Calc-silicate Layers Research Articles

Garnetite, garnet-quartz (‘coticule’) and calc-silicate layers in high-pressure metapelitic rocks, Venezuela: metamorphosed exhalites in a Cretaceous back-arc basin

ABSTRACT Massive, boudinaged, rose-coloured layers up to one metre in thickness composed of 60–80 vol% spessartine-rich garnet and 15–40 vol% quartz occur in high-pressure, garnet-mica schist on Margarita Island, Venezuela. In the past, such spessartine-garnet-rich rocks have often, although inaccurately, been called ‘coticule’. Apatite can be significant, reaching 20 vol%. Accessory minerals are allanite, rutile (commonly rimmed by ilmenite) and rare phengite. Within the same mica-schist sequence, intercalated calc-silicate layers from one to two metres in thickness exhibit alternating layers, on a sub-mm- to cm-scale, of quartz and greenish layers rich in zoisite, epidote-supergroup minerals and diopside. Hyalophane and barite are present in small amounts. Calc-silicates are commonly pitted, with yellow to dark-brown, gossanous, botryoidal crusts. The composition of garnet in the garnet-rich layers, the calc-silicate layers and their host metasediments is distinctly different. In the garnet-rich layers, compositions are almandine-dominant (Alm57-63), followed by spessartine (Sps18-24), grossular (Grs2-16), pyrope (Prp6-11) and andradite (Adr0-6). Garnet in calc-silicate layers can range from almost pure grossular to Grs45-49Alm32-34Sps13-15, and in the enclosing metasediments is typically Alm71-73Prp14-15Grs3-8Sps4-7And2. Chondrite-normalized REE patterns of all three rock types are similar in shape, differing only in absolute contents, so that a common sedimentary precursor is indicated. Major and trace element chemistry of the garnet-rich layers are best explained if their pre-metamorphic protolith originated as a mixture of 60–70% normal detrital input and a precipitate from a distal source of hydrothermal activity. The term meta-exhalite is appropriate. The calc-silicate layers are more enigmatic, but an origin as a mixture of a hydrothermal component with a Ca-rich sediment also appears likely. Numerous lines of evidence indicate that the protoliths of the high-pressure rocks of Margarita Island originated in an Early Cretaceous back-arc basin in NW Colombia, so that an exhalite origin for garnet-rich and calc-silicate rocks is a plausible scenario.

De Kraalen and Witrivier Greenstone Belts, Kaapvaal Craton, South Africa: Characterisation of the Palaeo-Mesoarchaean evolution by rutile and zircon U-Pb geochronology combined with Hf isotopes

Abstract The formation and evolution of Palaeoarchaean De Kraalen and Witrivier Greenstone Belts (DKGB and WGB) of the Kaapvaal Craton are poorly known. Here we report zircon and rutile in situ U-Pb ages and zircon Hf isotopic data from a variety of supracrustal rocks. The zircon cores from a metamafic amphibole-bearing gneiss from the DKGB give a protolith age of 3 441 ± 5 Ma, whereas the zircon mantle domains give a metamorphic age of 3 211 ± 16 Ma. The 176Hf/177Hft values for all zircon domains give a tight cluster around 0.280596 ± 0.00006 (2 SD). U-Pb analyses of zircon for an amphibolite intercalated with thin calc-silicate layers from the WGB give a single crystallisation age of 3 230 ± 3 Ma, but the Hf isotope ratios of these zircon grains define two different populations. The first population yields 176Hf/177Hf~3.23 Ga = 0.28064 ± 0.00004, corresponding to εHf~3.23 Ga = 2.4 ± 1.9 (2SD) and Hf model ages between ca. 3.51 to 3.30 Ga. These are Hf isotope characteristics for zircons from a relatively juvenile source extracted from a depleted mantle source ca. 0.28 to 0.07 Ga prior zircon crystallisation. The second population yields 176Hf/177Hf~3.23 Ga = 0.28093 ± 0.00004 with εHf~3.23 Ga = 8.1 ± 1.3 (2SD). These Hf data combined with the 206Pb/207Pb ages lead to isotope ratios that lie above those of Depleted Mantle. The unusually high Hf isotope signature for the cores of the zircons from the WGB amphibolite most likely represent a contribution from an early highly depleted mantle source. A rutile in situ U-Pb age of 3.085 Ga from a recrystallised quartzite indicate that the rocks from the DKGB experienced slow cooling following the 3.21 Ga metamorphic event or (partial) resetting due to elevated geothermal gradient caused by the ca. 3.1 Ga intrusions of the Vrede Granitiod Suite. The latter interpretation is preferred because ~145 Ma of slow cooling from the amphibolite facies conditions of peak metamorphism to the blocking temperature for mass diffusion of Pb in rutile is unlikely. While the Zr-in-rutile temperature of ca. 710°C at 7 kbar for DKGB most likely records the peak temperature of the ~3.23 to 3.21 Ga event. The trace element concentrations of the metamorphic rutile grains within the quartzite of the DKGB indicate that the source rock was enriched in Cr. Either due to silification during hydrothermal alteration of the (ultra)mafic country rock or during deposition in an atmosphere that allowed for chromite grains to be part of the sediment.

