Diapiric Uprise Research Articles

Geochemical characteristics of basaltic volcanism within back-arc basins

Summary Back-arc basins are formed by extensional processes similar to those occurring at mid-ocean ridges. However, whereas the magmas erupted along the major ocean ridges are predominantly LIL element-, Ta- and Nb-depleted N-type MORB, many back-arc basins are floored by basalts transitional between N-type MORB and island arc or even calc-alkaline basalts (viz. enrichment of LIL elements (K, Rb, Ba, Th) relative to HFS elements (Nb, Ta, Zr, Hf, Ti)). On a broad scale, it is possible to relate basalt composition, tectonic setting of the basin, and maturity of the adjacent subduction zone. Thus, the Parece Vela Basin, formed during the earliest stages of the Mariana subduction system, is floored by basalts indistinguishable from N-type MORB, whereas the later Mariana Trough is erupting N-type MORB and basalts with calc-alkaline characteristics, commonly in close spatial proximity. The calc-alkaline component is best developed in narrow, ensialic basins such as Bransfield Strait, where the extension is adjacent to mature, continent-based magmatic arcs. This range of compositions, from N-type MORB to calc-alkaline basalt, can be satisfactorily explained only by invoking chemical variations in the composition of the mantle material supplying the back-arc basin crust. Two major processes may be suggested: (i) selective contamination of the mantle wedge by LIL-enriched hydrous fluids, perhaps together with sediments, derived from the descending, dehydrating oceanic lithosphere; and (ii) repeated melt (and incompatible element) extraction during basalt genesis. The former process will enrich the mantle source of back-arc basalts with LIL elements; the latter will deplete the source in all incompatible elements, but the net effect of both processes is to increase the LIL/HFS element ratio of the source regions. Consequently, as the subduction zone matures, the LIL/HFS element ratio of successive back-arc basalts will be expected to increase, from initial N-type MORB ‘background’ values, to ratios more typical of island-arc basalts. The model has implications for mantle dynamics in back-arc regions, because transfer of material from the subducted slab may destabilize the overlying mantle, potentially leading to diapiric uprise when tectonic conditions permit extension.

Structural evolution of the Thor-Odin gneiss dome

The Thor-Odin dome is a large structural culmination along the eastern margin of the Shuswap terrain. Previous workers have presented divergent models for the origin of the dome. Some have proposed that it is a product of diapiric uprise of material from the lower crust; alternatively, the dome is interpreted to have formed from the interference of later folding. This study presents the results of a detailed analysis of new data on the structure, pattern of strain variation and metamorphic fabric of the dome. These data together with new geochronological and metamorphic phase assemblage information provide the basis for a revised interpretation of the evolution of the Thor-Odin gneiss dome. The pre-doming evolution of the Thor-Odin gneiss dome was characterized by the formation of large-scale nappe structures and the imbrication of Archean basement rocks with the Cambrian and younger cover rock sequence. The first period of deformation (Phase One) consisted of large-scale infolding of the cover rock sequence into the basement rocks. The Pingston fold in the core of the dome is a product of this event. The second period of deformation (Phase Two) was marked by the forcing of wedges of basement into the cores of northerly-directed nappes. These structures had no observable effect on the central core zone, which is here termed the Autochthonous Core Gneiss Domain. The third period of deformation (Phase Three) was co-axial with Phase Two and consisted of imbrication and refolding of the upper levels of the stack of Phase Two nappes. The actual domal shape was formed by fold interference of Phases Four, Five and Six, which were upright, chevron-style, flexural-slip folds with northwest, southwest and northerly-trending axial planes. The “doming” events occurred at upper crustal levels, probably during the Tertiary. Analysis of the strain pattern suggests that little of the total deformational strain is related to these dome-forming deformations. This outline of the events that formed the dome is consistent with the new geochronological results and interpretation of metamorphic phase assemblages. Cordierite-gedrite-sillimanite assemblages, previously interpreted as indicating 1000 MPa pressure, probably formed under reduced water fugacities and 500–700 MPa total pressure. The Thor-Odin gneiss dome thus provides no evidence for lower crustal upwelling related either to dome formation or to the formation of the Rocky Mountain thrust belt.

