Amphibolite Gneiss Research Articles

$$^{40}Ar/^{39}Ar$$ Mineral Age Constraints on the Paleozoic Tectonothermal Evolution of High-Grade Basement Rocks within the Ross Orogen, Central Transantarctic Mountains

The Precambrian to early Paleozoic orogenic history of the central Transantarctic Mountains is obscured by uncertainties in. the regional extent, relative intensities, and tectonic significance of three or more events: the Nimrod, Beardmore, and Ross orogenies. Significant problems exist in correlation of structural styles, structural geometry, and lithotectonic units across major tectonic boundaries, as well as in conflicting geochronological results. In an effort to resolve these disparities, ages were determined for hornblende and muscovite from several lithotectonic subunits of the Nimrod Group exposed in the Miller Range, including amphibolitic gneiss, ca. 1.7 Ga orthogneiss, and metasedimentary mica schist. These rocks display ductile deformation fabrics that formed prior to intrusion of ca. 500 Ma, post-kinematic granitic plutons. Hornblende concentrates from eight gneiss samples in the Miller Range display internally discordant apparent age spectra due to variable intracrystalline contamination with extraneous argon components (total-gas ages range between 510 and 738 Ma). vs. isotope correlations are generally well defined (MSWD < 2.0) and yield ages between ca. 525 and 487 Ma. Two muscovite concentrates from mica schist samples in the Miller Range record plateau ages of 508 and 486 Ma, which are interpreted to date cooling through temperatures required for intracrystalline retention of argon. The data indicate that conventional K-Ar dates (up to 1100 Ma) previously reported for hornblende from the Miller Range probably have no geologic significance because of the widespread presence of extraneous argon. They cannot, therefore, be regarded as dating any "pre-Ross" event(s). Intracrystalline argon systems in samples from this study appear to have been completely rejuvenated during granite emplacement associated with the Ross orogeny. Structural fabric elements were partially annealed in many dated Nimrod Group samples, likely reflecting contact effects imposed by emplacement of post-tectonic granite plutons at ca. 500 Ma.

Geochemistry, Petrogenesis, and Tectonic Setting of Amphibolites from the Southernmost Exposure of the Appalachian Piedmont

The Uchee belt of the southern Appalachian Piedmont consists of a sequence of amphibolite, amphibolitic gneiss, and gneissic calc-silicate surrounded by highly strained mylonitic rocks associated with motion along the Bartletts Ferry-Goat Rock fault system. Amphibolites from three major localities within the belt, the Hudson Mill Rapids, Davis Mill, and Lindsey Creek localities, have been analyzed for their major-and trace element chemistry. The mafic amphibolites are concluded to have been derived from intrinsically basaltic protoliths, as determined by the presence of patchy zoning in relict igneous plagioclase, normative mineralogy, Niggli trends, and a variety of diagnostic geochemical criteria. Davis Mill amphibolites are characterized by high REE abundances and flat REE pattern [], similar to volcanic rocks from Pacific-type island arcs. Geochemical parameters such as high La/Nb and V/Ni ratios, low Ti/V and Ti/Zr ratios, and discriminant trace element diagrams based on elements generally considered immobile during metamorphism, indicate an island-arc and back-arc basin affinity for the amphibolites. Hudson Mill Rapids amphibolites, characterized by a wide range of REE concentrations and strongly negative Ce anomaly are interpreted as having been derived from parental basalts produced in a back-arc basin by melting of a descending slab plus mantle wedge. The chemical diversity of the amphibolites and the presence of thrusts, strike-slip faults, and highly strained rocks are consistent with the premise that the western Uchee belt is a deformed composite part of an extensive arc system.

