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A geological map with cross-section of the Himalayan metamorphic belt along the Kaghan Valley transect. Abbreviations used in map are; MCT: Main Central Thrust; MMT: Main Mantle Thrust (for further details on the rock types and structures, see Rehman et al., 2007, 2008). The map is modified after Kaneko et al. (2003).
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... 4. Location of the Group I and II eclogites. The location map is zoomed area of the box shown on the upper right, which is a reduced/modified form of the geological transect map shown in Fig. 2 (modified from Kaneko et al., 2003; Rehman et al., 2007).
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... are generally considered as derived from basaltic or gabbroic rocks which have either been intensely metamorphosed during subduction-obduction related processes or, associated with continental crust and affected by major crustal thrusting. The advantage of petrological, geochemical, and geochronological study of eclogitic rocks is twofold. First, metabasic rocks are capable of preserving the original magmatic characteristics of igneous formations. Second, the study of eclogites enables us to appreciate the behaviour of isotopic tracers during high-grade metamorphism. In this chapter we report Sm-Nd and Lu-Hf isotope compositions of whole rock and their constituent minerals (garnet, clinopyroxene, phengite, and epidote) from the Himalayan high-pressure (HP) and ultrahigh-pressure (UHP) eclogites which are exposed in the Kaghan Valley of Pakistan ( Fig. 1). These eclogites were formed when the Indian Plate slab subducted beneath the Asian Plate to greater depths and experienced UHP metamorphism. The Kaghan Valley eclogites have been discussed previously by several authors (e.g. Greco et al., 1989; Pognante & Spencer, 1991; Tonarini et al., 1993; Spencer et al., 1995; Lombardo & Rolfo, 2000; O’Brien et al., 2001; Kaneko et al., 2003; Parrish et al., 2006; Rehman et al., 2008). Spencer et al. (1995) carried out geochemical work (major and trace elements and Sr isotope) on the Kaghan Valley whole rock eclogites. They interpreted based on their geochemical results that the protolith for these eclogites was the Permian Panjal Trap basalts. Similarly, Tonarini et al. (1993) reported an Sm-Nd isochron age of 49 and a Sm-Nd isochron age of 49 ± 6 Ma representing the eclogite facies event in the Himalayan region. They calculated the above age from a garnet-clinopyroxene pair. Spencer & Gebauer (1996) reported a U-Pb zircon SHRIMP age of 44 Ma for the eclogite facies event. In addition, Kaneko et al. (2003) reported 46 Ma as the peak time for the UHP event in the Himalayan region. Their result was based on the U-Pb age dating on the coesite-bearing zircon from felsic gneisses which surround the UHP eclogites in Kaghan Valley. Parrish et al. (2006) also reported similar age results deduced from the U-Pb zircon age dating. Recently, Wilke et al. (2010a) elaborated the multi-stage history of the Kaghan Valley eclogites based on the major and trace element geochemistry of the eclogites, and a U-Pb zircon and rutile geochronology from the felsic gneisses. Most of the above mentioned researchers and references therein elaborate the metamorphic history of the eclogites and surrounding felsic gneisses. However, (i) details on the protolith environment of these eclogites, (ii) fluids infiltration effects on these rocks during their subduction to mantle depths, and (iii) element mobility during eclogite facies and late-stage metamorphism remained unclear. To discuss the above problems we reinvestigated the Kaghan Valley eclogites by examining carefully petrological and textural features, and extending our work to multi-isotope (Sm-Nd and Lu-Hf) systematic. Our results enabled us to (i) differentiate between the types of eclogites, (ii) evaluate their source rock, and (iii) effect of fluid infiltration on these eclogites during subduction and later stages of exhumation. Our results provide significant evidence of isotopic disequilibrium during and after the eclogite facies metamorphism. Eclogites of the Himalayan chain are the products of continental basaltic flows and their preceding feeder dikes of Permian to Lower Triassic Panjal Trap (e.g. Honegger et al., 1982; Spencer et al., 1995). These basaltic extrusions at the northern margin of the Indian Plate mark start of the Himalayan cycle (Honegger et al., 1982). During the Himalayan cycle the volcanic activity began in the Upper Carboniferous, with the climax in Lower Permian, producing several hundred meters of intermediate to acid pyroclastic and welded tuffs overlying Carboniferous shales and shallow water limestone (Honegger et al., 1982). The onset of India-Asia collision ranging from 65 Ma to 40 Ma resulted in deep subduction of the Indian Plate beneath the Asian Plate (e.g. Patriat & Achache, 1984; Beck et al., 1995; Guillot et al., 2003). This subduction and related metamorphism resulted in a large-scale thrusting and piling up of the oceanic crust and sediments in the form of the Lesser Himalayan sequence, the Higher Himalayan crystalline and the Tethyan sediments as well as the ophiolitic mélanges along the suture zone between the Indian