Showing papers on "Metamorphism published in 1981"

Pressure, temperature, and time indicators in mafic schist; their application to reconstructing the polymetamorphic history of Vermont

Abstract: Mineral chemistry and overgrowth relationships in mafic schist from Vermont are shown to be sensitive indicators of pressure, temperature, and relative time and to be good chronicles of the Paleozoic history of this polymetamorphic terrane. Within the common assemblage, amphibole + chlorite + epidote + plagioclase +quartz+ Ti-phase ± carbonate ± K-mica ± Fe^(3+)-oxide, electron microprobe analyses show that increasing metamorphic grade (as defined by intercalated pelitic schist) is recorded by an increase in the edenite, glaucophane, and tschermakite contents of amphibole, in the anorthite content of plagioclase, and in the substitution of (Al^(VI),F^(2+), Ti), Al^(IV) for (Fe^(2+), Mg, Mn), Si in biotite, chlorite, and muscovite. With increasing pressure the glaucophane component of amphibole increases. For medium-pressure metamorphism the albite-oligoclase gap is in the garnet zone, where amphibole has between 1.2 and 1.8 formula proportion Al^(IV) and (Al^(VI) + Fe^(3+) + Ti + Cr). This gap is below the garnet isograd in low-pressure mafic schist where Al^(IV) and (Al^(VI) + Fe^(3+) + Ti + Cr) in the amphibole are both less than 0.6. Mineral growth periods observed are characterized by metamorphic grade and facies series and are assigned to two Ordovician (Taconic) and two Devonian (Acadian) events. Silurian-Devonian mafic schist in northeastern Vermont records two periods of low-pressure, Devonian metamorphism. These events are expressed by medium-pressure and low-pressure metamorphism in the Cambrian and Ordovician rocks to the west and south. In the pre-Silurian rocks high-pressure and medium-pressure metamorphism is assigned to the Ordovician. The observed high-pressure metamorphism is confined to a 110 by 40 km area along the Green Mountain anticlinorium axis in north-central Vermont.

Towards a paleogeography and tectonic evolution of Iran: Discussion

Abstract: Although Berberian and King must be complimented extensively in Iran and subtly casts doubts on the abil on their attempt to synthesize the geology and tectonic of other geologists, I must point out that many evolution of Iran, I must point out several errors in their Berberian's publications are in the form of unpublish paper concerning the Neyriz-Sirjan region. This area is internal reports, which are unavailable. Thus his fit critical to the tectonic history of Iran as it encompasses work cannot be evaluated. Also, Berberian and Ki the Zagros ophiolite belts, a paired metamorphic zone have completely ignored the extensive field work and a Mesozoic-Cenozoic volcanic/plutonic belt. Adib (1978) on the metamorphism of the Neyriz-Sirj They state (p. 241) that ophiolite emplacement took area, even though it was referenced many times in place only during the Late Cretaceous, because it seems Haynes and Reynolds (1980). In particular, they conthat there is no disruption in the sedimentation process of sider the metamorphic rocks west of Sirjan to be the Zagros and Central Iran during Middle Jurassic time, Abukuma type (p. 247). This is not true as south of t' and discount the idea proposed by Haynes and Reynolds Cheshmeh Anjir fault the metamorphic rocks are clea (1980) that ophiolites were emplaced during both Barrovian type with kyanite being locally abundz Middle Jurassic and Late Cretaceous times. Their first (Haynes and McQuillan 1974; Adib 1978; Haynes a error is that they claim our age of 170 Ma for initial Reynolds 1980). North of the Cheshmeh Anjir fault t ophiolite obduction is only based on one sample. This is metamorphism is indeed Abukuma type (Adib 19; untrue as we clearly stated (Haynes and Reynolds 1980) Haynes and Reynolds 1980). that our hornblende age, from the Makran, agreed The last error is Berberian and King's division of the remarkably with a previous WAr age of 170 + 8 Ma Quaternary volcanic rocks of the Aj-Bazman zone intc from the Neyriz area (Adib 1978). Berberian and King calc-alkaline and alkaline series. They claim the Aj(p. 241) discount our 170 + 5 Ma 40Ar-39~r hornblende Bidkhan zone (Fig. 18) is alkaline. The reference age because "It is possible that the radiometric age has (Dimitrijevic 1973) quoted by Berberian and King f o ~ been disturbed by later retrograde recrystallization and "Mosahim" and "Dehaj" volcanics actually states the] argon loss." Frankly, this is ridiculous because argon are calc-alkaline not alkaline (Dimitrijevic 1973, Table loss would mean that our 170Ma age is too young; 1). Also, the rocks of this belt are predominantly therefore, the real age of ophiolite emplacement would andesite to dacite. Admittedly, alkalic types are present, have to be older than Middle Jurassic and not younger as is common in most calc-alkaline belts, but the rocks (Late Cretaceous) as required by Berberian and King. are silica saturated, not unsaturated as would be required Moreover, the remarkably flat plateau of our sample for the true alkaline series. Petrochemical analysis indicates that little or no argon movement has occurred. (Ghorashi-Zadeh 1978) of the 2.8 + 0.2 Ma Late Dacite Berberian and King are guilty of "selecting" data as they dome at Sar Cheshmeh (situated between Bidkhan and do not dispute our Late Cretaceous ages for biotite and Mosahim) indicates that these dacites lie within t' muscovite (Haynes andReynolds 1980). As hornblende calc-alkaline field as defined by Irvine and Barag has a higher argon blocking temperature than micas, (1971). why did they not query the mica ages? Also, their paper It is not the intent of this discussion to comment on t contains references to dates obtained by other workers; tectonic significance of the above, rather to point out t why were these not disputed, particularly as several of significant errors in Berberian and King's data in ord these are K/Ar ages? that the interested reader can draw his own conclusion Berberian (P. 256) 'laims have worked his, D. 1978. Geology of the metamorphic a( t south-westem margin of the central-eastem Iranian mic~ 'paper by M. Berberian and G. C. P. King, 1981. Canadian plate (Neyriz area). Neues Jahrbuch fuer Geologie UI Journal of Earth Sciences, 18, pp. 210-265. Palaeontologie Abhandlungen, 156, pp. 393-409.

