Nephrite had long been mined as resources of gemstone in eastern Taiwan. It outcrops in the orogenic mountain, the Central Range, where the black schist dominates and the ultramafic serpentinites distribute sparsely. The orogeny has occurred when the subduction (South China Sea subducted to the Philippine Sea Plate) ceased and collision began at about 5 Ma. Observations shows the nephrite occurred at the interface of serpentinite and the Clinozoisite schist, enriched in Cr, Ni, but also Ca. The genesis of nephrite had been thought as a result of a series of complex reactions include the metasomatism of ultramafic rock and its surroundings and succeeding fluid interactions. This study conducts B isotopes, Sr isotopes and trace elemental measurement to give further geochemical constraints on the genesis of nephrite. Samples include rocks--black schist, clinozoisite schist, serpentinite, and associated minerals—nephrite, diopside, calcite, tremolite asbestos, cat’s eye nephrite and talc. The Sr element are enriched in clinozoisite schist, calcite, black schist (1005 ppm, 285 ppm~545 ppm, 150 ppm, respectively), and rather depleted in nephrite, diopside, cat’s eye nephrite, tremolite asbestos, serpentinite (4.3 ppm, 5.6 ppm, 3.2 ppm, 2.5 ppm, 2.0 ppm, respectively). Despite the huge difference in Sr contents, the ⁸⁷Sr/⁸⁶Sr ratios of all the samples are in the range of 0.71424 ~ 0.71815, with the highest in serpentinite (0.718151) and lowest in clinozoisite schist, nephrite and clacite (0.714240, 0.714788, 0.714951~ 0.715925, respectively), indicate the Sr source from continental crust majorly. The B concentrations and δ¹¹B values are: in serpentinite ~21 ppm and -0.5 ‰, in nephrite ~5 ppm and -6.1 ‰, in clinozoisite schist ~2.5 ppm and -5.9 ‰. The B isotopes characterize the serpentinite as of “subduction zone type”. The isotopes study provides constraints to the genesis of nephrite and thus a possible viewpoint: although the immobile elements, e.g. Cr, Ni, shows the nephrites origin from serpentinite, its different ⁸⁷Sr/⁸⁶Sr ratios from serpentinite indicates later flushing by fluids which are similar to those in clinozoisite schist and calcite. And the nephrite’s lower B concentrations and δ¹¹B values than in serpentinite may result from the flushing (replacement) of later fluids or dehydration processes, or both. Further discussions combining the viewpoints of mineralogy would be necessary to make more comprehensive interpretations.

How to cite: Pi, J., Chen, H.-F., and Chao, H.-C.: The Genesis of Nephrite— Geochemical Constraints by B isotopes, Sr isotopes and Trace Elements, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4889, https://doi.org/10.5194/egusphere-egu23-4889, 2023.

Over the last one decade, it has become increasingly clear that a distinct tectonothermal event has affected the entire Southern Granulite Terrane of India during the Cryogenian (850-635 Ma), however, the evidence is more predominant from the Madurai Block. Characterization of this tectonothermal event through multi-dimensional petrochronological studies is crucial in understanding the Proterozoic crustal evolution of southern India in particular, and the thermal evolution of continental crust, in general.

In the Madurai Block, the oscillatory-zoned elongated magmatic zircon grains, with unzoned metamorphic rims, from the porphyritic charnockites, intruding the massive mafic rocks and enderbites, yield a Cryogenian (~800 Ma) magmatic emplacement age and an Ediacaran-Cambrian metamorphic overprint (~570 Ma). Detailed geochemical study reveal that the precursors of these charnockites were ferroan A-type granite plutons that were most likely emplaced in a riftogenic setting. Texturally controlled in-situ dating of monazite grains from the associated garnet-biotite-sillimanite bearing metapelitic granulites, occurring north and west of the Sirumalai Hills near Dindigul city, yield weighted mean ages of 845-815 Ma from the core and mantle, dating the age of peak metamorphism. The chemically distinct, recrystallized thin rims, sometimes cutting across both core and mantle, yield a weighted mean age of ~615 Ma, signifying Ediacaran-Cambrian metamorphic overprint. Detailed petrological and thermobarometric study, complemented by thermodynamic modelling, constrain the peak P-T conditions of these rocks at ~800-850°C, 7.5-8.0 kbar. The age of the peak metamorphism, obtained from the monazite cores and mantles, is coeval with the extensive A-type felsic magmatism in the Madurai Block, suggesting that the metamorphic event was linked to the enhanced heat input through rift related felsic magmatism. However, the trigger behind the widespread Cryogenian thermal events needs to be ascertained to place them in context of the global tectonic framework.

