Early cretaceous ophiolites of the Yarlung Zangbo Suture Zone: insights from dolerites and peridotites from the Baer upper mantle suite, SW Tibet (China)

The Baer ophiolitic massif is located in the northern sub-belt of the western segment of the Yarlung Zangbo Suture Zone (YZSZ) and mainly consists of a lherzolite-dominant mantle suite, dolerite intrusions and limited crustal outcrops. The dolerites show sub-ophitic texture and light rare earth element-depleted chondrite-normalized rare earth element patterns similar to normal-mid-ocean ridge basalts (N-MORB); though, they display enrichments in fluid-mobile elements (Rb, Ba, and Sr) and marked depletions in Th and Nb. The U-Pb ages of several magmatic zircon grains recovered from two dolerite samples indicate that the intrusion of the dikes into the Baer lherzolitic mantle occurred at 125.6-126.3 Ma, consistent with the widespread mafic magmatism between 120 and 130 Ma in the Yarlung Zangbo ophiolites. The dolerites have slightly more radiogenic Sr-87/Sr-86 ratios (0.7043-0.7054) in comparison to N-MORB, whereas they show Nd-143/Nd-144 values (0.513067-0.513114) similar to N-MORB and high zircon Hf-isotope compositions. They have a limited range of Nd-isotope (epsilon(Nd)(t) values: + 8.2 to + 9.1) and juvenile Hf-isotope compositions (epsilon(Hf)(t) values: + 8.4 to + 14.2 and + 10.0 to + 15.1) indicating derivation from mantle melts. The moderate spread in the epsilon(Hf) (t) values of zircons indicates derivation of the dolerites parental magma from a weakly contaminated spinel-bearing mantle source. This is also corroborated by the geochemical signatures of the Baer dolerites (enrichment in LILE and depletion in HFSE) suggesting minor slab input to the mantle source of the dike-filling melt. We suggest that the genesis of the dolerite dike-forming melt happened at a stage of subduction initiation in a sub-oceanic mantle domain mildly affected by fluids emanating from the downgoing slab. Our data combined with literature data allow us to presume that the intrusion of the dolerites into the Baer mantle corresponds to an early phase of subduction initiation beneath a developing forearc basin.