Early Cretaceous metamorphic age of the South Fork Mountain Schist in the northern Coast Ranges of California
No abstract available.
California↗
Geology topics
Marvin A. Lanphere
Publications and source records attributed to Marvin A. Lanphere.
New potassium-argon data on the age of mineralization and metamorphism in the Willow Creek mining district, southern Talkeetna Mountains, Alaska
The now largely abandoned Willow Creek mining district, southern Talkeetna Mountains, Alaska, produced nearly $18,000,000 in gold and minor silver between 1909 and the early 1950's. Mineralized quartz veins, which contain gold and silver along with minor quantities of base metals (in pyrite, galena, chalcopyrite, sphalerite, molybdenite, and arsenopyrite), cut Late Cretaceous and early Tertiary tonalite and quartzmica schist of probable Jurassic age (Ray, 1954; Silberman and others, 1976; Bela Csejtey, Jr., unpub. data, 1978).
Alaska↗
Intrusive rocks of the Yakutat-St. Elias area, south-central Alaska
Twenty-three plutons, exposed over a total area of nearly 1200 km 2 , have been studied in the Alaska part of the St. Elias Mountains between long 138° and 141°W. Results of potassium-argon age determinations combined with field relations, petrography, and major- and trace-element chemistry suggest six major intrusive events: (1) late Paleozoic gabbro to quartz diorite intruded Paleozoic metamorphic rocks that are probably equivalent to the Kaskawulsh Group in adjacent areas of Canada, (2) Triassic quartz diorite formed one small pluton in undated metamorphic rocks near Mt. St. Elias, (3) Jurassic tonalite and granite intruded upper Paleozoic(?) and lower Mesozoic(?) metamorphic rocks, (4) Late Cretaceous or Tertiary altered tonalite formed three widely separated plutons in metasedimentary rocks of Jurassic(?) and Cretaceous age in the Yakutat Group, (5) Eocene granodiorite and granite, and (6) late Cenozoic tonalite and granodiorite intruded both the Yakutat Group and upper Paleozoic(?) and lower Mesozoic(?) metamorphic rocks. The Paleozoic, Jurassic, and Cretaceous or Tertiary plutonic suites are restricted to particular geologic terranes, and the Jurassic and Eocene suites correlate with regional plutonic belts present elsewhere in southern Alaska. The distribution of the Tertiary plutons does not require large-scale horizontal displacements along the Fairweather and other major high-angle faults. The available data indicate that the mineral resource potential of the Yakutat-St. Elias area is low for those deposits that are generally related to magmatic processes.
Alaska↗
Age and tectonic significance of volcanic rocks on St. Matthew Island, Bering Sea, Alaska
Reconnaissance investigations of the heretofore little known volcanic assemblage on St Matthew Island provide significant information on the tectonic history of the Bering Sea shelf. St. Matthew Island is made up of approximately 500 m of subaerial calc-alkaline volcanic rocks ranging in composition from high-alumina basalt to rhyolite. Four K-Ar analyses of samples from this volcanic sequence give Late Cretaceous ages of 65-77 m.y., and intercalated carbonaceous tuff layers yield Cretaceous pollen assemblages. Along the northeast coast of St. Matthew Island, the volcanic rocks are intruded by granodiorite that gives an early Tertiary K-Ar age of 61 m.y. Correlations with on-land geology in northeast Siberia and marine geophysical data from the western Bering Sea strongly suggest that St. Matthew Island represents a southeastward extension of the Okhotsk-Chukotsk belt, a Cretaceous and early Tertiary volcanic arc that borders the Pacific margin of Siberia for nearly 3,000 km. The apparent continuation of this volcanic arc along the margin of the Bering shelf at least as far east as St. Matthew Island supports suggestions by Burk and by Scholl and others that in late Mesozoic time the Pacific plate margin coincided with the present Bering shelf margin and did not shift to the Aleutian trench until the end of Cretaceous or the beginning of Tertiary time.