The Mesoproterozoic supercontinent Rodinia, which assembled between 1300 and 900 Ma, broke apart during the Cryogenian between 830 and 650 Ma. The Indian continent, being an integral part of all Rodinia reconstructions, was largely affected by the magmatic and metamorphic events related to Rodinia breakup, and the Southern Granulite Terrane is no exception. In summary, we suggest that the pre-Cryogenian crust of the Madurai Block has been affected by widespread and voluminous A-type magmatism and associated granulite facies metamorphism in response to rifting and crustal extension during the breakup of the Rodinia supercontinent. Subsequent compression and crustal thickening related to Gondwana amalgamation during Ediacaran-Cambrian resulted in high- to ultrahigh-temperature metamorphism. This metamorphic event was long and strong enough to overprint, and sometimes obliterate, the signals of the Cryogenian thermal event.

The Cryogenian thermal events have also been recorded from the Nilgiri-Namakkal Block, north of the Palghat Cauvery Shear Zone. The strikingly similar geochemical characteristic and close spatial association of the Cryogenian rocks across the perceived terrane boundary, i.e. the Palghat Cauvery Shear Zone, negates the hypothesis of Cambrian amalgamation of the Southern Granulite Terrane with the Dharwar craton along the Palghat Cauvery Shear Zone.

How to cite: Sarkar, T., Tiwari, A. K., and Singha, A.: Cryogenian tectonothermal events in the Madurai Block of the Southern Granulite Terrane, India: Characterization and implications., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8405, https://doi.org/10.5194/egusphere-egu23-8405, 2023.

The Cryogenian gabbros of the Ambatondrazaka region belong to the Imorona-Itsindro plutonic suite that originated from an upper mantle source after the eastward subduction of the Mozambican oceanic lithosphere beneath the Precambrian Malagasy basement from 0.8–0.7 Ga. These gabbros exhibit a particular coronitic texture where each corona consists of a core of forsteritic olivine surrounded by three successive rims. The first rim is formed by clinoenstatite, the second is formed by the clinoenstatite-diopside intergrowth with some exsolutions of pleonaste and pyrope garnet. However, the last is formed by symplectites of pargasite with exsolutions of pleonaste. Assuming that the temperature gradually decreases and that the pressure remains constant or also gradually decreases, the coronitic texture is the result of three successive stages of mineral reactions. In the first stage at rim one, the crystallization of clinoenstatites was favored by the diffusion of Fe²⁺ and Mg²⁺ from the forsteritic olivine being rich in Mg²⁺ while the supply of Si and Al comes from the surrounding labradorite. During the second stage in rim two, the formation of the clinoenstatite-diopside intergrowth follows the same crystallization process as that in rim one, but the calcium input from the surrounding labradorite favored the crystallization of diopside. Additionally, the supply of Mg and Fe from olivine and Al from labradorite resulted in the formation of pleonaste and pyrope garnet exsolutions. In the last stage at rim three, the formation of pleonaste exsolutions is identical as in stage two, while the supply of H₂O favored the crystallization of pargasite symplectites. Overall, the coronitic texture is the result of a solid-state metamorphic reaction due to orogenic uplift related to the Pan-African Orogeny (0.58 – 0.51 Ga). The anhydrous phases of the reaction in the upper mantle formed the pyroxenes, spinels, and garnet in rims one and two, while the hydrous phase in the continental crust favored the formation of pargasites in rim three. 

How to cite: Rarivoarison, H.: Petrographic and mineralogical studies of the formation of coronitic gabbros in the Ambatondrazaka region, central Madagascar, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5499, https://doi.org/10.5194/egusphere-egu23-5499, 2023.

