Joseph A. Vance

Steady-state exhumation of the European Alps

Fission-track grain-age distributions for detrital zircon are used in this study to resolve the late Cenozoic exhumation history of the European Alps. Grain-age distributions were determined for six sandstone samples and one modern river sediment sample, providing a record from 15 Ma to present. All samples can be traced to sources in the Western and Central Alps. The grain-age distributions are dominated by two components, P1 (8–25 Ma) and P2 (16–35 Ma), both of which show steady lag times (cooling age minus depositional age), with an average of 7.9 m.y. for P1 and 16.7 m.y. for P2. These results indicate steady-state exhumation in the source region at rates of ∼0.4–0.7 km/m.y. since at least 15 Ma.

Late Miocene exhumation and uplift of the Washington Cascade Range

The Washington Cascade Range is a complex, polygenetic mountain range that dominates the topographic, climatic, and cultural configurations of Washington State. Although it has been the locus of ongoing arc magmatism since the Eocene, most of the range is distinct from the southern part of the arc in Oregon and California in that bedrock uplift has produced high surface elevations and topographic relief, rather than volcanic burial or edifice construction. (U-Th)/He and fission-track ages of bedrock samples on the east flank of the range record relatively rapid cooling in the early Tertiary, but slow exhumation rates (∼0.2 km/m.y.) through most of the Oligocene. Samples on the west flank suggest rapid cooling in the late Miocene (8–12 Ma), and age variations in vertical transects are consistent with a pulse of rapid exhumation (0.5–1.0 km/m.y.) at that time. Apatite He ages as young as 1–5 Ma in several areas suggest that high cooling and possibly exhumation rates persist locally. Accelerated exhumation rates ca. 10 Ma are also observed in the Coast Mountains of British Columbia and southeast Alaska, ∼1500 km to the north, suggesting a large-scale mechanism for the exhumation pulse at that time.

Ages and stratigraphy of lower and middle Tertiary sedimentary and volcanic rocks of the central Cascades, Washington: Application to the tectonic history of the Straight Creek fault

In the central Cascade Range of Washington, three structural blocks of early Tertiary sedimentary and volcanic rocks help to define the position and history of the southern segment of the Straight Creek fault. East of the fault, in the Teanaway River block, the early Eocene fluviatile feld-spathic sandstone of the Swauk Formation is interbedded with largely dacitic volcanic rocks of the Silver Pass Volcanic Member. Zircon fission-track ages on the Silver Pass are about 52 m.y. Overlying the tightly folded Swauk and Silver Pass is the Teanaway Formation, a gently dipping accumulation of basalt, andesite, rare dacite, and rhyolite that yields middle Eocene (about 47 m.y.) whole-rock K-Ar ages. Conformably overlying the Teanaway is the Roslyn Formation, a coal-bearing fluviatile feld-spathic sandstone of probable middle to late Eocene age. The late Eocene and Oligocene(?) Naches Formation, exposed west of the Straight Creek fault in the Cabin Creek block, is rich in fluviatile feldspathic sandstone and rhyolite flows and tuffs in its lower part but grades upward into a sequence dominated by basalt but with some andesite. The basal Guye Sedimentary Member of the Naches Formation underlies and is interbedded with the Mount Catherine Rhyolite Member in the Snoqualmie Pass area. On the basis of zircon fission-track ages on silicic tuffs and whole-rock K-Ar ages on basalt, the basal part of the Naches is about 40 to 44 m.y. old. The formation is tightly folded and complexly faulted along the Straight Creek fault. The Straight Creek fault intersects the Olympic-Wallowa lineament in the strongly deformed Manastash River block. In this block, fluviatile coal-bearing feldspathic sandstone of the Manastash Formation is overlain by principally dacitic volcanic rocks of the Taneum Formation. Overlying the Taneum is the basalt of Frost Mountain. We correlate this threefold sequence, feldspathic sandstone-dacite-basalt, with the sequence in the Teanaway River block. Small patches of rhyolitic ash-flow tuff, dated at 33 m.y. (late Oligocene) and interbedded feldspathic sandstone overlie the Roslyn and Teanaway Formations in the Teanaway River block and are probably correlative with the late Oligocene Wenatchee Formation exposed in the Chiwaukum graben east of the Teanaway River block as well as with the late Oligocene volcanic rocks (30 m.y.) along the Cascade crest to the southwest. These latter rocks are little deformed and unconformably overlie the Naches Formation. The ages and depositional record in the three blocks indicate a Tertiary history of dominantly vertical movement along the Straight Creek fault and its southeasterly splays that merge with the Olympic-Wallowa lineament. The horst-and-graben structure of the blocks, as well as enechelon fold axes in the Swauk Formation, suggests some post–early Eocene, right-lateral shear along the fault, although there is no direct evidence of lateral offset. Vertical movement, significant since early Eocene Swauk deposition, followed late Eocene Naches deposition but tapered off by late Oligocene time and ceased by Miocene time, when the fault was intruded by the 25-m.y.-old Snoqualmie batholith and other plutons. However, structures with Olympic-Wallowa lineament trends appear to have influenced folding of the Miocene Yakima Basalt Subgroup.

