Mesozoic and cenozoic volcanic rocks from central and southern Tibet: 39Ar- 40Ar dating, petrological characteristics and geodynamical significance

Abstract

Field relations, radiometric ages and mineralogical and chemical characteristics of the orogenic volcanic rocks from the Lhasa block (Tibet) are studied. Volcanic activity started before the deposition of the Albian-Aptian sedimentary units (Takena Formation and Xigaze Group). Volcanism is volumetrically limited in the northern part of the Lhasa block: basalts, andesites and dacites are present while ignimbrites are scarce. Radiometric ages of these rocks range between 110 and 80 Ma. Conversely, in the southern part of the Lhasa block, volcanism is largely developed (Gangdese belt). There, paroxysmal activity occurred later than in the north, between 60 and 50 Ma (Lingzizong Formation). This volcanism is mainly represented by andesites and ignimbrites. A younger episode at about 15-10 Ma took place in this southern region. The mineralogical and geochemical characteristics indicate that, as a whole, the volcanic rocks from the Lhasa block belong to a calc-alkaline continental-margin series, enriched in K 20 and other incompatible elements. Subordinate potassic alkaline lavas are present in the northern margin of the Gangdese belt; some basalts exhibit a tholeiitic affinity (Daquin area, southern region). The geodynamic significance of these Cenozoic volcanic rocks lies in the relative motion of India and the Lhasa block, after accretion of the latter to the northern Qantang block. A geodynamic model based on changes of the type of subduction of the Indian oceanic crust beneath the Lhasa block, for the period 120-40 Ma, is proposed to account for the abundance, age, and magmatic affinity of the volcanic products.