Time-scales of partial melting in the Himalayan middle crust: insight from the Leo Pargil dome, northwest India (original) (raw)
Related papers
Crustal generation of the Himalayan leucogranites
Tectonophysics, 1987
Le Fort, P., Cuney, M., Detailed studies of the Himalayan two-mica leucogranites, such as the Manaslu pluton, indicate that they have very uniform mineralogical, petrological and structural characteristics. One can relate their occurrence to the thickest zones of the underlying Tibetan Slab. In these zones, migmatization attains its greatest development and vertical extension. The granite is emplaced at first along the main disharmonic plane above the Main Central Thrust (MCI'), at the top of the Tibetan Slab (infrastructure). Ductile deformation of the granite is variable; the granite being syn-to late-kinematic with regard to the functioning of the MCT. Major elements are very homogeneous (except for Na and K) implying that P-T conditions of melting were relatively uniform. The melted material was of a similar composition over a vast volume, and the percentage of melting was small (lo-15%). Trace elements are highly variable. Some are characteristic for very evolved material (Ta, Rb, Cs, U) or show the link with the Tibetan Slab (Ba-Sr), whilst others are problematic (Th, REE). REE and Th abundances, being much less in the granite than in the Tibetan Slab, imply that they have been extracted during one of the main stages of formation, possibly by monazite. Radiogenic (Pb, Sr, Nd) and stable (0) isotopes are consistent with the origin of the granite from the Tibetan Slab. However, the heterogeneous Sr isotopic ratios make age dating difficult and imply poor mixing or little fluid interaction during its evolution. H-isotope data indicate that magmatic compositions of the main body of the Manaslu granite have been preserved. Late or post-magmatic alterations are extremely local in the main pluton.
Gondwana Research, 2012
Late Oligocene-Miocene leucogranites within southern Tibet form part of an extensive intrusive igneous province within the Himalayan orogen. The main rock types are tourmaline leucogranites (Tg) and two-mica leucogranites (2 mg). They have high SiO 2 (70.56-75.32 wt.%), Al 2 O 3 (13.55-15.67 wt.%) and ( 87 Sr/ 86 Sr) i (0.724001-0.797297), and low MgO (0.02-0.46 wt.%) and ( 143 Nd/ 144 Nd) i (0.511693-0.511906). Chondrite-normalized rare earth element (REE) patterns display strong negative Eu anomalies. Whole-rock major and trace element and Sr-Nd isotope data for the leucogranites suggest that their source region was a two-component mixture between a fluid derived from the Lesser Himalayan (LH) crustal sequence and the bulk crust of the Higher Himalayan (HH) sequence. Trace element and Sr-Nd isotope modeling indicate that the proportion of fluid derived from the LH sequence varied from 2% to 19% and the resulting metasomatised source experienced 7-16% melting. The amount of fluid derived from the LH sequence increases from north to south. Northward underthrusting of the Indian continent resulted in infiltration of the LH-derived fluid into the overlying HH sequence. Subsequent decompression melting of this metasomatised crust, mostly during the Miocene (25-9 Ma), generated the leucogranites. This may be linked to steepening of the subducted slab of Indian lithosphere beneath the orogenic belt.
Lithos, 2012
The timing of partial melting and the pressure-temperature (P-T) paths in the High Himalayan Crystalline Sequence (HHCS) in far-eastern Nepal has been investigated using zircon chronology, rare earth element (REE) compositions, and P-T pseudosection analysis. Zircon from migmatites formed during Himalayan thermal events displays inherited magmatic core overgrown by two generations of metamorphic rims. The new rims are distinguished on the basis of their Tertiary ages, low MREE contents, and low Th/U ratios. The inner zircon rims from Sil + Grt + Bt + Kfs + Pl + Qtz and Ky + Sil + Grt + Bt + Ms + Pl + Qtz migmatites at different structural level of the HHCS display ages of c. 33-28 Ma (Early Oligocene) and c. 21-18 Ma (Early Miocene): these rims are characterized by flat MREE to HREE patterns and were overgrown by partial melt through muscovite dehydration melting under the stability of garnet, which occurred at P = c. 7-10 kbar and T = c. 730-780°C, and at P = c. 8-14 kbar and T = c. 720-770°C, respectively. The outer zircon rims are relatively enriched in HREE with respect to the inner rims and were overgrown at c. 27-23 Ma (Late Oligocene) and at c. 18-16 Ma (Early Miocene) during melt crystallization accompanying breakdown of garnet at P = c. 4-7 kbar and T = c. 650-725°C. Early Miocene Ms-Bt leucogranites with two successively overgrown zircon rims at c. 18.3 ± 0.3 Ma and c. 16.3 ± 0.2 Ma were intruded into Early Oligocene migmatite hosts. Microstructural observations and the corresponding P-T conditions associated with the two generations of zircon rims indicate that the Early Oligocene and Early Miocene migmatites show relatively isobaric and nearly isothermal P-T paths during exhumation, respectively. The inferences are consistent with higher average cooling rates for the Early Miocene (c. 30-40°C/My) than the Early Oligocene (c. 15-25°C/My) migmatites, inferred from peak-T conditions and FT (c. 6 Ma for both migmatites) and U-Pb zircon ages. The P-T-t paths of the two migmatites indicate that burial of the Early Miocene migmatites has been coeval with exhumation of the Early Oligocene migmatites, implying the formation of large-scale thrust within the HHCS.
