Burial Metamorphism Research Articles

Paleomagnetism of late Archean rocks of Hamersley basin, Western Australia and the paleointensity at early Proterozoic

We report the results of paleomagnetic and rock magnetic measurements of late Archean rocks from Hamersley basin, Western Australia, and the paleointensity determination for early Proterozoic. Basalts and banded iron formations have two to four components of remanent magnetization, and have consistent directions for different localities, indicative of geomagnetic origin. Rock magnetic measurements of basalts reveal that the main magnetic mineral is a fine-grained magnetite present in ∼10 ppm in mass, which do not alter when heated up to its blocking temperature. We interpret the stable component up to ∼390 °C as the post-tilting thermal overprint from uplift at ≃2.0 Ga, and the higher temperature component as the pre-tilting thermoviscous remanence during burial metamorphism. From the Thellier type paleointensity experiments using the thermal overprint component, we obtain a mean virtual dipole moment (VDM) estimate of (1.8–3.6)×10 22 A m 2. This suggests that early Proterozoic was characterized by a 1 weak geomagnetic field of less than one-half of the present.

Geothermal palaeogradients and metamorphic zonation from the conodont colour alteration index (CAI)

The conodont colour alteration index (CAI) is potentially valuable in thermal history investigations, but there are problems in its use. An approach is proposed herein to overcome the problem of establishing reliable geothermal palaeogradients in areas with burial metamorphism, based on knowledge of the stratigraphic succession thickness, the boundaries of chronostratigraphic units, and the age and duration of deformation episodes. Although also problematical that a correlation between CAI and Kübler index (KI) values does not exist, there are areas where CAI and KI values are notably consistent for the anchizone boundaries; however, anomalous correlations between CAI and KI values have also been found in some areas. A CAI anchizone (or ancaizone) is defined in order to enable CAI values to be used independently to establish a metamorphic zonation.

MODE OF OCCURRENCE, CHEMICAL VARIATION AND GENESIS OF MORDENITE AND ASSOCIATED ZEOLITES FROM THE MORDEN AREA, NOVA SCOTIA, CANADA

Selected zoned and homogeneous amygdales and veins of zeolite from the earliest Jurassic North Mountain Basalt, in Nova Scotia, have been examined in thin section and by SEM, XRD and electron-microprobe analysis. Zoned sequences of zeolites in amygdales and veins in the North Mountain basalt show a succession of clinoptilolite-Na → mordenite → heulandite, clinoptilolite-Ca, or epistilbite → stilbite, barrerite, or stellerite, and are interpreted as developing under falling temperatures. Locally, fractures within zeolites have been filled with smectite and at least three different Mg–Fe–Na-aluminosilicates. Barite, Cu, Ni and Au precipitates occur throughout some zoned amygdales. The high temperatures required for the precipitation of labradorite and mordenite, the repetitive zonation in some amygdales, and the occurrence of metal precipitates all suggest an origin for the zeolites by active hydrothermal circulation, not by burial metamorphism as implied by previous authors. Formation of mordenite was favored by hydrothermal circulation of alkaline lake waters and extraction of Na from underlying evaporites of the Blomidon Formation.

Pre-accretion metamorphism of the Teloloapan Terrane (southern Mexico): example of burial metamorphism in an island-arc setting

