Reaction Textures Research Articles

Symplectite growth in the presence of alkaline fluids: evidence from high-aluminous metasediments of the Highland Complex, Sri Lanka

High-Al garnet-sillimanite-graphite gneisses (khondalites) from the Matale area in central Sri Lanka show evidence for open system formation of symplectites. Three types of khondalite (type A, B, and C) were collected from two localities (Lo1 and Lo2) in this area. In Lo1, khondalite type A shows a stable mineral assemblage with garnet (Grt), ribbon quartz (Qtz), prismatic sillimanite (Sil), alkali feldspar (Akfs) and minor graphite. The abundant leucosome is interpreted as (modified) crystallised melt (Liq). Qtz is invariably rimmed by Akfs double corona. In type B, Grt and Sil are partially rimmed by spinel (Spl)-Akfs symplectites. Type C samples show the extensive development of Spl-Akfs symplectites, locally with Bt, as well as corundum (Crn)-Akfs symplectites after Sil in domains relatively remote from Grt. Mineral inclusions in Grt suggest advanced biotite dehydration melting producing Grt ± Spl + Liq, similar to other Sri Lankan metapelitic granulites. Peak P-T conditions are ~8–9 kbar and ~ 850 °C. The symplectites developed on the retrograde path, with Ti in Bt thermometry indicating 700–720 °C. The chemical potential calculations indicated that the chemical potential gradient from garnet to sillimanite could well have produced the AKfs+Spl symplectite. Hence the most critical point is the initiation of Grt breakdown reactions which can facilitate the formation of Spl + Akfs and Crn + Akfs symplectites after sillimanite via the chemical potential gradient. Textural observations coupled with mass balance of symplectitic reaction textures indicate that addition of external Na and K to reaction sites is needed to trigger the symplectite formation. Excess Fe and Mg from Grt breakdown probably diffused away from garnet along a chemical potential gradient. As a result, Sil in the vicinity of Grt has broken down to Spl-Akfs symplectites, whereas Sil further away from Grt produced Crn-Akfs symplectites. Spl-Akfs and Crn-Akfs symplectites after Grt and Sil in khondalites are restricted to the boundary zone between the Wanni and Highland Complexes. The formation of symplectites is inferred to have taken place during or just after a decompression stage with minor cooling, under the influence of an alkaline fluid derived from nearby pegmatites, when the assemblage Grt + Sil was already metastable. The combination of fluids and symplectites indicates a very fast process rate.

P-T-t evolution of sapphirine-bearing semipelitic granulites from Vadkampatti in Eastern Madurai Domain, southern India: Insights from petrography, pseudosection modelling and in-situ monazite geochronology

Migmatitic semipelitic granulites from the Eastern Madurai Domain of southern India preserves reaction textures indicating a clockwise P-T evolution, with near isothermal decompression. The prograde history is recorded by included phases within porphyroblastic garnet and orthopyroxene. Development of high XMg garnet (up to 0.69) + orthopyroxene (Al2O3 up to 9.6 wt%) assemblage indicates attainment of ultrahigh temperature (UHT) condition at the peak stage. Subsequent formation of a series of symplectic assemblages (Opx2 + Spr2 + Sil2 + Pl2 and Opx2 + Crd2) with aluminous orthopyroxene (Al2O3 up to 9.5 wt%), at the expense of garnet indicates decompression at ultrahigh temperatures. Conventional thermobarometry and feldspar thermometry indicate peak metamorphic P-T conditions of ~970 ± 30 °C, 10.3 ± 0.4 kbar and formation of symplectites at ~960 ± 20 °C, 9.2 ± 0.3 kbar for Opx2 + Spr2 + Sil2 and at ~890 ± 40 °C, 7.3 ± 0.6 kbar for Opx2 + Crd2. Obtained P-T conditions are in agreement with the P-T estimates from pseudosection modelling. Texturally controlled in-situ monazite dating shows that the Th-poor and extremely HREE depleted dark cores of matrix monazite with a weighted age of 586 ± 8 Ma formed simultaneously with garnet growth during the prograde evolution. On the other hand, Th-rich and HREE depleted bright rims with a weighted age of 532 ± 7 Ma formed from the dissolution-reprecipitation due to melt crystallization at or near T-max, dating the peak of metamorphism. The compositionally homogeneous HREE + Y enriched monazite in the symplectite with a weighted age of 475 ± 12 Ma, formed along with the symplectites during garnet breakdown. Our results are suggestive of a late Neoproterozoic to Cambrian long-lived (ca. 55 million years) hot orogenic plateau in the Madurai Block. UHT conditions were likely achieved by conductive heating, before being rapidly uplifted to shallower crustal depth during late Cambrian to early Ordovician (475 ± 12 Ma). This is consistent with the hot orogen model for the attainment of UHT conditions, which demonstrates that crusts that are moderately rich in heat producing elements can reach temperatures >900 °C when the duration of orogeny is sufficiently long (i.e. slow erosion rates) to allow conductive heating through radioactive decay.

