Showing papers on "Terrane published in 1979"

Petrology and Provenance of Neogene Sand from Nicobar and Bengal Fans, DSDP Sites 211 and 218

Abstract: The Bengal-Nicobar submarine fan complex is part of a linked sedimentary chain consisting of molasse, deltaic, and flysch deposits resulting from the sequential closing of a remnant ocean basin. Ultimate sources for turbidite sand from this fan complex are the uplifted gneissic, sedimentary, and metasedimentary terranes of the Himalayas. Detailed point-counts of lithic grains, as well as standard QFL percentages, of 22 Neogene sand samples from DSDP sites 211 and 218 reveal very uniform compositions. Typical QFL percentages are: 55-30-15. Plagioclase/total feldspar is typically near 0.7. Lithic types are dominated by quartz-mica tectonite. quartz-mica aggregate, polycrystalline mica, and other sedimentary and metasedimentary varieties. Andesitic volcanic lithic grains are absent. The indicated provenance ("tectonic highlands") for Bengal-Nicobar sands contrasts markedly with that of sand and sandstone derived from magmatic arcs and rifted continental margins. Lithic populations of magmatic arc sand and sandstone are dominated by volcanic grains. whereas lithic populations of rifted continental margin sand and sandston are dominated by polycrystalline quartz and sedimentary grains. Two triangular plots of lithic grains distinguish the provenance of sandstone derived from major tectonic settings. Detailed point-counts of lithic grains from thick sedimentary accumulations of unknown tectonic setting are a powerful paleogeographic tool when used in the manner outlined here.

Initial 87 Sr/ 86 Sr ratios of plutonic rocks from Japan

Abstract: Initial 87Sr/86Sr rations were determined for more than 80 plutonic rocks in Japan. The 87Sr/86Sr ratios of gabbroic and granitic rocks show no significant difference in plutonic terranes where both rocks occur closely associated, implying a genetic relationship between them (e.g., Green Tuff belt) or reequilibration at deep level (e.g., Ryoke belt). Wherever granitic rocks occur independently from gabbroic rocks, the granites have higher ratios than the gabbros. Initial 87Sr/86Sr ratios of the granitic rocks are low (<0.706) in Northeast Japan but high (<0.706) in Southwest Japan, the boundary being the Tanakura Tectonic Line. Within Southwest Japan, the ratios are low along the Japan Sea side of the southernmost area. This regional variation is generally correlated with thickness of the continental crust as deduced from the Bouguer anomaly. Initial 87Sr/86Sr ratios of the granitic rocks vary from 0.7037 to 0.7124. The low group (<0.706) is considered to consist of essentially mantle-derived magmas contaminated by crustal material in lesser but varying degree, because of its geological setting and initial 87Sr/86Sr values. The high group may have been formed by contamination of a deep-seated magmas by crustal material or by generation of the main part of the magmas within the continental crust. The ratios of individual belts reflect their own history depending upon age and Rb/Sr ratio of the crustal material. Initial 87Sr/86Sr ratios of granitic rocks are generally low for the magnetite-series but high for the ilmenite-series. Thus, a negative correlation is observed between initial ratios and δ34S for most Cretaceous-Paleogene granites. However, Neogene ilmenite-series granites are low in both initial 87Sr/86Sr and δ34S indicating interaction of the granitic magma with young sedimentary rocks enriched in 32S.

