High-pressure and ultrahigh-temperature metamorphism at Komateri,northern Madurai Block,southern India

We report for the first time the evidence for prograde high-pressure (HP) metamorphism preceding a peak ultrahigh-temperature (UHT) event in the northernmost part of the Madurai Block in southern India. Mg–Al-rich Grt–Ged rocks from Komateri in Karur district contain poikiloblastic garnet with numerous multi-phase inclusions. Although most of the inclusion assemblages are composed of gedrite, quartz, and secondary biotite, rare staurolite + sapphirine and spinel + quartz are also present. The X_Mg (=Mg/[Fe+Mg]) of staurolite (0.45–0.49) is almost consistent with that reported previously from Namakkal district in the Palghat–Cauvery Shear Zone system (X_Mg = 0.51–0.52), north of the Madurai Block. The HP event was followed by peak UHT metamorphism at T = 880–1040 °C and P = 9.8–12.5 kbar as indicated by thermobarometric computations in the Grt–Ged rock and associated mafic granulite. Symplectic intergrowth of spinel (X_Mg = 0.50–0.59, ZnO < 1.7 wt.%) and quartz, a diagnostic indicator of UHT metamorphism, probably formed by decompression at UHT conditions. The rocks subsequently underwent retrograde metamorphism at T = 720–760 °C and P = 4.2–5.1 kbar. The P–T conditions and clockwise exhumation trajectory of the Komateri rocks, comparable to similar features recorded from the Palghat–Cauvery Shear Zone system, suggest that the Madurai Block and the Palghat–Cauvery Shear Zone system underwent similar HP and UHT metamorphic history probably related to the continent–continent collision during the final stage of amalgamation of Gondwana supercontinent.     

The timing of ultrahigh-temperature metamorphism in Southern India: U–Th–Pb electron microprobe ages from zircon and monazite in sapphirine-bearing granulites

We report here U–Pb electron microprobe ages from zircon and monazite associated with corundum- and sapphirine-bearing granulite facies rocks of Lachmanapatti, Sengal, Sakkarakkottai and Mettanganam in the Palghat–Cauvery shear zone system and Ganguvarpatti in the northern Madurai Block of southern India. Mineral assemblages and petrologic characteristics of granulite facies assemblages in all these localities indicate extreme crustal metamorphism under ultrahigh-temperature (UHT) conditions. Zircon cores from Lachmanapatti range from 3200 to 2300 Ma with a peak at 2420 Ma, while those from Mettanganam show 2300 Ma peak. Younger zircons with peak ages of 2100 and 830 Ma are displayed by the UHT granulites of Sengal and Ganguvarpatti, although detrital grains with 2000 Ma ages are also present. The Late Archaean-aged cores are mantled by variable rims of Palaeo- to Mesoproterozoic ages in most cases. Zircon cores from Ganguvarpatti range from 2279 to 749 Ma and are interpreted to reflect multiple age sources. The oldest cores are surrounded by Palaeoproterozoic and Mesoproterozoic rims, and finally mantled by Neoproterozoic overgrowths. In contrast, monazites from these localities define peak ages of between 550 and 520 Ma, with an exception of a peak at 590 Ma for the Lachmanapatti rocks. The outermost rims of monazite grains show spot ages in the range of 510–450 Ma.While the zircon populations in these rocks suggest multiple sources of Archaean and Palaeoproterozoic age, the monazite data are interpreted to date the timing of ultrahigh-temperature metamorphism in southern India as latest Neoproterozoic to Cambrian in both the Palghat–Cauvery shear zone system and the northern Madurai Block. The data illustrate the extent of Neoproterozoic/Cambrian metamorphism as India joined the Gondwana amalgam at the dawn of the Cambrian.     

Ultrahigh-temperature metamorphism in the Achankovil Zone: Implications for the correlation of crustal blocks in southern India

The Achankovil Zone of southern India, a NW–SE trending lineament of 8–10 km in width and > 100 km length, is a kinematically debated crustal feature, considered to mark the boundary between the Madurai Granulite Block in the north and the Trivandrum Granulite Block in the south. Both these crustal blocks show evidence for ultrahigh-temperature metamorphism during the Pan-African orogeny, although the exhumation styles are markedly different. The Achankovil Zone is characterized by discontinuous strands of cordierite-bearing gneiss with an assemblage of cordierite + garnet + quartz + plagioclase + spinel + ilmenite + magnetite ± orthopyroxene ± biotite ± K-feldspar ± sillimanite. The lithology preserves several peak and post-peak metamorphic assemblages including: (1) orthopyroxene + garnet, (2) perthite and/or anti-perthite, (3) cordierite ± orthopyroxene corona around garnet, and (4) cordierite + quartz symplectite after garnet. We estimate the peak metamorphic conditions of these rocks using orthopyroxene-bearing geothermobarometers and feldspar solvus which yield 8.5–9.5 kbar and 940–1040 °C, the highest P–T conditions so far recorded from the Achankovil Zone. The retrograde conditions were obtained from cordierite-bearing geothermobarometers at 3.5–4.5 kbar and 720 ± 60 °C. From orthopyroxene chemistry, we record a multistage exhumation history for these rocks, which is closely comparable with those reported in recent studies from the Madurai Granulite Block, but different from those documented from the Trivandrum Granulite Block. An evaluation of the petrologic and geochronologic data, together with the nature of exhumation paths leads us to propose that the Achankovil Zone is probably the southern flank of the Madurai Granulite Block, and not a unit of the Trivandrum Granulite Block as presently believed. Post-tectonic alkali granites that form an array of “suturing plutons” along the margin of the Madurai Granulite Block and within the Achankovil Zone, but are absent in the Trivandrum Granulite Block, suggest that the boundary between the Madurai Granulite Block and the Trivandrum Granulite Block might lie along the Tenmalai shear zone at the southern extremity of the Achankovil Zone.     

