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Stable C, O, and S Isotope Record of Magmatic-Hydrothermal Interactions Between the Falémé Fe Skarn and the Loulo Au Systems in Western Mali
Towards resolving the metamorphic enigma of the Indian Plate in the NW Himalaya of Pakistan
Abstract The Pakistan part of the Himalaya has major differences in tectonic evolution compared with the main Himalayan range to the east of the Nanga Parbat syntaxis. There is no equivalent of the Tethyan Himalaya sedimentary sequence south of the Indus–Tsangpo suture zone, no equivalent of the Main Central Thrust, and no Miocene metamorphism and leucogranite emplacement. The Kohistan Arc was thrust southward onto the leading edge of continental India. All rocks exposed to the south of the arc in the footwall of the Main Mantle Thrust preserve metamorphic histories. However, these do not all record Cenozoic metamorphism. Basement rocks record Paleo-Proterozoic metamorphism with no Cenozoic heating; Neo-Proterozoic through Cambrian sediments record Ordovician ages for peak kyanite and sillimanite grade metamorphism, although Ar–Ar data indicate a Cenozoic thermal imprint which did not reset the peak metamorphic assemblages. The only rocks that clearly record Cenozoic metamorphism are Upper Paleozoic through Mesozoic cover sediments. Thermobarometric data suggest burial of these rocks along a clockwise pressure–temperature path to pressure–temperature conditions of c. 10–11 kbar and c. 700°C. Resolving this enigma is challenging but implies downward heating into the Indian plate, coupled with later development of unconformity parallel shear zones that detach Upper Paleozoic–Cenozoic cover rocks from Neoproterozoic to Paleozoic basement rocks and also detach those rocks from the Paleoproterozoic basement.
U-Pb monazite ages from the Pakistan Himalaya record pre-Himalayan Ordovician orogeny and Permian continental breakup
Rare-earth mobility as a result of multiple phases of fluid activity in fenite around the Chilwa Island Carbonatite, Malawi
Mineralogical Magazine – a move to online-only publication
The Geology and Mineralogy of the Loulo Mining District, Mali, West Africa: Evidence for Two Distinct Styles of Orogenic Gold Mineralization
A Fluid Inclusion and Stable Isotope Study at the Loulo Mining District, Mali, West Africa: Implications for Multifluid Sources in the Generation of Orogenic Gold Deposits
Two-phase exhumation of ultra high-pressure and medium-pressure Indian Plate rocks from the Pakistan Himalaya
Abstract The Indian Plate rocks of NW Pakistan contain evidence for both Eocene and Miocene phases of post peak metamorphic exhumation. The Eocene phase shortly followed peak synchronous ultra high-pressure (UHP) and Barrovian metamorphism and was driven by the rapid return towards the surface of deeply buried, positively buoyant coesite-bearing UHP rocks, flanked by thrusts below and extensional shears above. Uplift of the UHP rocks contributed to crustal thickening and resulted in internal imbrication of the Barrovian metamorphic rocks onto which they were thrust. The Eocene and Miocene events were separated by a phase of large-amplitude and -wavelength folding. Upright folds related to this event have shallow WNW or ESE plunges. Quartz c -axis data suggest that the maximum stretching direction paralleled the fold axes. During the Miocene the Main Mantle Thrust was reactivated as a major top-side-north extensional fault zone. Cascading folds on its hanging wall and cascading folds and a variety of ductile to brittle top-side-north meso- and microstructures on its footwall document significant top-side-north movement. The driving force for Miocene extension is unlikely to be channel flow as suggested for the central Himalaya. Instead, rapid shortening of the overriding plate following Late Oligocene slab break-off could have destabilized the wedge and driven extension in its upper parts.
Yoshida, M., Windley, B. F. & Dasgupta, S. (eds) 2003. Proterozoic East Gondwana: Supercontinent Assembly and Breakup. : Geological Society Special Publication no. 206. x + 472 pp. London, Bath: Geological Society of London. Price £110.00, US $183.00; GSL member price £55.00, US $92.00; AAPG/SEPM/GSA member price £66.00, US $110.00; hard covers. ISBN 1 86239 125 4.
