Late Progradation Research Articles

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).

Meta-rodingite dikes as recorders of subduction zone metamorphism and serpentinite dehydration: Voltri Ophiolite, Italy

The metamorphic devolatilization of serpentinites during subduction represents the largest potential source of fluids from the subducting slab and influences a range of important processes from subduction zone seismicity to arc magmatism. Obtaining a record of metamorphic dehydration directly from serpentinites, however, is challenging, as serpentinite mineral assemblages are less conducive to extracting a detailed pressure-temperature-time (P-T-t) history. In this study, we use meta-rodingites (metasomatized gabbros) from the central portion of the Voltri Ophiolite in the Ligurian Alps (Italy), to investigate the P-T and geochronological record of subduction metamorphism and dehydration within the adjacent serpentinites. These meta-rodingites underwent eclogite-facies metamorphic recrystallization to a garnet-clinopyroxene-chlorite-ilmenite assemblage, and are cross-cut by several generations of garnet-bearing veins. A comparison of the Voltri meta-rodingites with un-subducted seafloor rodingites from the Apennines reveals textural and geochemical evidence of complete metamorphic recrystallization of the original seafloor metasomatic assemblage during Alpine subduction. Phase equilibria modeling of the Voltri meta-rodingite sample suggests that the peak metamorphic assemblage recorded was stable in the range of ~450–600 °C, which is consistent with previous P-T estimates for gabbroic and ultramafic lithologies from the Voltri Ophiolite. A late prograde to peak metamorphic age of 40.6 ± 2.9 Ma was obtained from the meta-rodingite using SmNd garnet geochronology, which closely matches the ages of late prograde to peak metamorphism estimated by companion studies of associated eclogites from the central part of the Voltri Ophiolite. The overlap in P-T conditions and timing of peak metamorphism for samples across the area suggests subduction and exhumation of the central part of the Voltri Ophiolite as a coherent lithospheric slice. Dating of a garnetite vein cross-cutting the main meta-rodingite body suggests an influx of fluid at 37.7 ± 1.4 Ma, at or near peak eclogite-facies conditions. Enrichment in the vein selvage of both compatible transition metals (Cr, Ni, Cu) and fluid-mobile elements (Li, Rb, Cs, Ba) suggests the fluid carried both serpentinite and sedimentary geochemical signatures and interacted with the main meta-rodingite body. We suggest that the most probable explanation for this fluid composition, and thus the source of the fluid, is the dehydration of the Voltri serpentinites. These serpentinites likely experienced an influx of sedimentary-derived, fluid-mobile-element-bearing fluids earlier in their history, either on the seafloor or most likely during the early stages of subduction. The presence of a sedimentary signature in this serpentinite-derived fluid emphasizes the importance of subducted serpentinites and their dehydration to the geochemical cycling of fluid-mobile elements in subduction zones. The widespread development of such vein systems within multiple meta-rodingites across the Voltri Ophiolite points to a large-scale fluid release event, likely resulting from the partial dehydration of brucite and antigorite within the surrounding serpentinite hosts. By utilizing direct dating of vein mineralogy formed during serpentinite dehydration, we provide one of the first geochronological records of serpentinite dehydration and fluid release occurring during subduction.

Mineralogical and metasomatic evolution of the Jurassic Baoshan scheelite skarn deposit, Nanling, South China

The Baoshan scheelite deposit, located in the Nanling region, South China, is a skarn-type tungsten ore deposit. Orebodies occur in the contact zone between Carboniferous limestone and Baoshan granitic pluton. The pluton consists mostly of coarse-grained, porphyritic, and fine-grained biotite granites, which have zircon U-Pb ages of 172.4 ± 0.3 Ma, 166.6 ± 0.3 Ma, and 156.6 ± 1.4 Ma, respectively. These rocks have similar geochemical characteristics with high SiO2, total alkalis, high differentiation indexes and high Rb/Sr ratios of S-type granites formed by partial melting of upper continental crust. The porphyritic granites display the highest degree of fractionation, and are most closely associated with the formation of skarn ores.The skarns are zoned mineralogically outward from garnet, through clinopyroxene, to wollastonite, and overprinted by retrograde minerals. Three stages of skarnization are recognized: Early prograde, late prograde and retrograde. The early prograde stage is mainly composed of Al-rich garnet (Grs60–76) and Mg-rich clinopyroxene (Di41–62) with subordinate Mo-rich scheelite (Sch59–86), all of which are typically overgrown by late prograde assemblages of Fe-rich garnet (And67–95), Fe-rich clinopyroxene (Hd81–87), and Mo-depleted scheelite (Sch97–98). The retrograde stage is characterized by Al-rich garnet (Grs40–66) and pure scheelite (Sch100), accompanied by hydrous and sulfide minerals. Minor subcalcic garnet with magnetite sometimes occurs in the limited endoskarns. Fluid inclusions of the early prograde minerals homogenize at 390.5 to >550 °C, and one inclusion in wollastonite has a salinity of 13.0 equiv. wt% NaCl. Inclusions in minerals of the late prograde and retrograde stages have homogenization temperatures of 196.9–377.4 °C and 160.4–291.4 °C, respectively; their salinities 2.4–19.0 equiv. wt% NaCl and 1.4–11.6 equiv. wt% NaCl, respectively. Accordingly, the high-temperature and moderate- to high-salinity fluids in the early skarn stages are typical of magmatic fluids originating granites, whereas the lower temperature and variable salinity in the later stages suggest mixing with meteoric water.

