Gangdese Research Articles

Geochronology and petrogenesis of granitic rocks in Gangdese batholith, southern Tibet

Based on petrological and geochemical characteristics such as rock assemblage, petrogeochemistry, Sr-Nd isotope, zircon U-Pb age, and Hf isotope, we studied geochronological framework, magma types, source characters, and petrogenesis of different stages of magmatism of the granitic rocks from the Gangdese batholith in southern Tibet. The magmatic activities of the Gangdese batholith can be divided into three stages. The Mesozoic magmatism, induced by northern subduction of Neotethyan slab, was continuously developed, with two peak periods of Late Jurassic and Early Cretaceous. The Paleocene-Eocene magmatism was the most intensive, and resulted from a complex progress of Neotethyan oceanic slab, including subduction, rollback, and subsequent breakoff. And the Oligocene-Miocene magmatism was attributed to the convective removal of thickened lithosphere in an east-west extension setting after India-Asia collision. Isotopically, zircons from these granitic rocks are characterized by positive ɛ Hf(t) values, suggesting that the magmatic source of the Gangdese batholith might be an arc terrane, which was accreted to the southern margin of Asia during Late Paleozoic. Therefore, the chronological framework and Hf isotopic characteristics of the Gangdese batholith are distinct from the granitic rocks in adjacent areas, which can be served as a powerful tracer in studying source-to-sink relation of sediments during the uplift and erosion of Tibetan Plateau.

Gangdese arc detritus within the eastern Himalayan Neogene foreland basin: Implications for the Neogene evolution of the Yalu–Brahmaputra River system

In order to assess the spatial and temporal extent of sediment transport from the Gangdese batholith of Tibet to the eastern Himalayan Neogene foreland basin, we performed U–Pb and Lu–Hf analyses on eleven sandstone samples from three locations within the Arunachal and Sikkim Himalaya. We also analyzed detrital zircons from eight modern river sand samples of the Yalu–Brahmaputra River system and its major tributaries in the eastern Himalaya. Results from the river sands are used to contrast the provenance characteristics of the Gangdese arc in southern Tibet with nominally equivalent arc rocks east of the Himalaya in the northernmost Indo-Burma Ranges. Our results indicate that the deposition of Gangdese batholith-derived sediment within the eastern Himalayan foreland: (1) occurred throughout Late Miocene and Pliocene time (~ 10–3 Ma), (2) was limited to the Arunachal Himalaya, and (3) was sourced north of the Himalaya. This detritus may have been deposited by a transverse Himalayan river, such as the Subansiri River, as suggested by high percentages of the Gangdese-derived zircons within the Neogene samples (15–31%) and S- to SW-oriented paleocurrent directions from two of the Neogene sample localities. At this time, our preferred model to explain the data invokes capture of an originally westward-flowing Yalu River by the Subansiri River at ~ 10 Ma, followed by capture of the Yalu River by the Siang River at ~ 3–4 Ma.

Mantle input to the crust in Southern Gangdese, Tibet, during the Cenozoic: Zircon Hf isotopic evidence

The Quxu (曲水) complex is a typical intrusive among the Gangdese batholiths. Two sets of samples collected from the Mianjiang (棉将) and Niedang (聂当) villages in Quxu County, including gabbro, mafic micro-enclaves (MME), and granodiorites in each set, were well dated in a previous SHRIMP zircon U-Pb analysis (47-51 Ma). In this article, the same zircons of the 6 samples were ap- plied for LA ICP-MS Hf isotopic analysis. The total of 6 samples yields 176 Hf/ 177 Hf ratio ranging from 0.282 921 to 0.283 159, corresponding to eHf(t) values of 6.3-14.7. Their Hf depleted-mantle modal ages (TDM) are in the range of 137-555 Ma, and the zircon Hf isotope crustal model ages (TDM C ) range from 178 to 718 Ma. The mantle-like high and positive eHf(t) values in these samples suggest a mantle- dominated input of the juvenile source regions from which the batholith originated. The large varia- tions in eHf(t) values, up to 5-e unit among zircons within a single rock and up to 15-e unit among zir- cons from the 6 samples, further suggest the presence of a magma mixing event during the time of magma generation. We suggest that the crustal end-member involved in the magma mixing is likely from the ancient basement within the Lhasa terrane itself. The zircon Hf isotopic compositions further suggest that magma mixing and magma underplating at about 50 Ma may have played an important role in creating the crust of the southern Tibetan plateau.

