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 (Ն50 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 collisiontype 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.

Geochemical and geochronological evidence was obtained from granitoids of the South Tianshan orogen and adjacent regions, which consist of three individual tectonic domains, the Kazakhstan-Yili plate, the Central Tianshan Terrane and the Tarim plate from north to south.

No abstract

The pressure-temperature (P-T) conditions for producing adakite/tonalite-trondhjemite-granodiorite (TTG) magmas from lower crust compositions are still open to debate. We have carried out partial melting experiments of mafic lower crust in the piston-cylinder apparatus at 10-15 kbar and 800-1,050 °C to investigate the major and trace elements of melts and residual minerals and further constrain the P-T range appropriate for adakite/TTG formation. The experimental residues include the following: amphibolite (plagioclase ? amphibole ± garnet) at 10-15 kbar and 800 °C, garnet granulite (plagioclase ? amphibole ? garnet ? clinopyroxene ? orthopyroxene) at 12.5 kbar and 900 °C, two-pyroxene granulite (plagioclase ? clinopyroxene ? orthopyroxene ± amphibole) at 10 kbar and 900 °C and 10-12.5 kbar and 1,000 °C, garnet pyroxenite (garnet ? clinopyroxene ± amphibole) at 13.5-15 kbar and 900-1,000 °C, and pyroxenite (clinopyroxene ? orthopyroxene) at 15 kbar and 1,050 °C. The partial melts change from granodiorite to tonalite with increasing melt proportions. Sr enrichment occurs in partial melts in equilibrium with \20 wt% plagioclase, whereas depletions of Ti, Sr, and heavy rare earth elements (HREE) occur relative to the starting material when the amounts of residual amphibole, plagioclase, and garnet are [20 wt%, respectively. Major elements and trace element patterns of partial melts produced by 10-40 wt% melting of lower crust composition at 10-12.5 kbar and 800-900 °C and 15 kbar and 800 °C closely resemble adakite/TTG rocks. TiO 2 contents of the 1,000-1,050 °C melts are higher than that of pristine adakite/TTG. In comparison with natural adakite/ TTG, partial melts produced at 10-12.5 kbar and 1,000 °C and 15 kbar and 1,050 °C have elevated HREE, whereas partial melts at 13.5-15 kbar and 900-1,000 °C in equilibrium with [20 wt% garnet have depressed Yb and elevated La/Yb and Gd/Yb. It is suggested that the most appropriate P-T conditions for producing adakite/TTG from mafic lower crust are 800-950 °C and 10-12.5 kbar (corresponding to a depth of 30-40 km), whereas a depth of [45-50 km is unfavorable. Consequently, an overthickened crust and eclogite residue are not necessarily required for producing adakite/TTG from lower crust. The lower crust delamination model, which has been embraced for intra-continental adakite/TTG formation, should be reappraised.

No abstract

No abstract

No abstract

Traditionally the Chinese South Tianshan has been regarded as a late Paleozoic orogenic belt. However, little is known about the early Paleozoic tectonic architecture of the region. This paper presents the first evidence of Cambrian-Ordovician MORB-type basalts and adakitic diorites on the southern margin of the Yili plate in China. Basalts from Xiate in southwestern Tianshan show a typical transitional (T-) MORB and ferrobasalt composition, which indicate a formation at a propagating spreading ridge. The basalts give a weighted mean 206 Pb/ 238 U crystallization age of 516.3 ± 7.4 Ma by SHRIMP U-Pb zircon dating and have experienced contact metamorphism due to the intrusion of a dioritic pluton. The dioritic pluton has a weighted mean 206 Pb/ 238 U crystallization age of 470 ± 12 Ma and geochemical characteristics resembling that of adakitic rocks. The pluton is considered to have been formed by partial melting of garnet amphibolites from thickened lower crust in arc or continental collision settings. The basalts and diorites are considered to outline the eastern extension of the early Paleozoic suture zone, the Nikolaev Line, which stretches east-west for hundreds of kilometers between the Northern Tianshan and Central Tianshan terranes of Kyrgyzstan. Our findings substantiate that the Yili and Central Tianshan plates were separated by the early Paleozoic Terskey ocean. The Terskey ocean probably closed during the early stage of the late Ordovician (Lomize et al. in Geotectonics 31 (6): [463][464][465][466][467][468][469][470][471][472][473][474][475][476][477][478][479][480][481][482] 1997), resulting in the final amalgamation of the Yili and Central Tianshan plates. Consequently, an early Paleozoic suture zone is documented in the Chinese Tianshan region, which is most likely represented by the North Nalati fault.