Decompression and anatexis of Himalayan metapelites
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The Miocene leucogranites of the High Himalayas have been emplaced within a metasedimentary wedge, defined by the Main Central Thrust (MCT) and the South Tibetan Detachment System (STDS), a low‐angle detachment fault. Isotopic and tectonic constraints indicate that orogenic collapse along the South Tibetan Detachment System occurred at ∼20 Ma, synchronous with anatexis and emplacement of the leucogranites, thus suggesting that exhumation and anatexis were related. The isotope geochemistry of the Himalayan leucogranites indicates that their source lies within the metapelites of the metasedimentary wedge. Pelitic assemblages exhibit an inverted metamorphic geotherm consistent with wedge corner flow which stacked sillimanite‐grade thrust sheets onto kyanite‐grade rocks. However, the leucogranite protolith is not the sillimanite migmatites into which the melts have been emplaced but may be correlated with kyanite schists from near the base of the wedge. In the Langtang section of northern Nepal, leucogranite melts, formed from vapor absent incongruent melting of muscovite, were extracted from their source and migrated over distances >10 km before emplacement close to the STDS. A consequence of fractional melting in the hanging wall of the MCT is instability of the metasedimentary wedge. Decompression melting from depths >40 km can generate a melt fraction of ∼7%, depending on the initial temperature and muscovite content of the protolith. Enhancement of the available melt fraction during exhumation may have been critical in allowing the melt to migrate from its source under an extensional régime.Keywords:
Leucogranite
Anatexis
Sillimanite
Protolith
Migmatite
Main Central Thrust
Metamorphic core complex
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Andalusite
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In the Thor-Odin and Valhalla metamorphic core complexes, we have documented a remarkable uniformity of mineral δ18O values in the middle continental crust beneath the detachment faults. For example, in the Thor-Odin Complex, throughout an 8 km thick section of metasedimentary rocks and early Tertiary leucogranites in the hanging wall of the Monashee decollement (MD), quartz δ18O = 12.3 ± 0.5% (lσ S.D.) for metapelite (N = 11), 12.0 ± 0.1% for quartzite (N = 2), 12.6 ± 0.6% (N = 4) for < 1 m thick amphibolite layers, and 12.1 ± 0.4% (N = 24) for the concordant leucogranites. No exceptions have been found to this remarkable 18O/16O homogeneity except locally in a couple of thick amphibolites and within a ductile, relatively impermeable, marble-rich section. Similar zones of 18O/16O homogeneity associated with leucogranite genesis are observed throughout the mid-crustal section of the Valhalla Complex and just beneath the MD in the Monashee Complex, the only difference being that those rocks are overall 0.5 to 1.5% lower in δ18O than in the middle crust at Thor-Odin. These zones of pervasive homogenization in 18O/l6O must be a result of exchange with magmatic or metamorphic H2O, and these same volatiles appear to have been responsible for the leucogranite anatexis. A wide range in quartz δ18O from +8 to +16 within and below the MD suggests that this major thrust fault was impermeable to aqueous fluid flow during the partial melting stage; at that time, the basement appears to have been isolated from the mid-crustal metamorphichydrothermal system. LITHOPROBE crustal seismic profiles establish the MD as a W-dipping, crustal-scale ramp with 20 km of vertical relief, and Carr (1992) proposed an anatectic origin for the leucogranites during decompression melting associated with tectonic shortening as the mid-crustal section moved up this thrust ramp. Partial melting of metapelites and feldspathic grits from the Late Precambrian Windermere Supergroup began in response to influx of metamorphic H2O, aided by internal muscovite dehydration at ≍8 kbar and ≍750°C at the base of the Monashee ramp. Metapelites are volatile rich, but feldspar poor, whereas the opposite is true for the grit lithologies. Thus, at the base of the Monashee ramp large-scale (≍30°) partial melting of the metapelites produced magmas near H2O saturation (10 tol4 wt°), whereas the intercalated arkosic grit-derived magmas were undersaturated (5 to 6 wt°). As these H2O-rich, pelite-derived leucogranite melts moved upward to shallower depths, they cooled adiabatically and underwent decompressive exsolution of H2O. The released H2O was then able to exchange oxygen with lithologies infertile to melting as it concurrently migrated through the section toward the feldspathic grit layers, where it could act as a catalyst and be re-used, promoting further hydrothermal melting of the arkosic grits. Continued decompression melting and exsolution occurred simultaneously in different parts of the section during uplift, tectonic shortening, and buoyant uprise of the magma bodies, until final crystallization of all of the leucogranites took place much higher in the crust, where almost all of the H2O was released and again re-used for a final episode of 18O/l6O exchange with the unmelted metamorphic lithologies. In addition to the direct l8O/16O exchange that takes place between the metamorphic rocks and the migrating leucogranite magmas, this use and re-use of the same H2O during repeated episodes of partial melting and exsolution in different parts of the section seems adequate to explain the pervasive oxygen isotopic homogenization of these metasedimentary rocks. It is estimated that 25 to 30° partial melting of a typical section of the Windermere Supergroup occurred as a result of these cumulative processes, and this probably played a pivotal role in determining the susceptibility of this orogen to subsequent extensional collapse along the detachment faults.