Collided Paleozoic Microplate in the Western United States

Abstract: A region of the Cordillera is underlain by a Paleozoic microplate, Sonomia, that accreted to the passive margin of sialic North America in Triassic time Sonomia consists chiefly of sequential Paleozoic island arc terranes and is coupled in part to a deeper structure Sonomia migrated in the Permian with left-oblique convergence above a consuming boundary toward the continent and propelled before it an accretionary wedge of ocean floor strata Undersliding of the wedge by the continental slope and outer shelf created the Golconda allochthon, followed by an arrested subduction below Sonomia Pre-collision motions on Sonomia's outboard margin may have been kinematically related to the truncation of the sialic continent south of the Sonomia suture Sonomia may be related to other Paleozoic arc fragments that accreted at various times in the Pacific Northwest

Mesozoic forearc basin in central Oregon

Abstract: The Upper Triassic to Upper Jurassic sequence of the John Day inlier was deposited along the seaward flank of a major forearc basin. The clastic strata mask the buried contact between the coeval Huntington arc terrane on the southeast and an uplifted melange belt on the northwest. Counterparts elsewhere may be important mid-Mesozoic tectonic elements of the Cordillera.

Petrography of the Caples terrane of the Thomson Mountains, northern Southland, New Zealand

Abstract: Sediments of the Caples terrane in the Thomson Mountains have been mapped in five units, namely Bold Peak, Kays Creek, Upper Peak, Momus Sandstone, and Mt Campbell Formations. Each unit has a distinctive clastic petrography and a diagnostic heavy mineral assemblage, as well as being sedimentologically distinct. The Caples terrane as a whole is quartz poor, and clastic assemblages are dominated by volcanic (andesitic) and plutonic (dioritic) detritus. Bold Peak Formation contains microlitic volcanic and plutonic detritus; Atomodesma limestone and extrabasinal calcareous sediment clasts are minor, but significant, components. Lithologies and proportions of conglomerate clasts differ from those of clasts in the enclosing sandstones. Bold Peak Formation sandstones have an average detrital mode of Q10F15L75. The heavy mineral suite is dominated by hornblende, clinopyroxene, and epidote. Momus Sandstone and Mt Campbell Formations are petrographically indistinguishable. They are dominated by dioritic de...

Ophiolite and island-arc volcanism in Costa Rica

Abstract: The Pacific margin of Costa Rica is a very deformed basement terrane of ophiolite composed of pillowed and massive basalt, mafic and ultramafic plutonic rocks, volcanic breccia, hyaloclastite, radiolarian chert, and limestone. The ophiolite underwent changes through burial metamorphism after subduction of the Cocos plate under the southwestern margin of the Caribbean plate. In some areas the ophiolite is a melange. While the age of the emplacement of the ophiolite in northwestern Costa Rica is late Santonian to early Campanian, the period of accumulation of the ophiolite seems to be very long, possibly extending from middle Tithonian to late Santonian. New age determinations based on foraminifera and radiolaria support the previous dating. A relative scarcity of turbidite in the ophiolite of Costa Rica, compared to other similar terranes of the Pacific margin, has been associated with the intra-oceanic origin of the southern Central American arc. The island-arc suite clastic rocks first unconformably covered the ophiolite in Costa Rica in early Campanian time. Their origin is closely related to the intrusive and volcanic activity of a plutonic-volcanic arc located between South and North America since early Campanian time. The clastic rocks of the island-arc suite are low-porosity volcanogenic types. Limestone has accumulated from Cretaceous time to the present, in some areas forming porous biohermal bodies. Vertical tectonics originated marginal- and intra-arc sedimentary basins intermittently throughout the evolution of the arc. A fourfold geotectonic division of Costa Rica is proposed.

Correlation of middle and upper Proterozoic strata of the northern rim of the North Atlantic craton

Abstract: On the western and eastern sides of the North Atlantic craton, rocks between about 1·7 b.y. and 1·2 b.y. old are represented by thick basinal assemblages. Thin platformal successions cover the intervening region. The clastic component of these rocks was largely derived from the southeast where the Elsonian orogen might have been an important source terrane. Widespread mafic igneous activity (about 1·2 b.y. ago) in the central platformal region preceded orogenic movements in the basinal areas (Racklan-Carolinidian-Grenville orogeny). The Grenville orogen may have contributed significant amounts of clastic debris for a second cycle of sedimentary accumulation(∼ 1·2 b.y.–∼ 0·8 b.y.). The upper part of this cycle typically contains red beds and evaporites and, in the North American Cordillera, shows evidence of contemporaneous block faulting and extensional tectonism. These movements culminated in the East Kootenay and Hayhook orogenies and ushered in a third period of deposition about 850 m.y. ago. The third sequence includes glaciogenic rocks in the Cordilleran and East Greenland geosynclines. The basic geotectonic elements (platforms and geosynclines) of the North Atlantic craton were established as much as 1·7 b.y. ago, but continental fragmentation on the borders of the craton may not have taken place until early Phanerozoic times.