Petrology of arc volcanic rocks and their origin by mantle diapirs

Calc-alkalic chemical trends characteristic of arc volcanic rocks mainly result from three mechanisms which act additively: (1) fractional crystallization involving separation of titanomagnetite; (2) selective concentration of plagioclase phenocrysts and selective depletion of titanomagnetite phenocryst compared with the actually fractionated proportion; and (3) mixing of magmas on continuous fractionation trends. The association of calc-alkalic and tholeiitic trends in a single composite volcano may not represent different fractional crystallization processes or different chemistries of primary magmas, but the calc-alkalic chemical trend can be considered as a mixing trend resulting from mixing of various magmas on associated tholeiitic chemical trends. Chemical variations of most arc volcanic rocks, including calc-alkalic ones, can accordingly be essentially accounted for by the low-pressure fractional crystallization of phenocrystic phases from primary basaltic magmas. Crystallization sequences of arc magmas which are strongly dependent on water content in magmas are deduced from the phenocryst assemblages. The crystallization sequence changes laterally across-arc, suggesting increasing water contents in magmas toward the back-arc side, as is also seen for other incompatible elements such as K and Rb. Systematic differences in the characteristic crystallization sequence are also observed among arcs, roughly correlating with the crustal thickness. Water content in magma, like other incompatible elements, tends to increase with increasing crustal thickness. The variation of incompatible elements including water roughly represents that of the degree of partial melting of the upper mantle, which is broadly controlled by the crustal thickness. The variation of water content indicates that arc magmas are not saturated with water during differentiation to late differentiates such as dacite or rhyolite. This strongly constrains the maximum water contents in primary basaltic magma, at most 2.5 wt.%. This value suggests that magma generation beneath arcs is dependent on dry solidus of peridotite. Diapiric uprise of the hot deeper mantle and associated adiabatic decompression would be necessary for mantle peridotite to attain the temperature as high as dry solidus. Diapirs that begin to rise from the subduction zone may stop at or near the crust-mantle boundary because of the surrounding density change, and their degree of partial melting is roughly controlled by their stopped depth assuming their similar temperature. Across-arc variation is also explained by the stopped depth of diapirs, but is not controlled by crustal thickness.

Tectonic Implications of Tertiary Intrusion and Shearing within the Bitterroot Dome, Northeastern Idaho Batholith

The Bitterroot dome contains plutons of the northeastern Idaho batholith and their regionally metamorphosed country rocks. Several mesozonal to katazonal intrusions in the core of the dome have zircon U-Pb ages (lower intercept on concordia) which range from . Epizonal plutons in the northern Idaho batholith region have Eocene K-Ar ages. An extensive mylonite zone developed on the eastern margin of the dome during the latter stages of uplift. Zircon (lower intercept) ages from three sheared plutons in this zone are , and The fact that significant shallow to deep-seated intrusion and mylonitization occurred during Eocene time requires a careful evaluation of Early Tertiary tectonism in the northern Idaho batholith region. Cretaceous events appear to include multiple deformation, metamorphism, and intrusion of mesozonal to katazonal plutons into the evolving root zone of the northern Rocky Mountain overthrust belt. Early Tertiary events appear to include intrusion of mesozonal to epizonal plutons into the Late Cretaceous igneous-metamorphic complex and diapiric uprise of the Bitterroot dome accompanied by mylonitization on its eastern flank that was superimposed on the already existing thrust system.