Plate and the Kohistan island arc (Greco et al., 1989; Spencer et al., 1995; Rehman et al., 2007) (Fig. 2). In the Kaghan Valley transect a geological section is exposed which comprises the Lesser Himalayan sequence, the Higher Himalayan Crystalline, and the basement rocks of the Kohistan island arc (Fig. 2). Information on petrological, geochemical, and structural geology of the above sequences are explained in detail elsewhere (e.g. Rehman et al., 2007; 2008 and references therein). In this session, we will explain mafic rocks mainly. The mafic sheets exposed in the Higher Himalayan Crystalline in the Kaghan Valley, range in size from a few meters to few tens of meters within pelitic gneisses and marbles. Greenschist and amphibolite facies overprint on earlier eclogite facies assemblages. Coesite-bearing UHP eclogite is found as an isolated block (for locations see Rehman et al., 2007, 2008). The presence of coesite pseudomorphs in omphacite in eclogites, and in zircon from gneisses (Kaneko et al., 2003) indicate that these rocks reached a depth exceeding 100 km when the leading edge of the Indian Plate subducted beneath the Asian Plate. Eclogites in the Kaghan Valley occur as massive and extend for a few tens of meters (Fig 3a, b) and as an isolated boudines or lenses 2 to 3 m in diameter (Fig. 3c, d). They are hosted by felsic gneisses, marbles and amphibolite sheets. These eclogites were subdivided into Group I and II (Fig. 4). Both groups of eclogites are differentiated from each other on the basis of occurrence, mineralogy, and geochemistry. Group I eclogites are composed of garnet, clinopyroxene, quartz, amphibole with rare epidote and phengite. They contain rutile, ilmenite, apatite, and abundant zircons as accessory minerals. Group II eclogites are composed of garnet, clinopyroxene, quartz/coesite, phengite, epidote, and amphibole with accessory rutile, ilmenite, apatite, and rare zircon. Group I eclogites are HP and record pressure-temperature conditions at 704 ± 92 oC and 2.2 ± 0.3 GPa, whereas, Group II eclogites are UHP and recorded pressure-temperature conditions between 2.7 and 3.2 GPa and 757 and 786 oC respectively (Rehman et al., 2007). Based on petrographic features and mineral inclusion study three apparent stages of metamorphism have been reported in the Himalayan eclogites of Kaghan Valley (Rehman et al., 2008). The first stage represented a prograde garnet growth stage (jadeite + quartz + albite inclusions in garnet core). The second stage represented peak UHP stage (deduced from the presence of coesite inclusion in clinopyroxene and pressure-temperature conditions estimated from the chemical composition of the rim portions of adjacent garnet, clinopyroxene and phengite). The third stage was a decompression/retrogressive stage represented by the common occurrence of symplectitic augite, amphibole, and quartz after clinopyroxene. The third stage recorded texturally late-stage amphibolite facies overprint. Reaction textures and phase relations indicate that the metamorphic overprint was largely under hydrous conditions. Geochemical analytical procedures were performed at the Pheasant Memorial Laboratory (PML), Institute for the Study of the Earth's Interior (ISEI), Okayama University at Misasa, Japan, following the procedures of Nakamura et al. (2003), Lu et al. (2007), and Makishima & Nakamura (2007). Basaltic standard (JB3) from the Geological Survey of Japan was used as a standard. Five eclogite samples (whole rock powders of two samples from the Group I and three samples from the Group II eclogites) and 15 mineral separates of the same five eclogite samples from both groups were decomposed and analyzed for their Sm–Nd and Lu–Hf isotopic ratios and abundances. Garnet and clinopyroxene separates from Group I and II eclogites and epidote and phengite from Group II eclogites were handpicked (Group I did not contain phengite and epidote). To remove surface contamination, garnet and clinopyroxene were washed using ultrasonic bath, with 6 M HCl till the yellow color fainted away, whereas epidote and phengite were washed with 2 M HCl for 30–40 minutes, and rinsed with distilled water few times. Then distilled water was added to all mineral separates and put for washing in an ultrasonic bath for overnight for complete removal of any contamination. After drying, the separated mineral fractions were further pulverized using agate mill and mortar. The powdered fractions were further leached with 0.1 N HCl till the yellow color fainted away. Then rinsed with distilled water few times to remove any remaining contamination and dried at 70 °C. Powdered samples were decomposed in Teflon Bombs added with concentrated HF and HClO 4 at 245 °C for 4 days to get complete digestions. The solutions were then transferred to Teflon beakers, added with 0.1, 0.6 and 0.3 ml of HF, HClO 4 and HNO 3 respectively and put for agitation in ultrasonic bath for 8 hours to get complete homogenous solution. Fluoride residues produced by the initial acid attack were removed by repeated redissolution in HClO 4 following the procedures of Yokoyama et al. (1999). The analytical procedures for mass spectrometry following Yoshikawa & Nakamura (1993) for Nd isotopic ratios and abundances of Sm and Nd, employing ...
