Seminar

Adakites have distinctive geochemical characteristics and are generally considered as partial melting product of basaltic sources at high pressure conditions (above 1.5 GPa; > 50 km depth). Hence, the magma source of adakites with continental origin are commonly attributed to thickened lower crust. However, the compositions of primary adakitic magmas may be controlled by both of source materials and partial melting conditions, which have been underestimated in the previous studies. Here we compiled all published data of the adakites with continental origin that were strictly restricted to low Mg^#, Cr, Co and Ni concentrations from the Central Asian (CA) and Lhasa Terranes (LT), which represent typical accretionary-type and collisional-type orogenic belt, respectively. Both the CA and LT continental adakites exhibit high La/Yb and Sr/Y ratios, but low Nb/Ta ratios (12-13), which cannot be interpreted as a result of melting at rutile-bearing eclogite facies or high-pressure rutile differentiation. The Sr-Nd isotope compositions led to a general consensus that rocks chemically equivalent to the exposed Linzizong mafic volcanic successions are dominate source materials for the LT adakites. Similarly, the arc-like basalts are likely sources for the CA adakites. Using phase equilibrium calculations and trace-element modeling, we argue that the adakites from the CA and LT likely originated from fluids-fluxed lower crust melting by consuming plagioclase at normal crustal thickness (garnet amphibole facies without rutile), rather than thickened lower crust. Our study emphasized the importance of the source compositions and fluid contents in making the chemical signatures of adakites, possibly as well as Archean TTGs.

For Additional information: Mr. WANG Xiangsong, u3006391@hku.hk