利尻火山・くつ形溶岩流の内部構造

Three types of vesicular segregated bodies are recognized in the Kutsugata lava flows, Rishiri Volcano; namely, “segregation layer”, “segregation cylinder”, and “segregation stock”. In transverse section. the segregation layer is a flat-lying parallel banding from several mm to more than 10cm thick. The segregation cylinder is a slender vertical column which is connected with the segregation layer at the upper end of the column. The segregation stock is defined as a pulge or pulme segregated from the host lava, and is associated with the segregation layer. These dark-colored segregated bodies must have formed during consolidation of the lava flaws which had ceased to move on the flat ground, in the definite sequence with segregation layers first, then segregation cylinders and finally segregation stocks. During cooling of basaltic magma, crystal particles and residual 1iquid with bubbles must have composed a dispersed or framewarked salid-fluid system. The field evidence suggests that the segregation layer was derived from the extension fracture developed perpendicular to the 1east principa1 stress direction in the frameworked so1id-fluidcomp1ex. The segregation cy1inde and stock, however, must have originated in the f1ow by the thixotropic break dawn and the release of stress-hardening of the solid-fluid system. It is considered that there were three mechanisms of material transport in the Kutsugata 1ava f1ows, viz., f1ow of a viscous 1iquid in the extensioh fractures, vo1ati1e channel1ing in the three-phase f1uidized system, and diapiric uprise of low density plastic masses. Eight chemica-analyses of the segregated bodies are compared with six from their hostparts in a typical thick lava flow together with all published bulk rock analyses from the Rishiri Volcano in order to trace fractionation trends of the magma after extrusion.

The origin of kimberlite

A new diapiric model for kimberlite genesis takes into account recent interpretations of peridotite‐CO2‐H2O melting relationships. A minor thermal perturbation at depth might trigger release of reduced vapors with major components C‐H‐O. Where these volatile components cross the estimated solidus boundary near 260 km, partial melting occurs, the density inversion causes diapiric uprise along adiabats, and the partially melted diapirs begin to crystallize at 100 to 80‐km depth, where they reach a temperature maximum (thermal barrier) on the solidus. The released vapor enhances the prospects for crack propagation through overlying lithosphere in tension, and this could produce an initial channel to the surface. Magma separation could then occur from progressively greater depths, releasing CO2‐under‐saturated kimberlitic magma for independent uprise through the established conduit, quite unaffected by the thermal barrier on the solidus of peridotite‐CO2‐H2O. Cooler diapirs would cross the solidus at somewhat greater depth, solidifying to phlogopite‐dolomite‐peridotite with the release of aqueous solutions. These solutions are likely candidates for the mantle metasomatism commonly considered to be a precursor for the generation of kimberlites and other alkalic magmas. According to this model the metasomatism is a consequence of kimberlite magmatism rather than its precursory cause.

Horizontal tectonic interaction of an Archean gneiss belt and greenstones, Pilbara block, Western Australia

The Archean Pilbara block, Western Australia, is cited as a type example of a granite-greenstone terrane. New field evidence reveals that the internal structure of one of the major batholiths, the Shaw batholith, resembles an Archean gneiss belt. Greenstone intercalations within the gneiss belt can be traced into the surrounding greenstone sequence and were incorporated during subhorizontal thrusting and recumbent folding episodes. These early tectonic episodes preceded solid-state diapiric uprise of an overthickened crust. Granitic plutonism and partial melting of the sialic crust appear to have played an important role only in the later stages of the tectonic evolution of the Shaw batholith. Deformation and metamorphism are not a direct consequence of the emplacement of granitic magmas, contrary to the widely accepted magmatic model of batholith formation.

"Helium Spots": Caused by a Diapiric Magma from the Upper Mantle

"Helium spots," where a significant amount of helium is present in the soil [up to 350 parts per million with a high (3)He to (4)He ratio of (8.90 +/- 0.31) x 10(-6)], have been found along the fault zone formed by the 1966 Matsushiro swarm earthquakes. The formation of the "helium spots" and the occurrence of the earthquakes are interpreted as the results of a diapiric uprise of a magma approximately 1 kilometer in diameter.

