Thin Section Scale Research Articles

Interpreting fracture development from diagenetic mineralogy and thermoelastic contraction modeling

Four sets of thin-section scale, Mode I (open mode), cemented microfractures are present in sandstone from the Eocene Misoa Formation, Maracaibo basin, Venezuela. The first set of microfractures is intragranular (F1), formed early during compaction and are filled with quartz cement precipitated at temperatures equal to or higher than 100 °C. The second set of microfractures (F2) is cemented by bituminite–pyrite, formed at temperatures between 60 and 100 °C, and are associated with kerogen maturation and hydrocarbon migration from underlying overpressured source rocks. The third set of microfractures (F3) is fully cemented by either quartz cement or calcite cement. The former has fluid inclusion homogenization temperatures between 149 and 175 °C. These temperatures are mostly higher than maximum burial temperatures (∼160 °C), suggesting that upward flow, caused by a pressure gradient, transported silica vertically which crystallized into the fractures. Upward decompression may have also caused a P CO 2 drop, which, at constant temperature, allowed simultaneous carbonate precipitation into the third microfracture set. The fourth set of thin-section scale microfractures (F4) is open or partially cemented by siderite–hematite and other iron oxides. The presence of hematite and iron oxides in microfractures is evidence for oxidizing conditions that may be associated with the uplift of the Misoa formation. In order to time and place constraints on the depth of formation of the fourth set of microfractures, we have coupled published quartz cementation kinetic algorithms with uniaxial strain equations and determined if, in fact, they could be associated with the uplift of the formation. Our results suggest that thermoelastic contraction, caused by the formation's uplift, erosion, and consequent cooling is a feasible mechanism for the origin of the last fracture set. Hence, we infer that meteoric water invasion into the fractures, at the end of the uplift, cause the precipitation of oxides and the transformation of siderite to hematite.

AN OCCURRENCE OF SAPPHIRINE IN THE ARCHEAN SUPERIOR PROVINCE, NORTHERN QUEBEC

A clast containing sapphirine has been found in the Troie Complex in the Archean Minto Subprovince, in the northeastern part of the Superior Province, in northern Quebec. The area is dominated by synmagmatically deformed felsic plutonic rocks that host volcano-sedimentary slivers and rafts. The Douglas Harbour Domain contains the Faribault–Thury tonalite–trondhjemite complex, intruded by the 2740–2726 Ma enderbitic Troie Complex. Slivers and enclaves of volcano-sedimentary rocks in the Troie Complex are metamorphosed to granulite grade. A breccia in the core of the Troie Complex contains a heterogeneous population of xenoliths in an enderbitic matrix. The clast contains sapphirine as symplectitic rims with plagioclase or K-feldspar (or with both) developed around sillimanite and cordierite. Sapphirine also forms a corona around green hercynite, with K-feldspar rimming the sapphirine. Reaction textures, annealing and polygonization indicate localized equilibration, with mosaic equilibrium preserved on the thin-section scale. Various geothermobarometers yield estimates of high temperatures (755–1260°C) and pressures (7.5–14 kbar). The existence of localized equilibria is difficult to interpret in terms of simple prograde or retrograde metamorphism linked with orogenesis. Either static P–T conditions for a protracted period, or minimal attainment of granulite-facies conditions are required to produce and preserve these mosaic textures.

Compositional characteristics and paragenetic relations of magnesiohögbomite in aluminous amphibolites from the Belomorian complex, Baltic Shield, Russia

