Garnet-cordierite Gneisses Research Articles

Characterization of partial melting events in garnet-cordierite gneiss from the Kerala Khondalite Belt, India

Phase equilibria modelling coupled with U–Pb zircon and monazite ages of garnet–cordierite gneiss from Vallikodu Kottayam in the Kerala Khondalite Belt, southern India are presented here. The results suggest that the area attained peak P–T conditions of ~900 ​°C at 7.5–8 ​kbar, followed by decompression to 3.5–5 ​kbar and cooling to 450–480 ​°C, preserving signatures of the partial melting event in the field of high to ultra-high temperature metamorphism. Melt reintegration models suggest that up to 35% granitic melt could have been produced during metamorphism at ~950 ​°C. The U–Pb age data from zircons (~1.0 – ~0.7 ​Ga) and chemical ages from monazites (~540 ​Ma and ~941 ​Ma) reflect a complex tectonometamorphic evolution of the terrain. The ~941 ​Ma age reported from these monazites indicate a Tonian ultra-high temperature event, linked to juvenile magmatism/deformation episodes reported from the Southern Granulite Terrane and associated fragments in Rodinia, which were subsequently overprinted by the Cambrian (~540 ​Ma) tectonothermal episode.

Cordierite-bearing granulites from Ihosy, southern Madagascar: Petrology, geochronology and regional correlation of suture zones in Madagascar and India

Madagascar, a major fragment of Gondwana, is mainly composed of Precambrian basement rocks formed by Mesoarchean to Neoproterozoic tectono-thermal events and recording a Pan-African metamorphic overprint. The Ranotsara Shear Zone in southern Madagascar has been correlated with shear zones in southern India and eastern Africa in the reconstruction of the Gondwana supercontinent. Here we present detailed petrology, mineral chemistry, metamorphic P–T constraints using phase equilibrium modelling and zircon U-Pb geochronological data on high-grade metamorphic rocks from Ihosy within the Ranotsara Shear Zone. Garnet-cordierite gneiss from Ihosy experienced two stages of metamorphism. The peak mineral assemblage is interpreted as garnet + sillimanite + cordierite + quartz + plagioclase + K-feldspar + magnetite + spinel + ilmenite, which is overprinted by a retrograde mineral assemblage of biotite + garnet + cordierite + quartz + plagioclase + K-feldspar + magnetite + spinel + ilmenite. Phase equilibria modelling in the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO) indicates peak metamorphic conditions of 850–960 °C and 6.9–7.7 kbar, and retrograde P–T conditions of <740 °C and <4.8 kbar, that define a clockwise P–T path. Near-concordant ages of detrital zircon grains in the garnet-cordierite gneiss dominantly exhibit ages between 2030 Ma and 1784 Ma, indicating dominantly Paleoproterozoic sources. The lower intercept age of 514 ± 33 Ma probably indicates the timing of high-grade metamorphism, which coincides with the assembly of the Gondwana supercontinent. The comparable rock types, zircon ages and metamorphic P–T paths between the Ranotsara Shear Zone and the Achankovil Suture Zone in southern India support an interpretation that the Ranotsara Shear Zone is a continuation of the Achankovil Suture Zone.

Petrology and isotopic evolution of granulites from central Madurai Block (southern India): reference to Ediacaran crustal evolution

ABSTRACTThe Madurai Block, constituting part of the southern granulite terrain in southern India, has contributed significantly towards understanding the UHT (ultrahigh-temperature) granulites that serve as a window into the mid-lower continental crust. The dominant rock types are charnockites, sapphirine-bearing granulites, garnet cordierite gneisses, and quartzites. Significant textural relations reveal multiphase reactions responsible for the formation of diverse mineral parageneses during prolonged metamorphic history of the area. Prograde reaction is evident from the textural relationship where biotite/sillimanite relics are seen as inclusion in garnet/orthopyroxene, suggesting dehydration reactions. The symplectitic assemblages that formed during isothermal decompression involve a series of cordierite-forming reactions, followed by retrogression and cooling. Variety of mineral assemblages present in the rocks of this area offer a wide spectrum of P–T sensors that provide details on the physical conditions of metamorphism. For the rigorous interpretation of the P–T path in the Perumalmalai area, quantitative phase diagrams (P–T pseudosections) have been constructed and contoured for the compositional as well as modal isopleths of involved mineral phases. The rocks of Perumalmalai area document a clockwise decompression P–T trajectory, consistent with crustal thickening followed by extensional collapse. SHRIMP U–Pb ages from zircon associated with sapphirine-bearing granulite facies rocks of Perumalmalai area suggest a widespread Ediacaran tectonothermal event. The occurrence of Ediacaran UHT metamorphism followed by isothermal decompression in the Madurai Block is consistent with the timing and physical conditions associated with the formation of East African Orogen during the amalgamation of Gondwana.

