Petrogenetic Grid Research Articles

Competing Roles of Evolving P–T Conditions, Equilibration Volume and Chemical Potential Landscape in the Formation of Corona Texture: a Case Study from the Southern Granulite Terrane, India

Abstract Corona texture is defined by the development of partial or complete rim of one or more minerals around a central reactant mineral depicting limited mass transport (local equilibration) in the length scale of the coronitic layers. The mineral assemblages forming corona texture in a metamorphic rock are commonly used to trace the P-T-X conditions through which the rock evolved during various tectonic processes. However, without a proper assessment of the changes in the equilibration volume (EV) and its effect on the mineralogy, any petrological interpretation deduced from the coronitic texture may be incorrect. In this study, we demonstrate that the double corona texture, observed in a suite of Mg-Al rich ortho-amphibole cordierite-bearing rock from the Cauvery Shear System (Southern Granulite Terrane, India), developed in response to the continuously evolving EV. The studied rock contains aluminosilicate porphyroblasts that are set in a matrix of ortho-amphibole ± quartz. The aluminosilicate porphyroblasts are rimmed successively by an inner symplectic corona of sapphirine + cordierite and an outer mono-mineralic corona of cordierite. Locally, patches of corundum with a rind of cordierite grow preferentially along the interface of aluminosilicate and the inner symplectic corona. Based on detailed petrography and mineral composition analyses, the corona textures are interpreted to have formed through a sequence of different chemical reactions that occurred in local micro-domains. We calculated quantitative P-T pseudosection in a NCFMASHT (Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O-TiO2) system and activity-adjusted P-T petrogenetic grid in a MASH (MgO-Al2O3-SiO2-H2O) system which, together, suggest that the coronitic assemblages were formed in response to a steeply decompressive retrograde P-T path from >8.8 kbar to <6 kbar, at a nearly constant temperature of ~700°C. Changes in EV in response to the limited transport of chemical components during the formation of corona texture were investigated through isothermal P-μMgO, P-μSiO2 and P-μMgO-μSiO2 MASH diagrams. Our results quantitatively model the continuously changing chemical potential landscape (P-μMgO- μSiO2 evolution path) around the central aluminosilicate porphyroblast within the corona-bearing micro-domain. The path demonstrates that a gradually shrinking EV around the central aluminosilicate during retrogression led to the sequential change of mineral reactions and equilibrium mineral assemblages and resulted in the formation of multiple coronae. Unavailability of fluids and/or rapid exhumation is considered as the most dominant factors responsible for the decreasing elemental mobility and the consequent shrinking in EV in the studied rock.

Reply to Skublov et al. Comment on “Volodichev et al. Archean Zircons with Omphacite Inclusions from Eclogites of the Belomorian Province, Fennoscandian Shield: The First Finding. Minerals 2021, 11, 1029”

Early Precambrian retrogressed eclogites are abundant in the Archean Belomorian Province of the Fennoscandian Shield. Archean zircons with inclusions of omphacite have been found in these eclogites. Similar Archean zircons from the retrogressed eclogites also contain garnet inclusions. The Archean zircons display no negative Eu anomaly, which indicates their crystallization in plagioclase-free rock. Garnet, omphacite and clinopyroxene-plagioclase symplectite as a proxy of omphacite compose ≥75% of the studied rocks, with garnet and omphacite being major constituents and associating with rutile and quartz. These data strongly suggest that the studied rock is eclogite. In the majority of petrogenetic grids, P-T parameters calculated for these rocks fall in the eclogite-facies field. Thus, these findings and data provide evidence that eclogite-facies metamorphism occurred in the Neoarchean.

Paleoproterozoic emplacement and Cambrian ultrahigh-temperature metamorphism of a layered magmatic intrusion from the Central Madurai Block, southern India: From Columbia to Gondwana

The Madurai Block in the Southern Granulite Terrane (SGT) of Peninsular India is one of the largest crustal blocks within the Neoproterozoic Gondwana assembly. This block is composed of three sub-blocks: the Neoarchean Northern Madurai block, Paleoproterozoic Central Madurai block and the dominantly Neoproterozoic Southern Madurai Block. The margins of these blocks are well-known for the occurrence of ultrahigh-temperature (UHT) granulite facies rocks mostly represented by Mg-Al metasediments. Here we report a dismembered layered mafic–ultramafic intrusion occurring in association with Mg-Al granulites from the classic locality of Ganguvarpatti in the Central Madurai Block. The major rock types of the layered intrusion include spinel orthopyroxenite, garnet-bearing gabbro, gabbro and gabbroic anorthosite showing rhythmic stratification and cumulate texture. The orthopyroxene-cordierite granulite from the associated Mg-Al layer is composed of spinel, cordierite and orthopyroxene. The pyroxene in both rock units is high-Al orthopyroxene formed under UHT metamorphic conditions. Conventional thermobarometry yields near-peak metamorphic conditions of 9.5–10 kbar pressure and a minimum temperature of 980 °C. We computed P–T pseudosections and contoured for the compositional as well as modal isopleths of the major mineral phases, which yield temperature above 1000 °C. FMAS petrogenetic grid, Al-in-orthopyroxene isopleth, conventional thermobarometry and calculated pseudosection reveal a clockwise pressure–temperature (P–T) path and near isothermal decompression. The U–Pb data on zircon grains from the layered magmatic suite indicate emplacement of the protolith at ca. 2.0 Ga and the metamorphic overgrowths yield weighted 206Pb/238U mean ages ca. 520 Ma. Monazite from the garnet-bearing gabbro and Opx-Crd granulite yielded 206Pb/238U weighted mean ages of ca. 532 Ma and 523 Ma marking the timing of metamorphism. We correlate the layered intrusion to a Paleoproterozoic suprasubduction zone setting, defining the Ganguvarpatti area as part of a collisional suture assembling the Northern and Central Madurai Blocks. The Paleoproterozoic magmatism and late Neoproterozoic-Cambrian UHT metamorphism can be linked to the tectonics of the Columbia and Gondwana supercontinents.

