Showing papers on "Metamorphism published in 1980"

Carbonic metamorphism, granulites and crustal growth

Abstract: Stabilization of early crust against melting by high radioactivity and against resorption into the mantle by fast convective overturn requires that water and heat producers were flushed upwards within 50 Myr of accretion. Creation of a refractory base of granulite by metamorphism associated with CO2 vapour explains CO2-rich fluid inclusions in ancient high-grade rocks, minor-element depletions and local phenomena of arrested development of charnockite in Precambrian terrains. The hot-spot and plate-tectonic models of Precambrian crustal evolution lead to different schemes for CO2 delivery to continental roots. New tectonic concepts may be needed to explain carbonic metamorphism and other features of early crustal evolution.

Geology of Nepal and its regional frame

Abstract: Since the opening of Nepal in 1950, a wealth of new information on the geology of the Himalaya has emanated from this country. The sedimentary history of the Range is most reliably recorded in the richly fossiliferous ‘Tethyan’ or ‘Tibetan’ zone, which extends to the N from the summit region and has revealed an epicontinental to miogeosynclinal sequence, over 10 km thick, ranging from Cambrian to Cretaceous. Included are minor volcanic and glacial deposits and a Glossopteris flora of Permo-Carboniferous age, suggesting close palaeogeographic links with India and Gondwana-land. The absence of significant unconformities refutes allegations about a Hercynian or Caledonian orogenic prehistory for the Himalaya. The Mesozoic portion of the sequence passes northwards into the Indus-Tsangpo eugeosynclinal zone, where deep-sea sedimentation commenced in Triassic times and continued to the early Tertiary, with emplacement of ophiolites in the Cretaceous and thick flysch deposits in the Cretaceous-Eocene. Subduction of Tethyan oceanic crust and collision of India with Eurasia along the Indus-Tsangpo ophiolitic suture is a current hypothesis. The Central crystalline zone, which forms the High Range, appears at first sight to be the Precambrian crystalline basement of the Tibetan sediments. But the crystalline rocks (which in addition to gneisses and granites contain high- and low-grade metasediments) show a transitional relationship with the sediments of the Tibetan zone. There is an intimate relationship of metamorphism and granitization with late Tertiary deformation and thrusting, particularly with the Main Central Thrust (MCT) which separates the High Range from the Lesser Himalaya. Thrusting along the MCT is equated by many with continental subduction along a rupture in the Indian continental plate which was primarily responsible for the deformation, metamorphism and granitization in the Himalaya, but this is hardly conceivable without assuming prior consolidation of the plate fragments involved. Radiometric dating of gneisses and granites from the Central Crystalline zone has indicated Precambrian-Cambrian in addition to the predominant late Tertiary ages; together with stratigraphic data from the Lesser Himalaya they suggest that Indian shield elements are present in the crystalline masses of the Himalaya but have been largely obliterated by the Himalayan orogeny. Fundamental problems remain in the Lesser Himalaya. Stratigraphic work in the thick, slightly metamorphosed argillo-arenaceous and calcareous deposits is hampered by the almost total lack of palaeontological control. The sporadic and partly controversial discoveries of organic traces point to Tethyan affinities and a range from Precambrian to Tertiary, with a predominance of late Precambrian-early Palaeozoic and Permo-Carboniferous deposits, a widespread Middle Palaeozoic gap, and restricted Mesozoic-early Tertiary deposition