Journal of Metamorphic Geology最新文献

{"title":"Multi-episodic formation of baddeleyite and zircon in polymetamorphic anorthosite and rutile-bearing ilmenitite from the Chiapas Massif Complex, Mexico","authors":"Alejandro Cisneros de León, Axel K. Schmitt, Bodo Weber","doi":"10.1111/jmg.12683","DOIUrl":"10.1111/jmg.12683","url":null,"abstract":"<p>Massif-type anorthosite and comagmatic associations of rutile-bearing ilmenitite (RBI) and oxide-apatite-rich amphibolite (OARA) from the Chiapas Massif Complex (CMC) in southeastern Mexico display a protracted billion-year accessory mineral record encompassing magmatic crystallization at c. 1.0 Ga to recent ductile shear deformation at c. 3.0 Ma. Multiple discrete zircon populations between these age end-members resulted from neoformation/recrystallization during local to regional metamorphism that affected the southeastern portion of the CMC. The ubiquitous presence of relict baddeleyite (ZrO<sub>2</sub>), along with various zircon generations spatially associated with pristine to partly retrogressed Zr-bearing igneous and metamorphic minerals (e.g., ilmenite, rutile, högbomite and garnet), suggests significant Zr diffusive re-equilibration (exsolution) during slow cooling and mineral breakdown followed by crystallization of baddeleyite. The subsequent transformation of baddeleyite into zircon was likely driven by reaction with Si-bearing fluids in several geochronologically identified metamorphic stages. Strikingly contrasting compositional signatures in coeval zircon from anorthosite (silicate-dominated) and comagmatic RBI (Ti-Fe-oxide-dominated) indicate a major role of fluids locally equilibrating with the rock matrix, as indicated by distinct zircon trace element and oxygen isotopic compositions. A high-grade metamorphic event at c. 950 Ma is likely responsible for the formation of coarse-grained rutile (~0.1–10 mm in diameter), srilankite, zircon and garnet with rutile inclusions as well as metamorphic högbomite surrounding Fe-Mg spinel. Zr-in-rutile minimum temperatures suggest >730°C for this event, which may correlate to rutile-forming granulite facies metamorphism in other Grenvillian-aged basement rocks in Mexico and northern South America. A younger generation of baddeleyite exsolution occurred during post-peak cooling of coarse-grained rutile, reflected in rimward Zr depletion and formation of discontinuous baddeleyite coronas. Baddeleyite around rutile was then transformed into zircon possibly during subsequent metamorphism at c. 920 or 620 Ma, resulting from syn-kinematic and contact metamorphism, respectively. Regional metamorphism at c. 450 and 250 Ma extensively overprinted the existing zircon population, especially during the Triassic event, as suggested by a significant presence of zircon with this age. Nearly pristine baddeleyite occurring interstitial to ilmenite yielded an isochron age of c. 232 Ma according to in situ U–Pb secondary ion mass spectrometry (SIMS), suggesting either formation during metamorphic peak conditions or post-peak cooling. Zircon with ages of c. 80–100 Ma in anorthosite is identified for the first time within the CMC and coincides with cooling ages of c. 100 Ma for coarse-grained rutile. This age is similar to those of rocks occurring ~200 km further to the east in Guatemala, which are also b","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"40 9","pages":"1493-1527"},"PeriodicalIF":3.4,"publicationDate":"2022-06-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12683","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42669261","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Tectonic erosion and deep subduction in Central Tibet: Evidence from the discovery of retrograde eclogites in the Amdo microcontinent","authors":"Yinbiao Peng,&nbsp;Shengyao Yu,&nbsp;Sanzhong Li,&nbsp;Yongjiang Liu,&nbsp;M. Santosh,&nbsp;Pei Lv,&nbsp;Yunshuai Li,&nbsp;Chuanzhi Li,&nbsp;Yiming Liu","doi":"10.1111/jmg.12685","DOIUrl":"10.1111/jmg.12685","url":null,"abstract":"<p>The Amdo microcontinent which separates the Qiangtang