The New England Batholith, Eastern Australia: Geochemical Variations in Time and Space

Abstract: The New England Batholith, Australia, is part of the Upper Paleozoic New England Fold Belt, with most plutons intruded into the deformed trench-complex metasedimetary rocks in the southeast part of the Fold Belt. The Batholith was emplaced in two major periods of plutonism, the first during the Upper Carboniferous and the second during the Upper Permian and Triassic, with a major phase of metamorphism and deformation including westward overthrusting of the trench-complex sedimentary rocks between the two periods. On the basis of petrography, geochemistry and isotopic characteristics, the granitoids of the Batholith are subdivided into five named intrusive suites and a group of leucoadamellites. The differences between the six groups are considered to reflect differences in their source-rock types. The Carboniferous granitoids are peraluminous S-type and are divided into the Bundarra Plutonic Suite, a belt of very coarse-grained adamellites with cordierite ± garnet, and the Hillgrove Plutonic Suite, a belt of biotite-rich ± garnet deformed adamellites and granodiorites. Both suites have δ 18O greater than 10, negative δ34S, 87Sr/86Sr initial ratios about 0.706, are ilmenite-bearing and have low FeO3 ratios. The Bundarra Plutonic Suite, however, is consistently SiO2-rich (greater than 70%), contains cordierite and has higher δ18O than the Hillgrove Plutonic Suite. The two S-type suites are inferred to have formed by partial melting of the deepest parts of a wedge of trench-complex sedimentary rocks oceanwards of an ‘Andean’ volcanic chain to the west, analagous to the lower Tertiary S-type plutons of the Sanak-Baranof plutonic belt of southern Alaska. Of the remaining four groups of plutons emplaced during the Upper Permian and Triassic, two are metaluminous I-type granitoids, one the Clarence River Plutonic Suite (new name), a group of K-poor granodiorites and tonalites with low 87Sr/86Sr initial ratios (average 0.7035), and the other the Moonbi Plutonic Suite, a group of K-rich adamellites with low 87Sr/86Sr initial ratios (average 0.7045), δ18O less than 10 and δ34S positive. The Clarence River Plutonic Suite is inferred to have formed from the partial melting of a large-ion lithophile element-poor (gabbro?) source region and the Moonbi Plutonic Suite from a large-ion lithophile element-rich (shoshonite?) source region. A third group, the Uralla Plutonic Suite, is less metaluminous, has higher 87Sr/86Sr initial ratios (average 0.706), δ18O only just less than 10 and negative δ34S, and is considered to have formed in a region containing a physical mixture of metaluminous and peraluminous source rocks (gabbro and metasedimentary rocks?). Plutons of the fourth group are leucoadamellites, largely emplaced after the other suites, with low δ18O indicating an I-type source. It is argued that this group has resulted from low degrees of ‘dry’ partial metling of an I-type source region that had undergone an earlier melting event. These four groups of plutons are largely confined to a narrow belt parallel to, but 100 km west of, the present rifted margin of Australia. Their origin may be related to crustal thickening attendant on the overthrusting that occurred early in the Permian and/or a temperature increase in the lower crust due to the recovery of normal continental geotherms following the cessation of subduction.