in marginal basins in the S. The facies suggest continuity of shelf sedimentation from the Indian platform in the S across the Lesser Himalayan zone to the Tibetan zone in the N, with gradual thickening and completion of the section (closing of the Middle Palaeozoic gap) but with considerable differentiation in the Mesozoic. The ‘Lesser Himalayan crystallines’, which overlie the low-grade metasediments as klippen-like isolated masses or, in eastern Nepal, as extensive sheets merging with the Central Crystalline zone, pose the difficult problem of ‘reverse metamorphism’. Heat metamorphism by in situ granite intrusions, selected metamorphism and migmatization, inversion of stratigraphy by recumbent folding, block faulting, nappe structure and other explanations have been offered. The crystalline complex of Kathmandu in central Nepal, recently mapped in detail, consists primarily of a right way up sequence of regionally metamorphosed sediments displaying a metamorphic zonation roughly concordant with stratigraphy and a regular decrease in metamorphic grade from highly garnetiferous schists at the base to barely metamorphosed, fossiliferous Palaeozoic sediments on top. Banded gneisses and augen-gneisses have a restricted, laterally and vertically irregular distribution in this sequence, reflecting a superimposed migmatization that disrupts the primary (regional) metamorphic zonation. Small granite bodies are genetically related to the migmatites. The contact of the Kathmandu Crystalline zone with the underlying metasediments is marked by intense shearing and by a stratigraphic, metamorphic and structural discontinuity indicating a thrust plane. The Kathmandu Crystalline zone is interpreted as the remnant of a nappe, rooted in the Central Crystalline zone. The Himalayan orogeny also involved vast expanses of Trans-himalayan Tibet and Sinkiang. Studies by Chinese geologists show Tibet to be an intensely folded mountain country, forming part of a vast ‘Tethys-Himalayan Domain’ affected by Mesozoic-Tertiary folding and magmatism. It displays striking similarities with central Iran and appears linked with it through the Hindukush-Pamir-Karakorum system, a continuous orogenic belt to the N of the main Alpine-Himalayan ophiolitic suture. The Palaeozoic deposits of Central Tibet have the epicontinental facies of their Himalayan counterparts. The Sungpan-Kantze and Sankiang fold systems of northern and eastern Tibet are distinguished as a broad ‘Indosinian’ belt of intense late Triassic folding. Its axial zone, the Chinshakiang fault zone, is characterized by thick flysch deposits associated with basic and acid volcanic material of the Variscan-Indosinian cycle and accompanied by late Triassic-early Jurassic granite intrusions. This fold belt links the late Triassic (‘late Hercynian’) fold belt of northern Afghanistan and the northern Pamir with the classical Indosinian (late Triassic) fold belt of Yunnan and SE Asia. The mountains of Sinkiang, part of the ‘Pal-Asiatic Domain’ N of the Chinshakiang fault zone, bear the stamp of the Caledonian and Hercynian orogenies; however, the late Tertiary Himalayan movements strongly remoulded them as far N as the Tienshan Range, 1500 km N of the Himalaya. A southward migration of the centres of orogenic activity from the mountains of Sinkiang in Palaeozoic time to northern Tibet in late Triassic and to the Indus-Tsangpo line in Cretaceous-early Tertiary time, and further to the Himalayan Main Central Thrust in Middle Tertiary and to the Main Boundary Thrust and the Himalayan front in Pliocene-Pleistocene time, can be clearly recognized. It is tentatively explained in terms of continental drift by the breakaway of two large continental fragments—Tibet and India—from Gondwanaland and their successive collision with, and accretion to, Eurasia.

