M.A. Jirsa | Geology Reports

USGS2011

Neoarchean paleoweathering of tonalite and metabasalt: Implications for reconstructions of 2.69Ga early terrestrial ecosystems and paleoatmospheric chemistry

Field and laboratory investigations of a 2690.83 Ma ( 207 Pb/ 206 Pb age of Saganaga Tonalite) unconformity exposed in outcrop in northeastern Minnesota, USA, reveal evidence for development of a deep paleoweathering profile with geochemical biosignatures consistent with the presence of microbial communities and weakly oxygenated conditions. Weathering profiles are characterized by a 5–50 m thick regolith that consists of saprolitized Saganaga Tonalite and Paulson Lake succession basaltic metavolcanic rocks retaining rock structure, which is cross-cut by a major unconformity surface marking development of a successor basin infilled with alluvial deposits. The regolith and unconformity are overlain by thick conglomerate deposits that contain both intrabasinal (saprock) as well as extrabasinal detritus. Thin-section microscopy and electron microprobe analyses reveal extensive hydrolysis and sericitization of feldspars, exfoliation and chloritization of biotite, and weathering of Fe-Mg silicates and Cu-Fe sulfides; weathering of Fe-Ti oxides was relatively less intense than for other minerals and evidence was found for precipitation of Fe oxides. Geochemical analyses of the tonalite, assuming immobile TiO 2 during weathering ( τ Ti, j ), show depletion of SiO 2 , Al 2 O 3 , Na 2 O, CaO, MgO, and MnO, and to a lesser degree of K 2 O, relative to least-weathered parent materials. Significant Fe was lost from the tonalite. A paleoatmospheric pCO 2 of 10–50 times PAL is estimated based on geochemical mass-balance of the tonalite profile and assuming a formation time of 50–500 Kyr. Interpretations of metabasalt paleoweathering are complicated by additions of sediment to the profile and extensive diagenetic carbonate (dolomite) overprinting. Patterns of release of P and Fe and retention of Y and Cu in tonalite are consistent with recent laboratory experiments of granite weathering, and with the presence of acidic conditions in the presence of organic ligands (produced, for example, by a primitive microbial community) during weathering. Cu metal in the profile may document lower pO 2 than present day at the surface. Comparison with previous studies of weathered tonalite and basalt (Denison, 2.45–2.22 Ga) in Ontario, Canada, reveal general similarities in paleoweathering with our study, as well as important differences related to lower paleoatmospheric pO 2 and terrestrial biosignature for the older Minnesota profile. A falling water table in the Alpine Lake locality is presumed to have promoted formation of this gossan-like deep-weathering system that extends to 50-m depth.

Minnesota↗

USGS2007

Penokean tectonics along a promontory-embayment margin in east-central Minnesota

Recent geologic investigations in east-central Minnesota have utilized geophysical data, test drilling, and high-resolution geochronologic dating to produce a significantly improved map of a poorly exposed part of the 1880-1830 Ma Penokean orogen. These investigations have elucidated major changes in the structure of the orogen, as compared to its counterparts in northern Michigan and northwestern Wisconsin. Foreland basin, fold and thrust belt, and magmatic terrane components that are recognized to the east extend into east-central Minnesota, but they appear to be deflected southwards and truncated in proximity to Archean rocks of the Minnesota River Valley (MRV) subprovince. In contrast, the interior of the MRV subprovince to the southwest shows little sign of Penokean tectonism. In addition, the magmatic and metamorphic rocks of the internal zone of the orogen in east-central Minnesota are extensively invaded by ca. 1785-1770 Ma granitic rocks (the East-Central Minnesota Batholith), whereas, post-orogenic granites of this age occur sparingly to the east. These differences in orogenic structure may be related to their location near the juncture of an embayment (Becker embayment) and a promontory (MRV promontory) that formed the pre-Penokean continental margin. In this scenario, the MRV promontory, which at the surface consists chiefly of high-metamorphic-grade Mesoarchean gneisses, would have formed competent, high-standing crust that resisted deformation and did not host significantly thick continental margin sequences. In contrast, the part of the Becker Embayment adjoining the promontory would have involved relatively weak, low-standing crust that favored deposition of continental margin sequences and, during Penokean collision, would have accommodated tectonic loading of the cratonic margin through thin-skinned deformation. Thrusting of thick embayment sequences and possibly a block of Archean crust (Marshfield terrane) onto the embayment margin may have produced a greatly thickened crust that subsequently promoted crustal melting and generation of the geon 17 granites. Preliminary gravity and magnetic model studies of the present-day crust imply that rocks of the fold and thrust belt may sole out at 5-8 km depth; whereas, magmatic and high-metamorphic-grade rocks associated with the internal zone of the orogen could extend to mid-crustal depths. The tectonic model proposed here, implies that a collision between an embayment and an impinging continental mass may enhance tectonic thickening and subsequent generation of post-orogenic magmas. This and other hypotheses regarding the Penokean orogen need to be investigated further in the third dimension of depth, which will require a comprehensive suite of geophysical studies. ?? 2007 Elsevier B.V. All rights reserved.

Precambrian Research↗