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Photomicrograph (a) and semiquantitative X‐ray maps (b–e) of a thin section of garnet amphibolite sample LZ06‐20‐4, consisting mainly of wide leucosome bands and with narrow melanosome bands. The leucosomes consist dominantly of garnet, plagioclase, and quartz with minor amphibole, whereas the melanosomes contain abundant garnet and amphibole with minor biotite, plagioclase, and quartz. The elongated garnet, aligned amphibole, and plagioclase‐ and quartz‐rich ribbons define distinct foliation parallel to the margin of bands. The garnet grains commonly have a mineral inclusion‐rich core and an inclusion‐poor rim. Some garnet grains show distinct compositional zoning, characterized by increasing Mg and decreasing Fe, Ca, and Mn from core to rim. Warm colors indicate higher concentrations of each element in (b–e).
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The Himalayan orogen, which formed due to collision of the Indian and Asian continents during the Early Tertiary, is a prime example of a large, hot collisional orogen. Despite decades of study, the duration of partial melting of migmatitic rocks exposed in the Himalayan orogenic core remains highly controversial. As such, we have performed detaile...
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Citations
... M. Zhang et al. 2012Y. X. Zhang et al. 2022c;Palin et al. 2015;Peng et al. 2018;Kang et al. 2020). Notably, the metamorphic ages of the amphibolites in this study align with the ages of late-stage exhumation recorded in the lowtemperature eclogites near the Milin mélange ( Fig. 1) W. C. Li et al. 2022), signifying the upper limit of the formation age of the Milin mélange. ...
Tectonic mélanges are crucial for deciphering collision processes in orogenic belts. This study investigates the nature of metamorphosed mélange in the Milin area of the eastern Himalayan syntaxis to shed light on the India-Asia collision in the region. The mélange exhibits a “block-in-matrix” fabric in the lower and upper parts and a “lenticular-thrust-slices” fabric in the middle. Quartzite blocks in the lower part are proposed to be the Silurian or younger quartz arenites with a Tethyan affinity. Metacherts and amphibolites in the middle and upper parts may have originated from hydrothermal or terrestrial cherts and Nb-enriched basalts in the Asian forearc or syncollisional basin at ∼48-45 Ma. The diverse blocks/slices of the mélange, sourced from both the subducting and overriding plates, are suggested to be formed during Indian plate subduction. The protoliths of the amphibolite-metachert slice (∼45 Ma) in the mélange are distinctly different from the contemporary shallow sea (post-collisional) deposits but similar to the deep-water (initial collisional or pre-collisional) sediments in other parts of the suture zone. This implies that the amphibolite-metachert slice may represent the product of the initial India-Asia collision or even pre-collision and that (hard) collision likely lagged by ∼10-Myr in the eastern Himalaya syntaxis.
Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists
Supplementary material: https://doi.org/10.6084/m9.figshare.c.6896942
... The resultant granulites and migmatites provide important insights into crustal anatexis and associated magmatic activities in the Himalaya. Previous studies have focused on P-T paths, geochronology and/or partial melting history of these granulites (e.g., Guilmette et al., 2011;Kang et al., 2020). By contrast, studies on the melt compositions (e.g., Zhang et al., 2018) and related MI from the anatectic rocks (e.g., Bartoli et al., 2019;Carosi et al., 2014;Iaccarino et al., 2017) are rare, which limits our understanding on the origin and nature of anatectic melts in the Himalaya. ...
... According to previous studies, metapelite and metabasic rocks in the NBC experienced HP granulite-facies metamorphism under peak P-T conditions of 1.3-1.6 GPa and 825-900 C (e.g., Booth et al., 2009;Groppo et al., 2012;Guilmette et al., 2011;Iaccarino et al., 2015;Kang et al., 2020;Tian et al., 2016;Zhang et al., 2015), while some metapelites just reached upper amphibolite to near-granulite facies at the peak stage of 0.8-1.0 GPa and 700-750 C (Peng et al., 2021). ...
... A paragneiss sample (08LZ) was collected from a valley near the town of Pai in the NBC (Figure 1b), where orthogneiss, amphibolite and kyanite-bearing gneiss lenses (e.g., sample 6-11-2 in Zhang et al., 2012) are well exposed. According to previous studies, the rocks in this area underwent Eocene to Miocene HT and HP metamorphism at $1.5 GPa and $800-840 C, associated with extensive partial melting over 20 Ma from circa 40 to circa 20 Ma (e.g., Kang et al., 2020). ...