Intrusion of basaltic sills into highly porous sediments, and resulting hydrothermal activity

Abstract: Sill intrusions into highly porous sediments in the Guaymas Basin, Gulf of California, lead to low-grade metamorphism, thermal alteration and migration of organic compounds, marked changes in interstitial water chemistry, and large-scale expulsion of heated pore fluids. The latter process creates space for the intruding magma and initiates a hydrothermal system, which can explain the observed hydrothermal deposits around fault scarps on the basin floor.

Structure and metamorphism of rocks beneath the Semail ophiolite of Oman and their significance in ophiolite obduction

Abstract: Metamorphic rocks showing an inverted metamorphic zonation from upper amphibolite fades immediately beneath the peridotite to greenschist fades at lower levels, crop out discontinuously along the base of the Semail ophiolite thrust sheet in the Oman Mountains of eastern Arabia. These metamorphic rocks show polyphase deformation, mylonitic fabrics, and have been disrupted, folded and imbricated, and in places form tectonic inclusions in a serpentinite melange. In the more intact sequences, garnet-clinopyroxene amphibolites, with rare hornblende-bearing marbles and banded quartzites, occur at the higher levels, whilst a wide range of metasedimentary and metabasaltic rocks occur in the greenschist fades. Low glaucophane content of amphiboles and low jadeite content of clinopyroxenes suggest relatively low pressures of crystallisation. The distribution coefficient KD for co-existing garnet and clinopyroxene suggests a temperature range of 670 to 750°C. Residual heat from the recently formed ophiolite probably provided the dominant heat source for metamorphism, although frictional heating could have supplemented this. The metamorphic rocks were produced during Cenomanian-Turonian (late Cretaceous) times.The sub-ophiolite metamorphic rocks, together with basal serpentinite, Triassic alkaline and tholeiitic basalts (Haybi volcanic group), mountain-sized ‘exotic’ limestones and an upper Cretaceous sedimentary melange, comprise a distinct thrust slice termed the Haybi complex, which always overlies marine sediments of the allochthonous Hawasina complex and underlies the Semail ophiolite. The rocks of the Haybi complex are bounded by major thrust planes, the Semail thrust above and the Haybi thrust beneath, which truncate all schistosities, fold axes, imbricate thrusts and associated features. A model for ophiolite obducton is presented based on a palinspastic reconstruction of the allochthonous rocks on the Oman continental margin.

Tectonic relations of carbon dioxide discharges and earthquakes

Abstract: CO2-rich springs occur worldwide along major zones of seismicity. They are mostly in young orogenic belts, but some are in areas of rifting continental platforms. Analyses of 13C content indicate that much of the CO2 is derived from the mantle and that other important sources are the metamorphism of marine carbonate-bearing sedimentary rocks and the degradation of organic material. The presence of calc-silicate minerals, such as pumpellyite in metagraywacke, is evidence of former conversion of carbonate-bearing rocks into calc-silicate minerals and release of CO2. The CO2 pressure in fractured rocks of a fault zone reduces the effective normal stress and, if it is sufficiently great, allows the fault to slip. If the pressure were maintained at a sufficiently high level, the fault behavior might be characterized by frequent small earthquakes and aseismic creep such as occur along active segments of the San Andreas system. The presence of CO2-rich springs may indicate a potentially hazardous seismic region. Monitoring of CO2 discharges could be useful in earthquake prediction.

Caledonian Sm–Nd ages and a crustal origin for Norwegian eclogites

Abstract: Sm–Nd dating of minerals in several Norwegian country-rock eclogites reported here indicates that the rocks crystallized ∼425 Myr ago. The isotopic composition of both Nd and Sr in these eclogites suggests that they formed from much older crustal rocks. The high-pressure metamorphism of which the eclogites are relics probably involved subduction of the continental Baltic plate as it was overridden by the Greenland plate during the Caledonian orogeny.

U-Pb and Rb-Sr radiometric dates and their correlation with metamorphic events in the granulite-facies basement of the serre, Southern Calabria (Italy)

