Showing papers on "Terrane published in 1975"

Granitic to ultramafic rock complexes of the Indian Ocean ridge system, western Indian Ocean

Abstract: During five expeditions of the Scripps Institution of Oceanography to the western Indian Ocean, more than 4,500,000 sq km of the Central Indian Ridge and its branching Southeast Indian Ridge and Southwest Indian Ridge were explored by bathymetric, magnetic, and seismic-reflection profiling. In some 2,800,000 sq km of this region, igneous rocks of the crust, lower crust, and possible upper mantle are exposed by faulting or volcanism. Fifty-six dredge hauls of these igneous rocks were obtained, largely from the major cross-fractures (transform faults) or clefts trending athwart the volcanically active ridges. From north to south, the cross-fractures most intensively sampled were the Vema Fracture Zone, which crosses the crestal area near 9°S, Argo Fracture Zone near 13°30′S, Marie Celeste Fracture Zone near 17°30′S, and the newly delineated “Melville Fracture Zone” trending north-south for more than 600 km near 60°30′E on the Southwest Indian Ridge. Our field and laboratory studies indicate that under a capping of young flow basalt, there is a regional complex of igneous rocks produced by magma generated under the ridges, trapped and differentiated into sill-like, podiform, and larger, crudely stratified to well-stratified sheets. Rocks from the stratiform masses include abundant Iherzolite and minor harzburgite, orthopyroxenite, olivine- and two-pyroxene gabbros, Ti-ferrogabbros, norite, and anorthosite. Some associated diabase intrusions are granophyric and are cut by late-stage dikelets of quartz monzonite and Na-rich trondhjemite. Both calc-alkaline and alkalic lines of differentiation are indicated. The granitic dikelets contain clear, doubly-terminated crystals of zircon, unusual in a terrane of large-cation–depleted rocks. The overlying basalt flows are pillowed with chemical and mineralogical characteristics typical of olivine-bearing tholeiite from the ridge-rise systems of the world oceans. The ubiquitous nature of the crustal complex found throughout the western Indian Ocean, together with data from the Atlantic and Pacific Oceans, suggest that similar rock complexes, dominated in their lower parts by stratiform bodies, are characteristic of most of the igneous crust throughout the world oceans.

Metamorphic and deformational processes in the Franciscan Complex, California: Some insights from the Catalina Schist terrane

Abstract: On Santa Catalina Island, blueschist is structurally overlain by glaucophanic greenschist, which is overlain in turn by a unit of amphibolite and ultramafic rock. These three units are juxtaposed along sub-horizontal postmetamorphic thrusts; tectonic blocks of amphibolite are distributed along the thrust between the greenschist and the blueschist. Physical conditions of metamorphism are estimated to be approximately 300°C and 9 kb for blueschist, 450°C and 8 kb for greenschist, and 600°C and 10 kb for amphibolite. I suggest that metamorphism occurred in a newly started subduction zone, where an inverted thermal gradient developed below the hot hanging-wall peridotite. Postmetamorphic eastward underthrusting along surfaces of varying dip can explain the present structural relationships. Tectonic blocks of glaucophane-epidote schist, amphibolite, and eclogite elsewhere in the Franciscan Complex may be disrupted remnants of similar metamorphic zones. The inverted thermal gradient will only exist in the early stages of subduction, which explains why the blocks are the oldest rocks in the Franciscan Complex. The gross decrease in age and metamorphic grade westward across the Franciscan results from successive underthrusting and accretion of progressively younger slices of supercrustal material, concurrent with uplift and erosion. Pressure-temperature (P-T) conditions of metamorphism in each east-dipping tectonic slice will increase down-dip. At any given time, older, more easterly slices will have been uplifted further, hence metamorphic grade in the exposed edges will increase eastward and structurally upward. If erosion is faster than accretion for a time, younger slices will be metamorphosed at lower pressures than were the older higher ones. Simple reverse faulting can then produce the observed interleaving of rocks of different metamorphic grade.

