Dalai Zhong

Cenozoic structural and metamorphic evolution of the eastern Himalayan syntaxis (Namche Barwa)

Combined geological and geochronological investigations of the eastern Himalayan syntaxis in the Namche Barwa region of Tibet reveal the first-order elements of its Cenozoic tectonic evolution. The syntaxis is characterized by a northeast-plunging antiform and is bounded by two northeast-striking strike-slip shear zones: a left-slip shear zone on the western side and a right-slip shear zone on the eastern side. These strike-slip shear zones are linked by east^westtrending thrusts and served either as (1) a roof thrust to a large duplex system or (2) transfer faults to a south-directed thrust system that accommodated northward indentation of a folded Indian plate. An east^west-trending pop-up structure in the core of the antiform juxtaposes a granulite-bearing complex over sillimanite-bearing gneisses of Gangdese affinity to the north and of Indian affinity to the south. Previous studies suggest that mafic granulites in the complex record at least two episodes of metamorphism at V800‡C: the first at high pressures (14^15 kbar) followed by a second event at 8^10 kbar. Zircons from mafic granulites yield four populations of concordant U^Pb ion microprobe ages. Two groups are at V65 Ma and V160 Ma, and likely crystallized during Andean-type Gangdese magmatism prior to the Indo-Asian collision. A third cluster at V40 Ma exhibits very low Th/U ratios, and is interpreted to have crystallized in the presence of fluids associated with a high-pressure granulite facies metamorphic event during the early stages of the Indo-Asian collision, subsequent to high-pressure metamorphism in the western Himalaya syntaxis between V50 and 43 Ma. A fourth cluster of zircons yields ages between 11 and 25 Ma and Th/U ratios that decrease systematically with decreasing age. We interpret the youngest zircon age (V11 Ma) to represent the timing of moderatepressure high-grade metamorphism, with the older ages and higher Th/U ratios being a result of mixing with a restitic igneous component. This interpretation, coupled with a V 8M a 40 Ar/ 39 Ar age on hornblende from a metadiorite within the core of the antiform, suggests that the Namche Barwa syntaxis has been characterized by rapid cooling and exhumation since at least Late Miocene time. Despite its contrasting structural setting, the Miocene and younger metamorphism and cooling history in the Namche Barwa syntaxis are strikingly similar to those of the Nanga Parbat syntaxis of the western Himalaya. fl 2001 Elsevier Science B.V. All rights reserved.

Nd–Sr–Pb isotopes of Tengchong Cenozoic volcanic rocks from western Yunnan, China: evidence for an enriched-mantle source

Abstract Tengchong Cenozoic volcanic activity in the southeastern Himalayan belt of western Yunnan is considered to be rift-related after the Indo-Eurasia collision. In this study, Nd–Sr–Pb isotopic compositions were analyzed in order to understand the genesis of the Cenozoic volcanic rocks. Five basalts and andesitic basalts are characterized by high 87Sr/86Sr ratios (0.7057–0.7081), low ϵNd values (−1.1 to −5.7), and particularly high 208Pb∗/206Pb∗ ratios (1.08–1.12). Twenty samples of rocks, which include granite, granodiorite, and amphibolite from the same region, were analyzed to evaluate the role of contamination and assimilation in the volcanic rocks. Granites have high 87Sr/86Sr ratios and low ϵNd values, indicating a crustal origin. Granodiorites have 87Sr/86Sr ratios of 0.7069–0.7100 and ϵNd-values of −2.9 to −7.6 that indicate contribution of a mantle component. Amphibolites that are characterized by high ϵNd-values of 9.4 to −1.7 may represent remnants of obducted oceanic crust of the Neo-Tethyan basin. The volcanic rocks are distinguished from the granitoids and amphibolites in terms of Pb isotopic compositions. This observation probably implies that crustal contamination played a minor role in the origin of the volcanic rocks. Isotopic evidence for the volcanic rocks indicates that they probably originated from melting of an enriched-mantle source, that is, ascribed to assimilation of subducted oceanic crust and sediments.

