Journal of Metamorphic Geology最新文献

{"title":"Featured Cover","authors":"Shun Guo,&nbsp;Anping Chen,&nbsp;Xirun Cai,&nbsp;Yi Chen,&nbsp;Pan Tang,&nbsp;Qiuli Li","doi":"10.1111/jmg.12771","DOIUrl":"https://doi.org/10.1111/jmg.12771","url":null,"abstract":"<p>The cover image is based on the Original Article <i>Cold deep subduction of Indian continental crust and release of ultrahigh-pressure fluid during initial exhumation: Insights from coesite-bearing eclogite-vein systems in Kaghan Valley, Pakistan</i> by Shun Guo et al., https://doi.org/10.1111/jmg.12760\u0000 \u0000 <figure>\u0000 <div><picture>\u0000 <source></source></picture><p></p>\u0000 </div>\u0000 </figure></p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 3","pages":"i"},"PeriodicalIF":3.4,"publicationDate":"2024-03-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12771","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140114258","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":"Mechanisms and durations of metamorphic garnet crystallization in the lower nappes of the Caledonian Kalak Nappe Complex, Arctic Norway","authors":"Maria Thereza A. G. Yogi,&nbsp;Fred Gaidies,&nbsp;Olivier K. A. Heldwein,&nbsp;A. Hugh N. Rice","doi":"10.1111/jmg.12766","DOIUrl":"10.1111/jmg.12766","url":null,"abstract":"<p>The 3D microstructure and compositional zoning of garnet populations in micaschists from the Kolvik and Bekkarfjord nappes indicate the quasi-equilibration of their major components across the rock matrices during interface-controlled, size-independent garnet growth. There is microstructural evidence for foliation-parallel, small-scale resorption of garnet rims in the Kolvik Nappe, influencing the metamorphic peak conditions obtained from thermodynamic modelling. The local chemical compositions of rims less affected by resorption indicate a peak temperature of ~630°C, which is ~40°C higher than the temperature obtained from resorbed rims of the largest garnet crystal. Using a diffusion geospeedometry approach that considers the geometry of the compositional zoning of the garnet population, as well as the higher, more realistic peak temperature, a duration of 1 to 4.9 Myr is obtained for garnet growth in the Kolvik Nappe. This is approximately 1 order of magnitude faster than duration estimates obtained when using the apparent, lower temperature estimated from the resorbed garnet rims. Concomitantly to garnet growth in the Kolvik Nappe, garnet overgrowths formed in the Bekkarfjord Nappe for circa 2.5 Myr at metamorphic peak temperatures of ~560°C. The garnet growth durations obtained here are comparable with the uncertainty on the Lu–Hf garnet–whole rock isochron ages of 419.9 ± 2.4 Ma and 423.0 ± 1.9 Ma, previously obtained for these rocks. These results provide new insight into the timescales of repeated Barrovian-type metamorphic events experienced by the lower nappes of the Kalak Nappe Complex during the Caledonian Orogeny in Arctic Norway. This study emphasizes the importance of microstructural and chemical characterization of garnet populations in assessing metamorphic crystallization mechanisms and the extent of equilibration of garnet-forming components during prograde metamorphism. Moreover, our results provide means for reducing the uncertainty on metamorphic durations obtained via diffusion geospeedometry and, so, contributing to our understanding of geological timescales and processes.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 5","pages":"637-664"},"PeriodicalIF":3.4,"publicationDate":"2024-03-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12766","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140107985","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":"Metamorphic evolution and geological significance of Early Palaeozoic high-pressure granulites from the East Kunlun (NW China)","authors":"An-Ping Chen,&nbsp;Hong-Fu Zhang,&nbsp;Ming-Jie Zhang,&nbsp;Xiao-Qi Zhang","doi":"10.1111/jmg.12767","DOIUrl":"10.1111/jmg.12767","url":null,"abstract":"<p>Granulites and eclogites are useful for revealing the thermal and tectonic evolution of orogens. Early Palaeozoic granulites and associated eclogites in the East Kunlun Orogenic Belt (EKOB) display contrasting