Late Eocene Two-Pyroxene Trachydacites from the Southern Qiangtang Terrane, Central Tibetan Plateau: High-Temperature Melting of Overthickened and Dehydrated Lower Crust

Abstract

AbstractOrthopyroxene-bearing granitic rock (e.g. charnockite) is relatively rare but provides an excellent opportunity to probe the thermal and tectonic evolution of deep orogenic crust because of its distinct mineral assemblage. Here we present petrological, mineralogical, elemental, and Sr–Nd–Hf–O isotopic data for late Eocene (ca. 36 Ma; zircon U–Pb ages) volcanic rocks exposed in the Ejiu region in the southern Qiangtang Terrane to investigate how the central Tibetan crust evolved to its modern thickness and thermal state. The Ejiu volcanic rocks (EVRs) are trachydacites with anhydrous mineral assemblages (i.e. two pyroxenes, sanidine, plagioclase, and ilmenite, without amphibole and biotite) and geochemical characteristics (e.g. high P2O5 and TiO2) that resemble those of charnockite-type magmatic rocks. Mineral and whole-rock thermometry and hygrometry suggests that the parent magma crystallized under hot (~1000°C) and dry (H2O < 2 wt.%) condition. Besides, the EVRs display adakitic affinities according to their high SiO2 and Al2O3 contents, high Sr/Y, La/Yb, and Gd/Yb ratios, and low Y and Yb contents, without marked negative Eu anomalies. The calculated melts in equilibrium with pyroxenes also display adakitic compositions (e.g. high Sr/Y and La/Yb ratios), indicating that the adakitic compositions of the EVRs did not result from late-stage magmatic evolution. In addition, the melts of the EVRs were saturated in TiO2, as inferred from the high TiO2 contents of these rocks and the presence of ilmenite. An integrated analysis of the geochemical, petrological, and mineralogical data suggests that the EVRs were neither evolutional products nor partial melts of hydrous mafic materials at normal crustal pressures, but were formed by fusion of an eclogitized mafic protolith with residue containing garnet and rutile but lacking amphibole and plagioclase. The whole-rock Sr–Nd and zircon Hf isotope compositions of the EVRs [(87Sr/86Sr)i = 0.7053 to 0.7066; εNd(t) = −1.40 to −0.99; zircon εHf(t) = +1.08 to +5.31] indicate that the parental protolith was relatively juvenile in nature, but also contained some supracrustal materials given the high zircon δ18O values [zircon δ18O = +8.21‰ to +11.00‰]. The above arguments lead us to propose that of partial melting of a previously dehydrated—but chemically undepleted—mafic lower continental crust at high pressure (>1.5 GPa) and high temperature (>1000°C) generated the EVRs. Based on a synthesis of independent geological and geophysical data, we further suggest that the southern Qiangtang Terrane crust of the central Tibetan Plateau was thick, dry, and elevated during the Late Cretaceous to early Eocene time, and that it became abnormally hot owing to the ascending asthenosphere after lithospheric foundering during the middle Eocene.