Volcanic and interbedded volcaniclastic rocks of the lower Cretaceous island-arc series of the Teloloapan Terrane in southern Mexico contain metamorphic assemblages characteristic of the zeolite, prehnite–pumpellyite and lower greenschist facies produced by burial metamorphism prior to its accretion to nuclear Mexico. Distribution of secondary assemblages throughout the stratigraphic succession, together with the chemical evolution of metamorphic minerals, reveals a depth-controlled metamorphic zonation characterized by the presence of the diagnostic assemblages laumontite+pumpellyite+epidote and laumontite+celadonite+pumpellyite±epidote (zeolite facies) followed downward by assemblages containing prehnite+pumpellyite±white mica (prehnite–pumpellyite facies) and finally by the presence of the assemblages pumpellyite+actinolite+epidote and epidote+actinolite (greenschist facies). Analysis of assemblages in the Al–Fe 3+–FM–K system, reveals that facies boundaries are discontinuous, involving the disappearance of at least one phase and the appearance and/or extension of the field of equilibrium of other diagnostic minerals and assemblages. Application of empirically based thermobarometers, phase equilibria, mineral chemistry, and petrogenetic grids indicates that the P– T conditions of metamorphism ranged from 175 to 342°C and P<3 kbar. The data further indicate high geothermal gradients of about ∼55°C km −1. Seawater-derived fluids were characterized by high a K, high f O 2 and low X CO 2 .

The thermal history of the Bowen Basin, Queensland, Australia: vitrinite reflectance and clay mineralogy of Late Permian coal measures

The thermal history of the Bowen Basin (Queensland, Australia) has been investigated using vitrinite reflectance data and clay mineralogy. Vitrinite reflectance data combined with a study of clay mineral reactions indicates that the maximum temperatures which induced organic maturation of the Bowen Basin coals and extensive clay mineralisation are not related to deep burial metamorphism during the latest Middle Triassic–earliest Late Triassic as previously believed. The results of the present study indicate that the development of a zone of high heat flow in the latest Late Triassic had a major control on the thermal history of the Bowen Basin. High palaeogeothermal gradients estimated in the northern Bowen Basin are interpreted to result from convective heat transfer during a hydrothermal event. Variable heat distributions due to localised fracture-enhanced permeable zones acting as hot reservoirs in the deeper part of the basin may have been responsible for some significant local thermal anomalies in the lower coal measures. The estimated palaeogeothermal gradients in the southern Bowen Basin also indicate high heat flow in the lower sections of the stratigraphy. Sections in the southern Bowen Basin, however, are believed to reflect a rock dominated semi-closed system with low water/rock ratio, where rocks are impervious to circulating fluids and thus heat transfer may have occurred by conduction. The correlation between vitrinite reflectance and clay mineralogy shows a delay in illitisation reaction relative to organic maturity for many illite/smectite (I/S) mixed-layer clays in the northern Bowen Basin. This phenomenon can be explained as a result of insufficient time for the completion of mineral reactions and a variable potassium supply in relatively impermeable rocks. The relationship between I/S expandability and vitrinite reflectance for the Bowen Basin data compared to basins with known tectonic regimes suggests a thermal history in a rift setting for the Bowen Basin. The effect of thin igneous intrusions on clay mineral reactions is very limited. Intensive illitisation due to heating of intrusions can only be observed in narrow zones immediately adjacent to intrusive bodies. This further demonstrates that mineral reactions are too slow to record the effect of extremely short heating duration, in contrast to organic maturity indicators. These differences between mineral and organic parameters aid in the identification of local contact metamorphic effects.

CHEMICAL DEPOSITS ASSOCIATED TO METAVOLCANOSEDIMENTARY SEQUENCES OF THE CENTRAL PORTION OF THE SÃO FRANCISCO CRATON IN THE STATE OF BAHIA, BRAZIL: A REVIEW

Supracrustal suites of metavolcanosedimentary rocks (SC) occur in the central portion of the São Francisco Craton in the State of Bahia. They were primarily formed by calc- alkaline to perialkaline volcanic flows and tuffs, clastic and chemical sediments. The latter are referred to the banded iron (BIF), iron-manganese (BIMF) and manganese (BMF) formations, iron-poor magnesian-rich carbonate formation (IPMRCF), and minor metacherts. BIF and IPMRCF are present in all the reported SC. BIF is typified by the presence of the oxide, carbonate and silicate facies. Occurrences of the sulfide facies are reported for the Sobradinho-Salitre and the Boquira SC. The burial metamorphism and thermal effects pronounced by the Transamazonian migmatization and granitization are accountable for the generation of amphibolites, epidosites, steatite and talc schists, phyllites, sandstones, quartzites, itabirites, metamorphic limestones to include banded carbonates, marbles and calc-silicate rocks. It is suggested that the deposition was similarly processed throughout an extensive crustal segment and that it was part of the cratonic evolution occurred during the end of the Archean to the beginning of the Proterozoic.