Formation of peridotites by deserpentinization in the Darrington and Sultan areas, Cascade Mountains, Washington

In the Darrington and Sultan basin areas of the Cascade Mountains, mafic and ultramafic rocks occur as sheets and lenses that were tectonically emplaced along high-angle faults. The ultramafic rocks comprise pseudomorphic serpentinite (lizardite-chrysotile), antigorite serpentinite, and metamorphic peridotite (olivine-talc-tremolite). The peridotites are of two types: (1) subordinate black-weathering peridotite characterized by iron-poor olivine (Fo 98 −Fo 94 ) and essential magnetite, and (2) dominant orange-weathering peridotite with more iron-rich olivine (Fo 94 −Fo 85 ) and little magnetite. In contrast to upper-mantle peridotites, olivine composition in the orange-weathering peridotites is variable on all scales down to a few micrometres. Field relations, textural and mineralogical relics, mineral composition, and oxygen isotope values all reflect formation of the Darrington peridotites by deserpentinization. Field relations demonstrate the prograde metamorphic sequence lizardite-chrysotile serpentinite → antigorite serpentinite → metamorphic peridotite. Incipiently dehydrated serpentinites exhibit veinlets and scattered porphyroblasts of new-formed high-Mn olivine. Altered chromite, Fe and Ni-Fe sulfides, magnetite, and Mg carbonates in the peridotites have been inherited from the serpentinite parent. In the black-weathering peridotites, relict textures such as bastite pseudomorphs after pyroxenes are inherited from the parent serpentinites and survive as ghosts marked by the distribution of finegrained magnetite. In the black-weathering peridotites, iron is bound largely in magnetite inherited from the parent serpentinites; thus the olivine is iron poor. The variability of olivine composition in the orange-weathering rocks is thought to be related to the variable content of iron in antigorite of the parent serpentinites. Progressive metamorphism of these rocks reached middle amphibolite facies (500 to 600 °C). Deserpentinization occurred prior to Tertiary tectonic transport and emplacement into fault contact with the unmetamorphosed rocks of the present structural setting.

Epidote phenocrysts in dacitic dikes, Boulder County, Colorado

Epidote (Ps21%) crystallized early as elongate phenocrysts in Late Cretaceous rhyodacitic dikes in the vicinity of Ward, Boulder County, Colorado. Other unusual phenocryst phases are garnet (Gr17–24%) and muscovite. In a xenolith containing kyanite, corundum, biotite, and plagioclase, magmatic garnet grew as a rim around xenocrystic pyrope-rich (Py37%) garnet. The xenolith was derived from a granulite-facies zone, not represented at the present-day erosion surface which is composed of upper amphibolite-facies cordierite and sillimanite-bearing gneisses. The dike magmas were fed not from an immediately underlying batholith but from a magma chamber at a depth corresponding to a pressure of 8–13 kilobars. Phenocrysts cystallized in the temperature range 800 to 700° C, under H2O and O2 activities greater than normal for silicic magmas. This occurrence shows convincingly not only that epidote can be magmatic but that it is a possible early-crystallization phase in silicic magmas.