Petrography of Subduction Zone Sandstones from Nias Island, Indonesia

Abstract: Rocks on Nias Island, Indonesia can be divided into two tectonostratigraphic units: (1) highly deformed late Oligocene-early Miocene trench deposits (tectonic melange); and (2) Miocene-Pliocene trench slope deposits. Modal point counts of 24 representative samples of sandstones, half from the melange, and half from the slope strata, reveal that they are rich in quartz and sedimentary/metasedimentary lithic fragments. Melange and slope sandstone points are closely clustered on the QFL diagram, however several other differences serve to distinguish them petrographically. Trench slope sandstones contain significant amounts of K-feldspar, polycrystalline quartz, glauconite, and carbonate grains and cement, and the lithic fragment population averages Lv21 Lm 3 Ls26. Melange sandstones contain only small amounts of polycrystalline quartz, insignificant amounts of K-feldspar and glauconite, are cemented by phyllosilicate minerals rather than calcite, and the lithic population averages Lv37 Lm17 Ls46. Similar differences may characterize trench/trench slope deposits elsewhere. The quartzose nature of the Nias sediments indicates that they were derived from the west coast of Sumatra, where early Tertiary quartz-rich sediments and Paleozoic/Mesozoic metamorphic and plutonic rocks are exposed. Nias sandstones have much lower contents of volcanic lithic grains than do most other arc-derived sandstones. This may in part be due to the nonvolcanic source terrane on the west coast of Sumatra, and may also be due to volcanic inactivity in the late Oligocene and paleodrainage away from the trench.

Early Triassic (Smithian) Ammonites of Paleoequatorial Affinity from the Chulitna Terrane, South-Central Alaska

Abstract: -----------------.................... ---------........................... Bl Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Occurrence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Nature of fauna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Paleogeographic interpretation _ _ _ 3 References cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Ophiolitic detritus in the Upper Ordovician flysch of Notre Dame Bay and its bearing on the tectonic evolution of western Newfoundland

Abstract: Samples from the Upper Ordovician graywacke-shale sequence in western Notre Dame Bay, Newfoundland, contain trace quantities of detrital chromite. The age and provenance of these sedimentary rocks indicate that the chromite was derived from the general area of the ophiolite bodies now exposed in western Newfoundland. These data place a new constraint on tectonic reconstructions of the area for the Ordovician Period. Our analysis of the stratigraphy in Notre Dame Bay, together with recently published isotopic dates for both the Eastern Fleur de Lys terrane and Notre Dame Bay area is compatible with the hypothesis that ophiolite obduction in western Newfoundland resulted from the collision of an island arc with a passive continental margin during Middle Ordovician time.

Stratigraphy and sedimentology of the Caples terrane of the Thomson Mountains, northern Southland, New Zealand

Abstract: The Thomson Mountains west of Lake Wakatipu are underlain by sediments of the volcaniclastic Caples terrane, lying east of the Dun Mountain Ophiolite Belt and gradational into the western margin of the Haast Schist Zone. Although traditional formation-mapping methods are difficult to apply in the area, five formations have been differentiated. The Caples sediments can be divided into eight subordinate lithofacies. Bold Peak Formation is lithologically diverse, but dominated by quartz-poor, very thick bedded, coarse lithic sandstone. Upper Peak Formation consists of thin-bedded quartz-lithic sandstone and siltstone and is finer grained than other formations. Kays Creek Formation is a distinctively red-and green-coloured unit of quartz-deficient thick-bedded coarse volcanogenic sandstone. Momus Sandstone and Mt Campbell Formations are lithologically identical and consist of monotonous sequences of quartzose flysch-like medium sandstone. Bodies of Kays Creek-like lithologies occur within other forma...

