The pairing relationship between ophiolite and arc volcanic rocks along western Jinsha River suture zone and its geological significance: Evidence from geochemistry and LA-ICP-MS zircon U-Pb dating

A study of the field section of Duocai ophiolites mélange and Dangjiangrong volcanic rocks in Zhiduo area of western Jinsha River suture zone shows that ophiolites are mainly composed of gabbro, cumulate gabbro and basalt with the lack of the mantle peridotite unit. XRF tests of fine-grained gabbro and basic lava and LA-ICP-MS zircon U-Pb dating of cumulate gabbro from systematic sampling of ophiolite mélange show that basic lavas can be divided into two series, i.e., OIB type and MORB-IAT transitional type. The first series is not the member of ophiolite, and was involved in the mélange belt by tectonic emplacement. The second series is a transitional type and has the special geochemical characteristics of MORB-IAT ophiolites. The gabbros have obvious "TNT" slot in primitive mantle-normalized distribution patterns, evidencing that it has the "signal of island arc" feature; the gabbros, together with the second series of basic lava, belong to the ophiolite. The single zircon U-Pb age of cumulate gabbro is 252.5Ma±0.58Ma, which can be interpreted as the formation age of ophiolites. Duocai ophiolites and Dangjiangrong arc volcanic rocks are consistent well in geochemistry with each other, their formation age and a comparative study of their other aspects indicate that they have the paring relationship. Combined with the trench-arc-basin tectonic framework in the orogenic belt, the authors come to the conclusion that the Duocai ophiolites might have been formed in a fore-arc tectonic setting apart from the arc and close to the trench, resulting from the continual subduction of the Jinsha Ocean at the end of Permian period.