Ultramafic Rock Carbonation: Constraints From Listvenite Core BT1B, Oman Drilling Project

AbstractThe occurrence of the quartz‐carbonate alteration assemblage (listvenite) in ophiolites indicates that ultramafic rock represents an effective sink for dissolved CO2. However, the majority of earlier studies of ultramafic rock carbonation had to rely on the surface exposure of reaction textures and field relationships. Here we present the first observations on ultramafic rock alteration obtained from the 300 m deep BT1B drill hole, ICDP Oman Drilling Project, allowing for a continuous and high‐resolution investigation. Hole BT1B recovered continuous drill core intersecting surface alluvium, 200 m of altered ultramafic rock comprising mainly listvenite and minor serpentinite bands at 90 and 180 m depth, and 100 m of the underlying metamorphic sole. Textural evidence suggests that the carbonation of fully serpentinized harzburgite commenced by non‐equilibrium growth of spheroidal carbonate characterized by sectorial zoning resulting from radially oriented low‐angle boundaries. In the serpentinite, carbonate spheroids are composed of alternating magnesite cores and dolomite rims, whereas texturally similar carbonate in the listvenite is composed of Fe‐rich magnesite cores and Ca‐Fe‐rich magnesite rims. The distinct compositions and mineral inclusions indicate that the carbonation extent was controlled by fluid accessibility resulting in the simultaneous formation of limited carbonate in the serpentinite bands and complete carbonation in the listvenite parts of BT1B. The presence of euhedral magnesite overgrowing spheroidal carbonate in the listvenite suggests near‐equilibrium conditions during the final stage of carbonation. The carbonate clumped isotope thermometry constrains carbonate crystallization between 50 °C and 250 °C, implying repeated infiltration of reactive fluids during ophiolite uplift and cooling.

English

The Ikare area is underlain by rocks of the migmatite-gneiss-quartzite complex and older granite lithologic groups and charnockites. Categorization of these rocks in the literature and published map is generalized and the rocks are undifferentiated. Though reported in few literature as an area with rocks composed of granulite facie grade, the nature of transition is not well documented. Field and petrographic studies were carried out to properly characterize the different rock types, evaluate the migmatization processes and elucidate the nature of amphibolite to granulite facie transition. Major rock types recognized and mapped includes grey gneiss, charnockitic gneiss, granite gneiss, pelitic gneiss, porphyritic granite and quartzite. Minor rock types are the mafic and ultramafic components which are made up of basic schist and amphibolites. The area suffered at least four episodes of deformation (D1, D2, D3 and D4) and at least three episodes of metamorphism (M1, M2 and M3). Petrographic studies further show that the mineral assemblage within the quartzo-feldspathic rocks changed from hydrous to anhydrous phases as one traverses from the southern part of the study area to the northern part. Reaction textures indicate that the transition zone between the amphibolite and granulite facie rocks is achieved by a prograde dehydration reaction while rehydration reactions took place during the waning retrograde stage. Anatexis and metamorphic differentiation were mainly responsible for the migmatization process in Ikare area while the charnockitization process was a product of dehydration reactions aided by reduction in water fugacity due to influx of CO2. Key words: Structure, migmatites, granulite facies, deformational episodes, metamorphism, charnockitic gneiss.  

