特提斯喜馬拉雅多重基性巖漿事件：追溯新特提斯洋的生存時限

Supported by the SinoProbe Project (SinoProbe-2-6) and National Natural Science Foundation of China (41073024, 41273034).

E-mail: zls1970@gmail.com.

Processes accompanied the breakup of continents,spreading of ocean floor and continent-ocean transition could trigger large scale melting of the mantle beneath the continent as well as the ocean, and produce mafic magmas with distinct geochemical characteristics. Such rocks provide us an important record for unraveling the nature and the time of deep tectonic and magmatic processes during the tectonic evolution of large-scale orogenic belts, such as the Himalayan orogenic belt. As an integrated part of the Himalaya,the Tethyan Himalaya consists of well-developed early Paleozoic to Cenozoic sediments and is noted for development of spectacular semi-continuous, thousand kilometers long gneiss (or granitic) domes. It has preserved critical records to address the nature of deformation, magmatism, and metamorphism associated with the opening, spreading, and demise of the Neo-Tethyan Ocean and the final continental collision between the Indian and the Eurasian Plate at the early Cenozoic time. In addition, it also could be a type-example to address a number of first-order issues with regard to the tectonic dynamics of passive continental margin during the Wilson-cycle.

Recent systematic field observations combined with zircon U/Pb and geochemical data indicate that the Tethyan Himalaya had experienced at least four major stages of mafic magmatism at the Late Permian(～270 Ma), the Early Jurassic (～190-200 Ma), the Early Cretaceous (～135 Ma), and the Late Cretaceous(～70 Ma), respectively, and each of them is represented by mafic rocks with distinct geochemical characteristics. Implying the break-up of the northern margin of the Gondwana supercontinent and initial opening of the Neo-Tethyan Ocean, the late Permian mafic magmatism produced large-scale continental flood basalts (Panjal Traps) as well as variable sizes of diabases which intruded into pre-Permian sedimentary sequences. These rocks are characterized by enrichment in LREE, depletion in HREE, relatively high initial Sr (87Sr/86Sr(t)<0.7040 and up to 0.7078), Nd(εNd(t)>-2.4) and zircon Hf (εHf(t)>+2.5) isotope compositions. The newly identified Early Jurassic(～195 Ma, LA-ICP-MS zircon U/Pb ages) mafic rock suite occurs as large diabase plutons or sills within the Late Triassic shale and fine-grained sandstone. They formed at similar ages to those gabbros and granites in the southern margin and the central part of the Gangdese batholith. More primitive ones (MgO>8.0 wt%)from this suite are characterized by MORE-like REE distribution pattern and isotope (Sr and Nd) compositions, relatively high contents of mantle-compatible elements (e.g., Cr, Ni and Co), which indicates that they were derived from partial melting of depleted mantle during the spreading of the Neo-Tethyan ocean.The Early Cretaceous suite is more widespread and can be traced from the east of Yardoigneiss dome to the west of Dingri, and occur as diabase dikes or sills intruded into various formations of pre-Cretaceous sedimentary sequence. Some of them were involved in the intensive collision-related compression and shearing, and consequently formed large budding or recumbent folds. They are enriched in LREE but depleted in HREE, and contain variable mantle-compatible element ranging from MORE-like to continental basalts, consistent with derivatives from continental breakup of the India-Australia-Antarctic in the Early Cretaceous. In contrast, the Late Cretaceous(69.2±0.9 Ma, SHRIMP zircon U/Pb age) mafic rocks,previous considered as the Early Cretaceous, intruded into the Early Cretaceous Jiabula Formation of shale,sandstone, and carbonate. They display geochemical features (e.g., REE and mantle-compatible elements)between typical MORB and typical OIB, which suggests that they might result from the partial melting of the Indian continental mantle trigged by the break-off and detachment of subducted Neo-Tethyan oceanic slab during the incipient stage of Indian-Eurasian continental collision. Our new observations combined with available literature data demonstrate that mafic magmatism with distinct geochemical nature had accompanied the whole life-time, including birth, growth, and final demise,of the Neo-Tethyan Ocean. How to relate these mafic rocks of the Tethyan Himalaya to those of the “Yarlung Tsangpo Suture” is an open question that requires further investigation.

Multiple Mafic Magmatic Events along the Tethyan Himalaya:Tracing the Life-time of the Neo-Tethyan Ocean

ZENG Ling-sen1), GAO Li-e1), HE Ke-jun2), TANG Suo-han1), GUO Chun-li2)
1).State Key Laboratory of Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing100037,China;
2)Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing100037,China

Tethyan Himalaya; Neo-Tethys; India-Eurasia collision; Mafic Magmatism

10.3975/cagsb.2012.s1.32