Collision-deformed Paleozoic continental margin, western Brooks Range, Alaska

Abstract: A structural sequence of Carboniferous, Permian, and Triassic argillaceous and cherty rocks in the northwestern Brooks Range, herein named the Kagvik sequence, is a key to the tectonic history of Arctic Alaska. The Kagvik sequence lies structurally below thrust-faulted sheets of coeval carbonate rocks, including the Lisburne Group, and structurally above quartz mica schist and greenstone of the southern Brooks Range. Sedimentary features of the shelly fossil-rich carbonate strata of the Lisburne in the upper thrust sheets indicate deposition in a shallow-water shelf environment. The underlying shale and chert of the Kagvik is an attenuated section about 500 m thick repeated by imbricate thrusts. Ubiquitous pelagic fossils (radiolaria and Nereites trace-fossil assemblages) and volcanic material locally forming andesitic tuff and flows point to an oceanic environment of deposition. The presence of some limestone turbidites interbedded with the ocean-floor sediments of the Kagvik suggests that a carbonate shelf was nearby and that a south-facing continental margin existed during late Paleozoic and early Mesozoic time near the present-day southern Brooks Range. Collapse of this long-standing continental margin appears to be related to collision and accretion of Paleozoic island arcs and microcontinental blocks now represented by the metamorphic terrane along the south flank of the Brooks Range. The structurally higher thrust slices of mafic and ultramafic rocks emplaced on top of the collapsed continental margin represent ophiolitic basement of the Yukon-Koyukuk basin.

Geochemical evidence for paired arcs in the Permian volcanics of southern New Zealand

Abstract: Although remnants of a Permian volcanic arc in the South Island of New Zealand have hitherto been considered to comprise a single volcanostratigraphic unit, recent work in the southern part of the arc has revealed the presence of two lithologically distinct terranes, only one of which is definitely Permian in age. Major and trace element analyses of representative specimens show that these terranes are geochemically distinct. Comparison with modern volcanic arcs indicates that this ancient arc in the South Island was a paired arc system made up of primitive island arc tholeiites to the east and more evolved andesitic rocks to the west.

Geochronology and the Archean of the United States

Abstract: Archean rocks of the United States represent more than a billion years of earth history. Major episodes of igneous and metamorphic activity are recognized in the intervals of 3,700 to 3,500 m.y. 2,900 to 2,800 m.y., and 2,750 to 2,600 m.y. Additional events are identified between 3,100 and 3,000 m.y., but the extent of rocks of this age is not clear although ages in this range are now common in gneiss belts of the Superior Province. A younger period of granite emplacement (2,550 to 2,400 m.y.) is suggested by ages in Wyoming, but these require confirmation from additional dating.The late Archean episode of greenstone-granite formation (2,750 to 2,600 m.y.) was a major crust-forming event in both the Lake Superior region and the Wyoming age province. The abundance of ages of 2,800 m.y. and greater confirms the presence of an extensive sialic crust at this time. It follows that the greenstone belts formed on or adjacent to segments of this older crust.The presence of gneisses of early Archean age has been demonstrated conclusively at only two localities in the Lake Superior region--southwestern Minnesota and northern Michigan. Whether these very ancient rocks were originally of limited extent, or were more extensive but have been so modified by later metamorphism that their detection is difficult, is uncertain.Where not affected by younger metamorphism or tectonism, Archean terranes attained crustal stability in the latest Archean or earliest Proterozoic (2,400 to 2,600 m.y. ago). A granitic terrane in the northern Bighorn Mountains was not affected by significant thermal events after 2,700 m.y. even though major igneous activity and metamorphism occurred to the south approximately 100 m.y. later. Stability of this block prior to the development of abundant greenstone-granite complexes also supports the existence of a thick sialic crust by the end of middle Archean time.Much of the Archean terrane of the Lake Superior region was involved in the early Proterozoic Penokean orogeny, and the effects of this have greatly complicated geochronologic studies of the older rocks. Similar involvement of Archean rocks in early Proterozoic tectonism occurred marginally to the Wyoming age province. Vertical tectonism in middle Proterozoic time resulted in differential uplift of major blocks of Archean rocks in central and southern Wyoming.A connection between the Archean rocks of the Lake Superior region and those in the Wyoming age province in the buried basement of the northern Great Plains remains to be demonstrated. Both terranes can be projected substantial distances in the subsurface, but additional dating of basement samples is required to establish firmly a possible continuity between these exposed terranes.