Precambrian Tectonic Environments

Abstract: Since the concept of sea-floor spreading was first introduced by Dietz (1961) and Hess (1962) a voluminous literature has developed to support Mesozoic continental drift. Notable advances in paleomagnetism, polar wandering, and sea-floor spreading (Runcorn 1962, Vine & Matthews 1 963, Vine 1 966) were followed by formulation of the theory of plate tectonics (McKenzie & Parker 1 967, Le Pichon 1 968, Morgan 1 968, Isacks et al 1 968, McKenzie 1 972). Emergence of the plate tectonic or "new global tectonic" theory has provided a unifying worldwide explanation for tectonic processes (Dewey & Bird 1 970). New models for mountain building involve plate motions at divergent and convergent plate boundaries. Relegated to virtual obsolescence (Dickinson 1971) are the classical geosynclinal concepts widely held until the mid-1960s. The concept of plate tectonics compelled geologists to reexamine sedimentation, igneous activity, deformation, and metamorphism on both regional and global scales. Initially, attention was devoted to plate tectonic theory as it affected contemporary geosynclines, island arcs, mountain ranges, and oceanic domains (Mitchell & Reading 1 969, Bird & Dewey 1970, Dewey & Bird 1970, Coleman 1971, Dickinson 1971, Gilluly 197 1 , Oxburgh & Turcotte 1971), but the success of these efforts led to the application of the plate tectonic model to Paleozoic and late Precambrian orogenic terranes such as the Uralides and Appalachian/Caledonian Orogen (Hamilton 1 970, Bird & Dewey 1 970, McElhinny & Briden 1 97 1 ). A sequel has been a growing tendency to apply the theory to the Precambrian record and to evaluate the possible role of the plate tectonic mechanisms during early continental evolution (Dewey & Horsfield 1 970). Some believe that ancient mobile belts, and even the Archean greenstone belts, represent the impact scars of protocontinental fragments colliding with each other, with island arcs, or with oceanic crust (Gibb 1971, Gibb & Walcott 1971, White et al 1971, Condie 1972, Talbot 1973). How justified is application of the plate tectonic hypothesis to earlier geologic time? Before making judgment it may be advisable to examine critically the

Granitic Plutonism and Metamorphism, Eastern Alaska Range, Alaska

Abstract: Plutonic rocks in the eastern Alaska Range were emplaced in Late Pennsylvanian time (282 to 285 m.y. B.P.) and during two distinct intervals in Cretaceous time (105 to 117 and 89 to 94 m.y. B.P.) Development of a large plutonic-metamorphic complex, consisting of diorite and quartz diorite intimately associated with banded gneiss and other metamorphic rocks, apparently occurred during Late Triassic to Middle Jurassic time (163 to 199 m.y. B.P.). A smaller plutonic-metamorphic complex is Miocene in age (17 m.y.). The younger Cretaceous plutons are recognized only in the regionally metamorphosed Devonian and older terrane north of the Denali fault. Plutons of the older Cretaceous and Pennsylvanian events are restricted to Pennsylvanian and younger terrane south of the Denali fault and are associated with coeval volcanic rock assemblages. The major plutonic-metamorphic complex is also restricted to the terrane south of the Denali fault and may relate to collapse of an upper Paleozoic volcanic arc in Triassic time followed by syntectonic magmatism in Jurassic time. The Miocene plutonic-metamorphic complex may reflect the time of initial movement along the Denali fault. Porphyry copper deposits are associated with the Cretaceous plutons south of the Denali fault. The source of the copper may be subjacent copper-rich basalt flows (Nikolai Greenstone) of Triassic age.

The Twillingate Granite and nearby Volcanic Groups: An Island Arc Complex In Northeast Newfoundland

Abstract: The Twillingate Granite cuts mafic pillow lavas and silicic fragmental volcanic rocks of the Sleepy Cove Group. The granitic rocks are soda-rich and they vary from intensely foliated and mylonitic in the south to mildly foliated and massive toward the north. The Sleepy Cove volcanic rocks show similar structural and metamorphic variations from lineated amphibolitic pillow lavas, to elongated pillows of greenschist metamorphic grade, to slightly metamorphosed and relatively undeformed pillow lavas.The collective terrane occupied by the Twillingate Granite and Sleepy Cove Group is virtually surrounded by intrusive mafic dikes that are integral and coeval parts of the Moretons Harbour and Herring Neck Groups. The dikes decrease in abundance away from the contacts of the collective Twillingate – Sleepy Cove terrane. The essentially intrusive contact is modified by faults and locally, the profuse dike swarms are absent.Regional relationships, thickness, lithofacies, and petrochemistry all indicate that the Mor...

