Ophiolitic Melange Zone Research Articles

Field and Petrographic Characteristics of Photang Thrust Sheet of Zanskar Tethys Himalaya, Ladakh, India

The Photang thrust sheet is a distinct complex of rocks that lies under the Spongtang ophiolite, showing almost complete ophiolite sequences. The pre-collision tectonic setting, emplacement age, mechanism of association of diverse litho-units within Spongtang klippe, and their regional correlation is still debatable and partially known. The controversy exists as to whether the ophiolite was obducted onto the northern passive margin of the Indian plate during the Eocene or the late Cretaceous. Moreover, the fundamental question emerges whether the Photang thrust sheet is a part of the independent Spong arc, has been tectonically derived from the Ophiolitic Melange Zone in the Dras island arc, or is a representative of the accreted seamounts of the Indus suture zone. This paper presents the application of fundamentals of field geology and petrography to understand the pre-collision tectonic setting of the Photang thrust sheet, Spongtang ophiolite, and associated rock sequences in the Photoksar region of the Zanskar Himalaya. Keywords: Photang thrust sheet, Spongtang ophiolite, Tectonic setting, Obduction, Ophiolitic Melange Zone, Indus suture zone, Himalaya

Geochemistry and tectonic setting of Middle Ordovician MORB-like basalts in the Kunlun Orogen: implications for a back-arc environment

The Central Kunlun ophiolitic melange zone, northern Tibetan Plateau, represents a remnant of the Early Paleozoic oceanic lithosphere. The Middle Ordovician Longshigeng basalts (LBs) are located in south of this melange zone and have important significance for reconstructing the opening and subduction processes of the Proto-Tethys Ocean. This study presents petrological, whole-rock geochemical, and zircon U-Pb data for these basalts. LA-ICP-MS U-Pb zircon data indicate that these basalts formed ca. 463 Ma. Geochemically, the basalts are characterized by low SiO2 (47.00–50.63 wt%) and K2O (0.22–1.05 wt%), intermediate Mg# (47–57) and high TiO2 (1.51–2.72 wt%), and are identified as tholeiitic basalts. Their chondrite-normalized REE patterns feature slight enrichments in LREEs (LREE/HREE = 2.11–3.08) with mild Eu anomalies (δEu = 0.89–1.06), and their patterns are similar to the reference line of enriched mid-ocean ridge basalt (EMORB). The primitive mantle-normalized trace element diagrams also show similarities with EMORB and are characterized by overall enrichments in large ion lithophile and high field strength elements, without notable Nb, Ta, and Ti depletions. Combining these data with the tectonic discrimination diagrams, we infer that these rocks formed in a back-arc environment in response to the northward subduction of the Proto-Tethys Ocean (South Kunlun Ocean) in the East Kunlun region, northern Tibetan Plateau.

Evaluation of hydrochemical data using multivariate statistical methods to elucidate heavy metal contamination in shallow aquifers of the Manipur valley in Indo-Myanmar Range

Descriptive statistics, factor analysis, correlation matrices, and cluster analysis are used to gain insights on hydrochemical processes and contamination in the shallow aquifers of Manipur valley. Groundwater has remained as a prime source of water supply for a population of nearly 3 million people living in this valley. Sixteen variables (pH, ORP, TDS, Ti, V, Cr, Cu, Ge, As, Rb, Sr, Nb, Mo, Hf, Ta, and W) are monitored from 28 shallow wells. Mean pH and TDS values (6.8 and 800 mg/l, respectively) suggest fresh quality water in terms of its acidity, alkalinity, or salinity. Oxidation reduction potential values (mean − 6.75 mV) indicate dissolution of metals in anoxic condition. The order of abundance of metals is Sr > As > Rb > Ti > Cu > V > Cr > Mo > Ge > W > Hf > Ta > Nb. Sr, As, Cr, Cu, and Mo are elevated than the WHO limit. Elevation of Sr is attributed to weathering of gypsum, evaporite, and rock salt which reflects in factor 5 of the factor analysis. Factors 2 represents Cr, As, and Mo elevations and signifies geogenic weathering from ultramafic rocks of Manipur Ophiolite Melange Zone. Factor 3 represents Rb, V, and Cu elevations owing to natural weathering of clay and Fe-oxyhydroxides along with dissociation of solid organic carbons. Factor 4 is related to a reduced environment under low pH condition. Factor 1 reflects dissolution of Nb, Hf, Ta, and Ti under anoxic environment as insoluble oxides. Analysis on Pearson correlation and hierarchical clustering strongly support observation made by factor analysis. Thus, the present study shows the accountability of multivariate statistical techniques in interpreting and delineating the sources of contaminations in shallow groundwater.

