Late Cretaceous Intrusions Research Articles

Paleomagnetism of the Late Cretaceous intrusions from the Minusa trough (southern Siberia)

The paper summarizes paleomagnetic and rock-magnetic data on the Late Cretaceous diatremes and associated dikes from the Minusa trough located within the southwestern Siberian Platform. It is shown that the stable characteristic component of magnetization is superimposed magnetization (in physical sense). It is linked to Fe-rich titanomagnetite produced by the decay and oxidation of Ti-rich titanomagnetite derived from a primary magma. This process, however, coincides in time with the intrusion cooling, which is supported by paleomagnetic tests. Correlation of magnetic polarity with 39Ar/ 40Ar ages suggests that the acquired stable characteristic component of magnetization corresponds to magnetic Chrons C33-C32 and characterizes the Middle Campanian magnetic field (74–82 Ma). The mean paleomagnetic pole for this span is located at 82.8° N, 188.5° E, with α 95 = 6.1 and, within confidence intervals, coincides with the reference data from the European part of the Eurasian plate. The excellent agreement between virtual paleomagnetic poles testifies that the intraplate motions in the Mesozoic resulting in the crust deformation of Central Asia ceased in the late Cretaceous or were so small that elude detection by the paleomagnetic method.

New information on the thermal history of the southwestern Lower Saxony Basin, northern Germany, based on fission track analysis

The southwestern part of the Lower Saxony Basin (LSB) is characterized by gravity and magnetic anomalies and by an extremely high thermal maturity of organic matter. This was for many years attributed to a Late Cretaceous intrusion, but actually deep burial is being debated. The complex thermal history of the area has been studied by fission track analysis. Zircon data provide evidence for widespread (hydro)thermal activity during the Permian and Upper Jurassic/Lower Cretaceous. Apatite ages indicate a major cooling event in the mid Cretaceous (∼89–72 Ma) reflecting the time of inversion of the LSB. During the Cretaceous, the cooling of the basin centre was rapid compared to the basin margins. Apatite fission track ages from borehole samples which are recently within the upper part of the APAZ indicate a young heating of the sedimentary sequences until present.

3: Geochronology and geodynamics of Late Cretaceous magmatism and Cu–Au mineralization in the Panagyurishte region of the Apuseni–Banat–Timok–Srednogorie belt, Bulgaria

The Apuseni–Banat–Timok–Srednogorie (ABTS) belt, Europe's most extensive belt of calc–alkaline magmatism and Cu–Au mineralization, is related to the subduction of the Tethys ocean beneath the European continental margin during the Late Cretaceous phase of the Alpine–Himalayan orogeny. Economic deposits are restricted to certain segments along the belt, and all major porphyry-style and high-sulphidation ore deposits in Bulgaria are aligned on the Panagyurishte corridor, a narrow zone obliquely crossing the ABTS belt. This paper reviews the geology, geochemistry and geochronology of igneous and hydrothermal events along this corridor, in a profile extending from the European continental basement in the north to the accretionary complex of the Rhodopes in the south. Extensive U–Pb dating of zircons from subvolcanic intrusions and major plutons, supplemented by published age data for magmatic rocks and hydrothermal ore deposits obtained by other methods, reveals a general younging of magmatism from ca. 92 Ma in the north (Elatsite) to ca. 78 Ma in the south (Capitan Dimitrievo). Cu–Au deposits are restricted to the northern and central parts of the profile (ranging in age from ∼92 to ∼86 Ma), while the southernmost part exposes more deeply eroded mid-crustal plutons devoid of economic mineralization. The age progression correlates with a north-to-south geochemical trend of decreasing crustal input into mantle-derived magmas. Magmatism and ore formation in individual magmatic–hydrothermal complexes along the profile is much shorter lived. For example, subvolcanic intrusions and porphyry Cu–Au mineralization at the Elatsite deposit and andesitic volcanism and high-sulphidation Au–As–Cu mineralization at Chelopech all occurred within a time span of about 1 million years. The age progression of calc–alkaline magmatism within the Panagyurishte region, Bulgaria, from north to south is explained as a consequence of slab retreat during oblique subduction. This led to transtensional block faulting and subsidence, and thus to the preservation of near-surface magmatic–hydrothermal products, including economic Cu–Au deposits. Accretion of continental fragments in the Rhodopes south of the ABTS belt led to greater uplift of the youngest Late Cretaceous intrusions. This removed any ore deposits that might have been formed in this area.

