Tectonochemistry and p‐t Conditions of Ramgarh and Almora Gneisses from Askot Klippe, Kumaun Lesser Himalaya (original) (raw)
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Journal of Earth System Science, 2011
Uphalda gneisses (UG) is a crystalline klippe located near Srinagar in Garhwal Himalaya. These gneisses are compared with Debguru porphyroids (DP) (≈Ramgarh group) of Garhwal-Kumaun Himalaya and Baragaon mylonitic gneisses (BMG) of Himachal Himalaya. Petrographic study reveals that the deformation of UG was initiated at higher temperature (above 350 • C) and continued till lowering of temperature and deformation led to the mylonitization. Geochemically, these granitic gneisses (UG, DP and BMG) exhibit similar composition. Features such as high molecular A/CNK value (>1), presence of normative corundum and absence of normative diopside, enhanced Rb/Sr, Rb/Zr ratios, enrichment of Th and containing rounded zircons support their crustally-derived S-type granitic nature. The linear plot in major oxides is interpreted in terms of fractional crystallization processes. Mantle normalized multi-element spider diagram of UG illustrates depletion of Ba, Nb, Sr, P and Ti and enrichment of Th and show similarities with DP and BMG. Similarities were observed in lithology, petrographic characters and chemical composition of UG, DP, BMG and Ulleri augen gneisses (Nepal). Comparison with the rocks of Higher Himalayan crystallines (≈Vaikrita), suggests that these rocks (UG) are not transported from Higher Himalaya as understood earlier. This study however proposes that, these gneissic bodies represent an older basement occurring as a tectonic sliver which emplaced within the cover sequence as wedges at different structural levels. This is a regional phenomena observed throughout the Lesser Himalayan region.
Current Science, 2015
The present study reports the geochemical composition of Sewariya two-mica granites (SG) and Govindgarh granites (GG) intruding rocks of Delhi Supergroup along the western margin of South Delhi Fold Belt in Rajasthan, India. Both granite varieties are highly evolved possessing high SiO 2 , Al 2 O 3 , low MgO, CaO, Fe 2 O 3 , Ni, Cr and V and are calc-alkaline in nature. In chondrite normalized REE diagrams, SG are characterized by highly fractionated REE patterns (avg La N / Yb N = 21.45) and sharp negative Eu anomaly (Eu/ Eu* = 0.25), whereas GG do not show significant REE fractionated patterns (avg La N /Yb N = 3.31) and have variable Eu anomalies. From primitive mantle normalized multi-element diagrams, crustal signatures (low Nb, Ti and high Pb, U, Th) can be inferred for both the granitoid varieties. Also, strong peraluminous nature, high A/CNK, normative corundum and abundant mica content point towards a (meta)sedimentary protolith for them. An arc setting is indicated by their calc-alkaline nature; volcanic arc or syncollisional affinities in tectonic discriminant diagrams (Nb versus Y; Rb versus Nb + Y). They are peraluminous leucogranites derived from crustal melts with little mantle contribution as is evident from their mineralogy and geochemical characteristics. The anatectic conditions prevalent during the formation of these granites differed with SG being formed under dehydration conditions, while the formation of GG involved fluid-present melting reactions.
Journal of the Geological Society of India, 2009
The results of field, petrographic and geochemical work of the granitoids of Hutti-Gurgunta area in the northern part of Eastern Dharwar Craton (EDC) is presented in this paper. This crustal section comprises polyphase banded to foliated TTG gneisses, middle amphibolite facies Gurgunta schist belt and upper greenschist facies Hutti schist belt and abundant granite plutons. The focus of the present study is mainly on basement TTG gneisses and a granite pluton (~ 240 sq km areal extent), to discuss crustal accretion processes including changing petrogenetic mechanism and geodynamic setting. The TTGs contain quartz, plagioclase, lesser K-feldspar and hornblende with minor biotite while the granite contain quartz, plagioclase, K-feldspar and hornblende. Late stage alteration (chloritisation, sericitisation and epidotisation) is wide spread in the entire area. A huge synplutonic mafic body which is dioritic to meladioritic in composition injects the granite and displays all stages of progressive mixing and hybridization. The studied TTGs and granite show distinct major and trace element patterns. The TTGs are characterized by higher SiO 2 , high Al 2 O 3 , and Na 2 O, low TiO 2 , Mg#, CaO, K 2 O and LILE, and HFS elements compared to granite. TTGs define strong trondhjemite trend whilst granite shows calc-alkaline trend. However, both TTGs and granite show characteristics of Phanerozoic high-silica adakites. The granite also shows characteristics of transitional TTGs in its high LILE, and progressive increase in K 2 O with differentiation. Both TTGs and granite define linear to sub-linear trends on variation diagrams. The TTGs show moderate total REE contents with fractionated REE patterns (La/Yb N =17.73-61.73) and slight positive or without any significant Eu anomaly implying little amount of amphibole or plagioclase in residual liquid. On the other hand, the granite displays poor to moderate fractionation of REE patterns (La/Yb N = 9.06-67.21) without any significant Eu anomaly. The TTGs have been interpreted to be produced by low-K basaltic slab melting at shallow depth, whereas the granite pluton has been formed by slab melting at depth and these melts interacted with peridotite mantle wedge. Such changing petrogenetic mechanisms and geodynamic conditions explain increase in the contents of MgO, CaO, Ni and Cr from 2700 Ma to 2500 Ma granitoids in the EDC.