Zircon U–Pb and Hf isotopic constraints from eastern Transhimalayan batholiths on the precollisional magmatic and tectonic evolution in southern Tibet

Before the Indian collision with Asia, northward subduction of the Neo-Tethyan oceanic lithosphere produced an Andean-type convergent margin in South Asia characterized by arc magmatism starting from the early Jurassic and lasting until the Eocene. The magmatic arc is now represented by widespread intrusive bodies or the so-called Transhimalayan batholiths in the Lhasa terrane of southern Tibet that have been divided into two main magmatic suites, i.e., the northern plutonic belt and the southern Gangdese Batholith. Their temporal distribution, concerning how exactly the magmatic suites correlate eastwards and then southeastwards around the eastern Himalayan syntaxis, however, remains poorly constrained. Here we report the first combined zircon U–Pb and Hf isotopic study of the Transhimalayan batholiths from the eastern part of the Lhasa terrane (∼ 95–97.5°E and ∼ 28.5–30°N), SE Tibet. Zircon U–Pb dating results of 24 granitoids indicate that the rocks were emplaced principally in the Early Cretaceous (∼ 133–110 Ma) and subordinately in the Paleocene (∼ 66–57 Ma), long after an older granite intrusion in the earliest Jurassic (∼ 198 Ma). The zircon ε Hf( T) values range from + 5 to − 20, yielding Hf crustal model ages ( T DM C) between 0.8 and 2.4 Ga that peak at ∼ 1.7 Ga, suggesting a major episode of crustal growth in the Proterozoic and a predominantly, or even exclusively, crustal source for the granitoid petrogenesis. These zircon U–Pb and Hf isotopic constraints, together with whole-rock geochemical characteristics, allow us to correlate them to S-type granitoids in the northern plutonic belt, rather than to the I-type Gangdese Batholith from the central part of the southern Lhasa terrane. In contrast to the Gangdese magmatism that formed with a significant juvenile mantle input related to the Neo-Tethyan subduction, these S-type granitoids show geochemical similarities to the North American Cordilleran Interior batholiths that did not form directly from a subduction setting. Thus, we attribute the petrogenesis of the widespread Early Cretaceous granitoids in the northern belt and eastern Himalayan batholiths to a postcollisional regime due to the Late Jurassic–Early Cretaceous continental collision between the Lhasa and Qiangtang terranes. Under this framework, the S-type magmatism was generated in response to collision-induced crustal thickening, which may have capabilities of causing crustal anatexis by itself in the region. However, continued interplay with the Neo-Tethyan subduction zone processes such as back-arc extension could also have played a crucial, and long-lasting, role in the magma generation.

Zircon U–Pb geochronology and Hf isotopic constraints on petrogenesis of the Gangdese batholith, southern Tibet

During the Mesozoic–Cenozoic, northward Neotethyan subduction and subsequent India–Asia collision gave rise to the extensive Transhimalayan magmatism that stretches from Burma and western Yunnan through southern Tibet to the Ladakh and Kohistan complexes. To understand the age distribution and petrogenesis of the Gangdese batholith, the largest intrusive exposure along the Transhimalayan magmatic belt, fifty granitic samples were selected for in situ zircon U–Pb and Hf isotopic analyses. The U–Pb data suggest four discrete stages of magmatic activity, i.e., ~ 205–152, ~ 109–80, ~ 65–41 and ~ 33–13 Ma, respectively, with the 65–41 Ma stage being the most prominent. The Hf isotopic data indicate that the Gangdese batholith is overwhelmed by positive ε Hf( t) values, which are comparable to those of the Kohistan–Ladakh batholiths in the west but differ markedly from those of the Chayu–Burma batholiths in the east. Most of the Gangdese granites show similar and young Hf model ages (1000–300 Ma), indicating their derivation from juvenile crust. However, those formed in the 65–41 Ma stage exhibit more heterogeneous Hf isotopic ratios, with negative ε Hf( t) values being observed in some granites younger than 50 Ma, suggesting the involvement of old Indian continental crust in their petrogenesis. This age may thus mark the onset of the India–Asia collision. The newly established zircon U–Pb age and Hf isotope database of the Gangdese batholith can be used as a powerful tracer or “fingerprint” when studying the source-to-sink relation of the sediments eroded from the southern Tibetan Plateau.