Exploration Techniques at the Thorn Ag‐Au‐Cu High‐Sulfidation Epithermal Prospect, Northern British Columbia, Canada

Abstract. The Thorn prospect is an El Indio‐style high‐sulfidation epithermal prospect in a little‐explored belt of Late Cretaceous subaerial volcanics and subvolcanic intrusions in northwestern British Columbia, Canada. More than 30 massive sulfide (pyrite‐enargite‐tennantite/tetrahedrite) veins, quartz‐sulfide (quartz‐pyrite‐enargite‐tennantite/tetrahedrite) veins and quartz‐vuggy silica‐alunite veins/breccias fill ENE fractures and faults over an area of 1,600 times 1,900 m. They are hosted within a Late Cretaceous feldspar‐quartz‐biotite granodioritic porphyry stock and flanked by alteration envelopes a few meters to tens of meters wide: an inner envelope of intense sericite‐clay‐pyrite and an outer one of weak clay‐sericite‐chlorite. Where several veins are close together, the sericite‐clay‐pyrite alteration envelopes coalesce into zones up to 150 m wide.The most successful exploration techniques have been prospecting, silt and soil geochemistry, airborne magnetic and electromagnetic surveys and diamond drilling. Silt and soil geochemical anomalies for Au, Ag, As, Cu, Pb, Sb and Zn mark the known veins and indicate prospective areas where veins have not yet been discovered. Magnetics and resistivity outline the gross property‐wide structure and lithologies. Most of the sericite‐clay‐pyrite alteration and known veins are outlined by the <264 ohm‐m resistivity contour, which covers an area of 800 times 2,300 m. Weak EM conductors are thought to represent undiscovered massive sulfide and quartz‐sulfide veins, and their flanking sericite‐clay‐pyrite alteration.

Trace element and isotope (Sr, Nd) geochemistry of porphyry- and skarn-mineralising Late Cretaceous intrusions from Banat, western South Carpathians, Romania

Geochemical and isotopic (strontium, neodymium) data have been obtained from 24 samples of seven Late Cretaceous intrusions associated with skarn (Tincova, Bocsa, Ocna de Fier – Dognecea) and porphyry copper-molybdenum/skarn deposits (Oraviţa, Ciclova, Sasca and Moldova Nouă) in the South Carpathians of Romania. The intrusions cut the Supragetic-Getic nappes and form a 100-km-long, NNE-SSW-trending lineament in the Banat metallogenic province. The samples range in composition from gabbro to granodiorite and define, in major and trace element variation diagrams, a calc-alkaline to high-potassium calc-alkaline trend ranging from 50 to 67% SiO2. They are magnetite-bearing, I-type granitoids enriched in LILE/LREE and depleted in Nb, Ta and Zr, common features for magmas associated with subduction zones. The lack of cumulate textures and of positive europium anomalies seems to indicate that the samples represent liquid compositions. The observed trend thus approximates a liquid line of descent resulting from fractional crystallisation of parental magmas with similar major and trace element compositions. Initial strontium isotopic ratios (Sri) vary from 0.7042 to 0.7058 and eNd(t) values range from +3.9 to –0.2. Variations of Sri and eNd(t) values with SiO2 show minor crustal assimilation. The generally positive eNd(t) values, together with moderate Sri, indicate that the parental magmas result from partial melting of a rubidium-enriched and LREE-depleted source, such as a heterogeneous lithospheric mantle or a young mafic lower crust derived from it. Selective rubidium enrichment probably occurred during a previous subduction event by metasomatism of the overlying mantle wedge, either in the Early Cretaceous or earlier, as suggested by TDM model ages (ca. 600 Ma). The Cu-Mo(-Pb-Zn-Fe) deposits are intimately associated with the intrusions, and the Banat (Romania) region can be subdivided in two metallogenic zones. In northern Banat, Fe-Cu-Pb-Zn skarns occur whereas in southern Banat, porphyry-style copper and molybdenum deposits predominate. These differences result from a combination of several parameters: (1) magma composition, whereby copper- and molybdenum-rich deposits tend to be associated with calc-alkaline compositions; (2) an increase of the present-day erosion level, from south to north, as indicated by the presence of large equigranular plutons in northern Banat, and of porphyritic cupolas and apophyses associated with porphyry-style mineralisation in southern Banat; (3) the nature of the host rocks, with skarns preferentially developed in calcareous host rocks; and (4) local variations of conditions controlling the infiltration of fluids and the precipitation of ore minerals.

