amphibolite facies
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2021 ◽  
Vol 82 (3) ◽  
pp. 76-78
Author(s):  
Tzvetomila Vladinova ◽  
Milena Georgieva

The P-T evolution of carbonate-bearing metasedimentary rocks from the Sakar unit (Sakar-Strandzha Zone, SE Bulgaria) has been obtained using Perple_X modelling and conventional geothermometry. The metamorphic conditions vary from greenschist facies (250–350 °C/2–4 kbar) in the Klokotnitsa village area to amphibolite facies (550–650 °C/4.5–6.5 kbar) in the Topolovgrad town area, confirming a general increase of the metamorphic grade at east-west direction.


Author(s):  
Guibin Zhang ◽  
Jiaxing Wang ◽  
A. Alexander G. Webb ◽  
Lifei Zhang ◽  
Shuaiqi Liu ◽  
...  

Eclogites represent the highest pressure conditions yet observed from rocks thrust to the surface in the central Himalaya. A detailed investigation of the protolith nature of these eclogites is needed to better understand pre-Himalayan geological history. Retrogressed eclogites were collected from Thongmön (Dingri County) and Riwu (Dinggye County), central Himalaya, China. We investigated the bulk rock major and trace elements, Sr-Nd isotopes, zircon geochronology, and Hf-O isotopes. These retrogressed eclogites experienced five stages of metamorphic evolution from prograde amphibolite-facies to peak eclogite-facies, and high pressure granulites-facies, granulites-facies then final amphibolite-facies overprinting during exhumation. Geochemically, they are subalkaline basalts with high FeO contents and a tholeiitic affinity; trace elements show similarities with enriched mid-ocean ridge basalts. Bulk rocks have a wide range of εNd(t) values from −0.24 to +7.08, and an unusually wide range of initial 87Sr/86Sr ratios of 0.705961−0.821182. Zircon relict magmatic cores from both Thongmön and Riwu eclogites yield a consistent protolith age of ca. 1850 Ma, with enriched heavy rare earth element patterns and significant negative Eu anomalies. These relict cores have oxygen isotopes signatures of δ18O = 5.8−8.1‰, εHf(t) values of −4.85 to +9.59, and two-stage model ages (TDM2) of 1.91−2.81 Ga. Metamorphic overgrowth zircons yield much younger ages of ca. 14 Ma. Integration of all of the above data suggests that the protolith of these central Himalayan retrogressed eclogites might be Proterozoic continental flood basalts of the North Indian Plate, generated under a post-collisional extension setting during the assembly of the Columbia Supercontinent. Occurrence of both Neoproterozoic−early Paleozoic rocks and ca. 1.85 Ga rocks in the regional crystalline rocks may reflect either unrecognized sub-horizontal Main Central Thrust exposure(s) or exhumation of a deeply cut part of the Greater Himalayan Crystalline complex. In combination with previous reports of Late Cretaceous, Neoproterozoic, and similar but younger Paleoproterozoic protolith, it is clear that the central Himalayan eclogites originate from multiple sources of protolith.


