Garnet Zone Research Articles

Progressive Low-Grade Metamorphism Reconstructed from the Raman Spectroscopy of Carbonaceous Material and an EBSD Analysis of Quartz in the Sanbagawa Metamorphic Event, Central Japan

Low-grade metamorphic temperature conditions associated with the Sanbagawa metamorphic event were estimated by the Raman spectroscopy of carbonaceous material (RSCM) in pelitic rocks and an electron backscatter diffraction (EBSD) analysis of the quartz in siliceous rocks. Analytical samples were collected from the Sanbagawa metamorphic complex, the Mikabu greenstones, and the Chichibu accretionary complex in the eastern Kanto Mountains, central Japan. Previously, low-grade Sanbagawa metamorphism was only broadly recognized as pumpellyite–actinolite facies assigned to the chlorite zone. The RSCM results indicate metamorphic temperatures of 358 °C and 368 °C for the chlorite zone and 387 °C for the garnet zone of the Sanbagawa metamorphic complex, 315 °C for the Mikabu greenstones, and 234–266 °C for the Chichibu accretionary complex. From the EBSD analyses, the diameter of the quartz grains calculated by the root mean square (RMS) approximation ranges from 55.9 to 69.0 μm for the Sanbagawa metamorphic complex, 9.5 to 23.5 μm for the Mikabu greenstones, and 2.9 to 7.3 μm for the Chichibu accretionary complex. The opening angles of the c-axis fabric approximate 40–50°, presenting temperatures of 324–393 °C for the Sanbagawa metamorphic complex and the Mikabu greenstones. The temperature conditions show a continuous increase with no apparent gaps from these low-grade metamorphosed rocks. In addition, there exists an empirical exponential relationship between the estimated metamorphic temperatures and the RMS values of the quartz grains. In this study, integrated analyses of multiple rock types provided valuable information on progressive low-grade metamorphism and a similar approach may be applied to study other metamorphic complexes.

On exhumation velocities of high-pressure units based on insights from chemical zoning in garnet (Tianshan, NW China)

Exhumed high-pressure rocks formed during subduction provide insight into global geodynamic and plate tectonic processes. Their metamorphic history is partially encoded in their mineralogy and mineral chemistry, which can record the P-T evolution and rates of subduction and exhumation. Here, we study a garnet-glaucophane-albite schist from the SW Tianshan in China in which complexly zoned garnet displays a distinct and sharp oscillatory Mn-zonation and irregular, jagged Ca distributions with peak widths of ∼5 μm. Combined Raman quartz-in-garnet barometry and carbonaceous matter thermometry suggests a P-T path that reached ∼2.5 GPa and was followed by ∼0.9 GPa of nearly-isothermal decompression at temperatures of 530–540°C, associated with final garnet growth. This P-T path is largely confirmed by thermodynamic modeling. Three different approaches to modeling coupled Ca, Mn, Mg and Fe diffusion in garnet were used to estimate the duration between establishment of high-resolution Mn, Fe and Ca zoning during growth and cooling below nominal closure conditions upon exhumation. All models agree best with conditions of ∼530°C maintained for around 100 kyr, though slight differences are found depending on published diffusion coefficients. For the constrained exhumation P-T path, we infer that the studied sample experienced exhumation velocities in the range of 40–45 cm/yr from ∼80 km to ∼50 km depth. We propose an exhumation model involving diapiric Stokes flow along the plate interface and low viscosities around the exhumed unit that may facilitate such high exhumation velocities.

Pressure–temperature–time evolution of subducted crust revealed by complex garnet zoning (Theodul Glacier Unit, Switzerland)

AbstractCollisional orogens commonly include mono‐metamorphic and poly‐metamorphic units, and their different evolution can be difficult to recognize and reconcile. The Theodul Glacier Unit (TGU) in the Western Alps consists of an association of metasedimentary and metamafic rocks embedded within the Zermatt‐Saas tectonic unit. In spite of recent petrological studies, it remains unclear whether these rocks underwent one or multiple metamorphic cycles. In this study, different lithologies from the TGU unit (mafic schist, mafic granofels, and chloritoid schist) were investigated for petrography, quantitative compositional mapping of garnet, thermodynamic modelling, and Lu–Hf garnet dating. The data reveal a coherent mono‐metamorphic history with a β‐shaped pressure–temperature (P–T) path characteristic of oceanic subduction. Garnet Lu–Hf ages yield a restricted garnet crystallization time window between 50.3 and 48.8 Ma (±0.5%, 2SD). A prograde metamorphic stage recorded in garnet cores yields conditions of 490 ± 15℃ and 1.75 ± 0.05 GPa. Maximum pressure conditions of 2.65 ± 0.10 GPa and 580 ± 15℃ were reached at 50.3 ± 0.3 Ma. Initial exhumation was rapid and led to isothermal decompression to 1.50 ± 0.10 GPa within 1 Ma. This decompression was associated with lawsonite breakdown in mafic schist and in mafic granofels, causing intense fluid–rock interaction within and between different lithologies. This process is recorded in garnet textures and trace element patterns, and in the major element composition of K‐white mica. Initial exhumation was followed by re‐heating of ~30℃ at a pressure of 1.50 ± 0.10 GPa. Perturbation of the subduction‐zone thermal structure may be related to upwelling of hot asthenospheric mantle material and transient storage of the unit at the crust–mantle boundary.