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ABSTRACT Crustal thickening along the northern margin of the Indian plate, following the 50 Ma collision along the Indus Suture Zone in Ladakh, caused widespread high‐temperature, medium‐pressure Barrovian facies series metamorphism and anatexis. In the Zanskar Himalaya metamorphic isograds are inverted and structurally telescoped along the Main Central Thrust (MCT) Zone at the base of the High Himalayan slab. Along the Zanskar valley at the top of the slab, isograds are the right way‐up and are also telescoped along northeast‐dipping normal faults of the Zanskar Shear Zone (ZSZ), which are related to culmination collapse behind the Miocene Himalayan thrust front. Between the MCT and the ZSZ a metamorphic‐anatectic core within sillimanite grade rocks contains abundant leucogranite‐granite crustal melts of probable Himalayan age. A thermal model based on a crustal‐scale cross‐section across the Zanskar Himalaya suggests that M 1 isograds, developed during early Himalayan Barrovian metamorphism, were overprinted during high‐grade MCT‐related anatexis and folded around a large‐scale recumbent fold developed in the hanging wall of the MCT.
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Abstract Widespread anatexis was a regional response to the evolution of the Himalayan‐Tibetan Orogen that occurred some 30 Ma after collision between Asia and India. This paper reviews the nature, timing, duration and conditions of anatexis and leucogranite formation in the Greater Himalayan Sequence ( GHS ), and compares them to contemporaneous granites in the Karakoram mountains. Himalayan leucogranites and associated migmatites generally share a number of features along the length of the mountain front, such as similar timing and duration of magmatism, common source rocks and clockwise P–T paths. Despite commonalities, most papers emphasize deviations from this general pattern, indicating a fine‐tuned local response to the dominant evolution. There are significant differences in P–T– conditions during anatexis, and timing in relation to regional decompression. Further to that, some regions underwent a second event recording melting at low pressures. Zircon and monazite ages of anatectic rocks range between c . 25 and 15 Ma, suggesting prolonged crustal melting. Typically, a single sample may have ages covering most of this 10 Ma period, suggesting recycling of accessory phases from metamorphic rocks and early‐formed magmas. Recent studies linking monazite and zircon ages with their composition, have determined the timing of prograde melting and retrograde melt crystallization, thus constraining the duration of the anatectic cycle. In some areas, this cycle becomes younger down section, towards the leading front of the Himalayas, whereas the opposite is true in other areas . The relationship between granites and movement on the South Tibetan Detachment ( STD ) reveals that fault motion took place at different times and over different durations requiring complex internal strain distribution along the Himalayas. The nature and fate of magmas in the GHS contrast with those in the Karakoram mountains. GHS leucogranites have a strong crustal isotopic signature and migration is controlled by low‐angle foliation, leading to diffuse injection complexes concentrated below the STD . In contrast, the steep attitude of the Karakoram shear zone focused magma transfer, feeding the large Karakoram‐Baltoro batholith. Anatexis in the Karakoram involved a Cretaceous calcalkaline batholith that provided leucogranites with more juvenile isotopic signatures. The impact of melting on the evolution of the Himalayas has been widely debated. Melting has been used to explain subsequent decompression, or conversely, decompression has been used to explain melting. Weakening due to melting has also been used to support channel flow models for extrusion of the GHS , or alternatively, to suggest it triggered a change in its critical taper. In view of the variable nature of anatexis and of motion on the STD , it is likely that anatexis had only a second‐order effect in modulating strain distribution, with little effect on the general history of deformation. Thus, despite all kinds of local differences, strain distribution over time was such that it maintained the well‐defined arc that characterizes this orogen. This was likely the result of a self‐organized forward motion of the arc, controlled by the imposed convergence history and energy conservation, balancing accumulation of potential energy and dissipation, independent of the presence or absence of melt.
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The Miocene leucogranites of the High Himalayas have been emplaced within a metasedimentary wedge, defined by the Main Central Thrust (MCT) and the South Tibetan Detachment System (STDS), a low‐angle detachment fault. Isotopic and tectonic constraints indicate that orogenic collapse along the South Tibetan Detachment System occurred at ∼20 Ma, synchronous with anatexis and emplacement of the leucogranites, thus suggesting that exhumation and anatexis were related. The isotope geochemistry of the Himalayan leucogranites indicates that their source lies within the metapelites of the metasedimentary wedge. Pelitic assemblages exhibit an inverted metamorphic geotherm consistent with wedge corner flow which stacked sillimanite‐grade thrust sheets onto kyanite‐grade rocks. However, the leucogranite protolith is not the sillimanite migmatites into which the melts have been emplaced but may be correlated with kyanite schists from near the base of the wedge. In the Langtang section of northern Nepal, leucogranite melts, formed from vapor absent incongruent melting of muscovite, were extracted from their source and migrated over distances >10 km before emplacement close to the STDS. A consequence of fractional melting in the hanging wall of the MCT is instability of the metasedimentary wedge. Decompression melting from depths >40 km can generate a melt fraction of ∼7%, depending on the initial temperature and muscovite content of the protolith. Enhancement of the available melt fraction during exhumation may have been critical in allowing the melt to migrate from its source under an extensional régime.