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... are generally considered as derived from basaltic or gabbroic rocks which have either been intensely metamorphosed during subduction-obduction related processes or, associated with continental crust and affected by major crustal thrusting. The advantage of petrological, geochemical, and geochronological study of eclogitic rocks is twofold. First, metabasic rocks are capable of preserving the original magmatic characteristics of igneous formations. Second, the study of eclogites enables us to appreciate the behaviour of isotopic tracers during high-grade metamorphism. In this chapter we report Sm-Nd and Lu-Hf isotope compositions of whole rock and their constituent minerals (garnet, clinopyroxene, phengite, and epidote) from the Himalayan high-pressure (HP) and ultrahigh-pressure (UHP) eclogites which are exposed in the Kaghan Valley of Pakistan ( Fig. 1). These eclogites were formed when the Indian Plate slab subducted beneath the Asian Plate to greater depths and experienced UHP metamorphism. The Kaghan Valley eclogites have been discussed previously by several authors (e.g. Greco et al., 1989; Pognante & Spencer, 1991; Tonarini et al., 1993; Spencer et al., 1995; Lombardo & Rolfo, 2000; O’Brien et al., 2001; Kaneko et al., 2003; Parrish et al., 2006; Rehman et al., 2008). Spencer et al. (1995) carried out geochemical work (major and trace elements and Sr isotope) on the Kaghan Valley whole rock eclogites. They interpreted based on their geochemical results that the protolith for these eclogites was the Permian Panjal Trap basalts. Similarly, Tonarini et al. (1993) reported an Sm-Nd isochron age of 49 and a Sm-Nd isochron age of 49 ± 6 Ma representing the eclogite facies event in the Himalayan region. They calculated the above age from a garnet-clinopyroxene pair. Spencer & Gebauer (1996) reported a U-Pb zircon SHRIMP age of 44 Ma for the eclogite facies event. In addition, Kaneko et al. (2003) reported 46 Ma as the peak time for the UHP event in the Himalayan region. Their result was based on the U-Pb age dating on the coesite-bearing zircon from felsic gneisses which surround the UHP eclogites in Kaghan Valley. Parrish et al. (2006) also reported similar age results deduced from the U-Pb zircon age dating. Recently, Wilke et al. (2010a) elaborated the multi-stage history of the Kaghan Valley eclogites based on the major and trace element geochemistry of the eclogites, and a U-Pb zircon and rutile geochronology from the felsic gneisses. Most of the above mentioned researchers and references therein elaborate the metamorphic history of the eclogites and surrounding felsic gneisses. However, (i) details on the protolith environment of these eclogites, (ii) fluids infiltration effects on these rocks during their subduction to mantle depths, and (iii) element mobility during eclogite facies and late-stage metamorphism remained unclear. To discuss the above problems we reinvestigated the Kaghan Valley eclogites by examining carefully petrological and textural features, and extending our work to multi-isotope (Sm-Nd and Lu-Hf) systematic. Our results enabled us to (i) differentiate between the types of eclogites, (ii) evaluate their source rock, and (iii) effect of fluid infiltration on these eclogites during subduction and later stages of exhumation. Our results provide significant evidence of isotopic disequilibrium during and after the eclogite facies metamorphism. Eclogites of the Himalayan chain are the products of continental basaltic flows and their preceding feeder dikes of Permian to Lower Triassic Panjal Trap (e.g. Honegger et al., 1982; Spencer et al., 1995). These basaltic extrusions at the northern margin of the Indian Plate mark start of the Himalayan cycle (Honegger et al., 1982). During the Himalayan cycle the volcanic activity began in the Upper Carboniferous, with the climax in Lower Permian, producing several hundred meters of intermediate to acid pyroclastic and welded tuffs overlying Carboniferous shales and shallow water limestone (Honegger et al., 1982). The onset of India-Asia collision ranging from 65 Ma to 40 Ma resulted in deep subduction of the Indian Plate beneath the Asian Plate (e.g. Patriat & Achache, 1984; Beck et al., 1995; Guillot et al., 2003). This subduction and related metamorphism resulted in a large-scale thrusting and piling up of the oceanic crust and sediments in the form of the Lesser Himalayan sequence, the Higher Himalayan crystalline and the Tethyan sediments as well as the ophiolitic mélanges along the suture zone between the Indian Plate and the Kohistan island arc (Greco et al., 1989; Spencer et al., 1995; Rehman et al., 2007) (Fig. 2). In the Kaghan Valley transect a geological section is exposed which comprises the Lesser Himalayan sequence, the Higher Himalayan Crystalline, and the basement rocks of the Kohistan island arc (Fig. 2). Information on petrological, geochemical, and structural geology of the above sequences are explained in detail elsewhere (e.g. Rehman et al., 2007; 2008 and references therein). In this session, we will explain mafic rocks mainly. The mafic sheets exposed in the Higher Himalayan Crystalline in the Kaghan Valley, range in size from a few meters to few tens of meters within pelitic gneisses and marbles. Greenschist and amphibolite facies overprint on earlier eclogite