Proterozoic High-Temperature–Low-Pressure Metamorphism in the Mahakoshal Belt, Central Indian Tectonic Zone (India): Structure, Metamorphism, U-Th-Pb Monazite Geochronology, and Tectonic Implications

To understand the protracted accretionary evolution along the Central Indian Tectonic Zone (CITZ), we investigated garnet-staurolite schists and associated lithologies from the central Mahakoshal Belt (MB). Mesoscale structures, porphyroblast growth, garnet zoning, and pseudosection modeling were coupled with U-Th-Pb monazite dating to reconstruct a clockwise pressure-temperature-time (P-T-t) path for the garnet- and staurolite-bearing schists. The prograde path is characterized by near isobaric heating conditions, which initiates at pressure-temperature (P-T) conditions of 4.5–5.0 kbar and 550°–560°C and attains peak metamorphism at 5.5–6.0 kbar and 610°–620°C. The peak metamorphism was contemporaneous with the emplacement of southern-margin granitoids at ~1.70 Ga, resulting in the perturbation of geotherms in the collision setting. The retrograde arm (MR) of the P-T path passes through isobaric cooling at ~5.0 kbar and ~500°C. This late Paleoproterozoic P-T-t path is overprinted by hitherto uncharacterized tectonism that coincides with the Sausar orogeny and provides evidence for the northward extension of 0.95–0.85 Ga orogenic activity within the CITZ. The 1.80–1.55 and 0.95–0.85 Ga tectonothermal events identified in this study support that crustal evolution in the CITZ involved a mosaic of domains that were accreted together in the Neoproterozoic time.

Pervasive fluid-rock interaction in subducted oceanic crust revealed by oxygen isotope zoning in garnet

Dehydration reactions in the subducting slab liberate fluids causing major changes in rock density, volume and permeability. Although it is well known that the fluids can migrate and interact with the surrounding rocks, fluid pathways remain challenging to track and the consequences of fluid-rock interaction processes are often overlooked. In this study, we investigate pervasive fluid-rock interaction in a sequence of schists and mafic felses exposed in the Theodul Glacier Unit (TGU), Western Alps. This unit is embedded within metaophiolites of the Zermatt-Saas Zone and reached eclogite-facies conditions during Alpine convergence. Chemical mapping and in situ oxygen isotope analyses of garnet from the schists reveal a sharp chemical zoning between a xenomorphic core and a euhedral rim, associated to a drop of ~ 8‰ in δ18O. Thermodynamic and δ18O models show that the large amount of low δ18O H2O required to change the reactive bulk δ18O composition cannot be produced by dehydration of the mafic fels from the TGU only, and requires a large contribution of the surrounding serpentinites. The calculated time-integrated fluid flux across the TGU rocks is 1.1 × 105 cm3/cm2, which is above the open-system behaviour threshold and argues for pervasive fluid flow at kilometre-scale under high-pressure conditions. The transient rock volume variations caused by lawsonite breakdown is identified as a possible trigger for the pervasive fluid influx. The calculated schist permeability at eclogite-facies conditions (~ 2 × 10–20 m2) is comparable to the permeability determined experimentally for blueschist and serpentinites.

Rapid fluid-driven transformation of lower continental crust associated with thrust-induced shear heating

The rate and progress of regional-scale prograde metamorphism is commonly assumed to be controlled by conductive heat supply. In contrast, the rate of retrograde metamorphism is in most cases controlled by the rate of fluid supply. Crustal-scale heat conduction is a slow process, typically causing temperature changes at rates on the order of 10 °C/million years, but convective fluid fluxes can be substantial on much shorter time scales. Therefore, it is often assumed that prograde metamorphism usually takes place near thermodynamic equilibrium, whereas retrograde metamorphism may operate far from equilibrium. In many settings, including convergent plate margins, fluid-driven retrograde metamorphism is probably strongly coupled to fluid-producing prograde metamorphism. Here, we present new geochemical and geochronological results from the Lindås Nappe in the Bergen Arcs of Western Norway, which show that fluid-driven (retrograde) eclogite- and amphibolite-facies metamorphism of lower crustal granulites was synchronous with the emplacement of numerous pegmatites and dykes. These have geochemical characteristics suggesting an origin by fluid-present melting of the granulites themselves along with underlying schists. The morphology and chemical zoning of garnets from the exposed part of these schists suggest growth during rapid prograde metamorphism associated with emplacement of the Lindås Nappe. We present a simple model that indicates that rapid prograde metamorphism of the schists may have been driven by shear heating near the base of the Lindås Nappe, and that temperatures may have reached wet-melting conditions for both schists and granulites. The Lindås Nappe thus represents an example of how strong and dry lower crust may experience rapid retrograde metamorphic and structural reworking in direct response to shear-heating induced prograde metamorphism taking place near its base.

