Zircon U-Pb dating and geochemistry of mafic dike swarms in the Lhozag area, southern Tibet, China, and their tectonic implications.

Abstract： Distributed widely in the Lhozag area, southern Tibet, the nearly E-W-trending mafic dike swarms are mainly intruded into Late Jurassic-Early Cretaceous and earlier strata. The U-Pb dating of zircons from dike swarms yielded an age of 138.0±3.5 Ma. The dike swarms are mainly classified into two types. One type is low in K (K2O=0.06-0.54%) and total REE content (∑REE=58.95-115.5×10-6) with weak fractionation of LREE and HREE (La/Yb)N=0.23-2.94 and depletion of LREE. By contrast, the other type is rich in K (K2O=1.22-1.67%) and total REE content (∑REE=199.97-381.47×10-6) and has strong fractionation of LREE and HREE (La/Yb)N=6.57-11.5, exhibiting LILE enrichment and HFSE depletion. Geochemical study shows that these dike swarms were intrusions of continental within-plate magmas with the characteristics of continental tholeiitic basalt and may have been derived from the depleted asthenospheric mantle and enriched lithospheric mantle. They are the product of large-scale spreading of the Neo-Tethys ocean in its late stage. The dike swarms indicate that the passive margin of the Himalaya was in a tectonic environment of strong extension and splitting and a dynamic setting of lithospheric extension and thinning and asthenospheric upwelling during the Late Jurassic-Early Cretaceous.