Chemical deposits associated to metavolcanosedimentary sequences of the central portion of the São Francisco Craton the state of Bahia, Brazil: a review

Supracrustal suites of metavolcanosedimentary rocks (SC) occur in the central portion of the Sao Francisco Craton in the State of Bahia. They were primarily formed by calc- alkaline to perialkaline volcanic flows and tuffs, clastic and chemical sediments. The latter are referred to the banded iron (BIF), iron-manganese (BIMF) and manganese (BMP) formations, iron-poor magnesian-rich carbonate formation (IPMRCF), and minor metacherts. BIF and IPMRCF are present in all the reported SC. BIF is typified by the presence of the oxide, carbonate and silicate facies. Occurrences of the sulfide facies are reported for the Sobradinho-Salitre and the Boquira SC. The burial metamorphism and thermal effects pronounced by the Transamazonian migmatization and granitization are accountable for the generation of amphibolites, epidosites, steatite and talc schists, phyllites, sandstones, quartzites, itabirites, metamorphic limestones to include banded carbonates, marbles and calc-silicate rocks. It is suggested that the deposition was similarly processed throughout an extensive crustal segment and that it was part of the cratonic evolution occurred during the end of the Archean to the beginning of the Proterozoic.

Authigenic Illite and Organic Matter: The Principal Hosts of Vanadium in the Mecca Quarry Shale at Velpen, Indiana

AbstractThe Mecca Quarry Shale Member from Velpen, Indiana contains abundant vanadium which occurs in solid solution within illite-rich illite-smectite (I-S) having an average content of 1.65 wt. % V, and an overall composition of K0.8(Al2.8Mg0.5Fe0.4V0.3)(Si7.2Al0.8 g)O20(OH)4, analogous to the V-rich dioctahedral mica, roscoelite. The illite contains more than twice as much V as the associated kerogen. Detrital mica has a composition typical of 2M1, muscovite and contains no vanadium. The V-rich illite has a structure and composition typical of formation during normal prograde diagenesis and probably is widespread in the Mecca Quarry Shale because the bed is enriched in V throughout the Midwest. The smectite-to-illite reaction can not be a result of passive burial metamorphism because the host strata were buried no deeper than ~0.5 km at Velpen. The formation of illite occurred in unlithified sediments at shallow depths under the influence of pervasive 80–110°C basinal brines, possibly the same fluids that were responsible for the Mississippi Valley-type lead-zinc mineralization common in the Midwest. The presence of two types of K-rich phyllosilicates may be part of the reason for the lack of correlation between bulk V concentrations and the intensities of X-ray diffraction peaks of illite reported by others.

Zeolite Parageneses in the North Atlantic Igneous Province: Implications for Geotectonics and Groundwater Quality of Basaltic Crust