A palaeomagnetic study of charnockites from Madras Block, Southern Granulite Terrain, India

The South Indian Craton is composed of low-grade and high-grade metamorphic rocks across different tectonic blocks between the Moyar–Bhavani and Palghat–Cauvery shear zones and an elongated belt of eastern margin of the peninsular shield. The Madras Block north of the Moyar–Bhavani shear zone, which evolved throughout the Precambrian period, mainly consists of high-grade metamorphic rocks. In order to constrain the evolution of the charnockitic region of the Pallavaram area in the Madras Block we have undertaken palaeomagnetic investigation at 12 sites. ChRM directions in 61 oriented block samples were investigated by Alternating Field (AF) and Thermal demagnetization. Titanomagnetite in Cation Deficient (CD) and Multi Domain (MD) states is the remanence carrier. The samples exhibit a ChRM with reverse magnetization of D_m = 148.1, I_m = + 48.6 (K = 22.2, α₉₅ = 9.0) and a palaeomagnetic pole at 37.5 °N, 295.6 °E (dp/dm = 7.8°/11.8°). This pole plots at a late Archaean location on the Indian Apparent Polar Wander Path (APWP) suggesting an age of magnetization in the Pallavaram charnockites as 2600 Ma. The nearby St. Thomas Mount charnockites indicate a period of emplacement at 1650 Ma (Mesoproterozoic). Thus the results of Madras Block granulites also reveal crustal evolution similar to those in the Eastern Ghats Belt with identical palaeopoles from both the areas.     

Ultra high temperature metamorphism of mafic granulites from South Altyn Orogen,West China: A result from the rapid exhumation of deeply subducted continental crust

The South Altyn orogen in West China contains ultra high pressure (UHP) terranes formed by ultra‐deep (>150–300 km) subduction of continental crust. Mafic granulites which together with ultramafic interlayers occur as blocks in massive felsic granulites in the Bashiwake UHP terrane, are mainly composed of garnet, clinopyroxene, plagioclase, amphibole, rutile/ilmenite, and quartz with or without kyanite and sapphirine. The kyanite/sapphirine‐bearing granulites are interpreted to have experienced decompression‐dominated evolution from eclogite facies conditions with peak pressures of 4–7 GPa to high pressure (HP)–ultra high temperature (UHT) granulite facies conditions and further to low pressure (LP)–UHT facies conditions based on petrographic observations, phase equilibria modelling, and thermobarometry. The HP–UHT granulite facies conditions are constrained to be 2.3–1.6 GPa/1,000–1,070°C based on the observed mineral assemblages of garnet+clinopyroxene+rutile+plagioclase+amphibole±quartz and measured mineral compositions including the core–rim increasing anorthite in plagioclase (X_An = 0.52–0.58), core–rim decreasing jadeite in clinopyroxene (X_Jd = 0.20–0.15), and TiO₂ in amphibole (Ti^M2/2 = 0.14–0.18). The LP–UHT granulite facies conditions are identified from the symplectites of sapphirine+plagioclase+spinel, formed by the metastable reaction between garnet and kyanite at <0.6–0.7 GPa/940–1,030°C based on the calculated stability of the symplectite assemblages and sapphirine–spinel thermometer results. The common granulites without kyanite/sapphirine are identified to record a similar decompression evolution, including eclogite, HP–UHT granulite, and LP–UHT granulite facies conditions, and a subsequent isobaric cooling stage. The decompression under HP–UHT granulite facies is estimated to be from 2.3 to 1.3 GPa at ~1,040°C on the basis of textural records, anorthite content in plagioclase (X_An = 0.25–0.32), and grossular content in garnet (X_Grs = 0.22–0.19). The further decompression to LP–UHT facies is defined to be >0.2–0.3 GPa based on the calculated stability for hematite‐bearing ilmenite. The isobaric cooling evolution is inferred mainly from the amphibole (Ti^M2/2 = 0.14–0.08) growth due to the crystallization of residual melts, consistent with a temperature decrease from >1,000°C to ~800°C at ~0.4 GPa. Zircon U–Pb dating for the two types of mafic granulite yields similar protolith and metamorphic ages of c. 900 Ma and c. 500 Ma respectively. However, the metamorphic age is interpreted to represent the HP–UHT granulite stage for the kyanite/sapphirine‐bearing granulites, but the isobaric cooling stage for the common granulites on the basis of phase equilibria modelling results. The two types of mafic granulite should share the same metamorphic evolution, but show contrasting features in petrography, details of metamorphic reactions in each stage, thermobarometric results, and also the meaning of zircon ages as a result of their different bulk‐rock compositions. Moreover, the UHT metamorphism in UHP terranes is revealed to represent the lower pressure overprinting over early UHP assemblages during the rapid exhumation of ultra‐deep subducted continental slabs, in contrast to the cause of traditional UHT metamorphism by voluminous heat addition from the mantle.     

