GEOCHEMISTRY OF PRECAMBRIAN CARBONATES .7. BELT SUPERGROUP, MONTANA AND IDAHO, USA

摘要

Carbonates from the similar to 1100-1450 Ma old Proterozoic Belt Supergroup were collected from stratigraphic sections throughout Montana and Idaho, USA. The sampled sequences, in ascending stratigraphic order, include the Newland, Altyn, Spokane/Greyson transition, Empire, Wallace, Helena, Siyeh, Snowslip, Shepard, and Libby formations. An increase in the degree of postdepositional alteration of Belt limestones is reflected in a diminution of Sr and Mg contents, an increase in Mn, and depletion in C-13 and O-18. Two diagenetic trends can be resolved for the limestones. One, affecting the presumed originally aragonite-rich sediments, includes carbonates from the Lower Belt Newland Formation. In contrast, the Middle Belt Carbonate (Wallace, Helena, Siyeh formations) may have been originally of high-Mg calcitic mineralogy. Projection of the alteration trends for the Lower and Middle Belt limestones suggest similar to 21 parts per thousand SMOW and similar to +2.5 parts per thousand to +1.0 parts per thousand PDB as the best preserved values for the delta(18)O and delta(13)C of seawater, respectively; both comparable to results from other Mesoproterozoic carbonate sequences. The oxygen isotope data for limestones show a regional westward depletion of similar to 8 parts per thousand in O-18, possibly reflecting a higher temperature of postdepositional alteration in the western Belt basin. This depletion in O-18 is accompanied by a comparable decrease in delta(13)C values, most likely because a higher proportion of carbon was incorporated from CO2 generated by thermal cracking of hydrocarbons at depth. Dolostones in the Belt basin are dominantly micritic, with good preservation of depositional textures. Chemically and isotopically, their alteration trends mimic those of limestones, leading to comparable projected ''best'' values for delta(13)C. Sr-87/Sr-86 values of Belt carbonates range between 0.70484 and 0.74991. Progressive diagenesis, as indexed by decreasing concentrations of Sr and depletions in O-18 and C-13, results in an increase in Sr-87/Sr-86 values. The least radiogenic measurement, from a Lower Belt Newland limestone sample, appears to fit reasonably well into the general trend for Proterozoic seawater, as discussed in Mirota and Veizer (1994). This and the consistency of delta(18)O and delta(13)C in limestones with other coeval sequences suggests that the (Lower and Middle) Belt carbonate sections are marine, deposited in an environment that was not continuously separated from the open ocean. [References: 82]