The Pb-isotope geochemistry of granitoids from the Himalaya-Tibet collision zone: implications for crustal evolution

Details The Pb-isotope geochemistry of granitoids from the Himalaya-Tibet collision zone: implications for crustal evolution The Pb-isotope geochemistry of granitoids from the Himalaya-Tibet collision zone: implications for crustal evolution

Jul 1985

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1985e%26psl..74..220g&link_type=abstract

Earth and Planetary Science Letters, Volume 74, Issue 2-3, p. 220-234.

Computer Science

20

Scientific paper

Pb-isotopic data on 80 K-feldspars from six individual granitoid belts of the Himalaya-Tibet collision zone are reported. The isotopic trends observed in the Trans-Himalayan complex, which outcrops along the Indus-Zangpo suture zone, are best explained by a mixture of mantle-derived Pb component(s) with crustal-derived Pb; these confirm that the batholith was built upon sialic continental crust as a result of the northward subduction of Mesozoic oceanic crust beneath Southern Tibet. The Pb-isotopic compositions of K-feldspars from the three granitoid belts located within the Lhasa block, to the north of the Indus-Zangpo suture zone, are all radiogenic and indistinguishable from those of K-feldspars extracted from basement rocks. No pristine, mantle-derived, component was identified and the granitoids are interpreted as being wholly derived from the partial melting of the continental crust. Furthermore, the isotopic compositions of the granitoids are remarkably similar over the entire Lhasa block, implying that they were equilibrated by fluid remobilization during granulite-facies metamorphism in the lower crust.
To the south of the Indus-Zangpo suture zone and within the Indian Shield, the lead isotopic composition of K-feldspars from two granitoid belts, although significantly more radiogenic than those of the Lhasa block, are identical to that of the basement gneisses indicating also that the granitic melts are the products of the anatexis of the continental crust. In both the Lhasa block and the Indian Shield, the generation of granitic melts occurred as a consequence of large-scale intracontinental thrusting events that created crustal repetition and induced the development of thermal anomalies at depths due to the burial of meta-sedimentary rocks rich in water and radioactive heat-producing elements. The Lhasa block and the Indian Shield define two distinct Pb-isotopic provinces which have undergone complex episodes of U-Th-Pb fractionation and are largely composed of Precambrian material. Furthermore, the correspondence between distinct Pb-isotopic provinces and discrete continental segments might indicate that the blocks originated from different sectors of the Gondwanian continent.
Present address: Institute of Geology, Academia Sinica, Postbox 634, Beijing, People's Republic of China.

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