The porphyritic granitoids of Adoni area, located ~120 km west of Cuddapah basin, are intrusive into the Peninsular Gneissic Complex (PGC) of Dharwar craton. They contain outliers of Gulcheru sediments and are characterized by well-preserved primary igneous foliation. Petrographically, these granitoids are characterized by distinct porphyritic texture marked by the occurrence of prismatic microcline and lath shaped plagioclase phenocrysts within a groundmass of quartz, microcline, plagioclase, amphibole and biotite. Local development of myrmekitic texture is also noticed. The occurrence of perthites in the form of exsolution lamellae of albite within K-feldspar reflect a sub-solvous cooling of the parent magma during crystallization. Geochemical characteristics suggest that the studied granitoids have alkaline to peralkaline composition with prominent A-type signatures that collectively endorse a magmatic origin for their generation. Chondrite-normalized rare earth elements (REE) patterns exhibit enrichment in LREE and prominent negative Eu anomaly (Eu/Eu* = 0.13 to 0.27) indicating significant plagioclase fractionation from the parent magma. Trace and REE compositions combined with primitive mantle normalized trace element abundance marked by positive Rb and negative K, Nb-Ta, Zr-Hf anomalies suggest derivation of these granitoids by low pressure-high temperature partial melting of a tonalitic-granodioritic crust at mid to shallow crustal levels followed by fractional crystallization. The crustal melting has been induced by asthenospheric upwelling and minor basaltic underplating. Higher concentration of U (4-19 ppm) and Th (16-49 ppm) in these granitoids relative to the high field strength elements (HFSE) may reflect U and Th mineralization. Geochemical signatures corroborate a within plate tectonic realm for the origin and emplacement of these granitoids.
Journal of the Geological Society of India, 2010
Mafic dykes of Almora region intrude the Precambrian crystalline rocks of Kumaun Lesser Himalaya. Mafic dykes exhibit fine grained margin and medium to coarse grained core, melanocratic, low to highly ferromagnetic (MS=0.85-38.58×10-3 SI) in nature commonly showing subophitic to ophitic textures with ol-pl-cpx-hbl-bt-mt-ap-sp assemblage, and modally correspond to leucogabbro and olivinegabbro (sensu stricto). Olivine (Fo 61-Fo 33), clinopyroxene (Wo 46-En 42-Fs 22 to Wo 40-En 36-Fs 15) and plagioclase (An 58-An 12) have crystallized in the temperature range of ca1400-980 o C at pressure <2 kbar in an olivine tholeiitic basalt parent. Low acmite (Na pfu =0.033-0.025), (Mg # =0.64-0.82), Ti-Al contents of clinopyroxenes and their evolution along enstatite-ferrosilite join (i.e. MgºFe substitution) strongly suggest tholeiitic nature of mafic dyke melt with changing activities of alumina and silica. Clinopyroxene compositions of mafic dykes differ markedly as compared to those observed for adjoining Bhimtal volcanics but closely resemble to that crystallized in tholeiitic melts of Deccan province. Observed Cr vs Mg # variation, enriched LILE (Sr, Ba)-LREE and positive Eu-anomaly of the studied mafic dykes are indicative of fractional crystallization of olivine-clinopyroxene-plagioclase from a crustally-contaminated tholeiitic basalt magma derived from enriched mantle source. The mafic dykes of Almora are geochemically identical to mafic dykes of Nainital, but are unrelated to Precambrian mafic volcanic flow and dykes of NW Himalaya and dykes of Salma and Rajmahal regions.
