Ar-40/Ar-39 Evidence for Middle Proterozoic (1300-1500 Ma) Slow Cooling of the Southern Black Hills, South Dakota, Midcontinent, North America: Implications for Early Proterozoic P-T Evolution and Posttectonic Magmatism

Ar-40/Ar-39 total gas and plateau dates from muscovite and biotite in the southern Black Hills, South Dakota, provide evidence for a period of Middle Proterozoic slow cooling. Early Proterozoic (1600-1650 Ma) mica dates were obtained from metasedimentary rocks located in a synformal structure between the Harney Peak and Bear Mountain domes and also south of Bear Mountain. Metamorphic rocks from the dome areas and undeformed samples of the similar to 1710 Ma Harney Peak Granite (HPG) yield Middle Proterozoic mica dates (similar to 1270-1500 Ma). Two samples collected between the synform and Bear Mountain dome yield intermediate total gas mica dates of similar to 1550 Ma. We suggest two end-member interpretations to explain the map pattern of cooling ages: (1) subhorizontal slow cooling of an area which exhibits variation in mica Ar retention intervals or (2) mild folding of a Middle Proterozoic (similar to 1500 Ma) similar to 300 degrees C isotherm. According to the second interpretation, the preservation of older dates between the domes may reflect reactivation of a preexisting synformal structure (and downwarping of relatively cold rocks) during a period of approximately east-west contraction and slow uplift during the Middle Proterozoic. The mica data, together with hornblende data from the Black Hills published elsewhere, indicate that the ambient country-rock temperature at the 3-4 kbar depth of emplacement of the HPG was between 350 degrees C and 500 degrees C, suggesting that the average upper crustal geothermal gradient was 25 degrees-40 degrees C/km prior to intrusion. The thermochronologic data suggest HPG emplacement was followed by a similar to 200 m.y. period of stability and tectonic quiescence with little uplift. We propose that crust thickened during the Early Proterozoic was uplifted and erosionally(?) thinned prior to similar to 1710 Ma and that the HPG magma was emplaced into isostatically stable crust of relatively normal thickness. We speculate that uplift and crustal thinning prior to HPG intrusion was the result of differential thinning of the subcrustal lithosphere beneath the Black Hills. If so, this process would have also caused an increase in mantle heat flux across the Moho and triggered vapor-absent melting of biotite to produce the HPG magma. This scenario for posttectonic granite generation is supported, in part, by the fact that in the whole of the Black Hills, the HPG is spatially associated with the deepest exposed Early Proterozoic country rock.

The Role of Forest Soils in a Northern New England Effluent Management System

The Carrabassett Valley Sanitary District in Carrabassett Valley, Maine has utilized both a forest spray irrigation system and a Snowfluent™ system for the treatment of their wastewater effluent. This study was designed to evaluate potential changes in soil properties after approximately 20 years of treatment in the forested spray irrigation site and three years of treatment in the field Snowfluent™ site. In addition, grass yield and composition were evaluated on the field study sites. After treatment with effluent or Snowfluent™, soils showed an increase in soil exchangeable Ca, Mg, Na, and K, base saturation, and pH. While most constituents were higher in treated soils, available P was lower in treated soils compared to the controls. This difference was attributed to higher rates of P mineralization from soil organic matter due to an irrigation effect of the treatment, depleting available P pools despite the P addition with the treatment. Most of the differences due to treatment were greatest at the surface and diminished with depth. Depth patterns in soil properties mostly reflected the decreasing influence of organic matter and its decomposition products with depth as evidenced by significantly higher total C in the surface compared to lower horizons. There were decreasing concentrations of total N, and exchangeable or extractable Ca, Mg, Na, K, Mn, Zn, and P with depth. In addition, there was decreasing BS with depth, driven primarily by declining exchangeable Ca and Mg. Imgation with Snowfluent™ altered the chemical composition of the grass on the site. All element concentrations were significantly higher in the grass foliage except for Ca. The differences were attributed to the additional nutrients and moisture derived from the Snowfluent™. The use of forest spray imgation and Snowfluent™ as a wastewater treatment strategy appears to work well. The soil and vegetation were able to retain most of the applied nutrients, and do not appear to be moving toward saturation. Vegetation management may be a key tool for managing nutrient accumulation on the grass sites as the system ages.