Abstract: An approximately 7 km thick, continuous sequence of granulite-facies rocks from the lower crust, which contains a lower granulite-pyriclasite unit and an upper metapelite unit, occurs in the NW Serre of the Calabrian massif. The lower crustal section is overlain by a succession of plutonic rocks consisting of blastomylonitic quartz diorite, tonalite, and granite, and is underlain by phyllonitic schists and gneisses. Discordant apparent zircon ages, obtained from granulites and aluminous paragneisses, indicate a minimum age of about 1,900 m.y. for the oldest zircon populations. The lower intersection point of the discordia with the concordia at 296±2 m.y. is also marked by concordant monazites. Therefore, the age of 296±2 m.y. is interpreted as the minimum age of granulite-facies metamorphism. Concordant zircon ages were obtained from a metamorphic quartz monzogabbronorite sill (298±5 m.y.) and an unmetamorphosed tonalite (295±2 m.y.); they are interpreted as the intrusion ages. Discordant zircon ages from a blastomylonitic quartz diorite gneiss, situated between the lower crustal unit and the non-metamorphosed tonalite, reveal recent or geologically young lead loss by diffusion. The 207Pb/206Pb ages of the two analysed size-fractions point to an intrusion age similar to that of the overlying tonalite. Rb-Sr mineral ages are younger in the granulite-pyriclasite unit than in the overlying metapelite unit. Feldspars from the granulite-pyriclasite unit yield ages of about 145 m.y. and those from the metapelite unit 176±5 m.y. In the same way, the biotite cooling ages range between 108 and 114 m.y. in the granulitepyriclasite and between 132 and 135 m.y. in the metapelite unit and the tonalite. Some still younger biotite ages are explained by the influence of tectonic shearing on the Rb-Sr systems. A muscovite from a postmetamorphic aplite in the metapelite unit yields a cooling age of 203±4 m.y. The Rb-Sr isotopic analyses from migmatite bands do not lie on an isochron, perhaps due to limited isotopic exchange between the small scale layers during the long cooling period after the peak of metamorphism. In the phyllonitic gneisses and schists a Hercynian metamorphism is indicated by a muscovite age of 268±4 m.y., whereas the biotite age of 43±1 m.y. from the same sample can be correlated with an Alpine greenschist-facies metamorphism. On the basis of the radiometric dates and of the P-T path of the lower crustal section deduced petrologically, the following model is presented: the end of the Hercynian granulite-facies metamorphism was accompanied by an uplift of the lower crustal rocks into intermediate crustal levels and by synchronous plutonic intrusions into the lower crust and higher crustal levels, but essentially into the latter. Substantial further uplift did not occur until after cooling from the temperature of the granulite-facies metamorphism to the biotite closing temperature. This cooling lasted for about 185 m.y. in the lower part and for about 160 m.y. in the upper part of the lower crust section. A comparison between the geologic evolutions of the NW Serre of Calabria and the Ivrea Zone of the Alps demonstrates striking similarities. The activity of deep seated faults in both areas at least since late Hercynian time raises the possibility that a fault precursor of the boundary of the Adriatic microplate already existed at this time.

Conodonts as paleotemperature tools in Ordovician rocks of the Caledonides and adjacent areas in Scandinavia and the British Isles

Abstract: Based on collections from 110 localities, the temperature-induced color change associated with low-grade metamorphism in conodonts is used for the first regional assessment of the degree of heating...

Late Precambrian Evolution of Afro-Arabian Crust from Ocean Arc to Craton

Abstract: Large parts of northeast Africa and Saudi Arabia consist of a telescoped, Proterozoic island-arc ocean-basin complex less than 1 b.y. old. In the central Eastern Desert of Egypt, the oldest units in the complex are a mafic and ultramafic sequence representing an oceanic substrate. Concentrations of elements in constituent rocks least altered by metamorphism, including Cr, Ni, Ti, and REE in the ultramafic rocks and pillow basalts, are essentially those found in similar rocks of contemporary oceanic crust. Thick sequences of calc-alkaline volcanic rocks and related volcanogenic metasediments, including wackes, breccias, and banded iron formations, overlie the oceanic substrate. The andesitic volcanic rocks are similar to those in modern circum-Pacific island arcs, although amounts of Cr and Ni tend to be higher. Rare cobble beds in the metasediments in the Eastern Desert of Eygpt contain granitic and quartzitic clasts derived from old Proterozoic and Archean forelands, presumably those exposed west of the Nile River. Stratiform ultramafic sills as much as 1 km thick, as well as thinner gabbroic sheets, are intruded as magmas within the metasediments. The composition of the ultramafic sills approximates that of basaltic komatiites. In eastern Egypt, granitic plutons, ranging from syntectonic quartz diorites to a post-tectonic flood of LIL-enriched granite emplaced 550 to 570 m.y. B.P. partly engulf and surround much of the ocean-arc complex. Age studies suggest that the Egyptian segment of the ocean-arc complex could have evolved, been telescoped, and intruded by progressively more voluminous and fractionated granitic rocks between about 550 and 850 m.y. B.P. In southwest Saudi Arabia, limited radiometric data obtained by the U.S. Geological Survey suggest that these processes may have begun slightly earlier. Both the volcanic rocks within the complex and the granitic rocks intrusive into it have low initial ratios of Sr87/Sr86 (< 0.704), suggestive of a mantle origin. There is no evidence of older sialic roots beneath the complex.