Metamorphism at the base of the Samail Ophiolite, southeastern Oman Mountains

Abstract: Metamorphic rocks, showing an inverted metamorphic zonation from upper amphibolite facies to greenschist facies, occur beneath the Samail ophiolite. The metamorphic protolith was a section of marine basalts, Mn-rich cherts, and argillites. Garnet-clinopyroxene amphibolites occur within 2 m of the contact, hornblende-clinopyroxene assemblages occur within 80 m of the contact, and lower amphibolite facies rocks persist to 130 m from the contact. The higher-grade rocks show polyphase deformation, cataclastic fabrics, and partial retrogression to greenschist facies assemblages; however, lower-grade phyllites show evidence of only one episode of metamorphic crystallization. Unmetamorphosed sediments in contact with the basal metamorphic sheet do not show evidence of polyphase deformation. The distribution coefficient KD = FeO/MgO gar/FeO/MgO cpx ranges from 4.4 to 5.3, suggesting a T range of 755°– 865°C at 200 MPa. Low jadeite content in clinopyroxene and low glaucophane content of Ca amphiboles are consistent with moderate pressures of crystallization. The metamorphic sheet is inferred to be of composite origin. The highest-grade rocks were tectonically transported the greatest distance, and the lower-grade rocks were successively incorporated into the sheet as the peridotite was thrust over progressively cooler rocks. Residual heat from the ophiolite is the dominant heat source for the metamorphism, and this could have been augmented by a limited amount of frictional heat generated during thrusting. These thermal constraints suggest that the metamorphism occurred early in the tectonic transport history of the Samail ophiolite.

The behaviour of so-called immobile elements in hydrothermally altered rocks associated with volcanogenic submarine-exhalative ore deposits

Abstract: Evidence is available that some elements, notably Zr, TiO2, Y, Sc, Ce and Nb are largely immobile during the alteration of volcanic rocks owing to metamorphism, hydrothermal events and weathering (e.g. Floyd and Winchester, 1978). However, it is shown, by reference to analyses of rocks from the environment of five volcanogenic massive sulphide bodies, that while Zr, TiO2 (and Ce?) are mostly immobile even during intense hydrothermal alteration, Y and particularly Sc and Nb may be extremely mobile. When elements are removed by solution in a hydrothermal fluid it seems that reaction rates are such that these elements are almost totally removed from the rock. Therefore, of the so-called immobile trace elements, only Zr and TiO2 may be used with any reliability to identify the degree of magmatic differentiation in an hydrothermally altered rock. However, if an element has been mobile it is usually readily identified as having moved.

The geology of the Kambalda nickel field, Western Australia

Abstract: The Kambalda nickel field contains the major concentration of volcanic peridotitc-associated nickel ores in the Archean of Australia. It occupies an area of 400 km 2 underlain by two sequences of ultramafic, mafic, and felsic volcanics, and sedimentary rocks, which can be broadly correlated with previously erected regional stratigraphic sections. Most ores occur in the lower (Kambalda) sequence at the base of the Kambalda ultramafic rocks; the ultramafic rocks of the upper (Bluebush) sequence contain only minor mineralization. Four phases of deformation occurred over an interval of more than 300 m.y., resulting in a dominant north-northwest structural trend. Faulting was active during original volcanism and continued intermittently until after mafic-felsic dike intrusion. The ultramafic rocks have been serpentinized, and all rocks have exerienced low strain upper greenschist to lower amphibolite facies metamorphism and variable carbonation and potash metasomatism.The Kambalda ultramafic rocks are of variable thickness and in places exceed 1,000 m. The lower member of the ultramafic rocks, occupying approximately one-third of the sequence, comprises thicker high magnesium flows (i.e., generally > 36% volatile-free MgO) commonly separated by sulfidic quartz-albite sediments. Thinner, lower magnesium flow units characterize much of the upper member. The ores occur in 24 shoot complexes each containing multiple ore surfaces caused by structural dislocation of a few, grouped, ribbonlike original orebodies. Eighty percent is contact ore occurring at the base of the lowermost ultramafic flow and generally occupying elongate troughs in the footwall basalt-ultramafic contact; these troughs are thought to be original volcanic depressions which focused later deformation. The remainder is hanging-wall ore usually occurring directly above contact ore at the base of the second or third ultramafic flow. Hanging-wall orebodies occasionally grade into stratigraphically equivalent interflow sediments.The ores contain varying proportions of massive, matrix, and disseminated sulfides. The hypogene mineral assemblage is pyrrhotite-pentlandite-pyrite-(chalcopyrite), but millerite is locally significant. The massive ores in particular are commonly severely deformed and some have been remobilized tens of meters from their original contact position. Metamorphism has caused recrystallization of the ores and some redistribution of elements. There is commonly marked variation of mineralogy and S/Ni ratios between contact and overlying hanging-wall ores, and within and between original ore surfaces within shoots, but the relative contributions of primary composition and metamorphism to this variation are uncertain.In ore environments, the footwall basalt-ultramafic contact is more deformed than in non-ore environments, the basal ultramafic flows are thicker and richer in magnesium, the trend toward lower magnesium flows upward through the ultramafic formation is less well developed, and there is better textural development and compositional differentiation within flows. Non-ore environments usually contain interflow sediments, whereas there is generally a sediment-free prism of ultramafic rocks above ore environments.Most ores are interpreted to have originated by either simultaneous or sequential extrusion of sulfide melts and ultramafic lavas rich in olivine phenocrysts along active linear fissures in a basaltic ocean floor. Volcanic exhalations probably contributed to minor ore in sulfidic sediments.