Exceptionally Large Steeply Plunging Folds in the Torlesse Terrane, New Zealand

Abstract: A steeply plunging asymmetric fold system in the Ben Ohau Range of the Torlesse terrane, New Zealand, has a wavelength of up to 10 km and a length along the transverse axis (A axis) of at least 36 km. We interpret it as an originally recumbent structure that was strongly tilted "sideways" when the Torlesse sediment body collided with the eastern margin of Gondwanaland. The initial recumbent folding may have occurred on a strike-slip portion of the same margin.

Fission Track Evidence for a Miocene Cooling Event, Whipple Mountains, Southeastern California

Abstract: Fission track dating of metamorphic rocks from the Whipple Mountains, California indicates a. previously unreported cooling event affected the region during the early Miocene. The timing of this event coincides with basaltic volcanism and the development of an enigmatic low-angle fault terrane that seems to be of regional proportions. Metamorphosed granitic rocks and mylonitic gneisses lying along and structurally below the basal dislocation surface were sampled and their apatite, sphene, and zircon fractions dated. These mineral-thermometers have fission track retention temperatures that are sensitive within the range of 100° to 250° C. Data from all the mineral phases were concordant and therefore, indicate a rapid cooling interval between 18 and 20 m.y. ago. Cooling intervals in the geologic record of the magnitude described here are indicative of a major, rapid uplift of an orogenic terrane. Although at present it is not possible to place the proposed uplift into the tectonic framework of the area, the timing of the event does coincide with ma.jor southern Cordilleran developments such as Basin and Range extension and basaltic volcanism.

Primitive lead in deep crustal xenoliths from the Snake River Plain, Idaho

Abstract: The petrology and geochemistry of deep crustal rocks are poorly understood due to the inaccessibility of such materials. Most inferences of the lower crust are based on studies of ancient high-grade metamorphic terranes now exposed after a history of deep burial. However, direct sampling of deep crustal rocks is feasible when they are carried to the surface as accidental fragments (or xenoliths) in lava flows. Ferro-basalt to ferrolatite lavas from Craters of the Moon lava field (COM) in the Snake River Plain, southern Idaho are characterised by variable and high 87Sr/86Sr ratios (0.708–0.712) interpreted as reflecting variable degrees of crustal contamination1,2. Some of these lavas contain xenoliths of crustal derivation (silicic charnockites and pyroxene granulites) that are found nowhere in the vicinity in surface outcrops. I present here initial isotopic and trace element results for a suite of these crustal xenoliths. As part of a detailed Pb isotopic study of the lavas (in preparation), three granulites were analysed to test the contamination hypothesis. The results indicate an affinity between the xenoliths and many exposed granulite-facies metamorphic terranes, and the unusually primitive lead isotopic ratios recorded establish the presence of early Archean crust beneath south-central Idaho.