Melt inclusions (MIs) in high‐temperature metamorphic rocks provide a unique window into crustal anatexis in collisional orogenic belts and have been widely used to characterize compositions of anatectic melts as well as melting mechanisms. In this study, MIs hosted by peritectic garnet were for the first time identified in an Al 2 SiO 5 ‐free graywacke‐type paragneiss from the Namche Barwa Complex, the Eastern Himalaya, Southeast Tibet. These MIs occur as nanogranites in the rims of porphyroblastic garnet, exhibit negative crystal shapes with an average diameter of ~12 μm and consist of a mineral assemblage of biotite + quartz + plagioclase + K‐feldspar ± muscovite. Re‐homogenization experiments of these nanogranites were conducted at a pressure of 1.5 GPa and temperatures of 800°C, 850°C and 900°C and produced homogeneous glasses at 850°C. The homogenized glasses are strongly peraluminous and calc‐alkalic in composition, with 66.43–71.31 wt.% SiO 2 , 12.64–15.06 wt.% Al 2 O 3 , high alkaline (5.41–7.22 wt.%) and low ferromagnesian (2.72–4.46 wt.%) contents. They are lower in silica and CaO but higher in K 2 O compared with MI produced by fluid‐present melting of metasedimentary rocks, thus indicating fluid‐absent melting. These glasses are also characterized by enrichment of large ion lithophile elements (particularly Cs and Rb), depletion of Ba and Sr, low contents of light rare earth elements (3.6 to 33.7 ppm), high Rb/Sr ratios (6.19–37.3) and low Nb/Ta ratios (2.55–18.7). In combination with phase equilibrium modelling, these compositional features suggest that a sequential dehydration melting of muscovite and biotite was responsible for the production of MI during prograde metamorphism of the studied paragneiss. By compiling MI data published in the literature, we show that dehydration melting of metasedimentary rocks from the Himalayan orogen can produce initial melts with various peraluminous and granitic compositions.
... Petrologic observations and experimental studies have indicated that eclogitization is not only controlled by pressure and temperature conditions but also influenced by whole-rock composition (De Paoli et al., 2012;Green and Ringwood, 1972). The composition of crustal rocks (especially the lower crust) remains largely speculative in Tibet due to the rarity of xenoliths and exhumed lower crustal granulites, which are direct evidence for constraining the composition of the lower crust (Chan et al., 2009;Kang et al., 2020). ...
The relationship of the crustal contact between the Indian and Eurasian plates is a key issue in understanding crustal thickening and the subduction of the Indian lithosphere beneath the Qinghai-Tibetan Plateau. Across the middle of the Yarlung-Zangbo Suture (YZS), we deployed an ∼450-km-long SN-trending wide-angle reflection/refraction profile to observe the P-wave velocity (vP) structure beneath the northern Himalaya and the southern plateau. Our results show that, 1. the high vP (∼7.1 km/s) indicates that the Indian lower crust extends no more than 50 km north of the YZS. 2. The lower crust beneath the southern part of the plateau features an extremely low vP (
... In comparison with previously reported P-T paths (Figure 10), our study shows that HP (>14 kbar) rocks reached granulite to eclogite facies and mid-pressure rocks only reached upper amphibolite facies. Recently, Kang et al. (2020) reported amphibolitic rocks in the NBC core, which experienced a prolonged HP and high-temperature metamorphic and partial melting process from $40 to $20 Ma. This implies that HP and high-temperature metamorphism may have concomitantly occurred in NBC core. ...
... The reported peak metamorphic condition of their F I G U R E 1 1 Summary of reported metamorphic ages from the NBC. The reported ages are from Burg et al., 1997Burg et al., , 1998Ding et al., 2001;Booth et al., 2004;Booth et al., 2009;Gong et al., 2008;Seward & Burg, 2008;Lei et al., 2008;Xu et al., 2010;Zhang et al., 2010;Liu et al., 2011;Yu et al., 2011;Su et al., 2012;Zeng et al., 2012;Zhang et al., 2012;Xu et al., 2012;Zeitler et al., 2014;Liu & Zhang, 2014;Tian et al., 2016;Peng et al., 2018;Zhang et al., 2018;Kang et al., 2020 metapelitic sample was 720 760 C/8 10 kbar, and the Sm-Nd ages of garnet indicate that peak metamorphism occurred at $16.0 AE 2.5 Ma. In comparison, our metapelitic samples experienced peak metamorphism at $700-750 C/8-10 kbar, where garnet grew from >19 Ma to $14 Ma. ...