Mineralogy and metasomatic evolution of the Mianeh iron skarn deposit, Norduz-Agarak border, NW Iran

The Mianeh iron skarn deposit lies in the Arasbaran region within the Qaradagh metallogenic district, NW Iran. This high-grade massive magnetite skarn originated by the interaction of Upper Cretaceous limestone with metasomatic ore-bearing fluids associated with hypabyssal Oligo-Miocene quartz diorite. Mineral chemistry of the primary clinopyroxenes demonstrates the sub-alkaline, volcanic arc setting of magmatism. Two general stages of skarnification are recognized: (1) silicate skarn (stage I) is composed essentially of grossular and low-Fe diopside formed before the main mineralization and (2) magnetite-garnet skarn (stage II) composed of strongly anisotropic coarse-grained garnets with a narrow compositional zoning radially formed by addictive infiltrating of silica and iron-rich metasomatic fluids which overprint and/or crosscut the early stage silicate skarn. Anhydrous prograde calc-silicate assemblages were replaced by a series of hydrous calc-silicates (epidote, tremolite-actinolite) and/or quartz, calcite, magnetite, hematite, and pyrite. Magnetite (±hematite) is the dominant hypogene ore mineral that initially precipitated coincident with the late prograde to the early retrograde metasomatic stages. Mineralogical studies suggest that silicate skarn formation commenced at temperatures about 560 °C, X(CO2)fluid ≤ 0.15, αSiO2∼−1.0, and fluid pressure 1.0 kbar. The magnetite-garnet skarn formed from H2O-rich fluids [X(CO2)fluid < 0.1] at a temperature of 525 to 450 °C and maximum log ƒO2 between −20.2 and −23. During the late stages of prograde skarn development, the stability field of andradite shifted to low ƒO2 and ƒS2 conditions resulting in main iron ore deposition (as magnetite). The andradite replacement temperature and presence of pyrite (instead of pyrrhotite) suggest that logƒS2 remained constant at about −6 to −7 during cooling of the system.

Osumilite–melt interactions in ultrahigh temperature granulites: phase equilibria modelling and implications for the P–T–t evolution of the Eastern Ghats Province, India

AbstractThe exposed residual crust in the Eastern Ghats Province records ultrahigh temperature (UHT) metamorphic conditions involving extensive crustal anatexis and melt loss. However, there is disagreement about the tectonic evolution of this late Mesoproterozoic–early Neoproterozoic orogen due to conflicting petrological, structural and geochronological interpretations. One of the petrological disputes in residual high Mg–Al granulites concerns the origin of fine‐grained mineral intergrowths comprising cordierite + K‐feldspar ± quartz ± biotite ± sillimanite ± plagioclase. These intergrowths wrap around porphyroblast phases and are interpreted to have formed by the breakdown of primary osumilite in the presence of melt trapped in the equilibration volume by the melt percolation threshold. The pressure (P)–temperature (T) evolution of four samples from three localities across the central Eastern Ghats Province is constrained using phase equilibria modelling in the chemical system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO). Results of the modelling are integrated with published geochronological results for these samples to show that the central Eastern Ghats Province followed a common P–T–t history. This history is characterized by peak UHT metamorphic conditions of 945–955 °C and 7.8–8.2 kbar followed by a slight increase in pressure and close‐to‐isobaric cooling to the conditions of the elevated solidus at 940–900 °C and 8.5–8.3 kbar. In common with other localities from the Eastern Ghats Province, the early development of cordierite before osumilite and the peak to immediate post‐peak retrograde reaction between osumilite and melt to produce the intergrowth features requires that the prograde evolution was one of contemporaneous increasing pressure with increasing temperature. This counter‐clockwise (CCW) evolution is evaluated for one sample using inverse phase equilibria modelling along a schematic P–T path of 150 °C kbar−1 starting from the low P–T end of the prograde P–T path as constrained by the phase equilibria modelling. The inverse modelling is executed by step‐wise down temperature reintegration of sufficient melt into the residual bulk chemical composition at the P–T point of the 1 mol.% melt isopleth at each step, representing the melt remaining on grain boundaries after each prograde drainage event, to reach the melt connectivity transition (MCT) of 7 mol.%. The procedure is repeated until a plausible protolith composition is recovered. The result demonstrates that clastic sedimentary rocks that followed a CCW P–T evolution could have produced the observed mineral assemblages and microstructures preserved in the central Eastern Ghats Province. This study also highlights the role of melt during UHT metamorphism, particularly its importance to both chemical and physical processes along the prograde and retrograde segments of the P–T path. These processes include: (i) an increase in diffusive length scales during the late prograde to peak evolution, creating equilibration volumes larger than a standard thin section; (ii) the development of retrograde mineral assemblages, which is facilitated if some melt is retained post‐peak; (iii) the presence of melt as a weakening mechanism and the advection of heat by melt, allowing the crust to thicken; and (iv) the effect of melt loss, which makes the deep crust both denser and stronger, and reduces heat production at depth, limiting crustal thickening and facilitating the transition to close‐to‐isobaric cooling.

