Nanga Parbat-Haramosh Massif Research Articles

Detrital U–Pb rutile and zircon data show Indus River sediment dominantly eroded from East Karakoram, not Nanga Parbat

Rapid uplift and exhumation is hypothesized to occur within focused zones of orogenic syntaxes which may dominate sediment flux from a mountain belt. The Namche Barwa-Gyala Peri Massif (NBGPM) in the Himalayan Eastern syntaxis is an example of such a high sediment production zone. Here we apply detrital zircon and rutile U–Pb dating to modern river sediments sampled up- and downstream of the Nanga Parbat-Haramosh Massif (NPHM) in the Western Himalayan syntaxis. Mass balancing of these data show that the total amount of sediment contributed to the trunk Indus from NPHM is relatively small, accounting for 9–10% of the total zircons reaching the Arabian Sea, and potentially much less when source fertility is taken into account. The sediment productivity of NPHM is high but likely less than half of that seen at NBGPM (30–56 Mt/yr vs. 72–211 Mt/yr). This discrepancy may be explicable by the drier climate, less prominent syntaxial knick-zone, and weaker spatial relationship between NPHM and the Indus River compared to its eastern twin. Our revised sediment flux estimate for NPHM implies regional modern exhumation rates of ∼3–5 mm/y, lower than estimated from the core massif since 1.7 Ma.

Gemmological Characteristics of Gemstone Varieties Found in the Pegmatite of Haramosh Area, Gilgit-Baltistan

Haramosh valley is located in eastern side of Gilgitcity (Fig. 1). Geographically it is located between74°44' 17.37 " E and 35° 51 '8.97 " N. The area is asub range of Karakoram in the south-central region ofthe Rakaposhi–Haramosh mountains. Haramosh valleyis famous for its gemstones which are well known inthe world. The first gem mine was discovered in 1951in the area of Haramosh valley in Gilgit. Theprominent four localities of gem-bearing pegmatitesoccur in the vicinity of the Haramosh peak, Shengusalong Indus river, Haramosh Bulachi village, and thesetting of Khaltoro pasture. Along with glaciers,Hindukush Karakorum and Himalayan ranges host avariety of precious stones (Agheem et al., 2014). Inrecent decade Haramosh valley received moreattention because of the occurrence of a variety ofgems in pegmatite veins. Gemstone varieties related topegmatites are also found in other parts of GilgitBaltistan, like Shigar, Shengus, and Stuk Nala (Kazmiet al., Laurs et al., 1998).Pegmatites are found widelyin Gilgit-Baltistan Pakistan within the mountain rangesof Karakoram (Laurs et al. 1998). These macroliticpegmatite’s are mainly associated with leucogranites inHaramosh Massif, which is famous for recent rapiduplift (Laurs et al., 1998; Zeitler 1985). Variouscrystals of topaz, tourmaline, beryl, and quartz arebeing mined from these macrolitic pegmatite’s (Laurset al., 1998). Famous tourmaline crystal (10cm) ofPakistan is found in Stuk Nala of Nanga ParbatHaramosh Massif (Laurs et al., 1998).

Early Cretaceous to Eocene magmatic records in Ladakh arc: Constraints from U–Pb ages of Deosai volcanics, northern Pakistan

This research provides a detailed U–Pb geochronological record of Deosai volcanics in the western Himalaya. The volcanics comprise basalt, andesite, dacite, and rhyolite and are part of the Kohistan–Ladakh arc, exposed east of the Nanga Parbat–Haramosh Massif. Based on the U–Pb ages, this volcanic suite is classified into three chronostratigraphic units: (a) Chota Pani volcanics, (b) Shatung Nala volcanics, and (c) Bara Pani volcanics, with ages ranging between 123–115, 86–63, and 48–46 Ma, respectively. This study suggests that the Chota Pani volcanics are arc‐related, as their ages coincide well with the timing of initiation of the Kohistan–Ladakh arc. The Shatung Nala volcanics may have formed as a result of a collision between Kohistan–Ladakh arc and Karakoram Block (Eurasian Plate). Moreover, the U–Pb cluster age of Bara Pani volcanics coincides with the initial collision time between Indian and Eurasian plates in the western Himalaya, which strongly suggests its emplacement associated with the continental–continental collision. In addition, the dated granitic samples, yielding 120.9 ± 4.5, 80.2 ± 1.4, 50.42 ± 0.55, and 45.24 ± 0.64 Ma ages, provide further support that the emplacement of Deosai volcanic rocks took place as a consequence of closure of the northern and southern sutures, respectively.