COCORP seismic profiling of the Appalachian orogen beneath the Coastal Plain of Georgia

Abstract: A southeastward extension onto the Coastal Plain of an earlier COCORP traverse, which confirmed large-scale, thin-skinned thrusting of crystalline rocks of the southern Appalachians, has provided some of the most spectacular reflections yet seen in crustal seismic data. Most of the reflectors can be interpreted as either fault surfaces or as metamorphosed strata of late Precambrian—early Paleozoic age. They are consistent with the hypothesis that a major detachment extends eastward beneath this part of the orogen, although other interpretations with a more complex pattern of detachments or sutures are also possible. Large-scale overthrusting provides a mechanism for incorporating sedimentary rocks into the lower crust and may help to explain many of the layered features on crustal seismic data. Reflections from deep beneath the Coastal Plain indicate that the structural configuration of the rocks is complex and that the remains of a collision zone are being observed. Several east-dipping horizons, which bear strong similarities to thrust faults in Valley and Ridge sedimentary rocks, are seen in the basement at shallow and mid-crustal levels beneath the Coastal Plain. The Augusta fault, for example, displays a reflection which extends at a low angle some 80 km or more southeast of its surface position. In conjunction with surface geologic information, these new data demonstrate that late Paleozoic compressive deformation was pervasive and resulted in lateral movements in the upper crust extending from the Valley and Ridge to the crystalline rocks beneath the Coastal Plain — a distance of 400 km or more. A large antiform, cresting at about 2.3 sec, or about 6 km below the surface, and other structures beneath the Coastal Plain of Georgia deserve further consideration for petroleum exploration, although metamorphism may have eliminated petroleum from these rocks. Refracted arrivals and fault geometries indicate two Triassic rift basins beneath Coastal Plain sedimentary rocks, one of which has apparently not been recognized previously.

Water activity changes across an amphibolite-granulite facies transition, Broken Hill, Australia

Abstract: Phase compositions in pelitic and mafic gneisses place tight constraints on pressure (ranging from 3 up to 6 kb), and, to a lesser extent, on temperature (500° up to 800° C) during prograde regional metamorphism of the Willyama Complex, Broken Hill, SE Australia. These limits allow an evaluation of water activity across the terrain using various equilibria in pelitic and mafic gneisses. The stability of cummingtonite and biotite over much of the terrain places upper limits on temperature, and the presence of syn-metamorphic partial melts in the metasediments places lower limits on a(H2O). Garnet-biotitesillimanite-K feldspar-quartz relations combined with the partial melting data suggest a decrease in water activity from near 1.0 in the lower grade zones to 0.5±0.2 in the Broken Hill — Little Broken Hill part of the two pyroxene zone. This result is compatible with less precise hornblende-orthopyroxene-clinopyroxene-quartz relations.

Structural evolution of an early Proterozoic strata-bound Cu-Co-Zn deposit, Outokumpu, Finland

Abstract: The massive pyrite-pyrrhotite-chalcopyrite-sphalerite deposit of Outokumpu, comprising the Keretti and Vuonos orebodies, is a deformed and metamorphosed strata-bound mass associated with mineralised stockworks. Mobilisation of much of the ore followed formation of large recumbent isoclinal folds that are the major structures of the surrounding rocks and associated with the modification of originally flat saucer-shaped ore lenses into elongate ruler-shaped masses. Further modification of shape took place at the mobilisation stage with much of the pyrrhotitic ore, particularly, now occupying the thickest parts of the orebodies in the form of breccia or microbreccia. In many parts gross original characters still exist and the pyritic and pyrrhotitic constituents of the ore have survived as separate entities while locally the pyritic ore retains pre-deformational characteristics and consistent stratigraphic position within a thin horizon.Both ore and country rocks show evidence of extensive polyphase deformation with the effects of six fold phases shown in the ore. Mineral assemblages in the country rocks indicate a middle amphibolite facies peak of metamorphism. The serpentinite-black schist-carbonate-quartzite rock assemblage, with which the ore is associated, was tectonically incorporated within the regionally extensive mica schist by even earlier subhorizontal thrusting. This is related to the movement of a thrust nappe with the interdigitation of an ocean-floor ophiolite assemblage and flysch deposited during ocean closure associated with Svecokarelian tectonism.The original formation of the Keretti and Vuonos sulphide masses took place in a marine exhalative environment with a pyritic layer overlying a pyrrhotitic layer in each of the two c. 4 km diameter irregularly oval-shaped depressions whose centres were c. 8 km apart. The mineralised stockwork below each mass represents the upper parts of the conduit for metalbearing fluids in a convective system.

A geologic reconnaissance of the Cycladic blueschist belt, Greece

Abstract: The Cycladic blueschist belt consists of two distinctive segments separated by a broad zone of superposed granitic and high-temperature metamorphic rocks. The northern segment contains early metamorphic fold axes and parallel glaucophane lineations that trend ∼060° with a progressive increase in metamorphism toward the southeast. The southern segment contains similar fold axes and glaucophane lineations that trend ∼010° with an apparent increase in metamorphism toward the northwest. Radiometric dating of metamorphic minerals from both segments give apparent ages of about 40 to 80 m.y. These data suggest the existence of late Mesozoic or early Cenozoic subduction zones in the Aegean region that subsequently collided.