Sulfide Minerals as Potential Tracers of Isochemical Processes in Contact Metamorphism: Case Study of the Kochumdek Aureole, East Siberia

Marly limestones from the Lower Silurian sedimentary units of the Tunguska basin (East Siberia, Russia) underwent metamorphism along the contact with the Early Triassic Kochumdek trap intrusion. At ≤ 2.5 m from the contact, the limestones were converted into ultrahigh-temperature marbles composed of pure calcite and sulfide-bearing calcsilicate layers. The sulfide assemblages in the gabbro and marbles were studied as potential tracers of spurrite-merwinite facies alteration. The gabbro-hosted sulfides show Fe-Ni-Cu-Co speciation (pyrrhotite and lesser amounts of chalcopyrite, pentlandite, and cobaltite) and positive δ34S values (+2.7 to +13.1‰). Both matrix and inclusion sulfide assemblages of prograde melilite, spurrite, and merwinite marbles consist dominantly of pyrrhotite and minor amounts of troilite, sphalerite, wurtzite, alabandite, acanthite, and galena. In contrast to its magmatic counterpart, metamorphic pyrrhotite is depleted in Cu (3–2000 times), Ni (7–800 times), Se (20–40 times), Co (12 times), and is isotopically light (about –25‰ δ34S). Broad solid solution series of (Zn,Fe,Mn)Scub, (Zn,Mn,Fe)Shex, and (Mn,Fe)Scub indicate that the temperature of contact metamorphism exceeded 850–900 °C. No metasomatism or S isotope resetting signatures were detected in the prograde mineral assemblages, but small-scale penetration of magma-derived K- and Cl-rich fluids through more permeable calcsilicate layers was documented based on the distribution of crack-filling Fe-K sulfides (rasvumite, djerfisherite, and bartonite).

MN-ANDALUSITE, SPESSARTINE, MN-GROSSULAR, PIEMONTITE AND MN-ZOISITE/CLINOZOISITE FROM TRIKORFO, THASSOS ISLAND, GREECE

Mylonitized manganiferous schists and calc-silicate layers intercalated within amphibolite- to greenschist facies mica schists from the Trikorfo area (Thassos Island, Greece), host an unusual Mn-rich paragenesis of metamorphic silicate minerals, most of them in large, gemmy crystals. The silicates occur both in layers subparallel to the foliation and within discordant veins cross-cutting the metamorphic fabric. Piemontite (up to 12.7 wt. % Mn2O3), Mn-rich epidote (up to 7.8 wt. % Mn2O3), Mn-rich andalusite (up to 15.6 wt. % Mn2O3), Mn-poor pink clinozoisite-epidote (up to 0.87 wt. % Mn2O3), Mn-poor pink zoisite (up to 0.21 wt. % Mn2O3), spessartine (up to 47.7 wt. % MnO) and Mn-rich grossular (up to 3.6 wt. % MnO) are associated with diopside, hornblende, phlogopite, muscovite, tourmaline, hematite and iron-bearing kyanite. The studied assemblages are indicative of high fO2 conditions due to the presence of highly oxidized pre-metamorphic Mn-rich mineral associations. They developed during prograde metamorphism of a Mn-rich sedimentary protolith(s), followed by re equilibration to post-peak metamorphic conditions, vein formation and metasomatism during retrograde metamorphism accompanying the exhumation of the Thassos Island during the Oligocene-Miocene. Alternatively, the skarn similar mineralogy of the calc-silicate layers could have been formed by fluids released by granitoids during contact metamorphism. The studied area represents a unique mineralogical geotope. Its geological-mineralogical heritage should be protected through establishment of a mineralogical-petrological geopark that will also promote sustainable development of the area.