Petrology, pseudosection modelling and U-Pb geochronology of silica-deficient Mg-Al granulites from the Jagtiyal section of Karimnagar granulite terrane, northeastern Dharwar Craton, India

The Jagtiyal section exposes a part of the Precambrian granulite – facies terrane in the northeastern Dharwar Craton, specifically charnockite, orthogneiss, mafic granulite, garnet-cordierite gneiss and silica-deficient Mg-Al granulite. The silica-deficient Mg-Al granulites exhibit well-preserved metamorphic reaction microstructures and were used to elucidate the metamorphic history of the study area. Textural evidence exists for the prograde break-down of biotite and corundum and formation of a peak metamorphic assemblage of spinel, sapphirine, orthopyroxene, cordierite and K-feldspar. KFMASHTO system based P-T pseudosections as well as the application of multi-equilibrium calculation for mineral core compositions, gave P-T conditions of 7.5–8kbar and 800–840°C. The thermal peak was followed by near isothermal decompression that led to a variety of coronal textures including, successive rims on corundum, consisting of spinel+sapphirine+cordierite±orthopyroxene, and symplectitic intergrowth of sapphirine-cordierite, orthopyroxene-cordierite, spinel-cordierite, and biotite-cordierite. Modelled P-T conditions for this stage are 4.3–4.5kbar and 750–800°C. Few samples show textural evidence for further retrogression leading to the break-down of sapphirine and cordierite to form chlorite, corundum, spinel and andalusite. The proposed clockwise P-T path implies that granulites from the Jagtiyal section likely resulted from thickened continental crust undergoing decompression. New U-Pb zircon geochronologic data provide evidence for a Neoarchaean (2604±25Ma) age that likely corresponds to the timing of accretion of the terrane that was subjected to high temperature metamorphism at that time. Moreover, there is evidence of a Neoproterozoic thermal overprint in the area around 638Ma..

Nd isotope systematics in metamorphic rocks of the southern Russian Far East

Sm–Nd isotopic–geochemical investigations are widely used in the study of metamorphic sequences for estimating the age of continental crust formation and the lower age limit of protoliths of metamorphic com� plexes [1]. When the protolith is of primary magmatic origin, the Nd model age reflects in the first approxi� mation the onset of the continental crust formation from the mantle source or, in other words, the time of transformation of the oceanic crust into the continen� tal crust. The Nd isotope systematics of metamorphic terrigenous sedimentary rocks offers the opportunity to estimate the average model age and likely paleogeo� graphic provenances, as well as the lower age limit of sediment accumulation. In this communication, we discuss the data of the Nd isotopic composition in metamorphosed rocks from the three largest blocks located in the southern Far East areas: Matveevka–Nakhimovka, Sergeevka, and Anyui. It should be noted that the studied com� plexes were traditionally considered as representing inliers of the Early Precambrian crystalline basement and only recently have they been regarded as Neogean complexes that were formed by accretion, collision, or postcollision processes [2, 3]. The available geochro� nological data provide no grounds for answering the following question: what is the lag between metamor� phic transformation of rocks and their geological age? The Nd isotope measurements performed for meta� morphic complexes of the southern Far East bring us nearer to assessment of the lower age limits of their formation and to determination of the isotope crust composition for the whole region. The Khanka superterrane is located in southwestern Primor’e region (Fig. 1). It is formed by several ter� ranes of various origins. The southern and northern parts of the superterrane represent fragments of the passive margin of a craton and Early Paleozoic oro� genic belt, respectively. The last of them comprises the Matveevka and Nakhimovka metamorphic terranes that differ from each other in the constituting rocks and degree of their metamorphic transformation [6]. The most intensely metamorphosed rocks of the Matveevka terrane are united into the Iman Group, which is traditionally subdivided into the Ruzhino (diopside–calcite and forsterite–calcite marbles, biotite and biotite–cordierite gneisses with intercala� tions of quartzites and marbles), Matveevka (biotite– sillimanite and biotite–garnet–cordierite gneisses with quartzite and marble intercalations), and Tur� genevka (biotite–amphibole gneisses, crystalline schists, and amphibolites) formations. In the Nakhi� movka terrane, metamorphic rocks are united into the Ussuri Group, which consists of the Nakhimovka (biotite and biotite–amphibolite gneisses with lenses of marbles and amphibolites) and Tat’yanovka (biotite, diopside, and muscovite–graphite crystalline schists) formations. The relationships between the Iman and Ussuri groups remain unknown.