Probing the basement of southern Tibet: evidence from crustal xenoliths entrained in a Miocene ultrapotassic dyke

Abstract: A variety of felsic and mafic granulites and ultramafic rocks occur as xenoliths within a 12.7 Ma ultrapotassic dyke intruding Xigaze flysch immediately to the north of the Yarlung–Tsangpo suture zone in southern Tibet. Garnet–clinopyroxene–plagioclase–quartz thermobarometry on mafic granulite xenoliths gives temperatures of 1130–1330 °C and pressures between 22 and 26 kbar indicating equilibration in the high-pressure and ultrahigh-temperature granulite field and defining a geotherm of c . 16 °C km −1 . Ultramafic xenoliths consist mainly of hornblende and biotite, probably of restitic crustal rather than mantle origin, and attained peak metamorphic conditions of 920–1130 °C and 17–24 kbar, whereas felsic granulites equilibrated at 870–900 °C at an inferred pressure of 17 kbar. In situ U–(Th)–Pb laser ablation inductively coupled plasma mass spectrometry dating of zircons shows that protoliths may include Proterozoic basement rocks, Late Cretaceous calc-alkaline tonalites of the Gangdese batholith root and/or remnants of a Neo-Tethyan oceanic arc. Certain zircons from a felsic granulite and an ultramafic xenolith have mean 206 Pb/ 238 U ages of 16.8 ± 0.9 Ma and 15.6 ± 0.6 Ma, respectively, and monazites from a micaceous xenolith yielded a mean 208 Pb/ 232 Th age of 14.4 ± 0.4 Ma. These results show that the southern Tibet basement reached a thickness of c . 80 km by 17–14 Ma at the latest and has remained unchanged until the present day. Supplementary material: Mineral analyses used for pressure–temperature estimations and results of 40 Ar/ 30 Ar laser single-grain fusion experiments are available at http://www.geolsoc.org.uk/SUP18325 .

Detrital zircon evidence from Burma for reorganization of the eastern Himalayan river system

The first <i>in situ</i> Hf and U-Pb isotope analyses of detrital zircons from Upper Miocene sandstone in the Inner-Burma Tertiary Basin, together with data of igneous zircons from the eastern Transhimalayan batholith from southern Tibet to Burma, enable us to study the sedimentary source to sink relation and river evolution around the eastern Himalayas. Among 47 out of 62 dated zircons from the Miocene sandstone that exhibit Cretaceous and Paleogene ²⁰⁶Pb/²³⁸U ages, 24 grains have positive ε_Hf(T) isotope values up to +16. Whilst zircons of such ages are common in the Transhimalayan plutons, those showing high ε_Hf(T) values have been observed so far only in the Gangdese batholith. Our results, therefore, support the notion that by Late Miocene time the Yarlu Tsangpo, which flows past the Gangdese batholith in southeastern Tibet, drained into the Irrawaddy River. We attribute this river routing to not only regional topographic control but also the dextral movement of the Jiali-Gaoligong-Sagaing fault system that appears most active during the Middle Miocene. Subsequent reorganization of these mountain rivers was affiliated with headward erosion of the Brahmaputra River that eventually cut across the Namche Barwa Syntaxis and captured the Yarlu Tsangpo drainage to form the modern eastern Himalayan river system.

Zircon SHRIMP U–Pb ages of the Gangdese Batholith and implications for Neotethyan subduction in southern Tibet