Zeolites are among the most common products of chemical interaction between groundwaters and the Earth's crust during diagenesis and low-grade metamorphism. The unique crystal structures of zeolites result in large molar volumes, high cation-exchange capacities, and reversible dehydration. These properties influence both the stability and chemistry of zeolites in geologic systems, leading to complex parageneses and compositional relationships that provide sensitive indicators of physicochemical conditions in the crust. Observations of zeolite occurrence in Tertiary basaltic lavas in the North Atlantic region indicate that individual zeolite minerals are distributed in distinct, depth-controlled zones that parallel the paleosurface of the plateau basalts and transgress the lava stratigraphy. The zeolite zones are interpreted to have formed at the end of burial metamorphism of the lavas. Relative timing relations between various mineral parageneses and crustal-scale deformal features indicate that the minerals indicative of the zeolite zones formed within 1 million years after cessation of volcanism. Empirical correlation between the depth distribution of zeolite zones and the temperatures of formation of zeolites in geothermal systems provides estimates of regional thermal gradients and heat flow in flood-basalt provinces. Similarly, the orientations of zeolite zones can be used to distinguish synvolcanic and post-volcanic crustal deformation. Because zeolites that characterize the individual zones display different ion-exchange selectivities for various cations, reactions between groundwaters and zeolites in basaltic aquifers can result in depth-controlled zones where individual elements are concentrated in the crust. This is established for Sr, which is concentrated by at least an order of magnitude in heulandite, resulting in an overall Sr enrichment of lavas in the heulandite-stilbite zeolite zone.

Time interval between volcanism and burial metamorphism and rate of basin subsidence in a Cretaceous Andean extensional setting

40Ar/ 39Ar ages were obtained from basaltic flows belonging to a 9-km-thick sequence generated in an extensional ensialic setting of an arc/back-arc basin type during the Early Cretaceous and presently exposed along the Coastal Range of central Chile. The basalts have been affected by very low- to low-grade burial metamorphism, mostly under prehnite–pumpellyite facies. Age values obtained from primary (volcanic) and secondary (metamorphic) minerals permit to quantify the time interval between volcanism and burial metamorphism. A plateau age of 119±1.2 Ma from primary plagioclase represents the best estimation of the age of the volcanism, whereas adularia, in low-variance assemblages contained in amygdules, gave a plateau age of 93.1±0.3 Ma which is interpreted as the age of the metamorphism. Considering the P– T conditions estimated for this metamorphic event, the c. 25 Ma time interval between volcanic emplacement and prehnite–pumpellyite facies metamorphism, the rate of basin subsidence in this extensional geodynamic setting would be comprised in the interval 150–180 m/Ma.

Pre- and synorogenic burial metamorphism in the Ardenne and neighbouring areas (Rhenohercynian zone, central European Variscides)

The Rhenohercynian zone of the central European Variscides contains in the central Ardenne/Eifel/Brabant/Campine region of Belgium, France, Germany, and Luxemburg an omnipresent thermal overprint, which reaches in places uppermost epizonal conditions. Because of its distribution and structural relationships this metamorphism cannot be explained by a singular tectonometamorphic event. Field and microstructural observations, new illite crystallinity and other data relating to this metamorphism are presented. Together with a review of existing petrological, illite crystallinity, vitrinite reflectance, conodont alteration index, and fluid inclusion data, and their relationship to small- and regional-scale geological structures three distinct geotectonic situations can be described, which overlap and alternate in space and time. (1) The oldest metamorphism, which reached epizonal conditions, can be localized in the pre-Devonian basement rocks of the Brabant Massif and, with minor evidence, of the Stavelot Massif further south. It is clearly related to a pre-Variscan (Caledonian) deformation event and could be of burial origin. (2) The main regional metamorphism affected all Devonian and Carboniferous rocks and the lower Palaeozoic basement highs of the Ardenne/Eifel area. It does not exceed diagenetic conditions in some areas, but reaches maximal values of 500°C and 400 MPa in the Serpont and Rocroi Massif areas ( Theye and Fransolet, 1993; Robion et al., 1995). It is pre- to synkinematic to the main Variscan contractional penetrative deformation and correlates well with the age and thickness of the sedimentary cover rocks. The metamorphism is associated with the pre-Carboniferous preorogenic rifting stage of the Rhenohercynian basin and documents the peak of subsidence and sediment accumulation. We interpret it as diastathermal metamorphism. It reaches its maximum in the low-grade metamorphic zones of the Ardenne area, which are inverted synsedimentary structures. (3) The diastathermal metamorphism is overprinted in the central part of the Ardenne/Eifel area (Middle Devonian to Carboniferous rocks of the Dinant Synclinorium) by an anchizonal/beginning epizonal metamorphism. In the foreland basins north of the Midi Fault Zone it reaches diagenetic to lowermost anchizonal conditions. It is the expression of the burial by thick synorogenic mid- to Upper Carboniferous cover rocks, which were deposited in piggyback and foreland basins. This clastic wedge relates to the Carboniferous contraction of the Rhenohercynian basin, whose metamorphosed rocks were themselves affected by the deformation of the advancing orogenic front.