terrane to the north and the Lhasa terrane to the south is a key terrane for reconstructing the tectonic evolution of Central Tibet. We report the new finding of retrograde eclogites within the Amdo microcontinent in this study. The eclogites are characterized by peak metamorphic mineral assemblages of garnet, omphacite, rutile and quartz and underwent a four-stage metamorphic evolution, including a peak eclogite facies stage (M₁) at ~20–24 kbar and 580–620°C, followed by an HP granulite facies decompression stage (M₂) at ~13–15 kbar and 750–780°C, a subsequent MP-UHT granulite facies heating stage (M₃) at 8–10 kbar and &gt;840°C and a final amphibolite facies retrogression (M₄) at 5.3–6.0 kbar and 560–580°C. The eclogites exhibit rare earth element distribution patterns and trace element abundances similar to those of N-MORB and arc-related volcanics, with depleted whole-rock ε_Nd(t) values of 3.4 to 4.2, and are inferred to have formed in a back-arc basin tectonic setting. Zircon and rutile U–Pb dating yields a protolith age of 226 ± 5 Ma, a peak eclogite facies metamorphic age of 190 ± 1 Ma, an HP granulite facies metamorphic age of 179 ± 1 Ma and an amphibolite facies retrograde age of 172 ± 1 Ma. The clockwise P–T–t paths and the oceanic protolith signature of retrograde eclogites suggest that part of the back arc basin was subducted to depths of ~80 km. Tectonic erosion associated with the subduction of the Amdo microcontinent beneath the Tethys Ocean accounts for the deep subduction of the back-arc basin and the absence of arc magmatic rocks in the northern Amdo microcontinent.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"40 9","pages":"1545-1572"},"PeriodicalIF":3.4,"publicationDate":"2022-06-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44104880","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Effects of Fe3+ in sillimanite on mineral stabilities and parageneses in ultrahigh-temperature metapelites","authors":"Bin Wang,&nbsp;Chunjing Wei","doi":"10.1111/jmg.12684","DOIUrl":"10.1111/jmg.12684","url":null,"abstract":"Based on the nature of ferric sillimanite, an activity model for sillimanite containing Fe3+ is constructed, tested and adopted to calculate phase equilibria of pelitic compositions under ultrahigh‐temperature (UHT) conditions. The calculated P–T projections and pseudosections suggest that the incorporation of Fe3+ into sillimanite can fairly solve the current imperfectly topological match between thermodynamic calculations and synthetic experiments, especially at high oxygen fugacity. Fe3+ in sillimanite remarkably elevates the temperature to switch the parageneses of orthopyroxene + sillimanite (Opx + Sil) and sapphirine + quartz (Spr + Qz) in oxidized metapelites, with an increment around 50–70°C. The calculated compatibility diagrams show that the widely approbatory UHT‐diagnostic mineral assemblages of Opx + Sil and Spr + Qz usually occur in metapelites with high Mg/Fe2+ ratios, which depend on both bulk‐rock MgO and oxygen fugacity, whereas the metapelites with low Mg/Fe2+ ratios are characterized by the assemblage of Garnet + sillimanite (Grt + Sil) with or without spinel (Spl) in UHT conditions. Moreover, comprehensive comparisons suggest that the essential petrogenetic framework of natural UHT metapelites is mostly governed by the two metamorphic reactions of Opx + Sil = Spr + Grt and Grt + Sil = Spl + Spr.","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"40 9","pages":"1529-1544"},"PeriodicalIF":3.4,"publicationDate":"2022-06-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41983885","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Low-grade prehnite-pumpellyite facies metamorphism and metasomatism in basement rocks adjacent to the Permian Oslo rift: The importance of displacive reactions","authors":"Håkon Austrheim,&nbsp;Ane K. Engvik,&nbsp;Morgan Ganerød,&nbsp;Kristina G. Dunkel,&nbsp;Mari Roen Velo","doi":"10.1111/jmg.12682","DOIUrl":"10.1111/jmg.12682","url":null,"abstract":"&lt;p&gt;The Kongsberg and Bamble lithotectonic domains of