facies assemblages. Coesite-bearing UHP eclogite is found as an isolated block (for locations see Rehman et al., 2007, 2008). The presence of coesite pseudomorphs in omphacite in eclogites, and in zircon from gneisses (Kaneko et al., 2003) indicate that these rocks reached a depth exceeding 100 km when the leading edge of the Indian Plate subducted beneath the Asian Plate. Eclogites in the Kaghan Valley occur as massive and extend for a few tens of meters (Fig 3a, b) and as an isolated boudines or lenses 2 to 3 m in diameter (Fig. 3c, d). They are hosted by felsic gneisses, marbles and amphibolite sheets. These eclogites were subdivided into Group I and II (Fig. 4). Both groups of eclogites are differentiated from each other on the basis of occurrence, mineralogy, and geochemistry. Group I eclogites are composed of garnet, clinopyroxene, quartz, amphibole with rare epidote and phengite. They contain rutile, ilmenite, apatite, and abundant zircons as accessory minerals. Group II eclogites are composed of garnet, clinopyroxene, quartz/coesite, phengite, epidote, and amphibole with accessory rutile, ilmenite, apatite, and rare zircon. Group I eclogites are HP and record pressure-temperature conditions at 704 ± 92 oC and 2.2 ± 0.3 GPa, whereas, Group II eclogites are UHP and recorded pressure-temperature conditions between 2.7 and 3.2 GPa and 757 and 786 oC respectively (Rehman et al., 2007). Based on petrographic features and mineral inclusion study three apparent stages of metamorphism have been reported in the Himalayan eclogites of Kaghan Valley (Rehman et al., 2008). The first stage represented a prograde garnet growth stage (jadeite + quartz + albite inclusions in garnet core). The second stage represented peak UHP stage (deduced from the presence of coesite inclusion in clinopyroxene and pressure-temperature conditions estimated from the chemical composition of the rim portions of adjacent garnet, clinopyroxene and phengite). The third stage was a decompression/retrogressive stage represented by the common occurrence of symplectitic augite, amphibole, and quartz after clinopyroxene. The third stage recorded texturally late-stage amphibolite facies overprint. Reaction textures and phase relations indicate that the metamorphic overprint was largely under hydrous conditions. Geochemical analytical procedures were performed at the Pheasant Memorial Laboratory (PML), Institute for the Study of the Earth's Interior (ISEI), Okayama University at Misasa, Japan, following the procedures of Nakamura et al. (2003), Lu et al. (2007), and Makishima & Nakamura (2007). Basaltic standard (JB3) from the Geological Survey of Japan was used as a standard. Five eclogite samples (whole rock powders of two samples from the Group I and three samples from the Group II eclogites) and 15 mineral separates of the same five eclogite samples from both groups were decomposed and analyzed for their Sm–Nd and Lu–Hf isotopic ratios and abundances. Garnet and clinopyroxene separates from Group I and II eclogites and epidote and phengite from Group II eclogites were handpicked (Group I did not contain phengite and epidote). To remove surface contamination, garnet and clinopyroxene were washed using ultrasonic bath, with 6 M HCl till the yellow color fainted away, whereas epidote and phengite were washed with 2 M HCl for 30–40 minutes, and rinsed with distilled water few times. Then distilled water was added to all mineral separates and put for washing in an ultrasonic bath for overnight for complete removal of any contamination. After drying, the separated mineral fractions were further pulverized using agate mill and mortar. The powdered fractions were further leached with 0.1 N HCl till the yellow color fainted away. Then rinsed with distilled water few times to remove any remaining contamination and dried at 70 °C. Powdered samples were decomposed in Teflon Bombs added with concentrated HF and HClO 4 at 245 °C for 4 days to get complete digestions. The solutions were then transferred to Teflon beakers, added with 0.1, 0.6 and 0.3 ml of HF, HClO 4 and HNO 3 respectively and put for agitation in ultrasonic bath for 8 hours to get complete homogenous solution. Fluoride residues produced by the initial acid attack were removed by repeated redissolution in HClO 4 ...
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... 1. (a) Geographical location of the Himalayan range. (b) An enlarged portion of the boxed area shown in the upper left, representing main tectonic units of the Indian Plate, the Kohistan arc, and the southern margin of the Asian Plate. Black star and a rectangle surrounding it represent location of the eclogites in the Kaghan Valley transect (see Fig. 2 for details).
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... Occurrences of such UHP metamorphic rocks are related to the processes of subduction followed by exhumation in orogenic belts (de Sigoyer et al. 2000(de Sigoyer et al. , 2004. Various UHP metamorphic rocks have been reported from Dabie-Sulu terrain, eastern China (Tang et al. 2007); Tso Morari Complex, eastern Ladakh (Guillot et al. 1997); Kaghan Valley, Pakistan (Rehman et al. 2008(Rehman et al. , 2012; Sesia-Lanzo Zone, Italy (Inger et al. 1996); and Tavsanli Zone, Turkey (Sherlock and Arnaud 1999). Most of these UHP metamorphic belts have continental affinities (Carswell and Cuthbert 2003). ...