Alaska↗
K-Ar ages of Pleistocene rhyolitic volcanism in the Coso Range, California
Rhyolite domes and rhyolite and basalt flows near Coso Hot Springs in the Coso Range, California, give K-Ar ages that range from about 0.04 ± 0.02 m.y. to 0.96 ± 0.19 m.y. Most of the units appear to be between 0.05 and 0.15 m.y. old. The total volume of erupted rhyolitic magma is about 2.4 km 3 . The age, volume, and composition of these volcanic rocks indicate that the Coso volcanic field is a potential geothermal resource area.
California↗
Age and tectonic significance of volcanic rocks on St. Matthew Island, Bering Sea, Alaska
Reconnaissance investigations of the heretofore little known volcanic assemblage on St. Matthew Island provide significant information on the tectonic history of the Bering Sea shelf. St. Matthew Island is made up of approximately 500 m of subaerial calc-alkaline volcanic rocks ranging in composition from high-alumina basalt to rhyolite. Four K-Ar analyses of samples from this volcanic sequence give Late Cretaceous ages of 65-77 m.y., and intercalated carbonaceous tuff layers yield Cretaceous pollen assemblages. Along the northeast coast of St. Matthew Island the volcanic rocks are intruded by granodiorite that gives an early Tertiary K-Ar age of 61 m.y. Correlations with on-land geology in northeast Siberia and marine geophysical data from the western Bering Sea strongly suggest that St. Matthew Island represents a southeastward extension of the Okhotsk-Chukotsk belt, a Late Cretaceous and early Tertiary volcanic arc that borders the Pacific margin of Siberia for 3,000 km. The apparent continuation of this volcanic arc along the margin of the Bering shelf at least as far east as St. Matthew Island supports suggestions by Burk and by Scholl and others that in late Mesozoic time the Pacific plate margin coincided with the present-day Bering shelf margin and did not shift to the Aleutian trench until the end of Cretaceous or the beginning of Tertiary time. During the summer of 1971 the U.S. Geological Survey conducted a reconnaissance expedition to St. Matthew Island in the central Bering Sea aboard the Survey research vessel Don J. Miller II. The purpose of the expedition was to obtain basic geologic information on the age and lithologic character of the volcanic rocks on St. Matthew as part of a broad program of onshore and offshore investigations of the energy and mineral resources of the Bering Sea shelf. St. Matthew Island had not been mapped previously and available geologic information was confined to brief notes from early exploratory surveys of the Bering Sea region published more than 50 years ago (Dawson, 1894; Emerson, 1910). St. Matthew Island together with two small neighboring islands, Hall and Pinnacle, is situated 400 km west of mainland Alaska on the broad continental shelf that connects Alaska and Siberia (fig. 1). Although these islands have a combined area of only 350 2 km, they are important because they provide a rare subaerial exposure of the geology of the shelf and furnish new information on the tectonic history of the Bering Sea region. This report briefly describes the geology of the island and suggests how the island fits into the tectonic framework of the Bering Sea region.
Alaska↗
Offset plutons and history of movement along the McKinley segment of the Denali Fault system, Alaska
The Foraker and McGonagall plutons, bodies of granodiorite with nearly identical mineralogy and chemistry, are considered to be parts of a single igneous mass that has undergone right-lateral displacement of about 38 km along the McKinley segment of the Denali fault system since the igneous mass crystallized about 38 m.y. ago. These offset plutons place severe constraints on the amount and rate of movement along the McKinley segment since the beginning of the Oligocene Epoch. The 38-km displacement indicates an average rate of 0.1 cm/yr if movement began immediately after crystallization in early Oligocene time or an average rate of 0.4 cm/yr if movement began 10 m.y. ago in late Miocene time. These rates, however, are considerably less than Holocene movement rates measured along the fault further to the east, which suggests that the Holocene displacement rate is greater than the pre-Holocene rate or that right-lateral movement along the fault may diminish to the west.