The Flinton Group: a late Precambrian metasedimentary succession in the Grenville Province of eastern Ontario

Abstract: Clastic and carbonate metasediments, preserved in narrow synclines, have been correlated over an area of 2000 km2. These strata, the Flinton Group, lie unconformably on metamorphosed volcanic, clastic, and carbonate rocks, and on large granitic intrusions. The group, which comprises six formations, has undergone at least two major folding episodes and one main regional metamorphism of varying grade. The only post-Flinton intrusions are pegmatites at high grade and one tectonically emplaced ultramafic slice.Depositional environment ranged from fluvial to moderate-depth marine. Rapid facies changes, coupled with persistence of some units along strike and close relationships between facies and underlying lithology, point to local sources and local tectonic control of deposition basins. At the onset of sedimentation, a deeply weathered source terrain yielded mature basal redbeds, which were succeeded by less mature clastics as block faulting caused increase of relief between sources and basins. These facies p...

Petrology and 40Ar/39Ar geochronology of some hellenic sub-ophiolite metamorphic rocks

Abstract: Whole rock, electron microprobe analyses and 40Ar/39Ar geochronology of certain ophioliterelated metamorphic rocks from beneath the Pindos, Vourinos, Othris and Euboea ophiolites of Greece show that they were formed mainly from ocean-type basalts, in part under P-T conditions of the upper mantle and that they have ages between 170–180 m.y. The evidence presented is inconsistent with the view that these sub-ophiolite metamorphic rocks were produced by the obduction of ocean-type crust onto a continental margin, or that they are remnant slices of Palaeozoic ‘basement’, but is consistent with their formation by thrusting and related metamorphism occurring within ocean lithosphere during the Lower to Middle Jurassic. It is proposed that this intraoceanic metamorphism was caused by the inception of a fault zone which subsequently became the transport surface for the main phase of ophiolite emplacement that occurred in the Hellenides from the Late Jurassic to Early Cretaceous.

Incipient metamorphism of Cambro-Silurian clastic rocks from the Jåmtland Supergroup, Central Scandinavian Caledonides, Western Sweden: illite crystallinity and 'vitrinite' reflectance

