Palaeoproterozoic magnesite: lithological and isotopic evidence for playa/sabkha environments

Magnesite forms a series of 1- to 15-m-thick beds within the approximate to2.0 Ga (Palaeoproterozoic) Tulomozerskaya Formation, NW Fennoscandian Shield, Russia. Drillcore material together with natural exposures reveal that the 680-m-thick formation is composed of a stromatolite-dolomite-'red bed' sequence formed in a complex combination of shallow-marine and non-marine, evaporitic environments. Dolomite-collapse breccia, stromatolitic and micritic dolostones and sparry allochemical dolostones are the principal rocks hosting the magnesite beds. All dolomite lithologies are marked by delta C-13 values from +7.1 parts per thousand to +11.6 parts per thousand (V-PDB) and delta O-18 ranging from 17.4 parts per thousand to 26.3 parts per thousand (V-SMOW). Magnesite occurs in different forms: finely laminated micritic; stromatolitic magnesite; and structureless micritic, crystalline and coarsely crystalline magnesite. All varieties exhibit anomalously high delta C-13 values ranging from +9.0 parts per thousand to +11.6 parts per thousand and delta O-18 values of 20.0-25.7 parts per thousand. Laminated and structureless micritic magnesite forms as a secondary phase replacing dolomite during early diagenesis, and replaced dolomite before the major phase of burial. Crystalline and coarsely crystalline magnesite replacing micritic magnesite formed late in the diagenetic/metamorphic history. Magnesite apparently precipitated from sea water-derived brine, diluted by meteoric fluids. Magnesitization was accomplished under evaporitic conditions (sabkha to playa lake environment) proposed to be similar to the Coorong or Lake Walyungup coastal playa magnesite. Magnesite and host dolostones formed in evaporative and partly restricted environments; consequently, extremely high delta C-13 values reflect a combined contribution from both global and local carbon reservoirs. A C- 13-rich global carbon reservoir (delta C-13 at around +5 parts per thousand) is related to the perturbation of the carbon cycle at 2.0 Ga, whereas the local enhancement in C-13 (up to +12 parts per thousand) is associated with evaporative and restricted environments with high bioproductivity

Syn- and post-extensional tectonic activity in the Palaeoproterozoic sequences of Broken Hill and Mount Isa and its bearing on reconstructions of Rodinia

Palaeoproterozoic sequences deformed during the 1600 Ma Olary and Isa orogenies in the Broken Hill and Mount Isa terranes preserve an earlier record of syn-extensional magmatism, deformation and low-pressure/high-temperature metamorphism linked to basin formation and normal faulting at upper crustal levels. Crustal extension occurred nearly continuously from 1800 to 1640 Ma and produced several stacked sedimentary basins, the content of which was mainly controlled by growth faulting along the margins of NNW- and NW-trending half-graben. Mid-crustal magmatic rocks intruded and unroofed during the course of extension are commonly mylonitised and were carried to higher structural levels on extensional shear zones active at the time of sedimentation. Metamorphic monazite ages from deformed host rocks of the syn-extensional Sybella Granite at Mount Isa support this interpretation and include a 1675 Ma population that is much too old to be related to either later crustal shortening or the Isan orogeny. This record of near continuous extension at upper and mid-crustal levels is difficult to reconcile with existing reconstructions of Rodinia in which the Broken Hill and Mount Isa terranes are juxtaposed against rocks of similar age in southwest Laurentia that preserve a history of contractional deformation related to terrane accretion, amalgamation and collision. Equally difficult to reconcile are palaeogeographical reconstructions of Australia which place the Broken Hill and Mount Isa terranes adjacent to each other in a back-arc position while maintaining an along-strike continuity with the rocks of southwest Laurentia. In such a configuration, the Mojave, Yavapai, and Mazatzal provinces lie south of the proposed common Palaeoproterozoic suture (Cheyenne Belt) whereas their inferred age equivalents in the Broken Hill and Mount Isa terranes lie north of this former convergent plate margin. An alternative reconstruction of Proterozoic eastern Australia is proposed in which back-arc extension in the Broken Hill and Mount Isa terranes was linked to retreat of a west-dipping (present-day coordinates) subduction zone and associated magmatic arc that now resides partially or wholly within North America. Eastern Australia and Laurentia ceased to be part of a single continental landmass soon after 1800 Ma and thereafter followed increasingly divergent tectonic paths until re-amalgamated during collision at ca. 1600 Ma. Extension in the Broken Hill and Mount Isa terranes produced a North American Basin-and-Range-style crustal architecture that has no obvious counterpart in southwest Laurentia