Eastern Tethyan Himalaya Research Articles

Early Cretaceous (~ 140 Ma) aluminous A-type granites in the Tethyan Himalaya, Tibet: Products of crust-mantle interaction during lithospheric extension

A-type granites have been the focus of considerable research due to their distinctive major- and trace-element signatures and tectonic significance. However, their petrogenesis, magmatic source and tectonic setting remain controversial, particularly for aluminous A-type granites. The earliest Cretaceous (ca. 140Ma) Comei granite in the eastern Tethyan Himalaya is associated with coeval oceanic island basalt (OIB)-type mafic lava, and has A-type granite geochemical characteristics including high 10,000×Ga/Al (up to 6), FeOtotal/MgO (4.6–6.1) and (Na2O+K2O)/Al2O3 (0.50–0.61) ratios but low CaO (0.6–1.6wt%) and Na2O (1.8–2.6wt%) contents. The Comei granite also has variable peraluminous compositions (A/CNK=1.00–1.36) along with zircon δ18O, εNd(t) and initial 87Sr/86Sr values of 8.2‰ to 9.3‰, −13.0 to −12.4 and 0.7238 to 0.7295, respectively. This range of compositions can be interpreted as the interaction between high-temperature upwelling OIB type basaltic magmas and a shallow crustal (<5kbar) metapelitic source. The Comei granite and coeval OIB type basaltic rock could represent the earliest stage (145–140Ma) of a large igneous event in eastern Tethyan Himalaya, which may well have been triggered by pre-breakup lithospheric extension prior to the arrival of the Kerguelen plume head.

Petrogenesis and tectonic implications of ∼130 Ma diabase dikes in the western Tethyan Himalaya (western Tibet)

Early Cretaceous mafic dikes are widely exposed in the eastern Tethyan Himalaya. However, coeval mafic rocks are limited in the western Tethyan Himalaya. We report the major and trace elements and zircon U–Pb ages of the newly discovered diabase dikes in the Zhongba area in the western Tethyan Himalaya. The ICP-MS zircon U–Pb analyses from one representative diabase dike yields a crystallization age of 129.8±1.3Ma (MSWD=6.7). These mafic dikes are characterized by high concentrations of TiO2 (2.81–3.27%), Fe2O3T (10.79–12.14%), and MgO (5.11–6.71%). The chondrite-normalized diagram indicates the enrichment of LREE and the depletion of HREE. The primitive mantle-normalized trace element patterns do not display negative anomalies of Nb, Ta, and Ti. These characteristics imply an OIB affinity. Trace element modelling (Sm/Yb vs. Sm) suggests that the dikes originated from an enriched mantle source region, composed primarily of garnet and spinel (spinel<garnet), with a low degree of partial melting (4–8%). Comparing with coeval OIB-affinity mafic rocks from the areas of Gyangze, Kangma, Cona, and Laguila, we propose that the Cretaceous mafic rocks were generated in an extensional setting along a passive continental margin and were derived from similar magma sources with some crustal contamination in the case of the Zhongba intrusives. Because of the similar geochemical characteristics, ages and tectonic settings, of the Zhongba diabase rocks and other volcanic rocks from the Kerguelen LIP, we suggest that the Zhongba diabase rocks are likely also part of the Kerguelen LIP.

Multiple Phases of Mafic Magmatism in Gyangze‐Kangma Area: Implications for the Tectonic Evolution of Eastern Tethyan Himalaya

Multiple Phases of Mafic Magmatism in Gyangze‐Kangma Area: Implications for the Tectonic Evolution of Eastern Tethyan Himalaya

Petrogenesis of the Early Cretaceous Laguila bimodal intrusive rocks from the Tethyan Himalaya: Implications for the break-up of Eastern Gondwana

