, 1985;Shaw et al, 1991Shaw et al, , 1992, the initial cooling that followed the metamorphism to high-T and low-P granulite-facies conditions was slow and prolonged, with melt crystallization continuing over ca.…”

Metamorphic CO2 production from calc-silicate rocks via garnet-forming reactions in the CFAS–H2O–CO2 system

“…Although less studied than metapelitic and basic lithologies, the importance of calc-silicate assemblages for the reconstruction of the pressure -temperature -fluid composition evolution (P-T-X fluid ) of metamorphic terrains is widely recognized (e.g. Warren et al 1987;Harley and Buick 1992;Dasgupta 1993;Fitzsimons and Harley 1994;Sengupta et al 1997;Stephenson and Cook 1997;Buick et al 1998). The majority of studied calc-silicate rocks come from granulite-facies terrains and are characterized by peak assemblages consisting of wollastonite + scapolite, with grossular-rich garnet often occurring as retrograde coronas (e.g.…”