metamorphic age. Such asynchronous granulite–eclogite associations have rarely been reported, and the geological significance of their existence remains to be further explored. In this study, petrological and geochronological techniques were used to investigate two felsic and two mafic granulites collected from the Qingshuiquan area in the eastern section of the EKOB. These rocks record similar <i>P–T</i> paths, which are characterized by a peak stage within suprasolidus and high-pressure (HP) granulite facies conditions (750–832°C and 10.1–12.0 kbar), followed by an initial decompression and cooling stage to subsolidus conditions (600–748°C and 6.5–8.6 kbar), and then a stage of further retrogression under greenschist facies conditions. The protoliths to these granulites are of volcanic and sedimentary origin and suggested to be a component of the continental basement unit. Metamorphic <i>P–T</i> paths indicate that these rocks experienced peak metamorphism at a depth of ~40 km, then cooling and uplift to a depth of ~25 km, and eventually experienced low-grade retrogression at shallow crustal levels. Cathodoluminescence images and compositional data demonstrate that the zircons in these rocks are of metamorphic origin and they crystallized at or near peak conditions. SIMS U–Pb dating of representative zircon grains yield concordant metamorphic ages of c. 490–520 Ma, with a peak value of 505 Ma on the probability density curve. These ages are similar to other 480–530 Ma ages typically retrieved from EKOB granulites and associated rocks, and are markedly older than the 400–450 Ma ages retrieved from eclogites and their host rocks. The HP granulites and eclogites of the EKOB do not show overprinting relationships. Such asynchronous characteristics imply that the two rock types formed in distinct tectonic settings and at different stages of a protracted subduction–collision process. The studied granulites are suggested to have formed in the root of a continental arc during a stage of Proto-Tethys Ocean subduction. The formation of the eclogites could be attributed to subsequent deep continental subduction.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 4","pages":"583-608"},"PeriodicalIF":3.4,"publicationDate":"2024-03-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140070084","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":"Ultrahigh thermal gradient granulites in the Narryer Terrane, Yilgarn Craton, Western Australia, provide a window into the composition and formation of Archean lower crust","authors":"Naomi M. Tucker,&nbsp;Johannes Hammerli,&nbsp;Anthony I. S. Kemp,&nbsp;Matthew L. Rowe,&nbsp;Chris M. Gray,&nbsp;Heejin Jeon,&nbsp;Martin J. Whitehouse,&nbsp;Malcolm P. Roberts","doi":"10.1111/jmg.12752","DOIUrl":"10.1111/jmg.12752","url":null,"abstract":"<p>Granulites from the Narryer Terrane in the northern Yilgarn Craton, Australia, record evidence for high to ultrahigh thermal gradients during the Meso–Neoarchean. U–Pb zircon ages reflect a complex history of high-grade, prolonged and poly-phase metamorphism, with evidence for several thermal pulses at ca. 2745–2725, ca. 2690–2660, and ca. 2650–2610 Ma. Forward phase equilibrium modeling on rocks with varying bulk compositions and mineral assemblages suggests that peak temperatures reached 880–920°C at pressures of 5.5–6 kbar at ca. 2690–2665 Ma, followed by near-isobaric cooling. These new <i>P</i>–<i>T</i> results also indicate that these rocks experienced some of the hottest thermal gradient regimes in the metamorphic record (≥150°C/kbar). Based on <i>P</i>–<i>T</i> models, U–Pb ages, and geochemical constraints, our data suggest that the geodynamic setting for the formation of this unusual thermal regime is ultimately tied to cratonization of the Yilgarn Craton. Previous models have inferred that ultrahigh thermal gradients and coeval large-scale anatexis in the Narryer Terrane were primarily generated by mantle-driven processes, despite most of the lithological, isotopic, and geochemical observations being at odds with the expected geological expression of large-scale mantle upwelling. We re-evaluate the mechanisms for high-grade metamorphism in the Narryer terrane and propose that long-lived high crustal temperatures between ca. 2690 Ma and 2610 Ma were instead facilitated by elevated radiogenic heat production in thickened, highly differentiated ancient crust. Mantle-derived magma input and new crustal addition may not be the only drivers for high- to ultrahigh-temperature metamorphism and stabilization of ancient crustal blocks.