Tectonic evolution of the Himalayan syntaxes: the view from Nanga Parbat

Abstract Current tectonic understanding of the Nanga Parbat–Haramosh massif (NPHM) is reviewed, developing new models for the structure and deformation of the Indian continental crust, its thermorheological evolution, and its relationship to surface processes. Comparisons are drawn with the Namche Barwa–Gyala Peri massif (NBGPM) that cores an equivalent syntaxis at the NE termination of the Himalayan arc. Both massifs show exceptionally rapid active denudation and riverine downcutting, identified from very young cooling ages measured from various thermochronometers. They also record relicts of high-pressure metamorphic conditions that chart early tectonic burial. Initial exhumation was probably exclusively by tectonic processes but the young, and continuing emergence of these massifs reflects combined tectonic and surface processes. The feedback mechanisms implicit in aneurysm models may have been overemphasized, especially the role of synkinematic granites as agents of rheological softening and strain localization. Patterns of distributed ductile deformation exhumed within the NPHM are consistent with models of orogen-wide gravitation flow, with the syntaxes forming the lateral edges to the flow beneath the Himalayan arc.

Eocene to Oligocene retrogression and recrystallization of the Stak eclogite in northwest Himalaya

Highly retrogressed eclogite is present in the Stak massif located on the northern edge of the Indian continental margin in northern Pakistan. Garnet in foliated samples contains omphacite inclusions (Xjd=0.33–0.40) and quartz inclusions and latter retain Raman spectroscopic evidence for high residual pressures up to 0.52GPa. These garnet grains do not show apparent compositional zoning. By contrast, one sample contains euhedral grains of garnet with quartz inclusions that show residual pressures as low as 0.25GPa. These garnet grains do not contain omphacite inclusions, and show different compositional zoning compared to the omphacite-bearing garnet. The metamorphic condition of this sample was estimated to be 1.0–1.4GPa/650–710°C using residual pressure values of quartz inclusions in garnets and the garnet–clinopyroxene geothermometer. The U–Pb ages of zircon grains range from 158 to 28Ma with a cluster between at ca. 32Ma, which is younger than that of the peak ultrahigh-pressure metamorphic ages of eclogitic massifs in the northwestern Himalaya, e.g. Kaghan and Tso Morari. We suggest that the retrogressed eclogitic rocks in the Stak massif were heated by nearby Nanga Parbat Haramosh massif at ca. 32Ma, subsequent to peak eclogite facies conditions. During this heating, part of the eclogite was largely recrystallized to form euhedral garnet grains. These results suggest that the Stak massif resided at a lower crustal depth while other ultrahigh-pressure massifs were exhumed in western Himalaya.

Comparison between the tectonic movements of Nanga Parbat-Haramosh Massif and Mt. Everest

Available with full text.