Three S-type volcanic suites from the Lachlan Fold Belt, southeast Australia

Abstract: The concept that granitoids of the Lachlan Fold Belt in southeast Australia, are derived from either igneous or sedimentary source rocks (I- or S-type), can be extended to volcanic rocks of the same age. Three suites of late Silurian S-type volcanics are described, two of which can be matched quite closely with plutonic equivalents. A large part of the Paleozoic continental margin volcanic activity in southeast Australia consisted of the magmatic recycling of old metasedimentary crust, probably of late Proterozoic age. The three volcanic suites are moderately to strongly peraluminous, with the corresponding presence of Al-rich minerals. Variation within the volcanic suites is ascribed chiefly to progressive removal of restite, or source material residual from partial melting. The most mafic suite, the Hawkins Suite, contains plagioclase, cordierite, orthopyroxene, biotite and quartz as restite components, with less abundant almandine. These rocks are chemically equivalent to mafic biotite-rich and cordierite-bearing granitoids from parts of the Berridale and Murrumbidgee Batholiths. Garnet is absent from the Goobarragandra Suite volcanics, which are a little more felsic and close in compositon to granitoids of the Young and Maragle Batholiths. The S-type character of the Laidlaw Suite is less pronounced although a sedimentary source seems to be established. Such a source would be less mature than those for the other two volcanic suites. The Laidlaw Suite resembles, but cannot be closely identified with, felsic S-type granitoids of the Murrumbidgee and Berridale Batholiths. Low-grade regional metamorphism in most of the volcanic rocks has resulted in much mineralogical alteration and mobility of alkali and alkaline earth elements. Despite this alteration, unaltered rocks are present in all three suites. Compositional data for the phenocryst phases within the unaltered rocks has allowed estimation of some intensive parameters. Hawkins Suite volcanics were extruded directly from their source region and preserve phenocryst equilibria established at 5–6 kbar at 800°C. The Laidlaw and Goobarragandra Suites reequilibrated at lower pressures than their source before extrusion. The Laidlaw Suite reequilibrated at estimated temperatures of 725°–730°C, ƒO2 of 10−15.5 to 10−14 bars, and ƒ(H2O) of 2 to 2.5 kbar. Differing mineral compositions in the three suites are related to differing source rock compositons and oxygen fugacities. Relative biotite and orthopyroxene mg is possibly pressure dependent.

Contrasting high and intermediate pressures of metamorphism in the Archaean Sargur Schists of southern India

Abstract: The Archaean Karnataka craton of southern India contains Eastern and Western crustal blocks (separated by a major thurst) in which Sargur Schists occur as lenses within tonalitic Peninsular Gneisses. The Schist complex comprises pelites, quartzitic psammites, carbonates and calc-silicates, iron formations, and basic rocks, and thus provides many mineral assemblages ideal for the calculation of PT conditions. With their gneisses the Sargur rocks are unconformably overlain by the Dharwar greenstone belts, and are generally thought to be older than 3,000 my.

Eo-alpine metamorphism in the uppermost unit of the Cretan nappe system — Petrology and geochronology

Abstract: The uppermost unit of the Cretan nappe system consists of ophiolites on the top, and an ophiolitic melange at the base. Among the various constituents of the melange, there are slices of low-P/high-T metamorphics. They form a variegated series consisting of tholeiitic ortho-amphibolites, para-amphibolites, andalusite and sillimanite-cordierite-garnet bearing mica schists, calcsilicate rocks, and marbles. The metamorphic sequence is locally intruded by early tectonic magmatites of gabbroic, dioritic and granitic composition. Critical mineral assemblages lead to a maximum temperature of about 700° C reached during metamorphism, at a total pressure of 4–5 kilobars. K — Ar dating on 6 hornblendes, 7 biotites and 1 muscovite yielded cooling ages of 75–66 m.y. and confirmed earlier results according to which the metamorphism and related magmatism took place in Late Cretaceous times. In order to evaluate the age relationships between the hightemperature metamorphics within the ophiolitic melange and the ophiolites, hornblendes from ultramafic and mafic rocks of the ophiolite complex were dated by the K — Ar method. Hornblende from one schistose hornblendite forming a constituent of the ophiolites proper yielded 156 m.y. and thus provides a middle Jurassic minimum age for the formation of this piece of oceanic lithosphere. Four hornblendes of calc-alkaline gabbrodiorite dikes within the ophiolite complex gave distinctly lower K — Ar dates of about 140 m.y.. The dikes probably intruded after the detachment of the ophiolites in an island-arc or continental-margin environment. As a consequence, the high-temperature metamorphics and related intrusives in the ophiolitic melange of Crete are genetically unrelated to the overlying ophiolites. The paleogeographic position of the crystalline terrane, slices of which are now incorporated into the ophiolitic melange is still open to discussion.