The Trans-Himalayan magmatism, which occurred extensively in the Lhasa terrane of southern Tibet, has long been related to the Neotethyan subduction before the India–Asia collision. To better delineate the magmatic duration, we report a geochronological study with 25 SHRIMP zircon U–Pb ages from the Gangdese Batholith that represents the largest Trans-Himalayan plutonic complex. The results suggest two distinct stages of plutonism in the Late Cretaceous (ca. 103–80 Ma) and early Paleogene (ca. 65–46 Ma), respectively. Our new data confirm if not refine the notion that a Gangdese magmatic gap or quiescent period existed between ca. 80 and 70 Ma. It is furthermore identified that the early stage ended with adakitic intrusion and the latter stage is marked by a peak activity at ca. 50 Ma. We attribute the cessation of the early stage, and following magmatic gap, to a flattening of the northward Neotethyan subduction, and the initiation of the latter stage to rollback of the subducted slab. The proposed scenarios can also account for the southward migration and intensification of Cretaceous to Paleogene volcanism in the Lhasa terrane that demonstrates a coeval, eruptive “flare-up” event around 50 Ma, interpreted as the result of detaching the Neotethyan oceanic slab from the adherent, more buoyant Indian continental lithosphere owing to the India–Asia collision. Our model is, moreover, in general accord with sedimentary and structural geologic records from southern Tibet where subduction-related orogenesis appears to have evolved through time before India started colliding Asia.

Late Cretaceous Gangdese intrusions of adakitic geochemical characteristics, SE Tibet: Petrogenesis and tectonic implications

The Gangdese batholith emplaced from the Cretaceous to Eocene in southern Tibet has been widely regarded as the major constituent of an Andean-type convergent margin resulting from northward subduction of the Neo-Tethyan oceanic lithosphere under Asia. While the Gangdese batholith consists predominantly of calc-alkaline rocks, we identify from the eastern part of the batholith a suite of epidote-bearing granodiorites that shows adakitic geochemical characteristics, marked with apparently higher La/Yb and Sr/Y, and lower Y and HREE, than other Gangdese rocks and common arc magmas. SHRIMP zircon U–Pb analyses of two of the samples yielded 206Pb/ 238U dates of 80.4 ± 1.1 and 82.7 ± 1.6 (2 σ) Ma, which constrain the emplacement ages of the adakitic rocks. Trace element modeling suggests that these rocks originated from partial melting of a garnet amphibolite source that, on the basis of the Sr and Nd isotope data [I Sr = 0.7044–0.7048; ε Nd( T) = + 3.2 to + 0.9], we interpret to be a newly underplated, mafic lower crust, rather than the subducted Neo-Tethyan oceanic crust. This juvenile crust was produced by Cretaceous basaltic underplating above the mantle wedge and then thickened by the tectonic contraction owing to flattening of the Neo-Tethyan subduction, a process that also led to the adakitic magmatism. Our interpretation involving a Late Cretaceous flat-slab subduction and related orogenesis in southern Tibet is consistent with petrographic data, such as the occurrence of magmatic epidote and muscovite rimmed with resorption texture in the granodiorites, which indicate deep-seated emplacement followed by rapid tectonic exhumation.

Mantle contributions to crustal thickening during continental collision: Evidence from Cenozoic igneous rocks in southern Tibet

The Tibetan crust is twice as thick as average continental crust. Crustal compression and shortening as a result of Indian–Asian collision is often considered to be the primary cause for the crustal thickening. In this paper, we show that magmatic contribution is also important. We come to this conclusion by documenting the Paleogene Linzizong volcanic succession (LVS), its coeval granitoid batholiths and the Miocene adakitic rocks along the Gangdese magmatic belt in southern Tibet. It has been widely accepted that the Indian–Asian collision proceeded from a “soft” phase at ∼ 65–70 Ma to a “hard” phase at ∼ 45–40 Ma, followed by continued post-collisional convergence to the present. In response to the collision and post-collision convergence are a series of tectono-magmatic events recorded in the Gangdese magmatic belt. These include (1) the syn-collisional LVS volcanism (∼ 65–40 Ma) and the emplacement of southern Gangdese batholiths (a peak age of ∼ 50 Ma); (2) a period (∼ 40 Ma to 25 Ma) that is magmatically quiescent, yet tectonically dominated by active compression and crustal shortening; and (3) the emplacements of post-collisional adakitic rocks (∼ 25–12 Ma), potassic–ultrapotassic volcanics (∼ 25–10 Ma) and peraluminous muscovite-bearing granites (∼ between 24 and 18 Ma). These three major events contribute in different ways to the crustal thickness. Phase I, formation of the lower juvenile crust from ∼ 65 Ma to 50 Ma with crustal thickening largely concentrated at ∼ 50–40 Ma via input of mantle-derived magmas; Phase II, crustal thickening by tectonic shortening at ∼ 40–25 Ma; and Phase III, retaining crustal thickness, but thinning of the lithospheric mantle since ∼ 25 Ma in response to crustal extension and upwelling and lateral flow of asthenospheric mantle. We emphasize that collision-induced crustal thickening took place mainly in the period of ∼ 50–40 Ma and ∼ 25 Ma, i.e., the period between the late stage of the LVS volcanism and the beginning of the adakitic rock emplacement. Most of the LVS rocks and the collision related granitoids in southern Gangdese have ε Nd > 0, attesting to the significance of mantle input, most likely through re-melting of mantle-derived basaltic rocks, including the subducted Neo-Tethyan ocean crust. The petrologic and geochemical characteristics of the Miocene potassic adakitic rocks support the idea that the lower portion of the thickened Tibetan crust is mafic and is genetically associated with the earlier LVS magmatism. We estimate that the mantle material input contributed about 30% of the total thickness of the present-day Tibetan crust. By assuming a pre-collision crustal thickness of ∼ 35 km, then the tectonic contribution would be about 20 km.