Catastrophic rock slope failures and late Quaternary developments in the Nanga Parbat–Haramosh Massif, Upper Indus basin, northern Pakistan

The Nanga Parbat–Haramosh Massif has some of the greatest relief on Earth and highest measured rates of uplift, denudation, and river incision in bedrock. Many studies have sought to understand how its morphology relates to geotectonic evolution and glaciations. However, few catastrophic rock slope failures had been recognised and many of their impacts had been attributed to other processes. Recently more than 150 of these landslides have been found within a 100-km radius of Nanga Parbat (8125 m). New discoveries are reported east, north and west of Nanga Parbat along the Indus streams. Most generated long-run-out rock avalanches that dammed the Indus or its tributaries, some impounding large lakes. They initiated episodes of intermontane sedimentation followed by trenching and removal of sediment. Valley-floor features record a complex interplay of impoundment and sedimentation episodes, superimposition of streams in pre-landslide valley floors, and exhumation of buried features. These findings depart from existing reconstructions of Quaternary events. A number of the rock-avalanche deposits were previously misinterpreted as tills or moraine and their associated lacustrine deposits attributed to glacial lakes. Features up to 1000 m above the Indus, formerly seen as tectonically raised terraces, are depositional features emplaced by landslides, or erosion terraces recording the trenching of valley fill in landslide-interrupted river reaches. Unquestionably, tectonics and glaciation have been important but decisive and misread formative events of the Holocene involve a post-glacial, landslide-fragmented fluvial system. The latter has kept valley developments in a chronic state of disequilibrium with respect to climatic and geotectonic controls. Accepted glacial chronologies are put in doubt, particularly the extent and timing of the last major glaciation. The pace and role processes in the Holocene have been seriously underestimated.

Insights into the patterns and locations of erosion in the Himalaya — A combined fission-track and in situ Sm–Nd isotopic study of detrital apatite

Understanding the role that climate, erosion and tectonics play in determining the mean relief, elevation and general form of a mountain range over time has become the focus of much recent research in the field of tectonic geomorphology. A crucial constraint in these studies is the long-term exhumation rate, which is increasingly being examined using detrital minerals from the sedimentary rock record that represent the eroded remnants of the orogen. A classic field area of such studies is the Himalayan orogen — the Earth's largest mountain range. We describe here a novel approach that uses the in situ analysis of Nd isotopes by laser ablation multicollector inductively coupled plasma mass spectrometry of fission-track dated apatite grains to unambiguously tie them to their source regions. We demonstrate the capability of this new approach on several modern and Holocene river sand samples from the Himalayan orogen and determine, because of the large isotopic differences in the constituent tectonostratigraphic units, both the location and magnitude of long-term erosion. In agreement with previous studies we are able to show that in the Central and Eastern Himalaya long-term denudation is focussed within the High Himalayan Crystalline Series (HHCS) at average rates of ∼ 2 mm/yr. Average long-term rates are similar in the Western Himalaya sampled by the Indus River and largely reflect erosional denudation of the Western Indian Himalaya and the Western Syntaxis (the Nanga Parbat Haramosh Massif and South Karakoram Metamorphic Complex). Notably, the Asian continent supplies significantly more apatites to the mouth of the Indus than they do to the Bengal delta. The ability to reconstruct the patterns of long-term erosion in the Himalaya using detrital apatites from a few samples collected from major transverse drainages provides confidence in the utility of this approach to examine the locations and magnitude of long-term erosion in the past.

Ductile extrusion in continental collision zones: ambiguities in the definition of channel flow and its identification in ancient orogens

Abstract Field characteristics of crustal extrusion zones include: high-grade metamorphism flanked by lower-grade rocks; broadly coeval flanking shear zones with opposing senses of shear; early ductile fabrics successively overprinted by semi-brittle and brittle structures; and localization of strain to give a more extensive deformation history within the extrusion zone relative to the flanking regions. Crustal extrusion, involving a combination of pure and simple shear, is a regular consequence of bulk orogenic thickening and contraction during continental collision. Extrusion can occur in response to different tectonic settings, and need not necessarily imply a driving force linked to mid-crustal channel flow. In most situations, field criteria alone are unlikely to be sufficient to determine the driving causes of extrusion. This is illustrated with examples from the Nanga Parbat-Haramosh Massif in the Pakistan Himalaya, and the Wing Pond Shear Zone in Newfoundland.