Porosity evolution and mineral paragenesis during low-grade metamorphism of basaltic lavas at Teigarhorn, eastern Iceland

Low-grade alteration of basaltic lavas at Teigarhorn, eastern Iceland, resulted in three distinct stages of mineral paragenesis that correlate to events in the burial and intrusive history of Icelandic crust. Metasomatism and brittle deformation during the paragenetic stages dramatically affected the paleo- hydrology of the lavas and formed temporally distinct mineral assemblages. The lavas initially contained up to 22 percent total porosity concentrated near the tops and bottoms of individual lava flows. Celadonite and silica (Stage I) precipi- tated along the walls of primary pores prior to deep burial of the lavas and occluded 8 percent of the initial porosity. During burial (Stage II), hydrolysis of olivine and basaltic glass led to the formation of mixed-layer chlorite/smectite clays in the matrix of the lavas and as rims filling 40 percent of the volume of primary pores. Chlorite contents in Stage II mixed-layer clay rims increased from 20 to 80 percent during the infilling of individual vesicles, reflecting increas- ing temperatures with time as the lavas were buried. The end of Stage II occurred after burial and is represented by filling of 40 percent of total primary porosity by zeolites (scolecite or heulandite stilbite mordenite epistilbite) and re- placement of plagioclase by zeolites and albite. The Stage II zeolite assemblages are indicative of two regional metamorphic mineral zones in eastern Iceland, the mesolite scolecite and heulandite stilbite zones. The presence of the bound- ary between the mesolite scolecite and heulandite stilbite zones indicates that the maximum temperature during burial metamorphism was 90° 10°C. Localized areas of intense hydrothermal alteration associated with intrusion of basaltic dikes (Stage III) overprint Stages I and II. Extensive fracturing and hydrothermal brecciation during Stage III added 3 to 11 percent total porosity in which mm- to cm-scale museum-grade crystals of quartz, calcite (Iceland spar), stilbite, scolecite, heulandite, and laumontite precipitated. Estimates of the tem- perature during Stage III (based on fluid inclusion homogenization temperatures in calcite, chlorite geothermometry, and the zeolite assemblage) range from 120° to 180°C. Although the thermobarometric conditions during Stages II and III led to similar mineral assemblages, careful attention to textural and geologic rela- tions permits seemingly complex, multi-stage parageneses in metabasalts to be interpreted in a petrotectonic context.

Geochemical studies on Dachang antimony ore deposit in Qinglong, Guizhou Province

The Dachang antimony deposit in Qinglong, Guizhou Province, is strictly controlled by the “Dachang Layer” which is a complex altered rock occurring at unconformity between the Permian Emeishan basalt and the Maokou limestone. Based on the studies of the hanging- and foot-wall rocks, the trace elements and REE contents of the rocks and ores and heavy placer minerals in the basalt, this paper is focused on the relations between these data and the “Dachang Layer” and its hanging- and foot-wall rocks. The author pointed out that the “Dachang Layer” and basalt are the source-beds of antimony; ilmenite and magnetite are the major mineral carriers of antimony. In the processes of halmyrosis and burial metamorphism of the “Dachang Layer” and basalt, antimony was mobilized along with the mobilization of iron and was preliminarily concentrated in the “Dachang Layer”.