2021 ◽  
Author(s):  
◽  
Michael Fuller Gazley

<p>A significant proportion of the world's Au occurs in the metamorphosed mafic rocks of Archaean greenstone belts. In such deposits, the original stratigraphy and its possible role in localising Au mineralisation can be difficult to discern due to a lack of distinctive marker units and the mineralogically and texturally monotonous nature of the metabasaltic host rocks. Understanding the effects of metamorphism, deformation, and alteration on these largely uniform host rocks, which may have experienced multiple generations of Au mineralisation, is essential for finding and extracting Au from within those deposits, and for discovering new greenstone-hosted Au deposits. This study examines the effects of primary stratigraphy on Au mineralisation, the conditions and possible controls on metamorphism, and the timing of Au mineralising events at Plutonic Gold Mine (Plutonic), Plutonic Well Greenstone Belt (PWGB), Marymia Inlier, Western Australia. Questions that remain unresolved in over 20 years of mining can now be addressed utilising advances in portable X-Ray fluorescence (pXRF), thermodynamic modelling of mineral activities and geochronological techniques. The stratigraphy of the Au-mineralised amphibolite-facies metabasalts that comprise the mine sequence at Plutonic has been examined using pXRF techniques. The results illustrate a geochemical stratigraphy in which individual lava flows can be identified on the basis of element concentrations. The most evolved basalts are at the structural base of the succession, and the least evolved at the top of the sequence. This confirms previous geochemical interpretations and textural evidence that the sequence is overturned, and demonstrates for the first time that the presented section does not involve significant structural repetition. In conjunction with Au assay data, the pXRF data reveal that Au typically occurs along basalt flow boundaries. The elemental concentration data clearly demonstrate stratigraphic control on Au mineralisation that is not readily apparent at the macroscopic level. Results of P–T pseudosection calculations in the NCFMASHTOS (Na₂O-CaO-FeO-MgO-Al₂O₃-SiO₂-H₂O-TiO₂-O-SO₂) system are presented for two typical metabasaltic rocks from the Plutonic. Those results, together with changes in mineral compositions and mineral assemblages observed in the rocks, are used to argue that a previously-unrecognised steep pressure increase (from ~ 3–4 kbar at ~ 500 °C to ≥ 8 kbar at ~ 600 °C) accompanied metamorphism to peak temperatures. Existing models for the early evolution of the PWGB involve nappe stacking supported by relatively cold strong crust, with little overall change in thickness and with peak metamorphism at temperatures similar to those reported here, but with pressures of ~ 4 kbar. Prior to this study the main episode of Au mineralisation in the PWGB was interpreted to either have accompanied or shortly followed the attainment of peak metamorphic conditions in the late Archaean at ~ 2650 Ma. New Pb-isotope results reveal that the majority of Au-associated sulphides at Plutonic are Proterozoic in age, at ~ 2200 Ma, suggesting that Au-mineralisation may have been widespread in the inlier and associated cratonic areas at that time. Later Au-mineralising events have also been constrained at ~ 1830 Ma, and at 1730–1660 Ma. Rb-Sr data from a biotite from Plutonic possibly indicates that the metamorphism was followed by a protracted period of slow cooling. Alternatively, the biotite data may reflect some combination of resetting, probably related to metasomatic events associated with Au mineralisation at ~ 2200 Ma, or with the Capricorn Orogeny at ~ 1830 Ma, and cooling. A further metasomatic event at ~ 1720 Ma is dated by both U-Pb dating of zircon overgrowths, and a new ²⁰⁷Pb-²⁰⁶Pb age from a hydrothermal sphene in chlorite-carbonate vein of 1725 ± 26 Ma. This metasomatic event was probably associated with Au mineralisation, as the Pb-isotope ages for the final Au-mineralising event range from 1730–1660 Ma.</p>


