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Two types of K-rich magma of Eocene to Early Oligocene (ca. 40–30) and Plio-Pleistocene (ca. 5–0.1 Ma) age were emplaced prior to and following left-lateral slip on the Ailao Shan-Red River (ASRR) fault, a regional shear zone extending between southwest China and the Tonkin Gulf (South China Sea) that accommodated ‘escape’ of the Indochina block. The first type is exposed in the Dali-Lijiang and adjacent regions of western Yunnan and Sichuan and comprises ultramafic potassic to ultrapotassic ‘absarokites’ and their shoshonite, banakite, and SiO 2 -rich derivatives which were emplaced immediately prior to activation of the ASRR fault. They are characterized by high Mg.-nos, and low contents of fusible oxides (FeO*, CaO, Al 2 O 3 ), for equivalent MgO content, and pronounced primitive mantle-normalized high-field strength element (HFSE) depletions. In contrast, ‘post-escape’ K-rich magmas were erupted in the Puer, Maguan-Pingbian regions of south and southeast Yunnan. Apart from their relative enrichments in potassium they show typical HFSE-rich intra-plate compositional affinity. Geological and geomorphic evidence, and thermochronologic age dating of metamorphisc events, suggest that left-lateral shearing occurred between ca. 30 and 17 Ma; thereby accommodating the southeastward ‘escape’ of Indochina and (possibly) two episodes of spreading in the South China Sea. The southwestern part of Dali-Lijiang magmatic products was detached and offset by ca. 600 km and are now located in Phan Xi Pang in northern Viet Nam. The same is true for the Permo-Triassic Emeishan flood basalts, whose western exposures were likewise displaced by the same amount and are now represented by the Song Da complex, also in northern Viet Nam. Here, we report geochemical, isotopic, and 40 Ar/ 39 Ar age data for samples from both the ‘pre-escape’ Dali-Lijiang magmas and the ‘post-escape’ K-rich Puer, Maguan-Pingbian basalts and basanites, with a view to comparing and contrasting their interpolated source compositions, estimated conditions of upper mantle melt segregation and, by inference, their mantle dynamic and contamination histories insofar as these were conditioned by the India-Asia collision. Our interpretations yielded two complementary conclusions. The first contends that the pre-escape magmas result from adiabatic melting of crust-contaminated asthenosphere comprising a ‘melange’ of continental lithospheric mantle (CLM) (hydrated by sab-derived hysdrous fluids released at 0.2–0.5 GPa) and lower crust, delaminated from the overriding plate during mantle wedge corner flow and further enriched by metasomatic melts of subducted continental crust. We postulate that incipient H 2 O-saturated melting of the ‘melange’ occurs at depths of between ca. 100 and 200 km after being ‘dragged’ down by relict oceanic slab fragments, in response to the dehydration of supra-subduction amphibole- and phlogopite. The ensuing viscosity ‘crisis’ and buoyancy relative to ambient ‘fertile’ convecting mantle of such asthenospheric ‘pockets’, and the collision-related change from lithospheric compression to extension, almost certainly predisposes such a refractory yet crust-contaminated ‘pockets’ to rapid adiabatic melting. The second conclusion concerns the post-escape K-rich basalts and basanites and is based on the contention that decompression melting of thermally anomalous K-rich asthenospheric occurred in response to regional post-escape transtension, concomitant with the cessation Indochina escape and contiguous seafloor spreading. However, although these magmas share the HFSE-rich fertile source character of other, widely dispersed, post-escape Cenozoic basalts they more specifically resemble relatively rare examples of intra-plate, K-rich activity observed in northeast China, central Spain, and elsewhere in Asia and Europe, arguably (indirectly) reflecting mantle perturbations caused by major continental collisions.

article i nfo Ophiolite plays a key role in identifying paleo-ocean and paleo-plate and rebuilding the evolutionary history of ancient orogen. Mafic-ultramafic rocks are distributed in a broadly E-W direction in the middle of the Qiangtang terrane, northern Tibetan plateau. However, interpretation of these rocks as ophiolite and subsequent tectonic implications have been much disputed, and they were regarded by some to have an origin in a continental rift set- ting. A detailed zircon dating and geochemical and Sr-Nd-Hf isotopic study of this suite of rocks has been under- taken in order to clarify this important issue. The mafic-ultramafi cs uite was collected from the Gangma Co and Guoganjianian localities and is composed of cumulate and isotropic gabbro, basalt, actinolitite and plagiogranite. All basaltic rocks are tholeiitic and have low rare earth element (REE) abundances with variable REE patterns and slight negative Nb and Ti anomalies. These features are comparable with those of normal mid-ocean ridge ba- salts (N-MORB) and/or enriched mid-ocean ridge basalts (E-MORB). The positive whole-rock eNd(t) and zircon eHf (t) values indicate that these rocks were derivedfrom a long-term depleted mantle source. Zircon U-Pb dating using a sensitive high-resolution ion microprobe (SHRIMP) on two cumulate gabbros and two plagiogranites sam- ples yielded Carboniferous ages of 357±2.5 Ma, 356.1±3.0 Ma, 354.7±4.7 Ma and 345.4±4.6 Ma. The ophiolite marks a Paleo-Tethys Ocean basin in middle of the Qiangtang terrane, and it is interpreted as the western exten- sion of the Changning-Menglian Paleo-Tethys ophiolite in the eastern margin of the Tibetan plateau.

Felsic magmatic rocks of Triassic ages occur on the north side of the Qiangtang metamorphic belt. Zircon dating using the SHRIMP and LA-ICP-MS techniques reveals that these rocks were emplaced between 223 and 205 Ma. The emplacement time is similar to the Ar–Ar ages of phengites and sodic amphiboles of the eclogites and blueschist from the Qiangtang metamorphic belt. The felsic magmatic rocks can be divided into two groups with distinct geochemical and Sr–Nd–Hf isotopic compositions. The first and older group (223–219 Ma) includes diorite from the Baohu area and some volcanic rocks from the Juhuashan area. These rocks have high Sr (291–1367 ppm) and low Y (5.5–13.9 ppm) contents with high Sr/Y ratios (28–117). They are characterized by slightly negative eNd(t) values (−1.9 to −5.7), positive eHf(t) values (+3.2 to +5.4) and young zircon Hf model ages (644–729 Ma). The second and younger group (215–210 Ma) comprises volcanic rocks from the Jiangai and Guoganjianian areas. They show high initial 87Sr/86Sr ratios (0.708–0.714), distinct negative eNd(t) (−9.6 to −7.9) and eHf(t) values (−15.3 to −10.1) and much older bulk-rock Nd model ages (1658–2178 Ma) and zircon Hf model ages (1251–2012 Ma). Geochemical arguments led us to suggest that the rocks of the first group were generated by the melting of subducted oceanic crust and associated sediments, and the second group rocks were derived from anatexis of the Proterozoic continent crust. In conclusion, the Late Triassic magmatic rocks from the northern Qiangtang have recorded the process from subduction to break-off of an oceanic slab. The Qiangtang metamorphic belt to the south represents a northward oceanic subduction zone.