Petrogenesis of incipient charnockite in the Ikalamavony sub-domain, south-central Madagascar: New insights from phase equilibrium modeling

Incipient charnockites representing granulite formation on a mesoscopic scale occur in the Ambodin Ifandana area of Ikalamavony sub-domain in south-central Madagascar. Here we report new petrological data from these rocks, and discuss the process of granulite formation on the basis of petrography, mineral equilibrium modeling, and fluid inclusion studies. The incipient charnockites occur as brownish patches, lenses, and layers characterized by an assemblage of biotite+orthopyroxene+K-feldspar+plagioclase+quartz+magnetite+ilmenite within host orthopyroxene-free biotite gneiss with an assemblage of biotite+K-feldspar+plagioclase+quartz+magnetite+ilmenite. Lenses and layers of calc-silicate rock (clinopyroxene+garnet+plagioclase+quartz+titanite+calcite) are typically associated with the charnockite. Coarse-grained charnockite occurs along the contact between the layered charnockite and calc-silicate rock. The application of mineral equilibrium modeling on the mineral assemblages in charnockite and biotite gneiss employing the NCKFMASHTO system as well as fluid inclusion study on coarse-grained charnockite defines a P–T range of 8.5–10.5kbar and 880–900°C, which is nearly consistent with the inferred P–T condition of the Ikalamavony sub-domain (8.0–10.5kbar and 820–880°C). The result of T versus H2O activity (a(H2O)) modeling demonstrates that orthopyroxene-bearing assemblage in charnockite is stable under relatively low a(H2O) condition of 0.42–0.43, which is consistent with the popular models of incipient-charnockite formation related to the lowering of water activity and stabilization of orthopyroxene through dehydration of biotite. The occurrence of calc-silicate rocks adjacent to the charnockite suggests that the CO2-bearing fluid that caused dehydration and incipient-charnockite formation might have been derived through decarbonation of calc-silicate rocks during the initial stage of decompression slightly after the peak metamorphism. The calc-silicate rocks might have also behaved as a cap rock that trapped CO2 infiltrated from an external source. ‘CO2-rich fluid ponds’ formed beneath calc-silicate layers could have enhanced dehydration of biotite to orthopyroxene, and produced layers of coarse-grained charnockite adjacent to calc-silicate layers.

Genesis of alpinotype fissure minerals from Thasos Island, Northern Greece - Mineralogy, mineral chemistry and crystallizing environment.

Alpinotype fissure-minerals in Thasos Island are hosted in gneisses, amphibolites, Mn-rich schists and calc-silicate layers, and marbles of the Carboniferous-Permian Pangeon Unit, which represents the lower tectonostratigraphic unit of the southern Rhodope metamorphic core complex. Alpinotype fissures crosscut metamorphic fabrics and are closely related to the exhumation processes of the core complex during the Oligocene-Miocene. Most mineralized fissures occur close to a major detachment fault, which separates gneisses from marbles and amphibolites. The mineralogy of the alpinotype fissures is closely related to the host rocks: amphibolite-hosted fissures include adularia, albite, quartz, titanite, apatite, actinolite, chlorite, calcite, hematite and rutile. Fissures in para- and orthogneisses- and in metapegmatites are characterized by smoky and clear quartz, adularia, muscovite and hematite. Fissures within spessartite-piemontite schists contain quartz, chlorite, spessartite, hematite, rutile, albite, epidote and traces of zircon. Finally fissures in calc-silicate layers include Mn-grossular, quartz and Mn-clinozoisite. Hydrothermal alteration halos surrounding the fissures may suggest leaching of the wall rocks as a potential mechanism for mineral deposition. Scepter quartz crystals consist of a lower Tessinhabit crystal and several generations of upper prismatic quartz crystals, suggesting several stages of crystallization and changing P-T-x conditions with time. Chlorite geothermometry indicates temperatures of formation in the range between 286 and 366 °C. Tessin habit quartz was deposited from CO2-bearing fluids, probably at the transition from a compressional to an extensional tectonic regime and was later dissolved by meteoric water dominated fluids resulting in the formation of quartz scepters. Oxidizing conditions are indicated by the widespread occurrence of hematite in the mineralization. The studied area represents a unique mineralogical geotope. Its geological-mineralogical heritage should be protected through establishment of a mineralogical-petrological geopark that will also promote sustainable development of the area.