How long-lived is ultrahigh temperature (UHT) metamorphism? Constraints from zircon and monazite geochronology in the Eastern Ghats orogenic belt, India

Along the coast of Peninsular India, the Eastern Ghats expose a deep crustal section through a composite Proterozoic orogenic belt. To quantify the late Mesoproterozoic–early Neoproterozoic pressure (P)–temperature (T)–time (t) evolution of the Eastern Ghats Province, new SHRIMP U–Pb zircon and monazite age data from multiple localities are reported and integrated with the results of phase equilibria modelling. Samples of residual granulite, migmatite and enderbite yield a spread of weighted mean 207Pb/206Pb zircon and monazite ages between ca 970 and ca 930Ma. Based on ranges of spot ages from several samples, the late prograde to peak ultrahigh temperature (UHT) metamorphism (counter-clockwise evolution (CCW) to T >950°C at P >8kbar) and initial cooling is interpreted to have occurred between ca 1130 and ca 970Ma. Regionally extensive enderbite and charnockite magmas were emplaced into the hot, suprasolidus crust around the time of peak metamorphism. For the residual granulites and migmatites the retrograde P–T–t path is characterized by close-to-isobaric cooling to the variable but elevated solidi for different samples. Weighted mean ages between ca 970 and ca 930Ma in several samples are interpreted to record the timing of crystallization of melt trapped by the percolation threshold in each of these samples. Two additional weighted mean ages of ca 980Ma (from Korhonen F.J., Saw, A.K., Clark, C., Brown, M., Bhattacharya, S., 2011. New constraints on UHT metamorphism in the Eastern Ghats Province through the application of phase equilibria modelling and in situ geochronology. Gondwana Research 20, 764–781) extend this range back in time by 10My. The variability in the calculated weighted mean ages across the region is interpreted to be due mainly to differences in the temperature of the elevated solidus from sample to sample, suggesting a slow cooling rate of ∼1°C/My during the retrograde stage of this long-lived UHT metamorphism. These results demonstrate that the Eastern Ghats Province sustained UHT conditions (T >900°C) for ≫50My, and perhaps for as long as 200Ma from ca 1130 to 930Ma, during a single CCW tectono-metamorphic event.

Geomorphological and sequence stratigraphic variability in wave‐dominated, shoreface‐shelf parasequences

Abstract Physical stratigraphy within shoreface‐shelf parasequences contains a detailed, but virtually unstudied, record of shallow‐marine processes over a range of historical and geological timescales. Using high‐quality outcrop data sets, it is possible to reconstruct ancient shoreface‐shelf morphology from clinoform surfaces, and to track the evolving morphology of the ancient shoreface‐shelf. Our results suggest that shoreface‐shelf morphology varied considerably in response to processes that operate over a range of timescales. (1) Individual clinoform surfaces form as a result of enhanced wave scour and/or sediment starvation, which may be driven by minor fluctuations in relative sea level, sediment supply and/or wave climate over short timescales (101−103 years). These external controls cannot be distinguished in vertical facies successions, but may potentially be differentiated by the resulting clinoform geometries. (2) Clinoform geometry and distribution changes systematically within a single parasequence, reflecting the cycle in sea level and/or sediment supply that produced the parasequence (102−105 years). These changes record steepening of the shoreface‐shelf profile during early progradation and maintenance of a relatively uniform profile during late progradation. Modern shorefaces are not representative of this stratigraphic variability. (3) Clinoform geometries vary greatly between different parasequences as a result of variations in parasequence stacking pattern and relict shelf morphology during shoreface progradation (105−108 years). These controls determine the external dimensions of the parasequence.

Garnets as open systems during regional metamorphism

If a garnet fractures during metamorphism, its interior will be in chemical communication with matrix phases (solids and fluids) and may participate in late prograde or retrograde metamorphic reactions. The core of a growth-zoned garnet is unlikely to be in equilibrium with matrix phases and may react locally, in the vicinity of fractures, or extensively, resulting in dissolution and reprecipitation of the garnet interior and/or production of new phases. The results of petrologic techniques such as thermobarometry, pressure-temperature path calculations, and geochronometry may be misinterpreted if sites of garnet and mineral-inclusion compositional modification are not identified. Brittle fracturing of garnet during regional metamorphism may be facilitated by the presence of mineral inclusions. The conditions at which differential expansion of an inclusion relative to its host garnet are sufficient to cause radial cracking around the inclusion vary as a function of peak pressure, retrograde pressure-temperature path, and thermoelastic properties of the inclusion and garnet. Inclusion-induced fracturing is favored by nearly isothermal decompression, a common metamorphic pressure-temperature path. Other fractures may form as a result of propagation of an inclusion-host boundary or by mechanisms external to the garnet.