Journal of Asian Earth Sciences
This study presents new data sets on the petrological, fluid inclusion and U-Pb zircon geochronological aspects of the garnet-bearing amphibolites from the eastern Karakorum (Pangong-Tso), Ladakh. to delineate the P-T-t-fluid evolution and its tectonic implications. The calculated phase equilibria in the model system NCKFMASHTO reveal that the peak metamorphism occurred at 8.5-9.5 kbar, 630-655°C, and retrograde metamorphism crossed a stage at 5.5-5.7 kbar, 620-630°C. Monophase primary and secondary carbonic and carbonic-aqueous fluid inclusions occur in garnet and quartz. Trapping of the primary inclusion occurred at pressures lower than the estimated peak P-T conditions (5.8-6.1 kbar, 629-638°C) indicative of partially re-equilibrated fluid inclusions during exhumation. The primary inclusions were original as carbonic-aqueous fluid derived from prograde decarbonation of the amphibolite's protolith or adjacent carbonate-rich rocks and H 2 O leached out during deformation, leaving a pure CO 2 fluid. U-Pb dating of zircons yielded two age groups: (i) at 128 to 131 Ma interpreted as the age of detrital zircons of sedimentary derivation, and constrain the maximum depositional age of the sedimentary protolith at 131 Ma and (ii) at 114 ± 0.38 Ma is interpreted as the age of peak metamorphism remarkably similar to the metamorphic age of the surrounding metapelites. This metamorphic stage corresponds to the well-known Cretaceous phase of crustal thickening and heating along the active continental margin of South Asia.
Mineralogy and Petrology, 2010
The Madurai Block, southern India, lies between the Palghat-Cauvery and the Achankovil shear zones. The Karur area represents a portion of the granulite-facies terrain of the Madurai block. Highpressure (HP) and ultrahigh-temperature (UHT) mafic granulites have been found as enclaves within the gneisses. The peak assemblage (M1) consists of garnet, orthopyroxene, clinopyroxene, quartz, and plagioclase. Garnet breaking down during isothermal decompression is indicated by the development of pyroxene+plagioclase symplectites, which characterize the M2 stage of metamorphism. Late stage hornblende-plagioclase symplectites rimming garnet is related to the decompressioncooling M3 stage of metamorphism. Peak metamorphism M1 occurs at~12 kbar pressure and temperatures in excess of 1,000°C. This was followed by a retrograde M2 stage when the mafic granulites suffered isothermal decompression to 6 kbar to 7 kbar at 800-900°C. At the terminal retrograde stage M3 solid-melt back reaction took place at 4.5-5.5 kbar and 650-700°C. The proposed clockwise P-T path implies that rocks from the study area could have resulted from thickened continental crust undergoing decompression. The SHRIMP data presented here from the Karur area provide evidence for a Neoproterozoic (521±8 Ma) metamorphic event in the Madurai block. The formation of symplectic assemblages during near isothermal decompression can be attributed to tectonic activity coinciding with the Pan-African phase of a global orogeny.
Journal of Asian Earth Sciences, 2012
Geochemical and Nd isotope data on granitoids of the NW part of the late Archaean eastern Dharwar craton are presented to elucidate their petrogenesis and role in crust formation. The granitoids are divided into three suites viz. trondhjemite-granodiorite gneisses, biotite monzogranites and porphyritic biotite granodiorites. The gneisses are pre-to syn-kinematic (with respect to deformation in the adjacent Hungund-Kushtagi schist belt), which show variable SiO 2 and Al 2 O 3 , enriched LREE and depleted HREE with slightly negative to no Eu anomalies. They display unusual chemistry in having higher FeO(T), K 2 O, Ba, Cr and Ni compared to the typical Archaean tonalite-trondhjemite-granodiorite (TTG). The biotite monzogranites are mostly syn-to late-kinematic and exhibit evolved calc-alkaline compositions with high SiO 2 , K 2 O, LILE and LREE, depleted to undepleted HREE and strongly negative to no Eu anomalies. The porphyritic granodiorites show syn-to late-kinematic calc-alkaline, sanukitoid-like character with a wide range of SiO 2 , higher TiO 2 , P 2 O 5 , Sr, Ba, Cr and Ni, and lower Rb. They, however, uniquely display higher K 2 O, RREE and Th than typical sanukitoids. The trondhjemite-granodiorite gneisses are interpreted as product of melting of a subducted basaltic slab followed by slight contamination from the overlying metasomatized mantle wedge. Subsequent melting of the extremely metasomatized mantle wedge resulted in formation of the parental magma of the porphyritic granodiorites. Intrusion of the latter triggered melting of the TTG crust accreted earlier and generated the evolved monzogranites. The monzogranites occurring east of the Hungund-Kushtagi schist belt show higher eNd but lower T DM ages than those occurring to the west, indicating that terranes with different histories were juxtaposed by lateral accretion.