Synkinematic intrusion of peraluminous and associated metaluminous granitic magmas, Whipple Mountains, California

Abstract: ABsrRAcr The autochthon of the Whipple Mountains detachment terrain (California) contains two generations of peraluminous intrusive bodies. each one coeval with a distinct metamorphic event. Garnetbiotite granodiorite and two-mica adamellite plutons were generated during an early ductile metamorphism. Subsequent Late Cretaceous mylonitization was synchronous with the intrusion and formation of a sheeted sill complex. Peraluminous sills, which contain two-mica tonalite and garnet-two-mica granodiorite, form a maior central portion of the compositionally zoned sill complex. The structurally uppermost and lowermost sills, composed of biotite granodiorite and hornblende-biotite guartz diorite. respectively, are metaluminous. The primary white mica in the peraluminous rocks contains 19.5-32.5% of a titaniferous ferriceladonite end-member. K( Mgo " Ti,,.: ) Fe3 + (Si3.oAlo.4) Oro( OH ) u. Chemographic analysis shows that these'celadonitic muscovites are consistent with shallow levels of intrusion: a depth of 9.6-11.5 km is calculated. Shallow depth is also suggested by the Mn-rich composition of garnet, vhich contains 42.647.4 mole Vo almandine. the remainder being comprised primarily of subequal proportions of spessartine and grossular. Biotite compositions indicate crystallization under fairly oxidizing conditions (Ni-NiO to MnG-MnsOr buffer curves), an inference consistent with the abundance of magnetite and the near-absence of ilmenite. These peraluminous intrusive rocks are thus set apart from S-type granitoid rocks. The composition of the magmas that generated the older and younger peraluminous suites is only weakly aluminasaturated when compared with $type granitoid rocks of other orogenic belts; this is due to a high content of Na and Ca. not to low Al. The marked similarity of melts coeval with the metamorphic events is indicative of crustal anatexis involving exceedingly similar source materials. Although fractional crystallization was dominated by plagioclase. the removal of hornblende or allanite (or both) performed a major rote in increasing the peraluminous nature of one intrusive bodv.

Mineral-chemical and isotopic studies of Namaqualand granulites, South Africa: A grenville analogue

Abstract: The northwestern part of South Africa and southern South-West Africa/Namibia is amongst the most extensive granulite terranes in Africa. This work reports the results of electron microprobe studies of minerals from two-pyroxene, cordieriteorthopyroxene (-gedrite) (-sapphirine) and garnet and/or cordierite parageneses from Namaqualand, in the N.W. Cape Province of South Africa. Determined PT conditions of prograde metamorphism based on thermodynamic calculations are 800°–900° C and ca. 6–7 Kb; and it is argued that rocks of unusual composition, notably cordierite-orthopyroxene rocks, are restites after the extraction of granitic liquid from former argillites. This interpretation is consistent with previously published data on similar rocks, and with McCarthy's (1976) suggestion of extensive partial melting in the quartzofeldspathic rocks in the area.

Imbricate low-angle faulting in uppermost Franciscan rocks, South Yolla Bolly area, northern California

Abstract: The South Yolla Bolly area of northern California contains the intersection of the Franciscan complex, the Klamath Mountains, and the Great Valley sequence. Four distinct lithic units, among them the South Fork Mountain Schist, as well as several subunits, have been discerned in the Franciscan rocks of this area. These units are composed mostly of coherent or semi-coherent layered sedimentary and mafic igneous rocks and their metamorphic equivalents rather than melange, and are arranged in largely upright, parallel imbricate fault slices. Four major low-angle faults have been clearly recognized. Paleontologic data indicate that at least one fault places older material over younger. Lithic units exhibit various degrees of differences in metamorphic mineral assemblages, textural grade, and amount of internal deformation across the faults, with more intensely deformed and metamorphosed material occupying successively higher structural positions. These contrasts indicate that some of these faults served as surfaces by which deformed rocks were emplaced differentially upward during and/or after metamorphism. Original bedding and compositional layering in all of the coherent units generally parallel the thrust faults. Two fault-bounded units are each composed of a coherent sequence of basalt at the base overlain by bedded chert, and further overlain by mudstone and graywacke or their metamorphic equivalents. The faults which underlie these two sequences seem to have originated as decollements localized at or near a sedimentary rock – mafic-crust interface, perhaps as discrete underthrust slices. The distinct contrasts in original rock composition among these units plus the aforementioned characteristics seem to indicate that these faults had an early history in the accretionary prism apart from their later use as convenient surfaces for differential upward emplacement of metamorphosed material.