Amphibole 40Ar/ 39Ar Geochronology from the Okcheon Metamorphic Belt, South Korea and its Tectonic Implications

An understanding of the Okcheon Metamorphic Belt (OMB) in South Korea is central to unraveling the tectono-metamorphic evolution of East Asia. Amphibole-bearing rocks in the OMB occur as calcsilicate layers and lenses in psammitic rocks, in the psammitic rocks themselves, and in the mafic volcanic layers and intrusives. Most amphiboles fail to show 40Ar/ 39Ar plateau ages; those that do have ages ranging from 132 to 975 Ma. The disturbed age pattern and wide variation in 40Ar/ 39Ar ages can be related to metamorphic grade, retrograde chemical reactions, excess Ar and amphibole composition. The oldest age (975 Ma) can be interpreted either as an old igneous or metamorphic age predating sedimentation or a false age caused by excess Ar. The youngest age of 132 Ma and the disturbed age pattern found in amphiboles from rocks located close to Jurassic granitoids are the result of retrograde thermal metamorphic effects accompanying intrusion of the granitoids. Some medium- or coarse-grained amphiboles in the calcsilicates are aggregates of fine-grained crystals. As a result, they are heterogeneous and prove to be readily affected by excess Ar. A disturbed age pattern in amphiboles from the calcsilicates occurring in the high-grade metamorphic zone may also be the product of excess Ar. On the other hand, the disturbed pattern of amphiboles present in the calcsilicates from the low-grade metamorphic zone could arise from both excess Ar and mixed ages. However, amphiboles from psammitic rocks and some calcsilicates in the high-grade metamorphic zone and in intrusive metabasites display real plateau ages of 237 to 261 Ma. The temperature conditions in the high-grade metamorphic zone were higher than the argon closing temperature for amphibole, and the amphiboles in this zone give plateau ages only when they are homogeneous in composition, lack excess Ar, and have not been thermally affected by intrusion of the granitoids. The unmodified 40Ar/ 39Ar ages prove rather younger than the age of the Late Paleozoic metamorphic event of 280 to 300 Ma, but they are close to muscovite K-Ar ages of 263 to 277 Ma. These 40Ar/ 39Ar amphibole ages are interpreted as the time of cooling that followed the main regional, intermediate-P/T metamorphic climax. The results demonstrate that interpretation of 40Ar/ 39Ar amphibole ages in an area subjected to several metamorphic events can be accomplished only by undertaking a thorough tectono-metamorphic study, accompanied by detailed chemical analysis of the amphiboles.

The Massabesic Gneiss Complex, New Hampshire: a study of a portion of the Avalon Terrane