The metamorphic history of the Borrowdale Volcanic Group: a petrographic study

SUMMARY Petrographic studies of the secondary mineralogy of the Borrowdale Volcanic Group of north-west England suggest that these rocks were affected by at least four metamorphic events during the Palaeozoic. The first event was hydrothermal metamorphism which probably occurred during volcanism and resulted in deposition of chalcedony in veins and vesicles. Celadonite may be a result of this hydrothermal event or later burial metamorphism. The chalcedony was subsequently overprinted by contact and burial metamorphism. Contact metamorphism is defined by the presence of biotite and/or hornblende with or without hornfelsing and extends up a few km from the batholith/volcanic interface. Burial metamorphism is defined by two assemblages: calc-silicate hosted and calcite hosted, where their distribution is interpreted as being controlled by a change in the X CO 2 . content of the (predominantly hydrous) fluid associated with burial metamorphism. A fluid with lower X CO 2 was responsible for formation of the calc-silicate assemblage, whereas the fluid associated with the calcite hosted assemblage had a relatively higher X CO 2 . Burial metamorphic minerals replaced and infilled both hydrothermal chalcedony and primary igneous features (crystals, matrix and vesicles) and reached a maximum grade of prehnite-actinolite to actinolite-pumpellyite facies in the Devonian prior to the Acadian Orogeny. The last metamorphic event is regional metamorphism which is associated with the Acadian Orogeny and resulted in the formation of tectonically aligned white mica. Pervasive carbonate associated with the cleavage may be a result of regional metamorphism or a later post-metamorphic event. Post-metamorphic carbonate veins in the Lake District were probably coeval with post-Brockram carbonate veins in Nirex borehole volcanic sequences.

A pre-2.2 Ga age for giant hematite ores of the Hamersley Province, Australia?

One current model for the origin of giant hematite ores of the Hamersley province requires a two-step process: formation of martite and goethite by supergene enrichment, followed by burial metamorphism to convert goethite to microplaty hematite. Our new structural, stratigraphic, and geoehronological data suggest that at least some microplaty hematite ores were present before metamorphism owing to burial under the lower Wyloo Group. A SHRIMP U-Pb zircon age from volcaniclastic breccia within the lower Wyloo Group suggests that hematite detritus in that group was derived from hematite ores older than 2209 + or - 15 Ma. The action of pre-2.2 Ga oxidizing fluids is implied, although their origin (supergene versus hypogene) remains controversial.

Prograde metamorphism, strain evolution, and collapse of footwalls of thick thrust sheets: a case study from the Mesozoic Sevier hinterland U.S.A.

Analysis of prograde metamorphic fabrics in the exhumed footwall of the Mesozoic Windermere thrust, northeast Nevada, reveals fabrics and style of ductile flow developed during tectonic burial metamorphism of a sedimentary footwall of a thick thrust sheet. In response to structural burial, footwall strata underwent Barrovian metamorphism synchronous with development of bedding-parallel or near parallel S and S– L tectonites. Footwall rocks range from unmetamorphosed at high structural levels and progressively increase in metamorphic grade to upper amphibolite facies at deep levels. Attenuation of footwall stratigraphic units accompanied tectonite development whereby the amount of attenuation varies with metamorphic grade from ∼0% in lower greenschist facies to ∼30–50% in upper amphibolite facies rocks. Microstructures in metacarbonate, metapsammite, and metapelite, and analysis of crystallographic preferred orientations of quartz c-axes in quartzite, suggest a dominantly coaxial strain path with flattening strain predominating at lower metamorphic grade and plane strain more dominant at higher metamorphic grade. The microstructural data indicate that dominantly coaxial deformation during metamorphism accommodated extension of the footwall. Ductile extension most likely records footwall collapse induced by a loss of strength due to relative upward migration of isotherms during and or following burial. Extension may act as a mechanism that in part facilitates isostatic accommodation or sinking of the overlying load resulting in a reduction of topography. Attenuation of footwalls of thick thrust sheets and production of predominantly coaxial layer-parallel or near parallel fabrics may be an intrinsic process once sufficient structural overburden is developed and thermal relaxation and prograde metamorphism ensues.

