REE and HFSE mobility due to protracted flow of basinal brines in the mesoproterozoic Belt-Purcell Supergroup, Laurentia

Argillites from the Belt-Purcell Supergroup record three types of REE patterns. Type 1 (T1), near-primary compositional pattern is characterized by: (1) a PA-UCC-like REE pattern, and Th/Sc ratios reflecting a dominantly Paleoproterozoic catchment; (2) depletion of Ca, Sr and Na due to weathering in the provenance of the sediments; (3) variable depletion of Nb–Ta and Zr–Hf from winnowing of titanite and zircon, respectively, with Nb/Ta and Zr/Hf ratios similar to PA-UCC; and (4) K-enrichment, with variable Th/U and Ce/Ce* ratios from diagenesis. Sandstones with T1 patterns are compositionally equivalent to argillites diluted with quartz, and show the same four T1 compositional features, albeit at lower absolute abundances.Relative to T1 argillites, Type 2 (T2) and Type 3 (T3) patterns both feature enriched and fractionated HREE, but T2 exhibits LREE depletion. Relative to T1 these constitute T2 low − ΣLREE/ΣHREE (15 ± 3) and T3 high − ΣLREE/ΣHREE (24 ± 3) patterns showing progressive enrichment of HREE, erratic ratios of Zr/Hf, Ti/Sm, Y/Ho, Y/Yb and Al/Yb. Collectively, T2 and T3 patterns record diagenetic transfer of REE as well as REE/HFSE and HFSE/HFSE fractionations. Such fractionations are more pronounced in sandstones due to greater hydraulic conductivity for diagenetic fluids.Hence REE and HFSE patterns in the argillites and sandstones are interpreted as diagenetic processes superimposed on a PA-UCC provenance signature, specifically to pervasive and episodic migration of oxidizing-alkaline diagenetic basinal brines in the Belt-Purcell Supergroup. During these diagenetic processes Al, Th and Ga behaved isochemically.Dominantly positive Ce anomalies coupled with mobility of U (as recorded by variable Th/U ratios) is consistent with oxidizing conditions, which may have been internally generated within the basin by reactions between clays and carbonate. Alkalinity of the fluids could have been promoted by some combination of: (1) enrichment of K as indicated by mass balance calculations and the presence of illite; (2) hydrothermal and/or diagenetic alteration of mafic igneous units lower in the stratigraphic sequence; and (3) dissolution of evaporitic units, evident in relict halite casts, in the Belt-Purcell sequence.Supporting this interpretation, previous studies revealed two populations of monazite, differentiated in terms of texture, composition and age. Detrital monazites are >1400 Ma, whereas diagenetic monazites are euhedral and chemically distinctive, spanning ∼1400–∼300 Ma. The diagenetic monazites may record episodic brine activity for ∼1000 Ma that induced T2 and T3 compositional features, and redistributed HREE and Zr–Hf at a basin scale.