The stability and origin of sodicgedrite in ultrahigh-temperature Mg-Al granulites: a case study from the Gondwana suture in southern India

Mg-Al-rich rocks from the Palghat-Cauvery Shear Zone System (PCSZ) within the Gondwana suture zone in southern India contain sodicgedrite as one of the prograde to peak phases, stable during T = 900–990°C ultrahigh-temperature metamorphism. Gedrite in these samples is Mg-rich (Mg/[Fe + Mg] = X _Mg = 0.69–0.80) and shows wide variation in Na₂O content (1.4–2.3 wt.%, Na^A = 0.33–0.61 pfu). Gedrite adjacent to kyanite pseudomorph is in part mantled by garnet and cordierite. The gedrite proximal to garnet shows an increase in Na^A and Al^IV from the core (Na^A = 0.40–0.51 pfu, Al^IV = 1.6–1.9 pfu) to the rim (Na^A = 0.49–0.61 pfu, Al^IV = 2.0–2.2 pfu), suggesting the progress of the following dehydration reaction: Ged + Ky → Na-Ged + Grt + Crd + H₂O. This reaction suggests that, as the reactants broke down during the prograde stage, the remaining gedrite became enriched in Na to form sodicgedrite, which is regarded as a unique feature of high-grade rocks with Mg-Al-rich and K–Si-poor bulk chemistry. We carried out high-P-T experimental studies on natural sodicgedrite and the results indicate that gedrite and melt are stable phases at 12 kbar and 1,000°C. However, the product gedrite is Na-poor with only <0.13 wt.% Na₂O (Na^A = 0.015–0.034 pfu). In contrast, the matrix glass contains up to 8.5 wt.% Na₂O, suggesting that, with the progressive melting of the starting material, Na was partitioned into the melt rather than gedrite. The results therefore imply that the occurrence of sodicgedrite in the UHT rocks of the PCSZ is probably due to the low H₂O activity during peak P-T conditions that restricted extensive partial melting in these rocks, leaving Na partitioned into the solid phase (gedrite). The occurrence of abundant primary CO₂-rich fluid inclusions in this rock, which possibly infiltrated along the collisional suture during the final amalgamation of the Gondwana supercontinent, strengthens the inference of low water activity.     

Preservation of evidence for prograde metamorphism in ultrahigh-temperature, high-pressure kyanite-bearing granulites, South Harris, Scotland

Mineralogical and mineral chemical evidence for prograde metamorphism is rarely preserved in rocks that have reached ultrahigh‐temperature (UHT) conditions (>900 °C) because high diffusion and reaction rates erase evidence for earlier assemblages. The UHT, high‐pressure (HP) metasedimentary rocks of the Leverburgh belt of South Harris, Scotland, are unusual in that evidence for the prograde history is preserved, despite having reached temperatures of ～955 °C or more. Two lithologies from the belt are investigated here and quantitatively modelled in the system NaO–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O: a garnet‐kyanite‐K‐feldspar‐quartz gneiss (X_Mg = 37, A/AFM = 0.41), and an orthopyroxene‐garnet‐kyanite‐K‐feldspar quartzite (X_Mg = 89 A/AFM = 0.68). The garnet‐kyanite gneiss contains garnet porphyroblasts that grew on the prograde path, and captured inclusion assemblages of biotite, sillimanite, plagioclase and quartz (<790 °C, <9.5 kbar). These porphyroblasts preserve spectacular calcium zonation features with an early growth pattern overgrown by high‐Ca rims formed during high‐P metamorphism in the kyanite stability field. In contrast, Fe‐Mg zonation in the same garnet porphyroblasts reflects retrograde re‐equilibration, as a result of the relatively faster diffusivity of these ions. Peak P–T are constrained by the occurrence of coexisting orthopyroxene and aluminosilicate in the quartzite. Orthopyroxene porphyroblasts [y(opx) = 0.17–0.22] contain sillimanite inclusions, indicative of maximum conditions of 955 ± 45 °C at 10.0 ± 1.5 kbar. Subsequently, orthopyroxene, kyanite, K‐feldspar and quartz developed in equilibrated textures, constraining the maximum pressure conditions to 12.5 ± 0.8 kbar at 905 ± 25 °C. P–T–X modelling reveals that the mineral assemblage orthopyroxene‐kyanite‐quartz is compositionally restricted to rocks of X_Mg > 84, consistent with its very rare occurrence in nature. The preservation of unusual high P–T mineral assemblages and chemical disequilibrium features in these UHT HP rocks is attributed to a rapid tectonometamorphic cycle involving arc subduction and terminating in exhumation.     