Origin of multilayer corona textures in mafic granulites from the Sandmata Complex, Aravalli Craton (northwestern India): petrological characteristics and tectonic implications

In Late Proterozoic mafic granulites of the Sandmata Complex in Rajasthan, northwestern India, multilayer corona textures were formed along the interface between orthopyroxene and plagioclase. We examined the metamorphic conditions and processes of formation of these coronae, which provide an insight into the interplay between steady state and sequential diffusion-controlled mineral growth mechanisms. The individual corona–symplectite layers consist of clinopyroxene + quartz|garnet + clinopyroxene|garnet + quartz|K-feldspar, from the inner to the outer margins of the coronae. The single-value decomposition models suggest that the multilayered coronae were formed in a locally closed system, via sequential diffusion of Mg, Fe and Ca into the reaction zone, which has acted as the main driving force for the growth of corona textures. The relict orthopyroxene with exsolved clinopyroxene yields primary crystallization conditions of 8.4 ± 1.5 kbar and ~ 1100–1000 °C. Clinopyroxene in the innermost corona layer grew at ~ 9 kbar and 850–800 °C, whereas clinopyroxene + garnet grew outward at ~ 8 kbar and 700–600 °C. Subsequent hydrous retrogression (~ 6 kbar and 600–550 °C) resulted in the development of rimward zoning in garnet and the growth of amphibole. On combining textural relations and the above conventional P–T estimates, a near-isobaric cooling P–T path was reconstructed using phase equilibria modeling. Further, the near-isobaric cooling path is consistent with the magmatic underplating hypothesis in the Sandmata Complex, where the intrusion of magmatic bodies (i.e., Gyangarh–Asind igneous complex and Anjana granite) favored the development of granulite facies assemblage in norite and gabbronorite protoliths.

Thermal structure in subducted units from continental Moho depths in a palaeo subduction zone, the Asemigawa region of the Sanbagawa metamorphic belt, SW Japan

AbstractRaman CM geothermometry applied to 126 samples of pelitic schists collected over an area of 11 km × 7 km reveals the thermal structure of the Asemigawa region of the Sanbagawa metamorphic belt, southwest Japan in unprecedented detail. In general, the estimated temperatures gradually increase from south to north in the range of 288–553°C. However, a temperature gap from ~380 to ~440°C is identified near the boundary between the chlorite and garnet zones. This temperature region matches the depth of the continental Moho of the Sanbagawa subduction zone. The temperature gradient in the higher‐temperature domain is higher than that in the lower‐temperature domain, and large‐scale tight folds that affect the thermal structure are developed in the high‐grade units and in the vicinity of the temperature discontinuity. These geological structures probably reflect that the exhumed slab units was dammed at the Moho depth due to the upward movement being impeded by increase in the coupling strength of the overlying rocks associated with exhumation from beneath serpentinite rocks to a shallower domain overlain by crustal rocks. Changes in the coupling strength along the subduction boundary led the strong folding at the higher‐temperature domain and the pre‐formed foliation developed at the Moho depth may have acted as the tectonic boundary, resulting in a temperature discontinuity. These results will contribute to elucidating various geological phenomena occurring in the forearc regions of modern subduction zones.

Metapelite from the high‐ to ultrahigh‐pressure terrane of the Eastern Alps (Pohorje Mountains, Slovenia)—New pressure, temperature and time constraints on a polymetamorphic rock

AbstractThe Austroalpine nappe stack of the Pohorje Mountains (Mts.) in northeastern Slovenia comprises a suite of eclogite facies metamorphic rocks that were partially assigned to Eo‐Alpine ultrahigh‐pressure metamorphism (UHPM). We selected a micaschist, which was previously related to this metamorphism, for a detailed study including the chemical zonation of garnet and potassic white mica, the identification of mineral inclusion assemblages, pseudosection modelling with PERPLE_X, and monazite in‐situ dating with the electron microprobe. Polymetamorphism was revealed by (at least) two generations of garnet and phengite and four populations of monazite yielding ages of 283.6 ± 6.1 (2σ), 94.1 ± 3.7, 47.9 ± 10.8 and 26.2 ± 2.8 Ma. The Permian monazite population is characterized by relatively high Y contents (~1.15 wt% Y) and low La/Gd mass ratios (8.7) indicating its formation before the growth of porphyroblastic garnet. The Eo‐Alpine population, however, grew synchronously with garnet based on low Y contents (~0.05 wt%) and high La/Gd ratios (21.4). The older Tertiary population (48 Ma) shows also high Y contents (1.1 wt%) and low La/Gd ratios (10.6) whereas the younger Tertiary population is characterized by low Y contents. The Permian P–T conditions of 7.5–10 kbar at 600–650°C were obtained using the inclusion assemblage of staurolite+rutile+biotite in porphyroblastic garnet. High pressure (HP) but no UHPM was reconstructed for both Eo‐Alpine coarse phengite (Si = 3.22 per formula unit = pfu) and small Tertiary garnet+fine‐grained phengite (Si = 3.27 pfu) at peak pressures ~16 kbar and 18.5–23 kbar respectively. Maximum temperatures close to 650°C were likely reached during the Eo‐Alpine HP event, whereas those of the Tertiary HP event were probably ~580°C. These HP metamorphic events suggest that the Pohorje Mts. experienced both an Eo‐Alpine and a Tertiary subduction–exhumation history, the latter of which was mainly reported for underlying Penninic nappes so far.

