Distinct Metamorphic Events Research Articles

Three metamorphic events recorded in a single garnet: Integrated phase modelling, in situ LA‐ICPMS and SIMS geochronology from the Moine Supergroup, NW Scotland

Abstract In situ LA‐ICP‐MS monazite geochronology from a garnet‐bearing diatexite within the Moine Supergroup (Glenfinnan Group) NW Scotland records three temporally distinct metamorphic events within a single garnet porphyroblast. The initial growth of garnet occurred in the interval c. 825–780 Ma, as recorded by monazite inclusions located in the garnet core. Modelled P–T conditions based on the preserved garnet core composition indicate an initially comparatively high geothermal gradient regime and peak conditions of ∼650 °C and 7 kbar. Monazite within a compositionally distinct second shell of garnet has an age of 724 ± 6 Ma. This is indistinguishable from a SIMS age of 725 ± 4 Ma obtained from metamorphic zircon in the sample, which is interpreted to record the timing of migmatization. This second stage of garnet growth occurred on a P–T path from ∼6 kbar and 650 °C rising to ∼9 kbar and 700 °C, with the peak conditions associated with partial melting. A third garnet zone which forms the rim contains monazite with an age of 464 ± 3 Ma. Monazite in the surrounding matrix has an age of 462 ± 2 Ma. This corresponds well with a U–Pb SIMS zircon age of 463 ± 4 Ma obtained from a deformed pegmatite that was emplaced during widespread folding and reworking of the migmatite fabric. The P–T conditions associated with the final phase of garnet growth were ∼7 kbar and 650 °C. The monazite ages coupled with the phase relations modelled from this multistage garnet indicate that it records two Neoproterozoic tectonothermal events as well as the widespread Ordovician Grampian event associated with Caledonian orogenesis. Thus, this single garnet records much of the Neoproterozoic to Ordovician thermal history in NW Scotland, and highlights the long history of porphyroblast growth that can be revealed by in situ isotopic dating and associated P–T modelling. This approach has the potential to reveal much of the thermal architecture of Neoproterozoic events within the Moine Supergroup, despite intense Caledonian reworking, if suitable textural and mineralogical relationships can be indentified elsewhere.

Refining temporal constraints on metamorphism in the Nashoba terrane, southeastern New England, through monazite dating

Electron-microprobe dating of monazite grains within high-grade mylonitic rocks of the Nashoba terrane in eastern Massachusetts provides new temporal constraints on metamorphism in southeastern New England. In situ dating of monazite grains from three fault zones has allowed the timing of multistage events to be discerned. Three distinct metamorphic events were detected in the Nashoba terrane. The fi rst metamorphic event (M1) occurred from 435 to 400 Ma, with an average age of 423 Ma. A second metamorphic event (M2) occurred at ca. 390 Ma and was associated with widespread migmatization. A third metamorphic event (M3) occurred during the ca. 378‐ 371 Ma time interval and was possibly associated with the Neoacadian orogeny. Intermittent monazite growth during the 360‐305 Ma interval suggests that the main phase of metamorphism in the shear zones was complete, but the highly deformed fault zones acted as a conduit for fl uid migration, which was responsible for the production of young monazite grains. By at least 345 Ma, the Nashoba terrane had cooled below the stability of sillimanite.

Polymetamorphic evolution of pelitic schists and evidence for Permian low‐pressure metamorphism in the Vepor Unit, West Carpathians

AbstractPhase equilibrium modelling and monazite microprobe dating were used to characterize the polymetamorphic evolution of metapelites from the northern part of the Vepor Unit, West Carpathians. Three generations of garnet and associated metamorphic assemblages found in these rocks correspond to three distinct metamorphic events related to the Variscan orogeny, a Permian phase of crustal extension and the Alpine orogeny. Variscan staurolite‐bearing and Alpine chloritoid‐bearing assemblages record medium‐temperature and medium‐pressure regional metamorphisms reaching 540–570 °C/5–7.5 kbar and 530–550 °C/5–6.5 kbar respectively. The Permian metamorphic assemblage involves garnet, andalusite, sillimanite, biotite, muscovite, plagioclase and corundum and locally forms silica‐undersaturated andalusite‐biotite‐spinel coronas around older staurolite. The transition from andalusite to sillimanite indicates a prograde low‐pressure and medium‐temperature metamorphism characterized by temperature increase from 500 to 650 °C at ∼3 kbar. As accessory monazite is abundant in the rocks, an attempt was made to derive its age of formation by means of electron microprobe‐based Th‐U‐Pb chemical dating. Despite the polymetamorphic nature of the metapelites, the monazite yielded uniform Permian ages. Microstructures confirm that monazite was formed in relation to the low‐pressure and medium‐temperature paragenesis, and the weighted average ages obtained for two different samples are 278 ± 5 and 275 ± 12 Ma respectively. The virtual lack of Variscan and Alpine monazite populations points to interesting aspects concerning the growth systematics of monazite in metamorphic rocks.