Geochemical data from the 625 Ma Massabesic Gneiss Complex of southern New Hampshire show strong affinities for other Avalonian rocks of southern New England and suggest continental rifting in the Late Proterozoic. Migmatized paragneiss, the dominant rock type in the complex, has major and trace element compositions that are compatible with graywackes from continental arcs. The paragneiss also has strong lithologic, metamorphic, and isotopic similarities to the rocks of the Hope Valley zone of Connecticut Avalon, suggesting a possible Hope Valley— Massabesic correlation. At 625 Ma, the paragneiss ε_Nd values are similar to Avalonian crust in other locations of the orogen. Two types of amphibolite are present in minor amounts in the paragneiss of the Massabesic Gneiss Complex. The first type is a paramphibolite and consists of calc-silicate layers in the Massabesic paragneiss, the second type is metaigneous. Major and trace element abundances reveal that the protoliths of the orthoamphibolites range from continental rift alkaline basalts and tholeiites to N-type MORBs. Orthoamphibolite ε_Nd (625 Ma) values range from 2.4 to 4 as expected of rift-related magmas derived from partial melting of a depleted mantle source and have the same values as Iapetus ocean floor rocks of similar age. Orthoamphibolite major and trace element geochemical characteristics overlap those of the Middlesex Fells amphibolites of the Esmond-Dedham zone of eastern Massachusetts Avalon, which range from alkaline to transitional basalts erupted in a continental rift setting. The compositions of orthoamphibolites define a potential magmatic continuum produced by batch partial melting of the mantle initiated during continental rifting and proceeded to ocean basin formation. The inferred continuity of mafic magmatism from the Esmond-Dedham (Middlesex Fells Formation) to the Massabesic Gneiss Complex (and Hope Valley zone) suggests that these zones are not distinct lithotectonic zones but are parts of a single landmass. Massachusetts Avalon (Esmond-Dedham) represents the continental section of Avalon where the alkaline to transitional magmas of the early rifting stages are preserved. According to our tectonic reconstruction, the Massabesic Gneiss Complex is the oceanward, continental margin represented by volcanoclastic sediments with the MORBs representing the initiation of ocean basin development. The leading edge of this landmass, of which the Massabesic Gneiss Complex is the only observable remnant, collided with Laurentia during the Acadian Orogeny. The inboard, thicker, more continental trailing-edge, that is, platform Avalon (Esmond-Dedham) collided later during the Alleghanian Orogeny.

Why metasomatic fronts are really metasomatic sides

Back-scattered electron microscope images of metasomatic calc-silicate rocks demonstrate complex mineral growth patterns; often oscillatory zoning defines a euhedral or subhedral morphology. We infer from this the existence of an appreciable transient porosity (up to a few percent) during metasomatic calc-silicate growth, and argue that porous, reacting calc-silicate layers become conduits for irreversible metamorphic fluid loss. Because the layers are too permeable to sustain a steep gradient in hydraulic head downdip, fluid pressure in the calc-silicate must generally be lower than in adjacent low-permeability lithologies, so that steep local gradients in the hydraulic head will drive fluid from surrounding rocks up, down, or sideways into the transiently porous reacting layer. So-called “metasomatic fronts” separate transiently permeable layers that have undergone metasomatism from little-altered, low-permeability rocks. Thus they develop parallel to the flow path at its edge and should be termed metasomatic sides.

Comparative petrology of gneissic rocks in southwestern Newfoundland

In southwestern Newfoundland, pelitic migmatites of the Meelpaeg Subzone of the Gander Zone are separated by faults and plutons from metasedimentary rocks of the Port-aux-Basques gneiss complex (PBGC). The PBGC is a polymetamorphic sequence of amphibolite-facies, pelitic, semipelitic, and psammitic rocks (and associated metabasic dykes). Maximum metamorphic grade surpassed the first sillimanite (i.e., staurolite-consuming) isograd. Metamorphic conditions approached 650–700 °C at Pmax approximately 6.5–8.5 kbar (1 kbar = 100 MPa).The Meelpaeg gneisses also include sillimanite-grade, two-mica rocks, but they lack the Barrovian mineralogy (e.g., kyanite, staurolite, rutile) characterizing parts of the PBGC. The Meelpaeg rocks attained temperatures similar to those of the PBGC, but confining pressure was substantially lower (approx. 4 kbar), indicating uplift from relatively shallow structural levels.Both groups of paragneisses also differ in some aspects of their bulk chemistry (notably CaO/K2O ratios) and their lithologic associations. The Meelpaeg metapelites are less calcic and relatively potassic (mean CaO/K2O = 0.32) compared with their counterparts in the PBGC (mean CaO/K2O = 1.12), but both groups of rocks have similar bulk Fet/(Fet + Mg) ratios (mean XFe ≈ 0.75). In contrast with the PBGC, which contains abundant metabasites and thin coticule-like (garnet + quartz) seams, the Meelpaeg metapelites are associated with biotite + garnet "tonalitic" gneiss and, despite their relatively lime-poor composition, calc-silicate layers and pods.In terms of contrasting lithologic associations and bulk chemistry, paragneiss of the PBGC is distinct from gneissic rocks in the Meelpaeg Subzone. This underscores difficulties in relating rocks in the Port-aux-Basques area to well-established lithotectonic entities elsewhere in Newfoundland. Despite apparent differences in their protoliths and contrasts in metamorphic pressure, available U–Pb data suggest that high-grade metamorphism in both areas occurred during the middle Silurian.