Geothermobarometry shows that metapelite samples from Namche Barwa Complex (NBC) reached upper‐amphibolite to near‐granulite facies during the peak metamorphic stage, with similar conditions of ~700–750 °C/8–10 kbar, and then experienced retrograde metamorphism at ~630–700 °C/4–7 kbar. In‐situ monazite LA‐ICP‐MS U‐Th‐Pb dating suggests divergent metamorphism in the NBC: metapelite on the hanging wall of Namu‐La thrust preserved a continuous metamorphic record of >19–3 Ma, whereas metapelite on the footwall yielded age ranges of >18–14 Ma and 8–3 Ma, with a gap in recorded ages between 14 Ma and 8 Ma. Monazite grains in the garnet porphyroblasts, more depleted in the heavy rare earth elements (HREE), yielded the youngest age of ~14 Ma. This is interpreted as the timing of upper amphibolite facies peak metamorphism in the metapelite from the NBC, with the NBC being exhumed coherently thereafter. Furthermore, the discrepancy between reported peak metamorphic ages of high‐pressure granulite (~40–30 Ma, ~25–20 Ma) and mid‐pressure metapelite (~14 Ma, this study) indicate asynchronous subduction‐exhumation processes in the NBC. We suggest that crustal flow has played an essential role in exhumation since ~40 Ma, and recent surficial erosion (<8 Ma) intensified the exhumation of the NBC, with young leucogranite (<10 Ma) resulting from decompression melting. From ~3 Ma to the present, the interplay of erosion and tectonic movement caused ubiquitous rapid uplift, resulting in the concomitant exhumation of various types of rocks and the formation of the spectacular high relief between Yarlung Tsangpo gorge and Namche Barwa Peak.
... The granulitized eclogites in central to eastern Himalayas were metamorphosed at granulite facies under T max peak conditions of 6-12 kbar and 750-970 • C (e.g., Groppo et al., 2007;Corrie et al., 2010;Grujic et al., 2011;Wang et al., 2017dWang et al., , 2021Li et al., 2019;O'Brien, 2019), corresponding to thermobaric ratios of 808-1250 • C/GPa. The HP granulites in central and eastern Himalayas were generally metamorphosed under peak P-T conditions of 14-18 kbar and 800-890 • C (e.g., Kali et al., 2010;Guilmette et al., 2011;Zhang et al., 2015;Tian et al., 2016;O'Brien, 2019;Kang et al., 2020), corresponding to thermobaric ratios of 494-571 • C/GPa. Cordieritebearing migmatites in central Himalaya were produced by metamorphism under peak P-T conditions of 4-7 kbar and 700-877 • C (Goscombe and Hand, 2000; Imayama et al., 2010Imayama et al., , 2019Streule et al., 2010;Groppo et al., 2013), corresponding to thermobaric ratios of 1253-1750 • C/GPa. ...
Crustal anatexis in collisional orogens has great bearing on geochemical differentiation of the continental crust. However, it is often uncertain what kind of crustal rocks were partially melted for felsic magmatism at convergent plate boundaries. To address this issue, a combined study of in-situ monazite U–Th–Pb ages, in-situ monazite and allanite SmNd isotopes and whole-rock SmNd isotopes was carried out for Higher Himalayan metamorphic rocks and leucogranites in the Himalayan orogen. Metapelite, metagreywacke and granitic gneiss are the dominant constituents of the Higher Himalayan Crystallines, and they experienced upper amphibolite to granulite facies metamorphism at ca. 26–13 Ma. Although the three rock types show consistently negative εNd(t) values at t = 20 Ma, their Nd isotope compositions become less and less enriched from metapelite through metagreywacke to granitic gneiss. The metapelite has the lowest εNd(t) values of −19.9 to −15.7, the metagreywacke has intermediate εNd(t) values of −17.4 to −12.7, and the granitic gneiss has the highest εNd(t) values of −14.1 to −7.7. Leucogranitic magmatism occurred at ages from ca. 26 to 7 Ma, coeval with anatectic metamorphism of the three rock types during the Oligocene–Miocene. Relict zircon UPb age distributions, whole-rock trace element patterns, initial Nd isotope compositions and two-stage Nd model ages for the Higher Himalayan leucogranites are comparable with those for the Higher Himalayan metamorphic rocks, confirming that the metapelite, metagreywacke and granitic gneiss would have their compositional counterparts at structurally deeper positions to serve as the crustal sources of the leucogranites. This is also verified by forward phase equilibrium modelling for partial melting of the metamorphic rocks with respect to the differences in their composition and fertility. In addition, the leucogranites show a decreasing trend in εNd(t) values with their ages, indicating that the dominant crustal sources would gradually change from the least fertile granitic gneiss through the intermediate fertile metagreywacke to the most fertile metapelite during the protracted crustal anatexis from the late Oligocene to the Miocene. Therefore, the deep crust in the Himalayan orogen would consist of the metamorphic rocks with similar compositions to the shallow crust. The combined geochronological and geochemical study of accessory minerals and host rocks can provide the genetic link between the crustal sources and their melting products in collisional orogens. Nevertheless, the Nd isotope variation in the these leucogranites may also be related to incongruent melting of the crustal sources, which has a potential to result in the Nd isotope disequilibrium between melt and residue.