Constraining the metamorphic evolution of a cryptic hot Mesoproterozoic orogen in the Central Indian Tectonic Zone, using P– T pseudosection modelling of mafic intrusions and host reworked granulites

In this study, we reconstruct the metamorphic pathways of reworking of a deep crustal granulite terrane in the Central Indian Tectonic Zone, using Archaean/Palaeoproterozoic (?) polycyclic felsic granulites and two groups of intrusive Mesoproterozoic mafic granulites, a coarse-grained noritic gabbro and a fine-grained gabbroic norite. The granulite terrane, locally referred to as the Bhandara–Balaghat granulite domain, is bounded by the South Indian Block and a Grenville-aged, younger tectonic domain of the Central Indian Tectonic Zone. The granulites and the mafic intrusions were multiply deformed and metamorphosed in the early Mesoproterozoic. Using P– T pseudosection modelling of host felsic granulites and the mafic intrusions, two distinct metamorphic events (BM 2 and BM 3) with contrasting P– T paths have been established. The P– T path of BM 2 metamorphism has a clockwise sense, having an important prograde segment of heating of more than 250 °C with pressure fall, followed by cooling. The peak BM 2 metamorphism has been constrained at ∼6 kbar, ∼725 °C. Emplacement of the mafic intrusions was broadly coincident with the low-pressure metamorphism, and they underwent a phase of subsolidus cooling. During subsequent metamorphism, BM 3, the mafic intrusions and the host felsic granulites were re-metamorphosed along a counterclockwise P– T path. This led to tectonic burial of the mid-crust to ∼9.4 kbar, ∼760 °C, which was followed by cooling accompanying pressure decline. Although, the clockwise P– T path is generally interpreted in terms of thermal relaxation of crust following thrusting, the scale of heating (∼250 °C) during initial decompression, documented here, is much larger than modelled for tectonically thickened crust, but consistent with magmatically active extensional zones. Based on this evidence and also the syn-metamorphic mafic intrusions, we present an alternate interpretation in terms of mid-crustal extension for the BM 2 metamorphism. In contrast, the counter clockwise P– T path during BM 3, recording cooling following prograde burial is explained by tectonic thickening of a hot mid-crust. Collating available geochronological data, the two metamorphic events appear to indicate tectonic switching from lithospheric extension to contraction in the early Mesoproterozoic. The findings provide the first quantitative constraints on an early Mesoproterozoic hot orogen at the craton–mobile belt interface in the Central Indian Tectonic Zone.

Petrology and Geochronology of Granulites from the McKaskle Hills, Eastern Amery Ice Shelf, Antarctica, and Implications for the Evolution of the Prydz Belt