The Kerguelen mantle plume triggered the rift of Eastern Gondwana to open the eastern Indian Ocean, with the formation of ~132Ma Comei–Bunbury large igneous province (LIP). The Comei area is located in the eastern Tethyan Himalaya, paleogeographically belonging to Greater India. The Laguila bimodal intrusive rocks from the Comei area consist of mafic (gabbro-diabase) and felsic rocks (quartz monzonite-granodiorite). This paper presents detailed LA-ICP-MS zircon U-Pb chronology, major and trace elements, and Sr-Nd-Pb isotope geochemistry of the Laguila bimodal intrusive rocks, in order to constrain the early activity of the Kerguelen mantle plume. LA-ICP-MS zircon U-Pb dating shows that the Laguila intrusive rocks were emplaced in the Early Cretaceous (~134–130Ma). The Laguila mafic rocks are enriched in LREE, LILE and HFSE, similar to those of oceanic island basalts (OIB). Their 87Sr/86Sri (0.7054 to 0.7066), 143Νd/144Nd (T) (0.512548 to 0.512619) and (206Pb/204Pb)t ratios (18.492 to 18.859) are comparable with those basalts derived by the Kerguelen hot spot. Elemental and isotopic data suggest that they were likely derived by partial melting of the Kerguelen plume source in the spinel–garnet transition zone (~60–80km). The Laguila felsic rocks share most of the geochemical features of A-type granite and show different 87Sr/86Sri (0.7171 to 0.7204), 143Νd/144Nd (T) (0.511874 to 0.511956) and (206Pb/204Pb)t ratios (19.087 to 19.274) from those of the mafic rocks. They were likely derived by partial melting of crustal rocks at a shallow depth (<30km) triggered by underplating of the coeval basaltic magmas. The Laguila intrusive rocks were emplaced in a rift setting during the breakup of eastern Gondwana, associated with the Kerguelen plume activity.We calculated the magmatic volume of Comei–Bunbury basalts and the result is ~1.1×104km3. The small volume is not reconciled with those typical models for the initial magmatic eruption of mantle plume. It was possible that the thick lithosphere (>150km) underneath Greater India prevented the Kerguelen plume from large-scale melting because of the high pressure when the latter impinged onto the former at ~132Ma. The shallow source (~60–80km) of the Laguila mafic rocks indicate that they originated likely from a series of “diapirs” or “fingers” rising from the head of the Kerguelen plume, which penetrated into the thick lithospheric mantle of Eastern Gondwana at enough shallow depth for melting.

Geochemistry and detrital zircon U–Pb dating of Lower Cretaceous volcaniclastics in the Babazhadong section, Northern Tethyan Himalaya: Implications for the breakup of Eastern Gondwana

Lower Cretaceous volcaniclastic sandstones, widely distributed in the Tethyan Himalaya, provide insights into evolution of the Tethyan Himalaya and the continental breakup of Eastern Gondwana. Here we present the petrological, geochronological and geochemical data from the Lower Cretaceous volcaniclastic sandstones at the Babazhadong section, eastern Tethyan Himalaya. The geochronological data show that the youngest detrital zircons in the lowest volcaniclastic sandstones have U–Pb age of (134 ± 4 Ma), therefore were deposited post late Valanginian. The volcaniclastics are predominantly comprised by mafic detritus in the lower part, while felsic grains become more abundant up-section. The proportion of volcaniclastics in the sandstones also increases upwards. Geochemical data of mafic detritus indicate within-plate basalt affinity. Based on these observations, in combination with the stratigraphic correlation of coeval volcaniclastic sandstones deposited along the length of the Himalayas, we conclude that the volcaniclastics in the Babazhadong area are within-plate affinity, probably related to the opening of deep, crust cross-cutting fractures along the northern margin of Greater India.

Age and thermal history of Eo- and Neohimalayan granitoids, eastern Himalaya

Four distinct granitoid suites outcrop across a N–S transect in the eastern Himalaya between the Main Central Thrust (MCT) and the Indus Tsangpo suture. The Arunachal and Tsona leucogranites outcrop, respectively, in the MCT hanging-wall and adjacent the South Tibetan Detachment. Both crystallized at about 19Ma, and appear to be lateral equivalents of the High Himalayan Leucogranites. In the central Tethyan Himalaya, the Dala granitoids form a suite of undeformed plutons emplaced into deformed Tethyan Himalayan Sequence metasediments. In the northern Tethyan Himalaya, the Yala-Xiangbo granitoids are part of an igneous complex in the core of the Yala-Xiangbo dome. Both the Dala and Yala-Xiangbo granitoids have Eocene magmatic ages (44.1±1.2Ma and 42±5Ma, respectively) that are notably older than the vast majority of post-collisional granitoids along the main Himalayan Arc. Pelitic units from the core of the Yala-Xiangbo dome record two episodes of amphibolite-grade metamorphism and monazite growth coinciding with well-documented episodes of Eohimalayan and Neohimalayan metamorphism. The thermochronological histories of the Dala and Yala-Xiangbo units suggest that the eastern Tethyan Himalaya was tectonically quiescent from ca. 40Ma until ca. 15Ma. Miocene exhumation of the Dala and Yala-Xiangbo units, and all North Himalayan Domes studied to date, appears best explained by a structural control, such as concurrent north-directed motion on the Great Counter Thrust. We hypothesize that an extension of the Ninety East Ridge may have been responsible for Eocene high heat-flow beneath the eastern Himalaya and explain the localization of magmatism there.