Early-Middle Miocene paleodrainage and tectonics in the Pakistan Himalaya

The 18–14 Ma Kamlial Formation Himalayan foreland basin sedimentary rocks in the Chinji Village region, Potwar Plateau, Pakistan, are characterized by: (1) lithofacies indicative of deposition by a large river; (2) a dominant magmatic arc provenance completely unlike the ‘recycled orogen’ foreland basin deposits stratigraphically below, above, or coeval with these rocks; and (3) subordinate contribution from a rapidly exhuming source, interpreted as either the Nanga Parbat Haramosh Massif or the southern margin of the Asian crust. The start of Kamlial Formation deposition at this locality at 18 Ma marks a major break with the older Murree Formation rocks, which were deposited by rivers draining predominantly the Himalayan thrust stack south of the arc. We interpret this change as the result of diversion of the paleo-Indus River to its present position, which crosses the Kohistan arc and Himalayas and debouches into the foreland. If the rapidly exhuming subordinate source region were the Nanga Parbat Haramosh Massif, then initiation of its uplift would have resulted in significant arc detritus to the basin as the overlying arc carapace was exhumed. As the carapace was progressively breached, arc material would have become a less substantial component of detritus to the basin, consistent with the reported petrography of the overlying Siwalik deposits.

The Tertiary collision‐related thermal history of the NW Himalaya

AbstractGarnet‐whole rock Sm‐Nd data are presented for several samples from the Indian plate in the NW Himalaya. These dates, when combined with the P‐T evolution of the Indian plate rocks, allow a thorough reconstruction of the prograde thermal evolution of this region (including the Nanga Parbat Haramosh Massif) during the early Cenozoic. Combining these data with Rb‐Sr mineral separate ages, enables us to constrain the post‐peak cooling history of this region of the Himalaya.The data presented here indicate that the upper structural levels of the cover rocks of the Nanga Parbat Haramosh Massif, and similar rocks in the Kaghan Valley to the south‐west, were buried to pressures of c. 10 kbar and heated to temperatures of c. 650 °C at 46–41 Ma. The burial of the lower structural levels of the cover rocks of the Nanga Parbat Haramosh Massif, to similar depths but at higher temperatures of c. 700 °C, occurred slightly later at 40–36 Ma, synchronous with the imbrication and exhumation of the amphibolite‐ and eclogite‐grade rocks of the Kaghan Valley. In contrast, the cover rocks of the Nanga Parbat Haramosh Massif were not imbricated or exhumed at this time, remaining buried beneath the Kohistan‐Ladakh Island Arc until the syntaxis‐forming event that occurred in the last 10 Myr. The timing of tectonic events in the north‐western Himalaya differs from that experienced by the rocks of the Central Himalaya in that the earliest stage of burial in the NW Himalaya predates that of the Central Himalaya by c. 6 Myr. This difference may result from the diachronous nature of the Indo‐Asian collision or may simply be a reflection of differing timing at different structural levels.

Tracing the origins of the western Himalaya: an isotopic comparison of the Nanga Parbat massif and Zanskar Himalaya

