OCCURRENCE AND CHEMICAL COMPOSITION OF AMPHIBOLITE AND CARBONATE ROCKS IN THE NUVVUAGITTUQ SUPRACRUSTAL BELT, QUEBEC, CANADA: IMPLICATIONS FOR THE ORIGIN OF CA-POOR AMPHIBOLITE AND ITS ALTERATION HISTORY

The Nuvvuagittuq supracrustal belt (NSB) comprises one of the oldest supracrustal rocks; thus its geologic information can constrain the evolution of the early Earth. The supracrustal rocks are mostly composed of unique mafic rocks (Ujaraaluk Unit (UU)), which are highly depleted in Ca because they contain cummingtonite instead of hornblende. They are thought to be derived from hydrothermally altered basalts, but their distinctive Ca-poor composition is poorly explained. The supracrustal rocks comprise UU and low-grade greenstone as well as sedimentary rocks. We explored the origin and alteration history of UU based on geochemical data of carbonate-rich parts of the UU and greenstone as well as those of UU from literatures. Their major and trace element contents were obtained using XRF at the Tokyo Institute of Technology and ICP-MS at the University of Tokyo.

Calcium and K contents of UU range from 0.12 to 9.94 wt.% and from 0.1 to 7.04 wt.%, respectively, indicating that UU are depleted in Ca and enriched in K compared with the Eoarchean basalts in the Isua supracrustal belt. Their chondrite-normalized rare earth element (REE) patterns show negative Y (0.57 to 1.06) and positive Ce (0.92 to 1.69) anomalies. The Y and Ce anomalies are defined by [Y/Y*]_CH and [Ce/Ce*]_CH, where [Y*]_CH = 0.5 [Dy]_CH + 0.5 [Ho]_CH and [Ce*]_CH = 2 [Pr]_CH – [Nd]_CH. The geochemical characteristics of UU are similar to those of the Paleoarchean carbonated and silicified basalts, suggesting that most UU rocks don’t contain carbonate minerals but originated from carbonated and subsequently silicified basalt. In other words, the Ca-poor signatures are possibly due to silicification after the carbonation.

The carbonated UU and greenstone were classified as highly- and less-carbonated rocks based on loss on ignition values. REE contents of metamorphic fluids percolating through carbonated rocks are estimated from comparison of REE patterns between the highly- and less-carbonated rocks for UU and greenstone, respectively. The estimated contents of fluids were almost identical each other, suggesting that the carbonate minerals in UU and greenstone were formed at the same event. The results suggest that the carbonate minerals in UU aren’t product of initial carbonation, but were formed after intrusion of the greenstone protolith.