Petrological Studies and Interpretation of Reaction Textures in the Granulites of Bhavani Complex, Tamil Nadu, India

Petrological Studies and Interpretation of Reaction Textures in the Granulites of Bhavani Complex, Tamil Nadu, India

Microstructures reveal multistage melt present strain localisation in mid-ocean gabbros

In this work, we examine the tectono-metamorphic evolution of gabbroic rocks of the Atlantis Bank oceanic core complex, South West Indian Ridge, focusing on multistage strain localisation associated with exhumation. We study a sample from the core complex footwall below the depth of seawater hydrothermal fluid-rock interaction. We identify a succession of increasing strain localisation where early solid-state deformation of an olivine gabbro is succeeded by two episodes of melt present deformation resulting in increasing localisation of both strain and melt flux into a narrowing zone.The early melt-absent solid-state deformation occurs dominantly in the dislocation creep regime, and localises strain at the tens of metre scale. This evolved into melt present deformation characterised by (i) grain size reduction by replacement reactions, and (ii) dislocation creep and incipient grain boundary sliding in coarse- and fine-grained parts, respectively. This is based on (1) microstructures indicative of the former presence of melt, (2) a shift to lower calcium plagioclase, and (3) reaction textures involving partial replacement of olivine and diopside by fine grained enstatite and hornblende exhibiting only minor internal deformation features.A narrow (2–3 mm) high strain zone which cuts the earlier shear zone foliation at ~30°, exhibits strong and near complete grain size reduction by melt-rock reaction and tightly spaced foliation within a millimetre-wide high strain zone. Here, strain localisation is contemporaneous with a second melt influx shown by (1) complete olivine replacement by fine grained hornblende and enstatite exhibiting no crystallographic preferred orientation, (2) microstructures indicative of the former presence of melt, and (3) changes in the mineral chemistry of diopside, plagioclase, enstatite and hornblende. We suggest deformation was dominated by melt assisted grain boundary sliding allowing higher volumes of melt flux, a high degree of weakening and deformation focused in the narrow shear zone than during the porous melt flow in the wider, early shear zone.This combined microstructural and microchemical study highlights that the recognition of changes in deformation regime, their time relationships and potential melt related deformation are critical for understanding, and therefore modelling, progressive strain localisation, melt flux and the rheological evolution of oceanic detachment faults.

Geological and Geotechnical Characteristics of Ntem Formations: Insight of its Applications in the Memve’ele Dam Construction (Southern Cameroon)

This paper discusses the geological and geotechnical potentialities of Ntem formations from the Memve’ele dam construction. Stratigraphic formations on the dam site consist from top to bottom sand, silty sand, silty, silty clay, very to moderate weathered rocks with a thickness of about 0.5–3 m above a sound bed rock which are represented by Ntem formations. These rocks have main mineral associations represented by orthopyroxene, clinopyroxene, quartz, plagioclase and garnet, display reaction textures, idiomorphic crystals, straight boundaries between crystals, the iron bearing amphibole, biotite and epidote, quartz ribbons and iron minerals (ilmenite and magnetite) are also present. All these permit to class them from granulite to amphibolite facies metamorphic conditions; therefore they are very hard and appear good for civil applications. The permeability has thickness from 0 to 13 m in the depth with Lugeon values higher than 15 UL. The RQD values are permitted to class these rocks as being moderated to good resistant. These character features which improve their geophysical properties (e.i. hard quality) have permitted that the dam and powerhouse foundations refitting on them. According to geophysical properties and rock mass rating, the rock mass is fair and good and is classified to class II and III. All these indicate that the rock mass is well compacted and sound with sparsely spaced discontinuities. The Ntem formations have uniaxial compressive strength under saturated condition generally higher than 40 Mpa, dry density or specific weight not less than 2.7 g/cm3, low porosity (average from 0.69 to 0.83%), low coefficient of water absorption (average from 0.012 to 0.04%), deformation modulus from 82.3 to 87.86 Gpa, Los Angeles and Micro Deval values are lower than 40% and 20% respectively, swelling risks are very limited with low values (1.07–1.11%) of sulphate, under 4.5% of cement and low values (0–0.07%) obtained during alkali-reaction test. All these have permitted that to obtain concrete aggregate and rip rap by crushing and processing these rocks from quarry during the dam, spillway, powerhouse and road constructions. These results confirm use of these formations during Memve’ele dam constructions.

