Revised ages of blueschist metamorphism and the youngest pre-thrusting rocks in the San Juan Islands, Washington

2005 ◽  
Vol 42 (7) ◽  
pp. 1389-1400 ◽  
Author(s):  
E H Brown ◽  
T J Lapen ◽  
R Mark Leckie ◽  
Isabella Premoli Silva ◽  
Davide Verga ◽  
...  
Keyword(s):  
Subduction Zones ◽  
Late Jurassic ◽  
Volcanic Rocks ◽  
Sedimentary Rocks ◽  
San Juan ◽  
San Juan Islands ◽  
Regional Deformation ◽  
Facies Metamorphism ◽  
Thrust System ◽  
Western Washington

New ages of rocks in the San Juan Islands, northwest Washington, significantly change our understanding of the evolution of the San Juan Islands thrust system. Re-examination of foraminifera-bearing mudstones at Richardson on Lopez Island indicates a late Aptian (112–115 Ma), not late Albian (100 Ma) age as currently presented in the literature. The age brackets of thrusting, marked by these pre-thrusting mudstones and 84-Ma post-thrusting sedimentary rocks, span a much longer period than previously thought, diminishing controls on rates of displacement in the thrust system and the timing of regional deformation in western Washington. New 40Ar/39Ar plateau ages of phengite in blueschist-facies meta-volcanic rock, also at Richardson, are 124 ± 0.7 Ma (2σ, late Barremian). These blueschist-facies volcanic rocks are in fault contact with the fossiliferous mudstones. Therefore, the blueschist-facies metamorphism at Richardson, previously inferred to be associated with the thrusting, now appears to have occurred prior to thrusting. Further, the Ar ages demonstrate that blueschist-facies fabric formed earlier than the thrust event and is therefore not directly useful in analyzing the thrusting kinematics. The Richardson 40Ar/39Ar age is similar to isotopic ages found in the eastern San Juan Islands and in the Shuksan blueschist terrane in the northwest Cascades, and thus fits into an emerging regional age pattern of blueschist-facies metamorphism during Late Jurassic – Early Cretaceous (up to Barremian) but not late Albian – Cenomanian. If this pattern is more broadly confirmed for the San Juan Islands, all the blueschist-facies metamorphism can be regarded as having formed in subduction zones elsewhere along the continental margin rather than in the anomalous setting of an on-land thrust system, as in the San Juan Islands.

A Discussion on global tectonics in Proterozoic times - Late Proterozoic cratonization in southwest Saudi Arabia

1976 ◽  
Vol 280 (1298) ◽  
pp. 517-527 ◽  
Keyword(s):  
Saudi Arabia ◽  
Volcanic Rocks ◽  
Sedimentary Rocks ◽  
Granulite Facies ◽  
Greenschist Facies ◽  
Arabian Shield ◽  
Granulite Facies Metamorphism ◽  
Quartz Monzonite ◽  
Facies Metamorphism ◽  
Global Tectonics

Early cratonal development of the Arabian Shield of southwestern Saudi Arabia began with the deposition of calcic to calc-alkalic, basaltic to dacitic volcanic rocks, and immature sedimentary rocks that subsequently were moderately deformed, metamorphosed, and intruded about 960 Ma ago by dioritic batholiths of mantle derivation (87Sr/86Sr = 0.7029). A thick sequence of calc-alkalic andesitic to rhyodacitic volcanic rocks and volcanoclastic wackes was deposited unconformably on this neocraton. Regional greenschistfacies metamorphism, intensive deformation along north-trending structures, and intrusion of mantle-derived (87Sr/86Sr = 0.7028) dioritic to granodioritic batholiths occurred about 800 Ma. Granodiorite was emplaced as injection gneiss about 785 Ma (87Sr/86Sr = 0.7028- 0.7035) in localized areas of gneiss doming and amphibolite to granulite facies metamorphism. Deposition of clastic and volcanic rocks overlapped in time and followed orogeny at 785 Ma. These deposits, together with the older rocks, were deformed, metamorphosed to greenschist facies, and intruded by calc-alkalic plutons (87Sr/86Sr = 0.7035) between 600 and 650 Ma. Late cratonal development between 570 and 550 Ma involved moderate pulses of volcanism, deformation, metamorphism to greenschist facies, and intrusion of quartz monzonite and granite. Cratonization appears to have evolved in an intraoceanic, island-arc environment of comagmatic volcanism and intrusion.

