U–Pb Ages for Sphene from Early Precambrian Igneous Rocks in Northeastern Minnesota – Northwestern Ontario

1971 ◽  
Vol 8 (10) ◽  
pp. 1319-1324 ◽  
Author(s):  
E. J. Catanzaro ◽  
G. N. Hanson
Keyword(s):  
Hydrofluoric Acid ◽  
Igneous Rocks ◽  
Granitic Rocks ◽  
Low Grade ◽  
Early Precambrian ◽  
Grade Metamorphism ◽  
Northwestern Ontario ◽  
Good Agreement

Five sphene concentrates from early Precambrian igneous granitic rocks in northeastern Minnesota and northwestern Ontario give 207Pb–206Pb ages of 2680–2750 m.y., which are in good agreement with discordant U–Pb ages for zircon and whole-rock Rb–Sr ages (λβ = 1.39 × 10−11y−1) from these rocks. Three of the sphene concentrates give concordant ages, two discordant.Although the rocks have undergone at least one low-grade metamorphism, which has affected some of the K–Ar and Rb–Sr mineral ages from the region, the 207pb–206pb ages for sphene do not appear to have been affected.An electroplating technique was employed for recovering the lead from the sphenes after decomposition in perchloric and hydrofluoric acid.

A Rb–Sr isotopic study of the Archean rocks of the eastern Lac Seul region, English River subprovince, northwestern Ontario

1980 ◽  
Vol 17 (5) ◽  
pp. 569-576 ◽  
Author(s):  
Joseph L. Wooden ◽  
Alan M. Goodwin
Keyword(s):  
Source Region ◽  
Granitic Rocks ◽  
Isotopic Study ◽  
Zircon Age ◽  
Crustal Rocks ◽  
Northwestern Ontario ◽  
Plutonic Complex ◽  
History Of ◽  
Lower Crustal ◽  
Good Agreement

Rb–Sr whole-rock data for the gneissic and granitic rocks of the eastern Lac Seul region, when combined with the U–Pb zircon dating of Krogh, document a history of multiple intrusion for the area. The oldest rocks are the Sen Bay plutonic complex gneisses which have complex Rb–Sr systematics. Interpretation of the Rb–Sr data yields model ages of 3000–3100 Ma which are in good agreement with a zircon age of 3040 Ma. The next oldest rocks are trondhjemitic–granodioritic gneisses with a Rb–Sr age of 2780 ± 90 Ma. The initial Sr ratio (I) of 0.7009 ± 4 for these rocks suggests that this age approximates the time of intrusion and that the magma was derived from lower crustal rocks with a very short residence lime in the crust. Following a period of deformation and metamorphism, granodioritic to granitic dikes, sills, and small plutons were intruded between 2660 and 2560 Ma ago. I values for these racks range from 0.7019–0.7027. If the I values of these rocks represent the source region for the granitic magmas, then one explanation for the I values would be that the magmas were derived from a source region of mixed lithology and age. The Sen Bay plutonic complex is considered to represent an earlier cycle of crustal formation which is distinct from a later 2800–2550 Ma old cycle which dominates much of the Superior Province.

scholarly journals The ammonium content in the Malayer igneous and metamorphic rocks (Sanandaj-Sirjan Zone, Western Iran)

2011 ◽  
Vol 62 (2) ◽  
pp. 171-180 ◽  
Author(s):  
Vahid Ahadnejad ◽  
Ann Hirt ◽  
Mohammad-Vali Valizadeh ◽  
Saeed Bokani
Keyword(s):  
Colorimetric Method ◽  
Igneous Rocks ◽  
Granitic Rocks ◽  
Metamorphic Rocks ◽  
Low Grade ◽  
High Concentration ◽  
Western Iran ◽  
Metasedimentary Rocks ◽  
Sanandaj Sirjan Zone ◽  
External Sources

The ammonium content in the Malayer igneous and metamorphic rocks (Sanandaj-Sirjan Zone, Western Iran)The ammonium (NH4+) contents of the Malayer area (Western Iran) have been determined by using the colorimetric method on 26 samples from igneous and metamorphic rocks. This is the first analysis of the ammonium contents of Iranian metamorphic and igneous rocks. The average ammonium content of metamorphic rocks decreases from low-grade to high-grade metamorphic rocks (in ppm): slate 580, phyllite 515, andalusite schist 242. In the case of igneous rocks, it decreases from felsic to mafic igneous types (in ppm): granites 39, monzonite 20, diorite 17, gabbro 10. Altered granitic rocks show enrichment in NH4+(mean 61 ppm). The high concentration of ammonium in Malayer granites may indicate metasedimentary rocks as protoliths rather than meta-igneous rocks. These granitic rocks (S-types) have high K-bearing rock-forming minerals such as biotite, muscovite and K-feldspar which their potassium could substitute with ammonium. In addition, the high ammonium content of metasediments is probably due to inheritance of nitrogen from organic matter in the original sediments. The hydrothermally altered samples of granitic rocks show highly enrichment of ammonium suggesting external sources which intruded additional content by either interaction with metasedimentary country rocks or meteoritic solutions.