... The GHC, also referred to as the Namche Barwa Complex (NBC) by Zhang et al. (2012), consists of migmatitic orthogneiss, paragneiss, mafic granulite, amphibolite, schist, marble and calc-silicate rock. All the rocks of the GHC underwent high-grade metamorphism and partial melting during the Cenozoic (Zhong and Ding, 1996;Liu and Zhong, 1997;Burg et al., 1998;Ding et al., 2001;Booth et al., 2004Booth et al., , 2009Liu et al., 2007;Xu et al., 2010Xu et al., , 2012Zhang et al., 2010aZhang et al., , 2012Zhang et al., , 2015Zhang et al., , 2018Guilmette et al., 2011;Su et al., 2012;Liu and Zhang, 2014;Tian et al., 2016Tian et al., , 2019Tian et al., , 2020Peng et al., 2018;Kang et al., 2020). ...
... The pelitic and felsic HP granulites in the GHC are characterized by having peak metamorphic mineral assemblage of garnet + kyanite + plagioclase + K-feldspar + biotite + quartz + rutile, commonly with antiperthite (or ternary feldspar) (Liu and Zhong, 1997;Ding and Zhong, 1999;Liu et al., 2007;Zhang et al., 2010aZhang et al., , 2015Guilmette et al., 2011;Su et al., 2012;Xiang et al., 2013;Tian et al., 2016Tian et al., , 2019Tian et al., , 2020. Although migmatitic mafic rocks, including garnet-bearing amphibolite and garnet-free amphibolite, occur widely in the EHS, typical HP mafic granulites, containing garnet, clinopyroxene, plagioclase, quartz and rutile, are rarely reported (Zhong and Ding, 1996;Liu and Zhang et al., 2014;Zhang et al., 2018;Kang et al., 2020). Field observation reveals that the HP mafic granulites show transitional contacts with the hosting amphibolite, characterized by gradual decreasing of garnet and clinopyroxene contents, and increasing of amphibole and plagioclase contents from the granulite to the amphibolite. ...
... Recently, considering amphibole as one of peak metamorphic minerals, P-T conditions of 14-15.5 kbar and 780-790 °C and 15−17 kbar and 805−840 °C were estimated for the HP mafic granulites from the EHS by Zhang et al. (2018) and Kang et al. (2020), respectively. Our phase equilibrium modelling shows that the observed peak metamorphic mineral assemblage of the studied typical HP mafic granulites is stable at P-T conditions of 12-19 kbar and 780-900 °C (Fig. 6). ...