Kinematic evolution of the eastern Tethyan Himalaya: constraints from magnetic fabric and structural properties of the Triassic flysch in SE Tibet

Abstract Anisotropy of magnetic susceptibility (AMS) combined with structural analysis are used in this work with the aim to characterize the tectonic evolution of the Triassic flysch within the eastern Tethyan Himalaya Thrust Belt in SE Tibet. The attitude of the magnetic foliation and lineation are concordant with the planar and linear structures of tectonic origin defined by the preferred orientation of the iron-bearing silicates. Two different tectonic domains can be defined: (a) the southern domain is controlled by the Eohimalayan tectonic foliation (S1) recorded in the magnetic foliation which trends east–west and dips to the north; (b) the northern domain is dominated by the Neohimalayan magnetic foliation with WNW–ESE strike and dips to the south opposite to the vergence of the main structures. A slightly prolate magnetic ellipsoid has been found in between the two domains recording the intersection of S1 and the subtle development of the S2 tectonic foliation. Hinterland propagation of the deformation lead to the Great Counter backthrust generation, pointed out by the SSW steeply plunging magnetic lineation. Furthermore different orientations of magnetic foliation may indicate an Early Miocene c. 20° clockwise vertical-axis rotation.

Paleomagnetic evidence for clockwise rotation and tilting in the eastern Tethyan Himalaya (SE Tibet): Implications for the Miocene tectonic evolution of the NE Himalaya

Crustal movement around and away from the Namche Barwa syntaxis is indicated in the Asian velocity field inferred from GPS data and Quaternary fault slip rates. Nevertheless, there is a limited field-based control on the rotational history of the north-eastern Himalayan arc. Exploring the poly-phase nature of deformation, within the Cretaceous diorite dykes and their host-rock (Triassic flysch), in the eastern Tethyan Himalaya (90°–92°E), combined with new remote sensing data and existing thermo-geochronological data, allow us to unravel the kinematic relationship between paleomagnetic remanence vectors and the deformation phases. Decay at 325 °C in high temperature susceptibility curves and in the thermal demagnetization of the SIRM indicate that the characteristic remanent magnetization in the Cretaceous diorite dykes is carried by pyrrhotite. The pyrrhotite component unblocks at 280–350 °C, revealing normal and reverse polarities. It is of post-folding origin with a mean remanence direction of 019°/28° and 001°/20° in the eastern (Qonggyai valley) and western (Nagarze) part of the sampling area, respectively. The ~ 22 Ma K–Ar age of the last metamorphic event support that the remanence is post-Eohimalayan folding and likely of thermoremanent or thermo-chemical origin. Comparison of the declination with respect to the Early-Miocene reference direction, yields a trend from no apparent rotation in the west to 20° clockwise rotation in the east with respect to the stable Indian plate. This result can be kinematically related to the Middle to Late-Miocene strain partitioning between far-field southeast extrusion of SE Tibet and near-field strike-slip faulting and E–W extension. Furthermore the observed pattern of tilting around horizontal axis may reflect concealed North Himalayan doming.

The age of the Chuangde Formation in Kangmar, southern Tibet of China: Implications for the origin of Cretaceous oceanic red beds (CORBs) in the northern Tethyan Himalaya

The Chuangde Formation in the northern Tethyan Himalaya consists of the Upper Cretaceous oceanic red beds (CORBs). Due to the structural complexity and poor fossil preservation, the age of this unit remains an issue. In one interpretation, the age of the Chuangde Formation in the eastern Tethyan Himalaya is significantly older than in the western Tethyan Himalaya, suggesting that the formation of CORBs in the Late Cretaceous Tethys ocean may have been triggered by the ocean circulation pattern change in response to the changes associated with the collision between Indian and Asian plates. New foraminiferal biostratigraphic data from a well-preserved and fossil-rich section in Kangmar, southern Tibet, China, reveal that the Chuangde Formation in the eastern Tethyan Himalaya has an age from Early Campanian to Maastrichtian. Fossil assemblages from this section allow its correlation with other CORB-bearing sections and argue against the age discrepancy of CORBs between the eastern and western parts of the northern Tethyan Himalaya. Biostratigraphic correlation and paleogeographic reconstruction indicates that the CORBs in the Tethyan Himalaya are restricted to the slope and basinal environments but they are entirely missing in the shelf environments including the shelf margin. The similar thicknesses (and therefore potentially similar sedimentation rates) of time-equivalent units between shelf and slope/basinal sections argues against the formation of CORBs in the Tethyan Himalaya by transportation of terrigenous iron into a sediment-starved ocean. The lack of CORBs in shelf environments and the widespread occurrence CORBs in slope/basinal environments suggest that the CORBs were likely formed by oxidation of reduced iron in the upper ocean water column close to the chemocline rather than in oxic bottom waters.