GSA Bulletin↗
Chemical variations across the Alaska-Aleutian Range batholith
A study of 79 chemical analyses of plutonic rocks in the northern part of the Alaska-Aleutian Range batholith shows that K 2 O and SiO 2 in Late Cretaceous and Tertiary rocks decrease toward the Pacific margin and that Al 2 O 3 and CaO increase. Plots for Fe 2 O 3 , FeO, MgO, and TiO 2 suggest a possible increase toward the Pacific margin; Na 2 O, H 2 O+, and MnO show no significant trends across the batholith. Oxide trends for groups of plutons in the western and eastern parts of the batholith in general are opposite that of the groups combined. For the Jurassic plutonic rocks, K 2 O may decrease toward the Pacific margin; other oxides show considerable scatter, and meaningful trends are not readily apparent. The oxide trends across the batholith are similar to trends across the central Sierra Nevada batholith of California, with the exception of SiO 2 and A1 2 O 3 , which show no significant changes. In the Coast Range batholith of British Columbia, potassium also increases away from the Pacific margin. The increase in K 2 O toward the continent in the Alaska-Aleutian Range batholith is similar to that found across Quaternary volcanic island arcs, where K 2 O content increases directly with increasing depth to inclined seismic zones. This increase suggests that generation of magma may have taken place along, or above, a paleoseismic zone. Geologic evidence supports such a model for the Jurassic plutonic rocks, which represent the roots of an early Mesozoic magmatic arc that probably formed above a descending oceanic plate. However, the hypothesis that magma for Late Cretaceous and Tertiary plutonic rocks was generated along seismic zones does not fit available geologic evidence, and an anatectic model cannot be ruled out.
Alaska↗
Alaska-Aleutian range batholith: Geochronology, chemistry, and relation to circum-Pacific plutonism
Potassium-argon mineral ages and reconnaissance mapping of approximately 65,000 sq km in south-central Alaska indicate that the Mesozoic and Cenozoic plutonic rocks in the region were emplaced during three discrete intrusive epochs. Most of the plutonic rocks are part of the Alaska-Aleutian Range batholith; the remainder appear as outcrops in isolated plutons northeast of the main batholith. Maximum and minimum concordant mineral ages on coexisting biotite and hornblende are used to mark the beginning and ending, respectively, of each intrusive epoch. The oldest intrusive epoch, Early and Middle Jurassic, contains plutonic rocks emplaced between about 176 and 154 m.y. ago. Jurassic plutonism occurred along a magmatic arc at least 1,300 km long, extending from about 480 km southwest of Becharof Lake northeast to the Talkeetna Mountains. Aeromagnetic data suggest that the magmatic arc, which represents the roots of the arc portion of an early Mesozoic arc-trench system, also continues southwest into the Bering Shelf. The associated trench is thought to be represented by an imbricated mélange of ophiolite and submarine lava with associated chert and argillite that occupies a belt 140 km southeast of the magmatic arc. Clastic sediments more than 4.5 km thick occupy the 140-km-wide arc-trench gap. Late Cretaceous and early Tertiary plutonic rocks, emplaced between about 83 and 58 m.y. ago, are found mainly in the northern part of the batholith and in isolated plutons to the northeast toward Mount McKinley. Middle Tertiary plutonic rocks ranging in age from 38 to 26 m.y. occur in two areas within the batholith and also in the Mount McKinley area. Although some of the Late Cretaceous and Tertiary plutons are associated with extrusive rocks, these plutons are largely post-tectonic, and magma was generated beneath both stable platform areas as well as beneath former eugeosynclinal regions in which deformation had essentially ceased. Magma for the Jurassic plutonic rocks appears to have been generated along or above an early Mesozoic subduction zone. The region southeast of the zone is considered to represent a classic example of continental accretion of eugeosynclinal sediments caused by under-thrusting of oceanic crust beneath an island arc. Magma generation for the Cretaceous and Tertiary plutons in the northern part of the batholith does not appear related to the early Mesozoic subduction zone, for it would require that the zone shift inland, or toward the continent, from its position during the Jurassic and then shift away from the continent once again to its present position.