A combined petrological and geochronological study was carried out on mafic granulites and associated felsic gneisses from the McKaskle Hills, eastern Amery Ice Shelf, East Antarctica. Garnet-bearing mafic granulites exhibit reaction textures and exsolution textures that indicate two-stage metamorphic evolution. Thermobarometric estimates from matrix and symplectite assemblages yield peak and retrograde P^T conditions of 9 0^9 5 kbar and 880^9508C and 6 6^7 2 kbar and 700^7508C, respectively. Similar but slightly scattered peak P^Testimates of 7 9^10 1kbar and 820^9808C are obtained from the core compositions of minerals from felsic paraand orthogneisses. Evidence for the prograde history is provided by muscovite inclusions in garnet from a paragneiss. Sensitive highresolution ion microprobe U^Pb zircon dating reveals an evolutionary history for the granulites, including a mafic and felsic igneous intrusion at 1174^1019Ma, sedimentation after 932^916Ma, and a high-grade metamorphism at 533^529Ma. In contrast, Sm^Nd mineral^whole-rock dating mainly yields a single age population at 500Ma.This suggests that the McKaskle Hills form part of the Prydz Belt, and that the relatively high peak P^Tconditions and a decompression-dominated P^T path for the rocks resulted from a single Cambrian metamorphic cycle, rather than two distinct metamorphic events as formerly inferred for the granulites from Prydz Bay.The age data also indicate that the Precambrian history of the McKaskle Hills is not only distinct from that of the early Neoproterozoic terrane in the northern Prince Charles Mountains, but also different from that of other parts of the Prydz Belt. The existence of multiple basement terranes, together with considerable crustal thickening followed by tectonic uplift and unroofing indicated by the clockwise P^T^t evolution, suggests that the Prydz Belt may represent a collisional orogen that resulted in the assembly of Gondwana during the Cambrian period.

Domainal fabric development, associated microstructures and P-T records attesting to polymetamorphism in a granulite complex of the Eastern Ghats granulite belt, India

The granulite complex around Jenapore, Orissa, Eastern Ghats granulite belt, bears the imprint of two episodes of strong deformation (D1 and D2) attended with foliation (fabric) development (S1 and S2). Two distinct metamorphic events at P-T conditions of ∼900°C at ∼9 kbar and ∼600°C at ∼6 kbar are correlated with D1 and D2 respectively. The reaction textures in S1-microdomains are interpreted to be the product of near isobaric cooling at ∼9 kbar from 950°C to 600°C, whereas those in the S2-microdomains are considered to be the result of an up-pressure trajectory from ∼6 kbar at 600°C. The D1-M1 high P-T granulite event is interpreted to be Archean in age (ca. 3 Ga) on the basis of the isotopic data obtained from the charnockite suite of the area. The later relatively low P-T granulite facies event, attendant to D2-S2 is considered to be related to the Grenvillian orogeny as represented by the dominant isotopic record in the belt.

Polymetamorphism of Aktyuz eclogites (northern Kyrgyz Tien-Shan) deduced from inclusions in garnets

Garnet and omphacite in eclogites from the Aktyuz area, northern Kyrgyz Tien-Shan, preserve evidence of two distinct metamorphic events. Relic polyphase inclusions of Fe-rich staurolite (XMg < 0.29), Mg-taramite, paragonite, and oligoclase (An < 16) contained in garnet cores represent a separate pre-eclogitic amphibolite or epidote-amphibolite facies metamorphism (T = 560-650 °C, P = 4-10 kbar). This relatively low- to medium-pressure metamorphic event probably resulted from the heat of the hot hanging wall at the inception of subduction. Inclusions in the garnet cores and rims and matrix omphacite (glaucophane, sodic-calcic and calcic amphiboles, paragonite, phengite, epidote, albite, rutile, hematite, and quartz) suggest that epidote-blueschist facies conditions (T = 330-570 °C, P = 8-16 kbar) were experienced during the prograde stage of the eclogitic metamorphism. This event occurred within a subduction zone. The estimated peak metamorphic conditions of the eclogites are T = 600-710 °C and P = 15-25 kbar. The subsequent retrograde path through epidote-amphibolite and greenschist facies suggests a clockwise P-T path, in contrast to the anticlockwise P-T path proposed by previous work.

Interpreting prograde‐growth histories of Al2SiO5 triple‐point rocks using oxygen‐isotope thermometry: an example from the Truchas Mountains, USA

