Precambrian crustal blocks in Minnesota: Neodymium isotope evidence from basement and metasedimentary rocks
No abstract available.
Minnesota↗
Geology topics
G. N. Hanson
Publications and source records attributed to G. N. Hanson.
High-precision UPb ages of metamorphic rutile: Application to the cooling history of high-grade terranes
Metamorphic rutiles occurring in granulite and upper amphibolite facies metapelitic rocks of the Archean Pikwitonei granulite domain (Manitoba) and the Proterozoic Adirondack terrane (New York) give concordant and near concordant U Pb ages. The Pb concentrations in rutile range from 2.85 to 168 ppm, U concentrations range from 10.9 to 390 ppm and the measured 206 Pb/ 204 Pb ratios range from 182 to 22,100 corresponding to 238 U/ 204 Pb ratios of 398–75,100. The proportions of radiogenic 208 Pb are very low, ranging from 0.0 to 6.9% of total radiogenic Pb. The habits of the rutile crystals range from stubby to acicular, the physical properties vary from opaque/black to transparent/reddish-brown. Separate batches of black and reddish-brown rutile grains from the same samples have similar U and Pb concentrations, Pb-isotope ratios, and yield the same U/Pb ages within analytical uncertainty. No correlation of U concentration and 206 Pb/ 204 Pb ratios with morphology or color of the rutiles was observed among the samples analyzed. Most rutiles yield concordant U Pb ages which are reproducible within analytical uncertainty, i.e. generally ±2 Ma . The U Pb ages for prograde rutile are younger than the time of peak metamorphism given by U Pb ages for garnet and zircon, and also younger than U Pb ages for sphene and monazite, and 40 Ar/ 39 Ar and K Ar ages for hornblende but older than 40 Ar/ 39 Ar and K Ar ages for biotite from the same area. This suggests that the rutile ages reflect cooling below closure temperatures. Within a single hand-specimen, and thus for an identical thermal history, larger rutile grains give older ages than do smaller grains. This suggests that volume diffusion is the most probable mechanism responsible for the ages being younger than the time of peak metamorphism. It also suggests that the dimensions for such diffusion are directly related to the dimensions of the rutile crystal and not to the dimensions of sub-grain domains, as is the case for Ar diffusion in hornblende and feldspar. The concordant U Pb ages as well as the similar ages over a large area suggest that rutile has a well-defined temperature of closure. At a cooling rate of about 0.5–1°C/Ma the closure temperature for U Pb diffusion in rutile is about 420°C for grains with a radius of 0.009–0.021 cm, and about 380°C for grains with a radius of 0.007–0.009 cm. U Pb rutile ages obtained on different grain sizes indicate a cooling rate of about 0.5°C/Ma for the Pikwitonei granulite domain at 2460-2300 Ma. U Pb ages for rutile thus provide high-precision ages for evaluating the cooling history of high-grade terranes and can be critical to the quantitative development of thermal models for crustal evolution.
Earth and Planetary Science Letters↗
Re-Os, Rb-Sr, and O isotopic systematics of the Archean Kolar schist belt, Karnataka, India
The Re-Os, Rb-Sr, and O isotopic compositions of mafic and ultramafic amphibolites, gold ores, and granitic gneisses of the circa 2700 Ma Kolar schist belt reveal at least two episodes of post-magmatic alteration that affected these systems. The Re-Os isotopic systematics of many of the rocks of the belt indicate that Os was introduced to the area via fluids that carried very radiogenic Os ( 187 Os 186 Os 2.4Ga > 39 "> 187 Os 186 Os 2.4Ga > 39 ). The source of the radiogenic Os was likely ancient crust. On an outcrop scale, this alteration is also characterized by relatively minor additions of excess 87 Sr and δ 18 O values higher than magmatic. The Rb-Sr systematics of most of these rocks are consistent with closed-system behavior since a period between 2700 and 2400 Ma ago, indicating that the alteration event likely occurred no later than the early Proterozoic. In contrast to this late Archean or early Proterozoic alteration, samples of several komatiitic amphibolites have very 187 Os-depleted compositions, indicating that open-system behavior also occurred at a much later time. This alteration may have been caused by surficial weathering or the interaction of the rocks with fluids bearing unradiogenic Os. Results suggest that the Re-Os system may have only limited utility for geochronologic applications in regions for which post-crystallization noble metal mineralization is evident (e.g., gold ores). In such regions, however, the system may have an important application in assessing the timing and the ultimate sources of noble metal additions.
Geochimica et Cosmochimica Acta↗
Nd, O and Sr isotopic constraints on the origin of Precambrian rocks, southern Black Hills, South Dakota
The Nd, O and Sr isotopic characteristics of Precambrian metasedimentary, metavolcanic and granitic rocks from the Black Hills of South Dakota are examined. Two late-Archean granites (2.5-2.6 Ga) have T dm ages of 3.05 and 3.30 Ga, suggesting that at least one of the granites was derived through the melting of significantly older crust. Early-Proterozoic metasedimentary rocks have T dm ages that range from 2.32 to 2.45 Ga. These model ages, in conjunction with probable stratigraphic ages ranging from 1.9 to 2.2 Ga, indicate that mantle-derived material was added to the continental crust of this region during the early-Proterozoic. Previous studies of the Harney Peak Granite complex have reported U-Pb and Rb-Sr ages of about 1.71 Ga and most granite samples examined in this study have Sr isotopic compositions consistent with that age. Two granite samples taken from the same sill, however, give two-point Rb-Sr and Sm-Nd ages of 2.08 ±0.08 and 2.20 ±0.20Ga (∑ 2200 Nd = −15.5), respectively. In addition, whole-rock and apatite samples of the spatially associated Tin Mountain pegmatite give a Sm-Nd isochron age of 2000 ±100 Ma (∑ 2200 Nd = −5.8 ±1.8). The Sm-Nd, O and Rb-Sr isotopic systematics of these granitic rocks have been complicated to some degree by both crystallization and post-crystallization processes, and the age of the pegmatite and parts of the Harney Peak Granite complex remain uncertain. Processes that probably complicated the isotopic systematics of these rocks include derivation from heterogeneous source material, assimilation, mixing of REE between granite and country rock during crystallization via a fluid phase and post-crystallization mobility of Sr. The Nd isotopic compositions of the pegmatite and the Harney Peak Granite indicate that they were not derived primarily from the exposed metasedimentary rocks.
Geochimica et Cosmochimica Acta↗