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Past geodynamic events can be reconstructed using the spatial and temporal patterns of geochemical variability in igneous rocks. Here we apply this principle to define the timing and geometry of breakoff of the Neo-Tethyan slab after the India-Asia collision. Two episodes of mafic magmatism (∼53 Ma and ∼ 48 Ma) are preserved in the Quxu area of the southern Lhasa terrane (SL). We present their whole-rock geochemistry, zircon U-Pb ages, and in situ zircon Hf isotope ratios. The early gabbro (∼53 Ma) exhibits typical arc basalt characteristics, including enrichment of large-ion lithophile elements (LILEs), depletion of high-field strength elements (HFSEs), and radiogenic Nd and zircon Hf isotopes, implying that they were formed by partial melting of metasomatized lithospheric mantle. In comparison to the early gabbro, the late hornblende gabbro (∼48 Ma) contains higher contents of HFSEs (e.g., Nb, Zr) and relatively less radiogenic Nd and zircon Hf isotopes, suggesting interaction between the enriched sub-continental lithospheric mantle and asthenospheric mantle. It is likely that upwelling asthenosphere reached the central SL around 50 Ma and that the Neo-Tethyan slab tore off under this area at this time. The spatial and temporal distribution of mafic rocks across the whole southern Lhasa terrane show that this geochemical transition and hence, presumably, the tearing off of the Neo-Tethyan slab, propagated eastward along the collision zone throughout Paleocene-Eocene time.

Cathodoluminescence imaging, combined with SHRIMP U–Pb dating, demonstrates that zircons from ore-bearing porphyries of the Gangdese porphyry copper belt on the southern Tibetan Plateau are composed of inherited, metamorphic and magmatic zircon with ages of 41.5 to 62.5 Ma, 17.69 to 26.0 Ma and 11.39 to 17.20 Ma, respectively. LA ICP-MS analysis also shows that the inherited zircons are characterized by high concentrations of Y (1121 ppm), HREE (641 ppm) and MREE (182 ppm), and low concentrations of U (208 ppm), Th (171 ppm) and Hf (0.96%). Compared with the inherited and magmatic zircons, the metamorphic zircons have markedly lower Th/U ratios (average = 0.54). Although the three types of zircons all have negative Eu anomalies and positive Ce anomalies, the magmatic zircons display much larger variations in Ce anomalies. Importantly, these zircon types correspond to three events during the evolution of the Gangdese collisional orogen that allow us to propose a tectonic model. During the India–Asia collision (50–60 Ma), mantle-derived mafic magmas were underplated, forming a dense crustally-contaminated mafic to ultramafic layer at the base of the crust. At about 21 Ma, asthenospheric upwelling resulted in partial melting of the underplated mafic rocks under granulite facies conditions and rapid uplifting of the southern Plateau. Concomitantly with the extensional crustal collapse after the uplift at about 15 Ma, the ore-bearing magmas were emplaced, forming porphyry copper deposits.

The Jinshajiang orogenic belt (JOB) of southwestern China, located along the eastern margin of the Himalayan–Tibetan orogen, includes a collage of continental blocks joined by Paleozoic ophiolitic sutures and Permian volcanic arcs. Three major tectonic stages are recognized based on the volcanic–sedimentary sequence and geochemistry of volcanic rocks in the belt. Westward subduction of the Paleozoic Jinshajiang oceanic plate at the end of Permian resulted in the formation of the Chubarong–Dongzhulin intra-oceanic arc and Jamda–Weixi volcanic arc on the eastern margin of the Changdu continental block. Collision between the volcanic arcs and the Yangtze continent block during Early–Middle Triassic caused the closing of the Jinshajiang oceanic basin and the eruption of high-Si and -Al potassic rhyolitic rocks along the Permian volcanic arc. Slab breakoff or mountain-root delamination under this orogenic belt led to post-collisional crustal extension at the end of the Triassic, forming a series of rift basins on this continental margin arc. Significant potential for VHMS deposits occurs in the submarine volcanic districts of the JOB. Mesozoic VHMS deposits occur in the post-collisional extension environment and cluster in the Late Triassic rift basins.

