Direct dating of Adirondack massif anorthosite by U-Pb SHRIMP analysis of igneous zircon: Implications for AMCG complexes

2004 ◽  
Vol 116 (11-12) ◽  
pp. 1299-1317 ◽  
Author(s):  
J. M. McLelland ◽  
M. E. Bickford ◽  
B. M. Hill ◽  
C. C. Clechenko ◽  
J. W. Valley ◽  
...  
Keyword(s):  
Igneous Zircon ◽  
Massif Anorthosite

LATE MIOCENE XENOCRYST-BEARING LAVAS ON GREEN RIDGE, CENTRAL OREGON CASCADE RANGE: IMPLICATIONS FOR THE FORMATION OF PROTEROZOIC MASSIF ANORTHOSITE

2020 ◽  
Author(s):  
Richard M. Conrey ◽  
◽  
David G. Bailey ◽  
Drew Castronovo ◽  
Martin J. Streck ◽  
...  
Keyword(s):  
Late Miocene ◽  
Cascade Range ◽  
Massif Anorthosite

NEW U-PB DETRITAL AND IGNEOUS ZIRCON GEOCHRONOLOGICAL RESULTS FROM THE ST. CROIX TERRANE AND ISLESBORO BLOCK, PENOBSCOT BAY, MAINE

2019 ◽  
Author(s):  
David B. Cavagnaro ◽  
◽  
Justin V. Strauss ◽  
Douglas N. Reusch ◽  
John W.F. Waldron ◽  
...  
Keyword(s):  
Igneous Zircon ◽  
Penobscot Bay

scholarly journals Secondary magnetite in ancient zircon precludes analysis of a Hadean geodynamo

2018 ◽  
Vol 116 (2) ◽  
pp. 407-412 ◽  
Author(s):  
Fengzai Tang ◽  
Richard J. M. Taylor ◽  
Josh F. Einsle ◽  
Cauê S. Borlina ◽  
Roger R. Fu ◽  
...  
Keyword(s):  
Electron Microscopy ◽  
Transmission Electron Microscopy ◽  
Remanent Magnetization ◽  
Diffusion Mechanism ◽  
Aqueous Fluid ◽  
Pipe Diffusion ◽  
Transmission Electron ◽  
Igneous Zircon ◽  
Jack Hills ◽  
Magnetic Inclusions

Zircon crystals from the Jack Hills, Western Australia, are one of the few surviving mineralogical records of Earth’s first 500 million years and have been proposed to contain a paleomagnetic record of the Hadean geodynamo. A prerequisite for the preservation of Hadean magnetization is the presence of primary magnetic inclusions within pristine igneous zircon. To date no images of the magnetic recorders within ancient zircon have been presented. Here we use high-resolution transmission electron microscopy to demonstrate that all observed inclusions are secondary features formed via two distinct mechanisms. Magnetite is produced via a pipe-diffusion mechanism whereby iron diffuses into radiation-damaged zircon along the cores of dislocations and is precipitated inside nanopores and also during low-temperature recrystallization of radiation-damaged zircon in the presence of an aqueous fluid. Although these magnetites can be recognized as secondary using transmission electron microscopy, they otherwise occur in regions that are indistinguishable from pristine igneous zircon and carry remanent magnetization that postdates the crystallization age by at least several hundred million years. Without microscopic evidence ruling out secondary magnetite, the paleomagnetic case for a Hadean–Eoarchean geodynamo cannot yet been made.

scholarly journals U–Pb zircon geochronology and depositional history of the Montresor group, Rae Province, Nunavut, Canada

2017 ◽  
Vol 54 (5) ◽  
pp. 512-528 ◽  
Author(s):  
John A. Percival ◽  
William J. Davis ◽  
Michael A. Hamilton
Keyword(s):  
Detrital Zircon ◽  
Time Window ◽  
Depositional History ◽  
Tectonic Events ◽  
Density Peaks ◽  
Minimum Age ◽  
History Of ◽  
Igneous Zircon ◽  
Definition Of ◽  
Age Constraints

Paleoproterozoic metasedimentary successions of the northwestern Canadian Shield provide records of tectonic events, but the definition of depositional ages has proved elusive. Although previously poorly understood, the Montresor belt of western Nunavut yields new insight into the 2.2–1.8 Ga time window. On the basis of U–Pb analyses of detrital zircon in sedimentary rocks and igneous zircon in sills, we conclude that arenite of the lower Montresor group was deposited between 2.194 and 2.045 Ga, and arkose of the upper Montresor group after 1.924 Ga, adding constraints on the Rae cover sequence. The lower Montresor arenite yielded an older group (3.05–2.58 Ga) and a younger, more tightly constrained group (2.194 ± 0.014 Ga). Four of six zircon grains analyzed from a gabbro sill within the lower Montresor have discordant 207Pb/206Pb ages (2.71, 2.66, 2.53, and 2.39 Ga) and are considered to be inherited, whereas two grains provide an age of 2045 ± 13 Ma, interpreted to date crystallization and providing a minimum age for the lower Montresor package. Upper Montresor arkose contains detrital zircon with probability density peaks at 2.55–2.25 and 2.1–1.92 Ga, together with scattered older grains (3.8–2.65 Ga). The youngest grain yields an age of 1924 ± 6 Ma, establishing a maximum age for sandstone deposition. Provenance is inferred to have been from the west, where igneous sources of 2.5–2.3 Ga (Queen Maud block) and 2.03–1.89 Ga (Thelon orogen) are known. Collectively, the new ages suggest a minimum 120 million year gap between deposition of the pre-2045 ± 13 Ma lower and post-1924 ± 6 Ma upper parts of the Montresor group. Similar age constraints may apply to other parts of the Rae cover sequence.