Rapid fluid infiltration and permeability enhancement during middle–lower crustal fracturing: Evidence from amphibolite–granulite-facies fluid–rock reaction zones, Sør Rondane Mountains, East Antarctica

Permeability is a key control on fluid infiltration in the crust. However, quantitative geological constraints on crustal permeability are limited, particularly with regards to its temporal evolution. Here we constrain the permeability evolution in the middle–lower crust, based on metamorphic processes associated with fluid infiltration and crustal fracturing. We investigated mafic granulite and orthopyroxene–hornblende schist (opx–hbl schist) samples from Mefjell, Sør Rondane Mountains, East Antarctica. Millimetre-scale amphibolite-facies reaction zones occur along fractures in these rocks. In the mafic granulite, two zones were identified: (i) reaction zones (1–2 mm thick); and (ii) mafic granulite host rock. The opx–hbl schist sample can be divided into the following three zones: (i) actinolite–cummingtonite zones (1.4 mm thick); (ii) actinolite–orthopyroxene zones (1.6 mm thick); and (iii) host rock. These zones are evident from the modal mineralogy, reaction textures, and trace element profiles. The P–T conditions of fluid infiltration are estimated to be 0.55 GPa and 620 °C for the mafic granulite, and 0.3 GPa and 450 °C for the opx–hbl schist, respectively. Chlorine concentrations in apatite grains show a gradual decrease from the fractures towards the host rocks. Chlorine concentration profiles suggest that the dominant processes of chlorine transport were advection with minor diffusion for both samples. Based on these results, the timescales of fluid infiltration are constrained to be ~8 h for the mafic granulite and ~10 h for the opx–hbl schist. The pressure gradient across the reaction zones was estimated from the H2O activity in the reaction zones to be 2–15 MPa/mm. The permeability of the host rock and fractures were estimated to be 10−21–10−23 and 10−8–10−9 m2, respectively. Our results show that rapid infiltration of Cl-bearing fluids (~10 h) occurred due to a limited fluid flux from low-permeability (10−21–10−23 m2) host rocks. The low-permeability media led to fluid accumulation and further fracturing. The spatio-averaged permeability then increased by more than several orders of magnitude (10−10–10−16 m2) and the fluid pressure decreased. The contrasting permeability between the host rocks and fractures reveals permeability enhancements associated with crustal fracturing on timescales comparable to geophysical observations. Compared with average long-term (Myr) permeability estimates from previous studies (e.g., 10−18 m2), the permeability obtained from the fracture–reaction zone systems shows large fluctuations in the middle–lower crust (10−21–10−23 to 10−10–10−16 m2).

Multi-stage evolution of kimberlite melt as inferred from inclusions in garnet megacrysts in the Grib kimberlite (Arkhangelsk region, Russia)

To provide new insights into the origin of garnet megacrysts and evolution of kimberlite melts, we studied in detail the polymineralic and monomineralic inclusions and their host garnets from the Grib kimberlite (Arkhangelsk diamond province, Russia). Low-Cr and high-Cr garnet megacrysts and eclogitic garnets contain abundant polymineralic and rare monomineralic inclusions. Monomineralic inclusions presented by clinopyroxene, ilmenite, olivine replaced by serpentine, were found exclusively within the low-Cr megacrysts. The composition of clinopyroxene exhibits geochemical equilibrium with the host garnet, indicating its primary origin during the formation of the megacryst assemblage. The low-Cr garnet–clinopyroxene mineral assemblage formed as a result of high-temperature, melt-associated mantle metasomatism by failed kimberlite within the lithospheric mantle (T = 1150 °C and P = 5.5 GPa). Polymineralic inclusions are characterised by a silicate or silicate-sulphate matrix. The central part of the silicate inclusions is filled by serpentine and contains ilmenite, spinel, perovskite, calcite and apatite. At the contact with host garnets, phlogopite, spinel and amphibole occur as reaction minerals. Composition of spinel and other minerals within inclusions with silicate matrix suggests that kimberlite melt was trapped at mantle pressures. Inclusions with silicate matrix were found in all garnets. The matrix of silicate-sulphate inclusions consists of silicate cryptocrystalline phases and sulphate minerals (celestine–barite) and contains calcite grains. The inclusions are distributed in some low-Cr garnet megacrysts and eclogitic garnet. The silicate-sulphate inclusions were crystallised from the late-stage kimberlite melt. Diverse reaction textures are evidences of disequilibrium between the host crystals and polymineralic inclusions and indicate that garnet and the hosted inclusions reacted with the ascending kimberlite melt. The silicate-sulphate inclusions with a thin rim of epidote within eclogitic garnets indicate that a kimberlite melt invaded the garnet and induced partial melting. The studied inclusions allow us to propose three stages of the Grib kimberlite evolution: 1) generation of garnet megacrysts and primary inclusions due to melt metasomatism, 2) reaction of the high-Ti kimberlite melt with garnet megacrysts (including their dissolution) and 3) alteration of the inclusions in garnet after kimberlite ascent.

