Richard Lee Armstrong

Rb-Sr and K-Ar geochronometry of Mesozoic granitic rocks and their Sr isotopic composition, Oregon, Washington, and Idaho

Mesozoic orogeny and magmatism began in the northwestern United States soon after deposition of Permian strata, but no rocks have yet been dated from the Permian-Triassic orogenic period. Middle Triassic to Late Jurassic sediment sequences include major unconformities and evidence of several episodes of igneous activity. An early culmination of magmatism occurred in Late Triassic and Early Jurassic time (200–217 m.y. ago) in eugeosynclinal parts of far western Idaho. A widespread and intense culmination in Late Jurassic time was the final major orogenic event in the Oregon eugeosyncline. The Bald Mountain (147 to 158 m.y. old), Wallowa (probably 143 to 160 m.y. old but affected by Cretaceous metamorphism), Deep Creek (at least 137 m.y. old), and many other plutons in the Blue and Klamath Mountain regions in Oregon and in western Idaho were emplaced shortly before the end of Jurassic. The bulk of the Idaho batholith was emplaced during a Cretaceous culmination of igneous activity — the southern (Atlanta) lobe about 75 to 100 m.y. ago and the northern (Bitterroot) lobe about 70 to 80 m.y. ago. Much of the batholith was affected by Eocene magmatism which resulted in widespread resetting of isotopic dates for older rocks to values of 50 m.y. or less. Between 55 and 70 m.y. ago, there was a lull in igneous activity in the northwestern United States. Sr isotope initial ratios change abruptly across a boundary in western Idaho from ∼0.7040 or less, to the west, to ∼0.7060 or greater, to the east. This change marks the boundary between Precambrian crust and Phanerozoic eugeosyncline. The geologic setting of the observed transition and its time independence suggest that it is due to contamination and assimilation processes involving magmas from the mantle and enclosing crustal rocks. Contamination of magmas with radiogenic Sr renders the Sr whole-rock isochron technique useless in dating the Idaho batholith and other intrusive rocks in central and eastern Idaho, areas underlain by Precambrian basement.

Cenozoic volcanic rocks of eastern China — secular and geographic trends in chemistry and strontium isotopic composition

Abstract The Cenozoic volcanic rocks of eastern China are subalkalic to alkalic basalts erupted in an early Tertiary back-arc rift environment and from scattered late Tertiary and Quaternary volcanic centers in a continental area crossed by active faults, driven by subduction of the Pacific plate and the collision of India and Eurasia. Immobile trace elements and major elements conform very well to each other in classification of the 59 rocks for which complete data are reported and they correctly identify the tectonic setting. LIL-element enrichments of the basalts lie between those of P-MORB and ocean island alkalic basalts, and show a secular increase. 87Sr/86Sr ratios of basalts vary from 0.7029 to 0.7048. Alkalic basalts are systematically less radiogenic than geographically coextensive and contemporaneous tholeiitic basalts. Increase of radiogenic Sr with increasing crustal thickness and crustal age and with silica enrichment of the magmas suggests crustal contamination but this is inadequate to explain the LIL-element enrichment patterns and variable LIL-element enrichments. The preferred hypothesis is that the alkalic magmas come from a deeper source, with long-term LIL-element depletion and low Rb/Sr ratio but relatively recent LIL-element enrichment. Conversely the tholeiitic magmas are melts of subcontinental mantle lithosphere that is more LIL-element depleted than the alkalic source, at the time of magma genesis, but has had an elevated Rb/Sr ratio for much of its post-consolidation history.

Evolving geographic patterns of Cenozoic magmatism in the North American Cordillera: The temporal and spatial association of magmatism and metamorphic core complexes

Four maps are presented here that show the location and extent of magmatic fields between eastern Alaska and northern Mexico during the successive time intervals of 55–40, 40–25, 25–10, and 10–0 Ma, and four others show the distribution of metamorphic core complexes during the same Cenozoic time intervals. The maps are based on U.S. Geological Survey and Canadian Cordilleran data bases contining about 6000 isotopic dates and extensive literature review. For nearly 60 Ma the development of metamorphic core complexes has coincided with the locus of a really extensive and voluminous intermediate-composition magmatic fields. The association is suggestive of a close link between magmatism and core complex formation, namely that magma directly and indirectly lowers the strength of the crust. Magmatism thus controls the location and timing of core complex formation. The stresses responsible may be inherited from Mesozoic crustal thickening, locally created by uplift and magmatic thickening of the crust, and imposed by the global pattern of plate motions and driving forces. Since the Miocene, rates of magmatism, extension, and core complex formation have declined. The modern Basin and Range province is not a suitable model for the situation that existed during major magmatic culminations. The singular event of early Miocene time, the merging of two large magmatic fields, extinguishing the Laramide magmatic gap, explains several disconnected observations: the hyperextension episode of the Colorado River corridor, rapid reorientation of stress patterns across much of western North America, and subsequent rapid tectonic movements in California. Magma-triggered breakup of western North America lithosphere coincided with development of the San Andreas transform system. Thermal destruction of the Laramide magmatic gap created a California “microplate” about 22 Ma ago that moved rapidly away from North America. Thus two plate tectonic processes, thermal destruction of the lithosphere “bridge” and northward growth of a transform system, interacted to produce Miocene and later tectonic patterns and events.