40Ar/39Ar Data on Primary and Secondary Minerals of a Volcanic Series Affected by Burial Metamorphism: Example of a Cretaceous Andean Extensional Basin

Precise time constraints on both the formation of magmatic rocks and subsequent alteration or very low-grade metamorphism are difficult to obtain on the same rocks, mainly because of the difficulty to separate uncontaminated mineralogical components relative to these two events. We initiated a 4~ study on a thick Lower Cretaceous basaltic sequence in a regional subsiding basin affected by a very low to low-grade metamorphism due to burial, with the aim to obtain reliable ages on primary minerals (volcanism) and secondary phases (metamorphism) permitting to define the time interval between volcanism and burial metamorphism. The 9 km thick series, located at the latitude of Santiago, is part of a.c. 1000 km long volcanic belt of Early Cretaceous age exposed along the Coastal Range of central Chile (see more details in Aguirre et al., 1989). Most of the basic lavas belong to the highK calc-alkaline and shoshonite series. The rocks have been affected by very lowto low-grade, non deformational metamorphism, which preserved the primary structures and textures. Its grade increases with stratigraphic depth ranging from zeolite facies at the top to lower greenschist facies at the very bottom of the pile. Characteristic metamorphic assemblages in zeolite and prehnite-pumpellyite facies are: (a) laumontite + chlorite + pumpellyite + prehnite + epidote + K-feldspar and (b) pumpellyite + epidote + chlorite + prehnite + K-feldspar. The various characteristics of the series and of its geologic framework have been interpreted by different authors as representing an extensional ensialic setting either of a marginal basin or of an island arc type. Transparent plagioclases in bulk samples and single grains of phenocrysts, and the freshest whole rocks were carefully selected to provide the best estimate of the age of the volcanism. Adularia single grains, filling amygdales (together with pumpellyite, chlorite and low albite), and cloudy single grains of plagioclases (contaminated by secondary sericite, Kfeldspar and albite) were analysed to obtain the age of the metamorphism. Previous dating by K/Ar, 4~ and Rb/Sr displayed ages mainly ranging from 100 to 134 Ma (Aberg et al., 1984; Munizaga et al., 1988; Boric and Munizaga, 1994). The plagioclase bulk sample displayed a disturbed age spectrum because we failed to obtain uncontaminated phases on several hundreds of grains. Concordant plateau age (119.4 41.2 Ma) and high temperature apparent ages were obtained on a restricted population (29 and 37 grains, 96BU6 on Fig. 1) and single phenocrysts of transparent plagioclases. This represents the best estimate of the age of the volcanism. Adularia single grains gave a well defined plateau age at 93.1 _+ 0.3 Ma on one grain (96BU10 in Fig. 1) and slightly increasing apparent ages (probably due to a slight alteration) converging towards a value of 94.2 _+ 0.6 Ma on another grain. This corresponds to the age of the very lowgrade metamorphism. Between these extreme values characterizing the two events, intermediate apparent ages were obtained

Burial Metamorphism (Thermal Maturation) in Cretaceous Sediments of the Southern Benue Trough and Anambra Basin, Nigeria