Academic Reminiscences and Thermodynamics-Kinetics of Thermo-Barometry-Chronology

This article has three major components that include, in addition to the technical aspects, reminiscences of my academic upbringing, my move to the USA from India, and my professional career. I have recounted many stories that I hope convey some sense of time, especially in these two countries with vastly different cultures, my personal journey with its ups and downs and how I made the transition to an academic career path in USA even though that was not in my future plan as a young man. The development of the field of thermobarometry and its integration with diffusion and crystal kinetic modelling of compositional zoning (or lack thereof) and cation ordering in minerals have led to important quantitative constraints on the pressure-temperature-time evolution of terrestrial rocks and meteorites. I review the historical developments in these areas and a segment of my own research spanning the period of 1964-2021. The foundational works of the thermometry of metamorphic rocks and palaeothermometry were laid at the University of Chicago around 1950. Subsequently, the synergetic growth of thermodynamics and experimental studies in petrology in the 1960s and 1970s, along with the introduction of electron microprobe as a nondestructive analytical tool with micron scale resolution, gave a major boost to the field of thermobarometry. There were also significant new developments in the field of thermodynamics of solid solutions in the petrology community and demonstration from observational data, countering strong scepticism, that the principles of classical thermodynamics were applicable to “complex natural systems”. The section on thermodynamic basis of thermobarometry concludes with a discussion of the thermodynamics of trace element and single mineral thermometry. I further deal with the experimental protocols, along with selected examples, for phase equilibrium studies that provide the bedrock foundation for the field of thermobarometry based on elemental compositions of coexisting minerals in a rock. It is followed by an account of the controversies and international meetings relating to the aluminum silicate and peridotite phase diagrams that play crucial roles in the thermobarometry of metamorphic rocks and mantle xenoliths, respectively. The construction of quantitative petrogenetic grids to display stability relations of minerals in multicomponent–multiphase systems came into play in the field of metamorphic petrology in the mid-1960s and early 1970s. Augmented by experimental data, these petrogenetic grids led to important discoveries about the P-T-<em>f</em>(O<sub>2</sub>) and bulk compositional controls on the stability of certain “index” minerals that are used to define metamorphic isograds and different types of regional metamorphism; one such grid also opened up a new field that came to be known as ultra-high temperature metamorphism. The construction of petrogenetic grids has now evolved to computer based calculations of complex equilibrium P-T phase diagrams, commonly referred to as “pseudosections”, by minimisation of Gibbs free energy of a system with fixed bulk composition. I discuss these historical developments and modern advancements. Subsequently I highlight some aspects of thermobarometry and diffusion kinetic modelling of selected natural samples along with their broader implications and present a critical discussion of different protocols for thermobarometry of natural assemblages. Following up on the introductory historical perspective of development of palaeothermometry, I discuss the modern advancements using density functional theory (DFT). Examples of DFT based calculations have been shown for hydrogen isotope fractionation in mineral-water/hydrogen systems and “clumped isotope” thermometry. The hydrogen isotope fractionation data led the development of new low temperature palaeothermometers using serpentine-talc/brucite mineral pairs. These results enable simultaneous solutions of both temperature and source of fluid in the serpentinisation process of rocks. The final section is devoted to high temperature thermochronology dealing with the problems of closure temperature of decay systems in minerals and the use of bulk and spatial resetting of mineral age according to a specific decay system to determine cooling rates of the host rocks. Complications arise in the interpretation of mineral ages determined by such decay systems as <sup>176</sup>Lu-<sup>176</sup>Hf or the short- lived system <sup>53</sup>Mn-<sup>53</sup>Cr in which the parent nuclide has a much lower closure temperature than the corresponding daughter product. Numerical simulations help explain the discrepancy between the <sup>176</sup>Lu-<sup>176</sup>Hf and <sup>147</sup>Sm-<sup>143</sup>Nd ages of garnets in metamorphic rocks and enable construction of the entire T-t cycle from the discrepant ages and some additional constraints.