Garnet perspectives on the metamorphic history and tectonic significance of Paleoproterozoic high-pressure mafic granulites from the northern Hengshan, North China Craton

Reconstructing the pressure-temperature (P-T) evolution of early Precambrian high-pressure granulites and eclogites is critical for decoding the tectono-metamorphic evolution of the early Earth. However, debates continue regarding the P-T history of these rocks because of various modifications by peak and retrograde rehydration metamorphism. As a significant rock-forming mineral in high-grade metamorphic rocks, garnet not only grows with increasing pressure or temperature and develops variably zoned patterns along given P-T loops, but its properties are stable subsquent to its formation when compared to other minerals in the matrix. Consequently, garnet is excellent for monitoring the thermo-metamorphic history of its host rock during orogeny. Selected garnet grains from the Paleoproterozoic high-pressure mafic granulites in the northern Hengshan area of the North China Craton, which exhibit variable microstructural and chemical zoning, present an excellent example for extracting metamorphic evolution of the host rocks. Microstructural zoning displays a zonal distribution of frozen inclusion-type minerals in garnet. Chemically, the garnet shows pronounced compositional zoning, with bell-shaped XSps, initially flat and then increasing/decreasing XPyr and Fe#, initially increasing and then decreasing XGrs from core to mantle, and resorbed rims. Both zoning patterns document multiple generations of radial growth history. The progressive evolution of the host rocks can be further divided into early prograde (M1–1), late prograde (M1–2), near peak (M2–1), and peak (M2–2) stages. The resorption textures indicate the succeeding decompression (M3) and cooling (M4) stages.The pseudosection modeling indicates P-T conditions from M1–1 to M2–1 of 6–9 kbar/600–700 °C, 9–11.2 kbar/680–740 °C and 12–15 kbar/735–785 °C, respectively. Although the peak metamorphic information of M2–2 is lost because of the breakdown of the garnet rim, plausible P–T results are estimated at ~15–17 kbar/~790–810 °C. These conditions prevailed to the transitional eclogite-granulite facies through qualitative reconstruction of the original composition of clinopyroxene, modal proportion, and rim composition of the garnet. The subsequent conditions of decompression (M3) and cooling (M4) stages were roughly estimated at ~7–10 kbar/~800–850 °C and 3–5 kbar/ 650–680 °C. Together, they define a clockwise P-T path in response to the Paleoproterozoic orogeny involved in the assembly of the North China Craton. The zoned garnet suggests that the host mafic granulite documents a relatively complete progressive metamorphic history, which probably correlates with subduction processes. The resorbed rim sequentially records exhumation and cooling processes. Furthermore, the preservation of growth zoning in granulitic garnet and near-isothermal decompression P-T path suggests that the host rocks must have experienced a very short peak metamorphism and a rapid exhumation process to the middle crust. These findings suggest that the tectonic processes involved in the formation of the Paleoproterozoic northern Hengshan granulite terrane might be similar to those of the Phanerozoic continental collisional orogens under modern plate tectonic regimes; however, they record relatively higher apparent thermal gradients (~14–16 °C /km).

Crustal reworking and hydration: insights from element zoning and oxygen isotopes of garnet in high-pressure rocks (Sesia Zone, Western Alps)

Subduction zones represent one of the most critical settings for fluid recycling as a consequence of dehydration of the subducting lithosphere. A better understanding of fluid flows within and out of the subducting slab is fundamental to unravel the role of fluids during burial. In this study, major and trace element geochemistry combined with oxygen isotopes were used to investigate metasediments and eclogites from the Sesia Zone in order to reconstruct the effect of internal and external fluid pulses in a subducted continental margin. Garnet shows a variety of textures requiring dissolution–precipitation processes in presence of fluids. In polycyclic metasediments, garnet preserves a partly resorbed core, related to pre-Alpine high-temperature/low-pressure metamorphism, and one or multiple rim generations, associated with Alpine subduction metamorphism. In eclogites, garnet chemical zoning indicates monocyclic growth with no shift in oxygen isotopes from core to rim. In metasediments, pre-Alpine garnet relics show δ18O values up to 5.3 ‰ higher than the Alpine rims, while no significant variation is observed among different Alpine garnet generations within each sample. This suggests that an extensive re-equilibration with an externally-derived fluid of distinct lower δ18O occurred before, or in correspondence to, the first Alpine garnet growth, while subsequent influxes of fluid had δ18O close to equilibrium. The observed shift in garnet δ18O is attributed to a possible combination of (1) interaction with sea-water derived fluids during pre-Alpine crustal extension and (2) fluids from dehydration reactions occurring during subduction of previously hydrated rocks, such as the serpentinised lithospheric mantle or hydrated portions of the basement.

