John F. Sutter

Interpretation of discordant 40Ar/39Ar age-spectra of mesozoic tholeiites from antarctica

Conventional K-Ar ages of tholeiitic basalts of the Ferrar Group in the central Transantarctic Mountains indicate significant loss of radiogenic 40Ar from this unit over much of its outcrop area. Argon loss varies inversely with amount of devitrified matrix in the basalts, which have not been thermally or tectonically disturbed since extrusion. 40Ar/19Ar age-spectra of these tholeiites are generally discordant and indicate significant inhomogeneity in the distribution of radiogenic 40Ar with respect to 39Ar, but are distinctly different from release patterns of thermally disturbed samples. Amounts of argon redistribution vary directly with amounts of devitrification and are reflected in progressive modification of the age spectra. A model of redistribution of radiogenic 40Ar by devitrification of originally glassy matrix is suggested that is consistent with disturbance of the conventional K-Ar systematics as well as the 40Ar/39Ar age-spectra. Samples with substantial redistribution but minor loss of radiogenic argon yield age spectra whose apparent ages decrease from low-temperature to high-temperature steps, similar to those reported for some lunar basalts, breccias, and soils. Modification of all the age spectra is attributed to redistribution of radiogenic 40Ar during progressive devitrification, although 39Ar-recoil effects suggested by Turner and Cadogan (1974) may be a factor in some cases. Where devitrification involves most potassium sites within the basalt, 40Ar/39Ar age-plateaux may be formed that have no geologic significance.

Botanical and Geological Significance of Potassium-Argon Dates from the Juan Fernández Islands

Potassium-argon dating of five basalts from the three main islands of the Juan Fern�ndez (or Robinson Crusoe) Islands of Chile in the southeastern Pacific gives ages of 1.01 � 0.12 and 2.44 � 0.14 million years for Masafuera, 3.79 � 0.20 and 4.23 � 0.16 for Masatierra, and 5.8 � 2.1 for Santa Clara. These ages are much younger than that of the underlying oceanic plate and are consistent with the origin of the island-seamount chain from a mantle hot spot beneath the eastward moving Nazca plate. The young age for the archipelago suggests that speciation within endemic genera has occurred within the past 4 to 5 million years. Endemic genera of apparently more ancient origins, such as Lactoris and Thyrsopteris, have apparently dispersed to the islands and survive refugially.

Ductile strain and metamorphism in an extensional tectonic setting: a case study from the northern Snake Range, Nevada, USA

Summary In recent years considerable attention has focused on metamorphic core complexes of the Basin and Range Province of the western US Cordillera. These highly extended areas are characterized by an upper plate that has been brittlely attenuated by normal faults separated by a sub-horizontal detachment surface from a lower plate that has been ductilely thinned and stretched. A study of mesoscopic structures, finite strain, microstructures, quartz c-axis fabrics and 40Ar/39Ar geochronology was undertaken in order to characterize the nature, geometry, kinematic history and timing of ductile extension in the lower plate of the northern Snake Range metamorphic core complex in E-central Nevada. These data provide new insights into the processes of deep-seated ductile strain beneath supracrustal normal fault mosaics in highly extended regions. Mesoscopic structures and finite strain analyses indicate that the lower plate underwent plane strain with a sub-vertical Z-axis and a sub-horizontal WNW-ESE X-axis. The magnitude of strain increases dramatically from W-to-E, and is constant vertically. Bedding and foliation are everywhere parallel and bedding is thinned 30–90%. The nature and geometry of microstructures and c-axis fabrics changes progressively from W-to-E. Lower strain rocks on the W flank of the range are characterized by a single bedding-parallel foliation, defined by flattened detrital grains, globular grains which have their c-axes parallel to the Z-axis and symmetrical cross-girdle c-axis fabrics. Further E to slightly higher strains, the rocks are characterized by asymmetrical cross-girdle c-axis fabrics, although globular grains with their c-axes parallel to Z are still present. The high-strain rocks on the E flank of the range are characterized by C-planes, asymmetrical mica ‘fish’ and oblique quartz foliations, and asymmetrical single-girdle c-axis fabrics. These data suggest that the lower plate has not deformed entirely by either coaxial or non-coaxial strain, and we propose an evolutionary model whereby the lower plate deformed during an early period of coaxial strain followed by a later component of non-coaxial strain accompanied by coaxial strain on the E flank of the range. 40Ar/39Ar geochronology on lower-plate hornblendes and micas indicates that temperatures of deformation were >280°C, but <530°C, and increased with depth. Geochronological data suggest that lower-plate ductile strain is post-latest Cretaceous, and biotite and muscovite data indicate that ductile deformation was probably ongoing 22–26 Ma. Our data and regional geological relations suggest that ductile extensional deformation in the northern Snake Range occurred because of localized heat input to shallow levels of the crust, and together with seismic reflection data, indicate that lower-plate rocks probably represent the top of a regional metamorphic terrane of Tertiary age.