First field evidence of southward ductile flow of Asian crust beneath southern Tibet

There is lively debate on whether Asian plate material was involved in southward flow of mid-lower crust in a ductile channel beneath southern Tibet. One argument against such involvement is the apparent absence of material derived from Asian lithosphere within the High Himalayan Series (Indian plate) that could represent the putative channel. A north-south–trending mid-Miocene dike swarm that intrudes the Tethyan sedimentary cover of the Sakya gneiss dome (Indian plate) yields new Sr-Nd isotopic data (87Sr/86Sr = 0.7071–0.7079; ϵNd −4 to −6) indicating that these melts share the same source as Miocene dacitic dikes from north of the Indus-Tsangpo suture. Moreover, dikes on both sides of this suture represent crustal melts derived largely from mid-lower crust of the Asian plate, exposed today as the Nyainqentanglha gneisses that underlie the Gangdese batholith. We infer that melting of the Asian lithosphere extended south of the surface trace of the suture, requiring southward propagation of anatectic Asian middle crustal material during the Miocene. The emplacement ages of the southern dike swarm (12–9 Ma) thus delimit the timing of active southward ductile flow of Asian material.

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.

Re-Os age for molybdenite from the Gangdese porphyry copper belt on Tibetan plateau: Implication for geodynamic setting and duration of the Cu mineralization

The Gangdese porphyry copper belt consists of one large and five middle-small deposits in addition to dozens of ore-bearing porphyry bodies. The belt trends 350 km long along the EW-striking Gangdese batholith, and locally occurs as a string of beads extending about 50 km within the SN-trending rifting zones (grabens) on the Tibetan plateau. Monzonitic granite- porphyry and quartz monzonitic porphyry, as dominant host rocks, are shoshonitic and potassic calc-alkaline. Available dating data indicate that the ages of the shallow-level emplacement for these porphyries vary from 10 Ma to 18 Ma, which are identical to that of associated potassic calc-alkaline volcanic rocks (10—15 Ma) and mafic dykes (13—18 Ma). The timing and duration of Cu mineralization events are constrained by Re-Os ages for molybdenites from three porphyry copper deposits in the Gangdese porphyry copper belt. Five molybdenite samples from the Nanmu deposit yielded an 187Re-187Os isochron with a highly precise age of (14.76±0.22) Ma; six molybdenites from the Chongjiang deposit gave an isochron age of (14.04±0.16) Ma. Re-Os model ages for two molybdenite samples from Lakang’e deposit vary from 13.5 Ma to 13.6 Ma, which are basically identical to isochron ages mentioned above. All the thirteen samples from these three deposits yielded a linear array in the isochron diagram with a higher correlation coefficient of 0.99719 and an isochron age of (14.39±0.22) Ma (1 σ error), suggesting an coeval event of the Cu mineralization and a shared source of ore materials for the Gangdese porphyry copper belt. Compared with the longer-lived felsic magmatic-hydrothermal system, the Cu mineralization is a relatively short event with duration of about 0.5 Ma, and usually occurs in the later-stage of the complicate magmatic system. The emplacement age of the Gangdese porphyries indicates that they intruded after the rapid rising of the Gangdese Mountains at 21—23 Ma, and formed in a post-collision crustal extension environment. Constraint of the NS-trending rifting zones (grabens) on localization of these porphyry bodies implies that the minor east-west stretching dates back to approximately 18 Ma. Highly precise Re-Os age of the Cu mineralization event indicates that a significant east-west extension has commenced at about (14±0.5) Ma ago, which is identical to that of the normal faulting in central Tibet (13.5 Ma) and southern Tibet (14 Ma). The strong extension not only provided dilatant conduits for the migration and emplacement of the porphyry magmas pooling near the base of lithosphere, but also resulted in the rapid rising of a large volume of volatile, thus in turn constraining the temporal-spatial localization of the Gangdese porphyry Cu belt.