Abstract New Sr and Nd isotope data for basement gneisses and leucogranites are presented from two contrasting areas of the western Himalaya; the Nanga Parbat-Haramosh massif (NPHM) and Zanskar. Sr-isotope systematics of metapelites and anatectic migmatites from the Zanskar Himalaya are characterized by ε Sr of 515–930, typical of the High Himalayan Crystalline unit as exposed for more than 2000 km along strike. Moreover, Zanskar leucogranites are typical of the belt of Early Miocene granites intruding the High Himalayan Crystallines across the orogen (mean ε Sr = 834). In contrast, the NPHM leucogranites show an elevated average ε Sr of 2400, and basement samples show a wide range in ε Sr from 1850 to 8150. Errorchrons for the metasedimentary gneisses indicate isotopic homogenization of the basement at c. 500 Ma for the Zanskar samples compared with c. 1800 Ma from the NPHM, confirming that the two terrains have experienced contrasting pre-Himalayan histories. Nd isotopic data from the NPHM indicate model ages from 2300 to 2800 Ma, indicating the mean crustal formation ages of the protoliths from which the sediments were derived. A compilation of published Nd data from the Himalaya indicates average protolith formation ages of 2640 ± 220 Ma for the Lesser Himalaya lithologies, compared with 1940 ± 270 Ma for the High Himalaya unit. Gneissic lithologies from Zanskar and the NPHM have previously been correlated with the High Himalayan Crystalline Series, since both display high-grade Himalayan metamorphism and are intruded by syn- to post-tectonic tourmaline-bearing leucogranites. Isotopic systematics in the Zanskar region confirm this correlation. In contrast, the NPHM basement rocks are better correlated with Lesser Himalayan lithologies, exposed south of the Main Central Thrust. We conclude that the NPHM represents either a lower structural level of the Lesser Himalaya Series, or its protolith.

Resistive (dry?) lower crust in an active orogen, Nanga Parbat, northern Pakistan

Conductivity models beneath the Nanga Parbat Haramosh Massif (NPHM) derived from magnetotelluric soundings reveal that there is no widespread, interconnected, conductive aqueous fluid to minimum depths of 40 km below sea level. Given the continuing tectonic denudation, young granitic and migmatitic bodies indicating partial melt at shallow crustal depths, and active seismicity, this result is surprising in light of similar studies in active tectonic regimes elsewhere. Away from the NPHM, models reveal the usual conductive lower crust. We propose that deep magmatic and metamorphic fluids are produced beneath NPHM in isolated zones but that the active deformation permits their escape through the brittle–ductile transition. A magnetotelluric survey in an area as complicated as Nanga Parbat required the development of methodologies for utilizing standard two-dimensional inversions in a three-dimensional environment. We show here how to identify which parts of the magnetotelluric responses are adequately represented with two-dimensional approximations. Unlike previous efforts, we do not attempt to create a set of generic rules that may be applicable to all geologic environments. Instead, a procedure is outlined that can be tailored to each interpretation. One important result of this work is that magnetotelluric data along a profile can be used to constrain structure off the ends of the profile.

The evolution of the Main Mantle Thrust in the Western Syntaxis, Northern Pakistan

Abstract Neogene events in the Nanga Parbat-Haramosh massif have obscured much of its earlier evolution. However, structural mapping of the eastern margin reveals a ductile contact zone preserving many features of the original Main Mantle Thrust that emplaced the Ladakh island arc over the Indian margin in the late Cretaceous. The sequence of ductile deformation was controlled both by the contrasting rheologies of the Ladakh island arc and the Main Mantle Thrust footwall, and the changing thermal regime during subduction, collision and burial. Preliminary P-T estimates indicate conditions during southward thrusting on the Main Mantle Thrust of c. 650 °C and 9.5 kbar, with later deformation (post-dating garnet growth) in some units at c. 500 °C and 7.4 kbar. The concordant fabrics and lithological boundaries on either side of the contact are only disrupted by a NW-vergent, brittle thrust south of the village of Subsar (Indus gorge) which cross-cuts the steepened Main Mantle Thrust Zone. This thrust is related to the neotectonic Liachar Thrust on the western margin of the massif, and is an expression of the regional tectonics at the western termination of the Himalayan arc. This late thrusting followed formation of the syntaxial antiform in Neogene times.