Structural and metamorphic constraints on ca. 70 Ma deformation of the northern Valhalla complex, British Columbia: implications for the tectonic evolution of the southern Omineca belt

Penetrative deformation occurred ca. 70 Ma ago throughout the northern Valhalla complex in Valhalla and Passmore domes and in the Gwillim Creek shear zone, exposed at the deepest structural levels in both domes. Intense strain ( S T) in the Gwillim Creek shear zone (domain II) was synchronous with and outlasted deformation ( D 2) throughout the northern complex (domain I). Upper-amphibolite facies peak mineral assemblages define the predominant foliation. Temperature and pressure results, determined from microdomains with established relationships to reaction textures and microstructures, provide constraints on conditions under which deformation occurred. Deformation was synchronous with and outlasted peak metamorphic conditions at all structural levels. Peak conditions of 825°C and 730 MPa and 850°C and 840 MPa were determined for domains I and II, respectively. This was followed by cooling and retrograde garnet breakdown at conditions of 715°C and 490 MPa and 765°C and 730 MPa in domains I and II, respectively. The faster cooling rate per kilometer of exhumation for domain II relative to domain I is consistent with a model of conductive cooling via thrusting of domain II on to a cold footwall. Metamorphism is interpreted to have resulted from crustal thickening and burial to depths of ca. 25 km based on an inferred clockwise P– T path and the paucity of Late Cretaceous intrusions. Lack of retrograde metamorphism throughout the complex and the high degree of annealing of microstructures indicates that the rocks remained above greenschist-facies conditions until they were exhumed in the Early Tertiary on the Valkyr–Slocan Lake extensional shear zone system. Previous workers have determined that the peak of metamorphism occurred at 72–67 Ma in a restricted locality in the core of Passmore dome, near Vallican. Our study links this dated metamorphism with the structural evolution and metamorphic history throughout the area, and shows that supracrustal rocks at all structural levels in Valhalla and Passmore domes underwent the same metamorphic and deformation event as those near Vallican. Therefore, we assign a ca. 70 Ma age to the penetrative, high-temperature deformation in northern Valhalla complex and the Gwillim Creek shear zone. This coincides with a major period of shortening in the Rocky Mountains of the Foreland belt. Strain in northern Valhalla complex may represent a local transient shear zone that accommodated crustal thickening in the hinterland during orogen-scale compression, or it may be an exhumed part of the basal detachment of the Rocky Mountains.

Paleomagnetic data from the Caborca terrane, Mexico: Implications for Cordilleran tectonics and the Mojave‐Sonora megashear hypothesis