Journal of Petrology, 2006
In the Ranmal migmatite complex, non-anatectic foliated granite protoliths can be traced to polyphase migmatites. Structural^microtextural relations and thermobarometry indicate that syn-deformational segregation^crystallization of in situ stromatic and diatexite leucosomes occurred at 8008C and 8 kbar. The protolith, the neosome, and the mesosome comprise quartz, K-feldspar, plagioclase, hornblende, biotite, sphene, apatite, zircon, and ilmenite, but the modal mineralogy differs widely. The protolith composition is straddled by element abundances in the leucosome and the mesosome. The leucosomes are characterized by lower CaO, FeO þMgO, mg-number, TiO 2 , P 2 O 5 , Rb, Zr and total rare earth elements (REE), and higher SiO 2 , K 2 O, Ba and Sr than the protolith and the mesosome, whereas Na 2 O and Al 2 O 3 abundances are similar. The protolith and the mesosome have negative Eu anomalies, but protolith-normalized abundances of REE-depleted leucosomes show positive Eu anomalies. The congruent melting reaction for leucosome production is inferred to be 0Á325 quartz þ 0Á288 K-feldspar þ 0Á32 plagioclase þ 0Á05 biotite þ 0Á014 hornblende þ 0Á001 apatite þ 0Á001 zircon þ 0Á002 sphene ¼ melt. Based on the reaction, large ion lithophile element, REE and Zr abundances in model melts computed using dynamic melting approached the measured element abundances in leucosomes for 40Á5 mass fraction of unsegregated melts within the mesosome. Disequilibrium-accommodated dynamic melting and equilibrium crystallization of melts led to uniform plagioclase composition in migmatites and REE depletion in leucosome.
Journal of Asian Earth Sciences, 1999
The upper part of the High Himalayan slab in north central Nepal is comprised of a thick layer-parallel sheet of biotite + muscovite + tourmaline 2 garnet 2 sillimanite 2 cordierite leucogranite up to 3±4 km thick and dipping north at 5±208. These strongly peraluminous magmas were emplaced into high temperature±low-pressure sillimanite and cordierite bearing gneisses, calc-silicates and rare amphibolites which were metamorphosed at temperatures of 600±6508C some time during the Oligocene± early Miocene. Parallel stringers of black xenolithic gneisses within the leucogranites suggest passive magmatic intrusion along fractures parallel to the schistosity in the country rocks. In the mountains of Cho Oyu, Gyachung Kang, Pumori, Lingtren and the base of the Everest massif, these leucogranites form part of a single structural horizon bounded at the top by the Lhotse Detachment, the lower of two N-dipping normal faults of the South Tibetan Detachment (STD) system, and below by the Khumbu Thrust (KT), an out-of-sequence fault which was partly responsible for the uplift, erosion and exhumation of the leucogranites. A model for the emplacement of these leucogranites is proposed, where they represent viscous minimum melts, produced by melting of a pelitic protolith, similar to the underlying sillimanite grade gneisses, through muscovite breakdown, either during¯uid-absent melting at <7508C, or¯uid-saturated melting at <6508C. These leucogranites may have intruded up to H40 km horizontally from their source, but were emplaced by hydraulic fracturing along multiple sills into recently metamorphosed high temperature±low pressure rocks of the middle crust. The entire mid-crustal region where the granites were formed and emplaced was later uplifted along the hangingwall of the Khumbu Thrust, and by the structurally lower Main Central Thrust (MCT) to their present position. The location of the leucogranites at the top of the slab, but never intruding across the STD normal faults and the complete lack of leucogranites further down the slab rule out frictional heating along the MCT as a viable heat source and also rule out diapirism as a viable emplacement mechanism. High radioactivity of the crustal source, percolation of¯uid from the migmatitic source into sills and dykes during simple shear, heat focussing due to a large thermal conductivity contrast across the STD, and decompression during active low-angle normal faulting above, are all viable processes to explain leucogranite melting and emplacement.
2008
Effi cient extraction of granitic magma from crustal sources requires the development of an extensive permeable network of melt-bearing channels during deformation. We investigate rocks that have undergone deformation and melting within the Karakoram Shear Zone of Ladakh, NW India, in which leucosome distribution is inferred to record the permeable network for magma extraction. Delicate structures preserved in these rocks record the development of this permeable magma network and its subsequent destruction to form a mobile mass of melt and solids, resulting from the interplay between folding and magma migration. During folding, magma migrated from rock pores into layer-parallel and axial-planar sheets, forming a stromatic migmatite or metatexite with two communicating sets of sheets, intersecting parallel to the fold axis. Once the network was developed, folding and stretching was eased by magma migration and slip along axial planar magma sheets. Folding and magma migration led to layer disaggregation, transposition, and the formation of a diatexite where rock coherency and banding were destroyed. A number of structures developed during this process such as cuspate fold hinges, disharmonic folds, truncated layering, shear along axial planar leucosomes, and fl ow drag and disruption of melanosomes. In this system, magma migration was an integral part of deformation and assisted the folding and stretching of metatexites, while folding gave rise to a magma sheet network, now preserved as leucosomes, as well as the pressure gradients that drove magma migration and the breakup of the metatexite. Thus, metatexite folding increased melt interconnectivity, while magma mobility increased strain rate and released differential stresses.