The consideration of mass balance to loss of elements from metamorphic rocks during devolatilization and anatexis reveals some principal constraints that must be considered in any model of element redistribution in metamorphic processes. During devolatilization, the changes in rock composition with the increase of metamorphic grade are a result of loss of fluid, produced by devolatilization reactions. Fluid, characterised by low viscosity and density, can be effectively extracted from a rock. Metamorphic devolatilization on average results in loss of 1-4 wt. % of the rock mass to the fluid and typically the average loss is <2 wt. %. This relatively small mass fraction mandates that in order to decrease the content of an element significantly (small percentage loss will not be visible on sediment heterogeneity) the concentration of an element in fluids must be much greater than in the protolith. For example, for 50% extraction of an element by 2% fluid, the fluid should have 25 times higher content than the protolith and loss of 50% of element with 0.5% of fluid require fluid with 100 times enrichment (Stepanov 2021).

Anatexis produce granitic melt with high viscosity and density lower than restite. The experimental data suggest that melt extraction could occur when melting degrees >10%. For a completely incompatible element enrichment by 10 times relative to protolith could is maximum achievable in anatectic process. Many elements are concentrated in residual phases and completely incompatible behaviour is rarely observed, hence reducing the efficiency of enrichment. The closes examples of incompatible behaviour during anatexis are restites produced by high-T anatexis, when accessory minerals experienced complete dissolution in melt, such as restites of the Kokchetav complex and septa from Ivrea Verbano Zone (Ewing et al., 2014). However, higher melting degree produce less enriched melt even for incompatible elements. For compatible element melt loss increase content in restite, but loss of 10% melt increase only by 11%, and 50% of melt loss (which could be considered as maximum) increase incompatible element by factor of 2. The mass balance constraints show limits of the possible effect of fluid/melt loss on rock composition and suggests that fluid loss could produce higher enrichment factors than melt loss.

References

Stepanov A.S., A review of the geochemical changes occurring during metamorphic devolatilization of metasedimentary rocks. Chemical Geology, 568 v, 120080, 2021.

Ewing, T.A., Rubatto, D., Hermann, J., 2014. Hafnium isotopes and Zr/Hf of rutile and zircon from lower crustal metapelites (Ivrea–Verbano Zone, Italy): Implications for chemical differentiation of the crust. Earth and Planetary Science Letters 389, 106–118.

How to cite: Stepanov, A.: The mass balance constraints on the depletion of elements during metamorphic devolatilization and anatexis, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7897, https://doi.org/10.5194/egusphere-egu23-7897, 2023.