AbstractOxygen‐isotope compositions of kyanite, andalusite, prismatic sillimanite and fibrolite from the Proterozoic terrane in the Truchas Mountains, New Mexico differ from one another, suggesting that these minerals did not grow in equilibrium at the Al2SiO5 (AS) polymorph‐invariant point as previously suggested. Instead, oxygen‐isotope temperature estimates indicate that growth of kyanite, andalusite and prismatic sillimanite occurred at c. 575, 615 and 640 °C respectively. Temperature estimates reported in this paper are interpreted as those of growth for the different AS polymorphs, which are not necessarily the same as peak metamorphic temperatures for this terrane. Two distinct temperature estimates of c. 580 °C and c. 700 °C are calculated for most fibrolite samples, with two samples yielding clear evidence of quartz‐fibrolite oxygen‐isotope disequilibrium. These data indicate that locally, and potentially regionally, oxygen‐isotope disequilibrium between quartz and fibrolite may have resulted from rapid fibrolite nucleation. Pressures of mineral growth that were extrapolated from oxygen‐isotope thermometry results and calculated using petrological constraints suggest that kyanite and one generation of fibrolite grew during M1 at 5 kbar, and that andalusite, prismatic sillimanite and a second generation of fibrolite grew during M2 at 3.5 kbar. M1 and M2 therefore represent two distinct metamorphic events that occurred at different crustal levels. The ability of the AS polymorphs to retain δ18O values of crystallization make these minerals ideal to model prograde‐growth histories of mineral assemblages in metamorphic terranes and to understand more clearly the pressure–temperature histories of multiple metamorphic events.

Contrasting textural record of two distinct metamorphic events of similar P–T conditions and different durations

AbstractA structural, metamorphic and geochronological study of the Staré Město belt implies the existence of two distinct metamorphic events of similar peak P–T conditions (700–800 °C, 8–10 kbar) during the Cambro‐Ordovician and the Carboniferous tectonometamorphic events. The hypothesis of two distinct periods of metamorphism was suggested on the basis of structural discordance between an undoubtedly Carboniferous granodiorite sill intrusion and earlier Cambro‐Ordovician fabrics of a banded amphibolite complex. The analysis of crystal size distribution (CSD) shows high nucleation density (N0) and low average growth rate (Gt) for Carboniferous mylonitic metagabbros and mylonitic granodiorites. The parameter N0 decreases whereas the quantity Gt increases towards higher temperatures progressively approaching the values obtained from the Cambro‐Ordovician banded amphibolite complex. The spatial distribution of amphibole and plagioclase shows intense mechanical mixing for lower‐temperature mylonitic metagabbros. In high‐temperature mylonites a strong aggregate distribution is developed. Cambro‐Ordovician amphibolites unaffected by Carboniferous deformation show a regular to anticlustered spatial distribution resulting from heterogeneous nucleation of individual phases. This pattern, together with CSD, was subsequently modified by the grain growth and textural equilibration controlled by diffusive mass transfer during Carboniferous metamorphism. The differences between the observed textures of the amphibolites are interpreted to be a consequence of the different durations of the Carboniferous and Cambro‐Ordovician thermal events.

40Ar/ 39Ar mineral ages from basement rocks in the Eastern Kunlun Mountains, NW China, and their tectonic implications

40Ar/ 39Ar dating and estimates of regional metamorphic P– T conditions were carried out on the basement rocks of the Eastern Kunlun Mountains, Western China. Samples from the Jinshuikou, Xiaomiao, Kuhai, Wanbaogou, and Nachitai groups revealed distinct metamorphic events and four age groups. The age group in the range from 363 to 439 Ma is interpreted to represent cooling after Middle Silurian–Late Devonian granulite(?) and amphibolite facies metamorphism, which is dominated by low–middle pressure/high temperature conditions. This tectono-thermal event is related to the closure of an oceanic basin or marginal sea. An age group of 212–242 Ma represents cooling after Triassic metamorphic overprint, which is probably associated with magmatic intrusions. This thermal event, together with the Permo-Triassic ophiolite zone along the South Kunlun Fault, relates to the closure of a major ocean (between India and Eurasia) and the eventual N-ward accretion of the Qiangtang block in Permo-Triassic times. The significance of the age group of 104–172 Ma may be related to the ductile deformation along the Xidatan fault due to the northward-directed accretion of the Lhasa block. Biotites from Nachitai record a partial isotopic resetting at ca. 32 Ma that is interpreted to represent a late-stage exhumation caused by further crustal shortening.