Two ancient magnetizations have been isolated in rocks of the Caborca terrane, northwest Mexico. The characteristic magnetizations of Neoproterozoic and Paleozoic miogeoclinal shelf‐strata, arc‐derived Lower Jurassic marine strata, and Jurassic volcanic and volcaniclastic rocks are of dual polarity and east‐northeast declination (or south‐south‐west) and shallow inclination. Magnetizations in Neoproterozoic and Paleozoic miogeoclinal strata are interpreted as secondary (J*) and to be of similar age to those observed in Lower and Middle Jurassic rocks. Remanence acquisition is bracketed between about 190 and 160 Ma. The overall mean (D=15.0°, I=8.5°; n=38 sites; six localities; k=19.1, α95=5.5°) suggests a moderate to large clockwise rotation of 12 to 50° (depending on reference direction assumed) of the Caborca terrane, and rocks of the Sonoran segment of the Cordilleran volcanic arc, with respect to the North America craton. When compared with expected inclinations, observed values are not anomalously steep, arguing against statistically significant southward latitudinal displacement of the Caborca block after remanence acquisition. Late Cretaceous intrusions yield primary, dual‐polarity steep inclination “K” magnetizations (D=341.4°, I=52.3°; n=10 sites; five localities; k=38.3, α95=7.9°) and have locally remagnetized Neoproterozoic and Jurassic strata. When present, secondary (K*) magnetizations in Neoproterozoic strata are of higher coercivity and higher unblocking temperature than the characteristic (J*) magnetization. Importantly, the regional internal consistency of data for Late Cretaceous intrusions suggests that effects of Tertiary tilt or rotation about a vertical axis over the broad region sampled (∼5000 km2) are not substantial. Late Cretaceous primary (K) magnetizations and secondary (K*) magnetizations yield a combined mean of D=348.1°, I=50.7° (N=10 localities; 47 sites; k=53.5, α95=6.7°), indicating at most small (<∼10°) clockwise rotation of the Caborca region with respect to the craton. Permissible post‐Late Cretaceous latitudinal displacement is near or below the detection limit of paleomagnetism (<∼300 km). Limited data from Lower Cretaceous strata of the Bisbee Group (D=339.9°, I=47.9°; n=4 sites) suggest that the modest clockwise rotations inferred on the basis of J* magnetizations in Jurassic and older strata occurred in Jurassic time. Together, the lack of evidence for southward displacement, yet evidence for statistically significant clockwise rotation, and the overall similarity of Jurassic magnetizations in the Cordilleran arc with those of the Caborca block, despite the fact that some of them are clearly secondary, are not consistent with the Mojave‐Sonora megashear hypothesis of Late Jurassic left‐lateral strike‐slip motion of the crust of northern Mexico.

Petrogenesis of coexisting SiO 2-undersaturated to SiO 2-oversaturated felsic igneous rocks: The alkaline complex of Itatiaia, southeastern Brazil

The Itatiaia alkaline complex is a Late Cretaceous intrusion (72 Myr) made up of felsic differentiates, with syenitic rocks dominant throughout and with presence of both nepheline- and quartz-rich varieties. Dykes with phonolitic or trachytic composition cross-cut the coarse-grained facies. The rocks are arranged concentrically, with the core of the complex being formed by SiO 2-oversaturated syenites (with a small outcrop of granites), and are radially displaced by faults related to regional tectonic lineaments. The minerals show gradual but significant changes in composition (salitic and augitic to aegirine-rich pyroxenes, hastingsite and actinolite to richterite and arfvedsonite amphiboles, sodic plagioclase to orthoclase feldspars and so on) and the whole-rock trends are broadly consistent with fractional crystallization processes dominated by alkali feldspar removal. Sr-isotopic data indicate more radiogenic ratios for the SiO 2-oversaturated rocks (0.7062-0.7067 against 0.7048-0.7054 for the SiO 2-undersaturated syenites), consistent with small amounts of crustal input. The favored hypothesis for the petrogenesis of the different syenitic groups is the prolonged differentiation starting from differently SiO 2-undersaturated mafic parental magmas (potassic alkali basalts to ankaratrites, present in the Late Cretaceous dyke swarms of the area), accompanied by variable crustal contamination prior to the final emplacement. The lack of carbonatite as a significant lithotype, the potassic affinity of the Itatiaia complex, and the relatively high Sr-isotopic ratios match the characteristics of the other complexes of the Rio de Janeiro-Sa˜o Paulo states coastline and confirm the ultimate derivation of these differentiated rocks from an enriched lithospheric mantle source.