Organic matter reflectance, illite crystallinity, and fluid-inclusion techniques have been used to evaluate burial metamorphic conditions for the lithostratigraphic successions that accumulated during the Cretaceous in the southern Benue trough and Anambra basin of southern Nigeria. These successions were invaded by intrusives, volcanic rocks, and vein-type lead-zinc minerals, especially in the Albian-Cenomanian section of the Abakaliki anticline. The sequence includes lower Maastrichtian subbituminous coals in the Anambra basin. In exposed Cretaceous deposits on a northwest-southeast section from Enugu to Abakaliki, mean random vitrinite reflectance (Rom) in oil ranges from 0.55 to 0.67% in the lower Maastrichtian coals and shales, approximately 0.91% in the Coniacian shales, 0.97% in the Turonian shales, and up to 4.31% in the Albian shales. A corresponding increase in the illite content of the illite-smectite mixed-layer clay fractions is reflected in the low values of illite crystallinity indices coupled with a decrease from 32 to 0% in the percentage of smectite from the northwest-southeast Maastrichtian-Albian section. Fluid-inclusion pressure-corrected temperatures from vein quartz in the Albian shales range from 170 to 250°C. The results of this work show that thermal maturation in the Cretaceous successions increases from the post-Santonian (Campanian-Maastrichtian) Anambra basin into the older Benue trough where strong diagenetic to anchimetamorphic (i.e., very low grade metamorphism) conditions were reached. The data suggest that these sediments at the present outcrop levels originally were buried at higher maturity levels. Maximum erosion appears to have taken place on the axis of the Abakaliki anticline. The presence of bitumen in fractures and pores of the exposed Maastrichtian units in the Anambra basin suggests that the matured sediments generated some unknown quantity of petroleum. This finding, coupled with reported gas finds and some oil in previous exploration wells of the Anambra basin, enhances the possibilities of Cretaceous targets in the downdip regions.

Age of metamorphism of Otago Schist in eastern Otago and determination of protoliths from initial strontium isotope characteristics

Geochronological studies of the eastern Otago Schist and adjacent metagreywacke sequences are applied to their structural/metamorphic history and schist protoliths. Rb‐Sr whole‐rock isochron and K‐Ar total‐rock ages are inversely correlated with metamorphic grade (both in terms of mineral facies and textural zones), dating the time of peak metamorphism and/or postmetamorphic cooling. On the northern flank of the schists, older ages (200–230 Ma) date initial burial metamorphism and deformation in late Triassic ‐ early Jurassic times. Younger age patterns (to 115 Ma) reflect either (1) higher grade metamorphism during a single event followed by long‐continued uplift, or (2) early burial metamorphism at >200 Ma with a second, regional metamorphism between then and 115 Ma followed by rapid uplift. Initial Sr isotopic ratios derived from isochrons indicate that protoliths originate in two terranes. North of Dunedin, initial 87Sr/86Sr ratios range from 0.7064 to 0.7092, similar to Torlesse Terrane (Rakaia Subterrane) analogues in the Wellington region, and indicate a granitoid source dominated by I‐types. South of Dunedin, along the south Otago coast, schists have lower initial 87Sr/86Sr ratios, 0.7052 to 0.7064, unlike Caples‐type rocks elsewhere in Otago. They are thus of “uncertain” terrane affinity but indicate a dominantly silicic‐intermediate volcanic source.

Chlorite Thermometry in the Witwatersrand Basin: Constraints on the Paleoproterozoic Geotherm in the Kaapvaal Craton, South Africa

A series of post‐depositional alteration events can be distinguished in the late Archean Witwatersrand Basin. They range from thrust‐controlled metamorphism during pre‐Transvaal crustal thickening, burial metamorphism and hydrothermal infiltration during crustal thinning and associated deposition of the 2.6‐2.1 Ga Transvaal Supergroup sediments, thermal metamorphism related to the 2.06 Ga Bushveld Igneous Complex, and hydrothermal alteration as a consequence of the 2.023 Ga Vredefort impact event. The relative importance of these events varies locally, with different events responsible for the maximum temperatures achieved in different parts of the basin: the pre‐Transvaal metamorphism affected the northern margin of the basin, whereas the syn‐Bushveld thermal metamorphism is particularly pronounced in the lower parts of the Witwatersrand Supergroup. Chlorite geothermometry, in conjunction with fluid inclusion microthermometry and the thermodynamic modeling of equilibrium mineral assemblages, was applied ...