A conceptual hypothesis for the relation of differing tectonic terranes to plutonic emplacement

Abstract: Great batholiths such as those formed in western North America during the past 200 m.y. resulted from major additions of sialic rock to the crust and consequent volumetric and areal expansion of the sialic crust. Such crustal growth begins with the emplacement of rock of basaltic composition at the base of the crust. From this underplating, differentiated magmas rise as diapirs into the upper crust. Some of this material escapes upward into the zone of brittle deformation and forms shallow plutons and volcanic ejecta, but the greater portion comes to rest at depths of 5 to 15 km as essentially conformable plutons. The volumetric additions caused by the emplacement of great volumes of rock within these depth limits are made possible by lateral compression and transport of the pre-existing upper crustal rocks. To accommodate lateral compressive transport at intermediate depths, supra-crustal rocks undergo lateral extension by block faulting, and deep crustal rocks extend by lateral flow. Zones of extension, lateral compression, and so-called load flattening—where they exist within belts of batholith emplacement—have probably formed simultaneously one above the other.

The missing Precambrian crust

Abstract: Every judgement in science stands on the edge of error and is personal. –J. Bronowski Field evidence on the nature of the basement of early and middle Proterozoic domains is incomplete. With minor exceptions, few data are at hand for existence of simatic regimes 2.6 to 1.0 b.y. ago. Geochemical and isotopic parameters and paleomagnetic constraints require an essentially continuous sialic crust consisting of stable cratons and mobile belts during this period. However, considerations of crustal volume and structure suggest that the area of the Precambrian sialic crust could not have exceeded about one-fourth of the present-day Earth surface; this leaves the nature of the remaining three-fourths enigmatic. Four alternative models are tested, but only two are consistent with the early and middle Proterozoic crustal record: (1) a sialic megacontinent and a paleo-Pacific simatic regime; (2) a smaller radius of the Earth. The first interpretation is difficult to reconcile with plate tectonics; inherent in the latter is the production of large volumes of two-stage mantle-melting materials, as around the Phanerozoic Pacific, but corresponding volcanic-sedimentary assemblages are very rare in 2.6- to 1.0-b.y.-old terranes. An existence during this time of a tectonically inert simatic regime is difficult to reconcile with the intense tectonic and thermal activity in contemporaneous mobile belts. The difficulties require a re-examination of the question of the Precambrian Earth radius.

Petrology and provenance of Pleistocene deposits in the south Parengarenga-Te Kao district, northern New Zealand

Abstract: Pleistocene deposits in the south Parengarenga-Te Kao area are predominantly medium to fine, highly mature sandstones. They are comprised of 90–96% quartz and subordinate feldspar and rock fragments. Mafic minerals generally comprise less than 1% of these rocks and are opaque dominated. An unusually high proportion of the monocrystalline quartz, possibly up to 40%, has been derived from an acid volcanic terrane, the most likely source being acid volcanic rocks in the Central Volcanic Plateau. The remaining quartz is multicycle and has been derived from older sedimentary terranes in the Northland and Auckland areas. The mineralogy of the modern Parengarenga Facies in Great Exhibition Bay is almost identical to that of the Pleistocene sandstones from which they were probably derived.

K-ar ages of metamorphic rocks from the tsukuba district, ibaraki prefecture

Abstract: K-Ar dating on micas from six metamorphic rocks and two granitic rocks in the Tsukuba district were carried out. The ages of the metamorphic rocks are 58-62 m. y. showing no regular relationship to their metamorphic grade. Dating of the granitic rocks gives 53-63 m. y. The results, together with geological and petrological facts, support the idea that these metamorphic rocks were influenced by thermal effects of the associated mass of biotite granite. With regard to the existence of plutonism and metamorphism in early Tertiary time the Tsukuba metamorphics resemble the Ryoke terrane.

Occurrence of Atomodesma limestone near Dansey Pass, North Otago

Abstract: A thin lens of grey limestone containing abundant comminuted fragments of the molluscan bivalve Atomodesma was found in the Haast Schist terrane near Dansey Pass (North Otago). The find extends the...