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Abstract A specific question about the H imalayas is whether the orogeny grew by distributed extrusion or discrete thrusting. To place firm constraints on tectonic models for the orogeny, kinematic, thermobarometric and geochronological investigations have been undertaken across the Greater H imalayan Crystalline Complex ( GHC ) in the Nyalam region, south‐central T ibet. The GHC in this section is divided into the lower, upper and uppermost GHC , with kinematically top‐to‐the‐south, alternating with top‐to‐the‐north shear senses. A new thrust named the Nyalam thrust is recognized between the lower and upper GHC , with a 3 kbar pressure reversion, top‐to‐the‐south thrust sense, and was active after the exhumation of the GHC . Peak temperature reached ∼749 °C in the cordierite zone, and decreased southwards to 633–667 °C in the kyanite and sillimanite‐muscovite zones, and northwards to greenschist facies at the top of the South T ibetan Detachment System ( STDS ). Pressure at peak temperature reached a maximum value in the kyanite zone of 9.0–12.6 kbar and decreased northwards to ∼4.1 kbar in the cordierite zone. Zircon U‐Pb ages of a sillimanite migmatite and an undeformed leucogranite dyke cutting the mylonitized rocks in the STDS reveal a long‐lived partial melting of the GHC , which initiated at 39.7–34 Ma and ceased at 14.1 Ma. Synthesizing the obtained and collected results, a revised channel flow model is proposed by considering the effect of heat advection and convection by melt and magma migration. Our new model suggests that distributed processes like channel flow dominated during the growth of the H imalayan orogen, while discrete thrusting occurred in a later period as a secondary process.
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Tectonothermal evolution of the High Himalayan Crystalline Sequence, Langtang Valley, northern Nepal
The High Himalayan Crystalline Sequence in north‐central Nepal is a 15‐km‐thick pile of metasediments that is bound by the Main Central Thrust to the south and a normal fault to the north. The Langtang section through the metasediments shows an apparent inversion of metamorphic isograds with high‐ P , kyanite‐grade rocks exposed beneath low‐ P , sillimanite‐grade rocks. Textural evidence confirms that the observed inversion is a result of a polyphase metamorphic history and phase equilibria studies indicate that thermal decoupling has occurred within a mechanically coherent section of crust. Rocks now exposed at the base of the High Himalayan thrust sheet underwent Barrovian regional metamorphism (M1) prior to 34 Ma in the early stages of the Himalayan orogeny, recording metamorphic conditions of T = 710 ± 30° C, P = 9 ± 1 kbar. After the activation of the Main Central Thrust, which emplaced these metapelites southwards onto the lower grade Lesser Himalayan formations, the upper part of the thrust sheet was overprinted by a second heating event (M2), resulting in sillimanite‐grade metamorphism and anatexis of metapelites at T = 760 ± 30° C, P = 5.8 ± 0.4 kbar between 17 and 20 Ma. Crustally derived, leucogranite magmas have been emplaced into low‐grade Tethyan sediments on the hangingwall of the normal fault that bounds the northern limit of the metapelitic sequence. The cause of the selective heating of the upper section of the metasediments during M2 cannot be reconciled with either post‐thrusting thermal relaxation or advection models. The cause of M2 remains problematical but it is suggested that heat focusing has occurred at the top of the High Himalayan Crystalline Sequence as a result of movement on the normal fault blanketing metapelites of high heat productivity with low‐grade sediments of low thermal conductivity. This model implies that the normal fault was active before M2, consistent with decompression textures that formed during, or shortly after, sillimanite‐grade metamorphism.
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The Paiku composite leucogranitic pluton in the Malashan gneiss dome within the Tethyan Himalaya consists of tourmaline leucogranite,two-mica granite and garnet-bearing leucogranite.Zircon U-Pb dating yields that(1)tourmaline leucogranite formed at28.2±0.5 Ma and its source rock experienced simultaneous metamorphism and anatexis at 33.6±0.6 Ma;(2)two-mica granite formed at 19.8±0.5 Ma;(3)both types of leucogranite contain inherited zircon grains with an age peak at~480 Ma.These leucogranites show distinct geochemistry in major and trace elements as well as in Sr-Nd-Hf isotope compositions.As compared to the two-mica granites,the tourmaline ones have higher initial Sr and zircon Hf isotope compositions,indicating that they were derived from different source rocks combined with different melting reactions.Combined with available literature data,it is suggested that anatexis at~35 Ma along the Himalayan orogenic belt might have triggered the initial movement of the Southern Tibetan Detachment System(STDS),and led to the tectonic transition from compressive shortening to extension.Such a tectonic transition could be a dominant factor that initiates large scale decompressional melting of fertile high-grade metapelites along the Himalayan orogenic belt.Crustal anatexis at~28 Ma and~20 Ma represent large-scale melting reactions associated with the movement of the STDS.
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