Fluids in high- to ultrahigh-temperature metamorphism along collisional sutures: Record from fluid inclusions

摘要

Petrographic studies and microthermometric investigations on fluid inclusions associated with high- to ultrahigh-temperature metamorphic rocks in three major Precambrian suture zones on the globe demonstrate the dominant occurrence of CO_2-rich fluids. These rocks form part of hot orogens developed along collisional plate boundaries. The sapphirine-quartz-bearing Mg-Al-rich rock from the Palghat-Cauvery Suture Zone, a trace of the Cambrian Gondwana suture zone in southern India, preserves evidence for a prograde high-pressure event and subsequent peak ultrahigh-temperature metamorphism along a clockwise path, and contains abundant CO_2-rich inclusions in corundum, garnet, and sapphirine. Most of the fluid inclusions are either primary or secondary and preserve low-density CO_2-rich fluids (0.569-0.807 g/cm3). Similar low-density CO_2-rich fluid inclusions (0.853-0.953 g/cm3) are also present in pelitic granulites from the Limpopo Complex of southern Africa, a Neoarchean granulite-facies orogen formed by continent-continent collision. In contrast, the garnet-orthopyroxene granulite from Tonagh Island in the Neoarchean Napier Complex in East Antarctica contains very high-density primary (1.095-1.129 g/cm3) and secondary (0.960-1.179 g/cm3) carbonic inclusions in garnet and quartz. The calculated isochores for the fluid inclusions from the Palghat-Cauvery Suture Zone and the Limpopo Complex yield significantly lower-pressure estimates than those predicted from peak metamorphic conditions. We interpret this as a result of significant density decrease due to rapid decompression along a clockwise P-T trajectory. In contrast, the estimated isochores for primary inclusions in garnet-orthopyroxene granulites from the Napier Complex are consistent with the peak P-T conditions estimated from mineral phase equilibria for the Tonagh Island rocks, suggesting that most of the fluid inclusions in these rocks did not undergo any marked effect of volume change and density decrease. The contrasting fluid densities among the localities investigated in this study are probably related to the nature of the P-T trajectory; the Tonagh Island rocks had a near-isochoric exhumation history whereas the metamorphic orogens in the other two sutures witnessed rapid decompression. Our results suggest that whereas the composition of the syn-metamorphic fluids are preserved in all cases, density reversal occurs within inclusions as a function of the tectonic history and exhumation style.