SE-Norway are known as classical Precambrian high-grade metamorphic terrains. The area has undergone extensive metasomatism with formation of albitites and scapolite-rich rocks and numbers of previously economically important deposits including the Kongsberg Silver and the Modum Cobalt mines. We demonstrate here that the central part of the Bamble lithotectonic domain (Kragerø area) has locally developed low-grade metamorphic minerals (prehnite, pumpellyite, analcime, stilpnomelane and thomsonite) belonging to the prehnite-pumpellyite and zeolite facies. Structurally, the low-grade minerals occur as fracture fills, in the alteration selvages around fractures where the rock is albitized, and along shear zones and cataclastic zones. The fracture fill and the alteration selvages vary from millimetres scale to 1 m in thickness. The fractures with low-grade minerals are part of larger fracture systems. The low-grade minerals typically formed by both displacive (swelling) and replacive reactions and in a combination of these. Prehnite together with albite, K-feldspar, quartz, epidote and hydrogarnet form lenses along (001) faces in biotite and chlorite leading to bending of the sheet silicates through a displacive reaction mechanism. Numerous replacement reactions including the earlier minerals as well as the low-grade minerals occur. As albite, K-feldspar, talc, quartz, actinolite, titanite, calcite and hydrogrossular form in the same veins and in the same biotite grain as the classical low-grade minerals, they probably belong to the low-grade assemblage and some of the albitization in the region presumably occurred at low-grade conditions. Alteration of olivine (Fo69) at low-grade conditions results in the formation of clay minerals including ferroan saponite. Reconnaissance studies at the east (Idefjord lithotectonic domain) and the northwest (Kongsberg lithotectonic domain) sides of the Oslo rift together with reports of low-grade assemblages in south-western Sweden along the continuation of the rift into Skagerrak suggest that the low grade assembles occur in rocks adjacent to the Oslo rift along its full extent. Ar-Ar dating of K-feldspar from the low-grade assemblages gave an age of 265.2 ± 0.4 Ma (MSWD = 0.514 and P = 0.766), suggesting that the low-grade metamorphism and some of the metasomatism is induced by fluids and heat from the magmatic activity of the Permian Oslo rift, which requires transport of fluid over distances of several kilometres. The metamorphic conditions are constrained by stability fields of prehnite, pumpellyite and analcime to be less than 250°C and at a pressure less than 5 kbars. The displacive reactions created micro-fractures and porosity in the adjacent minerals that enhance fluid flow and low-grade mineral formation on a local scale. On a thin section scale, the displacive growth of albite in biotite results in a local volume increase of several 100%. Whether the","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"40 9","pages":"1467-1492"},"PeriodicalIF":3.4,"publicationDate":"2022-06-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12682","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43711489","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Multi-mineral petrochronology on a high-pressure mafic granulite reveals short-lived high-temperature metamorphism in the North China Craton","authors":"Xiao-Meng Li,&nbsp;Hao Cheng,&nbsp;Besim Dragovic,&nbsp;Kai-Yang Du,&nbsp;Ying Zhou","doi":"10.1111/jmg.12681","DOIUrl":"10.1111/jmg.12681","url":null,"abstract":"<p>Accurately defining the peak ages and timescales of high-temperature metamorphism is fundamental to unravelling tectonic dynamics. However, metamorphic constraints are frequently hampered by a large spread of zircon U–Pb ages without explicit textural relationships. Integrated garnet and zircon petrochronology may clarify ambiguous ages retrieved from ancient high-temperature metamorphic rocks. There is a long-standing debate on the interpretation of the spread of zircon ages from <i>c</i>. 