Abstract: The degree of incipient metamorphism in the Cambro-Silurian of the Jamtland Supergroup (Eastern Complex) of the Jamtland Caledonides, from the Caledonian front autochthon westwards through the parautochthon towards the thrust front of the metamorphic main (9Western Complex9) nappes, has been studied using: ( a ) determination of the illite crystallinity by X-ray powder diffraction on the K- and Mg-saturated –2 μm fractions; the low-grade limits of the anchizone and the 9opizone9 of Kubler correspond to peak widths of c. 0.38° and 0.21°Δ2 (illite 001 diffraction peak) under the experimental conditions used; ( b ) determination of the rank of coaly particles in the clastic rocks by measurement of the 9vitrinite9 reflectance using a reflected-light microscope with photomultipliers. In the Indalsalven segment S of Iat. 63°309 N between Brunflo in the E and Undersaker in the W, and further N to Ronnofors-Juvuln, 4 zones of illite crystallinity may be distinguished: (A) The Ostersund-Krokom zone: mean 9saturated9 10 A peak widths of ∅0.32°Δ2θ, the highest peak widths occurring in the easternmost part of the zone (9diagenetic9 zone of Kubler). (B) The Nordero-Naldsjon zone: mean peak widths between 0.27° and 0.35° Δ2θ middle-anchizone of Kubler. (C) The Mattmar–Alsensjon zone: predominance of peak widths between 0.19° and 0.25° 2θ. (D) The Morsil–Jarpen zone; predominance of peak widths of 0.21° Δ2θ characteristic of the 9epizone9 of Kubler. This zone extends N to Ronnofors–Juvuln, W of the Offerdal nappe. 9Vitrinite9 reflectance values were measured on 35 samples. In illite-crystallinity zone A the predominant mean maximum reflectance ranges from 1.4-2.9% R max oil , corresponding to low-volatile bituminous coal to low-rank anthracite. In zones B, C and D the ranges of R max oil are similar, from 3.7 to 4.3%, corresponding to high-rank anthracite. Similarly high reflectances (4.2-4.5% R max oil ) were found in 3 samples from the northern continuation of zone D. The comparative uniformity of the reflectances from zones B–D contrasts with the increase in illite crystallinities, particularly from zone B to C. Anatase was found in demineralization residues from zones A and B (together with rutile); in the residues from zones C and D rutile is the only titanium oxide. The results are consistent with the hypothesis that the incipient metamorphism in the Jamtland Supergroup was at least in part caused by the overthrust metamorphic allochthon, particularly the mainly medium- and locally medium to high-grade metamorphic Seve nappe. The association of the onset of the anchizone with semi-anthracitic to low-rank anthracitic coal ranks (c. 2.5–2.8% R max oil ) is in agreement with the relationships found in several other areas of incipient regional metamorphism.

Wall-rock alteration at the Millenbach Cu-Zn mine, Noranda, Quebec

Abstract: Pipe-shaped zones of altered rock containing disseminated and stockwork vein mineralization underlie each of the several Archean volcanogenic Cu-Zn ore lenses at the Millenbach mine. The rocks and mineralization have been metamorphosed first to hornblende hornfels assemblages, then retrograded to greenschist facies assemblages, but the metamorphism has not significantly affected the bulk composition of the rocks. The alteration pipes show a distinct mineralogical zoning in which a core zone containing either anthophyllite-bearing assemblages or massive chlorite grades outward into a biotite-rich zone characterized by a spotted texture. Normative calculations were used to show that alteration pipes initially had a chlorite core which graded laterally and vertically into a sericite-rich outer zone and finally into unaltered rocks. Similar zoning is present on the selvedges of individual sulfide veins within the alteration pipes. Volume was conserved, and a distinctive metasomatic trend developed across the alteration pipes and vein selvedges during the alteration process. The most characteristic features of the altered rocks are increased Fe and Mg, and decreased Ca and Na, relative to fresh rocks. K and Al have generally been added at the margins and leached at the cores of pipes. Chemical zoning across vein selvedges is similar. The zoning of alteration types on vein borders, and to a certain extent in the pipes as a whole, is interpreted as due to the progressive reaction of an initially homogeneous hydrothermal solution with the rocks as the fluid moved upward in, and outward from, permeable channelways in the discharge conduit of a submarine, sea water-dominated hydrothermal system. However, an additional cause of vertical, and to a lesser extent lateral, zoning is thought to have been the existence of a strong vertical thermal gradient, as well as lateral variations in the permeability in the pipes. The thermal gradient could have been caused by the discharge fluid column being under hydrostatic pressure and everywhere at its boiling point. The fluid was probably evolved sea water which was in equilibrium with the rocks at depth in the system and then degenerated, causing alteration and sulfide deposition as a result of cooling and chemical changes associated with boiling as it rose in the conduit systems.