Chapter 19 Jurassic-Cretaceous history

Jurassic-Cretaceous follows Triassic history with minor change. It was an interval dominated by deposition of marine muds, silts and sands, with occasional non-marine environments on advancing deltas (Parker 1967; Harland 1973a; Kelly 1988). Subdued topography contrasted with Triassic and Paleogene terrains. But there was also Late Jurassic and Early Cretaceous intrusion of basic sills and volcanism in eastern Svalbard. Figure 19.1 shows the distribution of Jurassic and Cretaceous deposits in Svalbard. The two periods (208-65 Ma) span 143 million years but the stratal record for this interval totals only 1700 m of which more than half was deposited in Albian time. The Jurassic-Cretaceous rocks of the eastern platform, presently cropping out on some islands, represent a relict of a once continuous sheet of strata, which is still preserved extensively across much of the Barents Shelf. The Triassic-Jurassic boundary is marked by seemingly continuous facies from Rhaetian to Toarcian; but then follows a contrast between the main Spitsbergen Basin (which hardly subsided) and the Eastern Platform. The contrasting areas of east and west Svalbard were divided by the continuing activity along the Billefjorden lineament. To the east, subsidence permitted a complex and variable sequence resulting from deltas from the east (marine and non-marine) through Liassic to mid-Bathonian time. To the west there was little evident subsidence and only condensed deposits of the uppermost Wilhelmoya Formation were washed by shallow seas. This part of the story concluded the history of the Kapp Toscana Group. A Late Bathonian marine transgression transformed east and

Mesozoic invasion of crust by MORB-source asthenospheric magmas, U.S. Cordilleran interior

Mafic and ultramafic xenoliths entrained in lavas of the Cima volcanic field have Nd and Sr isotopic ratios indicative of a source similar to that of mid-ocean ridge basalt (MORB). Nd and Sr internal isochrons demonstrate a Late Cretaceous intrusion age. These results, combined with evidence for emplacement in the lower crust and upper mantle, indicate invasion of the lower crust by asthenospheric magmas in the Late Cretaceous. Constituting the first prima facie evidence for depleted-mantle magmatism in the Basin and Range province prior to late Cenozoic volcanism, these results lend key support to models suggesting crustal heating by ascent of asthenosphere in the Mesozoic Cordilleran interior.

Tectonic Patterns and Cenozoic Basalts in the Western Margin of the South China Sea: ABSTRACT

The allochthonous fragments - Indosinia, Sibumasu, East Malaya, and Southwest Kalimantan - rifted from Gondwanaland and drifted northward. Indosinia collided with the Yangzi-Huanan terranes in Devonian or Early Carboniferous period and became part of the East Asia continent. Sibumasu collided· with East Asia continent and East Malaya during the Indosinian Orogeny (220-200 Ma). The Southwest Kalimantan terrane probably rifted from the northeast margin of Indosinia in the Cretaceous. ·At about 50 Ma the collision of the Indian continent into the Eurasian continent led to the fragmentation of Asia and was followed by the opening of the Andaman Sea, the clockwise rotation of the Indochina plate, and the rifting and the opening of the South China Sea. The Late Cretaceous alkaline intrusions in the Red River area of northern Vietnam formed during initial stage of rifting of the South China Sea. The Indian-Eurasian collision has successively pushed the Indochina Peninsula in the east­ southeast direction. Most of the Middle Tertiary movements probably occurred along the left-lateral Red River, Tonle Sap-Mekong faults concurrent with the opening of most of the eastern South China Sea. The extensional tectonics along these predominantly strike-slip faults may be responsible for the Pliocene­ Pleistocene alkaline basalts, which extend from the Mekong Delta northwestward into Thailand. Some basalt outcrops in Thailand, Kampuchea and Vietnam appear to trend north-south. The composition of basalts ranges from tholeiitic to alkalic. The basanitoid basalts contain abundant megacrysts of zircon, ruby, sapphire, and spinel. The megacrysts probably originated through assimilation of metamorphic rocks deep in the continental crust. The cores from the South China Sea indicate that the increase in subsidence rate appears in wells during the Quaternary. It may be related to the increase in Quaternary basalt and high heat flow in the western part of the South China Sea basin.

Accretion, collision and strike-slip faulting: the Woyla group as a key to the tectonic evolution of North Sumatra

