Contrasting Sr[sbnd]Nd[sbnd]Hf isotope systematics in Early Jurassic magmatic rocks from the Lhasa terrane: Implications for the lithospheric architecture of the Lhasa terrane

Abstract

Early Jurassic plutonic rocks from the Central Lhasa block that range from gabbro to granite yield similar 206Pb/238U ages ranging from 187.6 Ma to 196.0 Ma. Gabbros and diorites are characterized by similar radiogenic 87Sr/86Sr(t) ratios that range from 0.7053 to 0.7060, unradiogenic εNd(t) values ranging from −4.2 to −2.5 and εHf(t) values ranging from −8.8 to −2.8. Compared with the intermediate to mafic rocks, the granites have much higher 87Sr/86Sr(t) ratios, from 0.7102 to 0.7127, and lower εNd(t) values, from −8.7 to −7.5, and εHf(t) values from −11.9 to −7.5. Elemental and isotopic data suggest that the gabbros were derived from a melt-modified subarc mantle, diorites represent derivatives produced by hornblende fractional crystallization of gabbros, and granites were generated by the melting of ancient crustal basement rocks. Combined with literature data, these results suggest that the Central Lhasa block, as well as parts of the Southern and Northern Lhasa blocks, also contains ancient crystalline basement rocks.