The Himalayan orogen, resulting from the Early Cenozoic collision of the Indian and Asian plates, is an ideal vehicle to study active orogenic processes and test geodynamic models of how the crust responds to collisional orogeny. This paper focused on migmatitic high-pressure (HP) mafic granulite and associated leucosome from the Greater Himalayan Crystallines (GHC) in the Eastern Himalayan Syntaxis (EHS) in order to understand the conditions and timescales over which high-grade rocks and partial melts were produced during the Himalayan orogeny. Combining with previous study results from the Western and Central Himalayas and Trans-Himalayan magmatic arc, we obtained the following conclusions: (1) The mafic granulites from the EHS underwent HP and high-temperature (HT) granulite facies metamorphism and partial melting, with peak metamorphic conditions of 15–17 kbar and 820–880 °C. The GHC, at least its western part of the EHS, underwent coherent HP granulite-facies metamorphism. (2) The HP mafic granulites experienced long-lived dehydration melting of amphibole from ∼40 Ma to ∼20 Ma during prograde metamorphism and generated up to ∼16 vol.% partial melt. The variable degrees of dehydration melting of the HP mafic, pelitic and felsic granulites in the EHS generated voluminous granitic melts with distinct compositions, and provided the source for the Himalayan granites. (3) Peak metamorphic pressure of the GHC gradually decreases, whereas the metamorphic temperature progressively increases from the Western to Eastern Himalayas. This indicates that the Indian continental crust deeply subducted into the mantle in the Western Himalaya after the Indo-Asia collision, whereas the Indian crust underthrusted or relaminated beneath the Asian continental crust, and formed the thickened lower crust in the Central and Eastern Himalayas and Gangdese arc. (4) The melts derived from the underthrusted Indian crust probably resulted in isotopic compositional enrichment of the Early Cenozoic mantle- and juvenile crust-derived magmatic rocks of the Gangdese arc.
... Crustal flow developed pervasively in orogens associated with large-scale movement of melt-weakened crust (Beaumont et al., 2001;Whitney et al., 2004;Farias et al., 2020;Kang et al., 2020). Partial melting changes the rheology and density of the crust and leads to mechanical decoupling between molten layers and bounding lithologies because decreased viscosity enables crustal flow under weak driving forces (Holtzman et al., 2005;Rosenberg and Handy, 2005). ...
Flow of partially molten crust is a key contributor to mass and heat redistribution within orogenic systems, however, this process has not yet been fully understood in accretionary orogens. This issue is addressed in a Devonian migmatite-granite complex from the Chinese Altai through structural, petrological, and geochronological investigations presented in this study. The migmatite-granite complex records a gradual evolution from metatexite, diatexite to granite and preserves a record of two main Devonian phases of deformation designated D1 and D2. The D1 phase was subdivided into an early crustal thickening episode (D1B) and a later extensional episode (D1M) followed by D2 upright folding. The D1M episode is associated with anatexis in the deep crust. Vertical shortening, associated with D1M, gave rise to the segregation of melt and formation of a sub-horizontal layering of stromatic metatexite. This fabric was reworked by the D2 deformation associated with the migration of anatectic magma in the cores of F2 antiforms. Geochronological investigations combined with petro-structural analysis reveal that: (1) D1M partial melting started probably at 420−410 Ma and formed sub-horizontal stromatic metatexites at ∼30 km depth; (2) The anatectic magma accumulated and migrated when a drainage network developed, as attested by the pervasive formation of massive diatexite migmatites, at 410−400 Ma; (3) Soon after, massive flow of the partially molten crust from orogenic lower to orogenic upper crustal levels, assisted by the interplay between D2 upright folding and magma diapirism, led to migmatite-granite emplacement in the cores of regional F2 antiforms that lasted until at least 390 Ma; (4) a terminal stage was manifested by the emplacement of 370−360 Ma granite dykes into the surrounding metamorphic envelope. We propose that Devonian anatexis assisted by deformation governed first the horizontal and then the vertical flow of partially molten orogenic lower crust, which drove crustal flow, mass redistribution, and crustal differentiation in the accretionary system of the Chinese Altai.
... Thus, metamorphic rocks of a wide range of ages are found as records of this continuing orogeny. However, the duration of heating (prograde) or cooling (retrograde) of individual rock/rock-packages i.e., t rock are on timescales of < 10-20 million years (e.g., Anczkiewicz et al., 2014;Kang et al., 2020), implying t rock < t orogen in this case. Here, we mainly focus on the metamorphic cooling rates i.e., t rock . ...