... Therefore, we have ascertained the degree of mobility for individual elements in order to put constraints on their protolith, genesis, and tectonic setting. Postmagmatic element mobility is assessed by comparing elemental variations with that of the least mobile element, Zr (Winchester and Floyd 1977;Ahmad et al. 1996;Rehman et al. 2012). In TMC eclogites, we observe positive correlations of Zr against TiO 2 , Fe 2 O 3 t , P 2 O 5 , Al 2 O 3 , Na 2 O, MnO, La, Sm, and Eu, while expected negative correlations of Zr against MgO, CaO, and Cr probably reflect their magmatic characteristics ( fig. 4). ...
The Tso Morari Crystalline Complex (TMC), eastern Ladakh, is marked by the presence of eclogites as boudins and lenses within the Puga Formation. These eclogites are composed of garnet, omphacite, amphibole, phengite, glaucophane, quartz, and iron oxide, with rare coesite inclusions in garnet reflecting ultrahigh-pressure metamorphic characteristics. Geochemically, TMC eclogites have high Fe-Ti basaltic compositions and classify as subalkaline tholeiites. Rare earth element and multielement diagrams display enriched patterns similar to enriched mid-ocean ridge basalt coupled with perturbed large ion lithophile elements and higher whole-rock (87Sr/86Sr) ratios (0.70884 to 0.72721) reflecting the possible influence of postcrystallization processes rather than variable interaction with host granite gneisses (87Sr/86Sr ratio: ∼0.73901). To evaluate the existing protolith possibilities, we calculated εNd(t=289Ma) values (+1.9 to +9.5) and εNd(t=140Ma) values (+1.1 to +8.9) of TMC eclogites; both indicate their derivation from depleted-mantle sources. The εNd(t=289Ma) values of the early Permian enriched Panjal volcanics of Kashmir Valley (−5.3 to +1.3) and Phe volcanics of Zanskar Himalaya (−7.4 to −1.1) are very different from TMC eclogites. However, the εNd(t=289Ma) values of TMC eclogites are similar to the depleted Panjal volcanics (+0.3 to +4.3). Similarly, the εNd(t=140Ma) values of the TMC eclogites closely resemble those of the adjoining Ladakh ophiolites, such as the Nidar-Spongtang-Shergol-Dras ophiolitic mafic rocks (+5.1 to +9.9). These observations partly negate the existing hypothesis of enriched Panjal and Phe volcanics for being the protolith for the TMC eclogites. Thus, we propose that the protolith for the TMC eclogites could be represented by the subducted portion of the early Permian depleted Panjal volcanics and Late Jurassic to Early Cretaceous Ladakh ophiolitic mafic rocks, subducted to eclogite-grade metamorphism (around ~53 Ma) and were subsequently tectonically accreted to the obducting Indian continental crust during their exhumation.
... Occurrences of such UHP metamorphic rocks are related to the processes of subduction followed by exhumation in orogenic belts (de Sigoyer et al. 2000(de Sigoyer et al. , 2004. Various UHP metamorphic rocks have been reported from Dabie-Sulu terrain, eastern China (Tang et al. 2007); Tso Morari Complex, eastern Ladakh (Guillot et al. 1997); Kaghan Valley, Pakistan (Rehman et al. 2008(Rehman et al. , 2012; Sesia-Lanzo Zone, Italy (Inger et al. 1996); and Tavsanli Zone, Turkey (Sherlock and Arnaud 1999). Most of these UHP metamorphic belts have continental affinities (Carswell and Cuthbert 2003). ...
... Therefore, we have ascertained the degree of mobility for individual elements in order to put constraints on their protolith, genesis, and tectonic setting. Postmagmatic element mobility is assessed by comparing elemental variations with that of the least mobile element, Zr (Winchester and Floyd 1977;Ahmad et al. 1996;Rehman et al. 2012). In TMC eclogites, we observe positive correlations of Zr against TiO 2 , Fe 2 O 3 t , P 2 O 5 , Al 2 O 3 , Na 2 O, MnO, La, Sm, and Eu, while expected negative correlations of Zr against MgO, CaO, and Cr probably reflect their magmatic characteristics ( fig. 4). ...