A sapphirine-cordierite-garnet-sillimanite granulite from Enderby Land,Antarctica: implications for FMAS petrogenetic grids in the granulite facies

A quartz-absent magnesian paragneiss layer from Mount Sones, in the Archaean Napier complex of Enderby Land, Antarctica, contains the stable divariant FMAS assemblage sapphirine (X _Mg=78) — cordierite (X _Mg=87) — garnet (X _Mg=51) — sillimanite. Rare green spinel (X _Mg=53.5, ZnO=2.65wt%) occurs as inclusions mainly within sapphirine, but also within sillimanite and garnet. Late thin coronas of cordierite (X _Mg=90.5) mantle sapphirine in contact with extensively exsolved anorthoclase. The mineral textures are interpreted to indicate the former stability of a hypersthene-quartz absent assemblage followed by the development of the FMAS equilibrium assemblage sapphirine-cordierite-garnet-sillimanite (sp, hy, qz) and further divariant reaction involving the consumption of sapphirine. The (sp, hy, qz) assemblage uniquely defines the stable P-T reaction topology appropriate to granulites from the Napier Complex, as this paragenesis is allowed in the grids of Hensen (1971, 1986) but is not possible in other grids which assume the stability of a sapphirine-absent ([sa]) FMAS invariant point involving the phases spinel, garnet, hypersthene, cordierite, sillimanite and quartz. The observed mineral assemblages and textures are consistent with peak metamorphism between the [sp] and [hy] invariant points of Hensen (1971), at temperatures of 930–990° C, followed by cooling on a lower dP/dT trajectory towards the (sp, qz) univariant line. The initial spinel-bearing assemblage was stabilized by Zn and to a lesser extent by Ni and Cr, and hence does not require a marked decrease in temperature and increase in pressure to produce the (sp, hy, qz) assemblage. It is inferred that fO ₂ conditions substantially lower than those used in the experiments of Annersten and Seifert (1981) prevailed in the high-grade metamorphism in the Napier Complex.     

Newly discovered youngest UHT metamorphism on Earth,Western Sulawesi,Indonesia

Ultrahigh-temperature (UHT) metamorphism represents an extreme crustal thermal event with peak conditions exceeding 900 °C at 7–13 kbar. In the modern-style plate tectonic system, records of the UHT metamorphism are relatively rare due to the secular cooling of Earth. In the Palu region of Western Sulawesi, we newly discovered a series of HT-UHT metamorphic rocks including amphibolite, granulite, eclogites and gneiss. Of them, two granulite samples (18CS14-2, 18CS14-4) with high garnet content (>50 mol%) are chosen for petrographic observation, phase equilibrium modelling, and zircon U-Pb dating. These rocks are characterized by a relic M₁ assemblage of Grt + Ky + Bt + Rt and a M₂ assemblage of Grt + Sil + Pl + Spl + Crd ± Qtz + Ilm + melt. Phase equilibrium modelling based on effective bulk compositions yields UHT conditions of 7.2–8.5 kbar/940–1080 °C (18CS14-2) and 7.0–7.3 kbar/1000–1040 °C (18CS14-4). U-Pb analysis reveals two generations of metamorphic zircon with evolving REE content that is intimately related to garnet growth and decomposition. Zircon age of 36–5.3 Ma is ascribed to syn- to post-M₁ metamorphism, whereas the young zircon age of 5.1–3.8 Ma is linked to syn- and post-M₂ stage. The UHT metamorphism was probably the consequence of the upwelling of asthenospheric mantle triggered by post-collisional delamination of lithosphere in the Miocene-Pliocene (ca. 5 Ma). It could represent the youngest known UHT metamorphism on Earth.     

Anatomy of a Cambrian suture in Gondwana: Pacific-type orogeny in southern India?