GSA Bulletin↗
Timing of mesozoic and cenozoic plutonic events in circum-Pacific North America
Evaluation of isotopic ages of granitic intrusive rocks of large batholiths in circum-Pacific North America indicates that Mesozoic and Cenozoic plutonism was episodic but not periodic. Three intrusive epochs have been defined in the Alaska-Aleutian Range batholith of Alaska on the basis of concordant potassium-argon ages of coexisting biotite and hornblende or muscovite. Only two intrusive epochs based on concordant ages of mineral pairs and an older plutonic episode can be recognized in the Sierra Nevada and southern California batholiths. Detailed intrusive histories for other parts of Alaska, British Columbia, and Yukon Territory cannot be inferred from available data. The intrusive epochs in the western United States do not correlate with the epochs in the Alaska-Aleutian Range batholith. The spacing, duration, and synchroneity of intrusive epochs in the circum-Pacific batholiths of North America are not yet well established.
GSA Bulletin↗
Distribution and age of high-grade blueschists, associated eclogites, and amphibolites from Oregon and California
Isolated blocks of high-grade blueschist and amphibolite facies metamorphic rocks occur within the Jurassic and Cretaceous eugeosynclinal deposits of the Coast Ranges of southwestern Oregon and California. The blocks range in size from individual rock masses commonly 5 to 1,000 ft in diameter to a few larger masses as much as 7 mi long and 2 mi wide. The high-grade blocks are predominantly basaltic in composition and include glaucophane schists, eclogites, and gneissic rocks of the amphibolite facies. Field relationships indicate that the blocks are closely associated with serpentine, that high-grade blueschist and amphibolite blocks, lower grade blueschists, volcanic rocks, and cherts occupy disturbed zones that may be related to thrusting, and that there is no exposed in situ provenance for the high-grade blueschists, eclogites, and amphibolites. Potassium-argon mineral ages of white mica and actinolite from the blueschists and of hornblende from the amphibolites indicate that these minerals crystallized approximately 150 m.y. ago, but the ages measured on glaucophane from the blueschist blocks are commonly younger. These data suggest that the high-grade blue-schist and amphibolite blocks represent fragments of a cryptic metamorphic terrane of pre-Tithonian age that have been tectonically mixed with younger rocks of the Franciscan Formation in California and Otter Point Formation in Oregon. The younger ages for glaucophane probably reflect metamorphic episodes in which lower grade in situ blueschist facies mineral assemblages were developed in the blocks after their emplacement within the Franciscan Formation. This pre-Tithonian cryptic metamorphic terrane probably developed as a result of interaction between oceanic and continental plates. The occurrence of tectonic blocks of this terrane within mélange zones in Oregon and California may be related to later plate interaction.
California, Oregon↗
Age of the Mesozoic oceanic crust in the California Coast Ranges
K-Ar ages of approximately 155 m.y. were measured on minerals from ophiolites at two localities in the California Coast Ranges. Upper Jurassic rocks of the Great Valley sequence are in depositional contact with underlying ophiolites, which are interpreted as Mesozoic oceanic crust that formed during differentiation of mantle material at a spreading ridge some 15 m.y. before deposition of the Great Valley sequence began.
California↗
Age and chemistry of mesozoic and tertiary plutonic rocks in south-central Alaska
On the basis of potassium-argon mineral ages, plutonic rocks in an area of approximately 22,000 square miles in the southern Alaska Range and the Aleutian Range can be assigned to age groups that show differences in chemical characteristics and geographic distribution. The plutonic groups are Early and Middle Jurassic, Late Cretaceous and early Tertiary, and middle Tertiary in age. Most of the plutonic rocks in the Aleutian Range south of Iliamna Lake appear to be Jurassic, but north of Iliamna Lake, Jurassic plutonic rocks seem to be restricted to a belt on the southeast side of the Chigmit Mountains—Alaska Range. In the western or northwestern part of the Alaska Range north of Iliamna Lake, only Cretaceous and Tertiary plutonic rocks have been found. Rocks rich in K-feldspar are predominant in the Cretaceous and Tertiary plutons, but subordinate in the Jurassic plutons. Most of the mineralization in the region is associated with the Cretaceous and Tertiary plutons.
GSA Bulletin↗
Chronology of intrusion, volcanism, and ore deposition at Bingham, Utah— A reply
No abstract available.