Structure and Petrology of the Nagaland-Manipur Hill Ophiolite Melange Zone, NE India: A Fossil Tethyan Subduction Channel at the India – Burma Plate Boundary

Structure and Petrology of the Nagaland-Manipur Hill Ophiolite Melange Zone, NE India: A Fossil Tethyan Subduction Channel at the India – Burma Plate Boundary

Structure and Petrology of the Nagaland-Manipur Hill Ophiolite Melange Zone, NE India: A Fossil Tethyan Subduction Channel at the India – Burma Plate Boundary

Structure and Petrology of the Nagaland-Manipur Hill Ophiolite Melange Zone, NE India: A Fossil Tethyan Subduction Channel at the India – Burma Plate Boundary

Structure and petrology of the Nagaland-Manipur Hill Ophiolitic Mélange zone, NE India: A Fossil Tethyan Subduction Channel at the India – Burma Plate Boundary

The Nagaland-Manipur Hill ophiolite belt in NE India represents the southern extension of the Neotethyan Yarlung-Zhangbo suture zone in Southern Tibet, and connects this on-land exposure of the late Mesozoic collision front in the north with a modern trench-arc system in the Andaman Sea region in the south. Ophiolitic subunits in the Nagaland-Manipur Hill area in the Indo-Myanmar Ranges occur as blocks or thrust sheets within a melange with a serpentinite or fine-grained greywacke matrix, and are spatially associated with eclogitic and blueschist rock assemblages. This ophiolitic melange zone is tectonically sandwiched between an older (Triassic - Cretaceous) accretionary prism complex (Nimi Flysch) to the east and a younger (Late Cretaceous -Miocene) accretionary wedge (Disang Flysch) to the west. The Nagaland-Manipur Hill ophiolitic melange is thus part of a progressively westward migrated subduction-accretion complex, and it represents a typical subduction channel melange evolved during the fast subduction of the Neotethyan oceanic lithosphere beneath Asia - Sundaland. The exhumation of this melange was facilitated by return flow within a subduction channel at the plate interface between the downgoing Indian continental margin, subducting at a shallow angle during the Eocene, and the Triassic - Early Cretaceous accretionary prism of the overlying Burma plate. The Upper Eocene - Miocene Pokhpur sedimentary strata uncomformably overlying the ophiolitic melange and the accretionary prism metasedimentary rocks constitute a wedge-top or slope-basin sequence, rather than a postcollisional molasse deposit. The occurrence of ophiolitic subunits in the Nagaland-Manipur Hill area represents an accretionary-type ophiolite, derived mainly from the downgoing plate, and does not characterize a Penrose-type, complete ophiolite sequence.

