First report of Lower Permian basalts in South Tibet: tholeiitic magmatismduring break-up and incipient opening of Neotethys

The Neo-Tethys Ocean began to form at Early Permian times, when continental flood basalts were emplaced in various areas of the newly-formed Indian pa ssive margin, exposed today in the so-called Tibetan Sedimentary Zone of th e Himalaya. Lower Permian mafic volcanic rocks, which have long been known from various Himalayan localities from Kashmir to Arunachal Pradesh, are he re for the first time reported to occur also in South Tibet (Bhote Kosi Bas alts of the Gyirong County). The basalts unconformably overlie lowermost Pe rmian diamictites, with locally intervening black shales and debris flow de posits, and are followed in turn by chert-bearing quartzarenites and silty to phosphatic marls yielding brachiopods of Roadian-Wordian age. The age of the lavas can thus be bracketed as late Early Permian (post-Sakmarian and pre-Roadian). The geochemistry of these subalkalic tholeiites, akin to MORB s, testifies to their similarity not only with the adjacent Nar-Tsum Spilit es of central Nepal, but also with the Panjal Traps and Abor Volcanics of t he western and eastern Himalayas respectively. The geochemical signature of Lower Permian volcanic rocks is in fact unifor m all along the Himalayan Range, and markedly different from that of basalt ic-rhyolitic alkalic products sporadically emplaced during the previous rif ting stage. Rift volcanism in the Tethys Himalaya began in the Early Carbon iferous and came to an end in Sakmarian times. In the Early Permian, initia l submergence of the rift shoulders and sediment starvation were followed b y tholeiitic magmatism, which is therefore interpreted as following break-u p and incipient sea-floor spreading in the Neotethys Ocean. Roughly contemp oraneous emplacement of continental flood basalts of similar geochemical si gnature along a 2000 km long rift axis would in fact suggest extensive mant le melting at the transition from continental rifting to break-up and openi ng of the Neotethys between Northern Gondwana and the Peri-Gondwanian block s. (C) 1999 Elsevier Science Ltd. All rights reserved.