In-situ laser ablation ICP-MS/MS is becoming a widespread approach for white mica Rb/Sr geochronology. This technique allows determination of single-spot dates using an initial ⁸⁷Sr/⁸⁶Sr composition measured from Ca-bearing phases (Rösel & Zack 2022; GGR 46). The dates can be correlated with microstructural position and chemistry of white mica to discern complex tectonic histories. To demonstrate the power of in-situ white mica Rb/Sr geochronology, the technique was applied to four high-pressure/low-temperature (HP/LT) lithologies of the Cycladic Blueschist Unit (CBU) on Syros, Greece, which reached ~22 kbar and ~550°C at c. 53-45 Ma (e.g., Laurent et al. 2018; JMG 36). The CBU along the southern coast contains foliated eclogitic blocks that are wrapped by retrograde, foliated blueschists. At the western coast, the CBU possesses non-foliated HP skarn blocks similarly surrounded by retrograde, foliated blueschists. In the eclogite and blueschists, alignment of white mica defines the foliation along with glaucophane, epidote, and titanite. The southern blueschist also bears white mica grains with mineral cleavage oblique to the foliation. In the skarn, white mica are undeformed and sometimes exhibit a radial habit. White mica chemistry is relatively homogeneous in the eclogite (X_Cel: 0.33-0.39) and skarn (X_Cel: 0.36-0.50) compared to the blueschists from the western (X_Cel: 0.26-0.50) and southern (X_Cel: 0.33-0.57) exposures. Single-spot Rb/Sr dates are not correlated with microstructure nor chemistry for the eclogite and skarn, yielding weighted averages of 58.1 ± 4.3 Ma (MSWD: 1.3; n: 38) and 43.8 ± 2.8 Ma (MSWD: 1.1; n: 30), respectively. The blueschists show dispersions of dates that correlate with chemical variations, proxied by high-Ti (>1300 µg/g) and low-Ti (<1000 µg/g) domains. For the western blueschist, high-Ti domains yield a weighted average of 44.8 ± 3.4 Ma (MSWD: 0.93; n: 14), whereas low-Ti zones are 35.5 ± 2.9 Ma (MSWD: 1.4; n: 22). For the southern blueschist, high-Ti regions yield dispersed Cretaceous to Eocene dates, predominantly defined by the oblique white mica. The low-Ti domains gave a weighted average of 39.8 ± 2.1 Ma (MSWD: 0.99; n: 19). Altogether, white mica Rb/Sr geochronology records the timing of HP/LT metamorphism in the eclogitic block, followed by HP metasomatism in the skarn, and subsequent retrograde deformation events recorded by the low-Ti mica domains in both blueschist samples. The dates from high-Ti zones of the western blueschist reflect partial retention of the metasomatic history. The dates from high-Ti domains from the southern blueschist are older than HP/LT metamorphism and are interpreted as partial retention of ⁸⁷Sr from the blueschist’s protolith. The older events in the blueschist, and the metamorphic record of the eclogite, were not recorded by white mica ⁴⁰Ar/³⁹Ar geochronology on the equivalent rocks from the same exposures, which instead preserve the retrograde events (Laurent et al. 2021; GCA 311). These results demonstrate that Rb/Sr geochronology is a dynamic tool when coupled with structural and chemical data to extract metamorphic, metasomatic, deformation, and possibly detrital/magmatic records of white mica in rocks metamorphosed below ~600°C.

Funding provided by the National Science Center of Poland project nr. 2021/40/C/ST10/00264

How to cite: Barnes, C. J., Zack, T., Bukała, M., Rösel, D., and Schneider, D. A.: White mica Rb/Sr geochronological records of high-pressure/low-temperature rocks in the Cycladic Blueschist Unit (Syros, Greece), revealed by in-situ laser ablation ICP-MS/MS, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9493, https://doi.org/10.5194/egusphere-egu23-9493, 2023.

The Barrovian type metamorphism affecting the Precambrian microcontinents of peri-Siberian tract of the Central Asian Orogenic Belt is mostly dated indirectly on zircon from (syn-tectonic) magmatic rocks as Late Proterozoic – Ordovician. However, in-situ monazite geochronology in micaschists and migmatite gneisses at the northern part of the Precambrian Baidrag block, central Mongolia, revealed that the Baikalian Late Proterozoic – Early Cambrian cycle overprints an earlier Tonian phase of metamorphism. The apparent Barrovian-type zoning ranging from garnet, staurolite, kyanite to kyanite/sillimanite migmatitic gneisses is thus false and points to hidden metamorphic discontinuities and mixed metamorphic histories from different times. Therefore, to decipher and interpret the record of different tectono-metamorphic events it is necessary to unravel complete P–T–t paths from individual samples. Two localities with Tonian-age monazite show anticlockwise P–T paths: 1) Grt−Sil−Ky gneiss records burial to the sillimanite stability field (~720°C, 6.0 kbar) followed by burial to the kyanite stability field (~750°C, 9 kbar) and, 2) The Grt−St schist records burial to the staurolite stability field (~620°C, 6 kbar), further followed by almost isothermal burial (~590°C, 8.5 kbar). Based on monazite textural position, internal zoning, and REE patterns, the time of prograde burial under a thermal gradient of 27–32°C/km is estimated at c. 890−853 Ma and further burial under a geothermal gradient of 18–22°C/km is dated at c. 835−815 Ma. On the other hand three localities with Late Proterozoic to Cambrian monazite ages show clockwise metamorphic paths at variable P–T gradients: 3) P–T conditions of the Grt schist reaches ~5 kbar and 500 °C and 4) the Grt−St−Ky schist reaches conditions of 9 kbar and 670 °C, indicating burial under a geothermal gradient of 20–26 °C/km. 5) Grt–Sil gneiss shows peak of 6–7 kbar and 700–750 °C, indicating melting conditions at 30–32 °C/km gradient. Monazite included in porphyroblasts and in the matrix indicate that these P–T conditions reached under variable geothermal gradient were semi-contemporaneous and occurred between 570 and 520 Ma.  By correlation with published zircon ages of 600–530 Ma from granitoid magmatic rocks we suggest that the areas with higher geothermal gradient may be explained by closer vicinity of magmatic intrusions. These P−T and geochronology data from a continuous Barrovian metamorphic section suggest that anticlockwise P−T evolution from c. 930 to 750 Ma can be interpreted as a result of thickening of peri-Rodinian supra-subduction extensional and hot edifice.  This metamorphic event was followed by a clockwise P−T evolution from c. 570 to 520 Ma possibly related to shortening of the northern Baidrag active margin and incipient collision with with peri-Siberian continental mass further north.