Rare earth elements and stable isotope geochemistry of carbonates from the mélange zone of Manipur ophiolitic Complex, Indo-Myanmar Orogenic Belt, Northeast India

Carbonates that occur in the melange zone of Manipur ophiolitic complex (MOC), Indo-Myanmar Orogenic Belt, Northeast India contain diverse fauna with dominance of foraminifera assemblages (planktonic and benthic) and calcareous nanofossils. Rare earth element (REE) contents (~51 ppm average value) in the MOC carbonates are high compared with the average value of typical marine carbonate (~28 ppm). The Post-Archean Australian Shale (PAAS)-normalized REE patterns of these carbonates exhibit seawater-like REE patterns with light rare earth elements (LREE) depletion and relative heavy rare earth elements (HREE) enrichment with negative Ce anomalies (Ce/Ce* = 0.28–0.69) and positive Eu anomalies (Eu/Eu* = 1.07–2.08). The carbonates show depleted values of δ13C ‰ (PDB) (0.89–2.74 ‰) which characterize marine precipitates while depletion in δ18O ‰ (PDB), reaching minimum values (−6.29 to −11.40 %). The negative anomalies of Ce in these carbonates suggest that they were deposited under oxygenated environment. The observed shale-normalized positive Eu anomalies and spread in negative δ18O values to a lesser extent of δ13C ‰ (PDB) values of these carbonates suggest their formation were altered by diagenesis in shallow marine environment. Field, petrographical studies in conjunction with REE and stable isotope characteristics suggest that these carbonates form part of the ophiolitic melange zone that emplaced during subduction and obduction processes of the Indian plate and Myanmar plate collision.

Palaeozoic tectonics and evolutionary history of the Qinling orogen: Evidence from geochemistry and geochronology of ophiolite and related volcanic rocks

The tectonic framework and the evolutionary history of the Qinling orogenic belt are keys for understanding the convergent processes between the North China and South China blocks. The widely exposed ophiolitic and subduction-related volcanic melange along the Shangdan and Erlangping belts provides important constraints on the tectonic evolutionary processes of the Qinling orogen. The melange in the Shangdan zone is predominantly composed of ultramafic and mafic rocks that can be divided into three geochemical groups: (1) N- MORB type; (2) E-MORB type; and (3) island-arc/active continental margin-related basalts. The samples with N- and E-MORB affinity are characterized by depletion or slight enrichment of LREE without fractionation of HFSE and no negative Nb–Ta anomaly. The island-arc/active continental margin-related basalts are typically depleted in Nb–Ta and Ti. It is inferred that the melange within the Shangdan suture represents remnants of an oceanic crust and associated volcanics. An age for the melange can be constrained by a U–Pb zircon age of 517.8±2.8Ma, obtained from the gabbro within the E-MORB-type ophiolite in the Yanwan area.To the north of the suture zone, the Erlangping melange consists similarly of ultramafic and mafic rocks, andesites and rhyolites. The mafic and andesitic rocks exhibit strong depletion of Nb–Ta and Ti indicating a subduction-related affinity. However, their depletion in Nb–Ta is weaker than that of the island-arc/active continental margin-related volcanic rocks. Taken together, the two ophiolitic melange zones indicate the existence of an early Palaeozoic Shangdan Ocean that was associated with a back-arc basin on the northern North Qinling Island arc terrane, separating the South China from the North China blocks.

New significant advances of regional geological survey in the blank regions of Qinghai-Xizang Plateau

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp.136-7

Petrogenesis of basalts for Sangxiu Formation in the central segment from Tethyan Himalayas: Plume-lithosphere interaction

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp. 125-7

Geochemistry of the ophiolite and island-Arc volcanic rocks in the Mianxian-Lueyang suture zone, southern Qinling and their tectonic significance

Ultrabasic rocks in the Mianxian-Lueyang ophiolitic melange zone include harzburgite and dunite which exhibit LREE depletion with remarkable positive Eu anomaly. The diabase dike swarm shows LREE enrichment but slightly negative Eu anomaly. Metamorphosed volcanic rocks can be divided into two groups in terms of their REE geochemistry and trace element ratios of Ti/V, Th/Ta, Th/Yb and Ta/Yb. One is the MORB-type basalt with LREE depletion, representing the fragments of oceanic crust and implying an association of the MORB-type ophiolite and an ancient ocean basin between theQinling and Yangtze plates during the Middle Paleozoic-Early Mesozoic era. The other comprises the island-arc volcanic rocks including tholeiitic basalt and a large amount of calc-alkaline intermediate-acid volcanic rock, which could not be the component of the ancient oceanic crust but the result of magmatism at the continental margin. This indicates that the Mianxian-Lueyang limited ocean basin had undergone a whole process of development, evolution and vanishing from Devonian-Cretaceous to Permian. And the Qinling area had become an independent lithospheric microplate, on the southern side of which there were exhibited the tectonic characteristics of active continental margins during the Late Paleozoic-Early Mesozoic. That is to say, the Qinling cannot be simply considered as a result of collision between the Yangtze and North China plates.