Petrology and in situ U-Th-Pb Monazite Geochronology of Ultrahigh-Temperature Metamorphism from the Andriamena Mafic Unit, North-Central Madagascar. Significance of a Petrographical P-T Path in a Polymetamorphic Context

Petrological studies and electron microprobe dating of monazite from the mafic Andriamena unit, north–central Madagascar, indicate that an apparently continuous P–T path inferred for Mg-granulites is actually discontinuous, resulting from the superposition of two distinct metamorphic events at 2 5 Ga and 750Ma. The late Archaean event corresponds to an ultrahigh-temperature metamorphism (1000 C, 10 5 kbar) characterized by a sapphirine–garnet–orthopyroxene–quartz assemblage. Neoproterozoic ages are associated with the development of a sapphirine–cordierite-bearing assemblage, symplectites of orthopyroxene–sillimanite and partial melting at 850 C and 7 kbar. This sequence of reactions and mineral assemblages could be interpreted as the result of near-isothermal decompression to about 4 kbar followed by isobaric cooling to 650 C. However, geodynamic constraints suggest that the granulites underwent a phase of cooling to the stable geotherm following the ultrahightemperature metamorphism at 2 5 Ga. Consequently, we suggest that the ‘petrographical path’ inferred from the Mg-granulites is not representative of the actual P–T–t path. The decompression, in particular, is an artefact of the P–T path with no geological meaning; it results from the equilibration of the refractory late Archaean ultrahigh-temperature assemblages at a lower pressure during the middle Neoproterozoic event.

Multiple Metamorphic Events at Broken Hill, Australia. Evidence from Chloritoid-bearing Parageneses in the Nine-Mile Mine Region

Pressure (P) and temperature (T) conditions for two distinct metamorphic events are quantified in the Nine-Mile Mine region of the central Broken Hill Block, New South Wales, Australia. Parageneses of the earlier event formed contemporaneously with D2 which produced a pervasive foliation. The event occurred at conditions around 650°C and 4–5 kbar, but may have been preceded by higher peak temperatures, possibly during an early M1 event. Thus the event described here is called M2. This is supported by geochronological studies that report at least two high-grade partial melting events in the region. A later paragenesis, here called M3, overprinted statically the M2–D2 and D3 fabrics. PT conditions during M3 reached 480°C at 5 kbar. Possible PT paths between M2 and M3 are inferred from the interpretation of chloritoid-bearing parageneses using (1) a new calculated petrogenetic grid, (2) calculated compatibility diagrams and (3) thermodynamic pseudosections. In particular, two possible PT paths are discussed: (a) a direct path involving negligible pressure change during cooling and (b) cooling with minor decompression from M2 and much later compression and heating to M3. PT path (b) is interpreted from pseudosections. It is also considered more likely because: (1) the orientation and position of the regional isograds for M2 and M3 do not coincide across the Broken Hill Block and (2) D3 is interpreted as a substantial shortening event. Thus, a much larger P difference would be expected between M2 and M3 if the two events occurred on the same PT path. The interpretation of independent PT paths for M2 and M3 allows the possibility that the M3 event is much younger than the mid-Proterozoic high-grade events.

A revised Archaean chronology for the Napier Complex, Enderby Land, from SHRIMP ion-microprobe studies

The long and complex Archaean evolution of the Napier Complex of Enderby Land, characterized by high-grade metamorphism and several strong deformations, is reassessed in the light of new SHRIMPU–Pb zircon dating results bearing on the ages of protoliths and possible regional extents of distinct Archaean tectonothermal events. Initial felsic igneous activity occurred over a significant time interval c. 3800 Ma ago. An age of 2980±9 Ma for the emplacement of charnockite at Proclamation Island might date the oldest tectonothermal event to be recognized in the Napier Complex. An ensuing, very-high grade, previously imprecisely dated tectonothermal event occurred at 2837±15 Ma. U–Pb zircon ages ranging from 2456+8/−5 Ma to 2481±4 Ma date a subsequent, protracted high-grade tectonothermal event. Whereas the ~2840 Ma event is of regional importance in the Amundsen Bay-Casey Bay area, it is possible that the ~2980 Ma event was of only moderate grade, minor importance, or even absent, in that part of the Complex. If so, the apparent trend to very-high temperature metamorphism in the Tula and Scott mountains compared with the Napier Mountains may reflect two distinct metamorphic events rather than a simple baric and thermal gradient. The oldest crustal component in the Napier Complex appears to have been of igneous derivation. Zircon populations in paragneisses at Mount Sones are similar to those in the nearby orthogneisses, which therefore may have been basement. Another paragneiss, in the Casey Bay area, yields no zircons older than 2840 Ma, probably indicating that pre-3000 Ma crust, which is now located nearby, was not exposed at the time of sedimentation there. The isotopic data are quite complex, particularly in rocks that experienced postcrystallization metamorphic temperatures of 1000°C or more. It is postulated that this complexity, which was largely the product of migration of radiogenic Pb within the zircon grains in ancient times, and produced local excesses of this element with respect to its parent U, was caused by volume diffusion at these abnormally high regional crustal temperatures.

