POST-COLLISIONAL ULTRAPOTASSIC ROCKS IN LHASA BLOCK, TIBETAN PLATEAU: SPATIAL AND TEMPORAL DISTRIBUTION AND ITS' IMPLICATIONS

Abstract

Study of the postcollisional volcanics in Lhasa block in southern Tibet is one of the most important progresses in the Tibetan plateau in the past 10 years. This paper presents new 40Ar/39Ar dating results on biotites and sanndines of the ultrapotassic rocks from Dangreyongcuo and Xurucuo areas in central Lhasa block. The Dangreyongcuo trachyte yields a biotite isochron age of 13.2 ± 0.3Ma, and three sanidine isochron ages of 13.0 ± 0.3Ma, 13.7 ± 0.3Ma and 13.0 ± 0.3Ma, respectively. The pyroxene trachyte of Xurucuo volcanics yields a biotite isochron age of 11.2 ± 0.3Ma. The distribution feature of the ultrapotassic and potassic rocks in Lhasa block is discussed based on our Ar-Ar data, plus other literature ages. The ultrapotassic rocks erupted during the time between 25Ma and 8Ma ago. They are spatially limited to the west of Longitude E87 degree. These rocks erupted tectonicly related strongly to the N-S trending normal faults system, Cenozoic basins or N-S trending lakes. Their ages overlaped the time spans of the extension-related N-S trending normal faults system, mafic dike intrusive, adakatic intrusive and other tectonic events, which suggets that a lithepheric delamination in southern Tibet should have played an important role for all these united processes. The ultrapotsssic magmatism occurred synchronously both in Lhasa and Qiangtang blocks implies that they should have similar lithospheric structure and composition beneath these two tectonic units.