Abstract Young volcanic centres of the Bransfield Strait and James Ross Island occur along back-arc extensional structures parallel to the South Shetland island arc. Back-arc extension was caused by slab rollback at the South Shetland Trench during the past 4 myr. The variability of lava compositions along the Bransfield Strait results from varying degrees of mantle depletion and input of a slab component. The mantle underneath the Bransfield Strait is heterogeneous on a scale of approximately tens of kilometres with portions in the mantle wedge not affected by slab fluids. Lavas from James Ross Island east of the Antarctic Peninsula differ in composition from those of the Bransfield Strait in that they are alkaline without evidence for a component from a subducted slab. Alkaline lavas from the volcanic centres east of the Antarctic Peninsula imply variably low degrees of partial melting in the presence of residual garnet, suggesting variable thinning of the lithosphere by extension. Magmas in the Bransfield Strait form by relatively high degrees of melting in the shallow mantle, whereas the magmas some 150 km further east form by low degrees of melting deeper in the mantle, reflecting the diversity of mantle geodynamic processes related to subduction along the South Shetland Trench.

ABSTRACT High-resolution sand petrography and heavy mineral analyses help to frame U-Pb age and Hf isotope data from zircon grains, integrated in turn with geochemical data from detrital apatite, rutile, garnet, and monazite, and with Raman spectroscopy data from detrital amphibole, pyroxene, and epidote-group minerals. This multitechnique approach, including stream-profile analysis, was used to characterize components of the sediment flux and define erosion patterns across the Lhasa block, a complex continental arc terrane caught in the Himalayan collision. Litho-feldspatho-quartzose detrital modes and hornblende-dominated heavy mineral assemblages suggest that the majority (four fifths) of the sand bed load in the Lhasa River catchment is derived from erosion of granitoid batholiths. Gravel composition, however, is markedly different and dominated by volcanic pebbles in the trunk river, as in all of its four major tributaries, testifying to an order-of-magnitude difference in apparent erosion rates between granitoid batholiths and arc lavas. This marked contrast, partly explained by wide exposures of granitoid rocks in the rugged Nyainqêntanglha Range characterized by active incision, is notably amplified by the high sand-generation potential of granitoid rocks, which, in contrast to dense joint blocks of andesitic lavas, tend to disintegrate to sandy grus upon weathering. Sedimentary strata, making up a good half of exposed rocks, are also underrepresented in sand bed load, suggesting selective mechanical breakdown of nondurable shale/slate grains. This exposes a serious bias affecting estimates based on sand only, and it highlights the necessity for taking into account the entire size spectrum from mud to gravel in order to improve the accuracy of sediment budgets. Provenance analysis should involve multiple methods applied to multiple minerals, rather than be based solely on a single rare mineral, even if it is exceptionally laden with potential provenance information, such as zircon. We here divide arc-derived suites into those eroded from undissected arcs, in which nearly continuous volcanic cover is present, and those from dissected arcs, in which cogenetic plutons are widely exposed from erosional unroofing. —Dickinson and Suczek (1979, p. 2175)

Abstract: The Early Permian (290 Ma) Panjal Traps are the largest contiguous outcropping of volcanic (basaltic, andesitic and silicic) rocks within the Himalaya that are associated with the Late Palaeozoic break-up of Gondwana. The basaltic Panjal Traps have compositional characteristics that range from continental tholeiite to ocean-floor basalt but it is clear that crustal contamination has played a role in their genesis. The basalts that show limited evidence for contamination have Sr–Nd isotopes ( 87 Sr/ 86 Sr i = 0.7043–0.7073; ε Nd ( t ) = 0 ± 1) similar to a chondritic (subcontinental lithospheric mantle) source, whereas the remaining basaltic rocks have a wide range of Nd ( ε Nd ( t ) = −6.1 to +4.3) and Sr ( 87 Sr/ 86 Sr i = 0.7051–0.7185) isotopic values. The primary melt composition of the low-Ti Panjal Traps is picritic with mantle potential temperatures ( T P = 1400°C to 1450°C) similar to ambient mantle. The silicic volcanic rocks were derived by partial melting of the crust, whereas the andesitic rocks were derived by mingling between crustal and mantle melts. The Panjal Traps initially erupted within a continental rift setting. The rift eventually transitioned into a nascent ocean basin that led to seafloor spreading and the formation of the Neotethys Ocean and the ribbon-like continent Cimmeria.