Fluid-melt-rock interaction during the transition from magmatism to HT-UHT-granulite- and amphibolite-facies metamorphism in the Ediacaran Adrar-Suttuf Metamafic Complex, NW Margin of the West African Craton (Southern Morocco)

Abstract

Underplated mafic intrusions ponded at the base of the lower continental crust in extensional settings can experience ultra-high temperature (UHT) granulite-facies metamorphism during tens of My due to slow cooling rates, being also the source of heat and carbonic fluids for regional high-temperature (HT) granulite-facies metamorphism in the continental crust. This work analyses the fluid-melt-rock interaction processes that occurred during the magmatic to HT-UHT-granulite- and amphibolite-facies metamorphic evolution of high-grade mafic rocks from the Eastern Ediacaran Adrar-Suttuf Metamafic Complex (EASMC) of the Oulad Dlim Massif (West African Craton Margin, Southern Morocco). P-T conditions were determined using Ti-in-amphibole thermometry, two-pyroxene and amphibole-plagioclase thermobarometry, and phase diagrams calculations. The thermobarometric study reveals the presence of tectonically juxtaposed lower- and mid-crustal blocks in EASMC that experienced decompression-cooling paths from, respectively UHT and HT granulite-facies conditions at ca. 1.2 ±0.28 GPa and 975 ±50 °C, and ca. 0.82 ±0.15 GPa and 894 ±50 °C, to amphibole-facies conditions at ca. 0.28 ±0.28 GPa and 787 ±45 °C (precision reported for the calibrations at 1s level). An age for the magmatic to UHT granulite-facies metamorphic transition of 604 Ma was constrained from published SHRIMP Th-U-Pb zircon ages of the igneous protoliths. An amphibole 40Ar-39Ar cooling age of 499 ±8 Ma (precision at 2s level) was obtained for the lower-crustal blocks. Amphibole 40Ar-39Ar closure temperatures of 520-555 °C were obtained for an age range of 600-499 Ma and an average constant cooling rate of 4.2 °C/My, suggesting that the lower-crustal blocks cooled down to the greenschist-amphibolite facies transition in ca. 100 My. During the high-temperature stage, interstitial hydrous melts caused incongruent dissolution melting of olivine and pyroxenes, and, probably, the development of An-rich spikes at the grain rims of plagioclase, and assisted textural maturation of the rock matrix and local segregation of pargasite into veins. Subsequent local infiltration of reactive hydrous metamorphic fluids along mineral grain boundaries during cooling down to amphibolite-facies conditions promoted mineral replacements by coupled dissolution-precipitation mechanisms and metasomatism. Ubiquitous dolomite grains, with, in some cases, evidence for significant textural maturation, appear in the granoblastic aggregates of the high-grade mafic rocks. However, calculated phase relationships reveal that dolomite could not coexist with H2O-CO2 fluids at HT-UHT granulite- and low-medium P amphibolite-facies conditions. Therefore, it is proposed that it may have been generated from another CO2-bearing phase, such as an immiscible carbonatitic melt exsolved from the parental mafic magma, and preserved during cooling due to the prevalence of fluid-absent conditions in the granoblastic matrix containing dolomite. The lower-crustal mafic intrusions from EASMC can represent an example of a source of heat for granulitisation of the mid-crust, but a sink for carbon due to the apparent stability of dolomite under fluid-absent conditions.