This study investigates the compositional characteristics, parageneses, and stability relations of some högbomite-bearing assemblages in coarse-grained corundum-garnet amphibolites from a blackwall zone that separates troctolitic metagabbro from kyanite-bearing paragneiss at Diadina Mountain, Belomorian Belt, Russia. The blackwall zone presumably was formed through infiltration driven metasomatism during the Svecofennian metamorphic event at ~1.9 Ga. Euhedral högbomite grains (up to 15 mm in size) occur in domains of coarse tschermakitic amphibole, biotite, and spinel with minor rutile and ilmenite in the two blackwall varieties having contrasting bulk compositions. The other minerals in the two associations include corundum + garnet ± cordierite + chlorite + plagioclase + carbonate and spinel ± gedrite + sapphirine + chlorite + carbonate. The studied högbomite belongs to the magnesiohögbomite-2N3S polysome type [P3̅m1 with a = 5.721(1) Å, c = 23.045(1) Å] and exhibits compositions that are poor in Zn (0.05-0.42 wt% ZnO) and Ni (0.20-0.50 wt% NiO) [(Fe 2+ 2.7−3.1Mn 0.01 Ni 0.04-0.1 Zn 0.01-0.1 )Mg 4.8-3.7 (Al 18.1-18.8 Cr 0.1-0.2 Fe 3+ 0.4-0.9 )Ti 1.6-1.2 O 38 (OH) 2 ]. Compositional variation is controlled by the substitution Ti 4+ + R 2+ ↔ 2R 3+ . Systematic partitioning data for Fe 2+ , Mg, and Zn indicate attainment of chemical equilibrium between magnesiohögbomite and the associated minerals (Spl, Hbl, Ged, Grt, Spr, Bt) on the thin section scale. Textural relations suggest growth of magnesiohögbomite under amphibolite-facies conditions (6 ± 1 kbar, 600 ± 50 °C) through complex mineral-fluid equilibria involving oxide (Crn, Spl, Rt, Ilm), silicate (Am, Bt, Spr), and carbonate (Cal, Dol, Mgs) phases. A partial logf O₂ -logf S2 diagram for the system FeO-Al 2 O 3 -TiO 2 -O 2 -S 2 -H 2 O shows that growth of magnesiohögbomite from Crn + Ilm and Spl + Rt is restricted to a narrow f O₂ -window and low f S2 . The topological constraints, together with petrological data, suggest that magnesiohögbomite is formed in titanian and aluminous protoliths under greenschist-to amphibolite-facies conditions if f H2O is high, f S2 low, and f O₂ is defined by the paragenesis ilmenite + rutile + magnetite

Structural controls on Witwatersrand gold mineralisation

Structural controls on the distribution of gold in the Witwatersrand Basin can be seen at scales ranging from that of thin sections up to regional seismic data sets. At the thin section scale, gold occurs largely in fractures with hydrocarbons. These fractures are associated with pulses of thrusting deformation that occurred in the latter stages of the basin's development. The distribution of thrust–fracture networks (and therefore gold) is controlled by the mechanical attributes of the host stratigraphy. Thrust displacements are generally very low, with structures exploiting depositional contacts and mesoscale sedimentary bedforms. Many of the small faults are isolated from each other with bimodal vergence between larger-scale thrusts of the same age. Early-formed faults and fractures become folded and faulted by subsequent propagation of larger-scale thrusts. These characteristics are consistent with formation of the mineralised thrust–fracture networks in the frontal, low-displacement parts of a thrust system. In the Carletonville goldfield, the prospectivity of the Ventersdorp Contact Reef (VCR) has been evaluated by using underground observations to pinpoint the structural habitat of gold, and therefore characterise the size of prospective zones around structures intersecting the VCR, mapped within 3D seismic data. In the Welkom goldfield, the controls on gold distribution at the Tshepong mine have been characterised from 3D seismic data, drilling and underground observations, and used to develop an ore-body model that aids reserve estimation and production planning.

Clast-size analysis of impact-generated pseudotachylite from Vredefort Dome, South Africa

Clast sizes in pseudotachylite from Vredefort Dome, South Africa, which is generally regarded as being of impact-origin, were studied. Clast-size analyses were conducted on the outcrop and thin section scales, and the clast-size distribution is fractal on each scale. On the outcrop scale, the clast-size distribution of the impact-generated pseudotachylite has a small D-value ( D=1.29±0.02). On the thin section scale, the D-values ( D=1.68±0.22 and 1.85±0.13) may be slightly larger than those of fault-generated pseudotachylites and natural fault gouge, and a probable lower fractal limit (≈50 μm) is on the same order as that for fault-generated pseudotachylites. On the thin section scale, proportion of clasts for the impact-generated pseudotachylites is larger than that for fault-generated pseudotachylites, and on the outcrop scale, the impact-generated pseudotachylite has a remarkably high proportion of clasts.

An Elle-based 2-D model for cation exchange reaction between garnet and biotite

We have developed a model for diffusion zoning development in garnet employing a numerical modeling system, Elle [Jessell et al. 2001]. The Elle system simulates textural development at thin section scale with an assumption that long term textural changes can be achieved by adding incremental processes. Many numerical models have been developed for the diffusion zoning pattern of garnet, but these models considered only lattice diffusion without allowing grain boundary diffusion. However, our model simulates the diffusion zoning in garnets by considering not only lattice diffusion but also grain boundary/lattice reaction and grain boundary diffusion. Sub-processes for our garnet-biotite system include (1) lattice diffusion, (2) grain-boundary diffusion, and (3) grain boundary/lattice reaction. The finite difference explicit method was used for the numerical calculation of lattice and boundary diffusion. For diffusion calculation, chemical concentrations of individual nodal points are used to calculate the amount of transported cations with diffusion coefficients determined from experiments. For lattice/boundary reaction, the lowest energy compositions of garnet and biotite were calculated using the distribution coefficient. Experimental runs were performed to test the consistency and reproducibility of the results. The results demonstrate the consistency and reproducibility, but show somewhat different zoning from that of Spear (1991), due to reduced dimension (3D in nature vs 2D in our model) and the arbitrarily chosen reaction constant. In spite of these limitations, we feel that the model presented here is at least a good starting point for diffusive mass transfer in a material with grain boundary structure. Journal of the Virtual Explorer, 2004 Volume 15 Paper 6 http://virtualexplorer.com.au/ An Elle-based 2-D model for cation exchange reaction between garnet and biotite Page 2 Journal of the Virtual Explorer, 2004 Volume 15 Paper 6 http://virtualexplorer.com.au/ An Elle-based 2-D model for cation exchange reaction between garnet and biotite Page 3