New SHRIMP U-Pb zircon ages of the metapelitic granulites from NW of Madurai, Southern India

Zircon cathodoluminescent imaging and SHRIMP U-Pb dating were carried out for metapelitic rocks (sapphirine-bearing granulites and garnet-cordierite gneisses) from the NW of Madurai, Southern India. The cathodoluminescence images reveal the complex, inhomogeneous internal structure having irregular-shaped core and overgrowths. Zircon grains have obliterated oscillatory zoning. SHRIMP U-Pb chronological results yield ages of 550±15 Ma and 530±50 Ma as a time of metamorphic overprint, and the age of 2509±12 Ma and 2509±30 Ma corresponding to a timing of protolith formation for sapphirine-bearing granulites and garnet-cordierite gneisses respectively. Zircon ages reflect that continental crust in the NW of Madurai region resulted from the recycling of Archaean protolith of an igneous origin similar to the preserved crust in the southern part of Dharwar craton. The present SHRIMP U-Pb zircon ages are in close agreement with earlier published Nd isotopic data which suggest an extended precrustal history of their protoliths. The abraded zircon grains indicate multiple recycling and repeated metamorphism that has ultimately resulted in present day continental crust exposed in Madurai region. These SHRIMP U-Pb zircon ages from metapelitic UHT granulites are also significant to understanding the architecture of the SGT during the amalgamation of Gondwana in Neoproterozoic time.

Laser ablation ICP mass spectrometry for zircon U–Pb geochronology of ultrahigh-temperature gneisses and A-type granites from the Achankovil Suture Zone, southern India

Abstract We report results from U–Pb analyses by laser ablation ICP mass spectrometry of zircons in ultrahigh-temperature garnet-cordierite gneisses and A-type granite intrusions within the Achankovil Shear/Suture Zone in southern India, a region considered to mark the zone of the latest Neoproterozoic-Cambrian Gondwana collision. Field evidence and geochronologic data for the gneisses and granites suggests the possibility for the multi-heating of metamorphism in earlier stage and magmatism in later stage. Zircon U–Pb dating for 138 spots on 123 grains yielded well-defined age peaks at 480–560 Ma and less prominent age peaks at approximately 600–900 Ma in compiled age data histograms. The Neoproterozoic ages of ca. 600–900 Ma correlating with the protolith history might suggest the magmatic and metamorphic processes in a subduction-accretion regime prior to the final collision during latest Neoproterozoic-Cambrian. The ages of 480–560 Ma obtained from the granites correlate with the final suturing of the Gondwana supercontinent.

