SHRIMP-RG U-Pb isotopic systematics of zircon from the Angel Lake orthogneiss, East Humboldt Range, Nevada: Is this really Archean crust

Premo, Wayne R.; Castiñeiras, Pedro; Wooden, Joseph L.

doi:10.1130/ges00164.1

Cited by 26 publications

(67 citation statements)

References 19 publications

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“…Monazite in RM-4 and some zircon dates near 83 Ma may either be mixed ages (from beam overlap of more than one crystal zone) or suggest another possible episode of melt generation and crystallization ca. 83 Ma, an age of granite intrusion also reported from U-Pb zircon dating in the East Humboldt Range (McGrew et al, 2000;Premo et al, 2008Premo et al, , 2010). An age of 69 Ma for WRP02-7 and Rby111 does not match previously known events.…”

Section: Ages Of Crystallizationmentioning

confidence: 67%

“…The lack of dated inherited pre-Late Cretaceous cores in zircon in the pegmatitic granite contrasts with nonpegmatitic Paleogene and Cretaceous leucogranites and most other dated granitoids in the Ruby-East Humboldt Range complex, which typically include Proterozoic and/or Archean zircon cores or U-Pb evidence of inherited old zircon (Wright and Snoke, 1993;Premo et al, 2005Premo et al, , 2008. A sampling bias cannot explain the lack in the pegmatitic granite of pre-Cretaceous zircon cores either observed or age measured in our 73 geochronologic analyses and age estimated in another 49 probed zircon spots (Supplemental Table 1 [see footnote 1]).…”

Section: Lack Of Ancient Inherited Zircon Coresmentioning

confidence: 94%

“…The discordant older 206 Pb/ 238 U ages project distantly toward upper intercepts ages ca. 1.9 or 2.7 Ga, suggestive of possible inheritance from monazite comparable in age to Paleoproterozoic and Neoarchean zircon xenocrysts found in nonpegmatitic granitoids in the core complex (Wright and Snoke, 1993;Premo et al, 2005Premo et al, , 2008Premo et al, , 2010. Note that unlike the other dated members of the pegmatitic granite suite, samples Rby107 (zircon) and RM-27 (monazite) record Paleogene U-Pb dates and yielded no analyzed Cretaceous zircon or monazite.…”

Section: Pegmatitic Granite Gneiss In Lower Lamoille Canyon (Rby107 Amentioning

confidence: 95%

“…Relative proportions of the metamorphosed stratal rock types are similar to those in unmetamorphosed or weakly metamorphosed protoliths to which they are correlated in the southern Ruby Mountains and nearby ranges (Howard, 1971(Howard, , 2000Coats, 1987). Elsewhere in the core complex are older Neoproterozoic strata of the McCoy Creek Group, and possible (disputed) Neoarchean orthogneiss (Howard, 1971(Howard, , 1980(Howard, , 2000Snoke, 1980;Lush et al, 1988;McGrew et al, 2000;Howard and MacCready, 2004;Premo et al, 2008Premo et al, , 2010McGrew and Snoke, 2010).…”

Section: Introductionmentioning

confidence: 96%

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Gneissic pegmatitic leucogranite forms a dominant component (>600 km 3 ) of the midcrustal infrastructure of the RubyMountains-East Humboldt Range core complex (Nevada, USA), and was assembled and modifi ed episodically into a batholithic volume by myriad small intrusions from ca. 92 to 29 Ma. This injection complex consists of deformed sheets and other bodies emplaced syntectonically into a stratigraphic framework of marble, calc-silicate rocks, quartzite, schist, and other granitoids. Bodies of pegmatitic granite coalesce around host-rock remnants, which preserve relict or ghost stratigraphy, thrusts, and fold nappes. Intrusion infl ated but did not disrupt the host-rock structure. The pegmatitic granite increases proportionally downward from structurally high positions to the bottoms of 1-km-deep canyons where it constitutes 95%-100% of the rock. Zircon and monazite dated by U-Pb (sensitive high-resolution ion microprobe, SHRIMP) for this rock type cluster diffusely at ages near 92, 82(?), 69, 38, and 29 Ma, and indicate successive or rejuvenated igneous crystallization multiple times over long periods of the Late Cretaceous and the Paleogene. Initial partial melting of unexposed pelites may have generated granite forerunners, which were remobilized several times in partial melting events. Sources for the pegmatitic granite differed isotopically from sources of similar-aged interleaved equigranular granites. Dominant Late Cretaceous and fewer Paleogene ages recorded from some pegmatitic granite samples, and Paleogene-only ages from the two structurally deepest samples, together with varying zircon trace element contents, suggest several disparate ages of fi nal emplacement or remobilization of various small bodies. Folded sills that merge with dikes that cut the same folds suggest that there may have been in situ partial remobilization. The pegmatitic granite intrusions represent prolonged and recurrent generation, assembly, and partial melting modifi cation of a batholithic volume even while the regional tectonic environment varied dramatically from contractile thickening to extension and mafi c underplating.