40Ar/39Ar chronology of the Nain anorthosites, Canada

1993 ◽  
Vol 30 (6) ◽  
pp. 1166-1178 ◽  
Author(s):  
Yang Yu ◽  
S. A. Morse
Keyword(s):  
Early Stage ◽  
Local Heating ◽  
Total Duration ◽  
Field Studies ◽  
Contact Aureole ◽  
Effusion Rate ◽  
Rifted Margins ◽  
Massif Anorthosite ◽  
Nain Plutonic Suite

A hornblende 40Ar/39Ar age of 1328 ± 8 Ma has been obtained from the contact aureole of the Bird Lake Massif, confirming it as one of the oldest members of the Nain plutonic suite. This age constrains the timing of the early stage magma activity of the Nain plutonic suite.Plagioclase samples from the intrusions yield either U-shaped, or staircase-shaped 40Ar/39Ar age spectra, and most of them have two isochrons of similar age but greatly different initial argon composition. Most plagioclase closure ages are >95% of either the hornblende age from a contact aureole or coexisting biotite ages, and appear to reflect single cooling events for each intrusion. Two major stages of emplacement for the Nain anorthosites are identified: an older anorthosite event and a main anorthosite event, in agreement with results from earlier field studies. The Port Manvers Run, Paul Island, and Nukasorsuktokh Island intrusions yield much younger plagioclase ages than any previously established intrusion ages, probably reflecting previously unknown later, local heating events.The total duration of the major anorthositic magma activity in the Nain region is about 23 Ma, estimated from the Bird Lake massif margin at 1328 ± 8 Ma and the Kiglapait intrusion at 1305 ± 2 Ma (Sm/Nd, U/Pb), implying an emplacement rate of about 0.0022 km3/year. This rate is far less than the effusion rate of magma on some of the Phanerozoic rifted margins and supports the "aborted rifting" model for the generation of massif anorthosite.

Almandine garnet phenocrysts in a ~1 Ga rhyolitic tuff from central India

2008 ◽  
Vol 146 (1) ◽  
pp. 133-143 ◽  
Author(s):  
SARBANI PATRANABIS-DEB ◽  
JUERGEN SCHIEBER ◽  
ABHIJIT BASU
Keyword(s):  
Central India ◽  
Mineral Chemistry ◽  
Triple Junctions ◽  
Fine Grained ◽  
Mineral Inclusions ◽  
Major Oxide ◽  
Shale Formation ◽  
Igneous Zircon ◽  
Compositional Variability ◽  
Lower Crustal

AbstractWe report on the newly discovered almandine garnet phenocrysts in rhyolitic ignimbrites (Sukhda Tuff) in the Precambrian Churtela Shale Formation of the Chhattisgarh Supergroup in central India. SHRIMP ages of igneous zircon from the ignimbrites range from 990 Ma to 1020 Ma. These ignimbrites exhibit characteristic eutaxitic texture with compacted curvilinear glass shards with triple junctions. Quartz (commonly embayed; bluish cathodoluminescence) and albite (altered but retaining ghosts of twinning) are common phenocrysts; others are apatite, ilmenite, rutile, magnetite, zircon, monazite and garnet. There are no metamorphic or granitic xenoliths in the ignimbrites. Garnet grains occur as isolated broken isotropic crystals with sharp or corroded boundaries in a very fine-grained groundmass of volcanic ash that consists principally of albite, quartz, magnetite and glass. They do not have any systematically distributed inclusions. A few have penetratively intergrown phenocrysts of apatite, ilmenite, rutile and zircon, which we interpret as subophitic texture. Extensive SEM-BSE imaging of more than 100 grains and electron microprobe traverses across about 30 grains showed no zoning or systematic compositional variability. Common (metamorphic) garnets are usually zoned with respect to Fe–Mg–Mn and typically have mineral inclusions. We infer, therefore, that these observed garnets are not metamorphic xenocrysts. The average major oxide composition of analysed garnets from five different horizons within the Sukhda Tuff, spanning approximately 300 m of the stratigraphic section, have very small standard deviation for each element, which is suggestive of a single magmatic source. Phenocrysts of quartz, including those in contact with coexisting garnets, show blue scanning electron CL, indicating rapid cooling from high temperature; this suggests that adjacent coexisting garnets are not slowly cooled restites. We conclude, on the basis of texture, mineral chemistry and absence of any indicative xenoliths or xenocrysts, that these almandine garnets (Al78.7Py12.3Gr7.4Sp1.6) are phenocrysts within the Sukhda Tuff. Almandine of such composition is stable under high pressure. We infer that almandine crystallized at lower crustal depths in a magma that ascended very rapidly and may have erupted explosively.

Crystallization history of a massif anorthosite in the eastern Indian shield margin based on borehole lithology

2005 ◽  
Vol 25 (1) ◽  
pp. 77-94 ◽  
Author(s):  
Dipankar Mukherjee ◽  
Naresh C. Ghose ◽  
Nilanjan Chatterjee
Keyword(s):  
Indian Shield ◽  
History Of ◽  
Massif Anorthosite
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