2021 ◽  
Author(s):  
◽  
Michael Fuller Gazley

<p>A significant proportion of the world's Au occurs in the metamorphosed mafic rocks of Archaean greenstone belts. In such deposits, the original stratigraphy and its possible role in localising Au mineralisation can be difficult to discern due to a lack of distinctive marker units and the mineralogically and texturally monotonous nature of the metabasaltic host rocks. Understanding the effects of metamorphism, deformation, and alteration on these largely uniform host rocks, which may have experienced multiple generations of Au mineralisation, is essential for finding and extracting Au from within those deposits, and for discovering new greenstone-hosted Au deposits. This study examines the effects of primary stratigraphy on Au mineralisation, the conditions and possible controls on metamorphism, and the timing of Au mineralising events at Plutonic Gold Mine (Plutonic), Plutonic Well Greenstone Belt (PWGB), Marymia Inlier, Western Australia. Questions that remain unresolved in over 20 years of mining can now be addressed utilising advances in portable X-Ray fluorescence (pXRF), thermodynamic modelling of mineral activities and geochronological techniques. The stratigraphy of the Au-mineralised amphibolite-facies metabasalts that comprise the mine sequence at Plutonic has been examined using pXRF techniques. The results illustrate a geochemical stratigraphy in which individual lava flows can be identified on the basis of element concentrations. The most evolved basalts are at the structural base of the succession, and the least evolved at the top of the sequence. This confirms previous geochemical interpretations and textural evidence that the sequence is overturned, and demonstrates for the first time that the presented section does not involve significant structural repetition. In conjunction with Au assay data, the pXRF data reveal that Au typically occurs along basalt flow boundaries. The elemental concentration data clearly demonstrate stratigraphic control on Au mineralisation that is not readily apparent at the macroscopic level. Results of P–T pseudosection calculations in the NCFMASHTOS (Na₂O-CaO-FeO-MgO-Al₂O₃-SiO₂-H₂O-TiO₂-O-SO₂) system are presented for two typical metabasaltic rocks from the Plutonic. Those results, together with changes in mineral compositions and mineral assemblages observed in the rocks, are used to argue that a previously-unrecognised steep pressure increase (from ~ 3–4 kbar at ~ 500 °C to ≥ 8 kbar at ~ 600 °C) accompanied metamorphism to peak temperatures. Existing models for the early evolution of the PWGB involve nappe stacking supported by relatively cold strong crust, with little overall change in thickness and with peak metamorphism at temperatures similar to those reported here, but with pressures of ~ 4 kbar. Prior to this study the main episode of Au mineralisation in the PWGB was interpreted to either have accompanied or shortly followed the attainment of peak metamorphic conditions in the late Archaean at ~ 2650 Ma. New Pb-isotope results reveal that the majority of Au-associated sulphides at Plutonic are Proterozoic in age, at ~ 2200 Ma, suggesting that Au-mineralisation may have been widespread in the inlier and associated cratonic areas at that time. Later Au-mineralising events have also been constrained at ~ 1830 Ma, and at 1730–1660 Ma. Rb-Sr data from a biotite from Plutonic possibly indicates that the metamorphism was followed by a protracted period of slow cooling. Alternatively, the biotite data may reflect some combination of resetting, probably related to metasomatic events associated with Au mineralisation at ~ 2200 Ma, or with the Capricorn Orogeny at ~ 1830 Ma, and cooling. A further metasomatic event at ~ 1720 Ma is dated by both U-Pb dating of zircon overgrowths, and a new ²⁰⁷Pb-²⁰⁶Pb age from a hydrothermal sphene in chlorite-carbonate vein of 1725 ± 26 Ma. This metasomatic event was probably associated with Au mineralisation, as the Pb-isotope ages for the final Au-mineralising event range from 1730–1660 Ma.</p>


Author(s):  
Jie Dong ◽  
Chunjing Wei

Abstract The South Altyn ultrahigh-pressure (UHP) metamorphic belt is claimed to host the deepest subducted continental crust based on the discovery of former stishovite, and thus can provide unique insights into the tectonic evolution from deep continental subduction and exhumation to arc–backarc extension. In this paper, we present detailed studies of petrography, mineral chemistry, phase equilibria modelling and zircon U-Pb dating for three representative samples involving garnet amphibolite (A1531 & A1533) and associated garnet-biotite gneiss (A1534) from the UHP belt. Three phases of metamorphism are inferred for the rocks. The first phase high pressure (HP)–UHP-type eclogite facies is represented by the mineral assemblages of garnet and phengite inclusions in zircon and garnet cores with the high grossular (XGrs = 0.33–0.34). The Si contents of 3.40–3.53 and 3.24–3.25 p.f.u. in phengite inclusions yield pressure conditions of &gt;1.7–2.3 GPa for A1533 and 2.5–2.55 GPa for A1534 at a fixed temperature of 770 °C. The second phase medium-pressure (MP)-type overprinting of garnet amphibolite facies shows P–T conditions of 0.8–1.2 GPa/750–785 °C based on the stability fields of corresponding mineral assemblages, the measured isopleths of Ti contents in biotite and amphibole cores, and XGrs in garnet. The third phase low-pressure (LP) type overprinting includes early-stage heating to peak granulite facies followed by cooling towards a late-stage amphibolite facies. The peak granulite facies is represented by the high Ti amphibole mantle, high Zr titanite and the intergrowths of clinopyroxene + ilmenite in A1533 & A1531, with P–T conditions of 800–875 °C/0.80–0.95 GPa. The late-stage is defined by the solidus assemblages, giving P–T conditions of 0.5–0.7 GPa/720–805 °C. U-Pb geochronology on metamorphic zircons from A1533 and A1534 gives three ages of c. 500 Ma, c. 482 Ma and c. 460 Ma. They are interpreted to represent the HP–UHP, MP and LP types of metamorphism respectively, based on cathodoluminescence images, mineral inclusions and trace element patterns. Combining the regional geology and metamorphic evolution from the Altyn Orogen, a tectonic model is inferred, including the following tectonic scenarios. The small Altyn Microcontinent was subducted to great mantle depths with dragging of the surrounding vast oceanic lithosphere to undergo the HP–UHP eclogite facies metamorphism during the early subduction stage (c. 500 Ma) of the Proto-Tethys Ocean. Then, the subducted slabs were exhumed to a thickened crust region to be overprinted by the MP-type assemblages at c. 482 Ma. Finally, an arc–backarc extension was operated within the thickened crust region due to the retreat of subduction zones. It caused evident heating and the LP-type metamorphic overprinting at c. 460 Ma, with a fairly long interval of 30–40 Myr after the HP–UHP metamorphism, distinct from the short interval of &lt;5–10 Myr in the Bohemian Massif.


