anqing Ji

Adakites from continental collision zones: Melting of thickened lower crust beneath southern Tibet

Adakites are geochemically distinct intermediate to felsic lavas found exclusively in subduction zones. Here we report the first example of such magmas from southern Tibet in an active continental collision environment. The Tibetan adakites were emplaced from ca. 26 to 10 Ma, and their overall geochemical characteristics suggest an origin by melting of eclogites and/or garnet amphibolites in the lower part (≥50 km) of thickened Tibetan crust. This lower-crustal melting required a significantly elevated geotherm, which we attribute to removal of the tectonically thickened lithospheric mantle in late Oligocene time. The identification of collision-type adakites from southern Tibet lends new constraints to not only the Himalayan-Tibetan orogenesis—how and when the Indian lithosphere started underthrusting Asia can be depicted—but also the growth of the early continental crust on Earth that consists dominantly of the tonalite-trondhjemite-granodiorite suites marked by adakitic geochemical affinities.

Zircon U-Pb and Hf isotope constraints on the Mesozoic tectonics and crustal evolution of southern Tibet

The first in situ Hf and U-Pb isotope analyses of zircon separates from Mesozoic granites in southern Tibet identify a significant, previously unknown stage of magmatism. Igneous zircons (n = 34) from a granite within the Gangdese batholith show a weighted mean 206 Pb/ 238 U age of 188.1 ± 1.4 Ma and e Hf (T) (the parts in 10 4 deviation of initial Hf isotope ratios between the zircon sample and the chondritic reservoir) values between +10.4 and +16.8, suggesting predominantly Early Jurassic intrusive activity with a juvenile mantle contribution. Of 40 inherited zircons from two Cretaceous S-type granites in the northern magmatic belt, 23 delineate a slightly older 206 Pb/ 238 U age cluster between 188 and 210 Ma. These zircons have e Hf (T) values from −3.9 to −13.7, yielding crustal Hf model ages from ca. 1.4 to 2.1 Ga, suggesting a major episode of crustal growth in Proterozoic time and remelting of this crust in Early Jurassic time. Combining these with literature data, we interpret the Jurassic Gangdese magmatism as an early product of the Neo-Tethyan subduction that played a long-lasting role in the tectonic evolution of southern Tibet prior to the India-Asia collision.

SHRIMP zircon U-Pb ages of Kalatongke No. 1 and Huangshandong Cu-Ni-bearing mafic-ultramafic complexes, North Xinjiang, and geological implications

The SHRIMP zircon U-Pb dating was carried out and yielded 287±5 Ma (MSWD = 0.34) and 274±3 Ma (MSWD = 1.35) for the Kalatongke No. 1 and Huangshandong Cu-Ni-bearing mafic-ultramafic complexes. These ages are much more precise than pre-existing rock-mineral Rb-Sr, Sm-Nd and Re-Os isochron ages for the two complexes and constrain the timing of not only the complexes but also the magmatic Cu-Ni sulfide deposits more reliably. It is necessary to carefully reevaluate the previous chronological data for the complexes. The Cu-Ni-bearing mafic-ultramafic complexes have the ages similar to those of postcollisional A-type granites in the same area, implying that they could be related to the delamination of lithospheric mantle and upwelling and partial melting of asthenospheric mantle in postcollisional setting. Therefore, the Cu-Ni-bearing mafic-ultramafic complexes are a direct indicator of vertical growth of the continental crustal in the Central Asian Orogenic Belt.

Miocene Jiali faulting and its implications for Tibetan tectonic evolution

The Karakoram^Jiali Fault Zone (KJFZ) comprises a series of right-lateral shear zones that southerly bound the eastward extrusion of northern Tibet relative to India and stable Eurasia. Here we present new 40 Ar/ 39 Ar age data from the Puqu and Parlung faults, two easternmost branches of the Jiali fault zone, which indicate a main phase of the KJFZ shearing from V18 to 12 Ma. Thus, the Tibetan eastward extrusion bounded by principal strike-slip fault zones started and was probably most active around the middle Miocene, an interval marked also by active east^west extension in southern Tibet. The coincidence of these two tectonic events strongly suggests a common causal mechanism, which is best explained as oblique convergence between India and Asia. Under the framework of this mechanism, the extension in southern Tibet is not a proxy for the plateau uplift. The KJFZ activity was furthermore coincident with right-lateral displacements along the Gaoligong and Sagaing faults in southeast Asia. This defines a Miocene deformation record for the regional dextral accommodation zone that, in response to the continuing India^ Asia collision, may have accounted for the initiation and prolonged history of clockwise rotation of the Tibetan extrusion around the eastern Himalayan Syntaxis. ? 2002 Elsevier Science B.V. All rights reserved.

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.

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.

Determination of Eocene–Oligocene (30–40 Ma) deformational time by zircon U–Pb SHRIMP dating from leucocratic rocks in the Ailao Shan–Red River shear zone, southeast Tibet, China

The Ailao Shan–Red River (ASRR) shear zone, a long, narrow metamorphic belt that strikes NW–SE, is the continuation of the southeastern margin of the Tibetan Plateau. It mainly comprises amphibolite-facies mylonitic gneiss. In this study, we report zircon concordant U–Pb SHRIMP ages of 37.52 Ma ± 0.67 Ma [mean square weighted deviation (MSWD) = 1.4] and 30.8 Ma ± 0.3 Ma (MSWD = 3.2) from a granitic gneiss in the northern part of the Ailao Shan metamorphic belt, and from a leucogranitic dike in the middle of the zone, respectively. Zircon U–Pb ages between 35 Ma and 41 Ma were also obtained from a biotite gneissic granite in the southern part of the belt. Contrasting the internal structure of the zircon crystals with our new U–Pb dating suggests that their nucleation and growth initiated during a late Eocene tectonic uplift event between ca. 40 and 35 Ma. Detailed field observations and zircon U–Pb dating results suggest that the deformation time of mylonitic gneiss and granitic gneiss in the ASRR was 40–30 Ma, the formation age of the gneissic schistosity of the ASRR metamorphic belt was as early as ca. 40 Ma.

Zircon U–Pb dating constraints on the crustal melting event around 8 Ma in the eastern Himalayan syntaxis

Abstract We determined detailed U–Pb ages for zircons in migmatitic granites densely exposed in the Namche Barwa peak (NBP) area, the core area of the eastern Himalayan syntaxis (EHS). The cores of these zircons show ages ranging from 863 to 1,256 Ma, with high Th/U ratios. These ages correspond to the crystallization age of the Proterozoic Indian continental basement. The majority of zircon rims from these migmatitic granites yield a significant age peak at about 8 Ma with an increase in Th/U ratios, suggesting that a large-scale partial crustal melting event occurred at 8 Ma in the NBP area. Because thermochronologic results have demonstrated that a rapid denudation started around 8 Ma, we interpret this large-scale partial melting of the crust as a decompression melting caused by rapid denudation in the NBP area. The rapid denudation at 8 Ma probably resulted from the combination of tectonic activities and surface erosion.