Abstract
It has been proposed that anoxic and iron-rich (ferruginous) marine conditions were common through most of Earth history. This view represents a major shift in our understanding of the evolution of marine chemistry. However, thus far, evidence for ferruginous conditions comes predominantly from Fe-speciation data. Given debate over these records, new evidence for Fe-rich marine conditions is a requisite if we are to shift our view regarding evolution of the marine redox landscape. Here we present strong evidence for ferruginous conditions by describing a suite of Fe-rich chemical sedimentary rocks—banded iron formation (BIF)—-deposited during the Early Cambrian in western China. Specifically, we provide new U-Pb geochronological data that confirm a depositional age of ca. 527 Ma for this unit, as well as rare earth element (REE) data are consistent with anoxic deposition. Similar to many Algoma-type Precambrian iron formations, these Early Cambrian sediments precipitated in a back-arc rift basin setting, where hydrothermally sourced iron drove the deposition of a BIF-like protolith, the youngest ever reported of regional extent without direct links to volcanogenic massive sulphide (VMS) deposits. Their presence indicates that marine environments were still characterized by chemical- and redox-stratification, thus supporting the view that—despite a dearth of modern marine analogues—ferruginous conditions continued to locally be a feature of early Phanerozoic seawater.
Highlights
It is well established that the Archaean and Palaeoproterozoic Earth was characterized by anoxic, ferruginous oceans, as evidenced by the deposition, and subsequent cessation of, large-scale BIFs1,2
The Taxkorgan terrane occurs within the West Kunlun orogenic belt, southwest of the Tarim block
Previous research has argued that the West Kunlun orogenic belt, relevant to the Tarim block[58,59] and its neighboring regions, formed by a back-arc extensional tectonic event linked to the proto-Tethys Ocean subduction and seafloor spreading[58,60]
Summary
It is well established that the Archaean and Palaeoproterozoic Earth was characterized by anoxic, ferruginous oceans, as evidenced by the deposition, and subsequent cessation of, large-scale BIFs1,2. We couple geochronological data with mineralogy, sedimentary features, and REE systematics from a suite of Fe-rich chemical sedimentary rocks in the Taxkorgan terrane, western China, and provide strong evidence for ferruginous conditions in the Early Cambrian. These sedimentary rocks closely resemble Archaean Algoma-type BIF, sedimentary rocks that formed close to volcanic arcs and spreading centres, having been produced by exhalative hydrothermal processes related to submarine volcanism in either partially closed basins or open seawater systems[25]. Our newly described BIFs are the only regionally extensive examples of this type far reported from the Phanerozoic, without having spatial and stratigraphic links to known VMS deposits
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