The subducted slab of Yangtze Continental block beneath the Tethyan orogen in western Yunnan

The western Yunnan area is a natural laboratory with fully developed and best preserved Tethyan orogen in the world. Seismic tomography reveals a slab-like high velocity anomaly down to 250 km beneath the western Yunnan Tethyan orogen, to its west there is a low-velocity column about 300 km wide. In the region from Lancangjiang to Mojiang an obvious low velocity in the lower crust and uppermost mantle overlies on the slab. Synthesizing the available geological and geochemical results, the present paper demonstrates that this slab-like high velocity anomaly is a part of the subducted plate of Yangtze continental segment after the closure of Paleotethys. The collision of India and Eurasia continent starting from 50–60 MaBP might trigger thermal disturbance in the upper mantle and cause the uprising of asthenosphere, in that case the subducted Yangtze plate could be broken off, causing Cenozoic magmatic activities and underplating in the Lancangjiang-Mojiang region.

Metamorphic characteristics and geotectonic implications of the high-pressure granulites from Namjagbarwa, eastern Tibet

A large area of high-pressure garnet-kyanite granulite is exhumed in the Namjagbarwa area, which provides a window for observing the deep crust rocks and structures of the Tibetan Plateau. Three mineral assemblages can have been distinguished in the garnet-kyanite HP granulites by petrography, i.e. M1. Mus+Bi+P1+Q, M2. Gt+Ky +perphite/antiperphite+Rt+Q, M3. Gt+Sill+Cord+Sp+Ilm ± Opx. Metamorphic conditions of the peak granulite assemblages (M2) formatted by thickening of crusts, with available isotopic ages of 45–69 Ma, are at 1.4—1.8 Gpa and 750—850°. Their retrograde assemblages overprinted by decompressure during the uplift, with available isotopic ages of 18—23 Ma, were formed at 0.60—0.70 Gpa, 621—726°. The thermobarometric evaluation, petrogenetic grid and corresponding isotopic ages indicate a clockwise isothermal decompression metamorphic path. The HP granulite metamorphic history indicates that the collision of the Indian Plate with the Eurasian Plate had begun at 70 Ma, far earlier than the widely accepted 45 Ma.

Orogen-parallel extension in Himalaya: Is it the indicator of collapse or the product in process of compressive uplift?

Extensional structures developed extensively in Himalaya with their strikes perpendicular to that of the orogenic belt. The studies of such structures in Qusum, Burang, Lhozhag, Nyalam and Yadong show that they represent an orogen-parallel extension. The basins produced by the extension experienced orogen-perpendicular compression during their formation. The ages of the extension rang from 16 to 13 Ma and coincide with the intensive compressive stage in Southeastern Asia. Therefore, the extension is syn-shortening and the dominate tectonic movement in Himalaya was compression and uplift during the extension. The extension was the partition of compressive deformation rather than the indicator for the collapse and falling of the Tibetan Plateau.

Discovery of ophiolite in southeast Yunnan, China

The Babu ophiolite in Malipo County, SE Yunnan, consists of three units: ultramafic, mafic and basaltic rocks. Studies of geological mapping, petrology and geochemistry reveal that the gabbro is similar to that in the Troodos ophiolite, and the diabase and basalt belong to a normal MORB-type, analogous to the Shuanggou ophiolite in west Yunnan. The ophiolite studied as a thrust tectonic slab, was overthrust northwards onto the Late Paleozoic-Triassic deep marine deposits of continental margin. It is inferred to be relics of Paleotethyan ocean; namely, a branch of Paleotethys occurs in South China, where the tectonic nature and evolutional history of the area should be reconsidered.

The western boundary of extrusion blocks in the southeastern Tibetan Plateau

The model about block extrusion related to collision between Eurasia and India which is proposed by Tapponnier et al. has not been accepted so far in the field. Based on kinematics and chronology, the Gailigong dextral strike-slip fault in west Yunnan and Nabang dextral strike-slip fault which is the part of Mogok belt are the western boundary of extrusion blocks, as the eastern boundary Red River-Ailaoshan left-lateral fault. And more studies show that as the important way for absorbing the convergence between two continents, the age of blocks’ multistage extrusion is about 23 and 13 Ma.