Simple kinetic model for replacement reactions involving solid solutions: the significant role of geofluids

We develop a kinetic model that illustrates the relative role of intergranular diffusion and surface reactions in overall metamorphic net-transfer reactions involving solid solutions. The behavior and state of the system were observed to depend on the non-dimensional parameter(s), defined herein by gamma equiv (k cdot L cdot K^{n-1}) /D^{mathrm{eff}}, where k denotes the dissolution rate constant of the mineral, L denotes the distance between the two minerals participating in the reaction, K denotes the coefficient of the equilibrium abundance ratio of the precipitated component in the intergranular fluid to those in the mineral, n denotes the order of the dissolution reaction, and D^{mathrm{eff}} denotes the effective diffusion coefficient of the precipitated component in the intergranular fluid. When gamma is small, the system becomes homogeneous, which is controlled by the surface reaction. In contrast, a large gamma implies a heterogeneous system controlled by both surface reaction and diffusion. Geofluid is among the most important influencers of kinetic regimes and reaction textures in metamorphic and metasomatic rocks because the intergranular fluid distribution considerably affects the intergranular diffusivity (D^{mathrm{eff}}). Our conceptual model can potentially reveal the nature of the reaction kinetics and the fluid–rock interactions and capture the complex behaviors of natural systems in future extensions.

Metamorphic P–T evolution and tectonic implications of pelitic granulites in the Ji’an area, northeastern Jiao–Liao–Ji Belt, North China Craton

The Ji’an area in the Jiao–Liao–Ji Belt of the North China Craton contains garnet-cordierite-sillimanite pelitic granulites that preserve representative metamorphic reaction textures. Petrographic observations and quantitative phase equilibria modeling indicate clockwise P–T paths with near-isothermal decompression and decompressional cooling following the peak metamorphism. The peak metamorphic assemblage was garnet + biotite + K-feldspar + plagioclase + sillimanite + quartz + ilmenite + magnetite with P–T conditions of 9–11 kbar and 830–880 °C. The ensuing post-peak near-isothermal decompressional assemblage was garnet + biotite + cordierite + K-feldspar + plagioclase + quartz ± sillimanite ± magnetite ± ilmenite with P–T conditions of 4.5–7.5 kbar and 810–900 °C, and this assemblage was characterized by large amounts of matrix cordierite due to the biotite decompressional dehydration melting reaction of Bt + Sil + Qz ± Pl → Crd + Kfs ± Ilm + Melt. LA–ICP–MS U–Pb analyses of detrital and metamorphic zircons in the sillimanite–garnet–cordierite pelitic granulites yielded a protolith age of ~2000 Ma and a late Paleoproterozoic metamorphic age of 1890–1870 Ma. Moreover, LA–ICP–MS U–Pb dating of monazites revealed a similar metamorphic age of 1880–1840 Ma. It had been suggested in the earlier literature that the metapelitic rocks from the Ji’an area reached the amphibolite-facies with a counterclockwise P–T path. However, our results indicate a clockwise P–T path and a granulite-facies metamorphism. We find that granulite-facies metamorphism was widespread throughout the Jiao–Liao–Ji Belt, as at Ji’an, Southern Liaohe, Jiaobei, and Bengbu, and the clockwise P–T paths and similar metamorphic ages obtained for all those areas indicate that the granulite-facies metamorphism within the Jiao–Liao–Ji Belt was a response to Paleoproterozoic continent–continent subduction and the collision of the Longgang and Nangrim blocks of the North China Craton during the peak metamorphic stage, followed by post-collisional exhumation in the post-peak decompressional stage.

Phonolitic melt production by carbonatite Mantle metasomatism: evidence from Eger Graben xenoliths

