HAR•E• ET AL.' GE•ONOIX)O• OF THE JOSE•H•E O?H•oLrrE AND ITS EM•LACEMENTThe 4øAr/39Ar ages on igneous minerals will be similar to crystallization ages only if cooling was rapid (e.g., shallow level dikes). Many of the dated plutons are small and were intruded into rocks having ambient temperatures of <350øC. Such small intrusions would be expected to cool quickly below the -500øC closure temperature for hornblende. Many of these plutons, however, consist of two or more phases, so the temperature may have remained elevated by continual addition of heat from new intrusions, especially if substantial volumes of magma were transported through the plutonic complex.Zircon U/Pb age data are presented in Table 2, and the locations of the samples are given in Appendix A (available on microfiche). Notes on analytical procedures and assessment of uncertainties are given in the footnotes of Table 2 Table 2 with Table 1
The Josephine ophiolite is a large, complete ophiolite generated in a Late Jurassic back arc basin along western North America. The ophiolite underwent subseafloor metamorphism under a steep thermal gradient as indicated by downward changes in mineralogy and δ18O values. Characteristic mineral zonation includes mica±orthoclase and hematite in the lower extrusives, greenschist‐facies assemblages in the upper sheeted dikes, and amphibolite‐facies assemblages in the lower sheeted dikes and high‐level gabbro. Cumulate gabbros are only incipiently altered but are depleted in 18O indicating high‐temperature alteration with fluids that evolved at low water(W)/rock(R) ratios. A second, off‐axis circulation system is recorded by metalliferous sediments that occur 8–21 m above the ophiolite. Alteration in the upper sheeted dike complex and especially the extrusive sequence is heterogeneous at outcrop scale as indicated by large variations in mineralogy, chemistry, and δ18O values within individual pillows and dikes. The heterogenity is due to (1) variations in W/R ratio and starting material during alteration by downwelling seawater (e.g., pillow cores versus glassy rims) and (2) localized discharge mineralization (mostly epidosites) superimposed on the background recharge alteration. Epidosites occur largely as dike‐parallel stringers, irregular replacement of pillow and massive lava, and replacement of interpillow and pillow‐breccia matrices. The chemistry of most extrusives and sheeted dikes is characterized by loss of Ca and gain in Na and Mg, consistent with alteration by downwelling fluids. In addition, the sheeted dikes are depleted in K2O, and the lower sheeted dikes and high‐level gabbros are depleted in Zn and Cu. The δ18O and δD values for most samples from the extrusive sequence indicate alteration by seawater at <200°C and high W/R ratios, whereas alteration of the sheeted‐dike complex took place at 250°–450°C and involved fluids enriched in 18O relative to seawater. The δ18O values and chemistry of the sheeted‐dike complex indicate that downwelling seawater evolved by interaction with diabase at low W/R ratios into high‐Ca, δ18O‐rich, metal‐rich, low pH fluids similar to those venting at modern “black smokers.” Epidosites represent extreme metasomatism and are strongly enriched in Ca and depleted in Mg, Na, Zn, and Cu. “Albite epidosites” in the extrusive sequence show similar bulk changes, but to a lesser degree. Fluids calculated to be in exchange equilibrium with epidosites and albite‐epidosites are enriched in 18O. This feature and their chemistry imply that they represent pathways of discharging fluids. Cooling and/or mixing of fluids discharging upwards through the sheeted dikes occurred at the contact with pillow lavas, resulting in silicification and sulfide mineralization at about 350°C. Continued cooling of the discharging fluids upward in the extrusive sequence resulted in formation of albite‐epidosites, hematitic pillow lavas, and potassic alteration. At one locality (Turner‐Albright), hot dischar...
The geochronology, stratigraphy, and spatial relationships of Middle and Late Jurassic terranes of the Klamath Mountains strongly suggest that they were formed in a single westfacing magmatic arc built upon older accreted terranes. A Middle Jurassic arc complex is represented by the volcanic rocks of the western Hayfork terrane and consanguineous dioritic to peridotJtic plutons. New U/Pb zircon dates indicate that the Middle Jurassic plutonJc belt was active from 159 to 174 Ma and is much more extensive than previously thought. This plutonic belt became inactive just as the 157 Ma Josephine ophiolite, which lies west and structurally below the Middle Jurassic arc, was generated. Late Jurassic volcanic and plutonic arc rocks (Rogue Formation and Chetco intrusive complex) lie outboard and structurally beneath the Josephine ophiolite; U/Pb and K/Ar age data indicate that this arc complex Js coeval with the Josephine ophiolite. Both the Late Jurassic arc complex and the Josephine oph•ol•te are overlain by the "GalJce Formation," a Late Jurassic flysch sequence, and are intruded by 150 Ma dikes and sills. The
New geochronological data and published structural and stratigraphic data show that two distinctly different ophiolitic assemblages formed in general proximity to one another at nearly the same time and were subsequently imbricated along a regional thrust zone. The Josephine ophiolite constitutes a complete oceanic crust and upper mantle sequence which lies within the western Jurassic belt of the Klamath province. Within the study area the Josephine ophiolite was formed by seafloor spreading at about 157 m.y. before present. It was immediately covered by a thin pelagic and hemipelagic sequence which grades into a thick flysch sequence, both of which comprise the Galice Formation. The Galice flysch was derived from volcanic arc and uplifted continental margin orogenic assemblages. A major nonvolcanic so,urce for the Galice flysch appears to have been the western Paleozoic and Triassic belt of the Klamath province exposed to the east. Proximal volcanic arc activity migrated to the site of the Josephine-Galice section by 151 m.y. and is represented by numerous dikes and sills which intrude the ophiolite and Galice Formation. The Preston Peak ophiolite is a polygenetic assemblage consisting of (1)
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