Epidote Group Minerals in Low-Medium Pressure Metamorphic Terranes

Research Article| January 02, 2004 Epidote Group Minerals in Low–Medium Pressure Metamorphic Terranes Rodney H. Grapes; Rodney H. Grapes Institut für Mineralogie, Petrologie und Geochemie, Albert-Ludwigs-Universität Freiburg, Albertstrasse 23 b, D-79104 Freiburg, Germany, rodney.grapes@minpet.uni-freiburg.de, paul.hoskin@minpet.uni-freiburg.de Search for other works by this author on: GSW Google Scholar Paul W. O. Hoskin Paul W. O. Hoskin Institut für Mineralogie, Petrologie und Geochemie, Albert-Ludwigs-Universität Freiburg, Albertstrasse 23 b, D-79104 Freiburg, Germany, rodney.grapes@minpet.uni-freiburg.de, paul.hoskin@minpet.uni-freiburg.de Search for other works by this author on: GSW Google Scholar Author and Article Information Rodney H. Grapes Institut für Mineralogie, Petrologie und Geochemie, Albert-Ludwigs-Universität Freiburg, Albertstrasse 23 b, D-79104 Freiburg, Germany, rodney.grapes@minpet.uni-freiburg.de, paul.hoskin@minpet.uni-freiburg.de Paul W. O. Hoskin Institut für Mineralogie, Petrologie und Geochemie, Albert-Ludwigs-Universität Freiburg, Albertstrasse 23 b, D-79104 Freiburg, Germany, rodney.grapes@minpet.uni-freiburg.de, paul.hoskin@minpet.uni-freiburg.de Publisher: Mineralogical Society of America First Online: 03 Mar 2017 © The Mineralogical Society Of America Reviews in Mineralogy and Geochemistry (2004) 56 (1): 301–345. https://doi.org/10.2138/gsrmg.56.1.301 Article history First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Rodney H. Grapes, Paul W. O. Hoskin; Epidote Group Minerals in Low–Medium Pressure Metamorphic Terranes. Reviews in Mineralogy and Geochemistry 2004;; 56 (1): 301–345. doi: https://doi.org/10.2138/gsrmg.56.1.301 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyReviews in Mineralogy and Geochemistry Search Advanced Search Epidote group minerals are common in metamorphosed mafic to intermediate igneous rocks, quartzofeldspathic sediments and calc-alumina silicate (marl) rocks of higher grade zeolite to medium grade amphibolite facies of low–medium pressure contact and regional metamorphic terranes (i.e., pressure and temperature conditions below the calcite-to-aragonite transition). Within any one rock, epidote composition in terms of Fe3+/(Fe3+ + Al) can be variable, but in general, is limited by whole-rock composition, such that epidote group minerals in metabasite lithologies are more Fe-rich than those in marls that tend to be more Al-rich and typically include zoisite. Because of their wide... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Textural evidence for recent co-seismic circulation of fluids in the Nojima fault zone, Awaji island, Japan