Utah↗
Chronology of intrusion, volcanism, and ore deposition at Bingham, Utah
Potassium-argon dates for major igneous rock types in the Bingham mining district, Utah, range from 39 to 32 m.y. and suggest that:(1) Plutonism, volcanism, and hydrothermal activity were sequential stages in a magmatic history of about 7 m.y. duration.(2) Latitic volcanic rocks, in part, postdate emplacement of the Last Chance and Bingham stocks.(3) Sulfide mineralization and hydrothermal alteration followed emplacement of the monzonitic stocks and extrusion of at least the earliest units in the volcanic sequence; the time interval between intrusion and alteration was probably less than 1 m.y.(4) The rhyolites of Shaggy Peak, which may represent terminal differentiation products in a comagmatic series, are the youngest igneous rocks in the area.
Utah↗
Isotopic age of the Nevadan Orogeny and older Plutonic and metamorphic events in the Klamath Mountains, California
Several metamorphic and plutonic events have been recognized in the Klamath Mountains utilizing potassium-argon and rubidium-strontium mineral and whole-rock ages. The oldest known metamorphic event in the region produced the Abrams Mica Schist and the co-extensive Salmon Hornblende Schist. Strontium evolution diagrams indicate that the age of primary metamorphism of the Abrams Mica Schist is approximately 380 m.y. (Devonian). The Stuart Fork Formation of Davis and Lipman (1962), the schists of Condrey Mountain, and related schists were produced during a Middle and Late Jurassic metamorphic event. Granitic plutonic rocks are divided into four groups that are different in age and somewhat different in chemical characteristics. The oldest pluton, the Pit River stock, has a minimum age of 246 m.y. (Permian) and is grouped with the Castle Crags pluton. The other three plutonic groups are Middle and Late Jurassic and are characterized by the following ranges in isotopic age: 165 to 167 m.y., 145 to 155 m.y., and 127 to 140 m.y. The principal lode gold deposits in the California part of the Klamath Mountains seem to be related to the youngest group of plutons. If the Nevadan orogeny in this area is restricted to a Middle and Late Jurassic deformational, metamorphic, and plutonic event, the orogeny includes the emplacement of plutons of the three younger groups and the metamorphic development of the Stuart Fork Formation, schists of Condrey Mountain, and related schists.
California↗
Post-paleozoic radiometric ages and their relevance to fault movements, Northern Southeastern Alaska
Recently determined lead-alpha and potassium-argon ages from northern southeastern Alaska indicate major plutonic events in the Paleozoic, Mesozoic, and Tertiary; in contrast, previous studies suggested that only one complex Jurassic and Cretaceous event occurred. The ages presented in this paper indicate the following Mesozoic and Tertiary plutonic events: Middle or Late Jurassic (144–164 m.y.); Early Cretaceous (103–117 m.y.); Eocene (42–48 m.y.); and Oligocene to Miocene (24–31 m.y.). The present data show no distinctive a real pattern for the Mesozoic plutons, but those of known Tertiary age are restricted to Baranof and Kruzof islands, a distribution that suggests a belt of Tertiary plutonism along the margin of the Pacific Ocean. Stratigraphic evidence and radiometric ages indicate that Baranof Island and possibly Chichagof Island have been uplifted several kilometers since Miocene time, whereas Admiralty Island to the east appears to have been relatively stable since Paleocene time. This movement apparently took place on the north-striking Chatham Strait fault, which separates the islands, and probably also had a large right-lateral component. Northwest-striking faults in Chichagof and Baranof islands were probably active during at least part of the movement on the Chatham Strait fault. Movement on one of the northwest-striking faults, the Patterson Bay fault of Baranof Island, took place some time between the Eocene and the Miocene and produced a 5-km, right-lateral separation. The inferred uplift of Baranof Island relative to Admiralty Island is based on the present-day exposure on Baranof Island of mesozonal Tertiary plutons, which were probably intruded at a depth of several kilometers, contrasted with the present-day exposure on Admiralty Island of continental sedimentary and volcanic rocks that were being deposited near sea level during the Tertiary. The uplift of the Baranof Island plutons to the surface in post-Miocene time contrasts sharply with the stable or weakly negative tectonic conditions that have prevailed on Admiralty Island since the Paleocene.
Alaska↗