2.5–1.8 Ga for the granulites of the North China Craton. In order to clarify the timing and duration of (ultra)high-temperature metamorphism in the North China Craton, we investigated a mafic granulite and the adjoining gneiss from the Yinshan Block of the North China Craton using zircon and titanite U–Pb geochronology combined with garnet Lu–Hf and Sm–Nd geochronology. Pseudosection modelling and conventional thermobarometric calculations constrain the peak metamorphic conditions to be ~1.0 GPa and ~850°C. The near-complete lack of major-element zoning in garnet, aside from ~2 μm diffusion profiles at crystal rims, suggests complete re-equilibration at peak temperatures followed by fast cooling from high temperatures. The Lu–Hf garnet age of 1870 ± 4 Ma and Sm–Nd age of 1870 ± 7 Ma, determined on the same garnet fractions, are indistinguishable from the zircon U–Pb age of 1866 ± 11 Ma obtained from zircon that grew contemporaneously with garnet, evidenced by the chemical equilibrium of coexisting garnet and zircon, and are additionally consistent with a titanite U–Pb age of 1876 ± 7 Ma. We interpret this close agreement of ages, within uncertainty, coupled to the existence of flat Sm–Nd–Hf profiles in garnet that also has well-preserved Lu zoning, to reflect a short-lived high-temperature metamorphic event that was terminated by rapid exhumation and cooling. The short-lived (&lt;4 Myr) high-temperature metamorphism may be generated in the lowermost parts of the crust through magmatic underplating/intraplating during extension that follows collision of the Ordos and the Yinshan Blocks.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"40 9","pages":"1447-1466"},"PeriodicalIF":3.4,"publicationDate":"2022-06-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45815679","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Constraining the P–T path of (U)HP rocks with reaction overstepping during subduction; example from the Western Gneiss Region (Norway)","authors":"Shah Wali Faryad,&nbsp;Josef Ježek,&nbsp;Jan Kulhánek","doi":"10.1111/jmg.12680","DOIUrl":"10.1111/jmg.12680","url":null,"abstract":"<p>This paper presents the results of petrological observations and diffusion modelling on garnet from high-pressure to ultrahigh-pressure ((U)HP) metamorphic rocks of the Western Gneiss Region in the Nordfjord. Garnet from kyanite-bearing micaschist preserves two generations of garnet growth that are related to the Pre-Caledonian granulite facies and Caledonian eclogite facies metamorphic events. Mafic eclogite, forming lenses in the micaschist, contains only eclogite facies assemblages with partial recrystallization under amphibolite facies conditions. Caledonian garnet in both the micaschist and hosting eclogite indicates reaction overstepping and nucleation near or above 550°C/2.0 GPa. Maximum pressure and temperature, calculated using pseudosection modelling for the eclogite facies event, were ~2.6 GPa and 650°C. The interface between the Pre-Caledonian and Caledonian garnet in the micaschist shows a strong compositional gradient or possibly a compositional jump. The preservation of such a gradient together with the hummocky-shaped composition profiles in the Caledonian garnet from the eclogite indicates either no relaxation or a short-time of relaxation of the rocks at their peak temperature conditions, as well as their exhumation by cooling. Possibly, heating or exhuming of the rocks by isothermal decompression could have easily modified such compositional irregularities along the garnet profiles. A cooling rate of ~187°C/Ma and exhumation rate in the vertical direction of ~2.5 cm/year for the HP rocks were obtained by considering that the temperature and transport distance changes from their maximum depth and peak temperature to the surface were proportional to the time (3.5 Ma) calculated by modelling for the garnet.