Paleoproterozoic rift-related volcanism of the Karrat Group, west Greenland: New insights from lithofacies analysis and geochemistry of the Kangilleq Formation

Abstract

Mafic volcanic rocks of the Paleoproterozoic Kangilleq Formation of the Karrat Group of West Greenland represent a submarine mafic volcanic sequence, erupted and deposited on the Rae craton of Greenland. The formation ranges from <100 m-thick to ∼500 m-thick, and consists of mafic volcaniclastic rocks, pillow lavas and massive basalt lavas that have undergone greenschist facies metamorphism, yet preserve primary volcanic textures such as pillows, drainage cavities, lobes, flow banding, autoclastic breccias, hyaloclastite, scoria and layering.Rocks of the Kangilleq Formation overlie the Archean Qeqertarssuaq Complex and are interleaved with and overlain by the ca. 1950 Ma Nûkavsak Formation. Based on detailed (1:200) mapping and high-precision geochemistry, rocks of the Kangilleq Formation are subdivided into the Kangiusiap and Umiammakku members. The Kangiusiap member is dominated by mafic, scoria-rich breccias, has sulfide mineralization, and is bound at the bottom and top by the Nûkavsak Formation. This member has subalkaline, transitional to tholeiitic, mid-ocean ridge basalt (MORB) to back arc basin basalt (BABB) signatures with a Nd-isotope value similar to the depleted mantle at 1.88 Ga. The Umiammakku member is dominated by pillow lavas, with subordinate amounts of massive lavas and volcaniclastic rocks, overlies the Qeqertarssuaq Complex and is overlain by, and interleaved with, the Nûkavsak Formation. It is alkaline with characteristics similar to modern ocean island basalts and Nd-isotopic values slightly lower than the depleted mantle at 1.88 Ga.The rocks of the Kangilleq Formation represent non-arc suites or suites that have only minor subduction zone or crustal input, and display geochemical signatures similar to basalts found in modern intracontinental back arc settings. Lithofacies, bedforms, internal stratigraphy, and the identification of synvolcanic faults, suggest it is the product of volcanic eruptions during incipient rifting within a back arc setting, and seafloor mass wasting events at fault scarps related to rifting. Based on the geological characteristics and its petrogenetic and tectonic history, the Kangilleq Formation is interpreted to be correlative with the Bravo Lake Formation of the Piling Group of northeastern Canada and records incipient rifting of the southern margin of the Rae craton in the Paleoproterozoic.