Age of a K-feldspar megacrystic granite from the Burgeo intrusive suite, and timing of tungsten mineralization at Grey River, southern Newfoundland

A K-feldspar megacrystic biotite granite phase of the Burgeo intrusive suite, southern Newfoundland, has been dated by the Rb–Sr whole-rock method. The isochron age of 412 ± 5 Ma (initial 87Sr/86Sr ratio = 0.7086 ± 0.0004) is consistent with the younger of two U–Pb zircon ages obtained from other phases of the Burgeo intrusive suite. The southern margin of the granite is intensively deformed in a shear zone that also affects amphibolitic gneisses of the Grey River enclave. Data from deformed granite samples plot below the 412 Ma isochron and may have resulted from reaction and partial reequilibration of the samples with a hydrothermal fluid of low initial 87Sr/86Sr ratio during metasomatic reactions accompanying shear zone deformation. However, alternative hypotheses such as crustal contamination or derivation from two separate and isotopically distinct magmas cannot be discounted. Mica ages between 390 and 370 Ma associated with tungsten mineralization at Grey River confirm that the mineralization was unrelated to the Burgeo intrusive suite. They support previous conclusions, based on geological and geochemical data, that the mineralization was related to posttectonic leucogranite intrusions.

Structural evolution of basement gneisses and Hadrynian cover, Bulldog Creek area, Rocky Mountains, British Columbia

Two gneiss bodies are contained in thrust sheets on the west edge of the Rocky Mountain Main Ranges near Valemount, British Columbia. The Bulldog Gneiss comprises Aphebian or older paragneiss and amphibolitic gneiss intruded by Aphebian orthogneiss sheets. The Yellowjacket Gneiss is granodioritic orthogneiss of unknown age. Both gneiss bodies are basement highs with thin Hadrynian metasediment cover sequences. The cover sequences are assigned to the lower Miette Group and are correlated with Horsethief Creek Group.Internal shortening of gneiss thrust sheets is expressed by recumbent folding and stacking of thin thrust sheets of gneiss and cover. The Bulldog Gneiss and its cover were carried on the postmetamorphic Purcell Thrust. The Yellowjacket Gneiss and its cover were carried on the pre- to synmetamorphic Bear Foot Thrust. Northeast and northwest displacement is documented on the moderately southwest-dipping Bear Foot Thrust, and a dextral oblique-slip – thrust model is proposed to explain the duality of thrust and dextral strike-slip kinematic indicators in mylonite from the fault. An estimate of shortening in the fore-land suggests that basement thrust sheets were translated more than 200 km to the northeast.Correlation of gneisses and cover with the westerly adjacent Malton Gneiss and its cover precludes major dextral strike-slip motion on the Southern Rocky Mountain Trench (SRMT) during and after thrusting. The SRMT was the locus of post-thrusting and postmetamorphic, Eocene(?), brittle, west-side-down, normal faulting.

Characteristics of basement rocks exposed near the Cajon Pass Scientific Drill Hole

Basement rocks exposed near the drill site to the northeast of the San Andreas fault consist of Mesozoic granitoid intrusions and amphibolite facies migmatitic gneiss. A possible tongue or lobe‐shaped pluton of granodiorite is exposed on Cleghorn Ridge, Squaw Peak and Whale Mountain. Adjacent gneiss contains marble, calcsilicate rocks, quartzite, and aluminous schist of sedimentary origin. Metagabbro and dioritic gneiss of possible igneous origin are also present. Peak metamorphic temperatures during emplacement of granodiorite are estimated to have been between 600 to 750°C at a pressure between 3‐6 kb.

Early history and reactivation of the rand thrust, southern California

The Rand thrust of the Rand Mountains in the northwestern Mojave Desert separates an upper plate of quartz monzonite and quartzofeldspathic to amphibolitic gneiss from a lower plate of metagraywacke and mafic schist (Rand Schist). The Rand thrust is considered part of the regionally extensive Vincent/Chocolate Mountain thrust system, which is commonly believed to represent a Late Cretaceous subduction zone. The initial direction of dip and sense of movement along the Vincent/Chocolate Mountain thrust are controversial. Microfabrics of mylonites and quartzites from the Rand Mountains were analyzed in an attempt to determine transport direction for this region, but the results are ambiguous. In addition, the southwestern portion of the Rand thrust was found to have been reactivated as a low-angle normal fault after subduction. Reactivation might have occurred shortly after subduction, in which case it could account for the preservation of high-pressure mineral assemblages in the Rand Schist, or it could be related to mid-Tertiary extension in the western United States. In either event, the reactivation might be responsible for the complicated nature of the microfabrics. The Rand Schist exhibits an inverted metamorphic zonation. Isograds in the schist are not significantly truncated by the reactivated segment of the Rand thrust. This indicates that other segments of the Vincent/Chocolate Mountain thrust should be re-evaluated for the possibility of late movement, even if they show an apparently undisturbed inverted metamorphic zonation.