Metamorphic gradient modification in the Early Cretaceous Northern Andes subduction zone: A record from thermally overprinted high-pressure rocks

New field observations and petrological data from Early Cretaceous metamorphic rocks in the Central Cordillera of the Colombian Andes allowed the recognition of thermally overprinted high-pressure rocks derived from oceanic crust protoliths. The obtained metamorphic path suggests that the rocks evolved from blueschist to eclogite facies towards upper amphibolite to high-pressure granulite facies transitional conditions. Eclogite facies conditions, better recorded in mafic protoliths, are revealed by relic lawsonite and phengite, bleb- to worm-like diopside-albite symplectites, as well as garnet core composition. Upper amphibolite to high pressure granulite facies overprinting is supported by coarse-grained brown-colored Ti-rich amphibole, augite, and oligoclase recrystallization, as well as the record of partial melting leucosomes.Phase equilibria and pressure-temperature (P-T) path modeling suggest initial high-pressure metamorphic conditions M1 yielding 18.2–24.5 kbar and 465–580 °C, followed by upper amphibolite to high pressure granulite facies overprinting stage M2 yielding 6.5–14.2 kbar and 580–720 °C. Retrograde conditions M3 obtained through chlorite thermometry yield temperatures ranging around 286–400 °C at pressures below 6.5–11 kbar. The obtained clockwise P-T path, the garnet zonation pattern revealing a decrease in Xgrs/Xprp related to Mg# increment from core to rim, the presence of partial melting veins, as well as regional constraints, document the modification of the thermal structure of the active subduction zone in Northern Andes during the Early Cretaceous. Such increment of the metamorphic gradient within the subduction interface is associated with slab roll-back geodynamics where hot mantle inflow was triggered. This scenario is also argued by the reported trench-ward magmatic arc migration and multiple extensional basin formation during this period. The presented example constitutes the first report of Cretaceous roll-back-related metamorphism in the Caribbean and Andean realms, representing an additional piece of evidence for a margin-scale extensional event that modified the northwestern border of South America during the Early Cretaceous.

Regional geochemical variations in a metamorphosed black shale: a reconnaissance study of the Silurian Smalls Falls Formation, Maine, USA

A reconnaissance geochemical study of 21 samples of sulphidic black phyllite and schist from the Silurian Smalls Falls Formation in Maine was undertaken in order to evaluate compositional changes during regional metamorphism. These samples represent variably metamorphosed black shale. Analyzed samples come from the chlorite zone in northern Maine and the biotite, garnet, and staurolite-andalusite zones in west-central Maine. Strata of the Smalls Falls Formation are distinctive in containing abundant pyrite and/or pyrrhotite (total S = 1.2–9.7 wt%), but only minor organic matter or graphite (TOC = 0.43–1.85 wt%); TOC/S ratios are uniformly low (average = 0.37 ± 0.22). Median enrichment factors were calculated for each element by normalizing the concentration to Ti in each sample to the Ti-normalized median composition of global black shale. In the chlorite zone, moderate to large decreases in enrichment factors (-23.1 to -49.8%) are evident for V, Cr, Cu, Ni, Zn, Pb, Sb, and U, attributed here to various factors during sedimentation plus variable element mobility during diagenesis. With increasing metamorphic grade (biotite through staurolite-andalusite zones), systematic small to extreme decreases (-14.5 to -99.0%) were found for Ba, Sb, Au, and U, together with less-systematic moderate to large decreases (-35.4 to -61.1%) for V and As. Molybdenum shows an extreme decrease (-94.7%) from the garnet to staurolite-andalusite zones. Excluding Ba, these results are interpreted to mainly reflect mobility of trace elements during pyrite recrystallization, and during the metamorphic transformations of organic matter to graphite and of pyrite to pyrrhotite. Moderate to large increases for Rb (+28.1 to +61.5%) and Th (+39.1 to +47.3%) from the biotite to staurolite-andalusite zones likely record the introduction of alkalis and mass loss, respectively, during metamorphism. Three samples from one site in the garnet zone differ in having anomalously high Fe/Al and low La/Yb ratios, attributed here to epigenetic formation of pyrite and related leaching of light rare earth elements during syn-metamorphic, channelized fluid flow.Geologic and geochemical data indicate that strata of the Smalls Falls Formation were deposited during an interval of anoxia on the northwestern flank of the Central Maine Basin, for which detrital sources included an evolved continental arc. Onset of anoxia coincided with deposition of the Mayflower Hill Formation of the Vassalboro Group, on the basin’s southeastern flank, related to emergence of the Brunswick subduction complex. We suggest that this emergence played a role in promoting both lateral and vertical circulation changes, nutrient loading, and deoxygenation through subsequent basin closure that culminated with Acadian deformation and metamorphism. Based on the relatively high contents of total sulphur present in our Smalls Falls samples, sediments in the Black Sea represent the only known plausible candidate among those in modern suboxic to euxinic basins.

Decoding the oxygen fugacity of ore-forming fluids from garnet chemistry, the Longgen skarn Pb-Zn deposit, Tibet

