No abstract

Formation of andesitic rocks is crucial to understand the evolution of continental crust.Here we present an integrated study for the Paleocene Dianzhong Formation of the Linzizong volcanic succession (LVS) in the Lhasa terrane, southern Tibet. The andesites of the Dianzhong Formation have relatively high Mg # (38-60) and exhibit typical subduction-like geochemical signatures. They are characterized by slightly enriched ε Nd (t) values (−2.82-0.35) and have a range of δ 98 / 95 Mo isotopic compositions (−0.40 to 0.13 ‰) larger than mid-ocean ridge basalt. Their high Mg # , moderate light rare earth element-enriched patterns, and slightly enriched ε Nd (t) values clearly rule out an origin of continental crust-derived melts or partial melts of earlier underplated mafic rocks. The dacites show similar geochemistry to the andesites, indicating an origin from the parental magmas of the andesites. Based on the geochemical comparisons between the Dianzhong andesites and rocks of mélange, we suggest that the andesitic rocks of the LVS were derived from partial melts of a mantle source dominated by mélange that formed along the subduction channel. The mélange diapir melting was triggered by asthenospheric upwelling and corner flow during the Paleocene slab roll-back of northward subducted Neotethyan oceanic crust. Because the Early Paleocene andesitic rocks in southern Lhasa show an average composition of continental crust, the partial melting of mélange rocks most likely provide an important mechanism for the production of andesitic magmas within continental arcs.

The North Qilian Belt in northwestern China contains a large number of ophiolites. Most ultramafic rocks from the ophiolites have been entirely transformed into serpentinites because of extensive serpentinization, making it difficult to determine their origin and tectonic significance and further hindering advancements in the understanding of the evolutionary history of the Palaeozoic North Qilian Ocean. In this study, we present new mineral chemistry and whole‐rock geochemistry analyses (including rare earth elements, [REE]) for the serpentinites from the Bianmagou ophiolite to discuss their protoliths, geodynamic environment, melt extractions, and melt/fluid interactions. The mineral chemistry of the relic minerals (e.g., olivine, orthopyroxene, and spinel) and the whole‐rock geochemical compositions (e.g., Mg#, Ni, etc.) strongly demonstrate that the protoliths of the serpentinites were refractory mantle residues after melt extractions and probably emplaced into a tectonic regime of mid‐ocean ridge akin to abyssal peridotites. They experienced high degrees of partial melting (15–20%) recorded by spinel Cr# values and bulk incompatible elements (e.g., Ti, heavy rare‐earth element), resulting from multiple episodes of melt depletion in the lithospheric mantle. Moreover, the serpentinites commonly have U‐shaped chondrite‐normalized REE patterns. The significantly elevated LREEs probably reflect the early partial‐melting processes overprinted by later melt‐dominated metasomatism. The medium to strong enrichment of certain fluid‐mobile elements (e.g., U, Pb, Ba, Sr, As, etc.) in the studied serpentinites was a consequence of the fluid/rock interactions during serpentinization. Overall, despite the variable serpentinization they experienced, this study indicates that the mineral chemical and geochemical proxies of serpentinites provide reliable constraints on the petrogenesis and mantle metasomatism of ancient ophiolites.

No abstract

No abstract