2021 ◽  
Vol 11 (18) ◽  
pp. 8756
Author(s):  
Changming Wang ◽  
Shicheng Rao ◽  
Kangxing Shi ◽  
Leon Bagas ◽  
Qi Chen ◽  
...  

Rutile is an important ore mineral to meet the increasing demand of critical metal Ti in various sectors. Here we report a rare example of rutile deposits hosted within the Baishugang–Wujianfang amphibolite-facies metamorphic rocks in the East Qinling Orogen, central China. The rutiles are mostly located within or along the margins of biotite and show 94.6 to 99 wt% TiO2. Rutiles occur as chains, thin layers along the foliation, and dense clusters. The grains are coexisted with magnetite. Based on Zr-in-rutile thermometer the estimated crystallisation temperature is at 630 °C at 7.0 kba. Based on Cr/Nb ratio, the source of the rutile is correlated with Ti-bearing silicate minerals such as biotite from aluminous sedimentary protoliths. The rutile deposit formed during lower amphibolite-facies metamorphism, and is distinct from the eclogite- and granulite-related types elsewhere in the orogen. The LA-ICP-MS U–Pb analyses of rutiles from the deposit yield lower intercept 238U/206Pb ages of 386 ± 16 Ma at the Baishugang–Wujianfang district. These ages correspond to a Devonian arc–continent collisional event between the South and North Qinling domains in the East Qinling Orogen.


2021 ◽  
Vol 353 (1) ◽  
pp. 193-208
Author(s):  
Asmae El Bakili ◽  
Michel Corsini ◽  
Jean-Marc Lardeaux ◽  
Sylvain Gallet ◽  
Philippe Münch ◽  
...  

2021 ◽  
Vol 2021 ◽  
pp. 1-11
Author(s):  
Xu Kong ◽  
Xueyuan Qi ◽  
Wentian Mi ◽  
Xiaoxin Dong

We report zircon U–Pb ages and Lu-Hf isotopic data from two sample of the retrograded eclogite in the Chicheng area. Two groups of the metamorphic zircons from the Chicheng retrograded eclogite were identified: group one shows characteristics of depletion in LREE and flat in HREE curves and exhibit no significant Eu anomaly, and this may imply that they may form under eclogite facies metamorphic condition; group two is rich in HREE and shows slight negative Eu anomaly indicated that they may form under amphibolite facies metamorphic condition. Zircon Lu-Hf isotopic of εHf from the Chicheng eclogite has larger span range from 6.0 to 18.0, which suggests that the magma of the eclogite protolith may be mixed with partial crustal components. The peak eclogite facies metamorphism of Chicheng eclogite may occur at 348.5–344.2 Ma and its retrograde metamorphism of amphibolite fancies may occur at ca. 325.0 Ma. The Hongqiyingzi Complex may experience multistage metamorphic events mainly including Late Archean (2494–2448 Ma), Late Paleoproterozoic (1900–1734 Ma, peak age = 1824.6 Ma), and Phanerozoic (495–234 Ma, peak age = 323.7 Ma). Thus, the metamorphic event (348.5–325 Ma) of the Chicheng eclogite is in accordance with the Phanerozoic metamorphic event of the Hongqiyingzi Complex. The eclogite facies metamorphic age of the eclogite is in accordance with the metamorphism (granulite facies or amphibolite facies) of its surrounding rocks, which implied that the tectonic subduction and exhumation of the retrograded eclogite may cause the regional metamorphism of garnet biotite plagioclase gneiss.


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