The Use of Strontium-87/Strontium-86 Ratios to Measure Atmospheric Transport into Forested Watersheds

Strontium-87/strontium-86 ratios indicate the sources of strontium in samples of natural waters, vegetation, and soil material taken from watersheds in the Sangre de Cristo Mountains of New Mexico. More than 75 percent of the strontium in the vegetation is ultimately derived from atmospheric transport and less than 25 percent from the weathering of the underlying rock. Much of the airborne strontium enters the watersheds by impacting on coniferous foliage, but deciduous foliage apparently traps little, if any, strontium-bearing aerosol. The strontium and presumably other nutrients are continuously recycled in a nearly closed system consisting of upper soil horizons, forest litter, and the standing crop of vegetation.

Petrochemistry and Sr, Pb, and Nd isotopic geochemistry of early precambrian rocks, Wutaishan and Taihangshan areas, China

Abstract In the Wutaishan and Taihangshan region, China, the lowermost high-grade metamorphic complex of grey-gneiss and amphibolite, the Fuping Complex, gives a 2.2 ± 0.2 Ga Pb-Pb isochron age with a first stage growth μ = 7.73, and a 2.37 ± 0.07 Ga SmNd isochron age with (143Nd/144Nd)0 = 0.50963 ± 0.00005 or ϵNd(T) = 1.50. Nd-depleted mantle model dates (TDM) of amhibolites 2.48 to 2.60 Ga, those of gneisses are 2.43 to 2.46 Ga. The unconformably overlying medium- to low-grade metavolcanic complex, the Wutai Complex, gives a 2.0 ± 0.1 Ga RbSr isochron age with (87/86Sr) 0 = 0.7025 ± 2, a 2.27 ± 0.02 Ga PbPb isochron date with afirst stage growth μ = 7.73, and a 2.26 ± 0.06 Ga SmNd isochron first stage (143Nd/144Nd)0 = 0.5097±2 orϵNd(T)=1.08. Individual sample Nd depleted mantle model date are ∼ 2.5 Ga. Metavolcanic samples from low-grade metasedimentary rocks of the Hutuo Group all lie close to a 2.3 Ga reference line on a RbSr isochron plot, have Nd TDM = 2.32, 2.34, and 2.62 Ga. From previous geological work, published UPb zircon dating, and ourisotopic and geochemical results, we infer that the Fuping rocks formed ∼ 2.6 Ga ago in an environment like a modern island arc. The Wutai Complex formed by 2.5 Ga ago, mostly formed in a tectonic setting similar to that of the Fuping Complex, with the exception that one volcanic cycle formed in an environment like a modern MOR and one unit formed in an environment transitional between modern within-plate and plate margin settings. The Hutuo Group formed 2.4 Ga ago in a within-plate tectonic environment. There is no evidence for continental crust before 2.6 Ga in the Wutaishan and Taihangshan region. This contradicts previous assignment of the Fuping Complex to 2.8 Ga ol older. The Fuping Complex is not an early nucleus of the Sino-Korean Craton. The new data also confirm that the Wutai Complex is older than its original assignment as Early Proterozoic. The SmNd systems for metabasaltic rocks in the region, either in amphibolite facies or in greenschist facies, are all significantly disturbed, in contrast with the undisturbed SmNd reported in previous studies of the Sino-Korea Craton. This study provides further evidence that the Archean igneous rocks in China formed at different times, from heterogeneous and depleted mantle sources.

Miocene peralkaline volcanism in west-central British Columbia — Its temporal and plate-tectonics setting

The 600-km-long Anahim volcanic belt of upper Miocene-Quaternary alkalic and peralkalic volcanic centers trends east-west along approximately lat 52°N in British Columbia, in contrast to the Miocene Pemberton volcanic belt of calc-alkalic centers and the Pliocene-Quaternary Garibaldi volcanic belt of calc-alkalic centers, which follow the northwest-trending continental margin. Anahim belt rock types range from alkali basalt and nephelinite, found as small cinder cones and flows, to oversaturated (and undersaturated) peralkalic varieties found in evolved central volcanoes and in their erosion-exposed roots. In contrast to the usual subduction-related calc-alkaline volcanism in the Pemberton and Garibaldi belts, volcanic activity in the Anahim belt has been linked with lithospheric fracturing above the northern edge of the subducted Juan de Fuca plate or interpreted as an edge effect of the subducted plate in the mantle. Available isotopic ages from the oldest centers in the Anahim belt become younger eastward at a rate of 2 to 3.3 cm/yr, suggesting that volcanic activity there may well be related to a mantle hot spot beneath British Columbia. Volcanic chemistry and isotopic composition do not distinguish between either a rift or a hot-spot setting.