scholarly journals Eoalpine (Cretaceous) very low- to low-grade metamorphism recorded on the illite-muscovite-rich fraction of metasediments from South Tisia (eastern Mt Papuk, Croatia)

2010 ◽  
Vol 61 (6) ◽  
pp. 469-481 ◽  
Author(s):  
Vanja Biševac ◽  
Kadosa Balogh ◽  
Dražen Balen ◽  
Darko Tibljaš
Keyword(s):  
Pannonian Basin ◽  
Regional Metamorphism ◽  
Eastern Alps ◽  
Low Grade ◽  
Metamorphic Complex ◽  
Grade Metamorphism ◽  
Sedimentary Sequences ◽  
Surrounding Areas ◽  
Temperature Time ◽  
Good Agreement

Eoalpine (Cretaceous) very low- to low-grade metamorphism recorded on the illite-muscovite-rich fraction of metasediments from South Tisia (eastern Mt Papuk, Croatia)Eoalpine very low- to low-grade metamorphism related to Cretaceous orogenesis has been investigated in the Slavonian Mts, Croatia. Samples belonging to the Psunj metamorphic complex (PMC), the Radlovac metamorphic complex (RMC) and Permian-Triassic and Triassic sedimentary sequences (PTSS) were studied. The Kübler and Árkai indices of all the analysed samples indicate high-anchizonal to epizonal metamorphism. The degree of Eoalpine metamorphism tends to be constant in all samples implying that the different complexes passed through and recorded the same event. Measurements of illite-white K-micab0-parameter of the RMC samples imply transitional low- to medium-pressure character of the metamorphism. These data together with K-Ar ages (~100-80 Ma) measured on illite-white K-mica rich < 2 μm grain-size fractions point to Late Cretaceous very low- to low-grade regional metamorphism presumably related to the main nappe-forming compressional events in the Pannonian Basin and the Carpathians. The P-T-t (pressure-temperature-time) evolution of the studied area is in good agreement with similar scenarios in the surrounding areas of Tisia, but also in Eastern Alps, Carpathians and Pannonian Basin (ALCAPA).

Late Hadrynian and Helikian pole positions from the Spokane Formation, Montana

1977 ◽  
Vol 14 (1) ◽  
pp. 67-73 ◽  
Author(s):  
Icaro Vitorello ◽  
Rob Van der Voo
Keyword(s):  
Heat Flow ◽  
Geomagnetic Field ◽  
Pole Position ◽  
The Other ◽  
Low Grade ◽  
Thermal Event ◽  
Thermal Demagnetization ◽  
Grade Metamorphism ◽  
Laramide Orogeny ◽  
Good Agreement

The natural remanent magnetizations of Beltian argillites from the Spokane Formation (formerly Grinnell Formation) have been analyzed with alternating field and thermal demagnetization techniques. Two groups of magnetic directions have been observed. One is assumed to be the primary magnetization, representative of the Helikian (approx. 1300 Ma) geomagnetic field: D = 201.5°; I = +40°; k = 61; and α95 = 5.1° for N = 14 sites (51 samples), yielding a pole position at 15.5 °S. 134.5 °W, dp = 3.7°, dm = 6.2°. Comparisons with contemporaneous pole positions reveal good agreement. The other direction of magnetization corresponds to a Late Hadrynian age (approx. 600 to 750 Ma) and resulted from low-grade metamorphism. It is speculated that this thermal event resulted from enhanced heat flow during continental breakup in the Late Hadrynian. This direction (D = 267.5°; I = +17; k = 10; α95 = 9.1° for N = 28 specimens) yields a pole position at 5°N, 163.5°E, with dp = 4.9°, dm = 9.4°. Both groups of directions of magnetization are pre-folding, which occurred during the Laramide orogeny. Laramide deformation was only moderate, however, and the eastward thrusting of the Beltian sediments over the Lewis thrust was probably not accompanied by significant structural rotation.

scholarly journals Evolution of the Palaeozoic basement of the North Patagonian Andes in the San Martín de los Andes area (Neuquén, Argentina): petrology, age and correlations