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 4","pages":"425-470"},"PeriodicalIF":3.4,"publicationDate":"2024-02-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12752","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139950361","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":"Deformation-induced graphitization and muscovite recrystallization in a ductile fault zone","authors":"M. Rebecca Stokes,&nbsp;Aaron M. Jubb,&nbsp;Ryan J. McAleer,&nbsp;David L. Bish,&nbsp;Robert P. Wintsch","doi":"10.1111/jmg.12763","DOIUrl":"10.1111/jmg.12763","url":null,"abstract":"<p>A suite of slate samples collected along a 2 km transect crossing the Lishan fault in central Taiwan were evaluated to assess the role of ductile deformation in natural graphitization at lower greenschist facies metamorphic conditions. The process of natural aromatization, or graphitization, of an organic precursor is well established as a thermally driven process; however, experimental studies have shown that the energy provided by deformation can substantially reduce the activation energy required for graphitization. This study provides a natural example of deformation-induced graphitization. A strain gradient approaching the Lishan fault was established by scanning electron microscope imaging and X-ray diffraction analysis of phyllosilicates and quartz that showed an increase in the strength of slaty cleavage development via dissolution-precipitation processes. Thermal conditions were constrained to be near isothermal using calcite-dolomite geothermometry. Raman spectroscopic results from carbonaceous material, including D1-full width-at-half-maximum (FWHM), G-FWHM, Raman band separation (RBS), and a lesser-known vibrational mode B_2g-FWHM, showed robust linear trends across the same sampling transect. However, the G-FWHM parameter showed a trend opposite of that expected from thermally driven graphitization. The Raman results are interpreted to reflect a strain-driven reduction in graphite crystallite size (decrease in G-FWHM) but improvement in structural ordering in individual coherent domains. A multiple linear regression with an <i>R</i>² value of 0.92 predicts the graphite D1-FWHM values from the XRD-derived ratio of muscovite populations and muscovite microstrain, demonstrating the concomitant recrystallization of silicates and carbonaceous material across the strain gradient, despite the disparate processes accommodating the deformation. This study demonstrates the role of deformation in natural graphitization and provides a new perspective on the use of graphite as a geothermometer in strongly deformed greenschist facies rocks.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 4","pages":"529-550"},"PeriodicalIF":3.4,"publicationDate":"2024-02-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12763","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139778017","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":"A buried gneiss dome in the northern Gawler Craton: The record of early Mesoproterozoic (ca. 1600–1560 Ma) extension in southern Proterozoic Australia","authors":"Jie Yu,&nbsp;Martin Hand,&nbsp;Laura J. Morrissey,&nbsp;Justin L. Payne","doi":"10.1111/jmg.12762","DOIUrl":"10.1111/jmg.12762","url":null,"abstract":"<p>Mabel Creek Ridge, in the northern Gawler Craton, is a granulite-facies domain recording early Mesoproterozoic metamorphism, flanked by less metamorphosed rocks and dissected by crustal-scale divergent structures. The nature of early Mesoproterozoic events is poorly understood due to the lack of basement outcrop. Calculated metamorphic phase diagrams and geochronology are used to decipher the tectonic regime of a potential gneiss dome. Pressure–temperature (<i>P–T</i>) modelling of metapelites from five drill holes across Mabel Creek Ridge suggests it has experienced conditions of ~6.4–7.4 kbar and 800–850°C and the growth of suprasolidus cordierite after garnet indicates subsequent decompression. In situ U–Pb monazite and Lu–Hf garnet geochronology constrains the granulite-facies metamorphism of Mabel Creek Ridge to <i>ca</i>. 