The Tso Morari Crystalline Complex (TMC), eastern Ladakh, is marked by the presence of eclogites as boudins and lenses within the Puga Formation. These eclogites are composed of garnet, omphacite, amphibole, phengite, glaucophane, quartz, and iron oxide, with rare coesite inclusions in garnet reflecting ultrahigh-pressure metamorphic characteristics. Geochemically, TMC eclogites have high Fe-Ti basaltic compositions and classify as subalkaline tholeiites. Rare earth element and multielement diagrams display enriched patterns similar to enriched mid-ocean ridge basalt coupled with perturbed large ion lithophile elements and higher whole-rock (87Sr/86Sr) ratios (0.70884 to 0.72721) reflecting the possible influence of postcrystallization processes rather than variable interaction with host granite gneisses (87Sr/86Sr ratio: ∼0.73901). To evaluate the existing protolith possibilities, we calculated εNd(t=289Ma) values (+1.9 to +9.5) and εNd(t=140Ma) values (+1.1 to +8.9) of TMC eclogites; both indicate their derivation from depleted-mantle sources. The εNd(t=289Ma) values of the early Permian enriched Panjal volcanics of Kashmir Valley (−5.3 to +1.3) and Phe volcanics of Zanskar Himalaya (−7.4 to −1.1) are very different from TMC eclogites. However, the εNd(t=289Ma) values of TMC eclogites are similar to the depleted Panjal volcanics (+0.3 to +4.3). Similarly, the εNd(t=140Ma) values of the TMC eclogites closely resemble those of the adjoining Ladakh ophiolites, such as the Nidar-Spongtang-Shergol-Dras ophiolitic mafic rocks (+5.1 to +9.9). These observations partly negate the existing hypothesis of enriched Panjal and Phe volcanics for being the protolith for the TMC eclogites. Thus, we propose that the protolith for the TMC eclogites could be represented by the subducted portion of the early Permian depleted Panjal volcanics and Late Jurassic to Early Cretaceous Ladakh ophiolitic mafic rocks, subducted to eclogite-grade metamorphism (around ~53 Ma) and were subsequently tectonically accreted to the obducting Indian continental crust during their exhumation.
... Later studies report more occurrences and further varieties of eclogites, including coesite-bearing ultrahigh-pressure eclogites, in much wider extent within the Kaghan Valley (Kaneko et al., 2003;Lombardo, Rolfo, & Compagnoni, 2000;O'Brien et al., 2001;Rehman et al., 2007Rehman et al., , 2008Spencer, Tonarini, & Pognante, 1995;Treloar, 1997;Treloar, O'Brien, Parrish, & Khan, 2003), Neelum Valley in Kashmir (Fontan et al., 2000), (Bhat & Zainuddin, 1979;Jan & Karim, 1990;Rehman, Kobayashi, et al., 2013;Rehman, Yamamoto, & Shin, 2013b;Rehman et al., 2014;Shellnutt et al., 2014;Spencer et al., 1995;and (Greco et al., 1989;Spencer et al., 1995). The age of the eclogite-facies metamorphism in the western part of Himalaya (Kaghan Valley, Pakistan) is well constrained at around 49-45 Ma (Parrish et al., 2006;Rehman et al., 2012;Rehman et al., 2013aRehman et al., , 2016Spencer & Gebauer, 1996;Wilke et al., 2010). The end of Himalayan metamorphism accompanied by rapid cooling and unroofing occurred during 43-25 Ma (Ar-Ar cooling ages of hornblende and white mica from the gneisses (Chamberlain, Zeitler, & Erickson, 1991)). ...
A pictorial review and brief description of metabasites (mainly eclogites) exposed in the western part of Himalaya, north Pakistan are presented. The metabasites occur in the Kaghan and Neelum valleys in the form of thick sheets, dikes, lenses, and zoned bodies. On the basis of petrography and mineral paragenesis, they can be distinguished into greenschist, amphibolites, garnet-amphibolites, eclogites, amphibolitized eclogites, and garnetites. In this review, first a brief history of the term eclogite and its origin for understanding the process of eclogite formation is presented. Then, a description on the occurrence of various metabasites/eclogites outcrops in the western Himalaya is given. Later, detailed petrological and textural features observed in hand specimen and under the microscope are discussed for individual rock types. Finally, an overview of the Himalayan metabasites tracing back from their magmatic basaltic source to their transformation into amphibolites and eclogites via multi-stage metamorphism is provided. The field features, with the aid of petrological and geochemical evidence of these metabasites, help to constrain the tectono-metamorphic evolution of the continent–continent collision type orogenic belts such as the India–Asia collision-related Himalayan metamorphic belt.
... doi.org/10.1016/j.lithos.2016.06.001. The data of ε Hf for bulk rock eclogites is from Rehman et al. (2012) and is calculated with respect to the protolith emplacement age ca. 267 Ma of the Panjal Traps. ...