Evolution of the Sierras de Córdoba, Argentina

Located in the southeastern part of the Sierras Pampeanas (Argentina), the Pampean Ranges of Córdoba show the evolution at the western margin of the Rio de la Plata craton, from extensional stages during Neoproterozoic times to compressive stages during the Pampean orogeny. The Cumbre de Gaspar Group represents distal deep-sea clastic sedimentation deposited in a passive margin environment, very possibly during a time of incipient oceanization. The Santa Sabina Group, representing shelf sedimentation, rests conformably on the Cumbre de Gaspar Group. Stocks of granites and granodiorites of calc-alkaline affinity intruded the Cumbre de Gaspar Group during the early Pampean or earlier. The polyphase Pampean orogeny involves several distinct metamorphic and orogenic events. Metamorphism is discussed using the chemical composition of minerals. The major effect of the Pampean orogeny was the formation of a pile of westward thrust blocks. The Rio de la Plata craton, thrust above its marginal basin, forms the uppermost unit. The intrusion of the El Pilon granite (dated at 520 ± 5 Ma) post-dates these tectonic and metamorphic events. After this main orogenic event, the domain remained permanently located on the cratonic side of younger Famatinian and Andean orogenic events. The Mermela Group probably corresponds to the molasse of the Pampean orogeny and to the base of a new sedimentary cycle. It is difficult to distinguish Famatinian tectonics in the Sierras de Córdoba because the main orogeny occurred further west. However, this orogenic cycle is marked by the intrusion of the large Achala batholith. During the late Palaeozoic and Mesozoic the Sierras de Córdoba remained a stable platform. The whole region was rejuvenated by the Andean orogeny, which induced tilted blocks in its foreland and the Pocho volcanics of shoshonitic affinities.

Monazite U-Pb dating of staurolite grade metamorphism in pelitic schists

A study of the occurrence of and relations between rare-earth element (REE) minerals in pelitic schists indicates that monazite forms at or near the P and T of the staurolite isograd. Samples at staurolite grade from the Silurian Perry Mountain Formation in the Rumford quadrangle of Maine yield monazite in sufficient quantities to permit accurate dating of the metamorphic events forming the monazites. The bulk chemistry of the metapelites, as seen in the major element abundances and REE patterns, does not vary significantly across the study area. Thus the appearance and disappearance of REE phases is assumed to reflect changes in metamorphic grade. In a sample from the biotite zone, scanning electron microscope and microprobe studies show allanite and monazite intimately associated on a 10 μm scale. The texture suggest that metastable detrital monazite breaks down, distributing its REE components to allanite. From samples below staurolite grade in which monazite is not present, our observations suggest that REEs are partitioned into allanite. At or near the staurolite isograd monazite forms as a metamorphic mineral, initiating its role as a geochronometer. Garnet-biotite geothermometry on samples at this grade from this and other studies places constraints on the minimum temperature necessary to form monazite: 525° C±25°C at 3.1±0.25 kbar. A total of 15 separates from nine schist samples ranging up to sillimanite grade have been dated. Each date is remarkably concordant, even though petrologic and textural studies by previous workers have shown that the rocks in the area have been affected by at least three metamorphic episodes. Calculations indicate insignificant Th disequilibrium in these monazites. The conditions associated with the metamorphic events suggest that monazite remains closed to lead loss provided that subsequent metamorphisms are at or below sillimanite grade. Two distinct metamorphic events are resolved, one at around 400 Ma and one at about 370 Ma. The latter was due to thermal effects of a nearby pluton that yields concordant monazite ages of 363 Ma. This work suggests that in addition to dating plutonism and high-grade metamorphism, monazite should be viewed as a reliable geochronometer for moderate metamorphism of pelitic schists.