Focus on the use of soils for geochemical exploration in glaciated terrane

Abstract: Large portions of the Earth's surface have been glaciated several times during the last two million years. The overburden in these areas is made up of glacial drift, which has been laid down by the action of glaciers and their meltwaters, and thereafter subjected to postglacial processes. 1n glacial terrane, therefore, geochemical dispersion can be divided into two main classes, (1) syngenetic dispersion, i.e. principally mechanical or particulate dispersal which took place during glaciation and (2) epigenetic dispersion, i.e. chemical and mechanical dispersion which has taken place after glaciation. In combination these processes may result in intricate geochemical dispersion patterns and anomalies that are diffiCUlt to interpret. The sampling and analytical methods used should, therefore, be those which will disclose anomalies that are genetically not too complex. Interpretation of syngenetic patterns presupposes a thorough knowledge of the glacial history. To obtain meaningful results it is frequently necessary to sample tills in section to the bedrock surface. This often requires heavy equipment, and sampling costs may be relatively high. The analytical methods used should employ rigorous chemical digestion as well as mineralogical determination of resistant minerals. Epigenetic dispersion patterns in glacial overburden can be produced downslope due to metal dispersion in groundwater, or immediately over the bedrock source due to capillary forces, biological activity or gaseous movement of volatile compounds. Mineral deposits in contact with groundwater may act as natural galvanic cells which may result in electrochemical dispersion of metal into the overlying glacial drift. Epigenetic dispersion patterns may be detected in near-surface soils at relatively low sampling costs and by weak chemical extraction. Empirical evidence supporting these principles is provided by published and unpublished data. This paper reviews those data that have appeared in the western literature during the last decade, the intention being to outline the present state of art in utilizing analysis of soil samples as an exploration tool in a glacial terrane. If we are to advance Lhe applicability of soil geochemistry in glaciated environments, more research into dispersion mechanisms is required.

Pribilof segment of the Bering Sea continental margin: A reinterpretation of Upper Cretaceous dredge samples

Abstract: Samples of an Upper Cretaceous sandstone dredged from Pribilof Canyon, Bering Shelf margin, do not compare well with rocks of the Shumagin Formation of presumed Late Cretaceous age on Sanak Island. Contrary to repeated published inferences, the rocks from Pribilof Canyon do not appear to be strongly deformed. They show no evidence of slaty cleavage or penetrative deformation and were probably deposited in a shelf environment at neritic or possibly upper bathyal depth rather than in a trench or deep continental slope basin. The presence of shelf-facies rocks in Pribilof Canyon at the edge of the Bering Shelf leaves little room for a subduction-related accretionary terrane such as exists along the Shumagin shelf near Sanak Island. The concept of strongly folded deep-water trench-facies rocks in Pribilof Canyon has led to the development of Late Cretaceous subduction-related scenarios that include magmatic arcs parallel to the hypothetical subduction zone. Alternative hypotheses include a strike-slip plate boundary along the Pribilof segment of the Bering margin, which is consistent with the petrographic character of the Pribilof Canyon rocks, or a transitional trench-transform boundary–for example, the modern central to western Aleutian Trench.

The Minto copper deposit, Yukon Territory; a metamorphosed orebody in the Yukon crystalline terrane