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"40 9","pages":"1427-1446"},"PeriodicalIF":3.4,"publicationDate":"2022-06-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47905807","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Thermodynamic constraints on the composition of orogenically thickened lower crust","authors":"Yi-Bing Li,&nbsp;Yu-Wen Wu,&nbsp;Bin Su,&nbsp;Si Chen,&nbsp;Qing-Hua Zhang,&nbsp;Yi Chen","doi":"10.1111/jmg.12679","DOIUrl":"10.1111/jmg.12679","url":null,"abstract":"<p>Orogenically thickened lower crust is the key site of crustal differentiation, crustal deformation, and Moho modification. However, the composition of thickened lower crust is still highly debated. Here, we calculate a set of pseudosections with mafic lower crust compositions in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O₂ (NCKFMASHTO) system. Our modelling results show that the maximum thickness of the mafic lower crust increases with the Moho temperature (<i>T</i>_Moho). In addition, the lithologies of stable mafic crust are characterized by medium-pressure (MP) to high-pressure (HP) granulites at 40–50 km, HP granulites and garnet-omphacite granulites at 50–60 km, and garnet-omphacite granulites at 60–70 km. Under the Pamir geothermal conditions, mafic rocks with high SiO₂ (&gt;50.2 wt%), X_Mg (&gt;0.70), X_Ca (&gt;0.49), or low X_Al (&lt;0.11) could be stable at 70 km; however, only ~10% of global mafic granulite xenoliths lie within this compositional range. Further modelling indicates that if <i>T</i>_Moho reaches 900–1000°C, neither the lower crust nor the upper mantle has significant strength relative to the upper crust and that only ~5–37% of mafic materials are gravitationally stable at 70 km. This implies that the base of doubly thickened (70 km) crust is dominated by intermediate-felsic rocks, consistent with the low <i>V</i>_<i>p</i> and <i>V</i>_<i>p</i>/<i>V</i>_<i>s</i> values seismically observed in young orogenic crustal roots. Thus, most mafic materials at &gt;70 km could delaminate into the deep mantle. Our results provide insights on the formation of extremely thick crust with a predominantly intermediate-felsic base and the crustal thickness variation in continental collision zones.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"40 8","pages":"1405-1426"},"PeriodicalIF":3.4,"publicationDate":"2022-06-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42265281","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Garnet crystallization mechanisms and localized polymetamorphism in the southwestern Meguma Terrane, Nova Scotia, Canada","authors":"Alexandra B. Nagurney,&nbsp;Mark J. Caddick,&nbsp;Chris E. White","doi":"10.1111/jmg.12678","DOIUrl":"10.1111/jmg.12678","url":null,"abstract":"<p>We present data on the pressure and temperature (<i>P–T</i>) conditions experienced by metamorphic rocks of the Meguma Terrane, Nova Scotia, Canada, also utilizing three-dimensional microstructural data on one sample to better constrain the mechanisms that controlled garnet crystallization. Inverse and forward thermodynamic modelling place peak <i>P–T</i> conditions in the southwestern Meguma Terrane at ~650°C and 4.5 kbar. Interpretation of these results with petrographic observations and previous <i>P–T</i> constraints across the terrane suggests that amphibolite facies metamorphism occurred during the Devonian Neoacadian orogeny (406–388 Ma). Integration of quantitative 3D textural data with an estimated metamorphic heating rate of &lt;5°C/Myr is consistent with amphibolite facies metamorphism resulting from tectonic loading during the Neoacadian orogeny, though the exact nature of the orogeny is still not well understood. Further, the intrusion of granitic plutons into the Meguma metasediments at 373 Ma likely locally drove metamorphic recrystallization (polymetamorphism). The 3D size, shape, and location of garnet crystals in one sample reveal that the rate-limiting step for garnet crystallization was likely the diffusion of aluminium through the intergranular matrix at length scales less than the mean nearest neighbour distance between garnet crystals. Nucleation was aided by epitaxial overgrowth onto a muscovite substrate, though it appears there may have been a decoupling between minerals providing a substrate and those providing nutrients during garnet growth.