Geology topics

A.P. Nutman

Publications and source records attributed to A.P. Nutman.

4 recordsLinked to original sources

USGS1998

Caledonian eclogite-facies metamorphism of early Proterozoic protoliths from the North-East Greenland Eclogite Province

High-pressure metamorphic assemblages occur in mafic, ultramafic and a few intermediate rocks in a gneiss complex that covers an area of approximately 400 ?? 100 km in the North-East Greenland Caledonides. Detailed petrologic and geochronologic studies were carried out on three samples in order to clarify the P-T-t evolution of this eclogite province. Geothermobarometry yields temperature estimates of 700-800 ??C and pressure estimates of at least 1.5 GPa from an ecologite senu stricto and a high as 2.35 GPa for a garnet websterite. The eclogite defines a garnet-clinopyroxene-amphibole-whole rock Sm-Nd isochron age of 405 ?? 24 Ma (MSWSD 0.9). Isofacial garnet websterites define garnet-clinopyroxene-orthopyroxene-amphibole-whole rock-(biotite) ages of 439 ?? 8 Ma (MSWD = 2.1) for a coarse=grained sample and 370 ?? 12 Ma (MSWD = 0.6) for a finer-grained variety. Overgrowths on zircons from the fine-grained pyroxenite and the eclogite give a pooled 206Pb/238U SHRIMP age of 377 ?? 7 Ma (n = 4). Significantly younger Rb-Sr biotite ages of 357 ?? 8, 330 ?? 6 and 326 ?? 6 agree withyoung Rb-Sr, K-Ar and 40Ar/39Ar mineral ages from the gneiss complex and indicate slow cooling of the eclogitic rocks. High-pressure metamorphism may have been at least 439 Ma old (Siluro-Ordovician) with cooling through amphibolite-facies conditions in the Devonian and continued crustal thinning and exhumation well into the Carboniferous. Sm-Nd whole rock model ages indicate the eclogite protoliths are Early Proterozoic in age, while 207Pb/206Pb SHRIMP ages of 1889 ?? 8 from an-hedral zircon cores probably reflect Proterozoic metasomatism. The samples have negative ??Nd values (-5 to -16) and elevated 87Sr/86Sr ratios (0.708-0.715), consistent with field evidence that the eclogite protoliths were an integral part of the continental crust long before Caledonian metamorphism. The presence of a large Caledonian eclogite terrane in Greenland requires modification of current tectonic models that postulate subduction of Baltica beneath Laurentia during the Caledonian orogeny.

Contributions to Mineralogy and Petrology↗

USGS1998

Early Archean crust in the northern Wyoming province: Evidence from U-Pb ages of detrital zircons

U–Pb ages of individual detrital and metamorphic zircons from 12 Archean metasedimentary rocks, including quartzites, from the Beartooth, Ruby, and Tobacco Root uplifts of the northern Wyoming province indicate that they were deposited between 2.7 and 3.2 Ga. Younger, metamorphic zircons are found as overgrowths and new grains in some samples, and yield ages between 2.7 and 1.9 Ga. They are, however, much less abundant than detrital grains, which constitute >75% of the 355 grains analyzed. The majority of the detrital grains have ages between 3.2 and 3.4 Ga; none are younger than 2.9 Ga. Grains with 207 Pb/ 206 Pb ages between 3.4 and 4.0 Ga constituted approximately 15% of all grains with analyses within 10% of concordia, but are concentrated in samples from the eastern Beartooth Mountains. Comparison of the average of the Pb–Pb ages of individual zircons within 10% of concordia with previously published Lu–Hf chondritic model ages for some individual samples suggests that the age distribution recorded by the U–Pb system in these zircons has not been significantly disturbed by pre- or post-depositional Pb-loss. Collectively, these data suggest that the individual metasedimentary rocks did not completely share a common provenance and that a major crust-forming cycle occurred 3.2 to 3.4 Ga. In conjunction with previously published U–Th–Pb whole-rock data, these results suggest that rocks with a relatively high proportion of >3.4 Ga grains may have had crust of comparable age in their provenance.

Precambrian Research↗

USGS1996

Extended history of a 3.5 Ga trondhjemitic gneiss, Wyoming Province, USA: Evidence from U-Pb systematics in zircon

The Beartooth-Bighorn magmatic zone (BBMZ) and the Montana metasedimentary province (MMP) are two major subprovinces of the Archean Wyoming province. In the northwestern Beartooth Mountains, these subprovinces are separated by a structurally, lithologically and metamorphically complex assemblage of lithotectonic units that include: (1) a strongly deformed complex of trondhjemitic gneiss and interlayered amphibolites; and (2) an amphibolite facies mafic unit that occurs in a nappe that structurally overlies the gneiss complex. Zircons from a trondhjemitic blastomylonite in the gneiss complex yield concordant U-Pb ages of 3.5 Ga, establishing it as the oldest rock yet documented in the Wyoming province. Two younger events are also recorded by zircons in this rock: (1) an apparently protracted period of high-grade metamorphism and/or intrusion of additional magmas at ∼ 3.25 Ga; and (2) growth of hydrothermal zircon at ∼ 2.55 Ga, apparently associated with ductile deformation that immediately preceded structural emplacement of the gneiss. Although this latter event appears confined to areas along the BBMZ-MMP boundary, evidence of ∼ 3.25 Ga igneous activity is found in the overlying amphibolite (3.24 Ga) and throughout the MMP. These data suggest that this boundary first developed as a major intracratonic zone of displacement at or before 3.25 Ga. The limited occurrences of 2.8 Ga magmatic activity in the MMP suggest that it had a controlling influence on late Archean magmatism as well.

Precambrian Research↗

USGS1996

Contrasts between Sm-Nd whole-rock and U-Pb zircon systematics in the Tobacco Root batholith, Montana: Implications for the determination of crustal age provinces

Proper documentation of the extent and age of crust in the western US is critical for constraining a variety of geologic problems ranging from the growth rate of continents to Precambrian continental reconstructions. The secondary isotopic systematics of granitoids have been one of the principal means used to characterize continental crust in areas where the basement is covered. In southwestern Montana and eastern Idaho a group of Late Mesozoic to Cenozoic, dioritic to quartz monzonitic batholiths (e.g., Tobacco Root, Idaho, Pioneer, Boulder, etc.) share a limited range of Paleoproterozoic Sm-Nd depleted mantle model ages. The Tobacco Root batholith (TRB) has a Nd isotopic composition ( ϵ Nd = −17.9 to −19.1) and SmNd model age ( T DM = 1.63 to 1.90 Ga) typical of this group. The TRB, however, intruded Archean crust (∼3.3 Ga, ϵ Nd = ∼ −35), rather than the presumed Proterozoic crust intruded by the other plutons. The Archean heritage of the TRB is confirmed by the presence of premagmatic zircons which range from 2.2 to 3.0 Ga. The combination of U-Pb zircon and Nd model ages suggest that the batholith was derived from both Archean and Proterozoic crustal sources, as well as an ∼80 Ma mantle component. This contrasts with a sample from the northern Idaho batholith which exhibits concordancy between its Sm-Nd and premagmatic zircon systems at ∼1.74 Ga. These data point to the difficulties that can occur if crustal age provinces are defined solely on the basis of Nd model ages of younger plutons, particularly in areas such as the northwestern US where Archean and Proterozoic crust is poorly exposed and dispersed over a large area.

Idaho, Montana, Wyoming↗