The relationship between the compositional zonation of garnet in skarn deposits and the evolution of physicochemical conditions of ore-forming fluids is not clear and needs further investigation. This study presents the characteristics of trace elements of garnet from the Longgen skarn Pb-Zn deposit, Tibet with an aim to reveal the evolution trend of the physical and chemical conditions (fO2 and pH) of ore-forming fluids and to explore the fingerprint of garnet trace elements in different types of skarn deposits.Based on robust alteration halo, mineral assemblages and petrographic observation, the garnets of the Longgen skarn Pb-Zn deposit are divided into two types: (1) distal exoskarn garnet near limestone with ore-bearing and fine oscillatory zoning (Adr39.9-75.0; Grt1, Adr represents andradite) and (2) proximal exoskarn garnet (core: Adr32.9-70.7; rim: Adr83.3-99.5) with Pb-Zn-rich mineral and coarse oscillatory zoning (Grt2). According to the total rare earth element (ΣREE) content in the garnet compositional zone, Grt1 is further divided into three component zones: Grt1-1 (heavy rare earth element (HREE)-enriched chondrite-normalized REE distribution patterns and positive Eu-anomaly (δEu > 1, δEu = 2*(Eusample/Euchondrite)/(Smsample/Smchondrite + Gdsample/Gdchondrite)), Grt1-2 (HREE-enriched REE patterns with negative Eu-anomaly (δEu < 1)), and Grt1-3 (light rare earth element (LREE)-enriched REE patterns with δEu > 1). Similarly, five component zones were identified in Grt2: Grt2-1 (REE-rich REE patterns with δEu < 1), Grt2-2 (LREE-depleted REE patterns with δEu < 1), Grt2-3 (steady REE patterns with δEu < 1), Grt2-4 (LREE-enriched REE patterns with δEu > 1), and Grt2-5 (LREE-enriched REE patterns with δEu < 1) from the core to rim, respectively.The fractionation of LREE with HREE and the behavior of Eu indicate that the physical–chemical conditions during the formation of Grt1-1-Grt1-2 and Grt2-1-Grt2-3 were near neutral (pH = 6–7), and changed to mildly acidic in Grt1-3 and Grt2-4-Grt2-5 (pH < 6–7). In addition, the characteristics of low U (<1 ppm) and the linear relationship between Sn and Eu indicate that the fO2 of the ore-forming fluid was relatively oxidized in comparison to the garnets found in other skarn deposits, and Grt1-1-Grt1-3 and Grt2-1-Grt2-5 have a gradual increasing oxidative ability. The linear relationship between ΣREE and Y/Ca indicates that Grt1 and Grt2 grew in a near-closed system. The incorporation of REE3+ into the garnet was mainly controlled by a “yttrogarnet” (YAG)-type substitution, Ca site vacancy, fluid chemistry, and physicochemical conditions of precipitation. However, the compositional zone at the transition zone between Al-rich and Fe-rich segments in Grt2-4 may be formed in an open system, as suggested by the exchange of LREE and Cl−. In conclusion, garnet crystal growth was affected by the crystal chemistry and physicochemical conditions of the skarn deposit. This study demonstrates that factor analysis and discrimination diagrams (log Sn vs. log U, log Ce vs. log U, and log (Ce + Hf) vs. log U) are effective methods for distinguishing different types of skarn deposits by their contents of redox-sensitive trace elements in garnet.

Li isotope zoning in garnet from Franciscan eclogite and amphibolite: The role of subduction-related fluids

Fluids released within subduction zones affect many fundamental Earth processes, but very little is understood about processes of fluid transport within subduction zones and the chemistry of the fluids. Centimeter-scale variations in bulk-rock Li isotope compositions in subduction-related metamorphic rocks are observed associated with fluid-related features. The small spatial scales of these variations suggest very short duration fluid infiltration events – on the order of months to centuries. However, bulk-rock measurements record a time-integrated record, while in situ measurements in metamorphic porphyroblasts such as garnet have the potential to record individual events experienced by the rock. In this study, traverses across garnet in Franciscan eclogite and amphibolite were analyzed for Li isotopes in situ using SIMS to determine the potential for inferring the duration of metamorphic fluid flow and for deciphering fluid chemistry. The Li concentrations of these garnet crystals are very low, ∼5 µg/g. The measured range of δ7Li within some Franciscan garnets is 19‰, which falls well outside the uncertainty of the measurement (<∼4‰ 2σ). Observed variations in δ7Li occur within crystals over a scale of a few hundred microns, with crystal core-to-rim traverses exhibiting troughs of very low δ7Li measurements in the mantle region of the garnet crystals, surrounded by higher values in the garnet crystal rims and cores. Garnet crystal rims have lower δ18O values, which have previously been interpreted as a product of the infiltration of serpentinite-derived fluids. These results suggest a role for fluids fluxing through the slab to create the observed variability, and the unusual trough pattern suggests a role for intracrystalline diffusion. The low δ7Li troughs are observed only in garnet from one of two Franciscan localities. Nevertheless, garnets are similar in composition from the two localities. This contrast suggests that some aspect of the chemistry of the fluids may be responsible for the differences between the two localities; likely candidates are either pH or Eh of the fluid.

Regional Quartz Inclusion Barometry and Comparison with Conventional Thermobarometry and Intersecting Isopleths from the Connecticut Valley Trough, Vermont and Massachusetts, USA