1600–1560 Ma. In contrast, drill hole GOMA DH4 located to the north of Mabel Creek Ridge records conditions of 2.2–5.4 kbar and 710–740°C at <i>ca</i>. 1520 Ma, with no evidence for 1600–1560 Ma metamorphism. Our new <i>P–T</i> pseudosection results and geochronology data from Mabel Creek Ridge and adjacent crust, coupled with the regional seismic and airborne magnetic data, reveal that Mabel Creek Ridge represents a record of early Mesoproterozoic extension in the Gawler Craton, during which thermally perturbed lower crustal rocks were exhumed within a gneiss dome. Early Mesoproterozoic extension took place within a complex geodynamic regime resulting from the interplay between final Nuna convergence along the margin of northeast Australia at <i>ca</i>. 1600 Ma and subduction to the southwest at <i>ca</i>. 1630–1610 Ma.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 4","pages":"497-527"},"PeriodicalIF":3.4,"publicationDate":"2024-02-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12762","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139780939","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":"Paleoproterozoic ultrahigh-temperature mafic granulites with a high-pressure prograde path from the Alxa Block: Implications on the tectonic evolution of the Khondalite Belt, North China Craton","authors":"Lei Zou,&nbsp;Jing-Hui Guo,&nbsp;Li-Fei Zhang,&nbsp;Guang-Yu Huang,&nbsp;Shu-Juan Jiao,&nbsp;Zhong-Hua Tian,&nbsp;Da Wang,&nbsp;Ping-Hua Liu","doi":"10.1111/jmg.12764","DOIUrl":"10.1111/jmg.12764","url":null,"abstract":"<p>Ultrahigh-temperature (UHT) granulites, a prominent feature of Paleoproterozoic orogenic belts, preserve a record of geodynamic processes during the Precambrian (Archean–Paleoproterozoic). Quantitative pressure–temperature–time (<i>P</i>–<i>T</i>–<i>t</i>) paths of these UHT granulites can constrain the tectonic processes and metamorphic evolution in such a tectonic regime. Here, UHT mafic granulites with a high-pressure (HP) prograde path are first reported in the Diebusige Complex in the Alxa Block, western part of the Khondalite Belt (KB), North China Craton (NCC). The detailed petrographic studies show that two mafic granulite samples preserve corona textures around relict garnet or garnet pseudomorphs (completely replaced by plagioclase), and a third mafic granulite sample has a relatively simple mineralogy with a granoblastic-polygonal texture. These mafic granulites have similar peak (<i>T</i>_max) assemblages of clinopyroxene + orthopyroxene + plagioclase + ilmenite ± garnet ± amphibole ± quartz + melt. Phase equilibrium modelling and Ti-in-amphibole and rare earth element (REE)-based thermometries all constrain similar peak conditions of ~880–950°C/~8.5–10 kbar implying an ~100°C/kbar apparent geothermal gradient for these mafic granulites. Based on the corona textures or pseudomorphs of garnet and mineral assemblages, we identified a <i>P</i>_max (~14 kbar) prograde stage before the <i>T</i>_max stage. Thus, a clockwise <i>P</i>–<i>T</i> path with heating and decompression followed by near-isobaric cooling (IBC) is recorded from these UHT mafic granulites. In addition, zircon and apatite SHRIMP or LA–ICP–MS U–Pb dating yields an age interval of ~1.81–1.7 Ga, which is interpreted as representing the cooling time from ~900–800°C to ~575°C at the middle-upper crustal levels (&lt;25 km deep) for these mafic granulites, with an ~1.5–2.5°C/Myr cooling rate. The new <i>P</i>–<i>T</i>–<i>t</i> path of these rocks includes high-pressure prograde, UHT peak, and slow cooling retrograde processes, which implicates a post-collisional tectonic setting for UHT metamorphism in the KB and the processes of collision, exhumation, and cooling of the KB.