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"40 8","pages":"1383-1404"},"PeriodicalIF":3.4,"publicationDate":"2022-05-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46022517","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Late Famatinian (440–410 Ma) overprint of Grenvillian metamorphism in Grt-St schists from the Sierra de Maz (Argentina): Phase equilibrium modelling, geochronology, and tectonic significance","authors":"Sebastián O. Verdecchia,&nbsp;Carlos D. Ramacciotti,&nbsp;Cesar Casquet,&nbsp;Edgardo G. Baldo,&nbsp;Juan A. Murra,&nbsp;Robert J. Pankhurst","doi":"10.1111/jmg.12677","DOIUrl":"10.1111/jmg.12677","url":null,"abstract":"&lt;p&gt;The analysis of major and trace elements in zoned minerals is useful for deciphering parts of the tectonothermal evolution of polymetamorphic tarrain. We applied this approach to the Maz Metasedimentary Series in Western Sierras Pampeanas of Argentina, where polymetamorphism resulted in the overprinting of a Grenvillian basement (the Maz Complex) during the pervasive Rinconada tectonic phase of the Famatinian orogeny. The older metamorphism (M&lt;sub&gt;1&lt;/sub&gt;) is assigned to the youngest Grenvillian metamorphic event recognized in this basement at c. 1035 Ma, whereas the Rinconada metamorphism (M&lt;sub&gt;2&lt;/sub&gt;) was Silurian to early Devonian, essentially between 440 and 410 Ma. The latter resulted from oceanward migration of the orogenic front relative to earlier late Cambrian to Ordovician (490–470 Ma) tectonic phases of the Famatinian orogeny. The M&lt;sub&gt;1&lt;/sub&gt; and M&lt;sub&gt;2&lt;/sub&gt; metamorphic events have been recognized in a staurolite-garnet schist from the Maz Metasedimentary Series. Most metamorphic minerals from this rock were formed during the M&lt;sub&gt;2&lt;/sub&gt; event which was of the Barrovian type (±kyanite). Part of the metamorphic &lt;i&gt;P–T&lt;/i&gt; evolution is recorded in the complex compositional zoning of garnet porphyroblasts. Three types of garnet were identified based on texture and chemistry, including trace elements (REEs). Phase equilibrium analysis, compositional isopleth, and multi-equilibrium thermobarometry were applied in order to establish the &lt;i&gt;P–T&lt;/i&gt; history. M&lt;sub&gt;1&lt;/sub&gt; is represented by preservation of Grt&lt;sub&gt;1&lt;/sub&gt; ± Kfs ± Sil, with peak &lt;i&gt;P–T&lt;/i&gt; condition of 790°C and 5.2 kbar, that is, granulite facies. This early metamorphic event was related to a deformational D&lt;sub&gt;1&lt;/sub&gt; episode represented by a relict S&lt;sub&gt;1&lt;/sub&gt; foliation. The latter is preserved as aligned inclusions in staurolite porphyroblasts and as relics of an older crenulated foliation in microlithons from the matrix. M&lt;sub&gt;2&lt;/sub&gt; followed a clockwise &lt;i&gt;P–T&lt;/i&gt; path with three mineral growth stages. The earliest occurred at &lt;i&gt;~&lt;/i&gt;585°C and &lt;i&gt;~&lt;/i&gt;8.7 kbar and is represented by Grt&lt;sub&gt;2&lt;/sub&gt; ± St&lt;sub&gt;1&lt;/sub&gt; ± Bt&lt;sub&gt;1&lt;/sub&gt; + Qz. Grt&lt;sub&gt;2&lt;/sub&gt; was partially coeval with growth of St&lt;sub&gt;1&lt;/sub&gt;, which was stable at ~625°C and 9.0 kbar. Grt&lt;sub&gt;2&lt;/sub&gt; + St&lt;sub&gt;1&lt;/sub&gt; are syn-kinematic to the main S&lt;sub&gt;2&lt;/sub&gt; foliation (D&lt;sub&gt;2&lt;/sub&gt; episode). Subsequently, decompression (D&lt;sub&gt;3&lt;/sub&gt;) started as St&lt;sub&gt;2&lt;/sub&gt; (+ Bt&lt;sub&gt;2&lt;/sub&gt; + Ms&lt;sub&gt;1&lt;/sub&gt; + Qz + Pl) crystallized, and garnet was partially consumed at &lt;i&gt;~&lt;/i&gt;612–620°C and &lt;i&gt;~&lt;/i&gt;7.3–7.7 kbar. St&lt;sub&gt;3&lt;/sub&gt; + Grt&lt;sub&gt;3&lt;/sub&gt; crystallized at &lt;i&gt;~&lt;/i&gt;608°C and &lt;i&gt;~&lt;/i&gt;6.8 kbar at the end of D&lt;sub&gt;3&lt;/sub&gt;. Increasing &lt;i&gt;P–T&lt;/i&gt; conditions during the earlier M&lt;sub&gt;2&lt;/sub&gt; growth stage suggest burial of the Maz Metasedimentary Series, probably linked to