The Hirabayashi borehole (Awaji Island, Japan) was drilled by the Geological Survey of Japan (GSJ) 1 year after the Hyogo-ken Nanbu (Kobe) earthquake (1995, M JMA=7.2). This has enabled scientists to study the complete sequence of deformation across the active Nojima fault, from undeformed granodiorite to the fault core. In the fault core, different types of gouge and fractures have been observed and can be interpreted in terms of a complex history of faulting and fluid circulation. Above the fault core and within the hanging wall, compacted cataclasites and gouge are cut by fractures which show high apparent porosity and are filled by 5–50 μm euhedral and zoned siderite and ankerite crystals. These carbonate-filled fractures have been observed within a 5.5-m-wide zone above the fault, but are especially abundant in the vicinity (1 m) of the fault. The log-normal crystal size distributions of the siderite and ankerite suggest that they originated by decaying-rate nucleation accompanied by surface-controlled growth in a fluid saturated with respect to these carbonates. These carbonate-filled fractures are interpreted as the result of co-seismic hydraulic fracturing and upward circulation of fluids in the hanging wall of the fault, with the fast nucleation of carbonates attributed to a sudden fluid or CO 2 partial pressure drop due to fracturing. The fractures cut almost all visible structures at a thin section scale, although in some places, the original idiomorphic shape of carbonates is modified by a pressure-solution mechanism or the carbonate-filled fractures are cut and brecciated by very thin gouge zones; these features are attributed to low and high strain-rate mechanisms, respectively. The composition of the present-day groundwater is at near equilibrium or slightly oversaturated with respect to the siderite, calcite, dolomite and rhodochrosite. Taken together, this suggests that these fractures formed very late in the evolution of the fault zone, and may be induced by co-seismic hydraulic fracturing and circulation of a fluid with a similar composition to the present-day groundwater. They are therefore potentially related to recent earthquake activity (<1.2 Ma) on the Nojima fault.

The suevite of drill hole Yucatàn 6 in the Chicxulub impact crater

Abstract— The suevite breccia of the Chicxulub impact crater, Yucatàn, Mexico, is more variable and complex in terms of composition and stratigraphy than suevites observed at other craters. Detailed studies (microscope, electron microprobe, SEM, XRF) have been carried out on a noncontinuous set of samples from the drill hole Yucatàn 6 (Y6) located 50 km SW from the center of the impact structure. Three subunits can be distinguished in the suevite: the upper unit is a fine‐grained carbonate‐rich suevite breccia with few shocked basement clasts, mostly altered melt fragments, and formerly melted carbonate material; the middle suevite is a coarse‐grained suevite with shocked basement clasts and altered silicate melt fragments; the lower suevite unit is composed of shocked basement and melt fragments and large evaporite clasts. The matrix of the suevite is not clastic but recrystallized and composed mainly of feldspar and pyroxene. The composition of the upper members of the suevite is dominated by the sedimentary cover of the Yucatàn target rock. With depth in well Y6, the amount of carbonate decreases and the proportion of evaporite and silicate basement rocks increases significantly. Even at the thin section scale, melt phases of different chemistry can be identified, showing that no widespread homogenization of the melt took place. The melt compositions also reflect the heterogeneity of the deep Yucatàn basement. Calcite with characteristic feathery texture indicates the existence of formerly pure carbonate melt. The proportion of carbonate to evaporite clasts is less than 5:1, except in the lower suevite where large evaporite clasts are present. This proportion constrains the amount of CO2 and SOX released by the impact event.

Archaean cratonization and deformation in the northern Superior Province, Canada: an evaluation of plate tectonic versus vertical tectonic models

The Archaean Minto Block, northeastern Superior Province, is dominated by tonalite–trondhjemite, enderbite (pyroxene tonalite), granodiorite and granite, with subordinate mafic rocks and supracrustal belts. The plutons have been interpreted as the batholithic roots of Andean-type plate margins and intra-oceanic arcs. Existing horizontal-tectonic models propose that penetrative recrystallization and transposition of older fabrics during terrane assembly at ∼2.77 and ∼2.69 Ga produced a N-NW tectonic grain. In the Douglas Harbour domain (northeastern Minto Block), tonalite and trondhjemite dominate the Faribault–Thury complex (2.87–2.73 Ga), and enderbite constitutes 50–100 km-scale ovoid massifs (Troie and Qimussinguat complexes, 2.74–2.73 Ga). Magmatic muscovite and epidote in tonalite–trondhjemite have corroded edges against quartz+plagioclase, suggesting resorption during ascent of crystal-charged magma. Foliation maps and air photo interpretation show the common development of 2–10 km-scale ovoid structures throughout the Douglas Harbour domain. Outcrop and thin-section scale structures imply that many plutons experienced a phase of syn-magmatic deformation, typically followed by high temperature sub-magmatic overprints. Thermobarometric data for plutons indicate near-solidus recrystallization at 4–6 kbar pressures. The common preservation of syn-magmatic fabrics in plutons of different ages seems incompatible with the origin of these fabrics through superimposed regional orogenesis. The broad uniformity of intrusion ages and lithologies throughout the Minto Block, and the rarity of shallowly-dipping planar fabrics, also seem inconsistent with accretion of disparate older terranes, each of which should preserve distinct histories. A possible alternative explanation for these features is provided by vertical tectonic models, whereby buoyant felsic magmas ascended as crystal slurries, while dense supracrustal rocks (and solidified felsic intrusions emplaced into them) subsided as cold fingers (10–20 km-scale instabilities). Shear between upwelling and downwelling limbs would have concentrated in the weak intrusions, generating steeply-plunging syn-magmatic fabrics, and producing ductile overprints in solidified rocks.