We present an integrated study of LA-ICP-MS U–Pb age, Hf isotopes, and trace element geochemistry of zircons from the Himalayan eclogites (mafic rocks) and their host gneisses (felsic rocks) from the Kaghan Valley in Pakistan in order to understand the source and mode of their magmatic protoliths and the effect of metamorphism. Zircons from the so-called Group I (high-pressure) eclogites yielded U–Pb mean ages of 259 ± 10 Ma (MSWD = 0.74), whereas those of Group II (ultrahigh-pressure) eclogites yielded 48 ± 3 Ma (MSWD = 0.71). In felsic gneisses the central or core domains of zircons yielded ages similar to those from Group I eclogites but zircon overgrowth domains yielded 47 ± 1 Ma (MSWD = 1.9). Trace element data suggest a magmatic origin for Group I-derived (having Th/U ratios : > 0.5) and metamorphic origin for Group II-derived (Th/U < 0.07) zircons, respectively. Zircon Hf isotope data, obtained from the same dated spots, show positive initial 176Hf/177Hf isotopic ratios referred to as “ƐHf(t)” of(around + 10 in Group I eclogites; + 7 in Group II eclogites; and + 8 in felsic gneisses zircons, respectively. thus indicate a juvenile mantle source for the protolith rocks (Panjal Traps) with almost no contribution from the ancient crustal material. The similar ƐHf(t) values, identical protolith ages and trace element compositions of zircons in felsic (granites or rhyolites) and mafic (basalt and dolerite) rocks attest to a bimodal magmatism accounting for the Panjal Traps during the Permian. Later, during India-Asia collision in Eocene times, both the felsic and mafic lithologies were subducted to mantle-depths (> 90 km: coesite-stable) and experienced ultrahigh-pressure metamorphism before their final exhumation.
... Our study was conducted at both the outcrop scale (single UHP eclogite body of approximately 2 m in diameter) and the regional scale (one block of amphibolitized eclogites and four blocks of eclogites spreading in an area of about 10 km distance). Sample details are given in Table 1 and their locations are shown in Fig. 2. For most of these samples, thermobarometric information, whole rock major and trace element compositions, whole rock and mineral 147 Sm-143 Nd and 176 Lu-176 Hf isotopic data, and zircon U-Th-Pb ages have previously been reported (Rehman et al., 2007(Rehman et al., , 2008(Rehman et al., , 2012(Rehman et al., , 2013. Mineral abbreviations used throughout this study are after Whitney and Evans (2010) except symplectites are abbreviated as "Sym". ...
Oxygen isotope compositions are reported for the first time for the Himalayan metabasites of the Kaghan Valley, Pakistan in this study. The highest metamorphic grades are recorded in the north of the valley, near the India-Asia collision boundary, in the form of high-pressure (HP: Group I) and ultrahigh-pressure (UHP: Group II) eclogites. The rocks show a step-wise decrease in grade from the UHP to HP eclogites and amphibolites. The protoliths of these metabasites were the Permian Panjal Trap basalts (ca. 267±2.4Ma), which were emplaced along the northern margin of India when it was part of Gondwana. After the break-up of Gondwana, India drifted northward, subducted beneath Asia and underwent UHP metamorphism during the Eocene (ca. 45±1.2Ma). At the regional scale, amphibolites, Group I and II eclogites yielded δ18O values of +5.84 and +5.91‰, +1.66 to +4.24‰, and -2.25 to +0.76‰, respectively, relative to VSMOW. On a more local scale, within a single eclogite body, the δ18O values were the lowest (-2.25 to-1.44‰) in the central, the best preserved (least retrograded) parts, and show a systematic increase outward into more retrograded rocks, reaching up to +0.12‰. These values are significantly lower than the typical mantle values for basalts of +5.7±0.3‰. The unusually low or negative δ18O values in Group II eclogites potentially resulted from hydrothermal alteration of the protoliths by interactions with meteoric water when the Indian plate was at southern high latitudes (~60°S). The stepwise increase in δ18O values, among different eclogite bodies in general and at single outcrop-scales in particular, reflects differing degrees of resetting of the oxygen isotope compositions during exhumation-related retrogression.
... These eclogites are composed of garnet + omphacite + quartz + rutile + titanite + amphibole + apatite + epidote/allanite + symplectite ± with accessory ilmenite preserving no UHP imprints (Fig. 3a-c). They record P-T conditions at 2.2 ± 0.3 GPa and 704 ± 92°C (Rehman et al., 2012). At places these eclogites are strongly amphibolitized and contain abundant quartz-albite-amphibole symplectites (Fig. 3b). ...
We report ion microprobe U–Th–Pb geochronology of in situ zircon from the Himalayan high- and ultrahigh-pressure eclogites, Kaghan Valley of Pakistan. Combined with the textural features, mineral inclusions, cathodoluminescence image information and the U–Th–Pb isotope geochronology, two types of zircons were recognized in Group I and II eclogites. Zircons in Group I eclogites are of considerably large size (>100 μm up to 500 μm). A few grains are euhederal and prismatic, show oscillatory zoning with distinct core–rim luminescence pattern. Several other grains show irregular morphology, mitamictization, embayment and boundary truncations. They contain micro-inclusions such as muscovite, biotite, quartz and albite. Core or middle portions of zircons from Group I eclogites yielded concordant U–Th–Pb age of 267.6 ± 2.4 Ma (MSWD = 8.5), have higher U and Th contents with a Th/U ratio > 1, indicating typical magmatic core domains. Middle and rim or outer portions of these zircons contain inclusions of garnet, omphacite, phengite and these portions show no clear zonation. They yielded discordant values ranging between 210 and 71 Ma, indicating several thermal or Pb-loss events during their growth and recrystalization prior to or during the Himalayan eclogite-facies metamorphism. Zircons in Group II eclogites are smaller in size, prismatic to oval, display patchy or sector zoning and contain abundant inclusions of garnet, omphacite, phengite, quartz, rutile and carbonates. They yielded concordant U–Th–Pb age of 44.9 ± 1.2 Ma (MSWD = 4.9). The lower U and Th contents and a lower Th/U ratio (<0.05) in these zircons suggest their formation from the recrystallization of the older zircons during the Himalayan high and ultrahigh-pressure eclogite-facies metamorphism.