Internal control of fluid compositions in amphibolite-facies scapolitic calc-silicates, Mary Kathleen, Australia

Unaltered metasediments of the Mary Kathleen Fold Belt are composed predominantly of layered amphibolite-facies scapolitic calc-silicate rocks in which minimal infiltration of externally derived fluids occurred during regional metamorphism. There were substantial differences in volatile activities between different layers in the layered sequences, in particular: a CO2/a H 2 O inferred from reaction progress estimates and analysis of biotite-clinopyroxene-fluid phase relations; a NaCl/a H 2 O inferred from scapolite compositions; and a HCl/a H 2 O inferred from biotite compositions. In one outcrop in which a clinopyroxene-producing reaction dominated, differences in approximate X CO 2of up to 0.25 occurred between several samples collected over 50 metres. Variations in a H 2 O/a HCl of up to one order of magnitude are inferred at 1 to 50 m scales from biotite-Cl contents, and variations in NaCl contents of scapolite from 0.0 to up to 0.6 Cl atoms in the Cl−CO3−SO4 site reflect a large variation of a NaCl in the coexisting fluid at similar scales. Most calcsilicate layers internally buffered fluid compositions in the H2O−CO2−NaCl−HCl system. Local occurrences of NaCl-rich scapolite suggest that some layers may have been in equilibrium with halite during early prograde metamorphism. At peak metamorphic temperatures, disolution of halite was complete but layers containing high-NaCl scapolite continued to buffer fluid at high values of a NaCl. Fluid immiscibility does not appear to have affected the progress of the devolatilization reactions. Although fluid was predominantly internally buffered, moderate quantities of fluid were released by prograde mineral reactions in many layers, up to 30 cm3 fluid per 100 cm3 rock. Numerous episodes of fluid escape were required, probably via microfractures, such that the released fluid did not obviously influence reaction progress in the layers through which it passed. The anomaly of beautifully preserved internal buffering signatures and the requirement for produced fluid locally to pass across layers in a deforming rock sequence suggest that the escaping fluid did not leave any readily observable tracks. This is explained by rapid rates of fracture propogation and fluid migration therein. This internally buffered system contrasts strongly with adjacent calc-silicate rocks that show evidence for infiltration of externally derived fluids at high fluid/rock ratios, and highlights the broad range of fluid behaviour that can be expected in deforming, heterogeneous rock sequences.

Strain and folding mechanisms in a similar style fold from the northern Selkirks of the Canadian Cordillera

Minor structures and intragranular strain in calcite have been examined in a folded marble unit near Mica Dam, 130 km north of Revelstoke, British Columbia. The fold forms part of a train of polyclinal folds in the core of the major Mica Dam Antiform which formed in a third folding phase, F 3 Metamorphism had reached kyanite grade before F 3, biotite quartz and calcite were recrystallizing early in F 3 but no longer did so in the later stages. Pressure of 4.0–5.0 kbar and temperatures of 300–400°C are estimated to have prevailed during folding. The fold is an asymmetric, approximately cylindrical antiform with axial surface dipping southwest and hinge plunging southeast at 12°. The parameters t′ α and T′ α determined on a right section indicate the fold to be approximately similar. Calc-silicate layers and pegmatite sheets which outline the fold are both buckled and boudiné and the adjacent synform is limited by an upper detachment surface. Other F 3 minor folds, related to the Mica Dam Antiform, exhibit the same characteristics and all this indicates that there was a buckle component in the folding history. Using the least-squares strain-gage technique, intragranular twinning strains have been determined for five samples of marble. The maximum extensional strain, ϵ 3, is subparallel to layering and at a high angle to the fold hinge. All of the samples exhibit a compression strain nearly parallel to the fold hinge. These strains do not reflect the buckling mechanism suggested by field observations. The late-stage three-dimensional strains observed in this fold are not compatible with existing two-dimensional, theoretical models for the origin of similar style folds.