Mineral equilibrium modelling and calculated chemical potential relations of reaction textures in the ultrahigh‐temperature In Ouzzal terrane (In Hihaou area, Western Hoggar, Algeria)

AbstractMg–Al‐rich orthopyroxene–sillimanite‐bearing granulites from In Hihaou, In Ouzzal terrane, NW Hoggar were previously determined to have experienced ultrahigh‐temperature metamorphism at 1,050°C and 10 kbar. Multilayer corona and symplectite reaction textures that pervasively replace peak metamorphic porphyroblasts were interpreted to have formed during near‐isothermal decompression. Pseudosection calculations using new and realistic activity–composition models give estimates of 900 ± 20°C and 8 ± 0.5 kbar for the reaction texture assemblage. Calculated chemical potential relations show that reaction textures formed due to very minor decompression from peak conditions, when the removal of intracrystalline melt led to the development of local equilibrium domains around garnet and sillimanite porphyroblasts. The growth of initial monomineralic coronas further isolated the porphyroblasts from the matrix, leading to the later development of the symplectites. These results indicate that the derivation and interpretation of UHT conditions using the occurrence of diagnostic assemblages on classic petrogenetic grids can overestimate temperature and/or pressure conditions. Similarly, the interpretation of reaction textures to derive retrograde conditions and P–T paths without considering their spatial development is unlikely to be reliable, as textures do not typically form in this way or routinely record this information.

Evolution of metamorphic fluid recorded in granulite facies metacarbonate rocks from the middle segment of the Mogok metamorphic belt in central Myanmar

AbstractThe Mogok metamorphic belt of Palaeogene age, which records subduction‐ and collision‐related events between the Indian and Eurasian plates, lies along the western margin of the Shan plateau in central Myanmar and continues northwards to the eastern Himalayan syntaxis. Reaction textures of clinohumite‐ and scapolite‐bearing assemblages in Mogok granulite facies metacarbonate rocks provide insights into the drastic change in fluid composition during exhumation of the collision zone. Characteristic high‐grade assemblages of marble and calcsilicate rock are clinohumite+forsterite+spinel+phlogopite+pargasite/edenite+calcite+dolomite, and scapolite+diopside+anorthite+quartz+calcite respectively. Calculated petrogenetic grids in CaO–MgO–Al2O3–SiO2–H2O–CO2 and subsets of this system were employed to deduce the pressure–temperature–fluid evolution of the clinohumite‐ and scapolite‐bearing assemblages. These assemblages suggest higher temperature (>780–810°C) and [=CO2/(CO2+H2O) >0.17–0.60] values in the metamorphic fluid for the peak granulite facies stage, assuming a pressure of 0.8 GPa. Calcite grains commonly show exsolution textures with dolomite particles, and their reintegrated compositions yield temperatures of 720–880°C. Retrograde reactions are mainly characterized by a reaction zone consisting of a dolomite layer and a symplectitic aggregate of tremolite and dolomite grown between clinohumite and calcite in marble, and a replacement texture of scapolite by clinozoisite in calcsilicate rock. These textures indicate that the retrograde reactions developed under lower temperature (<620°C) and (<0.08–0.16) conditions, assuming a pressure of 0.5 GPa. The metacarbonate rocks share metamorphic temperatures similar to the Mogok paragneiss at the peak granulite facies stage. The values of the metacarbonate rock at peak metamorphic stage are, however, distinctly higher than those previously deduced from carbonate mineral‐free paragneiss. Primary clinohumite, phlogopite and pargasite/edenite in marble have F‐rich compositions, and scapolite in calcsilicate rock contains Cl, suggesting a contrast in the halogen compositions of the metamorphic fluids between these two lithologies. The metamorphic fluid compositions were probably buffered within each lithology, and the effective migration of metamorphic fluid, which would have extensively changed the fluid compositions, did not occur during the prograde granulite facies stage throughout the Mogok metamorphic belt. The lower conditions of the Mogok metacarbonate rocks during the retrograde stage distinctly contrast with higher conditions recorded in metacarbonate rocks from other metamorphic belts of granulite facies. The characteristic low conditions were probably due to far‐ranging infiltration of H2O‐dominant fluid throughout the middle segment of the Mogok metamorphic belt under low‐amphibolite facies conditions during the exhumation and hydration stage.

Dissolving dolomite in a stable UHP mineral assemblage: Evidence from Cal-Dol marbles of the Dora-Maira Massif (Italian Western Alps)

In deep and cold subduction such as that experienced by the UHP Units of the Western Alps, carbon dissolution is a relevant mechanism for carbon transfer from the slab into the mantle. The UHP impure Cal-Dol-marbles from the Dora-Maira Massif are studied to investigate the poorly known evolution of dolomite during deep subduction. Dolomite shows four stages of growth, from pre-Alpine to early-retrograde Alpine, coupled with chemical variations and distinct included mineral assemblages. To explain the evidence for growth and partial reabsorption of dolomite through HP prograde, UHP peak, and UHP early-retrograde Alpine metamorphism, a chemically simple marble (Cal, Dol, Di, Fo, and retrograde Atg, Tr, Mg-Chl) has been studied in detail. Microstructural relationships, coupled with mineral chemistry, indicate the growth of the assemblage dolomite+diopside+forsterite±aragonite during HP prograde, UHP peak, and UHP early-retrograde evolution. Mixed-volatile P-T projection modeled in the simple CaO-(FeO)-MgO-SiO 2 -H 2 O-CO 2 system and T - P - X CO 2 petrogenetic grids and pseudosections predict the prograde (1.7 GPa, 560 °C) growth of dolomite in equilibrium with diopside and forsterite through the breakdown of antigorite+aragonite. In a H 2 O-CO 2 -saturated system, the subsequent HP-UHP evolution is predicted in the Di+Fo+Dol+Arg stability field in equilibrium with a dominantly aqueous COH fluid [ 0 . 0003 X CO 2 0 . 0008 ], whose composition is internally buffered by the equilibrium assemblage. Thermodynamic modeling indicates that neither the consumption nor the growth of new dolomite generations at UHP conditions can have been induced by metamorphic reactions. The abundant primary H 2 O+Cal+Dol+Cl-rich Tr+Cl-rich Tlc±chloride fluid inclusions present in UHP Cpx indicate that a dominantly aqueous, saline (salinity >26.3 wt% of NaCl eq ) COH fluid, containing Ca, Mg, and Si as dissolved cations was present during the growth of the UHP assemblage Dol+Cpx+Ol+Arg. The complex zoning of dolomite is therefore interpreted as due to protracted episodes of dissolution and precipitation in saline aqueous fluids at HP/UHP conditions. Kinetics of dolomite dissolution in aqueous fluids is poorly known, and experimental and thermodynamic data under HP conditions are still lacking. Data on calcite indicate that dissolution at HP is enhanced by a prograde increase in both P and T , by high salinity in aqueous fluids, and/or low-pH conditions. In the studied marble, the P-T path and the occurrence of free high-saline fluids represent favorable conditions: (1) for the inferred dissolution-precipitation processes of the stable dolomite in a closed system, and (2) for possible migration of the dissolved carbonate, if the system would have been open during subduction.