Geochemical and geochronological constraints on the origin and emplacement of the Yarlung Zangbo ophiolites, Southern Tibet

Abstract The Indus-Yarlung Zangbo suture zone in southern Tibet marks the Eocene collision of the Indian continent and the Lhasa Block of Eurasia. It is characterized, particularly in its central portion, by an east-west belt of ophiolitic and related oceanic volcanic and sedimentary rocks that form a number of structurally juxtaposed geological terranes. Although tectonically disrupted in many places, almost complete ophiolite sequences exist at Luobusa and Zedong in the east and near Xigaze in the west. In Luobusa, the ophiolite sequence is thrust over the Tertiary molasse deposits of the Luobusa Formation or onto plutonic rocks of the Gangdese batholith. A mantle sequence dominates the ophiolite massif and consists chiefly of harzburgite and clinopyroxene-bearing harzburgite with abundant podiform chromitites enveloped by dunite. The Luobusa ophiolite formed the basement to an intra-oceanic volcanic arc, the Zedong terrane, which developed between the Mid-Jurassic and Mid-Cretaceous. Farther to the west, complete ophiolite sequences exist at Dazhuqu and near Xigaze. These ophiolites have suprasubduction zone geochemical signatures but there is no apparent development of a volcanic arc. Sensitive high-resolution ion microprobe U-Pb zircon analyses yield an age of 126 Ma for the crystallization of a quartz diorite from the Dazhuqu massif. Amphibolites that occur as large blocks in mélanges at the base of the ophiolites are considered to be remnants of dynamothermal metamorphic soles produced early in the ophiolite obduction process. Ar/Ar geochronology on amphibole and biotite separates from these rocks yields ages of 80 and 90 Ma, respectively, for this event, which is considered to have occurred as the Indian continental margin entered the intra-oceanic subduction zone. Continued northward subduction of the remaining portion of the Neo-Tethyan ocean floor beneath the southern margin of Eurasia produced the Gangdese continental arc on the southern margin of the Lhasa Block and led to the final closure of the ocean with the collision of India and Eurasia in the Eocene.

The Zedong Window: A record of superposed Tertiary convergence in southeastern Tibet

Determining the timing, magnitude, and location of deformation due to the Indo‐Asian collision is widely acknowledged as an important step in understanding how the lithosphere responds during continental collision. A puzzling result of geological investigations of the Lhasa Block over the past 2 decades has been the apparent lack of significant Tertiary deformation there. Perhaps the most important structural feature of the Lhasa Block is the south directed Gangdese Thrust System, which developed along its southern edge. The thrust system, which separates the Andean‐type batholith of southern Asia from rocks of Indian affinity, is obscured at most locations across southeastern Tibet by back thrusts of the younger, north directed Renbu Zedong Thrust System. The best documented site where both thrusts are exposed occurs near Zedong (Zedong Window). Systematic geochronologic analyses were conducted in this area. U‐Pb zircon dating of three samples of a synkinematically to postkinematically deformed hanging wall granitoid (the Yaja granodiorite) cut by the Gangdese Thrust indicates a crystallization age of 30.4±0.4 Ma (2σ), thus placing an upper bound on the initiation of the thrust. U‐Pb zircon dating of granitoid samples structurally higher in the Gangdese hanging wall yields emplacement ages of 42.5±1.0 Ma and 48.9±0.8 Ma (2σ), similar to other magmatic complexes within the Gangdese arc. Geochemical results are consistent with these plutons forming in the same environment as precollisional intrusions within the Gangdese batholith, suggesting a significant post‐collisional input of juvenile heat. The40Ar/39Ar thermochronology of samples from a vertical section through the hanging wall within the Yaja granodiorite, coupled with results of a numerical thermal model, indicate an average slip rate along the Gangdese Thrust of 7 mm/yr between 30 and 23 Ma and a minimum displacement of ∼50 km. Farther east in the Zedong Window, thermal effects produced by later north directed thrust sheets of the Renbu Zedong system appear to have obscured thermal history signatures in the Gangdese hanging wall related to earlier south directed thrusting. Thermochronological results from this region indicate that thermal overprinting related to the north directed thrusting occurred between 25 and 10 Ma, consistent with previous estimates.