Geotectonic significance of gneissic amphibolites from the Vema Fracture Zone, equatorial Mid‐Atlantic Ridge

A large collection of gneissic amphibolites was recovered by two close dredge hauls from the deepest part of the north facing slope of the transverse ridge forming the south wall of the Vema Fracture Zone. Serpentinites and various types of gabbroic rocks ranging from undeformed slightly uralitized gabbros and norites to flaser and mylonitic gabbros were associated with the gneissic amphibolites (in the shallowest of the dredge hauls). Petrological studies indicate that the amphibolites were derived from similar gabbroic rocks that reequilibrated under stress in the conditions of the amphibolite facies. On the other hand, the associated meta‐gabbros display sequences of secondary minerals, indicating complex cooling and cataclastic histories without reaching metamorphic equilibrium. We tentatively suggest that the gneissic amphibolites and associated metagabbros formed in a vertical shear zone generated in oceanic layer 3 by tectonism associated with the Vema Fracture Zone. Hydrothermal circulation of seawater along the highly permeable shear zone was activated by magmatic intrusions. K/Ar dating suggests, within relatively large analytical uncertainties, that the amphibolite metamorphism took place 10 m.y. ago, i.e., at a time when the dredging sites were located in the vicinity of the spreading center.

A tectonic window in central Newfoundland? Geological evidence that the Appalachian Dunnage Zone may be allochthonous

In the Through Hill area of central Newfoundland, mafic to ultramafic complexes, which preserve varying amounts of ophiolite stratigraphy, mark the trace of a major fault zone that outcrops in a roughly elliptical pattern. The major, northeast-trending axis is about 60 km and the shorter axis is about 30 km in length. The most complete ophiolite stratigraphy is preserved in the Coy Pond and Pipestone Pond complexes, which have steep dips and face east and west, respectively, outwards from the centre of the ellipse.The ophiolitic rocks are bounded on the outside by rocks typical of the Dunnage Zone of central Newfoundland, principally Lower to Middle Ordovician volcanic and volcaniclastic rocks interpreted to be of island-arc affinity (Davidsville, Victoria Lake, and Baie d'Espoir groups, Cold Spring Pond Formation) and Upper Ordovician to Silurian clastic sediments (Botwood Group). The volcano-sedimentary sequences are interpreted to have been deposited on the ophiolite conformably, but the contacts are not exposed. Both ophiolite and volcano-sedimentary sequences have similar structural and metamorphic histories, exhibiting one principal deformation and the formation of folds with subhorizontal axes, local development of second-generation folds and associated cleavage, and an intervening period of metamorphism in the greenschist facies.The elliptical area enclosed by the ophiolite belt is referred to as the Mount Cormack Terrane, and is underlain by variably metamorphosed shale, quartz-rich sandstone, quartzo-feldspathic to amphibolitic gneisses, and granite. A limestone occurrence contains shelly fossils of Early to Middle Ordovician age. An early deformation formed folds with steep axes, and subsequent metamorphism resulted in a progression from greenschist facies to upper amphibolite facies, with the production of migmatite and granodiorite.The preferred interpretation of the geology is that the elliptical Mount Cormack Terrane is exposed as a window through an overlying allochthon composed of ophiolitic and volcano-sedimentary rocks of the Dunnage Zone. The emplacement of the allochthon probably postdated deposition of the Silurian Botwood Group. Paleontologic, lithologic, and structural considerations suggest that the sediments of the Mount Cormack Terrane were deposited at the eastern margin of Iapetus and are perhaps correlatives of rocks exposed in the Gander Zone. This implies that the Dunnage Zone has been thrust, probably in an eastwards direction, on a scale comparable with the allochthons mapped in the Scandinavian Caledonides and proposed for the Appalachians of Quebec and the United States.

Occurrence and significance of gneissic amphibolites in the Vema fracture zone, equatorial Mid-Atlantic Ridge

Summary Numerous gneissic amphibolites were recovered together with variably deformed and metamorphosed gabbros and serpentinites in the Vema fracture zone during two separate cruises. Petrological studies and K/Ar dating show that these rocks result from recrystallization of gabbros under stress, at temperatures consistent with amphibolite facies, probably in the vicinity of the ridge axis. We suggest that gneissic amphibolites were formed in a large subvertical shear zone related to transform fault movement which favoured penetration of seawater in oceanic layer 3. Less deformed metagabbros recrystallized away from the fault zone under static conditions from interaction with evolved seawater-derived fluids.