The occurrence of fluor-wagnerite in UHT granulites and its implications towards understanding fluid regimes in the evolution of deep crust: a case study from the Eastern Ghats Belt, India

We report the occurrence of a rare phosphate mineral, fluor-wagnerite (Mg1.91–1.94Fe0.06–0.07Ca<0.01) (P0.99–1.00O4)(OH0.02–0.17F0.98–0.83) from the Eastern Ghats Belt of India, an orogenic belt evolved during Meso- to Neoproterozoic time. The host rock, i.e. high- to ultrahigh temperature (UHT) granulites (~1000 °C, 8–9 kbar) of the studied area was retrogressed after emplacement to mid-crustal level (800–850 °C, 6–6.5 kbar) as deduced from their pressure-temperature histories. Based on mineral chemical data and micro-Raman analyses, we document an unusual high Mg-F-rich chemistry of the F-wagnerite, which occur both in peak metamorphic porphyroblastic assemblages as well as in the retrograde matrix assemblage. Therefore, in absence of other common phosphates like apatite, fluor-wagnerite can act as an indicator for the presence of F-bearing fluids for rocks with high X Mg and/or fO2. The occurrence of F-rich minerals as monitors for fluid compositions has important implications for the onset of biotite dehydration melting and hence melt production in the deep crust. We propose that fluor-wagnerite can occur as an accessory mineral associated with F-rich fluids in lower-mid crustal rocks, and F in coexisting minerals should be taken into consideration when reconciling the petrogenetic grid of biotite-dehydration melting.

Experimental subsolidus phase relations in the system CaCO3–CaMg(CO3)2 up to 6.5 GPa and implications for subducted marbles

The dissociation of dolomite to aragonite + magnesite and the reaction of dolomite + aragonite to Mg-calcite have been experimentally determined by piston cylinder experiments in the range of 650–1000 °C and 2–6.5 GPa. Friction decay within the salt sleeves used as pressure medium was monitored by continuously logging the travel distance of the piston. Piston movement ceased after the first 24 h of the experiment, and all experiments were kept at the steady-state conditions for an additional 24–196 h. The reaction of dolomite to aragonite + magnesite has a positive slope and has been bracketed at 650 °C between 5.1 and 5.8 GPa and at 750 °C between 5.7 and 6.5 GPa. The reaction is sluggish with a 0.6-GPa interval where all three phases coexist. The reaction dolomite + aragonite to Mg-calcite has been determined in 0.5-GPa intervals from 660 °C at 2 GPa to 940 °C at 6.5 GPa. The composition of Mg-calcite at the reaction curve is buffered and changes systematically from X(Ca) of 0.90 at 2 GPa to 0.57 at 6.5 GPa. The solvus between dolomite and Mg-calcite shrinks slightly with increasing pressure, providing evidence for a small negative slope of the critical curve dolomite = Mg-calcite. The degree of disorder in dolomite has been determined in some of the retrieved samples by Rietveld refinement based on X-ray diffraction patterns. The order parameter S decreases gradually from 1 to 0.7 as a function of temperature as the critical curve is approached and then drops dramatically after crossing it. No ordering on quench has been observed. The experimental results permit the construction of a petrogenetic grid for the system dolomite, aragonite, Mg-calcite, and magnesite, which has an invariant point at ~7.5 GPa and 980 °C, representing the maximum pressure stability of dolomite. X(Ca) isopleths of Mg-calcite coexisting with dolomite serve mainly as a thermometer, whereas in the presence of aragonite they are a useful barometer. The experimental results have been applied to diamond-bearing marbles of the Kokchetav Massif, where Mg-calcite inclusions in garnet and zircon with X(Ca) of 0.76 most likely formed during prograde metamorphism close to the graphite–diamond transition.