Rb-Sr and U-Pb isotopic studies of the northeastern Idaho batholith and border zone

Border-zone rocks of the northeastern Idaho batholith have been subjected to multiphase penetrative deformation, regional metamorphism up to sillimanite-orthoclase grade, and multiple intrusion. Intrusion was accompanied by diapiric uprise of the plutonic-metamorphic complex, with extensive cataclasis and flow folding along the eastern margin of the rising gneiss dome. Normal faulting followed, with mylonitization and greenschist-grade metamorphism in local shear zones. Ages of some thermal events are estimated as follows: 85 ± 35 m.y. for a metamorphic event that affected quartzofeldspathic gneiss (Rb/Sr whole-rock line of best fit); 82 ± 10 m.y. for the emplacement of a quartz-diorite orthogneiss (estimate based on U/Pb isotopic ratios from a single zircon fraction); 66 ± 10 m.y. for the main stage of batholithic emplacement (U-Pb Concordia–lower intercept age); 46 ± 5 m.y., 42 ± 8 m.y., and 39 ± 2 m.y. for Rb/Sr isotopic equilibration late in the thermal evolution of the plutons (Rb/Sr mineral isochrons). The older dates conform generally to the range of intrusive ages assigned to the Flint Creek plutons and the Boulder batholith to the east. The younger dates reflect the igneous-hydrothermal-tectonic event of Eocene-Oligocene time which has reset K-Ar and fission-track (apatite) dates within the general Idaho batholith igneous complex. Magmas that formed the northeastern Idaho batholith were derived from, or contaminated with, older crustal material. This statement is supported by a 1,900- to 2,250-m.y. upper intercept age for batholithic zircons, some of which show euhedral overgrowths on anhedral cores, and by relatively high whole-rock Sr 87 /Sr 86 ratios from samples with low Rb 87 /Sr 86 ratios.

Degrees of melting in mantle diapirs and the origin of ultrabasic liquids

The temperature and degree of melting in an upwelling diapir in the mantle may be considerably less than that anticipated from an adiabatic cooling curve. Several geological and thermodynamic parameters may be incorporated to produce a more realistic melting model in diapirs. The latent heat of fusion of mantle material is the greatest buffer on degrees of melting. Models are presented which suggest that an uprising diapir intersecting the anhydrous solidus of mantle material at 50 kbars may be only 29% melted on reaching the surface. A diapir initiated at 100 kbars may be 69% melted. These are maximum values. These calculations imply that the generation of komatiitic liquids by diapiric uprise alone demands that the diapir originate at depths in excess of 300 km. Melting of mantle with an irregular geotherm is preferred for the origin of these liquids.

Petrologic and geophysical significance of mantle xenoliths in basalts

Data on the nature of the earth's upper mantle are derived from a combination of direct and indirect techniques. Indirect techniques, which include investigations of the seismic, thermal, electrical, and magnetic structures, monitor bulk properties of extensive regions of the upper mantle. The indirect measurements lead to estimates of the vertical and lateral structures in terms of the acoustic velocities, density, and thermal and electrical properties of the mantle. In contrast, certain geologic processes bring mantle samples directly to the surface. At these locations mantle rocks can be examined in fine detail and serve as a storehouse of descriptive data to calibrate the estimates of lateral and vertical structures given by the indirect geophysical methods. Geologic examples where mantle rocks have been brought through the crust to the surface include two major types of occurrences. The first group consists of large massifs of mafic and ultramafic rocks such as the ophiolites, alpine peridotites, and high‐temperature peridotites. They are brought to the surface by diapiric uprise of mantle material or tectonic forces resulting in the faulting or thrusting of mantle material into crustal environments. The second group includes inclusions of mantle material transported to the surface by host magmas that have their origin in the mantle. The alkalic magmas such as alkaline‐olivine basalts, mugiarites, nephelinites, and kimberlites are most commonly the host magmas. The mantle inclusions are predominantly accidental and represent direct samples torn from the walls of the conduits channeling the magma to the surface. In this sense they represent reverse churn drill holes into the mantle and offer the most direct technique for getting a single profile at a single locality. This report focuses on the progress made in studies concerned with mantle inclusions, particularly insofar as they contribute to knowledge of the physical structure of the mantle.