Southern India occupies a central position in the Late Neoproterozoic–Cambrian Gondwana supercontinent assembly. The Proterozoic mosaic of southern India comprises a collage of crustal blocks dissected by Late Neoproterozoic–Cambrian crust-scale shear/suture zones. Among these, the Palghat–Cauvery Suture Zone (PCSZ) has been identified as the trace of the Cambrian suture representing Mozambique Ocean closure during the final phase of amalgamation of the Gondwana supercontinent. Here we propose a model involving Pacific-type orogeny to explain the Neoproterozoic evolution of southern India and its final amalgamation within the Gondwana assembly. Our model envisages an early rifting stage which gave birth to the Mozambique Ocean, followed by the initiation of southward subduction of the oceanic plate beneath a thick tectosphere-bearing Archean Dharwar Craton. Slices of the ocean floor carrying dunite–pyroxenite–gabbro sequence intruded by mafic dykes representing a probable ophiolite suite and invaded by plagiogranite are exposed at Manamedu along the southern part the PCSZ. Evidence for the southward subduction and subsequent northward extrusion are preserved in the PCSZ where the orogenic core carries high-pressure and ultrahigh-temperature metamorphic assemblages with ages corresponding to the Cambrian collisional orogeny. Typical eclogites facies rocks with garnet + omphacite + quartz and diagnostic ultrahigh-temperature assemblages with sapphirine + quartz, spinel + quartz and high alumina orthopyroxene + sillimanite + quartz indicate extreme metamorphism during the subduction–collision process. Eclogites and UHT granulites in the orogenic core define P–T maxima of 1000 °C and up to 20 kbar. The close association of eclogites with ultramafic rocks having abyssal signatures together with linear belts of iron formation and metachert in several localities within the PCSZ probably represents subduction–accretion setting. Fragments of the mantle wedge were brought up through extrusion tectonics within the orogenic core, which now occur as suprasubduction zone/arc assemblages including chromitites, highly depleted dunites, and pyroxene bearing ultramafic assemblages around Salem. Extensive CO₂ metasomatism of the ultramafic units generated magnesite deposits such as those around Salem. High temperature ocean floor hydrothermal alteration is also indicated by the occurrence of diopsidite dykes with calcite veining. Thermal metamorphism from the top resulted in the dehydration of the passive margin sediments trapped beneath the orogenic core, releasing copious hydrous fluids which moved upward and caused widespread hydration, as commonly preserved in the Barrovian amphibolite facies units in the PCSZ. The crustal flower structure mapped from PCSZ supports the extrusion model, and the large scale north verging thrusts towards the north of the orogenic core may represent a fold-thrust belt. Towards the south of the PCSZ is the Madurai Block where evidence for extensive magmatism occurs, represented by a number of granitic plutons and igneous charnockite massifs of possible tonalite–trondhjemite–granodiorite (TTG) setting, with ages ranging from ca. 750–560 Ma suggesting a long-lived Neoproterozoic magmatic arc within a > 200 km wide belt. All these magmatic units were subsequently metamorphosed, when the Pacific-type orogeny switched over to collision-type in the Cambrian during the final phase of assembly of the Gondwana supercontinent. One of the most notable aspects is the occurrence of arc magmatic rocks together with high P/T rocks, representing the deeply eroded zone of subduction. The juxtaposition of these contrasting rock units may suggest the root of an evolved Andean-type margin, as in many arc environments the roots of the arc comprise ultramafic/mafic cumulates and the felsic rocks represent the core of the arc. The final phase of the orogeny witnessed the closure of an extensive ocean — the Mozambique Ocean — and the collisional assembly of continental fragments within the Gondwana supercontinent amalgam. The tectonic history of southern India represents a progressive sequence from Pacific-type to collision-type orogeny which finally gave rise to a Himalayan-type Cambrian orogen with characteristic magmatic, metasomatic and metamorphic factories operating in subduction–collision setting.     

Low-density CO2-rich fluid inclusions from charnockites of southwestern Madurai Granulite Block, southern India; implications on graphite mineralization

Characterization of fluid inclusions in graphite-bearing charnockites from the southwestern part of the Madurai Granulite Block in southern India reveals a probable relation with the formation and break down of graphite during the high-grade metamorphism. The first-generation monophase pure CO₂ inclusions, the composition of which is confirmed by laser Raman spectroscopy, recorded moderate density (0.77–0.87 g/cc) corresponding to low tapping pressure (around 2 kb) than that of the peak granulite-facies metamorphism. The precipitation of graphite, as inferred from graphite inclusions and δ¹³C values of the graphite from the outcrops, is interpreted as the cause of this lowering of fluid density. An intermediate generation of pseudosecondary inclusions resulted from the re-equilibration or modification of the first-generation fluids and the CO₂ formed is interpreted to be the oxidation product from graphite. The youngest generation of fluids which caused widespread retrogression of the granulites is a low-temperature (350 °C) high-saline (32.4–52.0 wt% NaCl equivalent) brine. Carbon isotope data on the graphite from the charnockites show δ¹³C values ranging from −11.3 to −19.9‰, suggesting a possibility of mixing of carbon sources, relating to earlier biogenic and later CO₂ fluid influx. Combining the information gathered from petrologic, fluid inclusion and carbon stable isotope data, we model the fluid evolution in the massive charnockites of the southwestern Madurai Granulite Block.     

Charnockite genesis across the Archaean–Proterozoic terrane boundary in the South Indian Granulite Terrain: Constraints from major–trace element geochemistry and Sr–Nd isotopic systematics

Major, trace element compositions and Sr–Nd isotopic characteristics of charnockitic gneisses from the Southern Granulite Terrain (SGT), South India are presented. The study region encompasses the central segment of the Cauvery Shear Zone system (CSZ) and regions within the Madurai Block (MB) immediately south of it (designated here as the CSZ/MB and MB domains). Differences in the compositions and source characteristics between charnockitic rocks of the CSZ vis-à-vis those of the CSZ/MB and MB regions are highlighted. Foremost, the charnockites and enderbites of the CSZ show highly fractionated REE patterns with positive Eu-anomalies, depleted HREE, Y and near chondritic εNd₀ and initial-⁸⁷Sr/⁸⁶Sr at ca. 2.5 Ga, consistent with hydrous partial melting of amphibolitic crust with residual garnet and hornblende for the parental melts. By contrast, modeled at ca. 1.8 Ga and 0.8 Ga, the CSZ/MB and MB charnockitic rocks, which show a wider range of Ti and P, relatively lower degree of HREE depletion, commonly negative Eu-anomalies and undepleted Y, present clear evidence for involvement of Archaean crustal components in sources of their magmatic protoliths. There is also evidence for significant intracrustal melting processes within a thickened crust at elevated temperatures between 800 and 1000 °C. Implications to the controversial Archaean–Neoproterozoic terrane boundary problem of the SGT are discussed.     

