The origin of zircons in Isua supracrustal belt (ISB), Greenland

Abstract

Composition and tectonic environment for first-formed crust are always the missing piece of puzzle of the Earth’s crustal history. The only clue to that is the detrital zircons from Hadean eon (4.0-4.6 Ga), they are the only remnant that left on Earth. It is well-known that crustal product in Archean eon (2.5- 4.0 Ga) was dominated with tonalite-trondjhemtite-granodiorite (TTG) suites of rocks, whilst granitoids were prevailed in Proterozoic eon (541Ma-2.5 Ga). Numerous studies have been dedicated to find out whether Hadean has distinctive crust forming mechanism or a similar mechanism with subsequent Archean eon. In other words, people are desperate to know whether Hadean eon was composed with TTG-like rock or a completely different crustal product. The primary goal of this paper is to perform a direct investigation on zircons geochemistry and inclusion population, targeted zircons are hosted in Isua Supracrustal Belt (ISB), which is one of the Eoarchean geological record. A comparative study is also carried out to determine whether or not Hadean zircons could have originally eroded from comparable metavolcanics rocks. Results show that Hadean zircons are displaying similarities with ISB zircons, which implies that only two crust forming mechanisms were operating during Precambrian period. Of the 9 Isua samples, JZ5A and JZ5B are taken as the best estimate of the parental magma to Hadean zircons in perspective of geochemistry and inclusion population. Solely based on the result acquired from this study, vertical tectonic model is preferred to explain the formation of ISB and perhaps the tectonic setting in the first 600 million years.