Geochemistry of the Shuksan greenschists and blueschists, North Cascades, Washington: Variably fractionated and altered metabasalts of oceanic affinity

The Shuksan schist comprises a structurally coherent, metabasaltic member of the Easton Formation, the uppermost allochthon (Shuksan thrust plate) in the thrust system of the western North Cascades of Washington State. Late Jurassic metamorphism at moderately high P/T produced interlayering of actinolite-bearing greenschist assemblages with blue amphibole-bearing rocks. Major and trace element analyses of twelve greenschist and blueschist samples have been used to establish similarities between the basaltic protolith and moderately to strongly fractionated Type I MORB, to distinguish the effects of seafloor alteration superimposed on the primary igneous chemistry, and to evaluate the origin and nature of the chemical controls which produced the two mineral assemblages.The twelve analyzed samples exhibit moderate to strong LREE depletion, and characteristically low concentrations of other non-labile trace elements such as Nb, Th and Hf. The highly to moderately incompatible elements Ti, P, Nb, Zr, Hf, Y, Sc, and the REE vary by factors of 1.5 to 3.5 within the suite in a systematic pattern, increasing smoothly with increasing total iron. The relative enrichments of these elements are inversely proportional to bulk partition coefficients estimated for fractionation of basaltic magmas. The magnitude of the negative europium anomaly increases with overall incompatible element enrichment. These variations are consistent with the production of a wide spectrum of compositions by different degrees of low pressure fractionation of similar Type I MORB parent magmas.The concentrations of Sr, Rb, Na, and K vary irregularly and do not correlate with the non-labile trace elements. K and Rb are substantially elevated over typical MORB values in most samples and exhibit a consistently lower ratio (K/Rb=400 vs 1000) than fresh MORB. Concentrations of these four elements are believed to have been modified by low temperature seafloor alteration (pre-metamorphic) characterized by the formation of K-rich celadonitic clays, palagonite and minor potassium feldspar.The critical chemical variables that control the occurrence of actinolite and blue amphibole in the Shuksan schists are total iron, Fe2O3-content and Na/Ca (all high in blueschists). The chemical features were largely established by magmatic processes and inherited from the igneous parent rocks; the chemically more evolved samples are blueschists. The Fe2O3-content and Na/Ca, however, may be modified during alteration, rendering initial bulk compositions near the chemical boundary susceptible to changes which may shift rock compositions from one compatibility field to the other. Heterogeneous alteration of pillow lavas and other fragmental deposits, followed by intense flattening during metamorphism, provides a mechanism for generating blueschists and greenschists interlayered on the cm scale.

ZONED GRANITIC INTRUSIONS—AN ALTERNATIVE HYPOTHESIS OF ORIGIN

Certain granitic intrusions exhibit a gradational compositional zoning from relatively mafic margins to more leucocratic and potassic cores. Previous explanations of this zoning, including that of assimilation, are discussed and rejected. The proposed hypothesis involves: (1) crystallization from the margins inward, sealing in the released volatile phase which is forced downward with further crystallization; (2) migration downward of alkalis and silica, with the volatiles, and their enrichment in the magma at depth, leaving behind a more mafic residuum.

Significance of coexisting lawsonite, prehnite, and aragonite in the San Juan Islands, Washington

Aragonite and prehnite are widely associated in low-grade sedimentary, volcanic, and metaplutonic rocks of the San Juan Islands, Washington. The abundance of prehnite and the apparent absence of the high-pressure mineral lawsonite were thought to indicate that the aragonite had formed metastably. Our work now shows that lawsonite, commonly turbid and with a fibrous habit, also occurs in some of these rocks. The discovery of lawsonite indicates that the San Juan aragonite is probably a stable phase that indicates high-pressure metamorphism. The association of aragonite and prehnite also implies that the stability range of prehnite may extend to pressures above those of the calcite-aragonite transition. The occurrence of lawsonite is compatible with the interpretation that the bed rock of some of the San Juan Islands was subjected to deep and rapid tectonic burial by subduction at a convergent plate boundary.

Stable isotope compositions of olivine and dolomite in peridotites formed by deserpentinization, Darrington area, North Cascades, Washington

Abstract Oxygen isotope analyses of five olivines from the Darrington peridotite, Washington, yield δO 18 values of +7.3 to +8.9%. which are consistent with derivation of these rocks from a serpentinite precursor. The isotopic data are compatible with mineralogical, textural and chemical evidence that most of the Darrington peridotites have formed by deserpentinization. Olivine from a single, petro-graphically distinct peridotite sample has a δO 18 -value of +5.2%. which is within the field of high-temperature olivines. The isotopic and textural evidence indicate that this is a partially recrystallized peridotite tectonite. Oxygen and carbon isotope analyses of dolomites from olivine-carbonate rocks indicate that they could have originated by introduction of atmospheric CO 2 via meteoric waters during the formation of ophidolomites or ophicalcites. Subsequent metamorphism and reequilibration have modified the δO 18 -values.