Scapolite-bearing and related calc-silicate layers from the alpujarride series. (Betic cordilleras of Southern Spain). A discussion on their origin and some comments

Generalized development of calc-silicate bands in several alpujarride units of the central and western segments of the Betic Zone, is interpreted as the result of metamorphic differentiation from original calcium-rich sedimentary beds in the sequence. Presence of Scapolite suggests that this phenomenon may have been made possible by the catalizing action of high anion concentrations in the pore fluid medium during metamorphism, probably caused by the existence of evaporic material in the original sediments. This hypothesis, which is further supported by anomalous high Ca, Na and K contents of the enclosing metapelites, leads to the correlation of the present calc-silicate bearing metamorphic formations with well-known permo-werfenian evaporitic series in other units of the Betic Zone. The use of this new criterion of stratigraphic correlation would indicate that highly metamorphosed permo-triassic sequences have often been misidentified as more ancient formations, thus causing an underestimation of the importance of alpine HT-LP metamorphism in the alpujarride tectonic domain.

Folds and Strain in Granville Metamorphic Rocks, Bancroft, Ontario, Canada

East of Bancroft, Ontario, metasedimentary and metavolcanic rocks lie tightly folded between a syntectonic granitic batholith in the southeast and a complex of feldspathic gneisses and plutons to the north. At least three generations of folds are well displayed in marble and calc-silicate layers. F 1 folds are isoclines with few hinges exposed and an axial-surface foliation that closely parallels stratification. F 2 folds are dominant on all scales. These folds bend the stratiform foliation, vary in tightness, and are upright to overturned northwest toward the gneiss complex. During and after F 2 deformation, minerals of the amphibolite facies crystallized. F 3 folds, only locally present, are open and recumbent; folding was virtually postmetamorphic. Axes of minor F 2 folds and parallel mineral lineations commonly plunge at low angles along the axial traces; but toward the gneiss complex and near the granitic batholith, the axes progressively increase in rake until, as in the gneiss, folds are reclined and plunge approximately down the dip of axial surfaces. Variations in thickness of folded layers indicate that folds have undergone flattening and that this strain progressively increased with increase in rake. The evidence supports the concept that during medium-grade metamorphism, constriction near plutons or feldspathic massifs led to areal changes in shape and orientation of the dominant folds.

Development of an augen gneiss terrain

Augen gneisses and related migmatites commonly form extensive areas in deep crystalline terrains and are interlayered with horizons of undoubted metasedimentary rocks. Augen gneiss, migmatite, and hornblende gneiss near McMurdo Sound, Antarctica, is interlayered with calcareous metasedimentary rocks on all scales and passes gradationally into quartzo-feldspathic gneiss. Numerous discontinuous lenses and layers of metasedimentary rocks in augen gneiss have been disrupted by transposition of layering and form tectonic inclusions. Calc-silicate layers have reacted with the surrounding rocks and become partially amphibolitized. Field evidence suggests that augen gneiss formed from quartzo-feldspathic gneiss by local metasomatism that accompanied strong deformation and smoothed out original metasedimentary differences. Locally partial melting took place. The final result is a sequence of augen gneiss, hornblende gneiss, and migmatite with thin amphibolite lenses and layers. The metasedimentary rocks are correlated with lower grade limestones and calcareous arenites including graywackes 125 km to the south. The quartzo-feldspathic gneiss would thus represent highly metamorphosed graywacke and sandstone. Augen gneiss and quartzo-feldspathic gneiss have similar chemical compositions, but augen gneiss seems to be somewhat more mafic. The composition of augen gneiss is more mafic than average continental shield surface rock and less mafic than the average crustal rock. Evidence indicates that augen gneiss and related rocks formed by homogenization of rocks that were originally sedimentary and its composition may reflect the bulk composition of material supplied to the crust earlier in the earth's history.