Petrogenesis of the earliest Early Cretaceous mafic rocks from the Cona area of the eastern Tethyan Himalaya in south Tibet: Interaction between the incubating Kerguelen plume and the eastern Greater India lithosphere?

The relationship between the breakup of eastern Gondwanaland and the Kerguelen plume activity is a subject of debate. The Cona mafic rocks are widely exposed in the Cona area of the eastern Himalaya of south Tibet, and are studied in order to evaluate this relationship. Cona mafic rocks consist predominantly of massive basaltic flows and diabase sills or dikes, and are divided into three groups. Group 1 is composed of basaltic flows and diabase sills or dikes and is characterized by higher TiO 2 and P 2O 5 content and OIB-like trace element patterns with a relatively large range of ɛNd( T) values (+ 1.84 to + 4.67). A Group 1 diabase sill has been dated at 144.7 ± 2.4 Ma. Group 2 consists of gabbroic sills or crosscutting gabbroic intrusions characterized by lower TiO 2 and P 2O 5 content and “depleted” N-MORB-like trace element patterns with relatively higher, homogeneous ɛNd( T) values (+ 5.68 to + 6.37). A Group 2 gabbroic diabase dike has been dated at 131.1 ± 6.1 Ma. Group 3 basaltic lavas are interbedded with the Late Jurassic–Early Cretaceous pelitic sediments; they have compositions transitional between Groups 1 and 2 and flat to slightly enriched trace element patterns. Sr–Nd isotopic data and REE modeling indicate that variable degrees of partial melting of distinct mantle source compositions (enriched garnet–clinopyroxene peridotite for Group 1 and spinel-lherzolite for Group 2, respectively) could account for the chemical diversity of the Cona mafic rocks. Geochemical similarities between the Cona mafic rocks and the basalts probably created by the Kerguelen plume based on spatial–temporal constraints seem to indicate that an incubating Kerguelen plume model is more plausible than a model of normal rifting (nonplume) for the generation of the Cona mafic rocks. Group 1 is interpreted as being related to the incubating Kerguelen plume–lithosphere interaction; Group 2 is likely related to an interaction between anhydrous lithosphere and rising depleted asthenosphere enriched by a “droplet” originating from the Kerguelen plume, while Group 3 may be attributed to thermal erosion resulting in the partial melting of lithosphere during the long-term incubation of a magma chamber/pond at a shallow crustal level. The Cona mafic rocks are probably related to a progressively lithospheric thinning beneath eastern Gondwanaland from 150–145 Ma to 130 Ma. Our new observations seem to indicate that the Kerguelen plume may have started its incubation as early as the latest Jurassic or earliest Cretaceous period and that the incubating Kerguelen plume may play an active role in the breakup of Greater India, eastern India, and northwestern Australia.

The Earliest Jurassic psiloceratids in the Eastern Tethyan Himalaya, South Tibet

Recent collection of ammonites in the Germig area, South Tibet, allows the establishment of an Early to Middle Hettangian psiloceratids succession, documenting the Planorbis and Liasicus Zones. Five ammonite assemblages, ranging from the Planorbis to Laqueus Subzone have been recognized. They are the Neophyllites sp.– Psiloceras ex. gr. pacificum ass., the Psiloceras cf. polymorphum– Psiloceras cf. pacificum ass., Caloceras crassicostatum– Psiloceras distinctum ass., Kammerkarites cf. haploptychus ass. and Discamphiceras silberlingi ass. The Lower and Middle Hettangian ammonite succession together with the Rhaetian Choristoceras cf. marshi (Hauer) offers a basis for the study of the Triassic and Jurassic boundary section, only recently known of in the Eastern Tethyan Himalaya.

The first report of the Late Triassic-Early Jurassic passage beds in the Eastern Tethyan Himalaya

A good exposure of Triassic-Jurassic strata, named the Germig section in the Lanongla area, south Tibet has been investigated recently. Non-marine conditions of the Upper Triassic represented by the Zhamure formation existed in most of south Tibet, but they terminated in the area by the Germig formation which represents a marine transgression from the Late Norian-Rhaetian to the Early and Middle Hettangian. It is reported for the first time that Triassic-Jurassic passage beds with ammonite evidence have been found in Tethyan Himalaya. The earliest Jurassic faunas exhibit affinity with those of Nevada, implying a trans-Pacific dispersal in the early stage of biotic recovery after the end-Triassic mass extinction.