Metamorphic and geochronologic constraints on the tectonic evolution of the Central Eastern Desert of Egypt

The Central Eastern Desert (CED) of Egypt which is part of the Arabian–Nubian Shield, is characterized by the unique occurrence of small bodies of banded iron formation (BIF) intercalated with volcanic and volcaniclastic rocks of an “ophiolitic – island arc succession”. All BIFs and their host rocks were regionally metamorphosed under greenschist to amphibolite facies conditions during the Pan-African Orogeny and were intruded by several syn- to post-tectonic magmas. Multiequilibrium calculations, conventional thermobarometry, and petrogenetic grid considerations indicate a northward decrease in metamorphic grade from 520±30°C, 5±2kbar at Um Nar in the south, to 400±50°C, 4±2kbar at Abu Marwat to the north. Areas south of Wadi El Dabbah are characterized by early overturned to recumbent folds verging north and northeast, whereas areas to the north are predominated by southwest-directed folds and thrusts. Cathodoluminescence imaging and LA-ICP-MS analysis lead to the identification of four types of zircon from felsic intrusive and metavolcaniclastic rocks: (i) crystals with oscillatory or sector zoning, and ages of 2.6–1.1Ga; (ii) crystals with oscillatory zoning, and three age populations of 828±5; 772±5; and 728±4Ma; (iii) overgrowths with bright luminescence and ages of 720–650Ma; and (iv) crystals from felsic intrusions with oscillatory zoning, and four age populations in the range of 680–544Ma.These results lead us to suggest that CED represents remnants of an island arc system formed above a southwest-dipping subduction zone. Oceanic crust in the fore- and back-arc basins formed at 850–700Ma, contemporaneously with arc activity. During periods of arc magmatism, these basins filled with volcaniclastic debris, which imposed anoxic conditions leading to increased concentrations of soluble Fe2+ in basin fluids. During periods of arc quiescence, Fe2+ was oxidized and precipitated as BIF precursor minerals in the fore- and back-arc basins. The BIFs and volcaniclastic rocks were affected by localized hydrothermal alteration that began on the ocean floor c. 710Ma. During the Pan-African orogeny, back-arc basin serpentinites, basalts, and overlying volcaniclastic rocks and BIFs were emplaced northward onto the island arc as ophiolitic material. North-directed stacking of wedge shaped thrust sheets resulted in regional metamorphism with highest grade conditions reached in the southern parts of the CED. Transpression along the northeastern margin of the arc during the waning stages of the Pan-African Orogeny (614–550Ma) led to the closure of the fore-arc basin and the emplacement of ophiolites in the northern CED with southwest verging folds. Syntectonic mostly felsic magma intruded the thrust stacks in pulses 680–544Ma. The proposed model for the evolution of the CED provides insights into the origin of small Neoproterozoic BIFs.

Recognizing and quantifying metamorphosed alteration zones through amphibolite facies metamorphic overprint at the Key Anacon Zn–Pb–Cu–Ag deposits, Bathurst Mining Camp, New Brunswick, Canada

The Key Anacon deposits, Bathurst Mining Camp, New Brunswick, are hosted in upper greenschist- to amphibolite-facies felsic volcanic rocks. The occurrence of cordierite–biotite and garnet–biotite–muscovite assemblages parallel to the regional tectonic fabric in the metamorphosed hydrothermal alteration zones point to a pre-metamorphic mineralization event that was synchronous with sub-aqueous volcanism. Modeling the altered felsic volcanic rocks in the system K2O–Fe2O3–MgO–Al2O3–SiO2–H2O–TiO2 (KFMASHT) and comparing the observed peak metamorphic assemblages with those produced in a petrogenetic grid allows us to interpret the style of pre-metamorphic hydrothermal alteration related to deposit formation. The compositional change in the stratigraphic footwall (structural hanging wall) is characterized by mass gains of 0.1 to 4.0 wt.% Fe2O3 (Total), 0.7 to 22.2 wt.% MgO, and 0.5 to 55.2 wt.% CaO, and mass losses of 25.1 to 56.7 wt.% SiO2, 0.2 to 2.0 wt.% Na2O, and 0.3 to 3.8 wt.% K2O (the values of the mass changes cited here are in absolute terms).Variable gains and losses of Zn, Pb, and Cu are characteristic of the footwall alteration zones with Zn displaying gains proximal to the massive sulfide lens, and losses distal to the sulfide lens. The alteration indices (AI) values increase as the massive sulfide lens are approached from either the footwall or hanging wall, whereas the Ghandi index (GI) discriminates the intensely chlorite-altered rocks proximal to mineralization from the sericitic altered rock in more distal areas. Overall, there is an increase of the GI from the weakly to moderately altered zone (GI=1.3 to 6.0) to the more intensely altered zone (GI=6.1 to 60). These physical and geochemical observations are consistent with early feldspar-destructive alteration followed by chloritization proximal to the sulfide lens and accompanied by sericitization alteration distal prior to sulfidation and oxidation during prograde metamorphism.