2018 ◽  
Vol 46 (1) ◽  
pp. 102 ◽  
Author(s):  
Samanta Serra-Varela ◽  
Pablo D. González ◽  
Raúl E. Giacosa ◽  
Nemesio Heredia ◽  
David Pedreira ◽  
...  
Keyword(s):  
Partial Melting ◽  
Melting Process ◽  
Shear Zones ◽  
Igneous Rocks ◽  
Low Grade ◽  
Metamorphic Evolution ◽  
Grade Metamorphism ◽  
The North ◽  
Plutonic Rocks ◽  
San Martín

In San Martín de los Andes area (Argentinian Patagonia) the Palaeozoic basement consists of metamorphic and plutonic rocks. The metamorphic rocks include strongly deformed schists, gneisses and migmatites. Their geochemical and petrographic characteristics suggest that the protholith could have been a sequence of pelites and greywackes. Detrital zircon analysis (U-Pb Q-ICP-LA-MS) yielded a maximum depositional age of 501±14 Ma (Series 3 Cambrian) for this sedimentary protolith. Metasedimentary rocks are affected by a regional foliation defined by the minerals of the metamorphic peak. This is a S2 foliation, since relics of a former foliation are present in some samples. This regional foliation is locally affected by open folds that develop an incipient crenulation cleavage (S3). The high-grade metamorphism includes partial melting processes, where the incipient segregates intrude parallel to the regional foliation and also cut it in presence of abundant melt. Zircons from anatectic granites formed during this partial melting process yielded a U-Pb Concordia age of 434.1±4.5 Ma (Llandovery-Wenlock, Silurian). The age of maximum sedimentation and the anatectic age constrain the metamorphic evolution of the basement into the lower Palaeozoic (between upper Cambrian and lower Silurian). The igneous rocks of the basement are granodiorites, tonalities, and some gabbros that cut the metamorphic basement and contain xenoliths and roof pendants from the country rocks. These plutonic rocks are affected by low-grade metamorphism, with the development of discrete, centimetric to hectometric, brittle-ductile shear zones. The age of these igneous rocks has been constrained through U-Pb zircons analysis, carried out by several authors between ca. 370 and 400 Ma (Devonian). The maximum sedimentation age for the protolith and its metamorphic evolution seems to be related to an early Palaeozoic orogenic event, probably the Patagonian Famatinian orogeny. In contrast, the Devonian igneous rocks of San Martín de los Andes could represent a Devonian magmatic arc, related to subduction processes developed at the beginning of the Gondwanan orogenic cycle, which culminates with the Gondwanan orogeny.

Low-grade metamorphism and accretion tectonics: Southern Uplands terrain, Scotland

1985 ◽  
Vol 49 (352) ◽  
pp. 335-344 ◽  
Author(s):  
A. E. S. Kemp ◽  
G. H. J. Oliver ◽  
J. R. Baldwin
Keyword(s):  
Subduction Zones ◽  
Structural Data ◽  
Low Grade ◽  
Structural Level ◽  
Concomitant Increase ◽  
Illite Crystallinity ◽  
Grade Metamorphism ◽  
Trench Slope ◽  
Reflectance Data ◽  
Trajectory Characteristic

AbstractPrevious studies of low-grade metamorphism in the Southern Uplands accretionary terrain indicated prehnite-pumpellyite facies/anchizone conditions developed throughout the area, except for local preservation of trench-slope sediments and an accreted seamount at zeolite facies/advanced diagenetic grade. New graptolite reflectance data are presented that show a general northward increase in temperature in the Southern Uplands. The results from two cross-strike traverses in the southern and central belts in contemporaneous sequences, using illite crystallinity, illite lateral spacing (bo) , and graptolite reflectance, indicate the development of systematic accretion-related low-grade metamorphism. Well-developed and constant anchizone conditions occur throughout the NE (Langholm) traverse, associated with common, F1 accretion-related folding and a regionally penetrative S1 cleavage. In the SW (Kirkcudbright) traverse, however, the youngest, last accreted packets are preserved at a transitional diagenetic stage and lack a penetrative S1 cleavage. Illite crystallinity, graptolite reflectance, and bo increase systematically northward through earlier accreted packets, reaching values of the NE traverse only at the northern end. The concomitant increase of bo with illite crystallinity suggests the relatively high P-low T trajectory characteristic of subduction zones. Integration of metamorphic and structural data relates increasing intensity of aceretion-related F1 folding, developmertt of S1 fabric, and onset of later fold phases to grade of metamorphism and structural level within the accretionary pile.

Neodymium and strontium isotopic dating of diagenesis and low-grade metamorphism of argillaceous sediments

1994 ◽  
Vol 58 (5) ◽  
pp. 1471-1481 ◽  
Author(s):  
Urs Schaltegger ◽  
Peter Stille ◽  
Naoual Rais ◽  
Alain Piqué ◽  
Norbert Clauer
Keyword(s):  
Low Grade ◽  
Isotopic Dating ◽  
Grade Metamorphism
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