Obducted nappe sequence in the San Juan Islands – northwest Cascades thrust system, Washington and British Columbia

2012 ◽  
Vol 49 (7) ◽  
pp. 796-817 ◽  
Author(s):  
E.H. Brown
Keyword(s):  
British Columbia ◽  
Detrital Zircons ◽  
San Juan ◽  
San Juan Islands ◽  
Tectonic History ◽  
Plutonic Complex ◽  
Coast Plutonic Complex ◽  
History Of ◽  
Accreted Terranes ◽  
Thrust System

The San Juan Islands – northwest Cascades thrust system in Washington and British Columbia is composed of previously accreted terranes now assembled as four broadly defined composite nappes stacked on a continental footwall of Wrangellia and the Coast Plutonic Complex. Emplacement ages of the nappe sequence are interpreted from zircon ages, field relations, and lithlogies, to young upward. The basal nappe was emplaced prior to early Turonian time (∼93 Ma), indicated by the occurrence of age-distinctive zircons from this nappe in the Sidney Island Formation of the Nanaimo Group. The emplacement age of the highest nappe in the thrust system postdates 87 Ma detrital zircons within the nappe. The nappes bear high-pressure – low-temperature (HP–LT) mineral assemblages indicative of deep burial in a thrust wedge; however, several features indicate that metamorphism occurred prior to nappe assembly: metamorphic discontinuities at nappe boundaries, absence of HP–LT assemblages in the footwall to the nappe pile, and absence of significant unroofing detritus in the Nanaimo Group. A synorogenic relationship of the thrust system to the Nanaimo Group is evident from mutually overlapping ages and by conglomerates of Nanaimo affinity that lie within the nappe pile. From the foregoing relations, and broader Cordilleran geology, the tectonic history of the nappe terranes is interpreted to involve initial accretion and subduction-zone metamorphism south of the present locality, uplift and exhumation, orogen-parallel northward transport of the nappes as part of a forearc sliver, and finally obduction at the present site over the truncated south end of Wrangellia and the Coast Plutonic Complex.

Late Jurassic (Kimmeridgian-Tithonian) macrofossil assemblage from Jason Peninsula, Graham Land: evidence for a significant northward extension of the Latady Formation

1997 ◽  
Vol 9 (4) ◽  
pp. 434-442 ◽  
Author(s):  
T.R. Riley ◽  
J.A. Crame ◽  
M.R.A. Thomson ◽  
D.J. Cantrill
Keyword(s):  
Antarctic Peninsula ◽  
Late Jurassic ◽  
Volcanic Rocks ◽  
Sedimentary Rocks ◽  
Coarse Grained ◽  
Areal Extent ◽  
South Eastern ◽  
Back Arc ◽  
The Antarctic ◽  
Major Sequence

New exposures of fossiliferous sedimentary rocks at Cape Framnes, Jason Peninsula (65°57′S, 60°33′W) are assigned to the Middle–Late Jurassic Latady Formation of the south-eastern Antarctic Peninsula region. A sequence of fine to coarse-grained sandstones of unknown thickness has yielded a molluscan and plant macrofossil assemblage rich in the following elements: perisphinctid ammonites, belemnopseid belemnites, oxytomid, trigoniid and astartid bivalves, and bennettitalean fronds and fructifications. The overwhelming age affinities are with the Kimmeridgian–early Tithonian part of the Latady Formation, as exposed on the Orville and Lassiter coasts. The Cape Framnes sedimentary rocks help to constrain the age of a major sequence of acid volcanic rocks on Jason Peninsula, and show that the Latady Basin was geographically much more extensive than recognized previously. It was the principal depositional centre of Middle–Late Jurassic sedimentation in the Antarctic Peninsula back-arc region and in areal extent may have rivalled the essentially Cretaceous Larsen Basin.

Upper Paleozoic to lower Mesozoic strata and their conodonts, western Coast Plutonic Complex, British Columbia

1985 ◽  
Vol 22 (9) ◽  
pp. 1329-1344 ◽  
Author(s):  
G. J. Woodsworth ◽  
M. J. Orchard
Keyword(s):  
Volcanic Rocks ◽  
Sedimentary Rocks ◽  
Late Triassic ◽  
Western Coast ◽  
Upper Paleozoic ◽  
Facies Metamorphism ◽  
Volcanic Suite ◽  
Plutonic Complex ◽  
Coast Plutonic Complex ◽  
Greenschist Facies Metamorphism

Six lithologic units, including two newly named formations, were mapped on Randall, Dunira, and nearby islands. The islands are characterized by greenschist-facies metamorphism and westerly directed thrusting. The oldest unit is a Late Mississippian, massive limestone on Ducie Island. The Dunira Formation, composed of thin-bedded limestone and siltstone, is Early and Middle Pennsylvanian in age. It is unconformably overlain by limestone and dolomite of the Upper Triassic Randall Formation. The Randall Formation grades upwards into a green phyllitic unit of Late Triassic(?) age. Rhyolitic and more mafic volcanic rocks may represent a bimodal volcanic suite of Early Jurassic age, based on a U–Pb date of 188 Ma on zircons. These five units correlate with rocks in the Alexander Terrane in southeastern Alaska. The sixth and presumed youngest unit consists of flysch-like sedimentary rocks of probable Middle Jurassic to Early Cretaceous age that may correlate with rocks of the Gravina–Nutzotin belt. The three older units yielded 15 conodont genera from 29 localities. The 13 Paleozoic genera are described and illustrated.