Zirconosilicates in the kakortokites of the Ilímaussaq complex, South Greenland: Implications for fluid evolution and high-field-strength and rare-earth element mineralization in agpaitic systems

AbstractThe layered agpaitic nepheline syenites (kakortokites) of the Ilímaussaq complex, South Greenland, host voluminous accumulations of eudialyte-group minerals (EGM). These complex Na-Ca-zirconosilicates contain economically attractive levels of Zr, Nb and rare-earth elements (REE), but have commonly undergone extensive autometasomatic/hydrothermal alteration to a variety of secondary mineral assemblages. Three EGM alteration assemblages are recognized, characterized by the secondary zirconosilicates catapleiite, zircon and gittinsite. Theoretical petrogenetic grid models are constructed to assess mineral stabilities in terms of component activities in the late-stage melts and fluids. Widespread alteration of EGM to catapleiite records an overall increase in water activity, and reflects interaction of EGM with late-magmatic Na-, Cl- and F-rich aqueous fluids at the final stages of kakortokite crystallization. Localized alteration of EGM and catapleiite to the rare Ca-Zr silicate gittinsite, previously unidentified at Ilímaussaq, requires an increase in CaO activity and suggests post-magmatic interaction with Ca-Sr bearing aqueous fluids. The pseudomorphic replacement of EGM in the kakortokites was not found to be associated with significant remobilization of the primary Zr, Nb and REE mineralization, regardless of the high concentrations of potential transporting ligands such as F and Cl. We infer that the immobile behaviour essentially reflects the neutral to basic character of the late-magmatic fluids, in which REE-F compounds are insoluble and remobilization of REE as Cl complexes is inhibited by precipitation of nacareniobsite-(Ce) and various Ca-REE silicates. A subsequent decrease in F– activity would furthermore restrict the mobility of Zr as hydroxyl-fluoride complexes, and promote precipitation of the secondary zirconosilicates within the confines of the replaced EGM domains.

Structural style and metamorphic conditions of the Jinshajiang metamorphic belt: Nature of the Paleo-Jinshajiang orogenic belt in the eastern Tibetan Plateau

The Jinshajiang metamorphic belt is a Barrovian sequence distributed within the Jinshajiang Suture Zone (JSZ), which was formed in the thickened Paleo-Jinshajiang orogenic belt after closure of the Jinshajiang Ocean. We have identified three metamorphic phases, M1, M2 and M3, corresponding to deformation stages D1, D2 and D3, in the Barrovian sequences. The metamorphic belt exhibits a metamorphic field gradient from chlorite to biotite, garnet, staurolite–kyanite and sillimanite grades. Inclusions in garnet and staurolite (chlorite, mica, quartz, feldspar, ilmenite and graphite) indicate that M1 reflects greenschist facies metamorphism. Pervasive M2 metamorphism formed a dominant S1 schistosity within the Barrovian sequence. Peak metamorphic conditions for metapelites of the garnet–staurolite and staurolite–kyanite grade were ∼580°C and ∼0.65GPa according to petrogenetic grids. Peak metamorphism was in conditions of ∼635°C and 0.50GPa for the metapelites and ∼650°C and ∼0.61GPa for amphibolites in sillimanite grade. Greenschist facies retrograde metamorphism, M3, followed D2 deformation, a top-down-to-southeast shear in the JSZ. The D3 deformation is characterized by well-developed brittle faults with fault gouge and breccia. Zircon grains from an amphibolite sample have cores with igneous oscillatory zoning and metamorphic rims. However, the metamorphic rims are too narrow to analyze. Laser ablation-inductively coupled plasma mass-spectrometry (LA-ICPMS) analyses of the igneous cores of zircons yielded a crystallization age of 242Ma. 40Ar/39Ar dating of white mica from a garnet-schist gave a plateau age of 224Ma. The peak metamorphism is thus limited to be between 242 and 224Ma. We thus suggest a tectonic shift from collision to extension of for the Permian to Triassic Paleo-Jinshajiang orogenic belt.

UHP Metamorphism Documented in Ti-chondrodite- and Ti-clinohumite-bearing Serpentinized Ultramafic Rocks from Chinese Southwestern Tianshan

The southwestern Tianshan (China) metamorphic belt records high-pressure (HP) to ultrahigh-pressure (UHP) conditions corresponding to a cold oceanic subduction-zone setting. Serpentinites enclosing retrogressed eclogite and rodingite occur as lenses within metapelites in the UHP unit, which also hosts coesite-bearing eclogites. Based on the petrology and petrography of these serpentinites, five events are recognized: (1) formation of a wehrlite–harzburgite–dunite association in the mantle; (2) retrograde metamorphism and partial hydration during exhumation of the mantle rocks close to the seafloor; (3) oceanic metamorphism leading to the first serpentinization and rodingitization; (4) UHP metamorphism during subduction; (5) retrograde metamorphism during exhumation together with a second serpentinization. The peak metamorphic mineral assemblage of the serpentinized wehrlite comprises Ti-chondrodite + olivine + antigorite + chlorite + magnetite + brucite. A computed pseudosection for this serpentinized wehrlite shows that the Al content in antigorite is mostly sensititive to temperature but can also be used to constrain pressure. The average XAl = 0·204 ± 0·026 of antigorite (XAl = Al (a.p.f.u.)/8, where Al is in atoms per formula unit for a structural formula M48T34O85(OH)62, and M and T are octahedral and tetrahedral sites, respectively) included in Ti-chondrodite and average XAl = 0·203 ± 0·019 of antigorite in the matrix result in a well-constrained peak metamorphic temperature of 510–530°C. Peak pressures are less precisely constrained at 37 ± 7 kbar. The Tianshan serpentinites thus record UHP metamorphic conditions and represent the deepest subducted serpentinites discovered so far. The retrograde evolution occurs within the stability field of brucite + antigorite + olivine + chlorite and formation of Ti-clinohumite at the expense of Ti-chondrodite has been observed, suggesting isothermal decompression. The resulting P–T path is in excellent agreement with the metamorphic evolution of country rocks, indicating that the UHP unit in Tianshan was subducted and exhumed as a coherent block. To refine the metamorphic path of the ultramafic rocks, we have investigated the stability fields of Ti-chondrodite and Ti-clinohumite using piston-cylinder experiments. A total of 11 experiments were conducted at 25–55 kbar and 600–750°C in a F-free natural system. Combined with previous experiments and information from natural rocks we constructed a petrogenetic grid for the stability of Ti-chondrodite and Ti-clinohumite in F-free peridotite compositions. The formation of Ti-chondrodite in serpentinites requires a minimum pressure of about 26 kbar, whereas in Ti-rich systems it can form at considerably lower pressures. A key finding is that at UHP conditions, F-free Ti-chondrodite or Ti-clinohumite breaks down in the presence of orthopyroxene between 700 and 750°C, at temperatures that are significantly lower than those of the terminal breakdown reactions of these humite minerals. These breakdown reactions are an additional source of fluid during prograde subduction of serpentinites.