Ultramafic xenoliths and green-core clinopyroxene xenocrysts from a basanite from Spitzberg, Cotta, Germany were investigated. The Spitzberg volcano on the northern shoulder of the Eger (Ohře) Graben is part of the Central European Cenozoic Volcanic suite. Inclusions of (tephri-)phonolitic, carbonatitic and sulfidic melt and associated reaction textures in the ultramafic xenoliths show evidence for metasomatic alteration of lherzolite to wehrlite by carbonatitic melt, which produced the (tephri-)phonolitic melt as a reaction byproduct. The investigated xenoliths and xenocrysts were sampled and brought to the surface by a basanitic eruption. Green-core clinopyroxene (gc-Cpx) xenocrysts found abundantly in the samples have similar inclusions as the wehrlite and are interpreted to have crystallized from the aforementioned (tephri-)phonolitic melt during a later interaction with additional carbonatite melt. Major and trace element chemistry as well as a complex suite of inclusion phases clearly link the melts in both cases. The growth textures of the gc-Cpx indicate that it grew in two stages: first quickly as a result of interaction between (tephri-)phonolitic silicate melt and carbonatite melt; then more slowly and free-floating in magma with no contact to Mantle or Crustal rocks. We conclude that Mantle metasomatism has in this case directly produced (tephri-)phonolitic magma of sufficient volume and proportion to allow extraction from the original host rock, not merely some interstitial phonolitic melt. However, surface outcrops of such phonolite have so far not been identified in continental settings.

Evidence for early impact on a hot differentiated planetesimal from Al-rich micro-inclusions in ungrouped achondrite Northwest Africa 7325

Abstract The ungrouped achondrite NWA 7325 is a cumulate olivine gabbro (Irving et al., 2013). It contains abundant and unique micro-inclusions of Ca-pyroxene (Bischoff et al., 2013) and spinel-like Al-Mg oxide (Goodrich et al., 2017) in plagioclase, indicating a remelting event induced either by impact (Goodrich et al., 2017) or by magma intrusion (Bischoff et al., 2013; Weber et al., 2016). In this work, a combined FIB-TEM study has been conducted on these micro-inclusions to address their petrogenesis and the related history of NWA 7325. TEM study revealed that micro-inclusions in the interiors of large plagioclase grains are Al-enriched spinel (Mg/Al atomic ratio: 0.03–0.4) with minor needle-like α-corundum, whereas those in the margins are predominantly Al-rich diopside (En44.5-46.6Fs1.2-1.5Wo31.2-36.7CaTs17.6-22.4) with minor forsterite (Fo94.6-94.7). The Mg/Al atomic ratios of the spinel micro-inclusions are negatively correlated with the distance away from the interface of plagioclase-pyroxene. Large plagioclase grains also exhibit a decrease in the Mg/Al atomic ratio from the rims to the cores. Based on the reaction texture at the interfaces of plagioclase-pyroxene, we infer that the Mg concentration gradient in large plagioclase grains could have resulted from Mg diffusion from the remelted rims of pyroxene into plagioclase. In addition, TEM observations showed that large plagioclase grains are not single crystals, but assemblages of submicron to micron-sized crystals. The preservation of Mg concentration gradients, submicron-sized polycrystalline plagioclases, and the consistent presence of micro-inclusions within large plagioclase grains likely indicate complete remelting of plagioclase and partial remelting of pyroxene (only rims of pyroxene with plagioclase) followed by fast cooling. We propose that micro-inclusions of diopside, forsterite, Al-rich spinel and corundum crystallized from the melt, which developed a Mg concentration gradient during the remelting of NWA 7325. The heating temperatures of pyroxene and plagioclase in the remelting event were estimated to be 1274–1327 °C and ≥1530 °C, respectively. A subsequent cooling rate of ∼500–650 °C/h at 1300 °C was found by fitting the measured Mg concentration gradient in large plagioclase grains with a Fick’s second law model that incorporated the diffusion coefficients of Mg in plagioclase-melt. These results are better explained by a shock event; a magmatic intrusion process is ruled out. To achieve the coexistence of shock-induced high temperature (≥1274 °C) in-situ melting and only undulatory extinction of forsterite grains, an ambient temperature of 1000–1100 °C in the surrounding, parent rock of NWA 7325 was required prior to impact. This work suggests a very early shock event when NWA 7325 was hot and buried in the crust of its parental planetesimal, which is a scenario consistent with its magma crystallization age (∼4.3 Ma after CAIs, e.g., Koefoed et al., 2016). This work also implies that impacts are a potential heat source for melting hot planetesimals in the early Solar System.