AbstractA comparative analysis of Raman shifts of quartz inclusions in garnet was made along two traverses across the Connecticut Valley Trough (CVT) in western New England, USA, to examine the regional trends of quartz inclusion in garnet (QuiG) Raman barometry pressure results and to compare this method with conventional thermobarometry and the method of intersecting garnet core isopleths. Overall, Raman shifts of quartz inclusions ranged from 1·2 to 3·5 cm–1 over all field areas and displayed a south to north decrease, matching the overall decrease in mapped metamorphic grade. Raman shifts of quartz inclusions typically did not show systematic variation with respect to their radial position within a garnet crystal, and indicate that garnet probably grew at nearly isothermal and isobaric pressure–temperature (P–T) conditions. The P–T conditions inferred from conventional thermobarometry were in the range of ∼500–575 °C and ∼7·4–10·3 kbar over the sample suite and are in good agreement with previous published thermobarometry throughout the CVT. These P–T results are broadly consistent with QuiG barometry and also suggest that garnet grew isothermally and isobarically at near peak P–T conditions. However, P–T conditions and P–T paths inferred using either garnet core thermobarometry or garnet core intersecting isopleths yield results that are internally inconsistent and generally disagree with the pressure results from QuiG barometry. Garnet core isopleth intersections consistently plotted between the nominal garnet-in curve on mineral assemblage diagrams and the P–T conditions constrained by QuiG isomekes for the majority of the sample suite. Additionally, most samples’ P–T results from QuiG barometry and rim thermobarometry show marked disagreement from those derived from garnet core thermobarometry, compared with the minority that showed agreement within uncertainty. Pressures calculated from QuiG barometry ranged from 8·5 to 9·5 kbar along the traverses in western Massachusetts (MA) and central Vermont (VT) and from 6·5 to 7·5 kbar in northern VT indicating an increase in peak burial of 3–6 km from north to south. Along the western end of the central VT traverse, there are differences in measured Raman shifts and inferred peak pressures of up to 1 kbar across the Richardson Memorial Contact (RMC), indicating a possible fault contact with minor post-peak metamorphic shortening of up to ∼3 km. In contrast, along an east–west traverse in the vicinity of the Goshen Dome, MA, there was little observed variation in Raman shifts across the contact. By contrast, QuiG barometry clearly indicates significant discontinuities in peak pressure east of the Strafford Dome in central VT. This supports the interpretation that post-peak metamorphic shortening was necessary to juxtapose upper staurolite–kyanite zone rocks next to lower garnet zone pelites. Overall, it is concluded that garnet core thermobarometry and garnet core isopleths may provide unreliable results for the P–T conditions of garnet nucleation and inferred P–T paths during garnet growth unless independently verified. The consistency of QuiG results with rim thermobarometry indicates that peak metamorphic conditions previously reported for the CVT using garnet rim thermobarometry are robust and that variation in QuiG barometry results is a valuable tool to analyze structural features within a metamorphic terrane.

Revaluation of “equilibrium” P-T paths from zoned garnet in light of quartz inclusion in garnet (QuiG) barometry

Metamorphic P–T paths provide valuable constraints on the tectonics of collisional orogens and many such paths have been deduced from analysis of chemical zoning in garnet. Inclusion barometry (e.g. quartz-in-garnet or QuiG) provides complementary and sometimes contradictory information regarding the interpretation of garnet zoning and the P–T paths derived therefrom. For example, QuiG barometry on several generations of garnet from well-characterized samples from Fall Mountain, New Hampshire, is consistent with the P–T path inferred from other methods. However, QuiG barometry is inconsistent with P–T paths inferred from garnet zoning from the Orfordville Belt (Vermont), Townshend Dam (Vermont), the Connecticut Valley Trough (Vermont), and Sifnos (Greece). New data from the Perry Mountain Formation, southeastern New Hampshire, suggests that QuiG may preserve a record of polymetamorphic events. Specifically, QuiG barometry in coticule samples is interpreted to record a previously unrecognized early medium pressure (ca. 0.9 GPa) metamorphism and a later low pressure (ca. 0.3 GPa) metamorphism.

Eocene migmatite formation and diachronous burial revealed by petrochronology in NW Himalaya, Zanskar

AbstractIn this contribution, we highlight the importance of in‐situ monazite geochronology linked to P−T modelling for identification of timescales of metamorphic processes. Barrovian‐type micaschists, migmatites and augengneiss from the Gumburanjun dome in the southeastern extremity of the Gianbul dome, NW Himalaya, have been studied in order to correlate the early stages of Himalayan metamorphism at different crustal levels and infer the timing of anatexis. P−T−t paths are constrained through combined pseudosection modelling and in‐situ and in‐mount monazite and xenotime laser ablation–split‐stream inductively coupled plasma‐mass spectrometry. Petrography and garnet zoning combined with pseudosection modelling show that garnet‐staurolite schists record burial from ~530 to 560°C and 5.5 kbar to ~630 to 660°C and 7 kbar; staurolite‐kyanite schists from ~530 to 560°C and 5 kbar to ~670 to 680°C and 7−9 kbar; and garnet‐kyanite migmatites from 540−570°C and 5 kbar to ~680 to 750°C and 7−10 kbar, probably also to &gt;750°C and &gt;9 kbar above the muscovite stability field. The decompression paths of garnet‐staurolite schists indicate cooling on decompression, while garnet rim chemistry and local sillimanite growth point to a stage of re‐equilibration at ~600 to 670°C and 4−6 kbar in some of the staurolite‐kyanite schists, and at ~670 to 700°C and 6 kbar in garnet‐kyanite migmatites. Some of the staurolite‐kyanite schists and garnet‐kyanite migmatites also contain andalusite or andalusite‐cordierite. Monazite and xenotime were analysed in thin sections in garnet, staurolite and kyanite, and in the matrix; and in mounts. BSE images and compositional maps of monazite (xenotime was too small) show variable internal structures from homogeneous through patchy zoning with embayed to sharp boundaries. Two groups of samples can be identified on the basis of the presence or absence of c. 44 − 37 Ma ages. The first group of samples—two garnet‐staurolite schists—recorded only c. 31 − 27 Ma ages in porphyroblasts and no c. 40 Ma ages. The second group (samples of staurolite‐kyanite schist, garnet‐kyanite migmatites, augengneiss) have both the older, c. 44 − 37 Ma monazite ages in porphyroblasts and younger ages down to c. 22 Ma. These significantly different ranges of ages from porphyroblasts of 44−37 Ma, and 31−27 Ma, are interpreted as the duration of prograde P−T paths in Eocene and Oligocene, and indicate diachronous two‐stage burial of rocks. Early migmatization occurred at 38 Ma. The c. 29 Ma is interpreted as the time when rocks from the lower and middle crustal levels were partially exhumed and came in to contact with rocks that were downgoing at this time. Localized monazite recrystallization is as young as 26−24 Ma. The youngest ages of 23−22 Ma are related to leucogranite emplacement.