Structural and Metamorphic Relatioms Between Sargur and Dharwar Supracrustal Rocks and Peninsular Gneiss in Central Karnataka

Abstract: The Dharwar Supergroup and its basement of Peninsular Gneiss and Sargur supracrustal rocks in the areas of Ghatti Hosahalli and southeast Bababudan display certain textural, structural and unconformable relations which have important implications for the Archaean chronology of the Karnataka craton. In the first. instance these relations show that certain tonalitic-granitic parental rocks of the Peninsular Gneiss basement to the Dharwar supracrustal rocks were formed as a series beginning with polyphase gneisses and ending with discordant plutons such as the Chikmagalur granite s.l. The Sargur rocks were deformed and metamorphosed to medium-high grade during intrusion of the polyphase gneisses. After cooling, uplift and erosion of the Peninsular Gneiss and the tracts and enclaves of Sargur rocks, the Dharwar supracrustal association was deposited unconformably on the medium-high grade basement. The pre-Dharwar metamorphic minerals in the Sargur rocks were partly retrogressed and then overprinted by a second major metamorphism, mainly low grade, whose climax was attained after the main deformation of the belts and basins of the Dharwar supracrustal rocks. This major low grade metamorphism in central Karnataka is correlated with the later Archaean high grade terrane (ca. 2600 Ma) in southern Karnataka and elsewhere in Peninsular India.

Geology of the crystalline basement complex, Eastern Transverse Ranges, Southern California: Constraints on regional tectonic interpretation

Abstract: About 3000 km2 within the crystalline basement complex of the Eastern Transverse Ranges in the Chuckwalla, Orocopia, Eagle, Cottonwood, Hexie, Little San Bernardino, and Pinto Mountains of Riverside County, California were mapped at scales of 1:36,000 and 1:62,500 and compiled at 1:125,000 (Plate I). Pre-Jurassic(?) (i.e., older than the Mesozoic batholiths) rocks of the crystalline complex comprise two lithologically distinct terranes. These terranes are called the Joshua Tree and San Gabriel terranes for regions of southern California in which their lithologies were initially characterized. The two terranes are superposed along a previously unrecognized low-angle fault system of regional extent, the Red Cloud thrust. During the course of this study, the structurally lower Joshua Tree terrane has been defined as a stratigraphically coherent group of crystalline rocks that consists of Precambrian granite capped by a paleo-weathered zone and overlain nonconformably by orthoquartzite that interfingers westward with pelitic and feldspathic granofelses. The quartzite contains near-basal quartz/quartzite clast conglomerates, and has well-preserved cross-bedding that appears upright wherever it has been observed. Pelitic and feldspathic granofelses crop out to the west of the quartzite exposures in four lithologically different belts that trend northnorthwest throughout the area mapped. These lithologic belts are interpreted to have been derived from stratigraphically interfingering sedimentary protoliths deposited in a basin offshore from a quartzose beach-sand protolith. In proximity to the early Red Cloud thrust, this whole stratigraphic package was pervasively deformed to granite gneiss, stretched pebble conglomerate, lineated quartzite, and schist. A northeast-trending pattern of metamorphic isograds was orthogonally superimposed on the northnorthwest-trending protoliths of the Pinto gneiss. A central andalusite zone, located in the southern Little San Bernardino and Hexie, and northern Eagle Mountains, is flanked to the northwest and southeast by sillimanite zones. Coincident with this symmetrical distribution of aluminosilicates is an asymmetrical distribution of other pelitic mineral zones, with prograde cordierite-aluminosilicate-biotite- and K-feldspar-aluminosilicate-bearing assemblages to the northwest in the northern Little San Bernardino and Pinto Mountains, staurolite-bearing assemblages in a narrow zone in the southern Little San Bernardino-Hexie and northern Eagle Mountains, and retrograde chlorite-muscovite-bearing assemblages in the southernmost Little San Bernardino, Cottonwood, southern Eagle, Orocopia, and Chuckwalla Mountains. One occurrence of chloritoid-sillimanite in the central Eagle Mountains is apparently also retrograde. The crossing isograds are interpreted to result from a temporal increase in PH2O relative to PT from south to north through the field area. Comparison of the pelitic assemblages with experimental studies suggests peak conditions of PT ≈ 3.5 to 4 kb, T ≈ 525 to 625°C. The early prograde metamorphism pre-dated the thrusting event; the retrograde stage may have overlapped in time with the emplacement of the San Gabriel terrane allochthon. Cordierite-orthoamphibole-bearing assemblages are present in one stratigraphic zone of the Pinto gneiss. In this study, the Precambrian lithologies of the San Gabriel terrane are viewed as a three-part deep crustal section, with uppermost amphibolite grade pelitic (Hexie) gneiss intruded by granodioritic (Soledad) augen gneiss at the highest level, retrograded granulite (Augustine) gneiss at an intermediate level, and syenite-mangerite-jotunite at the lowest level exposed in the Eastern Transverse Ranges. The Hexie gneiss, characterized by sillimanite-garnet-biotite-bearing assemblages, is thrust over andalusite-bearing granofels of the Pinto gneiss. The Red Cloud thrust system is inferred to have developed in three or four sequential structural events: 1) early thrusting that probably moved parallel to the ENE mineral lineations recorded in both plates; 2) regional folding of the initial thrust surface around NNE-trending axes; 3) later thrusting that broke with some component of westward movement across a fold in the older thrust surface to produce a stacking of crystalline thrust plates of the two terranes; 4) continued or renewed folding of both thrust faults with eventual overturning toward the SW. It is consistent with all observations to date to link these structural events into a single regional tectonic episode that resulted in westward-vergent allochthonous emplacement of the San Gabriel terrane over Joshua Tree terrane. The thrust timing can only be loosely bracketed in time between 1195 m.y. and 165 m.y. ago. The pre-batholithic terranes and the westward-vergent Red Cloud thrust are considered to be exotic with respect to the pre-batholithic rocks and structures exposed to the north and east of the field area. The bounding discontinuity has been obliterated by intrusion of both suites of Mesozoic batholithic rocks. The Mesozoic plutonic rocks comprise two batholithic suites, both of which intrude the Joshua Tree and San Gabriel terranes and the Red Cloud thrust system. NW-SE trending belts of plutonic lithologies have been mapped within each suite: the oldest lithology of the younger suite intrudes the youngest lithology of the older suite. The older suite, Jurassic(?), lying to the NE, appears to have an alkalic character; the younger suite, Cretaceous(?), appears calc-alkaline. The older suite consists of biotite- and K-feldspar-bearing gabbro-diorites intruded by low-quartz monzogranites. The younger suite includes hornblende-biotite-sphene granodiorite intruded by porphyritic monzogranites, intruded in turn by nonporphyritic monzogranite. The Eastern Transverse Ranges south of the Pinto Mountain fault are defined by several Cenozoic E-W left-lateral strike-slip faults that have a cumulative westward displacement from S to N of about 50 km. The left-lateral faults are interpreted to form part of a conjugate fault set with complementary right-lateral faults in the Mojave and Colorado Deserts. Along the western boundary of the Eastern Transverse Ranges in the Little San Bernardino Mountains, the crystalline rocks have been pervasively cataclasized by an event that post-dates intrusion of the Cretaceous(?) plutonic rocks. The cataclasis is attributed to the Vincent-Orocopia-Chocolate Mountain thrust that is thought to superpose the diverse pre-batholithic and batholithic rocks of the Eastern Transverse Ranges above Pelona-type schist. The cataclastic foliation is folded along the length of the Little San Bernardino Mountains in an antiform that is inferred to be cored with Pelona-type schist. This fold may have formed a single antiformal feature comprising all the crystalline-rock antiforms now recognized along the San Andreas fault that are cored by Pelona-type schist. Displacements of the piercing points formed by the antiformal axis apparently indicate 220 km of right-lateral offset on the present San Andreas strand and about 80 km of right-lateral offset along a fragmented older San Andreas strand that consisted of the San Francisquito, Fenner, and Clemens Well faults and a buried extension of this fault beneath the alluvial fill of the valley between the Chocolate and Chuckwalla Mountains.