The pressure-temperature-time (P-T-t) evolution of metamorphic rocks is directly related to geodynamics as different tectonic settings vary in their thermal architecture. The shapes of P-T paths and thermobaric ratios (T/P) of metamorphic rocks have been extensively used to distinguish different tectonic domains. However, the role of metamorphic timescales in constraining tectonic settings remains underutilized. This is because of the poorly understood relationship between them, and the difficulty in accurately constraining the onset and end of a particular metamorphic event. Here,we show why and how the intrinsic relationship between thermal regime, rheology
and rate of motion controlled by the heat, mass and momentum conservation laws translate to differences in heating, cooling, burial, exhumation rates of metamorphic rocks and thereby, to the duration of metamorphism.We compare the P-T-t paths of the orogenic metamorphic rocks of different ages and in particular, analyse their retrograde cooling rates and durations. The results show that cooling rates of the metamorphic rocks are variable but are dominantly <50 °C/Ma during most of the Precambrian before increasing by an order of magnitude (>100 °C/Ma) during the late Neoproterozoic to Phanerozoic. To seek what controlled this secular change in
metamorphic cooling rates, we use thermomechanical modelling to calculate the P-T-t paths of crustal rocks in different types of continental orogenic settings and compare them with the rock record. The modelled P-T-t paths show that lithospheric peel-back driven orogenic settings, which are postulated as an orogenic mode
operating under the hotter mantle conditions of late Archean to early Proterozoic, are characterised by longer durations of metamorphismand slower cooling rates (a few10s of °C/Ma) as compared to the modern orogenic settings (a few 100s of °C/Ma) operating under relatively colder mantle conditions. This is because peel-back orogens feature: (1) hot lithospheres with very high crustal geotherms being sustained by high mantle heat flow and profuse magmatism, and (2) distributed deformation patterns that limit vertical extrusion (exhumation) of the metamorphic rocks along localized deformation zones and instead, trap them in the orogenic core
for a long time. In contrast, modern orogens mostly involve colder lithospheres and allow rapid exhumation through localized deformation, which facilitates faster cooling of hot, exhumed metamorphic rocks in a colder ambience. Thus, we propose that the secular change in metamorphic cooling rates indicates a changing regime of orogenesis and thereby, of plate tectonics through time. Predominance of the slower metamorphic cooling rates before the Neoproterozoic indicate the occurrences of peel-back orogenesis and truncated hot (collisional) orogenesis during that time, while the appearance of faster cooling rates since the late Neoproterozoic indicates the transition to modern style of orogenesis. A transition between these orogenic styles also accounts for the prolonged longevity (>100 million years) of many Precambrian orogenic belts as compared to the Phanerozoic ones. This study underscores the strength of timescales in combination with P-T paths to distinguish tectonic settings of different styles and ages.
... Importantly, since mafic granulite sample AKP occurs as boudinaged enclaves within charnockite and gneiss in Mercara, this granulite must have experienced the same metamorphic conditions as the host rocks. Thus, we infer a clockwise P-T path for this terrane (Fig. 16), fitting evolutionary paths expected during collisional orogenesis at a convergent plate margin (England and Thompson, 1984;Weller et al., 2013;Palin et al., 2018;Kang et al., 2020). ...
Early continent-building processes on Earth are challenging to investigate, particularly since juvenile felsic crust formed during the Early Archean (4.0–3.2 Ga) is rarely preserved. Thus, associated sedimentary records are of fundamental importance, although in many cases these have been metamorphosed and reworked to various degrees since deposition. Here we present new petrological and zircon and monazite U–Pb age data from one of the Earth's oldest ‘khondalite’ (granulite-facies aluminous metapelite) belt, which we define as the Mercara khondalite belt, and associated charnockite and mafic granulite from the Mercara suture, the collision zone welding the Coorg and Western Dharwar Blocks in southern India. Petrologic analyses and phase equilibria modelling of the khondalites and associated charnockite and mafic granulite reveal a clockwise pressure–temperature (P–T) path with a peak temperature of above ca. 900 °C, and pressures up to 12 kbar. Detrital zircon grains in the metasedimentary rocks have magmatic cores with oscillatory zoning and ages up to ca. 3.5 Ga, and metamorphic overgrowths with ages of 3.1–3.0 Ga. Monazite in the khondalites yield identical metamorphic ages in the range of 3.1 to 3.0 Ga. Some of these rocks are overprinted by a younger thermal event at ca. 2.8–2.6 Ga. We correlate the high P–T metamorphism with the subduction-collision history between the northern margin of the Mercara block and the Western Dharwar Craton during the Mesoarchean, which indicates that plate tectonics had been established on Earth by at least ca. 3.1 Ga, in agreement with many independent lines of evidence. The Mesoarchean Mercara khondalite belt signals emergence of continents on the early Earth with active drainage systems leading to the deposition of voluminous detritus. The ca. 3.1 to 3.0 Ga (ultra) high temperature and high pressure metamorphism also coincides with the timing of assembly of the Earth's oldest supercontinent Ur