The geological significance of a Paleozoic mafic granulite found in the Yiyang area of northeastern Jiangxi Province

A mafic granulite has been found in southern Yiyang county in northeastern Jiangxi Province. This granulite has a mineral assemblage of garnet+plagioclase+hypersthene+biotite+magnetite. Based on the composition variation of minerals and relationships among them, pre-peak metamorphism (M1) and peak metamorphism (M2) are recognized. A petrogenetic grid and geothermobarometry measurements yield pre-peak metamorphic conditions of 830-870℃ and ~1.0 GPa, and peak metamorphic conditions of 850-900℃ and 0.7-0.8 GPa, suggesting a near isothermal decompression clockwise PTt path. LA ICPMS zircon U-Pb dating indicates that granulite facies metamorphism occurred about 436 Ma, and its protolith was possibly formed in the early Neoproterozoic. Coupled with available data, we suggest that early Paleozoic (Caledonian) high-grade metamorphism in the Cathaysia Block is related to collisional orogeny, which probably results from the collision-compression between the Yangtze Block and Eastern Gondwana.

Taking the temperature of Earth's hottest crust

The limitations of conventional thermobarometry and petrogenetic grids for determining the peak P–T conditions of granulites are well known. These limitations have been overcome during the past decade with the calibration of single mineral thermometers, particularly Al-in-orthopyroxene, Zr-in-rutile and Ti-in-zircon, and the increased use of P–T pseudosection thermobarometry. Most recent studies of ultrahigh temperature (UHT) granulites (those formed at >900 °C) have used one or other of these methods to argue for peak metamorphic temperatures up to or beyond 1000 °C. Since models for the thermal evolution of orogens generally do not predict such extreme temperatures it is important to confirm their veracity. Here we combine in a single study single mineral thermometry with P–T pseudosection thermobarometry to provide a robust determination of peak temperature and tight constraints on the retrograde P–T path for one UHT granulite locality in the Eastern Ghats Province. This is the first study to apply the most recent update of the internally consistent thermodynamic dataset of Holland and Powell (2011) and the re-parameterized a–x models of White et al. (2014) and Wheller and Powell (2014) to UHT granulites. For two samples, we report Zr-in-rutile temperatures of >1000 °C and Ti-in-zircon temperatures of ∼900 °C, supported by Al-in-orthopyroxene temperatures of ∼900 °C, that correspond closely to those estimated using P–T pseudosections for conditions at the thermal peak and at the solidus on the retrograde P–T path, respectively. The P–T path is counter-clockwise in common with other UHT granulite localities in the Eastern Ghats Province. By demonstrating that UHT metamorphism at T>1000°C is real we provide a robust constraint that must be met by geodynamic models for the development of ultrahot orogens.

Reaction textures and metamorphic evolution of sapphirine–spinel-bearing and associated granulites from Diguva Sonaba, Eastern Ghats Mobile Belt, India

AbstractThe Diguva Sonaba area (Vishakhapatnam district, Andhra Pradesh, South India) represents part of the granulite-facies terrain of the Eastern Ghats Mobile Belt. The Precambrian metamorphic rocks of the area predominantly consist of mafic granulite (±garnet), khondalite, leptynite (±garnet, biotite), charnockite, enderbite, calc-granulite, migmatic gneisses and sapphirine–spinel-bearing granulite. The latter rock type occurs as lenticular bodies in khondalite, leptynite and calc-granulite. Textural relations, such as corroded inclusions of biotite within garnet and orthopyroxene, resorbed hornblende within pyroxenes, and coarse-grained laths of sillimanite, presumably pseudomorphs after kyanite, provide evidence of either an earlier episode of upper-amphibolite-facies metamorphism or they represent relics of the prograde path that led to granulite-facies metamorphism. In the sapphirine–spinel-bearing granulite, osumilite was stable in addition to sapphirine, spinel and quartz during the thermal peak of granulite-facies metamorphism but the assemblage was later replaced by Crd–Opx–Qtz–Kfs-symplectite and a variety of reaction coronas during retrograde overprint. Variable amounts of biotite or biotite+quartz symplectite replaced orthopyroxene, cordierite and Opx–Crd–Kfs–Qtz-symplectite at an even later retrograde stage. Peak metamorphic conditions of c. 1000°C and c. 12 kbar were computed by isopleths of XMg in garnet and XAl in orthopyroxene. The sequence of reactions as deduced from the corona and symplectite assemblages, together with petrogenetic grid and pseudosection modelling, records a clockwise P–T evolution. The P–T path is characteristically T-convex suggesting an isothermal decompression path and reflects rapid uplift followed by cooling of a tectonically thickened crust.