Tabular intrusion and folding of the late Archaean Murehwa granite, Zimbabwe, during regional shortening
Front Matter
Abstract Often described as a natural laboratory, the Himalaya are probably the ideal place in which to study ongoing continent-continent collision. This volume focuses on the geology of the northwestern part of the Himalaya which provides the most complete and best-exposed transect across the range. Here, in northern Pakistan and in Ladakh in northwest India, the full profile across the south Asian continental margin, and the north Indian margin is superbly exposed in mountains reaching as high as K2 (8611 m) and Nanga Parbat (8125 m). The south Asian geology is exemplified in the Karakoram and Hindu Kush ranges along the north and northwestern frontiers of Pakistan. The unique Kohistan-Dras island-arc terrane is sandwiched within the Tethyan suture zone between India and Asia. Rocks of the northern margin of the Indian Plate are exposed in both the Zanskar and the Pakistan Himalaya. The northern sedimentary carbonate platform of the Indian Plate, magnificently exposed in the mountains of Zanskar and Ladakh, is largely missing in Pakistan where the Kohistan arc has been obducted southward onto the metamorphosed rocks of the internal crystalline zones of the Indian Plate. The Nanga Parbat syntaxis represents an orogenic bend developed within a convergent zone in the thrust belt where the south-vergent thrusts of the central and eastern Himalaya swing around through 300 degrees.
The gravity field of the Karakoram Mountain Range and surrounding areas
Abstract A ‘blank on the map’ only 60 years ago, the Karakoram Range has been explored and surveyed with greater difficulty than the Himalaya and Tibet due to its rugged terrain and extensive glaciation. In the past ten years we have succeeded in doubling the number of gravity stations. A substantial improvement in coverage and overall quality was obtained by concentrating on previously unsurveyed areas and by validating older data with more accurate measurements. Our data were merged with earlier data, converted to full Bouguer anomalies and gridded. The resulting Bouguer anomaly map defines very precisely the gravimetric low associated with the Nanga Parbat-Haramosh syntaxis, and the huge negative anomalies between the Karakoram Fault and the Main Karakoram Thrust. Large negative values are now visible also in the Ghujerab-Khunjerab areas. Correlation of the topography and Bouguer anomaly shows that a plate of flexural rigidity with D = 2 × 10 24 Nm fits the coherence data in the Karakoram at all but two distinct frequency ranges centred at wavelengths of 80 and 300 km. In a rheologically layered lithosphere developing a buckling instability under horizontal compression, the observed spectral features of the topography and Bouguer gravity anomalies constrain the depth of the competent layers to be in the range 13–20 km and 50–75 km respectively.
Abstract Indian plate, granulite facies, migmatitic basement gneisses exposed within the Nanga Parbat syntaxis host at least two generations of mafic sheets. In the southern part of the syntaxis, concordant sheets yield Palaeo-Proterozoic model ages of 2.2–2.6 Ga, which probably date early stages of continental growth. In the northern part of the syntaxis the sheets include a suite of discordant, silica-saturated or oversaturated sub-alkaline basalts extracted from a slightly depleted sub-continental mantle. Nd model ages and an imprecise Sm-Nd isochron yield an age of emplacement at between 1.6 and 1.8 Ga. That these dykes cross-cut granulite facies migmatitic fabrics implies that peak metamorphism in the Indian plate gneisses was, at latest, Meso-Proterozoic and not Tertiary in age. Zircon and amphibole ages published elsewhere suggest that this metamorphism was probably c. 1850 Ma in age. That the basement gneisses were refractory by the Tertiary has implications for the derivation of leucogranite sheets during the Neogene. Although the gneisses experienced a Tertiary-aged metamorphism, it was to lower temperatures than the Meso-Proterozoic metamorphism. Unless the gneisses were rehydrated during the Tertiary, the leucogranites need to have been sourced from more fertile rocks underplating the granulite facies basement complex.