FIGURE 1. Geological distribution map for basalts from Sangxiu Formation in the central segment of Tethyan Himalayas. a. Tectonic location (modified from Mahoney et al. 2002). b. Distributions (after Pan Gui-tang et al. 2004, in press). c. Geological map for studied area (Regional Geological report (1: 250, 000) for Luoza County, 2003, unpublished) Sangxiu Formation, which is only distributed in the Yangzuoyong Tso area (south to Lhasa, Figure 1b), is a special stratum unit that is mainly characterized by volcanic interlayers. Tectonically, it is located in the eastern part of central segment of Tethyan Himalayas, and paleogeography belongs to the northern margin of Greater India. Here one finds not only the southern transitional part of space-time framework of the central-eastern Yarlung Zangbo ophiolitic melange zone, but also the supposedly the extended end-point of Ninetyeast Ridge (Figure 1a). The age of the Sangxiu Fm is from Late Jurassic to Early Cretaceous, which is constrained by fossils, though precise isotopic age of basaltic magmatic effusion is not known at present. Regionally, the Rajmahal traps on eastern Indian margin are explained by the consequences of early activity of the Kerguelen hotspot (Kent et al. 2002), and the track of Kerguelen hotspot since 120 Ma was interpreted from Rajmahal traps via Ninetyeast Ridge to the Kerguelen hotspot (O’Neill et al. 2003). If the early volcanic activity of the Kerguelen hotspot is extended much farther to the north and northeast, can this hotspot influence the volcanic activity in Tethyan Himalayas− the northern margin of Greater India during Late Jurassic to Early Cretaceous? The goal of this paper is to show our recent work on the geochemical and isotopic compositions and to discuss the characteristics of magmatic source, especially to shed light on the petrogenetic processes and to suggest a possible genetic relationship with Kerguelen hotspot in the light of systemically petrological and geochemical data (including REE, incompatible trace element and isotopic data) for basalts from Sangxiu Fm in Tethyan Himalayas. Stratigraphy and petrography The underlying stratum of Sangxiu Fm is the Late Jurassic Weimei Fm., which is mainly consists of quartz sandstones and dark grey argillaceous rocks with high content of quartz, shallow water ripple marks and relic fossils, is thought to the sedimentary records of shore-shallow sea (Jiang et al. 2003). The overlying stratum is the Early Cretaceous Jiabula Fm. that is characterized by accumulates of collapse and turbid sediments. The lower parts of Sangxiu Fm are mainly composed of apogrites, conglomerates, the middle parts are mainly basalts and shales gripped, and the upper parts are aleuritic shales and calcipulverite lens gripped. The massive, amygdaloidal, pillow basalts can be observed in different outcrops. The phenocrysts (about 5-10%) in basalts are mainly consist of plagioclase and pyroxene, which have experienced variable degree of alteration and metamorphism, however, the original textures appear to have been preserved; the plagioclase groundmass also underwent partly alteration; the accessory mineral assemblages include ilmenite, titanite and magnetite.

FIGURE 1. The distributions of peraluminous granites in Tibet IBaingoin-Baxoila Ling belt IICoqen-Xainza belt IIISouth Gandise belt IVYarlung Zangpo suture VLhagoi Kangri belt VInorthern Himalayan belt VIIHigh Himalayan belt; 1~58 rock number There are many outcrops of peraluminous granites in QinghaiTibet plateau. It is an important format of stronger magmatic activity. The Himalayan and Gandise belts are the famous for research bases of peraluminous granites. In the Himalayan belt, muscovite granite provide petrological evidence that India subducted northward below the Tibetan Continent (Deng Jinfu et al. 1994). Based on summary of past research data, the goal of present paper is to study temporal-spatial distribution and basic character of peraluminous granites in Tibet, and to discuss the implication of peraluminous granites to Eurasia– India collision and the uplift of Tibetan Plateau.