How to cite: Stipska, P., Peřestý, V., Soejono, I., Schulmann, K., Collett, S., Kylander Clark, A. R. C., and Aguilar, C.: Hidden metamorphic discontinuities in the NE Baidrag block, Mongolia, reveal anticlockwise metamorphic paths at c. 890−790 Ma indicating peri-Rodinian back-arc compression followed by c. 560–520 Ma burial, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15332, https://doi.org/10.5194/egusphere-egu23-15332, 2023.

Precise thermobarometric and geochronologic data are crucial to correctly interpret the timing of metamorphism and identify complex polymetamorphic histories. We present new P-T-t-D data from samples collected in two Austroalpine nappes exposed in the Eastern Alps, Austria: the structurally upper greenschist-facies Schöckel Nappe (“Graz Paleozoic,” Drauzug-Gurktal Nappe System) and the structurally lower amphibolite-facies Waxenegg Nappe (Koralpe-Wölz Nappe System). In the latter, polymetamorphism was previously inferred. However, the timing of metamorphism is poorly resolved and only limited geochronology exists in the Schöckel Nappe.

Detailed petrographic investigations of chloritoid-bearing phyllite and micaschist samples collected at two localities at the base and in a higher structural level of the Schöckel Nappe revealed complex phase relations of REE-minerals, involving multiple REE-epidote generations that may be associated with monazite, xenotime, apatite and zircon. In garnet-bearing micaschist of the Waxenegg Nappe, we observed large (up to 500 µm) monazite exhibiting distinct core-rim chemical zoning. From careful documentation of the microstructural phase relations, thermodynamic modeling, Raman spectroscopy of carbonaceous matter and in-situ LA-ICPMS U-(Th)-Pb dating of REE-epidote and monazite we show that rocks in all three localities were affected by LP metamorphism (0.3 – 0.4 GPa) during the Permian event (250 – 282 Ma) with peak temperatures decreasing from 560°C in the lower to 475°C in the upper nappe. During the Eo-Alpine event, overprinting at c. 90 Ma occurred under conditions of ~550°C and 1.0 – 1.1 GPa in the Waxenegg Nappe. At the base of the Schöckel Nappe, peak metamorphism at ~450 – 470°C and 0.4 – 0.7 GPa and cooling below 300°C likely took place before 110 Ma. Towards higher structural levels, only limited Eo-Alpine overprinting at low P-T conditions (<400°C, 0.3 – 0.5 GPa) is evident, thus the observed mineral assemblage reflects mostly Permian metamorphism.

Our results demonstrate that the main metamorphic signature in the Schöckel Nappe can be resolved as the Permian event and that the Eo-Alpine overprint is relatively lower grade than previously proposed. We observe a marked increase in Eo-Alpine peak conditions (~80 – 100°C, 0.3 – 0.5 GPa) across the nappe contact with higher grade rocks in the footwall compared to the hanging wall. The metamorphic pattern is consistent with the existence of a major normal fault between the Drauzug-Gurktal Nappe and Koralpe-Wölz Nappe systems in the easternmost part of the Austroalpine Unit, as already identified in its central and western parts. Finally, our study highlights that coupling modern thermobarometric analytical approaches with high spatial resolution geochronology on accessory minerals is critical to improve our understanding of the fundamentally important low-grade units of orogens.