Selective Elemental Depletion During Metamorphism of Archaean Granulites, Scourie, NW Scotland

Abstract: The low abundances of large ion lithophile elements (LIL), K, Rb, U, Th, Cs, and high K/Rb ratio in rocks varying in composition from gabbro to granite in the Scourian complex, NW Scotland, are interpreted as due to depletion during granulite facies metamorphism. Depletion was controlled by the mineralogy of the rock, the composition of the associated fluid phase and its volume relative to the volume of the rock. K-feldspar granites and granodiorites were not depleted in K and only moderately in Rb, but tonalites and trondhjemites were strongly depleted in both K and Rb. Published mineral-fluid partition coefficients for LIL in aqueous systems indicate that between 0.075 and 2.0 rock volumes of fluid phase passed through the host rock in order to achieve the observed selective elemental depletion.

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

Abstract: 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.

On the thermal stability of Rb-Sr and K-Ar biotite systems: Evidence from coexisting Sveconorwegian (ca 870 Ma) and Caledonian (ca 400 Ma) biotites in SW Norway

Abstract: The Caledonian orogeny has imposed a zone of greenschist facies metamorphism on the high-grade Sveconorwegian basement along the front of the Caledonian nappe system in S.W. Norway. In this zone a Caledonian generation of green biotite (ca 400 Ma old) has developed, indicating a metamorphic temperature of about 400° C. This Caledonian biotite occurs side by side with relicts of a Sveconorwegian generation of brown biotite (ca 870 Ma old). The somewhat younger ages obtained from a number of brown biotites can be related to a partial transformation of the old biotite to titanite+green biotite during the Caledonian metamorphic recrystallization. Loss of radiogenic Ar and Sr from the biotite by volume diffusion apparently has not been operative, even at a temperature as high as 400° C. The Sveconorwegian biotite appears to have remained virtually closed to K-Ar and Rb-Sr up to the break-down due to metamorphic recrystallization.

The Geological Evolution of Northern Sumatra

Abstract: . . . . the causes we know everything about depend on causes we know very little about, which depend on causes we know absolutely nothing about. — Tom Stoppard, Tavesties. In northern Sumatra four major volcano-sedimentary sequences, separated by unconformities, are recognised. Three are pre-Tertiary in age and the other is Tertiary to Recent. The oldest rocks belong to the Late Palaeozoic Tapanuli Group which is primarily clastic and probably largely of glaciomarine origin. Similar rocks occur in northwestern Malaysia and southern Thailand. Two periods of deformation, accompanied by magmatism and metamorphism, preceded the deposition of the overlying Peusangan Group. This group is related to an eastward dipping palaeo-subduction zone and it consists of two main components a Late Permian volcanic arc assemblage and a Middle to Late Triassic back-arc succession which is equivalent to the Semanggol Formation of Malaysia. The Late Mesozoic Woyla Group includes a volcanic arc assemblage and portions of a dismembered ophiolite together with a cover sequence which formed in a back-arc basin. Subsequent closure of this basin in the Late Cretaceous and the low angle of plate convergence during the Tertiary has resulted in severe deformation of the ophiolitic portion of the Woyla Group. Since at least the Late Eocene northern Sumatra has been the locus of periodic volcanic arc activity with sedimentation concentrated in the adjacent fore-arc and back-arc basins. A marked unconformity in the Late Oligocene and a tectonic event initiated in the late Middle Miocene have been used to divide the Tertiary into three supergroups (1, 2 & 3). The later event, comtemporary with the commencement of sea-floor spreading in the Andaman Sea, led to the rise of the Barisan Mountains by the Pleistocene and the growth of the dextrally transcurrent Sumatran Fault System Serpentinites from the Woyla Group ophiolite were emplaced from the latest Miocene.

An inner Cordilleran belt of muscovite-bearing plutons

Abstract: Muscovite-bearing granitic rocks, rare in the great coastal batholiths of the western margin of North America, are common in an inland belt that stretches from Sonora to British Columbia. Compositional characteristics of these inland plutons (strongly peraluminous, high 87Sr/86Sr) ally them with the S-type granitoids, whereas the coastal batholiths are best described as I-type. Although a simple S-type origin by anatexis of metasediment is not required, compositions of these plutons imply a far greater contribution from ancient crustal material than do those of marginal batholiths. The distinction between margin and inner Cordillera in nature of plutonism suggests important regional differences in magmagenic processes; these differences must have been a function of variations in composition of and/or conditions in the preexisting crust. It is noteworthy that the muscovite-bearing belt is located at the edge of inferred Precambrian crust and generally corresponds to areas of intense deformation and regional metamorphism of Mesozoic through early Cenozoic time.