The Effect of Reaction Overstep on Garnet Microtextures in Metapelitic Rocks of the Ilesha Schist Belt, SW Nigeria

Garnet-bearing assemblages of K-rich and K-poor metapelites from the Ilesha Schist belt, SW Nigeria, are investigated. K-rich samples contain the assemblages (A) garnet±staurolite±muscovite±chlorite±magnetite, (B) andalusite±garnet±staurolite±muscovite±chlorite±magnetite and (C) sillimanite±andalusite±garnet±muscovite±chlorite± magnetite. K-poor samples contain the assemblages (D) garnet±staurolite±cordierite±chlorite and (E) garnet±cordierite±chlorite staurolite. All assemblages contain quartz, plagioclase, biotite and ilmenite. P±T pseudosections calculated in the system CaO±Na2O±K2O±TiO2±MnO±FeO±MgO± Al2O3±SiO2±H2O O2 suggest peak metamorphism at 590 20 C at 5 0 5 kbar, followed by retrogression to 550 C at 3 0 kbar, in agreement with field evidence, domain assemblages, mineral compositions, modes and geothermobarometry. The absence of compositional zonation shows that garnet in all investigated rocks nucleated and grew at constant P±T±X in equilibrium with associated minerals on the thin-section scale. However, the garnet-in reaction did not begin until the establishment of a significant temperature overstep of 80 C, with consequent rapid dehydration of chloriteand muscovite-bearing assemblages resulting in interface-controlled garnet growth under locally hydrostatic pressure conditions. Garnet in rock types A and B formed with a characteristic pattern of biotite and quartz inclusionsÐa `house of cards' texture. Additionally, clusters and aggregates of unzoned garnet grains formed in rock types A and D. Garnet resorption in rock types A±C resulted from prograde stauroliteand retrograde andalusite-forming reactions.

Melt Generation and Fluid Flow in the Thermal Aureole of the Bushveld Complex

Granite sheets emplaced into the migmatite zone of the eastern contact aureole of the Bushveld Complex resulted from fluid-enhanced, incongruent biotite melting of the underlying Silverton Formation shales during prograde metamorphism. Ba concentrations are extreme in both the sheets (>1000 ppm) and the hornfels (>800 ppm) into which they have been emplaced. We conclude that a Ba-rich, hydrothermal fluid induced melting in the aureole, and that fluid transport of Ba2+, and to a lesser extent, Sr2+ and Eu2+, persisted in the melt zones under subsolidus conditions. Sr-isotope systematics from high-Ba localities define an errorchron of 2161 ± 106 Ma with an initial (87Sr/86Sr) ratio of 0·705 ± 0·001. Metasedimentary rocks unaffected by fluid infiltration were homogenized at the same time but with an increased initial ratio, suggesting that whereas isotope homogenization was achieved between outcrops permeated by fluids, there is no evidence of regional homogenization. Oxygen-isotope compositions of psammitic metasediments in the aureole are uncorrelated with distance from the contact, suggesting the infiltrating fluid equilibrated isotopically with the metasediments. Their elevated δ18O values (11·3–12·1‰) are consistent with a fluid source from devolatilization of the sedimentary lithologies. Textural analysis at both outcrop and thin-section scale of arkose and psammites in the aureole (Lakenvalei and Magaliesberg Formations) shows that the extent of melting was highly heterogeneous, even on the grain scale, and resulted from heterogeneously distributed infiltration of aqueous fluid on dilatant cracks and grain boundaries. Cathodoluminescence imaging of quartz shows a marked difference in the amount of fine structure, with that in the melted rocks having uniform luminescence in contrast to that in rocks containing little or no melt, which preserve textures inherited from the regionally metamorphosed protolith. Simple finite-difference thermal modelling of the aureole suggests that the width of the melt zone (>500 m) is inconsistent with conductive heat transfer, and hence that the thermal structure has been modified by fluid advection.