New Occurrence of Sapphirine–spinel–bearing Granulite from NW of Chilka Lake, Eastern Ghats Belt, Odisha

Abstract A new locality of sapphirine-spinel-bearing granulites from the Kaithapalli area which lies NW of Chilka Lake, Odisha is reported. The area tectonically forms a northern part of Eastern Ghats belt. It occurs as small enclaves within the khondalite and pelitic granulite. The mineral assemblage includes spinel-sapphirine-garnet-cordierite-orthopyroxene-sillimanite-biotite-k-feldspar-plagioclase-quartz. Development of reaction textures and symplectites are common in the sapphirine-spinel granulites which have been used to describe reaction history. The relative XMg values among various minerals are as follows: cordierite > biotite > sapphirine > orthopyroxene > garnet > spinel. The P-T evolution of these sapphirine-spinel granulites constrained through the pseudosection modelling in the NCKFMASH model system using Perple_X software indicate conditions of ultra-high temperature (UHT) metamorphism. The P-T estimates computed by isopleths define a retrograde trajectory with decompression of c.2.5 kbar from P-Tmax of c.10.5 kbar at c.950 °C. The sequence of reactions as deduced from the symplectite assemblages, together with pseudosection modelling, from the Kaithapalli area, offer greater opportunities for providing a better picture of petrological evolution of northern part of the Eastern Ghats Belt (EGB).

High-pressure, ultrahigh-temperature 1.9 Ga metamorphism of the Kramanituar Complex, Snowbird Tectonic Zone, Rae Craton, Canada

The high-pressure (P) granulite–facies Kramanituar Complex, dominated by a metagabbroic–anorthositic suite with subordinate sillimanite- and kyanite-bearing diatexite and charnockite, is one of several complexes that demarcate the 1000 km-long Snowbird Tectonic Zone. Ti-in-quartz (TiQ) and Zr-in-rutile (ZiR) thermometry on inclusions in garnet in mafic and pelitic rocks establish that temperatures (T) exceeded 990–1000 °C, ~ 100 °C higher than recorded by Fe–Mg exchange thermometers. At 1000 °C, thermobarometry and forward modelling of both mafic and pelitic rocks from the complex define 14–15 kbar pressures, where TiQ and ZiR temperatures are in good agreement. The occurrence in diatexite of sparse prograde F-bearing biotite and high-T rutile inclusions in garnet rims, not cores, can be accounted for with a high-Ti solubility biotite model and the presence of fluorine. Forward modelling suggests that heterogeneities in both the composition and zoning of garnet reflect the effects of melt loss on bulk composition. Gabbroic rocks with coronitic and symplectitic reaction textures record high-T decompression to 8 kbar and 800 °C, also reflected by matrix quartz ribbons in diatexite which yield TiQ values between 655 and 824 °C at 8 kbar. Combined results define a clockwise P–T path for the complex at 1910–1896 Ma. South of the complex, a contemporaneous clockwise P–T path is recorded in semipelitic rocks that reached peak conditions of 690 °C and 8.1 kbar. Overall, results support tectonic reworking of the thickened (> 50 km) Rae margin from ca. 1910 Ma, culminating in ca. 1902 Ma mantle-derived mafic magmatism and rapid exhumation, potentially triggered by slab breakoff or lithospheric delamination.

Petrology, phase equilibria modelling and zircon U-Pb geochronology of garnet-bearing charnockites from the Miyun area: Implications for microblock amalgamation of the North China Craton

The North China Craton (NCC) was amalgamated through Late Neoarchean amalgamation of seven microblocks along zones of ocean closure represented by granite-greenstone belts (GGB). Typical basement rocks of the Zunhua GGB between the Qianhuai and Jiaoliao microblocks are exposed in the Miyun area of the NCC. Here we report petrology, mineral chemistry, metamorphic P–T estimation and zircon U-Pb geochronology of newly identified garnet-bearing charnockites from the Miyun area. The prograde mineral assemblages are composed of quartz + biotite + clinopyroxene + plagioclase occurring as inclusions in garnet. The peak metamorphic mineral assemblages are identified as garnet + clinopyroxene + orthopyroxene + quartz + plagioclase + K-feldspar + ilmenite ± hornblende whereas the retrograde assemblages are characterized by the breakdown of garnet and formation of quartz + biotite association, together with K-feldspar moats around garnet and magnetite in the matrix. The prograde metamorphic P–T condition is defined at <770 °C and < 9.2 kbar. The peak metamorphic P–T conditions of the charnockite are constrained by a combination of quantitative pseudosection modelling and conventional geothermobarometry, and the results show peak conditions of 835–905 °C and 9.1–11.4 kbar indicating relatively high temperature and high pressure metamorphism. The zircon grains in the garnet-bearing charnockite display clear core-mantle-rim texture, with partial melting event recorded at 2499 ± 12 Ma and subsequent Early Paleoproterozoic retrograde cooling and decompression stage at 2463 ± 11 Ma. The clock-wise P–T trajectories inferred from mineral reaction textures and quantitative P–T pseudosection modelling, in combination with the widespread Late Neoarchean to Early Paleoproterozoic metamorphic ages (2525–2446 Ma) reported from elsewhere in the NCC suggest that the collisional assembly of microblocks and initial cratonization might have occurred during the Neoarchean-Paleoproterozoic transition.