Tectonic implications of juxtaposed high- and low-pressure metamorphic field gradient rocks in the Turvo-Cajati Formation, Curitiba Terrane, Ribeira Belt, Brazil

The Turvo-Cajati Formation (TCF) is an important unit forming the Curitiba Terrane, a major segment of the southern Ribeira Belt, SE Brazil. It is composed of rocks of greenschist (LTCF), amphibolite (MTCF) and granulite (HTCF) facies conditions. Previous studies in the HTCF indicate that the unit underwent extensive partial melting under high-pressure conditions (670–810 °C and 9.5–12 kbar), within the kyanite stability field. In this paper, a study of the metamorphic zoning within the LTCF and MTCF is undertaken using pseudosection modeling in the NCKFMASHTO and MnNCKFMASHTO model systems coupled with detrital zircon U–Pb geochronology. Four metamorphic zones are recognized for the LTCF and MTCF: biotite, garnet, staurolite and sillimanite zones, with predominance of sillimanite zone and pressures lower than 8 kbar, as staurolite breaks down straight to sillimanite, without formation of a kyanite zone. Pseudosections yielded metamorphic peak conditions of ~530–560 °C and ~6–7.5 kbar (garnet zone) and ~660–690 °C and ~6–7.5 kbar (sillimanite zone). The metamorphic field gradient is flat and of low to medium pressure, below the typical Barrovian-type baric regime, and different from the HTCF. Available petrological and geochronological data suggest that the TCF comprises a paired low-P and high-P metamorphic belt, associated with a major Ediacaran suture zone in the southern Ribeira Belt. Probability density plots from detrital zircon U–Pb ages indicate late-Cryogenian-Ediacaran arc-related and Rhyacian sources for all TCF sub-units. This scenario suggests that the TCF is made up of a collisional juxtaposition of an accretionary wedge (HTCF) and a back-arc basin (LTCF and MTCF) on the border of a microplate, which includes a Rhyacian basement microcontinent, the Atuba Complex. It is inferred the high metamorphic gradient recorded in the LTCF and MTCF was related with asthenospheric upwelling in the back-arc region, which also produced extensive partial melting in the Atuba Complex basement.

Mineralogy, isotope geochemistry and ore genesis of the miocene Cuonadong leucogranite-related Be-W-Sn skarn deposit in Southern Tibet

The Himalayan orogenic belt is characterized by widespread leucogranites, which are mostly peraluminous with less than 5% mafic minerals. Previous studies mainly focused on the petrological characteristic, genetic mechanism, magmatic evolution and tectonic significance of these leucogranites. However, recently, a skarn-type Be–W–Sn polymetallic deposit, which is spatially related to the Cuonadong leucogranites of the Himalayan leucogranites belt, was discovered. The orebodies occur in the inner and outer contact zones between the Cuonadong leucogranites and their surrounding marble. This study focuses on the the characteristics of the skarn mineral assemblages. The deposit shows a typical skarn mineral zonation, from inside to outside, including dark pyroxene-garnet skarn zone, red garnet zone, dark green amphibole-specularite zone, light-green vesuvianite-epidote ribbon, and vesuvianite-chlorite zone. Zircon U-Pb dating shows that the ore-related leucogranite was emplaced at ca. 18 Ma. Geochemically, the leucogranite has high SiO2, Al2O3, Na2O, K2O and low CaO contents belonging to peraluminous granite. The Cuonadong leucogranite is characterized by right-dipping chondrite-normalized REE patterns with distinct negative Eu anomalies. Importantly, all samples show Lanthanide tetrad effect on chondrite-normalized REE patterns, indicating significant melt-fluid interaction has occurred due to strong fractional crystallization. Additionally, the Cuonadong leucogranite is enriched in large ion lithophile elements (such as Rb) and relatively depleted in high field strength elements (such as Nb, Zr, Hf), suggesting a crustal source region. High F and B contents in the magma could have significantly increased the solubility of water in the magma, while the magmatic-hydrothermal solution enriched in F and B should have remarkably enhanced the metal-carrying capacity and increased the permeability of surrounding rocks, which in turn effectively facilitated water–rock reaction.