Developing concepts of high-resolution diagenetic stratigraphy for Pliocene cool-water limestones in New Zealand, and their sequence stratigraphy

The diagenetic evolution of calcitic cool-water limestones can be markedly different from that for aragonite-dominated tropical carbonates. This petrographic study develops a "sequence stratigraphic" approach to cool-water carbonate diagenesis based on the chronological occurrence of all diagenetic features (not only cement phases) within sedimentary successions, their vertical and lateral distribution, and their grouping into discontinuity-bounded (by dissolution surfaces, fractures, renucleation events) suites at a thin-section scale. The progression of diagenetic environments that moved through a succession is related to diagenetic discontinuities correlatable throughout the study area. In our limestones a critical discontinuity event to decipher is the first evidence of mechanically-induced skeletal fracturing, enabling the definition of precompaction and post-compaction diagenetic features. A comparison of just the pre-compaction diagenetic suites in samples from above and below significant outcrop disconformities enables isolation of the "specific diagenesis" of unconformity-bounded depositional units, which is that suite of diagenetic events associated with the evolution of the sedimentation realm of a unit, exclusive of diagenetic overprints shared with any overlying unit through an unconformity since these relate to some subsequent, different depositional stage. In turn, the specific diagenesis of a succession of depositional facies between consecutive unconformities may then be interpreted within a sequence stratigraphic framework. Sections of cool-water Pliocene skeletal limestones in eastern North Island, New Zealand, comprise deepening-and then sometimes shallowing-upwards, unconformity-bounded depositional packages, or sequences. We recognize ten different specific diagenetic signatures in these rocks that are correlatable to depositional lithofacies. The vertical distribution of specific diagenetic signatures within the sequences exhibits a trend from mainly constructive to mainly destructive diagenesis in the deepening-upwards member of the sequences, and constructive in their shallowing-upwards portion, that can be integrated in a sequence stratigraphic framework. Constructive diagenesis occurs in the lower part of the transgressive system tract (TST) and in the upper part of the regressive system tract (RST), whereas destructive diagenesis is dominant in the upper part of the TST, the highstand system tract (HST), and the lower part of the RST.

Anticlockwise P-T Path of Granulites from the Monte Castelo Gabbro (Ordenes Complex, NW Spain)

The study of mafic and aluminous granulites from the Monte Castelo Gabbro (Ordenes Complex, NW Spain) reveals an anticlockwise P–T path that we interpret as related to the tectonothermal activity in a magmatic arc, probably an island arc. The P–T path was obtained after a detailed study of the textural relationships and mineral assemblage succession in the aluminous granulites, and comparing these with an appropriate petrogenetic grid. Additional thermobarometry was also performed. The granulites are highly heterogeneous, with distinct compositional domains that may alternate even at thin-section scale. Garnets are generally idiomorphic to subidiomorphic, and in certain domains of the aluminous granulites they show overgrowths forming xenomorphic coronas around a more or less idiomorphic core. Both types of garnets show significant Ca enrichment at the crystal rims, which, together with the other mineralogical and textural characteristics, is compatible with a pressure increase with low T variation. P–T estimations indicate a peak of T > 800°C and P ∼ 9·5 kbar, attained after a significant increase in pressure that took place at high temperatures (in the sillimanite field). We suggest that this kind of trajectory, probably anticlockwise, is compatible only with a terrane heated by an intense magmatic activity after or during tectonic crustal thickening (magmatic injection at the base and/or into the crust), which is characteristic of magmatic arcs.

K-Ar Age and Chemistry of Phengite from the Sanbagawa Schists in the Kanto Mountains, Central Japan, and their Implication for Exhumation Tectonics

EPMA analyses and K-Ar age determinations were carried out on phengite in pelitic schist from the Sanbagawa metamorphic belt of the Kanto Mountains, Central Japan. Phengite from the Sanbagawa pelitic schist in the Kanto Mountains generally occurs as aggregates of fine-grained crystals. It is extremely fine-grained in domains adjacent to relatively rigid garnet and albite porphyroblasts. This suggests that deformation-induced grain-size reduction took place in phengite during the ductile deformation accompanying the exhumation of the host schists. EPMA analysis shows that phengite is chemically heterogeneous at the thin-section scale, suggesting that it formed during retrograde metamorphism in restricted equilibrium domains. The retrograde chemical reaction was promoted by the ductile deformation. K-Ar ages of phengite get younger from the Southern Unit (82 Ma) to the Northern Unit (58 Ma) in the Kanto Mountains. The age range is similar to that in Central Shikoku. The older schists occur in the higher metamorphic grade zone in Central Shikoku and in the lower-grade zone in the Kanto Mountains. The thermal structures in Central Shikoku are inverted, so that the highest-grade zone occurs in the upper or middle parts of the apparent stratigraphic succession. In contrast, the Kanto Mountains have a normal thermal structure: the higher-grade zone is in the lower part of the apparent stratigraphic succession. The different tectonic features in exhumation produced the two contrasting age-temperature-structure relations at the western side of Sanbagawa belt in Central Shikoku and the eastern end of the Sanbagawa belt in the Kanto Mountains that are 800 km distant from each other. Namely, the western Sanbagawa belt in Central Shikoku underwent longer ductile deformation during the exhumation than the eastern Sanbagawa belt in the Kanto Mountains.