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 4","pages":"551-581"},"PeriodicalIF":3.4,"publicationDate":"2024-02-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139752239","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":"Cold deep subduction of Indian continental crust and release of ultrahigh-pressure fluid during initial exhumation: Insights from coesite-bearing eclogite-vein systems in Kaghan Valley, Pakistan","authors":"Shun Guo,&nbsp;Anping Chen,&nbsp;Xirun Cai,&nbsp;Yi Chen,&nbsp;Pan Tang,&nbsp;Qiuli Li","doi":"10.1111/jmg.12760","DOIUrl":"10.1111/jmg.12760","url":null,"abstract":"&lt;p&gt;The ultrahigh-pressure (UHP) eclogites from the Kaghan Valley in Pakistan, which formed by the deep subduction of the Indian plate beneath the Asian plate in the Eocene, contain complex metamorphic vein systems (including both isolated veins and vein networks), with mineral assemblages of epidote + quartz + kyanite + phengite ± omphacite ± garnet. The investigations on the Kaghan UHP eclogite-vein systems provide important insights into the mechanism and timing of metamorphic dehydration, fluid flow, and fluid–rock interaction in the deeply subducted Indian continental slab as well as the chemical characteristics of slab-derived, aqueous fluids. Abundant lawsonite pseudomorphs, characterized by prismatic aggregates of epidote, kyanite, and quartz porphyroblasts, are first recognized in the Kaghan eclogites. This observation, in combination with the occurrence of coesite pseudomorphs in epidote porphyroblasts as well as the coexistence of epidote and coesite in the eclogite zircon, indicates the previous existence of UHP lawsonite in these eclogites. Petrological studies and phase equilibrium modelling reveal clockwise &lt;i&gt;P&lt;/i&gt;–&lt;i&gt;T&lt;/i&gt; trajectories for the Kaghan eclogites that are featured by prograde vectors in lawsonite-stability regions with peak conditions of 3.0–3.4 GPa/650–690°C, followed by isothermal decompression and lawsonite breakdown under UHP conditions during the initial exhumation stage. The results of metamorphic evolution, together with in situ epidote and bulk Sr isotopic analyses, indicate that the fluids responsible for vein systems are most likely derived from the breakdown of UHP lawsonite in the eclogites. SIMS U–Pb dating of metamorphic zircons from the eclogites, integrated with the Raman analysis of inclusions in zircons, indicates that the UHP dehydration of eclogites occurred at 46.4 ± 1.2 and 46.8 ± 0.9 Ma. Analyses of hydrothermal zircons from the veins yielded slightly younger ages of 44.7 ± 1.0 and 44.9 ± 1.4 Ma, which represent the timing of fluid flow and/or vein crystallization during exhumation of the UHP rocks. Mass-balance calculation results, in combination with the vein compositions, show that the fluid flow and fluid-eclogite interaction led to the transfer of Si, Al, Ca, K, and incompatible trace elements from the eclogites into the fluids, from which the vein systems crystallized. This study indicates cold deep subduction of Indian continental crust along low geothermal gradients (6–7°C/km). The UHP fluid liberation and channelized fluid flow occurred during the initial exhumation of the cold Indian slab and are expected to induce the transfer of H&lt;sub&gt;2&lt;/sub&gt;O and incompatible trace elements from the Indian slab to the Asian lithosphere, which potentially contributes to the formation of post-collisional magmas. Moreover, we suggest that metamorphic vein systems in UHP lawsonite eclogites offer important constraints on the occurrence and timing of fast slab exhumation in continental subduction-collis","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 3","pages":"395-424"},"PeriodicalIF":3.4,"publicationDate":"2024-02-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139752320","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":"Formation of multistage garnet grains by fragmentation and overgrowth constrained by microchemical and microstructural mapping","authors":"Paola Manzotti,&nbsp;Daniele Regis,&nbsp;Duane C. Petts,&nbsp;Riccardo Graziani,&nbsp;Matthew Polivchuk","doi":"10.1111/jmg.12761","DOIUrl":"10.1111/jmg.12761","url":null,"abstract":"<p>Garnet is an exceptionally useful mineral for reconstructing the evolution of metamorphic rocks that have experienced multiple tectonic or thermal events. Understanding how garnet crystallizes and its mechanical behaviour is important for establishing a petrological and