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Section: Ages Of Crystallizationmentioning

confidence: 67%

Section: Lack Of Ancient Inherited Zircon Coresmentioning

confidence: 94%

Section: Pegmatitic Granite Gneiss In Lower Lamoille Canyon (Rby107 Amentioning

confidence: 95%

Section: Introductionmentioning

confidence: 96%

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“…Zircons were picked and mounted on 1 disks and then polished to reveal cross-sections. Analyses were performed at the SUMAC facility at Stanford University following the methods of Premo et al (2008). AS3 (Schmitz et al, 2003) and MAD were run as standards.…”

Section: Geochronologymentioning

confidence: 99%

High resolution tephra and U/Pb chronology of the 3.33–3.26Ga Mendon Formation, Barberton Greenstone Belt, South Africa

Decker

Byerly

Stiegler

et al. 2015

Precambrian Research

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a b s t r a c tThe Mendon Formation in the Barberton Greenstone Belt of South Africa marks the boundary between the Onverwacht and Fig Tree Groups. These groups are characterized by mafic to ultramafic volcanism and felsic volcanism with related epiclastic sedimentation, respectively. This transition marks the end of komatiitic volcanism in the Barberton Greenstone Belt and is accompanied by numerous impact-related spherule layers. This study characterizes the upper Mendon Formation texturally and geochemically over a wide areal extent and across structure and facies changes in an attempt to better understand the evolution of tectonic processes at this boundary. A suite of whole rock and handheld X-ray fluorescence analyses are presented in conjunction with textural information, stratigraphic relationships, and U/Pb ages to create a temporal and chemostratigraphic framework for the Mendon Formation. Local and regional stratigraphic variations, including absence of distinctive layers and variation in layer thickness, seen across the Mendon preclude ascription of a single stratigraphy that accurately describes the >1.2 km of section present in this formation. These variations indicate diachronous deposition of the Mendon Formation over a wide areal extent and into multiple basins or sub-basins by more than one magmatic source. 204 Pb-corrected 206 Pb/ 238 U and 207 Pb/ 235 U concordia model ages of 3279 ± 9.1 Ma and 3287.3 ± 2.9 Ma for two samples from upper portions of the Mendon Formation provide temporal context for deposition. Two samples from the basal 10 m of the Fig Tree Group, above the S2 spherule bed that marks the boundary between the Onverwacht and Fig Tree Groups, give model ages of 3267.8 ± 6.9 Ma and 3261 ± 18 Ma. These ages provide added constraints for the Onverwacht-Fig Tree boundary and confirm that the Weltevreden Formation is roughly age-correlative with the uppermost Mendon Formation.While the Mendon and Weltevreden Formations are in part age-correlative and have similar lithologies, they do not appear to be genetically related. The dominance of ultramafic volcanic rocks and the paucity of felsic volcanic and terrigenous sedimentary rocks within the Mendon and Weltevreden Formations indicate that the primary mode of crustal formation was likely plume-related magmatic accretion and not subduction. The relatively sharp transition within the Barberton Greenstone Belt from ultramafic volcanic sequences to more felsic volcanic and epiclastic sedimentary sequences is everywhere marked by impact-related spherule layers, which suggest that major impacts may have played a role in the evolution of early tectonics to more modern, subduction-related styles.

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