Tectonics of the SW margin of the Nanga Parbat-Haramosh massif

Abstract We present an analysis of the tectonic evolution of the southwestern portions of the Nanga Parbat massif, Pakistan Himalaya, based upon detailed mapping and structural analyses from the Bunar, Biji, Diamir, Airl, Niat and SW Rupal valleys. Mainly metasedimentary cover rocks of the Indian plate are divided into upper and lower cover. There is a marked structural thinning of the cover in the main Bunar valley from south to north, and this is attributed to a major frontal ramp in the original Main Mantle Thrust (MMT). A hitherto unmapped shear zone, the Diamir Shear Zone, is identified, that is associated with a syn-kinematically intruded belt of granitic rocks, the Jalhari Granite. The shear zone is a several kilometre thick, generally W-vergent, ductile to brittle shear zone that is associated with local overturning of the entire MMT section, typified by the Gashit Fold. 40 Ar/ 39 Ar cooling ages from across the area indicate a steep cooling age gradient across the Diamir Shear Zone from >40 to <5 Ma. The Diamir Shear Zone is mechanically linked to part of the Raikhot Fault System and, together, they are seen to be a crustal-scale reverse fault that has allowed relative uplift and overthrusting of the core of Nanga Parbat.

Mazeno Pass Pluton and Jutial Pluton, Pakistan Himalaya: age and implications for entrapment mechanisms of two granites in the Himalaya

Zircon and monazite U-(Th)-Pb ion microprobe analysis were performed on the Mazeno Pass pluton and the Jutial pluton, two leucogranite bodies within the Nanga Parbat-Haramosh massif (NPHM), Pakistan Himalaya. Zircon rim ages and monazite ages indicate the Mazeno Pass pluton in southwest NPHM intruded at 1.40 ± 0.05 Ma; the Jutial pluton, to the north, similarly yields concordant zircon and monazite ages suggesting crystallization at 9.45 ± 0.06 Ma. The Jutial pluton was subsequently intruded by leucogranite dikes at 5.3 Ma, as revealed by monazite ages. Concordancy of U-Pb and Th-Pb accessory mineral ages demonstrates the robustness of the technique on young rocks. Both plutons, some of the youngest in the Himalaya, have a general association with nearby shear zones that we interpret to have played an integral role in granite evolution and emplacement setting (`deformation enhanced ascent'). Together with new field observations, these results provide an insight on the spatial and temporal relationship between plutonism and deformation relating to the development of the massif.

Incision and differential bedrock uplift along the Indus River near Nanga Parbat, Pakistan Himalaya, from 10Be and 26Al exposure age dating of bedrock straths

Well preserved, river-cut bedrock surfaces (straths) abandoned by the progressive incision of the Indus River in northern Pakistan were dated using in situ cosmogenic 10Be and 26Al exposure age dating. Measurements on young and modern straths show no significant ingrowth of 10Be and 26Al, implying that strath exposure ages closely approximate their abandonment age. Strath exposure ages range up to 65 ka and increase with height above the river. The straths can be divided into two groups: low straths dated at about 7 ka, which yield very rapid bedrock incision rates (9–12 m/ka), and high straths >7 ka in age, which yield somewhat slower incision rates (1–6 m/ka). Incision rates for both groups increase by ∼1–3 m/ka downstream toward an active reverse fault, the Raikot fault, that forms the western border of the Nanga Parbat–Haramosh massif (NPHM). The downstream increases in incision rates for the low straths mimic downstream increases in the modern gradient, suggesting that bedrock incision rates have been proportional to stream power over the last 7 ka. Gradient increases are probably the result of the creation of knickpoints by bedrock uplift along the Raikot fault. The longitudinal profile and steep gorge of the Indus River in the study area indicate that bedrock incision appears to keep pace with differential vertical motions between the NPHM and surrounding regions. Thus, the 1–3 m/ka difference in incision rates likely reflects the differential bedrock uplift rate between the NPHM and the Skardu Basin. The differential incision rate is consistent with differential exhumation rates obtained from apatite fission track ages assuming geotherms of ≤80–90°C/km in the NPHM and 35°C/km in the Skardu Basin. This relation suggests that rapid bedrock incision of at least 1–3 m/ka has persisted within the NPHM for the last 0.5 Ma. The acceleration of incision rates throughout the study area since 15 ka is likely to be a climatic signal that records an increase in the discharge and/or sediment load of the Indus River in the study area related to deglaciation of the surrounding terrane.