Isothermal decompression history in the “Western Granulite” terrain, central Tanzania: Evidence from reaction textures and trapped fluids in metapelites

The Mozambique Belt (MB) of the East Africa Orogen contains large areas of granulite-facies migmatitic gneisses with Archaean and Palaeoproterozoic protolith ages and that were recycled during the Neoproterozoic Pan-African orogeny. The study area is situated along the Great Ruaha River and within the Mikumi National Park in central Tanzania where migmatitic gneisses and mafic to intermediate granulites are interlayered with Neoproterozoic granulite-facies migmatitic metapelites. Mineral textures suggest isothermal decompression, with the peak mineral assemblage comprising Grt–Bt–Ky–Kfs–Pl–Qtz ± Phn ± Ti-Oxide ± melt and amphibolite-facies retrograde assemblage Grt–Bt–Sil–Ms–Kfs–Pl–Qtz ± Fe–Ti-Oxide. The near isothermal retrograde overprint is seen in well-developed formation of pseudomorphs after garnet. The HP granulite-facies assemblages record P–T conditions of 13–14 kbar at 760–800 °C. Retrogression and the release of fluids from crystallizing melts occurred at 7 kbar and 650–700 °C. A fluid inclusion study shows three types of fluid inclusion consisting of nearly pure CO₂, as well as H₂O–NaCl and H₂O–CO₂ mixtures. We suggest that a immiscible CO₂-bearing brine represents the fluid composition during high-grade peak metamorphism, and that the fluid inclusions containing H₂O–NaCl or nearly pure CO₂ represent trapped fluids from in situ crystallised melt. The results suggest strong isothermal decompression, which is probably related to a fast exhumation after crustal thickening in the central part of the Mozambique Belt in Tanzania.     

Regional-scale Grenvillian-age UHT metamorphism in the Mollendo–Camana block (basement of the Peruvian Andes)

The Mollendo–Camana Block (MCB) is a 50 × 150 km Precambrian inlier of the Andean belt that outcrops along the Pacific coast of southern Peru. It consists of stromatic migmatites of Paleoproterozoic heritage intensely metamorphosed during the Grenville event (c. 1 Ga; U‐Pb and U‐Th‐Pb ages on zircon and monazite). In the migmatites, aluminous mesosomes (FMAS) and quartzofeldspathic leucosomes (KFMASH), contain various amounts of K‐feldspar (Kfs), orthopyroxene (X_Mg Opx = 0.86), plagioclase (Pl), sillimanite (Sil; exceptionally kyanite, Ky) ilmenite (Ilm), magnetite (Mag), quartz (Qtz), and minor amounts of garnet (X_Mg Grt = 0.60), sapphirine (X_Mg Spr = 0.87), cordierite (X_Mg Crd = 0.92) and biotite (X_Mg Bt = 0.83). The ubiquitous peak mineral assemblage is Opx‐Sil‐Kfs‐Qtz‐(± Grt) in most of the MCB, which, together with the high Al content of orthopyroxene (10% Al₂O₃) and the local coexistence of sapphirine‐quartz, attest to regional UHT metamorphism (> 900 °C) at pressures in excess of 1.0 GPa. Fluid‐absent melting of biotite is responsible for the massive production of orthopyroxene that proceeded until exhaustion of biotite (and most of the garnet) in the southern part of the MCB (Mollendo‐Cocachacra areas). In this area, a first stage of decompression from 1.1–1.2 to 0.8–0.9 GPa at temperatures in excess of 950 °C, is marked by the breakdown of Sil‐Opx to Spr‐Opx‐Crd assemblages according to several bivariant FMAS reactions. High‐T decompression is also shown by Mg‐rich garnet being replaced by Crd‐Spr‐ and Crd‐Opx‐bearing symplectites, and reacting with quartz to produce low‐Al‐Opx‐Sil symplectites in quartz‐rich migmatites. Neither osumilite nor spinel‐quartz assemblages being formed, isobaric cooling at about 0.9 GPa probably followed the initial decompression and proceeded with massive precipitation of melts towards the (Os) invariant point, as demonstrated by Bt‐Qtz‐(± pl) symplectites in quartz‐rich migmatites (melt + Opx + Sil = Bt + Grt + Kfs + Qtz). Finally, Opx rims around secondary biotite attest to late fluid‐absent melting, compatible with a second stage of decompression below 900 °C. The two stages of decompression are interpreted as due to rapid tectonic denudation whereas the regional extent of UHT metamorphism in the area, probably results from large‐scale penetration of hot asthenospheric mantle at the base of an over‐thickened crust.     