The Woyla Group exposed in the Batang Natal River, North Sumatra, is made up of fault-bounded silvers of serpentinite, pillow basalt, ribbon chert, pelagic silstones, mélanges, andesitic volcanics, volcaniclastic turbidites and massive limestones. This assemblage has previously been interpreted as fragments of a collapsed Jurassic-Late Cretaceous marginal basin. In the present study the Woyla Group is re-interpreted as part of an accretionary complex formed from ocean floor materials of Triassic to Early Cretaceous age, incorporating collided sea-mounts, plateaux and volcanic arc fragments accumulated during the subduction of a major ocean, Tethys III, prior to India's collision with Asia. Early Cretaceous fossils and Late Cretaceous intrusions constrain the time of accretion to mid-Cretaceous. The Langsat volcanics, associated volcaniclastics and intrusive granodiorites at the western end of this section, have been dated by the K/Ar method as Late Oligocene, demonstrating that they are unrelated to the rest of the complex and were emplaced along strike-slip faults prior to the mid-Miocene. The Woyla Group is considered in the context of the tectonic evolution of North Sumatra, resulting from the break-up of Gondwanaland and the accumulation of its fragments along the margin of Asia, the collision of India, the opening of the Andaman Sea and the initiation of the present arc and the Sumatran strike-slip fault zone.

The Scotia–Quaal metamorphic belt: a distinct assemblage with pre- early Late Cretaceous deformational and metamorphic history, Coast Plutonic Complex, British Columbia

The Scotia–Quaal metamorphic belt extends from Hawkesbury Island to Work Channel between the early Late Cretaceous Ecstall and the Paleogene Quottoon plutons. The belt consists of a Proterozoic?–Paleozoic metasedimentary and metavolcanic sequence, the Middle Devonian Big Falls orthogneiss, Early Jurassic orthogneiss, and Jurassic or Cretaceous mafic and ultramafic intrusive rocks. The assemblage may be correlative with Nisling terrane lithologies and may have shared a common history, at least from Early Jurassic time on, with rocks of Stikine terrane.Strong planar and linear fabrics, abundant folds, and scarcity of kinematic indicators characterize the belt's deformational style. Development of a strong foliation followed by three episodes of folding occurred between emplacement of the Middle Devonian Big Falls orthogneiss and early Late Cretaceous Ecstall intrusion. Paleogene fabrics occur in the Quottoon pluton and in the easternmost 1.5 km of the Scotia-Quaal belt. If a major Paleogene shear zone is postulated to explain the disparity in cooling dates, metamorphic histories, and structural styles between the western and central Coast Plutonic Complex, then it does not traverse the Scotia–Quaal metamorphic belt, but must be located within or at the western edge of the Quottoon pluton.Medium-pressure, epidote–amphibolite to upper amphibolite facies metamorphic conditions are preserved in the central region. Metamorphic grade increases gradually across the belt from west to east and from south to north. Regional metamorphism outlasted regional deformation. Contact metamorphism associated with Quottoon and Ecstall intrusion is apparent only in rocks of the southern region where regional metamorphic grades are lowest.

Sequential Mesozoic and Cenozoic Tectonic Events in Grant Range and Railroad Valley, Nye County, Nevada: ABSTRACT

A series of palinspastic geologic cross sections were constructed for the Grant Range-Railroad Valley (GR-RRV) area. This palinspastic reconstruction differs from previously published interpretations in two major ways. First, new granite age dates and a chronological framework deduced by Fryxell in 1984 in the central and southern Grant Range were applied, where possible, to the northern Grant Range. Second, recent drilling was correlated to outcrop interpretations. Results of this reconstruction suggest that significant tectonism affected the GR-RRV area earlier than previously believed. Mesozoic compression produced east-vergent folding in the GR-RRV area, followed by Late Cretaceous intrusion. Local, possibly blind thrusting may have been associated with the intrusion. Above the granite dome, shallow, brittle, listric faulting and deeper low-angle attenuation faulting accommodated intrusion. After intrusion, relative quiescence prevailed during the deposition of the Sheep Pass Formation and the lower Garrett Ranch Group ignimbrite. Uplift in the middle and late Oligocene produced pervasive listric, normal, east-dipping faulting that may in part be represented by apparent growth faults within volcanics. Subsequent west-dipping listric faulting was in part gravity driven. High-angle Miocene normal faulting initiated the present structure. All commercial oil fields in Railroad Valley are producing from Miocene and younger structures. However, anmore » understanding of older structures will allow more accurate prediction of reservoirs within a particular, prospective, younger structure.« less