Fabrics and Deformation of Archean Metasedimentary Rocks, Ross Lake – Gordon Lake Area, Slave Province, Northwest Territories

Metagreywackes, which lie stratigraphically above metavolcanics fringing a granitic complex (part basement), exhibit late Archean multiphase fold structures. F1 folds are large linear depressions and anticlinal culminations trending partly concordant with margins of the granitic complex. Open to isoclinal F2 folds vary and curve in trend. Though usually near upright, some F2 folds are overturned away from the complex and others towards F1 depressions. F3 folds are small-scale structures not present in most outcrops, but throughout a large region they are accompanied by a steep S3 axial-plane schistosity striking NW to N.Original orientations of an S2 schistosity, axial planar to F2 folds, are preserved as inclusion trails in biotite porphyroblasts. Angular deflections from the trails show that regional horizontal flattening across the S3 schistosity formed the F3 folds and tightened curved trends of F2 folds. Estimates of strain from the fabrics indicate no increase near the complex.As for folds in other Archean terrains, F1 and F2 folds could have developed mainly as gravity structures during diapiric uprise of a granitic basement. In contrast, the F3 structures may reflect horizontal movements of early crustal plates.

Geophysical Interpretation of Crustal Structure along the Southeastern Coast of Norway and Skagerrak

Seismic, magnetic, and geologic data are used to provide constraints on gravity models of the crust from southeastern Norway into the Skagerrak. Seismic data indicate that the Moho rises from more than 39 km under central Norway to approximately 30 km under the Skagerrak and that a basin with about 5 to 6 km of sediments is present in the Skagerrak. Magnetic data show a maximum depth of 6 km to magnetic basement in the sedimentary basin, and characteristic magnetic anomalies are found over basement along the coast and under the Skagerrak. The Great Friction Breccia fault zone, separating migmatitic and supracrustal rocks along the southeast coast, is a major tectonic feature on land. Bouguer gravity anomalies increase from slightly negative values northwest of the breccia zone to as high as +50 mgal along the coast and offshore. Major features of this interpretation are the following: (A) recognizable supracrustal rocks are more mafic than underlying mobilized migmatites, (B) a probable crustal low-velocity zone is formed by the above compositional inversion, (C) intrusive granites in the supracrustal rocks may be formed by anatexis and diapiric uprise of the migmatites, (D) the Great Friction Breccia is a major tectonic feature, and (E) continuation of the Oslo Graben and associated igneous rocks under the Skagerrak seems likely.

Late Cenozoic Alkalic Basaltic Magmas in the Western Colorado Plateaus and the Basin and Range Transition Zone, U.S.A., and Their Bearing on Mantle Dynamics

Late Cenozoic volcanism along the eastern margin of the Basin and Range province in northern Arizona and Utah has created a suite of essentially alkalic basaltic lavas similar to those occurring in other regions of recent uplift, extensional tectonism, and high heat flow. The lavas were derived from a series of partial melts of the mantle, modified to varying degrees by polybaric fractionation of olivine and possibly plagioclase and clinopyroxene during ascent to the surface. Basanite and alkali olivine basalt melts originated at depths of at least 65 km and possibly as much as 95 km (20 to 30 kb) by variable but generally small degrees of partial melting. More voluminous hawaiite magmas originated at shallower depths by a somewhat greater degree of partial melting and were more substantially modified by crystal fractionation prior to extrusion. Magmas parental to the lava extrusions became increasingly divergent in composition with time, reflecting a broadening depth interval of partial melting in the mantle. Concurrent eruptive activity and block faulting generally shifted eastward with time at a rate of approximately 1 cm/yr. These time-space-composition variations, in combination with the observed essentially marginal localization of basaltic volcanism for the whole Basin and Range province, are explained in terms of upper mantle dynamics. We envisage upwelling mantle or plume activity beginning sometime in the middle to late Cenozoic in the core area of the Basin and Range province and causing progressive thinning or “erosion” of the lithosphere. Erosion ultimately produced a steep, keellike asthenosphere-lithosphere boundary beneath the Colorado Plateaus and the Basin and Range transition zone. Eastward-flowing mantle peridotite from the core of the plume was throttled against this keel, causing localized shear heating. This heat enhanced partial melting so that sufficient liquid was created to separate from the refractory residuum and to rise to the surface. Diapiric uprise of magma, or partially melted mantle in the uppermost mantle, or injection of a swarm of dikes into the lower crust, might have weakened and attenuated the overlying brittle crust, causing concurrent faulting. Eastward erosion of the keel and site of partial melting as time progressed allowed eruptive and fault activity to also migrate.