Geochemistry and tectonic significance of alkali granites along the Ulungur River, Xinjiang

In North Xinjiang there is an alkali granite belt extending in the NW-SE direction along the southern bank of the Ulungur River and running parallel to the suture zone, i.e., Aermantai-Zhaheba Ophiolitic Melange Zone, between the Junggar Plate and the Altay Orogenic Belt. Whole-rock Rb-Sr isochron ages of the Ulungur alkali granites are within the range of 292–309Ma, showing that they were genetically connected with the latest episode of Hercynian magmatism subsequent to the syncollision S-type and post-collision uplifting I-type granitoids in the Altay region. The alkali granites are mineralogically characterized by the occurrence of aegirine and arfvedsonite and chemically by high silicon and alkali, low calcium and magnesium and abundant high-field elements, being typical A-type granites. The alkali granites were formed in the final stage of the Hercynian calc-alkaline magmatic cycle in a very short period of time. They are in line with the post-orogenic A-type (PA-type) granites, implying that the tectonic regime was changed from compression to extension.

Tectonic setting and pre-orogenic sedimentation in the Indus-Tsangpo (Yarlung) suture zone of Ladakh Himalaya, India

There are three tectonically distinguishable zones along the Indus Tsangpo Suture of the Ladakh Himalaya: The Lamayuru Formation of Triassic-Palaeogene age representing accreted oceanic basin sediments on the passive margin of the Indian subcontinent; Ophiolitic Melange Zone of Jurassic_Late Cretaceous age; and the Nindam Formation of Late Cretaceous-Early Palaeogene age consisting of trench and trench slope sediments. The plate tectonic setting of the Lamayuru Formation and the Nindam Formation could be correlated with the modern analogue for the passive setting (trailing-edge) and the subduction (leading-age) related sedimentary deposits respectively. Pre- and syn-tectonism related to subduction of the Indian plate with the Eurasian plate, produced favourable sites for deposition of sediments. The rocks exposed help elucidate the relationship between the ongoing convergence and sediments being shed into the basins; they record the existence of rock masses removed by erosion from tectonic highlands.

Mesozoic neo‐tethyan pre‐orogenic deep marine sediments along the Indus–Yarlung Suture, Himalaya

Shelf, forereef and basin margin (slope) olistoliths (Exotic blocks of limestone) of Permian–Jurassic age are tectonically juxtaposed within the Triassic to Eocene age pre‐orogenic, deep abyssal plain turbidites of the Lamayuru. The pre‐collision tectonic setting and depositional environment of the limestone olistoliths can be reconstructed from within the neighbouring Zanskar range. The disorganized Ophiolitic Melange Zone, an association of different tectonic rock slivers of Jurassic–Eocene age, is tectonically underlain by the overthrusted Lamayuru Formation and tectonically overlain by the Nindam Formation. Tectonic slivers of Late Jurassic–Early Cretaceous age red radiolarian cherts represent a characteristic lithotectonic unit of the Ophiolitic Melange Zone, those occurring near the contact zone with the Lamayuru Formation, were deposited within the neo‐Tethyan deep‐ocean floor of the Indian passive margin below the carbonate compensation depth. These tectonic slivers accumulated along the northern margin of the Indus–Yarlung Suture Zone of the Ladakh Indian Himalaya during subduction accretion associated with the initial convergence of the Indian plate beneath the Eurasian plate.