Rift metamorphism of bimodal magmatic rocks during breakup of supercontinent Rodinia

Abstract

The identification of supercontinental breakup processes in fossil continental rift zones is crucial for the reconstruction of supercontinents. However, it remains a challenge due to intensive obliteration by later geological events. In general, supercontinental breakup is realized through successful continental rifting, developing from lithospheric rupturing to seafloor spreading. It is closely associated with asthenospheric upwelling in both space and time, leading to enhancement of both conductive and advective heat transfers from the asthenospheric mantle through the lithospheric mantle into the continental crust. This gives rise to bimodal magmatism, high-temperature (high-T) water–rock reaction, and rift metamorphism at high geothermal gradients. Although such abundant processes are substantial to study supercontinental breakup, it is difficult to decipher the sequence of their operation that provides valuable clues for the relationship between magmatism and metamorphism during continental rifting. This issue is addressed by the present study of petrology and geochronology for plagioclase amphibolite and metagranite from the northern margin of the South China Block. This margin was successfully rifted from supercontinent Rodinia in the middle Neoproterozoic, making it suitable for studying rifting-related processes. The protoliths of the plagioclase amphibolite and metagranite are the bimodal magmatic rocks that formed during the supercontinental breakup in the middle Neoproterozoic. They experienced high-T hydrothermal alteration by continental glacier meltwater at first, and then were re-heated to undergo high-temperature/low-pressure (high-T/low-P) metamorphism during the continental rifting. The plagioclase amphibolites record high geothermal gradients of ca. 50 °C/km, and the metagranites preserve higher geothermal gradients of > 80 °C/km since muscovite in the metagranites was dehydrated to form metamorphic andalusite during further decompressional heating. The two kinds of metamorphic rocks were produced at the high geothermal gradients, corresponding to high T/P Buchan type amphibolite facies characteristic of continental rifting along divergent plate margins and continental rift zones. The bimodal magmatism, high-T hydrothermal alteration and high-T/low-P metamorphism would operate sequentially over an interval of approximately 20 Myr, revealing that the bimodal magmatism and the high-T/low-P metamorphism are the two manifestations of regional thermal anomaly due to lithospheric extension during the supercontinental breakup. Therefore, the lithospheric extension plays a significant role in driving the rift metamorphism.