Strain partitioning along the Himalayan arc and the Nanga Parbat antiform

Research Article| September 01, 1998 Strain partitioning along the Himalayan arc and the Nanga Parbat antiform Leonardo Seeber; Leonardo Seeber 1Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964 Search for other works by this author on: GSW Google Scholar Arnaud Pêcher Arnaud Pêcher 2UJF Laboratoire de Geologie, 38031 Grenoble, France Search for other works by this author on: GSW Google Scholar Geology (1998) 26 (9): 791–794. https://doi.org/10.1130/0091-7613(1998)026<0791:SPATHA>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Leonardo Seeber, Arnaud Pêcher; Strain partitioning along the Himalayan arc and the Nanga Parbat antiform. Geology 1998;; 26 (9): 791–794. doi: https://doi.org/10.1130/0091-7613(1998)026<0791:SPATHA>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Shortening along the Himalayan arc of continental convergence is approximately in the radial direction. If the underthrusting foot-wall block (India) is not deformed, the hanging-wall block (Tibet) needs to stretch along the arc, as suggested by radial grabens in southern Tibet. In contrast, the Nanga Parbat–Haramosh massif and the western Himalayan syntaxis are part of a 250-km-long antiform that strikes in the radial direction (northeast) and verges northwest. The Nanga Parbat antiform is the structural and topographic expression of arc-parallel shortening that compensates for arc-parallel extension in southern Tibet. This shortening is predicted to be as high as 12 mm/yr. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

HP metamorphic belt along the Indus suture zone of NW Himalaya: new discoveries and significance

Two new occurrences of HP rocks have been found in the Indus suture zone on the eastern side of the Nanga Parbat-Haramosh massif: a retrogressed eclogitic assemblage in pyroxenitic boudins within the Ladakh island-arc sequence, and an HP granulitic paragenesis in mafic alternations of High Himalaya banded gneisses. PT estimates on five samples with garnet-clinopyroxene-amphibole-plagioclase-quartz-FeTi oxide-biotite mineral assemblage have respectively yielded 13 ± 1 kbar for 610 ± 30 °C, and 10 ± 1 kbar for 700 ± 50 °C. This metamorphism is linked to the continuous subduction of continental and arc material after India-Eurasia collision. The granulitic retrogression corresponds to a phase of collage, with prograde granulites during exhumation.

A microstructural and argon laserprobe study of shear zone development at the western margin of the Nanga Parbat-Haramosh Massif, western Himalaya

A sample of banded amphibolite from the western margin of the Nanga Parbat–Haramosh Massif as Sassi has been studied using microstructural and 40Ar/39Ar laserprobe techniques to investigate the relationship between deformation and argon isotope variations in a natural system. Amphibolite-grade deformation occurred during south-directed overthrusting of the Kohistan arc over India along the Main Mantle Thrust and was overprinted by extensional reactivation of the earlier fabric and the formation of biotite-rich shear zones. Subsequent deformation along discrete fine-grained fault zones was characterised by the formation of scapolite, chlorite and K-feldspar, early plastic deformation and later cataclasis. Different minerals developed during this history show a wide range in apparent 40Ar/39Ar ages. Biotite, chlorite and scapolite exhibit much lower concentrations of excess argon, indicating their equilibration in a fluid relatively poor in excess argon. A `true' age of ca. 8 Ma from biotite represents a minimum age for deformation associated with formation of the Nanga Parbat Syntaxis and also precludes Pliocene metamorphism in this area of the syntaxis. Both high- and low-closure temperature minerals (amphiboles and feldspars) record apparent ages which are associated with the incorporation of excess argon within the mineral lattice. Although differential thermal resetting of minerals at different closure temperatures is important, variations in the inherited 40Ar/36Ar ratio throughout the sample is dominated by deformation and fluid infiltration. Consequently it appears that within deforming metamorphic rocks, areas with significantly different argon isotope compositions may be present and need not be homogenised by diffusion.