Cathodoluminescence record of K-metasomatism in ash-flow tuffs: Grain-scale mechanisms and large-scale geochemical implications

Near-surface, low-temperature, K-metasomatism of rhyolitic to dacitic ash-flow tuffs is a volumetrically and petrologically significant feature of the western United States and may be a primary alteration path in arid lands. Cathodoluminescence (CL) petrography of K-metasomatized ash-flow tuffs quickly yields information about the mineralogical changes and reaction mechanisms that take place during alteration for large sample suites and links these properties to geochemical changes. Tertiary volcanic rocks from Creede (Colorado), Socorro (New Mexico), and the Harcuvar Mountains (Arizona), were altered in different geologic settings, yet have very similar CL textures and chemistry. Original igneous feldspars were replaced by adularia primarily by dissolution and crystallization that occurred preferentially along preexisting fractures, surfaces, compositional zoning features, and cleavages. Fluid flow was primarily along grain boundaries, but was in some cases inhomogeneous at the thin-section scale.

Construction of an intergranular volume compaction curve for evaluating and predicting compaction and porosity loss in rigid-grain sandstone reservoirs

To evaluate compaction as a factor in porosity evolution, a plot of intergranular volume vs. depth was constructed using data from relatively uncemented reservoir sandstones from a variety of depths, ages, and geographic locations. The resulting intergranular-volume-decline curve reveals that sands compact mechanically and intergranular volume declines rapidly, from about 40 to 42% at the surface, to about 28% at 1500 m. Between about 1500 and 2500 m, intergranular volume continues to decline slowly, until the framework stabilizes at around 26% (maximum potential porosity in the absence of cement or matrix). No further significant decrease in intergranular volume is observed to the depth limits of the data set at 6700 m. Comparison of intergranular volume and volume of quartz cement for different formations reveals no obvious balance between intergranular pressure solution (as monitored by intergranular volume) and quartz cementation. This indicates that grain-to-grain pressure solution and quartz cement precipitation do not proceed concomitantly on the thin-section scale. Moreover, grain compaction is limited (to about 26% intergranular volume) in rigid-grain sandstones, which suggests that the occurrence and distribution of deep porosity is a function of the volume of cement available to fill the intergranular pores. Therefore, deep, porous sandstones are relatively uncemented rather than undercompacted.

Influence of lithological heterogeneity, mechanical anisotropy, and magmatism on the rheology of an arc, North Cascades, Washington

Many aspects of crustal dynamics are dependent on changes in rheology and strength with depth in the lithosphere. Several of the controlling factors for rheology are difficult to study experimentally, particularly lithological heterogeneity, mechanical anisotropy, and magmatism, and we focus on these in a study of the deformation patterns in a thick crustal section (∼5- to 40-km paleodepth) through the Cretaceous Cascades core of the NW Cordillera. This magmatic arc consists of metamorphosed oceanic and arc terranes intruded by magmatic bodies ranging from <10-cm-thick sheets to large plutons. Heterogeneous brittle deformation marked by serpentinite melange characterizes the shallowest part of the crustal section, and the remainder of the section is characterized by heterogeneous, fold-dominated ductile deformation. Early tight to isoclinal recumbent folds and associated axial-planar fabrics are refolded by one or more cycles of nearly coaxial, open to isoclinal, upright to overturned folds. Layering played a mechanically active role during folding at all levels, as indicated by cleavage refraction, boudinaged layers, and kinematic indicators that record fold-related shear. Ductile deformation intensifies in the narrow structural aureoles of plutons, and SW-directed, reverse shear was partitioned into some of the aureoles. The poor strain memory of these magmatic bodies makes it difficult to determine if deformation was focused in the pluton magma chambers before they reached the solidus, as commonly predicted. All of the plutons have magmatic foliations that at least in part reflect regional strains, and these foliations are strong in the deeper plutons. The thinner sheets acted as competent bodies during folding and boudinage, after they reached the solidus, but generally did not cause marked strain gradients in their hosts. A relative strength profile constructed for the Cascades crustal section shows an overall decrease in strength with depth for the ductile part of the arc that fits idealized strength profiles. However, in more detail relative strengths are markedly variable. Units were able to accumulate large ductile strains, but even small variations in the physical properties of interlayered rock types exerted a strong influence on deformation patterns throughout the mid- to deep-crustal part of the profile. This profile thus emphasizes the complex vertical rheological stratification of arcs at the crustal to thin section scale, and should be applicable to many other magmatic arcs.