How to cite: Hollinetz, M. S., Huet, B., Schneider, D. A., McFarlane, C. R. M., Rantitsch, G., and Grasemann, B.: Unravelling polymetamorphism in greenschist- and amphibolite-facies rocks using thermodynamic modeling and in situ U-Pb dating of REE-minerals (Austroalpine Unit, Eastern Alps, Austria), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11972, https://doi.org/10.5194/egusphere-egu23-11972, 2023.

The Inthanon Zone is regarded as the main suture between the Indochina and Sibumasu blocks and comprises ultramafic rocks, marine sediments and crystalline basement rocks. The gneissic basement is exposed in two different structural domains: (1) the Inthanon core complex located to the west of the Chiang Mai basin, and (2) the Mae Ping shear zone located to the south of the core complex. Here, we present new petrological and geochronological results from gneisses and schists of the Inthanon zone. Four different mineral assemblages can be recognised in gneisses and schists: (1) garnet–muscovite–biotite±sillimanite±chlorite schist, (2) garnet–muscovite–biotite–plagioclase–K-feldspar gneiss, (3) tourmaline-bearing muscovite–biotite– orthogneiss, and (4) migmatitic biotite gneiss. These rocks typically contain accessory ilmenite, pyrite, apatite, tourmaline, monazite, xenotime, and zircon. In-situ Th-U-total Pb dating of monazite reveals at least two metamorphic events, one in the Early Jurassic and another one in the Early Paleocene. A garnet–muscovite–biotite–sillimanite schist sample shows matrix micas and fibrolithic sillimanite wrapped around garnet porphyroblasts. Multi-equilibrium thermobarometry using Tweequ (Berman, 1996) yields metamorphic peak conditions of 0.5 GPa and 570 °C. Monazite dating yields two age populations at 189±5 and 61±7 Ma. A second sample belonging to this group contains chlorite instead of sillimanite and has a main schistosity with tightly folded relicts of a former fabric. Garnet porphyloblasts exhibit pressure shadows with quartz and mica.  Monazite dating gives a single age population of 65±6 Ma. Garnet–muscovite–biotite–plagioclase–K-feldspar gneiss samples show corona textures with plagioclase, quartz, biotite, and muscovite around garnet porphyroblasts, indicative of pressure decrease. P–T conditions of 0.6–0.7 GPa and 680–700 °C were calculated using the garnet-biotite-plagioclase-quartz and garnet-biotite geothermobarometers. The formation of coronae around garnet occurred during exhumation at slightly lower conditions of 0.4–0.5 GPa and 640–660 °C. Monazite dating yields a main population at 189±5 Ma with few 50-70 Ma dates. Tourmaline-bearing muscovite–biotite–plagioclase–K- feldspar gneiss samples are characterized by an ultramylonitic texture. Large K-feldspar augen and tourmaline porphyroclasts are surrounded by a fine-grained, foliated matrix of quartz, and feldspar. The mineral assemblage indicates middle amphibolite grade. Monazite dating of this sample yields two populations at 192±3 and 58±4Ma.  Migmatitic biotite–gneiss samples preserve a biotite–plagioclase–K-feldspar–quartz assemblage in both the melanosome and leucosome. Monazite dating provides a single population of 61±2 Ma. Two tectono-metamorphic events are revealed by our data: a widespread medium P-T regional metamorphic phase, and a younger overprint of unclear grade (low to high T assemblages are found) but significant spatial extent. While the first event was coeval with abundant plutonism during Sukhothai-Sibumasu collision (~185 Ma), the second one (~60 Ma) does not appear to be connected with regional plutonic activity and might be related to large scale shearing as seen in the Mae Ping and Three Pagoda shear zones.