temporal record of metamorphism and deformation and for recognizing multiple geologic stages within the growth history of an individual crystal. Here, we integrate fine-scale microstructural (electron backscatter diffraction [EBSD]) and microchemical (Laser Ablation Inductively Coupled Plasma Mass Spectrometry [LA-ICP-MS] mapping) data obtained on a polycyclic garnet-bearing micaschist from the Alpine belt. Results suggest that fragmentation of pre-Alpine garnet porphyroblasts occurred during the late pre-Alpine exhumation and/or the onset of the Alpine burial, such that the older pre-Alpine garnet fragments were transported/redistributed during Alpine deformation and acted as nucleation sites for Alpine garnet growth. These processes produced a bimodal garnet size distribution (millimetre- and micrometre-sized grains). Thermodynamic modelling indicates that Alpine garnet grew during the final stage of burial (from 1.9 GPa 480°C to 2.0 GPa 520°C) and early exhumation (down to 1.6 GPa 540°C) forming continuous idioblastic rims on and sealing fractures in pre-Alpine garnet grains. We propose that fragmentation–overgrowth processes in polycyclic rocks, coupled with ductile deformation, may produce a bimodal garnet size distribution in response to fragmentation and re-distribution of pre-existing grains; these clasts can act as new nucleation sites during a subsequent orogenic cycle.</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 4","pages":"471-496"},"PeriodicalIF":3.4,"publicationDate":"2024-02-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12761","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139885254","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":"Paleoarchean metamorphism in the Acasta Gneiss Complex: Constraints from phase equilibrium modelling and in situ garnet Lu–Hf geochronology","authors":"Jonas Kaempf,&nbsp;Tim E. Johnson,&nbsp;Chris Clark,&nbsp;Julian Alfing,&nbsp;Michael Brown,&nbsp;Pierre Lanari,&nbsp;Kai Rankenburg","doi":"10.1111/jmg.12759","DOIUrl":"10.1111/jmg.12759","url":null,"abstract":"<p>The oldest known evolved (felsic) rocks on Earth (<i>c</i>. 4.03 Ga) are found in the Acasta Gneiss Complex (AGC) in north-western Canada and represent a fundamental keystone in unravelling the geological processes governing crustal growth and differentiation during the Hadean and early Archean. Although the timing of multiple episodes of magmatism, metamorphism and deformation in these tonalitic gneisses has been investigated extensively, the metamorphic pressure–temperature (<i>P–T</i>) conditions recorded by the rocks are poorly constrained. Here, we use phase equilibrium modelling coupled with in situ garnet Lu–Hf geochronology and trace element analysis for two garnet-bearing tonalitic gneisses to decipher the metamorphic history of the AGC. The observed mineral assemblages are consistent with peak metamorphic conditions of <i>T</i> = 725–780°C and <i>P</i> = 4.5–6.2 kbar and the generation of a small amount of melt (&lt;7 vol.%). Garnet geochronology constrains the age of metamorphism to 3.3–3.2 Ga, consistent with previous evidence for a late Paleoarchean tectono-metamorphic event in the AGC. Subsequent isotopic disturbance of garnet at <i>c</i>. 1.9 Ga is interpreted to correspond to a modification of the primary Lu–Hf systematics in response to garnet resorption/recrystallization during the Paleoproterozoic Wopmay orogeny, resulting in significant scatter between these two age components. Our study adds to the small number of published <i>P–T</i> data for metamorphic rocks older than 2.8 Ga and shows that tonalitic gneisses in the AGC record a high apparent thermal gradient of ~140°C/kbar in the late Paleoarchean. This thermal gradient is the highest among the limited dataset, but is broadly similar to data from other Paleoarchean-Mesoarchean crustal rocks in recording high <i>T/P</i> ratios (&gt;77.5°C/kbar).</p>","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"42 3","pages":"373-394"},"PeriodicalIF":3.4,"publicationDate":"2024-01-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/jmg.12759","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139588903","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}