tectonic thickening by underthrusting (tectonic phase D&lt;sub&gt;2&lt;/sub&gt;). Peak metamorphic conditions were attained during thrust stacking. The tec","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"40 8","pages":"1347-1381"},"PeriodicalIF":3.4,"publicationDate":"2022-05-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46468112","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Garnet, zircon, and monazite age and REE signatures in (ultra)high-temperature and high-pressure rocks: Examples from the Caledonides and the Pamir","authors":"Lorraine Tual,&nbsp;Matthijs A. Smit,&nbsp;Ellen Kooijman,&nbsp;Melanie Kielman-Schmitt,&nbsp;Lothar Ratschbacher","doi":"10.1111/jmg.12667","DOIUrl":"10.1111/jmg.12667","url":null,"abstract":"&lt;p&gt;Rare earth element (REE) signatures of high-U/Pb and high-Th/Pb accessory minerals are typically used to link their ages to specific petrological processes (‘petrochronology’)—most notably the growth or breakdown of garnet. Although this approach is powerful, gaps in our understandings of REE systematics in high-grade rocks exist, particularly regarding the degree to which these minerals chemically equilibrate at extreme conditions. To investigate this, we performed comparative chronology and REE analysis of garnet, zircon, and monazite in (1) a fluid-rich, ultrahigh-pressure (UHP) migmatite from the Western Gneiss Region, Norway, and (2) dry felsic granulite xenoliths from the Pamir, Tajikistan. Zircon and monazite from the hydrous migmatite provided ages of 450–370 Ma and a range of Gd/Yb values across this time span. A Lu-Hf garnet age of 422 ± 2 Ma coincides with the age at which zircon and monazite exhibit the highest Gd/Yb values, as well as the largest range therein. The degree of dispersion in these values is substantial, especially for monazite. Zircon and monazite in the dry xenoliths provide age clusters between 50 and 11 Ma, recording pulsed growth and recrystallization. The Lu/Hf garnet ages for these samples are 41–38 Ma. The accessory minerals of that age are texturally associated with garnet, yet have the lowest, not the highest, Gd/Yb values. In both cases, there is evidence that co-genetic garnet and accessory minerals achieved REE equilibrium during growth. However, the age and compositional record of chronometric accessory minerals that were co-genetic with garnet are distinctly different between the two case examples. In the hydrous migmatite, supply-limited garnet growth in the presence of fluid phase resulted in strongly zoned garnet and a correspondingly large range in Gd/Yb values among co-genetic zircon and monazite. The range in Gd/Yb values match among these phases and collectively capture the strong fluctuations in the REE composition of matrix micro-domains caused by garnet growth. In this case, it is the range in Gd/Yb values, rather than a specific composition that is diagnostic for garnet growth. In the felsic granulites, garnet and accessory minerals growth occurred in a fluid-limited regime, in which short fluid pulses triggered reactions that had been likely significantly overstepped. The seemingly contradictory pattern of increasing Gd/Yb &lt;i&gt;after&lt;/i&gt; garnet growth in these samples is the result of garnet having high Gd/Yb and continued zircon growth forcing further HREE depletion of the matrix with time. The situation that high, rather than low, Gd/Yb values in zircon and monazite indicate equilibrium with garnet may be common in dry granulites and other anhydrous high-temperature rocks. Our findings provide an improved framework for the reliable identification of the accessory minerals that equilibrated with garnet, even when the REE-age record of these minerals is complex, dispersed, and seemingly contra","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"40 8","pages":"1321-1346"},"PeriodicalIF":3.4,"publicationDate":"2022-05-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12667","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48064372","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}