... These eclogites are composed of garnet + omphacite + quartz + rutile + titanite + amphibole + apatite + epidote/allanite + symplectite ± with accessory ilmenite preserving no UHP imprints (Fig. 3a-c). They record P-T conditions at 2.2 ± 0.3 GPa and 704 ± 92°C (Rehman et al., 2012). At places these eclogites are strongly amphibolitized and contain abundant quartz-albite-amphibole symplectites (Fig. 3b). ...
We present U-Pb geochronology and trace element geochemistry of zircon from the Himalayan high- and ultrahigh-pressure eclogites, Kaghan Valley, Pakistan, by the in situ zircon using ion microprobe technique. These eclogites were previously divided into two groups on the basis of whole-rock and multi-isotope geochemistry. Combined with the textural features, mineral inclusions, cathode luminescence image information, and rare earth element patterns, two contrasting populations of zircons were recognized from within thin sections of Group I and II eclogites. Zircons in Group I eclogites are large (50 ~ 500 μm), euhederal to subhederal, and contain abundant inclusions of quartz, plagioclase, and muscovite in the core or middle portions, whereas omphacite, muscovite, and rutile in the rim portions. Several zircons are zoned (bright-thick cores and dark-thin rims), whereas most of the others lack zoning or any internal structures when studied under cathode luminescence imaging system. Zircon grains in Group II eclogites are small (~ 50 μm) with few exceptionally large grains (> 250 μm). They are homogenous, unzoned, and lack inclusion or contain rare inclusion of phengite, omphacite, quartz and rutile. The U-Pb isotope ratio yielded a concordant age of 267 ± 2 Ma from the core portion of zircon of Group I eclogites. Several zircon grains gave U-Pb discordant values of 180 and 114 Ma. The rim portion of one of the zircon grain in Group I eclogites gave a U-Pb discordant value of 71 ± 6 Ma. Zircons of Group II eclogites (UHP phase) did not give age due to their very low U and Pb concentrations. On the basis of trace element geochemistry and U-Pb age dating, we interpret that the zircons of Group I eclogites are of igneous origin. Their growth show decrease in heavy REE from core- to rim-portion. The boundary truncations, embayment of structurally younger rims and growth phases into older cores suggest that the U-Pb integrity was disturbed during several thermal events. This observation supports the protolith-related concordant U-Pb age of 267 Ma and the discordant values of 180 and 114 Ma as thermal events or Pb-loss during the recrystallization of the older zircons. The REE data from zircon rim portion, in Group I eclogites, has substantially lower average REE abundance than the core portions. The lower REE contents imply that these portions of zircon crystallized from a reservoir which was more depleted in REE. Zircons from Group II eclogites are of metamorphic origin. They have clearly distinctive REE patterns, which lack a strong negative Eu anomaly, and show a strong positive Ce anomaly. These depleted REE contents suggest that the formation of metamorphic zircons was from the recrystallization of the older zircons during subduction-related metamorphism.
The subduction of Indian plate lithosphere during its collision with Asian plate in the Eocene resulted in a regional metamorphic belt along the strike of the Himalayan orogen. High-/Ultrahigh-pressure (HP/UHP) metamorphic rocks (eclogites and host gneisses) confirm the metamorphic event in western Himalaya (Kaghan ca. 46 Ma and Tso Morari at ca. 47 Ma) at mantle depths (>90 km: coesite-stable). In contrast, HP/UHP rocks have not been reported from central and eastern Himalaya and only highly retrogressed eclogites and granulites (ca. 25 to 13 Ma) occur. The presence of UHP rocks in western Himalaya and highly retrogressed eclogites and granulites in central and eastern Himalaya was regarded for a diachronous India-Asia collision. Despite the along-strike regional homogeneity in major lithotectonic units of the Himalayan orogen metamorphic diachroneity is enigmatic. It is unlikely to have a subduction-related prolonged progressive metamorphic event. In contrast, the age difference and preservation of UHP phases in the west and their transformation into granulites in central and eastern Himalaya could be associated with their prolonged residing times at crustal levels in the central and eastern Himalaya whereas the rocks exhumed rapidly in the west. The higher thermal events relating to melting of the subducting Indian lithosphere in central and eastern Himalaya evidenced from ultra-potasic volcanics in southern Tibet, likely decompressed the early metabasites into granulitized eclogites even resetting their geologic clock that is why eclogites and granulites in the east show younger ages compared with their UHP counterparts in the west.