Turonian (Upper Cretaceous) lithostratigraphy and biochronology, southern Gulf Islands, British Columbia, and northern San Juan Islands, Washington State

2005 ◽  
Vol 42 (11) ◽  
pp. 2001-2020 ◽  
Author(s):  
James W Haggart ◽  
Peter D Ward ◽  
William Orr
Keyword(s):  
British Columbia ◽  
Washington State ◽  
San Juan ◽  
Submarine Fan ◽  
Island Formation ◽  
Shallow Marine ◽  
San Juan Islands ◽  
Thrust System ◽  
Gulf Islands ◽  
Facies Succession

Clastic strata preserved on Sidney Island, Barnes Island, and adjacent islands of the southernmost Gulf Islands of British Columbia and the northern San Juan Islands of Washington State are assigned to new stratigraphic units: the Sidney Island Formation and the Barnes Island Formation. The Sidney Island Formation consists of basal conglomerate and sandstone that grades upward through planar-stratified sandstone into hummocky cross-stratified sandstone and siltstone, all of which are deposited in relatively shallow-marine environments. The Barnes Island Formation, in contrast, consists of deep-marine conglomerate, sandstone, and mudstone that was deposited in a submarine-fan setting. Mollusk fossils from the Sidney Island Formation are of Early to Middle Turonian age, whereas ammonites and foraminifers from the Barnes Island Formation indicate a Late Turonian age. The Sidney Island Formation thus records initial marine transgression and inundation of basement rocks, followed by basin deepening that is transitional to the deep-marine submarine-fan deposits of the Barnes Island Formation. Sidney Island Formation strata have been considered previously as derived from uplift along the nearby San Juan thrust system in mid-Cretaceous time. However, the shallow-marine strata are internally well organized, and the facies succession is persistent across the formation's outcrop area. In addition, the formation lacks the distinctive detrital metamorphic mineral assemblages that are characteristic of older rocks of the San Juan Islands. These observations suggest that strata of the Sidney Island Formation did not accumulate immediately adjacent to active thrusting but rather in a more distal setting relative to the thrust system.

scholarly journals Age and Geochemistry of Late Jurassic Mafic Volcanic Rocks in the Northwestern Erguna Block, Northeast China

Minerals ◽  
2021 ◽  
Vol 11 (9) ◽  
pp. 1010
Author(s):  
Yan Li ◽  
Feng-Jun Nie ◽  
Zhao-Bin Yan
Keyword(s):  
Subduction Zones ◽  
Late Jurassic ◽  
Volcanic Rocks ◽  
Magmatic Evolution ◽  
Compositional Evolution ◽  
Ti Oxides ◽  
Lithospheric Extension ◽  
Wide Range ◽  
Magma Sources ◽  
Light Rare Earth Elements

The northwestern Erguna Block, where a wide range of volcanic rocks are present, provides one of the foremost locations to investigate Mesozoic Paleo-Pacific and Mongol-Okhotsk subduction. The identification and study of Late Jurassic mafic volcanic rocks in the Badaguan area of northwestern Erguna is of particular significance for the investigation of volcanic magma sources and their compositional evolution. Detailed petrological, geochemical, and zircon U-Pb dating suggests that the Late Jurassic mafic volcanic rocks formed at 157–161 Ma. Furthermore, the geochemical signatures of these mafic volcanic rocks indicate that they are calc-alkaline or transitional series with weak peraluminous characteristics. The rocks have a strong MgO, Al2O3, and total alkali content, and a SiO2 content of 53.55–63.68 wt %; they are enriched in Rb, Th, U, K, and light rare-earth elements (LREE), and depleted in high-field-strength elements (HFSE), similar to igneous rocks in subduction zones. These characteristics indicate that the Late Jurassic mafic volcanic rocks in the Badaguan area may be derived from the partial melting of the lithospheric mantle as it was metasomatized by subduction-related fluid and the possible incorporation of some subducting sediments. Subsequently, the fractional crystallization of Fe and Ti oxides occurred during magmatic evolution. Combined with the regional geological data, it is inferred that the studied mafic volcanic rocks were formed by lithospheric extension after the closure of the Mongol-Okhotsk Ocean.

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