Evolution of the Yunkai Terrane, South China: Evidence from SHRIMP zircon U–Pb dating, geochemistry and Nd isotope

The Yunkai Terrane is one of the most important pre-Devonian areas of metamorphosed supracrustal and granitic basement rocks in the Cathaysia Block of South China. The supracrustal rocks are mainly schist, slate and phyllite, with local paragneiss, granulite, amphibolite and marble, with metamorphic grades ranging from greenschist to granulite facies. Largely on the basis of metamorphic grade, they were previously divided into the Palaeo- to Mesoproterozoic Gaozhou Complex, the early Neoproterozoic Yunkai ‘Group’ and early Palaeozoic sediments. Granitic rocks were considered to be Meso- and Neoproterozoic, or early Palaeozoic in age. In this study, four meta-sedimentary rock samples, two each from the Yunkai ‘Group’ and Gaozhou Complex, together with three granite samples, record metamorphic and magmatic zircon ages of 443–430 Ma (Silurian), with many inherited and detrital zircons with the ages mainly ranging from 1.1 to 0.8 Ga, although zircons with Archaean and Palaeoproterozoic ages have also been identified in several of the samples. A high-grade sillimanite–garnet–cordierite gneiss contains ∼242 Ma metamorphic zircons, as well as ∼440 Ma ones. Three of the meta-sedimentary rocks show large variations in major element compositions, but have similar REE patterns, and have t DM model ages of 2.17–1.91 Ga and ε Nd (440 Ma) values of −13.4 to −10.0. Granites range in composition from monzogranite to syenogranite and record t DM model ages of 2.13–1.42 Ga and ε Nd (440 Ma) values of −8.4 to −1.2. It is concluded that the Yunkai ‘Group’ and Gaozhou Complex formed coevally in the late Neoproterozoic to early Palaeozoic, probably at the same time as weakly to un-metamorphosed early Palaeozoic sediments in the area. Based on the detrital zircon population, the source area contained Meso- to Neoproterozoic rocks, with some Archaean material. Palaeozoic tectonothermal events and zircon growth in the Yunkai Terrane can be correlated with events of similar age and character known throughout the Cathaysia Block. The lack of evidence for Palaeo- and Mesoproterozoic rocks at Yunkai, as stated in earlier publications, means that revision of the basement geology of Cathaysia is necessary.

Evaluation of dimension stone in gneissic rocks — a case history from southern Finland

A dimension stone prospect in southern central Finland was assessed by a detailed mapping, geo-radar survey, and core drilling. The prospect is a veined and bedded garnet–cordierite gneiss, consisting of a dark schistose medium-grained palaeosome and a light coarse-grained granitic leucosome. Both components are found mainly as thin units, but the leucosome can occur as individual veins several metres thick, which leads to disturbing variations in the appearance of the stone. The soundness of the stone is defined by the amount of tight, but open cracks in the palaeosome and weakness zones in the weathered leucosome. As a whole, the soundness is diminished down to 10 m below the outcrop surface. Furthermore, a subhorizontal body of younger granite is identified at approx. 8–20 m depth. Consequently, the prospect is not feasible for production of dimension stone. The core drilling was decisive in the final evaluation of the prospect because the true density of the cracks and the subsurface granite were identified by the method. Only with the use of core drilling could the proper interpretation of the characteristics of the prospect be made, demonstrating the importance of three-dimensional investigation of a dimension stone prospect.

Conditions of Metamorphism of the Garnet-Cordierite Gneisses from Malai Mahadeswara Hills, South India and the Origin of Zn-Cr-Rich Spinel

Abstract A spinel phase with unusually high ZnO (8.8-11.0) and Cr2O3 (13.2-15.8%) contents is found in garnet-cordierite-sillimanite bearing migmatised pelitic bands interbedded with tonalitic charnockites of Malai Mahadeswara (MM) Hills. The textural and microprobe data indicate that this spinel phase is associated with late iron-rich garnet (rim)-cordierite-sillimanite assemblage. P-T conditions of cordierite-garnet assemblages vary from peak metamorphic conditions of greater than 8 kbar and 750°C to 6.7±1.4 kbar and 685 ±20°C. The last retrograde phase of metamorphism took place at P of 4.5 kbar and at T of 580°C. The formation of Zn and Cr rich spinel phase must have taken place during this waning stages of metamorphism by the activity of hydrothermal fluids.