Potassium–Argon Age Determinations of Metamorphic and Plutonic Rocks in the Yukon Crystalline Terrane

Abstract: Forty-four new potassium–argon age determinations on minerals of metamorphic and igneous rocks from the Yukon Crystalline Terrane define the timing of the three most recent thermal events affecting this region. The oldest, 160 to 170 Ma ago, involved weak retrograde metamorphism of igneous and metamorphic rocks and coincides with the intrusion of batholiths of pink quartz monzonite. The next event, 90 to 100 Ma ago, reflects the emplacement of batholiths of the Coffee Creek quartz monzonite suite. The youngest thermal episode, 50 to 60 Ma ago, marks the time when the Nisling Range alaskite, with its porphyry dyke swarms and explosive acid volcanic rocks, was emplaced and when the K–Ar system of the Ruby Range Batholith was thermally reset. The data provide a younger limit to the age of the oldest Mesozoic plutonic rocks, the Klotassin suite, but they do not define its time of emplacement.

Timing of Kenoran Metamorphism in the Eastern Abitibi Greenstone Belt, Quebec: Evidence From 40Ar/39Ar Ages of Hornblende and Biotite From Post-Kinematic Plutons

Abstract: The Opemisca and La Ronde Lake plutons are typical of post-Kenoran felsic plutons which intrude the Archean greenstone terrane of the eastern Abitibi orogen. Hornblendes from the plutons record und...

Alternative Interpretations for the Casino Complex and Klotassin Batholith in the Yukon Crystalline Terrane

Abstract: A small part of the Klotassin batholith, that intrudes the Yukon Metamorphic Complex, has been studied in detail near the Casino porphyry copper–molybdenum deposit, Yukon Territory. Eleven potassium–argon model ages, including concordant biotite and hornblende ages, indicate a mid-Cretaceous (100 Ma) age for this batholith. All ages are indistinguishable statistically. This age, therefore, is interpreted as the age of emplacement of the Klotassin batholith. Older dates can be expected from rocks of the Yukon Metamorphic Complex, the Carmacks batholith, and Aishihik batholith.The Casino complex, host to the Casino deposit, is dated at latest-Cretaceous (70 Ma), based on potassium–argon model ages from two samples of biotite. The complex is clearly intrusive into and younger than the Klotassin batholith. Previously, this unit was interpreted as Early Tertiary. Possibly the several so called Early Tertiary volcanic and hypabyssal rocks in Yukon are not strictly contemporaneous.

Metamorphism of the moke creek lode, otago, new zealand

Abstract: A load pressure of 6.4 ± 0.4 kbar is estimated for the metamorphism and reequilibration of the Moke Creek sulphide lode near Queenstown (S132) on the basis of a recently published calibration of the iron content of sphalerite as a geobarometer, and the recognition of equilibrium sulphide textures. This estimate confirms previous estimates of a geothermal gradient of 15–25° c/km during greenschist facies metamorphism in this area of the Haast Schist Terrane. The author suggests a syngenetic origin for the lode.

On the isotopic ages of some granites and metamorphic rocks from northwest china

Abstract: With more than 110 data of isotopic ages (most of which are new) at his disposal, the writer discusses, with the help of other geological data, the age, distribution, degree of development and interrelationship of the magmatism and metamorphism occurring in Central Tsinlingshan, Chilienshan, Tienshan and Altai mountains in Northwest China.The data available show that there were at least six periods of magmatism and five major periods of metamorphism or migmatization in Northwest China. They may be preliminarily divided as follows: Magmatism—1) 1150--1780 m.y. (early Proterozoic), 2) 645—880 m.y. (late Proterozoic), 3) 375—550 m.y. (Caledonian), 4) 240—365 m.y. (Hereynian), 5) 180—226 m.y. (Indo-sinian), 6) 80—170 m.y. (Yenshan-Jan); metamorphism (or migmatization)—1) 700—900 m.y. (late Proterozoic), 2) 490—550 m.y. (early Caledonian), 3) 400—440 m.y. (late Caledonian), 4) 230—380 m.y. (Hercynian), 5) 200 m.y. or so (Indo-sinian). In many regions, magmatism is closely related to metamorphism in time. This paper also sums up the age deviations between micas and feldspars in the same terrane, and this may serve as references for further study on the problem of mineral adaptation.