Early Miocene Subglacial Basalts, the East Antarctic Ice Sheet, and Uplift of the Transantarctic Mountains

Subglacially erupted volcanic rocks from Mount Early and Sheridan Bluff, Antarctica, yield whole-rock potassium-argon dates and argon-40/argon-39 release spectra of Early Miocene age. Field associations suggest the existence of the East Antarctic ice sheet and significant uplift of the Transantarctic Mountains by that time.

Episodic Volcanism in the Central Oregon Cascade Range

Radiometric dating of late Cenozoic igneous rocks of the central Oregon Cascade Range has shown that volcanic activity has been strongly episodic, with most lavas erupting during short intervals of 1 or 2 m.y. The strongest activity occurred during middle Miocene time concurrently with eruption of the Columbia River Basalt. Subsequent episodes followed at about 5-m.y. intervals. Preliminary data from other parts of the circum-Pacific area suggest that these same episodes occurred in unison over a large part of the Earth.

U-Th-Pb, Rb-Sr, and Ar-Ar mineral and whole-rock isotopic systematics in a metamorphosed granitic terrane, southeastern California

Mesozoic structural domes are developed in an older Proterozoic crystalline basement of granitic to granodioritic foliate metaplutonic rocks in the Halloran Hills, southeastern California. Isotopic analyses of whole rocks and mineral separates from these rocks by U-Th-Pb, Rb-Sr, and Ar-Ar techniques yield a complex pattern of discordance that is the result of a fairly simple geologic history. Individual mineral isotopic systems have variably equilibrated with each other in response to Mesozoic regional metamorphism and locally to later heating during Mesozoic batholith emplacement. Discordant U-Th-Pb zircon data indicate that the granitic core rocks are 1,710 Ma and that one dioritic phase may be slightly older. Rb-Sr whole-rock model dates scatter about 1,700 Ma Rb-Sr amphibole–whole-rock and U-Th-Pb amphibole dates are also Proterozoic. Potassium feldspars retain a 207 Pb/ 206 Pb signature of their Proterozoic age. Ar-Ar amphibole spectra from the flank of the main dome reveal disturbed dates of 1,450 Ma to 1,100 Ma, and the dates become younger toward the structurally deeper core of the dome. All remaining isotopic determinations yield Mesozoic or younger dates for mineral–whole-rock systems. Rb-Sr whole-rock–apatite–feldspar–biotite analyses show nonequilibration of strontium isotopes, with resultant mineral pair dates from 4 foliate plutonic rocks ranging from 200 to 50 Ma. No single metamorphic age is indicated by the Rb-Sr data. Rb-Sr whole-rock–biotite dates are consistently younger than any other determinations and may be reduced by weathering or gain of nonradiogenic strontium from ground water. U-Pb sphene and apatite analyses from rocks that yield 1,710-Ma zircon dates are nearly concordant at 140 Ma. An amphibole from the structurally deepest rocks of the main dome that yield 140- to 150-Ma U-Pb sphene dates has an Ar-Ar plateau date of 144 Ma. The U-Pb sphene and Ar-Ar amphibole analyses are believed to be the best age estimate for the end of the highest-temperature phase of regional metamorphism. Th-Pb sphene and apatite dates and Ar-Ar biotite dates cluster at 90 ± 5 Ma as a consequence of regional cooling during Late Cretaceous time following extensive Mesozoic plutonism in the region at 97 to 90 Ma. We interpret the discordant mineral date patterns to have resulted from metamorphism of ∼1,700-Ma plutonic rocks during the Jurassic (≥ 140–50 Ma) and subsequent uplift and cooling to ∼200 °C at about 90 Ma. On the basis of this study, the isotope dating systems ranked in decreasing order of resistance to resetting are: U-Th-Pb zircon (concordia intercept) ≥ Rb-Sr whole rock ∼Rb-Sr amphibole ∼U-Th-Pb amphibole ∼Pb-Pb whole rock > Ar-Ar amphibole ≥ Rb-Sr sphene ≥ U-Pb sphene and apatite > Rb-Sr plagioclase-potassium feldspar-apatite > Th-Pb sphene and apatite ∼Ar-Ar biotite ∼U-Pb feldspars > Rb-Sr biotite.

40Ar-39Ar ages of some pre-Tertiary plutonic and metamorphic rocks of eastern Oregon and their geologic relationships

The “central melange” terrane of eastern Oregon, consisting mainly of an argillite-chert sequence and ophiolitic rocks, and the Seven Devils volcanic-arc terrane of northeastern Oregon and western Idaho probably formed in a volcanic island-arc and back-arc basin environment. New 40Ar-39Ar ages indicate that the ophiolitic rocks formed during the Permian and Late Triassic and that metamorphism synchronous with pervasive orogenic deformation occurred at the end of Triassic time. Accretion of the terrane to the North American continent in Late Jurassic time was accompanied by a second major orogenic event.