Granulites and garnet–cordierite gneisses from Dronning Maud Land, Antarctica

Abstract The central sector of Mühlig‐Hofmannfjellet (3°E/71°S) in western Dronning Maud Land (East Antarctic shield) is dominated by large intrusive bodies of predominantly orthopyroxene‐bearing quartz syenites (charnockites). Metasedimentary rocks are rare; however, two distinct areas with banded gneiss–marble–quartzite sequences of sedimentary origin were found during the Norwegian Antarctic Research Expedition NARE 1989/90. Cordierite‐bearing metapelitic gneisses from two different localities contain the characteristic mineral assemblage: cordierite + garnet + biotite + K‐feldspar + plagioclase + quartz ± sillimanite ± spinel. Thermobarometry indicates equilibration conditions of about 650°C and 4 kbar. Associated orthopyroxene–garnet granulites, on the other hand, revealed pressures of about 8 kbar and temperatures of 750°C. The earlier granulite facies metamorphism is not well preserved in the cordierite gneisses as a result of excess K‐feldspar combined with interaction with an H2O‐rich fluid phase, probably released by the cooling intrusives. These two features allowed the original high‐grade K‐feldspar + garnet assemblages to recrystallize as cordierite–biotite–sillimanite gneisses, completely re‐equilibrating them. Phase relationships indicate that the younger metamorphic event occurred in the presence of a fluid phase that varied in composition between the lithologies.

Charnockites, pyroxene granulites, and garnet-cordierite gneisses at a boundary between Early Svecofennian rocks and Småland-Värmland granitoids, Karlskoga, southern Sweden

Charnockites, pyroxene granulites, and garnet-cordierite gneisses at a boundary between Early Svecofennian rocks and Småland-Värmland granitoids, Karlskoga, southern Sweden

Charnockites, pyroxene granulites, and garnet-cordierite gneisses at a boundary between Early Svecofennian rocks and Småland-Värmland granitoids, Karlskoga, southern Sweden

Abstract A lobe of Early Svecofennian high-grade, metamorphic rocks surrounded and intruded by rocks of the Smaland-Varmland batholith east of Karlskoga, central southern Sweden, has been studied. Application of geothermobarometry reveals that these rocks have suffered granulite facies metamorphism at conditions constrained to 670–770°C, 4.0–4.5 kbar and aH2O0.1–0.3. The metamorphism has transformed biotite granite into charnockite, intermediate volcanite into pyroxene granulite, and lower grade presumably semipelitic gneiss into garnet-cordierite gneiss. Extensive partial melting accompanied the metamorphism in the garnet-cordierite gneisses and granulites, but not in the charnockites. The metamorphism is attributed to a local contact metamorphic peak, associated with the emplacement of the Smaland-Varmland granitoids and related mafic plutonics, in the penecontemporaneous, amphibolite facies, regional “serorogenic Svecofennian” episode.

Geochemistry of a peraluminous granitoid suite from North-eastern Victoria, South-eastern Australia

The Koetong Suite of Silurian, 2-mica granitoids was derived from a metasedimentary source and emplaced into Ordovician sediments and metasediments along the eastern margin of the Western Metamorphic Belt of South-eastern Australia. Whole-rock geochemical considerations preclude derivation of the magmas represented by the granitoids from exposed Ordovician metasediments. The magmas were generated by partial melting of material similar in composition to garnet-cordierite gneisses exposed in the adjacent metamorphic belt. Melting at pressures in excess of 5 Kb and temperatures about 750°C produced peraluminous magmas and, when the degree of partial melting approached 25–30%, these magmas became mobile and moved vertically into the overlying Ordovician sediments. During movement from the source region to the zone of emplacement, separation of the melt and refractory residue components of the magma resulted in a range of compositions so that whole-rock analyses of the granitoids are linearly related on major and trace element variation diagrams. Processes such as crystal fractionation and crystal accumulation may have operated locally. The magmas were largely composed of solid material throughout their emplacement histories and the amount of melt may not have exceeded 30–45% at any stage. Metasedimentary inclusions are a reflection of source heterogeneity. After emplacement of the magmas, in situ crystallization of a relatively anhydrous assemblage of minerals led to water contents in residual, intercrystalline, melts sufficiently high for muscovite to begin crystallization at pressures around 4 Kb. Subsequent saturation of intercrystalline residual melt and loss of the resultant volatile phase caused the development of eutectoid intergrowths involving muscovitebiotite-quartz and alkali feldspar.