U-Pb zircon and monazite dating of a mafic-ultramafic complex and its country rocks

Abstract: U-Pb data on zircons from the largest mafic-ultramafic body (6×2 km) of the French Central Massif (Sauviat-sur-Vige) yield the following age results: Primary magmatic crystallization of the gabbroic and peridotitic protoliths took place in the Cambro-Ordovician (496±25/17 m.y.). Variable transformation under eclogite facies conditions was Hercynian (320±29/36 m.y.). The same age pattern, derived by U-Pb monazite analyses, was found also for the immediate country rocks, i.e. kyanite bearing, coarse-grained metagranites occurring to the W and N of the Sauviat massif. Due to the fact that there is no regional Hercynian high-grade metamorphism in this part of the French Central Massif (e.g. Duthou 1977; Bernard-Griffiths 1975), both mafic-ultramafic complex as well as immediate felsic country rocks must have been emplaced tectonically into pre-Hercynian (Acadian±Caledonian) crustal rocks. The cause for such a Hercynian tectonism is thought to be due to continent-continent collision of the Spanish with the Armorican plate.

Geochronology of the Arabian-Nubian Shield in Southern Israel and Eastern Sinai

Abstract: The metamorphic and igneous orogenic basement rocks of southeastern Israel and eastern Sinai evolved as part of the Arabian-Nubian Shield during latest Proterozoic time Pelitic schist of the "Elat Association" from Wadi Nahal Shlomo south of Eilat (Israel) is dated at about 800 my, but the geologic significance of this date is not clear In southeastern Sinai, metapelitic rocks of the Kid Group have a date of diagenesis or metamorphism of about 615 my Syn-orogenic and post-orogenic plutons which intrude the layered metamorphic complex(s) are dated at ~600 my near Eilat and ~590 my at Wadi Kid Other dated plutons include the Timna Granite at 592 ± 7 my, in the area of Nevi'ot at 595 my, and in the vicinity of Dahab at 588 ± 2 my and 595 ± 03 my The radiometric dates and low initial $^{87}Sr/^{86}Sr$ ratios (~07034) imply that pre-1000 my continental sialic crust was not involved in the formation of the shield in southeast Israel and eastern Sinai