Cratonic Reactivation in the Precambrian Basement of Western Canada. III. Crustal Evolution

Relative to the Superior and Slave provinces, the Churchill province is enriched in K and Rb. The inferred mechanism of alkali enrichment involves diapiric uprise of mantle material from the base of the low velocity zone, in response to upward movement of water from the deep mantle. Intergranular fluid in the peridotite is enriched in K and Rb. At the base of the crust this fluid separates to enter deep-seated crustal shear zones. At upper crustal levels, reaction with permeable cataclasites causes K-metasomatism, involving especially the change of plagioclase into K-feldspar.Eclogite bodies within the peridotite, on moving upward to the base of the crust yield andesitic magma which separates to form sialic underplating. The existence of a discrete lower crust beneath southern Alberta, western Ontario, and northern Manitoba is shown by deep crustal reflection and refraction seismic studies.Generation of juvenile sial during the Hudsonian orogeny is indicated by initial ratios of whole-rock Rb–Sr isochrons for igneous rocks. During metasomatism, potassium and rubidium were added in the ratio of about 350:1. This ratio makes it unlikely that these alkalis were derived by anatexis of Kenoran crystalline basement.

Behavior of an Archean Granitic Batholith

The Bamaji–Blackstone granite has 'invaded' volcanic rocks of the Lake St. Joseph area, northwestern Ontario. The resulting deformation, close to the granite, is substantially pure shear in three dimensions. This is indicated by flattened but not noticeably unidirectionally extended pillows, and by chocolate-tablet boudinage. Both features imply extension in all directions parallel to the granite margin and marked compression on horizontal axes normal to the granite margin. The flattening (compression) is also suggested by buckled granitic dikes enclosed in the flattened pillows. The discrepancy between the strains in pillows and dikes is explained as due to the late arrival of the dikes. Flattening persists away from the granite, but is increasingly accompanied by marked extension on sub-vertical axes, a deformation pattern akin to die drawing.The flattening mode is ascribed largely to the diapiric uprise of the granite, much modified by inflation of the granite due to the arrival of granitic magma late in the 'invasion' process. The granitic dikes presumably represent some magma that escaped.

Role of water in magma generation and initiation of diapiric uprise in the mantle

The content and distribution of water is a critical factor in determining mantle properties, especially in the low-velocity and Benioff zones. Estimated temperature distributions vary widely, depending on assumptions regarding the relative significance for heat transfer of conduction, radiation, and convection. Comparison of estimated geotherms with known or inferred phase relationships in the system peridotite-eclogite-water provides information about the possible physical state of the mantle at various depths for comparison with geophysical measurements. Water is stabilized in minerals such as amphibole and phlogopite in the upper mantle; at greater depths, it may exist as intergranular fluid probably adsorbed on mineral surfaces, or it is dissolved in interstitial silicate magma. The most satisfactory explanation for the low-velocity zone involves incipient melting due to traces of water. Water rising from the deep mantle in preferred zones would augment the interstitial silicate magma at the base of the low-velocity zone, initiating the diapiric uprise necessary for basaltic magma generation. A petrological model for the suboceanic mantle suggests that a lens of gabbro-rich material may account for the gravity anomaly and seismic refraction measurements at mid-oceanic ridges. A petrological model for the area beneath island arcs suggests that the seismic low-velocity zone should not be continuous across it. Magma may be generated beneath island arcs by partial fusion of down-going oceanic crust by frictional heating, by migration of water from the lithosphere into overlying mantle, or by diapiric uprise of material from the downgoing lithosphere. Diapiric uprise could be initiated by influx of water into the layer including; the mantle-upper lithosphere boundary over a down-going slab. The composition of the liquid generated depends on many variables, including the water content and the stability of hydrous minerals. Magma generation may depend on the regime of water, rising from the deep mantle in preferred zones and carried down with the lithosphere in Benioff zones.