Prograde metamorphic zoning in amphibolite facies pelitic gneisses from Kemiö, Southwest Finland

The metamorphism of the Svecofennides in southwestern Finland has been studied in pelitic gneisses from the Kemio region. Three metamorphic zones have been distinguished which are characterized, in order of increasing grade, by the parageneses muscovite + quartz, biotite + sillimanite + K feldspar, and cordierite + K feldspar. Temperature and load pressure estimates of metamorphism range from 560±25° C, 3±0.5 kb for the upper boundary of the muscovite + quartz zone to 670±25° C, 4±0.5 kb for the cordierite + K feldspar zone at a partial water pressure of 0.4±0.1 Ptotal

Metamorphism of the Thompson nickel belt gneisses: Paint Lake, Manitoba

Gneisses mapped in the Thompson nickel belt are the product of a Hudsonian metamorphic (M3) overprinting of an Archean granulite terrain. Pikwitonei region granulites may be partially preserved in amphibolite facies migmatite gneisses of the Thompson nickel belt. Temperatures attending the earlier Archean (M2) metamorphic event have been estimated to be 910 ± 70 °C. More detailed work on the younger overprinting suggests that the metamorphic conditions were relatively constant during the M3 metamorphism. Estimated temperature and pressure during the Hudsonian metamorphism are 650 ± 50 °C and 4–5 kbar (4–5 × 105 kPa), respectively.Although total pressure may have been constant, calculated values for [Formula: see text] during the retrograde overprinting indicate a variability due to lithologic control. This variability in [Formula: see text] is shown to be a controlling factor in the development of hydrous retrograde mineral assemblages. Metapelitic gneisses are characterized by [Formula: see text] values of 0.05Ptotal, which may explain the absence of muscovite as a stable M3 mineral phase. Enderbitic gneisses exhibit higher values of [Formula: see text] (0.25Ptotal) due to the emplacement of syn-M3 pegmatite dykes producing a more hydrous stable M3 mineral assemblage.

Ancient crustal metamorphism at low pH2O:charnockite formation at Kabbaldurga, south India

Arrested charnockitic conversion of amphibolitic gneiss at Kabbaldurga, Karnataka State, south India, was studied mineralogically. Iron-rich pyroxenes were generated from amphibole in patches and stringers without melting. The dark colour of charnockite arises from numerous tiny veins of chlorite and manganese-bearing calcite, particularly in feldspars. The metamorphism was effected by very local, mainly grain-boundary, migration of volatiles low in H2O, and probably dominantly CO2. This was followed by vein alteration at lower temperatures from volatiles richer in H2O. The volatiles are ascribed to massive liberation from the mantle in upwelling areas, and this may have been an important process in the evolution of the deep continental crust.

Precambrian Evolution of the Spanish Peaks Area, Montana

Precambrian crystalline rocks in the Spanish Peaks area of southwestern Montana are composed primarily of granitic and amphibolitic gneiss, quartzite, and amphibolite. Sillimanite gneiss, ultrabasic intrusions, marble, and post-Precambrian basalt are also present. Almandine-amphibolite metamorphic facies and sillimanite-almandine-orthoclase subfacies are widely distributed in the area. Two major Precambrian orogenic events followed accumulation of a thick sedimentary sequence. An early Precambrian orogenic deformation produced isoclinal folds, folded pegmatites, and amphibolite boudins. These structural features were deformed in a second orogeny, which was characterized by large, open northeast-trending folds that developed under metamorphic conditions favorable for ductile deformation and a second generation of pegmatites. Cross folding and flexure of the second generation folds and development of kinks accompanied or followed the second orogeny. Mafic dikes and sills were emplaced following the second orogenic event and subsequently metamorphosed. They cut across the second-generation folds. Young, probably post-Precambrian quartz veins, plus felsic porphyry dikes, and basalt dikes cut all earlier structures. Alternate correlation of thermal events whose ages have been determined radiometrically at 2.6, 1.9, and 1.6 b.y. with metamorphic and structural events are made as follows: Amphibolitization of unfolded dikes and sills: I, 1.9 b.y; II, 1.6 b.y. Open, subhorizontal folds: I, 2.6 b.y.; II, 1.9 b.y. Isoclinal folds: I, age uncertain; II, 2.6 b.y.