Great Stone Dome--Great Disappointment?: ABSTRACT

Baltimore Canyon Trough's Great Stone dome is one of the largest, most prominent subsurface structural features in the Atlantic offshore. The dome, with nearly 140,000 ac under closure and more than 11,000 ft of structural relief, was formed by the early Late Cretaceous intrusion of a large mafic igneous body. The uplifting resulted in a major Barremian unconformity across the dome along with minor higher unconformities. The dome was the site of intense bidding during 1976 when $651 million was bid for 37 tracts including $107.8 million for one crestal tract. Seven wells were subsequently drilled on the dome to total depths from 12,000 to 17,449 ft. The wells penetrated Cretaceous and Jurassic sandstones with porosities ranging up to 30%, but none encountered pay or significant shows. Lack of hydrocarbons in the section tested appears to be a result of low organic matter content of shales. Thermal maturation profiles show source rocks changing abruptly from immature to overmature across the Barremian unconformity. If hydrocarbons were generated (probably gas), lack of adequate seals at or above the unconformity prevented entrapment in crestal parts of the structure. The seven wells mainly tested structural traps on the dome. Additional prospects remain to be tested, such as possible flank fault traps and stratigraphic traps formed by sands pinching out around the dome's flanks where mature source rocks are likely to occur. End_of_Article - Last_Page 1433------------

Dynamothermal Contact Metamorphism Superposed on Regional Metamorphism in the Pelitic Rocks of the Chiwaukum Mountains Area, Washington Cascades

Research Article| July 01, 1980 Dynamothermal Contact Metamorphism Superposed on Regional Metamorphism in the Pelitic Rocks of the Chiwaukum Mountains Area, Washington Cascades Charles C. Plummer Charles C. Plummer 1Geology Department, California State University, Sacramento, California 95819 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1980) 91 (7_Part_II): 1627–1668. https://doi.org/10.1130/GSAB-P2-91-1627 Article history received: 17 Aug 1979 rev-recd: 21 Jan 1980 accepted: 05 Feb 1980 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Charles C. Plummer; Dynamothermal Contact Metamorphism Superposed on Regional Metamorphism in the Pelitic Rocks of the Chiwaukum Mountains Area, Washington Cascades. GSA Bulletin 1980;; 91 (7_Part_II): 1627–1668. doi: https://doi.org/10.1130/GSAB-P2-91-1627 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Chiwaukum Schist, in the Washington Cascade Mountains, has undergone two distinct episodes of metamorphism: (1) Regional Barrovian-type metamorphism and (2) subsequent “contact” metamorphism, associated with the Late Cretaceous intrusion of the Mount Stuart batholith, comparable to part of the Buchan-type progression.The later metamorphic event is unusual because the effects are predominantly dynamothermal, although in contact with and genetically related to plutonic intrusion. Some indications that penetrative deformation accompanied recrystallization during “contact” metamorphism are (1) preferred orientation of minerals such as cordierite, andalusite and sillimanite; (2) alignment of mafic minerals within granitic rocks near contacts; (3) granitic contacts being generally parallel to the predominant s-plane of the schist; (4) biotite within the schists showing more pronounced alignment the closer it is to granitic contacts; and (5) minerals in the schists near contacts commonly being microboudinaged and showing evidence for rotation during and subsequent to crystallization. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Dynamothermal contact metamorphism superposed on regional metamorphism in the pelitic rocks of the Chiwaukum Mountains area, Washington Cascades: Summary

The Chiwaukum Schist, in the Washington Cascade Mountains, has undergone two distinct episodes of metamorphism: (1) Regional Barrovian-type metamorphism and (2) subsequent “contact” metamorphism, associated with the Late Cretaceous intrusion of the Mount Stuart batholith, comparable to part of the Buchan-type progression. The later metamorphic event is unusual because the effects are predominantly dynamothermal, although in contact with and genetically related to plutonic intrusion. Some indications that penetrative deformation accompanied recrystallization during “contact” metamorphism are (1) preferred orientation of minerals such as cordierite, andalusite and sillimanite; (2) alignment of mafic minerals within granitic rocks near contacts; (3) granitic contacts being generally parallel to the predominant s -plane of the schist; (4) biotite within the schists showing more pronounced alignment the closer it is to granitic contacts; and (5) minerals in the schists near contacts commonly being microboudinaged and showing evidence for rotation during and subsequent to crystallization.