How to cite: Santitharangkun, S., Hauzenberger, C., Skrzypek, E., and Gallhofer, D.: Petrology and Th-U-Total Pb Monazite Ages from The Inthanon Core Complex, Thailand, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13576, https://doi.org/10.5194/egusphere-egu23-13576, 2023.

The Dacia megaunit in the Eastern part of Serbia comprises Getic and Supragetic nappe systems and corresponds to E-W striking Balkan Mountains (Sredna Gora and East Balkan units; sensu Schmid et al., 2020). The area of our study is located between Danube River to the East and Mlava to the West (Homoljske Mts., a part of Balkan Mts.) and consist of low to medium grade metamorphic rocks of Late Proterozoic to Early Paleozoic ages.

Two different metamorphic units were sampled:

(1) northern, low-grade metamorphic sequence is characterized by numerous types of chlorite sheets containing chlorite, epidote, muscovite, actinolite, hornblende and garnets together with quartz, albite and secondary calcite and fine-grained illite. Accessory minerals are titanite, rutile, ilmenite and apatite.   

The sampled schists were recognized as belonging to low and lower part of medium grade Barrovian metamorphic assemblages, characterized by zonal distribution of the index–minerals: chlorite, epidote, biotite, amphibole and garnet.

(2) southern, medium-grade metamorphic sequence is characterized by different amphibolite rocks, with amphiboles (28-60 vol.%) ranging from tchermakite and magnesiohornblende to actinolite. Additionally, these rocks contain 17 – 40 vol.% of oligoclase, 5-22 vol% of quartz; 5 – 13 vol% chlorite (ripidolite), 0,4 – 13 vol% of Al-Fe epidote and 0,1-0,7 vol% of andradite garnet.

Multielement diagrams normalized to N-MORB of low-grade metamorphic sequence show enrichment of LILE relative to HFSE with negative Nb and positive K, U and Pb anomalies, while medium-grade metamorphic sequence shows a disturbed pattern with LILE >> HFSE, positive Pb anomaly and in some cases U, Th, while Nb, Ti and Sr are negative. Both sequences show significant crustal influence.

Medim-grade metamorphic sequence originate from an igneous precursor (andesite-subalkaline basalt protolith). Using Zr-Ti plot after Pearce, these rocks belong to volcanic arc basalts and within plate tholeiites. According to Meschede (1986) Zr/4-2Nb-Y and Wood (1980) Th-Hf/3-Ta plots, they display normal to enriched MORB characteristics similar to basalts from volcanic arc setting.

Geothermobarometric calculations were made for garnet-amphibole-plagioclase assemblage from medium-grade metamorphic sequence using values of titanium in amphibole and aluminum in chlorites. Obtained temperature range between 600 and 750 °C while pressure range between 7 and 9 Kb, corresponding to the recognized amphibolite facies of medium grade metamorphism. A direction of increase of pressure and temperature conditions within the prograde metamorphic sequence towards the south is proposed.

References:

Schmid SM, Fügenschuh B, Kounov A, Maţenco L, Nievergelt P, Oberhänsli R, Pleuger J, Schefer S, Schuster R, Tomljenović B, Ustaszewski K, van Hinsbergen DJJ (2020) Tectonic units of the Alpine collision zone between Eastern Alps and western Turkey. Gondwana Res 78:308–374.

Meschede, M. (1986) A Method of Discrimination between Different Types of Mid-Ocean Ridge Basalts and Continental Tholeiites with the Nb-Zr-Y Diagram. Chemical Geology, 56, 207-218.

Wood, D.A. (1980) The Application of a Th-Hf-Ta Diagram to Problems of Tectonomagmatic Classification and to Establishing the Nature of Crustal Contamination of Basaltic Lavas of the British Tertiary Volcanic Province. Earth and Planetary Science Letters, 50, 11-30.

How to cite: Borojević Šoštarić, S. and Anzulović, A.: Getermobarometry of the late Proterozoic to Paleozoic Barrovian metamorphic sequence in the Dacia megaunit: case study Eastern Serbia, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16783, https://doi.org/10.5194/egusphere-egu23-16783, 2023.