Geological Journal, 2013
The Higher Himalayan Leucogranites (HHLG) intruded into the high grade rocks of the Higher Himalayan Crystallines (HHC) in Arunachal Himalaya of the Eastern Himalaya, yield distinctive field data, petrography, microstructures, geochemical and mineral chemistry data. The Arunachal HHLG are characterized by the presence of two micas; normative corundum; high contents of SiO 2 (67-78 wt.%), Al 2 O 3 (13-18 wt.%), A/CNK (0.98-1.44) and Rb (154-412 ppm); low contents of CaO (0.33-1.91 wt.%) and Sr (19-171 ppm), and a high ratio of FeO (tot) /MgO in biotite (2.54-4.82). These distinctive features, along with their strong depletion in high field strength elements (HFSE), suggest their affinity to peraluminous S-type granite generated by the partial melting of crustal material. Geothermobarometric estimations and mineral assemblages of the HHC metapelites confirm that the HHLG were probably generated in the middle crust (~20 km) and the produced melts intruded the HHC in the form of sills/dykes. Microstructurally, the HHLG shows high temperature deformation features including chessboard extinction in quartz and cuspate/lobate grain boundaries between quartz and feldspars (plagioclase and K-feldspar). The deformation microstructures suggest that the HHLG was deformed under early high temperature ductile deformation conditions. These fabrics were subsequently superimposed by later brittle deformation features associated with decreasing temperatures during the exhumation of the HHLG towards shallow structural levels at the time of Himalayan orogeny.
Lithos
Garnet-bearing migmatites and associated leucogranites and leucosomes of the Trivandrum Block in the Kerala Khondalite Belt were formed through granulite facies dehydration melting of metasedimentary protoliths. Significant trace element depletions in Cs, Zr, Nb and the compatible transition metal elements V, Cr, Ni, Cu and Zn relative to average post-Archaean shales and model middle and upper crust, recorded in all samples, require their sedimentary protoliths to have been impure sandstones and greywackes, rather than shales. Leucogranites (70-75wt %) and leucosomes (SiO 2 : 68-70wt %), which are uniformly peraluminous and classed as S-type on the basis of their A/NK and ASI relations, form a compositional array that shows strong correlated variations between TiO 2 and Fe 2 O 3 , and TiO 2 or Fe 2 O 3 with Co and Y. These reflect coupled variations in modal garnet and ilmenite and require the presence of up to 15-20 wt% of entrained peritectic garnet in the higher Y and HREE leucosomes. The leucosomes have REE patterns with normalised La/Sm of 10, negative Eu anomalies (Eu* < 0.8) and flat HREE at 6-20 times chrondrite, whereas leucogranites range to much lower HREE (1-5 times chondrite) with higher La/Sm and large positive Eu anomalies (Eu* = 1.4-3.4). Despite broadly similar major element compositions that lie within the granite minimum melt field in terms of felsic components, leucogranite Zr contents are highly variable, ranging down to 4-20 ppm in the lowest HREE and high Eu* cases, resulting in Zr saturation temperatures (544-647ºC) that are lower than any feasible melt. These geochemical features, coupled with covariations between Nb,-Ta and Y-Yb, collectively support petrological and field observations that the leucosomes are mixtures between former melt and entrained peritectic garnet and ilmenite. The leucogranites are the products of melt extraction and migration on metres to several metres lengthscales. Leucogranite Nb-Y, Ta-Yb, Eu* and Sr-Ba relationships demonstrate that their chemical
In the uppermost parts of the Higher Himalayan Crystallines (HHC) of the Great Himalaya, widespread in situ partial melting of sillimanite+K-feldspar gneiss resulted in the formation of migmatite and resultant melt accumulation near the South Tibetan Detachment System (STDS) during various deformation events along the Dhauli Ganga valley in Garhwal. The oldest migmatite phase, designated as the Me1, parallels the main foliation S m as the stromatite layers and concordant leucogranite bands. Younger melt phases Me2, Me3 and Me5 are recorded along small-scale ductile thrusts, extensional fabric and structureless patches, respectively. It is only the Me4 melting phase that is evidenced by largescale melt migration along cross-cutting irregular veins. These were possible conduits for migration and accumulation of melt into larger leucogranite bodies like the Malari granite (19.0± 0.5 Ma). mutual relations between the Himalayan migmatite and leucogranite have been thoroughly investigated for their petrogenesis and geochronology along the Himalaya (see . However, structural control on the Himalayan migmatite has not been well understood within the HHC along with the different stages of melt formation, and is the subject matter of this paper.