5.4.7 Discussion on M3
The metapelites from the kyanite zone preserve petrographic information about their retrograde metamorphic history. The first stage of the retrograde evolution for the kyanite zone metapelites corresponds to the appearance of late fibrolitic sillimanite in some samples (Fig 5.27, point R1). The growth of sillimanite on the shearing surfaces of extensional C/S fabrics indicates extension-induced decompression. Subsequent partial replacement of kyanite by cordierite in some Mg-rich (staurolite-free) rocks confirms a decreasing pressure path (Fig 5.27, point R2). Small-scale quartz and K-feldspar segregation suggest incipient partial melting, but the presence of stable muscovite in the samples indicates that the P-T conditions reached in the kyanite zone did not induce pervasive melting. The later stages of the retrograde evolution are characterized by the appearance of andalusite as the stable aluminosilicate, which is itself subsequently replaced by margarite (Fig 5.27, points R3 and R4). These mineral changes suggest a retrograde P-T path starting with more or less isothermal decompression, and continuing with a more pronounced cooling. This retrograde P-T path, based on petrographic observations for the kyanite zone is very similar to the P-T path that can be deduced from the thermobarometric data of the underlying migmatitic zone (Fig 5.22). In this zone, the metabasic granulites have retained peak metamorphic conditions associated with M2 ( T ~ 800°C and P ~ 1200 MPa) but the metapelitic samples have reequilibrated at low-P / medium-T (T ~ 600°C and P ~ 350 MPa) during M3 which indicates a significant pressure drop associated with this last thermal event.
The reaction textures and mineral assemblages of the kyanite zone and sillimanite zone, together with thermobarometric data, indicate that the HHCS must have undergone isothermal decompression. Isothermal decompression of the migmatitic zone played a major role in the production of leucogranitic melts inasmuch as it maintained these high-grade rocks above the muscovite dry melting curve on a large part of the retrograde path.
The occurrence of retrograde minerals in the kyanite zone is intimately associated with the presence of leucogranitic intrusions. In the area of Mune and Reru, 30 kilometres to the NW of the studied area, leucogranitic dikes intrude the ZSZ but no large leucogranitic bodies are present in close contact with the ZSZ. The kyanite zone in this area is characterized by the presence of prograde kyanite in the metapelitic schists as well as in association with quartz veins, but none of the above-mentioned retrograde minerals could be observed within this zone. Only sillimanite occurs in this area but at a lower structural level. This sillimanite is found in migmatites showing a weaker stage of partial melting than along the Kurgiakh valley, for kyanite is still preserved as relict grains within these rocks and muscovite was never totally consumed.
There is thus a close relation between the leucogranites and the growth of late retrograde minerals. The continuous production and intrusion of leucogranites must have contributed to maintain the kyanite zone at high temperature during exhumation and thus favoured isothermal decompression and the growth of retrograde minerals in this zone. The growth of retrograde minerals was certainly also favoured by the input into the kyanite zone of enriched fluids and volatiles derived from the last stages of crystallization of the magma. A late fluid rich phase of crystallization of the leucogranites is indeed testified by the presence of late, undeformed, pegmatitic dikes.
It should, however, be emphasized that the order of crystallization of the retrograde minerals goes from high-temperature minerals (sillimanite) towards lower temperature minerals (andalusite) and that they have thus not crystallized along a prograde metamorphic path as would be expected with contact metamorphism (Buchan type). The intrusion of leucogranites within the metapelitic rocks however had a catalytic effect on the growth of retrograde minerals.