Petrologic, structural, and age relations of serpentinite, amphibolite, and blueschist in the Shuksan Suite of the Iron Mountain–Gee Point area, North Cascades, Washington

Part of the Shuksan blueschist terrane, near Iron Mountain and Gee Point, North Cascades, Washington, has associated serpentinite, amphibolite, barroisite schist, blueschist, rare eclogite, and blackwall-type metasomatic rock. Field, petrographic, and microprobe observations indicate that the amphibolite and barroisite schists were metamorphosed in contact with peridotite and suggest that the peridotite may have been a heat source. The serpentinite and associated rocks are structurally concordant with the regional blueschists and have been overprinted by blueschist metamorphism. Isotopic dating gives metamorphic ages of 148 ± 5 to 164 ± 6 m.y. for the amphibolite and barroisite schist and 129 ± 5 m.y. for nearby regional Shuksan blueschists. The origin of the serpentinite + amphibolite + blueschist assemblage is interpreted to be the result of sequential events in a subduction zone. As subduction began, oceanic crustal materials underwent high-temperature metamorphism along the hot ultramafic hanging wall and were converted to amphibolites; materials that were subducted later came in contact with a cooler hanging wall and recrystallized as blueschist. This hypothesis may be applicable to the origin of similar rock associations in the Franciscan terrane and other orogenic belts.

Isotopic ages of glaucophane schists on the Kodiak Islands, southern Alaska, and their implications for the Mesozoic tectonic history of the Border Ranges fault system

Elongate fault-bounded blocks of an Early Jurassic subduction complex are preserved along a major tectonic boundary in southern Alaska, the Border Ranges fault system (BRFS). The high-P/low-T metamorphic rocks of the subduction complex are juxtaposed along one strand of the BRFS with an approximately coeval pluton and its associated thermal aureole. New U-Pb and Rb-Sr isotopic dates from the subduction complex are 204 ± 8 Ma and 195 ± 10 Ma (2 sigma errors), respectively. The similarity of these dates to previously reported K-Ar dates supports petrographic and petrologic observations that the subduction complex did not experience any metamorphism related to the intrusion of the now adjacent pluton. Petrographic and petrologic data also show that the pluton did not experience a high-P/low-T metamorphism. The pluton and the metavolcanics that it intrudes are interpreted as part of a Late Triassic to Early Jurassic primitive island-arc complex. If the subduction complex is related to this coeval island arc, then the forearc and back part of the accretionary prism have been tectonically eroded, either by subduction erosion, strike-slip faulting, or a combination of these processes. If, however, the subduction complex was juxtaposed with the island arc during strike-slip faulting that accompanied and postdated the Cretaceous accretion of terranes of southern Alaska with North America, then the subduction complex could be related to an Early Jurassic island arc exposed on Vancouver Island, where it is part of Wrangellia. The new isotopic ages and petrologic data indicate that the BRFS probably began its history as a major fault zone as early as the Middle Jurassic, prior to the well-documented Cretaceous megathrust history.

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.

Chronology of the peralkaline, late Cenozoic Mount Edziza Volcanic Complex, northern British Columbia, Canada

The late Cenozoic Mount Edziza Volcanic Complex covers an area of about 1,000 km 2 in north-central British Columbia, 300 km east of the transcurrent boundary between the North American and Pacific plates. It is made up of a group of overlapping basaltic shields and intermediate to salic peralkaline composite domes, flows, and central volcanoes that are associated with extensional structures in the underlying basement. New K-Ar and fission-track dates (45) and Rb-Sr and Sr isotope analyses (12) from the Mount Edziza Volcanic Complex are reported. The age dates are for the most part consistent with the stratigraphy and indicate that frequent eruptive activity occurred during the past 8 m.y. Five major magmatic cycles each began with the eruption of basalt and culminated with the eruption of oversaturated peralkaline magma. Low 87 Sr/ 86 Sr initial ratios (0.7028 ± 0.0001) indicate a mantle source for the basalts. Low Sr contents and high 87 Sr/ 86 Sr ratios in the salic end members suggest that the oversaturated rocks were derived from the basaltic magma by crystal fractionation in crustal reservoirs. Rb-Sr isochrons suggest that residence times for the fractionating magma were about 0.7 to 1 m.y. Early removal of large amounts of plagioclase, followed by fractionation of potash feldspar, can account for most of the observed petrological and isotopic relationships. A few individual compositions and one suite of mainly intermediate samples contain anomalously large amounts of both 87 Sr and radiogenic argon. This indicates that contamination with crustal material and possibly mixing of parental basalt with partly fractionated magma from previous events may have produced the relatively small volume of intermediate rocks.