Triassic paragonite- and garnet-bearing epidote-amphibolite from the Hida Mountains, Japan

Paragonite- and garnet-bearing high-grade epidote-amphibolite (PGEA) in the Ise area of the Hida Mountains, Japan is characterized by the high-pressure (HP) epidote-amphibolite facies parageneses (M₁), garnet + hornblende + clinozoisite + paragonite + quartz + rutile. Paragonite and garnet of the peak M₁ stage are locally replaced by retrograde albite (+ oligoclase) and chlorite (M₂), respectively. Phase equilibria constrain peak metamorphic conditions of P = 1.1–1.4 GPa and T = 530–570 °C, and a decompressional P–T path for this rock. Mineral parageneses of prograde epidote-amphibolite facies are comparable to some HP rocks from the Hongan region of western Dabie, but differ from other HP mafic schists with cooling ages of c. 330 Ma in the Hida Mountains. New paragonite K–Ar dating for the PGEA yields a Triassic cooling event at 210 Ma that is coeval with regional cooling and exhumation of the Sulu–Dabie–Qinling (SDQ) belt. Both petrological and geochronological data of the Triassic HP epidote-amphibolite in Hida Mountains support our earlier hypothesis that the SDQ belt extends across the Korean Peninsula to SW Japan.     

Crustal architecture beneath Madurai Block,southern India deduced from magnetotelluric studies: Implications for subduction–accretion tectonics associated with Gondwana assembly

The Madurai Block in southern India is considered to represent the eroded roots of an arc-accretionary complex that developed during the subduction–collision tectonics associated with the closure of the Mozambique Ocean and final suturing of the crustal fragments within the Gondwana supercontinent in the Late Neoproterozoic–Cambrian. Here we present a magnetotelluric (MT) model covering the main collisional suture (Palghat–Cauvery Suture Zone) in the north into the central part of the Madurai Block in the south comprising data from 11 stations. Together with a synthesis of the available seismic reflection data along a N–S transect further south within the Madurai Block, we evaluate the crustal architecture and its implications on the tectonic development of this region. According to our model, the predominantly south dipping seismic reflectors beneath the Madurai Block define a prominent south-dipping lithological layering with northward vergence resembling a thrust sequence. We interpret these stacked layers as imbricate structures or mega duplexes developed during subduction–accretion tectonics. The layered nature and stacking of contrasting velocity domains as imaged from the seismic profile, and the presence of thick (>20 km) low resistivity layers ‘floating’ within high resistivity domains as seen from MT model, suggest the subduction of a moderately thick oceanic crust. We identify several low resistivity domains beneath the Madurai Block from the MT model which probably represent eclogitised remnants of oceanic lithosphere. Their metamorphosed and exhumed equivalents in association with ultrahigh-temperature metamorphic orogens have been identified from surface geological studies. Both seismic reflections and MT model confirm a southward subduction polarity with a progressive accretion history during the northward migration of the trench prior to the final collisional assembly of the crustal blocks along the Palghat–Cauvery Suture Zone, the trace of the Gondwana suture in southern India.     

Analysis of airborne magnetic and gravity anomalies of peninsular shield, India integrated with seismic and magnetotelluric results and gravity anomalies of Madagascar, Sri Lanka and East Antarctica

Modelling of gravity and airborne magnetic data integrated with seismic studies suggest that the linear gravity and magnetic anomalies associated with Moyar Bhavani Shear Zone (MBSZ) and Palghat Cauvery Shear Zone (PCSZ) are caused by high density and high susceptibility rocks in upper crust which may represent mafic lower crustal rocks. This along with thick crust (44–45 km) under the Southern Granulite Terrain (SGT) indicates collision of Dharwar craton towards north and SGT towards south with N–S directed compression during 2.6–2.5 Ga. This collision may be related to contemporary collision northwards between Eastern Madagascar–Western Dharwar Craton (WDC) and Eastern Dharwar Craton (EDC). Arcuate shaped N and S-verging thrusts, MBSZ-Mettur Shear and PCSZ-Gangavalli Shear, respectively across Cauvery Shear zone system (CSZ) in SGT also suggest that the WDC, EDC and SGT might have collided almost simultaneously during 2.6–2.5 Ga due to NW–SE directed compressional forces with CSZ as central core complex in plate tectonics paradigm preserving rocks of oceanic affinity. Gravity anomalies of schist belts of WDC suggest marginal and intra arc basin setting.The gravity highs of EGFB along east coast of India and regional gravity low over East Antarctica are attributed to thrusted high-density lower crustal/upper mantle rocks at a depth of 5–6 km along W-verging thrust, which is supported by high seismic velocity and crustal thickening, respectively. It may represent a collision zone at about 1.0 Ga between India and East Antarctica. Paired gravity anomalies in the central part of Sri Lanka related to high density intrusives under western margin of Highland Complex and crustal thickening (40 km) along eastern margin of Highland Complex with several arc type magmatic rocks of about 1.0 Ga in Vijayan Complex towards the east may represent collision between them with W-verging thrust as in case of EGFB. The gravity high of Sri Lanka in the central part falls in line with that of EGFB, in case it is fitted in Gulf of Mannar and may represent the extension of this orogeny in Sri Lanka.     