Autochthonous granodiorites north-west of Frederikshåbs Isblink, Fiskenæsset region

The gneiss complex immediately north-west of Frederikshåbs Isblink is composed mostly of amphibolite facies, grey, streaky, quartzo-feldspathic, biotite and hornblende gneisses. Subordinate amphibolites, homblendites, thin anorthosites, peridotite lenses up to 200 m across, rare concordant marble bands and associated calc-silicate gneisses and kyanite and sillimanite gneisses make up the bulk of the remainder, and the whole has been complexly deformed. Occupying the higher ground within the complex, and comprising the largest single lithological unit there is a considerable volume of massive weathering, porphyroblastic granodiorite and granodiorite gneiss. Field relationships show this to be unquestionably autochthonous and almost certainly of palingenetic origin and while on a small scale the 'contact' with the surrounding gneisses may be locally discordant, overall it follows closely the large scale structure and this suggests its control by preexisting lithology.

STATISTICAL STRUCTURE OF AEROMAGNETIC FIELD

A study of the statistical structure of an aeromagnetic field over approximately four and a half thousand square miles of the Parry Sound and Huntsville area of southern Ontario has revealed the following: From the point of first‐order statistics, that is, mean, variance, skewness, and probability distribution function, the right half of the area which is occupied by hornblende or biotite migmatite is homogeneous and Gaussian, whereas the left half, which is mainly occupied by amphibolite gneiss, is generally inhomogeneous and non‐Gaussian. From the point of view of second‐order statistics, that is, the spectrum, the entire field appears to be inhomogeneous. This difference between statistical pictures is attributed to the fact that there are at least two preferred directions of strike, which influence the shape of the spectrum significantly.

Gneissic Amphibolite at Las Palmas, Puerto Rico, and Its Significance in the Early History of the Greater Antilles Island Arc

The basal complex of Puerto Rico consists principally of serpentinite, minor amounts of chert and spilite, and locally small blocks of amphibolite. A detailed structural and metamorphic study of a relatively large block of gneissic amphibolite at Las Palmas reveals that the rock has undergone repeated deformation and regional metamorphism prior to contact metamorphism by intrusive serpentinite. The first event recorded by the amphibolite is amphibolite facies regional metamorphism. Foliation and some hornblende lineation were formed at this time. An increase in the intensity of regional metamorphism then produced quartzo-feldspathic segregations from the host rock, and was accompanied by small-scale folding and development of several types of lineation oblique (o the early hornblende lineation. The metamorphic intensity then decreased slightly and tectonic movement radically decreased, as nonfoliated but metamorphosed mafic dikes cut the gneissic amphibolite. As the metamorphic grade decreased further, late-stage kink-bands were developed at high angles to the earlier small-scale folding. Later, the amphibolite was intruded by serpen-tinite, causing incipient low-grade contact metamorphism and some calcium metasomatism. H. H. Hess has maintained that Puerto Rico and other islands in the Greater Antilles rest directly on oceanic crust, and that the intrusive serpentinite is derived from the bottom layer of the crust. In the Las Palmas area, the amphibolite formed prior to intrusion of serpentinite, indicating a complex preserpentinite history. Three interpretations of the gneissic amphibolite are considered. It may represent: (1) metamorphosed gabbroic rocks genetically related to the serpentinite, (2) regionally metamorphosed mafic rocks that are part of the oceanic crust, or (3) older mafic rocks regionally metamorphosed as part of an extensive pre-Late Cretaceous orogeny. The evidence from Las Palmas is not conclusive, but when considered in the regional framework, either of the last two interpretations seems reasonable.