Structural evolution of the western margin of the Nanga Parbat massif, Pakistan Himalaya: insights from the Raikhot–Liachar area
Abstract There are several competing interpretations of the structure of the margins of the Nanga Parbat massif: that the massif is bounded by the original suture between the Indian continent and the Kohistan-Ladakh island arc―the Main Mantle Thrust; that the massif is entirely bounded by neotectonic faults; that it is bounded by a combination of early and late faults and shear zones. If the marginal structures of the massif are to be related to local and regional geo-tectonic evolution then their correct characterization is critical. The Raikhot Bridge area on the western margin of the massif is useful in this regard, as it provides accessible and near-continuous outcrops. This contact, sometimes called the Raikhot Fault, is composite. Sheared metagabbros of the Kohistan arc are juxtaposed tectonically against metasediments and orthogneisses of the Nanga Parbat massif along an early ductile shear contact, developed under amphibolite facies conditions. In this regard it may be a preserved segment of the Main Mantle Thrust. However, this ductile shear zone has been strongly modified, flattened and rotated, and is cut by younger shears and faults. The original kinematics of the shear zone have been largely overprinted by these subsequent deformations. The younger structures include NE-SW striking, dextral strike-slip faults and a major top-to-NW thrust and shear zone. A sequence of metamorphism, deformation and igneous emplacement may be used to study the history of structural evolution within the massif. The use of a single name (e.g. Raikhot Fault) for the present-day map contact between the Nanga Parbat massif and neighbouring Kohistan is misleading. The early contact (termed here the Phuparash Shear Zone, possibly the northeastern continuity of the Main Mantle Thrust) is modified by the Buldar Fault Zone (dextral strike-slip) and the Liachar Thrust Zone (top-to NW carriage of the Nanga Parbat massif across the Phuparash Shear Zone and onto Kohistan). The activity of the Buldar Fault and Liachar Thrust Zone continued during exhumation of the massif, through amphibolite facies to the Earth's surface. The interaction between these structures is at present unknown. However, establishing these and similar interactions within the Nanga Parbat area remain central to establishing the role of regional NE-SW dextral transpression in the modern structure of the 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.
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.
Abstract The Nanga Parbat massif lies in the core of the major north-south trending, broadly upright antiform that marks the NW syntaxis of the Himalayan arc. However, this antiformal structure is not evident in the trend of foliation and banding within the central and southern parts of the massif. Reconnaissance field studies in this region (Astor, Rama and Rupal areas) have delineated an important shear zone with top-to-the-south overthrust kinematics. This Rupal Shear Zone carries the migmatitic core of the massif onto non-migmatitic metasediments locally termed the Tarshing Group. The shear zone traces north into a broad high strain zone of steep foliation with gently plunging mineral elongation lineations with no consistent sense of shear. A tentative model is proposed whereby top-to-the-south overshear in the Rupal area passes northwards into a steep belt of apparently constrictional N–S elongation. This type of large-scale transpression may record the early growth of the syntaxis. However, relating these structures to Himalayan orogenesis and the amplification of the NW syntaxis is problematic. The Nanga Parbat massif displays a long and complex history of polyphase deformation, metamorphism and magmatism, as might be expected of a terrane derived from the basement of the Indian sub-continent. Although at least the later part of the constrictional steep belt developed with syn-kinematic leucogranite intrusions (< 10 Ma), the old age limit on the Rupal Shear Zone remains unconstrained.
Geochronological constraints on the evolution of the Nanga Parbat syntaxis, Pakistan Himalaya
Abstract New amphibole, muscovite and biotite Ar-Ar and K-Ar data and zircon and apatite fission track data are presented from the western margin of the Nanga Parbat syntaxis as well as from the Indus and Astor valley sections which cross the syntaxis. Amphibole data date a regional cooling through 500 °C at 25 ± 5 Ma and are inconsistent with earlier suggestions that the peak of regional metamorphism was Neogene in age, although there is no doubt that some rocks were still at upper amphibolite facies temperatures as recently as 5 Ma. The data can be used to constrain structural models for syntaxial uplift. After an initial phase of crustal-scale buckling, bodily uplift of the syntaxis was along subvertical shear zones developed along its margins, although with a significantly higher time-averaged strain rate for shears developed along the western margin than along the eastern margin. The latter may be antithetic to the former. These shears were operative from 10 to < 1 Ma. In the southwestern part of the syntaxis, this subvertical uplift was superseded, since 6 Ma, by uplift along moderately SE-dipping NW-vergent shears on the hanging wall of which are located Neogene-aged migmatites.