Metamorphic carbonate ejecta from vesuvius plinian eruptions: Evidence of the occurrence of shallow magma chambers

Abstract: Petrological, mineralogical and chemical data of 46 ejecta deriving from the sedimentary basement beneath Somma-Vesuvius volcano are reported. The ejecta samples were collected in pumice deposits formed during two major Plinian eruptions. One of these pumice deposits was formed during the well-known 79 A.D. eruption, and the other one — the so-called Avellino pumice — during an eruption occurred about 3,500 years B.P. Most of the ejecta from both the layers are fragments of contact-metamorphosed carbonate rocks. For the ejecta of the 79 A.D. Plinian eruption, the mineralogical parageneses of the metamorphosed carbonate rocks (dolomite-Mg calcite-periclase, and dolomite-Mg calcite-brucite) allow the evaluation of the conditions under which contact metamorphism developed. Temperatures, estimated by the Mg content in the calcite coexisting with dolomite, ranged from 360° to 790°C, whereas total fluid pressure would not have exceeded 1,500–2,000 bars with a maximum depth of metamorphism (and hence of the magma chambers) of 5,000–6,000 m. The ejecta from the so-called Avellino pumice layer (characzerized prevalently by a dolomite-Mg calcite assemblage) show that contact metamorphism occurred under the same temperature range as that of the 79 A.D. ejecta, but at an higherP CO2 partial pressure and probably at an higher total fluid pressure. These differences in physico-chemical conditions of metamorphism seem to indicate that the two Plinian eruptions were fed probably by two different magma chambers. Comparison between chemical composition of the carbonate ejecta and carbonate samples of the Mesozoic sedimentary series outcropping near the volcano indicates that fragmentation of almost all the sequence were brought to the surface by the explosive Plinian eruptions. Although the data at our disposal do not provide any information on the size of the 79 A.D. eruption magma chamber, this probably had an important vertical length component of at least 2,000 m

The Sveconorwegian Orogenic Belt, a case of Late-Proterozoic Plate-Collision.

Abstract: A geotetctonic model is proposed for the Sveconorwegian orogeny (1200-850 Ma), based on compilation of lithological and structural evidence. This analysis suggests a N—S trending, asymmetrical orogenic belt with longitudinal zonation. In the western-most core zone, synkinematic intrusions dominate, while the central zone consists primarily of reworked basement with subordinate intrusions and metasediments. In the easternmost marginal zone, series of pre-Sveconorwegian mega-units are separated by thrust zones and finally bordered by the Sveconorwegian frontal thrust towards the east. Graben formation in the Telemark area preserves the deposits of previous epicratonic stages and marks the initiation of Sveconorwegian activities at about 1190 Ma. The subsequent orogenic development, associated with lithospheric subduction, shows tight folding about N—S axes with steep axial planes in the west, and more open NW—SE folding with gentle westward dipping axial planes towards the east. Regional metamorphism of low-P, high-T conditions imply steep geothermal gradients and suggests hercynotype crustal evolution, in agreement with a Cordilleran-type plate collision. In the Bamble district, important structural alignment forms a zone of crustal anisotropy which was repeatedly reactivated and ultimately became the site for Permian rifting forming the Oslo Graben. The lithological and structural characters as well as the crustal evolution for the Sveconorwegian orogeny show strong similarities with the Grenville development in southeastern Canada. Structural vergence and orogenic zonation suggest that the Sveconorwegian front and the Grenville front are equivalent and form different parts of a single orogenic belt, permitting a tentative reconstruction of the Late-Proterozoic, North-Atlantic Shield.