Isograds and the Role of H2O in Metamorphic Facies of Orthogneisses of the Northwest Adirondack Area, New York

Four isograds are shown on a geologic map of the northwest Adirondack area based on the first development of: (1) regional metamorphic garnet in biotite-quartz-oligoclase paragneiss; (2) orthopyroxene as a major member of the mafic assemblage in amphibolitic gneiss, and the disappearance of sphene in hornblende quartz syenite and old granite orthogneisses; (3) garnet in amphibolitic orthogneiss and metagabbro; and (4) garnet in pyroxene quartz syenitic and hornblende quartz syenitic orthogneisses. The successive new mineral assemblages are inferred to represent development under conditions of increasing temperatures and load pressures. An attempt is made to correlate them with the metamorphic facies of progressive metamorphism. The orthogneisses between any two isograds show a variety of mineral assemblages which are best interpreted in terms of development under a wide range of P H 2 O . The hypothesis of partial pressure of H 2 O about equivalent to load presure for the origin of the Adirondack orthogneisse gives results inconsistent with the data. In each of the metamorphic zones from that of almandine amphibolite facies to that of the pyroxene granulite subfacies, igneous rocks that were pyroxene syenite or quartz syenite have been metamorphosed to equivalent pyroxenic gneisses, and hornblende quartz syenite or hornblende granite to equivalent hornblende gneisses. The rocks have, in general, been impermeable either to access or to loss of H 2 O during plastic flow and any subsequent recrystallization. Evidence is presented that most of the hornblende-hypersthene amphibolites, with or without garnet, associated with rocks of the pyroxene granulite subfacies were formed at the same T and Pi as this facies, but at relatively high P H 2 O . The late-stage development of a network of rescrystallized garnetiferous shear zones in otherwise nongarnetiferous host rock is discussed.

Iron ore at Bomi hill, Liberia

The iron deposit exposed on the crest of Bomi hill north of Monrovia, Liberia, consists of massive magnetite and hematite. The ore zone overlies granitic gneiss and is overlain locally by amphibolite gneiss and elsewhere by banded iron-rich siliceous rocks of probable sedimentary origin. Surface induration and recrystallization of the ore is attributed to the action of meteoric waters.

The mother lode mine in British Columbia: R. H. Allen. Eng. Min. J., lxxxviii, 1101

Copper–gold–bismuth–tellurium mineralization in the Stanos area, Chalkidiki Peninsula, Greece, occurs in the Proterozoic- to Silurian-aged Serbomacedonian Massif, which tectonically borders the Mesozoic Circum-Rhodope metamorphic belt to the west and crystalline rocks of the Rhodope Massif to the east. This area contains the Paliomylos, Chalkoma, and Karambogia prospects, which are spatially related to regional NW–SE trending shear zones and hosted by marble, amphibolite gneiss, metagabbro, and various muscovite–biotite–chlorite–actinolite–feldspar–quartz schists of the Silurian Vertiskos Unit. Metallic minerals occur as disseminated to massive aggregates along foliation planes and in boudinaged quartz veins. Iron-bearing sulfides (pyrite, arsenopyrite, and pyrrhotite) formed prior to a copper-bearing stage that contains chalcopyrite along with galena, sphalerite, molybdenite, and various minerals in the system Bi–Cu–Pb–Au–Ag–Te. Fluid inclusion homogenization temperatures of primary aqueous liquid–vapor inclusions in stage I quartz veins range from 170.1 °C to 349.6 °C (peak at ~ 230 °C), with salinities of 4.5 to 13.1 wt.% NaCl equiv. Calculated isochores intersect P–T conditions associated with the upper greenschist facies caused by local overpressures during late-stage tectonic movement along the shear zone in the Eocene, which produced stretching and unroofing of rocks in the region. Values of δ34S for sulfides in the Stanos shear zone range from 2.42 to 10.19‰ and suggest a magmatic sulfur source with a partially reduced seawater contribution. For fluids in equilibrium with quartz, δ18O at 480 °C varies from 5.76 to 9.21‰ but does not allow for a distinction between a metamorphic and a magmatic fluid.A 187Re–187Os isochron of 19.2 ± 2.1 Ma for pyrite in the Paliomylos prospect overlaps ages obtained previously from intrusive rocks spatially-related to the Skouries porphyry Cu–Au, the Asimotrypes Au, and the intrusion-related Palea Kavala Bi–Te–Pb–Sb ± Au deposits in northern Greece, as well as alteration minerals in the carbonate-replacement Madem Lakkos Pb–Zn deposit. Ore-forming components of deposits in the Stanos area were likely derived from magmatic rocks at shallow depth that intruded an extensional shear environment at ~ 19 Ma.