Incremental 40Ar/39Ar ages of biotite and hornblende from the northeastern Reading Prong: Their bearing on late Proterozoic thermal and tectonic history

K-Ar and 40 Ar- 39 Ar incremental-release ages have been determined for biotite and hornblende from Grenville basement gneiss units of the northeastern Reading Prong west of the Hudson River. Biotite ages range from 768 ± 15 to 819 ± 18 m.y. (average of 790 m.y.), and those for unaltered hornblende range from 869 ± 20 to 949 ± 24 m.y. (average of 900 m.y.). Individual biotite and hornblende release spectra show negligible variations in 40 Ar/ 39 Ar ratios, and total-gas and incremental-release ages are similar. These ages are discordantly younger than zircon ages of about 1,060 m.y. from this area; however, the consistent correlation of K-Ar, total-gas, and incremental-release ages argues against the discordancy resulting from partial argon loss during a distinct post-Grenville thermal event. The correlation suggests that the ages date times during post-Grenville metamorphic cooling when temperatures dropped below those required for argon retention in the minerals. The ages place constraints on post-metamorphic cooling history and allow development of a model for the late Proterozoic uplift of this segment of the Appalachian Grenville terrane.

Structural geology and 40Ar-39Ar geochronology of the Goldstone–Lane Mountain area, Mojave Desert, California

Metasedimentary rocks near Goldstone, California, consist in ascending structural order of (1) a calc-silicate hornfels unit; (2) a quartzite, schist, and radiolarian chert unit; and (3) a calcareous and pelitic sequence of rocks that contains minor basaltic or andesitic composition dikes, sills, and flows(?). Unit 1 is of unknown, but probably pre-Mesozoic age; unit 2 is lower Paleozoic, in part Ordovician, and is correlative to lower Paleozoic eugeoclinal rocks in the El Paso Mountains north of the Garlock fault; unit 3 is probably correlative to upper Paleozoic (Permian) basinal sequences in the El Paso Mountains. These rocks were strongly deformed during a progressive deformational event that caused isoclinal to tight folding about northwest-southeast–trending fold axes and northeast-dipping axial planes. Tectonic transport during this deformation was in a southwesterly direction. Similarity of structural trends to those in the El Paso Mountains suggests that the deformation in these two areas is the same age and predates intrusion of Permo-Triassic plutons in the El Paso Mountains. Regional relationships suggest that deep-water metasedimentary rocks in the northwestern Mojave Desert and in the El Paso Mountains are allochthonous with respect to miogeoclinal and cratonal Paleozoic rocks elsewhere in the Mojave. The emplacement of the allochthonous rocks postdates deposition of Permian strata and predates Permo-Triassic to earliest Triassic deformation and plutonism. This deformation involved both eugeoclinal and miogeoclinal sequences and took place within an Andean arc setting. The subsequent Mesozoic history of these two parts of the Mojave Desert has been the same. 40 Ar- 39 Ar data on hornblende, biotite, and muscovite in the Goldstone and Lane Mountain quadrangles indicate that a plutonic complex consisting of gabbro, quartz diorite, and tonalite was intruded during the Late Jurassic, probably slightly earlier than 148 m.y. ago. Voluminous granitic rocks were intruded during the Late Cretaceous, about 85–90 m.y. ago. These younger plutonic rocks caused variable resetting of all K-Ar systems in older rocks. Cooling of Late Cretaceous plutons to about 280 ± 40 °C occurred by about 78.4 m.y. Regional uplift of these rocks to a crustal level where no further diffusive loss of argon took place occurred at about 45 m.y. ago.

Early Cretaceous basement rocks in Hispaniola

A medium-grained, foliated amphibolite found in the Cordillera Central of the Dominican Republic near the Haitian border yields concordant K-Ar ages of 123 ± 2 m.y. for hornblende and plagioclase. The rock exhibits a low Rb/Sr ratio (0.0217) and present-day 87Sr/86Sr ratio (0.7024) but differs in chemical composition from metamorphosed mafic rocks of the Duarte Formation. A previously reported K-Ar age of 127 ± 6 m.y., obtained on a hornblendite body from the Duarte Formation, agrees closely with the amphibolite age. The hornblendite also differs in composition from the metamorphosed mafic rocks of the Duarte Formation and from the amphibolite. The 123- to 127- m.y. ages are interpreted to represent a minimum date for a metamorphic event that affected the basement complex of Hispaniola. An unpublished K-Ar date for the Bermeja complex in Puerto Rico falls within this range, but older ages have been obtained from rocks in Cuba. A date of about 120 m.y. indicated by one interpretation of a previously reported conformable lead isotopic composition for galena from the Pueblo Viejo gold deposit in the upper part of the Los Ranches Formation volcanic rocks in the Dominican Republic also agrees closely with this 123- to 127-m.y. range; together with other data, it suggests that the extrusion of Los Ranchos volcanic rocks and metamorphism of the Duarte Formation could be genetically related.