Early subduction–exhumation and late channel flow of the Greater Himalayan Sequence: implications from the Yadong section in the eastern Himalaya

Based on metamorphic studies of the Yadong high-pressure (HP) granulite and multiple thermochronological investigations of granitoids from both upper and lower parts, the Yadong section in the eastern Himalaya constrains the Cenozoic tectonic evolution of the Greater Himalayan Sequence (GHS). The Yadong HP granulite, located at the top of the GHS, underwent a peak-stage HP granulite facies metamorphism and two stages of retrograde metamorphism. Granulite and hornblende facies retrograde metamorphism took place at 48.5 and 31.8 Ma, respectively, marking the time of exhumation of the subducted Indian slab to lower and middle crustal levels. Subsequently, an average young zircon U–Pb age obtained from the Yadong HP granulite indicated that this unit was captured by its surroundings in a partially molten condition at 16.9 Ma. In addition, three granitoids from both the lower and the upper parts of the GHS yielded biotite 40Ar/39Ar ages of 11.0, 11.3, and 11.5 million years. These consistent ages suggest that the GHS along the Yadong section was laterally extruded and synchronously cooled to ∼300°C at ∼11.3 Ma. Furthermore, the granitic gneisses yield apatite fission track ages of ∼7 million years, documenting the cooling of the GHS to ∼110°C. A two-stage model describes the Cenozoic tectonic evolution of the GHS: (1) the Indian slab had subducted under Tibet before ∼55 Ma, and was exhumed to the lower crust (50-40 km) at 48.5 Ma, and to the middle crust (22-15 km) at 31.8 Ma; and (2) the partial melting occurred at middle crustal levels during the period 31.8 to 16.9 Ma, causing channel flow. In the late stage, the GHS was laterally extruded by ductile mid-crustal flow during the period 16.9 to ∼7 Ma, characterized by a fast cooling rate of ∼2 mm per year.

Midcrustal shearing and doming in a Cenozoic compressive setting along the Ailao Shan-Red River shear zone

The Cenozoic Xuelong Shan antiformal dome is located along the northern segment of the Ailao Shan-Red River shear zone in Yunnan, China. Subhorizontal foliation in the gneiss core is recognized, representing a broad top-to-NE shear initiated under amphibolite facies conditions and propagating into greenschist facies in the mantling schist and strike-slip shear zone. Microfabrics of crystallographic-preferred orientations (CPOs) in quartz suggest that the deformation temperatures increased with increasing structural depth from the upper crust (300–500°C) in the mantling schist to the midcrust (15 km or more, ≥650°C) in the gneissic core. This trend is mirrored by variations in the metamorphic grade of the syn-kinematic mineral assemblages and microstructures, which range from garnet + amphibole + biotite + sillimanite + rutile + feldspar in the core to garnet + staurolite + biotite + epidote + muscovite within the limb units. The dome experienced the following deformation history: (1) a broad top-to-NE shear in the subhorizontal foliation of the gneiss core during the first stage of deformation (D1); (2) opposing reverse-sense shear along the two schist limbs of the dome during contraction-related doming (D2–D3); (3) sinistral strike-slip shearing within the eastern limb (D4); and (4) extensional deformation (D5). The structural-thermal patterns suggest the antiformal dome formation was roughly coeval with top-to-NE ductile shearing in the midcrust of Tibet at 32 Ma or earlier. A major implication is that there was a phase of contractional ductile deformation in the region prior to the initiation of strike-slip deformation.

Late Miocene Thermal Evolution of the Eastern Himalayan Syntaxis as Constrained by Biotite 40Ar/39Ar Thermochronology

We conducted biotite 40Ar/39Ar thermochronological research in the eastern Himalayan syntaxis and the neighboring Mesozoic and early Cenozoic Gangdese batholith belt with the aim of exploring the overall cooling pattern and thermal evolution of this remote region in the Himalayan orogen. A compilation of our newly acquired ages with existing data reveals a new temporal-spatial pattern. First, a temporal gap at 13–7 Ma exists in the cooling history of the study area, with ages <7 Ma within the syntaxis and ages >13 Ma in the Gangdese belt. The gap could be a manifestation of a renewed rapid cooling of the syntaxis since ∼7 Ma within a general region of slow cooling. Second, the isochron contours of the cooling ages have an annulus shape, with a younging trend toward the Namche Barwa–Gyala Peri peaks at the core of the syntaxis at 7–4 and 4–2 Ma and along the margins of the syntaxis at 2–1 Ma. This pattern may be a result of a regionally progressive cooling from the margin to the core of the syntaxis that was punctuated by locally focused accelerated cooling and exhumation. Our new findings imply that the Cenozoic faults bounding the syntaxis controlled the development of the syntaxis. Additionally, new estimates of the exhumati rate suggest that the development of the syntaxis has been accelerating since 7 Ma.