Melting of fluorphlogopite-plagioclase pairs at 1 atmosphere

Melting experiments on fluorphlogopite + plagioclase (An68) pairs have been performed at 1 atmosphere, as a function of temperature and run duration. Run products include, in addition to melt, spinel, forsterite and a newly formed calcic plagioclase. The solidus was located between 1100 and 1150°C and the amount of melt produced increases sharply at 1200°C. From this temperature, liquids are compositionally zoned between the two mineral interfaces, and the mica reacts out after sufficient time indicating the existence of a critical temperature of melting located between 1175 and 1200°C. These two temperature domains are also characterized by differences in plagioclase reaction textures, melt proportions and reaction rates. Textural and Sr isotopic studies of the plagioclases show that at all temperatures the first step of the melting reaction is characterized by congruent dissolution of the starting plagioclase followed by precipitation of the new plagioclase. At all temperatures the melt compositions are initially located on the intersection of the cotectic line and the starting plagioclase-fluorphlogopite tie-line. Below the critical temperature of melting these melts are metastable undercooled liquids. Above the critical temperature of melting there is an array of stable melt compositions located on the plagioclase- fluorphlogopite tie-line. In both cases the melt compositions evolve with time toward more albite-rich compositions in conjunction with the new plagioclase precipitation. The system fluorphlogopite-plagioclase thus provides an example where liquid compositions vary with time although plagioclase-melt interface equilibrium is satisfied. This is due to the involvement of a solid solution in the reaction. Above the critical temperature of melting, the rate-controlling process for melting is chemical diffusion in the melt. Below the critical temperature of melting the rate-limiting step is the formation of the metastable undercooled melt. The sluggishness of the melting reaction in this temperature regime is explained by the difficulty of forming such a melt.

Petrogenesis and tectonic significance of Sardasht syenite–monzonite–gabbro–appinite intrusions, NW Iran

The Middle Eocene intrusive rocks of Sardasht are located in the north-western part of the Sanandaj–Sirjan Zone (SSZ), NW of Iran. The Sardasht complex is a massive or occasionally layered intrusion composed of mafic (gabbro, appinitic gabbro, and appinite) and felsic (syenite and monzonite) units with respective calc-alkaline and alkaline-shoshonite affinities. Mafic mineral constituents are plagioclase, clinopyroxene, olivine, amphibole and opaque minerals which display granular, poikilitic, ophitic, and reaction textures. Amphibole is the dominant mafic mineral displaying an extraordinary variability in texture and chemical composition characterizing the appinite suites. The felsic units are mainly composed of perthitic alkali feldspar along with low abundances of plagioclase, biotite, amphibole and opaque minerals. The occurrence of mafic minerals with felsic minerals in the syenites are typical of agpaitic texture indicating that mafic aggregates and felsic units of the Sardasht intrusions were injected simultaneously. The abundance of major and trace elements show that mafic intrusions were originated from a metasomatized spinel lherzolite mantle with high concentrations of light rare earth elements (LREEs), Ti, Sr and Ba and low contents of Hf, Zr, Nb, and Ta. Mafic and felsic rocks showed the 87Sr/86Sr ratios ranging from 0.7044 to 0.7049 and from 0.7079 to 0.7095 with e(Nd)40.7 Ma ≈ + 4.1–7.6 and + 4.0–4.6, respectively. Combining petrological, geochemical, and isotopic findings, it is suggested that the Sardasht mafic and felsic intrusions were formed as a result of Neotethys slab break-off and transtension along the SE-trending lateral strike-slip fault zones related to oblique subduction of the Neotethys plate underneath east the SSZ in Early to Middle Eocene. The felsic intrusions were resulted from partial melting of the arc crust base due to the injection of hydrous-mafic magma formed in the mantle wedge into the base.

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 (&gt;780–810°C) and [=CO2/(CO2+H2O) &gt;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 (&lt;620°C) and (&lt;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.