Contrasting parageneses in the manganese silicate-carbonate rocks from Parseoni,Sausar Group,India and their interpretation

Mn silicate-carbonate rocks at Parseoni occur as conformable lenses within metapelites and calc-silicate rocks of the Precambrian Sausar Group, India. The host rocks are estimated to have been metamorphosed at uppermost P-T conditions of 500–550°C and 3–4 kbar. The Mn-rich rocks contain appreciable Fe, reflected in the occurrence of magnetite₍₁₎ (MnO 1%), magnetite₍₂₎ (MnO 15%) and magnetite₍₃₎ (MnO 10%). Two contrasting associations of pyroxmangite, with and without tephroite, developed in the Mn silicate-carbonate rocks under isothermal-isobaric conditions. The former assemblage formed in relatively Fe-rich bulk compositions and equilibrated with a metamorphic fluid having a low X _{CO 2} (<0.2), and the latter equilibrated with a CO₂-rich fluid. Rhodochrosite+magnetite₍₁₎+quartz protoliths produced the observed mineral assemblages on metamorphism. Partitioning of major elements between coexisting phases is somewhat variable. Fe shows preference for tephroite over pyroxmangite at the ambient physical conditions of metamorphism. Oxygen fugacity during metamorphism was monitored at or near the QFM buffer in tephroite bearing domains, and the fluid composition was buffered by mineral reactions in respective domains. As compared to other metamorphosed Mn deposits of the Sausar Group, the Mn silicate-carbonate rocks at Parseoni were, therefore, metamorphosed at much lower f _{O 2} through complex mineral-fluid interactions.     

Relative geobarometry from high-pressure rocks of quartzofeldspathic composition from the Sesia Zone,Western Alps,Italy

In the Sesia Zone of the Western Alps, Italy, early Alpine blueschist to eclogite facies metamorphism of rocks of quartzofeldspathic composition has produced the same high-pressure assemblage of; quartz, Na-pyroxene, Na-amphibole, paragonite, phengite, zoisite, garnet, magnetite, sphene and Fe-sulphide (=the QFS assemblage) over an area (> 150 km². Relative gradients in pressure and temperature over this region are reflected in the variations in mineral chemistries of the individual phases of the quartzofeldspathic assemblage through continuous reactions. Mineralogical discontinuities do not occur in the QFS assemblage of this region. Increases in the Jadeite content of the pyroxenes (X _{Jd 0.48} to X _{Jd 0.93}) and in the glaucophane content of the amphiboles (X _{Gl 0.89} to X _{Gl 0.96}) occur from the southwest to the northeast of the region studied. Analysis of coexisting garnets and pyroxenes indicate that the compositional variation of amphiboles and pyroxenes is associated with a decrease in the grossular component of the coexisting garnet. Zoned pyroxenes and garnets, together with the regional trends in mineral chemistries suggest that the evolution of the QFS assemblage with increasing pressure may be modelled by pressure-sensitive continuous reactions in which amphibole, zoisite and the more jadeitic pyroxene constitute the high-pressure assemblage. Chemographic constraints permit the positioning in pressure/temperature space of the compositional isopleths of those model continuous reactions involving these phases which meet the textural and chemical criteria observed in the natural assemblages. The low dP/dT slope (–20 bars/° C) of these isopleths causes the continuous reactions to be useful for geobarometric calculations at pressures above the absolute breakdown of albite to jadeite plus quartz. In addition the pseudobinary loops for the other continuous reactions which are potentially useful geobarometers and involve either the NaAlCa_–1Mg_–1 exchange or the MgCa_–1 exchange are calculated. Comparison of mineral chemistries with the isopleths yields a relative barometric scheme for the localities studied. With these barometric observations, it is possible to show that the P-T path which the Sesia body travelled towards the final recorded state was one of increasing pressure. Other blueschist and eclogite occurrences from Syros and Sifnos which contain rocks of quartzofeldspathic composition are also examined.     

Retrogressive hydration of calc-silicate xenoliths in the eastern Bushveld complex: evidence for late magmatic fluid movement

Two calc-silicate xenoliths in the Upper Zone of the Bushveld complex contain mineral assemblages which permit delineation of the metamorphic path followed after incorporation of the xenoliths into the magma. Peak metamorphism in these xenoliths occurred at T=1100–1200°C and P <1.5 kbar. Retrograde metamorphism, probably coinciding with the late magmatic stage, is characterized by the breakdown of akermanite to monticellite and wollastonite at 700°C and the growth of vesuvianite from melilite. The latter implies that water-rich fluids (X_CO2 <0.2) were present and probably circulating through the cooling magmatic pile. In contrast, calc-silicate xenoliths within the lower zones of the Bushveld complex, namely in the Marginal and Critical Zones, also contain melilite, monticellite and additional periclase with only rare development of vesuvianite. This suggests that the Upper Zone cumulate pile was much ‘wetter’ in the late-magmatic stage than the earlier-formed Critical and Marginal Zone cumulate piles.