Successive overprinting granulite facies metamorphic events in the Anmatjira Range, central Australia

The Anmatjira Range and adjacent Reynolds Range, central Australia, comprise early Proterozoic metasediments and othogneisses that were affected by three, and possibly four, temporally distinct metamorphic events, M1–4, and deformation events, D1–4, in the period 1820–1590 Ma. The north‐western portion of the range, around Mt Stafford, preserves the effects of ±1820 Ma M1‐D1, and shows a spectacular lateral transition from muscovite + quartz‐bearing schists to interlayered andalusite‐bearing migmatites and two‐pyroxene granofelses that reflect extremely low‐pressure granulite facies conditions, over a distance of less than 10 km. Orthopyroxene + cordierite + garnet + K‐feldspar + quartz‐bearing gneisses occur at the highest grade, implying peak conditions of ±750°C and 2.5 ± 0.6 kbar. An anticlockwise P–T path for M1 is inferred from syn‐ to late‐D1 sillimanite overprinting andalusite, petrogenetic grid considerations and quantitative estimates of metamorphic conditions for inferred overprinting assemblages. The effects of M1 have been variably overprinted to the south‐east by a c. 1760 Ma M2–D2 event. Much of the central Anmatjira Range, around Ingellina Gap, comprises orthogneiss, deformed during D2, and metapelites that have M1 andalusite and K‐feldspar overprinted by M2 sillimanite and muscovite. The south‐eastern portion of the range, around Mt Weldon, comprises metasediments and orthogneisses that were completely recrystallized during M2–D2, with metapelitic gneisses characterized by spinel + sillimanite + K‐feldspar + quartz‐bearing assemblages that suggest peak M2 conditions of &gt;750°C and 5.5 ± 1 kbar. Overprinting parageneses in metapelitic gneisses imply that D2 occurred during essentially isobaric cooling. A third granulite facies event, M3, affected rocks in the Reynolds Range, immediately to the south of the Anmatjira Range, at c. 1730 Ma. A possible fourth event, M4, with a minimum age of c. 1590 My affected both Ranges, but resulted in only minor overprinting of M1–3 assemblages. The superimposed effects of M1–4, mapped for the entire Anmatjira–Reynolds Range area, indicate that only minor or no dislocation of the regional geology occurred during any of the metamorphic and accompanying folding, events. Although the immediate cause of each of the metamorphic events involved advection, the ultimate causes were external to the metasediments and most probably external to the crust.

Thermal evolution of the Greater Himalaya, Garhwal, India

The hanging wall of the Main Central Thrust (MCT) in Garhwal, India (roughly 79°N–80°E; 30°N–31°N), exhibits an inverted metamorphic gradient: sillimanite ± potassium feldspar assemblages near the top of the hanging wall, or Greater Himalayan sequence, are underlain by kyanite grade rocks near the fault. Textural relationships in pelitic samples from the Alaknanda and Dhauli river valleys indicate that the “inversion” is the product of two distinct metamorphic events: an early Harrovian event (M1), which affected the entire Greater, Himalayan sequence and a later Buchan event (M2), the effects of which are most obvious in the upper part of the sequence. Rim thermobarometry, garnet inclusion thermobarometry, and thermodynamic modeling of garnet zoning reveal that the basal portions of the metamorphic sequence experienced peak M1 conditions of &gt;900 K and &gt;960 MPa (roughly 36 km depth) before following an “erosion controlled” uplift path (e.g., England and Richardson, 1977). M2 metamorphic temperatures in the upper part of the sequence also exceeded 900 K, but maximum pressures (317–523 MPa) indicate paleodepths of only 12–19 km. Calculated pressure‐temperature paths indicate that M2 was characterized by temperature increases of &gt;80 K and roughly 5 km of tectonic burial We attribute M1 to tectonic burial of the Greater Himalayan sequence during the early stages of India‐Eurasia collision. We believe that the uplift and cooling path of the sequence was interrupted in late Oligocene(?) ‐ Miocene time by a second burial and heating event (M2) related to thrust imbrications in southern Tibet. This burial was coincident with the generation of leucogranites, which are abundant near the top of the Greater Himalayan sequence but are virtually absent near the MCT. Field relations do not constrain whether the leucogranites were derived from some presently unexposed portion of the Greater Himalayan sequence and were injected at their present structural level, or were melted in situ. If the granites were injected, then they may have provided some of the heat necessary for M2 metamorphism. Although our data suggest no direct relationship between the Main Central Thrust (as mapped in Garhwal) and metamorphism in the Greater Himalaya, anatectic melting of an unexposed portion of the Greater Himalayan sequence could have been associated with movement along a blind thrust with characteristics similar to the mapped MCT in central Nepal (cf. Le Fort, 1981). If the granites were produced by in situ M2 melting, then we must appeal to a heat source within the upper part of the Greater Himalayan sequence such as locally high concentrations of heat‐producing elements (cf. Pinet and Jaupart, 1987).