Petrology of the Fort Smith - Great Slave Lake radiometric high near Pilot Lake, N.W.T.

Near Pilot Lake, the east boundary of the Fort Smith – Great Slave Lake radiometric high coincides with the contact of a well-foliated, porphyroblastic microcline–plagioclase–quartz-garnet–biotite gneiss1 (Pilot Lake Gneiss) with a hybrid assemblage of quartzite, mica schist, garnet–cordierite gneiss, and minor amphibolite (Variable Paragneiss). Anomalously high concentrations of uranium and thorium are associated with mafic-rich, lenticular bodies with a mineral assemblage biotite + monazite + zircon + ilmenite + hematite ± apatite ± plagioclase ± quartz. The mafic pods occur within both the Variable Paragneiss and the Pilot Lake Gneiss. Corundum and spinel occur in the mafic lenses and sillimanite, kyanite, and hypersthene in other inclusions in the Pilot Lake Gneiss.The ilmenite–magnetite–monazite–zircon–apatite assemblage is interpreted as a "black sand" concentration in a clastic sedimentary sequence subsequently metamorphosed by a regional granulite facies event. A granitic pluton intruded during the same orogenic cycle assimilated the clastic metasedimentary rocks containing black sand interlayers, becoming enriched in thorium from the monazite. A second metamorphic event at lower P–T conditions, accompanied by strong cataclasis, developed the texture of the Pilot Lake Gneiss as now observed. Shearing within the gneiss locally concentrated hematite + quartz + uranium.Regional tectonic extrapolations suggest that the pyroxene granulite event was Kenoran and the later amphibolite event Hudsonian.

Garnet-cordierite gneisses near the Egersund-Ogna anorthositic intrusion, southwestern Norway

Graphite-bearing garnet-cordierite rocks from the Sjelset-Vikesá region, south Bogaland, Norway have been analysed by microprobe methods. The garnets belong to the pyropealmandine series and contain 28 mole% pyrope, while the cordierites contain about 71% of the Mg end-member. A regional temperature of about 720–750°C and a lithostatic pressure in the order of 6–7 kb have been inferred from the garnet and cordierite compositions. The petrographic observation suggests that such PT conditions might reflect a retrograde metamorphic event. A tentative estimation of the fluid phase composition, based on the study of opaque minerals, is presented.

Prograde and retrograde equilibria in garnet-cordierite gneisses in south-central Massachusetts

Partial electron microprobe analyses of garnet, biotite and cordierite in sillimanite-K feldspar gneisses of the Brimfield Formation in south-central Massachusetts indicate that the compositions of these minerals are not constant in a thin section. The FeO/MgO mol ratio of biotite is sensitive to the nature of other FeO-MgO minerals occurring in close proximity. The most iron-rich biotites are those that do not contact either cordierite or garnet. The most iron-poor biotites occur as inclusions in garnet. Biotites in direct contact with either cordierite or garnet have intermediate FeO/MgO ratios. The bulk of a given grain of garnet or cordierite is homogeneous in composition. Chemical zoning is absent. All grains of garnet and cordierite in a thin section are constant in composition. However, where garnet and cordierite abut biotite, the FeO/MgO ratio of the garnet rim is increased and that of cordierite is decreased. The FeO/MgO ratios of garnet, cordierite and biotite bare a regular relation to each other indicating a possible equilibrium state. However the distribution coefficient defined by the compositions of minerals in direct contact are greater than those defined by the compositions of the interiors of garnet and cordierite matched with the compositions of biotites removed from these phases. This pattern is believed to be the result of two thermal events. The first event produced the mineral assemblages and widespread equilibrium was obtained. A subsequent retrograde event left the mineralogy intact but caused cation exchange reactions at immediate contacts between garnet, cordierite and biotite. The physical conditions of the first event are estimated at P=5–6 kb, T=700–750° C. The retrograde event occurred at lower temperatures and very low activities of H2O since no muscovite is developed at microcline-sillimanite contacts.