From microscope to mountain belt: 150 years of petrology and its contribution to understanding geodynamics, particularly the tectonics of orogens

Thirty-five years ago the introduction of the plate tectonics paradigm led to a new understanding of orogeny. Subsequently, the development of advanced instruments for remote collection of information and for analysis of elemental and isotopic composition of materials, and the increases in computing power have enabled an unprecedented number of high-precision data about the Earth to be collected, analyzed, modelled and displayed. Within this revolution in global tectonics, the metamorphic petrologist has developed methods to unravel the depth, thermal, temporal and deformational history of orogens using detailed observations at map, hand sample and thin-section scales in combination with elemental and isotope data, and using inverse and forward modelling. Two exciting new directions in metamorphic petrology in relation to geodynamics concern the kinship between earthquakes and metamorphic reactions in subduction zones, and the petrology of the Earth’s mantle. Evidence of the changes in pressure ( P) and temperature ( T) in the Earth’s crust and upper mantle during the break up, movement, and collision of pieces of the continental lithosphere is sporadically recorded by the mineralogy and microstructures preserved in rocks exhumed to the surface. Better calibration of phase equilibria, the use of internally-consistent thermodynamic data sets and the development of techniques to retrieve close-to-peak P–T conditions from metamorphic rocks have yielded more precise P–T data that enhance our ability to characterize the path followed by individual rocks in P–T space. An improved ability to date segments of the P–T path, and to separate the length of time associated with the prograde (increasing T) evolution from the age of close-to-peak P–T conditions has enabled better understanding of the rates and processes involved in lithosphere thickening. At the same time, better constraints on the retrograde thermal history have contributed to our knowledge of the several tectonic processes that may operate during exhumation, although these are less well understood. The expanding database of key information, combined with predictions from modelling, has allowed the identification of characteristic P–T–t evolutions expected for rocks that have undergone distinct tectono-metamorphic histories. However, relating structural events recorded by rocks to specific points along the P–T evolution remains problematic, particularly regarding complex overprinting patterns of inclusion trails in porphyroblasts. These advances have improved our understanding of the tectonic evolution of orogens. At the extreme of conditions for crustal metamorphism are the recently discovered ultra-high pressure (UHP) and ultra-high temperature (UHT) facies of metamorphism. Both are problematic given our limited knowledge of processes at these conditions, particularly the return of UHP rocks from peak- P conditions and the mechanism for extreme heat in the crust in UHT metamorphism. The extreme depth inferred for metamorphism in some UHP terranes raises the issue of whether theoretically plausible tectonic overpressures can be dynamically maintained to affect metamorphic reactions. If the pressure gradient recorded by UHP rocks is greater than lithostatic, the UHP metamorphism may have occurred at depths shallower than currently believed. These studies have provided a reliable first-order framework for the comparison of rocks of ancient suture zones where the plate tectonics situation is less certain. However, orogens are spatially and temporally extended nonlinear systems with feedback relations. Such complex systems generate apparently simple behavior by self-organization, and the influence of unique histories must be respected.

SPODUMENE PETALITE EUCRYPTITE: MUTUAL RELATIONSHIPS AND PATTERN OF ALTERATION IN Li-RICH APLITE PEGMATITE DYKES FROM NORTHERN PORTUGAL

The co-occurrence, at the thin section scale, of the three anhydrous Li-aluminosilicates spodumene, petalite and eucryptite, is not a common feature. Such an association occurs in some aplite–pegmatite dykes of the Covas de Barroso district, northern Portugal. We describe their mutual relationships: where spodumene is an early magmatic phase, petalite precipitates directly fr om a late orthomagmatic fluid, the spodumene remaining metastably, whereas eucryptite is obviously hydrothermal and secondary. Replacement of the primary Li-minerals is the rule, but is diversified and selective: spodumene is mainly replaced by albite an d muscovite, petalite by K-feldspar and eucryptite. Muscovite is widespread, and the late hydrothermal paragenesis is dominated by quartz associated with various mixtures of phosphates. The assemblage eucryptite, K-feldspar and albite is obviously in chemical disequilibrium with the early phases. The compositional evolution of the hydrothermal fluids (Na, K, Li) is tentativel y bracketed. There is disequilibrium among thermodynamically conflicting mineral phases in terms of pressure, temperature and fluid:rock ratio, in a mobile and partly open system. A genetic affiliation of the Li-enriched pegmatites with the nearby two-m ica granites is difficult to prove. The tectonically controled intrusion of biotite granites to the east of the pegmatite belt woul d seem to be responsible of the drop in pressure that accounts for the spodumene–petalite transition and the development of the widespread replacement by feldspars, not present in the western part of the belt.