Abstract: The original host rocks and form of the Minto copper deposit (lat 62 degrees 36' N; long 137 degrees 15' W) have been disguised by a succession of postmineralization events. These were: (1) regional metamorphism in the Early to Middle Triassic which generated the foliated and isoclinally folded fabric of the deposit and mobilized the sulfides, destroying all primary features except, probably, the mineral zonation (bn-cp-mt/cp-bn/cp + or - py); (2) the Late Triassic intrusion (ca 203 m.y.) of the Klotassin granodiorite, which engulfed and dispersed the deposit; (3) endoblastic potash metasomatism, as the intrusion cooled, forming orthoclase porphyroblasts throughout the deposit and its wall rock--chemical equilibrium was attained between silicates in the ore zone rocks and in the wall rocks; (4) recrystallization of biotite throughout the ore and wall rocks, an event recorded by two concordant K-Ar ages (177 + or - 9 and 180 + or - 9 m.y.; Early Jurassic); (5) cataclastic deformation, possibly related to the rapid uplift of the Klotassin suite during Laberge Group sedimentation (Early Jurassic), which fractured feldspars, strained quartz, arid probably sheared orthoclase porphyroblasts in the ore zone; and (6) intrusion of pegmatite and alaskite dikes, probably related to Middle Jurassic or younger intrusive groups of the Yukon Crystalline Terrane. Subsequently, following extensive erosion, the deposit was faulted, possibly in the Eocene, intruded by andesitic dikes related to the Eocene or younger Carmacks Group volcanics, and affected by supergene chlorite-sericite-hematite, kaolinite, and laumontite alteration.The genesis of the deposit remains in doubt. Whereas the highest grade (Cu-, Ag-, and Au-rich) quartz and K-feldspar-bearing rocks could have originated as a zone of silicification and potash silicate alteration associated with a pre-Klotassin granodiorite hydrothermal deposit, a sedimentary (i.e., red-bed copper) origin cannot be ruled out.

Post-orogenic peralkaline and calc-alkaline granites and associated mineralization of the Arabian Shield, Kingdom of Saudi Arabia

Abstract: Following a long period of island arc-like orogenic activity, post-orogenic granites of Pan-African age (670-550 Ma) were emplaced throughout the Arabian Shield. Most of these rocks are typical subsolvus calc-alkaline biotite monzogranite. The early Pan-African granites are associated with widespread small tholeiitic gabbro intrusions, many of which are layered. The calc-alkaline granites are not evenly distributed and their area of exposure relative to other rock types increases from S.W. to north and N.E. In the north-eastern part of the shield, they are associated spatially with rhyolites of similar composition and with molasse derived from granitic terrane.

Role of the Melones fault zone in the structural chronology of the North Yuba River area, western Sierra Nevada, California: Summary

Abstract: INTRODUCTION The Melones fault zone is the easternmosi major element of the Foothills Fault system (Clark, 1960, 1964) of the Western Sierra Nevada metamorphic belt, California. In the southern part of the belt, it commonly separates. Mesozoic rocks on the west from Paleozoic rocks on the east, but to the north, the fault is bounded on both sides largely by Paleozoic rocks. Accordingly, E. M. Moores considers the northern segment to be significantly older than that farther south (Schweickert and Cowan, 1975). Sense of displacement along the zone is uncertain, as is the role of the fault in the tectonic evolution of the belt. For example, the zone has been interpreted as being both of possible dip-slip origin (Knopf, 1929; Russell and Cebull, 1914) and strike-slip origin (Chandra, 1953; Clark, 1960; Baird 1962). Probably, movement was complex and includes both components of displacement (Cebull, 1972), which may be related either to a single oblique-slip motion or to changes of sense with time. Recently, the Melones fault has been interpreted in terms of regional plate-tectobics models. Schweickert and Cowan (1974, 1975) depict the Melones, with some of the other Foothills faults, as sutures between collided arcs. At one place along the Melones fault zone, along the north fork of the Yuba River west of Downieville (Fig. 1), Schweickert (1976) reports lawsonite blueschist and cites such rocks as supporting the proposal that the zone is one of suturing “between diverse terranes juxtaposed by subduction” (p. 409). According to Schweickert and Cowan (1975), the rock succession along the North Yuba River, from Downieville westward for about 30 km, is a melange. If this is correct, the Melones fault zone there represents only the easternmost portion of the melange succession (see Figure 1 of Schweickert and Cowan, (1975)