New evidence II for MDD model of thermal evolution history of Gangdese batholith

The cooling curves are obtained using specific40Ar/39Ar stage heating procedure and MDD modeling. In addition to the results of dating hornblende (K-Ar) and apatite (FT), the widespread existence of rapid cooling events has further been confirmed. A new knowledge of the starting time of Gangdese overthrust faulting in the east of Zedang, which was earlier in south and later in north is acquired.

Tertiary deformation history of southeastern and southwestern Tibet during the Indo-Asian collision

Geologic mapping and geochronological analysis in southwest (Kailas area) and southeast (Zedong area) Tibet reveal two major episodes of Tertiary crustal shortening along the classic Indus-Tsangpo suture in the Yalu River valley. The older event occurred between ca. 30 and 24 Ma during movement along the north-dipping Gangdese thrust. The development of this thrust caused extensive denudation of the Gangdese batholith in its hanging wall and underthrusting of the Xigaze forearc strata in its footwall. Examination of timing of major tectonic events in central Asia suggests that the initiation of the Gangdese thrust was approximately coeval with the late Oligocene initiation and development of north-south shortening in the eastern Kunlun Shan of northern Tibet, the Nan Shan at the northeastern end of the Altyn Tagh fault, the western Kunlun Shan at the southwestern end of the Altyn Tagh fault, and finally the Tian Shan (north of the Tarim basin). Such regionally synchronous initiation of crustal shortening in and around the plateau may have been related to changes in convergence rate and direction between the Eurasian plate and the Indian and Pacific plates. The younger thrusting event along the Yalu River valley occurred between 19 and 10 Ma along the south-dipping Great Counter thrust system, equivalent to the locally named Renbu-Zedong thrust in southeastern Tibet, the Backthrust system in south- central Tibet, and the South Kailas thrust in southwest Tibet. The coeval development of the Great Counter thrust and the North Himalayan granite-gneiss dome belt is consistent with their development being related to thermal weakening of the north Himalayan and south Tibetan crust, due perhaps to thermal relaxation of an already thickened crust created by the early phase of collision between India and Asia or frictional heating along major thrusts, such as the Main Central thrust, beneath the Himalaya.

Crustal deformation of the Lhasa terrane, Tibet plateau from Project INDEPTH deep seismic reflection profiles

International Deep Profiling of Tibet and the Himalaya (INDEPTH) deep reflection data in the Yangbajain‐Damxung graben of southern Tibet yield evidence of magmatism and deformation beneath the southern Lhasa terrane and Yarlung‐Zangbo suture. Shallow reflections and low‐velocity first arrivals indicate a thin, Quaternary graben fill of generally less than a few hundred meters thickness. Underlying stratified reflections, extending to a maximum of about 11.5 km depth, likely originate from deformed Paleozoic‐Cenozoic supracrustal strata of the Lhasa terrane. A prominent, undulatory band of reflections within the crystalline basement extends beneath the length of the Yangbajain‐Damxung graben (depth ranges from ∼12 km to ∼18 km). This horizon has been interpreted to mark the top of a midcrustal partial‐melt zone underlying southern Tibet. The undulatory character of this horizon additionally suggests that it may be tectonically deformed. A somewhat deeper, subhorizontal, wide‐angle reflection extends southward beneath the outcrop of the Yarlung‐Zangbo suture, indicating that the suture is cut off or superposed by a younger structure at depth. Three gently north dipping reflections at 19 km, 24 km, and 27 km depth beneath the Gangdese batholith are suggestive of a midcrustal duplex and may mark the northward (downdip) extension of the late Oligocene‐early Miocene Gangdese thrust system. A prominent ∼40° north dipping reflection imaged in the deep crust beneath the Gangdese batholith, between 40 and 60 km depth, might mark the downdip expression of the Yarlung‐Zangbo suture or, alternatively, a younger reverse fault in the lower crust (or both). Viewed in aggregate, the reflection data are suggestive of moderate, postcollisional shortening of the upper crust of the Lhasa terrane, accompanied by melting of the middle crust. Although the data are permissive of wholesale underthrusting or fluid injection of Indian continental crust beneath the Lhasa terrane, they show no direct evidence for this having occurred.