Small grains and big implications: Accessory Ti- and Zr-minerals as petrogenetic indicators in HP and UHP marbles

The paper by Proyer et al. (2014; this issue) demonstrates that Ti- and Zr-bearing accessory minerals (rutile, titanite, zircon) record through their reaction textures part of the metamorphic history of a UHP marble. Calculation of relevant petrogenetic grids has the power to constrain the retrograde P-T path based on phase stability fields and the geothermobarometric evaluation of H 2 O-independent mineral reactions involving these Ti- and Zr-bearing minerals. They calculated simple petrogenetic grids in the system TiO 2 -ZrO 2 -CaO-MgO-Al 2 O 2 -SiO 2 -CO 2 -H 2 O (TZCMASCH) for calcite-dolomite marbles with forsterite/antigorite in excess and including those Ti- and Zr-bearing minerals for which thermodynamic data are known (rutile, titanite, geikielite, zircon, baddeleyite) and delineated their stability fields as well as the succession of their stability regions. This approach allowed the authors to infer the shape of the retrograde P-T path. Thus the combination of very careful petrography, calculated simple petrogenetic grids and the application of geothermometry involving these Ti- and Zr-bearing accessory minerals becomes an indispensible tool when reconstructing a metamorphic rock’s evolution.

Ti- and Zr-minerals in calcite-dolomite marbles from the ultrahigh-pressure Kimi Complex, Rhodope mountains, Greece: Implications for the P-T evolution based on reaction textures, petrogenetic grids, and geothermobarometry

Rutile, titanite, and zircon formed as relatively coarse-grained accessory minerals in several samples of high-grade calcite-dolomite marble with an early ultrahigh-pressure history. These minerals decomposed to a texturally complex set of secondary minerals during subsequent stages of retrograde metamorphism. The reactions involve several generations of geikielite-ilmenite as well as zirconolite [(Ca,Th,U)Zr(Ti,Fe,Nb,Ta) 2 O 7 ], kassite/cafetite [CaTi 2 O 4 (OH) 2 /CaTi 2 O 5 ·H 2 O], Ti-bearing humite group minerals, thorianite, and sometimes euxenite [(Ca,U,Th,REE)(Nb,Ta,Ti) 2 (O,OH) 6 ]. Stable coexistence of zircon and olivine is observed and stably coexisting titanite with olivine and/or humite-group minerals is reported here for the first time outside of carbonatites, kimberlites, or lamprophyres. Petrogenetic grids constructed for Ti- and Zr-bearing olivine/antigorite-saturated calcite-dolomite marbles show that geikielite is stable at highest pressures, followed by titanite and rutile, and that baddeleyite + diopside replaces zircon + calcite to higher pressures. The observed reaction textures are consistent with an earlier derived P-T path for the Kimi Complex. They corroborate a period of heating during decompression from 25 to 20 kbar and ca. 800 °C, where the assemblage olivine-diopside-spinelrutile- zircon formed. This assemblage partially re-equilibrated during subsequent decompression and cooling, thus forming the observed reaction textures. Even though no memory of the UHP path is preserved in the accessory minerals, their reaction relationships turn out to be potentially very useful for geothermobarometry over a large range of metamorphic conditions.

Trace element mass balance in hydrous adiabatic mantle melting: The Hydrous Adiabatic Mantle Melting Simulator version 1 (HAMMS1)

AbstractA numerical mass balance calculation model for the adiabatic melting of a dry to hydrous peridotite has been programmed in order to simulate the trace element compositions of basalts from mid‐ocean ridges, back‐arc basins, ocean islands, and large igneous provinces. The Excel spreadsheet‐based calculator, Hydrous Adiabatic Mantle Melting Simulator version 1 (HAMMS1) uses (1) a thermodynamic model of fractional adiabatic melting of mantle peridotite, with (2) the parameterized experimental melting relationships of primitive to depleted mantle sources in terms of pressure, temperature, water content, and degree of partial melting. The trace element composition of the model basalt is calculated from the accumulated incremental melts within the adiabatic melting regime, with consideration for source depletion. The mineralogic mode in the primitive to depleted source mantle in adiabat is calculated using parameterized experimental results. Partition coefficients of the trace elements of mantle minerals are parameterized to melt temperature mostly from a lattice strain model and are tested using the latest compilations of experimental results. The parameters that control the composition of trace elements in the model are as follows: (1) mantle potential temperature, (2) water content in the source mantle, (3) depth of termination of adiabatic melting, and (4) source mantle depletion. HAMMS1 enables us to obtain the above controlling parameters using Monte Carlo fitting calculations and by comparing the calculated basalt compositions to primary basalt compositions. Additionally, HAMMS1 compares melting parameters with a major element model, which uses petrogenetic grids formulated from experimental results, thus providing better constraints on the source conditions.