Metamorphic studies: research in progress

Report of a meeting of the Metamorphic Studies Group held at the University of Sheffield, 11 March 1987. The meeting was organized by K. H. Brodie and D. A. Carswell. A Metamorphic Studies Group meeting on ‘Metamorphic Studies: Research in Progress’ and the sixth Annual General Meeting of the Group were held on 11 March 1987. For the first time the annual meeting was held outside London, in the Department of Geology at the University of Sheffield. It was very well attended, and 10 papers were presented on a range of metamorphic topics, including an invited contribution from T. Bell (James Cook University, Australia). The morning session concentrated on high grade and regional metamorphic topics. C. R. L. Friend, in a joint contribution with A. P. Nutman and V. R. McGregor, presented the results of a reinvestigation of an area of amphibolite and granulite facies metamorphism in SW Greenland, involving systematic isotopic studies. The area can be divided into zones with significantly different tectono-metamorphic histories that have been tectonically juxtaposed prior to amphibolite acies metamorphism and deformation which affected the whole area. P. J. Treloar continued the topic of P–T histories of granulite facies terranes in a joint paper with J. Carney and M. J. Crow, involving an example from NE Zimbabwe. Attempts have been made to date two distinct metamorphic events that have affected the rocks, the second of which involved decompression associated with uplift of the area. D. A. Carswell discussed the reality of the postulated inflection

Metamorphism of the Spokane Dome Mylonitic Zone, Priest River Complex: Constraints on the Tectonic Evolution of Northeastern Washington and Northern Idaho

Metamorphic rocks in the core of the southern Priest River Complex record two distinct metamorphic events. The earliest metamorphism occurred above 5 kb near the amphibolite-granulite boundary and accompanied the formation of a steep, crystalloblastic foliation. The second, less pronounced, metamorphism was at lower pressure (<3.5 kb), in the middle amphibolite facies, and accompanied mylonitization within a 4 km thick zone. Retrograde reactions during the second event include the formation of andalusite, muscovite, secondary green biotite, and the breakdown of garnet. Biotite-garnet geothermometry yielded widely variable and unrealistically high temperatures. Plagioclase-garnet geobarometry is partially reset by the retrograde event, and therefore provides minimum pressures. The P-T path is not known precisely but may include two episodes of prograde metamorphism, perhaps related to continued accretion of terranes to the west. The first metamorphism probably accompanied the Jurassic accretion of Quesnellia, whereas the second may represent crustal thickening during late Cretaceous intracontinental, compressional, ductile shearing.

39Ar- 40Ar laser probe dating of terrestrial rocks

We have shown that for metamorphosed granites, as well as for metamorphosed sediments, it is possible to obtain a reliable K-Ar age from a sample as small as 0.2 μg. Samples with ages as young as 40 m.y. are easily studied. There is no reason to believe that the method could not be used for samples considerably younger. The main application of the laser will probably be for cases where the same mineral species has been formed during two distinct metamorphic events in the same rock. In this case, it is difficult, if not impossible, to make a mineral separate suitable for a 39 Ar-40Ar study. We have shown that in the case of an old granite the biotite in one sample distinctly shows thermal events at 1600 m.y. whileits primary cooling age is 2100 m.y. The thermal release study yields a good plateau age, roughly an average age of the two thermal events. An other interest of this method is demonstrated by argon dating of two different kinds of glaucophane, only one of theseshowing argon in excess.