Thermal evolution and slip history of the Renbu Zedong Thrust, southeastern Tibet

Cretaceous granitoids of the Gangdese batholith, southeastern Tibet, were overthrust by upper greenschist to epidote‐amphibolite facies Tethyan rocks derived from the Indian shelf along the north directed (∼30° dip) Renbu Zedong Thrust (RZT). Thermochronological results obtained from a NE‐SW transect near Lian Xian show evidence of thermal effects related to thrusting. Granitoids immediately beneath the RZT exhibit considerable recrystallization to greenschist facies assemblages. Biotite and K‐feldspar 40Ar/39Ar ages measured along the traverse into the footwall increase systematically away from the RZT. The timing of initial upward displacement of the RZT hanging wall is constrained to have occurred at ∼18 Ma from the hornblende ages and the two K‐feldspar samples closest to the fault. K‐feldspars up to 15 km from the thrust yield 9–12 Ma ages for the initial ∼20% of 39Ar release. Distal samples yield 40Ar/39Ar ages nearly as old as the 70–110 Ma ion microprobe 206Pb*/238U ages determined for coexisting zircons. We have integrated our thermal history results with numerical heat flow models and found that while reheating to 320–280°C at shallow (∼7 km) levels due to rapid (&gt;15 mm/yr) slip along the RZT at ∼10 Ma is capable of explaining the initial portion of the K‐feldspar age spectra, a prior common thermal history experienced by all samples cannot satisfactorily account for all the 40Ar/39Ar results. Instead, we find our thermal history constraints to be more completely explained by a numerical model in which (1) rocks currently at the surface originated from different depths,(2) footwall samples in close proximity to the RZT experienced fault drag from 19 to 15 Ma and (3) postthrusting denudation of the region involving localized tilting occurred at ∼10 Ma. The minimum average slip rate and displacement along the ramp during this period are 2 mm/yr and 12 km, respectively, but are likely to have been greater. The cooling episode recorded in all the K‐feldspar age spectra beginning at ∼10 Ma may either reflect denudation following regional uplift due to displacement along the ramp of the Main Himalayan Thrust or topographic collapse following cessation of RZT thrusting.

Thermal evolution of the Gangdese batholith, southern Tibet: A history of episodic unroofing

The Gangdese batholith, southern Tibet, was part of an Andean‐type arc at the southern margin of Asia prior to the collision of India and Asia at approximately 50 to 40 Ma. Fission‐track and 40Ar/39Ar analyses of 28 rocks from 10 Gangdese granitoid plutons along an ∼250 km length of the batholith in the Lhasa region provide a detailed understanding of the age and the postcrystallization erosional and tectonic history of these rocks. These data suggest a range of ages for these plutons of 94 to 42 Ma, with the majority being of Tertiary age. The postcrystallization cooling histories of all of these plutons are characterized by marked discontinuities. We conclude that most of these discontinuities, and all of them after 40 Ma, reflect tectonic changes that produced brief pulses of rapid erosion which were distributed in both space and time. In addition to the initial cooling of hot magma against cold country rock, all of the rocks we studied showed evidence for at least one subsequent episode of rapid cooling, dropping many tens of degrees in a few million years. Conversely, these plutons all experienced intervals during which they cooled very slowly or not at all; these slow‐cooling intervals lasted from 5 to 50 million years. Our data indicate that since the collision between India and Asia began, response to continued convergence has been quite variable in even this relatively small area. The data reported here are consistent with a recently proposed model of Oligo‐Miocene crustal shortening along the Gangdese Thrust system in this area.