Petrology of the basic granulites from Kodaikanal, South India
Gondwana Research, 1999
Basic granulites occurring as small enclaves and pods within charnockites contain predominantly orthopyroxene, clinopyroxene, hornblende, plagioclase feIdspar and quartz. Chemical composition of coexisting orthopyroxene, clinopyroxene, plagioclase and hornblende has been represented in ACF and AFM diagrams. The mineral assemblages and the textural relationships of the basic granulites have been described. Garnet is notably absent in the basic granulites and this is explained as due to lower (< 8 kbar) pressure and relatively magnesian bulk composition.
Precambrian Research, 2013
The Khasi mafic rocks of Shillong Plateau represent the Proterozoic mafic magmatism of Shillong Cratonic Block (SCB) in Northeastern India. The Khasi mafics are best exposed at Laitlynkot, East Khasi Hills, Shillong Plateau, where they occur as lensoidal bodies that are intrusive into the Shillong group of rocks (represented by sediments or low-grade metasediments). Relict high-grade basement rocks (∼Archean) and younger porphyritic granite intrusive also occur in this area. The investigated mafic magmatic rocks petrographically belong to hornblende-gabbro clan and bear imprints of metamorphism. Geochemical characters and variation trends of major and trace elements as well as selected trace element ratios suggest that the Khasi mafic rocks have a tholeeitic lineage of arc affinity controlled by fractional crystallization with a prominent iron enrichment trend. Incompatible trace element abundance patterns of the investigated rocks marked by LILE and LREE enrichments, relative depletion of HFSE, and pronounced negative Nb anomalies conform to a subduction-related arc signature and indicate the generation of the parent melt by slab dehydration and wedge melting processes. Chondrite-normalized REE profiles with flat HREE patterns indicate melting above the stability field of garnet. Low Nb/Ta (7.67-13.7) and high Zr/Hf (44-95.2) and Zr/Sm (8.92-55.9) ratios, with respect to primitive mantle values (Nb/Ta: 17, Zr/Hf: 36, Zr/Sm: 25) are consistent with parent magma derivation from a metasomatized, enriched mantle source. Critical trace element ratios corroborate an enriched mantle source (EMI) for the magma that produced the Khasi mafics and indicate a prominent role of crustal contamination. The isotopic fingerprint of an enriched mantle reservoir (EMI) has been documented in terms of high 87 Sr/ 86 Sr ratios ranging from 0.703769 to 0.711712, while 143 Nd/ 144 Nd ratio varies between 0.511536 and 0.511596. The Khasi mafic are characterized by high Al 2 O 3 , Rb and Th enrichment and Nb/Th < 8 which altogether reflect crustal assimilation and fraction crystallization (AFC) of the magma. Geochemical data suggest that partial melting of a subduction-modified, metasomatized, enriched lithospheric mantle wedge of spinel-peridotite composition produced the parental melt to the Khasi mafics, which evolved and attained a continental arc signature through assimilation of crustal materials. Metasomatism of mantle wedge by slab-dehydrated, LILE-rich fluids and incorporation of subduction-derived sediments account for the enrichment of the source. The role of the subducted sediments for mantle enrichment has also been attested by Ce/Ta vs. Ce relations and Th/Ce ratios. Tectonic affiliation and petrogenetic characters of these mafic rocks point towards their generation and emplacement in a continental margin arc setting involving subduction of oceanic plates beneath continental lithosphere, slab dehydration and mantle wedge melting associated with assimilation of crustal components and fractional crystallization (AFC) processes.